Tantalum-Catalyzed Peptide Elongation of Unprotected Amino Acids Using N-Trimethylsilylimidazole

Unprotected amino acids are completely insoluble in organic solvents, limiting their use in organic synthesis. To address this issue, we report a Ta-catalyzed strategy for the elongation of unprotected amino acids using N-trimethylsilylimidazole. More specifically, silylation of the unprotected amino acids was carried out at both termini to generate a linear intermediate that was soluble in organic solvents, thereby permitting the efficient introduction of amino acid esters at the C-terminus. Furthermore, the in situ silicon protection approach employed herein was applicable to varying lengths of amino acid chains, leading to the generation of N-unprotected β-, γ-, and δ-peptides without any loss of enantio- or diastereopurity. These innovative results have facilitated the elimination of several steps in the preparation of desired peptides and have simplified the synthesis of peptides containing long-chain amino acids and branched peptides.