Rh(III)-Catalyzed Switchable [4 + 1] and [4 + 2] Annulation of <i>N</i>-Aryl Pyrazolones with Maleimides: An Access to Spiro Pyrazolo[1,2-<i>a</i>]indazole-pyrrolidine and Fused Pyrazolopyrrolo Cinnolines
Chih‐Yu Lin, Wan‐Wen Huang, Ying‐Ti Huang, Sandip Dhole, Indrajeet J. Barve, Chung‐Ming Sun
Abstract
A rhodium(III)-catalyzed controllable [4 + 1] and [4 + 2] annulation of N -aryl pyrazolones with maleimides as C1 and C2 synthon has been explored for the synthesis of spiro[pyrazolo[1,2- a ]indazole-pyrrolidines] and fused pyrazolopyrrolo cinnolines. The product selectivity was achieved through time-dependent annulation. The [4 + 1] annulation reaction involves sequential Rh(III)-catalyzed C–H alkenylation of N -aryl pyrazolone, followed by an intramolecular spirocyclization via aza-Michael-type addition to afford spiro[pyrazolo[1,2- a ]indazole-pyrrolidine]. However, prolonged reaction time converts in situ formed spiro[pyrazolo[1,2- a ]indazole-pyrrolidine] into fused pyrazolopyrrolocinnoline. This unique product formation switch proceeds via strain-driven ring expansion through a 1,2-shift of the C–C bond.