Enantioselective Syntheses and Absolute Configuration of the Ladybird Defence Alkaloids (+)-Calvine and (+)-2-Epicalvine

Enantiomerically pure (+)-calvine (1a) and (+)-2-epicalvine (1b), two piperidine alkaloids isolated from ladybird beetles of the genus Calvia (Coccinellidae), were synthesized by two different strategies starting from (−)-2,3,6,7,8,8a-hexahydro-3-phenyl-5H-[1,3]oxazolo[3,2-a]pyridine-5-carbonitrile (2). The key steps of these syntheses are the stereocontrolled formation of an asymmetric centre α to the nitrogen atom of 2 and the subsequent introduction of the methoxycarbonylmethyl substituent at the α′-position. Comparison of the optical rotations of the synthetic benzoates (12a) and (12b) with those of the corresponding benzoates derived from the natural compounds has revealed the absolute configuration of (+)-calvine to be (2S,6S) and that of (+)-2-epicalvine to be (2R,6S).