Peptide coupling of unprotected amino acids through in situ p-nitrophenyl ester formation

Several series of dipeptides and tripeptides were prepared via an activation–coupling method involving the in situ formation of a p-nitrophenyl ester of an (N-protected) amino acid, followed by coupling with an unprotected amino acid in partially aqueous solutions. The resulting peptide is easily isolated by precipitation. In general, the yields obtained are good to excellent and racemization is minimal. This method is particularly advantageous with respect to its simplicity and lack of obligatory side chain protection/deprotection steps.