Rh(III)-Catalyzed (4 + 1) Annulation of Pyrazol-3-ones with Alkynoates via Ortho-Alkenylation/Cyclization Cascade: Synthesis of Indazole-Fused Pyrazoles.

A facile synthesis of novel indazole-fused pyrazoles from pyrazol-3-ones and alkynoate esters/amides via Rh(III)-catalyzed sequential C-H activation/ortho-alkenylation/intramolecular cyclization cascade is reported. The important characteristic of this method is that the resulting scaffold bearing quaternary carbon has been obtained through unusual [4 + 1] rather than expected [4 + 2] addition where alkynoate acts as a one-carbon unit.