Dimethylaluminum methide complex Tf2CHAlMe2: an effective catalyst for Diels–Alder reaction of α,β-unsaturated lactone derivatives with cyclopentadiene

Lewis acid derived by mixing Tf2CH2 and Me3Al was found to be an effective catalyst system for the catalytic DA reaction of less reactive α,β-unsaturated lactone derivative with cyclopentadiene (CP). In this catalyst system, Tf2CHAlMe2 is an active species and an excess amount of Me3Al plays an important role to lower the catalyst loading. Substituent effect of the lactone framework on π-facial selectivity was also examined. In the reactions of both γ-substituted 5-membered lactone derivatives and γ- or δ-methylated 6-membered lactone derivatives with CP, selective attack on the anti face of γ- or δ-substituent was observed. On the other hand, in the cases of γ- or ɛ-methylated 7-membered lactone derivatives, CP favorably attacked on the syn face.