To a solution of triethylsilane (88 mg, 0.76 mmol) and TMSOTf (0.004 ml) in methylene chloride (2 ml) was added dropwise a solution of the compound (237 mg, 0.63 mmol) obtained in Step 2 in methylene chloride (2 ml) and the mixture was stirred at room temperature for 4 hours. The reaction mixture was washed with saturated saline, dried, and then the solvent was distilled off. The residue was purified by a silica gel column chromatography (ethyl acetate:hexane=1:6) to yield the title compound (160 mg, 0.45 mmol, yield 71%).
