An Alternative Approach to Synthesize Sildenafil via Improved Copper-Catalyzed Cyclization

A facile synthetic pathway for sildenafil has been developed. This approach is characterized by a ligand-free Ullmann-type copper-catalyzed coupling reaction to construct sildenafil and its derivative, pyrrazolo[4,3-d]pyrimidin-7-one ring, with yields of 79% and 82%, respectively, in a convergent fashion by connecting key building blocks halo-pyrazole moiety 16c with 2-ethoxybenzamidine and 2-ethoxy-5-[(4-methylpiperazin-1-yl)sulfonyl]benzamidine in a one-pot reaction. Thus, this approach circumvents the need to use nitric/sulfuric acid for nitration, a costly Pd-catalyst for reduction, and coupling agents encountered in the reported processes.