Synthesis of the Pentacyclic Core of Sanguinone B Enabled by Alkyne Functionalization.

Sanguinones A and B are recently isolated pyrroloiminoquinone alkaloids possessing a rare C4 carboxylic acid. We report a synthetic approach toward the full carbon and nitrogen pentacyclic core of sanguinone B, with the correct oxidation at C4. Highlights of this approach include the application of a Larock/Buchwald-Hartwig annulation cascade to access a key tricyclic intermediate and alkyne functionalization using Rh(I) or Cu(I) catalysis to access hindered benzylic ketones.