[H-].[Na+].[O:3]=[C:4]1[C:10]2[CH:11]=[CH:12][CH:13]=[CH:14][C:9]=2[N:8]2[CH:15]=[N:16][C:17]([C:18]([O:20][CH2:21][CH3:22])=[O:19])=[C:7]2[CH2:6][NH:5]1.[CH3:23]I.O>CN(C)C=O.C(O)(=O)C>[CH3:23][N:5]1[C:4](=[O:3])[C:10]2[CH:11]=[CH:12][CH:13]=[CH:14][C:9]=2[N:8]2[CH:15]=[N:16][C:17]([C:18]([O:20][CH2:21][CH3:22])=[O:19])=[C:7]2[CH2:6]170 mg (1.6 mmol) of sodium hydride (55 percent oil dispersion) are suspended in 5 ml of dry dimethylformamide and treated with 135 mg (0.5 mmol) of ethyl 5,6-dihydro-6-oxo-4H-imidazo[1,5-a][1,4]benzodiazepine-3-carboxylate. After completion of the gas evolution, 0.15 ml (2.3 mmol) of methyl iodide is added and the mixture is stirred at room temperature for 1 hour. The mixture is poured into ca 50 ml of water, neutralised with glacial acetic acid and extracted three times with ca 30 ml of chloroform each time. The combined chloroform phases are washed with ca 20 ml of water, dried over magnesium sulphate and evaporated. After column chromatography and recrystallisation from ethyl acetate, there is obtained ethyl 5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a][1,4]benzodiazepine-3-carboxylate of melting point 166°-167° C.
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