300 mg (0.71 mmol) of ethyl 6-[N-(2-chloro-4-fluorophenyl)sulfamoyl]-3,3-dimethoxy-1-cyclohexene-1-carboxylate obtained in Example (16a) and 436 mg (1.42 mmol) of (4R,5R)-2,2-dimethyl-4,5-bis[(trimethylsilyl)oxy]methyl[1.3]dioxolane were dissolved in 12 ml of dichloromethane and 26 μl (0.142 mmol) of trimethylsilyl trifluoromethanesulfonate was added thereto with stirring under ice-cooling, followed by stirring for 90 hours at room temperature. Saturated aqueous sodium hydrogencarbonate was added to the reaction solution and the mixture was extracted with dichloromethane. The organic layer was washed with water and dried over anhydrous magnesium sulfate, followed by concentration under reduced pressure. The residue was subjected to silica gel column chromatography (solvent; hexane:ethyl acetate=4:1) to give 90 mg of ethyl (2R)-8-[N-(2-chloro-4-fluorophenyl)sulfamoyl]-2-((4R)-2,2-dimethyl[1.3]dioxolan-4-yl)-1,4-dioxaspiro[4.5]dec-6-ene-7-carboxylate as an amorphous substance (yield: 24%).
