A Structure for 261C, a Novel Tricylic Alkaloid from the Madagascan Poison Frog, Mantella betsileo

Based upon 1 H-NMR, MS and IR spectra, a perhydro 2-allyl-5-ethyl-7-npropylpyrrolo[2,1,5-cd]indolizine structure (1) is proposed for a major tricyclic alkaloid 261C isolated from skin extracts of a Madagascan poison frog of the mantellid genus Mantella.Brightly colored poisonous frogs of the mantellid genus Mantella are found only in Madagascar.We published a survey in 1993 1 of alkaloids in seven species and this included many of the pumiliotoxin and homopumiliotoxin classes, many bicyclics of the "izidine" class and several decahydroquinolines.Most have been reported in NewWorld dendrobatid frogs although others, as yet, have been found only in the Madagascan frogs.One quinolizidine alkaloid, 217A, was isolated in sufficient amounts for a complete structure determination by 1 H-NMR spectrometry. 2We also published in 1996 3 a tabulation of alkaloids present in skins extracts from various populations of nine Mantella species but characterization details of a number of previously unreported alkaloids were not presented. Recent gas chromatographic-EI-MS spectral studies of skin extracts from a population of the Madagascan frogMantella betsileo indicated the presence of significant amounts of a tricyclic alkaloid of molecular weight 261 with the following EIMS spectrum: m/z 261 (19%) with major fragments at 260 (27), 232 (77), 220 (64), 218 (100), i.e.ions consistent with losses of hydrogen, ethyl, allyl and propyl radicals from the molecular ion.The code designation 261C had been assigned to this alkaloid, when it was first detected as a major alkaloid in skin extracts of another mantellid frog, Mantella expectata.At that time it was designated as having an unclassified structure.