Acetic Acid → 7-Methylthieno[3,2-d]pyrimidine-2,4(1H,3H)-dione

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Methyl 3-amino-4-methylthiophene-2-carboxylate IX (100 g, 0.584 mol) and acetic acid (750 mL, 13.1 mol) were stirred for 5 min to obtain a clear solution. A solution of potassium cyanate (56.8 g, 0.70 mol) in water (120 mL) was slowly added over 20 min, and the mixture was stirred for 1.5 h. Additional potassium cyanate (56.8 g, 0.70 mol) in water (120 mL) was slowly added over 20 min and the mixture was stirred for 2 h. Water (600 mL) was added and the mixture was cooled to 10° C. and stirred for 2 h. The solid was collected by filtration and washed with cold water (250 mL). The solid was then stirred for 12 h in a solution of sodium hydroxide (79.4 g, 1.99 mol) in water (1.4 L). The pH was adjusted to 6-7 by slow addition of aqueous concentrated hydrochloric acid solution (35 wt %, 110 mL) and was then stirred for 5 min. The resulting solid was collected by filtration, washed with water (2×250 mL) and dried under reduced pressure at 50° C. for 24 h to afford 7-methylthieno[3,2-d]pyrimidine-2,4(1H,3H)-dione VIII as an off-white solid (89.6 g, 84% yield). 1H NMR (500 MHz, DMSO-d6) δ 7.68 (s, 1H), 2.20 (s, 3H); LCMS (ESI pos) m/z [M+H] 183.

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