Several 6-phenyl-3(2H)-pyridazinones bearing different alkenyl groups at position 5 have been prepared in the search for novel antiplatelet agents. The target compounds were synthesised by a palladium-catalysed Heck cross-coupling reaction. Variable amounts of 4-phenyl-6-substituted-2-phthalazinones were isolated as by-products during these experiments. The crucial issues for successful Heck coupling in these systems concern the protection of position 2 of the heterocyclic ring and the use of tris(o-tolyl)phosphine as a ligand. (C) 2004 Elsevier Ltd. All rights reserved.