Allylic alcohols photooxidation

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... 

Furthennore, the presence of electnxi-withdrawing substituents, such as an allylic hydroxy group, is deactivating, an effect intensified by esterification to a degree such that acylation of an allylic alcohol may be sufficient to protect the double bond during photooxidation at another site. ... 

Among oxi tions producing allylic alcohols or their derivatives the modem variants of selenium dioxide oxidations are by far the most popular. Systems based on metal acetates, particularly palladium tri-fluoroacetate, can be very useful and are receiving increasing attention but the Kharasch-Sosnovsky reaction, once very common for allylic oxidation, is now rarely used. Sensitized photooxidation with singlet oxygen, a very well-known procedure, is still somewhat unpredictable and has periu K received less consideration than it deserves. 

Alternatively, some of the natural kaurenes may function as free radical scavengers against oxidative stress. These compounds are amenable to photooxidation in the presence of photosensitizers under moderate UV light to give allylic alcohol derivatives via the corresponding hydroperoxides 50 and 51 (Banerjee et al., 1973). This reaction may explain at least in part the synthesis in vivo of various hydroxyl kaurenes naturally found in Espeletiinae and other plant groups. 

Buchi and co-workers have reported a total synthesis of th norsesquiterpene khusimone (3), the last step of which required the isomerization of the more stable isomer, isokhusimone (1). This conversion was achieved in two steps by sensitized photooxidation of 1 followed by reduction with triethyl phosphite to give the allylic alcohols 2. Reduction of 2 with zinc and hydrogen chloride gave a separable mixture of 3 and epikhusimone 4. ... 

The issues of sensitizer degradation and purification of the oxidation products were taken into account by DiMagno et al., who used the electron-deficient fluorous porphyrin 15 (5,10,15,20-tetrakis(heptafluoropropyl)porphyrin) as a sensitizer in the photooxidation of cyclohexene and allylic alcohols in CHjCN/perfluorohexanes [26]. 

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