1-Benzyloxycarbonyl-3-[1-(tert-butoxycarbonyl)amino-1-cyclopropyl]methylpyrrolidine (originated from F3 of Reference Example 4; 100 mg, 0.267 mmol) was dissolved in anhydrous methanol (20 ml), and the solution was mixed with 10% palladium on carbon catalyst (water content 50%, 100 mg) and stirred at room temperature for 1 hour in an atmosphere of hydrogen under atmospheric pressure. After filtration of the reaction solution through celite, the resulting filtrate was concentrated under a reduced pressure. The thus obtained residue and triethylamine (2 ml) were added to dry acetonitrile (8 ml), and the mixture was further mixed with 5-amino-6,7,8-trifluoro-1-[2-(S)-fluoro-1-(R)-cyclopropyl]-1,4-dihydro-4-oxoquinoline-3-carboxylic acid (80 mg, 0.222 mmol) and heated under reflux for 16 hours. After cooling of the reaction solution, the thus precipitated crystals were collected by filtration, washed with acetonitrile, mixed with concentrated hydrochloric acid (10 ml) under ice-cooling and then stirred at room temperature for 5 minutes. This was mixed with distilled water (15 ml), and the thus obtained acidic aqueous solution was washed with dichloromethane (20 ml×2), adjusted to pH 11 with sodium hydroxide aqueous solution under ice-cooling and then washed with chloroform (20 ml). The resulting basic aqueous solution was adjusted to pH 7.4 with 1 N hydrochloric acid and extracted with chloroform (80 ml×4). After drying over anhydrous sodium sulfate, the solvent was evaporated under a reduced pressure. Thereafter, the resulting residue was purified by recrystallizing from ethanol-28% aqueous ammonia and then dried under a reduced pressure, thereby obtaining 59 mg (60.8%) of the title compound as yellow needle crystals.
