Substrate-Controlled Novel Peptide Bond Formation and Oligopeptide Synthesis from Unprotected Amino Acids

Peptides, which are composed of a wide variety of amino acids, have attracted considerable attention particularly in the pharmaceutical industry. Ever since the pioneering works of Curtius and Fischer more than a century ago, efficient methodologies for forming peptide bonds have been developed and reported. While most of these methods involve reagent-controlled additive-based reactions, we have established several substrate-controlled reactions that show enormous potential for practical use because they do not require excess amounts of additives and enable selective reactions to proceed at specific positions. And, the conventional peptide synthesis employs condensation of N- and C-terminal protected amino acids and deprotection. Protective groups are important to suppress side reactions and carry out the desired reaction selectively, but there are concerns about an increase the amount of waste and a decrease in total yield due to increase of steps. We developed novel methods for forming peptide bonds using substrate-controlled systems, that include: 1) Catalytic peptide bond formation using silyl esters, 2) remote condensation reaction of dipeptides, 3) peptide bond formation in unprotected amino acids, 4) silacyclic dipeptide synthesis, 5) peptide elongating both termini of a silacyclic dipeptide. In this paper, we present these novel substrate-controlled reactions that we have developed and the history and current techniques of peptide bond formation.