To a solution of 1-tert-butoxycarbonyl-3-azabicyclo[3.2.0]heptane (1.02 mmol) in dimethyl sulfoxide (1.5 ml) were added 180 mg (0.5 mmol) of difluoroboric acid ester of 1-[2-(S)-fluoro-1-(R)-cyclopropyl]-6,7-difluoro-8-methoxy-1,4-dihydro-4-oxoquinoline-3-carboxylic acid and 0.5 ml of triethylamine, followed by 110 hours of stirring at room temperature. After evaporation of triethylamine, the resulting residue was mixed with water, and the thus precipitated crystals were collected by filtration. To the thus collected crystals were added 15 ml of 90% methanol and 3 ml of triethylamine, subsequently carrying out 4 hours of reflux. This was cooled to room temperature and then concentrated. The resulting residue was mixed with 10% citric acid and extracted with chloroform. The organic layer was washed with water and saturated brine and dried on anhydrous sodium sulfate, subsequently evaporating the solvent. To the thus obtained residue dissolved in dichloromethane (5 ml) and cooled in an ice bath was added trifluoroacetic acid (5 ml), followed by 1 hour of stirring at room temperature. After concentration, the resulting residue was mixed with concentrated hydrochloric acid and washed with chloroform. The resulting solution was adjusted to pH 7.5 with concentrated sodium hydroxide aqueous solution and then extracted with chloroform. The extract was dried over anhydrous sodium sulfate, subsequently evaporating the solvent. By recrystallizing the resulting residue from ethanol, 75.9 mg of the title compound was obtained.
