Q2(a). Labelled allyl aryl ether: Claisen, Cope and a Diels–Alder trap

Introduction. In allyl 2,6-dimethylphenyl ether both ortho positions are blocked. The Claisen [3,3]-sigmatropic shift still occurs, but the dienone cannot tautomerise; a second [3,3] (Cope) shift carries the allyl group to the para position (para-Claisen rearrangement). Each [3,3] shift bonds the far end of the allyl group, so the label alternates between the two ends.

Scheme: Two concerted chair-like [3,3] shifts; the labelled carbon is marked *.

1. E (Claisen, concerted, chair TS): the C–O bond breaks and the labelled terminal *CH₂ bonds to C-6 of the ring. E = 6-allyl-2,6-dimethylcyclohexa-2,4-dien-1-one; * is now the CH₂ attached to the ring (allyl inversion).
2. F (Cope): the 1,5-diene C=C–C(6)–*CH₂–CH=CH₂ shifts the allyl group from C-6 to C-4; again the far end bonds, so * returns to the terminal =CH₂. F = 4-allyl-2,6-dimethylcyclohexa-2,5-dien-1-one.
3. G: F has an H on C-4 and enolises to the aromatic phenol: G = 4-allyl-2,6-dimethylphenol, label on the terminal =*CH₂ (two inversions = no net inversion).
4. H: E is a cyclohexa-2,4-dienone, a reactive cis-diene. Maleic anhydride traps it by a Diels–Alder reaction (endo) before the Cope step: H = bicyclo[2.2.2]octenone anhydride, * on the ring–CH₂. This proves E is an intermediate.
5. On heating H undergoes retro-Diels–Alder to E + maleic anhydride; E then goes on to F and G, with the same label position.

Scheme: Trapping product of the dienone E.

Conclusion. E: 6-allyl dienone with * on the ring-bound CH₂; F: 4-allyl dienone with * on =CH₂; G = 4-allyl-2,6-dimethylphenol with * on the terminal =CH₂; H = maleic anhydride adduct of E. The label shows one inversion per [3,3] step.

Application. Labelling and maleic anhydride trapping experiments of this kind proved the two-step, concerted nature of the para-Claisen rearrangement; Claisen rearrangements are used to make C-allyl phenols such as eugenol analogues.