To a solution of potassium carbonate (1.52 g) in ethanol (40 ml) which was kept at 90°-95° C., was added ethyl 7-(4-acetyl-1-piperazinyl)-6-chloro-1-(2-chloroethyl)-1,4-dihydro-4-oxo-1,8-naphthyridine-3-carboxylate (4.0 g) and the mixture was heated to reflux for 1.5 hours. After the mixture was cooled, the resulting potassium 7-(4-acetyl-1-piperazinyl)-6-chloro-1,4-dihydro-4-oxo-1-vinyl-1,8-naphthyridine-3-carboxylate was collected, and dissolved in 30 ml of water and 20 ml of 1 N sodium hydroxide. The mixture was heated to reflux for 2 hours and acidified with acetic acid, and then adjusted to pH 8 with ammonia solution to give a precipitate. The precipitate was dissolved in diluted hydrochloric acid and adjusted again to pH 8 with ammonia water to give 1.56 g of 6-chloro-1,4-dihydro-4-oxo-7-(1-piperazinyl)-1-vinyl-1,8-naphthyridine-3-carboxylic acid, m.p. 272°-274° C.
