[OH:1][CH:2]([C:6]1[C:14]2[S:13][CH:12]=[CH:11][C:10]=2[C:9]([OH:15])=[CH:8][CH:7]=1)C(O)=O.C(O)C.S(=O)(=O)(O)O.C(OC(C)C)(=O)C>O>[OH:15][C:9]1[C:10]2[CH:11]=[CH:12][S:13][C:14]=2[C:6]([CH:2]=[O:1])=[CH:7][CH:8]=1Into a 100-mL 3-necked round-bottom flask purged and maintained with an inert atmosphere of nitrogen, was placed 2-hydroxy-2-(4-hydroxy-1-benzothiophen-7-yl)acetic acid (500 mg, 2.01 mmol, 1.00 equiv, 90%), Fe2(SO4)3 (750 mg, 1.88 mmol, 1.15 equiv), ethanol (0.75 mL), sulfuric acid(0.4N) (3.75 mL). The resulting solution was stirred for 5 h at 55-60° C. in an oil bath. After cooling to 25° C., 15 mL of isopropyl acetate and 5 ml of water were added under stirring, then the organic phase was separated and organic phase was diluted by 10 ml of water (pH was 3), then used NaOH (2N) solution were added dropwise at 20 degree C. until a pH of 12-12.5 was reached. The organic phase was removed and to the aqueous phase were added H2SO4 (2N) solution until a pH was 4-4.5. The product precipitated during the addition. The suspension was stirred overnight at 25° C., then for 1.2 h in an ice bath and then filtered. This resulted in 200 mg (53%) of 4-hydroxy-1-benzothiophene-7-carbaldehyde as a gray solid.
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