Uses
4-hydroxy-3,5-dimethylbenzoic acid is widely used as a reagent for potent TTR amyloidogenesis inhibitors. It plays an important role as an intermediate in agrochemicals and dyestuffs. It is also used in the preparation of mexiletine derivatives.
Synthesis
(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.
