PURPOSE: To prepare the titled compound easily, in high yield, in a short time, by dissolving 3,4,5-trimethoxytoluene in water-lower alcohol mixture containing a strong acid, adding an aqueous solution of a persulfate to the solution, and extracting the objective compound with an aqueous solution of sulfuric acid after reaction.

CONSTITUTION: 3,4,5-Trimethyoxytoluene (A) is dissolved in a mixture of water and a lower alcohol added with a strong acid such as sulfuric acid. An aqueous solution of a persulfate is added to the above solution and made to react at 40W70°C to obtain the titled compound. The amount of the strong acid is pref. equimolar to the compound (A). The objective compound can be separated from the reaction mixture containing unreacted compound (A) by (1) extracting the reaction mixture with chloroform, (2) distilling off the chloroform from the extract, and (3) extracting the residue with aqueous solution of sulfuric acid. The concentration of sulfuric acid in the aqueous solution is pref. 20W40%. The amount of the aqueous solution of sulfuric acid is about 25W50 times (W/V) based on the extract left after distilling off the chloroform.