Formal Cycloaddition Reactions of Vinylogous Amides with α,β-Unsaturated Iminiums. A Strategy for Constructing Piperidinyl Heterocycles

A formal [3 + 3] cycloaddition strategy for constructing piperidinyl heterocycles from vinylogous amides and α,β-unsaturated iminiums is described here. These reactions proceed well, leading to various heterocycles that could serve as useful synthetic building blocks. Comparative studies between the reactivities of vinylogous amides and other 1,3-diketo systems have also been examined. Preferences for the nitrogen atom during the electrocyclic ring closure step are noted in these reactions, and preliminary calculations suggest that these preferences could result from electronic factors.