[F:1][C:2]1[C:3]([N:23]2[CH2:28][CH2:27][N:26]([C:29]3[CH:34]=[CH:33][C:32]([F:35])=[CH:31][CH:30]=3)[CH2:25][CH2:24]2)=[CH:4][C:5]2[N:14]=[CH:13][C:12]3[N:11]([CH3:15])[CH:10]=[C:9]([C:16]([O:18]CC)=[O:17])[C:8](=[O:21])[C:7]=3[C:6]=2[CH:22]=1.[OH-].[K+]>C(O)C>[F:1][C:2]1[C:3]([N:23]2[CH2:24][CH2:25][N:26]([C:29]3[CH:30]=[CH:31][C:32]([F:35])=[CH:33][CH:34]=3)[CH2:27][CH2:28]2)=[CH:4][C:5]2[N:14]=[CH:13][C:12]3[N:11]([CH3:15])[CH:10]=[C:9]([C:16]([OH:18])=[O:17])[C:8](=[O:21])[C:7]=3[C:6]=2[CH:22]=1A suspension of 0.70 g of ethyl 9-fluoro-8-[4-(4-fluorophenyl)piperazin-1-yl]-4-methyl-1-oxo-1,4-dihydrobenzo[f][1,7]naphthyridine-2-carboxylate in a mixture of 250 cm3 of ethanol at 40% and 4 cm3 of a 1 N aqueous potassium hydroxide solution was heated, with stirring, for 8 hours at a temperature of 80° C. After cooling to a temperature of 60° C., the light insoluble material was removed by filtration. The solution was neutralized with 4.1 cm3 of a 1 N aqueous hydrochloric acid solution; the insoluble material formed was dewatered, washed 3 times with 20 cm3 of water and twice with 20 cm3 of ethanol and dried under vacuum at 100° C. 0.56 g of 9-fluoro-8-[4-(4-fluorophenyl)piperazin-1-yl]-4-methyl-1-oxo-1,4-dihydrobenzo[f][1,7]naphthyridine-2-carboxylic acid was obtained in the form of yellow crystals, which melted at 346° C.
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