The Cycloaddition of Allyl Cations to 1,3‐Dienes: General Method for the Synthesis of Seven‐Membered Carbocycles. New Synthetic Methods (40)

The reaction of allyl cations and 1,3‐dienes affords seven‐, five‐, and six ‐membered rings as well as products of electrophilic substitution and linear 1:1‐adducts. In this review article, preparative and mechanistic advances are summarized with special emphasis on the synthesis of seven‐membered carbocycles. For the first time, a comprehensive mechanistic description is presented: Three classes A, B, and C correlate product type and stereochemistry with the nucleophilicity of the diene, the electrophilicity of the allyl cation intermediate, and the donor capacity of the group Y attached to the central carbon of the allylic moiety. The problem “allylic resonance versus nucleophilic participation of Y” is discussed and experimental evidence is adduced for some participation of Y as a function of its nucleophilicity. For cyclopentenyl cations, enhanced charge separation and electrophilicity are postulated, even when Y is a good donor, due to the structurally enforced polarity of the π‐allyl system.