The 5-[2-ethoxy-5-(4-methylpiperazin-1-ylsulfonyl) phenyl]-1-methyl-3-propyl-1, 6-dihydro-7 H-pyrazolo [4, 3-d] pyrimidin-7-one, sildenafil, is a cGMP-specific phosphodiesterase-5 (PDE5) inhibitor used for penile erectile dysfunction. In the search for more potent and selective PDE5 inhibitors, new sildenafil analogues (6a− v), characterized by the presence on the sulfonyl group in the 5′ position of novel N-4-substituted piperazines or ...