Highly stereoselective conversion of aryl peptidyl ketones into the corresponding peptide alcohols

In this paper we describe the conversion of aryl peptidyl ketones, by a hydride reduction, into the corresponding peptide alcohols. The developed methodology is highly stereoselective and represents a very important application in peptide chemistry for obtaining peptide alcohols. It provides peptide alcohols with definite stereochemistry and in moderate, but satisfactory, yields. The reducing procedure, performed with NaBH3CN and TiCl4, probably proceeds via two diastereomeric cyclic intermediates that show different reactivity. The stereochemistry of the resulting alcohols was established after obtaining them by an alternative synthetic procedure. Furthermore, the methodology adopted keeps the urethane protecting group on the amino function of the N-terminal amino acid residue.