Peptides are attracting much attention as a therapeutic modality that both have the advantages of small molecules and biologics. A liquid phase chemical synthesis of peptides can freely introduce natural or non-natural amino acid and suitable for mass production compared with biochemical genetic expression. Conventional peptide synthesis usually elongates the peptide chain from the C-terminus to the N-terminus. However, this method generates a large amount of waste derived from protecting groups of the peptide main chain and condensation reagents, thus increases the environmental burden and manufacturing cost. If the C-terminal carboxylic acids of the elongating peptide/starting amino acid are distinguishable and suppressing epimerization are possible, the waste could be minimized by utilizing the main chain-unprotected amino acid and elongating the peptide chain from the N-terminus to the C-terminus, as opposed to conventional methods.

We previously reported the catalytic conversion of a peptide C-terminal carboxylic acid to a peptide thiocarboxylic acid (peptide thioacid) using potassium thioacetate as a sulfur source in high yields and low epimerization level.¹ Because the thioacid can be regarded as an activated acyl surrogate, the coupling with the main chain-unprotected amino acid could be achieved through the peptide thioacid-selective activation. As a result of investigations, coupling of peptide thioacid with a main chain-unprotected amino acid proceeded. Aerobic dimerization of peptide thioacid produced diacyl disulfide intermediate, which is the active species for the subsequent coupling. Using a nucleophilic additive greatly suppressed the problematic epimerization. This optimized protocol could be applied to various amino acids even with large steric hindrance, and also extensible to iterative and convergent peptide coupling.²