[CH3:1][N:2]([CH3:29])[CH:3]1[CH2:6][N:5]([C:7]2[C:8]([F:28])=[CH:9][C:10]3[C:11]([CH:27]=2)=[N:12][C:13]2[N:14]([CH3:26])[CH:15]=[C:16]([C:21]([O:23]CC)=[O:22])[C:17](=[O:20])[C:18]=2[CH:19]=3)[CH2:4]1.CS(O)(=O)=O>C(O)C.[OH-].[K+]>[CH3:1][N:2]([CH3:29])[CH:3]1[CH2:4][N:5]([C:7]2[C:8]([F:28])=[CH:9][C:10]3[C:11]([CH:27]=2)=[N:12][C:13]2[N:14]([CH3:26])[CH:15]=[C:16]([C:21]([OH:23])=[O:22])[C:17](=[O:20])[C:18]=2[CH:19]=3)[CH2:6]1A suspension of 1.9 g of 8-(3-dimethylamino-1-azetidinyl)-3-ethoxycarbonyl-7-fluoro-1-methyl-4-oxo1,4-dihydrobenzo[b][1,8]naphthyridine in 20 cm3 of ethanol and 19 cm3 of 0.5N aqueous potassium hydroxide is heated with stirring to a temperature in the region of 80° C. for 5 hours. After cooling to approximately 5° C., the reaction mixture is treated with 9.5 cm3 of N aqueous methane sulphonic acid solution. The insoluble matter is drained, washed with twice 10 cm3 of water and 3 times 25 cm3 of ethanol and recrystallized in 125 cm3 of dimethylformamide. 1.4 g of 8-(3-dimethylamino-1-azetidinyl)-7-fluoro-1-methyl-4-oxo-1,4-dihydrobenzo[b][1,8]naphthyridine-3-carboxylic acid are obtained in the form of a yellow solid, which decomposes at 312° C.
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