[Cl:1][C:2]1[C:3]([F:26])=[CH:4][C:5]2[C:6]([CH:25]=1)=[N:7][C:8]1[N:9]([N:21](C=O)[CH3:22])[CH:10]=[C:11]([C:16]([O:18]CC)=[O:17])[C:12](=[O:15])[C:13]=1[CH:14]=2.Cl.[OH-].[Ca+2].[OH-]>C(O)(=O)C>[Cl:1][C:2]1[C:3]([F:26])=[CH:4][C:5]2[C:6]([CH:25]=1)=[N:7][C:8]1[N:9]([NH:21][CH3:22])[CH:10]=[C:11]([C:16]([OH:18])=[O:17])[C:12](=[O:15])[C:13]=1[CH:14]=2A suspension of 16.4 g of 8-chloro-3-ethoxycarbonyl-7-fluoro-1-(N-formyl-N-methylamino)-4-oxo-1,4-dihydro-benzo[b][1,8]naphthyridine in 164 cm3 of acetic acid and 164 cm3 of an aqueous 17.5% hydrochloric acid solution is heated at a temperature close to 100° C., with stirring, for 4 hours. After cooling to a temperature close to 10° C., 165 cm3 of 30% slaked lime is added at between 10° and 20° C. The product is drained and washed with 3 times 150 cm3 of water, 3 times 150 cm3 of ethanol and 3 times 150 cm3 of diethyl ether. 13.64 g of 8-chloro-7-fluoro-1-methylamino-4-oxo-1,4-dihydro-benzo[b][1,8]naphthyridine-3-carboxylic acid are obtained in the form of a yellow solid melting at 354°-356° C.
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