Tris(2,2,2-trifluoroethoxy)silane-Enabled Peptide Bond Formation between Unprotected Amino Acids and Amino Acid <i>t</i>-Butyl Esters

Conventional peptide synthesis involves multiple protection and deprotection steps, and typically relies on stoichiometric amounts of coupling reagents and additives. This makes the process cumbersome, and results in poor atom economy and hazardous waste generation. Therefore, direct peptide bond formation using unprotected amino acids is a promising alternative. However, this approach presents some challenges: 1) Solubility of unprotected amino acids in organic solvents; 2) Control of undesired side reactions; 3) Chemo-selective activation of the carboxylic acid group in the presence of an amine functionality; and 4) Epimerization. To address these challenges, we developed tris(2,2,2-trifluoroethoxy)silane [H-Si(OCH₂CF₃)₃], a cost-effective and accessible coupling reagent. This single reagent efficiently synthesizes N-terminal free peptides from unprotected amino acids and amino acid tert-butyl esters, without the need for any additives. H-Si(OCH₂CF₃)₃ enhances amino acid solubility through coordinating with both termini and plays a dual role, serving as a transient amine-protecting group and as a carboxylic acid activating or promoting reagent for peptide bond formation. This method is operationally simple and versatile, enabling the efficient synthesis of N-terminal free peptides from unprotected amino acids and amino acid tert-butyl esters, with good yields, high optical purity, and broad side-chain compatibility.