Sulfonate esters as coupling reagents for peptide synthesis

A number of N-tert-Butoxycarbonyl (Boc) and N-9-fluorenylmethoxycarbonyl (Fmoc) dipeptide esters were prepared by the direct coupling of N-protected amino acids and amino acid esters using aryl 4-nitrobenzenesulfonates as coupling reagents in the presence of 1-hydroxybenzotriazole (HOBt) as a catalyst. In the absence of the amino acid esters as external nucleophiles, aryl esters of N-protected amino acids were obtained in good yield. Model racemization tests suggested that these reagents provided racemization-free coupling reaction for N-alkoxycarbonyl amino acid but not for N-acyl amino acid. These reagents are stable crystalline solids, provide fastreaction, the procedure is easy to perform and isolation of the products is simple. Therefore aryl 4-nitrobenzenesulfonates should be generally useful for peptide synthesis.