2-Hydroxy-3,6-di-hydroxycarbonylnaphthalene (2.4 g) was dissolved in dimethylformamide (50 g), followed by heating to 100° C. Potassium carbonate (4.6 g) was slowly added and benzyl chloride (4.2 g) was added dropwise. After heating for about 20 hours, the reaction solution was poured into a mixed solution of water (300 g) and methanol (100 g). The deposit was filtered and then washed with water to obtain 3.5 g of 2-benzyloxy-3,6-di-benzyloxycarbonylnaphthalene as whitish yellow powder (DSC analysis value: 100.5° C.).
