Mild conditions for the synthesis of amino thioacid and peptide thiocarboxylates (e.g., 55) were developed employing potassium thioacetate (AcSK) as a cost-effective source of sulphur and catalytic diacetylsulfide (20 mol% Ac2S) in DMF (Figure ). In the proposed mechanism, mixed anhydride 58 is formed on reaction of the peptide with Ac2S and undergoes anhydride exchange with AcSK to yield the mixed diacylsulfide 59. Regeneration of the Ac2S catalyst upon thioacid-anhydride exchange generates the potassium salt of the peptide thioacid (e.g.,