L-Methionine (L-Met) reacted with dichloroacetic acid in concentrated hydrochloric acid under refluxing to give (4S)-1,3-thiazane-2,4-dicarboxylic acid hydrochloride [(4S)-TDC . HCl]. L-Homocysteine (L-Hcy) was obtained in an optically pure form by treatment of (4S)-TDC . HCl with hydroxylamine hydrochloride. D-Hcy was also synthesized starting from D-Met via (4R)-TDC . HCl intermediate.