Example IV Synthesis of 3,4,5-trimethoxycinnamic acid (by microwave radiation method): in a 100 mL Erlenmeyer flask, a mixture of 3,4,5-trimethoxybenzaldehyde (5.0 g, 0.025 mol) and malonic acid (5.30 g, 0.050 mol) was added, followed by piperidine (4-8 mL) and acetic acid (25-35 mL). The flask was fitted with a loose funnel at the top and the mixture was shaken well and placed in a microwave oven and microwaved in batches for 4-8 min. Upon completion of the reaction, the cooled mixture was poured into ice-cold water and acidified with 5% HCl solution. The precipitated yellow solid was collected by filtration and recrystallized by aqueous ethanol to give 3,4,5-trimethoxycinnamic acid in 88% yield; the melting point was 127 °C (literature value 126-128 °C). The spectral data of the product were consistent with those reported in the literature: 1H NMR (CDCl3) δ 7.73 (1H, d, J = 16.0 Hz, -CH=CH-COOH), 6.78 (2H, s, H-2 and H-6), 6.38 (1H, d, J = 16.0 Hz, CH=CH-COOH), 3.91 (9H, s, 3-OCH3, 4- OCH3, 5-OCH3); 13C NMR: δ 172.6 (COOH), 153.8 (C-3 and C-5), 147.4 (C-4), 140.8 (CH=CH-COOH), 129.8 (C-1), 116.8 (CH=CH-COOH), 105.8 (C-2 and C-6), 61.4 (4- OCH3), 56.5 (3-OCH3 and 5-OCH3).
