4-Hydroxycoumarin is prepared by the reaction of o-hydroxyacetophenone and Diethyl carbonate. The steps are as follows:
General procedure: To a stirred mixture (1.15 mmol) and sodium hydride (138 mg, 5.75 mmol) in 5 ml dry toluene, a solution of the diethyl carbonate (1.15 mmol) in dry toluene was dropped. The mixture was reacted at 0°C for 30 min, then heated to reflux and reacted for another 4 h. The reaction mixture was quenched with water (20 mL) at ice bath and neutralized to pH 7 with 2 N HCl. The precipitate was filtered, washed with cold water and dried to get 6a-f. 1.2.1.1. 4-Hydroxy-2H-chromen-2-one (6a) White solidyield: 95; 1H NMR (600 MHz, DMSO-d6) δ 12.53 (s, 1H), 7.83 (d, J = 7.2 Hz, 1H), 7.65 (d, J = 7.8 Hz, 1H), 7.38-7.34 (m, 2H), 5.60 (s, 1H).
