A process for chemically converting a peptide chain into a peptide thioester includes, when a —C(═X)—R1 group is introduced to the thiol group of the cysteine residue and then the resulting peptide is reacted with a compound having a leaving group represented by the formula: —NH—C(═Y)NHR3 in an organic solvent, the —NH—C(═Y)NHR3 group binds via addition reaction to the carboxyl group of the N-terminal-side peptide bond of the cysteine residue, whereby the peptide bond is cleaved and the C-terminal-side peptide fragment is cut off. Further, when the resulting peptide chain having the —NH—C(═Y)NHR3 group is reacted with a thiol in a buffer solution, a thiol exchange reaction occurs, namely, the thiol group of the thiol binds to the carbonyl carbon to which the —NH—C(═Y)NHR3 group has bound, whereby the —NH—C(═Y)NHR3 group is eliminated.
