Synthesis of Sildenafil by the O-alkylation of 5-[2-hydroxy-5-(4-methylpiperazin-1- ylsulphonyl) phenyl]-1-methyl-3-n-propyl-1, 6-dihydro-7H-pyrazolo-[4, 3-d] pyrimidin-7-one 8 proved difficult because concomitant alkylation of the pyrimidine moity also occurred. The selective O-alkylation was carried by DCC assisted decarboxilative etherification of the carbonate 9.
