Example 8 Synthesis of 4-oxo-4H-chromene-2-carboxylic acid Procedure: a mixture of diethyl oxalate (110 mL, 810 mmol) and 2'-hydroxyacetophenone (44 mL, 365 mmol) was added slowly and dropwise to a solution of sodium ethanolate (76 g, 1.11 mol) in ethanol (600 mL) over a period of 20 minutes. The reaction mixture was heated to 80 °C and maintained for 1 h, followed by cooling to room temperature. Water (500 mL) and ether (600 mL) were added to the reaction system and the pH was adjusted with concentrated hydrochloric acid to 2. The organic phase was separated and the aqueous phase was further extracted with ether (2 x 600 mL). The organic phases were combined, washed with saturated aqueous sodium chloride solution (2 x 600 mL), dried over anhydrous magnesium sulfate and concentrated to give a brown oily solid. The solid was mixed with glacial acetic acid (440 mL) and concentrated hydrochloric acid (110 mL) and heated to 85 °C for overnight reaction. After the reaction was completed, it was cooled to room temperature, diluted with water (550 mL) and the solid was collected by filtration. The solid was washed with water (2 x 125 mL) and dried in a vacuum oven to give a purple solid product (58 g, 83% yield). Melting point: 260-261 °C. 1H NMR (300 MHz, DMSO-d6) δ 8.03 (m, 1H), 7.85 (m, 1H), 7.71 (m, 1H), 7.51 (m, 1H), 6.89 (s, 1H).
