To a solution of the compound (4) (25.1 g, 51.2 mmol) in anhydrous pyridine (250 ml) were added p-toluenesulfonyl chloride (14.6 g, 1.5 equivalents) and 4-dimethylaminopyridine (626 mg, 0.1 equivalents), and the reaction liquid was stirred at room temperature for 16 hours. After the sufficient progress of the reaction was confirmed, water (10 ml) was added to stop the reaction, and the reaction liquid was concentrated under reduced pressure. The residue was suspended in a minor amount of ethyl acetate, poured to 0.5 M hydrochloric acid (200 ml), and extracted with ethyl acetate (3×200 ml). The organic layers were combined, washed with a saturated sodium hydrogen carbonate solution (2×100 ml) and saturated saline (2×100 ml), dried with sodium sulfate, filtered, and concentrated under reduced pressure. The obtained residue was crystallized twice from heated ethanol to obtain the title compound (5) in the form of colorless needle crystals (25.0 g). The filtrate was concentrated, purified with silica gel chromatography (hexane-ethyl acetate, 5:1→4:1→3:1) to obtain the compound (5) (4.00 g). {29.0 g (45.0 mmol) in total, 87.9%}.
