Diene

A compound containing two carbon–carbon double bonds; conjugation is a separate structural qualification.

A five-carbon cyclic conjugated diene. Cycloaddition across its two double bonds produces a norbornene framework while retaining the original five-membered carbon ring.

Buta-1,3-diene is $\mathrm{C}{\mathrm{H}}_2=\mathrm{CH}-\mathrm{CH}=\mathrm{C}{\mathrm{H}}_2$, a four-carbon diene with a conjugated system. Adjacent p orbitals allow electron delocalization across the two double bonds and the intervening single bond.

A suitable cisoid conformation can react with a dienophile in a Diels-Alder reaction. The need for the reactive conformation connects internal rotation with cycloaddition reactivity.