To a solution of racemic (2′R,3R,4′S)-6-chloro-4′-(3-chlorophenyl)-2′-(5-fluoro-2-methylphenyl)-2,3-dihydro-2,6′-dioxospiro[indole-3,3′-piperidine]-1-methoxyethyl trimethylsilane (1.0 g, 1.67 mmol) prepared in example 55b in N,N-dimethyl-formamide (20 mL) at room temperature was added bromo-acetic acid tert-butyl ester (0.8 g, 4.1 mmol) and cesium carbonate (3.0 g, 9.20 mmol). The reaction mixture was stirred under nitrogen for 4 h, then poured into saturated aqueous NH4Cl solution. The mixture was extracted with ethyl acetate. The organic layers were combined, dried over Na2SO4 and concentrated. The residue was purified by chromatography (EtOAc:Hexanes=1:4) to give racemic (2′R,3R,4′S)-6-chloro-4′-(3-chlorophenyl)-2′-(5-fluoro-2-methylphenyl)-2,3-dihydro-1′-[(tert-butoxycarbonyl)methyl]-2,6′-dioxospiro[indole-3,3′-piperidine]-1-methoxyethyl trimethylsilane as a white foam (Yield 0.58 g, 48.7%).
