Application of Lewis Acid Catalyzed Tropone [6+4] Cycloadditions to the Synthesis of the Core of CP-225,917

The carbocyclic core of CP-225,917 and CP-263,114 is accessible through the [6+4] cycloaddition of a tropone with a 2-substituted cyclopentadiene. Examination of this reaction has revealed for the first time that this cycloaddition process is catalyzed by Lewis acids, including lanthanide triflates. Cycloadditions of several mono-, di-, and trisubstituted tropones with 2-silyloxycyclopentadienes using ZnCl2 catalysis are found to proceed in good yield and, in many cases, with excellent diastereoselectivity. Subsequent transformation to the core of the CP-molecules involves a site-selective Baeyer−Villiger oxidation of a tricyclic diketone, followed by a syn-elimination process.