C([O:4][CH:5]([C@@H:12]([NH:18][C:19]([O:21][C:22]([CH3:25])([CH3:24])[CH3:23])=[O:20])[CH2:13][CH:14]1[CH2:17][CH2:16][CH2:15]1)[C:6]([NH:8][CH:9]1[CH2:11][CH2:10]1)=[O:7])(=O)C.[OH-].[Na+]>CO>[CH:14]1([CH2:13][C@H:12]([NH:18][C:19](=[O:20])[O:21][C:22]([CH3:24])([CH3:23])[CH3:25])[CH:5]([OH:4])[C:6]([NH:8][CH:9]2[CH2:10][CH2:11]2)=[O:7])[CH2:17][CH2:16][CH2:15]1To a solution of (3S)-3-(tert-butoxycarbonylamino)-4-cyclobutyl-1-(cyclopropylamino)-1-oxobutan-2-yl acetate (3.8 g, 10.7 mmol) in methanol (50 mL) was added sodium hydroxide aqueous solution (1N, 15 mL) at room temperature. After 2 hours, methanol was removed under reduced pressure, and the concentrate was extracted with ethyl acetate. The ethyl acetate was washed with brine and dried over magnesium sulfate. The solvent was removed under reduced pressure, and the residue was crystallized from 100 mL of ethyl acetate and hexane(v/v=3/1) to give tert-butyl (2S)-1-cyclobutyl-4-(cyclopropylamino)-3-hydroxy-4-oxobutan-2-ylcarbamate (2.9 g) as a white solid.
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