Add compound II (10 mmol), acetic acid (50 ml), potassium bromide (1 mmol), and manganese acetate (1 mmol) to an autoclave. Seal the autoclave, introduce air, and raise the pressure to 1.8 MPa. Heat the system to 120°C. After stirring for 4 hours, restore the pressure to atmospheric pressure, open the lid, and filter to separate the insoluble matter. Pour the filtrate into a vacuum distillation apparatus. Recover acetic acid under reduced pressure at 75–80°C until dry. Continue heating to 145–150°C under high vacuum to collect the product, m-Phthalaldehyde (9.2 mmol), with a yield of 92%.
