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In a dry three-necked flask, p-toluidine (5.0 g, 46.7 mmol), diethyl malonate (14 mL, 93.3 mmol) and polyphosphoric acid (22.5 mL, 140 mmol) were added sequentially. The reaction mixture was stirred at 150 °C for 3 hours. Upon completion of the reaction, the reaction system was cooled to room temperature, 500 mL of ice water was slowly added, and the pH was adjusted to 5-6 with alkali. the resulting yellow solid product was collected by filtration. The solid was dissolved in an appropriate amount of sodium hydroxide solution and filtered again to yield 7.5 g of 6-methyl-2,4-dihydroxyquinoline as a yellow solid in 92% yield.

[1] Patent: CN107098901, 2017, A. Location in patent: Paragraph 0082; 0083; 0084; 0085
[2] British Journal of Pharmacology, 2015, vol. 172, # 5, p. 1195 - 1221
[3] European Journal of Medicinal Chemistry, 2010, vol. 45, # 10, p. 4467 - 4472