title: Preparation of 2,3,4-Trisubstituted Piperidines by a Formal Hetero-Ene Reaction of Amino Acid Derivatives.
abstract: 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)-4-isopropenyl piperidines with FeCl(3) and 2-alkyl-3-(benzylideneamino)-4-isopropylpiperidines with TiCl(4). Tosyl-protected imines show a decreased level of selectivity. The relative configurations of the piperidines were established by NMR and X-ray crystal structure analyses. Iminium ion cyclization followed by two competitive ionic pathways, i.e. either proton elimination or hydride transfer are discussed for these reactions.
authors.affiliation: Organisch-Chemisches Institut der Universität Münster, Corrensstrasse 40, D-48149 Münster, Germany.
authors.forename: Sabine
authors.initials: S
authors.lastname: Laschat
authors.forename: Roland
authors.initials: R
authors.lastname: Fröhlich
authors.forename: Birgit
authors.initials: B
authors.lastname: Wibbeling