Rapid and Enantioselective Synthetic Approaches to Germanicol and Other Pentacyclic Triterpenes

Two exceedingly short synthetic routes to the key intermediate 2 for the synthesis of the pentacyclic triterpene germanicol 1 have been developed. In the first, the ( S)-epoxide of farnesyl bromide is transformed in just three steps to the tetracyclic intermediate 7, which is converted to chiral 2 by treatment with polyphosphoric acid. The second synthetic route to 2 involves the coupling of the ( S)-epoxide 8 with vinyl iodide 9 to give 10 and two-stage acid-catalyzed cyclization of 10 to form 2. During the course of this work we have also discovered a very unusual intramolecular 1,5-proton shift from a carbocation to a C-C double bond. The details of the process have been confirmed by (2)H-labeling experiments.