Alkaloids from dendrobatid poison frogs: Further cis-decahydroquinolines and 8-methylindolizidines

Skin extracts from one population of the poison frog Dendrobates auratus contain a variety of alkaloids, including 2,5-disubstituted-cis-decahydroquinolines and 5-substituted-8-methylindolizidines. Three of the major alkaloids are cis-decahydroquinolines, whose structures based on NNR spectral analyses are 2,5-diallyl-cis-decahydroquinoline (cis- 219A ), 2-allyl-5(pent-2-en-4-ynyl)-cis-decabydroquinoline (cis- 243A ) and 5-methyl-2-propyl-cis-decahydroquinoline (cis- 195A ; the last is identical with “pumiliotoxin C” previously isolated from the poison frog Dendrobates pumilio . Alkaloids cis- 219A and cis- 243A differ fron cis- 195A in configuration at the C-2 position. Another poison frog, Dendrobates histrionicus , was previously shown to produce nearly exclusively trans-decahydroquinoline isomers of 219A and 243A . NMR analyses indicate that the structure of a minor alkaloid from Dendrobates auratus is 5-(pent-2-en-4-ynyl)-8-methylindolizidine ( 203A ). Two additional 8-methylindolizidines isolated from Dendrobates pumilio are 5-(hept-4,6-dienyl)-8-methylindolizidine ( 233D ) and 5-(hept-6-hydroxy-4-enyl)-8-methylindolizidine ( 251B ). FTIR spectral analyses distinguish cis- and trans-decahydroquinolines and provide evidence for configurations of indolizidines.