[Synthesis]
General procedure for the synthesis of 4,5,6-trimethoxy-2,3-dihydro-1H-inden-1-one from 3-(2,3,4-trimethoxyphenyl)propionic acid: 3-(2,3,4-trimethoxyphenyl)propionic acid (4.73 g, 19.68 mmol) and 40.0 mL of Eaton's reagent (purchased from Acros) were added to a flask, and the reaction was stirred for 12 hours. After completion of the reaction, the mixture was poured onto ice, and after the ice melted, the aqueous phase was extracted with CH2Cl2 (2 x 100 mL). The organic phase was combined and washed with saturated NaHCO3 solution (2 x 200 mL). The organic phase was dried over Na2SO4 and the solvent was evaporated under reduced pressure to give the crude product 4,5,6-trimethoxy-2,3-dihydro-1H-inden-1-one as a white solid. The crude product was purified by flash chromatography on a prepacked 50 g silica gel column with EtOAc (solvent A) and hexane (solvent B) as eluents, using a gradient elution: 10% A/90% B was maintained for 3.18 min (1 column volume), the gradient change from 10% A/90% B → 50% A/50% B was more than 33.0 min (10 column volumes), and the gradient change from 50% A/50% B was maintained for 6.36 min (2 column volumes) at a flow rate of 40.0 mL/min and detected at wavelengths of 254 and 280 nm.The purified 4,5,6-trimethoxy-2,3-dihydro-1H-inden-1-one (2.12 g, 11.90 mmol, 78% yield) was finally obtained as a white solid.1H NMR (500 MHz, CDCl3) δ 7.02 (1H, s, H-7), 3.96 (3H, s, OCH3-4), 3.95 (3H, s, OCH3-5), 3.88 (3H, s, OCH3-6), 3.04 (2H, t, J=5.7Hz, H-3), 2.66 (2H, t, J=5.7Hz, H-2).13C NMR (126MHz, CDCl3 ) δ 206.0 (C, C-1), 154.2 (C, C-6), 150.0 (C, C-4), 147.6 (C, C-5), 141.6 (C, C-3a), 132.5 (CH, C-1a), 100.6 (CH, C-7), 61.1 (CH3, OCH3-5), 60.6 (CH3, OCH3-4) 56.2 (CH3, OCH3-6), 36.1 (CH2, C-2), 22.4 (CH2, C-3). |