PURPOSE: To obtain the subject compound useful as an intermediate for synthesis of medicines such as vitamin E at an improved reaction rate in a high yield by oxidizing 2,3,6-trimethylphenol with, oxygen in the presence of a catalyst composed of copper chloride and a vanadium compound in a solvent.

CONSTITUTION: 2,3,6-Trimethylphenol is oxidized by oxygen or an oxygen- containing gas in the presence of copper chloride and a vanadium compound as a catalyst in a solvent such as ethylene glycol dimethyl ether, DMF or acetone at 20-100°C, preferably 50-80°C to obtain the objective compound. As the amount of the catalyst used, copper chloride is preferably used within a range of 0.01-10 times, especially 0.1-0.5 time based on the number of moles of the raw compound and the vanadium compound 0.001-5 times, especially 0.01-1.0 time.