Rose-Bengal-Sensitized Photooxidation of Quadricyclane. A (2σ + 2σ + 2π)Cycloaddition of Singlet Oxygen.

Quadricyclane was photooxygenated with singlet oxygen generated under Rose-Bengal-sensitized conditions to a dioxetane, which in turn gave 2-cyclopentene-1,4-dicarbaldehyde and 5-norbornene-cis-2,3-exo-diol. For the mechanism to form the dioxetane, an involvement of a [2σ+2σ+2π] electrocyclic process is proposed.