The general procedure for the synthesis of 3-formylbenzofuran from 3-methylbenzofuran was as follows: 3-methylbenzofuran (200 mg, 0.19 mL, 1.5 mmol) and selenium dioxide (SeO2, 200 mg, 1.8 mmol, 1.2 eq.) were suspended in 1,4-dioxane (3 mL) and heated and refluxed for 24 hours. Upon completion of the reaction, the reaction mixture was allowed to cool to room temperature, diluted with ethyl acetate/hexane (2:1 v/v, 10 mL), and filtered through a short silica gel plug to remove the selenium residue. The silica gel plugs were washed with ethyl acetate (2 x 3 mL), and the organic filtrates were combined and evaporated to dryness. Purification by silica gel column chromatography (eluent: hexane/ethyl acetate 96:4) afforded the yellow oily product 3-formylbenzofuran (162 mg, 1.1 mmol, 74% yield). The product was characterized by 1H NMR (300 MHz, CDCl3): δ 10.18 (s, 1H), 8.26 (s, 1H), 8.19 (m, 1H), 7.54 (m, 1H), 7.41 (m, 2H); ESI-MS m/z = 147.0 ([MH]+), calculated value C9H7O2 = 147.04.
