In 7 ml of toluene was suspended 0.70 g of ethyl 7-bromo-1-cyclopropyl-8-difluoromethoxy-1,4-dihydro-4-oxoquinoline-3-carboxylate, followed by adding thereto 3 ml of ethanol, 1.74 ml of a 2 M aqueous sodium carbonate solution, 0.80 g of (+)-1-methyl-2-trityliso-indoline-5-boronic acid and 0.05 g of bis(triphenyl-phosphine)palladium(II) chloride, and the resulting mixture was heated under reflux for 2 hours under a nitrogen atmosphere. The reaction mixture was added to a mixed solvent of 10 ml of ethyl acetate and 10 ml of water, and the organic layer was separated. The organic layer obtained was washed with a saturated aqueous sodium chloride solution, dried over anhydrous magnesium sulfate, and then distilled under reduced pressure to remove the solvent. The resulting residue was purified by a column chromatography (eluent; n-hexane:ethyl acetate=1:1) to obtain 0.72 g of ethyl (+)-1-cyclopropyl-8-difluoromethoxy-7-(1-methyl-2-trityliso-indolin-5-yl)-1,4-dihydro-4-oxoquinoline-3-carboxylate as colorless crystals.
