A mixture consisting of 5-amino-6,7-difluoro-1-[(1R,2S)-2-fluoro-1-cyclopropyl]-1,4-dihydro-8-methyl-4-oxoquinoline-3-carboxylic acid (45 mg, 0.144 mmol), (3S,4R)-4-(1-tert-butoxycarbonylaminocyclopropyl)-3-fluoropyrrolidine (52.8 mg, 0.216 mmol), 1-methylpiperidine (18.5 mg, 0.186 mmol) and dimethyl sulfoxide (1.8 ml) was stirred at an outer temperature of 80° C. for 7 hours. When analysis was carried out by a high performance liquid chromatography after completion of the reaction, formed ratio of the compound of interest 5-amino-7-[(3S,4R)-4-(1-tert-butoxycarbonylaminocyclopropyl)-3-fluoropyrrolidinyl]-6-fluoro-1-[(1R,2S)-2-fluoro-1-cyclopropyl]-1,4-dihydro-8-methyl-4-oxoquinoline-3-carboxylic acid was 4%. Also, residual ratio of the starting material was 90%.
