A New Route to Cyclopentenones via Ruthenium-Catalyzed Carbonylative Cyclization of Allylic Carbonates with Alkenes

[RuCl₂(CO)₃]₂/Et₃N and (η³-C₃H₅)RuBr(CO)₃/Et₃N are highly effective catalyst systems for carbonylative cyclization of allylic carbonates with alkenes to give the corresponding cyclopentenones in high yields. For example, treatment of allyl methyl carbonate (1a) with 2-norbornene (2a) in the presence of a catalytic amount of [RuCl₂(CO)₃]₂ (2.5 mol %) and Et₃N (10 mol %) at 120 °C for 5 h under 3 atm of carbon monoxide gave the corresponding cyclopentenone, exo-4-methyltricyclo[5.2.1.0^2,6]dec-4-en-3-one (3a), in 80% yield with high stereoselectivity (exo 100%).