To 80 mL of tetrahydrofuran, there was added 31.3 g (content: about 50 wt %) of the wet crystals of Nα-(2,6-dichlorobenzoyl)-4-nitro-L-phenylalanine isopropyl ester at room temperature in a nitrogen gas atmosphere and the mixture was then stirred. To the mixture, there were added 80 mL of 2-propanol and 0.86 g of 3% Pt—S/C (wet product, obtained by applying platinum onto carbon and then poisoning the same with sulfur), hydrogen gas was then introduced into the reaction system and the reaction system was stirred at ordinary pressure. The reaction system was stirred at room temperature overnight as it was, and then the catalyst was removed from the reaction solution by filtration through Celite after the confirmation of the progress of the reaction according to HPLC. The resulting filtrate (initial amount of 178 g) was concentrated by distilling the solvent off under reduced pressure till the amount of the filtrate reached 91 g and 40 mL of 2-propanol was added to the concentrate to thus adjust the concentration of the condensate. To this liquid whose concentration had been adjusted, there was dropwise added 150 mL of water at room temperature to thus separate out a crystalline solid and the resulting suspension was maintained at a temperature of not higher than 10° C. overnight. The resulting crystals were filtered off, washed with 100 mL of water and the resulting wet crystals were dried at 60° C. under reduced pressure to thus give 13.16 g of the title compound as a white crystalline solid (overall yield of these two steps: 86%).
