[CH3:1][O:2][C:3]([C:5]1[C:10]([C:11]([OH:13])=O)=[CH:9][CH:8]=[CH:7][N:6]=1)=[O:4].S(Cl)([Cl:16])=O>>[CH3:1][O:2][C:3]([C:5]1[C:10]([C:11]([Cl:16])=[O:13])=[CH:9][CH:8]=[CH:7][N:6]=1)=[O:4]Pyridine-2,3-dicarboxylic acid 2-methyl ester (500 mg, 2.76 mmol) was dissolved in thionyl chloride (8.21 g, 68 mmol) and heated to reflux for 4 h. Excess thionyl chloride was removed under vacuum. The obtained crude product (620 mg, assumed purity 60%, 68% yield) was used in the next step without further purification.
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