Effect of Tether Length on endo/exo Stereoselectivity in Alkene–Arene meta‐Photocycloaddition Reactions towards the Aphidocolin/Stemodin Scaffolds

Intramolecular alkene‐arene meta‐photocycloadditions are powerful transformations that use the enhanced reactivity of photoexcited benzene rings to facilitate addition of an alkene 1,3 across donor groups and form complex three‐dimensional fused‐ring systems from readily accessible starting materials. Intramolecular examples have traditionally been restricted to three‐membered tethers, with cycloaddition resulting from exo‐conformation. However, by judicious tether design we have demonstrated that a four‐membered tether can also proceed in good yield; interestingly, via an endo exciplex (1.2 : 1) enabling access to both natural product skeletons and interesting scaffolds for medicinal chemistry research.