Formaldehyde → 8-(3,4-Dimethyl-1-piperazinyl)-1-ethyl-7-fluoro-4-oxo-1,4-dihydro-benzo[b][1,8]naphthyridine-3-carboxylic acid

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8-(3,4-Dimethyl-1-piperazinyl)-1-ethyl-7-fluoro-4-oxo-1,4-dihydro-benzo[b][1,8]naphthyridine-3-carboxylic acid is prepared under the conditions of Reference Example 21 but starting from 2.3 g of 1-ethyl-7-fluoro-8-(3-methyl-1-piperazinyl)-4-oxo-1,4-dihydro-benzo[b][1,8]naphthyridine-3-carboxylic acid, 2.26 cm3 of 98% formic acid and 5.6 cm3 of a 30% aqueous solution of formaldehyde. 1.75 g of 8-(3,4-dimethyl-1-piperazinyl)-1-ethyl-7-fluoro-4-oxo-1,4-dihydro-benzo[b][1,8]naphthyridine-3-carboxylic acid are obtained in the form of a yellow solid melting at 293°-294° C.

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