Hydrochloric Acid → Hexanamide, 3-amino-N-cyclopropyl-2-hydroxy-, hydrochloride (1:1), (2S,3S)-

Isopropanol (250 mL) was added to (2S,3S)—N-cyclopropyl-3-(tert-butoxycarbonyl)amino-2-hydroxyhexanoic acid amide (26.5 g, 92.54 mmol), and the mixture was heated up to about 60° C. To the solution, 35% hydrochloric acid aqueous solution (14.5 g, 138.8 mmol) was dropwise added, and the mixture was stirred for 24 hours. After the disappearance of the starting material, the reaction liquid for the remove of Boc function group was cooled from 60° C. to 2° C. at a constant cooling rate, taking about 6 hours. As a result, no solid aggregated and the flowability thereof was good. The precipitated crystals were collected by filtration under reduced pressure, using a Kiriyama funnel (diameter 6 cm, filter pore size 4 μm). At the time, the filterability was good and the time for the separation of the crystals from the mother liquid was about 60 seconds. By the isolation and drying in vacuum, (2S,3S)—N-cyclopropyl-3-amino-2-hydroxyhexanoic acid amide hydrochloride (16.53 g) was obtained as white crystals (yield: 80 mol %, isopropanol content: no isopropanol detected, chemical purity: 100.0 area %, optical purity: 100% ee).