Water → 1-isobutyl-1H-thieno[2,3-d][1,3]oxazine-2,4-dione

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A mixture of methyl 2-(isobutylamino)thiophene-3-carboxylate (2.1 g, 9.9 mmol), sodium hydroxide (0.39 g, 9.8 mmol), water (20 mL) and methanol (10 mL) was heated at reflux for 2 h and cooled to room temperature. Methanol was removed under reduced pressure; the residue was diluted with dichloromethane (10 mL) and cooled to 0° C. Triphosgene (5.85 g, 19.7 mmol) was added portionwise; the reaction mixture was stirred for 18 h and diluted with dichloromethane. The organic layer was separated, dried over anhydrous sodium sulfate and concentrated to provide the crude product as a light brown solid (2.0 g). This material was used in the next step (Step D) without any purification. 1H NMR (400 MHz, CDCl3): δ 7.25 (d, 1H), 6.95 (d, 1H), 3.79 (d, 2H), 2.40-2.36 (m, 1H), 1.02 (d, 6H).

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