Peptide-Chain Elongation Using Unprotected Amino Acids in a Micro-Flow Reactor.

Conventional peptide synthesis requires a deprotection step after each amidation step, which decreases synthetic efficiency. Therefore, peptide synthesis using unprotected amino acids is considered an ideal approach. Here, we report peptide chain elongation using unprotected amino acids via a mixed carbonic anhydride. Micro-flow technology enabled rapid mixing of an organic layer containing a protected amino acid or dipeptide and an aqueous layer containing an unprotected amino acid or dipeptide to accelerate the desired amidation, and this approach successfully suppressed undesired racemization/epimerization (≤0.4 %). Various di-, tri-, and tetra-peptides were obtained in good to high yields. This is the first report on peptide chain elongation that proceeds without severe racemization from unprotected amino acids using inexpensive, nonexplosive, less wasteful, and less toxic reagents.