General procedure for the synthesis of 2-chloro-3-nitro-4-pyridinecarboxylic acid from 2-chloro-4-methyl-3-nitropyridine: 2-chloro-4-methyl-3-nitropyridine (4.00 g, 23.00 mmol) was dissolved in sulphuric acid (40 ml) at 0 °C and potassium dichromate (8.85 g, 30.00 mmol) was added in batches. The reaction mixture was heated and stirred at 60°C for 8 hours. After completion of the reaction, the mixture was allowed to cool to room temperature and slowly poured into ice water. Multiple extractions were carried out with ethyl acetate (5 x 30 ml). The organic phases were combined, dried over anhydrous sodium sulfate, filtered and concentrated under reduced pressure. The resulting residue was ground with pentane (3 × 5 ml) to afford 2-chloro-3-nitro-4-pyridinecarboxylic acid (4.00 g, 19.70 mmol), which was used for subsequent reactions without further purification.LCMS analysis (Method C): 1.51 min; mass spectrometry (MS): ESI m/z 201.1 [M-1 ]; 1H NMR (400 MHz. DMSO-d6) δ 8.83 (d, J = 5Hz, 1H), 8.03 (d, J = 5Hz, 1H).
