Cycloaddition with singlet oxygen

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). 

Salamci, E., Secen, H., Siitbeyaz, Y. and Bald, M. (1997) A condse and convenient synthesis of dl-proto-quercitol and dl-golo-quercitol via ene reaction of singlet oxygen combined with [2 + 4] cycloaddition to cyclohexadiene. Journal of Organic Chemistry, 62 (8), 2453-2457. 

Salamci, E., Secen, H., Sutbeyaz, Y., Balci, M., A Concise and Convenient Synthesis of dl proto Quercitol and dl gala Quercitol via Ene Reaction of Singlet Oxygen Combined with [2 + 4] Cycloaddition to Cyclohexadiene, J. Org. Chem. 1997, 62, 2453 2457. 

Singlet oxygen reacts with polyacenes and electron-rich benzenes via [4+2] cycloaddition to afford endoperoxides (EP) that release O upon heating (Eq. 14.32) [138]. 

Recently the solvent effect on the [4+2] cycloaddition of singlet oxygen to cyclic dienes has been subjected to a multiparameter analysis. A pre-equilibrium with charge-transfer character is involved, which is affected by the solvent through dipolarity-polarisability (n ) and solvophobic interactions ( Sjf and Another multiparameter analysis has been published by Gajewski, demonstrating the... 

Characteristic reactions of singlet oxygen lead to 1,2-dioxetane (addition to olefins), hydroperoxides (reaction with aHyhc hydrogen atom), and endoperoxides (Diels-Alder "4 -H 2" cycloaddition). Many specific examples of these spectrally sensitized reactions are found iu reviews (45—48), earlier texts (15), and elsewhere iu the Engchpedia. 

Cycloaddition reactions can occur with retention of configuration in the pseudoexcitation band (Sect 1.1) whereas [2jt H-2jtJ reactions are symmetry-forbidden in the delocalization band. Experimental evidence is available for the stereospecific [2-1-2] cycloaddition reactions between A and olefins with retention of configuration (Scheme 14) [82]. A perepoxide intermediate was reported to be trapped in the epoxide form [83] in the reaction of adamantylideneadamantane with singlet oxygen affording dioxetane derivatives [84]. 

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