Elimination–addition. Part XVI. Elimination in 2-sulphonylethyl carboxylates: a method for the protection of carboxy-groups in peptide synthesis

Elimination of carboxylate ions from 2-sulphonylethyl esters of carboxylic acids is rapid in mildly alkaline conditions. The reaction provides the basis of a method for the protection of carboxy-groups during peptide synthesis. The sequence involves formation of 2-toluene-p-sulphonylethyl esters of amino-acids or peptides which are then coupled with N-protected amino-acids to give fully protected peptides. These, on mild treatment with bases, liberate N-protected peptides. 2-Toluene-p-sulphonylethyl esters are stable under the conditions used for coupling by the mixed carbonic anhydride, p-nitrophenyl ester, and dicyclohexycarbodi-imide methods.