Lewis acid catalysed rearrangement of triterpenoids

From the whole plant of Opuntia vulgaris Mill (Cactaceae), four known triterpenoids friedelin, friedelan, 3-ol, taraxerone and taraxerol, have been isolated. Friedelan 3β-ol and friedelan-3-ol undergo a novel rearrangement with boron tribromide. Taraxeryl-acetate gives the expected β-amyrin acetate which undergoes further reactions to δ-amyrin acetate. Friedelan 3β-ol rearranges on treatment with boron trifluoride to give mainly olean 13 (18)-ene and olean 12-ene, together with an unexpected epimeric product. Mechanisms are suggested for these various reactions.