[O:1]=[C:2]1[NH:7][C:6]2[CH:8]=[C:9]([C:11]([O:13][CH3:14])=[O:12])[S:10][C:5]=2[NH:4][CH2:3]1>C1COCC1.[O-2].[Mn+4].[O-2].[O-2].[Mn+2]>[O:1]=[C:2]1[NH:7][C:6]2[CH:8]=[C:9]([C:11]([O:13][CH3:14])=[O:12])[S:10][C:5]=2[N:4]=[CH:3]1To a suspension of methyl 2-oxo-1,2,3,4-tetrahydrothieno[3,2-b]pyrazine-6-carboxylate 20 (515 mg, 2.427 mmol) in THF (30 mL) was added manganese(IV) oxide (2.11 g, 24.27 mmol). The reaction was stirred at rt for 30 min. manganese oxide was filtrated and washed with EtOAc. The filtrate was evaporated to give crude methyl 2-oxo-1,2-dihydrothieno[3,2-b]pyrazine-6-carboxylate 21 (483 mg, 95%). NMR (400 MHz, DMSO-d6) 3.90 (s, 3H), 7.62 (br s, 1H), 8.13 (s, 1H), 12.80 (br s, 1H). (m/z)=211 (M+H)+.
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