6-(5-Chloro-1,8-naphthyridin-2-yl)-5-hydroxy-7-oxo-2,3,6,7-tetrahydro-1,4-dithiino[2,3-c]pyrrole (11.9 g.) is added, at 0° C., to a suspension of sodium hydride (1.2 g.) in anhydrous dimethylformamide (120 cc.). The reaction mixture is stirred for 1 hour at 5° C., and 1-chlorocarbonyl-4-methylpiperazine (16.6 g.) dissolved in anhydrous dimethylformamide (80 cc.) is then added. After stirring for 21/2 hours at 5° C., the crystals which have appeared are filtered off and washed twice with diethyl ether (60 cc.). The product thus isolated is dissolved in an ice-cold 2N aqueous solution of methanesulphonic acid (200 cc.). The aqueous acid solution is washed with diethyl ether (80 cc.), and rendered alkaline by adding an excess of 10N sodium hydroxide solution. The oil which separates out is extracted twice with methylene chloride (total 400 cc.). The organic solution is washed three times with distilled water (total 240 cc.), dried over anhydrous sodium sulphate, treated with decolourising charcoal (0.2 g.) and evaporated. The crystals obtained (10.1 g.), m.p. 238° C., are dissolved in a boiling mixture of acetonitrile (200 cc.) and methylene chloride (180 cc.). After distilling off the methylene chloride and then cooling the residue for 1 hour at 2° C., the crystals which have appeared are filtered off, washed twice with ice-cold acetonitrile (total 40 cc.) and dried under reduced pressure (20 mm. Hg). 6-(5-Chloro-1,8-naphthyridin-2-yl)-5-(4-methylpiperazin-1-yl)carbonyloxy-7-oxo-2,3,6,7-tetrahydro-1,4-dithiino[2,3-c]pyrrole (8.0 g.), melting at 240° C., is thus obtained.
