C([O:3][C:4](=O)[CH2:5][NH:6][C:7]1[S:11][C:10]([C:12]([O:14][CH3:15])=[O:13])=[CH:9][C:8]=1[N+:16]([O-])=O)C>C(O)(=O)C.O.[Fe]>[O:3]=[C:4]1[NH:16][C:8]2[CH:9]=[C:10]([C:12]([O:14][CH3:15])=[O:13])[S:11][C:7]=2[NH:6][CH2:5]1A mixture of methyl 5-(2-ethoxy-2-oxoethylamino)-4-nitrothiophene-2-carboxylate 19 (7.02 g, 24.35 mmol) and iron powder (4.08 g, 73.1 mmol) in acetic acid (100 mL) and water (15 mL) was stirred at 70° C. for 2 h. Quenched with ice/water and the resulting precipitate was collected by filtration to give crude methyl 2-oxo-1,2,3,4-tetrahydrothieno[3,2-b]pyrazine-6-carboxylate 20 (4.74 g, 92%). NMR (400 MHz, DMSO-d6) 3.69 (s, 3H), 3.87 (s, 2H), 7.06 (s, 1H), 7.21 (br s, 1H), 10.28 (br s, 1H). (m/z)=213 (M+H)+.
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