Effects of the π and π* Orbitals of the 5-Unsaturated Substituents on the π-Facial Selectivities in the Diels-Alder Reactions of Cyclopentadienes

The exclusive anti-π-facial selectivity in the Diels-Alder reaction of 5-formylpentamethylcyclopentadine, Cp*-CHO, was proposed to come from the π-HOMO distortion of the diene due to the perturbation by low-lying π*orbital on the substituent. The role of the π* orbital was experimentally substantiated by a considerable amount of syn-attack products in the reactions of Cp*-CH=NOH and Cp*-CH=CH2 having high-lying substituent π* orbitals.