We synthesized various 4-benzylamino-1-chloro-6-substituted phthalazines (15) and 4- benzylamino-1-chloro-7-substituted phthalazines (16) and evaluated their inhibitory activity toward phosphodiesterase 5 (PDE5) purified from porcine platelets. The PDE5-inhibitory activities of 15 were greater than those of the isomers (16). The preferred substituent at the 4- position of phthalazine was a (3-chloro-4-methoxybenzyl) amino group, and those at the 6 ...
