Sequential peptide ligation by using a controlled cysteinyl prolyl ester (CPE) autoactivating unit

A peptide building block containing a cysteinyl prolyl ester (CPE) autoactivating unit was ligated with a cysteinyl peptide under native chemical ligation conditions. The CPE autoactivating function can be controlled by the protection of the thiol group, permitting the selective ligation of the peptide building block at either the C or N terminus.