Caffeic acid (0.5 g, 2.77 mmol) was dissolved in dry ethanol (20 mL) followed by the addition of concentrated sulfuric acid (0.5 mL, 96%, v/v). The reaction mixture was stirred at room temperature for 2 hours. Upon completion of the reaction, the mixture was cooled to room temperature, diluted with ethyl acetate (100 mL) and washed with 5% aqueous sodium bicarbonate to neutral pH. the organic layer was subsequently washed with distilled water and dried with anhydrous magnesium sulfate. The solvent was removed by distillation under reduced pressure and the residue was purified by silica gel column chromatography (40% ethyl acetate-hexane as eluent) to afford the target product ethyl 3,4-dihydroxycinnamate (382 mg, 81%) as a white solid. The product characterization data were as follows: melting point 128-140 °C; 1H NMR (400 MHz, DMSO-d6, 25 °C) δ (ppm): 9.58 (br s, 1H, OH), 9.14 (br s, 1H, OH), 7.47 (d, J = 15.93 Hz, 1H, CH=CH), 7.05 (s, 1H, ArH), 7.00 (dd, J = 8.24, 1.16 Hz, 1H, ArH), 6.76 (d, J = 8.12 Hz, 1H, ArH), 6.26 (d, J = 15.89 Hz, 1H, CH=CO), 4.16 (q, J = 7.04 Hz, 2H, CH2CH3), 1.24 (t, J = 7.08 Hz, 3H, CH2CH3) ; 13C NMR (101 MHz, DMSO-d6, 25 °C) δ (ppm): 167.01, 148.83, 146.02, 145.45, 125.97, 121.80, 116.19, 115.26, 114.51, 60.15, 14.73; HRMS m/z calculated for C10H12O4 (H+): 209.0808; found: 209.0811.
