Methods and reagents for the formation of amide bonds between an activated carboxylic acid derivative and an azide useful in the synthesis of peptides, proteins and derivatized or labeled amino acids, peptide or proteins. The method involves the formation of a phosphinothioester which reacts with an azide resulting in amide formation. The invention provides phosphinothiol reagents which convert activated carboxylic acid derivatives to phosphinothioesters which then react with azides to form an amide bond. The methods and reagents of the invention can be used for stepwise synthesis of peptides on solid supports or for the ligation to two or more amino acids, two or more peptide or two or more protein fragments.