The mechanisms of the rearrangements of allylic hydroperoxides: 5α-hydroperoxy-3β-hydroxycholest-6-ene and 7α-hydroperoxy-3β-hydroxycholest-5-ene

Abstract

The rearrangement of 5α-hydroperoxy-3β-hydroxycholest-6-ene in solution under ¹⁸O₂, gives isotopically normal 7α-hydroperoxy-3β-hydroxycholest-5-ene, whereas the epimerization of this product to give 7β-hydroperoxy-3β-hydroxycholest-5-ene involves incorporation of 73–83% of ¹⁸O₂ into the hydroperoxy group. These two reactions proceed through the corresponding hydroperoxyl radicals, which have different e.s.r. spectra and which therefore must exist as separate and distinct species.

The former reaction shows a first-order dependence on hydroperoxide concentration, and a half order dependence on t-butyl hyponitrite which was added as an initiator.

It is suggested that the first reaction involves a sigmatropic [2,3]-rearrangement, whereas the second reaction proceeds through a dissociative mechanism.