To conc. sulfuric acid (10 ml) is added 1-(1-methyl-4-piperidinyl)-1,3-dihydro-5-(2-fluorophenyl)-2H-1,4-benzdiazepin-2-one (2.09 g; 5.95 m mol) under cooling at temperature of -8° C. A solution of potassium nitrate (571 mg, 5.95×0.95 m mol) in conc. sulfuric acid (5 ml) is dropwise added to the mixture at the same temperature, which is allowed to stand at -5° C. over a period of 6 hours and then at room temperature over a period of 15 hours. The reaction mixture is poured onto ice (200 g), neutralized with conc. aqueous ammoniac solution, and shaken with methylene chloride. The organic layer is washed with water, dried over anhydrous magnesium sulfate and concentrated in vacuo. The residue is chromatographed on a column of alumina, which is eluted with ethyl acetate. The eluate is concentrated in vacuo and the residue is crystallized from methylene chloride-ether to give 7-nitro-1-(1-methyl-4-piperidinyl)-1,3-dihydro-5-(2 -fluorophenyl)-2H-1,4-benzdiazepin-2-one (544 mg) as crystals melting at 199° to 201° C. (dec.). The yield is 23%.
