Cyclopentadiene + maleic anhydride give a bicyclic adduct — the anhydride ends up endo.
1. Two products differ only in which way the substituent points.
Endo tucks the electron-withdrawing group under the newly formed ring; exo swings it outward, away from the bridge.
2. Endo is the kinetically favored product — the endo rule.
The dienophile's carbonyl π system overlaps the diene's developing π system in the transition state, a stabilizing secondary orbital interaction.
3. Exo is usually the more stable, thermodynamic product.
With the bulky group pointing outward, exo has less steric strain, so it sits lower in energy but forms more slowly.
4. Normal Diels-Alder conditions give mostly endo.
Because the reaction is run under kinetic control at moderate temperature, the faster-forming endo adduct dominates the product mixture.
Endo = substituent under the ring · kinetic, favored by secondary orbital overlap · Exo = substituent out, more stable but slower · normal conditions → mostly endo.
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Endo — the transition state is stabilized by secondary orbital overlap between the dienophile's carbonyl and the diene, so it forms faster.
Under the newly formed ring, tucked toward the bridge / the diene's π system.
Exo — the substituent points outward, reducing steric strain, but it forms more slowly.
Mostly the endo product, because the reaction runs under kinetic control.
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