Iridium(III)-catalysed annulation of pyrazolidinones with propiolates: a facile route to pyrazolo[1,2-a] indazoles

A facile synthesis of various pyrazolo[1,2-a] indazoles from pyrazolidinones and propiolates via Iridium(III)-catalysed C-H bond activation/subsequent [4+1] cyclization has been developed. The reaction proceeds smoothly under the mild reaction conditions and propiolates act as a novel C1 synthon. This transformation represents a redox-neutral process and exhibits a highly regioselectivity, tolerates various functional groups.