[CH2:1]([N:3]1[C:12]2[C:7](=[CH:8][C:9]([F:14])=[C:10](Cl)[N:11]=2)[C:6](=[O:15])[C:5]([C:16]([OH:18])=[O:17])=[CH:4]1)[CH3:2].[NH3:19].C(O)C>C(O)(=O)C>[CH2:1]([N:3]1[C:12]2[C:7](=[CH:8][C:9]([F:14])=[C:10]([NH2:19])[N:11]=2)[C:6](=[O:15])[C:5]([C:16]([OH:18])=[O:17])=[CH:4]1)[CH3:2]1 Gram of 1-ethyl-1,4-dihydro-4-oxo-6-fluoro-7-chloro-1,8-naphthyridine-3-carboxylic acid (for example, described in European Patent Application No. 0,027,752) is mixed with 25 ml of concentrated ammonia solution containing 20% of ethanol. The mixture is maintained in a sealed tube for 4 hours at 120°-125° C. The mixture is cooled, and acetic acid added until the pH is slightly acid, when the precipitate formed is filtered off and washed with water. The product is dried, 0.8 gram of 1-ethyl-1,4-dihydro-4-oxo-6-fluoro-7-amino-1,8-naphthyridine-3-carboxylic acid being obtained, of melting point 299°-303° C.
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