Dimerization of an intermediate during the sodium in liquid ammonia reduction of l -thiazolidine-4-carboxylic acid

Reduction with sodium in liquid ammonia of l-thiazolidine-4-carboxylic acid (II) leads, depending on the reaction procedure followed, to N-methyl-l-cysteine (I) or a mixture of N-methyl-l-cysteine (I) (30%) and a dimer (50%), the structure of which is shown to be VI. Contrary to what has been expressed in the literature, the reaction of 1,2-dichloroethane with cysteine (III) at a pH of 8–10 does not lead to compound VI but to compound VIII.