The Stereochemistry of the Addition of Chlorotitanium Enolates of N-Acyl Oxazolidin-2-ones to 5- and 6- Membered N-Acyliminium Ions

Abstract

The stereoselective addition of chiral and achiral titanium enolates derived from the corresponding N-acyl oxazolidin-2-ones to 5- and 6- membercd N-acyliminium ions afforded 2-substituted pyrrolidines in moderate to good diastereoisomeric ratio (5:1 to 14:1) while lower diastercoselection was generally observed in the formation of the corresponding 2-substituted piperidines. The stereochemical outcome was found to be modulated by the nature of the cyclic N-acyliminium ion (5- or 6-membered) and of its carbamate and by the N-acyl group in the enolate precursor. The preferential lk approach seems to be dictated mainly by the minimization of non-bonding interactions between the N-acyl group in the chlorotitanium (IV) enolate and the carbamate and methylene groups in the cyclic N-acyliminium ion.