Singlet oxygen photooxygenation

Photochemically produced singlet oxygen, PHOTOOXYGENATION PHOTOCHEMICAL REACTION... 

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... 

Low-temperature autoxidation of 3,4-dihydro-2//-pyrazoles (e.g. 10, R = Me) provides unstable a-azo hydroperoxides (Scheme 30) [61a,bj 3,4-dihydro-2//-pyrazoles that are inert towards autoxidation (cf. 10, R = Ph) can be converted to hydroperoxides by reaction with singlet oxygen (photooxygenation) at 0- 5 °C [61b], a-Azo hydroperoxides readily decompose at room temperature to -keto radicals which can be entrapped with oxygen to furnish 3-hydroxy-1,2-dioxolanes (e.g. 11 —> 12, Scheme 30) [61cj. The preparation and thermal decomposition of several ether, silyl ether and ester derivatives of 3-hydroxy-1,2-dioxolanes have been reported [62]. 

In most cases, only a little activation energy is necessary for the described transformations (1-5 kcal/mol), which accounts for the insensitivity of the rates of singlet-oxygen photooxygenations to reaction temperature, i.e., the reactions may be also carried out at low temperatures (in order to circumvent secondary reactions of the peroxidic products). 

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