A series of novel pyridone carboxylic acids having a 4-hydroxypiperazinyl group at the 7-position of norfloxacin and ciprofloxacin were prepared. The in vivo antibacterial efficacies of these compounds were superior to those of corresponding piperazinyl derivatives. From the results of the studies on the pharmacokinetic profile and toxicity, the 4-hydroxypiperazinyl derivatives were confirmed to be pharmacologically superior to corresponding piperazinyl derivatives. Thus, a 4-hydroxypiperazinyl group was revealed to be a beneficial substituent for potential use in future quinolone antibacterials.

1‐Hydroxypiperazine dihydrochloride 7 was prepared and it was applied to the syntheses of new pyridone carboxylic acid antibacterial agents (PCA‐antibacterial agents). 1‐Cyclopropyl‐6,8‐difluoro‐1,4‐dihydro‐7‐(4‐hydroxypiperazin‐1‐yl)‐4‐oxoquinoline‐3‐carboxylic acid 13 showed the most potent antibacterial activity.

A series of 6-fluoro- and 6,8-difluoro-7-(azole substituted)-1,4-dihydro-4-oxo-3-quinolinecarboxylic acids were prepared. Structure-activity relationship studies indicated that the antibacterial potency was better when the 6,8-substituents were fluorine atoms and the 7-substituent was either 1-imidazolyl, 20, or 4-methyl-1-imidazolyl, 25. From the results of studies on pharmacokinetic profile and toxicity, 20 and 25 were found to possess excellent antibacterial activities and to show high blood levels after oral administration to mice with low toxicity.

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