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Step b) Synthesis of 2-chloro-4-(chloromethyl)pyridine: 11.3 g of (2-chloropyridin-4-yl)methanol obtained in step a) was dissolved in 200 mL of dichloromethane, to which 6.896 mL of thionyl chloride was slowly added, followed by the addition of 2.1 mL of dimethylformamide as catalyst. The reaction mixture was stirred at room temperature for 3 hours. Upon completion of the reaction, 50 mL of water was added slowly and dropwise to quench the reaction. The organic layer was separated, dried with anhydrous magnesium sulfate, filtered to remove the desiccant, and the filtrate was concentrated under reduced pressure to afford 12.8 g (100% yield) of 2-chloro-4-(chloromethyl)pyridine as an amber liquid, which could be used in the subsequent reaction without further purification. Thin layer chromatography (TLC) analysis showed an Rf value of 0.44 (unfolding agent: dichloromethane).

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