A Phosphine‐Catalyzed Regioselective [3+2] Cycloaddition of Ethyl 5,5‐Diarylpenta‐2,3,4‐trienoate with Aromatic Aldehydes and α,β‐Unsaturated Carbonyl Compounds

Tributylphosphine‐catalyzed regioselective [3+2] cycloadditions between ethyl 5,5‐diarylpenta‐2,3,4‐trienoate 1 and various aromatic aldehydes 2 to produce a wide variety of polysubstituted 2,5‐dihydrofurans 3 , and between 1 and β‐unsubstituted α,β‐unsaturated carbonyl compounds 5 to give polysubstituted cyclopentenes 6 with a quaternary carbon center, are reported. In both cases the reaction partners approach each other via the sterically less hindered orientation to afford the target products in excellent regioselectivity. The reaction mechanism involved first the generation of a zwitterionic intermediate between the butatriene 1 and PBu₃. For the formation of 2,5‐dihydrofurans 3 , the preferred cyclization mode encompassed the nucleophilic attack of the α‐position of butatriene to the aldehydic carbon of 2 , followed by the ring closure between the aldehydic oxygen of 2 and the γ‐position of butatriene, which is the first report of a normal [3+2] cycloaddition between cumulenes and aldehydes. For the formation of cyclopentenes 6 , the reaction involved attack of the γ‐position of the butatriene to the electron‐deficient β‐position of the α,β‐unsaturated carbonyl compounds 5 , followed by the ring closure between the α‐position of 5 and the α‐position of butatriene, which shows a different regioselectivity to the previously reported [3+2] cycloadditions between butatriene and olefins.