Novel ofloxacin derivatives: Synthesis, antimycobacterial and toxicological evaluation Murugesan Dinakaran, a Palaniappan Senthilkumar, a Perumal Yogeeswari, a Arnab China, b Valakunja Nagaraja b and Dharmarajan Sriram a, * a Medicinal Chemistry Research Laboratory, Pharmacy group, Birla Institute of Technology and Science, Pilani 333031, India b Department of Microbiology and Cell Biology, Indian Institute of Science, Bangalore, Karnataka 560012, India Received 8 October 2007; revised 23 November 2007; accepted 28 November 2007 Available online 4 December 2007 Abstract—Thirty novel 9-fluoro-2,3-dihydro-8,10-(mono/di-sub)-3-methyl-8-nitro-7-oxo-7H-[1,4]oxazino[2,3,4-ij]quinoline-6-car- boxylic acids were synthesized from 2,3,4,5-tetrafluoro benzoic acid and evaluated for in vitro and in vivo antimycobacterial activ- ities against Mycobacterium tuberculosis H37Rv (MTB), multi-drug resistant Mycobacterium tuberculosis (MDR-TB), and Mycobacterium smegmatis (MC 2 ) and also tested for the ability to inhibit the supercoiling activity of DNA gyrase from mycobac- teria. Among the synthesized compounds, 10-[2-carboxy-5,6-dihydroimidazo[1,2-a]pyrazin-7(8H)-yl]-9-fluoro-2,3-dihydro-3- methyl-8-nitro-7-oxo-7H-[1,4]oxazino[2,3,4-ij]quinoline-6-carboxylic acid was found to be the most active compound in vitro with MIC99 of 0.19 lM and 0.09 lM against MTB and MTR-TB, respectively. In the in vivo animal model also the same compound decreased the bacterial load in lung and spleen tissues with 1.91 and 2.91 À log 10 protections, respectively, at the dose of 50 mg/ kg body weight. Compound 10-[(4-((4-chlorophenyl)(phenyl)methyl)piperazin-1-yl)]-9-fluoro-2,3-dihydro-3-methyl-8-nitro-7-oxo- 7H-[1,4]oxazino[2,3,4-ij]quinoline-6-carboxylic acid was found to be the most active in the inhibition of the supercoiling activity of DNA gyrase with an IC 50 of 10.0 lg/mL. The results demonstrate the potential and importance of developing new oxazino quin- olone derivatives against mycobacterial infections. Ó 2007 Elsevier Ltd. All rights reserved. Fluoroquinolones exhibit potent in vitro and in vivo antimycobacterial activity. 1 There is a significant effort to include fluoroquinolones as new front-line agents (ofloxacin and moxifloxacin) and second-line agents (ciprofloxacin) to combat tuberculosis (TB). 2 Experts from the World Health Organization favor a third-line regimen containing four drugs (an aminoglycoside, ethi- onamide, pyrazinamide, and ofloxacin) during the initial phase and two drugs (ethionamide and ofloxacin) during the continuation phase. 3 Quinolones inhibit bacterial type II topoisomerase, DNA gyrase, and topoisomerase IV, 4 which are essential enzymes that maintain the supercoiling of the DNA by carrying out two opposing reactions. The incidence of mycobacterial resistance to fluoroquinolones is relatively low at the present time, and there are no reports of cross-resistance or antago- nism with other classes of antimycobacterial agents. 5 There is also a considerable effort to discover and devel- op newer fluoroquinolones, and some of them might have value in the treatment of TB. 6,7 As a part of the study attempting to further optimize the quinolones against Mycobacterium tuberculosis (MTB) 8,9 and multi-drug resistant M. tuberculosis (MDR-TB) as MTB is getting resistance to many quinolones, 10–12 here- with we report the synthesis, antimycobacterial and toxicological evaluation of novel 9-fluoro-2,3-dihydro- 8,10-(mono/di-sub)-3-methyl-8-nitro-7-oxo-7H-[1,4]oxaz- ino[2,3,4-ij]quinoline-6-carboxylic acids. Thirty new oxazinoquinolone carboxylic acids with var- iation at C 9 and C 10 positions were synthesized from 2,3,4,5-tetrafluoro benzoic acid (1) according to the lit- erature method 13 with modification in some steps (Scheme 1). Briefly, compound 1 on reaction with 1, 1 0 -carbonyldiimidazole in tetrahydrofuran afforded the corresponding imidazolide, which, in situ, was treated with neutral magnesium salt of ethyl potassium malon- ate in the presence of tri-ethyl amine to yield 82% of ethyl 3-(2,3,4,5-tetrafluorophenyl)-3-oxopropanoate (2). Ethyl 2,3,4,5-tetrafluoro-a-[[(2-hydroxy-1-methyl- 0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.bmcl.2007.11.110 Keywords: Oxazino[2,3,4-ij]quinoline-6-carboxylic acids; Antimyco- bacterial activity; Tuberculosis. * Corresponding author. Tel.: +91 1596 244684; fax: +91 1596 244183; e-mail: dsriram@bits-pilani.ac.in Available online at www.sciencedirect.com Bioorganic & Medicinal Chemistry Letters 18 (2008) 1229–1236