Intramolecular Diels-Alder

Regioselectivity reversal

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The usual regioselectivity may be compromised in intramolecular reactions. In some cases, it is impossible for the reagents to get together in the 'correct' orientation. The ester (methyl-2,7,9-decatrienoate) above has a short chain of carbons between its diene and dieneophile. This restricts the orientation and produces an endo transition state which leads to a trans ring fusion.