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Among those bicyclic alkaloids possessing a bridgehead nitrogen atom, the indolizidine alkaloids, having a fused 6/5 ring system, occupy a structurally intermediate position between the pyrrolizidine and quinolizidine classes. In contrast to the latter groups, however, the indolizidines have not generally been identified so clearly as a specific class of alkaloids. This may be due in part to the occurrence of the indolizidine ring system in a number of alkaloids as a structural moiety associated with additional heterocyclic sys-