Cycloaddition protocol for the assembly of the hexacyclic framework associated with the kopsifoline alkaloids.

ABSTRACT

An approach to the hexacyclic framework of the kopsifoline alkaloids has been developed and is based on a Rh(II)-catalyzed cyclization-cycloaddition cascade. The resulting [3+2]-cycloadduct was readily converted into the TBS enol ether 23. Oxidation of the primary alcohol present in 23 followed by reaction with CsF afforded compound 24 that contains the complete hexacyclic skeleton of the kopsifolines. [reaction see text]