[OH:1][C:2]1[C:11]([C:12]([OH:14])=[O:13])=[CH:10][C:9]2[C:4](=[CH:5][CH:6]=[C:7]([C:15]([OH:17])=[O:16])[CH:8]=2)[CH:3]=1.CI.N12CCCN=C1CCCC[CH2:21]2.C(=O)([O-])O.[Na+]>C(#N)C.CN1CCCC1=O.C(OCC)(=O)C>[OH:1][C:2]1[C:11]([C:12]([OH:14])=[O:13])=[CH:10][C:9]2[C:4](=[CH:5][CH:6]=[C:7]([C:15]([O:17][CH3:21])=[O:16])[CH:8]=2)[CH:3]=12-Hydroxy-3,6-di-hydroxycarbonylnaphthalene (11.6 g) was dispersed in anhydrous acetonitrile (116.0 g) and N-methyl-2-pyrrolidone (39.0 g). Then, methyl iodide (7.85 g) was added and 1.8-diazabicyclo[5.4.0]undec-7-ene (hereinafter referred to as “DBU”) (8.37 g) was added dropwise over 5 minutes, followed by heating at 50° C. overnight. Further methyl iodide (2.32 g) and DBU (2.42 g) was added and mixture was stirred overnight. The deposited crystal was filtered and then dried under reduced pressure to obtain 7.29 g of a crude crystal of 2-hydroxy-3-hydroxycarbonyl-6-methoxycarbonylnaphthalene. Aliquot of this crystal (1.52 g) was dissolved in ethyl acetate (30.2 g) and an aqueous 5% sodium hydrogencarbonate. The solution was separated into the organic layer and the aqueous layer, and then the organic layer was extracted with an aqueous 5% sodium hydrogencarbonate solution (10.3 g) and aqueous extract was combined with the above aqueous layer. To this aqueous layer was added dropwise 10% hydrochloric acid until the pH becomes 7 while ice-cooling. The deposited crystal was filtered and then dried under reduced pressure to obtain 1.02 g of 2-hydroxy-3-hydroxycarbonyl-6-methoxycarbonylnaphthalene (DSC analysis value: 295.1° C.).
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