3,5-Bis(trimethylsilyl)benzoic acid (hereinafter referred to as “BTMB”; 700 mg) was suspended in acetonitrile (5.25 ml) in a nitrogen atmosphere. Pyridine (0.212 ml) and Compound 1a (603 mg) were added to the suspension, and phosphorus oxychloride (0.131 ml) was added dropwise under ice-cooling. The mixture was stirred while refluxing under heating for 3.5 hours. After the reaction mixture was allowed to cool, water and ethyl acetate were added thereto. The organic layer was washed three times with water and once with a saturated sodium chloride solution, and then dried over anhydrous sodium sulfate. The solvent was distilled off, and the residue was purified by neutral silica gel column chromatography (5% ethyl acetate/n-hexane) to obtain Compound 1b (232 mg, 20%) as a white foam.
