Myrmicarin 663: A New Decacylic Alkaloid from Ants

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Ang‘u. Chem. In! . Ed. Engl. 1997,36, No. 112 VCH VerlagsgeseNschaft mbH, D-694Sl Weinherm. 19Y7 OS70-083319713601-0077$ IS.OO+ ,2510 77

[l] a) R Koster. G. W. Rotermund, Tetrahedron Lett. 1964, 1667-1671; b) R. Koster, G. Benedikt, M. A. Grassberger, Justus Liebigs Ann. Chem. 1968, 719, 187- 209: c) R. Koster, H J. Horstschifer, P. Binger, K. Mattschei, ibid. 1975, 1339- 1356, d) P. Binger, Tetrahedron Letl. 1966, 2675-2680.

[2] A leading review covering the early development of carboranechemistry: R. N. Grimes. CurhorunrP, Academic Press, London, 1970

[3] N. S. Hosmane. H. Zhang, J. A. Maguire, Y. Wang. C. J. Thomas, T. G. Gray, Angr.n. Chrm. 1996, 108, 1093-1095; Angeu Chem. I n / . Ed. Engl. 1996, 35, 1000 1002.

[4] W. Jiang. C. B. Knobler. M. D. Mortimer, M. F. Hawthorne, Angeu Chem. 1995. 107. 1470-.1473; Angen Chem. In / . Ed. Engl. 1995,34, 1332-1334.

[5] a) R. Koster, R. Boese, B. Wrackmeyer, Angew. Chem. 1994. 106,2380-2382, A n g w Chem. h i t . Ed. Engl. 1994,33, 2294-2296: b) B. Gagnus, H. Stock, W. Sieberl. M. Hofmann, P. von Rague Schleyer, ibid. 1994. 106,2384-2385 and 1994.33.2296 - 2298; c) M. A. Fox, R. Greatrex. M. Hofmann, P. von Rague Schleyer, ihid. 1994, IO6, 2384-2385 and 1994.33. 2298-2300.

[6] R. KGsrer, R. Boese. 6. Wrackmeyer, H.- J. Schanz, J Chem. Soc. Chem Commun. 1995. I961 - 1962.

[7] The formula “Et,BH” IS used for simplicity: Tetraethyldiborane(6) is usually obtained and used as a mixture with triethylborane and small amounts of other ethyldiboranes(6): R. Koster, G. Bruno, P Bmger, Jusrus Liebigs Ann. Chem. 1961. 644, 1 -72

[XI R. E Williams. in Elecrron Deficienr Boron and Carbon Clusters (Eds.: G. A Olah. K. Wade. R. E. Williams), Wiley, New York, 1991, Chapter 2.

[9] T .0nak .B Lockman,(j.Haran,JChem Soc. Dalton Trans. 1973,2115-2118. [lo] Crystal structure analysis of dimeric 7 (C,,H,,B,Na,; M , = 531.2; colorless

cryslal of irregular shape, 0.40 x 0.30 x 0.25 mm): monoclinic, P2,/n: a = 10.347(2), h =13.791(2), c =12.365(2)A, p =10022(2?; V=1736.4(5)A3, Z = 4: pc4,cd = 1.016 gcm-’; data were collected with a Siemens P4 diffrac- tometer (Mo,,: graphite monochromator; i. = 0.71073 A; 2 0<28<55”); T = 173 K , 5165 reflections, 3954 independent reflections (R,,, = 0.0355). 2731 observed reflections, [ F > 3.Ou(F)]: Lorentz and polarization correction; structure solution by direct methods followed by Fourier synthesis using the SHELXTL-Plus program and refined against F (non-hydrogen atoms an- isotropic; the position of the B(S)-H-B(6) hydrogen resulted unarnbigously from difference Fourier syntheses; all other hydrogen positions were calculated and refined using the “nding model” with fixed isotropic temperature factors). The refinement (full matrix least squares), using 176 parameters, converged a t R : n R = 0.06XiO 050. The max./min. residual electron density was 0.45/ - 0.3X e k - .%. Further details of the crystal structure analysis can be obtained

from the Fachiniormationszentrum Karlsruhe, D-76344 Eggenstein-Leopolds- hdfen (Germany) on quoting the depository number CSD-405767.

[ I l l N S. Hosmane. U . Siriwardane, G. Zhang, H. Zhu, J. A. Maguire. J Chem. So(. . Chem. Commun. 1989. 1128-1130.

Myrmicarin 663: A New Decacyclic Alkaloid from Ants** Frank Schroder, Volker Sinnwell, Horst Baumann, Manfred Kaib, and Wittko Francke*

Dedicated to Professor Jerrold Meinwald on the occasion of his 70th birthday

Alkaloids with unbranched carbon skeletons, which are derived from the acetate pool, are widespread among the plant and animal kingdoms. Whereas the ladybird alkaloid precoccinellin (l), which consists of a single unbranched chain, was discovered some time ago,”’ only recently have the more com- plex structures 2 and 3 been described.‘” They may be regarded as “dimers” of precoccinellin (1). So far, a large variety of monocyclic and bicyclic alkaloids typically bearing 13 or 15 carbon atoms has been identified in We now report on

[*I Prof Dr. W Frdncke. F. Schroder, Dr. V. Sinnwell Instjtut fur Organische Chemie der Universitat Martin-Luther-King-Platz 6, D-20146 Hamburg (Germany) Fax: Int. code +(40)4123-3834 e-mail. fschroed(~chemie.uni-hamburg.de H. Baumann. Dr. M Kaib Lehrstuhl Tierphysiologie der Universitat Bayreuth, Germany

I**] This research was supported by the Dentsche Forschungsgemetnschaft (Fr507/ 8-3 and Ka526/4-4)

H

H Q“

1 2

U

3 4

the isolation and identification of a new decacyclic alkaloid, myrmicarin 663, made up of three unbranched chains of 15 carbon atoms each. Myrmicarin663 may be regarded as a “trimer” of a disubstituted indolizidine like 4, previously identified from the African ant. Mvrmicaria eumenoide~.‘~] , <

Ants of the genus Myrmicaria have an exceptionally large poison gland reservoir, which can contain up to 0.5 pL of a lipophilic secretion.[’] Apart from a group of simple monoterpene hydrocarbons, GC/MS analysis of the secretion from the species Myrmicaria striata revealed a new alkaloid of very low volatility as the main component. The compound is highly temperature- and air-sensitive. Since a sample of pure alkaloid showed more than 50% decomposition within 2 h when stored under air, all isolation steps and the identification procedure had to be carried out strictly under argon.

The molecular weight (Mr = 663) of the alkaloid was determined by CI mass spectroscopy. Apart from an intense molecular ion for C,,H,,N,O (myrmicarin 663), high-resolution EI mass spectrometry showed two diagnostic peaks corresponding to loss of C,H,O (m/z 606; base peak) and C,H, (m/z 634). Isolation of the alkaloid could be accomplished either by column chromatography on neutral alumina or by extraction of a pentane solution of the secretion with hydrochloric acid. From more than 300 poison gland reservoirs, 14 mg of myrmicarin 663 was isolated as a viscous oil of 95

indicated the presence of carbonyl and enamine functions. Hydrogenation resulted in a mixture of compounds with M , = 671, corresponding to the hydrogenation of four double bonds. Thus, assuming the presence of four C-C double bonds and one carbonyl group, the fifteen double bond equivalents implied by the

purity (NMR). Strong IR bands at 1712 cm-‘ and 1651 cm

molecular formula suggested a decacyclic structure. To obtain more detailed structural information, a sample of

pure myrmicarin 663 was analyzed by NMR spectroscopy (Fig- ure 1). Apart from the signals of a single proton at 6 = 4.21 and a multiplett appearing at 6 = 3.3-3.5, the one-dimensional ‘H NMR spectrum exclusively showed severely overlapping signals of aliphatic protons. Six methyl, eighteen methylene, and eleven methine groups could be assigned from 13C NMR and ( 13C,l H) correlation experiments. Furthermore, the presence of ten quaternary carbon atoms was established. including seven carbon atoms appearing in the olefinic region and a carbonyl carbon atom at 6 = 211.5 (Table 1). The 65 protons were assigned to four larger spin systems (I-IV in Scheme 1 ) and three isolated ethyl groups (see Table 2) by phase-sensitive DQ (‘H,’ H) COSY experiments. Spin systems I and I1 are linked by small, long-range couplings of 1-3 Hz between the protons at C-6 and C-22. Because of the small difference between the chemical shift values of the protons 20-H and 21-H. the vicinal coup-

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c 10

- 1 5

- 20 : I 2 5

- 30

T . . , . . . . , . . l , , r

30 2 5 20 1.5 1.0

- 6 Figure 1. Section of the one-dimensional ‘H NMR spectrum (top) and the phase- sensitive NOESY spectrum (bottom) of myrmicarin663.

Scheme 1. Connection of the ‘H spin systems I-IV in myrmicarin663 as deduced from long-range correlations of the quaternary carbon atoms.

ling J,o,21 in spin system I1 could not be directly detected in (’H,’H) COSY spectra. Therefore, a two-step RELAY experiment was performed,[6’ which showed a crosspeak between 20-H and 22-H, corresponding to magnetization transfer from 20-H via 21-H to 22-H,. The connection of the two methine groups at positions 20 and 21 is further corrobora- ted by (13C,‘H) long-range correlations of the carbon atoms C-19, C-20, C-21, and C-22 in HMBC spectra (Table 1).

As a next step, the four ‘H spin systems were combined according to the information from long-range correlation signals of the quaternary carbons, which resulted in the decacyclic structure shown in Scheme 1. The structure of the tricycli6sub-

Table 1. I3C NMR chemical shlft values (C,D,, reference (CH,),Si) and HMBC signals of the carbon atoms in myrmicarin663.

6 HMBC

c - l c-2 C-3 c-4 c-5 C-6 c-7 c-8 c-9 C-10 10-CH,CH, 10-CH2CH3 c-11 1 I-CH, c-12 C-13 I 3-CH2CH, IS-CH,CH, C-14 C-15 C-16 C-17 c-18 C-19 c-20 c-2 1 21-CH2CH, 21-CH2CH, c-22 C-23 COCH,CH, COCH,CH, COCH,CH, C-1’ C-2’ C-2a‘ (2-3’ C-4 C-4a‘ C-5’ C-6‘ C-7’ C-7a’ 1’-CH,CH, 1’-CH,CH,

60.87 25.00 21.07 29.46 58.39 37.69

108.17 74.32 58.1 1 48.73 29.93 12.78 42.88 18.77 55.08 61.77 23.29 9.36

148.62 90.35 20.38 28.37 54.74 30.71 49.39 34.42 28.99 12.28 25.21

143.24 211.54

32.60 8.29

124.30 111.96 127.48 27.39 37.25 54.81 29.99 22.86 21.01

11 1.77 18.77 16.39

2-H,, 3-H I-H, 3-H, 4-H,, 4-H,

2-H,, 3-H, 6-H,, 6-H,

4-H,, 5-H 5-H, 6-H,, 6-H,, 9-H, 20-H, 22-H,, 22-H,

lO-H, 20-H, lO-CH,H,, 13-CH,Hb

9-H, lO-H, 10-CH,CH3, 11-H

9-H, IO-CH,H,, Il-CH,, 12-H

9-H, 11-H, 13-CH,Hb 9-H, 12-H, 13-CH,HbCH,, 15-H 9-H, 12-H, 13-CH,CH3

15-H, 18-H, 12-H, 9-H, 16-H,, 16-H,, 13-CH,Hb

1-H, 2-H,, 5-H

I-H, 3-H, 4-H,

9-H, 10-H, 19-H,, 20-H

9-H, lO-CH,CH,, ll-CH,, 12-H

lO-H, 10-CH,CH,

10-H, 11-H, 12-H

1 3-CH,CH3

16-H,, 16-H,, 17-H,, 17-H, 15-H, 17-H,, 17-H,, 18-H 15-H, 16-H,, 16-H,, 18-H, 19-H,, 19-H, 16-H,, 16-H,, 17-H,, 19-H,, 19-H, 20-H, 21-H, 17-H, 9-H, 19-H,, 19-H,, 21-H, 21-CH2CH,, 22-H, 19-H,, 20-H, 21-CH,CH3, 22-H,, 22-H, 20-H, 21-H, 21-CH,CH3, 22-H, 21-H, 21-CH2CH, 20-H, 21-H, 21-CH2CH, I-H, 5-H, 6-H,, 6-H,, 22-H,, 22-H,

COCH,CH, COCH,CH, 12-H, l‘-CH.H,, 7’-H,, 7’-H, 11-H, 12-H, 1’-CH,Hb, 3‘-H,, 3’-H, 12-H, 3’-H,, 3’-H,, 4-H, 4‘-H,, 4‘-H, 3’-H,, 3-H,, 5’-H, 3‘-H,. 4-H,, 5’-H,, 5‘-H,, 6-H, 4-H,, 7’-H, 4a’-H, 5’-H,, 5’-H,, 7’-H,, 7’-H, 5’-H,, 5’-H, l’-CH,H,,, TH, , T H , , 6’-H, 1‘-CH,CH,

I-H, COCH,HbCH3

l’-CHzCH3

unit followed from HMBC signals of C-2’/C-2ar and C-1’/C-7af with 3’-H/4-H and 6’-H/7’-H, respectively. Analogously, the linkage of spin systems I1 and I11 resulted from correlation signals of the bridgehead atoms C-8, C-13, and C-14. Further- more, the observation of crosspeaks between C-7 and 6-H, 9-H, 20-H, and 22-H, as well as between (2-23 and 6-H and 22-H enabled us to connect spin systems I with I1 and 111. The positions of the three nitrogen atoms could be deduced from the chemical shift values of the attached carbons and corresponding protons as well as from a number of (13C,lH) long-range correlations via nitrogen, for example, from C-5 to 1-H, C-14 to 18-H, or (2-23 to 1-H and 4-H. Thus, myrmicarin663 consists of two oligocychc subunits, a hexahydropyrrolo[2,1 ,S-c,dJin- dolizine and an entirely new heptacyclic system. Long-range correlation signals observed for the proton 12-H of the heptacycle with C-l’, C-2’, and C-2a’ in the pyrrole ring of the tricycle finally revealed that the two subunits are connected through C-12 and C-2‘. Extraordinarily high chemical shift values are shown by carbon atoms C-9 through C-13, which are not bond- ed to a heteroatom. Like in certain steroids that bear an additional 5-membered ring fused to the D-ring of the steroid skele-

78 8 VCH Verlagsgesellschafi mbH, D-6945/ Weinheim. 1997 OS?O-O833/9?/360I-O078 S 15.00+ ,2510 Angew. Chem. Int. Ed. Engl. 1997, 36, No. I12

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Table 2 ' H NMR chemical shift values (C,D,, reference (CH,),Si), f'H,'H)-coupling constants and selected NOE effects in mymicarin663 [a]

served for 9-H and 20-H, which indicated cis orientation of these protons, and setting the configuration of C-8 arbitrarily to (R),

NOE 6 J [Hzl

1 -H

2-H,

4-H, 4-H, 5-H 6-H, 6-H, 9-H 10-H 10-CH,H, lO-CH,H, 10-CH,CH, 11-H 11-CH, 12-H 1 3-CHAH, 13-CH,Hb 1 3-CH,CH3 15-H 16-H, 16-H, 17-H, 17-H, 18-H 19-H, 19-H, 20-H 21-H 21 -CH,CH , 21-CH2CH, 22-H, 22-H, COCH,H, COCH,H, COCH ,CH I '-C H,H, l'-CH,H,

2-n,

~-H,IH,

I'-CH,CH3 3'-H, 3'-H, 4-H, 4-H, 4a'-H 5'-H, 5'-H, 6-H' 6-H, 7-H, 7'-H,

3 474 1201 J(2,,2,)=13.1,J(2,,3,)=4.8,J(2,,3,)=11.0 2.195 1.37- 1.49 1.732 1.070 J(4,,5) = 3.4 3.350 2.394 2.Xi0 2.218 J(9,lO) = 5.0 2.076

1.726 1.1 24 2.210 J(11,12)=127, J ( l I , l l -CH,)= 6.5 1.192

1.552 J(l3-CH,Hb,13-CH,H,) = 1 3 6 1.789 1.100 4.211 2.138 2.058 J(16,,17,) =7.1 1.308 J(17,,17,) =12.8, J(17,.18) =7.6 2.023 J(17,,18) = 5 2 3 368 1.429 1.579 J(19,,20) = I 0 2 2.006 J(20,21) = 3.2 2.065 1.26 1 37 0.955 J(21-CH,CH3,21-CH2CH,) =7.5 1.873 J(22,,22,) =16 7 1.442 2.339 J(CH,H,,CH.H,) = 17.8 2.483 I .06X J(COCH,CH,,COCH,CH,) = 7 3 2.671 J(l'-CH,H,,I'CH,H,) = 14 6 2.83X 1.381 J(I'-CH,CH,,l'-CH,CH,) =7.5 2.793 2.660 2 I29 1 731 J(4,,4a') ~ 1 0 . 2 3.434 1.608 0.930 1.418 5(6,.6',)=13.6, 3(6,,7',)=6.5,J(6,,7',)=11.9 1.717 2.692 J(7',.7',) = 15.8 2.474

J ( 1,2J = 5.4, J( 1,2J = 1.9

J(2,,3,) = 4.0, J(2,,3,) = 4.0, 4J(2,,4,) = 1.0

J(4,,4,) ~ 1 2 . 6 , J(4,,5) =11.4, J(4,,3,) =11.2

J(5.6,) = 2.8, J(5,6,) = 9.0 J(6,,6,) =15.3, J(6,,22,) = 2.7, J(6,,22,) =1.2 J(6,,22,) = 2.0, J(6,,22,) = 3.4

J(10,ll) = 9 4, J(lO,CH,H,) =7.3, J(lO,CH,H,) = 4.9 1 529 J(10-CH,Hb.10-CH,H,) r 1 3 . 0

J( 10-CH,CH,,10-CH,CH3) = 7.6

3 33x

J ( l 3-CH2CH, ,1 3-CH2CH3) = 7.3 J(15,16,) = 5.1, J(15,16,) = 3.5 J(16,,16,) =15.6, J(16,,17,) = 3.6

J(18.19,) = 4.8, J(18,19,) = 9.0 5(19,,19,) =12.3, J(19,,20) = 8.9

421.22,) = 6.1, 421.22,) = 10.6

J(3',.3',) =14.5, J(3',,4',) = 6.4, J(3',,4,) =10.8 J(3',,4,) = 0.5, J(3',,4',) = 8.0 J(4',.4',) = 11.2, J(4,,4a') = 5.3

J(4a'.5',) = 3.5, J(4a',5',) = 11.0 J(5',,5',) = I 2 8, J(5',,6,) = 2.8, J(5' , ,6,) = 4.2 J(5',,6,) =13.1, 3(5',.6,) = 2.6

J(6,.7',) =1.2, J(6,,7',) = 6.5

22-H', 22-H: 4-HI"

11-CH;, 6-H: 6-H:, 5-H' 6-H:, C O C K H , 4-H:, 4-H:, 10-H', 1 I-CH;, 12-Hm 18-H" lO-CH,CK, 13-CH2C&', 20-H" ll-CHy, 12-H" 3'-H:

21-H" 3'-Hy 12-H", 3'-H: l'-CHI,Cfl

15-H' 15-H", 20-H" 16-HF, 16-Hy 17-Hp

19-H," 18-H' 19-H: 20-Hs

2 1 - C H , C e 22-HF

22-H,"

4-H:, 4a'-Hm 4-HP 4a'-H' 5'-H: 5'-H:, 6-H:

7-H: 7'-H' 7'-HY

we finally arrived at the structure shown in Scheme 2. The configu- rations at C-10, C-13, and C-21 followed directly from the NOE effect of the methyl groups 10- CH,CH, and 13-CH,CH3 with 9-H, 20-H, and 21-H, whereas those of C-11 and C-12 are determined from the NOE effects 10- H/12-H, lO-H/lI-CH,, and 10- CH,CH,/ll-H. As there is no positive NOE 11-H/12-H, the large coupling J, i z indicated a 1,2-trans diaxial configuration of these protons, which corrobora- ted the assignments given for C-I 1 and C-12. Furthermore, the relative configuration of these stereogenic centers is confirmed by NOE effects between 6-H, and the protons of the "upper face" of the G-ring (10-H, 11-CH,, and 12-H). The configuration at C-5 followed from observation of the NOE effects 4-HJ1 K H , and 4- HC/6-H,, as well as from the coupling constants and internal NOE effects of the protons of the A- ring, which suggested a slightly twisted chair conformation for this ring, thus indicating an 1,2- trans diaxial configuration of 4- H, and 5-H. Analogously, the as- signment of an axial position for the propionyl side chain at C-I resulted from the small coupling constants of I-H and from the NOE effect between one of the protons of I-COCH, and 5-H. Finally, the configuration at C-18 followed from the NOE 6-H,/18- H and from the coupling con-

of spin system 11.

[a] The crosspeak volume of aliphatic geminal proton pairs (corresponding to an interatomic distance of 178 pm) was used asthereferencestandard(lOO%): V'>25%, 2 5 % > V m > 1 0 % , V"<lO%. Thesubscriptscforcisand t for wansfor the protons refer to their position relative to I-H in the heptacycle or to 4a'-H in the tricycle.

stants and internal NOE effects

ton,"] chemical shift values up to 6 = 60 are observed for carbon atoms of aliphatic methine groups.

The relative configuration of myrmicarin 663 was determined by NOE effects (phase-sensitive 2D NOESY experiments) and the 'H coupling constants (Table 2). Due to the high complexity of the spin systems and considerable overlap in the aliphatic regions, especially small coupling constants could not always be directly extracted from DQ ('H,' H) COSY spectra. In these cases, accurate values for the 'H couplings were obtained by additional evaluation of E. COSY spectra .[81 An impression of the huge number of observable NOE effects is given by the spectrum displayed in Figure 1. The NOE effects were classified

crosspeak volume of geminal proton pairs (for details see foot- note [a] of Table 2). Starting with the strong NOE effect ob-

into strong (')' medium (m) and weak (w) to the Scheme 2. Relative configuration in myrmicarin663. For clarity the positions of a few substructures have been slightly changed from those in the energetically most favorable conformation.

Angew Chem. In!. Ed. Engl. 1997, 36. No. / / 2 VCH Verlugsgesellschuft mhH, 0-69451 Weinhrim. 1997 0570-0833/97/36Ot-0079 S 15 00+.25/0 79

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Myrmicarin 663 Myrmicarin 237A Myrmicarin 645

Scheme 3. Structural relationships between myrmicarin 237A (constructed from one unbranched chain of 15 carbon atoms) and the “trimers” myrmicarin 663 and myrmicarin 645. Tbe central eight carbon atoms of each of the carbon C , ,-chains form indolizine subunits. Carbon-carbon bonds appear only between certain carbon atoms of the C,, chains.

To determine the relative configuration at C-4a‘ in the tri- cyclic subunit, first of all the torsion angle about the 12-2‘ bond (1 1-12-2-1’) had to be considered. Molecular modeling (SYBYL, Tripos inc.) yielded two energetic minima at 131” (70 kcalmol-’) and 312” (72 kcalmol-’) separated by a rota- tional barrier of about 30 kcalmol-’. As evident from the NOE effects 1 1-H/3’-Ht, 1 l-CH3/3’-H, and 1 3-CH,CH3/3‘-H,, as well as 12-H/1’-CH2CH,, myrmicarin663 exists in a conformation that corresponds to the more stable minimum of the model. On the basis of this information, the coupling constants and internal NOE effects of spin system IV indicated that 3’-H, is trans to 4a’-H, which corresponds to an (R*) configuration at C-4a’.

Thus, the constitution and relative configuration of the new alkaloid from Myrmicaria are established. Attempts to crystal- lize myrmicarin 663 or a suitable derivative for determination of the absolute configuration by single-crystal X-ray diffraction analysis have not yet been successful.

Myrmicarin663 is the first example of a new class of alkaloids consisting of condensed indolizine subsystems. Scheme 3 shows the close relationship between the simple indolizine alkaloid mymicarin 237A from M.eumenoide~[~] and myrmicarin 663 from M.striata. Thus, myrmicarin 663 can be regarded to con- sist of three unbranched C,,-chains. In addition to myrmi- carin663, the poison gland secretion of M . striata contains traces (1 %) of a structurally related alkaloid with M , = 645 (C45H63N3, “mymicarin 645”) showing ‘H-spin systems that are consistent with the structure given in Scheme 3. Due to the sensitivity of myrmicarin 645 and the small amounts available (50 pg), heteronuclear NMR experiments could not be performed.

The accumulation of enamine moieties in the considerably strained structures of myrmicarin 663 and myrmicarin 645 may account for their high temperature- and air-sensitivity. In view of this lability, it seemed to be necessary to confirm that no structural alterations had occurred during isolation. For this purpose, a sample of freshly collected poison gland secretion was submitted to NMR analysis without any purification or workup. Due to the comparatively simple composition of the ants’ secretion, the characteristic signals of the monoterpene hydrocarbons as well as those of myrmicarin663 could be directly identified in the DQ (‘H,’H) COSY spectrum. Further- more, no additional components with a comparable concentra- tion were detected. The signals of myrmicarin645 could not be detected in these experiments. possibly due to the very low con- centration of this alkaloid.

Similar to myrmicarin 237AJ4] myrmicarin663 acts as a poison on prey such as termites. The secretion is applied topical- ly by the sting apparatus, which is modified to a brush- or spatulumlike organ.[’] The presence of rnonoterpenes is crucial for the activity of the secretion, since it lowers its viscosity and improves the spreading properties. Similar alkaloids with related structures comprising one or two unbranched C, ,-chains occur in other Myrmicaria species and have analogous functions.[’. ‘‘1

Received. July 5 , 1996 [Z9299IE] German version Angew. Chem 1997, 109. 161 - 164

Keywords: alkaloids - indolizine NMR spectroscopy structure elucidation

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