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With toluene-4-sulfonic acid; In di-isopropyl ether; cyclohexane; for 12h;Dean-Stark; Reflux;
10.0 g of the compound represented by the formula (I-8-4), 11.0 g of the compound represented by the formula (I-8-5), 0.8 g of p-toluenesulfonic acid monohydrate, 100 mL of cyclohexane and 100 mL of diisopropyl ether were put into a reactor equipped with a Dean-Stark apparatus. While removing water, this was heated under reflux for 12 hours. This was diluted with dichloromethane, and washed sequentially with an aqueous solution of 5percent sodium hydrogencarbonate and salt water. Purification through column chromatography (silica gel, dichloromethane) gave 11.1 g of the compound represented by the formula (I-6-8).