C([O:3][C:4]([C:6]1[C:7](=[O:24])[C:8]2[CH:9]=[C:10]3[CH:21]=[C:20]([F:22])[C:19]([F:23])=[CH:18][C:11]3=[N:12][C:13]=2[N:14]([O:16][CH3:17])[CH:15]=1)=[O:5])C.Cl.C(O)(=O)C>O>[F:22][C:20]1[C:19]([F:23])=[CH:18][C:11]2=[N:12][C:13]3[N:14]([O:16][CH3:17])[CH:15]=[C:6]([C:4]([OH:5])=[O:3])[C:7](=[O:24])[C:8]=3[CH:9]=[C:10]2[CH:21]=1A suspension of 3-ethoxycarbonyl-7,8-difluoro-1-methoxy-4-oxo-1,4-dihydrobenzo[b][1,8]-naphthyridine (2.78 g) in 17.5% strength hydrochloric acid (30 cc) and acetic acid (30 cc) is heated to a temperature in the region of 100° C. for 1 hour. After cooling to approximately 20° C., the reaction mixture is poured into water (100 cc). The precipitate formed is drained and washed with water (3×+cc) and ethanol (2×5cc). After 1 recrystallization in dimethylformamide (100 cc) containing ethanol (20%), 7,8-difluoro-1-methoxy-4-oxo-1,4-dihydrobenzo[b][1,8]-naphthyridine-3-carboxylic acid (2.03 g) is obtained in the form of a yellow solid, m.p. 325°-327° C.
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