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

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Abstract
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.

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