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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.

Over the past years it was found that modification of the 3-carboxylic acid group of quinolones generally produced compounds with a substantial decrease in antibacterial activity. The 3-carboxylic acid moiety together with the 4-carbonyl function are believed to be the most structurally critical sites for this class of compounds to DNA gyrase. The authors have designed and synthesized a series of quinolone analogues in which the 3-carboxylic acid group has been modified. These compounds, 2,3,4,9-tetrahydroisothiazolo[5,4-b]quinoline-3,4-diones, possess biological activities far superior to their parent counterparts. For example, the MICs (microgram/ml) for A-62824 (ciprofloxacin 3-carboxylic acid modified analogue) and ciprofloxacin against some organisms are as follows: S. aureus ATCC 6538P (0.02, 0.2); S. epidermidis 3519 (0.05, 0.2); E. coli Juhl (0.005, 0.01); P. aeruginosa A5007 (0.05, 0.1) and Acinetobacter sp. CMX 699 (0.05, 0.78). This investigation has produced the first successful modification of the 3-carboxylic acid group of quinolones resulting in a series of extremely potent antibacterials. The design and synthesis as well as the biological activities of these new derivatives are described.