Preparative vis‐laser photochemistry. Qinghaosu‐type 1,2,4‐Trioxanes by molecular oxygen trapping of paterno‐büchi triplet 1,4‐diradicals derived from the bicyclic enol lactones Δ1,6‐ and Δ1,10‐2‐oxabicyclo[4.4.0]decen‐2‐one and p‐benzoquinone

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Abstract
The VIS‐laser irradiation of enol lactones 1a, b and benzoquinone in an argon atmosphere led to the fenestrane‐type oxetane 2a and the propellane‐type oxetane 2b, with regioselective preference for the acetal structure. Under an oxygen atmosphere both regioisomeric fenestrane‐type 1,2,4‐trioxanes 4a and 5a were obtained from enol lactone 1a, while no oxygen trapping to the propellane‐type trioxanes was observed with 1b. Significant quantities of photooxygenation products were obtained from both enol lactones 1a, b due to singlet oxygen derived from oxygen quenching of benzoquinone triplets. The present study demonstrates that Qinghaosu analoga are accessible via trapping of Paterno‐Büchi triplet 1,4‐diradicals by molecular oxygen.