General procedure for the synthesis of 6-chloro-3-nitropyridin-2(1H)-one from 2-amino-3-nitro-6-chloropyridine: 6-chloro-3-nitropyridin-2-ylamine (Intermediate 4, 5 g, 28 mmol) was added to ice-cold concentrated sulfuric acid, and the reaction mixture was stirred for 10 min at 0 °C. Subsequently, NaNO2 solution (4 g, 58 mmol, dissolved in 30 mL of water) was added slowly over 10 min, keeping the reaction temperature at 0 °C and continued stirring at the same temperature for 30 min. Upon completion of the reaction, the mixture was poured into 150 mL of ice-cold water, the precipitate formed was collected by filtration, washed with cold water (2 x 50 mL), and dried under vacuum to afford 6-chloro-3-nitropyridin-2(1H)-one as a yellow solid in 80% yield (4 g, 23 mmol). Mass spectrum (ES): 172.5 (M-1) C5H3ClN2O3. 1H-NMR (DMSO-d6, 400MHz): δ 7.06 (d, J=8.3Hz, 1H), 8.40-8.43 (m, 1H).
