Studies on Cardiotonic Agents. VI. Synthesis of Novel 4, 5-Dihydro-3(2H)-pyridazinone Derivatives Carrying Some Benzoheterocycles at the 6-Position

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Several benzothiazolyl, imidazobenzothiazolyl, benzothienyl, benzothienopyrimidinyl and quinazolinyl 4, 5-dihydro-3(2H)-pyridazinones were synthesized and examined for cardiotonic activity in anesthetized dogs after i.v. administration. Among them, 4-methylamino-7-(2, 3, 4, 5-tetrahydro-5-methyl-3-oxo-6-pyridazinyl)quinazoline (36) showed potent and long-lasting inotropic activity (relative potency=2.11, milrinone=1). The activity of 36 was more potent than indolidan (2) (relative potency=1.53) which is one of the most potent inotropic agents to date.