Conformationally constrained congeners of 6-aryl-5-methyl-4,5-dihydro-3(2H)-pyridazinones active on the cardiovascular system: conformational studies by molecular mechanics calculations and 1H NMR spectroscopy.

Unsubstituted phenylpyridazinones 1a and 2a and their tricyclic analogues indenopyridazinone 3a, benzocinnolinone 4a, and benzocycloheptapyridazinone 5a were submitted to conformational analysis with Allinger's MM2(85) program in order to better define the relationship between the cardiovascular properties of some derivatives and their preferred conformations. Structures 1-4, giving rise to highly active compounds, were found to exist in a conformation showing a near-planar arrangement of the phenyl and the pyridazinone ring. On the contrary, 5, whose derivatives were inactive, shows two significantly populated conformations both markedly deviated from planarity. 1H NMR analysis of the tricyclic systems 3-5 was in full agreement with the molecular mechanics calculations.