[CH2:1]([O:3][C:4](=[O:15])[C:5]([N:12]=[N+]=[N-])=[CH:6][C:7]1[S:8][CH:9]=[CH:10][CH:11]=1)[CH3:2].O>C1(C)C=CC=CC=1>[CH2:1]([O:3][C:4]([C:5]1[NH:12][C:11]2[CH:10]=[CH:9][S:8][C:7]=2[CH:6]=1)=[O:15])[CH3:2]The resulting azide, 2-azido-3-thiophen-2-yl-acrylic acid ethyl ester (650 mg, 2.88 mmol) was dissolved in toluene (40 ml) and the solution was refluxed for 2 hours. The solution was thereafter cooled, and water (80 ml) was added. After separation of the phases the aqueous phase was extracted with ethyl acetate (3×20 ml), the combined organic phase was dried over Na2SO4 and the organic solvents were evaporated under reduced pressure. The crude product was purified by silica gel flash chromatography to give 450 mg of pure 4H-thieno[3,2-b]pyrrole-5-carboxylic acid ethyl ester (80% yield).
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