1. Resorcinol (3 mmol) and 3-hydroxy-4-methoxyphenylacetic acid (3 mmol) were dissolved in freshly distilled boron trifluoride ethyl ether solution (2 mL), heated to 85-90 °C and stirred magnetically for 6-10 hours (monitored by TLC until the raw material essentially disappears). Upon completion of the reaction, cooled to 10 °C, N,N-dimethylformamide (5 mL) was added slowly and dropwise to give mixture I. The reaction was carried out in the presence of N,N-dimethylformamide (5 mL).
2. Preparation of Vilsmeier-Haack reagent: N,N-dimethylformamide (8.1 mL) was cooled to 10 °C, phosphorus pentachloride (5 mmol) was added in batches, followed by heating to 55 °C and magnetic stirring for 1 h to give a light red or yellow mixture II.
3. Within 30 minutes, mixture II was added to mixture I in batches and stirred at room temperature for 3 hours. Subsequently, the reaction solution was poured into methanolized hydrochloric acid (0.1 mol/L, 30 mL), heated to 70 °C, kept at a constant temperature for 50 min, and then left to stand.
4. The reaction solution was concentrated by distillation under reduced pressure, washed with 150 mL of water and subsequently extracted with ethyl acetate (150 mL × 3 times). The organic phase was washed with water (150 mL x 2 times) and dried by rotary evaporation (filtered over a large amount of ethyl acetate extract to remove some impurities).
5. Some of the pigments were precipitated by suspension in a small amount of ethanol, centrifuged and filtered, and crystallized with hot ethanol, the operation being carried out under nitrogen protection.
6. Finally, the target product gross isoflavones (1.6 mmol) was obtained in 50-60% yield by silica gel column chromatography using methanol: dichloromethane = 1:35 as the unfolding agent.
