[4+1]/[2+1] Cycloaddition Reactions of Fischer Carbene Complexes with α,β‐Unsaturated Ketones and Aldehydes

Put two and two together: Fischer carbene complexes and α,β-unsaturated carbonyl compounds smoothly react under thermal conditions to give 2,3-dihydrofurans, and from them furans or 1,4-dicarbonyl compounds were easily accessible (see scheme). This cycloaddition process involves a cyclopropanation reaction followed by rearrangement from the acylcyclopropane to a dihydrofuran.