A convergent synthesis of the cardenolide skeleton: intramolecular aldol condensation via reduction of alpha-bromoketones

Synthesis of the highly biologically valuable cardenolide backbone was achieved via anionic polycyclization. Bromoketone 18, obtained from double-Michael cycloaddition between cyclohexenone 14 and gamma,delta-unsaturated beta-ketoester 16, was efficiently aldolized under reductive conditions. The highly functionalized tetracyclic compound 52 is an important synthetic intermediate that is potentially amenable to natural cardenolide total synthesis.