The reaction of 7-chloro-5-(o-chlorophenyl)-13-dihydro-1-methyl-2H-1,4-benzodiazepin-2-one N-oxide (I) with refluxing acetic anhydride gives 7-chloro-5-(o-chlorophenyl)-1,3-dihydro-3-acetoxy-1-methyl-2H-1,4-benzodiazepin-2-one (II), which is hydrolyzed with NaOH in ethanol.