PURPOSE: To obtain the subject compound useful as an intermediate for producing antiasthmatic agent, a therapeutic agent for liver diseases, etc., in high yield in excellent acquirement of raw materials, corrosion resistance and maintenance of environment by reacting 3,4,5-trimethoxytoluene with an oxidizing agent in the presence of a catalyst.

CONSTITUTION: 3,4,5-Trimethoxytoluene is reacted with an oxidizing agent such as hydrogen peroxide in the presence of an extremely strong resin catalyst such as a strongly acidic perfluoro resin of sulfonic acid type to give the objective compound. The amount of the extremely strong acidic resin catalyst is 0.1-20 pts.wt., preferably 0.5-10wt.% based on 1 pt.wt. of 3,4,5-trimethoxytoluene and the amount of the oxidizing agent is 1-2C equivalent, preferably 1.5-10 equivalent based on 3,4,5-trimethoxytoluene.