Enyne-2-pyrone [4 + 4]-Photocycloaddition: Sesquiterpene Synthesis and a Low-Temperature Cope Rearrangement

Intramolecular
[4 + 4] photoreaction of 2-pyrones with a 1,3-enyne
yields an unstable 1,2,5-cyclooctatriene product. Without a C4 pyrone
substituent, 1,3-hydrogen migration converts the allene to a 1,3-diene,
with a skeleton related to dactylol. With methoxy substitution, Cope
rearrangement yields a nine-membered ring fused to a cyclobutane.
Both structures were confirmed by X-ray crystallography. The Cope
rearrangement is apparently reversible, reforming the allene which
undergoes a proton shift to the more stable 1,3-diene product.