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.