Divergent synthesis of fused N-heterocycles via rhodium-catalysed [4 + 2] cyclization of pyrazolidinones with iodonium ylides

Abstract

A catalytic system-controlled divergent strategy is developed for the precise synthesis of cinnolines and pyrazolo[1,2-a]cinnolines via the rhodium-catalysed [4 + 2] cyclization of readily available pyrazolidinones and iodonium ylides. A range of cinnolines were obtained in useful to good yields by employing HFIP as the solvent, while the combination of DCE and an organic base selectively delivered pyrazolo[1,2-a]cinnoline frameworks. The remarkable features of this reaction include simple operation, broad scope and switchable fused N-heterocycle synthesis. Mechanistic studies indicated a plausible C–H bond activation/intramolecular cyclisation/aromatization cascade pathway.