Inverse Peptide Synthesis Using Transient Protected Amino Acids.

Peptide therapeutics have experienced a rapid resurgence over the past three decades. While a few peptide drugs are biologically produced, most are manufactured via chemical synthesis. The cycle of prior protection of the amino group of an α-amino acid, activation of its carboxyl group, aminolysis with the free amino group of a growing peptide chain, and deprotection of the N-terminus constitutes the principle of conventional C → N peptide chemical synthesis. The mandatory use of the Nα-protecting group invokes two additional operations for incorporating each amino acid, resulting in poor step- and atom-economy. The burgeoning demand in the peptide therapeutic market necessitates cost-effective and environmentally friendly peptide manufacturing strategies. Inverse peptide chemical synthesis using unprotected amino acids has been proposed as an ideal and appealing strategy. However, it has remained unsuccessful for over 60 years due to severe racemization/epimerization during N → C peptide chain elongation. Herein, this challenge has been successfully addressed by ynamide coupling reagent employing a transient protection strategy. The activation, transient protection, aminolysis, and in situ deprotection were performed in one pot, thus offering a practical peptide chemical synthesis strategy formally using unprotected amino acids as the starting material. Its robustness was exemplified by syntheses of peptide active pharmaceutical ingredients. It is also amenable to fragment condensation and inverse solid-phase peptide synthesis. The compatibility to green solvents further enhances its application potential in large-scale peptide production. This study offered a cost-effective, operational convenient, and environmentally benign approach to peptides.

Ynamide-Mediated Peptide Bond Formation: Mechanistic Study and Synthetic Applications.

Ynamides, a class of novel coupling reagents for peptide synthesis, facilitated peptide bond formation in a one-pot, two-step manner with α-acyloxyenamide active esters of amino acids as stable intermediates. Ynamide-mediated peptide synthesis proceeded by a reaction mechanism that is completely different from that of conventional coupling reagents and exhibited superiority in addressing the issue of racemization/epimerization during peptide bond formation. Herein, we present a systematic mechanistic analysis, including kinetics and Brønsted-type structure-reactivity studies and density functional theory calculations, providing unprecedented mechanistic insight into ynamide-mediated peptide bond formation. Based on these mechanistic studies, significant improvements were made, and the applicability of ynamide-mediated peptide bond formation was successfully expanded to peptide fragment condensation, head-to-tail cyclization and solid-phase peptide synthesis.