Alcohols photooxygenation

Vinylsilanes — silyl epoxy alcohols. Photooxygenation of vinylsilancs in the presence of Ti(0-(-Pr)4 affords silyl epoxy alcohols with high regio- and diastcreosclcctivity. The conversion involves as the first step an enc reaction with O2 to provide a /3-silyl allylic alcohol, which then undergoes epoxidation. 

In a different type of reaction, alkenes are photooxygenated (with singlet O2, see 14-8) in the presence of a Ti, V, or Mo complex to give epoxy alcohols formally derived from allylic hydroxylation followed by epoxidation, for example, ... 

Fig. 35 Mechanism of formation of hydrogen peroxide in the Wagnerova Type II photooxygenation of alcohols and its in-situ use to epoxidize alkenes and oxidized sulfides.

For example, such a photooxygenation of alcohols 569 lead to unsaturated alcohol-hydroperoxides 570, which react with ketones and aldehydes to give 1,2,4-trioxanes 571 characterized by a-styryl-substituents at the 6-position (Scheme Geraniol... 

This problem was solved by Adam and coworkers in 1994-1998. They presented a high-yielding and diastereoselective method for the preparation of epoxydiols starting from enantiomerically pure allyhc alcohols 39 (Scheme 69). Photooxygenation of the latter produces unsaturated a-hydroxyhydroperoxides 146 via Schenck ene reaction. In this reaction the (Z)-allylic alcohols afford the (5, 5 )-hydroperoxy alcohols 146 as the main diastereomer in a high threo selectivity (dr >92 8) as racemic mixmre. The ( )-allylic alcohols react totally unselectively threolerythro 1/1). Subsequent enzymatic kinetic resolution of rac-146 threolerythro mixture) with horseradish peroxidase (HRP) led to optically active hydroperoxy alcohols S,S) and (//,5 )-146 ee >99%) and the... 

Selective fomation of allylic hydroperoxides via singlet oxygen ene reaction 837 TABLE 2. Syn/anti regioselectivity in the photooxygenation of aUyUc alcohols 10- and 10-Z... 

SCHEME 33. Threo/erythro diastereoselectivity in the photooxygenation of aUyhc alcohols 115-117... 

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... 

The photooxygenation of 2,5-dimethylfuran in alcoholic solution at 20°C sensitized by xanthene dyes has been kinetically studied by Schenck and Gollnick.7,81 The rate constants of the various reaction steps have been determined according to the following procedure. 

Beside xanthene dyes, methylene blue,127 porphyrins and por-phins,128-130 zinc-tetraphenylporphin,128 a-hydroxyanthraquinones in alkaline alcoholic or pyridine solutions,112,131 vitamin A, /9-carotene, hypericin, rubrene, 3,4-benzpyrene, 20-methylcholanthrene, chlorophyll, and many other compounds have been found to be sensitizers of photooxygenation reactions.70,132,133 Some groups of dyes, however, such as porphyrins and porphins containing transition metals (e.g., Co), azo dyes, cyanine dyes, and triphenylmethane dyes either did not show any or only very poor sensitizer capabilities in the photooxygenation of a-terpinene.128,134... 

Dimethylfuran has been chosen as a substrate for the photooxygenation reactions. This substrate yields the ozonide of 1,2-dimethyl cyclobutadiene which undergoes a facile cleavage by the alcoholic solvent to the corresponding 2-alkoxy-5-hydroperoxy-2,5-dimethylfuran derivative.104,105... 

An improved preparation of artemisinin 9a and 10-deoxoartemisinin 108 has been described which uses the photooxygenation of protected keto ester 295 and alcohol 296, respectively (Scheme 50) <1998JNP633, 1998T319>. The mechanism of these cyclizations has been investigated. 

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