Cycloaddition between Electron-Deficient π-Systems, Photochemical and Radical-Induced Reactions:  A Novel, General, and Stereoselective Route to Polyfunctionalized Bridged Bicyclo[2.2.2]octanes, Bicyclo[3.3.0]octanes, Bicyclo[4.2.0]octanes, and Tricyclo[4.3.1.03,7]decanes

A novel, general, and stereoselective route to functionalized bridged bicyclo[2.2.2]octanes, bicyclo[3.3.0]octanes, bicyclo[4.2.0]octanes, and tricyclo[4.3.1.03,7]decanes has been described. Various functionalized and sustituted bicyclo[2.2.2]octanes endowed with a β,γ-enone chromophore were synthesized via cycloaddition of in situ generated cyclohexa-2,4-dienones with electron-deficient 2π partners and manipulation of the resulting adducts. Triplet sensitized irradiation of bridged bicyclooctenones led to synthesis of bicyclo[3.3.0]octanoids, whereas the direct irradiation furnished bicyclo[4.2.0]octanes in stereoselective fashion as a result of modulation of reactivity in excited states. Further, manipulation of the adducts led to appropriately appended and functionalized bicyclo[2.2.2]octanes that upon radical induced cyclization provided an efficient and stereoselective route to the tricyclo[4.3.1.03,7]decane (isotwistane) framework of pupukeananes.