A solution of racemic (2′S,3S,4′R)-6-bromo-4′-[5-chloro-2-(1-ethyl-1-hydroxycarbonyl-propoxy)-phenyl]-2′-(5-fluoro-2-methyl-phenyl)spiro[3H-indole-3,3′-piperidine]-2,6′(1H)-dione (480 mg, 0.75 mmol) and CDI (242 mg, 1.5 mmol) in DMF (5 mL) was heated at 60° C. for 2 h. Then to this solution was added a mixture of methanesulfonamide (712 mg, 7.5 mmol) and NaH (300 mg, 60%, 7.5 mmol) in DMF (5 mL), which had been stirred at room temperature for 2 h. After the resulting mixture was stirred at room temperature for 1 h, it was poured into water and the aqueous solution was acidified by concentrated HCl solution. After the aqueous phase was extracted with EtOAc twice, the combined organic layers were dried over anhydrous Na2SO4, concentrated and the residue was purified by flash column to give the title compound as a white solid (300 mg).
