Oxygen as a Dienophile in 1,4-Cycloaddition Reactions

Since two reviews on this topic have appeared recently,5,6 this section will be restricted to a short discussion of the mechanism and a few examples. 

4-Cycloaddition reactions occur between singlet oxygen and cisoid 1,3-diene systems yielding cyclic peroxides with the skeleton of 1,2-dioxenes according to the following scheme  

4-Cycloadditions with 1,3-cyclohexadienes proceeding with very low activation energies (Table IV) bear a close relationship to thermal Diels-Alder reactions (see ref. 5 and references cited therein). Hoffmann and Woodward237 have developed selection rules for thermal and photochemical concerted cycloaddition reactions according to which Diels-Alder reactions can occur in a concerted fashion with singlet ground-state 

Cyclopentadienes, 1,3-cyclohexadienes, 1,3-cycloheptadienes, as well as furan and aklyl-substituted furans, have been investigated as substrates of photosensitized oxygenation reactions, while aromatic compounds such as anthracenes and tetracenes as well as aryl-substituted carbo-and heterocyclic pentadienes were studied in direct and indirect (photosensitized) photooxygenation reactions. 

Two endoperoxides, 351 and 352, were obtained from optically active a-phellandrene (550),242,243 indicating that no steric hindrance is exerted by the isopropyl group of 350 on an oxygen attack. Stereoselective oxygenation is observed with ergosterol (555),19,22,23 lumisteryl acetate (555),244 levopimaric acid (557),245,246 and the benzoic ester of 6/3-hydroxy-5,5,9-trimethylhexahydronaphthalene (360),247 to mention 

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K. Gollnick and G. O. Schenck, Oxygen as a Dienophile, in 1,4-Cycloaddition Reactions The Diels-Alder Reaction in Heterocyclic Syntheses, J. Hamer, Ed., Academic Press, New York, 1967, p. 255, chap. 10. 

Polycyclic aromatic hydrocarbons such as anthracenes, tetracenes, and pentacenes, as well as cyclopentadienes, cyclohexa-l,3-dienes, cyclo-hepta-l,3-dienes, and furans, have been found to be suitable diene systems to which the singlet oxygen adds as a dienophile in a 1,4-cycloaddition reaction. Thus, endoperoxides (transannular peroxides) and, in the case of furans, ozonides of the corresponding cyclobutadienes are the primarily produced, more or less stable addition products (2, 21, 22). 

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