The general procedure for the synthesis of methyl 4-chloro-2,6-dipyridinecarboxylate from methanol and 4-oxo-1,4-dihydro-2,6-pyridinedicarboxylic acid was as follows: phosphorus pentachloride (PCl5, 11.8 g, 56.8 mmol) was slowly added to chloroform containing 4-oxo-1,4-dihydro-2,6-pyridinedicarboxylic acid (2.5 g, 13.8 mmol) ( CHCl3, 30 mL) solution. The reaction mixture was refluxed for 3 days. Upon completion of the reaction, it was cooled to 0°C and methanol (MeOH) was added dropwise until gas escape ceased. Subsequently, the solution was stirred at room temperature for 2 hours and then concentrated. To the concentrate, saturated aqueous sodium bicarbonate (NaHCO3) was added and the pH was adjusted to 7. The organic layer was separated, dried over anhydrous sodium sulfate (Na2SO4), filtered and the solvent was evaporated under vacuum. The residue was purified by column chromatography (eluent: cyclohexane/ethyl acetate, 8:2) to afford the target product methyl 4-chloro-2,6-dipyridinecarboxylate (2.57 g, 93% yield) as a white solid. The structure of the product was confirmed by 1H NMR (400 MHz, CDCl3) and 13C NMR (100 MHz, CDCl3).
