Conformational aspects of polypeptide structure XIV. Synthesis of L‐glutamate oligomers via active ester and azide coupling reactions

Abstract
In this paper we have once again shown the general utility of the modified active ester synthesis using a dipeptide active ester. We have also demonstrated that the acyl azide coupling method using the modifications of Rudinger and Honzl, and Schwyzer make a good general method for the synthesis of complex oligopeptides. This method employs reactions known to give products of high optical purity in reasonable yields. The use of the t‐butyloxycarbonyl hydrazyl (CBH) blocking group provides a good route to hydrazides of esters of glutamic acid. The classical route to prepare hydrazides involves the displacement of an ester with hydrazine. Such a pathway is clearly impossible because of the γ‐esters. Finally we demonstrated that trifluoroethanol is an excellent solvent for catalytic hydrogenation reactions.