Studies on epoxides. Part III. Epoxidation of allylic alcohols with chromium trioxide

Abstract

The action of chromium trioxide under acidic conditions on steroid allylic alcohols has been studied. Whereas the equatorial isomers are always oxidised to the corresponding enones, the axial counterparts can undergo either a similar oxidation to the enone or epoxidation of the double bond and oxidation of the alcohol, to lead ultimately to an epoxy-ketone.

With the axial allylic alcohols in which the rate of oxidation of the secondary alcohol is faster than that of epoxidation of the double bond, enones will be formed, whereas with those in which the relative rates of these two processes are reversed epoxy-ketones will be produced. Usually unsaturated alcohols which yield transoid enones belong to the first group, whereas those which would result in cisoid enones are of the second.

Whenever a compound first undergoes an allylic rearrangement, the stereochemistry of the product determines the fate of the reaction with chromium trioxide.

Epoxidation of the double bond in axial allylic alcohols is stereospecific and takes place always on the same side of the molecule as the hydroxy-group; it proceeds presumably through formation of a chromate ester followed by the transfer of an oxygen atom to the double bond.