To a solution of 3,4-dimethoxy-6-(4-pyridyl) hydroxymethylbenzaldehyde dimethylacetal (18.4 g) in a mixture of acetic acid (50 ml) and toluene (50 ml) is added dimethyl maleate (8.64 ml), and the mixture is refluxed for one hour. To the mixture is added methanesulfonic acid (9.33 ml), and the mixture is refluxed for 8 hours while removing the resulting water with a Dean-Stalk apparatus. The mixture is cooled to room temperature, and concentrated. The residue is dissolved in chloroform, and the pH value thereof is adjusted with aqueous potassium carbonate solution to pH 8. The mixture is extracted twice with each 100 ml of chloroform, and the extracts are washed with a saturated sodium chloride solution, dried over magnesium sulfate, and concentrated. The resulting residue is crystallized from diethyl ether to give a diester compound, 1-(4-pyridyl)-2,3-bis(methoxycarbonyl)-6,7-dimethoxynaphthalene (13.5 g).
