To a solution of 3.47 g of 2-methylthio-5-(1-hydroxy-6-methoxycarbonylhexyl)-4-(3-t-butyldimethylsilyloxy-1-octenyl)-2-cyclopentenone obtained in Example 6 in dichloromethane (30 ml) was added dimethylaminopyridine (1.54 g) and the mixture was cooled to 0° C. To the solution was dropwise added 0.59 ml of methanesulfonyl chloride, and the mixture was stirred at room temperature for 15 hours. To the reaction mixture were added ethyl acetate and an aqueous potassium hydrogensulfate, and the product was extracted into an organic layer. The extract was washed with saturated aqueous sodium hydrogencarbonate and saturated sodium chloride, dried over anhydrous magnesium sulfate, and filtered, followed by concentration. The concentrate was subjected to silica gel chromatography to give 2.15 g (yield 64%) of 2-methylthio-5-(6-methoxycarbonylhexylidene)-4-(3-t-butyldimethylsilyloxy-1-octenyl)-2-cyclopentenone.
