Condensation of 8-chloro-7-fluoro-2,3-dihydro-4H-benzothiazine (I) with diethyl ethoxymethylenmalonate at 120 C and subsequent cyclization with polyphosphoric acid yields ethyl 10-chloro-9-fluoro-7-oxo-2,3-dihydro-7H-pyrido[ 1,2,3-de][1,4]benzothiazine 6 carboxylate (II). Hydrolysis of the latter with 10% NaOH affords the corresponding free acid (III). Oxidation of (III) with H2O2 under mild conditions gives the sulfoxide (IV), which, reacting by means of N-methylpiperazine in DMF at 90 C, gives the 9-fluoro-10 [N-(4 methyl)piperazinyl]-7-oxo- 2,3-dihydro-7H-pyrido[1,2,3-de][1,4]benzothiazine-1-oxide-6-carboxylic acid (V). Finally, the reduction of sulfoxide (V) with PCl3 yieids title compound.