The condensation of 2-(2-hydroxyphenyl)-4-thiazolidinecarboxylic acid (I) with S-benzoyl-3-mercaptopropanoyl chloride (II) by means of triethylamine in acetone gives 3-(S-benzoyl-3-mercaptopropanoyl)-2-(2-hydroxyphenyl)-4-thiazolidinecarboxylic acid (III), which is debenzylated by a treatment with NH3.