Because the hydrazide method leads to the release of one equivalent of azide anion in situ during the acyl azide thiolysis step, the effect of adding one equivalent of sodium azide to the thiol-thioester exchange reaction on the rate of peptide alkyl thioester 6a formation was examined (see Supplementary Information). Sodium azide was found to catalyze the formation of peptide alkyl thioester 6a, but the rate achieved was again below that recorded for adapted hydrazide method, whether the reaction mixture was extracted or not. The reaction was also accompanied by byproduct formation which was not clearly identified. Our data clearly suggest that hydrazide method generates in situ a type of compound able to catalyze the thiol-thioester exchange process and those exact nature remains to be established.