Regio- and Diastereoselective Synthesis of Polysubstituted Piperidines Enabled by Boronyl Radical-Catalyzed (4+2) Cycloaddition.

Piperidines are widely present in small molecule drugs and natural products. Despite many methods have been developed for their synthesis, new approaches to polysubstituted piperidines are highly desirable. This work presents a radical (4+2) cycloaddition reaction for synthesis of piperidines featuring dense substituents at 3,4,5-positions that are not readily accessible by known methods. Using commercially available diboron(4) compounds and 4-phenylpyridine as the catalyst precursors, the boronyl radical-catalyzed cycloaddition between 3-aroyl azetidines and various alkenes, including previously unreactive 1,2-di-, tri-, and tetrasubstituted alkenes, has delivered the polysubstituted piperidines in generally high yield and diastereoselectivity. The reaction also features high modularity, atom economy, broad substrate scope, metal-free conditions, simple catalysts and operation. The utilization of the products has been demonstrated by selective transformations. A plausible mechanism, with the ring-opening of azetidine as the rate-limiting step, has been proposed based on the experimental and computational results.