Studies on Peptides. CLI. Syntheses of Cystine-Peptides by Oxidation of S-Protected Cysteine-Peptides with Thallium (III) Trifluoroacetate

Thallium (III) trifluoroacetate, a mild oxidant with a soft-acid character, was found to cleave various S-protecting groups of cysteine in trifluoroacetic acid, with spontaneous formation of cystine. Except for unmasked Trp and Met, other amino acids, including His and Tyr, remained intact in the presence of this oxidant. The usefulness of this oxidant for intramolecular disulfide bond-forming reactions was demonstrated by direct conversion of three model S-protected cysteine-peptides into cystine-peptides, i.e., oxytocin, urotensin II and human calcitonin generelated peptide.