Disulfide bond-forming reaction using a dimethyl sulfoxide/aqueous HCl system and its application to regioselective two disulfide bond formation

Disulfide bond formation in S-acetamidomethyl (Acm) cysteine-containing peptides by successive treatments with silver trifluoromethanesulfonate (AgOTf) and dimethyl sulfoxide (DMSO)/aqueous HCl is described. An S-Acm cysteine was found to be quantitatively converted into cystine by deprotection of the Acm group with AgOTf followed by DMSO/aqueous HCl treatment. Under these reaction conditions, no significant side reactions were observed with oxidation-sensitive amino acids such as Met, Tyr and Trp. Oxytocin and a Trp-containing peptide, urotensin II, were prepared by this method. Furthermore, regioselective two disulfide bond formation was found to be feasible by the combination of air oxidation and the AgOTf-DMSO/HCl system. This strategy has been successfully applied to the syntheses of tachyplesin I and endothelin I, which have two disulfide bonds and a Trp residue in the molecule.