To a mixture of 4.84 g of p-chlorobenzoic acid and 4 ml of triethylamine with 50 ml of dimethylformamide, was added dropwise a solution of 3.87 g of isobutyl chloroformate with 2 ml of dimethylformamide, the reaction mixture was stirred at a room temperature for 30 minutes. To the reaction mixture was added dropwise a solution of 5.72 g of 2-amino-3-(oxindol-3-yl)propionic acid hydrochloride prepared in reference example 1 with 3 ml of dimethylformamide, then the whole reaction mixture was stirred at at a room temperature for 30 minutes, next was stirred at 50°-60° C. for 1 hour. After the reaction was completed, the reaction mixture was poured into a large amount of a saturated sodium chloride aqueous solution, and extracted with chloroform. The chloroform layer was washed with water, and dried. The solvent was removed by evaporation to obtain crude crystals, then recrystallized from methanol-water to obtain 3.4 g of 2-(4-chlorobenzoylamino)-3-(oxindol-3-yl)propionic acid 1/4-hydrate in the form of colorless powdery substance. .Melting point: 133°-135° C. (decomp.) [α]D25 -50.3° (C=1, methanol)
