PURPOSE: To produce the titled compound useful as an intermediate of pharmaceuticals and agricultural chemicals such as coenzyme Q (a metabolic cardiotonic), economically in an industrial scale, by oxidizing 2,3-di(lower alkoxy)-5- (lower alkyl)phenol in the presence of a specific cobalt complex salt.

CONSTITUTION: The compound of formula III is produced by oxidizing the compound of formula I (R1WR3 are lower alkyl) in the presence of preferably 5W 50pts.wt., based on 100pts.wt. of the compound of formula I , of a cobalt complex salt of formula II (R4 is H, alkyl or alkoxy; R5WR8 are H or alkyl; L is pyridine; n is 0 or 1) preferably at 20W60°C and an oxygen partial pressure of 0.5W20kg/cm2. Various disadvantages in the conventional process such as too many reaction steps, etc. can be eliminated in the present process. The complex salt of formula II used as the catalyst can be synthesized easily e.g. from cobalt acetate, an o-formylphenol such as salicylaldehyde, etc. and a diamine such as ethylenediamine, etc.