[CH2:1]([O:3][C:4]1[C:13]2[C:8](=[CH:9][CH:10]=[CH:11][CH:12]=2)[C:7]([O:14][CH2:15][CH3:16])=[C:6]([C:17]([OH:19])=O)[C:5]=1[C:20]([OH:22])=[O:21])[CH3:2].S(Cl)(Cl)=O>C(Cl)(Cl)Cl>[CH2:15]([O:14][C:7]1[C:8]2[C:13](=[CH:12][CH:11]=[CH:10][CH:9]=2)[C:4]([O:3][CH2:1][CH3:2])=[C:5]2[C:20]([O:21][C:17](=[O:19])[C:6]=12)=[O:22])[CH3:16]1,4-Diethoxy-2,3-naphthalenedicarboxylic acid (25 g, 82 mmol) was added to a solution of thionyl chloride (23.3 g) in chloroform (150 ml) and stirred at reflux for 1 h. The resulting solution was cooled and evaporated to dryness. Further chloroform was added and evaporation repeated to give the title compound as a yellow solid (23.3 g, 99%).
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