Hydrazonoyl Chlorides as Reagents for Preparative Carboxy-Group Modification of Peptides in Aqueous Media.

Hydrazonoyl chlorides were developed as reagents for carboxy-group-selective modification under aqueous conditions. In aqueous buffered media, diverse carboxylic acids were converted to stable diacylhydrazines with high chemoselectivity over various functional groups found in amino acid residues. An alkyne-tagged hydrazonoyl chloride, which was readily synthesized and easy to handle, was applied to peptides and selectively modified carboxy groups in Glu and Asp residues and at the C-terminus, affording isolated products in high purity and good yields.