Cesium carbonate → 2,2,7-Trimethyl-2H-benzo[b][1,4]oxazin-3(4H)-one

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6-Nitro-m-cresol (30.6 g) was dissolved in acetonitrile (400 ml), and thereto was added cesium carbonate (163.3 g), and the mixture was warmed to 80° C. To the mixture was added dropwise ethyl 2-bromoisobutyric acid (60 ml), and the mixture was stirred at 80° C. for 6 hours. To the mixture was added dropwise again ethyl 2-bromoisobutyric acid (60 ml), and the mixture was further stirred at 80° C. for 8 hours. The reaction solution was cooled to room temperature, and filtered through celite. The filtrate was concentrated under reduced pressure. The residue was dissolved in ethyl acetate, washed with water (twice) and saturated aqueous sodium chloride solution (once), and dried over sodium sulfate. The mixture was filtered, and concentrated under reduced pressure to give a crude product. The obtained crude product was dissolved in ethanol (100 ml), and added dropwise in to a solution of iron (90 g) in acetic acid (300 ml), wherein the suspension was previously and separately warmed to 75° C. After the addition was completed, the mixture was stirred at 80° C. for 5 hours, and filtered through celite. The filtrate was concentrated under reduced pressure, and the residue was dissolved in ethyl acetate, washed successively with water and saturated aqueous sodium hydrogen carbonate solution, dried over sodium sulfate, filtered and concentrated under reduced pressure to give a crude product. The obtained crude product was washed with hexane, and collected by filtration to give the title compound (32 g) as whited crystal.

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