Ring-Contraction Disproportionation/Spirocyclization Cascade Reaction of Isochromeno4,3-bindol-5(11H)-ones: Synthesis of N-Unsubstituted Spirocycles

An efficient ring-contraction reaction of isochromeno4,3-bindol-5(11H)-ones via a nucleophile-induced disproportionation/spirocyclization cascade process has been developed under mild conditions. The process realized the conversion of isochromeno4,3-bindol-5(11H)-ones into N-unsubstituted spiroindoline-2,1'-isobenzofuran-3,3'-diones and spiroindoline-2,1'-isoindoline-3,3'-diones in the absence of a transition-metal catalyst or oxidant. Gram-scale reaction further demonstrated the practicability of the protocol.