Some derivatives of azaglycine and azaphenylalanine are described. Esters of acetyl-and benzoyl-aza-amino-acids rapidly cyclise to stable oxadiazolones and it is concluded that similar derivatives would be unsuitable for aza-peptide synthesis. t-Butyloxycarbonyl- azaglycine azide was too unreactive for use in peptide synthesis. Benzoyl- azaglycylphenylalanine ethyl ester and acetyl-azaphenylalanylphenylalanine ethyl ester ...