Asymmetric Total Synthesis of Bufospirostenin A

The first and asymmetric total synthesis of bioactive bufospirostenin A, an unusual spirostanol with rearranged A/B rings, was accomplished. The synthetically challenging [5-7-6-5] tetracyclic ring system, found in bufospirostenin A and some other natural products, was efficiently constructed by the unique intramolecular rhodium-catalyzed Pauson-Khand reaction of an alkoxyallene-yne. The 11 stereocenters in the final product, including the 10 contiguous stereocenters, were installed diastereoselectively.