To a cooled mixture of 7-chloro-2-(5-formylbenzofuran-2-yl)quinoline (0.77 g), N,N-dimethyl-3-mercaptopropionamide (0.37 g) and methyl 4-mercaptobutyrate (0.37 g) in acetonitrile (15 ml), boron trifluoride etherate (1.38 ml) was added dropwise at 0° C. After being stored in refrigerator for 18 hours, the resulting mixture was poured into ice-water, adjusted to pH 7 with aqueous sodium hydrogen carbonate solution and extracted with diethyl ether. The organic layer was washed with brine, dried over magnesium sulfate and concentrated under reduced pressure to give a mixture containing 3 compounds. The mixture was subjected to column chromatography on silica gel and eluted with toluene, successively a mixture of toluene and ethyl acetate (8:2) and at last a mixture of chloroform and methanol (10:1). The fractions eluted with toluene were concentrated under reduced pressure to give an oil of 7-chloro-2-{5-[bis(3-methoxycarbonylpropylthio)methyl]-benzofuran-2-yl}quinoline (0.25 g).
