Oxidation of Unsaturated Alcohols at Multiple Bonds

The enol form of mesitylphenylacetaldehyde, a vinylic alcohol, when dissolved in acetic acid and treated with lead tetraacetate at 40 °C, gives 2-acetoxy-2-mesitylphenylacetaldehyde in 86% yield 1151. The result can be interpreted as the addition of acetoxyls across the enol double bond followed by elimination of one molecule of acetic acid (equation 275), 

The oxidation of the alcoholic groups of allylic alcohols and phenyl-allylic alcohols was thoroughly discussed in previous sections on the oxidation of primary and secondary alcohols. In this section, only such reactions in which the double bond is oxidized while the alcoholic group is preserved will be discussed. 

The regio- and stereospecificity of epoxidation depend on the structure of the alcohols and on the oxidants. Most of the examples of epoxidation include allylic alcohols. In geraniol, where both allylic and a nonallylic double bonds are present, epoxidation occurs almost exclusively at the allylic double bond [215]. 

Stereoselectivity in cyclic allylic alcohols depends on the ring size and on the reagent used. 2-Cyclohexen-l-ol, on treatment with peroxybenzoic acid, gives a cis epoxide, probably because of the hydrogen bond of the hydroxyl hydrogen in the transition state. If the acetate of the alcohol is treated with peroxybenzoic acid, the trans epoxide predominates over the cis epoxide in a ratio of 4 1 (equation 276) [298]. 

In c -2-cycloocten-l-ol, oxidation with rt-butyl hydroperoxide catalyzed by vanadium acetylacetonate yields almost exclusively the cis epoxide, whereas /n chloroperoxybenzoic add produces exclusively the trans epoxide. In fra/i5-2-cycloocten-l-ol, both oxidants furnish predominantly the cis epoxide (equation 277) [216]. 

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