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Background  Prior work from these laboratories has centred on the development of enaminones as versatile intermediates for the synthesis of alkaloids and other nitrogen-containing heterocycles. In this paper we describe the enantioselective synthesis of indolizidine and quinolizidine analogues of bicyclic amphibian alkaloids  via  pyrrolidinylidene- and piperidinylidene-containing enaminones.    Results  Our previously reported synthesis of racemic 8- epi -indolizidine 209B has been extended to the laevorotatory enantiomer, (−) -9 . Attempts to adapt the synthetic route in order to obtain quinolizidine analogues revealed that a key piperidinylidene-containing enaminone intermediate (+) -28  was less tractable than its pyrrolidinylidene counterpart, thereby necessitating modifications that included timing changes and additional protection–deprotection steps. A successful synthesis of [(1 S ,4 R ,9a S )-4-pentyloctahydro-2 H -quinolizin-1-yl]methanol (−) -41  from the chiral amine  tert -butyl (3 R )-3-{benzyl[(1 R )-1-phenylethyl]amino}octanoate (+) -14  was achieved in 14 steps and an overall yield of 20.4%.    Conclusion  The methodology reported in this article was successfully applied to the enantioselective synthesis of the title compounds. It paves the way for the total synthesis of a range of  cis -5,8-disubstituted indolizidines and  cis -1,4-disubstituted quinolizidines, as well as the naturally occurring  trans -disubstituted alkaloids.

beilstein journal of organic chemistry

Analogues of amphibian alkaloids: total synthesis of (5R,8S,8aS)-(−)-8-methyl-5-pentyloctahydroindolizine (8-epi-indolizidine 209B)and[(1S,4R,9aS)-(−)-4-pentyloctahydro-2H-quinolizin-1-yl]methanol