486 mg of NaBH4 are added, in portions, to a suspension of 700 mg of benzyl [4-(1-methyl-3-oxopiperidin-4-yl)-2-(propan-2-yloxy)phenyl]carbamate in 35 ml of ethanol brought to 5° C. The reaction medium is stirred at ambient temperature for 50 minutes. 4 ml of water, 15 ml of a saturated aqueous sodium chloride solution and 40 ml of ethyl acetate are added. The two phases are separated, and the aqueous phase is washed with twice 20 ml of ethyl acetate. The combined organic phases are dried over magnesium sulfate, filtered and evaporated. Purification is carried out by flash chromatography on silica gel (40-63 μm), elution being carried out with a mixture of dichloromethane and (MeOH+10% NH4OH) (99/1 to 90/10). 183 mg of benzyl [4-(3-hydroxy-1-methylpiperidin-4-yl)-2-(propan-2-yloxy)phenyl]carbamate (trans diastereoisomers) and 183 mg of benzyl [4-(3-hydroxy-1-methylpiperidin-4-yl)-2-(propan-2-yloxy)phenyl]carbamate (cis diastereoisomers) are obtained.
