4.95 g of the 6-((4-chlorobutyl)thiomethyl)-2-methylimidazo[1,2-a]pyridine prepared in the Preparative Example 17 was dissolved in 50 ml of dichloromethane, followed by the cooling of the obtained solution with ice. A solution of 3.18 g of m-chloroperbenzoic acid in 20 ml of dichloromethane was dropped into the resulting solution in 20 minutes. After one hour, 0.16 g of m-chloroperbenzoic acid was further added to the obtained mixture. The obtained mixture was additionally stirred for 30 minutes, followed by the addition of an aqueous solution of sodium thiosulfate. The obtained mixture was vigorously stirred at room temperature for 15 minutes, followed by the addition of an aqueous solution of potassium carbonate. The obtained mixture was extracted with dichloromethane. The organic phase was dried over anhydrous potassium carbonate and distilled to remove the solvent. The residue was purified by silica gel column chromatography [solvent: dichloromethane/methanol (50:1 to 30:1)] to give 4.18 g of the title compound as a pale-pink solid (yield: 79%).
