Quinapril hydrochloride (Accupril®) is an angiotensin converting enzyme (ACE) inhibitor for the treatment of hypertension and congestive heart failure 34. The original manufacturing route contained a variety of undesired qualities, such as the use of methylene chloride, the potentially explosive hydroxybenzotriazole, the sensitizer dicyclohexylcarbodiimide (DCC), and copious toluene volumes for removing acetic acid by solvent exchange. The key to the redesign of the quinapril process was the loss of yield due from an intramolecular cyclization that formed a diketopiperazine impurity. The rate of this side reaction of the product was greatly enhanced, when the reaction mixture was held at >55°C during the three acetic acid/toluene azeotropic distillations.