Potassium carbonate is added to dimethyl sulfoxide, and the mixture is stirred at 70° C. The mixture is cooled to room temperature, and thereto is added (2S,3S)-3-(2-aminophenylthio)-2-hydroxy-3-(4-methoxyphenyl)propionamide, and stirred. To the reaction mixture is added dropwise 2-dimethylaminoethyl chloride, and the mixture is stirred at room temperature. The reaction mixture is poured into ice-water, and extracted with ethyl acetate. The extract is dried, and concentrated under reduced pressure to remove the solvent to give (2S,3S)-3-[2-(2-dimethylaminoethylamino)phenylthio]-2-hydroxy-3-(4-methoxyphenyl)-propionamide.
