Solvent-Dependent Divergent Functions of Sc(OTf)3 in Stereoselective Epoxide-Opening Spiroketalizations

A stereocontrolled synthesis of benzannulated spiroketals has been developed using solvent-dependent Sc­(OTf)3-mediated spirocyclizations of exo-glycal epoxides having alcohol side chains. In THF, the reaction proceeds via Lewis acid catalysis under kinetic control with inversion of configuration at the anomeric carbon. In contrast, in CH2Cl2, Brønsted acid catalysis under thermodynamic control leads to retention of configuration. The reactions accommodate a variety of aryl substituents and ring sizes and provide stereochemically diverse spiroketals.