Triple Function of 4‐Mercaptophenylacetic Acid Promotes One‐Pot Multiple Peptide Ligation.

One‐pot multiple peptide ligation is a key technology to improve the efficiency of chemical protein synthesis. One‐pot repetitive peptide ligation requires a cycle of three steps: peptide ligation, removal of a protecting group, and inactivation of the deprotection reagent. However, previous strategies are not sufficient because of harsh deprotection conditions, slow deprotection rates, and difficulty in quenching the deprotection reagent. To address these issues, we developed a rapid, efficient deprotection and subsequent quenching strategy using an allyloxycarbonyl group to protect the N‐terminal cysteine residue. 4‐Mercaptophenylacetic acid (MPAA), a thiol additive for native chemical ligation, functioned not only as a scavenger for π‐allyl palladium complexes, but also as a quencher of palladium(0) complexes. By utilizing the multifunctionality of MPAA, we carried out a one‐pot five‐segment ligation to afford histone H2AX (142 amino acids), which was isolated in 59 % yield. [ABSTRACT FROM AUTHOR]