2-methyl 2-butene (1.14 ml), sodium dihydrogen phosphate (389 mg) and sodium chlorite (305 mg) were added to an acetone-water mixed solvent (8 ml-2 ml) of the compound 326-8 (779 mg). The obtained mixture was stirred at room temperature for 1 hour. Thereafter, water was added to the reaction solution, and chloroform was added to the reaction solution to carry out liquid separation and extraction. A small amount of 1 N hydrochloric acid was added to the obtained extract, and chloroform was further added to the reaction solution to carry out liquid separation and extraction. The obtained organic layer was dried over magnesium sulfate. The organic layer was dried over anhydrous magnesium sulfate and concentrated under reduced pressure, so as to obtain a crude product of the title carboxylic acid (814 mg).
