Synthesis
Example 13 (a) Procedure for the synthesis of 4-chloro-3-nitropyridin-2-amine (Aust. J. Chem. 1982,35,2025.): 4-chloro-2-aminopyridine (10 g, 77.5 mmol) was dissolved in concentrated sulfuric acid (100 mL) and cooled to about -8°C using a salt-ice bath. Fuming nitric acid was added slowly with stirring, keeping the reaction temperature below 0°C. After addition, the reaction mixture was stirred at room temperature for 20 minutes, followed by careful pouring onto ice. A 32% ammonium hydroxide solution was slowly added while the temperature was controlled with an ice bath below 25 °C until the solution pH reached 3. The solid product was collected by filtration, washed with water and recrystallized from a 1:1 aqueous solution. The resulting solid was added to ice-cold concentrated sulfuric acid (200 mL) in batches, controlling the rate of addition to maintain the temperature below 4 °C. After addition, the reaction mixture was allowed to come to room temperature. After 2.5 hours of reaction, two regional isomers (1:1 ratio), 3-nitro and 5-nitro compounds, were detected by LCMS. The reaction mixture was poured into ice and alkalized with 32% ammonium hydroxide solution. It was filtered and washed with water to obtain a mixture of the two regional isomers. The mixture was dissolved in ethyl acetate and heptane was added to precipitate the unwanted regional isomer. The isomer is removed by filtration and the ethyl acetate is evaporated to give the target product and 15-20% of the impure isomer. In addition, the two isomers could be separated by rapid chromatography on silica gel (Combiflash system) and an appropriate ethyl acetate/heptane gradient. The final product was obtained as 5 g in 37% yield. m/z 172 (M-1) by MS (ESI), 174. 2.2 min RT (LCMS, 254 nm).
References
[1] Tetrahedron Letters, 2012, vol. 53, # 42, p. 5597 - 5601
[2] Patent: WO2007/40438, 2007, A2. Location in patent: Page/Page column 59
[3] Journal of Medicinal Chemistry, 2012, vol. 55, # 20, p. 8721 - 8734,14
[4] Journal of Medicinal Chemistry, 2012, vol. 55, # 20, p. 8721 - 8734
[5] Patent: WO2013/190320, 2013, A1. Location in patent: Paragraph 00117; 00118
