Synergistic Effect of the TiCl4/p-TsOH Promoter System on the Aza-Prins Cyclization.

A novel aza-Prins cyclization promoted by a synergistic combination between a Lewis acid and a Brønsted acid to efficiently afford piperidines is described. Contrary to what has been previously reported in the literature, the generality of the reaction employing N-alkyl, N-aryl, and nonprotected homoallylamines has been demonstrated. The reaction is highly diastereoselective depending on the homoallylic amine used, N-PMP homoallyl amine leading preferentially to the trans diastereomer, and free homoallylamine affording the deprotected piperidine as single cis diastereomer.