After adding 3-(R)-[1-[N-(tert-butoxycarbonyl)amino]cyclopropyl]pyrrolidine (788 mg, 3.48 mmol) and triethylamine (2 ml) to dried dimethyl sulfoxide (1 ml, 5-amino-7-fluoro-1-[2-(S)-fluoro-1-(R)-cyclopropyl]-1,4-dihydro-8-methoxy-4-oxoquinoline-3-carboxylic acid (621 mg, 2.00 mol) was added thereto and stirred at 90° C. for 168 hours while sealing in a sealed tube under a nitrogen atmosphere. After letting the reaction solution cool and then concentrating the reaction solution under a reduced pressure, the residue obtained was dissolved in chloroform (200 ml) and washed with a 10% aqueous citric acid solution (100 ml). The organic layer was then dried over anhydrous sodium sulfate, and after filtering, the filtrate was concentrated under a reduced pressure. After adding dropwise concentrated hydrochloric acid (10 ml) to the obtained residue while cooling with ice, it was stirred at the same temperature for 30 minutes. 1 mol/l Hydrochloric acid (20 ml) was then added to the reaction solution, and after washing the yellow acidic aqueous solution with chloroform (50 ml×3), the pH was adjusted to 12.0 with an aqueous sodium hydroxide solution. After adjusting the pH of the basic aqueous solution to 7.8 with 1 mol/l hydrochloric acid, extraction with chloroform (100 ml×3) was performed. The organic layers were then combined and dried over anhydrous sodium sulfate, and then the solvent was evapolated under a reduced pressure. The residue thus obtained was purified by recrystallization from a mixed solvent of ethanol and diethyl ether, and then dried under a reduced pressure. 74 mg (9%) of the title compound was thereby obtained in the form of yellow crystals.
