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Acetic anhydride (70 ml) was added to a mixture of 3-methoxyphthalic acid (51.0 g, 0.26 mol) in anhydrous tetrahydrofuran (250 ml) and the mixture was strirred and held at reflux for 4 hours. Upon cooling to room temperature the solvent was removed in vacuo and the resulting solid material was dried in a vacuum oven at 50° C. overnight to afford 3-methoxyphthalic anhydride (45.9 g, 99%) as a colourless solid. 1H NMR (DMSO-d6) 7.97 (1H, dd), 7.63 (1H, d), 7.60 (1H, d), 4.02 (3H, s). MS: [M+H]+ 179.

1. temperatureat reflux for 4 hours
2. customwas removed in vacuo
3. customthe resulting solid material was dried in a vacuum oven at 50° C. overnight