Ψ[CH2O] pseudodipeptide synthesis An improved approach which allows absolute configuration determination

An improved way to obtain Ψ[CH2O] pseudodipeptide units is proposed, involving an intramolecular Williamson's reaction, with displacement of bromine by an alkoxide, instead of the classical intermolecular one. Until now, Ψ[CH2O] pseudodipeptide synthesis done by this new method, has used a protected form of the amino alcohol hydroxyl group to prepare the acyclic precursor. In the present paper, the use of an active ester of the brominated carboxylic acid avoids this protection step. The pseudodipeptides Ac-GlyΨ[CH2O]-d,l-Ala-OH and Ac-Ser(Bzl)Ψ[CH2O]-d,l-Ala-OH were obtained in high yields, through a delta-lactam intermediate, which furthermore allows the determination of the absolute configuration of compounds using HPLC and appropriate nuclear magnetic resonance (NMR) techniques.