Complementary enantioselective approaches to the quinolizidine alkaloids lupinine and epilupinine by enolate Claisen rearrangements or direct allylation of piperidin-2-ylacetic acid derivatives

Enolate Claisen rearrangement of the piperidinylacetic acid allyl ester 11 leads mainly to the diastereoisomer 12, whereas direct allylation of the lithio enolate of the corresponding methyl ester 14 gives a preponderance of the alternative diastereoisomer 13. Hydroboration and cyclisation of the latter isomer has been used to obtain (+)-lupinine 17; in principle, this approach is adaptable to the synthesis of either enantiomer of both lupinine and epilupinine, given the correct choice of C-allylation method.