Asymmetric Synthesis of Carbocyclic Propellanes.

A modular synthesis of functionalized carbocyclic propellanes was developed. Formation of the first of two quaternary bridgehead centers has been achieved by desymmetrization of prostereogenic ketones by either Hajos-Parrish-Eder-Sauer-Wiechert-type processes or Werner's catalytic asymmetric Wittig reaction. The obtained bicyclic enones were subjected to conjugate additions upon which the remaining ring was formed by olefin metathesis. All bridges are amenable to further derivatization, which renders those compounds useful as central units in fragment-based drug discovery or as ligand scaffolds.