The condensation of 7-chloro-1,3-dihydro-3-hydroxy-1-methyl-5-phenyl-2H-1,4-benzodiazepin-2-one (I) with phenyl chlorocarbonate (II) in pyridine yields 7-chloro-1,3-dihydro-3-phenoxycarbonyl-1-methyl-5-phenyl-2H-1,4-benzodiazepin-2-one (III), m.p. 176-8 C, which is then condensed with dimethylamine.