N-Benzyl-or N-tosyl-N-(4-methyl-3-pentenyl) amino aldehyde benzylimines, which are obtained from alanine, leucine, or phenylalanine methyl esters in five steps, can be cyclized diastereoselectively in the presence of Lewis acids to give 3-amino-2, 4-dialkyl-substituted piperidines. The product distribution and diastereoselectivity depends on the type of Lewis acid and nitrogen-protecting group. Benzyl-protected imines give 2-alkyl-3-(benzylamino)- ...