Triethylamine is added to thiolactic acid (I) and 2-thiophencarbonyl chloride (II) in tetrahydrofuran. Mix is cooled at -5 C under agitation and thionyl chloride is added keeping the temperature between 0 and 10 C. A solution of homocysteine thiolactone hydrochloride and triethylamine in water is then added to the reaction, keeping the temperature below 10 C. The reaction is carried out for 1 h at room temperature under agitation, after which the solvent is evaporated at reduced pressure. Water is used to wash and diethyl ether to extract. The organic phase is dried on Na2SO4 anhydrous and the solvent is eliminated at reduced pressure. The raw product obtained from a mix of dichloromethane/diethyl ether (60/40) is crystallized (MR-889).