Photocycloaddition of Cyclohex‐2‐enones to Tetramethoxyethylene: Formation of 2,2,3,3‐Tetramethoxy‐1‐oxaspiro[3.5]non‐5‐enes

Cyclohex‐2‐enones 1a–1c undergo photocycloaddition to tetramethoxyethylene in benzene to afford 1‐oxa‐spiro[3.5]non‐5‐enes 3 in very good yields. In MeCN as solvent, higher relative amounts of bicyclo[4.2.0]octan‐2‐ones 4 are formed. Results from similar experiments with the same enones and 2,3‐dimethylbut‐2‐ene or 1,1‐dimethoxyethene indicate that the driving force for oxetane formation is a sufficiently large difference in redox potentials between E red of the enone and E ox of the alkene. A mechanism is proposed for cycloalk‐2‐enone + alkene photocycloadditions wherein the cyclobutane adducts arise from an exciplex intermediate and the oxetanes from a subsequently formed contact ion‐pair.