PAPER www.rsc.org/obc | Organic & Biomolecular Chemistry Synthesis and structure of azole-fused indeno[2,1-c]quinolines and their anti-mycobacterial properties† Ram Shankar Upadhayaya, a Popat D. Shinde, a Aftab Y. Sayyed, a Sandip A. Kadam, a Amit N. Bawane, a Avijit Poddar, b Oleksandr Plashkevych, c Andras F¨ oldesi c and Jyoti Chattopadhyaya* c Received 19th July 2010, Accepted 27th August 2010 DOI: 10.1039/c0ob00445f Prompted by our discovery of a new class of conformationally-locked indeno[2,1-c]quinolines as anti-mycobacterials, compounds 2a and 3a (Fig. 1; MIC < 0.39 mg mL -1 and 0.78 mg mL -1 , respectively) 14 with a freely rotating C2-imidazolo substituent, we herein describe the synthesis of pentacyclic azole-fused quinoline derivatives 4 and 5, in which we have restricted the rotation of the C2-imidazolo moiety by fusing it to the adjacent quinoline-nitrogen to give a five-membered fused azole heterocycle. The idea of locking the flexibility of the system by conformational constraint was simply to reduce its entropy, thereby reducing the overall free-energy of its binding to the target receptor. Out of 22 different azole-fused indeno[2,1-c]quinoline derivatives, seven structurally distinct compounds, 9, 15, 17, 25, 27, 28 and 29, have shown 79–99% growth inhibition of Mycobacterium tuberculosis H37Rv at a fixed dose of 6.25 mg mL -1 . The efficacies of these compounds were evaluated in vitro for 8/9 consecutive days using the BACTEC radiometric assay upon administration of single dose on day one. Of these, two compounds, 9 and 28, inhibited growth of M. tuberculosis very effectively at MIC < 0.39 mg mL -1 (0.89 mM and 1 mM, respectively). These active compounds 9, 15, 17, 25, 27, 28 and 29 were screened for their cytotoxic effect on mammalian cells (human monocytic cell line U937), which showed that the human cell survival is almost unperturbed (100% survival), except for compound 25, hence these new compounds with new scaffolds have been identified as potent anti-mycobacterials, virtually with no toxicity. Thus these “hit” molecules constitute our important “leads” for further optimization by structure–activity relationship against TB. Introduction Many natural and synthetic biologically active compounds are found to be nitrogen containing heterocycles and they constitute an important class of pharmacophores in medicinal chemistry. 1–4 Within this group of heterocycles, quinoline derivatives have been well known in medicinal chemistry as anti-malarials, 5 anti-bacterials, 6 anti-cancer 7 as well as anti-mycobacterials. 8–10 Quinoline-based anti-TB compound TMC207 11 (Fig. 1) bearing a bulky biaryl side chain at position C3, is a highly potent anti-TB agent, has novel mode of action and is currently in phase II clinical trials with very promising activity against MDR-TB. 12 Based on molecular dissection of TMC207, we have recently reported the design, synthesis and biological activity of relatively less complex molecules possessing potent anti-TB activity, 13,14 among which conformationally-locked indeno[2,1-c]quinolines 14 2a and 3a showed effective inhibition of Mycobacterium tuber- culosis H37Rv with MIC 99 < 0.39 mg mL -1 (1 mM) for the former and MIC 99 < 0.78 mg mL -1 (2 mM) for the latter in the a Institute of Molecular Medicine, Pune, 411 057, India b Institute of Molecular Medicine, Calcutta, 700 091, India c Bioorganic Chemistry, Department of Cell and Molecular Biology, Biomed- ical Centre, Uppsala University, SE-75123, Uppsala, Sweden. E-mail: jy- oti@boc.uu.se; Fax: +46-18-554495; Tel: +46-18-4714577 † Electronic supplementary information (ESI) available: Spectral data (1D- NMR, 2D-NMR, LCMS, Mass, HPLC and IR) for all new compounds is included. See DOI: 10.1039/c0ob00445f whole cell assay. We assumed that by covalently restricting the rotation of the C2-imidazolo moiety in 2a and 3a, by fusing to the adjacent quinoline-nitrogen in the form of pentacyclic azole- fused quinoline derivatives 4 and 5, we might be able to reduce the entropy of the system without imposing enthalpy penalty, hence directly contributing to the reduction of the overall free-energy of binding to the target receptor. We also argued that this would give us scope to explore the pharmacological role of the free-rotating C2-imidazolo substituent in compounds 2a and 3a for the anti- tuberculotic activity (Fig. 1). Literature survey revealed that the tetrazolo-, 1,2,4-triazole- and dihydroimidazole-fused quinolines were found to posses important biological activities. Tetrazolo-fused quinolines have anti-inflammatory, anti-bacterial properties 15 and platinum(II) complexes of tetrazolo-quinolones were found to possess anti- tumor properties, 16,17 whereas condensed 1,2,4-traizoles are found to be excellent anti-depressants. 18a–g Synthesis of azole-fused quinolines 19 and isoquinolines 20 is reported in literature and these compounds have been studied for the spectral characteristics like proton-magnetic resonance 19 and photochemical properties, 21 but no conclusive NMR data that unambiguously substantiates the ring-closure to the fused heterocycles has been presented so far. 22–26 Inspired by the interesting biological activities of fused quino- lines and the potent anti-TB activity displayed by our confor- mationally locked indeno[2,1-c] quinolines 2a and 3a, in which the imidazolo group at the C2 position of the quinoline ring is important for biological activity. In order to examine the role of the This journal is © The Royal Society of Chemistry 2010 Org. Biomol. Chem., 2010, 8, 5661–5673 | 5661 Downloaded by Uppsala University on 25 November 2010 Published on 15 December 2010 on http://pubs.rsc.org | doi:10.1039/C0OB00445F View Online