Sildenafil chlorosulfonyl is a synthetic intermediate in the synthesis of the phosphodiesterase 5 (PDE5) inhibitor sildenafil .1It is also a potential impurity found in commercial preparations of sildenafil. 1.Nagaraju, V., Sreenath, D., Rao, J.T., et al.Separation and determination of synthetic impurities of sildenafil (Viagra) by reversed-phase high-performance liquid chromatographyAnal. Sci.19(7)1007-1011(2003)