On the Diels–Alder reactions and the Lewis acid induced rearrangements of 6-fumaryl 1,3,8-nonatrienes

6-Fumaryl 1,3,8-nonatrienes substituted at the C5 position by a vinyl group were found to undergo competing tandem sigmatropic rearrangement/Diels–Alder cyclisation when heated under standard Diels–Alder cyclisation conditions. This rearrangement became the exclusive pathway when the reactions were performed in the presence of a Lewis acid.