Hydrochloric Acid → 6-(1H-indol-6-yl)hex-5-ene-2,4-dione

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Ethyl acetate (7 mL), 2,4-pentanedione (9.35 mL, 90.9 mmol) and boron trioxide (5.76 g, 82.7 mmol) was placed in a 200 mL reaction vessel with a reflux condenser. To the stirring mixture at 85° C. was added dropwise a solution of 1H-indole-6-carboxaldehyde (2.00 g, 13.8 mmol) and trimethyl orthoformate (1.6 mL, 14 mmol) in 14 mL of ethyl acetate. After the reaction mixture was stirred for 30 min at 95° C., n-butylamine (6.8 mL, 69 mmol) was added dropwise with additional stirring for 2 h. The reaction mixture was cooled to 50° C. before addition of 3N HCl (32 mL). After being stirred at 50° C. for 30 min, the mixture was filtered to remove solids. The filtrate was diluted with ethyl acetate, washed with brine twice, and dried over MgSO4. After filtration, the filtrate was concentrated in vacuo, and the residue was purified by silica gel column chromatography (hexane/ethyl acetate=95/5 to 75/25) followed by recrystallization (hexane/ethyl acetate) to obtain the title compound as a pale yellow crystal (0.92 g, 29%).

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