Cyclopropylamine (1.99 g, 34.94 mmol) was dropwise added to a mixed solution of (2S,3S)-3-(tert-butoxycarbonyl)amino-2-hydroxyhexanoic acid (7.2 g, 29.12 mmol), ethyl acetate (144 g) and water (7.2 g). The mixture was cooled by ice. HOBt monohydrate (4.68 g, 30.58 mmol) and EDC hydrochloride (6.14 g, 32.03 mmol) were added thereto, and the mixture was stirred with cooling by ice for 22 hours. Next, 5% NaHCO3 aqueous solution (72 g) was added thereto, and the mixture was heated up to 40° C. Then, the aqueous layer was removed. The organic layer was washed twice with 5% NaHCO3 aqueous solution (72 g) at 40° C., and finally the organic layer was washed with water (72 g), to obtain an ethyl acetate solution of (2S,3S)—N-cyclopropyl-3-(tert-butoxycarbonyl)amino-2-hydroxyhexanoic acid amide (yield: 90 mol %, chemical purity: 96.1 area %, optical purity: 99.3% ee). The obtained organic layer (140.2 g) was concentrated under reduced pressure to be 62.5 g, using a rotary evaporator. The obtained slurry was heated up to 60° C. so as to dissolve the contents, and then cooled to 51° C. Seed crystals of (2S,3S)—N-cyclopropyl-3-(tert-butoxycarbonyl)amino-2-hydroxyhexanoic acid amide (2 mg) were added to the solution. As a result, crystals were gradually precipitated. The mixture was aged at the same temperature for 1 hour, and then was cooled to 2° C., taking about 5 hours. The precipitated crystals were separated and dried to obtain (2S,3S)—N-cyclopropyl-3-(tert-butoxycarbonyl)amino-2-hydroxyhexanoic acid amide (6.45 g, isolation yield: 78 mol %, chemical purity: 100.0 area %, optical purity: 100% ee).
