FC1C(F)=C(F)[C:5]2=[N:6][C:7]3[N:8](C)[CH:9]=[C:10]([C:15]([OH:17])=[O:16])[C:11](=O)[C:12]=3[CH:13]=[C:4]2C=1.CN1CCNCC1.O.C(O)(=O)C>CS(C)=O>[N:8]1[C:7]2[C:12](=[CH:13][CH:4]=[CH:5][N:6]=2)[CH:11]=[C:10]([C:15]([OH:17])=[O:16])[CH:9]=1A suspension of 4 g of 7,8,9-trifluoro-1-methyl-4-oxo-1,4-dihydro-benzo[b][1,8]naphthyridine-3-carboxylic acid in 60 cm3 of dimethyl sulphoxide and 3 g of 1-methylpiperazine is heated at 80° C. for 1 hour and a half. After cooling to about 20° C., 150 cm3 of water are added. 18 cm3 of 10% acetic acid are added to the solution obtained. The precipitate formed is drained, washed with 3 times 50 cm3 of water and recrystallized from 50 cm3 of dimethylformamide. 4 g of 7,9-difluoro-1-methyl-8-(4-methyl-1-pip-erazinyl)-4-oxo-1,4-dihydrobenzo[b]1,8]naphthyridine-3-carboxylic acid are obtained in the form of a yellow solid melting at 316° C..
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