A highly stereocontrolled route to 2-(2′-oxiranyl)piperidines and pyrrolidines: enantioselective synthesis of (+)-α-conhydrine

The first enantio- and diastereoselective approach to both 2-(2′-oxiranyl)piperidines and to 2-(2′-oxiranyl)pyrrolidines is reported. The method relies on the Sharpless asymmetric epoxidation of allyl alcohols as the sole source of chirality, and involves as the key step the base-mediated cyclization of (α-aminoalkyl)oxiranes functionalized at the ϵ (or δ) position. The asymmetric synthesis of (+)-α-conhydrine illustrates the applicability of this strategy to the preparation of biologically active 2-(1-hydroxyalkyl)piperidine alkaloids.