Benzene oxides cycloaddition reaction with singlet oxygen

The cycloaddition reaction of benzene oxide-oxepin 1 with singlet oxygen to yield an endo-peroxy adduct has been studied in several laboratories. 

While both hydrogenation and epoxidation reactions of (7) (and substituted forms) occur on the oxepin valence tautomer, cycloaddition reactions proceed more readily on the arene oxide form (where the diene is closer to planarity). Thus the dienophiles DM AD and maleic anhydride (MA) readily yielded [4 + 2] cycloadducts with (7) as shown in Scheme 22 (67AG(E)385). A similar type of singlet oxygen cycloaddition reaction gave an unstable endoperoxide (106) which upon heating yielded trans-benzene trioxide quantitatively (equation 14). (75JOC3743). 

The possibility of valence isomerization also affects the reactions of the oxepins [5]. For instance, cycloadditions involve the benzene oxide, as shown by the Diels-Alder reaction with activated al-kynes, and give the epoxybicyclo[2.2.2]octatriene 9 or, in the presence of singlet oxygen, afford the peroxide 10. The latter isomerizes thermally yielding the ra .s-benzene trioxide 11 ... 

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