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Chemical Synthesis of Oligosaccharides related to the Cell Walls of Plants and Algae
Kinnaert, Christine; Daugaard, Mathilde; Nami, Faranak; Clausen, Mads Hartvig
Published in:Chemical Reviews
Link to article, DOI:10.1021/acs.chemrev.7b00162
Publication date:2017
Document VersionPeer reviewed version
Link back to DTU Orbit
Citation (APA):Kinnaert, C., Daugaard, M., Nami, F., & Clausen, M. H. (2017). Chemical Synthesis of Oligosaccharides relatedto the Cell Walls of Plants and Algae. Chemical Reviews, 117(17), 11337-11405.https://doi.org/10.1021/acs.chemrev.7b00162
1
Chemical Synthesis of Oligosaccharides related to the Cell Walls of Plants and
Algae
Christine Kinnaert, Mathilde Daugaard, Faranak Nami, Mads H. Clausen*
Center for Nanomedicine and Theranostics, Department of Chemistry,
Technical University of Denmark, Kemitorvet, Building 207, 2800 Kgs. Lyngby, Denmark
Abstract
Plant cell walls are composed of an intricate network of polysaccharides and proteins that varies
during the developmental stages of the cell. This makes it very challenging to address the functions
of individual wall components in cells, especially for highly complex glycans. Fortunately,
structurally defined oligosaccharides can be used as models for the glycans, to study processes such
as cell wall biosynthesis, polysaccharide deposition, protein-carbohydrate interactions, and cell-cell
adhesion. Synthetic chemists have focused on preparing such model compounds, as they can be
produced in good quantities and with high purity. This review contains an overview of those plant and
algal polysaccharides, which have been elucidated to date. The majority of the content is devoted to
detailed summaries of the chemical syntheses of oligosaccharide fragments of cellulose,
hemicellulose, pectin, and arabinogalactans, as well as glycans unique to algae. Representative
synthetic routes within each class are discussed in detail and the progress in carbohydrate chemistry
over recent decades is highlighted.
2
Contents
Introduction ........................................................................................................................... 3 Architecture and composition of plant cell walls ............................................................... 6
2.1 Polysaccharides in plant and algal cell walls ................................................ 8 2.2 The study of cell wall composition, function and evolution ....................... 10
Cellulose ............................................................................................................................... 12 Hemicellulose ....................................................................................................................... 18
4.1 Xyloglucans ................................................................................................. 20 4.2 β-(1→3)-D-Glucans ..................................................................................... 28 4.3 Mixed-linkage [β-(1→3), β-(1→4)] glucans .............................................. 35 4.4 Xylans (arabinoxylans, glucuronoxylans and glucurono-arabinoxylans) ... 38 4.5 Mannans ...................................................................................................... 47
Pectin .................................................................................................................................... 56 5.1 Homogalacturonan ...................................................................................... 59 5.2 Xylogalacturonan ........................................................................................ 66 5.3 Apiogalacturonan ........................................................................................ 66 5.4 Rhamnogalacturonan I ................................................................................ 70
RG-I backbone ............................................................................................................... 71 RG-I side chains ............................................................................................................. 80
5.5 Rhamnogalacturonan II ............................................................................... 81 RG-II side chain A ......................................................................................................... 83 RG-II side chain B ......................................................................................................... 89 RG-II side chain C ......................................................................................................... 94 RG-II side chain D ......................................................................................................... 95 RG-II side chain E/F ...................................................................................................... 95
(Arabino)galactans and arabinans .................................................................................... 96 6.1 Type I AGs and galactans ........................................................................... 97 6.2 Type II AGs ............................................................................................... 107 6.3 Arabinans .................................................................................................. 119
Polysaccharides exclusive to algae ................................................................................... 124 7.1 Glucuronans .............................................................................................. 124 7.2 Carrageenans ............................................................................................. 125 7.3 Agarans ...................................................................................................... 127 7.4 Porphyrans ................................................................................................. 127 7.5 Alginates .................................................................................................... 128 7.6 Fucans ........................................................................................................ 137 7.7 Ulvans ........................................................................................................ 150
Conclusion .......................................................................................................................... 151 References ................................................................................................................................ 154 TOC Graphic ........................................................................................................................... 183
3
Introduction Plant cell walls are highly sophisticated fiber composite structures. They are dynamic systems, which
have evolved to fulfill the wide range of biological functions necessary to support plant life. The wall
is the outer layer of the cell, providing a strong and protective supporting boundary, and defining the
size and shape of the cell. Beyond its structural function, the cell wall also plays an important role in
cell expansion during plant growth, and in plant-microbe interactions, in particular the defense
response against pathogens.1,2 The cell wall is not just a boundary between plant cells, but forms the
interface to neighboring cells, contributing to intercellular communication.
Plant cell walls are highly heterogeneous, with a wide range of constituent components. Table 1
compares the monosaccharides found in mammalian cells with the ones occurring in plant and/or algal
cell wall, together with the symbols used here and in the CFG nomenclature. The diversity can make
it challenging to distinguish the function of individual components and how they interact. Researchers
have therefore used well-defined oligosaccharides as model compounds to represent domains from
the larger, more complex polysaccharides. Many groups have succeeded in synthesizing such
oligosaccharides using different strategies. This review will introduce the glycans found in plant cell
walls and describe all relevant chemical oligosaccharide synthesis, up to and including February 2017.
The material is organized in tables showing the structures according to glycan nomenclature given in
Figure 1. With the exception of some rare disaccharide combinations found in pectin, we present only
trisaccharides and longer structures, and only oligosaccharides that have been deprotected
successfully. Representative syntheses are discussed in detail, in order to include different approaches
to plant cell wall glycan assembly. Some of the syntheses described here have been covered in
4
previous reviews; we are endeavoring to provide a comprehensive overview of the field, and therefore
have chosen not to exclude these.
Table 1. Comparison between mammalian monosaccharides and plant/ algae cell wall monosaccharides.
Monosaccharide CFG symbol (existing or suggested)
Symbol used here Plant/algal cell wall glycans
Mammalian glycans
Glucose
D D
Galactose
D and L D
N-Acetyl-glucosamine
D
N-Acetyl-galactosamine
D
Mannose
D D
Galacturonic acid
D D
Xylose
D D
Fucose
L L
Sialic acid
D
Iduronic acid
L L
Rhamnose
L
Gulose
L
Arabinose
L
5
Apiose
D
3-Deoxy-lyxo-heptulosaric acid
(Dha)
D
2-Keto-3-deoxy- manno-
octulosonic acid (Kdo)
D
Aceric acid
Me
L
L-gulose
D-glucose
D-xylose
D-mannose
D-galactose L-arabinose
L-rhamnose OAc
COOH
SO3H
β-notation
α-notation
= O1
23
46
L-fucoseCOOMe
D-apiose
L-aceric acid less common enantiomer (D/L)
D-3,6-anhydrogalactose
D-2-Keto-3-deoxy-D-manno-octulosonic acid (Kdo)
D-3-deoxy-D-lyxo-heptulosaric acid (Dha)
OMe
Example:
=
OCOOHHO
HOHO
OMeO
OHO
OOH
HO OH
O
= α-D-GalpA-(1→2)-[α-L-Araf-(1→4)]-β
-L-RhapOMe
L-iduronic acid
Me
Figure 1. Glycan notation.
6
Architecture and composition of plant cell walls As a simple model, plant cell walls can be grouped in two types: the primary and the secondary cell
wall. The primary cell wall is a thin and flexible structure deposited when the cell is growing, while
the secondary cell wall, much thicker and stronger, is deposited on certain cells when they have ceased
to grow. Primary cell walls are typically composed of around 70% water. The dry material is made up
of polysaccharides (approximately 90%) and (glyco)proteins (~ 10%). In the primary cell wall,
cellulose microfibrils are cross-linked by non-cellulosic polysaccharides (often referred to as
hemicellulose), and these are intertwined with pectic polysaccharides. The chemical composition of
the main polysaccharides found in land plants is summarized in Table 2.
There is strong evidence that hemicellulose, mostly xyloglucans, bind to the cellulose microfibrils via
two different mechanisms: being entwined in the microfibril during crystallization, which occurs
immediately after synthesis; and by hydrogen bonding between the glycan chains.3–5 The
polysaccharide network in the cell wall helps the plant resist turgor pressure and other deformations.
Less is known about the interaction of pectic polysaccharides with the other wall components, but
there is evidence that neutral glycans side chains also interact with cellulose, a factor that is usually
not accounted for in models describing plant cell wall architecture.6–8 Pectic polysaccharides are
usually divided into four different domains (HG, RG-I, RG-II, and XGA / AGA, see Table 1), which
are believed to be covalently linked to form the pectin macromolecule. Evidence suggests that some
pectic polysaccharides are also crosslinked to hemicelluloses, phenolic compounds and wall proteins,
which provides an even higher degree of complexity in the structure and function of the wall.5,9 A
2009 paper detailing the structure, function and biosynthesis of plant cell wall pectic polysaccharides
by Mohnen & Caffall provides a good overview.6
7
Table 2. Cellulose, hemicellulose and pectin structures. Cellulose β-(1→4)-Glup Hemicellulose Xyloglucans (XyG) β-
(1→4)-Glup with mainly Xylp branching
Glucans β-(1→3)-Glup with β-(1→6) branching
Mixed-linkage glucans (MLGs) β-(1→3)-β-(1→4)-Glup
Xylans β-(1→4)-Xylp (potentially with Araf and GlupA branching)
Mannans β-(1→4)-Manp and β-(1→4)-Glup/Manp with Galp branching
Pectin Homogalacturonan (HG) α-(1→4)-GalpA
Rhamnogalacturonan I (RG-I) α-(1→4)-GalpA-α-(1→2)-Rhap backbone with side chains of Galp and Araf
Rhamnogalacturonan II (RG-II) α-(1→4)-GalpA with conserved side chains of mainly rare sugars
Xylogalacturonan (XGA) α-(1→4)-GalpA with Xylp branching
Apiogalacturonan (AGA) α-(1→4)-GalpA with Apif branching
In addition to polysaccharides, the primary cell wall also contains about 10% protein, which is mostly
glycosylated. Arabinogalactan proteins (AGPs) are some of the most abundant and most studied
glycoproteins in plant cell walls and they are thought to be involved in recognition and signaling
events at the cell surface.10 However, as the focus of this review is on polysaccharides, we will not
describe the plant cell wall proteome. We invite the reader to look at some comprehensive reviews
covering this subject.11,12
In secondary cell walls, the embedding material is not pectin, but rather the phenolic polymer lignin,
which constitutes up to 30% of the dry weight of the wall.13 The secondary cell wall also contains
cellulose and a network of connecting hemicellulosic polysaccharides with a lower degree of
backbone branching (mainly xylan and mannans). The deposition of lignin results in dehydration of
the wall, rendering it more hydrophobic and less dynamic. Other components such as the aliphatic
polyesters cutin and suberin can also be found in the plant cell wall as minor components.14 Plant cell
walls display a considerable degree of diversity in their composition and molecular architecture
depending on plant species but also related to cell type and cell wall microstructure.15 An interesting
8
review of the heterogeneity in the chemistry, structure and function of plant cell walls has been
published by Fincher and co-workers recently.15
2.1 Polysaccharides in plant and algal cell walls
The primary cell walls of seed plants and especially angiosperms (flowering plants) are by far the
most studied. However, since land plants have algal ancestors, the cell walls of algae can be interesting
from an evolutionary point of view. Land plants originate from freshwater green algae of the
Charophytes class, which emerged from their aquatic habitat approximately 450 million years
ago.16,17,18 As shown in the phylogenetic tree of eukaryotic organisms (Figure 2), land plants
(Embryophytes), red algae (Rhodophytes) and green algae (Chlorophytes) all belong to the
Archaeplastida clade, while brown algae (Phaeophytes) belong to the separate Chromalveolata clade.
The phylogenetics can explain why the cell walls of brown algae share polysaccharides with both
plants (cellulose) and animals (sulfated fucans), but also with some bacteria (alginates).19
9
Figure 2. Phylogenetic tree of eukaryotic organisms.
The nature of the different cell wall components from the major plant and algal families has been the
topic of several reviews and is summarized in Table 3.16,20,21 The unique presence of different sulfated
polysaccharides in the Rhodophytes and Phaeophytes is noteworthy. Agar, carrageenan and porphyran
are the most important sulfated polysaccharides found in the red algae, while brown algae produce
homofucans. The structure of these polysaccharides will be described in detail later in this review.
10
Table 3. Major cell wall polymers present in different plant and algal taxa.22
Taxa Polymer
Chloroplastida Rhodophytes
(red algae)
Phaeophytes
(brown algae) Embryophytes (land plants)
Charophytes (green algae)
Chlorophytes (green algae)
Crystalline polysaccharides
cellulose cellulose cellulose
cellulose mannan xylan
(1→3)-β-D-xylan
cellulose
Hemicellulose
xyloglucan mannans xylans MLG
glucan (callose)
xyloglucan mannans xylans
glucan (callose)
xyloglucan mannans
glucuronan glucan (callose)
mannan sulfated MLG
(1→3), (1→4)-β-D-xylan
sulfated xylofucoglucan
sulfated xylofuco- glucuronan
glucan (laminarin) Matrix
carboxylic polysaccharides
pectins pectins ulvans alginates
Matrix sulfated
polysaccharides ulvans
agars carrageenans
porphyran fucans
2.2 The study of cell wall composition, function and evolution
The study of the plant cell wall requires a multi-disciplinary approach and there are many challenges
in trying to elucidate cell wall architecture, function, and evolution. The field is being advanced by
technologies such as atomic force microscopy, immunofluorescence microscopy and the development
of molecular probe sets.23 In a 2012 review, Willats and co-workers discussed a multiple-level strategy
for correlating the occurrence of cell wall polysaccharides with genetic diversity in order to get an
insight into underlying evolutionary mechanisms.24 Many of the most efficient techniques employ
molecular probes that interact specifically with glycans, such as monoclonal antibodies (mAbs) and
carbohydrate-binding modules (CBMs). Well-defined oligosaccharides provide the highest resolution
possible when characterizing protein-carbohydrate interactions, and they contribute significantly to
11
the toolkit available for plant research. Figure 3 shows an example of in situ mapping of cellulose,
rhamnogalacturonan I and methyl esterified homogalacturonan in seed coat epidermal cells and
mucilage in Arapidopsis thaliana.
Figure 3. Labelling of polysaccharidic components of seed coat epidermal cells and mucilage in Arabidopsis thaliana. (A) Composite image of a section through whole seed and mucilage. Scale bar = 100 µm. (B) Magnification of region in (A). Scale bar = 50 µm. Blue: calcofluor-labeled cellulose; red: INRA-RU1-immunolabelled rhamnogalacturonan; green: JIM7-immunolabelled highly methyl esterified homogalacturonan.23 Images used with permission from Adeline Berger.
A B
12
Cellulose
Figure 4. Structure of cellulose.
Cellulose is the main component of the primary cell wall in plants. It is the most abundant organic
compound on earth and consists of an unbranched homopolymer of β-(1→4)-glucan units (Figure 4).
The biopolymer is currently used in a wide variety of applications including the production of biofuels,
paper products, and textile fibers.25 Cellulosic chains are held together tightly by both intra- and
intermolecular hydrogen bonds facilitated by the conformation of the polymer. The chain length
depends on the source of the cellulose; the degree of polymerization (DP) is believed to reach up to
8,000 glucose residues in primary cell walls and 15,000 in secondary cell walls. This makes cellulose
one of the longest bio-macromolecules known.
Table 4 shows all chemically synthesized β-(1→4)-glucans with at least three glucose residues. It is
worth noting that the majority of the reports on the preparation of β-(1→4)-glucans detail enzymatic
procedures.
Table 4. The cellulose fragments synthesized chemically.
Structure R Year Reference
OR
Me
H
(CH2)3(CH)2O
1981
1983
1983
1990
Takeo et al.26
Takeo et al.27
Takeo et al.27
Rodriguez and Stick28
13
CH2CH2C10H7
(CH2)5NHCOO(CH2)3C8F17
(CH2)5NH2
1999
2010
2016
Xu and Vasella29
Chang et al.30
Dallabernardina et al.31
OR
Me
(S)-CH-CH3CH2OH
(R)-CH-CH3CH2OH
(S)-CH2CH-OHCH3
(R)-CH2CH-OHCH3
1983
1983
1986
1983
1986
1983
1985
1986
1983
1985
1986
Takeo et al.27
Sugawara et al.32
Sugawara et al.33
Sugawara et al.32
Sugawara et al.34
Sugawara et al.32
Nicolaou et al.33
Sugawara et al.34
Sugawara et al32
Nicolaou et al.33
Sugawara et al.34
OR
H
(CH2)5NHCOO(CH2)3C8F17
CH2CH2C10H7
1983
1996
2010
1999
Takeo et al.27
Nishimura and Nakatsubo35
Chang et al.30
Xu and Vasella29
OR (S)-CH2CH-OHCH3
(R)-CH2CH-OHCH3
1985
1985
Nicolaou et al.33
Nicolaou et al.33
OR2
(CH2)5NH2 2016 Dallabernardina et al.31
14
OR2
(S)-CH2CH-OHCH3
(R)-CH2CH-OHCH3
1985
1985
Nicolaou et al.33
Nicolaou et al.33
OR5
H 1996
1999
Nishimura and Nakatsubo35
Xu and Vasella29
In 1981 Takeo and co-workers reported the synthesis of β-methylcellotrioside using a Koenigs-Knorr
glycosylation36 between building blocks 1 and 2 (Scheme 1).26 This produced the trisaccharide in 39%
yield alongside unreacted 1 and 2. The trisaccharide was converted into the bromide 4 before
methanolysis in the presence of mercuric cyanide, which gave 84% of 5. Zemplén deacetylation37 then
resulted in the targeted cellotrioside 6 in 92% yield.
15
Scheme 1. Synthesis of β-methyl cellotrioside reported by Takeo and co-workers in 1981.26
AcO OOAc
AcOAcO
O OOAc
AcOAcO
Br
HO OOAc
AcOAcO OAc
1 2
+
O
AcOAcO O O
OAc
AcOAcO
O OOAc
AcOAcO
AcOOAc
OAc
3
O
AcOAcO O O
OAc
AcOAcO
O OOAc
AcOAcO
AcOOAc
Br4
O
AcOAcO O O
OAc
AcOAcO
O OOAc
AcOAcO
AcOOAc
OCH3
5
O
OHHO O O
OH
HOOH
O OOH
OHHO
HOOH
OCH3
6
AgOTf(CH3)2NCON(CH3)2
CH2Cl2,
39%
HBr in AcOH
84%
Hg(CN)2
CH3OH:C6H6 1:10
84%
NaOMe
MeOH
92%
Nishimura and Nakatsubo published the first synthesis of cellooctaose using a convergent synthetic
strategy in 1996.35 The synthesis is depicted in Scheme 2. The tetrasaccharide 8 (derived from allyl
3-O-benzyl-4-O-(p-methoxybenzyl)-2,6-di-O-pivaloyl-β-D-glucopyranoside 7 using a linear
synthetic method described by Nishimura et al.38) was used as a common precursor for the
tetrasaccharide acceptor 9 and Schmidt39 donor 11. These two were coupled under standard conditions
(BF3⋅OEt2) giving the octasaccharide 12 in 95% yield. The glycosylation was conducted in a
high-vacuum system allowing for better control of the anhydrous reaction conditions. Protecting
group manipulations afforded the fully protected oligosaccharide 14. The allyl group was removed
using a one-pot procedure first reported by Kariyone and Yazawa40 before the anomeric position was
acetylated. Catalytic hydrogenolysis followed by acetylation yielded the fully acetylated
16
octasaccharide 18. After purification, deacetylation afforded cellooctaose 19 in 87% yield. In a similar
sequence, tetrasaccharide 8 was converted into cellotetraose 25 (see Scheme 2). More recently,
Pfrengle and co-workers produced linear β-(1→4)-glucans using solid-phase oligosaccharide
synthesis.31 This work was part of a larger study of xyloglucan synthesis and will be described in
section 4.1.
17
Scheme 2. Synthesis of cellotetraose and cellooctaose published by Nishimura and Nakatsubo.35
O
PivOBnO O O
OPiv
BnOPivO
O OOPiv
PivOBnO
OOPiv
OAllAcO OOPiv
BnOPivO
PMBO OOPiv
PivOBnO OAll
O
PivOBnO O O
OPiv
BnOPivO
O OOPiv
PivOBnO
OOPiv
OAllHO OOPiv
BnOPivO
O
PivOBnO O O
OPiv
BnOPivO
O OOPiv
PivOBnO
OOPiv
ORAcO O
OPiv
BnOPivO
10: R = H11: R = C(NH)CCl3
9
7 8
O
ROBnO O O
OR
BnORO
O OOR
ROBnO
OOR
OAllRO OOR
BnORO
O
AcOR2O O O
OAc
R2OAcO
O OOAc
AcOR2O
OOAc
OR1AcO O
OAc
R2OAcO
O
OHHO O O
OH
HOOH
O OOH
OHHO
OOH
OHHO O
OH
HOOH
25
O
PivOBnO O O
OPiv
BnOPivO
O OOPiv
PivOBnO
OOPiv
OAllAcO OOPiv
BnOPivO
12
O
ROBnO O O
OR
BnORO
O OOR
ROBnO
OOR
OAllRO OOR
BnORO
13: R = H14: R = Ac
O
AcOR2O O O
OAc
R2OAcO
O OOAc
AcOR2O
OOAc
OR1AcO O
OAc
R2OAcO
15: R1 = H, R2 = Bn16: R1 = Ac, R2 = Bn17: R1 = Ac, R2 = H18: R1 = R2 = Ac
3
3
3
O
OHHO O O
OH
HOOH
O OOH
OHHO
OOH
OHO O
OH
HOOH
19
O
OHHO O O
OH
HOOH
O OOH
OHHO
OOH
HO OOH
HOOH
SeO2, AcOH, dioxane
79%
DBU, CCl3CN, CH2Cl2, 96%
DBU, MeOH
65%
BF3·OEt2,
CH2Cl2
95%
NaOMeMeOH
78%
Ac2O, py, 50 oC, quant.
SeO2AcOH, dioxane
77%
Ac2O, py, quant.Pd(OH)2/C, H2
, THF
Ac2O, py, 64%
DBUMeOH:CH2Cl2
1:4
87%
NaOMeMeOH
quant.
20: R = H21: R = Ac
Ac2O, py, 82%
SeO2AcOH, dioxane
54%
22: R1 = H, R2 = Bn23: R1 = R2 = H24: R1 = R2 = Ac
Pd/C, H2, EtOH:AcOH 4:1
92%Ac2O, py, 72%
DBUMeOH:CH2Cl2
1:4
75%
18
Hemicellulose Hemicellulose was first described in 1891 by Ernst Schulze, as the components of the cell wall that
are easily dissolved in hot diluted mineral acids.41 Such a definition based on extractability was not,
however, found to be useful, and these days the term is commonly used to refer to a group of non-
starch, non-pectic polysaccharides found in association with cellulose in the cell walls of higher plants.
Based on current knowledge, hemicellulose is generally divided into four classes of structurally
different cell-wall polysaccharides: xylans, mannans, xyloglucans and glucans including mixed
linkage glucans (β-(1→3),(1→4)-glucans, MLGs) as shown in Figure 5.42 These are the types of
polysaccharides described in most reviews about hemicellulose, but Peter Ulvskov and Henrik
Scheller, in their review of hemicellulose, further narrow down the definition to glycans sharing a β-
(1→4)-linked backbone with an equatorial configuration at the C-4 position. This excludes MLGs,
callose and laminarin (β-(1→3)-linked glucans) from the hemicelluloses.4,42 Other polysaccharides,
such as galactans, arabinans, and arabinogalactans are sometimes included in this group as well, but
these will be discussed separately in section 6.42
19
X X S
G L X F G
Xyloglucans β-1,3-Glucans
n = 0,1,2
Mixed Linkage Glucans (MLGs)
MeO MeO
Glucuronoxylan (GX)
MeO
Glucuronoarabinoxylan (GAX)
Arabinoxylan (AX) Galactomannans
Galactoglucomannans
Xylans Mannans
L-gulose
D-glucose
D-xylose
D-mannose
D-galactose L-arabinose
L-rhamnose OAc
COOH
SO3H
β-notation
α-notation
= O1
23
46
L-fucoseCOOMe
D-apiose
L-aceric acid less common enantiomer (D/L)
D-3,6-anhydrogalactose
D-2-Keto-3-deoxy-D-manno-octulosonic acid (Kdo)
D-3-deoxy-D-lyxo-heptulosaric acid (Dha)
L-iduronic acid
Me
Figure 5. Overview of the different families of hemicellulose polysaccharides.
20
4.1 Xyloglucans
X X S
G L X F G
Figure 6. Structure of XyGs with the most common side chains.
Xyloglucans (XyGs) constitute the most abundant class of hemicellulose in the primary cell wall of
higher plants. They have been found in every species of land plant that has been analyzed, except
charophytes.4 An exemplary structure of XyG with the different side chains is shown in Figure 6.
However, this schematic representation does not indicate the frequency of each side chain and many
variations in this general pattern can be observed. XyGs have a backbone of β-D-(1→4)-glucans
substituted by α-D-(1→6)-linked xylose residues. A special one-letter code is used to label the
different XyG side chains. G denotes an unbranched Glcp residue, while X corresponds to an α-D-
Xylp-(1→6)-Glcp. The xylosyl residues can be further branched at O-2 with α-L-Araf (S) or β-Galp
(L), which can be further substituted at O-2 with α-L-Fucp (F). The galactose residues can be
acetylated. The branches dictate the physical properties of the molecules; the fewer the branches, the
less soluble the XyG. Only a few syntheses of XyGs with substituted xylose (side chains S, L or F)
have been reported to date, including an LG fragment produced by Jacquinet and co-workers43 and
an XXFG fragment synthesized by Ogawa and co-workers.44 In 2016, Pfrengle and co-workers
achieved the automated solid phase synthesis of a variety of XyG oligomers without xylose
substitution.31 Table 5 shows all the XyG fragments synthesized to date.
21
Table 5. The synthetic XyG fragments.
Structure R Year Reference
OR
(CH2)5NH2 2016 Dallabernardina et al.31
OR
Pr 1990 Wotovic et al.43
OR
(CH2)5NH2 2016 Dallabernardina et al.31
OR
(CH2)5NH2 2016 Dallabernardina et al.31
OR
H 1990 Sakai et al.44
OR
Me 2000 Watt et al.45
OR
H 1990 Sakai et al.44
OR
(CH2)5NH2 2016 Dallabernardina et al.31
OR
(CH2)5NH2 2016 Dallabernardina et al.31
22
In 1990, Jacquinet and co-workers synthesized tetrasaccharide LG (38) by condensation of the two
key disaccharides galactoxylose and cellobiose 33 and 36 respectively, as shown in Scheme 3. The
building blocks 33 and 36 were synthesized through standard protection group manipulations and
glycosylation reactions. Coupling of donor 33 and diol acceptor 36 afforded tetrasaccharide 37 in 50%
yield as a single regio- and stereoisomer. However, the authors showed that steric hindrance around
the O-6’ position of the acceptor had a dramatic impact on the regioselectivity as well as the yield of
the reaction. Indeed, when using a diol acceptor with benzyl protecting groups in lieu of acetyl groups,
only 10% of the undesired β-linked tetrasaccharide could be isolated, while none of the desired
stereoisomer was observed.
23
Scheme 3. Synthesis of LG tetrasaccharide by Jacquinet and co-workers.43
OAcO
OAllOAc
AcOO
HO
OAcO
AcOHO
O
OAc
BnO
BnO
OBn
O
ClBnO
BnOO
OAcO
OAllOAc
AcOO
O
OAcO
AcOHO
O
OAc
BnO
BnO
OBn
OBnOBnO
O
OHO
OPrOH
HOO
O
OOH
HOHO
O
OH
HO
HO
OH
OHOHO
O
OAcOAcO
OO
OEtMe
OBnOBnO
OO
OEtMe
OBnOBnO
OHOAll
OBnO
OO
OMeMe
BnO OBn
OBnO
AcOCl
BnO OBn
O
OAc
BnO
BnO
OBn
OOAllBnO
BnOO
1) 95% aq. AcOH2) Ac2O, py
3) BnNH2, Et2O4) (COCl)2
, DMF, CH2Cl2
57%
AgOTfClCH2CH2Cl
83%
AgOTfClCH2CH2Cl
50%
1) NaOMe, MeOH
2) Pd/C, H2
88%
OAcO
OAllOAc
AcO
OAcO
OAcO
AcOAcO O
HO
OAllOH
HO
OOO
OHHOO
Ph1) NaOMe, MeOH
2) benzaldehyde dimethylacetal DMF
75%
1) Ac2O, py
2) 60% aq. AcOH
85%
26 27 28
29 30
31 33
34 35 36
3633
37 38 (LG)
1) NaOMe, MeOH
2) NaH, BnBr, DMF
79%
1) 2,6-dimethylpyridinium perchlorate,ClCH2CH2Cl, AllBr, PhCl
2) NaOMe, MeOH
74%
3028 1) NaOMe, MeOH
2) KOtBu, DMSO
80%
O
OH
BnO
BnO
OBn
OOBnO
BnOO
32
1) Ac2O, pyridine2) HgCl2/HgO
acetone/water
3) (COCl)2
ClCH2CH2Cl, DMF
70%
The same year, Ogawa and co-workers44 reported the synthesis of both the heptasaccharide XXXG
(48) and the branched nonasaccharide XXFG (57) using a convergent approach as shown in Scheme
4. To reach the two targets, they used the key glycohexaosyl acceptor 46 (XXGG) as a common
precursor. This resulted from a coupling between fragment 43 (XX) and cellobiose acceptor 44 (GG).
Glycosylation with xylopyranose donor 40 and hexose acceptor 46 afforded the heptasaccharide 47
in 64% yield as a 4:3 ratio of separable α:β stereoisomers. Finally, saponification and hydrogenolysis
24
afforded the desired unprotected XXXG target 48. In the same manner, coupling between
trisaccharide donor 55 and 46 afforded the nonasacharide 56 as a 3:1 mixture of stereoisomers which,
after separation and deprotection, gave the XXFG xyloglucan oligosaccharide 57. Even though the
crucial glycosylation steps were plagued by low yields and poor stereoselectivity, both XyG targets
could be reached by way of a common advanced intermediate using this convergent strategy.
25
Scheme 4. Synthesis of XXXG and XXFG oligomers by Ogawa and co-workers.44
OHO
OBnOBn
BnO
OHO
OBnO
BnOBnO
OOR
ORRO
OO
ORO
RORO
O
OBn
BnOBnO SMe
O
RO
RORO
O
O
RO
RORO
41: R = Bn42: R = H
OO
OAcAcO
OO
OAcO
AcOAcO
O
AcO
AcOAcO
O
O
AcO
AcOAcO
CuBr2, Bu4NBr, HgBr2CH3NO2
62%
1) Ac2O, py2) NH2NH2·AcOH, DMF
3) CCl3CN, DBU, CH2Cl2
72%NH
CCl3
OBnO
OBnOBn
BnO
OPMBO
OBnO
BnOHO
39
40
43
43BF3·OEt2
52% OO
OAcAcO
OO
OAcO
AcOAcO
O
AcO
AcOAcO
O
O
AcO
AcOAcO
4445
R = PMB46
R = H
OBnO
OBnOBn
BnO
ORO
OBnO
BnO
40, CuBr2, Bu4NBr, HgBr2
CH3NO2
64% O
OOAc
AcO
OO
OAcO
AcOAcO
O
AcO
AcOAcO
O
O
AcO
AcOAcO
47
OBnO
OBnOBn
BnO
O
O
OBnO
BnO
O
AcO
AcOAcO
α:β 4:3 - 25% α isolated
1) NaOMe, MeOH
2) H2, Pd/C, MeOH
63%
OO
OHHO
OO
OOH
HOHO
O
HO
HOHO
O
O
HO
HOHO
48 (XXXG)
OHO
OHOH
HO
O
O
OOH
HO
O
HO
HOHO
O
OH
BnO OAllBnOO
OBnBnO
BnOAcOO NH
CCl3
OOBnBnO
BnORO
O
O
BnO OAllBnOO
OAcAcO
AcO
O
O
AcOAcO
51: R = Ac52: R = H
BF3·OEt2
73%NaOMe, MeOH
94%
MeOTf
81%
OOBnBnO
BnOO
O
O
BnO OAllBnOSMe
7 steps
31%
49 54 55
55
OO
OAcAcO
OO
OAcO
AcOAcO
O
AcO
AcOAcO
O
O
AcO
AcOAcO
OBnO
OBnOBn
BnO
OO
OBnO
BnO
CAN, CH3CN, H2O56%
OOAc
OAcOAc
O
OO
OAcAcO
1) NaOMe, MeOH
2) H2, Pd/C, MeOH, H2O
54%
46, CuBr2, Bu4NBr, HgBr2 Et2O 26%
56α:β 3:1 - 20% α isolated
57 (XXFG)
O SMeOBn
OBnBnO
Me
53
O OBnOBnBnO
Me O OAcOAcAcO
Me
O
O
OAc
AcOAcOMe
50
OO
OHHO
OO
OHO
HOHO
O
HO
HOHO
O
O
HO
HOHO
OHO
OHOH
HO
O
O
OHO
HO
OO
HOHO
OOH
HO OHO
O
OH
Me OHOH
26
More recently, Pfrengle and co-workers reported an automated glycan assembly of five XG
oligosaccharides using monosaccharide 58 and disaccharide 59 as building blocks (Scheme 5).31 This
solid phase synthesis also enabled the production of linear cellotriose and cellopentaose (see Table 4).
The authors used a Merrifield resin46 with a photocleavable linker47 and two phosphate glycosyl
donors48 with either Fmoc or a Lev groups as temporary O-4 protection. Using this strategy, the five
XyG oligomers (60–64) were produced in overall yields of 2–16%.
27
Scheme 5. Automated glycan assembly of five XyG oligosaccharides by Pfrengle and co-workers.31
HO
O
ON
O2N
O
OBnO
BnOOBz
OP
OBuOBu
O
FmocOO
O
BnOOBz
OP
OBuOBu
O
LevO
OBnO
OBnO
BnO
OR2O
BnOOBn
OHO
n
R2 = Bn, α-xylopyranosyl
OR2O
BnOOBz
OR1O
n
R1 = Fmoc, LevR2 = Bn, α-xylopyranosyl
OHO
HOOH
HO O OO
HOOH
O OO
HOOH
O OHO
HOOH
O
5859
Start
1a) 1 or 2 × 3.7 equiv. 58,
TMSOTf, CH2Cl2 or1b)
2 or 3 ×
3.7 equiv. 59,
TMSOTf, CH2Cl2
2a) 3 × 20% Et3N in DMF or
2b) N2H4·HOAc py, AcOH, H2O
Cleavage and global deprotection3) hν, CH2Cl24) NaOMe, MeOH, THF5) Pd/C, H2, EtOAc, MeOH, H2O, HOAc
OHO
HO
HOO
HOHO
HO
NH23
OHO
HOOH
HO O OO
HOOH
O OHO
HOOH
O NH23
OHO
HO
HO
OO
HOOH
O O OO
HOOH
O OHO
HOOH
O NH23
OHO
HOOH
HO O OO
HOOH
OHO
HO
HOO
HOHO
HOO
HOHO
HO
OHO
HOOH
O O OO
HOOH
O OHO
HOOH
O NH23
OHO
HOOH
HO O OO
HOOH
OHO
HO
HOO
HOHO
HOO
O
HOOH
O O OHO
HOOH
O OHO
HOOH
O NH23
HO OO
HOOH
OHO
HO
HOO
HOHO
HO
60: 15% (GXG)
61: 16% (GXXG) 62: 2% (GXXXG)
63: 10% (GXGXG) 64: 9% (XXGG)
HO
Merrifieldresin
28
4.2 β-(1→3)-D-Glucans
Figure 7. Structure of β-(1→3)-D-glucan (callose).
β-(1→3)-D-Glucans with various short β-(1→3)-D-glucan chains branched at the 6-position of the
backbone are found in the cell walls of land plants, algae and also fungi.49 Callose is in this class of
polysaccharide and has been isolated from all multicellular green algae as well as some higher plants
(embryophytes).50 This polymer plays an important role in maintaining cell wall integrity as well as
in plant defense. In response to pathogen attack, callose is deposited between the plasma membrane
and the pre-existing cell wall at the sites of attack.51 Another member of this class, laminarin, has been
found in brown algae. As certain types of β-glucans containing (1→3) and (1→6) linkages exhibit
biological activity by stimulating the innate immune system in man, their synthesis has attracted much
interest.52 Several groups have successfully accomplished syntheses of linear as well as branched
oligosaccharides mostly with the motivation of synthesizing fungal glycans. The synthesized
fragments are summarized in Table 6.
Table 6. The β-(1→3)-D-glucan fragments synthesized to date.
Structure R Year Reference
OR
Me
H
1993
2011
2005
Takeo et al.53
Adamo et al.54
Jamois et al.55
29
(CH2)3S(CH2)2NH2
Cy-(C5H9)
2011
2013
Adamo et al.54
Jong et al.56
OR
Me 2011 Adamo et al.54
OR
All 2003 Zeng et al.57
OR
2
Me
H
1993
2003
2005
Takeo et al.53
Zeng et al.57
Jamois et al.55
OR
3
Me
CH2CHOCH2
H
1993
2005
2005
Takeo et al.53
Huang et al.58
Jamois et al.55
2
OR
(CH2)3NH2 2016 Yashunsky et al.59
OR
4
Me
H
(CH2)3S(CH2)2NH2
(CH2)5COOCH3
(CH2)2NHCO(CH2)3COOSu
1993
2009
2011
2015
2015
Takeo et al.53
Mo et al.60
Adamo et al.54
Elsaidi et al.61
Liao et al.62
OR
5
Me 1993 Takeo et al.53
30
OR
6
Me
(CH2)6NH2
(CH2)2NHCO(CH2)3COOSu
1993
2010
2015
Takeo et al.53
Tanaka et al.63
Liao et al.62
3
OR
3
n = 0, 2
n
(CH2)2NH2 2015 Liao et al.64
OR
n = 4, 8, 12
(CH2)6NH2 2010 Tanaka et al.63
ORn = 1, 4, 8
(CH2)5NH2 2017 Weishaupt et al.65
OR
8
(CH2)2NHCO(CH2)3COOSu 2015 Liao et al.62
OR
10
(CH2)6NH2
(CH2)5NH2
(CH2)2NHCO(CH2)3COOSu
2010
2013
2015
Tanaka et al.63
Weishaupt et al.66
Liao et al.62
OR
12
(CH2)6NH2 2010 Tanaka et al.63
OR
14
Me
(CH2)6NH2
2010
2010
Tanaka et al.63
Tanaka et al.63
Several strategies have been developed to produce both linear and branched β-(1→3)-D-glucan
oligosaccharides through solution phase synthesis. For oligosaccharides up to the pentasaccharide
31
level, an iterative approach has been used, which has enabled the synthesis of tri-, tetra- and
pentasaccharides in the same sequence, as exemplified by the preparation of β-(1→3)-D-glucan
oligosaccharides reported by Vetvicka and co-workers (Scheme 6).55,56 The group used an iterative
deprotection-glycosylation approach with monosaccharide 65 to elongate the chain. This key
monosaccharide could act successfully as both donor and acceptor to afford linear glucans 66–69. In
common with most other reported syntheses of linear β-(1→3)-D-glucans, the glucoside building
block 65 carried a benzoyl group at the 2-position to invoke neighboring group participation and a
4,6-benzylidene acetal as a permanent protecting group. The short length oligosaccharides were easily
deprotected using Zemplén conditions and catalytic hydrogenolysis to afford the corresponding
reducing oligosaccharides.
Scheme 6. Synthesis of protected di, tri, tetra- and penta-β-D-(1→3)-glucans by Vetvicka and co-workers.55
OOOPh
SEtOBz
NAPO
65
1) NIS, BnOH, TESOTf 89%
2) DDQ, CH2Cl2, MeOH
85%
3) 65, NIS, TESOTf 83%
OOOPh
OBzNAPO
OOOPh
OBnOBz
O
OOOPh
OBzNAPO
OOOPh
OBzO
OOOPh
OBnOBz
O
n
66
67 : n = 1
68 : n = 2
69 : n = 3
1) DDQ, CH2Cl2, MeOH
86%
2) 65, NIS, TESOTf 78%
1) DDQ, CH2Cl2, MeOH
2) 65, NIS, TESOTf 70%
1) DDQ, CH2Cl2, MeOH
2) 65, NIS, TESOTf 64%
For the synthesis of longer β-D-glucan oligosaccharides in solution, the use of convergent strategies
employing di-53,59–62,64, tri-54,57–59,64, tetra-52,64 and pentasaccharide52,59 building blocks has been
demonstrated to be advantageous, as it reduces the number of steps. This type of approach has been
32
used to synthesize long linear52,62 and branched52,59,64 oligosaccharides. In 2015, Guo and co-workers62
reported a convergent synthesis of tetra-, hexa-, octa-, deca- and dodeca-β-(1→3)-D-glucans, which
they achieved through pre-activation-based iterative glycosylation with p-tolylthioglycosides as
donors and disaccharide 70 as the key building block. The latter was used as a donor to elongate the
chain from disaccharide to tetra-, hexa-, octa-, deca- and finally dodecasaccharides.
Scheme 7. Convergent linear synthesis of tetra-, hexa-, octa-, deca-, and dodecasaccharides by Guo and co-workers.62
OOOPh
OBzNAPO
OOOPh
STolOBz
O
70
OOOPh
OBzHO
OOOPh
OOBz
O
N3
1) 70, AgOTf, TTBP, p-TolSCl,
CH2Cl2, - 78 οC
2) DDQ
CH2Cl2, H2O
90%
OOOPh
BzOO
OOOPh
OOBz
O
N3
OOOPh
BzOHO
7271
2
70OO
OPh
BzOO
OOOPh
OOBz
O
N3
OOOPh
BzOHO
n
73a-d n = 4, 6, 8, 10
OHOHO
OHO
OHOHO
OOH
O
H2N
OHOHO
OHHO
n
74a-d n = 4, 6, 8, 10
1) 2-azidoethanol, AgOTf, TTBP
p-TolSCl, CH2Cl2, - 78 οC
2) DDQ, CH2Cl2, H2O
91%
Using another highly convergent and efficient synthesis, the same group prepared two branched
oligosaccharides consisting of a nonaglucan backbone carrying a β-(1→3)-D-glucodiose or -tetraose
at the 6’’’’-position.64 The backbone was assembled in a [4 + 1 + 4] fashion, in which each
tetrasaccharide building block was generated using an iterative one-pot glycosylation as shown in
Scheme 8.
33
Scheme 8. Retrosynthesis of branched glucans by Guo and co-workers.64
OOOPh
STolOBz
HOOO
OPhSTol
OBzNAPO
Preactivation-based iterative one-pot glycosylation
OOOPh
BzOO
OOOPh
OOBz
O
N3
OOOPh
BzOHO
2
OOOPh
BzOO
OOOPh
STolOBz
OOOOPh
BzOBzO
2
Preactivation-based iterative one-pot glycosylation
OOOPh
STolOBz
HOOO
OPhSTol
OBzBzO
OBnO STolOBz
HO
OLev
OOOPh
BzOO O
4
OBnO
OBzO
OH
OOOPh
BzOOBz
4
N3
OHOHO
OHO O
4
OHO
OHOOHO
HO
OHOH
4
NH2
RO
R =
1) 2 glycosylations2) selective removal of Lev
1) glycosylation of the branch R2) Zemplén deprotection and hydrogenolysisOHO
HO
OHHO
OHOHO
OHO
OHOHO
OHO
n = 0, 2
75a-b
76
77 78 79
80 81 82 81
+ +
Takahashi and co-workers achieved the impressive syntheses of linear hexadeca- and branched
heptadeca-saccharides by conducting a convergent synthesis employing a tetrasaccharide as the key
building block.52 In this case, 2,3-diol glycosides with a 4,6-O-benzylidene protecting group were
effective as glycosyl acceptors, made possible by the higher reactivity of the 3-position over the 2-
position. This afforded the desired (1→3)-linked regioisomer in each glycosylation reaction.
As shown by these examples, the use of pre-assembled building blocks is efficient but only gives
access to glucans of limited chain lengths. In 2013, the group of Seeberger developed an iterative
automated solid-phase synthesis of linear β-D-glucan oligosaccharides giving great flexibility in terms
of the length of the targeted oligosaccharides. They showed the efficiency of the approach by
synthesizing a linear β-(1→3)-D-glucan dodecasaccharide, as shown in Scheme 9.66 Glycosyl
34
phosphate building block 83 in combination with the resin-linker construct 84 (see also Scheme 5)
proved to be a suitable choice for the automated synthesis of linear glucans. The efficiency of this
glycosylation sequence was proven by the average glycosylation yield over 12 cycles: 89%. In 2017,
the group further optimized the conditions to synthesize a series of branched β-(1→3)-D-glucans.65
Scheme 9. Automated solid-phase synthesis of β-glucan dodecasaccharide 87 by Seeberger and co-workers.66
OBnO
FmocOOPiv
OP
OBuOBu
O
Merrifieldresin
83
12 ×3 ×
3 equiv. of 83
TMSOTf, CH2Cl2, dioxane3 ×
piperidine, DMF
O
O2N
NCBzO
BnO
OBnO
OOPiv
BnOOBnO
OOPiv
BnOOBnO
HOOPiv
BnO
10
hν, CH2Cl2
NHCBzOOBnO
OOPiv
BnOOBnO
OOPiv
BnOOBnO
HOOPiv
BnO
10
86
OOHO
OOH
HOOHO
OOH
HOOHO
HOOH
HO
10
1) NaOMe, MeOH, CH2Cl22) Pd(OH)2
, H2, THF, H2O, AcOH
NH2
87
85
O
O2N
NCBzHO
84
35
4.3 Mixed-linkage [β-(1→3), β-(1→4)] glucans
n = 0,1,2
Figure 8. Structure of MLGs.
Mixed-linkage glucans (MLGs) are usually composed of short stretches of β-(1→4)-glucans
connected through β-(1→3)-linkages. They are a type of hemicellulose characteristically found in
both the primary and secondary cell walls of grasses. These glucans have also been isolated from
bryophytes67 and from the horsetail plant (Equisetum).68 More recently, in 2000, Lahaye and co-
workers discovered MLG 6-sulfate in the cell wall of the red alga Kappaphycus alvarezii. No chemical
synthesis of MLG oligosaccharides had been reported before the automated synthesis achieved by
Pfrengle and co-workers in 2017.69
Table 7. The synthetic MLG fragments prepared by Pfrengle and co-workers.
Structure R Reference
OR
(CH2)5NH2 Dallabernardina et al.69
OR
(CH2)5NH2 Dallabernardina et al.69
OR
2
(CH2)5NH2 Dallabernardina et al.69
OR
(CH2)5NH2 Dallabernardina et al.69
36
OR
2 (CH2)5NH2 Dallabernardina et al.69
OR
(CH2)5NH2 Dallabernardina et al.69
OR
2
(CH2)5NH2 Dallabernardina et al.69
This group performed the automated synthesis of seven MLG oligosaccharides using glucose building
blocks 58 and 88 as shown in Scheme 10. After optimizing the conditions for trisaccharide 89, they
concluded that two glycosylation cycles were required for the formation of β-(1→3)-linkages while
one coupling sufficed to form the β-(1→4)-linkages. After completion of the solid-phase synthesis,
the oligomers were cleaved from the resin in a continuous flow reactor, and deprotected under
standard conditions to afford the targets (89–95).
37
Scheme 10. Automated synthesis of seven MLG oligosaccharides by Pfrengle and co-workers.69
OBnO
FmocOOBz
OP
OBuOBu
O
O
O2N
NCBzHO
84
BnOOBnO
BnOOBz
OP
OBuOBu
O
FmocO
58 88
StartCleavage and global deprotection3) hν, CH2Cl24) NaOMe
MeOH, THF5) Pd/C, H2,
EtOAc
MeOH, H2O, AcOH
1a) 2 × 3.7 equiv. of 58
TMSOTf, CH2Cl2
or1b) 1 ×
3.7 equiv. of 88
TMSOTf, CH2Cl2
2) 3 × 20% Et3N
DMF
OHO
HOOH
OHO O
HO
OHO
HO OHO
HOOH
O NH23
OHO
HOOH
OO
HO
OHO
HO OHO
HOOH
O NH23
OHO
HOOH
OHO
OHO
HOOH
OO
HO
OH
HOOHO
HOOH
OOO NH2
3
OHO
HOOH
OHO O
HO
OHO
HO OHO
HOOH
O OHO
HOOH
O NH23
OHO
HOOH
OO O
HO
OH
HO OO
HO
OHO
HO OHO
HOOH
OHO
HOOH
HO
OHO
HOOH
OO
HO
OHO
HO OHO
HOOH
O NH23
OHO
HOOH
OO
OHO
HOOH
OO
HO
HOOH
HO
OHO
HOOH
OO
HO
OH
HOO O NH23
OHO
HOOH
OO
HO
OH
HOOHO
HOOH
OO
OHO
HOOH
HO
OHO
HOOH
OO
HO
OH
HOOHO
HOOH
OOO NH2
3
O OHO
HOOH
OHO
HOOH
OO O
HO
OH
HOOHO
HOOH
OHO O
HO
OH
HO
89: 13%
90: 26%
91: 12%
92: 12%
93: 34%
94: 18%
95: 23%
= Merrifield resin
84
38
4.4 Xylans (arabinoxylans, glucuronoxylans and glucurono-
arabinoxylans)
MeO MeO
Glucuronoxylan (GX)
MeO
Glucuronoarabinoxylan (GAX)
Arabinoxylan (AX)
Figure 9. Schematic representation of different common heteroxylans.
Xylan, the most prevalent non-cellulosic polysaccharide in plant cell walls, binds to cellulose
microfibrils and to lignin in order to strengthen the cell wall.4 Recently, Dupree and co-workers used
13C solid-state magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy to
show that xylans, which form a threefold helical screw in solution, flatten into a twofold helical screw
ribbon to bind intimately to cellulose microfibrils in the plant cell wall.70 Xylans have a backbone of
β-(1→4) linked D-xylose and they are known to have highly complex and heterogeneous structures
due to the number, distribution and differentiation of side chains. Xylans are usually classified into
homoxylans and heteroxylans, where the latter includes arabinoxylans (AX), glucuronoxylans (GX)
and glucuronoarabinoxylans (GAX) as shown in Figure 9. Arabinoxylans present L-arabinofuranosyl
substituents α-(1→2) and/or α-(1→3) linked to the xylose residues of the backbone. Additionally, it
39
has been found that in grasses, the O-5 position of arabinose can be esterified with for example ferulic
acid that can form covalent bonds to lignin.71,72 In glucuronoxylans, 4-O-methyl-D-glucuronic acid
and/or D-glucuronic acid residues are generally α-(1→2)-linked to xylose substituents, with an
average substitution ratio of 1:10.73 Finally, GAX has 4-O-methyl-D-glucuronic acid in the O-2-
postitions and arabinofuranosides in the O-3-position with a ratio of xylan:substituents of
approximately 10:1.4.73 In addition to these substituents, most xylans are acetylated to a varying
degree. Esterification is mainly found in the O-3 position, and more rarely in the O-2 position. An
overview of the xylan fragments that have been synthesized is presented in Table 8.
Table 8. Synthetic xylan fragments.
Structure R Year Reference
OR H 1982 Hirsch et al.74
OR
2 Me
H
Bn
(CH2)5NH2
1981
1982
1995
2015
Kováč et al.75
Hirsch et al.74
Takeo et al.76
Schmidt et al.77
OR
3 H
Bn
1982
1995
Hirsch et al.74
Takeo et al.76
OR
4 (CH2)5NH2
Bn
2015
1995
Schmidt et al.77
Takeo et al.76
OR
5 Bn 1995 Takeo et al.76
40
OR
6 (CH2)5NH2
Bn
2015
1995
Schmidt et al.77
Takeo et al.76
OR
7 Bn 1995 Takeo et al.76
OR
8 Bn 1995 Takeo et al.76
OR
MeO
H 2001 Oscarson et al.78
OR
Me 1978 Kováč et al.79
OR
(CH2)5NH2 2015 Schmidt et al.77
OR
(CH2)5NH2 2017 Senf et al.80
OR
MeO
Me 1980 Kováč et al.79
OR
(CH2)5NH2 2015 Schmidt et al.77
OR
(CH2)5NH2 2017 Senf et al.80
OR
(CH2)5NH2 2015 Schmidt et al.77
OR
(CH2)5NH2 2017 Senf et al.80
41
OR
(CH2)5NH2 2017 Senf et al.80
OR
(CH2)5NH2 2015 Schmidt et al.77
OR
(CH2)5NH2 2015 Schmidt et al.77
OR
(CH2)5NH2 2015 Schmidt et al.77
OR
(CH2)5NH2 2017 Senf et al.80
OR
(CH2)5NH2 2017 Senf et al.80
OR
(CH2)5NH2 2017 Senf et al.80
OR
(CH2)5NH2 2015 Schmidt et al.77
The chemical synthesis of well-defined homoxylans has been reported over recent decades by several
groups using a range of synthetic strategies.74–77 Fewer have described the synthesis of heteroxylans,
77–79 probably due to the difficulty in producing differentially-protected xylose building blocks needed
to introduce side chains. Xylobiose was first synthesized by Myrhe and Smith in 1961, and the reaction
was carried out via Helferich coupling81 between benzyl 2,3-di-O-benzyl-D-xylopyranoside and a
peracetylated xylosyl bromide in the presence of Hg(CN)2.82 Twenty years later, Hirsch and Kováč
synthesized a series of oligoxylans in the same fashion by using a sequential approach to elongate the
42
chain and reach up to a pentaxylan as shown in Scheme 11.74,75 1,2,3-Tri-O-acetyl-4-O-benzyl-β-D-
xylopyranose 96 was used as common building block to form both the bromide donor 97 and acceptor
98. Mercury cyanide-mediated coupling of the two afforded the disaccharide product as a 1:1.5
mixture of α:β stereoisomers, which could be separated by column chromatography to afford 23% of
the desired β-(1→4)-linked disaccharide 99. Subsequent hydrogenolysis afforded the 4’-OH
disaccharide acceptor 100 ready for coupling with donor 97. Repetition of the two last steps enabled
the researchers to reach tri-, tetra- and pentaxylans (103a-c) after saponification. Using this procedure,
the yields of the glycosylation reactions were all quite low (< 25%). The same iterative strategy was
subsequently employed to obtain a series of methyl β-xylosides up to a xylohexaoside.83
Scheme 11. Hirsch and Kováč strategy for the synthesis of oligohomoxylans.75
OBnO
OAcOAcAcO
OBnO
AcOAcO
96
97 Br
OHO
OAcOAcAcO
98
Hg(CN)2
MeCN
23%
OO
OAcOAcAcO
99: R = Bn100: R = H
ORO
AcOAcO
1) 97, Hg(CN)2
2) Pd/C, MeOH acetone OO
AcOAcO
ORO
AcOAcO OO
OAcOAcAcO
n = 1,2,3
R = Bn 101a-cR = H 102a-c
OO
HOHO
OHO
OHHO OO
HO OHHO
103a-c n = 1,2,3
NaOMe
MeOH
HBrtoluene CH2Cl2
Pd/CMeOH
acetone
Pd/CMeOH, acetone
Pd/CMeOH, acetone
More recently, Takeo and co-workers developed a blockwise approach to a series of linear xylo-
oligosaccharides up to xylodecaose using suitably protected thio-xylobiosides as building blocks and
43
N-iodosuccinimide (NIS) in combination with silver triflate as promoting agent. This strategy afforded
glycosylation yields of around 80%.76
The first synthesis of heteroxylan oligosaccharides was reported by Hirsch and Kováč in 1982. 75,79
The fragment was a xylotriose backbone bearing a β-(1→3)-linked xylose unit and an α-(1→2)-linked
glucuronic acid. The researchers started a stepwise synthesis from methyl 2,3-anhydro-β-D-
ribopyranoside 104 as shown in Scheme 12. This epoxide was coupled with a xylosyl bromide donor
105 and the resulting disaccharide 106 was then selectively deprotected at positions O-3 and O-4 and
further glycosylated with xylosyl bromide 108 to afford the tetrasaccharide 109 in 40% yield. The
epoxide was then opened selectively with benzyl alcohol following the Fürst-Plattner rule84, in order
to produce the xylose tetrasaccharide 112. The free hydroxyl group in position 2 was then glycosylated
with glucoronate donor 113 in the presence of silver perchlorate and 2,4,6-collidine affording the
desired α-branched pentasaccharide in 57% yield.
Scheme 12. Synthesis of a branched 4-O-methyl-α-D-glucuronic acid-containing xylotetraose by Hirsch and Kováč.75,79
OAcO
BnOAcO
Br105
104
ORO
BnORO
106: R = Ac107: R = H
OAcO
AcOAcO
Br108
OO
BnOO
109: R = Ac110: R = H
ORO
RORO
ORO
RORO
Hg(CN)2
MeCN
62%
Hg(CN)2
MeCN
40%
BnBrNaH
DMF
83%
OO
BnOO
111
OBnO
BnOBnO
OBnO
BnOBnO
OO
BnOO
OBnO
BnOBnO
OBnO
BnOBnO
O
OHBnO OMeOBnONa
BnOH
OO
OHO
OHOOH
HOOHO
OHHO
112
OOHO
HOO
OHO
MeOMeOOC
HO
OBnO
MeOMeOOC
BnO Cl
1) AgClO4, Et3N CH2Cl2
, 57%
2) H2, Pd/C, H2O
acetone
113
114
NaOMe, MeOH 98%
NaOMe, MeOH 98%
OHO
O
OMe OO
O
OMe OO
O
OMe
OO
O
OMe
44
In 2001, Oscarson and Svahnberg reported the synthesis of two glucuronoxylan fragments based on
xylobiose. Using dimethyl(methylthio)sulfonium trifluoromethane sulfonate (DMTST) in diethyl
ether, they were able to couple the glucuronic acid donor 116 with complete α-selectivity to the
disaccharide acceptor 115 (Scheme 13).78
Scheme 13. Synthesis of a glucuronoxylan trisaccharide by Oscarson and Svahnberg.78
O
OBzBzO OPMBO
O
OBnBnO SEtMeO
OMeO
O
OBzBzO OPMBO
OOO
OMe
OMe
O
O
BnOBnO
MeO
OMeO
O
OHHO OHO
OHOHO
O
O
HOHO
MeO
OHO
DMTSTEt2O
89%
deprotection
32%
115 117
116
118
OOO
OMe
OMe
OH
Recently, Pfrengle and co-workers developed an automated synthesis of arabinoxylans.77,80 They first
reported the synthesis of a set of xylan oligosaccharides up to an octasaccharide decorated in different
positions with either one or two α-(1→3)-linked arabinofuranosides as shown in Scheme 14.65
45
Scheme 14. Automated synthesis of arabinoxylan fragments by Pfrengle and co-workers.76 O
BnOOBz
OP
OBuOBu
O
FmocO
119
ONapO
OBzO
POBu
OBuO
FmocO
120
OOBz
OBzBzO
SEt
121
OOBz
OFmocBzO
SPh
122
OO
HOHO
OHO
OHHO OO
OHHO
2 123 : 27%
O NH23
OO
HOHO
OHO
OHHO OO
OHHO
4
O NH2
3
124 : 20%
OO
HOHO
OHO
OHHO OO
OHHO
6
O NH2
3
125 : 43%
OO
OHO
OHO
OHHO OO
OHHO O NH2
3OOH
OHHO 126 : 23%OO
OHHO
OHO
OHO OO
OHHO O NH2
3O
OH
OHHO 127 : 19%
OO
OHO
OHO
OHHO OO
OHHO O NH2
3OOH
OHO 128 : 42%
OO
OHO
OO
OHHO
OOH
OHHO 129 : 8%
OHO
OHHO OO
OHHO OO
OHHO O NH2
3
OO
OHHO
OO
OHO
OO
OHHO OO
OHHO O NH2
3OOH
OHHO 130 : 15%
OO
OHO
OHO
OHHO
OOH
OHHOOO
OHO
OO
OHO OO
OHHO O NH2
3OOH
OHHO 131 : 21%
OO
OHHO
OHO
OHHO
OOH
OHHO
OO
OHO
OO
OHHO OO
OHHO O NH2
3OOH
OHHO 132 : 7%
OO
OHHO
OO
OHO
OOH
OHHO
OHO
OHHO
O
HO OH
OH
Automated solid-phase synthesis
Coupling:
TMSOTf, CH2Cl2 (119, 120)
NIS, TfOH, CH2Cl2, dioxane (121, 122)
Deprotection:
20% Et3N in DMF (Fmoc) DDQ, ClCH2CH2Cl (NAP)
Capping:
Ac2O, pyCleavage:
hν, CH2Cl2
Deprotection:
NaOMe, CH2Cl2, MeOH Pd/C, H2,
EtOAc, MeOH, H2O, AcOH
To achieve their synthesis, two xylosides 119 and 120, differentially protected at their O-3 position,
were used as building blocks for the assembly of the linear xylan backbone, while perbenzoylated and
2-O-Fmoc-L-arabinofuranosides 121 and 122 were used for substitution. The sequential synthesis
provided, within short timeframes and with overall yields of 7-43%, a collection of arabinoxylan
46
fragments (123–132) as shown in Scheme 14. Very recently, they optimized this strategy in order to
supplement this set of α-(1→3)-substituted glycans with α-(1→2)-substituted arabinoxylan
oligosaccharides 135 to 139 (see Figure 10). By using the 2-O-(azidomethyl)benzoyl (Azmb)-
protected donor 133 the team could make oligosaccharides containing monosubsituted xylose
residues. With the inclusion of the orthogonally protected donor 134, synthesis of the disubstituted
arabinoxylan 135 was possible.
OBnO
AzmbOO
POBu
OBuO
FmocO
133
ONapO
AzmbOO
POBu
OBuO
FmocO
134
OO
OO
OHO
OHHO OO
OHHO O NH2
3OOH
OHHO135 : 4%
OOH
OHHO
OO
OHO
OHO
OHHO OO
OHHO O NH2
3
137 : 13%
OOH
OHHO
OO
OHHO
OHO
OHO OO
OHHO O NH2
3
139 : 41%
OOH
OHHO
OO
OHO
OO
OHHO
136 : 7%
OHO
OHHO OO
OHHO OO
OHHO O NH2
3
OO
OHHO
OOH
OHHO
OO
OHHO
OO
OHHO
138 : 4%
OHO
OHO OO
OHHO OO
OHHO O NH2
3
OO
OHHOO
OH
OHHO
OO
OHO
OO
OHHO
OHO
OHHO OO
OHHO OO
OHHO O NH2
3
OO
OHHO
O
HOOH
HO 140 : 11%
Figure 10. A set of substituted (arabino)xylan oligosaccharides synthesized in 2017 by Pfrengle and co-workers.80 The strategy is presented in Scheme 14.
47
4.5 Mannans
Galactomannans
Galactoglucomannans Figure 11. Schematic representation of different mannans.
Mannans are widely distributed and the main hemicellulose in Charophytes. 67 They have been
intensively studied due to their role as seed storage compounds, but they are found in variable amounts
in all cell walls.4 The mannan class of polysaccharides in higher plants can be divided into two
categories: galactomannans and glucomannans. Galactomannans have a homologous backbone made
up of β-(1→4)-linked mannose residues only, and can be substituted at some of the O-6 positions by
β-D-galactose units. The glucomannan backbone is composed of β-(1→4)-linked glucose and
mannose units assembled in a random pattern with a higher number of mannose than glucose
monosaccharides. Galactoglucomannans are further substituted with galactose at the O-6 positions of
some backbone mannose residues. Furthermore, mannans are often acetylated at C-2, C-3 and more
rarely at C-6 positions depending on the plant species.85 Mannans and glucomannans have been highly
conserved throughout plant evolution, however their role and function in the plant cell wall remain
largely unclear. The exception is the galactoglucomannans which are often described as seed storage
compounds.4 The formation of β-linked mannosides is an exceedingly difficult glycosylation reaction
since neighboring group participation, the steric hindrance from a protecting group at the O-2 position
and the anomeric effect uniformly favor the formation of α-D-mannopyranosides. Nevertheless,
48
several groups have developed strategies to circumvent the challenges and have successfully
synthesized certain mannan fragments, as shown in Table 9. Numerous reviews dealing with advances
in the construction of β-D-mannosides have been published. Among those, a review by Gridley and
co-workers86 deals with most of the strategies. A more recent one, by Lichtenthaler, presents the use
of 2-oxoglycosyl donors in assembling oligosaccharides with β-D-mannose, β-L-rhamnose, N-acetyl-
β-D-mannosamine, and N-acetyl-β-D-mannosaminuronic acid residues.87
Table 9. Synthetic mannan fragments.
Structure R Year Reference
OR Bn 2003 Twaddle et al.88
OR2
Me 2008 Kim et al.89
OR4
Me
H
Pr
2004
2013
2015
Crich et al.90
Ohara et al.91
Tang et al.92
OR
H 2013 Ohara et al.91
OR
H 2013 Ohara et al.91
OR5
H 2013 Ohara et al.91
OR
2 H 2013 Ohara et al.91
OR
2 H 2013 Ohara et al.91
49
OR6
C6H11
H
2004
2013
Crich et al.90
Ohara et al.91
OR
2 2 H 2013 Ohara et al.91
OR Me 2012 Ekholm et al.93
OR
Me 2012 Ekholm et al. 93
In 2004, Crich and co-workers developed a direct approach for the synthesis of β-mannosides using
4,6-O-benzylidene-directed glycosylation via in situ generation of α-mannosyl triflates from their
thio- or sulfoxidoglycoside precursors. They employed this strategy in combination with a three step
iterative glycosylation protocol to synthesize a β-linked hexamannoside as shown in Scheme 15.90
Benzylidene deprotection, followed by selective Piv protection of the primary alcohol in each case
provided the acceptor ready for the glycosylation with 4,6-O-benzylidene-thiomannoside 141. High
glycosylation yields were obtained in each case (71–82%) and very good β-selectivity was achieved
(α:β ~1:10).
50
Scheme 15. Three-step iterative glycosylation protocol to synthesize hexamannoside 146d by Crich and co-workers.90
OOO
Ph
BnO
OBn
SPh
SO
N
BSP, Tf2OTTBP
MeOH
82%
OOO
Ph
BnO
OBnOMe
1) neopentyl glycol, CSA,CH2Cl2 89%
2) PivCl, DMAP, Et3N, CH2Cl2 98%
OHOBnO
OBnOMe
OPiv
141 142 143
141, BSPTf2O, TTBP
CH2Cl2
80%
OOBnO
OBnOMe
OPivOO
OPh
BnO
OBn
144
3-step protocol: A) removal of benzylideneB) Piv protectionC) glycosylation with donor 141
OOBnO
OBnOMe
OPivOO
BnO
OBnOOO
Ph
BnO
OBnOPiv
n145a-d n = 1, 2, 3, 4
OOHO
OHOMe
OHOO
HO
OHOHOHO
OHOH
4
OH
OOBnO
OBnOMe
OPivOO
BnO
OBnOOO
Ph
BnO
OBnOPiv
4145d 146d
1) NaOMe, MeOH 94%
2) H2, Pd/C, MeOH
87%
N
N
2-benzenesulfonyl piperidine
(BSP)
2,4,6-tri-tert-butylpyrimidine
(TTBP)
In 2008, Jeon and co-workers reported an efficient one-pot, three-step glycosylation strategy starting
from a reducing sugar. A 4,6-O-benzylidene mannopyranose was treated with phthalic anhydride in
the presence of DBU, and the α-mannosyl phthalate intermediate was further activated with triflic
anhydride to generate a mannosyl triflate. Like Crich and co-workers, the group used 4,6-O-
benzylidene protected mannose to impart stereoselectivity and form the desired β-mannosidic
linkage.89
Another approach was taken by Pohl and co-workers in an automated solution phase synthesis of a β-
(1→4)-linked hexamannoside, shown in Scheme 16.92 Inspired by the work of van der Marel and co-
workers in their formation of β-mannosidic linkages using a mannuronate donor (section 7.5), Pohl
and co-workers used a β-directing C-5 carboxylate strategy and synthesized the protected β-(1→4)-
hexamannoside 149 in an average yield of 75% per step. Post-assembly, the mannuronic acid residues
were reduced to mannans with DIBAL-H.
51
Scheme 16. Automated solution phase synthesis of β-hexamannoside by Pohl and co-workers.92
OMeO2C
TBSOOBn
BnOO
NH
CCl3
R = H
148, TMSOTf CH2Cl2
TBAF, Et3N · 3HFDMSO, THFReduction
DIBAL-H, CH2Cl2, toluene
1) TMSOTf, 148 CH2Cl2
2) TBAF, Et3N⋅3HF
DMSO, THF
R = TBS
Start Coupling
Deprotection
148
OMeO2CO
OBn
BnO O O
C8F17R
n
OMeO2CO
OBn
BnO O O
C8F17R
n
HO O
C8F17
OOOBn
BnO O O
C8F17H
6
HO
149
OOOH
HO OPrH
6
HO
150
1) Grubbs' cat. 2nd gen. ethylene, CH2Cl2
2) H2, Pd/C, MeOH
73%
147
Indirect approaches requiring epimerization at C-2 in readily available β-glucosides have been used
by other groups to synthesize linear β-(1→4)-linked mannans.88,91 Nikolaev and co-workers accessed
β-(1→4)-mannotriose from commercially available and cheap D-cellobiose octa-acetate 151 via an
epimerizing SN2 reaction as shown in Scheme 17.88 After some protecting group manipulations
required to reach benzyl 4’,6’-O-benzylidene-cellobioside diol 153, simultaneous epimerization of C-
2 and C-2’ was performed via triflation followed by reaction with Bu4NOBz, affording β-(1→4)-
linked mannobiose 154 in 76% yield. Mannobiose 156 was further coupled to glucose donor 157,
forming a trisaccharide, which was then epimerized to give mannotriose 160 in a reasonable overall
yield.
52
Scheme 17. Synthesis of β-mannotriose through C-2 epimerization of glucoside by Nikolaev and co-workers.88
O
OAc
AcO
OAcOAcO O
OAcAcO
OAc
OAc
151
O
OH
HO
OHOHO O
OHHO
OH
OBn
1) HBr, AcOH, CH2Cl2
2) BnOH, Hg(OAc)2
3) NaOMe, MeOH
70%
OO
OHOBzO O
OHBzO
OBz
OBnOPh
1) PhCH(OMe)2,
TsOH·H2O, DMF 68%
2) BzCN, Et3N 33%
1) Tf2O, py CH2Cl2
2) Bu4NOBz,
toluene
76%
OO OBzO OBzO
OBz
OBnOPh
OBz
OBz
OHO OBzO OBzO
OBz
OBnOBz
OBz
OH
OHO OBzO OBzO
OBz
OBnOBz
OBz
OBzBzCN, Et3N, MeCN, CH2Cl2
92%
AcOH
76%
O
OAc
AcO
AcOAcO
O NH
CCl3
TMSOTf, CH2Cl247%
OO OBzO OBzO
OBz
OBnOBz
OBz
OBzO
OAc
AcO
AcOAcO
OO OBzO OBzO
OBz
OBnOBz
OBz
OBzOO
OHBzO
OPh1) HCl,
MeOH, 61%
2) PhCH(OMe)2,
TsOH·H2O, MeCN
3) BzCN, Et3N MeCN 65% (two steps)
1) Tf2O, py CH2Cl2
2) Bu4NOBz,
toluene
88%
OO OBzO OBzO
OBz
OBnOBz
OBz
OBzOO
BzO
OPhOBz
OO OHO OHO
OH
OBnOH
OH
OHOHO
HO
OHOH
1) TFA, H2O, CH2Cl2
2) NaOMe, MeOH, THF
70%
152 153
154 155 156
157
158 159
160 161
A combination of direct and indirect strategies was used by Wong and co-workers in their synthesis
of β-(1→4)-linked hexa- to octamannans, where some of the targets were further acetylated at one or
two positions along the backbone.91 An example of the convergent synthetic strategy taken to the three
hexamannan targets 162, 163, 164 is presented in Scheme 18. The three oligosaccharides can be
derived from a common protected hexamannan intermediate 165. As direct β-selective mannosylation
remains challenging, the group chose to synthesize the hexamannoside 165 by an indirect method
involving β-selective glucosylation of trimannoside acceptor 167 with mannoglucose donor 166. The
subsequent epimerization of the glucose C-2 hydroxyl group was achieved via an oxidation-reduction
sequence affording the desired β-mannoside. This methodology simultaneously allowed for
53
acetylation at the designated positions. The trisaccharide fragments 166 and 167 could be obtained
from monosaccharides 168 and 171 respectively by repeating direct β-selective mannosylations using
169 or 170 as the donors.
Scheme 18. Retrosynthetic analysis of hexamannans 162–164 by Wong and co-workers.91
O
OH
HOHO
OHO O
OHOH
HO O
OH
OHO
OR1
O OOR2
OH
HO O
OH
OHO
OHO O
OHOH
HOOH
162: R1 = R2 = H163: R1 = Ac, R2 = H164: R1 = R2 = Ac
OOBnO
OBnO O
OBnOBn
BnO O
OBn
OBnO
OHO O
OPMBOBn
BnO O
OBn
OBnO
OBnO O
OBnOBn
BnOOBn
165
OPh
OOBnO
OBnO O
OBnOBn
BnO O
OBn
OBnO
OPh
BzO O
Cl3C NH
+ HO OOPMB
OBn
BnO O
OBn
OBnO
OBnO O
OBnOBn
BnOOBn
166 167
OOBnO
OROPh
SPh
O
OBn
HOBnO
BzOOAll O
OBnOBn
BnOOBn
HO
168 171169: R = Bn170: R = PMB
The authors used the same strategy, with a late β-glucosylation in [4 + 3] and [4 + 4] couplings
followed by epimerization to get the hepta- and octamannans shown in Figure 12.
54
O
OH
OHO
OHO O
OHOH
HO O
OH
OHO
OR1
O OOR2
OH
HO O
OH
OHO
OHO O
OHOH
HOOH
172: R1 = R2 = H173: R1 = Ac, R2 = H174: R1 = R2 = Ac
HO OOH
OH
HO
O
OH
OHO
OHO O
OHOH
HO O
OH
OHO
ORO O
OHOH
HO O
OH
OHO
OHO O
OHOH
HOHO O
OHOH
HO O
OH
OHO
OH
OH
175: R = H176: R = Ac
Figure 12. Hepta- and octamannans synthesized by Wong and co-workers.91
To date, only one synthesis of a galactoglucomannan fragment has been reported. The tetrasaccharide
fragment 185 was synthesized by Leino and co-workers in 2012, using a linear synthesis involving a
β-selective mannosylation with 4,6-O-benzylidene thiomannosyl donor 177, Scheme 19.93 A β-
selective glucosylation afforded trisaccharide 181, which was further selectively deprotected at the C-
6’’ position. Finally, and following Ando and co-workers’ discovery,94 α-selective galactosylation
with di-tert-butylsilylene galactoside donor 183 afforded tetrasaccharide 184 ready for deprotection
to reach the target 185.
55
Scheme 19. Synthesis of galactoglucomannan tetrasaccharide by Leino and co-workers.93
OHO
OAcAcO SPhBnO
OOO
Ph
BnO SPh
OBnBSP, TTBP, Tf2O
CH2Cl2
53%177
178
OOO
Ph
BnO
OBnOO
OAcAcO SPh
BnO
179
OBnOHOBnO
OBn
OMe180NIS, TMSOTf,
CH2Cl2
77%
OOO
Ph
BnO
OBnOO
AcOAcO
BnOO
BnOO
BnO
OBn
OMe
OHOBnO
BnO
OBnOO
AcOAcO
BnOO
BnOO
BnO
OBn
OMe
Cu(OTf)2, BH3
·THFCH2Cl2
85%
OOBnO
BnO
OBnOO
AcOAcO
BnOO
BnOO
BnO
OBn
OMe
O
BzOBzO
OOSiO
OBzBzO
OOSi
SPh
NIS, TMSOTf,CH2Cl2
68%
181
182 184
183
OOHO
HO
OHOO
HOHO
HOO
HOOHO
OH
OMe
O
HOHO
HOOH
185
1) HF·py, THF, 68%2) NaOMe, MeOH, THF, 96%
3) H2, Pd/C, MeOH 96%
56
Pectin Pectin was first discovered in 1790 by Vauquelin,95 being first described as ‘pectin’ in 1825 by
Braconnot.96 The name derives from the Greek word pektikos, meaning ‘congeal, solidify or curdle’,
and accurately describes the main characteristic of this group of polysaccharides. As a result of their
ability to form gels, pectic polysaccharides have many industrial applications. They are used as gelling
and stabilizing ingredient in a wide range of food products. They also fulfil important biological roles,
including imparting mechanical strength and flexibility to the cell wall. Pectin therefore has multiple
functions in plant growth, morphology, development, and defense.97 Pectic polysaccharides also play
an important role in cell-cell adhesion.6
Pectins make up around 35% of the dry weight of the primary cell wall in dicots and non-graminaceous
monocots, 2–10% in grasses and other commelinoid primary walls, and up to 5% in woody tissue.98
Although pectin is structurally and functionally the most complex and heterogeneous class of
polysaccharides in nature, it is relatively well-defined, being characterized by a high content of
galacturonic acid (see Figure 13). It is widely believed that four to five pectic domains, namely,
homogalacturonan (HG), rhamnogalacturonan I (RG-I), rhamnogalacturonan II (RG-II), and
xylogalacturonan (XGA) and/or apiogalacturonan (AGA) are covalently linked to one another to form
the pectin complex in muro. As yet, there is no sufficiently compelling evidence, as to how the
different blocks are assembled in the macromolecule. Therefore, the fine-structure of pectin is still a
matter of controversy and several models have been proposed. A review by Yapo in 2011 describes
the two traditional models, one with alternating ‘smooth’ (HG) and ‘hairy’ (XGA, RG-I/RG-II)
regions, the other proposing a backbone of RG-I, with side chains of HG, XGA and RG-II. The author
argues that in the face of more recent analysis, the traditional models become less and less convincing
57
as comprehensive descriptions of pectin.99 Irrespective of which model of polysaccharide structure
one subscribes to, it is undisputed that pectins consist of distinct domains. HG and RG-I represent the
two major kinds of backbone in pectin. While HG is a linear homopolymer of partially methyl
esterified and/or acetylated α-(1→4)-linked D-galacturonic acid, the RG-I backbone is made up of
alternating galacturonic acids and L-rhammnose monosaccharides, with various L-arabinan, D-
galactan and arabinogalactan side chains. Three more domains of pectin, xylogalacturonan (XGA),
apiogalacturonan and rhamnogalacturonan II (RG-II) can be seen as structurally modified HG as they
share the same backbone. While 25–75% of the galacturonic acid residues of XGA bear xylose
substitution, RG-II is a complex branched HG having four different side chains, with a total of up to
29 different monosaccharides, including a number of rare sugars.100 More detail on the structure of
the five pectic domains will be given further on in this section. Only few pectic oligosaccharides are
available from the chemical or enzymatic degradation of polysaccharides. Although degradation
techniques have been widely used for elucidating the structure of pectin (especially RG-I), the
oligosaccharide fragments prepared in this way required extensive purification and were generally
only obtained in analytical quantities. Furthermore, this approach has mostly been limited to isolation
of linear backbone fragments since the techniques generally involve the removal of side chains.97 This
fact has prompted chemists to devise synthetic approaches to pectic oligosaccharides, albeit to date
only for a limited number of structures. A review by Nepogodiev and co-workers summarized the
fragments of HG, RG-I and RG-II, which had been synthesized before 2011.101 These are included in
the following discussion.
58
Rhamnogalacturonan I Homogalacturonan
XylogalacturonanRhamnogalacturonan II
OMe
MeO
L-gulose
D-glucose
D-xylose
D-mannose
D-galactose L-arabinose
L-rhamnose OAc
COOH
SO3H
β-notation
α-notation
= O1
23
46L-fucose COOMe
D-apiose
L-aceric acidless common enantiomer (D/L)
D-3,6-anhydrogalactose
D-2-Keto-3-deoxy-D-manno-octulosonic acid (Kdo)
D-3-deoxy-D-lyxo-heptulosaric acid (Dha)
L-iduronic acid
Me
Figure 13. Schematic representation of the four domains of pectin.
59
5.1 Homogalacturonan
Figure 14. Homogalacturonan (HG) structure.
Homogalacturonan (HG), which is referred to as the ‘smooth region’ of pectin constitutes
approximately 65% of the macromolecule. HG consists of a linear chain of α-(1→4)-linked
galacturonic acid, which can be methyl esterified or acetylated at the C-2 and C-3 positions depending
on the plant as well as the type of tissue. HG has been shown to be present in stretches of
approximately 100 GalpA residues.102 It is believed that HG is synthesized as a highly methyl
esterified form in the Golgi apparatus and then de-esterified enzymatically, once it has been deposited
in the cell wall.98,103,104 The degree of methylation is well known to influence gelling capability as
well as other physical properties of pectin. C-6 carboxylates of HG residues can interact with calcium
ions to form a stable gel when more than 10 consecutive un-esterified galacturonic acid residues are
present.6 Furthermore, the methyl esterified GalpA residues of HG can associate through hydrophobic
interactions.100
Efforts to synthesize different fragments of HG, in some cases with a distinct pattern of methyl
esterification have provided a variety of oligomers ranging from tri- to dodecasaccharides, presented
in Table 10.
Table 10. Overview of the synthesized homogalacturonan fragments.
Structure R Year Reference
60
OR H 1990 Nakahara et al.105
OR Pr 2011 Pourcelot et al.106
OR H 1999
2001
Kester et al.107
Clausen et al.108
OR H 1999
2001
Kester et al.107
Clausen et al.108
OR H 1999
2001
Kester et al.107
Clausen et al.108
OR H 2003 Clausen and Madsen.109
OR H 2003 Clausen and Madsen.109
OR H 2003 Clausen and Madsen.109
OR H 2003 Clausen and Madsen.109
OR H 2003 Clausen and Madsen.109
OR8
Pr 1989 Nakahara et al.110
10OR
H 1990 Nakahara et al.111
61
As is typical for the synthesis of glycuronides, two distinct approaches have been employed to reach
oligomers of α-(1→4)-linked D-GalpA: a post-glycosylation oxidation and a pre-glycosylation
oxidation. A comprehensive microreview by van der Marel and co-workers explains the rational
behind these two strategies.112 With post-glycosylation oxidation, often the preferred approach, the
backbone of the target is assembled from aldose building blocks, followed by oxidation of the primary
alcohols to carboxylic acids prior to or after global deprotection. The alternative pre-glycosylation
oxidation strategy involves the glycosylation between protected uronic acid donors and acceptors.
Due to the electron-withdrawing carboxylic acid group, galacturonic acid building blocks are
substantially less reactive than the corresponding galactose derivatives, which makes this second
approach challenging. In fact, only two syntheses of trigalacturonides have been reported using the
pre-glycosylation oxidation method, and no oligomer longer than this has been produced.
The first was performed by Anker and co-workers in 1997 using thiophenyl galacturonides as glycosyl
donors, as shown in Scheme 20.113 Good stereoselectivity and a high yield was obtained for the first
glycosylation affording disaccharide 188. However, attempts to elongate the chain to a trisaccharide
by coupling the thiogalacturonide 190 with the disaccharide acceptor 189 only afforded the target
molecule in 45% yield. The trisaccharide 191 was not deprotected. Similar results were obtained by
Kramer et al., also by glycosylating a disaccharide acceptor with a thiogalacturonide donor. Despite
slight variations in the protecting group pattern, this group obtained the targeted trisaccharide in a
similarly disappointing 48% yield.114
62
Scheme 20. Protected trigalacturonide synthesized through a pre-glycosylation oxidation strategy by Anker and co-workers.113
OCO2BnPMBO
BnO SPhOBn
OCO2BnHO
BnO OBnOBn
78% α:β > 95:5
NIS, TfOHCH2Cl2
OCO2BnO
BnO OBnOBn
OCO2BnPMBO
BnOBnO
OCO2BnO
BnO OBnOBn
OCO2BnHO
BnOBnO
DDQCH2Cl2
, H2O
86% OCO2BnO
BnO OBnOBn
OCO2BnO
BnOBnO
OCO2MeBnO
BnO SPhOBn
45% α:β = 95:5
OCO2MeBnO
BnOBnO
186
187
188 189
190
191
NIS, TfOHCH2Cl2
These results encouraged other groups to turn their attention to the post-glycosylation oxidation
approach. Galactopyranose-based building blocks used for the synthesis of the neutral α-(1→4)-linked
galactan backbone require a temporary protecting group at the C-6 position, which can release the
primary alcohol prior to oxidation at a late stage of the synthesis. This method includes early work
done by Nakahara and Ogawa in the 1990s.105,110,111 Among their elegant syntheses, the pinnacle
remains the total synthesis of dodecagalacturonic acid using a highly convergent approach as shown
in Scheme 21.111
They chose galactosyl fluorides as donors, which afforded highly stereoselective glycosylations and
gave good yields (62–95%). Acetyl groups were used as temporary protecting groups for all the
primary alcohols, which were then selectively unmasked to afford the α-(1→4)-linked dodecagalactan
102. The alcohols were further oxidized to the corresponding carboxylic acids using a Swern115
followed by a Lindgren-Kraus-Pinnick oxidation116–118 affording the dodecagalacturonic acid product
in 50% yield. Final TBDPS deprotection and hydrogenolysis gave the target (203) in a good yield
overall.
63
Scheme 21. Synthesis of dodecagalacturonic acid by Nakahara and Ogawa.111
OO
BnOBnO
OBnO
BnOBnO
OAc
F
OO
BnOBnO
OHO
BnOBnO
OAc
OTBDPS
OAc
OAc
SnCl2,
AgClO4, Et2O
92% OO
BnOBnO
OO
BnOBnO
OAc
R
OAc
OO
BnOBnO
OBnO
BnOBnO
OAc
OAc
194: R = OTBDPS
195: R = OH (90%)
196: R = F (97%)
TBAF, AcOH, THF
Et2NSF3, MeOH
OO
BnOBnO
OHO
BnOBnO
OAc
OTBDPS
OAc
OO
BnOBnO
OO
BnOBnO
F
OAc
O
SnCl2,
AgClO4, Et2O
77% OO
BnOBnO
OO
BnOBnO
OAc
OTBDPS
OAc
OO
BnOBnO
OO
BnOBnO OAc
O
198
192
193
197
193 1) TBAF, AcOH, THF 2) Et2NSF3
, MeOH
93%
OO
BnOBnO
OO
BnOBnO
OAc
OAc
OO
BnOBnO
OO
BnOBnO OAc
O
OO
BnOBnO
OO
BnOBnO
OAc
OAc
OO
BnOBnO
OHO
BnOBnO OAc
F199
200OTBDPS
OAc
1) 80% aq. AcOH2) AcCl, py
86%
199
1) 200, SnCl2,
AgClO4 Et2O
2) 80% aq. AcOH 62%
3) AcCl, py 82%
O
O
BnOBnO
OHO
BnOBnO
OAc
O
O
BnOBnO
OAc
OTBDPS
OAc
6
O
O
BnOBnO
OBnO
BnOBnO
OH
O
O
BnOBnO
OH
OTBDPS
OH
1) 196, SnCl2,
AgClO4 Et2O 64%
2) NaOMe, MeOH 81%
O
O
HOHO
OHO
HOHO
COOH
O
O
HOHO
COOH
COOH1) Me2SO, (COCl)2, i-Pr2EtN
CH2Cl2
2) NaClO2, 2-methyl-2-butene tBuOH, NaH2PO4, H2O
50%3) TBAF, AcOH, THF
75%
4) 10% Pd/C in 80% aq. MeOH
75%
OH201 202 203
10 10
64
In order to synthesize some partially methyl-esterified hexagalacturonates, Madsen and co-workers
introduced two orthogonal protecting groups at the C-6 positions of D-galactose: acetyl and para-
methoxyphenyl (PMP). This enabled them to oxidize the C-6 position to the carboxylic acid or to the
methyl ester selectively, and thereby synthesize tri- and hexagalacturonans with different methylation
patterns.108,119 Their methodology was based on the repeated coupling of galactose mono- and
disaccharide donors onto a galactose acceptor to produce hexagalacturonan backbones. They chose n-
pentenyl donors, which provided good yields of the desired α-anomers. An example of their synthesis
of two partially-esterified hexamers is shown in Scheme 22. Tetrasaccharide acceptor 211, formed
from disaccharide 208 by sequential glycosylation with pentenyl donor 209 (ClAc = chloroacetate)
was coupled with disaccharide donor 207 to form the pure α-(1→4)-linked hexagalactoside, which
was further deacetylated to afford 212, the hexamer precursor of both targets. The oxidation to esters
was carried out in a one-pot three-step procedure. The primary alcohols were first oxidized by Dess-
Martin periodinane120 to the aldehydes, which underwent a sodium chlorite oxidation to afford the
corresponding carboxylic acids. The resulting acids were then esterified with either TMSCHN2
affording 213 or with PhCHN2 giving 214. The benzyl protection masked the positions destined to
become carboxylic acids, and aided the purification of the intermediates. To complete the synthesis,
the PMP-ethers were removed with cerium(IV) ammonium nitrate (CAN) and the primary alcohols
were oxidized and esterified using the same one-pot three-step procedure, affording both benzyl and
methyl esters. Hydrogenolysis afforded the two targets 215 and 216.
65
Scheme 22. Example of two partially methyl esterified hexagalacturonic acids prepared by Madsen and co-workers.109
O
OAcBnO
BnOOBn
OO
OAcBnO
BnOBnO F
NBS, Et2NSF3CH2Cl2
74%
O
OAcBnO
BnOBnO
O
OAcHO
BnOOBn
O
O
OAcO
BnOOBn
O
SnCl2,
AgClO4CH2Cl2
55%
204 205
206
207
O
OPMPHO
BnOBnO
O
OAcO
BnOOBn
OBn
O
OPMPClAcO
BnOOBn
O
1) 209, NIS, TESOTf CH2Cl2
2) thiourea, NaHCO3,
Bu4NI, THF 88%
209
O
OPMPO
BnOBnO
O
OAcO
BnOOBn
OBn
O
OPMPHO
BnOBnO
O
OPMPO
BnOBnO
O
OAcO
BnOOBn
OBn
O
OPMPO
BnOBnO
O
OPMPHO
BnOBnO
208 210 211
OCOORBnO
BnOBnO
OCOORO
BnOBnO
O
OPMPO
BnOBnO
OCOORO
BnOOBn
OBn
O
OPMPO
BnOBnO
O
OPMPO
BnOBnO
a: 213: R = Me (44%)b:
214: R = Bn (59%)
a) Dess-Martin periodinane, CH2Cl2then NaClO2
, NaH2PO4, THF, tBuOH, H2Othen TMSCHN2
, THF, MeOH
or
b) Dess-Martin periodinane, CH2Cl2then NaClO2
, NaH2PO4, THF, tBuOH, H2Othen PhCHN2
, EtOAc
OCOOR1HO
HOHO
OCOOR1O
HOHO
OCOOR2O
HOHO
OCOOR1O
HOHO OH
OCOOR2O
HOHO
OCOOR2O
HOHO
1) CAN, MeCN, H2O
2a) Dess-Martin periodinane, CH2Cl2then NaClO2
, NaH2PO4, THF, tBuOH, H2Othen TMSCHN2
, THF, MeOH (for 214)
or
2b) Dess-Martin periodinane, CH2Cl2then NaClO2
, NaH2PO4,THF, tBuOH, H2O
then PhCHN2, EtOAc (for 213)
3) H2, Pd/C, THF, MeOH, H2O 215: R1 = Me, R2 = H (48%)
216: R1 = H, R2 = Me (30%)
O
OHBnO
BnOBnO
O
OHO
BnOBnO
O
OPMPO
BnOBnO
O
OHO
BnOOBn
OBn
O
OPMPO
BnOBnO
O
OPMPO
BnOBnO1) 207, NIS, TESOTf
CH2Cl2
2) NaOMe, MeOH, THF 71%
212
212
1) 209, NIS, TESOTf CH2Cl2
2) thiourea, NaHCO3,
Bu4NI, THF 90%
66
5.2 Xylogalacturonan
Figure 15. Example of XGA structure.
Xylogalacturonan (XGA) is similar to HG except that the backbone is substituted with single β-
(1→3)-linked xylose residues or short β-(1→4)-linked xylan side chains as shown in Figure 15.121
XGA-based glycans have not yet received attention from synthetic chemists.
5.3 Apiogalacturonan
Figure 16. Example of AGA structure showing various types of branching points.
Apiogalacturonan (AGA) is found in the cell walls of aquatic plants such as duckweeds (Lemnaceae)
and marine seagrasses (Zosteraceae).122 In AGA, β-D-apiofuranosyl residues are attached to the 2-
and/or 3-positions of selected GalpA monosaccharides in the HG chain in the form of mono- or
sometimes short oligo-saccharides, as shown in Figure 16.6 While several groups have focused on the
synthesis of apiose-containing oligosaccharides found in the RG-II region of pectin (see section 5.5),
only two groups have reported the synthesis of oligosaccharides related to apiogalacturonan (see Table
11).
67
Table 11 Three different synthetic AGA oligosaccharides.
Structure R Year Reference
OR
Me 2004
2011
Buffet et al.123
Nepogodiev et al.124
OR
Me 2011 Nepogodiev et al.124
OR
Me 2011 Nepogodiev et al.124
As D-apiose is a rare sugar, all syntheses of D-apiose-containing oligosaccharides start with the
preparation of the appropriate apiose derivative, usually from D-mannose, L-arabinose or D-xylose.125–
128 Reimer and co-workers chose a pre-glycosylation oxidation approach to synthesize AGA
disaccharide 225 as shown in Scheme 23. They started by preparing 2,3-O-isopropylidene
apiofuranose 219 from D-mannose in five steps.126,127 Four additional transformations afforded
thioapiofuranoside donor 222. Glycosylation of the 2-OH galacturonate acceptor 223 proceeded
smoothly, giving disaccharide 224 in 83% yield. Deprotection afforded the disaccharide 225 in 76%
yield.
68
Scheme 23. Synthesis of disaccharide 225 by Reimer and co-workers.123
O
O
OH
O
HO
O OHOOO
O
O OHOOO
O
OH
37% aq. HCOH K2CO3
, MeOH
86%
H2SO4, aq. MeOH
then NaBH4then NaIO4
79%
O
O
OBn
O
BnOBnBr, NaHDMF
92%
1) 80% HCOOH2) Ac2O, py
81%
O
OAc
R
AcO
BnO
221: R = OAc222: R = STol
217 218 219 220 TolSH, BF3.Et2O
90%
OO
O
CO2Me
OMeHO
222, NIS, AgOTfCH2Cl2
84%
OO
O
CO2Me
OMeOO
OAcAcO
BnO
223 224
1) Pd(OAc)2, H2
EtOAc, MeOH, AcOH 90%
2) NaOMe, MeOH3) H2O, Amberlite H+
4) NaOH (aq.) 85% (three steps)
OHO
HO
CO2H
OMeOO
OHHO
HO
225
Some years later, in 2011, Field and co-workers synthesized the AGA fragments 225, 234 and 237
shown in Scheme 24.124 Like Reimer and co-workers, they prepared apiofuranoside donor 226 from
D-mannose according to previously published procedures. Treatment of arylthio glycoside 226 with
90% TFA followed by acetylation afforded triacetate 227 as an anomeric mixture. Coupling between
227 and 223 afforded the β-linked disaccharide 228 exclusively in 79% yield, which was further
deprotected to give the target disaccharide 225. Galacturonate acceptors with unprotected 3-OH or
both 2- and 3-OH groups (239 and 235, respectively) were prepared and coupled with 227, leading to
di- and tri-saccharides 233 and 236 in good yields. Cleavage of the ester protecting groups afforded
the targeted apiogalacturonans 234 and 237.
69
Scheme 24. Synthesis of di- and trisaccharide AGA fragments by Field and co-workers.124
O
OO
AcO STol
O
OAcAcO
AcOSTol
1) 90% TFA2) Ac2O, DMAP
py
76%
OCO2MeO
OHOOMe
226 227
223
NIS, HClO4/SiO2CH2Cl2
79% O
OAcAcO
AcO
OCO2MeO
OOOMe
O
OHHO
HO
OCO2HHO
HOOOMe
1) 80% aq. AcOH 2) NaOMe, MeOH3) NaOH, H2O
80%
228 225
OCO2MeHO
HOHOOMe
OCO2MeO
OBzOOMe
MeC(OEt)3, TsOH
THF
73%
EtO OCO2MeAcO
HOBzOOMe
80% AcOH
79%
BzCl, py
78%
OCO2MeO
OHOOMe
EtO
OCO2MeAcO
HOHOOMe
80% AcOH95%
227, NIS, HClO4/SiO2, CH2Cl2
84%
227, NIS, HClO4/SiO2
CH2Cl2
74%
OCO2MeAcO
OOOMe
O
OAcAcO
AcO
O
OAcAcO
AcOO
CO2HHO
OOOMe
O
OHHO
HO
O
OHHO
HO1) NaOMe, MeOH2) Et3N, MeOH, H2O
73%
OCO2MeAcO
OBzOOMeO
OAcAcO
AcO
OCO2HHO
OHO
OMeO
OHHO
HO
1) NaOMe, MeOH2) NaOH, H2O, MeOH
66%
229 230 231 232 233
234
235
236 237
70
5.4 Rhamnogalacturonan I
Figure 17. Representation of RG-I structure.98
RG-I (Figure 17) makes up 20–35% of pectin. Its average length has not been determined, since
isolated RG-I domains are generally accompanied by HG. However, the RG-I backbone has generally
been found to be shorter than HG. Furthermore, the RG-I backbone is predicted to have a three-fold
helix conformation.129 RG-I polysaccharides have repeating disaccharide units of (1→2)-α-L-
rhamnopyranosyl-(1→4)-α-D-galactopyranuronic acid. The GalpA residues may be O-acetylated on
C2-O and/or C3-O as seen for HG, but no methyl esterification has been observed.98 The diversity of
RG-I stems from the variety of side chains that can decorate the C-4 positions of rhamnose. Depending
on the plant source and the method of isolation, 20–80% of the rhamnosyl residues are found to be
substituted. These side chains consist of three different types of branched oligosaccharides of varying
length (1-20 residues): galactans, arabinogalactans (AG), and arabinans.98,130 Pectic arabinogalactans
are usually divided into two main groups.131 Type I arabinogalactan (AG-I) is the most prevalent
neutral pectic glycan and is especially abundant in citrus fruits and apples. It is characterized by a
71
linear β-(1→4)-linked D-Galp chain, which can be substituted at the C-3 position by L-Araf residues
and occasionally by arabinosyl oligosaccharides. It sometimes also carries galactan at the C-6
position. Type II arabinogalactan (AG-II) are highly branched polysaccharides composed of a
backbone of β-(1→3) or (1→6)-linked D-galactan, which can be (arabino)galactan-substituted at the
C-6 or C-3 positions, respectively.97 Galactans are simply β-(1→4)-linked D-galactan chains and have
been observed in lengths of up to 40 residues. Finally, the arabinans consist of α-(1→5)-linked L-
Araf residues with some degree of (1→3)-branching. The glycosyl residues α-L-fucosyl, β-D-
glucuronosyl and 4-O-methyl β-D-glucuronosyl may also be present.98 The (arabino)galactans and
arabinans will be discussed separately in section 6.
RG-I backbone
Figure 18. RG-I backbone structure.
Several syntheses of fragments of the RG-I backbone (Figure 18) have been carried out and an
overview of the different oligosaccharides produced is shown in Table 12.
Table 12. Synthetic fragments of RG-I backbone.
Structure R Year Reference
72
OR
Pr 2013 Ma et al.132
OR
Pr
H
2000
2008
2010
Maruyama et al.133
Nemati et al.134
Scanlan et al.129
OR
Me
2008 Reiffarth & Reimer135
OR
Pr 2013 Ma et al.132
OR
H 2013 Zakharova et al.136
OR
Pr 2013 Ma et al.132
As with HG fragment syntheses, and more generally all oligosaccharides containing uronic acids, both
pre-glycosylation oxidation and post-glycosylation oxidation approaches have been investigated by
different groups. The former approach was chosen by Reiffarth & Reimer135, also Vogel and co-
73
workers134 and Yu and co-workers.132 In 2008, Reiffarth and Reimer135 used a block synthesis
approach, coupling two disaccharide repeating units derived from the same disaccharide precursor, to
form their target rhamnogalacturonan tetrasaccharide 244 as shown in Scheme 25. The C-4 positions
of the rhamnosyl residues were orthogonally protected with an allyl group in order to allow potential
introduction of RG-I side chains. An acetyl group was used to protect their C-2 positions to enable
elongation of the backbone. Rhamnosyl thioglycoside donor 237 and methyl galacturonate acceptor
238 were coupled to form the key disaccharide intermediate 239. This was then further transformed
in a three-step sequence to generate a new disaccharide acceptor 240, by removal of the 2-acetyl
group, and a new trichloroacetimidate disaccharide donor 241. However, glycosylation of 240 with
donor 241 turned out to be more difficult than expected. Tetrasaccharide 242 was obtained in only
39% yield and attempts to optimize the reaction by changing the promoter and donor types were
unsuccessful. The fully deprotected tetrasaccharide methyl ester 243 was then reached over four steps
in 33% overall yield and ester hydrolysis using NaOH followed by acidification afforded the desired
rhamnogalacturonic acid tetrasaccharide 244 in 77% yield.
74
Scheme 25. Synthesis of an RG-I backbone tetrasaccharide fragment by Reiffarth & Reimer.135
O
SEt
OAc
AllOBzO
OCOOMeHO
BnOBnOOMe
NIS, TfOHCH2Cl2
78%237
238
OCOOMeO
BnOBnOOMe
O
OAcBzOAllO
OCOOMeO
BnOBnOOMe
O
OHBzOAllOHCl,
MeOH, CH2Cl2
80%
239 240
OCOOMeO
BnOBnO
O
OAcBzOAllO
1) H2SO4, AcOH, Ac2O (81%)
2) 4Å MS, CH2Cl2,
MeOH (89%)
3) CCl3CN, DBU, CH2Cl2 (81%)
241
240 + 241
AgOTfCH2Cl2
39%
O CCl3
NH
OCOOMeO
BnOBnO
O
OAcBzOAllO
OCOOMeO
BnOBnOOMe
O
OBzOAllO
242
OCOOMeO
HOHO
O
OHHOHO
OCOOMeO
HOHOOMe
O
OHOHO
OCOOHO
HOHO
O
OHHOHO
OCOOHO
HOHOOMe
O
OHOHO
1) Rh(PPh3)3Cl, DABCO EtOH, toluene, H2O
2) HgO, HgCl2 acetone, H2O
3) NaOMe, MeOH
4) H2,
Pd(OAc)2 EtOAc, MeOH, AcOH
33% (4 steps)
243 244
aq. NaOH, thenaq. HCl
77%
In 2008, Vogel and co-workers134 used a similar synthetic approach. Unlike Reiffarth & Reimer,
trimethylsilyl trifluoromethanesulfonate-promoted glycosylation with a similar trichloroacetimidate
disaccharide donor and disaccharide acceptor (O-3’ and O-4’ protection was modified compared to
240 and 241),provided them with the desired tetrasaccharide in 60% yield. More recently, in 2013,
Yu and co-workers132 reported the synthesis of tri-, penta-, and heptasaccharide fragments (251, 254
and 257) of the RG-I backbone using the pre-glycosylation oxidation approach with an iterative
elongation of the chain, using disaccharide building block 248 (Scheme 26). The syntheses feature
highly stereoselective formation of the α-D-GalpA-linkage, with very good yields, when using N-
phenyltrifluoroacetimidates as donors.
75
Scheme 26. Synthesis of tri-, penta- and heptasaccharide fragments of RG-I backbone by Yu and co-workers.132
OCOOMeAcO
BnOBnO
O
BnOBnO
O
OCOOMeO
BnOOBn
OAll
1) LiOH MeOH, H2O, 87%
2) H2,
Pd(OH)2/C MeOH, EtOAc, 92%
OCOOHHO
HOHO
O
HOHO
O
OCOOHO
HOOH
OPr
250 251
NaOMeMeOH
87%
OCOOMeHO
BnOBnO
O
BnOBnO
O
OCOOMeO
BnOOBn
OAll
252
OCOOMeAcO
BnOBnO O CF3
NPh
245
O
OAll
OHBnOBnO
TBSOTfCH2Cl2
68%
246O
COOMeAcO
BnOBnO
O
BnOBnO
O
OAll247
OCOOMeAcO
BnOBnO
O
BnOBnO
O
O
NPh
CF3
248
OCOOMeHO
BnOOBn
OAll
2491) PdCl2 THF, MeOH
2) PhN=C(Cl)CF3 K2CO3,
acetone
87%
248TBSOTftoluene
99%
OCOOMeO
BnOBnO
O
BnOBnO
O
OCOOMeO
BnOOBn
OAll
OCOOMeAcO
BnOBnO
O
BnOBnO
O
OCOOHO
HOHO
O
HOHO
O
OCOOHO
HOOH
OPr
OCOOHHO
HOHO
O
HOHO
O
1) LiOH MeOH, H2O, 82%
2) H2,
Pd(OH)2/C MeOH, EtOAc, 99%
253 254
OCOOMeO
BnOBnO
O
BnOBnO
O
OCOOMeO
BnOOBn
OAll
OCOOMeHO
BnOBnO
O
BnOBnO
O
NaOMeMeOH
86%
248TBSOTftoluene
92%
OCOOMeO
BnOBnO
O
BnOBnO
O
OCOOMeO
BnO OAll
OCOOMeO
BnOBnO
O
BnOBnO
O
OCOOMeAcO
BnOBnO
O
BnOBnO
O
OBn
TBSOTfCH2Cl2
88%
OCOOHO
HOHO
O
HOHO
O
OCOOHO
HO OPr
OCOOHO
HOHO
O
HOHO
O
OCOOHHO
HOHO
O
HOHO
O
OH
1) LiOH MeOH, H2O, 82%
2) H2,
Pd(OH)2/C MeOH, EtOAc, 87%
255 256 257
76
The three other groups who have synthesized RG-I backbone fragments used the post-glycosylation
approach, i.e. synthesizing a neutral backbone and oxidizing the primary hydroxyl groups to the
desired carboxylic acid functionality afterwards. 129,133,136 In 2000, Takeda and co-workers prepared
the unprotected propyl RG-I backbone tetrasaccharide 269 by employing trichloroacetimidate
glycosyl donors (see Scheme 27).133 Selected rhamnose residues bore PMB protective groups at C-4,
allowing for potential introduction of RG-I side chains.
77
Scheme 27. Synthesis of fully unprotected propyl glycosyl tetrasaccharide of RG-I backbone by Takeda and co-workers.133
OOAll
OBnHO
BnOOBn
O
O
NH
CCl3
AcOAcO OAc
258
259AgOTfCH2Cl2
97% OOAll
OBnO
BnOOBn
O
ORRORO
260: R = Ac261: R = H
NaOMe, MeOH70%
OOAll
OBnO
BnOOBn
O
OORO2,2-dimethoxypropane
TsOH
93%
262: R = H263a: R = PMB
PMBCl, NaH, DMF65%
OOAll
OBnO
BnOOBn
O
OR2R3O
R1O
80% AcOH
7a 67%7b
94%
264a: R1 = PMB, R2 = H, R3 = H
265a: R1 = PMB, R2 = H, R3 = Bn266a: R1 = PMB, R2 = Ac, R3 = Bn
1) Bu2SnO, benzene2) BnBr, TBABr
93%Ac2O, py, 71%
OOBnO
BnOBnO
O
OAcBnORO
262: R = H263b
R = BnBnBr, NaH, DMF
82%
1) Bu2SnO, benzene2) BnBr, TBABr
91%Ac2O, py, 99%
264b: R1 = Bn, R2 = H, R3 = H
265b: R1 = Bn, R2 = H, R3 = Bn266b: R1 = Bn, R2 = Ac, R3 = Bn
1) PdCl2, NaOAc,
90% aq. AcOH
2) CCl3CN, DBU CH2Cl2 O CCl3
NH267a: R = PMB (17%)267b: R = Bn (30%)
265a + 267b
AgOTfCH2Cl2
67%
OOBnO
BnOBnO
O
OAcBnOBnO
OOBnO
BnOOBn
O
OBnOPMBO
OAll
1) NaOMe, MeOH, 93%2) Pd/C, H2
, MeOH, 74%
3) TEMPO, KBr, NaClO aq. NaHCO3
, 50%
OCOOHO
HOBnO
O
OHHOHO
OCOOHO
HOOH
O
OHOHO
OPr
268 269
AgOTf-catalyzed glycosylation between allyl galactoside acceptor 258 and trichloroacetimidate
donor 259 afforded the α-linked disaccharide 260 in 97% yield as a single stereoisomer. Protecting
group manipulations afforded four differently protected disaccharides: two trichloroacetimidate
donors 267a and 267b (PMB and Bn protected at the Rhap C-4, respectively) and two 2’-OH acceptors
265a and 265b. One noteworthy aspect is that the two-step conversion into trichloroacetimidate
donors 267a and 267b resulted in modest yields (17% and 30%, respectively). Coupling of 265a and
267b gave the α-linked tetrasaccharide 268 in 67% yield. Deprotection followed by selective oxidation
78
of the primary hydroxyl groups with 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO), KBr and NaClO
afforded the desired unprotected propyl tetrasaccharide 269 in 34% yield over the three steps.
In the same year, Davis and co-workers used an orthogonal approach, combined with a late-stage
oxidation strategy, to synthesize a fully unprotected RG-I backbone tetrasaccharide 281 and its
corresponding methyl ester (see Scheme 28).129 Surprisingly, initial attempts to couple a
galactorhamnosyl disaccharide donor to Galp C4’-OH of a disaccharide acceptor failed. This was
probably due to low reactivity of the acceptor, and forced the authors to change strategy and assemble
the RG-I tetrasaccharide through galactosylation instead of rhamnosylation. The group used both
trichloroacetimidates and ethylthioglycosides as donors, affording good glycosylation yields and α-
stereoselectivity.
79
Scheme 28. Davis and co-workers’ synthesis of an RG-I backbone tetrasaccharide 281.129
O
O
NH
CCl3
OAcBnOBnO
OSEt
OBzHO
BnOOBn
270
271TMSOTfCH2Cl2
65% OSEt
OBzO
BnOOBn
O
OAcBnOBnO
272
O
OAcBnOBnO
SEt
OOBn
OBzHO
BnOOBn
274NIS, TMSOTf
CH2Cl2
75%
273
OOBn
OBzO
BnOOBn
O
ORBnOBnO
275: R = Ac276: R = H
NaOMe, MeOH, THF84%
272
276NIS, TMSOTf
CH2Cl2
83%
OOBn
R2O
BnOOBn
O
OBnOBnO
OR2O
BnOBnO
O
OR1BnO
BnO
OOH
COOHO
HOOH
O
OHOHO
OCOOHO
HOHO
O
OHHOHO
Pd/C, H2, H2O
62%
277: R1 = Ac, R2 = CH2OBz278: R1 = H, R2 = CH2OH279: R1 = H, R2 = COOH280: R1 = H, R2 = COOBn
281
NaOMe, MeOH, THF 78%
Cs2CO3, BnBr, DMF
72% over 2 steps
TEMPO, NaClO2, NaClO
In 2013, Clausen and co-workers synthesized a fully unprotected hexasaccharide backbone of RG- I
using a strategy relying on iterative coupling of a common pentenyl disaccharide glycosyl donor
284.136 The latter was prepared by an efficient chemoselective armed-disarmed coupling of a
thiophenyl rhamnoside donor 282, with a pentenyl galactoside acceptor 283 bearing the strongly
electron-withdrawing pentafluorobenzoyl ester (PFBz) protective group, as shown in Scheme 29. Two
iterative glycosylations using pentenyl disaccharide 284 afforded hexasaccharide 289 in reasonable
yields. Removal of the PFBz groups under Zemplén conditions afforded the Galp primary alcohols,
80
which were oxidized with Dess-Martin periodinane to the aldehydes and then with NaClO2 to the
desired carboxylic acids. As in many other cases, the carboxylic acids were protected with benzyl
esters in order to facilitate purification. Palladium catalyzed hydrogenolysis gave the fully unprotected
hexasaccharide 291.
Scheme 29. Fully unprotected hexasaccharide backbone of RG-I synthesized by Clausen and co-workers.136
O
BnOBnO
ONAP
SPh
OHO OPFBz
BnOOBn
O
NIS, TESOTfEt2O
78%
282
283O
BnOBnO
ONAP
OO OPFBz
BnOOBn
O
284
O
BnOBnO
OR
OO OPFBz
BnOBnOOBn
1) Br2, CH2Cl2
2) BnOH, TBABr
90%
285: R = NAP286: R = H
DDQ, H2O, CH2Cl2, MeOH
74%
284NIS, TESOTf
Et2O
71% O
BnOBnO
O
OO OPFBz
BnOBnOOBn
O
BnOBnO
OR
OO OPFBz
BnOBnO
287: R = NAP288: R = H
DDQ, H2O, CH2Cl2, MeOH
76%
O
BnOBnO
O
OO OH
BnOBnOOBn
O
BnOBnO
O
OO OH
BnOBnO
O
BnOBnO
ONAP
OO OH
BnOBnO
1) 284, NIS, TESOTf, Et2O
2) MeONa, MeOH
40%
O
BnOBnO
O
OOCOOBn
BnOBnOOBn
O
BnOBnO
O
OOCOOBn
BnOBnO
O
BnOBnO
ONAP
OOCOOBn
BnOBnO1) DMP, CH2Cl2
2) NaClO2, NaH2PO4
2-methyl-2-butene tBuOH, H2O, THF
3) PhCHN2, EtOAc
60%
O
HOHO
O
OOCOOH
HOHO
O
HOHO
O
OOCOOH
HOHO
O
HOHO
OH
OOCOOH
HOHO
H2, Pd/C, MeOHTHF, H2O
95%
OH289 290 291
RG-I side chains
As explained in the introduction to rhamnogalacturonan I (section 5.4), RG-I has three main types of
side chain: galactans, arabinogalactans and arabinans. Since (arabino)galactans are also found in
81
arabinogalactan proteins (AGPs), oligosaccharides related to these structures will be discussed
separately in Chapter 6.
5.5 Rhamnogalacturonan II
OMe
MeO
A B
CD
D-apiose
L-aceric acid
D-2-keto-3-deoxy-D-manno-octulosonic acid (Kdo)
D-3-deoxy-D-lyxo-heptulosaric acid (Dha)
OHOOC
OH
OH
O
OHHO
HOOH
OH
Me
OHO
HO COOHOH
OHOCOOH
HO COOHOH
OHOH
Rare sugar structures:
α-stereochemistry discovered in 2017
Figure 19. Schematic representation of RG-II based on the findings of Gilbert and co-workers.137
Rhamnogalacturonan II (RG-II) constitutes ~10% of pectin in the cell wall of higher plants and is a
structurally complex branched pectic polysaccharide.97 It is present in above-average quantities in
wine and other beverages obtained by fermentation of fruits and vegetables (one liter of red wine
contains between 100 and 150 mg of RG-II while around 20 mg is present in a liter of white wine).138
Therefore, red wine has been used as a source of RG-II in several studies of this polysaccharide.139
Unlike RG-I, the RG-II structure has been regarded to have remarkably little variation and to be
evolutionarily conserved in all vascular plants. Variations are minor and are limited to methylation
and/or acetylation of the side chains.140 However, recent discoveries have shown evidence of variation
also in the monosaccharide composition of the side chains.141 RG-II has an HG backbone, made up of
82
seven to nine residues, and substituted by four different side chains (A–D), which can contain up to
29 monosaccharides, including a number of rare sugars (see Figure 19).100,101,142 These comprise L-
aceric acid (AcefA; 3-C-carboxy-5-deoxy-L-xylose), 2-keto-3-deoxy-D-manno-octulosonic acid
(Kdop), 2-keto-3-deoxy-D-lyxo-heptulosaric acid (Dhap), D-apiose (Apif; 3-C-(hydroxymethyl)-β-D-
erythrose), L-galactose, 2-O-methyl-D-xylose and 2-O-methyl-L-fucose. The monosaccharides are
connected by over 20 different glycosidic linkages. Side chains A and B are the largest and have been
described as highly ramified octa- and heptasaccharides, respectively.143,144 The C and D side chains
are disaccharides, and two extra side chains E and F, composed of a single L-arabinose residue linked
to a mono- or di-substituted galacturonic acid residue respectively, have been isolated from red wine
RG-II recently.100,137,145
Before 2000, knowledge of the primary structure of RG-II was mostly based on the determination of
the glycosidic linkage composition (using methylation analysis) and identification of various
oligosaccharide fragments by mass spectroscopy (after acid and/or enzymatic hydrolysis of RG-II).
Recently, several NMR solution studies have helped determine the fine- and three-dimensional
structure of the oligosaccharide.144,146,147 In 2000, Vidal and co-workers published a convincing NMR
study of an RG-II fragment, which contained a hexasaccharide homogalacturonan backbone
substituted by side chains B and D.146 Their structural studies showed that the original configurations
postulated for the anomeric centers of two residues in the B-chain were incorrect. This led some other
groups to look in more detail at the structure of RG-II. In 2003, Pérez and co-workers reported a
thorough NMR investigation of RG-II. They reassigned most of the RG-II primary structure and used
NOESY data to propose a conformational model of the oligosaccharide in solution.145 Nevertheless,
several NMR assignments remained ambiguous. In 2017, Gilbert and co-workers published a revised
83
model of the RG-II structure based on an impressive work including crystal structure of side chain B
in complex with a glycosyl hydrolase combined with enzymatic degradation studies. This revised
structure is the one shown in Figure 19.137 The main difference compared to previously proposed
structures of RG-II is highlighted by pink circles: the L-Rhap-D-Apif linkage in chains A and B is α
and not β. It has been observed that more than 90% of the RG-II in the cell wall self-assembles into a
covalently cross-linked dimer in the presence of boron. RG-II is dimerized by borate ester formation
involving the apiosyl residues of side chain A belonging to two different RG-II monomers, which
results in networks of pectic polysaccharides.100 This secondary structure has been confirmed by 11B-
NMR.148 Several studies support the idea that RG-II and its borate crosslinking support cell adhesion
and wall mechanical strength and are therefore significant for plant growth and development.149,150
All the knowledge about RG-II structure and function accumulated before 2004 is summarized in a
comprehensive review by Darvill and co-workers.100
Most synthetic efforts have focused on side chain A and B, which also constitute most of the RG-II
structure, as mentioned previously. Since many of the synthesized fragments are only disaccharides,
the goal of synthesizing them was often not so much to study RG-II function, but rather to develop
synthetic methods. Furthermore, as the chemical synthesis of some of the rare sugars present in RG-
II is not straightforward, we will refer the reader to selected articles reporting methods that have been
developed for their preparation.
RG-II side chain A
Despite the fact that RG-II side chains were thought to be highly conserved in the plant kingdom,
some variations in the composition of side chain A were reported by Léonard and co-workers in
84
2013.141 They showed that in wild-type Arabidopsis plants, up to 45% of L-fucose was replaced by L-
galactose (see Figure 20). The study also provided evidence that a large proportion of the GlcpA
residues were methyl esterified and some of the β-GalpA residues were O-methylated at positions 3
and 4 (these variations are highlighted in red on Figure 20). Furthermore, until the work published by
Gilbert and co-workers in 2017, it was believed that the Rhap-Apif linkage (shown in pink-dot circle
on Figure 20) was a β-linkage, which was proved to be the wrong stereochemistry by Gilbert’s group.
MeO
A
MeO
AOMe
MeO
Or COOMe
OMe
MeO
Or COOMe
α-stereochemistry discovered in 2017
Figure 20. Schematic representation of side chain A including the variations across different plant species observed by Léonard and co-workers.141,145
Side chain A is attached to the homogalacturonan backbone residue via a D-Apif residue.151,152 One-
and two-dimensional 1H NMR spectroscopic analysis showed that apiose is β-(1→2)-linked to
GalpA.146 The synthesized fragments belonging exclusively to side chain A are reported in Table 13,
while fragments found in both side chain A and B are presented in Table 14.
Table 13. Synthetic fragments found exclusively in RG-II side chain A.
Structure R Year Reference
OR Me 1990 Backman et al.153
85
OR
Me 2005 Chauvin et al.154
OR
Me 2005 Chauvin et al.154
OR
Me 2005 Chauvin et al.154
Jansson and co-workers synthesized a Fucp-Rhap disaccharide by MeOTf-promoted glycosylation
between 2,3,4-tri-O-benzyl-1-thio-β-L-fucopyranoside 292 and methyl 2,3-isopropylidene-α-L-
rhamnopyranoside 293 affording the desired α-linked disaccharide 294 in 77% yield as shown in
Scheme 30.153
Scheme 30. α-L-Fucp(1→2)-α-L-RhapOMe fragment synthesized by Jansson and co-workers.153
O SEtOBn
OBnBnO
Me
O
O
Me
OMe
OHO
292
293MeOTfEt2O
77% OOBn
OBnBnO
Me
O
O
MeOMe
OO
294
90% aq. AcOHthen H2
, Pd/C
72% OOH
OHHO
Me
O
HO
MeOMe
OHO
295
In 2005, Nepogodiev, Field and co-workers achieved the synthesis of branched tetrasaccharide
fragment 308 following the route shown in Scheme 31. As this tetrasaccharide contains a combination
of two 6-deoxy sugar residues (Rhap and Fucp) and two uronic acids (GalpA), they chose a post-
86
glycosylation oxidation strategy in order to introduce the carboxylic acids in the very last step of their
synthesis. The method was based on the use of the differentially protected rhamnoside 299. Sequential
glycosylation afforded the neutral tetrasaccharide 307 in reasonable yields and stereoselectivities
overall. Following deprotection of 307, the primary hydroxyl groups were successfully oxidized using
TEMPO affording the target 308 in 47% yield.
Scheme 31. Synthesis of a branched tetrasaccharide fragment from side chain A by Nepogodiev, Field and co-workers.154
O
OMe
OHHOHO
Me O
OMe
OORO
Me
OEtMe
297: R = H298: R = PMB
296
MeC(OEt)3
TsOH
MeCNO
OMe
OAcHOPMBO
Me80% aq. AcOH
299 (70% 3 steps)
PMBCl, NaH, MeCN
O
OBzBzO
OBzBzO SMe
300NIS, TfOH
CH2Cl2
70%
O
OMe
OROPMBO
Me
OOBz
BzOOBz
BzO
301: R = Ac302: R = H
0.4 M HCl in MeOH45%
O
OMe
OORO
Me
OOBz
BzOOBz
BzO
O
OBnBnO
OBnBnO SMe
303NIS, TMSOTf
CH2Cl2
87%
O
OBn
OBnBnOBnO
304: R = PMB305: R = H
CAN, MeCN, H2O93%
OOBn
Me
BnO
OBn
306NIS (2 equiv.),TfOH (2 equiv.)
Et2O, CH2Cl2
87%
O
OMe
OO
Me
OOBzBzO OBz
BzO
O
OBn
OBnBnOBnO
O
OOBn
Me
BnO
OBnSPh
OOH
Me
HO
OH
O
OMe
OO
Me
ORHOOH
HO
OR
OHHOHO
O1) 0.05% NaOMe, MeOH
then H2, Pd/C, EtOH
61%
2) TEMPO, NaOCl, KBr H2O 47%
307 308: R = COOH
The linear trisaccharide α-L-Rhap-(1→3’)-β-D-Apif-(1→2)-α-D-Galp is a fragment that is common to
both side chain A and B. However, since the linkage between the rhamnose and the apiose residues
was believed to be β until 2017, chemists focused on synthesizing fragments with this configuration.
Reimer and co-workers123 and Nepogodiev and co-workers155 both reported the synthesis of
disaccharide elements of the fragment in 2004 (β-D-Apif(1→2)-α-D-Galp and β-L-Rhap-(1→3’)-β-D-
Apif, respectively). In 2011, Nepogodiev, Field and co-workers124 reported the synthesis of the full
structure (see Table 14).
87
Table 14. Synthetic fragments found in both side chain A and side chain B of RG-II.
Structure R Year Reference
OR
Me
Me
2004
2011
Buffet et al.123
Nepogodiev et al.124
OR
Me 2004 Chauvin et al.155
OR
Me 2011 Nepogodiev et al.124
All these syntheses required access to the rare sugar apiose. As explained in section 5.3, D-apiose
derivatives are usually produced from D-mannose, L-arabinose or D-xylose.125–128 The approach by
Reimer and co-workers was described earlier (see Scheme 23). The two other fragments synthesized,
required the challenging β-rhamnosylation to form β-L-Rhap-(1→3)-β-D-Apif disaccharide. Unlike
for β-mannosylation, only few methods for β-rhamnosylation have been reported.156,157 The first
report by Hodosi and co-workers in 1997 described the SN2 coupling between a sugar triflate and an
unprotected 1,2-O-stannylene acetal of L-rhamnopyranose.157 In 2003, Crich and Picione reported a
second method enabling the direct formation of a β-L-rhamnoside by means of thiorhamnoside donors
protected with a 2-O-sulfonate ester and, ideally, a 4-O-benzoyl ester.156 After attempting in vain to
apply Hodosi’s method for the synthesis of β-L-Rhap-(1→3)-β-D-Apif, Nepogodiev and co-workers
decided to use a cyclic 2,3-carbonate rhamnosyl bromide donor as shown in Scheme 32.124 It is worth
mentioning that the synthesis of thiophenyl apiofuranoside residue 310 required extensive
88
optimization and was achieved through a tedious 9-steps synthesis from L-arabinose. The authors did
test simpler protecting group schemes, but these strategies were unsuccessful. Koenigs-Knorr β-
rhamnosylation between 313 and apiogalacturonate acceptor 312 afforded the desired core
trisaccharide 314 in 59% yield, which was deprotected in four steps to reach the target molecule. In
2012, Yasomanee and Demchenko demonstrated a third approach by forming a β-rhamnoside
employing a picoloyl protecting group in the 3-position of a thioethyl rhamnose donor.158
Scheme 32. Synthesis of trisaccharide β-L-Rhap-(1→3’)-β-D-Apif(1→2)-α-D-Galp by Nepogodiev, Field and co-workers.124
O
Br
OOAcO
Me
O
O
OO
HPh
ClAcO
310
STol
OCOOMeO
OHOOMe
309
NIS, TfOHCH2Cl2
62%
OCOOMeO
OO
OMeO
OO
HPh
RO
311: R = ClAc312: R = H
Ag2OCH2Cl2
59%
313O
COOMeO
OO
OMeO
OO
HPh
OO
OOAcO
Me
O
OCOOHHO
HOO
OMeO
OHHO
OO
OHHOHO
Me
314 315NaOMe, MeOH, 96%
1) H2, Pd/C, EtOAc
2) 80% aq. AcOH3) NaOMe, MeOH
56%
4) Et3N, H2O, MeOH
78%
89
RG-II side chain B
OMe
B
not conserved
α-stereochemistry discovered in 2017
Figure 21. Side chain B of RG-II.
The most common structure of side chain B is represented in Figure 21. However, some variations
have been observed depending on the plant species, which mainly involve the presence or absence of
substituents at the O-2 and/or O-3 positions of the Arap residue.100 The different sugar substituents,
which have been observed at these two positions by Pérez and co-workers, are summarized in Table
15. Furthermore, mono-O- or di-O-acetylation can occur at the O-3 position of the AcefA residue as
well as the O-3/O-4 positions of 2-O-Me-Fucp.145
Table 15. Plant dependent variations observed by Pérez and co-workers for side chain B of RG-II.145
Glycose linked to Arap Plant Source
O-2 O-3
β-L-Araf-(1→2)-α-L-Rhap α-L-Rhap Grape (wine)
Ginseng
Sycamore
3-O-Me α-L-Rhap 3-O-Me α-L-Rhap P. bifurcatum
P. nudum
90
α-L-Rhap 3-O-Me α-L-Rhap C. thalictroides
L. tristachyum
P. bifurcatum
α-L-Rhap OH A. thaliana
E. hyemale
S. kraussiana
The synthetic fragments of side chain B that have been reported to date are summarized in Table 16
and Table 14.
Table 16. Synthetic fragments found exclusively in RG-II side chain B.
Structure R Year Reference
OMe
OR
C8H17
All
2000
2001
Mukherjee et al.159
Amer et al.160
OR
(CH2)3NH2 2008 Rao et al.161
OMe
OR
(CH2)3NH2 2007 Rao & Boons162
91
OMe
OR
(CH2)3NH2 2007 Rao & Boons162
Although aceric acid (AcefA) has only been found in RG-II, and the development of synthetic routes
to this unusual sugar started over five years ago, no RG-II fragments including aceric acid have yet
been reported. Two main approaches have been developed for its synthesis: a 4-step approach from
L-xylose163 and a 6-step synthesis from D-arabinose.164 Both Hindsgaul and co-workers159 and Kosma
and co-workers160 reported the synthesis of 2-O-Me-α-L-Fucp(1→2)-β-D-Galp early in the 21st
Century. The synthetic route employed by Hindsgaul and co-workers159 afforded the desired β-linked
disaccharide 321 in a straightforward manner (Scheme 33).
Scheme 33. Synthesis of 2-O-Me-α-L-Fucp(1→2)-β-D-Galp by Hindsgaul and co-workers159
OBnO
BnOOH
OBn
O(CH2)7CH3
O SEtOBnBnO
OPMB
OBnO
BnOO
OBn
O(CH2)7CH3
OOBnBnO
OR
318: R = PMB319: R = H320: R = Me
DMTST, DTBMPCH2Cl2
67%
316
317
CAN, MeCN, H2O, 70%MeI, NaH, DMF, 86%
OHO
HOO
OH
O(CH2)7CH3
OOHHO
OMe
H2,
Pd(OH)2
MeOH
89%
321
In 2007 Rao & Boons reported the synthesis of two highly challenging targets: tetra- and
hexasaccharide portions of side chain B containing the central L-Arap (338 and 337, respectively,
Scheme 34).162 The tetrasaccharide 338 was chosen as a second target because some plant species,
92
such as Arabidopsis, lack the β-L-Araf-(1→3)-L-Rhap substituent on the 2-position of the L-Arap unit.
This chemical synthesis was challenging because it involved a number of unusual monosaccharides,
due to the steric hindrance around the central arabinopyranoside as well as the presence of both
(1→2)-cis-linked 2-O-methyl-α-L-fucoside and β-L-arabinofuranoside residues, which are difficult to
install stereoselectively. The authors first attempted a convergent [2+2+2] approach. However, the
final glycosylation between tetrasaccharide acceptor 331 and a β-L-Araf-(1→3)-L-Rhap donor gave
an unacceptably low yield (8%). The strategy was therefore revised and the final two monosaccharides
were added sequentially as shown in Scheme 34.
Glycosylation between 2-O-methyl thioethylfucoside donor 322 and galactosyl acceptor 323 carrying
an electron withdrawing benzoyl ester group at the C-3 position afforded the desired α-(1→2)-linked
disaccharide 324 in high yield and with high α-selectivity. Reductive opening of the benzylidene
acetal followed replacement of the ester protecting groups with benzyl ethers, in order increase the
reactivity of acceptor 326.
93
Scheme 34. Tetra- and hexasaccharide portions of side chain B synthesized by Rao & Boons.162
O SEtOMe
OAcAcO
OO
OPh
O(CH2)3N3OH
BzO
NIS,TfOHCH2Cl2
, Et2O
88%
323
322
OO
OPh
O(CH2)3N3O
R1O
O OMeOR2
R2O
324: R1 = Bz, R2 = Ac325: R1 = R2 = Bn (80%)
OHO OBn
O(CH2)3N3O
BnO
O OMeOBnBnO
1) NaOMe, MeOH2) NaH, BnBr, TBAI, DMF
NaCNBH3, THF
2 M HCl, Et2O
85%
326
O O
OAcAcOAcO
NH
CCl3
OOBz
SPhOLev
HO
328
TMSOTf, CH2Cl2
91%
OOBz
SPhOLev
O
O
OAcAcOAcO
329
326NIS, TfOH
CH2Cl2
71%
OO OBn
O(CH2)3N3O
BnO
O OMeOBnBnO
OOBz
ORO
O
OAcAcOAcO
330: R = Lev331: R = H (85%)
NH2NH2⋅AcOH
CH2Cl2, MeOH
O
OBzLevOBzO
SPh
332NIS, TfOH
CH2Cl2
60%
OO OBn
O(CH2)3N3O
BnO
O OMeOBnBnO
OOBz
OO
O
OAcAcOAcO
O
OBzROBzO
333: R = Lev334: R = H (90%)
NH2NH2⋅AcOH
CH2Cl2, MeOH
OO
OOBn
O CCl3
NHtBu2Si
335TMSOTf, CH2Cl2
92%
OO OBn
O(CH2)3N3O
BnO
O OMeOBnBnO
OOBz
OO
O
OAcAcOAcO
O
OBz
BzOO
O
OOBn
tBu2Si
O
OO OH
O(CH2)3NH2O
HO
O OMeOHHO
OOH
OO
O
OHHOHO
O
OH
HOO
OH
OHO
330
OO OH
O(CH2)3NH2O
HO
O OMeOHHO
OOH
OHO
O
OHHOHO
HO
1) TBAF, THF
2) NaOMe, MeOH3) Pd/C, H2
, tBuOH, AcOH, H2O
51%336
337
338
1) NaOMe, MeOH2) Pd/C, H2
, tBuOH, AcOH, H2O
65%
327
Chemoselective TMSOTf-promoted glycosylation between rhamnopyranosyl trichloroacetimidate
donor 327 and thiophenyl arabinopyranoside 328 gave the desired α-(1→3)-linked disaccharide donor
94
329 in 91% yield, with high stereoselectivity, thanks to neighboring group participation from the
acetyl ester of 327. This disaccharide was coupled with acceptor 326 using thiophilic promoter
NIS/TfOH, affording the desired β-(1→3)-linked tetrasaccharide 330 in 71% yield. Selective removal
of the Lev group, followed by coupling with rhamnopyranoside donor 332 afforded α-(1→2)-linked
pentasaccharide 333. After Lev removal, the pentasaccharide was glycosylated with the
conformationally-constrained trichloroacetimidate arabinofuranosyl donor 335 affording
hexasaccharide 336 in 91% yield and, gratifyingly, exclusively as the β-anomer. Deprotection of both
tetrasaccharide 330 and hexasaccharide 336 proceeded smoothly to afford the two target
oligosaccharides.
RG-II side chain C
C
RG II backbone
Figure 22. Side chain C of RG-II.
The C side chain of pectin, consisting of the disaccharide α-L-Rhap-(1→5)-D-KDOp, was first isolated
and characterized in 1985.165 The disaccharide is linked to C3-O of a GalpA residue from the RG-II
backbone (see Figure 22). Synthesis of this side chain has not yet been reported. However, the rare
eight-carbon sugar α-D-KDOp is found in non-plant glycans, and therefore various synthetic
procedures have been developed.165–168
95
RG-II side chain D
D
RG-II backbone
Figure 23. Side chain D of RG-II.
RG-II side chain D, consisting of disaccharide β-L-Araf-(1→5)-D-Dhap, was first isolated from pectin
plant cell wall by Albersheim and co-workers in 1988.169 As for side chain C, this disaccharide is
attached to the C-3 position of a galacturonic acid in the backbone (see Figure 23), and has not been
synthesized either. However a procedure to synthesize the rare sugar 3-deoxy-D-lyxo-heptulosaric
acid (Dhap) was reported in 1995 by Banaszek.170
RG-II side chain E/F
E RG-II backbone
Figure 24. Side chain E of RG-II.
Several independent studies suggest that a single α-L-Araf residue is linked to O-3 of one of the GalpA
backbone residues of RG-II (see Figure 24).147,151,171 However, it has not yet been proven, whether
this residue is linked to O-3 or O-2 of GalpA and no synthesis of this linkage has been reported.
96
(Arabino)galactans and arabinans Arabinogalactans, galactans and arabinans are the three main types of side chain in the pectin RG-I
domain. However, arabinogalactans are also present in many plants as a constituent of hemicellulose
and in arabinogalactan proteins.172
Type II AGs (three examples of possible structures)
Type I AGs Galactans
Figure 25. Exemplary structures of arabinogalactans (AGs) and galactans.
Arabinogalactans (AGs) are cell wall polysaccharides having high proportions of galactose and
arabinose. They are widely present in the plant kingdom and occur as variable complex structures,
whose composition is highly dependent on the plant and even cells from which they originate. They
are not found to be well conserved. Arabinogalactans are usually divided into two groups: type I and
type II, which are schematically represented in Figure 25.131 Type I AGs are characterized by a linear
β-(1→4)-linked D-Galp chain, which may be substituted with α-(1→3)-linked single L-Araf, short α-
(1→3)- or α-(1→5)-linked arabinan oligosaccharides, or with β-(1→6)-linked galactose residues.97
97
Type I AGs are found in pectic complexes in seeds, bulbs and leaves and are mostly covalently bound
to the RG-I backbone.172 Galactans are simply β-(1→4)-linked D-galactan chains. They have been
observed in lengths of up to 40 residues.173 Type II AGs are highly branched polysaccharides
composed of a β-(1→3) or (1→6)-linked D-galactan backbone frequently substituted at C-3 and/or C-
6 with β-galactan or arabinogalactan branching, and occasionally carrying C-2 substitution.97 This
second type of AG exists in various plant tissues, including coniferous woods, mosses as well as gums,
saps and exudates of angiosperms.131 Type II AGs make up the carbohydrate part of arabinogalactan
proteins (AGPs) and account for over 90% of the molecular mass of these complexes. Although the
structure of arabinogalactans appears to be well established, new structural diversity is found in this
class of polysaccharides occasionally. Huisman et al., for example, reported the presence of arabinose
residues in the galactan backbone as well as a terminal L-Arap residue in the side chains of type I AG
from soybean pectin.174 NMR analysis has clearly demonstrated that type I AGs are covalently bound
to rhamnose in RG-I and this is the main type of pectic arabinogalactan.172,175 However, Cordeiro and
co-workers analyzed pectin from starfruit and found evidence of both type I and II AGs among the
RG-I side chains.176 The next section will give an overview of the AG fragments prepared by chemical
synthesis.
6.1 Type I AGs and galactans
β-(1→4)-linked galactan synthesis is challenging, due to the low accessibility and reactivity of the
axially disposed C4-OH. There is a requirement for protecting groups at both C-3 and C-6, which
increases steric hindrance around the nucleophile. Until 2016, only four groups had reported syntheses
of β-(1→4)-D-galacto-oligosaccharides, of which, three only reached the trisaccharide level.177-178
98
Lately a small library of oligogalactans including linear tetra-, penta-, hexa-, and heptasaccharides
have been synthesized by Clausen and co-workers.179 As commented on by Sakamoto and Ishimura
in 2013, the low availability of pure type I AG oligosaccharides has hindered the study of
galactonolytic enzymes.180 No synthesis of type I AGs had been reported until early 2017, when
Pfrengle and co-workers produced a set of type I AGs and determined the substrate specificities of
three β-(1→4)-endogalactanases.181 The galactans and type I AGs that have so far been synthesized
are presented in Table 17.
Table 17. Synthetic β-(1→4)-linked galactans including type I AGs.
Structure R Year Reference
OR Pr
Me
H
1984
1987
2001
El-Shenawy & Schuerch177
Kováč & Taylor182
Lichtenthaler et al.183
OR2
PMP
(CH2)5NH2
2001
2017
Lichtenthaler et al.183
Bartetzko et al.181
OR3
PMP
H
2001
2016
Lichtenthaler et al.183
Andersen et al.179
OR4
PMP
H
(CH2)5NH2
2001
2016
2017
Lichtenthaler et al.183
Andersen et al.179
Bartetzko et al.181
OR5
H 2016 Andersen et al.179
99
OR
(CH2)5NH2 2017 Bartetzko et al.181
OR
(CH2)5NH2 2017 Bartetzko et al.181
OR
(CH2)5NH2 2017 Bartetzko et al.181
OR
(CH2)5NH2 2017 Bartetzko et al.181
OR
(CH2)5NH2 2017 Bartetzko et al.181
OR4
H 2016 Andersen et al.179
OR
H 2016 Andersen et al.179
OR
H 2016 Andersen et al.179
OR
H 2016 Andersen et al.179
OR
H 2016 Andersen et al.179
OR
H 2016 Andersen et al.179
100
The first reported synthesis of a linear β-(1→4)-D-galactan trisaccharide was achieved by Schuerch
& El-Shenawy177 and is shown in Scheme 35.
Scheme 35. Synthesis of protected β-(1→4)-D-galactotriose 16 by Schuerch & El-Shenawy.177
OAcO
BzOBnO
OBn
OAll
1) Rh(PPh3)3Cl iPr2NEt, EtOH
2) HgO, HgCl2 acetone, H2O
89%
OAcO
BzOBnO
OBn
OH
339
1) Ac2O, py
2) TiBr4 EtOAc, CH2Cl2
78%
OAcO
BzOBnO
OBn
Br340 341
OHO
BzOBnO
OBn
OAll342
AgOTresCH2Cl2 65%
OAcO
BzOBnO
OBn
OO
BzOBnO
OBn
OAll343
1) Rh(PPh3)3Cl iPr2NEt, EtOH
2) ZnCl2, ZnO
acetone, H2O
85%
OAcO
BzOBnO
OBn
OO
BzOBnO
OBn
344OH
OAcO
BzOBnO
OBn
OO
BzOBnO
OBn
345 Cl
1) PhNCO N,N'-diphenylurea CH2Cl2
2) HCl (g), CH2Cl2
80%
OAcO
BzOBnO
OBn
OO
BzOBnO
OBn
OO
BzOBnO
OBn
OAll346
AgOTresCH2Cl2
85%
342
1) KCN in 95% EtOH
2) Pd/C, H2 acetone, H2O
70%
OHO
HOHO
OH
OO
HOHO
OH
OO
HOHO
OH
OPr347
The key building block 339 carried an allyl group as an exchangeable protecting group at the anomeric
center and an acetyl group for temporarily blocking the 4-position. A benzoyl group at the C-2 position
ensured neighboring group participation to form the desired β-linkage. Finally, benzyl groups were
used to block the remaining hydroxyl groups. Acceptor 342 was coupled to glycosyl bromide 340 in
a silver tresylate (2,2,2-trifluoroethanesulfonate, OTres) catalyzed glycosylation reaction providing
the disaccharide 343 in 65% yield. This disaccharide was then converted into the corresponding
glycosyl chloride 345, over four steps, with an overall yield of 69%. A second glycosylation of
acceptor 342 afforded the trisaccharide 346 in 85% yield, and this could easily be deprotected to afford
the target 347. Further selective deacetylation of the di- and trisaccharide did not proceed well, and
this strategy did not enable the authors to elongate the chain in a convergent way as they had hoped.
101
To improve the efficiency of this galactotriose synthesis, Kovác & Taylor suggested another approach
starting from acceptor 348.182 This was converted to the glycosyl chloride 349 in two steps (Scheme
36). Coupling of acceptor 348 and donor 349 promoted by silver trifluoromethanesulfonate gave the
desired disaccharide 350 in 62% yield. The bromoacetyl group was selectively removed with thiourea
to give acceptor 351, which was once again glycosylated with glycosyl chloride 349. However, in this
case, the glycosylation went slower and the trisaccharide 352 could only be obtained in 35% yield.
Further elongation was thus not feasible in this case either.
Scheme 36. Synthesis of β-(1→4)-D-galactriose by Kovác & Taylor.182
OHO
BzOBzO
OBz
OMeO
BzOBzO
OBz
Cl
BrAcO+ O
OBzBzO
OBzRO
350: R = AcBr351: R = H (85%)
348 349
O
OBzBzO
OBzO
OMe
O
OHHO
OHO
O
OHHO
OHO
OMe
O
OHHO
OHHO
352
1) 1,1,3,3-tetramethylurea BrAcBr 1,2-dimethoxyethane
2) ZnCl2, Cl2HCOMe
80%
O
OBzBzO
OBzO
O
OBzBzO
OBzO
OMe
O
OBzBzO
OBzBrAcO349, AgOTf
CH2Cl2
35%
AgOTfCH3NO2
, toluene
62%
thiourea, MeOH
NaOMe, MeOHtoluene
82%
353
Lichtenthaler and co-workers published the first synthetic route by which longer β-(1→4)-linked
galactans, up to a hexasaccharide, could be produced (Scheme 37).183 An iterative block strategy and
easy transformation of the PMP acceptors into thioglycoside donors really did make this a convergent
synthesis. Furthermore, the protecting group pattern was judiciously designed to minimize steric
hindrance around the unreactive galactosyl-4-OH. Hence, allyl and/or benzyl groups were chosen for
C-3 and C-6 in order to enhance the nucleophilicity of the acceptor. In this case, a pivaloyl ester was
chosen for C-2 protection. The acetyl group could then be used for blocking C-4, as mild Zemplén
102
conditions will not remove the Piv ester. MeOTf-mediated glycosylation of thioglycoside building
block 354 and acceptor 355 afforded the disaccharide 356 in a good yield (79%). This could then be
turned both into a new disaccharide donor 357 (by glycosylation with TMSSPh) or a new disaccharide
acceptor 358 (by deacetylation). Two rounds of glycosylation then afforded the hexasaccharide 361
on a gram scale. In comparison with the two previous studies, this strategy afforded remarkably high
glycosylation yields, considering the low reactivity of the Galp C-4 position. However, the removal
of the protecting groups, especially the allyl group, turned out to be challenging, and a five-step
procedure was necessary to reach the target 362.
103
Scheme 37. Synthesis of β-(1→4)-D-hexagalactoside 362 by Lichtenthaler and co-workers.183
OAcO OBn
AllOOPiv
SPhO
HO OBn
AllOOPiv
OPMP+
MeOTf, DTBMPCH2Cl2
79%
354 355
OO OBn
AllOOPiv
OPMP
OAcO OBn
AllOOPiv
NaOMeMeOH85%
OO OBn
AllOOPiv
OPMP
OHO OBn
AllOOPivO
O OBn
AllOOPiv
SPh
OAcO OBn
AllOOPiv
PhSSiMe3BF3
.OEt2
92%
356
357 358
357 + 358
MeOTf 2,6-di-tert-butyl-4-
methylpyridineCH2Cl2
76%
OO OBn
AllOOPiv
ORO OBn
AllOOPiv
OO OBn
AllOOPiv
OPMP
OO OBn
AllOOPiv
359 R = Ac
360 R = H
357, MeOTf2,6-di-tert-butyl-4-
methylpyridineCH2Cl2
78%
OO OBn
AllOOPiv
OO OBn
AllOOPiv
OO OBn
AllOOPiv
OPMP
OO OBn
AllOOPiv
OO OBn
AllOOPiv
OAcO OBn
AllOOPiv
361NaOMe,MeOH
86%
OO OH
HOOH
OO OH
HOOH
OO OH
HOOH
OO OH
HOOH
OO OH
HOOH
OHO OH
HOOH
OPMP
1) LiOH, MeOH, 81%2) DIBAL, [NiCl2(dppp)], THF, Et2O3) Ac2O, DMAP, py4) H2
, Pd/C, AcOH5) NaOMe, MeOH
54% (4 steps)
362
Clausen and co-workers achieved the synthesis of a set of galactooligosaccharides with a β-(1→4)-
linked backbone and the potential for (1→6)-branching. The group conducted a convergent block
synthesis, using a disaccharide building block to create linear oligosaccharides of varying lenghts.
TMSOTf-mediated coupling of trifluoroacetimidate donor 363 and acceptor 364 afforded the key
pentenyl disaccharide 365 in 83% yield (Scheme 38).179
104
Scheme 38. Synthesis of linear β-(1→4)–D-penta- and heptasaccharides by Clausen and co-workers.179
OClAcO
OPivBnO
OBn
O CF3
NPh
OHO
OPivBnO
OBn
O
363 364
+
OClAcO
OPivBnO
OBn
OO
OPivBnO
OBn
O
365
TMSOTfCH2Cl2
83%
OHO
OBnBnO
OBn
OBn
366
OO
OBnBnO
OBn
OBn
ORO
OPivBnO
OBn
OO
OPivBnO
OBnNIS,TESOTfMeCN,CH2Cl2
85%
367: R = ClAc368: R = H
NIS,TESOTfMeCN, CH2Cl2
83%
OO
OBnBnO
OBn
OBn
OO
OPivBnO
OBn
OO
OPivBnO
OBn
369
365O
ClAcO
OPivBnO
OBn
OO
OPivBnO
OBn
NIS,TESOTfMeCN,CH2Cl2
80%
365
OO
OBnBnO
OBn
OBn
OO
OPivBnO
OBn
OO
OPivBnO
OBn
O
OPivBnO
OBn
OO
OPivBnO
OBn
OClAcO
OPivBnO
OBn
OO
OPivBnO
OBn
O
372
NaOMe, MeOH91%
NaOMe, MeOH93%
OO
OHHO
OH
OH
OO
OHHO
OH
OO
OHHO
OH
O
OHHO
OH
OO
OHHO
OH
OHO
OHHO
OH
OO
OHHO
OH
O1) Et4NOH, MeOH, THF2) H2,Pd(OH)2/C, THF, MeOH, H2O
75%
373
OO
OHHO
OH
OH
OO
OHHO
OH
OO
OHHO
OH
OHO
OHHO
OH
OO
OHHO
OH
OO
OBnBnO
OBn
OBn
OO
OPivBnO
OBn
OO
OPivBnO
OBn
OHO
OPivBnO
OBn
OO
OPivBnO
OBn
1) Et4NOH, MeOH, THF2) H2,Pd(OH)2/C, THF, MeOH, H2O
78%
370
371
NIS-TESOTf-mediated glycosylation of the monosaccharide acceptor 366, with disaccharide 365,
afforded the trisaccharide 367. Selective deprotection of the chloroacetyl group, followed by coupling
with 365, enabled the synthesis both of a linear penta- and heptasaccharide (369 and 372, respectively)
105
in high yields. Finally, de-esterification by treatment with Et4NOH, followed by hydrogenolysis gave
the targets 370 and 373 in 78% and 75% yield, respectively.
In 2017, Pfrengle and co-workers reported the synthesis of a small library of type I AG
oligosaccharides through an automated solid phase sequence, using the six monosaccharide building
blocks 121, 374–378 shown in Scheme 39. Fmoc-protected galactose building blocks 374 and 375
were required for the synthesis of the β-(1→4)-linked backbone. A temporary Lev protecting group
was used at the C-3 position of building block 375, allowing for substitution. A third galactose
building block (376) with an Fmoc group in the C-3 position was used to prepare a mixed-linkage
galactan (383). Both benzyl ethers and benzoyl esters were used as permanent protecting groups for
the remaining positions. Finally, the three L-arabinofuranosides 121, 377 and 378 were used for the
side chains.
106
Scheme 39. Automated assembly of type I AGs by Pfrengle and co-workers.181
OBnO
OBzO
POBu
OBuO374
OBnFmocOO
LevOOBz
OP
OBuOBu
O375
OBnFmocOO
FmocOOBz
OP
OBuOBu
O376
OBnBnO
OOBz
BzO OBz
SEt
121
OFmocO
BzO OBz
SEt
377
OOBz
FmocO OBz
SEt
378
OO
HOHO
OH
OO
HOHO
OH
OO
OHHO
OH
O NH23
OHO
HOHO
OH
OO
HOHO
OH
OO
HOHO
OH
OO
OHHO
OH
O NH23
OO
HOHO
OHOO
HOHO
OHOHO
HOHO
OHO
HO
HOHO
OH
OO
HOO
OH
OO
OHHO
OH
O NH23
OOH
HO OH
OO
HOHO
OH
OO
HOHO
OH
OO
OHHO
OH
O NH23
OHO
HOO
OH
OOH
HO OH
OO
HOO
OH
OO
HOHO
OH
OO
OHHO
OH
O NH2
OO
HOHO
OHOHO
HOHO
OH
OOH
HO OH
OHO
HOHO
OH
OO
HOO
OH
OO
OHHO
OH
O NH23
OO
HOOH
OOH
HO
HO
O
HOO
OH
OO
OH
O OH
HO
NH23
OOH
O OHO
OH
HO OH 3
379: 15% 381: 2%380: 6%
382: 7% 384: 8%
385: 38% 386: 5%
OHO
HOHO
OH
OO
HO
OH
OO
HO
HO
OH
OO
HOHO
OH
OOH
NH23
383: 16%
Automated solid-phase synthesis
Coupling:
TMSOTf, CH2Cl2 (374, 375, 376)
NIS, TfOH, CH2Cl2, dioxane (121, 377, 378)
Deprotection:
20% Et3N in DMF (Fmoc) N2H4
, py, AcOH, H2O (Lev)Capping:
BzCl, DMAP, py
Cleavage:
hν, CH2Cl2Deprotection:
NaOMe, THF Pd/C, H2
, EtOAc, MeOH, H2O, AcOH
107
6.2 Type II AGs
Various different type II arabinogalactans have been produced by several groups and the synthesized
fragments are presented in Table 18 and Table 19. For the most part, arabinogalactans with a β-(1→6)-
galactan backbone have been synthesized (Table 19), but two groups reported the synthesis of some
fragments containing a β-(1→3)-galactan backbone, which is mostly found in arabinogalactan
proteins (AGPs, Table 18).184,185
Table 18. Synthetic type II AG fragments containing a β-(1→3)-linked backbone.
Structure R Year Reference
OR
Me 2002 Yang & Du185
OR
(CH2)5NH2 2015 Bartetzko et al.184
OR
(CH2)5NH2 2015 Bartetzko et al.184
OR
(CH2)5NH2 2015 Bartetzko et al.184
OR
(CH2)5NH2 2015 Bartetzko et al.184
108
OR
(CH2)5NH2 2015 Bartetzko et al.184
OR
(CH2)5NH2 2015 Bartetzko et al.184
OR
(CH2)5NH2 2015 Bartetzko et al.184
OR
(CH2)5NH2 2015 Bartetzko et al.184
OR
(CH2)5NH2 2015 Bartetzko et al.184
OR
(CH2)5NH2 2015 Bartetzko et al.184
OR
(CH2)5NH2 2015 Bartetzko et al.184
109
OR
(CH2)5NH2 2015 Bartetzko et al.184
The only solution phase synthesis of β-(1→3)-linked type II arabinogalactan was achieved by Yang
& Du in 2002 and is presented in Scheme 40.185 Regioselective glycosylation of diol galactoside 388
with arabinose donor 387 afforded the desired α-linked disaccharide 389 in 76% yield. Standard
protecting group manipulations gave acceptor 391, which could be further selectively coupled with
trichloroacetimidate donor 392, affording protected trisaccharide 393 in 94% yield. Deprotection gave
the desired trisaccharide 394.
Scheme 40. Synthesis of type-II arabinogalactan trisaccharide 394 by Yang & Du.185
OOBz
BzO OBz
OC(NH)CCl3
OOHO
O
OMeHO
OO
OBz
OBzBzO
OR3O
R2O
OMeR1O
TMSOTf, CH2Cl2
76%
387
388
389: R1 = H , R2, R3 = (CH3)2C390: R1 = Ac , R2, R3 = (CH3)2C391: R1 = Ac , R2 = R3 = H
Ac2O, py
90% TFA
OOBzBzO
BzO
OC(NH)CCl3BzO
TMSOTf, CH2Cl2
94%
392
OO
OBz
OBzBzO
OHO
O
OMeAcO
OOBzBzO
BzOBzO
OO
OH
OHHO
OHO
O
OMeHO
OOHHO
HOOH
393 394
NH3MeOH
74%
In 2015, Pfrengle and co-workers published the synthesis of a small library of 14 type II AGPs through
an automated solid phase approach using the four monosaccharide building blocks 121, 376, 395 and
396 (Scheme 41).184 Both linear and branched oligomers were synthesized as well as arabinogalactan
fragments that had either a β-(1→3)- or a β-(1→6)-galactan backbone.
110
Scheme 41. Examples of the type II AG fragments obtained by Pfrengle and co-workers.184
OFmocO
OBzO
POBu
OBuO376
OBnBnOO
FmocOOBz
OP
OBuOBu
O395
OLevBnOO
BnOOBz
OP
OBuOBu
O396
OFmocBnOO
OBz
BzO OBz
SEt
121
OHO
OHO
OOHHO
OOH
OO
OH
OHHOOH
O NH23
OOHHO
OOHO
OH
HO OH
OHO
OHO
OHO
OOH
OH
O NH23
OOHHO
HOOH
O
OHO
OHO
OHO
OOH
OH
O NH23
OOHHO
HOOH
OOH
OHHO
O
OH
OHHO
O
O
OHO
OHO
OHO
OOH
OH
O NH23
OOHHO
HOOH
OOH
OHHO
O
OH
OHO
OO HO
OH
OHHO
O
OHO
OHO
OHO
OOH
OH
O NH23
OOHHO
HOOH
O
OH
OHO
OHO O
OH
OHO
OOH
OHHO
O
HOHO
OH
OH
OOHHO
OOH
OOHO
OOHO
OH
HO OH
O NH2
3
O
OH
OHHO
OH
OOHO
HOOH
O NH23
O
OHO
HO
OO
OH
OHHO
OH
OOH
HO
HO397:17% 398: 17%
399: 10% 400: 16%
401: 16% 402: 11% 403: 9%
Automated solid-phase synthesis
Coupling:
TMSOTf, CH2Cl2 (376, 395, 396)
NIS, TfOH, CH2Cl2, dioxane (121)
Deprotection:
20% Et3N in DMF (Fmoc) N2H4
, py, AcOH, H2O (Lev)Capping:
BzCl, DMAP, py
Cleavage:
hν, CH2Cl2Deprotection:
NaOMe, THF Pd/C, H2
, EtOAc, MeOH, H2O, AcOH
Table 19. Synthetic type II AG fragments containing a β-(1→6)-linked backbone.
Structure R Year Reference
OR
(CH2)5NH2 2015 Bartetzko et al.184
111
OR
H
Me
2002
2010
Ning et al.186
Kaji et al.187
OR
(CH2)11CO2Me
C12H25
1998
2000
2003
Timmers et al.188
Du et al.189
Ning et al.190
OR
(CH2)11CO2Me
H
C12H25
1998
2002
2003
Timmers et al.188
Csávás et al.191
Ning et al.190
OR
(CH2)3NH2
(CH2)5NH2
2009
2013
2015
Fekete et al.192
Liang et al.193
Bartetzko et al.184
OR
(CH2)11CO2Me
C12H25
1998
2003
Timmers et al.188
Ning et al.190
OR
H
PMP
2002
2003
Csávás et al.191
Zeng et al.194
OR
(CH2)3NH2 2009 Fekete et al.192
112
OR
C12H25 2003 Ning et al.190
OR
PMP 2003 Zeng et al.194
OR
Me
H
2001
2002
Gu et al.195
Ning et al.186
OR
PMP 2005 Li et al.196
OR
(CH2)3NH2
2014 Huang et al.197
OR
PMP 2004 Li et al.198
113
OR
PMP 2005 Li et al.196
OR
PMP 2004 Li et al.199
OR
PMP 2004 Li et al.199
OR
2
PMP 2005 Li et al.196
The first synthesis of a tetrameric arabinogalactan with a β-(1→6)-linked backbone was achieved in
1998 by van Boom and co-workers, based on 1,2-anhydrosugar building blocks, as shown in Scheme
114
42.188 The group synthesized three different tetrasaccharide analogues, differing in the position of the
arabinofuranosyl side chain, which was linked either to the 2-, 2’- or 2’’-position of the trisaccharide
backbone. To afford the three targets, an oxidative coupling of galactals was undertaken. This
methodology involved a two-step procedure starting with stereoselective epoxidation of a suitably-
protected galactal 404 with 3,3-dimethyldioxirane, followed by ZnCl2-catalyzed condensation of the
resulting 1,2-anhydrosugar with a galactal acceptor. This glycal approach afforded an unprotected 2-
hydroxyl function, ready for glycosylation with an arabinosyl donor in order to introduce the
branching.
115
Scheme 42. Example of one of the three tetrameric arabinogalactans containing a β-(1→6)-linked backbone prepared by van Boom and co-workers.188
OOTBDPSBnO
BnOO
OTBDPSBnO
BnO
O
OOBz
BzO OBz
SEt
3,3-dimethyldioxiraneCH2Cl2
, acetone
quant.
HO OMe
O
10
OBnO
BnO O(CH2)11CO2MeOH
OTBDPS OBnO
BnO O(CH2)11CO2MeO
OR
OOBz
BzO OBz
OOTBDPSBnO
BnO
O
OBnO
BnO O(CH2)11CO2MeO
OOBz
BzO OBz
OBnO
BnOOH
OR
O
ZnCl2, THF
78%
NIS, TfOHClCH2CH2Cl, THF
91%
408: R = TBDPS409: R = H
410: R = TBDPS411: R = H
OBnO
BnO O(CH2)11CO2MeO
OOBz
BzO OBz
OBnO
BnOOH
O
OBnO
BnOOH
OTBDPS
O
OHO
HO O(CH2)11CO2MeO
OOH
HO OH
OHO
HOOH
O
OHO
HOOH
OH
O
ZnCl2, THF
68%
404 405
406
407
121
405
412
413
TBAF, THF, 80%
TBAF, THF, 76%
1) TBAF, THF2) tBuOK, CH2Cl2
, MeOH3) H2
, Pd/C, tBuOH, H2O
64%
OOTBDPSBnO
BnO
O
ZnCl2, THF
60%
405
Kong and co-workers later reported several syntheses of well-defined arabinogalactans carrying
different lengths of arabinan side chain.189,194–196,198,199 They started by producing a tetrasaccharide in
2000, and by 2005, they had reached a chain length of up to 20 residues. As one example of their
work, the synthesis of octaarabinogalactan 429 is presented in Scheme 43.198 This target consists of a
β-(1→6)-linked pentagalactan carrying arabinofuranosyl and diarabinofuranosyl branches at its 2’’’-
and 2’-positions, respectively.
116
Scheme 43. Synthesis of octaarabinogalactan by Kong and co-workers.198
O
OH
OPMPOAc
BzO
BzO
O
OBz
BzOBzO
BzO
OC(NH)CCl3
O
OBz
BzOBzO
BzO
OOROAc
BzO
BzOO
415: R = PMP (β)416: R = C(NH)CCl3
(α)
392
414TMSOTf, CH2Cl24-methoxyphenol
81%
1) CAN, H2O, CH3CN2) CCl3CN, K2CO3
, CH2Cl2 79%
O
OH
OPMPOBz
BzO
BzO
417
TMSOTf, CH2Cl24-methoxyphenol
80%
O
OBz
BzOBzO
BzO
O
ROBzO
BzOO
OOPMP
OBzBzO
BzOO
418: R = Ac419: R = H (79%)
1% MeCOCl, MeOHCH2Cl2
OOBz
BzO OBz
OC(NH)CCl3
O
OBz
BzOBzO
BzO
O
OBzO
BzOO
OOROBz
BzO
BzOO
387
TMSOTf, CH2Cl2
78%
OOBz
BzO OBz
420: R = PMP (β)421: R = C(NH)CCl3
(α)
1) CAN, H2O, CH3CN2) CCl3CN, K2CO3
, CH2Cl2 81%
O
OH
OPMPOBz
BzO
BzO
417
O
OAc
AcOBzO
BzO
OC(NH)CCl3422
TMSOTf, CH2Cl24-methoxyphenol
81% OOPMP
OBzBzO
BzO
O
OR
AcOBzO
BzO
O
423: R = Ac424: R = H (82%)
0.1% MeCOClMeOH, CH2Cl2
�421+ 424
O
OBz
BzOBzO
BzO
O
OBzO
BzOO
O
BzOBzO
BzOO
OOBz
BzO OBz
OOPMP
OBzBzO
BzO
O
ROBzO
BzO
O
O
425: R = Ac426: R = H (72%)
1% MeCOClMeOH, CH2Cl2
TMSOTf, CH2Cl2
75%
OOBz
BzO OBz427
TMSOTf, CH2Cl2
75%
OOBz
O OBz
OC(NH)CCl3O
OR
RORO
RO
O
ORO
ROO
O
RORO
ROO
OOR
RO OR
OOPMP
ORRO
RO
O
ORO
RO
O
O
OOR
RO OR
OOR
O OR
428: R = Bz429: R = H (84%)
NH3, MeOH
Branching at both the C-2 and C-3 positions is a well-known characteristic for AGs. In order to
evaluate differences in immunomodulating activity between C-2 and C-3 branched variants, the same
group synthesized a series of model AGs in 2005. To illustrate their method, the retrosynthetic analysis
of the arabinogalactan 20-mer is shown in Scheme 44.196 The synthesis of the key tetrasaccharide
building blocks 431–433 was achieved from the monosaccharides 387, 417, 422, 434 and 435.
117
Through coupling reactions between the tetrasaccharides, the authors reached arabinogalactans of
varying lengths, up to the longest (430), with twenty monosaccharides.
Scheme 44. Retrosynthetic analysis of AG 430 synthesized by Kong and co-workers.196
OBzO
OBzO
O
OAcBzO
BzOBzO
O
OBzO
BzOBzO
O
OC(NH)CCl3
OOBz
BzO OBz
OBzO
BzOO
O
OAcBzO
BzOBzO
O
OBzO
BzOBzO
O
OC(NH)CCl3
OOBz
BzO OBz
O
OAcBzO
BzOAcO
OC(NH)CCl3
OBzO
OBzO
O
OHBzO
BzOBzO
O
OBzO
BzOBzO
O
OPMPO
OBz
BzO OBz
O
OHBzO
AllOOBz
SiPrO
OAcBzO
BzOBzO
OC(NH)CCl3
OBzO
BzOOBz
OPMP
OHO
OBz
BzO OBz
OC(NH)CCl3
OHO
HOO
OHO
HOHO
O
OHO
HOHO
O
OOH
HO OH
OHO
OHO
OHO
HOHO
O
OHO
HOHO
O
OPMPO
OH
HO OH
O
OHO
OHO
OHO
HOHO
O
OHO
HOHO
O
OOH
HO OH
O
O
H
2
434 435 417 387 422
431 432 433
430
118
Separately, the groups of Yang and Zhang have developed one-pot glycosylation strategies for the
synthesis of 3,6-branched tetra- and hexaarabinogalactan. Both groups relied on the same
galactopyranosyl thioglycoside diol 436 as a key glycosylating agent, as shown in Scheme 45. Zhang
and co-workers, achieved the synthesis of the hexasaccharide 441 from the assembly of two
disaccharides and two monosaccharides, in a one-pot procedure involving three glycosylation
reactions, with an overall yield of 60%.197
OBzO
BzOBzO
O
OBzBzO
BzOBzOO
BzO
HOOBz
SPh
OHO
OBz
BzO OBz
SPhO
OC(NH)CCl3
OBzO
BzOOBz
O
OHBzO
BzOBzO
O
O(CH2)3N3
436 439437 438
OBzO
HOOBz
SPh
OH
436
1) 437, TMSOTf, CH2Cl22) 438, NIS, TfOH, CH2Cl23) 439, NIS, TfOH,
CH2Cl2
60% over 3 steps
OBzO
OBzO
OBzO
BzOBzO
O
OBzBzO
BzOBzO
O
OBzO
BzOOBz
OBzO
BzOBzO
O
O(CH2)3N3
O
O
OOBz
BzO OBz
OHO
OHO
OHO
HOHO
O
OHHO
HOHO
O
OHO
HOOH
OHO
HOHO
O
O(CH2)3NH2
O
O
OOH
HO OH
440 441
1) NaOMe, MeOH, 88%
2) Pd/C, H2, MeOH, 81%
Scheme 45. One-pot synthesis to reach a hexaarabinogalactan by Zhang and co-workers.197
In the same manner and starting from the same key monosaccharide building block 436, Yang and
co-workers achieved the synthesis of their tetrasaccharide in a one-pot manner, reaching the target in
52% yield over three glycosylation steps.
119
6.3 Arabinans
linear arabinan
branched arabinan Figure 26. Example of linear and branched arabinan structures.
Arabinans consist of α-(1→5)-linked L-Araf residues, with some degree of α-(1→3) and/or α-(1→2)
branching, as shown in Figure 26. In many schematic representations of pectin, the arabinan side
chains are shown as being attached directly to the rhamnose units of the RG-I backbone. 6,99,200–202
Very limited evidence for this claim has been reported, however. Most arabinan has been observed as
being linked to galactose. A great deal of work has been carried out on synthesizing oligomers of D-
arabinofuranosides from bacterial cell walls highlighted by a highly efficient assembly of a 92-mer
polysaccharide published in 2017 by Xin-Shan and co-workers.203 Significantly fewer syntheses of L-
arabinofuranosides have been reported and they are presented in Table 20.
Table 20. Synthetic α-L-arabinofuranosides.
Structure R Year Reference
OR
Me 1986 Nepogodiev et al.204
OR
Me 1995 Kaneko et al.205
120
OR
6 All 1999 Du et al.206,207
The first synthesis of a diarabinofuranoside was carried out by Arnd & Graffi in 1976.208 Ten years
later, Nepogodiev and co-workers reported the synthesis of a linear α-(1→3)-linked L-arabinotrioside
as shown in Scheme 46.204 TrClO4-mediated coupling of disaccharide 444 with monosaccharide 445
afforded trisaccharide 446 in 85% yield. This was further deesterified to obtain the target triarabinan
447.
Scheme 46. Synthesis of a linear α-(1→3)-linked L-arabinotrioside by Nepogodiev and co-workers.204
+TrClO4
, CH2Cl2
85%O
OBz
BzO OBz
BrO
OH
AcO OOCN
OO
AcO OOCN
OOBz
OBzBzO
OOTr
AcO OAc
OMe
OO
AcO OAc
OMeOOBz
BzO OBz
O
OAcAcO
O
OO
HO OH
OMeOOH
HO OH
O
OHHO
O
442 443 444
445
446 447
4Å MSMeCN
44%
NaOMe, MeOH
64%
In 1995, Kusakabe and co-workers synthesized all three α-L-diarabinofuranosides as well as the
trisaccharide shown in Table 20.205,209 Koenigs-Knorr glycosylations promoted by AgOTf afforded
the four targets. The synthesis of the trisaccharide also involved simultaneous couplings onto the 3-
and 5-positions of the reducing end monosaccharide. A year later, Kong and co-workers reported the
synthesis of a linear octaarabinan, bearing an allyl group at the anomeric position, as shown in Scheme
47.206 They were able to perform a regioselective glycosylation between the perbenzoylated
trichloroacetimidate donor 387 and the unprotected allyl α-L-arabinofuranoside 448 to obtain partially
protected disaccharide 449 in 78% yield. The latter was converted both to the diol disaccharide 450
and the trichloroacetimidate disaccharide donor 451, which were further coupled in a selective manner
121
to form arabinotetrasaccharide 452 in 79% yield. This could then be converted into both a new
acceptor 453, holding five free hydroxyl groups and a new donor 454. By coupling these, an
octasaccharide was obtained, which, after deprotection, afforded the desired target 455 in 71% over
the four last steps.
122
Scheme 47. Synthesis of linear allyl octa-α-(1→5)-L-arabinofuranoside by Kong and co-workers.206
OOBz
OOH
BzO OBz
O CCl3
NH
HO OH
OAll
TMSOTfCH2Cl2
78% OOBz
BzO OBz
OOH
O OH
OAll 1) BnOC(NH)CCl3
TMSOTf
2) NaOMe, MeOH
70% OOH
HO OH
OOBn
O OBn
OAll
OOBz
BzO OBz
OOBz
O OBz
O
1) BzCl, py2) PdCl2
, NaOAc, HOAc3) Cl3CCN, K2CO3
, CH2Cl2 70%
NH
CCl3
387
448
449 450
451
451
450, TMSOTfCH2Cl2
79% OOH
O OH
OOBn
O OBn
OAll
OOBz
BzO OBz
OOBz
O OBz 452
1) BnOC(NH)CCl3
TMSOTf
2) NaOMe, MeOH 67%
OOBn
O OBn
OOBn
O OBn
OAll
OOH
HO OH
OOH
O OH
1) Pd(OH)2/C, H2, EtOAc
2) Ac2O, py3) PdCl2
, NaOAc, HOAc4) Cl3CCN, K2CO3
, CH2Cl2 65%
OOAc
O OAc
OOAc
O OAc
O
OOBz
BzO OBz
OOBz
O OBz
NH
CCl3
453
454
454
1) 453, TMSOTf, CH2Cl22) Pd(OH)2/C, H2
, EtOAc3) BzCl, py4) NaOMe, MeOH
71% OOH
O OH
OOH
O OH
OOH
HO OH
OOH
O OH
OOH
O OH
OOH
O OH
OAll
OOH
O OH
OOH
O OH
455
123
Most recently, Yang and co-workers have synthesized a fully-protected pentaarabinan composed of a
linear α-(1→5)-linked arabinotrioside substituted with an α-L-arabinose unit at the 3- and 3’-positions
through an elegant four-component one-pot synthesis.210
124
Polysaccharides exclusive to algae This section focuses on the most common polysaccharides found exclusively in algal cell walls:
glucuronans, carrageenans, agarans, alginates, fucans and ulvans. β-(1→3)-D-Xylans have also been
isolated from the cell wall of green algae.211,212 So far, only one paper by Kong and Chen reports the
synthesis of such structures.213 A few other, more complex and highly branched sulfated
heteropolysaccharides (xylofucoglucans, xylofucoglucuronans, etc.) have been identified as minor
components in polysaccharide isolates from brown algae. However, their structures have not yet been
fully elucidated, and they have never been the subject of chemical synthesis, therefore they will not
be discussed further in this section.214
7.1 Glucuronans
Figure 27. Glucuronan structure.
Glucuronans are β-(1→4)-D-polyglucuronic acids, i.e. anionic homopolymers as shown in Figure 27.
These polysaccharides have been extracted from the cell wall of green algae such as Ulva and
Enteromorpha compressa.215,216 The biological activity of the glucuronans has only been considered
recently, and no efforts to isolate them were reported before the year 2000.217 In 2009, Redouan and
co-workers reported a rapid process to extract glucuronans selectively in large amounts from the green
algae Ulva lactura, using enzymatic depolymerization. The process could potentially be used on an
industrial scale to produce bioactive glucuronic acid oligosaccharides.218 In 1998, Isogai and co-
125
workers prepared some glucuronan oligosaccharides from cellulose using TEMPO-mediated
oxidation, work which could open up a new field of cellulose exploitation.219 No chemical synthesis
of a glucuronan longer than a disaccharide has been reported to date, and these disaccharides were
produced to optimize the TEMPO-oxidation of cellobiose.
7.2 Carrageenans
The carrageenans are a family of highly-sulfated D-galactans found in the cell walls of certain red
seaweeds of the Rhodophyte class. This family of polysaccharides has the ability to form thermo-
reversible gels or viscous solutions in the presence of salts. They are organized into ten different types
of carrabiose repeating units that differ by their degree of sulfation, and the presence or absence of
3,6-anhydrogalactose residues (see Figure 28).
126
β-carrageenan
α-carrageenanδ-carrageenan
µ-carrageenan κ-carrageenan
ν-carrageenan ι-carrageenan
λ-carrageenan θ-carrageenan
Enzymesor
KOH
γ-carrageenan
Figure 28. Schematic representation of the ten types of idealized disaccharide repeating units of carrageenans.
Natural carrageenans are non-homologous polysaccharides. This diverse family of glycans shares a
common galactan backbone (carrabiose) of alternating 3-linked-β-D-galactopyranose and 4-linked-α-
D-galactopyranose. It has been shown that hot alkaline treatment of the carrageenan having a 4-linked-
α-D-galactopyranose residue sulfated at the 6 position leads to cyclization, forming the 3,6-anhydro
rings.220,221 During carrageenan biosynthesis, this transformation is catalyzed by enzymes. No
synthesis of this type of algal polysaccharide has been reported to date.
127
7.3 Agarans
Enzymesor
KOH
Figure 29. Agarobiose backbone units.
The agarans are another family of sulfated galactans found in red seaweeds. As with carrageenans,
agarans have linear chains of alternating 3-linked β-galactopyranosyl residues and 4-linked-α-
galactopyranosyl residues. They differ from carrageenans in that the latter are L-Galp. As in
carrageenans, these residues can have 3,6-anhydro bridges (Figure 29). In 1991, Lahaye reported that
at least eleven different agarobiose structures could be identified depending on the type of red alga.222
The common modifications observed were mainly the presence of sulfate substituents, pyruvate,
urinate and methoxy groups. Red seaweeds producing agaran were named agarophytes, while the ones
producing carrageenans were called carrageenophytes. However, it has recently been shown that some
carrageenophytes also synthesize considerable amounts of agaran or DL-hybrid galactans.223 No
organic synthesis of agaran has yet been reported.
7.4 Porphyrans
OMe OMe
Enzymesor
KOH
Figure 30. Porphyran repeating unit.
128
The porphyrans are polysaccharides found in the red alga Porphyra umbilicalis, which was
characterized by Rees in 1961.224 This family of glycans shares the same backbone as agarans, but are
substituted with 6-O-methylation of the D-galactose units as shown in Figure 30.225 Similar to
carrageenans, Rees and co-workers showed that suitably linked L-galactose 6-sulphate units in
porphyrans can be converted quantitatively by alkaline treatment into 3,6-anhydrogalactose.224 This
type of oligosaccharide has not received attention from synthetic chemists yet.
7.5 Alginates
The alginates are a family of polysaccharides present in both brown algae and some gram-negative
bacteria. They are composed of two uronic acids: D-mannuronic acid (M) and L-guluronic acid (G,
the C5 epimer of M). The monomers are connected by (1→4) glycosidic linkages, with a 1,2-cis
configuration. The residues are arranged in either homopolymeric (polymannuronate -MM-, or
polyguluronate -GG-) or heteropolymeric (-MG-) segments, as shown in Figure 31.226,19
-MM--MG--GG-
Figure 31. Block composition of alginates.
These anionic polysaccharides can be isolated from marine brown algae. They have widespread
application in the biomaterial and food industries due to their gelling properties. It has been shown
that G-block-rich alginates increase gel strength by forming links with calcium, bridging two
129
antiparallel chains, as described earlier for homogalacturonan (see section 5.1).227 Alginates also offer
putative medical benefits, so there is considerable interest in generating chemically well-defined
alginate fragments. The assembly of mixed alginate sequences -MG- is particularly challenging, as it
requires the synthesis of β-D-mannuronic acid and α-L-guluronic acid, both of which have 1,2-cis
linkages. Only one synthesis of mixed sequence alginate oligosaccharides has in fact been reported,
by Codée and co-workers in 2015.228 Table 21 gives an overview of all the alginate fragments
synthesized to date.
Table 21. Synthetic alginates fragments.
Structure R Year Reference
OR (CH2)3NH2 2006
2008
van den Bos et al.229
Codée et al.230
OR2
(CH2)3NH2
Pr
2008
2012
Codée et al.230
Walvoort et al.231
OR3
(CH2)3NH2 2008 Codée et al.230
OR4
Pr 2015 Tang et al.92
OR6
Pr 2012 Walvoort et al.231
OR10
Pr 2012 Walvoort et al.231
OR (CH2)3NH2 2008 Dinkelaar et al.232
130
Pr 2009 Chi et al.233
OR2
Pr 2009 Chi et al.233
ORn = 1,2,3
(CH2)2NHAc 2015 Zhang et al.228
OR
n = 1,2
(CH2)2NHAc 2015 Zhang et al.228
OR (CH2)2NHAc 2015 Zhang et al.228
As mentioned earlier (section 4.5), forming the β-mannosidic linkage is considered one of the biggest
challenges in carbohydrate chemistry. Lately, syntheses of –MM- fragments up to dodecamers have
been published. In 2006, van der Marel and co-workers discovered the use of 1-thiomannuronate ester
donors in the stereocontrolled formation of 1,2-cis mannuronic linkages.229 They proved that the β-
selectivity of the mannuronate ester comes from the stereoelectronic influence of the C-5 ester and
demonstrated the efficiency of this strategy in the generation of longer alginate oligosaccharides (up
to a pentasaccharide) using a block coupling strategy (Scheme 48).230 Thioglycosyl donors were pre-
activated prior to coupling, and it was shown that judicious choice of promoter was crucial in ensuring
high yields as the oligomers grew in length. Excellent β-selectivity was maintained in all glycosylation
reactions and was not compromised by the size of the coupling partners.
131
Scheme 48. Assembly of mannuronate ester oligomers (-MM-) by van der Marel and co-workers.230
OMeO2CRO
OBn
BnOO
SPh
MeO2CO
OBn
BnO
O
X
OBnMeO2CROBnO
OOBnMeO2C
ROBnO O
CO2H
OOBnMeO2C
HOBnO SPh
456: X = β-SPh, R = Lev457: X = α-SPh, R = Bn
458: R = Lev (66%)459: R = Bn (65%)
4601) benzyl 2-
(hydroxymethylbenzoate) Ph2SO, TTBP, CH2Cl2
2) H2, Pd/C, NH4OAc, EtOAc, MeOH
461: R = Lev (65%, α:β = 1:10) 462: R = Bn (65%, α:β = 1:10)
OMeO2CHO
OBn
BnOO
O
MeO2CO
OBn
BnO
N3O
OBnMeO2CHOBnO O
N3
463 464
461 + 463 OMeO2C
OOBn
BnOO
O
MeO2CO
OBn
BnO
N3O
OBnMeO2CLevO
BnO
a) Ph2SO, TTBP, CH2Cl2 28% (α:β = 1:10)
or
b) BSP, Tf2O, TTBP, CH2Cl2
74% (α:β = 1:10)
Ph2SO, TTBP, CH2Cl2
BSP, Tf2O, TTBP, CH2Cl2
67% (α:β = 1:10)
OMeO2C
OOBn
BnOO
O
MeO2CO
OBn
BnO
N3O
OBnMeO2CBnO
BnO
OMeO2C
OOBn
BnOO
O
MeO2CO
OBn
BnO
N3O
OBnMeO2CBnO
BnO
BSP, Tf2O, TTBP, CH2Cl2
69% (α:β = 1:10)
2
3
467
468
OMeO2C
OOBn
BnOO
O
MeO2CO
OBn
BnO
N3O
OBnMeO2CHOBnO
H2NNH2py, HOAc
78%
465 466
OMeO2C
OOBn
BnOO
O
MeO2CO
OBn
BnO
N3O
OBnMeO2CROBnO
n
464: n = 0, R = H466: n = 1, R = H467: n = 2, R = Bn468: n = 3, R = Bn
OHO2CO
OH
HOO
O
HO2CO
OBn
HO
H2NO
OHHO2CHO
HO
469: n = 0 (59%)470: n = 1 (86%)471: n = 2 (65%)472: n = 3 (76%)
n
1) KOH, THF, H2O
2) H2, Pd/C, MeOH, HOAc
(tBuOH, H2O)
462 + 464
462 + 466
Mannuronic acid donors were also used by Codée and co-workers in an automated solid-phase
synthesis, to build alginates up to a dodecasaccharide. Trifluoroacetimidate donors were used as
building blocks, since pre-activation of donors was not possible on their solid phase synthesizer and
because the nature of the linker prohibited the use of soft electrophiles required for the activation of
132
thioglycosides. The stereodirecting effect of the C-5 carboxylate again provided the desired (1→2)-
cis glycosides with excellent stereoselectivity.
The synthesis of short L-guluronic acid oligomers (-GG- blocks up to a tetrasaccharide) has been
achieved by only two groups, Hung and co-workers233 and van der Marel, Codée and co-workers.232
As L-gulose and L-guluronic acid are rare sugars, they are not commercially available and need to be
synthesized prior to use, as shown in Scheme 49.
Scheme 49. Synthesis of L-gulose.232,233
OOH
OH
OH
HOOH
L-gulose476
OOHOH
O
OHHO
L-gulonic acidγ-lactone
474100 g scale
OOO
OH
OO
1) H2SO4 dimethoxypropane acetone
2) DIBAL-H toluene
84%
20% TFA in H2O
quant.
OOHOH
O
OHHOL-ascorbic acid
Hung and co-workers
van der Marel, Codée and co-workers
473
475
In contrast to D-mannopyranose, L-gulopyranose has an intrinsic preference for the formation of 1,2-
cis-glycosidic bonds without the need to incorporate stereodirecting groups. Furthermore, Codée and
co-workers demonstrated that L-guluronic acid donors are less stereoselective in glycosylation
reactions than L-gulose donors.232 Therefore, a post-glycosylation oxidation strategy has been the
preferred approach for the synthesis of -GG- block oligosaccharides.
133
After screening the efficiency of several gulose donors, Codée and co-workers found that a
dehydrative glycosylation234 with hemiacetal 479 gave the best yield, and α-selectivity, in the
glycosylation step. They achieved the synthesis of a guluronic acid trisaccharide, as shown in Scheme
50.232
Scheme 50. Synthesis of guluronic acid trisaccharide (-GGG-) by Codée and co-workers.232
O ROBn
OBn
OO
SitBu
tBu
O SPhOH
OH
OH
HO
OOBn
OBn
OHTBSO
ON3
OOBn
OBn
OO
478: R = SPh479: R = OH
O OBnOBn
OTBSO
ON3Ph2SO, TTBP, Tf2O
CH2Cl2
84% (α:β = 10:1)
481
O OBnOBn
OR1O
OOBn
OBn
OR1O
ON3
O OBnOBn
OR2R2O
484: R1, TBS, R2 = Si(tBu)2485: R1, R2 = H
O OHOH
HOOC
O
O OHOH
HOOC
O
ONH2
O OHOH
HOOC
OH
NIS, TFA, CH2Cl2
95%
477
480
TBAF, THF83%
1) tBuOH, H2O, TEMPO, BAIB
2) tBuOH, Pd/C, H2
85%
OOBn
OBn
OR3R1O
O OBnOBn
OR2O
ON3
TBAF, THF
87%
SitButBu
482: R1, R2 = R3 = H483: R1, R2 = TBS, R3 = H
TBSCl, imidazole, DMF78%
479, Ph2SO, TTBP, Tf2O, CH2Cl2then 483
42% only α
486
Hung and co-workers employed 1,6-anhydrogulose synthons to lock the C4-OH in a more accessible
environment and increase the reactivity of the nucleophile.233
As mentioned above, only one synthesis of alginate sequences containing both M and G residues has
been reported, by Codée and co-workers in 2015.228 They decided on an approach employing GM
building blocks, thus featuring a mannuronic acid donor, and pre-glycosylation oxidation in order to
134
minimize functional-group manipulation at a late stage of the synthesis. The preparation of the
building blocks required for the assembly of the longer oligosaccharides is presented in Scheme 51.
Glycosylation between the silylene-protected gulose imidate donor 487 and mannuronic acid acceptor
488 or 463 led to GM dimers 489 and 495, respectively, in good yields and with high
stereoselectivities. A desilylation/oxidation/methyl ester formation sequence afforded GM acceptor
building blocks 490 and 496. The trifluoroacetimidate GM donor 493 was then obtained from 490 in
an 83% yield over three steps.
Scheme 51. GM donor and acceptor building blocks for the synthesis of mixed alginate oligosaccharides by Codée and co-workers.228
O OBnOBn
OO
SitBu
tBu
OCF3
NPhO
OBnMeO2CHOBnO
STol
O OBnOBn
MeO2C
OOBn
OBn
O
STol
CO2Me
OR
490: R = H491: R = Lev
TMSOTf, CH2Cl2
91%
OOBnMeO2C
HOBnO
O OBnOBn
OO
SitBu
tBu
OCF3
NPhO
N3
O OBnOBn
MeO2C
OH
O OOBnMeO2C
BnOO
N3
O OBnOBn
MeO2C
OLev
O OOBnMeO2C
BnOOR
492: R = H493: R = C(=NPh)CF3
487
488
487
463
O OBnOBn
MeO2C
OH
O N3
494
1) HF·py, THF2) TEMPO, PhI(OAc)2
tBuOH, THF, H2O3) MeI, K2CO3
DMF
83%
O OBnOBn
OOBn
OBn
O
STol
CO2Me
OO
SitBu
tBu
489
O OBnOBn
O
O OOBnMeO2C
BnOO
N3
OSitBu
tBu
TMSOTf
CH2Cl2
-78 οC to -20 οC
58%495 496
NIS, TFA, CH2Cl291%
1) HF·py, THF2) TEMPO, PhI(OAc)2
tBuOH, THF, H2O3) MeI, K2CO3
DMF
84%
LevOH, EDCI, DMAPCH2Cl2
, 92%
F3CC(=NPh)Cl, K2CO3acetone, 98%
135
With the GM donors and acceptors in hand, the assembly of larger -GM- oligomers could proceed.
This was successfully achieved for structures ranging from the GMG trisaccharide to a GMGMGMG
heptasaccharide (Scheme 52).
136
Scheme 52. Assembly of -GM- oligomers up to heptasaccharide by Codée and co-workers.228
OOBn
OBnMeO2C
OR
O OOBnMeO2C
BnO
OOBn
OBnMeO2C
O
O N3
497: R = Lev498: R = H
493 + 494
TBSOTfCH2Cl2
84%α:β
> 20:1
GMG
N2H4, H2O
AcOH, py, 89%
493TBSOTf, CH2Cl2
31%
493TBSOTf, CH2Cl2
34%
OOBn
OBnMeO2C
O
O OOBnMeO2C
BnO
OOBn
OBnMeO2C
O
O N3
GMGMG
R
2
OOBn
OBnMeO2C
O
O OOBnMeO2C
BnO
OOBn
OBnMeO2C
O
O N3
Lev
3
501
GMGMGMG
493 + 496
TBSOTf CH2Cl2
26%α:β
> 20:1
493TBSOTf, CH2Cl2
30%
GMGM
OOBn
OBnMeO2C
O
O OOBnMeO2C
BnOO
N3
Lev
3OOBn
OBnMeO2C
O
O OOBnMeO2C
BnOO
N3
R
2
GMGMGM
493 + 490
TBSOTf CH2Cl2
91%α:β
> 20:1O
OBn
OBnMeO2C
OLev
O OOBnMeO2C
BnO O
OOBn
OBnMeO2C
OOBn
OBn
O
R
CO2Me
505: R = α-STol506: R = O(C=NPh)CF3
(92%)
505
1) NIS, TFA, CH2Cl22) F3CC(=NPh)Cl, K2CO3
, acetone
GMGM
OOBn
OBnMeO2C
O
O OOBnMeO2C
BnO O
OOBn
OBnMeO2C
OOBn
OBn
O
R
CO2Me
490TBSOTf, CH2Cl2
73%α:β
> 20:1
Lev
2
GMGMGM
499: R = Lev500: R = H
N2H4, H2O
AcOH, py, 98%
503: R = Lev504: R = H
N2H4, H2O
AcOH, py, 78%
507: R = α-STol508: R = O(C=NPh)CF3
(80%)
1) NIS, TFA, CH2Cl22) F3CC(=NPh)Cl, K2CO3
, acetone
OOH
OHNaOOC
O
O OOHNaOOC
HO
OOH
OHNaOOC
O
O NHAc
H
3
1) N2H4, H2O, AcOH, py
2) LiOH, H2O2, H2O, THF
3) Pd/C, H2, tBuOH, THF, H2O
4) Ac2O, NaHCO3,THF, H2O
60%
502
GMGMGMG
Surprisingly, the nature of the reducing end anomeric center of the GM acceptor had a tremendous
effect on the efficiency of glycosylation by influencing the conformation of the acceptor. The rigid
137
GM acceptor 496, bearing a β-linked anomeric azidopropyl spacer, led to lower glycosylation yields
(26–34%), whereas the conformationally-flexible GM-α-STol acceptor 490 gave much better yields
(73–91%).235 The GMGGMG hexasaccharide was synthesized using a [2+3+1] approach with
mannuronic acid donors, as summarized in Error! Reference source not found..
Scheme 53. Convergent assembly of GMGGMG hexasaccharide by Codée and co-workers.228
GGMreducing end
acceptor
GM donor:493
GMGGM GMGGM donor
G acceptor:494
87% 43%GMGGMG
All the oligomers were deprotected by removal of the levulinoyl groups, saponification of the methyl
esters, debenzylation plus azide reduction and finally acetylation of the spacer amine group as shown
for heptamer 502 in Scheme 52.
7.6 Fucans
In 1913, Kylin reported the first extraction of “fucoidin” from marine brown algae.236 Most of the
fucose containing sulfated polysaccharides (FCSPs) found in brown algae are fucoidans, which are
heteropolymers of sulfated fucans. These polysaccharides can account for more than 40% of the dry
weight of isolated cell walls in brown algae.237 Fucoidans have been found to have two types of
homofucose backbone chains as shown in Figure 32. Type I consists of (1→3)-linked α-L-
fucopyranoside residues, while type II is made up of alternating α-(1→3)- and α-(1→4)-L-
fucopyranoside residues.238 The complexity and heterogeneity of fucoidans is due to the sulfation
pattern, which differs depending both on the type of alga as well as between cell types. Sulfation can
138
occur at the O-2, O-3, and/or O-4 position of the fucose residues. Acetylation has also been observed
at various positions of the backbone, as has the presence of other sugar residues such as α-D-
glucuronopyranosyl or α-L-fucopyranosyl.237
Type I Type II
Figure 32. The two types of homofucose backbone chains found in brown seaweed fucoidans with examples of sulfation patterns.
It has been known for some time that fucoidans act as modulators of coagulation.239 But how their
sulfation pattern and conformation influence biological activity has not been studied. Early in the 21st
Century, synthetic chemists began showing interest in these targets, aiming to synthesize certain well-
defined oligofucans and help elucidate their structure-activity relationship. An overview of the type I
and type II fucan and fucoidan fragments synthesized are shown in Table 22 and Table 23,
respectively.
Table 22. Synthetic type I fucan/fucoidan fragments.
Structure R Year Reference
OR
Pr
Pr
2003
2011
Gerbst et al.240
Krylov et al.241
139
OR
Pr 2003 Gerbst et al.240
OR
Pr 2016 Vinnitskiy et al.242
OR
Pr 2016 Vinnitskiy et al.242
OR
Pr 2016 Vinnitskiy et al.242
OR
Pr 2016 Vinnitskiy et al.242
OR
2
C8H17
Pr
2004
2015
2006
2011
Hua et al.243
Kasai et al.244
Ustyuzhanina et al.238
Krylov et al.241
140
OR
2
Pr 2006 Ustyuzhanina et al.238
OR
2
C8H17
2010
2015
Zong et al.245
Kasai et al.244
OR
2
Pr
C8H17
2006
2010
Ustyuzhanina et al.238
Zong et al.245
OR
2
C8H17 2015 Kasai et al.244
OR
2
Pr
2011
Krylov et al.241
OR
2
C8H17
2004
2015
Hua et al.243
Kasai et al.244
OR
3
C8H17 2010 Zong et al.245
141
OR
3
C8H17 2010 Zong et al.245
OR
4
Pr
2006
2011
Ustyuzhanina et al.238
Krylov et al.241
OR
4
Pr
C8H17
2006
2010
Ustyuzhanina et al.238
Zong et al.245
OR
4
Pr 2006 Ustyuzhanina et al.238
OR
4
C8H17 2010 Zong et al.245
OR
5
C8H17 2010 Zong et al.245
OR
6
Pr 2011 Krylov et al.241
142
OR
6
Pr
C8H17
2006
2010
Ustyuzhanina et al.238
Zong et al.245
OR
6
Pr 2011 Krylov et al.241
OR
10
Pr 2011 Krylov et al.241
OR
10
Pr 2011 Krylov et al.241
OR
14
Pr 2011 Krylov et al.241
OR
14
Pr 2011 Krylov et al.241
Table 23. Synthetic type II fucan/fucoidan fragments.
Structure R Year Reference
OR Pr 2011 Krylov et al.241
143
OR
Pr 2011 Krylov et al.241
OR
Pr
C8H17
2006
2011
2014
2015
Ustyuzhanina et al.238
Krylov et al.241
Arafuka et al.246
Kasai et al.244
OR
Pr 2006 Ustyuzhanina et al.238
OR
C8H17
2014
2015
Arafuka et al.246
Kasai et al.244
OR
C8H17
2014
2015
Arafuka et al.246
Kasai et al.244
OR
C8H17
2014
2015
Arafuka et al.246
Kasai et al.244
OR
Pr
C8H17
2011
2014
2015
Krylov et al.241
Arafuka et al.246
Kasai et al.244
OR
2
Pr
2006
2011
Ustyuzhanina et al.238
Krylov et al.241
2
OR
Pr 2006 Ustyuzhanina et al.238
144
OR
2
Pr 2011 Krylov et al.241
In 2003, Nifantiev and co-workers reported the first synthesis of the two type I fucoidan trisaccharides
515 and 516 (Scheme 54).240 Both fucotriosides were prepared by use of the readily accessible
disaccharide acceptor 509.247,248 Regio- and stereoselective glycosylation with fucosyl bromide donor
510, promoted by mercury salts afforded the protected α-linked trisaccharide 511 in 65% yield.
Saponification and subsequent catalytic hydrogenolysis provided fucotrioside target 515 in a good
yield overall. Protecting group manipulations and sulfation gave trisaccharide 514. The latter was
deprotected to afford the sulfated fucotrioside target 516.
Scheme 54. First type I fucan and fucoidan synthesized by Nifantiev and co-workers.240
OMe
HO
OBnO
OAll
OMe
HO
OBnOH
OMe
BzO
OBnOBz
Br
Hg(CN)2,
HgBr2CH2Cl2
65%
509
510
OMe
HO
OBnO
OAll
OMe
HO
OBnO
OMe
RO
OBnOR
511: R = Bz512: R = H (90%)
OMe
HO
OBnO
OAll
OMe
HO
OBnO
OMe
HO
OBnOBz
NaOMe, MeOH 513
Bu2SnO, toluenethen BzCl
62%
H2, Pd/C, MeOH
75%
OMe
HO
OHO
OPr
OMe
HO
OHO
OMe
HO
OHOH
515
SO3.py
DMF
80%
OMe
NaO3SO
OBnO
OAll
OMe
NaO3SO
OBnO
OMe
NaO3SO
OBnOBz
1) H2, Pd/C, MeOH
2) NaOMe, MeOH
78%
OMe
NaO3SO
OHO
OPr
OMe
NaO3SO
OHO
OMe
NaO3SO
OHOH
516
514
145
By applying a stepwise elongation strategy with thioglycoside donors, Du and co-workers
subsequently reported the synthesis of a fully sulfated tetrasaccharide with a β-configured octyl group
at the reducing end.243 In 2006, Nifantiev and co-workers developed a method using [2+2], [2+4] and
[4+4] coupling reactions to produce oligofucoidans of both type I and type II, employing
trichloroacetimidates as donors.238 Inspired by these syntheses and in order also to generate type I
oligofucoidans of uneven length, Li and co-workers decided to apply a [2+2] coupling strategy to
produce the tetrasaccharide, followed by an iterative assembly of the remaining targets, as shown in
Scheme 55.245 They used 4-O-benzoyl protecting groups, claiming this improved the α-
stereoselectivity of the glycosylation steps through remote participation, in combination with 3-O-
acetyl and 2-O-benzyl groups. This way, they could produce two different sulfation patterns from
each oligofucoidan precursor.
Scheme 55. Retrosynthetic analysis of sulfated tetra- to octafucosides by Li and co-workers.245
OMe
R2O
OR1
O
OC8H17
OMe
R2O
OR1
O
OMe
R2O
OR1
O
OMe
R2O
OR1
OR3
n
OMe
BzO
OBnO
OC8H17
OMe
BzO
OBnO
OMe
BzO
OBnO
OMe
BzO
OBnOAc
n
517a-e n = 1-5 518b-e n = 2-5
OMe
BzO
OBnO
OC8H17
OMe
BzO
OBnO
OMe
BzO
OBnO
OMe
BzO
OBnOH 518a
OMe
BzO
OBnOAc
OC(NH)CCl3
+
iterative strategy
OMe
BzO
OBnO
OC8H17
OMe
BzO
OBnOH
OMe
BzOOBn
O
OMe
BzO
OBnOAc
SEt
+
OMe
BzOOBn
OAc
SEt
2 + 2assembling
strategy
R1 = SO3Na, R2 = R3 = H
or R1 = H, R2 = R3 = SO3Na
519
520
521
522
Per-O-sulfation of organic compounds containing many hydroxy groups can require lengthy reaction
times (of up to several days) and high temperatures, as well as substantial excess of the sulfating
146
reagent. Even so, one may still produce mixtures of partially sulfated products. This happened to
Nifantiev and co-workers as they tried to synthesize a fully sulfated tetrafucoside. As a result, they
developed an improved TfOH-promoted per-O-sulfation protocol with sulfur trioxide/pyridine,
enabling the reaction to take place at 0 °C and over a shorter period of time.249 They then adapted this
method for use in their previously developed block strategy, synthesizing per-O-sulfated type I and II
oligofucoidans from two to sixteen fucose residues. To their surprise, when using TfOH to promote
the sulfation, they observed an unusual rearrangement of the reducing pyranose residue into the
corresponding furanose. As this pyranoside-into-furanoside rearrangement is of great interest in the
synthesis of furanoside containing oligosaccharide targets, the group later explored its mechanism and
optimized this reaction.250 To avoid the rearrangement, they exchanged TfOH for TFA and this
produced the desired targets without by-products.
In 2014 and 2015, Toshima and co-workers designed a series of type I and type II fucoidan
tetrasaccharides with different sulfation patterns.246,251 An orthogonal deprotection strategy utilizing
the common key intermediates 528 and 531 (Scheme 56), carrying three types of protecting groups
(Bn, Bz and PMB) enabled an effective synthesis of the eight targets. The synthesis of the
tetrafucoside precursor 528 for the type II fragments is presented in Scheme 56.
147
Scheme 56. Key tetrasaccharides precursor 528 and 531 by Toshima and co-workers.246
OMe
BzOOBn
O
OMe
O
OBnOPMB
OMe
O
OBnOPMB
OC8H17
OMe
BzO
OBnOH
OMe
HO
OBnOPMB
S
OMe
BzOOBn
OClAc
OC(NH)CCl3
OMe
O
OBnOPMB
S
OMe
BzOOBn
OClAc
OMe
O
OBnOPMB
OMe
BzOOBn
OClAc
+
523
524
OC(NH)CCl3
Yb(OTf)3, Et2O
99%
525
1) NIS, Sc(OTf)3 H2O, MeCN 82%
2) DBU, CCl3CN CH2Cl2 76%
526
1) Yb(OTf)3, 1-octanol
CH2Cl2, 89%
2) thiourea, 2,6-lutidine DMF, 94%
OMe
O
OBnOPMB
OMe
BzOOBn
OH
OC8H17
527
1) 527, TMSOTf, Et2O2) thiourea, 2,6-lutidine
DMF
81% (2 steps)
528
OMe
BzOOBn
O
OMe
BzO
OBnO
OMe
BzO
OBnO
OMe
BzO
OBnOPMB
OC8H17
OMe
BzO
OBnOH
S
OMe
BzOOBn
OPMB
OC(NH)CCl3
529
530531
(type II precursor) (type I precursor)
Chemoselective glycosylation of 523 with 524, promoted by Yb(OTf)3, provided the desired α-
(1→4)-linked disaccharide 525 with very good stereoselectivity. The latter could be converted into a
trichloroacetimidate donor 526 and a new disaccharide acceptor 527 in two steps. Condensation of
527 with 1-octanol using Yb(OTf)3 afforded the corresponding octyl fucoside with surprisingly high
β-stereoselectivity. TMSOTf-promoted coupling of the two difucosides, followed by deprotection of
the ClAc group provided the key intermediate 528 as a single isomer in 81% yield over two steps. For
the five type II fucoidan targets, the deprotection and sulfation steps are presented in Scheme 57.
148
Scheme 57. Deprotection and sulfation of 528 to afford five type II tetrafucoidans 532–536.246
1) Pd(OH)2/C, H2
MeOH, AcOEt 88%
2) NaOMe, MeOH 97%
3) SO3.NEt3
, DMF
then 3 M aq. NaOH 81%
1) NaOMe, MeOH
80%2) DDQ, CH2Cl2
phosphate buffer 80%
3) SO3.NEt3
, DMF
then 3 M aq. NaOH
87%4) Pd(OH)2/C, H2
MeOH, H2O 99%
1) Pd(OH)2/C, H2
MeOH, AcOEt 98%
2) SO3.NEt3
, DMF
then aq. NaOH 97%
529
OMe
NaO3SO
OSO3NaO
OMe
O
OSO3NaOSO3Na
OMe
O
OSO3NaOSO3Na
OC8H17
OMe
NaO3SO
OSO3NaOSO3Na
OMe
HOOSO3Na
O
OMe
O
OSO3NaOSO3Na
OMe
O
OSO3NaOSO3Na
OC8H17
OMe
HO
OSO3NaOSO3Na
OMe
NaO3SOOH
O
OMe
O
OHOSO3Na
OMe
O
OHOSO3Na
OC8H17
OMe
NaO3SO
OHOSO3Na
OMe
NaO3SOOH
O
OMe
O
OHOH
OMe
O
OHOH
OC8H17
OMe
NaO3SO
OHOSO3Na
OMe
HOOH
O
OMe
O
OHOH
OMe
O
OHOH
OC8H17
OMe
HO
OHOH
533 534 535 536 537
1) NaOMe, MeOH
80%2) SO3
.NEt3
, DMF
then 3 M aq. NaOH
quant.3) Pd(OH)2/C, H2
MeOH, H2O 84%
1) Pd(OH)2/C, H2 MeOH, AcOEt 88%
2) NaOMe, MeOH 97%
A recent discovery showed that the fucoidan from the brown seaweed Chordaria flagelliformis
contains an α-(1→3)-linked polyfucopyranoside backbone with one third of the fucopyranosyl
residues bearing α-(1→2)-linked D-glucuronic acid substituents, and with half of the uronic acids
carrying α-(1→4)-L-fucofuranosides.252 In 2016, Nifantiev and co-workers achieved the synthesis of
a selectively O-sulfated branched pentasaccharide of this type (548) as shown in Scheme 58.242
149
Scheme 58. Synthesis of two branched type I fucoidan pentasaccharides by Nifantiev and co-workers.242
OCOOMe
TBSOBnO
BnO OC(NH)CCl3
O OC(NH)CCl3OBz
OBnMe OBn
OMe
BzO
OBnO
OAll
OMe
BzO
OPMBOClAc
OMe
BzO
OBnO
OAll
OMe
BzO
OHOClAc
OMe
BzO
OBnO
OAll
OMe
BzO
OOClAc
O COOMeOR
OBnBnO
540: R = TBS (α:β = 4:1)541: R = H (α
only)
537 538
539
OMe
BzO
OBnO
OAll
OMe
BzO
OOR
O COOMeO
OBnBnO
O
OBz
BnOMe OBn
543: R = ClAc 544: R = H
542
OMe
ClAcO
OBnOClAc
OC(NH)CCl3OMe
BzO
OBnO
OAll
OMe
BzO
OO
O COOMeO
OBnBnO
O
OBz
BnOMe OBn
OMe
ClAcO
OBnOClAc
OMe
HO
OHO
OPr
OMe
HO
OO
O COONaO
OHHO
O
OH
HOMe OH
OMe
HO
OBnOH
OMe
NaO3SO
OHO
OPr
OMe
NaO3SO
OO
O COONaO
OHHO
O
OSO3Na
HOMe OH
OMe
NaO3SO
OHOSO3Na
TFA (90%), CH2Cl2
96%
TMSOTf, CH2Cl2
87%
HF, MeCN, 77%
TMSOTf, CH2Cl2
79%
thiourea, MeOH, 80%
545
TMSOTf, CH2Cl2
86%
1) H2, Pd/C
THF, EtOAc, EtOH2) NaOH 2 M (aq.), MeOH
82%
546 547
548
1) LiOH 2 M (aq.), THF 2) Bu4NOH, THF 3) SO3·py, DMF4) H2
, Pd/C, THF, EtOAc, EtOH
76%
The assembly of both pentasaccharide targets 547 and 548 started with glycosylation between
difucopyranoside acceptor 538 and glucuronate donor 539, affording the trisaccharide 540 as a 4:1
α:β-mixture. The pure α-isomer 540 could be isolated after removal of the TBS group. This acceptor
was then glycosylated with fucofuranosyl donor 542 to give a mixture of α- and β-linked
tetrasaccharides in a ratio of 10:1. The desired α-anomer (isolated in 79% yield) was deprotected and
150
subsequently coupled with fucopyranoside donor 545, giving the core pentasaccharide 546 in 86%
yield. The target 547 was obtained after removal of all protecting groups. Alternatively, saponification
of 546, followed by O-sulfation and hydrogenolysis afforded the partially sulfated target 548.
7.7 Ulvans
Type A
Type B
= L-iduronic acid =OHO
HOOH OHHOOC
Rare sugar structure:
Figure 33. The two major types of disaccharide repeating units in ulvans.
Ulvans are water-soluble sulfated heteropolysaccharides, which have been extracted from the cell
walls of green marine algae. They represent between 8 and 29% of the dry weight of the cell wall.253,254
A review dealing with the structure and functional properties of ulvans was written by Lahaye and
Robic in 2007.254 The determination of the sugar sequences in ulvans represents a big challenge since
the polysaccharide composition varies widely and is dependent on algal source and ecophysiological
variation.254 However, acid hydrolysis and / or enzymatic degradation of cell walls from different
green seaweeds has shown the presence of L-rhamnose, L-xylose, D-glucuronic and L-iduronic acid.
These sugars are mostly distributed in disaccharide repeating units. Among these, the two most
frequent are ulvanobiuronic acid 3-sulfate Type A (β-D-GlcpA-(1→4)-α-L-Rhap3S-(1→4)) and Type
B (α-L-IdopA-(1→4)-α-L-Rhap3S-(1→4)), represented in Figure 33. Other less frequently occurring
units containing glucuronic acid on O-2 of rhamnose 3-sulfate or partially sulfated xylose replacing
151
the uronic acid have been found.255 No chemical synthesis of this type of oligosaccharide has been
undertaken to date.
Conclusion Plant cell wall polysaccharides include some of the most complex macromolecules found in nature.
For this reason, synthetic chemists have focused their attention on synthesizing well-defined
oligosaccharide fragments, as set out in this review. Not only are these targets extremely valuable in
studying protein-carbohydrate interactions, but they also help advance the development of new
methodologies in carbohydrate chemistry. Interest in this area is increasing, as witnessed by the many
contributions over the past twenty years. The more recent progress in automated synthesis holds
particular promise, as it is well suited to producing libraries of closely-related fragments of larger
polysaccharides. There is clearly no lack of unexplored structures to serve as inspiration for synthetic
chemists, especially within the marine polysaccharides. As this review makes clear, there is no
consensus as to the types of aglycone chosen for the targets: everything from reducing sugars over
simple alkyl groups to linkers predisposed to immobilization and / or conjugation is represented in the
review. More homogeneity in this choice, along with the establishment of a central repository for
synthetic oligosaccharides, represent areas, where we believe there is room for benefiting future
research. Nonetheless, we remain convinced that synthetic glycans will continue to provide
breakthroughs in the understanding of plant cell wall biology and physiology.
Author Information
152
Corresponding Author
E-mail: [email protected]
ORCID
Mads H. Clausen: 0000-0001-9649-1729
Notes
The authors declare no competing financial interest.
Biographies
Christine Kinnaert graduated from the Ecole Nationale des Industries Chimiques. Her thesis research
was carried out at Glaxo Smith-Kline and encompassed conformational analysis of polysaccharides.
She obtained her Ph. D. from the group of Prof. Mads H. Clausen at the Technical University of
Denmark in 2017. The subject of her thesis research was chemical synthesis of oligosaccharides
related to algal cell wall glycans. Christine is currently pursuing postdoctoral research at DTU.
Mathilde Daugaard obtained her M. Sc. (2011) and Ph. D. (2016) from the Technical University of
Denmark under the supervision of Prof. Thomas E. Nielsen and Prof. Mads H. Clausen, respectively.
The Ph. D. thesis research involved carbohydrate synthesis, more specifically preparation of α-L-
arabinans from rhamnogalacturonan I. She is currently employed by the Danish biopharmaceutical
company Nuevolution.
Faranak Nami obtained her M. Sc. in 2012 from Aarhus University under the supervision of Associate
Professor Henrik H. Jensen. The work centered on the synthesis of ibogaine analogues inducing an
153
inward conformation of the human serotonin transporter. Her Ph. D. was obtained in 2016 from the
Technical University of Denmark under the supervision of Professor Mads H. Clausen. Faranak
worked on oligosaccharide synthesis with the aim of carrying out enzymatic studies.
Mads H. Clausen obtained his Ph.D. from the Technical University of Denmark in 2002 under the
supervision of Prof. Robert Madsen. He was a postdoctoral fellow at Harvard University from 2002–
2004 in the group of Prof. Andrew G. Myers, where he worked on total synthesis of enediyne
antibiotics. In 2004, he started his independent career at the Department of Chemistry, Technical
University of Denmark, where he has been Professor of Chemical Biology since 2014. His research
interests center on chemical biology and bioorganic chemistry and include medicinal chemistry,
oligosaccharide and lipid synthesis and developing methods for library synthesis.
Acknowledgments
We are highly grateful to Ms. Adeline Berger and Dr. Helen North for providing the image used for
Figure 3. We acknowledge financial support from the Danish Council for Independent Research “A
biology-driven approach for understanding enzymatic degradation of complex polysaccharide
systems” (Grant Case no.: 107279), the Carlsberg Foundation, the Danish Strategic Research Council
(GlycAct and SET4Future projects), the Villum Foundation (PLANET project) and the Novo Nordisk
Foundation (Biotechnology-based Synthesis and Production Research).
154
References
(1) Ivakov, A.; Persson, S. Plant Cell Walls. In eLS; John Wiley & Sons: Chichester, 2012.
(2) Keegstra, K. Plant Cell Walls. Plant Physiol. 2010, 154, 483–486.
(3) Whitney, S. E. C.; Gothard, M. G. E.; Mitchell, J. T.; Gidley, M. J. Roles of Cellulose and
Xyloglucan in Determining the Mechanical Properties of Primary Plant Cell Walls. Plant
Physiol. 1999, 121, 657–664.
(4) Scheller, H. V.; Ulvskov, P. Hemicelluloses. Annu. Rev. Plant Biol. 2010, 61, 263–289.
(5) Gillis, P. P.; Mark, R. E.; Tang, R. C. Elastic Stiffness of Crystalline Cellulose in the Folded-
Chain Solid State. J. Mater. Sci. 1969, 4, 1003–1007.
(6) Caffall, K. H.; Mohnen, D. The Structure, Function, and Biosynthesis of Plant Cell Wall Pectic
Polysaccharides. Carbohydr. Res. 2009, 344, 1879–1900.
(7) Zykwinska, A.; Thibault, J. F.; Ralet, M. C. Competitive Binding of Pectin and Xyloglucan
with Primary Cell Wall Cellulose. Carbohydr. Polym. 2008, 74, 957–961.
(8) Zykwinska, A. W.; Ralet, M. J.; Garnier, C. D.; Thibault, J. J. Evidence for in Vitro Binding of
Pectin Side Chains to Cellulose. Plant Physiol. 2005, 139, 397–407.
(9) Popper, Z. A.; Fry, S. C. Widespread Occurrence of a Covalent Linkage between Xyloglucan
and Acidic Polysaccharides in Suspension-Cultured Angiosperm Cells. Ann. Bot. 2005, 96, 91–
99.
(10) Ellis, M.; Egelund, J.; Schultz, C. J.; Bacic, A. Arabinogalactan-Proteins (AGPs): Key
Regulators at the Cell Surface. Plant Physiol. 2010, 153, 403–419.
(11) Jamet, E.; Albenne, C.; Boudart, G.; Irshad, M.; Canut, H.; Pont-Lezica, R. Recent Advances
in Plant Cell Wall Proteomics. Proteomics 2008, 8, 893–908.
155
(12) Lee, S. J.; Saravanan, R. S.; Damasceno, C. M. B.; Yamane, H.; Kim, B. D.; Rose, J. K. C.
Digging Deeper into the Plant Cell Wall Proteome. Plant Physiol. Biochem. 2004, 42, 979–
988.
(13) Doblin, M. S.; Pettolino, F.; Bacic, A. Evans Review: Plant Cell Walls: The Skeleton of the
Plant World. Funct. Plant Biol. 2010, 37, 357–381.
(14) Fry, S. C. Plant Cell Walls. In eLS; John Wiley & Sons, Ltd, 2001; pp 1–11.
(15) Burton, R. A.; Gidley, M. J.; Fincher, G. B. Heterogeneity in the Chemistry, Structure and
Function of Plant Cell Walls. Nat. Chem. Biol. 2010, 6, 724–732.
(16) Sørensen, I.; Domozych, D.; Willats, W. G. T. How Have Plant Cell Walls Evolved? Plant
Physiol. 2010, 153, 366–372.
(17) Kenrick, P.; Crane, P. R. The Origin and Early Evolution of Plants on Land. Nature 1997, 389,
33–39.
(18) Becker, B.; Marin, B. Streptophyte Algae and the Origin of Embryophytes. Ann. Bot. 2009,
103, 999–1004.
(19) Michel, G.; Tonon, T.; Scornet, D.; Cock, J. M.; Kloareg, B. The Cell Wall Polysaccharide
Metabolism of the Brown Alga Ectocarpus Siliculosus. Insights into the Evolution of
Extracellular Matrix Polysaccharides in Eukaryotes. New Phytol. 2010, 188, 82–97.
(20) Popper, Z. A.; Tuohy, M. G. Beyond the Green: Understanding the Evolutionary Puzzle of
Plant and Algal Cell Walls. Plant Physiol. 2010, 153, 373–383.
(21) Popper, Z. A. Evolution and Diversity of Green Plant Cell Walls. Curr. Opin. Plant Biol. 2008,
11, 286–292.
(22) Popper, Z. A.; Michel, G.; Herve, C.; Domozych, D. S.; Willats, W. G.; Tuohy, M. G.; Kloareg,
B.; Stengel, D. B. Evolution and Diversity of Plant Cell Walls: From Algae to Flowering Plants.
156
Annu. Rev. Plant Biol. 2011, 62, 567–590.
(23) Popper, Z. A.; Ralet, M. C.; Domozych, D. S. Plant and Algal Cell Walls: Diversity and
Functionality. Ann. Bot. 2014, 114, 1043–1048.
(24) Fangel, J. U.; Ulvskov, P.; Knox, J. P.; Mikkelsen, M. D.; Harholt, J.; Popper, Z. A.; Willats,
W. G. T. Cell Wall Evolution and Diversity. Front. Plant Sci. 2012, 3, 1–8.
(25) Tolonen, L. K.; Juvonen, M.; Niemelä, K.; Mikkelson, A.; Tenkanen, M.; Sixta, H.
Supercritical Water Treatment for Cello-Oligosaccharide Production from Microcrystalline
Cellulose. Carbohydr. Res. 2015, 401, 16–23.
(26) Takeo, K.; Yasato, T.; Kuge, T. Synthesis of α- and β-Cellotriose Hendecaacetates and of
Several 6,6’,6’’-tri-Substituted Derivatives of Methyl β-Cellotrioside. Carbohydr. Res. 1981,
93, 148–156.
(27) Takeo, K.; Okushio, K.; Fukuyama, K.; Kuge, T. Synthesis of Cellobiose, Cellotriose,
Cellotetraose and Lactose. Carbohydr. Res. 1983, 121, 163–173.
(28) Rodriguez, E.; Stick, R. The Synthesis of Active-Site Directed Inhibitors of Some β-Glucan
Hydrolases. Aust. J. Chem. 1990, 43, 665–679.
(29) Xu, J.; Vasella, A. Oligosaccharide Analogues of Polysaccharides. Helv. Chim. Acta 1999, 82,
1728–1752.
(30) Chang, S.; Han, J.; Tseng, S. Y.; Lee, H.; Lin, C.; Lin, Y.; Jeng, W.; Wang, A. H.; Wu, C.;
Wong, C. Glycan Array on Aluminum Oxide-Coated Glass Slides through Phosphonate
Chemistry. J. Am. Chem. Soc. 2010, 132, 13371–13380.
(31) Dallabernardina, P.; Schuhmacher, F.; Seeberger, P. H.; Pfrengle, F. Automated Glycan
Assembly of Xyloglucan Oligosaccharides. Org. Biomol. Chem. 2016, 14, 309–313.
(32) Sugawara, F.; Nakayama, H.; Ogawa, T. Synthetic Studies on Rhynchosporoside:
157
Stereoselective Synthesis of 1-O- and 2-O-Cellotriosyl-3-Deoxy-2(R)-and -2(S)-Glycerol.
Carbohydr. Res. 1983, 123, C25–C28.
(33) Nicolaou, K. C.; Randall, J. L.; Furst, G. T. Stereospecific Synthesis of Rhynchosporosides, a
Family of Fungal Metabolites Causing Scald Disease in Barley and Other Grasses. J. Am.
Chem. Soc. 1985, 107, 5556–5558.
(34) Sugawara, F.; Nakayama, H.; Strobel, G. A.; Ogawa, T. Stereoselective Synthesis of 1- and 2-
O-α-D-Cellotriosyl-3-Deoxy-2(R)- and 2(S)-Glycerols Related to Rhynchosporoside. Agric.
Biol. Chem. 1986, 50, 2261–2271.
(35) Nishimura, T.; Nakatsubo, F. First Synthesis of Cellooctaose by a Convergent Synthetic
Method. Carbohydr. Res. 1996, 294, 53–64.
(36) Koenigs, W.; Knorr, E. Über Einige Derivate Des Traubensuckers Und Der Galactose. Berichte
der Dtsch. Chem. Gesellschaft 1901, 34, 957–981.
(37) Zemplén, G.; Pacsu, E. Über Die Verseifung Acetylierter Zucker Und Verwandter Substanzen.
Berichte der Dtsch. Chem. Gesellschaft 1929, 62, 1613–1614.
(38) Nishimura, T.; Nakatsubo, F.; Murakami, K. Synthetic Studies of Cellulose XI. High Yield
Synthesis of Cellotetraose Derivative by Convergent Synthetic Method from an Acyl Glucose
Derivative. Mouzai Gakkaishi 1994, 40, 44–49.
(39) Schmidt, R. R.; Michel, J. Facile Synthesis of α- and β-O-Glycosyl Imidates; Preparation of
Glycosides and Disaccharides. Angew. Chem. Int. Ed. 1980, 19, 731–732.
(40) Kariyone, K.; Yazawa, H. Oxidative Cleavage of Β, γ-Unsaturated Ether. Tetrahedron Lett.
1970, 33, 2885–2888.
(41) Schultze, E. Zur Kenntniss Der Chemischen Zuzammensetzung Der Pflanzlichen
Zellmembranen. Ber. Dtsch. Chem. Ges. 1891, 24, 2277–2287.
158
(42) Ebringerová, A.; Hromádková, Z.; Heinze, T. Hemicellulose. In Polysaccharides I; Springer-
Verlag: Berlin, 2005; Vol. 186, pp 1–67.
(43) Wotovic, A.; Jacquinet, J. C.; Sinaÿ, P. Synthesis of Oligosaccharins: A Chemical Synthesis of
Propyl O-β-D-Galactopyranosyl-(1,2)-O-α-D-Xylopyranosyl-(1,6)-O-β-D-Glucopyranosyl-
(1,4)-β-D-Glucopyranoside. Carbohydr. Res. 1990, 205, 235–245.
(44) Sakai, K.; Nakahara, Y.; Ogawa, T. Total Synthesis of Nonasaccharide Repeating Unit of Plant
Cell Wall Xyloglucan: An Endogenous Hormone Which Regulates Cell Growth. Tetrahedron
Lett. 1990, 31, 3035–3038.
(45) Watt, D. K.; Brasch, D. J.; Larsen, D. S.; Melton, L. D.; Simpson, J. Oligosaccharides Related
to Xyloglucan: Synthesis and X-Ray Crystal Structure of Methyl α-L- Fucopyranosyl-(1→2)-
β-D-Galactopyranosyl-(1→2)-α-D-Xylopyranoside and the Synthesis of Methyl α-L-
Fucopyranosyl-(1→2)-β-D-Galactopyranosyl-(1→2)-β-D-Xylopyranosid. Carbohydr. Res.
2000, 325, 300–312.
(46) Merrifield, R. B. Solid Phase Peptide Synthesis. I. The Synthesis of a Tetrapeptide. J. Am.
Chem. Soc. 1963, 85, 2149–2154.
(47) Eller, S.; Collot, M.; Yin, J.; Hahm, H. S.; Seeberger, P. H. Automated Solid-Phase Synthesis
of Chondroitin Sulfate Glycosaminoglycans. Angew. Chemie Int. Ed. 2013, 52, 5858–5861.
(48) Plante, O. J.; Andrade, R. B.; Seeberger, P. H. Synthesis and Use of Glycosyl Phosphates as
Glycosyl Donors. Org. Lett. 1999, 1, 211–214.
(49) Manners, D. J.; Masson, A. J.; Patterson, J. C. The Structure of a β-(1→3)-D-Glucan from
Yeast Cell Walls. Biochem. J. 1973, 135, 19–30.
(50) Scherp, P.; Grotha, R.; Kutschera, U. Occurrence and Phylogenetic Significance of
Cytokinesis-Related Callose in Green Algae, Bryophytes, Ferns and Seed Plants. Plant Cell
159
Rep. 2001, 20, 143–149.
(51) Ellinger, D.; Voigt, C. A. Callose Biosynthesis in Arabidopsis with a Focus on Pathogen
Response: What We Have Learned within the Last Decade. Ann. Bot. 2014, 114, 1349–1358.
(52) Tanaka, H.; Kawai, T.; Adachi, Y.; Hanashima, S.; Yamaguchi, Y.; Ohno, N.; Takahashi, T.
Synthesis of β-(1,3)-Oligoglucans Exhibiting a Dectin-1 Binding Affinity and Their Biological
Evaluation. Bioorg. Med. Chem. 2012, 20, 3898–3914.
(53) Takeo, K.; Maki, K.; Wada, Y.; Kitamura, S. Synthesis of the Laminara-Oligosaccharide
Methyl β-Glycosides of Dp 3-8. Carbohydr. Res. 1993, 245, 81–96.
(54) Adamo, R.; Tontini, M.; Brogioni, G.; Romano, M. R.; Costantini, G.; Danieli, E.; Proietti, D.;
Berti, F.; Costantino, P. Synthesis of Laminarin Fragments and Evaluation of a β-(1→3) Glucan
Hexasaccaride-CRM 197 Conjugate as Vaccine Candidate against Candida Albicans. J.
Carbohydr. Chem. 2011, 30, 249–280.
(55) Jamois, F.; Ferrières, V.; Guégan, J. P.; Yvin, J. C.; Plusquellec, D.; Vetvicka, V. Glucan-like
Synthetic Oligosaccharides: Iterative Synthesis of Linear Oligo-β-(1,3)-Glucans and
Immunostimulatory Effects. Glycobiology 2005, 15, 393–407.
(56) de Jong, A. R.; Volbeda, A. G.; Hagen, B.; van den Elst, H.; Overkleeft, H. S.; van der Marel,
G. A.; Codée, J. D. C. A Second-Generation Tandem Ring-Closing Metathesis Cleavable
Linker for Solid-Phase Oligosaccharide Synthesis. Eur. J. Org. Chem. 2013, 29, 6644–6655.
(57) Zeng, Y.; Kong, F. Synthesis of β-D-Glucose Oligosaccharides from Phytophthora Parasitica.
Carbohydr. Res. 2003, 338, 2359–2366.
(58) Huang, G.; Mei, X.; Liu, M. Synthesis of (R)-2,3-epoxypropyl(1→3)-β-D-Pentaglucoside.
Carbohydr. Res. 2005, 340, 603–608.
(59) Yashunsky, D. V.; Tsvetkov, Y. E.; Nifantiev, N. E. Synthesis of 3-Aminopropyl Glycoside of
160
Branched β-(1 → 3)-D-Glucooctaoside. Carbohydr. Res. 2016, 436, 25–30.
(60) Mo, K.; Li, H.; Mague, J. T.; Ensley, H. E. Synthesis of the β-1,3-Glucan, Laminarahexaose:
NMR and Conformational Studies. Carbohydr. Res. 2009, 344, 439–447.
(61) Elsaidi, H. R. H.; Paszkiewicz, E.; Bundle, D. R. Synthesis of a 1,3 β-Glucan Hexasaccharide
Designed to Target Vaccines to the Dendritic Cell Receptor, Dectin-1. Carbohydr. Res. 2015,
408, 96–106.
(62) Liao, G.; Zhou, Z.; Burgula, S.; Liao, J.; Yuan, C.; Wu, Q.; Guo, Z. Synthesis and
Immunological Studies of Linear Oligosaccharides of β-Glucan As Antigens for Antifungal
Vaccine Development. Bioconjug. Chem. 2015, 26, 466–476.
(63) Tanaka, H.; Kawai, T.; Adachi, Y.; Ohno, N.; Takahashi, T. β(1,3) Branched Heptadeca- and
Linear Hexadecasaccharides Possessing an Aminoalkyl Group as a Strong Ligand to Dectin-1.
Chem. Commun. 2010, 46, 8249–8251.
(64) Liao, G.; Burgula, S.; Zhou, Z.; Guo, Z. A Convergent Synthesis of 6- O -Branched β-Glucan
Oligosaccharides. Eur. J. Org. Chem. 2015, 2942–2951.
(65) Weishaupt, M. W.; Hahm, H. S.; Geissner, A.; Seeberger, P. H. Automated Glycan Assembly
of Branched β-(1→3)-Glucans to Identify Antibody Epitopes. Chem. Commun. 2017, 53,
3591–3594.
(66) Weishaupt, M. W.; Matthies, S.; Seeberger, P. H. Automated Solid-Phase Synthesis of a β-
(1,3)-Glucan Dodecasaccharide. Chem. Eur. J. 2013, 19, 12497–12503.
(67) Popper, Z. A.; Fry, S. C. Primary Cell Wall Composition of Bryophytes and Charophytes. Ann.
Bot. 2003, 91, 1–12.
(68) Sørensen, I.; Pettolino, F. A.; Wilson, S. M.; Doblin, M. S.; Johansen, B.; Bacic, A.; Willats,
W. G. T. Mixed-Linkage (1→3),(1→4)-β-D-Glucan Is Not Unique to the Poales and Is an
161
Abundant Component of Equisetum Arvense Cell Walls. Plant J. 2008, 54, 510–521.
(69) Dallabernardina, P.; Schuhmacher, F.; Seeberger, P. H.; Pfrengle, F. Mixed-Linkage Glucan
Oligosaccharides Produced by Automated Glycan Assembly Serve as Tools To Determine the
Substrate Specificity of Lichenase. Chem. A Eur. J. 2017, 23, 1–7.
(70) Simmons, T. J.; Mortimer, J. C.; Bernardinelli, O. D.; Pöppler, A.-C.; Brown, S. P.;
DeAzevedo, E. R.; Dupree, R.; Dupree, P. Folding of Xylan onto Cellulose Fibrils in Plant Cell
Walls Revealed by Solid-State NMR. Nat. Commun. 2016, 7, 1–9.
(71) Rennie, E. A.; Scheller, H. V. Xylan Biosynthesis. Curr. Opin. Biotechnol. 2014, 26, 100–107.
(72) Faik, A. Xylan Biosynthesis: News from the Grass. Plant Physiol. 2010, 153, 396–402.
(73) Balakshin, M.; Capanema, E.; Berlin, A. Isolation and Analysis of Lignin-Carbohydrate
Complexes Preparations with Traditional and Advanced Methods: A Review. In Studies in
Natural Products Chemistry; Atta-ur-Rahman, Ed.; Elsevier B.V., 2014; Vol. 42, pp 83–115.
(74) Hirsch, J.; Kováč, P.; Petráková, E. An Approach to the Systematic Synthesis of (1→4)-β-D-
Xylo-Oligosaccharides. Carbohydr. Res. 1982, 106, 203–216.
(75) Kováč, P.; Hirsch, J. Systematic, Sequential Synthesis of (1→4)-β-D-Xylo-Ligosaccharides
and Their Methyl β-Glycosides. Carbohydr. Res. 1981, 90, C5–C7.
(76) Takeo, K.; Ohguchi, Y.; Hasegawa, R.; Kitamura, S. Synthesis of (1→4)-β-D-Xylo-
Oligosaccharides of Dp 4–10 by a Blockwise Approach. Carbohydr. Res. 1995, 278, 301–313.
(77) Schmidt, D.; Schuhmacher, F.; Geissner, A.; Seeberger, P. H.; Pfrengle, F. Automated
Synthesis of Arabinoxylan-Oligosaccharides Enables Characterization of Antibodies That
Recognize Plant Cell Wall Glycans. Chem. Eur. J. 2015, 21, 5709–5713.
(78) Oscarson, S.; Svahnberg, P. Synthesis of Uronic Acid-Containing Xylans Found in Wood and
Pulp. J. Chem. Soc. Perkin Trans. 1 2001, 100, 873–879.
162
(79) Kováč, P.; Hirsch, J.; Kováčik, V.; Kočiš, P. The Stepwise Synthesis of an Aldopentaouronic
Acid Derivative Related to Branched (4-O-Methylglucurono)xylans. Carbohydr. Res. 1980,
85, 41–49.
(80) Senf, D.; Ruprecht, C.; de Kruijff, G. H. M.; Simonetti, S. O.; Schuhmacher, F.; Seeberger, P.
H.; Pfrengle, F. Active Site Mapping of Xylan-Deconstructing Enzymes with Arabinoxylan
Oligosaccharides Produced by Automated Glycan Assembly. Chem. - A Eur. J. 2017, 23, 3197–
3205.
(81) Helferich, B.; Schmitz-Hillebrecht, E. Eine Neue Methode Zur Synthese von Glykosiden Der
Phenole. Ber. Dtsch. Chem. Ges. 1933, 66, 378–383.
(82) Myhre, D. V.; Smith, F. Synthesis of Xylobiose (4-O-β-D-Xylopyranosyl-D-Xylose) 1. J. Org.
Chem. 1961, 26, 4609–4612.
(83) Kováč, P.; Hirsch, J. Sequential Synthesis and 13C-N.M.R. Spectra of Methyl β-Glycosides of
(1→4)-β-D-Xylo-Oligosaccharides. Carbohydr. Res. 1982, 100, 177–193.
(84) Fürst, A.; Plattner, P. A. 2-α-, 3-α- Und 2-Β, 3-β-Oxido-Chlolestane; Konfiguration Der 2-Oxy-
Cholestane. Helv. Chim. Acta 1949, 32, 275–283.
(85) Moreira, L. R. S.; Filho, E. X. F. An Overview of Mannan Structure and Mannan-Degrading
Enzyme Systems. Appl. Microbiol. Biotechnol. 2008, 79, 165–178.
(86) Gridley, J. J.; Osborn, H. M. I. Recent Advances in the Construction of β-D-Mannose and β-
D-Mannosamine Linkages. J. Chem. Soc. Perkin Trans. 1 2000, 10, 1471–1491.
(87) Lichtenthaler, F. W. 2-Oxoglycosyl (“Ulosyl”) and 2-Oximinoglycosyl Bromides: Versatile
Donors for the Expedient Assembly of Oligosaccharides with β- D-Mannose, β- L-Rhamnose,
N-Acetyl-β- D-Mannosamine, and N-Acetyl-β-D-Mannosaminuronic Acid Units. Chem. Rev.
2011, 111, 5569–5609.
163
(88) Twaddle, G. W. J.; Yashunsky, D. V; Nikolaev, A. V. The Chemical Synthesis of β-(1→4)-
Linked D-Mannobiose and D-Mannotriose. Org. Biomol. Chem 2003, 1, 623–628.
(89) Kim, K. S.; Fulse, D. B.; Baek, J. Y.; Lee, B.-Y.; Jeon, H. B. Stereoselective Direct
Glycosylation with Anomeric Hydroxy Sugars by Activation with Phthalic Anhydride and
Trifluoromethanesulfonic Anhydride Involving Glycosyl Phthalate Intermediates. J. Am.
Chem. Soc. 2008, 130, 8537–8547.
(90) Crich, D.; Banerjee, A.; Yao, Q. Direct Chemical Synthesis of the β-D-Mannans: The β-(1→2)
and β-(1→4) Series. J. Am. Chem. Soc. 2004, 126, 14930–14934.
(91) Ohara, K.; Lin, C.-C.; Yang, P.-J.; Hung, W.-T.; Yang, W.-B.; Cheng, T. J. R.; Fang, J.-M.;
Wong, C.-H. Synthesis and Bioactivity of β-(1→4)-Linked Oligomannoses and Partially
Acetylated Derivatives. J. Org. Chem. 2013, 78, 6390–6411.
(92) Tang, S.-L.; Pohl, N. L. B. Automated Solution-Phase Synthesis of β-1,4-Mannuronate and β-
1,4-Mannan. Org. Lett. 2015, 17, 2642–2645.
(93) Ekholm, F. S.; Ardá, A.; Eklund, P.; André, S.; Gabius, H.-J.; Jiménez-Barbero, J.; Leino, R.
Studies Related to Norway Spruce Galactoglucomannans: Chemical Synthesis, Conformation
Analysis, NMR Spectroscopic Characterization, and Molecular Recognition of Model
Compounds. Chem. Eur. J. 2012, 18, 14392–14405.
(94) Imamura, A.; Ando, H.; Korogi, S.; Tanabe, G.; Muraoka, O.; Ishida, H.; Kiso, M. Di-Tert-
Butylsilylene (DTBS) Group-Directed α-Selective Galactosylation Unaffected by C-2
Participating Functionalities. Tetrahedron Lett. 2003, 44, 6725–6728.
(95) Vauquelin, M. Analyse Du Tamarin. Ann. Chim. 1790, 5, 92–106.
(96) Braconnot, H. Recherche Sur Un Nouvel Acid Universellement Répandu Dans Tous Les
Végétaux. Ann. Chim. Phys. 1825, 2, 173–178.
164
(97) Mohnen, D. Pectin Structure and Biosynthesis. Curr. Opin. Plant Biol. 2008, 11, 266–277.
(98) Ridley, B. L.; O’Neill, M. A.; Mohnen, D. Pectins: Structure, Biosynthesis, and
Oligogalacturonide-Related Signaling. Phytochemistry 2001, 57, 929–967.
(99) Yapo, B. M. Pectic Substances: From Simple Pectic Polysaccharides to Complex Pectins-A
New Hypothetical Model. Carbohydr. Polym. 2011, 86, 373–385.
(100) Neill, M. A. O.; Ishii, T.; Albersheim, P.; Darvill, A. G. Rhamnogalacturonan II : Structure and
Function of a Borate Cross-Linked Cell. Annu. Rev. Plant Biol. 2004, 55, 109–139.
(101) Nepogodiev, S. A.; Field, R. A.; Damager, I. Chapter 3:Approaches to Chemical Synthesis of
Pectic Oligosaccharides. In Annual Plant Reviews, Volume 41: Plant Polysaccharides,
Biosynthesis and Bioengineering; Ulvskov, P., Ed.; Blackwell Publishing Ltd: West Sussex,
United Kingdom, 2011; Vol. 41, pp 65–92.
(102) Yapo, B. M.; Lerouge, P.; Thibault, J. F.; Ralet, M. C. Pectins from Citrus Peel Cell Walls
Contain Homogalacturonans Homogenous with Respect to Molar Mass, Rhamnogalacturonan
I and Rhamnogalacturonan II. Carbohydr. Polym. 2007, 69, 426–435.
(103) Willats, W. G. T.; McCartney, L.; Mackie, W.; Knox, J. P. Pectin: Cell Biology and Prospects
for Functional Analysis. Plant Mol. Biol. 2001, 47, 9–27.
(104) Petersen, B. O.; Meier, S.; Duus, J. Ø.; Clausen, M. H. Structural Characterization of
Homogalacturonan by NMR Spectroscopy-Assignment of Reference Compounds. Carbohydr.
Res. 2008, 343, 2830–2833.
(105) Nakahara, Y.; Ogawa, T. Synthesis of (1 ->4)-Linked Galacturonic Acid Trisaccharides, a
Proposed Plant Wound-Hormone and a Stereoisomer. Carbohydr. Res. 1990, 200, 363–375.
(106) Pourcelot, M.; Barbier, M.; Landoni, M.; Couto, A.; Grand, E.; Kovensky, J. Synthesis of
Galacturonate Containing Disaccharides and Protein Conjugates. Curr. Org. Chem. 2011, 15,
165
3523–3534.
(107) Kester, H. C. M.; Magaud, D.; Roy, C.; Anker, D.; Doutheau, A.; Shevchik, V.; Hugouvieux-
Cotte-Pattat, N.; Benen, J. A. E.; Visser, J. Performance of Selected Microbial Pectinase on
Sythetic Monomethyl-Esterified Di- and Trigalacturonates. J Biol Chem 1999, 274, 37053–
37059.
(108) Clausen, M. H.; Jørgensen, M. R.; Thorsen, J.; Madsen, R. A Strategy for Chemical Synthesis
of Selectively Methyl-Esterified Oligomers of Galacturonic Acid. J. Chem. Soc. Perkin Trans.
1 2001, 543–551.
(109) Clausen, M. H.; Madsen, R. Synthesis of Hexasaccharide Fragments of Pectin. Chem. Eur. J.
2003, 9, 3821–3832.
(110) Nakahara, Y.; Ogawa, T. Stereocontrolled Synthesis of the Propyl Glycoside of a
Decagalacturonic Acid, a Model Compound for the Endogenous Phytoalexin Elicitor-Active
Oligogalacturonic. Carbohydr. Res. 1989, 194, 95–114.
(111) Nakahara, Y.; Ogawa, T. Stereoselective Total Synthesis of Dodecagalacturonic Acid, a
Phytoalexin Elicitor of Soybean. Carbohydr. Res. 1990, 205, 147–159.
(112) van den Bos, L. J.; Codée, J. D. C.; Litjens, R. E. J. N.; Dinkelaar, J.; Overkleeft, H. S.; van der
Marel, G. A. Uronic Acids in Oligosaccharide Synthesis. Eur. J. Org. Chem. 2007, 3963–3976.
(113) Magaud, D.; Grandjean, C.; Doutheau, A.; Anker, D.; Shevchik, V.; Cotte-Pattat, N.; Robert-
Baudouy, J. An Efficient and Highly Stereoselective α(1→4) Glycosylation between Two D-
Galacturonic Acid Ester Derivatives. Tetrahedron Lett. 1997, 38, 241–244.
(114) Kramer, S.; Nolting, B.; Ott, A.; Vogel, C. Synthesis of Homogalacturonan Fragments. J.
Carbohydr. Chem. 2000, 19, 891–921.
(115) Omura, K.; Swern, D. Oxidation of Alcohols by “activated” Dimethyl Sulfoxide. A Preparative,
166
Steric and Mechanistic Study. Tetrahedron 1978, 34, 1651–1660.
(116) Bal, B. S.; Childers, W. E.; Pinnick, H. W. Oxidation of Α,β-Un Saturated Aldehydes.
Tetrahedron 1981, 37, 2091–2096.
(117) Kraus, G. A.; Roth, B. Synthetic Studies toward Verrucarol. 2. Synthesis of the AB Ring
System. J. Org. Chem. 1980, 45, 4825–4830.
(118) Lindgren, B. O.; Nilsson, T.; Husebye, S.; Mikalsen, Ø.; Leander, K.; Swahn, C.-G. Preparation
of Carboxylic Acids from Aldehydes (Including Hydroxylated Benzaldehydes) by Oxidation
with Chlorite. Acta Chem. Scand. 1973, 27, 888–890.
(119) Clausen, M. H.; Madsen, R. Synthesis of Hexasaccharide Fragments of Pectin. Chemistry 2003,
9, 3821–3832.
(120) Dess, D. B.; Martin, J. C. Readily Accessible 12-I-5 Oxidant for the Conversion of Primary
and Secondary Alcohols to Aldehydes and Ketones. J. Org. Chem. 1983, 48, 4155–4156.
(121) Schols, H. A.; Bakx, E. J.; Schipper, D.; Voragen, A. G. J. A Xylogalacturonan Subunit Present
in the Modified Hairy Regions of Apple Pectin. Carbohydr. Res. 1995, 279, 265–279.
(122) Hart, D. A.; Kindel, P. K. Isolation and Partial Characterization of Apiogalacturonans from the
Cell Wall of Lemna Minor. Biochem. J. 1970, 116, 569–579.
(123) Buffet, M. a J.; Rich, J. R.; McGavin, R. S.; Reimer, K. B. Synthesis of a Disaccharide
Fragment of Rhamnogalacturonan II. Carbohydr. Res. 2004, 339, 2507–2513.
(124) Nepogodiev, S. A.; Fais, M.; Hughes, D. L.; Field, R. a. Synthesis of Apiose-Containing
Oligosaccharide Fragments of the Plant Cell Wall: Fragments of Rhamnogalacturonan-II Side
Chains A and B, and Apiogalacturonan. Org. Biomol. Chem. 2011, 9, 6670–6684.
(125) Koós, M.; Mosher, H. S. A Convenient Synthesis of Apiose. Carbohydr. Res. 1986, 146, 335–
341.
167
(126) Nachman, R. J.; Hönel, M.; Williams, T. M.; Halaska, R. C.; Mosher, H. S. Methyl 3-Formyl -
2,3-O-Isopropylidene-D-Erythrofuranoside (D-Apiose Aldal) and Derivatives. J. Org. Chem.
1986, 51, 4802–4806.
(127) Ho, P.-T. Branched-Chain Sugars. Reaction of Furanoses with Formaldehyde: A Simple
Synthesis of D- and L-Apiose. Can. J. Chem. 1979, 57, 381–383.
(128) Williams, D. T.; Jones, J. K. N. The Chemistry of Apiose. Part I. Can. J. Chem. 1964, 42, 69–
72.
(129) Scanlan, E. M.; Mackeen, M. M.; Wormald, M. R.; Davis, B. G. Synthesis and Solution-Phase
Conformation of the RG-I Fragment of the Plant Polysaccharide Pectin Reveals a Modification-
Modulated Assembly Mechanism. J. Am. Chem. Soc. 2010, 132, 7238–7239.
(130) Mossine, V. V; Glinsky, V. V; Mawhinney, T. P. Food-Related Carbohydrate Ligands for
Galectins. In Food and Feed Chemistry; Columbia, USA., 2008; pp 235–270.
(131) Clarke, A. E.; Anderson, R. L.; Stone, B. A. Form and Function of Arabinogalactans and
Arabinogalactan-Proteins. Phytochemistry 1979, 18, 521–540.
(132) Ma, Y.; Cao, X.; Yu, B. Synthesis of Oligosaccharide Fragments of the Rhamnogalacturonan
of Nerium Indicum. Carbohydr. Res. 2013, 377, 63–74.
(133) Maruyama, M.; Takeda, T.; Shimizu, N.; Hada, N.; Yamada, H. Synthesis of a Model
Compound Related to an Anti-Ulcer Pectic Polysaccharide. Carbohydr. Res. 2000, 325, 83–
92.
(134) Nemati, N.; Karapetyan, G.; Nolting, B.; Endress, H.-U.; Vogel, C. Synthesis of
Rhamnogalacturonan I Fragments by a Modular Design Principle. Carbohydr. Res. 2008, 343,
1730–1742.
(135) Reiffarth, D.; Reimer, K. B. Synthesis of Two Repeat Units Corresponding to the Backbone of
168
the Pectic Polysaccharide Rhamnogalacturonan I. Carbohydr. Res. 2008, 343, 179–188.
(136) Zakharova, A. N.; Madsen, R.; Clausen, M. H. Synthesis of a Backbone Hexasaccharide
Fragment of the Pectic Polysaccharide Rhamnogalacturonan I. Org. Lett. 2013, 15, 1826–1829.
(137) Ndeh, D.; Rogowski, A.; Cartmell, A.; Luis, A. S.; Baslé, A.; Gray, J.; Venditto, I.; Briggs, J.;
Zhang, X.; Labourel, A.; Terrapon, N.; Buffetto, F.; Nepogodiev, S.; Xiao, Y.; Field, R. A.;
Zhu, Y.; O’Neill, M. A.; Urbanowicz, B. R.; York, W. S.; Davies, G. J.; Abbott, D. W.; Ralet,
M-C.; Martens, E. C.; Henrissat, B.; Gilbert, H. J. Complex Pectin Metabolism by Gut Bacteria
Reveals Novel Catalytic Functions. Nature 2017, 544, 65–70.
(138) Pellerin, P.; Neill, M. A.; Pierre, C.; Cabanis, M.-T.; Darvill, A. G.; Albersheim, P.; Moutounet,
M. Leat Complexation in Wines with the Dimers of the Grape Pectic Polysaccharide
Rhamnogalacturonan II. Int. J. Sci. Vigne Vin 1997, 31, 33–41.
(139) Pellerin, P.; Doco, T.; Vidal, S.; Williams, P.; Brillouet, J. M.; O’Neill, M. A. Structural
Characterization of Red Wine Rhamnogalacturonan II. Carbohydr. Res. 1996, 290, 183–197.
(140) Whitcombe, A. J.; O’Neill, M. A.; Steffan, W.; Albersheim, P.; Darvill, A. G. Structural
Characterization of the Pectic Polysaccharide, Rhamnogalacturonan-II. Carbohydr. Res. 1995,
271, 15–29.
(141) Pabst, M.; Fischl, R. M.; Brecker, L.; Morelle, W.; Fauland, A.; Köfeler, H.; Altmann, F.;
Léonard, R. Rhamnogalacturonan II Structure Shows Variation in the Side Chains
Monosaccharide Composition and Methylation Status within and across Different Plant
Species. Plant J. 2013, 76, 61–72.
(142) Scheller, H. V.; Jensen, J. K.; Sørensen, S. O.; Harholt, J.; Geshi, N. Biosynthesis of Pectin.
Physiol. Plant. 2006, 129, 283–295.
(143) Glushka, J. N.; Terrell, M.; York, W. S.; Neill, M. A. O.; Gucwa, A.; Darvill, A. G.;
169
Albersheim, P.; Prestegard, J. H. Primary Structure of the 2-O-Methyl-a- L -Fucose-Containing
Side Chain of the Pectic Polysaccharide , Rhamnogalacturonan II. Carbohydr. Res. 2003, 338,
341–352.
(144) Penhoat, H. C.; Gey, C.; Pellerin, P.; Perez, S. An NMR Solution Study of the Mega-
Oligosaccharide, Rhamnogalacturonan II. J. Biomol. NMR 1999, 14, 253–271.
(145) Pérez, S.; Rodríguez-Carvajal, M. A.; Doco, T. A Complex Plant Cell Wall Polysaccharide:
Rhamnogalacturonan II. A Structure in Quest of a Function. Biochimie 2003, 85, 109–121.
(146) Vidal, S.; Doco, T.; Williams, P.; Pellerin, P.; York, W. S.; O’Neill, M. A.; Glushka, J.; Darvill,
A. G.; Albersheim, P. Structural Characterization of the Pectic Polysaccharide
Rhamnogalacturonan II: Evidence for the Backbone Location of the Aceric Acid-Containing
Oligoglycosyl Side Chain. Carbohydr. Res. 2000, 326, 277–294.
(147) Rodrıguez-Carvajal, M. A.; Hervé du Penhoat, C.; Mazeau, K.; Doco, T.; Pérez, S. The Three-
Dimensional Structure of the Mega-Oligosaccharide Rhamnogalacturonan II Monomer: A
Combined Molecular Modeling and NMR Investigation. Carbohydr. Res. 2003, 338, 651–671.
(148) Ishii, T.; Matsunaga, T. Isolation and Characterization of a Boron-Rhamnogalacturonan-II
Complex from Cell Walls of Sugar Beet Pulp. Carbohydr. Res. 1996, 284, 1–9.
(149) Funakawa, H.; Miwa, K. Synthesis of Borate Cross-Linked Rhamnogalacturonan II. Front.
Plant Sci. 2015, 6, 1–8.
(150) Ishii, T.; Matsunaga, T.; Hayashi, N. Formation of Rhamnogalacturonan II-Borate Dimer in
Pectin Determines Cell Wall Thickness of Pumpkin Tissue. Plant Physiol. 2001, 126, 1698–
1705.
(151) Melton, L. D.; McNeil, M.; Darvill, A. G.; Albersheim, P. Structural Characterization of
Oligosaccharides Isolated from the Pectic Polysaccharide RG-II. Carbohydr. Res. 1986, 146,
170
279–305.
(152) Spellman, M. W.; McNeil, M.; Darvill, A. G.; Albersheim, P.; Dell, A. Characterization of a
Structurally Complex Heptasaccharide Isolated from the Pectic Polysaccharide
Rhamnogalacturonan II. Carbohydr. Res. 1983, 122, 131–153.
(153) Backman, I.; Jansson, P.; Kenne, L. Synthesis, NMR and Conformational Studies of Some 1,
4-Linked Disaccharides. J. Chem. Soc., Perkin Trans. 1 1990, 50, 1383–1388.
(154) Chauvin, A.-L.; Nepogodiev, S. A.; Field, R. A. Synthesis of a 2,3,4-Triglycosylated
Rhamnoside Fragment of Rhamnogalacturonan-II Side Chain A Using a Late Stage Oxidation
Approach. J. Org. Chem. 2005, 70, 960–966.
(155) Chauvin, A.-L.; Nepogodiev, S. A.; Field, R. A. Synthesis of an Apiose-Containing
Disaccharide Fragment of Rhamnogalacturonan-II and Some Analogues. Carbohydr. Res.
2004, 339, 21–27.
(156) Crich, D.; Picione, J. Direct Synthesis of the β-L-Rhamnopyranosides. Org. Lett. 2003, 5, 781–
784.
(157) Hodosi, G.; Kovac, P. A Fundamentally New, Simple, Stereospecific Synthesis of
Oligosaccharides Containing the β-Mannopyranosyl and β-Rhamnopyranosyl Linkage. J. Am.
Chem. Soc. 1997, 119, 2335–2336.
(158) Yasomanee, J. P.; Demchenko, A. V. Effect of Remote Picolinyl and Picoloyl Substituents on
the Stereoselectivity of Chemical Glycosylation. J. Am. Chem. Soc. 2012, 134, 20097–20102.
(159) Mukherjee, A.; Palcic, M. M.; Hindsgaul, O. Synthesis and Enzymatic Evaluation of Modified
Acceptors of Recombinant Blood Group A and B Glycosyltransferases. Carbohydr. Res. 2000,
326, 1–21.
(160) Amer, H.; Hofinger, A.; Puchberger, M.; Kosma, P. Synthesis of O-Methylated Disaccharides
171
Related to Excretory/secretory Antigens of Toxocara Larvae. J. Carbohydr. Chem. 2001, 20,
719–731.
(161) Rao, Y.; Buskas, T.; Albert, A.; O’Neill, M. a; Hahn, M. G.; Boons, G.-J. Synthesis and
Immunological Properties of a Tetrasaccharide Portion of the B Side Chain of
Rhamnogalacturonan II (RG-II). Chembiochem 2008, 9, 381–388.
(162) Rao, Y.; Boons, G.-J. A Highly Convergent Chemical Synthesis of Conformational Epitopes
of Rhamnogalacturonan II. Angew. Chem. Int. Ed. Engl. 2007, 46, 6148–6151.
(163) Jones, N. A.; Nepogodiev, S. A.; MacDonal, C. J.; Hughes, D. L.; Field, R. A. Synthesis of the
Branched-Chain Sugar Aceric Acid: A Unique Component of the Pectic Polysaccharide
Rhamnogalacturonan-II. J. Org. Chem. Notes 2005, 70, 8556–8559.
(164) Timmer, M.; Stocker, B. L.; Seeberger, P. H. De Novo Synthesis of Aceric Acid and an Aceric
Acid Building Block. J. Org. Chem. Notes 2006, 71, 8294–8297.
(165) York, W. S.; Darvill, A. G.; McNeil, M.; Albersheim, P. 3-Deoxy-D-Manno-2-Octulosonic
Acid (KDO) Is a Component of Rhamnogalacturonan II, a Pectic Polysaccharide in the Primary
Cell Walls of Plants. Carbohydr. Res. 1985, 138, 109–126.
(166) Unger, F. M.; Stix, D.; Schulz, G. The Anomeric Configurations of the Two Ammonium
(Methyl 3-Deoxy-D-Manno-2-Octulopyranosid)onate Salts (Methyl α- and β-Ketopyranosides
of Kdo). Carbohydr. Res. 1980, 80, 191–195.
(167) van der Klein, P. A. M.; Boons, G. J. P. H.; Veeneman, G. H.; van der Marel, G. A.; van Boom,
J. H. An Efficient Route to 3-Deoxy-D-Manno-2-Octulosonic Acid (KDO) Derivatives via a
1,4-Cyclic Sulfate Approach. Tetrahedron Lett. 1989, 30, 5477–5480.
(168) Feng, Y.; Dong, J.; Xu, F.; Liu, A.; Wang, L.; Zhang, Q.; Chai, Y. Efficient Large Scale
Syntheses of 3-Deoxy-D-Manno-2-Octulosonic Acid (Kdo) and Its Derivatives. Org. Lett.
172
2015, 17, 2388–2391.
(169) Stevenson, T. T.; Darvill, A. G.; Albersheim, P. 3-Deoxy-D-Lyxo-2-Heptulosaric Acid, a
Component of the Plant Cell-Wall Polysaccharide Rhamnogalacturonan-II. Carbohydr. Res.
1988, 179, 269–288.
(170) Banaszek, A. The First Synthesis of Acid ( DHA ) Derivatives. Tetrahedron 1995, 51, 4231–
4238.
(171) Thomas, J. R.; Darvill, A. G.; Albersheim, P. Isolation and Structural Characterization of the
Pectic Polysaccharide Rhamnogalacturonan II from Walls of Suspension-Cultured Rice Cells.
Carbohydr. Res. 1989, 185, 261–277.
(172) Heredia, A.; Jiménez, A.; Guillén, R. Composition of Plant Cell Walls. Z. Lebensm. Unters.
Forsch. 1995, 200, 24–31.
(173) Nakamura, A.; Furuta, H.; Maeda, H.; Takao, T.; Negamatsu, Y. Structural Studies by Stepwise
Enzymatic Degradation of the Main Backbone of Soybean Soluble Polysaccharides Consisting
of Galacturonan and Rhamnogalacturonan. Biosci. Biotechnol. Biochem. 2002, 66, 1301–1313.
(174) Huisman, M. M. H.; Brüll, L. P.; Thomas-Oates, J. E.; Haverkamp, J.; Schols, H. A.; Voragen,
A. G. J. The Occurrence of Internal (1→5)-Linked Arabinofuranose and Arabinopyranose
Residues in Arabinogalactan Side Chains from Soybean Pectic Substances. Carbohydr. Res.
2001, 330, 103–114.
(175) Hinz, S. W. A.; Verhoef, R.; Schols, H. A.; Vincken, J. P.; Voragen, A. G. J. Type I
Arabinogalactan Contains β-D-Galp-(1→3)-β-D-Galp Structural Elements. Carbohydr. Res.
2005, 340, 2135–2143.
(176) Leivas, C. L.; Iacomini, M.; Cordeiro, L. M. C. Pectic Type II Arabinogalactans from Starfruit
(Averrhoa Carambola L.). Food Chem. 2016, 199, 252–257.
173
(177) El-Shenawy, H.; Schuerch, C. Synthesis and Characterization of Propyl O-β-D-
Galactopyranosyl-(1→4)-O-β-D-Galactopyranosyl-(1→4)-α-D-Galactopyranoside.
Carbohydr. Res. 1984, 131, 239–246.
(178) Oberthur, M.; Peters, S.; Kumar Saibal, D.; Lichtenthaler, F. W. Synthesis of Linear β-(1→4)-
Galacto Hexa- and Heptasaccharides and Studies Directed towards Cyclogalactans. Carbohydr.
Res. 2002, 337, 2171–2180.
(179) Andersen, M. C. F.; Kračun, S. K.; Rydahl, M. G.; Willats, W. G. T.; Clausen, M. H. Synthesis
of β-1,4-Linked Galactan Side-Chains of Rhamnogalacturonan I. Chem. - Eur. J. 2016, 22,
11543–11548.
(180) Sakamoto, T.; Ishimaru, M. Peculiarities and Applications of Galactanolytic Enzymes That Act
on Type I and II Arabinogalactans. Appl. Microbiol. Biotechnol. 2013, 97, 5201–5213.
(181) Bartetzko, M. P.; Schuhmacher, F.; Seeberger, P. H.; Pfrengle, F. Determining Substrate
Specificities of β-1,4-Endogalactanases Using Plant Arabinogalactan Oligosaccharides
Synthesized by Automated Glycan Assembly. J. Org. Chem. 2017, 82, 1842–1850.
(182) Kovác, P.; Taylor, R. B. Alternative Syntheses and Related NMR Studies of Precursors for
Internal β-D-Galactopyranosyl Residues in Oligosaccharides, Allowing Chain Extension at O-
4. Carbohydr. Res. 1987, 167, 153–173.
(183) Lichtenthaler, F. W.; Oberthur, M.; Peters, S. Directed and Efficient Syntheses of β-(1->4)-
Linked Galacto-Oligosaccharides. Eur. J. Org. Chem. 2001, 3849–3869.
(184) Bartetzko, M. P.; Schuhmacher, F.; Hahm, H. S.; Seeberger, P. H.; Pfrengle, F. Automated
Glycan Assembly of Oligosaccharides Related to Arabinogalactan Proteins. Org. Lett. 2015,
17, 4344–4347.
(185) Yang, F.; Du, Y. Synthesis of Oligosaccharide Derivatives Related to Those from Sanqi, a
174
Chinese Herbal Medicine from Panax Notoginseng. Carbohydr. Res. 2002, 337, 485–491.
(186) Ning, J.; Wang, H.; Yi, Y. A Simple Approach to 3,6-Branched Galacto-Oligosaccharides and
Its Application to the Syntheses of a Tetrasaccharide and a Hexasaccharide Related to the
Arabinogalactans (AGs). Tetrahedron Lett. 2002, 43, 7349–7352.
(187) Kaji, E.; Nishino, T.; Ishige, K.; Ohya, Y.; Shirai, Y. Regioselective Glycosylation of Fully
Unprotected Methyl Hexopyranosides by Means of Transient Masking of Hydroxy Groups
with Arylboronic Acids. Tetrahedron Lett. 2010, 51, 1570–1573.
(188) Timmers, C. M.; Wigchert, S. C. M.; Leeuwenburgh, M. A.; van der Marel, G. A.; van Boom,
J. H. Synthesis of Tetrameric Arabinogalactans Based on 1,2-Anhydrosugars. European J. Org.
Chem. 1998, 1, 91–97.
(189) Du, Y.; Pan, Q.; Kong, F. Synthesis of a Tetrasaccharide Representing a Minimal Epitope of
an Arabinogalactan. Carbohydr. Res. 2000, 323, 28–35.
(190) Ning, J.; Yi, Y.; Yao, Z. An Efficient Method for the Synthesis of 2,6-Branched Galacto-
Oligosaccharides and Its Applications to the Synthesis of Three Tetrasaccharides and a
Hexasaccharide Related to the Arabinogalactans (AGs). Synlett 2003, 14, 2208–2212.
(191) Csávás, M.; Borbás, A.; Szilágyi, L.; Lipták, A. Successful Combination of
(Methoxydimethyl)methyl (MIP) and (2-Naphthyl)methyl (NAP) Ethers for the Synthesis of
Arabinogalactan-Type Oligosaccharides. Synlett 2002, 6, 887–890.
(192) Fekete, A.; Borbás, A.; Antus, S.; Lipták, A. Synthesis of 3,6-Branched Arabinogalactan-Type
Tetra- and Hexasaccharides for Characterization of Monoclonal Antibodies. Carbohydr. Res.
2009, 344, 1434–1441.
(193) Liang, X.-Y.; Liu, Q.-W.; Bin, H.-C.; Yang, J.-S. One-Pot Synthesis of Branched
Oligosaccharides by Use of Galacto- and Mannopyranosyl Thioglycoside Diols as Key
175
Glycosylating Agents. Org. Biomol. Chem. 2013, 11, 3903–3917.
(194) Zeng, Y.; Li, A.; Kong, F. A Concise Synthesis of Arabinogalactan with β-(1→6)
Galactopyranose Backbone and α-(1→2) Arabinofuranose Side Chains. Tetrahedron Lett.
2003, 44, 8325–8329.
(195) Gu, G.; Yang, F.; Du, Y.; Kong, F. Synthesis of a Hexasaccharide That Relates to the
Arabinogalactan Epitope. Carbohydr. Res. 2001, 336, 99–106.
(196) Li, A.; Kong, F. Concise Syntheses of Arabinogalactans with β-(1→6)-Linked
Galactopyranose Backbones and α-(1→3)- and α-(1→2)-Linked Arabinofuranose Side Chains.
Bioorg. Med. Chem. 2005, 13, 839–853.
(197) Huang, H.; Han, L.; Lan, Y.-M.; Zhang, L.-L. One-Pot Synthesis of a 3,6-Branched
Hexaarabinogalactan Using Galactopyranosyl Thioglycoside Diol as a Key Glycosylating
Agent. J. Asian Nat. Prod. Res. 2014, 16, 640–647.
(198) Li, A.; Zeng, Y.; Kong, F. An Effective Synthesis of an Arabinogalactan with a β-(1→6)-
Linked Galactopyranose Backbone and α-(1→2) Arabinofuranose Side Chains. Carbohydr.
Res. 2004, 339, 673–681.
(199) Li, A.; Kong, F. Syntheses of Arabinogalactans Consisting of β-(1→6)-Linked D-
Galactopyranosyl Backbone and α-(1→3)-Linked L-Arabinofuranosyl Side Chains.
Carbohydr. Res. 2004, 339, 1847–1856.
(200) Vincken, J.-P.; Schols, H. A.; Oomen, R. J. F. J.; Mccann, M. C.; Ulvskov, P.; Voragen, A. G.
J.; Visser, R. G. F. If Homogalacturonan Were a Side Chain of Rhamnogalacturonan I .
Implications for Cell Wall Architecture. Plant Physiol. 2003, 132, 1781–1789.
(201) Sakamoto, T.; Sakai, T. Analysis of Structure of Sugar-Beet Pectin by Enzymatic Methods.
Phytochemistry 1995, 39, 821–823.
176
(202) Øbro, J.; Harholt, J.; Scheller, H. V.; Orfila, C. Rhamnogalacturonan I in Solanum Tuberosum
Tubers Contains Complex Arabinogalactan Structures. Phytochemistry 2004, 65, 1429–1438.
(203) Wu, Y.; Xiong, D.-C.; Chen, S.-C.; Wang, Y.-S.; Ye, X.-S. Total Synthesis of Mycobacterial
Arabinogalactan Containing 92 Monosaccharide Units. Nat. Commun. 2017, 8, 14851.
(204) Nepogodiev, S. A.; Backinowsky, L. V.; Kochetkov, N. K. No Title. Bioorg. Khim. 1986, 12,
940–946.
(205) Kaneko, S.; Kawabata, Y.; Ishii, T.; Gama, Y.; Kusakabe, I. The Core Trisaccharide of a-L-
Arabinofuranan : Synthesis of Methyl L-Arabinofuranoside. Carbohydr. Res. 1995, 268, 307–
311.
(206) Du, Y.; Pan, Q.; Kong, F. Regio- and Stereoselective Synthesis of 1-5-Linked a-L-
Arabinofuranosyl Oligosaccharides. Synlett 1999, 10, 1648–1650.
(207) Du, Y.; Pan, Q.; Kong, F. An Efficient and Concise Regioselective Synthesis of α-(1→5)-
Linked L-Arabinofuranosyl Oligosaccharides. Carbohydr. Res. 2000, 329, 17–24.
(208) Arndt, D.; Graffi, A. The Synthesis of P-Nitrophenyl 5-O-α-L-Arabinofuranosyl-α-L-
Arabinofuranoside. Carbohydr. Res. 1976, 48, 128–131.
(209) Kawabata, Y.; Kaneko, S.; Kusakabe, I.; Gama, Y. Synthesis of Regioisomeric Methyl a-L-
Arabinofuranobiosides. Carbohydr. Res. 1995, 267, 39–47.
(210) Deng, L.; Liu, X.; Liang, X.; Yang, J. Regioselective Glycosylation Method Using Partially
Protected Arabino- and Galactofuranosyl Thioglycosides as Key Glycosylating Substrates and
Its Application to One-Pot Synthesis of Oligofuranoses. J. Org. Chem. 2012, 77, 3025–3037.
(211) Yamagaki, T.; Maeda, M.; Kanazawa, K.; Ishizuka, Y.; Nakanishi, H. Structures of Caulerpa
Cell Wall Microfibril Xylan with Detection of β -1,3-Xylooligosaccharides as Revealed by
Matrix-Assisted Laser Desorption Ionization/Time of Flight/Mass Spectrometry. Biosci.
177
Biotechnol. Biochem. 1996, 60, 1222–1228.
(212) Umemoto, Y.; Shibata, T.; Araki, T. D-Xylose Isomerase from a Marine Bacterium, Vibrio Sp.
Strain XY-214, and D-Xylulose Production from β-1,3-Xylan. Mar. Biotechnol. 2012, 14, 10–
20.
(213) Chen, L.; Kong, F. An Efficient and Practical Synthesis of β-(1→3)-Linked
Xylooligosaccharides. Carbohydr. Res. 2002, 337, 2335–2341.
(214) Synytsya, A.; Copiková, J.; Kim, W. J.; Park, Y. I. Cell Wall Polysaccharides of Marine Algae.
In Springer Handbook of Marine Biotechnology; Se-Kwon, K., Ed.; 2015; pp 543–590.
(215) Ray, B. Polysaccharides from Enteromorpha Compressa: Isolation, Purification and Structural
Features. Carbohydr. Polym. 2006, 66, 408–416.
(216) Domozych, D. S.; Ciancia, M.; Fangel, J. U.; Mikkelsen, M. D.; Ulvskov, P.; Willats, W. G. T.
The Cell Walls of Green Algae: A Journey through Evolution and Diversity. Front. Plant Sci.
2012, 3, 1–7.
(217) Abouraïcha, E. F.; El Alaoui-Talibi, Z.; Tadlaoui-Ouafi, A.; El Boutachfaiti, R.; Petit, E.;
Douira, A.; Courtois, B.; Courtois, J.; El Modafar, C. Glucuronan and Oligoglucuronans
Isolated from Green Algae Activate Natural Defense Responses in Apple Fruit and Reduce
Postharvest Blue and Gray Mold Decay. J. Appl. Phycol. 2017, 29, 471–480.
(218) Redouan, E.; Cedric, D.; Emmanuel, P.; Mohamed, E. G.; Bernard, C.; Philippe, M.;
Cherkaoui, E. M.; Josiane, C. Improved Isolation of Glucuronan from Algae and the Production
of Glucuronic Acid Oligosaccharides Using a Glucuronan Lyase. Carbohydr. Res. 2009, 344,
1670–1675.
(219) Isogai, A.; Kato, Y. Preparation of Polyuronic Acid from Cellulose by TEMPO-Mediated
Oxidation. Cellulose 1998, 5, 153–164.
178
(220) Distantina, S.; Wiratni; Fahrurrozi, M.; Rochmadi. Carrageenan Properties Extracted From. Int.
J. Chem. Mol. Nucl. Mater. Metall. Eng. 2011, 5, 487–491.
(221) Hoffmann, R. A.; Gidley, M. J.; Cooke, D.; Frith, W. J. Effect of Isolation Procedures on the
Molecular Composition and Physical Properties of Eucheuma Cottonii Carrageenan. Food
Hydrocoll. 1995, 9, 281–289.
(222) Lahaye, M.; Rochas, C. Chemical Structure and Physico-Chemical Properties of Agar.
Hydrobiologia 1991, 221, 137–148.
(223) Perez Recalde, M.; Canelón, D. J.; Compagnone, R. S.; Matulewicz, M. C.; Cerezo, A. S.;
Ciancia, M. Carrageenan and Agaran Structures from the Red Seaweed Gymnogongrus Tenuis.
Carbohydr. Polym. 2016, 136, 1370–1378.
(224) Rees, D. A. Estimation of the Relative Amounts of Isomeric Sulphate Esters in Some Sulphated
Polysaccharides. J. Chem. Soc. 1961, 5168–5171.
(225) Rees, D. A.; Conway, E. The Structure and Biosynthesis of Porphyran: A Comparison of Some
Samples. Biochem. J. 1962, 84, 411–416.
(226) Haug, A.; Larsen, B.; Smidsrød, O. Uronic Acid Sequence in Alginate from Different Sources.
Carbohydr. Res. 1974, 32, 217–225.
(227) Draget, K. I.; Smidsrød, O.; Skjåk-Broek, G. Alginates from Algae. In Polysaccharides and
Polyamides in the Food Industry; Steinbuchel, A., Rhee, S. K., Eds.; 2005.
(228) Zhang, Q.; van Rijssel, E. R.; Walvoort, M. T. C.; Overkleeft, H. S.; van der Marel, G. A.;
Codée, J. D. C. Acceptor Reactivity in the Total Synthesis of Alginate Fragments Containing
α-L-Guluronic Acid and β-D-Mannuronic Acid. Angew. Chem. Int. Ed. 2015, 54, 7670–7673.
(229) van den Bos, L. J.; Dinkelaar, J.; Overkleeft, H. S.; van der Marel, G. a. Stereocontrolled
Synthesis of β-D-Mannuronic Acid Esters: Synthesis of an Alginate Trisaccharide. J. Am.
179
Chem. Soc. 2006, 128, 13066–13067.
(230) Codee, J. D. C.; van den Bos, L. J.; de Jong, A.-R.; Dinkelaar, J.; Lodder, G.; Overkleeft, H.
S.; van der Marel, G. A. The Stereodirecting Effect of the Glycosyl C5-Carboxylate Ester:
Stereoselective Synthesis of β-Mannuronic Acid Alginates. J. Org. Chem. 2009, 74, 38–47.
(231) Walvoort, M. T. C.; van den Elst, H.; Plante, O. J.; Kröck, L.; Seeberger, P. H.; Overkleeft, H.
S.; van der Marel, G. A.; Codée, J. D. C. Automated Solid-Phase Synthesis of β-Mannuronic
Acid Alginates. Angew. Chem. Int. Ed. 2012, 51, 4393–4396.
(232) Dinkelaar, J.; Van Den Bos, L. J.; Hogendorf, W. F. J.; Lodder, G.; Overkleeft, H. S.; Codée,
J. D. C.; Van Der Marel, G. A. Stereoselective Synthesis of L-Guluronic Acid Alginates. Chem.
Eur. J. 2008, 14, 9400–9411.
(233) Chi, F. C.; Kulkarni, S. S.; Zulueta, M. M. L.; Hung, S. C. Synthesis of Alginate
Oligosaccharides Containing L-Guluronic Acids. Chem. Asian J. 2009, 4, 386–390.
(234) Garcia, B. A.; Poole, J. L.; Gin, D. Y. Direct Glycosylations with 1-Hydroxy Glycosyl Donors
Using Trifluoromethanesulfonic Anhydride and Diphenyl Sulfoxide. J. Am. Chem. Soc. 1997,
119, 7597–7598.
(235) Zhang, Q.; van Rijssel, E. R.; Overkleeft, H. S.; van der Marel, G. A.; Codée, J. D. C. On the
Reactivity of Gulose and Guluronic Acid Building Blocks in the Context of Alginate Assembly.
European J. Org. Chem. 2016, 14, 2393–2397.
(236) Kylin, H. Zur Biochemie Der Meeresalgen. Physiol. Chemie 1913, 83, 171–197.
(237) Berteau, O.; Mulloy, B. Sulfated Fucans, Fresh Perspectives: Structures, Functions, and
Biological Properties of Sulfated Fucans and an Overview of Enzymes Active toward This
Class of Polysaccharide. Glycobiology 2003, 13, 29–40.
(238) Ustyuzhanina, N.; Krylov, V.; Grachev, A.; Gerbst, A.; Nifantiev, N. Synthesis, NMR and
180
Conformational Studies of Fucoidan Fragments, 8: Convergent Synthesis of Branched and
Linear Oligosaccharides. Synthesis (Stuttg). 2006, 23, 4017–4031.
(239) Chargaff, E.; Bancroft, F. W.; Stanley-brown, M. Studies on the Chemistry of Blood
Coagulation. J. Biol. 1936, 4, 155–161.
(240) Gerbst, A. G.; Ustuzhanina, N. E.; Grachev, A. A.; Khatuntseva, E. A.; Tsvetkov, D. E.;
Shashkov, A. S.; Usov, A. I.; Preobrazhenskaya, M. E.; Ushakova, N. A.; Nifantiev, N. E.
Synthesis, NMR and Conformational Studies of Fucoidan Fragments. V. Linear 4,4′,4″‐Tri‐O‐
Sulfated and Parent Non‐sulfated (1→3)‐Fucotrioside Fragments. J. Carbohydr. Chem. 2003,
22, 109–122.
(241) Krylov, V. B.; Kaskova, Z. M.; Vinnitskiy, D. Z.; Ustyuzhanina, N. E.; Grachev, A. A.;
Chizhov, A. O.; Nifantiev, N. E. Acid-Promoted Synthesis of per-O-Sulfated
Fucooligosaccharides Related to Fucoidan Fragments. Carbohydr. Res. 2011, 346, 540–550.
(242) Vinnitskiy, D. Z.; Krylov, V. B.; Ustyuzhanina, N. E.; Dmitrenok, A. S.; Nifantiev, N. E. The
Synthesis of Heterosaccharides Related to the Fucoidan from Chordaria Flagelliformis Bearing
an α-L-Fucofuranosyl Unit. Org. Biomol. Chem. 2016, 14, 598–611.
(243) Hua, Y.; Gu, G.; Du, Y. Synthesis and Biological Activities of Octyl 2,3,4-Tri-O-Sulfo-α-L-
Fucopyranosyl-(1→3)-2,4-Di-O-Sulfo-α-L-Fucopyranosyl-(1→3)-2,4-Di-O-Sulfo-α-L-
Fucopyranosyl-(1→3)-2,4-Di-O-Sulfo-β-L-Fucopyranoside. Carbohydr. Res. 2004, 339, 867–
872.
(244) Kasai, A.; Arafuka, S.; Koshiba, N.; Takahashi, D.; Toshima, K. Systematic Synthesis of Low-
Molecular Weight Fucoidan Derivatives and Their Effect on Cancer Cells. Org. Biomol. Chem.
2015, 13, 10556–10568.
181
(245) Zong, C.; Li, Z.; Sun, T.; Wang, P.; Ding, N.; Li, Y. Convenient Synthesis of Sulfated
Oligofucosides. Carbohydr. Res. 2010, 345, 1522–1532.
(246) Arafuka, S.; Koshiba, N.; Takahashi, D.; Toshima, K. Systematic Synthesis of Sulfated
Oligofucosides and Their Effect on Breast Cancer MCF-7 Cells. Chem. Commun. 2014, 50,
9831–9834.
(247) Gerbst, A. G.; Ustuzhanina, N. E.; Grachev, A. A.; Zlotina, N. S.; Khatuntseva, E. A.; Tsvetkov,
D. E.; Shashkov, A. S.; Usov, A. I.; Nifantiev, N. E. Synthesis, NMR, and Conformational
Studies of Fucoidan Fragments 4: 4-Mono-and 4,4’-disulfated (1→3)-α-L-Fucobioside and 4-
Sulfated Fucoside Fragments. J. Carbohydr. Chem. 2002, 21, 313–324.
(248) Gerbst, A. G.; Ustuzhanina, N. E.; Grachev, A. A.; Khatuntseva, E. A.; Tsvetkov, D. E.;
Whitfield, D. M.; Berces, A.; Nifantiev, N. E. Synthesis, NMR, and Conformational Studies of
Fucoidan Fragments. III. Effect of Benzoyl Group At O-3 on Stereoselectivity of Glycosylation
by 3-O- and 3,4-Di-O-Benzoylated 2-O-Benzylfucosyl Bromides. J. Carbohydr. Chem. 2001,
20, 821–831.
(249) Krylov, V. B.; Ustyuzhanina, N. E.; Grachev, A. A.; Nifantiev, N. E. Efficient Acid-Promoted
per-O-Sulfation of Organic Polyols. Tetrahedron Lett. 2008, 49, 5877–5879.
(250) Krylov, V. B.; Argunov, D. A.; Vinnitskiy, D. Z.; Verkhnyatskaya, S. A.; Gerbst, A. G.;
Ustyuzhanina, N. E.; Dmitrenok, A. S.; Huebner, J.; Holst, O.; Siebert, H.-C.; et al. Pyranoside-
into-Furanoside Rearrangement: New Reaction in Carbohydrate Chemistry and Its Application
in Oligosaccharide Synthesis. Chem. - A Eur. J. 2014, 20, 16516–16522.
(251) Ustyuzhanina, N. E.; Fomitskaya, P. A.; Gerbst, A. G.; Dmitrenok, A. S.; Nifantiev, N. E.
Synthesis of the Oligosaccharides Related to Branching Sites of Fucosylated Chondroitin
Sulfates from Sea Cucumbers. Mar. Drugs 2015, 13, 770–787.
182
(252) Bilan, M. I.; Vinogradova, E. V.; Tsvetkova, E. A.; Grachev, A. A.; Shashkov, A. S.; Nifantiev,
N. E.; Usov, A. I. A Sulfated Glucuronofucan Containing Both Fucofuranose and
Fucopyranose Residues from the Brown Alga Chordaria Flagelliformis. Carbohydr. Res. 2008,
343, 2605–2612.
(253) Stadnik, M. J.; De Freitas, M. B. Algal Polysaccharides as Source of Plant Resistance Inducers.
Trop. Plant Pathol. 2014, 39, 111–118.
(254) Lahaye, M.; Robic, A. Structure and Function Properties of Ulvan, a Polysaccharide from
Green Seaweeds. Biomacromolecules 2007, 8, 1765–1774.
(255) Robic, A.; Gaillard, C.; Sassi, J. F.; Leral, Y.; Lahaye, M. Ultrastructure of Ulvan: A
Polysaccharide from Green Seaweeds. Biopolymers 2009, 91, 652–664.
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TOC Graphic OHO
HOOH OH
OH
OOH
OHOHHO
Me
OOH
HO OH
OHOHO
HOOH OH
OHO
HOOH OH
OHChemical synthesis
S
X
F
G