[Synthesis]
General procedure for the synthesis of 2-hydroxy-3,4-dimethoxybenzaldehyde (29) and 2,3-dihydroxy-4-methoxybenzaldehyde (29') from 2,3,4-trimethoxybenzaldehyde: 2,3,4-trimethoxybenzaldehyde (5.00 g, 25.5 mmol) was dissolved in anhydrous dichloromethane (15 mL) at 0 °C and under nitrogen protection. Anhydrous boron trichloride (28.0 mL, 1.0 M solution in dichloromethane) was added slowly through a dropping funnel and the reaction mixture was stirred at this temperature for 5 hours. Upon completion of the reaction, the reaction was quenched with distilled water (10 mL), the organic phase was separated, and the aqueous phase was extracted with dichloromethane (2 x 25 mL). The organic phases were combined, washed with saturated saline, dried over anhydrous sodium sulfate, filtered and concentrated to dryness under reduced pressure. 2-Hydroxy-3,4-dimethoxybenzaldehyde (29) (3.75 g, 20.6 mmol, 81% yield) was obtained as a white powder, as well as 2,3-dihydroxy-4-methoxybenzaldehyde (29') (0.20 g, 1.2 mmol, 5% yield) as an off-white powder.1H NMR (500 MHz, CDCl3) δ 11.2 (1H, s 2-OH), 9.75 (1H, s, 1-CHO), 7.29 (1H, d, J=9.0Hz, 6-H), 6.61 (1H, d, J=9.0Hz, 5-H), 3.95 (3H, s, 4-OCH3), 3.91 (3H, s, 3-OCH3).13C NMR (126MHz, CDCl3) δ 194.9 (1-CHO), 159.4 (C-4), 155.7 (C-2), 136.2 (C-3), 130.2 (6-CH), 116.6 (C-1), 104.0 (5-CH), 60.8 (3-OCH3), 56.3 (4-OCH3). |