(1) In a 500 mL three-necked flask, p-hydroxybenzoic acid (138 g, 1 mmol) and methanol (160 g, 5 mmol) were added, followed by solid acid catalyst (50 g). The mixture was stirred until homogeneous. The reaction flask was placed in an ultrasonic reactor and fitted with a reflux condensing unit. The reaction temperature was adjusted to 40-60°C and the ultrasonic power was set to 500 W for 1-3 hours. During the reaction, the color of the reaction solution gradually changed from white to yellow and finally to orange-red. The reaction process was monitored by high performance liquid chromatography (HPLC), and the reaction was judged to be complete when the residual amount of p-hydroxybenzoic acid was less than 5%. After stopping the reaction, the solid acid catalyst was removed by filtration (the catalyst can be recycled for subsequent reactions). The filtrate was allowed to crystallize at 5-10°C for 6-10 hours, after which the crystals were collected by filtration. The crystals were dried at 50-60°C for 6 h. The intermediate 4-hydroxy-3,5-dimethylbenzoic acid (141 g, 85% yield) was obtained by HPLC with impurity content of less than 0.1% in the product.
