Calcium-Catalyzed Asymmetric Synthesis of 3-Tetrasubstituted Oxindoles: Efficient Construction of Adjacent Quaternary and Tertiary Chiral Centers

Chiral Ca-catalyzed asymmetric addition reactions of 3-substituted oxindoles with N-Boc-imines afford 3-tetrasubstituted oxindole derivatives bearing adjacent quaternary and tertiary chiral centers, which are key structures for biological activities. Ubiquitous and nontoxic Ca catalysts (1-10 mol %) work well in this reaction, and high yields (up to >99%) and selectivities (up to >99% ee) of the products with wide substrate scope have been attained. The structures of the chiral Ca catalysts and intermediary Ca enolates are also discussed.