Synthesis of enantiopure azetidine 2-carboxylic acids and their incorporation into peptides

Enantiopure azetidine 2-carboxylic acids were prepared by hydrolysis of the corresponding 2-cyano azetidines, without ring cleavage of the azetidine or epimerization. The produced amino acids, which are conformationally constrained analogues of phenylalanine. can be cleanly debenzylated and used for the synthesis of tripeptides. In the course of the synthesis of new enantiopure 2-cyano azetidines through intramolecular alkylation of a metallated amino nitrile, it was found that the involved anionic cyclisation can be thermodynamically controlled, thus enhancing its diastereoselectivity. (C) 2003 Elsevier Ltd. All rights reserved.