A mixture of 6.7 parts of 1-(4-chlorobutyl)-1H-benzimidazole, 5.5 parts of 1,3-dihydro-1-(4-piperidinyl)-2H-benzimidazol-2-one, 5.3 parts of sodium carbonate, 0.1 parts of potassium iodide and 200 parts of 4-methyl-2-pentanone is stirred and refluxed overnight. The reaction mixture is cooled, water is added and the layers are separated. The organic phase is dried, filtered and evaporated. The residue is purified twice by column-chromatography over silica gel using first a mixture of trichloromethane and methanol (95:5 by volume) and then a mixture of trichloromethane and methanol (90:10 by volume) as eluent. The pure fractions are collected and the eluent is evaporated. The residue is crystallized from 4-methyl-2-pentanone. The product is filtered off and dried, yielding 1.8 parts of 1-{1-[4-(1H-benzimidazol-1-yl)butyl]-4-piperidinyl}-1,3-dihydro-2H-benzimidazol-2-one; mp. 157.1° C.
