To a solution of 5-methylthio-1-benzenesulfonylindole (200 mg ) in tetrahydrofuran (5 ml), n-butyl lithium (0.49 ml, 1.61 M) was added at −78° C. under nitrogen atmosphere and the mixture was stirred at −78° C. for 30 minutes, followed by addition of hexamethylphosphoric triamide (0.23 ml), and the mixture was stirred for 15 minutes. Subsequently, tetrahydro-4H-pyran-4-one (0.12 ml) was added thereto and the mixture was stirred at −78° C. for 15 minutes and heated to 15 to 30° C. The reaction solution was then poured into a saturated aqueous ammonium chloride solution, extracted with dichloromethane, dried over anhydrous magnesium sulfate, filtered and concentrated under reduced pressure. The resulting residue was separated using silica gel preparative thin-layer chromatography (hexane:ethyl acetate=1:1) and (methanol:chloroform=1:20) to obtain 39 mg of the desired product as a colorless oil.
