Enantioselective syntheses and absolute configuration of the ladybird defence alkaloids (+)-calvine and (+)-2-epicalvine

Enantiomerically pure (+)-calvine (1a) and (+)-2-epicalvine (Ib), two piper idine alkaloids isolated from ladybird beetles of the genus Calvia (Coccine llidae), 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 a to the nitrogen atom of 2 and the subsequent introd uction of the methoxycarbonylmethyl substituent at the u'-position. Compari son of the optical rotations of the synthetic benzoates (12a) and (12b) wit h those of the corresponding benzoates derived from the natural compounds h as revealed the absolute configuration of (+)-calvine to be (2S,6S) and tha t of (+)-2-epicalvine to be (2R,6S).