To a solution of 15 g (46.8 mmol) of ((5R,6S)-5-(3-Chlorophenyl)-6-(4-chlorophenyl)piperidin-2-one (Example 1, Step E) in 140 mL of DMF was added 3.75 g (94 mmol) of a dispersion of 60% sodium hydride in mineral oil at 0° C. After being stirred for 20 min, ethyl 2-bromobutanoate (17.2 mL, 117 mmol) was added at 0° C. and the resulting solution was stirred at 25° C. for 12 h until completion of the reaction. Then sat. aq. NH4Cl solution was added and the mixture was extracted with ethyl acetate. The combined organic layers were washed with water and sat. NaCl solution, dried over Na2SO4, filtered and the filtrate was concentrated under reduced pressure. Purification of the residue by flash chromatography on silica gel (eluent: 30% EtOAc/hexanes, gradient elution) provided the title compound as the faster eluting isomer.
