The general procedure for the synthesis of benzofuran-3-carboxylic acid from 3-(methoxymethylenyl)-2(3H)-benzofuranone was as follows: 152 g of o-hydroxyphenylacetic acid was added to a reaction flask, 500 g of acetic anhydride and 250 g of trimethyl orthoformate were added. The temperature of the reaction mixture was raised to 80-100 °C and the temperature was maintained for 6 hours. Upon completion of the reaction, the solvent was removed by distillation under reduced pressure to give a yellow solid product. To this solid product, 500 g of methanol was added, stirred until completely dissolved, the temperature of the solution was controlled at 0-10 °C, and 180 g of 30% sodium hydroxide solution was slowly added dropwise. Subsequently, the reaction mixture was heated to 50-60°C and kept at this temperature for 3-6 hours. At the end of the reaction, methanol was recovered by distillation under reduced pressure. To the residue, 300 g of drinking water was added, the temperature of the solution was adjusted to below 30 °C, and 160 g of 30% hydrochloric acid was added slowly and dropwise to adjust the pH to 1-2. The mixture was cooled to 0-10 °C and allowed to stand for 3 hours for decantation. The precipitate was collected by filtration and the solid product was washed with cold water. Finally, the product was dried under vacuum to give 158 g of white solid benzofuran-3-carboxylic acid in 97.5% yield.
