[N:1]1([C:7]2[C:16]([F:17])=[C:15]3[C:10]([C:11](=[O:24])[C:12]([C:21]([OH:23])=[O:22])=[CH:13][N:14]3[CH:18]3[CH2:20][CH2:19]3)=[C:9]([CH3:25])[C:8]=2[F:26])[CH2:6][CH2:5][NH:4][CH2:3][CH2:2]1.[C:27](OC(=O)C)(=[O:29])[CH3:28].Cl>[OH-].[Na+]>[C:27]([N:4]1[CH2:5][CH2:6][N:1]([C:7]2[C:16]([F:17])=[C:15]3[C:10]([C:11](=[O:24])[C:12]([C:21]([OH:23])=[O:22])=[CH:13][N:14]3[CH:18]3[CH2:19][CH2:20]3)=[C:9]([CH3:25])[C:8]=2[F:26])[CH2:2][CH2:3]1)(=[O:29])[CH3:28]To a solution of 7-(1-piperazinyl)-1-cyclopropyl-6,8-difluoro-5-methyl-1, 4-dihydro-4-oxoquinoline-3carboxylic acid (42 mg) in 5% sodium hydroxide (2 ml) is added acetic anhydride (0.1 ml) at room temperature. After the mixture is made acidic with dilute hydrochloric acid, the resultant is extracted with dichloromethane and the extract is dried over magnesium sulfate. After concentrating, the obtained residue is recrystallized from ethanol to give 7-(4-acetyl-1-piperazinyl)-1-cyclopropyl-6,8-difluoro-5-methyl-1, 4-dihydro-4-oxoquinoline-3-carboxylic acid (30 mg), as white powder, m.p. 217°-220° C.
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