Stereoselective synthesis of 2,5,6-trisubstituted piperidines.

[reaction: see text] A short and efficient synthesis of 2,5,6-trisubstituted piperidines was achieved by a combination of an aza-Achmatowicz oxidation of a furyl benzenesulfonamide, conjugate addition to the resulting 2H-pyridinone, and subsequent addition of various nucleophiles to a transient N-sulfonyliminium ion. The steric bulk of the tosyl group directs attack of the nucleophile from its opposite side, thereby leading to the formation of cis-substituted products.