General procedure for the synthesis of 2-oxo-1,2-dihydropyridine-4-carbonitrile from 2-amino-4-cyanopyridine: Sodium nitrite (NaNO2, 0.99 g, 14.3 mmol) was added in batches to a well-stirred 2-amino-4-cyanopyridine (0.96 g, 8.1 mmol) dissolved in concentrated sulfuric acid (H2SO4, 1.2 mL) and water (11 mL) in a premixed solution. During the reaction, the temperature of the reaction mixture was maintained between 0-5 °C and strictly controlled in the range of 5 ± 5 °C. With the release of nitrogen (N2), the originally clarified solution gradually became turbid. The mixture was slowly warmed to room temperature with stirring, followed by reflux heating in a water bath for 30 min and finally cooled to room temperature. The solid product formed was collected by filtration, washed with water (H2O) and dried under vacuum to afford the target product 2-oxo-1,2-dihydropyridine-4-carbonitrile (0.9 g, 92% yield) as a colorless solid. The structure of the product was confirmed by 1H NMR (DMSO-d6): δ 8.21 (d, 1H, J = 2.4 Hz), 7.61 (dd, 1H, J = 2.4, 9.6 Hz), 6.38 (d, 1H, J = 9.6 Hz), 3.3 (broad peak, 1H + 2H2O).
