[CH3:1][C:2]1[S:9][C:8]2[CH:7]=[C:6]([C:10]([O:12]CC)=[O:11])[NH:5][C:4]=2[C:3]=1[N:15]([CH3:24])[S:16]([C:19]1[S:20][CH:21]=[CH:22][CH:23]=1)(=[O:18])=[O:17].O1CCCC1.[OH-].[Na+]>C(O)C>[CH3:1][C:2]1[S:9][C:8]2[CH:7]=[C:6]([C:10]([OH:12])=[O:11])[NH:5][C:4]=2[C:3]=1[N:15]([CH3:24])[S:16]([C:19]1[S:20][CH:21]=[CH:22][CH:23]=1)(=[O:18])=[O:17]To a mixture of ethyl 2-methyl-3-[methyl(2-thienylsulfonyl)amino]-4H-thieno[3,2-b]pyrrole-5-carboxylate (8.39 g), tetrahydrofuran (50 mL) and ethanol (50 mL) was added 8N aqueous sodium hydroxide solution (10 mL), and the mixture was stirred at 60° C. overnight. The reaction mixture was concentrated, 6N hydrochloric acid was added to the obtained residue, and the mixture was extracted with ethyl acetate. The ethyl acetate layer was washed with saturated brine, dried (MgSO4) and concentrated. Crystallization from diethyl ether gave 2-methyl-3-[methyl(2-thienylsulfonyl)amino]-4H-thieno[3,2-b]pyrrole-5-carboxylic acid (7.07 g, yield 91%) as brown crystals.
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