The reaction of 4-acetyldiphenylsulfide (I) with carbon disulfide by means of sodium tert-amyloxide in benzene gives 4-phenylthiobenzoyldithioacetic acid (II), which is cyclized with o-phenylenediamine (III) in refluxing toluene yielding 4-(p-phenylthiophenyl)-1,3-dihydro-2H-1,5-benzodiazepine-2-thione (IV). The condensation of (IV) with diethylaminoethyl chloride (V) by means of NaH in refluxing benzene affords 2-[beta-(N-diethylamino)ethylthio]-4-(p-phenylthio)phenyl-3H-1,5-benzodiazepine (VI), which is finally quaternized with methyl iodide in isopropanol. (1,2)