To a solution of (2R)-2-[N-(chloroacetyl)amino]-3-(3-methoxy-4-methylphenyl)propionic acid methyl ester (1.85 g) in N,N-dimethylformamide (15 ml) were added successively benzylamine (1.65 g) and potassium carbonate (1.28 g) at 20° C. After stirring at 35° C. for 1.5 hour, the mixture was poured into a mixture of ice-water (20 ml) and dichloromethane (20 ml). After adjusting the mixture to pH 9 with a diluted aqueous hydrochloric acid solution with stirring, the organic layer was separated, washed with an aqueous sodium chloride solution (20 ml), dried over magnesium sulfate and the solvent was evaporated under reduced pressure to give (2R)-2-[N-(benzylaminoacetyl)amino]-3-(3-methoxy-4-methylphenyl)-propionic acid methyl ester in the form of an oily substance. A solution of (2R)-2-[N-(benzylaminoacetyl)-amino]-3-(3-methoxy-4-methylphenyl)propionic acid methyl ester obtained by the above process and acetic acid (0.18 ml) in isopropyl alcohol (10 ml) was stirred under reflux for 12 hours. After cooling the mixture to room temperature, isopropyl ether was added to the mixture. The produced precipitates were collected by filtration and washed with isopropyl ether to give (3R)-1-benzyl-3-(3-methoxy-4-methylbenzyl)piperazine-2,5-dione (1.45 g) in the form of colorless crystals.
