[O:1]=[C:2]([C:19]1[CH:20]=[CH:21][C:22]2[S:27][C:26]3[N:28]=[CH:29][CH:30]=[N:31][C:25]=3[NH:24][C:23]=2[CH:32]=1)[CH2:3][N:4]1[CH2:9][CH2:8][CH:7]([CH2:10][CH2:11][CH:12]([CH3:18])[C:13]([O:15][CH2:16][CH3:17])=[O:14])[CH2:6][CH2:5]1.[BH4-].[Na+].C(OCC)(=O)C>C(O)C>[OH:1][CH:2]([C:19]1[CH:20]=[CH:21][C:22]2[S:27][C:26]3[N:28]=[CH:29][CH:30]=[N:31][C:25]=3[NH:24][C:23]=2[CH:32]=1)[CH2:3][N:4]1[CH2:5][CH2:6][CH:7]([CH2:10][CH2:11][CH:12]([CH3:18])[C:13]([O:15][CH2:16][CH3:17])=[O:14])[CH2:8][CH2:9]1To a solution of 0.8 g of ethyl 4-[1-[2-oxo-2-(10H-pyrazino[2,3-b][1,4]benzothiazin-8-yl]ethyl]piperidin-4-yl]-2-methylbutanoate in ethanol (30 ml) was added 70 mg of sodium borohydride and the resulting mixture was reacted at room temperature for 30 minutes. After adding ethyl acetate, the reaction mixture was washed with water and a saturated aqueous solution of sodium chloride and dried over anhydrous magnesium sulfate. After distilling off the solvent under reduced pressure, the residue was purified by silica gel column chromatography (eluted with n-hexane/ethyl acetate) to thereby give 0.53 g of the title compound as a yellow oily substance.
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