Synthesis
To a reaction flask equipped with a stirrer was added 107 g of 2,6-diaminopyridine and 1070 ml of phosphoric acid, to which 241 g of 1,1,3,3-tetramethoxypropane was slowly added dropwise at room temperature. Subsequently, the reaction mixture was heated in an oil bath maintaining the internal temperature at 70-75°C and the reaction lasted for 40 minutes. Upon completion of the reaction, the mixture was poured into 5 liters of ice-cooled aqueous 5 M sodium hydroxide solution and the pH was adjusted to greater than 10. The precipitate was collected by filtration and washed twice with 200 ml of water. The filtrate was extracted twice with dichloromethane, 300 ml each. the combined dichloromethane phases were dried over 100 g of anhydrous sodium sulfate, filtered and concentrated to dryness. The crude product was purified by column chromatography using alumina as stationary phase and a solvent mixture of dichloromethane and methanol (100:1, v/v) as eluent, resulting in 102 g of red solid 1,8-naphthyridin-2-amine in 72% yield and 96% HPLC purity.
References
[1] Patent: CN105399739, 2016, A. Location in patent: Paragraph 0019; 0020
[2] Angewandte Chemie - International Edition, 2017, vol. 56, # 12, p. 3349 - 3353
[3] Angew. Chem., 2017, vol. 129, p. 3397 - 3401,5
[4] Journal of Heterocyclic Chemistry, 1997, vol. 34, # 3, p. 765 - 771
