Singlet oxygen hydroperoxide synthesis

SCHEME 12. Synthesis of hydroperoxides from silyl enol ethers TABLE 3. Preparation of alkyl hydroperoxides via singlet oxygen oxygenation... 

Several natural products (Scheme 60) have been synthesized in vitro via singlet oxygen ene reactions. For example, clavukerin C was prepared in racemic form, in a biomimetic synthesis by Kim and Pak, whereas a-farnesene hydroperoxide was obtained starting from geraniol. 

Photooxygenation of vinylsilane in the presence of acetic anhydride and pyridine furnishes a convenient procedure for the synthesis of a-trimethylsilylenones 185 in moderate to good yields (equation 156)278. Dye-sensitized photooxidations of allylsilane with singlet oxygen produces a mixture of (E)- and (Z)-3-trimethyl-silylallyl hydroperoxides 186 (equation 157)279. 

Disubstituted peroxides are cleaved reductively by the same reagents as are used for hydroperoxides. The usual products are alcohols derived from the two substituents, but considerable selectivity is possible with some substrates. This is illustrated by the reduction of cyclic endoperoxides of the general formula (50) these compounds are available from the cycloaddition of singlet oxygen to cyclic conjugated dienes. The reduction of such compounds provides a good method of synthesis of cis-1,4-diols, which can be formed with retention of the double bond or with reduction of it (Scheme 29). It is also possible to prepare unsaturated epoxides by reduction with triphenylphosphine or other phosphorus(III) reagents. 

Often, the hydroperoxides generated in the oxidation of alkenes are not the ultimately desired product. They can be reduced without extensive prior purification to the corresponding allylic alcohol. The sequence of singlet-oxygen oxidation followed by reduction constitutes a reasonably general method of synthesis of allylic alcohols from olefins with allylic migration of the double bond. Scheme 9.14 records some specific examples. 

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