Phosphorus pentachloride (PCl5, 39 g, 0.186 mol, 3 eq.) and 2,4-pyridinedicarboxylic acid (11.5 g, 6.2 x 10^-2 mol) were added to a 500 mL round bottom flask. The mixture was stirred at room temperature for 45 minutes and the mixture was observed to transform into a green liquid. Subsequently, the reaction mixture was diluted with dichloromethane (80 mL) and methanol (20 mL) was added slowly. The reaction mixture was poured into water (300 mL) containing sodium bicarbonate (NaHCO3, 60 g), the aqueous phase was extracted with dichloromethane (4 x 50 mL) and the combined organic phases were washed with 1 M sodium carbonate (Na2CO3, 2 x 50 mL). The crude product was purified by fast column chromatography (eluent: ethyl acetate/cyclohexane, 75:25). A solid product of 11.71 g was obtained in 96% yield. The product was stored at room temperature.1H NMR (200 MHz, CDCl3): δ 4.03 (s, 3H, -OCH3), 4.09 (s, 3H, -OCH3), 8.07 (dd, J = 6 Hz, 1H, H-3), 8.68 (s, 1H, H-5), 8.91 (d, 1H, H-2).13C NMR (75 MHz. CDCl3): δ 53.28 (-OCH3), 124.49 (C-5), 126.29 (C-3), 138.64 (C-4), 149.06 (C-6), 150.92 (C-2), 162.45 (C=O), 164.96 (C=O).
