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Example 2.4 Synthesis of 2-amino-5-fluoro-3-nitropyridine: 2-amino-5-fluoropyridine (8.6 g, 77 mmol) was dissolved in concentrated hydrochloric acid. A mixed acid solution consisting of nitric acid (3.25 ml, 77 mmol) and sulfuric acid (40 ml) was added slowly and dropwise (over 30 min) at 3°C. The reaction mixture was first stirred at room temperature for 1 hour, then warmed up to 55°C and continued stirring for 1 hour. Upon completion of the reaction, the mixture was decanted into ice water, neutralized with 10 M sodium hydroxide solution and subsequently extracted with dichloromethane. The organic layer was separated, dried with anhydrous sodium sulfate and concentrated under reduced pressure. The crude product was purified twice by silica gel column chromatography, the first time using methanol: dichloromethane (1:20) as eluent and the second time using ether: petroleum ether (1:1) as eluent, resulting in 2-amino-3-nitro-5-fluoropyridine (0.44 g, 3.6% yield).1H-NMR (300 MHz, CDCl3): δ 6.65 (broad single peak, 2H), δ 8.20 (double peak, 1H), 8.35 (single peak, 1H).