A process for the preparation of Compounds of the formula (1): ##STR00001## wherein R.sup.1, R.sup.2, R.sup.4 and R.sup.13 are as defined or a pharmaceutically or veterinarily acceptable salt or polymorph thereof, or a pharmaceutically or veterinarily acceptable solvate or pro-drug thereof: are potent and selective inhibitors of type 5 cyclic guanosine 3',5'-monophosphate phosphodiestbrase (cGMP PDE5) and have utility in the treatment of, inter alia, male erectile dysfunction (MED) and female sexual dysfunction (FSD).