The Diene Nu

The Diels-Alder reaction requires the diene to have an s-cis conformation, to allow the ends of the diene and dienophile to overlap in a concerted manner. Any substituents on the diene that would cause steric strain in the required s-cis conformation would slow down, or even prohibit, the Diels-Alder reaction. 

Diels-Alder Diene Must Have s-cis Conformation 

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