Synthesis of 2-Phenylthiazolidine Derivatives as Cardiotonic Agents. II. 2- (Phenylpiperazinoalkoxyphenyl) thiazolidine-3-thiocarboxamides and the Corresponding Carboxamides

A large number of 2- (phenylpiperazinoalkoxyphenyl) thiazolidine-3-thiocarboxamides and the corresponding carboxamides (II) were synthesized and tested for inotropic activity in anesthetized dogs. Compounds II were prepared from a hydroxybenzaldehyde (III) through the intermediates (IV, V, and X). Structure-activity relationships (SAR) were investigated by varying the structural parameters. Transposition of the piperazinoalkoxyl group to the meta or para position from the ortho position caused a marked fall in activity. Conversion of the thiocarboxamido to a carboxamido group caused a marked increase in activity. This tendency was generally observed in this series of compounds and constitutes a major deviation from the SAR in the simple 2-phenylthiazolidine series. With regard to effects of the length of the aminoalkoxy chain, the ethoxy derivatives were generally more potent than higher analogues. Lengthening of the N-alkyl group in the (thio) carboxamido group generally caused a decrease in activity. Among the various derivatives synthesized, II15 was found to be approximately one hundred times more potent than amrinone with a long duration of action.