[C:1]([O:5][C:6](=[O:25])[CH2:7][C:8](=[O:24])[C@@H:9]([NH:13][C:14]([O:16][CH2:17][C:18]1[CH:23]=[CH:22][CH:21]=[CH:20][CH:19]=1)=[O:15])[CH:10]([CH3:12])[CH3:11])([CH3:4])([CH3:3])[CH3:2].[BH4-].[K+].C(O)(=O)C>C(O)C>[C:1]([O:5][C:6](=[O:25])[CH2:7][C@@H:8]([OH:24])[C@@H:9]([NH:13][C:14]([O:16][CH2:17][C:18]1[CH:19]=[CH:20][CH:21]=[CH:22][CH:23]=1)=[O:15])[CH:10]([CH3:12])[CH3:11])([CH3:3])([CH3:4])[CH3:2]To a solution of t-butyl-(4S)-4-(N-benzyloxycarbonylamino)-5-methyl-3-oxohexanoate (6.0 g, 17.1 mmol) in 70 ml ethanol at 0° C. was added potassium borohydride (3.23 g, 58.8 mmol). After stirring for 4 h at 0° C. and 12 h at room temperature, the reaction mixture was acidified with glacial acetic acid to pH 4 and concentrated in vacuo. The residue was dissolved in a mixture of 200 ml ethyl acetate and 200 ml water. After additional washings of the aqueous phase with ethyl acetate (3×50 ml), the combined organic extracts were dried over magnesium sulfate and concentrated in vacuo. The crude product was purified by column chromatography on silica gel using ethyl acetate-hexane (1:4) as eluent to give the alcohol as a white solid (5.31 g).
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