443-69-6

351-09-7

443-69-6

Concentrated sulfuric acid (24.0 mL) was added to a dry 150 mL three-necked flask and heated to 50 °C. The intermediate synthesized above (2a-2d, 30.0 mmol) was slowly added and the color of the solution gradually deepened to black as the intermediate was added. After the addition was completed, the reaction temperature was raised to 80 °C and the reaction was maintained for 20 min. Subsequently, crushed ice (100 g) was slowly added to the reaction system, at which time the ice-water mixture showed a reddish brown color. After standing, the reaction mixture was filtered and washed with deionized water until neutral. The resulting solid was dissolved in 10.0% NaOH solution (90 mL), the pH was adjusted to 4 with concentrated hydrochloric acid and filtered. The filtrate was continued to adjust the pH to 2 with concentrated hydrochloric acid, at which time a large amount of brick red solid precipitated. The solid was collected by filtration and dried to give the reddish brown target products 3a-3d. (Elution conditions: petroleum ether: ethyl acetate, v/v = 1:2, Rf = 0.4-0.6).