Direct oxidation of (E)-4-(6-substituted-1-cyclohexenyl)-3-buten-2-ones (4a–7a) by selenium dioxide afforded 2-acetyltetrahydrobenzofurans (4b–7b) in moderate to good yields. An inverse, stepwise [2+ 4] cycloaddition mechanism of the intermediate formation was proposed from conformational and electronic requirements of conjugated dienones and solvent effects of SeO2 oxidation.
