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.

2-吡喃酮与1,3-烯炔的分子内[4+4]光化学反应，可生成不稳定的1,2,5-环辛三烯产物。当吡喃酮的C4位无取代基时，1,3-氢迁移会将该丙二烯转化为1,3-二烯烃，其分子骨架与达科特醇（dactylol）相关。若吡喃酮C4位带有甲氧基取代，则发生科普重排（Cope rearrangement），生成与环丁烷稠合的九元环产物。上述两种产物的结构均通过X射线晶体学（X-ray crystallography）得到确证。该科普重排过程表观上可逆，可重新生成丙二烯，后者经质子迁移得到更稳定的1,3-二烯烃产物。