Synthesis of a tripeptide with a C‐terminal nitrile moiety and the inhibition of proteinases

The synthesis of the tripeptide D-Phe-Pro-Arg with the nitrile group instead of the carboxylgroup is described. Initially, the corresponding peptide amide was synthesized by conventional methods in solution using Boc and Fmoc as the protecting group for D-Phe. The dehydration in order to create the nitrile moiety was achieved by treating the peptide amide with phosphorus oxichloride or trifluoroacetic anhydride. Best results were obtained by the use of phosphorus oxichloride in pyridine as the solvent in the presence of imidazole. After deprotection of the N-terminal amino acid the crude product was purified by chromatography on Butyl-Fractogel HW-40 (S). The purity of the final product was checked on a RP18 phase by hplc. The existence of the nitrile group was demonstrated by i.r. and 13C-n.m.r. spectra. The peptide nitrile exhibited a strong inhibition of thrombin compared to the tripeptide amide.