title: Synthesis of 3,4-disubstituted piperidines by carbonyl ene and Prins cyclizations: a switch in diastereoselectivity between Lewis and Brønsted acid catalysts.
abstract: [reaction: see text] A novel diastereoselective approach to cis and trans 3,4-disubstituted piperidines is described. Carbonyl ene cyclization of aldehydes 6a-e catalyzed by the Lewis acid methyl aluminum dichloride in refluxing chloroform affords trans piperidines 8a-e with diastereomeric ratios of up to 93:7. By contrast, Prins cyclization of 6a-e catalyzed by hydrochloric acid at low temperatures affords cis products 7a-e with diastereomeric ratios of up to 98:2.
authors.affiliation: School of Chemical Sciences, The University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK.
authors.forename: Jodi T
authors.initials: JT
authors.lastname: Williams
authors.forename: Perdip Singh
authors.initials: PS
authors.lastname: Bahia
authors.forename: John S
authors.initials: JS
authors.lastname: Snaith