Triterpenoids reduction/rearrangement

Steroids are heavily modified triterpenoids that are biosynthesized in living organisms from farnesyl diphosphate (Cl5) by a reductive dimerization to the acyclic hydrocarbon squalene (C30), which is converted into lanosterol (Figure 27.12). Further rearrangements and degradations then take place to yield various steroids. The conversion of squalene to lanosterol is among the most... 

Sometimes several of these rearrangements occur in one molecule, either simultaneously or in rapid succession. A spectacular example is found in the triterpene series. Friedelin is a triterpenoid ketone found in cork. Reduction gives 3p-friedelanol (47). When this compound is treated with acid, 13(18)-oleanene (48) is formed. In this case seven 1,2 shifts take place. On removal of H2O from position 3 to leave a positive charge, the following shifts occur hydride from 4 to 3 methyl... 

Rearrangements of nonconjugated alkenes have also been observed. Thus, a triterpenoid (partial structure 38) afforded the alkene (39 32% yield) upon reduction (Barton modification), a result attributed to initial base-induced isomerization of (38) to the corresponding a,3-unsaturated ketone followed by rearrangement during reduction. ... 

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