We report on the synthesis and the antimicrobial activity of 9-cyclopropyl-4-fluoro-6oxo[1,2,5]thiadiazolo[3,4-h]quinoline-5-carboxylic acid (16), representative of a new class of antibacterial agents structurally related to the clinically successful fluoroquinolones. The novel 6-fluoroquinolone (16) is herein prepared, in good yield, by thermal cyclocondensation of ethyl 7,8-diamino-9-cyclopropyl-4-fluoro-6-oxoquinoxaline-7-carboxylate with thionyl chloride, followed by acid-catalysed

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... s of the ester intermediate (15) . Their structures were characterized by IR, MS, 1 H-and 13 C NMR spectra and X-ray crystal structure of 16 is presented. The antimicrobial evaluation against a broad panel of wild and resistant Grampositive and Gram-negative bacteria and fungal species demonstrated the high antibacterial activity of 16, even at the concentration of 0.015 µg mL -1 . Chemistry Condensed 1,2,5-thiadiazoles are readily prepared via direct interaction of the appropriate orthodiaminoarene/heteroarene with thionyl chloride. 22,23,36 Following this versatile and general route, ethyl 6-oxothiadiazolo[3,4-h]quinoline-7-carboxylate (15) is herein prepared by thermal cyclocondensation of ethyl 7,8-diamino-9-cyclopropyl-4-fluoro-6-oxoquinoxaline-7-carboxylate (14) 37 with thionyl chloride (Scheme 1). Compound 14 was obtained by reduction of ethyl 7-