Structure-activity relationship of quinolone antibacterial agents: the effects of C-2 substitution

Very little is known about the structure-activity relationship of quinolone antibacterials at the 2-position. Because of the loss of biological activity with 2-methyl and 2-hydroxyl substitution, modifications at C-2 were generally considered to be unfavourable. Quinolone derivatives having a ring between positions 1 and 2 were recently shown to have biological activity. The sulfur-bridged analogs such as the benzothiazolo[3,2-a]quinolone, KB-5246 and NAD-394 have been reported to be highly active in vitro. The authors have synthesized 2-methylthiociprofloxacin, 2-methylofloxacin, the 5-oxopyrrolo[1,2-a]quinoline and isothiazolonaphthyridine to assess the importance of the sulfur atom on biological activity as well as the effect of C-2 substituent on the spatial arrangements of N-1 or the 3-carboxylic group. It was found that the planarity between the 4-keto and 3-carboxylic acid groups of quinoline molecules is the most important criterion for biological activity. The syntheses of the above four compounds are also described.