Diels-Alder – Stereochemistry of the Z,E-diene (cis-trans)

Click the structures and reaction arrows in sequence to view the 3D models and animations respectively

The two CH₃ substituents end up on opposite sides of the 6-membered ring from one another. One of the CH₃ groups may appear to get in the way of the reaction, however, the dienophile is approaching from underneath the diene, not in the plane.

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J. Sauer, Angew. Chemie Int. Ed. English, 1967, 6, 16–33.