Cyclopropylamine (13.9 g, 242.6 mmol) was dropwise added to a mixed solution of ethyl acetate (250 g)/THF (250 g)=1/1 and (2S,3S)-3-(tert-butoxycarbonyl)amino-2-hydroxyhexanoic acid (50.0 g, 202.2 mmol). The mixture was cooled by ice. A mixed solution of HOBt monohydrate (32.5, 212.3 mmol), EDC hydrochloride (42.6 g, 222.4 mmol) and water (43 g) was dropwise added thereto, and the mixture was stirred for 14 hours with cooling by ice. Next, 5% NaHCO3 aqueous solution (250 g) was added thereto, and the mixture was heated up to 40° C. Then, the aqueous layer was removed, and the organic-layer was washed twice with 5% NaHCO3 aqueous solution (250 g) at 40° C. and finally washed with water (250 g), to obtain a solution of (2S,3S)—N-cyclopropyl-3-(tert-butoxycarbonyl)-amino-2-hydroxyhexanoic acid amide (reaction yield: 95 mol %, extraction yield: 89 mol %, chemical purity: 96.9 area %).
