The development of an environmentally benign synthesis of sildenafil citrate (Viagra™) and its assessment by Green Chemistry metrics

The process development and scale-up of the sildenafil citrate process is reviewed. Key environmental metrics are reported at several time points during the development process. Significant achievements during the development of sildenafil citrate were (i) discovering a convergent, efficient synthetic route (ii) designing 7 process steps so that there was no extractive work-up in any step (iii) implementing efficient solvent recovery early in the product's commercial lifetime. The result of this work is. that the E-factor for the process is very low with just 6 Kg of waste per kilogram of product compared with an industry average of 25-100 Kg. Pfizer received the 2003 UK Award for Green Chemical Technology (Best Process category), for the sildenafil citrate process.