N-Alkyl bis(2-selanylethyl)amines catalyze the synthesis of peptide thioesters or peptide ligation from bis(2-sulfanylethyl)amido (SEA) peptides. These catalysts are generated in situ by reduction of the corresponding cyclic diselenides by tris(2-carboxyethyl)phosphine. They are particularly efficient at pH 4.0 by accelerating the thiol-thioester exchange processes, which are otherwise rate-limiting at this pH. By promoting SEA-mediated reactions at mildly acidic pH, they facilitate the synthesis of complex peptides such as cyclic O-acyl isopeptides that are otherwise hardly accessible.