Development of effective Lewis acids for the catalytic Diels–Alder reaction of α,β-unsaturated lactones with cyclopentadiene

We found that ‘Tf2CH2 + Me3Al’ systems are effective catalytic systems for the DA reaction of less reactive α,β-unsaturated lactone derivatives, compared to α,β-unsaturated ester derivatives, with cyclopentadiene. Mononuclear aluminum methide complex, Tf2CHAlMe2, as an active species is formed in these catalytic systems. Effects of lactone ring-size on the reactivity and stereoselectivity were also examined. By expanding ring-size, reactivity of α,β-unsaturated lactones reduced but endo-selectivity notably increased.