Development of a sufficiently reactive thioalkylester involving the side-chain thiol of cysteine applicable for kinetically controlled ligation.

N(α) -Trifluoroacetyl-Cys-Leu-NH2 (TfaC-Leu-NH2 ) was incorporated into thioesters through its side-chain thiol group to develop a more reactive peptide-thioester than the commonly used peptide-3-mercaptopropionic acid (MPA)-thioester. The TfaC-thioester could be readily synthesized by solid-phase peptide synthesis (SPPS) with Boc chemistry using in situ neutralization protocols in sufficient yield without any side reaction associated with the use of TfaC. This thioester proved to display a much higher reactivity in the thiol-free native chemical ligation (NCL) reaction than the MPA-thioester and to be comparable to the thioarylester, such as the 4-mercaptophenylacetic acid (MPAA)-thioester, in terms of the ligation rate. We were able to demonstrate the usefulness of the TfaC-thioester by using it to synthesize neuromedin S via a one-pot sequential NCL approach followed by desulfurization.