Catalytic Asymmetric Synthesis of Cyclopentene-spirooxindoles Bearing Vinylsilanes Capable of Further Transformations

We report a scandium-catalyzed [3 + 2] annulation of alkylideneoxindoles with allenylsilanes for the enantioselective formation of cyclopentene-spirooxindoles containing vinylsilanes. Using a Sc(OTf)₂/PyBOX/BArF complex, the spiroannulation of allenylsilanes affords products with >94:6 dr and >90:10 er. The effect of the counterion and ligand to control selectivity is discussed. The transformation of the vinylsilane is demonstrated using cross-coupling, epoxidation, and Tamao-Fleming oxidation reactions. A series of competition experiments provide a comparison of nucleophilicity between allyl- and allenylsilanes.