The general procedure for the synthesis of 5-methoxyisobenzofuran-1,3-dione using 4-methoxyphthalic acid as starting material was as follows: acetic anhydride (40 mL) was slowly added to a solution of anhydrous tetrahydrofuran (150 mL) containing 4-methoxyphthalic acid (30.8 g, 0.16 mol). The reaction mixture was refluxed with stirring for 4 hours. Upon completion of the reaction, the mixture was cooled to room temperature and the solvent was subsequently evaporated under reduced pressure to afford 4-methoxyphthalic anhydride (27.8 g, 99% yield) as an off-white solid. The structure of the product was confirmed by 1H NMR (DMSO-d6): δ 8.02 (1H, d), 7.59 (1H, d), 7.49 (1H, dd), 3.97 (3H, s). Mass spectrometry analysis showed the [M + H]+ peak at 179.
