Cascade Ring-Opening/Cyclization Reaction of Spiro(nitrocyclopropane)oxindoles with Huisgen Zwitterions and Synthesis of Pyrazolo[3,4- b ]indoles.

Herein, we report a ring-opening/cyclization cascade reaction of spiro(nitrocyclopropane)oxindoles with in situ generated Huisgen zwitterions (HZs) from PPh 3 and azodicarboxylates. This reaction provides an array of polyfunctionalized pyrazolo[3,4- b ]indole derivatives in moderate-to-excellent yields and generally high stereoselectivities with a broad substrate scope. The annulation products obtained from di- tert -butyl azodicarboxylates can be readily transformed into aromatic-substituted pyrazolo[3,4- b ]indoles in moderate yields upon treatment with trifluoroacetic acid, thus providing a new entry to this fused heterocycle skeleton. In terms of nitro-substituted donor-acceptor cyclopropane, this work significantly broadens the substrate scope for the annulation reaction of nitrocyclopropanes and HZs. The dual roles of the oxindole moiety in the ring opening of cyclopropane and a plausible mechanism for the cascade reaction are also discussed.