Solvent-Dependent Divergent Functions of Sc(OTf)_3 in Stereoselective Epoxide-Opening Spiroketalizations

摘要

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 CH_2Cl_2, Br?nsted 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.