Synthesis of the Pentacyclic Skeleton of Cortistatins.

We report a concise synthesis of the pentacyclic skeleton of the unique steroidal alkaloids cortistatins in 13 steps. The synthesis features a [5 + 2] cycloaddition to forge the oxabicyclo[3.2.1]octene B/C ring framework and a Pauson-Khand reaction to construct the D/E ring moiety, which simultaneously installs both a carbonyl group and an isoquinoline side chain at the E-ring.