Behaviour of S-substituted cysteine sulfoxide under acidolytical deprotecting conditions.

Treatment of S-p-methoxybenzylcysteine sulfoxide with hydrogen fluoride or methanesulfonic acid in the presence of anisole afforded S-p-methoxyphenylcysteine as a major product, while S-benzylcysteine sulfoxide resisted to the action of these deprotecting reagents in peptide synthesis. Thiophenol was found to be a powerful reducing reagent of the sulfoxides.