Catalyst-free light-mediated polyene cyclization

Polyene cyclization reactions are highly efficient and unique biomimetic transformations that yield complex polycyclic molecules from their acyclic precursors. Various cation-mediated and radical-mediated synthetic methods have been developed to obtain highly diastereoselective cyclizations, along with the use of visible-light in the recent years. In this paper, we report a visible-light-mediated, catalyst-free cascade cyclization of geranyl phenols containing bromo- and chloro-groups at different positions of the phenolic ring. The desired cyclization product was obtained with high stereoselectivity using a blue LED light in hexafluoro-2-propanol without any photocatalyst. The presence of a halogen, and particularly chlorine or bromine is essential for the success of this cyclization, as is the use of blue instead of green LED light. Although the exact reason for this success is unclear, some possible explanations are discussed and further control studies of photoreactions should be a part of any efforts in this area.