10 g of (+)-5-(o-chlorophenyl)-1,3-dihydro-3-methyl-7-nitro-2H-1,4-benzodiazepin-2-one are dissolved in 100 ml of acetone and treated with 6.3 g of potassium carbonate and 3.5 ml of dimethyl sulphate. After stirring for 2 hours at room temperature, a further 1 g of potassium carbonate and 1 ml of dimethyl sulphate are added, after which the mixture is stirred for a further hour at room temperature. The mixture is treated with 3 ml of glacial acetic acid and the acetone is distilled off. The residue is treated with 10% sodium bicarbonate solution and extracted with methylene chloride. The methylene chloride solution is dried over sodium sulphate, filtered and concentrated. The residue is purified on a column of silica gel (eluant: methylene chloride and methylene chloride/ethyl acetate 10:1) and crystallized from ether/ petroleum ether. There is obtained (+)-5-(o-chlorophenyl)--1,3-dihydro-1,3-dimethyl-7-nitro-2H-1,4-benzodiazepin-2-one which melts at 156°-158° C. and exhibits a rotation of [α]25 D =+376.2° (in methylene chloride, 1%).
