Sequential peptide chemical ligation by the thioester method and extended chemical ligation

The sequential chemical ligation of peptide thioesters by a combination of the thioester method and extended chemical ligation using a photoremovable auxiliary, 2-mercapto-1-(2-nitrophenyl)ethyl group, is described. The thiazolidine ring was used as a protecting group for the N-terminal 1,2-aminoethanethiol moiety of the auxiliary in the middle peptide thioester. After the first thioester coupling, the thiazolidine ring was opened by treatment with O-methylhydroxylamine. Second coupling by extended chemical ligation followed by UV irradiation gave the target polypeptide.