Piperacetildenafil was prepared as follows: piperidine (0.22 ml, 0.0022 mol) was added to a stirred 5-(5-bromoacetyl-2-ethoxyphenyl)-1-methyl-3-n-propyl-1,6-dihydro-7H-pyrazolo[4,3 -d]pyrimidin-7- one (Preparation 8, 0.95 g, 0.0022 mol) and anhydrous potassium carbonate (0.6 g , 0.0044 mol) in acetonitrile (50 ml). After 18 h, the mixture was evaporated under vacuum, the residue was dissolved in water (50 ml) and the solution was extracted with ethyl acetate (3 x 30 ml). The organic extracts were combined, washed with brine (3 x 20 ml), dried (Na2SO4 ) and vacuum evaporated. The resulting yellow solid was chromatographically separated on silica gel (12 g) using an elution gradient of methanol in dichloromethane (0-2% methanol) to give an off-white solid. Crystallization with ethyl acetate-hexane gave the title compound piperacetildenafil as an off-white powder (0.27 g, 28%), m.p.
