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A. A solution of [1,8]naphthyridine (0.2 mmol) and potassium amide (0.8 mmol) in liquid ammonia (20 mL) is stirred at -40 C. for 3 h. The solution is concentrated. Residue is partitioned between EtOAc and water. EtOAc layer is separated and washed successively with water and brine. EtOAc layer is dried over sodium sulfate, filtered and concentrated to give [1,8]naphthyridin-2-ylamine which is used as is for the next step.
In the reaction flask with a stirrer was added 107 g of 2,6-diamino pyridine and 1070 ml phosphoric acid at room temperature was slowly added dropwise 241 g 1,1,3,3-tetramethoxypropane to the resulting solution, dropwise after an oil bath to heat up, control the internal temperature 70-75 C, for 40 minutes. Then the reaction mixture was poured into 5 liters of ice 5M aqueous sodium hydroxide solution, to ensure that pH> 10, the filter cake (200 ml × 2), and the filtrate was washed with methylene chloride and extracted with dichloromethane (300 ml × 2) the combined dichloromethane phases were washed with 100 g of anhydrous sodium sulfate, filtered, and concentrated to dryness, the resultant crude product was purified by column chromatography (packing agent is alumina, eluting with methylene chloride: methanol (v / v ) = 100: 1), to obtain 102 g of a red solid 1,8-naphthyridin-2-amine, yield 72%, HPLC purity 96%.
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