Biosynthetic studies on adaline and adalinine, two alkaloids from ladybird beetles (Coleoptera: Coccinellidae)

The biosynthesis of the homotropane alkaloid (−)-adaline in the coccinellid beetle Adalia bipunctata has been studied by incorporation experiments with [1-14C] and [2-14C]acetate. A degradation scheme was developed which selectively afforded benzoic acid containing the carbonyl carbon atom of adaline. The labelling pattern thus obtained indicated that the alkaloid is biosynthesized via a polyacetate pathway. Moreover, feeding A. bipunctata adults with (−)-[10,10,11,11,12,12,13,13,14,14,14-2H11]adaline·HCl afforded (−)-[7,7,8,8,9,9,10,10,11,11,11-2H11]adalinine, demonstrating a biogenetic relationship between the two alkaloids. Possible mechanisms for this conversion are discussed. Finally, the synthetic scheme devised to obtain (±)-6-acetonyl-2-pentyl-1-piperideine did not yield the target compound, but afforded instead (±)-adaline in moderate yields. This unexpected result gives support to the hypothesis that this piperideine could be a key intermediate in the biosynthesis of adaline.