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.