(E)-pent-2-enal. The reaction is initiated by a regioselective metallation at the γ site of the unsaturated system that immediately induces 1,4-eliminative ring fission, and stereoselectively affords hydroxy-functionalized E-1,3-dienes. The esterification of those hydroxy dienes with acryloyl chloride gives activated trienes, suitable for intramolecularDiels–Alderreaction that yields medium-ring lactones. This