[BH4-].[Na+].[CH3:3][O:4][C:5](=[O:28])[CH2:6][C:7](=[O:27])[C@H:8]([N:18]([C:20]([O:22][C:23]([CH3:26])([CH3:25])[CH3:24])=[O:21])[CH3:19])[CH2:9][C:10]1[CH:15]=[CH:14][C:13]([Cl:16])=[C:12]([Cl:17])[CH:11]=1.O.CO>COC(C)(C)C>[CH3:3][O:4][C:5](=[O:28])[CH2:6][C@@H:7]([OH:27])[C@H:8]([N:18]([C:20]([O:22][C:23]([CH3:24])([CH3:25])[CH3:26])=[O:21])[CH3:19])[CH2:9][C:10]1[CH:15]=[CH:14][C:13]([Cl:16])=[C:12]([Cl:17])[CH:11]=1Sodium borohydride (0.227 g, 6 mmol) is added to the stirred solution of (R)-4-(tert-butoxy-carbonyl-methyl-amino)-5-(3,4-dichloro-phenyl)-3-oxo-pentanoic acid methyl ester (4) in tert-butyl methyl ether at 18-22° C. Water (3 ml) is added over a period of approx. 20 minutes at 18-22° C., then stirring is continued for approx. 15 minutes. Methanol (3 ml) is added over a period of approx. 30 minutes at 18-22° C., then stirring is continued for approx. 30 minutes to give a solution of (3R,4R)-4-(tert-butoxycarbonyl-methyl-amino)-5-(3,4-dichloro-phenyl)-3-hydroxy-pentanoic acid methyl ester (5), which is used as such in the next step.
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