Rh(III)-Catalyzed Spiroannulation Reaction of N -Aryl Nitrones with 2-Diazo-1,3-indandiones: Synthesis of Spirocyclic Indole- N -oxides and Their 1,3-Dipolar Cycloaddition with Maleimides.

An efficient strategy for the preparation of spirocyclic indole- N -oxide compounds through a Rh(III)-catalyzed [4 + 1] spiroannulation reaction of N -aryl nitrones with 2-diazo-1,3-indandiones as C1 synthons under extremely mild conditions is presented. From this reaction, 40 spirocyclic indole- N -oxides were easily obtained in up to 98% yield. In addition, the title compounds could be successfully used for the construction of structurally intriguing maleimide-containing fused polycyclic scaffolds via a diastereoselective 1,3-dipolar cycloaddition reaction with maleimides.