Hydrochloric Acid → 8-Chloro-7-fluoro-1-methylamino-4-oxo-1,4-dihydro-benzo[b][1,8]naphthyridine-3-carboxylic acid

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A suspension of 16.4 g of 8-chloro-3-ethoxycarbonyl-7-fluoro-1-(N-formyl-N-methylamino)-4-oxo-1,4-dihydro-benzo[b][1,8]naphthyridine in 164 cm3 of acetic acid and 164 cm3 of an aqueous 17.5% hydrochloric acid solution is heated at a temperature close to 100° C., with stirring, for 4 hours. After cooling to a temperature close to 10° C., 165 cm3 of 30% slaked lime is added at between 10° and 20° C. The product is drained and washed with 3 times 150 cm3 of water, 3 times 150 cm3 of ethanol and 3 times 150 cm3 of diethyl ether. 13.64 g of 8-chloro-7-fluoro-1-methylamino-4-oxo-1,4-dihydro-benzo[b][1,8]naphthyridine-3-carboxylic acid are obtained in the form of a yellow solid melting at 354°-356° C.

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