Sildenafil [1] (1, VIAGRA®, Scheme 1) is a potent inhibitor of phosphodiesterase type 5 (PDE 5) [2] whose effectiveness in treating male erectile dysfunction was recently discovered. PDE5 is the primary cGMP hydrol- yzing enzyme present in the corpus cavernosal smooth muscle of the penis. Upon sexual stimulation, nitric oxyde (NO) is released and activates guanylyl cyclase which pro- duces cGMP [3]. The latter initiates a protein phosphoryla- tion cascade that ...