Synthesis of Pentacyclic Framework of Herquline A

Abstract

The highly strained bowl-shaped pentacyclic structure of herquline A has rendered it one of the most difficult problems in organic synthesis yet to be solved. The challenges associated with the synthesis of herquline A have been well documented in four Ph.D. dissertations and in multiple reports regarding syntheses of its structurally simpler congeners. Herein, we report the construction of the pentacyclic core of herquline A that contains both N10–C2 and C3–C3′ bonds. The key for success was the development of the tandem aza-Michael addition/enolate capture protocol that set the stage for subsequent palladium catalyzed C3(sp2)-C3′(sp2) coupling reaction. Ensuing oxidative dearomatization of the left aryl ring allowed the formation of the pentacyclic diketone core of herquline A.