Use of tetrabutylammonium salts of amino acids in peptide synthesis

Tetrabutylammonium (TBA) salts of amino acids and peptides have increased solubility, as compared with that of alkali metals salts, in organic solvents. We have compared the reaction rates for tripeptide formation in methylene chloride from Boc‐Gly‐Phe activated with various phenols, N‐oxysuccinimide and azide, and TBA‐salt of tryptophan, as well as Trp‐OCH3. H‐Trp‐O− TBA+ as an amino component significantly accelerates the rate of reaction. Although a significant degree of racemization has been found, the use of TBA‐salt of amino acids and peptides is justified in many cases due to high conversion rates.