Intramolecular Ene Reactions
Intramolecular ene reactions benefit from less negative entropies of activation than their intrermolecular counterparts, so are usually more facile, occurring even in the case of simple enophiles, such as unactivated alkenes and alkynes. The high regio- and stereoselectivities that can be obtained in these reactions can offer considerable control in the synthesis of intricate ring systems.
Considering the position of attachment of the tether connecting the ene and enophile, Oppolzer has classified both thermal and Lewis acid-catalyzed intramolecular ene reactions as types I, II and III, and Snider has added a type IV reaction (Figure 7). In these reactions, the orbital overlap between the ene and enophile is largely controlled by the geometry of the approach of components.
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