Acid-Catalysed Backbone Rearrangement of cholesta-6,8(14)-dienes

Rearrangement of 5α- and 5β-cholesta-6,8(14)-dienes ( 13a and 13b , resp.) in the presence of anhydrous toluene-4-sulfonic acid in acetic acid leads to 5α- and 5β-12(13 → 14)-abeo-cholesta-8,13(17)-dienes ( 15a and 15b , resp.) via 5α- and 5β-cholesta-8,14-dienes ( 14a and 14b , resp.), respectively. Epimerization at C(20) of the spirosteradienes 15a and 15b occurs with increasing reaction time. Molecular-mechanics calculation of the relative stabilities of these compounds and of congeners thereof is in agreement with the observed reaction pathway.