Rearrangements of Optically Pure Hydroperoxides

The mechanism of autoxidation of lipid materials has been the focus of much investigation. Hydroperoxides are the primary products formed in lipid autoxidation and these hydroperoxide products undergo free radical rearrangements. For example, the hydroperoxides formed in the autoxidation of linoleate rearrange by a mechanism thought to involve formation of the peroxyl radical from the corresponding hydroperoxide and fragmentation of this peroxyl radical to the delocalized carbon radical. Readdition of oxygen to the delocalized radical leads to mixtures of products. Simple allylic hydroperoxides also rearrange by a mechanism involving formation of a peroxyl radical intermediate. The mechanism of this rearrangement has been shown to be concerted. This was demonstrated by the fact that the rearrangement carried out under 180 labelled oxygen does not lead to products that have incorporated labelled oxygen. Furthermore, rearrangement of optically pure allylic hydroperoxides leads to rearrangement products that are also optically pure. A concerted mechanism is consistent with these results. The optically pure allylic hydroperoxides have not been previously prepared and a major focus of our work has been the development of methods for the synthesis of these compounds. We have succeeded in developing methods for the resolution of allylic and dienylic hydroperoxides. Our method involves conversion of the hydroperoxides to perketal derivatives that are formed from the hydroperoxide and α-methyl vinyl ethers derived from (-)-2-phenyl-cyclohexanol. The diastereomeric perketal derivatives can be separated by chromatographic techniques and the perketal protecting group then removed with mild acid. In this way, seven different optically active hydroperoxides have been resolved. In fact, we have yet to encounter a chriral hydroperoxide that cannot be resolved by this approach. Rearrangement of optically pure allylic hydroperoxides occurs with chirality transfer to the new stereocenter formed in the rearrangement.