Synthesis and antibacterial activity of a new series of tetracyclic pyridone carboxylic acids.

A novel tetracyclic pyridone carboxylic acid with a thiazolidine ring, 1,2-dihydro-9,1-(epoxymethano)-7-fluoro-8-(4-methyl-1-piperazinyl)-5-oxo -5H- thiazolo[3,2-a]quinoline-4-carboxylic acid (4a), and variants with a nitrogen atom (4b) or carbonyl group (4c) in the place of the 10-position oxygen atom of 4a were prepared and tested for antibacterial activity and inhibitory activity on DNA gyrase from Escherichia coli KL-16. The in vitro antibacterial potency with regard to the 10-position atom was found to be of the following order; O > NCH3 = C = O. The IC50 values for DNA gyrase inhibition activity for the 4a, 4b, and 4c compounds were 0.33, 0.53, and 0.67 g/mL, respectively. The activity of 4a, in which the C-3 methyl group and C-5 of ofloxacin (2a) were connected with a sulfur atom to restrict the conformation of 2a, was more potent than that of 2a against both Gram-positive and -negative bacteria, except for Pseudomonas aeruginosa. Compared to the tetracyclic pyridone carboxylic acid 1a, which has a flat thiazole ring, compound 4a showed comparable or slightly more potent activity against both Gram-positive and -negative bacteria, except for P. aeruginosa.