Practical synthesis of (S)-pyroglutamic acid

A simple and rapid synthesis of (S)-pyroglutamic acid was developed by the thermal cyclodehydration of (S)-glutamic acid under solvent-free conditions, with continuous swirling during heating (220°C) to ensure homogeneity of the melt. The reaction proceeds until the bubbles disappear, not exceeding 5 min. The heating immediately ceases, then cooling the reaction vessel to prevent degradation and racemization. (S)-pyroglutamic acid is obtained in 70% yield, and the enantiomeric ratio is 97:3. The same reaction under microwave heating in an aqueous solution afforded a low yield of racemic product.