Enantioselective Synthesis of Tetrahydropyridines/Piperidines via Stepwise [4+2]/[2+2] Cyclizations

A phospbine-catalyzed novel enantioselective [4 -17]-arinulation reaction between allene ketones and 1-azadienes has been, developed, and tetrahydropyridines were obtained in good yields and with excellent enantioselectivities. Subsequent exposure of tetrahydropyridines to benzyne leads to a [2 + 2 ]-cyclization, creating optically enriched polycyclic pipericlines With a quaternary tereogeriic center and a cyclobutene moiety. The reported stepwise [4 +]/[2 + 2]-eyeloadditions represent a new approach to access enantiomerically enriched nitrogen-containing six-membered ring systems.