In DMF (5 ml) was dissolved 5-t-butyl-1H-pyrazolo-[3,4-d]pyridazin-4(5H)-one (288 mg) followed by addition of 3-(4-chlorobenzoyl)benzyl bromide (712 mg) and potassium carbonate (318 mg), and the mixture was stirred at room temperature for 15 hours. This reaction mixture was extracted with ethyl acetate and the organic layer was serially washed with water and saturated aqueous NaCl solution, dried over anhydrous magnesium sulfate, and concentrated. The residue was purified by silica gel column chromatography (n-hexane: ethyl acetate=1:1), washed with the following solvent, and dried. As the washing solvent, diethyl ether was used for 5-t-butyl-1-[3-(4-chlorobenzoyl) benzyl]-1H-pyrazolo[3,4-d]pyridazin-4(5H)-one and n-hexane-diethyl ester was used for 5-t-butyl-2-[3-(4-chlorobenzoyl)benzyl]-2H-pyrazolo [3,4-d]pyridazin-4(5H)-one.
