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There is no conjugating group on the alkene to stabilise the transition state by overlap between the orbitals of the conjugating group and the back of the diene – the reaction is only able to occur because it is intramolecular.
Because of this there is no endo effect – the molecule simply folds up in the most sterically favourable way, leading to the trans ring junction. Where the alkene has a conjugating group, the endo product may be formed.
Intramolecular Diels-Alder reactions may, or may not, give endo, exo or a mixture of products.
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