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Dedicated to Prof. Paul dr Muyo on the occasion of his 65th birthday (26.111.90) Cyclohex-2-enones la-lc undergo photocycloaddition to tetramethoxyethylene in benzene to afford 1 -oxaspiro[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,ldimethoxyethene indicate that the driving force for oxetane formation is a sufficiently large difference in redox potentials between Ered of the enone and E,, 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.

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