Ring-Contraction Disproportionation/Spirocyclization Cascade Reaction of Isochromeno[4,3-b]indol-5(11H)-ones: Synthesis of N-Unsubstituted Spirocycles.

An efficient ring-contraction reaction of isochromeno[4,3-b]indol-5(11H)-ones via a nucleophile-induced disproportionation/spirocyclization cascade process has been developed under mild conditions. The process realized the conversion of isochromeno[4,3-b]indol-5(11H)-ones into N-unsubstituted spiro[indoline-2,1'-isobenzofuran]-3,3'-diones and spiro[indoline-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.