S-Tritylamino acid p-nitrophenyl esters, with the exception of the glycine derivative, do not couple with amino acid esters. On the other hand, K-tritylsulfenyl (TRS) or No- nitrophenylsulfenyl (NPS) amino acid p-nitrophenyl esters show no steric hindrance and have been used to synthesize N-protected dipeptide esters. The So-nitrophenylsulfenyl derivatives of L-amino acids themselves have also been prepared and isolated in the pure ...