Q4(a). Which dienes cannot take part in a Diels–Alder reaction

Introduction. In the Diels–Alder reaction both new σ bonds form at C-1 and C-4 of the diene at the same time, from the same face. The two termini must be close, so the diene must react in the s-cis (cisoid) conformation. A diene that is locked s-trans by a ring cannot react; a diene that can rotate into s-cis reacts; a diene locked s-cis reacts fastest.

Figure: The two diene ends must meet the dienophile together; only the s-cis form can do this.

1. (i) 3-Methylenecyclohexene: the ring C=C and the exocyclic =CH₂ are linked by the ring C2–C3 bond; the ring holds C-1 and the CH₂ on opposite sides of that bond. Locked s-trans: does not react.
2. (ii) (E)-Penta-1,3-diene: open chain, rotation about C2–C3 is easy; the s-cis form is easily reached (the CH₃ is on the outside). Reacts.
3. (iii) 1,2-Dimethylenecyclohexane: both C=C are exocyclic on adjacent ring carbons; the ring holds the diene permanently s-cis. Reacts very readily.
4. (iv) 1,2,3,7,8,8a-Hexahydronaphthalene: one C=C (C4=C4a) is in the left ring, the other (C5=C6) in the right ring; the single bond between them is a ring bond, which fixes them anti (a heteroannular, transoid diene, as in steroid 3,5-dienes). Locked s-trans: does not react.
5. (v) 1-Vinylcyclohexene: the vinyl group rotates easily about the C1–C(vinyl) bond into s-cis. Reacts (it gives octahydronaphthalenes).

Conclusion. Dienes (i) and (iv) do not undergo the Diels–Alder reaction: each is held s-trans by a ring. (ii) and (v) react after rotating into the s-cis form; (iii) is locked s-cis and is the most reactive.

Application. The same test separates homoannular (s-cis, reactive) from heteroannular (s-trans) dienes in steroids; ergosterol acetate, a homoannular diene, gives a maleic anhydride adduct, while cholesta-3,5-diene does not.