Stereoselective Sequential [4+2]/[2+2] Cycloadditions Involving 2-Alkenylindolenines: An Approach to Densely Functionalized Benzo[ b]indolizidines.

A stereoselective sequential [4+2]/[2+2] cycloaddition process involving 2-alkenylindolenines has been developed. This unprecedented protocol allows a rapid access to densely functionalized benzo[ b]indolizidines containing a fully substituted piperidine ring with five contiguous stereogenic centers in good yields with excellent diastereoselectivities. This finding demonstrated the unique synthetic utility of the 2-alkenylindolenine species in the construction of complex polycyclic N-heterocycles.