The use of the aza-Diels–Alder reaction in the synthesis of pinidine and other piperidine alkaloids

The imine Ph2CHNCHCO2Et, generated from benzhydrylamine and ethyl glyoxylate, provides a Diels–Alder adduct with 1,3-pentadiene from which a range of cis-2,6-disubstituted piperidines can be accessed; the benzhydryl group confers high diastereocontrol for derivatising the six-membered ring, allowing access to 2,5,6-trisubstituted piperidines, and to 2,6-disubstituted piperidines such as pinidine.