Endoperoxides photooxygenation

In this case, the benzene dioxide 5 is obtained from a [2,2]paracyclophane diene by photooxygenation and rearrangement of the derived endoperoxide. 

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 . 

Wittig reaction of 10 with pentyl phosphonium salt in the presence of NaH gave Z-olefin 11 selectively. Dye-sensitized photooxygenation of 11 proceeded exclusively to give the endoperoxides 12a and 12b in almost quantitative yield as a roughly 1 1 mixture of two diastereomers. 

The only examples of 1,2,4-trioxolanes containing A-linked substituents arise from photooxygenation of oxazoles. The endoperoxides thus produced are bicyclic, with a 1,2,4-trioxolane ring bridged by an imine moiety and are unstable at ambient temperature. They have been characterized at low temperature and thermal rearrangement is described in Section 4.16.5.1.1. 

Singlet oxygen reacts with many carbocyclic 1,3-dienes to form the corresponding bridged endoperoxides, which can be further transformed into miscellaneous polyoxy functionalized carbocycles . The rate of photooxygenation of n-cyclic 1,3-dienes is gradually decreasing in the order n = 5) >6>7 8 While the rate... 

A sequence of transformations involving diastereoselective singlet oxygen photooxygenation of TBS-protected 3,5-cycloheptadienol 525 to predominantly 5yw-endoperoxide syn-526 and some anti-526, followed by reduction into the all-ci5 triol 527 as described in Scheme 146, was used by Johnson for synthesis of enantiopure methyl 2,4-dideoxyhexa-pyranosides D-528 and L-5284°5.406... 

As discussed in Section II.A.4, photooxygenation of l-aryl-2-methylpropenes in solution proceeds slowly, and affords a complex mixture of products arising mainly from [4 + 2] or [2 + 2] addition to the double bond. The ene pathway is less favourable or even absent. For example, -dimethylstyrene (20) affords the ene adduct in approximately 20% yield, benzaldehyde (from a [2 - - 2] pathway), and mainly two diastereomeric di-endoperoxides (from a [4-1-2] pathway) in a 2/1 ratio (Scheme 48). Similarly, for l-(2-methylpropenyl)naphthalene (137), the 1,4-endoperoxide is mainly formed. ... 

The mechanism of photooxygenation has been studied for a range of substituted imidazoles 45 (Scheme 14). It proceeds via the endoperoxide 46, which has been detected by H-NMR spectroscopy in certain instances, giving 47 for triarylimidazoles, 48 reversibly at low temperatures for R = alkyl, R = R = and a variety of different products for other... 

Mechanistic studies were started by Gaffron15-17 in 1926. At this time Moureu, Dufraisse, and Dean18 discovered the direct photooxygenation of rubrene and Windaus and Brunken19 discovered the photosensitized oxygenation of ergosterol. The structures of both the products formed were elucidated some years later and shown to be those of transannular peroxides (endoperoxides).20-23... 

Already in his early investigations on type II direct and indirect photooxygenation reactions, Gaffron18,32 33 showed that the quantum yields of these processes are independent of the intensity as well as of the exciting wavelength of light. He also showed that in the direct photooxygenation of rubrene, two rubrene molecules have to take part in the reaction sequence which bads to the stable endoperoxide. 

However, even in the latter case (e.g., reaction of 1Oa + anthracene -> anthracene-endoperoxide), there seems to be no direct path from 1X to the stable endoperoxide, since all kinetic results have revealed the participation of two molecules of anthracene in the photooxygenation reaction. This also indicates that the excess energy, contained in 1X after transition from the F state, is used as kinetic energy with which 1S0 and 1Oa fly apart. 

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