1-(5-Amino-2,4-difluorophenyl)-8-methyl-6,7-difluoro-4-oxo-1,4-dihydroquinoline-3-carboxylic acid (500 mg), 3-amino-1-propanol (400 mg) and N-methylpyrrolidine (170 mg) were added to pyridine (1,500 mg), and the mixture was stirred at 70° C. for 15 hours. Since the raw material were not dissolved, N,N-dimethylformamide (1 ml) was added to the mixture, followed by stirring at 70° C. for 43 hours. The reaction mixture was concentrated under reduced pressure. A process of adding distilled water (1 ml) to the residue and then concentrating the mixture under reduced pressure was conducted 3 times repeatedly. Another process of adding toluene (1.5 ml) to the resultant residue and then concentrating the mixture under reduced pressure was conducted 3 times repeatedly. A further process of adding ethanol (1.5 ml) to the resultant residue and then concentrating the mixture under reduced pressure was conducted 3 times repeatedly. Ethanol (1 ml) was added to the resultant residue, and the mixture was heated under reflux for 5 minutes and allowed to cool. Deposits were collected by filtration and washed with ethanol and diisopropyl ether in that order to obtain the title compound (155 mg) as a colorless powder.
