(3′R,4′S,5S,5′S,6′R)-3′,4′,5′-tris-benzyloxy-6′-benzyloxymethyl-3-[(4-ethylphenyl)methyl]-3′,4′,5′,6′-tetrahydro-spiro[thieno[2,3,f]isobenzofuran-5(7H),2′-[2H]pyran] (34.1 mg, 0.042 mmol) was dissolved in dichloromethane (1.4 mL). To the resultant solution was added pentamethylbenzene (94.3 mg, 0.636 mmol), and then the solution was cooled to −78° C. Boron trichloride (1.0 M dichloromethane solution, 0.212 mL) was added dropwise, and the solution was stirred for 3 hours. To the solution was then added methanol (1.5 mL) to stop the reaction. The temperature of the solution was raised to room temperature. The reaction solution was concentrated under reduced pressure. The resulting residue was purified by silica gel flash column chromatography (developing solution=ethyl acetate:n-hexane (2:25)), and the resultant product was then further isolated and purified by reverse phase HPLC (developing solution=acetonitrile:water (35:65 to 100:0)), to thereby obtain the titled compound (13.3 mg, 70%).
