A process is provided for preparing the dipeptidyl peptidase of the structure I 1 (also referred to as saxaglipitin) by direct dehydration, in one pot, of the amide II 2 by reacting amide II with phosphorus oxychloride in an organic solvent such as dichloromethane, quenching the reaction mixture with water to form the hydrochloric acid salt of I, and treating the hydrochloric acid salt with base to form the free base of I.