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 mild reaction conditions and propiolates act as a novel C1 synthon. This transformation represents a redox-neutral process, exhibits a highly regioselectivity, and tolerates various functional groups.