UV irradiation of arylidene-β-ionones in the presence of dioxygen: regioselective formation of stable endoperoxides

UV irradiation of (E,E)-arylidene-β-ionones 1 leads to photoisomerization resulting in (Z,E) arylidene-β-ionones 2, which undergo electrocyclization to 1,7,7-trimethyl-3-(E-2′-arylethenyl)-2-oxabicyclo[4.4.0]deca-3,5-dienes 3, and the latter in the excited state add regioselectively, involving only the electron-rich endocyclic diene, to molecular oxygen affording highly stable endoperoxides 6.