General procedure for the synthesis of 4-benzodioxane-2-carboxylic acid from (2,3-dihydrobenzo[b][1,4]dioxohexen-2-yl)methanol: Accurately weigh (2,3-dihydrobenzo[b][1,4]dioxohexen-2-yl)methanol, and dissolve it in 0.3 N aqueous KOH (1.3 equiv). Potassium permanganate (KMnO4, 2.0 eq.) was slowly added under ice bath conditions at 0 °C. The reaction mixture was first stirred at low temperature for about 10 minutes, then brought to room temperature and continued to stir overnight. Upon completion of the reaction, the organic phase was extracted with dichloromethane, the organic phase was washed with water sequentially, dried over anhydrous sodium sulfate, and concentrated under reduced pressure to give a white solid, i.e., the crude product of 4-benzodioxane-2-carboxylic acid. The crude product was purified by silica gel fast column chromatography, and the final pure product was obtained in 92.5% yield.
