Photooxygenation 1,4-cyclohexadiene

The keto endoperoxide 90 has been synthesised from 1,4-cyclohexadiene through its photooxygenation, reduction of the resulting diastereoisomeric mixture of endoperoxides 89 and subsequent oxidation (Scheme 60). Some chemistry of 90 is described . 

Singlet oxygen affords a variety of regio and diastereoselective reactions with chiral allylic alcohols amines - (e.g. substrate 161, Scheme 58) and chiral cyclohexadienes - that are useful for synthetic transformations. For example, the photooxygenation of a chiral allylic alcohol was used recently as the key step in the total syntheses of plakorin 162 and ewawfio-chondrilin (Scheme 58). If the photooxidation... 

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. 

In this regard, clear chemical and spectroscopic evidence for the disproportionation of the intermediate radical cations, photochemically and/or thermally generated, were achieved on 2,3-diphenyl-5,6-dihydro-1,4-dioxin, and derivatives 22a - c [90, 111, 57-159]. In fact, the 2,4,4,6-tetrabromo-2,5-cyclohexadien-l-one (TBCHD)-sensitized photooxygenation of 22a affords the corresponding 1,2-ethanedioldibenzoate, 67, the cleavage product of the intermediate 1,2-dioxetane, 24a, together with minor amounts of 4a,8a-diphenyl-2,3,4a,6,7,8a-hexahydro-p-dioxino[2,3-b]-p-dioxin 68 [158] ... 

Linker, T. and Frohlich, L., Regioselective and diastereoselective photooxygenation of chiral 2,5-cyclohexadiene-l-carboxyHc acids, Angew. Chem. Int. Ed. Engl., 33, 1971, 1994. 

A synthetically useful example for perfect regioselective control was reported for furan photooxygenation in the presence of a tricarbonyliron-protected cyclohexadiene. Numerous examples are known for terpene and steroid substrates with 1,3-cyclohexadiene subunits in which endoperoxides have been isolated in good to excellent yields, e.g., dehydro-6-ionone 25 gives the endoperoxide 26 in 50% yield by using rose bengal as the sensitizer in methanol. ... 

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