Ene-like Reactions of Singlet Oxygen

Olefins containing at least one allylic hydrogen atom react with 02 to form an allylic hydroperoxide. The analogous formal reaction of such alkenes with ethylene is known as ene-reaction. The tricyclic lactone peroxypartheno-lide has been prepared by such a reaction (6.13)619). 

Usually the allylic peroxides are not the final products but they are reduced to allylic alcohols as in the synthesis of E-5-cyclodecenone (6.14)620). 

Alternatively the allylic hydroperoxide can undergo the so called Hock-type fragmentation621 under acid-catalyzed hydrolysis with formation of two carbonyl compounds (or moieties). The sesquiterpene (—)-geijerone has been obtained from the triene (+)-y-elemene by such a sequence (6.15) 622 . 

In special cases allylic hydroperoxides decompose directly to a,P-un-saturated ketones. The allylic hydroperoxide formed in the photooxydation of calarene gives the three types of products discussed above (6.16) 623). 

Singlet oxygen reacts with electron rich or highly strained alkenes to form 1,2-dioxetanes. These four-membered ring peroxides decompose on warming to two carbonyl compounds (or moieties), usually with appearance of light emission (chemiluminescence). The macrocyclic bis-lactone in (6.17)608 , a musk fragrance, has been synthesized via such a sequence. 

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