Original Article

Synthesis and Biological Evaluation of Some New Amide Moiety Bearing Quinoxaline Derivatives as Antimicrobial Agents

Affiliations

Key words ▶ quinoxaline ● ▶ amide ● ▶ antimicrobial activity ●

U. Abu Mohsen1, L. Yurttaş2, U. Acar2, Y. Özkay2, Z. A. Kaplacikli2, H. Karaca Gencer3, Z. Cantürk3 1

Faculty of Pharmacy, Department of Pharmaceutical Chemistry, Al-Azhar University, Gaza, Palestine Faculty of Pharmacy, Department of Pharmaceutical Chemistry, Anadolu University, Eskisehir, Turkey 3 Faculty of Pharmacy, Department of Pharmaceutical Microbiology, Anadolu University, Eskisehir, Turkey 2

Abstract

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In this study, we aimed to synthesize some new quinoxaline derivatives bearing amide moiety and to evaluate their antimicrobial activity. A set of 16 novel compounds of N-[2,3-bis(4-methoxy/ methylphenyl)quinoxalin-6-yl]-substituted benzamide derivatives were synthesized by reacting 2,3-bis(4-methoxyphenyl)-6-aminoquinoxaline or 2,3-bis(4-methylphenyl)-6-aminoquinoxaline with benzoyl chloride derivatives in tetrahy-

Introduction

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received 27.03.2014 accepted 09.05.2014 Bibliography DOI http://dx.doi.org/ 10.1055/s-0034-1377004 Published online: 2014 Drug Res © Georg Thieme Verlag KG Stuttgart · New York ISSN 2194-9379 Correspondence Y. Özkay Faculty of Pharmacy Department of Pharmaceutical Chemistry Anadolu University 26470 Eskisehir Turkey Tel.: + 90/222/3350 580/3779 Fax: + 90/222/3350 750 [email protected]

Resistance of bacteria to antibiotics has become a serious health problem in the treatment of infectious disease. For decades, we were protected by a wide range of antibiotics, but now, increasingly, bacteria can resist the desired effects of these medications. Thus, development of new antibiotics and new diagnostic agents for resistant bacteria become an emergence [1, 2]. There are 2 way for discovering new antimicrobial agents: i) synthesis of new compounds being analogues for already existing drugs ii) discovery of new drugs with unique structure which the pathogen organism has never seen and recognized before [3]. Quinoxaline, also called as benzopyrazine, ring system is found in the fungal metabolite aspergillic acid and in dihydro form in luciferin of several bettles including the fire fly is responsible for the chemiluminescence of this ostracod. Quinoxaline derivatives are important nitrogen containing heterocyclic compounds of various biologically interesting properties with several pharmaceutical applications. It constitutes the building blocks of wide range of pharmacologically active compounds having antibacterial [4–6] and antifungal activity [7, 8]. In addition to quinoxalines, a considerable amount of research has been carried out on the discovery of new antimicrobial agents

drofuran and investigated for their antimicrobial activity. The structures of the obtained final compounds were confirmed by spectral data (IR, 1 H-NMR, 13C-NMR and MS). The antimicrobial activity of the compounds were determined by using the microbroth dilution method. Antimicrobial activity results revealed that synthesized compounds exhibited remarkable activity against Candida krusei (ATCC 6258) and Candida parapsilosis (ATCC 22019).

bearing amide moiety [9–12]. Cyclic amide represents the main scaffold with acetamide moiety as the side chain, which are present in famous antibiotics such as pencillins and cephalosporins for the treatment of systemic infections [13]. The synthesis and biological evaluation of amide derivatives bearing quinoxaline moiety can lead to new therapeutic agents possessing interesting medicinal properties [14]. Hence, in this paper we reported the synthesis and antimicrobial evaluation of novel molecules that bear quinoxaline ring and amide group.

Experimental

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All chemicals were purchased from SigmaAldrich Chemical Co (Sigma-Aldrich Corp., St. Louis, MO). All melting points (m. p.) were determined by Electrothermal 9100 digital melting point apparatus (Electrothermal, Essex, UK) and were uncorrected. All of the reactions were monitored by thin-layer chromatography (TLC) using Silica Gel 60 F254 TLC plates (Merck KGaA, Darmstadt, Germany). Spectroscopic data were recorded with the following instruments: IR, Shimadzu 8400S spectrophotometer (Shimadzu, Tokyo, Japan); 1H-NMR, Bruker DPX 500 NMR spectrometer (Bruker Bioscience, Billerica, MA, USA), 13C-NMR Bruker DPX 125 NMR spectrome-

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Authors

Original Article

Synthesis of 2,3-Bis(4-methoxyphenyl)-6nitroquinoxaline (1a) and 2,3-bis(4-methylphenyl)-6nitroquinoxaline (1b) Equimolar quantities of 4-methyl/4-methoxybenzil and 4-nitro1,2-phenylenediamine were refluxed in acetic acid for 3 h. After cooling precipitated residue was washed with water, filtered, dried and recrystallized from ethanol to obtain title products. 1a: m. p. 192–193 °C, Lit. m. p. 192–194 °C [15]. 1b: m. p. 169– 170 °C, Lit. m. p. 168–169 °C [16].

Synthesis of 2,3-Bis(4-methoxyphenyl)-6aminoquinoxaline (2a) and 2,3-bis(4-methylphenyl)-6aminoquinoxaline (2b) Nitro quinoxaline derivative (1a or 1b) (0.075 mmol) was dissolved in EtOH and Tin (II) chloride (0.375 mmol) was added. The mixture was refluxed for 1 h, allowed to cool down and adjusted to pH:10 by using 10 % NaOH solution. After filtration, the residue was refluxed in ethanol for 1 h and filtrated, warmly. The ethanolic solution was evaporated to gain title compounds [17]. 2a: m. p. 198–199 °C, Lit. m. p. 194–196 °C [18]. 2b: m. p. 185–186 °C.

General methods for synthesis of N-[2,3-bis(4-methoxy/ methylphenyl)quinoxalin-6-yl]-substituted benzamide derivatives (3a–p) Amino quinoxaline derivative (2a or 2b) (0.003 mol), appropriate benzoyl chloride (0.0035 mol) and triethylamine (0.0035 mol) were stirred in tetrahydrofuran (80 mL) at room temperature for 6 h. After TLC control, the solvent was evaporated. The raw product was washed with water, dried and then recrystallized with from ethanol.

N-[2,3-Bis(4-methoxyphenyl)quinoxalin-6-yl]benzamide (3a) Yield 82–84 %, m. p. 170–172 °C. IR (KBr, νmax, cm − 1): 3 287 (amide N-H), 1 674 (amide C = O). 1H NMR (500 MHz, DMSO-d6, ppm): δ 3.71 (6 H, s, OCH3), 6.89–8.05 (14 H, m), 8.14 (1 H, t, J: 8.80 Hz), 8.74 (1 H, s), 10.73 (1 H, s, N-H). 13C NMR (125 MHz, DMSO-d6): δ 55.06, 55.07, 113.46, 116.25, 116.37, 124.25, 127.82, 128.41, 128.48, 128.72, 129.23, 130.29, 130.74, 130.99, 131.07, 131.31, 131.79, 132.77, 134.69, 135.02, 137.33, 140.25, 140.89, 150.93, 152.59, 159.50, 159.64, 162.27, 166.16, 167.32. ES-MS (m/z): M + 1: 462 (36 %). Anal. calcd. For C29H23N3O3: C, 75.47; H, 5.02; N, 9.10. Found: C, 75.08; H, 5.01; N, 9.13.

N-[2,3-Bis(4-methoxyphenyl)quinoxalin-6-yl]-4chlorobenzamide (3b) Yield 78–80 %, m. p. 170–171 °C. IR (KBr, νmax, cm − 1): 3 285 (amide N-H), 1 674 (amide C = O). 1H NMR (500 MHz, DMSO-d6, ppm): δ 3.76 (6 H, s, OCH3), 6.90–6.93 (m, 3 H), 7.43 (2 H, dd, J:3.85, 9.52 Hz), 7.55 (2 H, d, J:8.50 Hz), 7.63 (2 H, d, J:8.45 Hz), 7.94 (2 H, d, J = 8.50 Hz), 8.05 (2 H, d, J:8.20 Hz), 8.12 (1 H, d, J:8.70 Hz), 8.68 (1 H, s), 10.77 (1 H, s, N-H). 13C NMR (125 MHz, DMSO-d6): δ 55.10, 55.12, 113.49, 113.50, 116.37, 124.25, 128.50, 128.67, 128.76, 129.58, 129.77, 130.99, 131.09, 131.27, 133.28, 136.70, 137.36, 137.76, 140.09, 140.79, 151.07, 152.66, AbuMohsen U et al. Some New Quinoxaline Derivatives … Drug Res

159.53, 159.67, 164.99, 166.43. ES-MS (m/z): M + 1: 496.5 (32 %), M + 2: 497.5 (34 %). Anal. calcd. For C29H22ClN3O3: C, 70.23; H, 4.47; N, 8.47. Found: C, 70.68; H, 4.46; N, 8.44.

N-[2,3-Bis(4-methoxyphenyl)quinoxalin-6-yl]-4fluorobenzamide (3c) Yield 80–82 %, m. p. 166–167 °C. IR (KBr, νmax, cm − 1): 3 288 (amide N-H), 1 674 (amide C = O). 1H NMR (500 MHz, DMSO-d6, ppm): δ 3.77 (6 H, s, OCH3), 6.90–6.92 (m, 3 H), 7.26–7.45 (6 H, m), 7.98–8.22 (5 H, m) 8.70 (1 H, s), 10.74 (1 H, s, N-H). 13C NMR (125 MHz, DMSO-d6): δ 55.05, 55.07, 113.45, 113.47, 115.24, 115.41, 115.58, 116.29, 116.34, 124.27, 124.56, 127.29, 127.31, 128.70, 130.56, 130.63, 130.98, 131.07, 131.27, 131.99, 132.07, 133.44, 133.52, 137.33, 140.17, 140.81, 150.98, 152.59, 159.52, 159.66. ES-MS (m/z): M + 1: 480 (29 %). Anal. calcd. For C29H22FN3O3: C, 72.64; H, 4.62; N, 8.76. Found: C, 72.45; H, 4.60; N, 8.77.

N-[2,3-Bis(4-methoxyphenyl)quinoxalin-6-yl]-4methoxybenzamide (3d) Yield 85–87 %, m. p. 115–117 °C. IR (KBr, νmax, cm − 1): 3 287 (amide N-H), 1 668 (amide C = O). 1H NMR (500 MHz, DMSOd6,ppm): δ 3.81 (3 H, s, OCH3), 3.86 (6 H, s, OCH3), 6.92 (1 H, t, J:9.25 Hz), 7.0 (2 H, d, J:8.80 Hz), 7.12 (4 H, d, J:8.90 Hz), 7.44 (1 H, t, J:9.90 Hz), 7.90 (2 H, d, J:8.85 Hz), 8.05 (4 H, d, J:8.85 Hz), 8.71 (1 H, s), 10.61 (1 H, s, N-H). 13C NMR (125 MHz, DMSO-d6): δ 55.11, 55.34, 55.70, 113.48, 113.73, 114.59, 120.15, 122.92, 129.84, 131.08, 131.30, 132.64, 162.13, 164.49, 166.99. ES-MS (m/z): M + 1: 492 (35 %). Anal. calcd. For C30H25N3O5: C, 73.30; H, 5.13; N, 8.55. Found: C, 73.11; H, 5.11; N, 8.53.

N-[2,3-Bis(4-methoxyphenyl)quinoxalin-6-yl]-4(methylthio)benzamide (3e) Yield 84–86 %, m. p. 190–193 °C. IR (KBr, νmax, cm − 1): 3 285 (amide N-H), 1 682 (amide C = O). 1H NMR (500 MHz, DMSO-d6, ppm): δ 2.53 (3 H, s, SCH3), 3.76 (6 H, s, OCH3), 6.75–6.82 (3 H, m), 7.23–7.44 (7 H, m), 7.86–8.15 (4 H, m), 8.71 (1 H, s), 10.64 (1 H, s, N-H). 13C NMR (125 MHz, DMSO-d6): δ 13.80, 13.91, 14.07, 25.08, 30.35, 34.31, 38.96, 55.05, 55.08, 66.98, 113.42, 113.45, 116.21, 123.64, 124.26, 124.75, 124.80, 125.09, 128.32, 128.65, 129.67, 130.43, 130.53, 130.98, 131.07, 131.33, 137.29, 140.29, 140.89, 143.54, 148.19, 150.86, 152.55, 159.49, 159.63, 162.04, 165.44. ES-MS (m/z): M + 1: 508 (31 %). Anal. calcd. For C30H25N3O3S: C, 70.98; H, 4.96; N, 8.28. Found: C, 71.16; H, 4.93; N, 8.23.

N-[2,3-Bis(4-methoxyphenyl)quinoxalin-6-yl]-4cyanobenzamide (3f) Yield 78–80 %, m. p. 130–131 °C. IR (KBr, νmax, cm − 1): 3 286 (amide N-H), 2 238 (cyano C≡N), 1 673 (amide C = O). 1H NMR (500 MHz, DMSO-d6, ppm): δ 3.79 (6 H, s, OCH3), 6.87–6.95 (4 H, m), 7.43–7.48 (4 H, m), 8.05–8.19 (6 H, m), 8.69 (1 H, s), 10.97 (1 H, s, N-H). 13C NMR (125 MHz, DMSO-d6): δ 30.38, 55.16, 113.55, 116.58, 128.69, 131.01, 131.09, 131.25, 132.52, 159.71. ES-MS (m/z): M + 1: 487 (33 %). Anal. calcd. For C30H22N4O3: C, 74.06; H, 4.56; N, 11.52. Found: C, 73.91; H, 4.53; N, 11.56.

N-[2,3-Bis(4-methoxyphenyl)quinoxalin-6-yl]-2,4difluorobenzamide (3g) Yield 75–77 %, m. p. 113–115 °C. IR (KBr, νmax, cm − 1): 3 289 (amide N-H), 1 671 (amide C = O). 1H NMR (500 MHz, DMSO-d6, ppm): δ 3.76 (6 H, s, OCH3), 6.88–7.47 (9 H, m), 7.15–7.84 (4 H,

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ter (Bruker Bioscience, Billerica, MA, USA), in DMSO-d6 using TMS as internal standard; M + 1 peaks were determined by AB Sciex-3200 Q-TRAP LC/MS/MS system (AB Applied Biosystems Co., MA, USA); Elementary analyses were carried on a Leco CHNS-932 analyser (LECO Co., Michigan, USA).

Original Article

N-[2,3-Bis(4-methoxyphenyl)quinoxalin-6-yl]-2,6difluorobenzamide (3h) Yield 79–81 %, m. p. 90–94 °C. IR (KBr, νmax, cm − 1): 3 287 (amide N-H), 1 668 (amide C = O). 1H NMR (500 MHz, DMSO-d6, ppm): δ 3.77 (6 H, s, OCH3), 6.91 (4 H, d, J: 8.50 Hz), 7.16–7.66 (7 H, m), 7.99 (1 H, d, J: 7.55 Hz), 8.10 (1 H, d, J: 9.0 Hz), 8.68 (1 H, s), 11.36 (1 H, s, N-H). 13C NMR (125 MHz, DMSO-d6): δ 55.03, 66.92, 111.94, 112.06, 112.09, 112.22, 112.25, 113.46, 115.11, 115.85, 123.29, 129.29, 131.0, 131.09, 131.16, 131.19, 132.73, 133.81, 132.89, 137.54, 139.32, 140.80, 151.35, 152.88, 158.21, 158.26, 158.77, 159.59, 159.73. ES-MS (m/z): M + 1: 498 (32 %). Anal. calcd. For C29H21F2N3O3: C, 70.01; H, 4.25; N, 8.45. Found: C, 69.96; H, 4.28; N, 8.44.

N-[2,3-Bis(4-methoxyphenyl)quinoxalin-6-yl]-2(trifluoromethyl)-4-fluorobenzamide (3i) Yield 82–84 %, m. p. 162–165 °C. IR (KBr, νmax, cm − 1): 3 286 (amide N-H), 1 673 (amide C = O). 1H NMR (500 MHz, DMSO-d6, ppm): δ 3.85 (6 H, s, OCH3), 6.90–6.92 (4 H, m), 7.42–8.08 (9 H, m), 8.65 (1 H, s), 11.13 (1 H, s, N-H). 13C NMR (125 MHz, DMSOd6): δ 54.99, 113.44, 114.27, 114.31, 114.48, 114.52, 115.97, 119.26, 121.47, 123.65, 128.69, 129.07, 130.97, 131.06, 131.24, 131.45, 132.42, 133.01, 133.08, 137.45, 139.68, 140.82, 151.16, 152.75, 159.56, 159.71, 161.08. ES-MS (m/z): M + 1: 548 (31 %). Anal. calcd. For C30H21F4N3O3: C, 65.81; H, 3.87; N, 7.67. Found: C, 65.29; H, 3.86; N, 7.65.

N-[2,3-Bis(4-methylphenyl)quinoxalin-6-yl]-3,4dimethoxybenzamide (3j) Yield 76–78 %, m. p. 104–105 °C. IR (KBr, νmax, cm − 1): 3 289 (amide N-H), 1 669 (amide C = O). 1H NMR (500 MHz, DMSO-d6, ppm): δ 2.30 (6 H, s, CH3), 3.85–3.88 (6 H, s, OCH3), 7.10–7.15 (5 H, m), 7.33–7.75 (6 H, m), 8.06 (1 H, d, J:7.55 Hz), 8.19 (1 H, d, J:8.30 Hz), 8.71 (1 H, s), 10.59 (1 H, s, N-H). 13C NMR (125 MHz, DMSO-d6): δ 20.77, 45.47, 55.35, 55.56, 55.62, 55.63, 55.85, 66.97, 110.84, 111.11, 111.34, 112.35, 116.18, 120.07, 121.33, 124.67, 124.95, 126.59, 128.58, 129.50, 129.54, 136.14, 136.19, 137.33, 137.95, 138.16, 140.63, 141.02, 148.34, 148.78, 151.21, 151.89, 152.97. ES-MS (m/z): M + 1: 490 (36 %). Anal. calcd. For C31H27N3O3: C, 76.05; H, 5.56; N, 8.58. Found: C, 75.88; H, 5.53; N, 8.60.

N-[2,3-Bis(4-methylphenyl)quinoxalin-6-yl]-3,4dichlorobenzamide (3k) Yield, 81–83 %, m. p. 203–204 °C. IR (KBr, νmax, cm − 1): 3 292 (amide N-H), 1 665 (amide C = O). 1H NMR (500 MHz, DMSO-d6, ppm): δ 2.37 (6 H, s, CH3), 6.98–7.25 (4 H, m), 7.32–8.24 (8 H, m), 8.65 (2 H, s), 11.12 (1 H, s, N-H). 13C NMR (125 MHz, DMSO-d6): δ 20.89, 46.56, 56.38, 57.42, 111.42, 111.89, 113.56, 119.54, 120.25, 122.46, 124.15, 128.23, 129.74, 136.89, 137.29, 137.59, 138.96, 139.47, 141.09, 147.24, 148.54, 151.75, 151.96, 152.97. ES-MS (m/z): M + 1: 498 (32 %), M + 2: 499 (65 %), M + 4 501

(11 %). Anal. calcd. For C29H21Cl2N3O: C, 69.89; H, 4.25; N, 8.43. Found: C, 69.81; H, 4.24; N, 8.42.

N- [2,3-Bis(4-methylphenyl)quinoxalin-6-yl]-4(methylthio)benzamide (3l) Yield 83–85 %, m. p. 175–177 °C. IR (KBr, νmax, cm − 1): 3 288 (amide N-H), 1 674 (amide C = O). 1H NMR (500 MHz, DMSO-d6, ppm): δ 2.38 (6 H, s, CH3), 2.56 (3 H, s, SCH3), 7.27–7.44 (7 H, m), 7.84–8.0 (8 H, m), 11.74 (1 H, s, N-H). 13C NMR (125 MHz, DMSO d6): δ 20.81, 45.23, 114.16, 116.59, 124.17, 128.25, 128.69, 128.71, 129.0, 129.53, 136.89, 137.08, 141.18, 147.54, 148.29, 151.70, 151.42, 152.97. ES-MS (m/z): M + 1: 476 (32 %). Anal. calcd. For C30H25N3OS: C, 75.76; H, 5.30; N, 8.84. Found: C, 75.49; H, 5.31; N, 8.82.

N-[2,3-Bis(4-methylphenyl)quinoxalin-6-yl]-4cyanobenzamide (3m) Yield 78–80 %, m. p. 131–135 °C. IR (KBr, νmax, cm − 1): 3 285 (amide N-H), 2 234 (cyano C≡N), 1 671 (amide C = O). 1H NMR (500 MHz, DMSO-d6) δ 2.28 (6 H, s, CH3), 7.13–7.55 (8 H, m), 7.84–8.20 (6 H, m), 8.72 (1 H, s), 11.04 (1 H, s, N-H). 13C NMR (125 MHz, DMSO-d6) δ 20.82, 45.39, 114.08, 116.59, 128.66, 128.71, 129.0, 129.53, 129.68, 131.97, 132.49, 136.08, 151.15, 152.56. ES-MS (m/z): M + 1: 455 (35 %). Anal. calcd. For C30H22N4O: C, 79.27; H, 4.88; N, 12.33. Found: C, 79.78; H, 4.89; N, 12.35.

N-[2,3-Bis(4-methylphenyl)quinoxalin-6-yl]-2,4difluorobenzamide (3n) Yield 80–82 %, m. p. 161–164 °C. IR (KBr, νmax, cm − 1): 3 289 (amide N-H), 1 669 (amide C = O). 1H NMR (500 MHz, DMSO-d6, ppm): δ 2.31 (6 H, s, CH3), 7.14–7.48 (10 H, m), 7.84–8.67 (4 H, m), 10.94 (1 H, s, N-H). 13C NMR (125 MHz, DMSO-d6): δ 20.73, 104.71, 105.03, 105.23, 105.33, 105.44, 105.74, 111.69, 111.72, 111.86, 111.89, 112.09, 112.12, 112.26, 112.29, 114.65, 115.84, 115.89, 123.94, 128.57, 128.59, 129.13, 129.50, 129.55, 133.82, 133.89, 133.91, 133.97, 133.99, 136.08, 136.11, 137.50, 138.03, 138.23, 140.96, 161.03, 162.99, 163.09, 163.72, 163.81, 164.01, 164.04. ES-MS (m/z): M + 1: 466 (30 %). Anal. calcd. For C29H21F2N3O: C, 74.83; H, 4.55; N, 8.16. Found: C, 74.42; H, 4.52; N, 8.18.

N-[2,3-Bis(4-methylphenyl)quinoxalin-6-yl]-2,6difluorobenzamide (3o) Yield 77–79 %, m. p. 73–76 °C. IR (KBr, νmax, cm − 1): 3 287 (amide N-H), 1 670 (amide C = O). 1H NMR (500 MHz, DMSO-d6, ppm): δ 2.29 (6 H, s, CH3), 7.12–7.67 (9 H, m), 8.02 (2 H, d, J: 8.85 Hz), 8.12 (2 H, d, J: 8.52 Hz), 8.66 (1 H, s), 11.38 (1 H, s, N-H). 13C NMR (125 MHz, DMSO-d6): δ 20.69, 25.03, 45.41, 66.92, 109.93, 112.01, 112.04, 112.17, 112.20, 112.36, 112.39, 112.52, 112.55, 115.86, 115.93, 123.45, 123.53, 128.57, 129.38, 129.49, 129.54, 132.62, 132.70, 132.78, 134.10, 134.18, 134.26, 136.03, 137.65, 138.07, 138.26, 138.26, 139.40, 139.52, 140.97, 151.66, 153.25, 158.23, 158.59, 158.64, 160.24, 160.29, 160.45, 160.67, 162.16. ES-MS (m/z): M + 1: 466 (32 %). Anal. calcd. For C29H21F2N3O: C, 74.83; H, 4.55; N, 8.16. Found: C, 74.70; H, 4.56; N, 8.15.

N-[2,3-Bis(4-methylphenyl)quinoxalin-6-yl]-2(trifluoromethyl)-4-fluorobenzamide (3p) Yield 77–79 %, m. p. 78–80 °C. IR (KBr, νmax, cm − 1): 3 296 (amide N-H), 1 672 (amide C = O). 1H NMR (500 MHz, DMSO-d6, ppm): δ

AbuMohsen U et al. Some New Quinoxaline Derivatives … Drug Res

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m), 8.64 (1 H, s), 10.90 (1 H, s, N-H). 13C NMR (125 MHz, DMSOd6): δ 54.97, 104.43, 104.64, 104.93, 105.14, 105.35, 105.71, 105.92, 106.14, 111.61, 111.78, 111.92, 112.60, 112.76, 113.41, 115.89, 121.23, 123.94, 130.96, 131.04, 131.25, 131.86, 133.87, 133.95, 134.83, 134.92, 137.42, 139.69, 140.85, 151.06, 152.67, 157.69, 159.52, 159.66, 160.92, 161.02. ES-MS (m/z): M + 1: 498 (30 %). Anal. calcd. For C29H21F2N3O3: C, 70.01; H, 4.25; N, 8.45. Found: C, 70.14; H, 4.26; N, 8.41.

Original Article

2.31 (6 H, s, CH3), 7.50–8.11 (13 H, m), 8.72 (1 H, s), 11.15 (1 H, s, N-H). 13C NMR (125 MHz, DMSO-d6): δ 20.65, 20.99, 45.39, 66.88, 114.26, 114.30, 114.46, 114.51, 114.72, 114.93, 114.98, 119.25, 119.42, 119.57, 119.74, 121.43, 123.59, 126.54, 128.55, 129.21, 129.28, 129.47, 129.54, 129.80, 133.04, 133.11, 133.48, 133.55, 136.11, 138.04, 161.66, 162.14, 163.65, 164.15, 164.57, 166.52. ES-MS (m/z): M + 1: 466 (33 %). Anal. calcd. For C20H21F4N3O: C, 69.90; H, 4.11; N, 8.15. Found: C, 69.78; H, 4.09; N, 8.14.

recorded to represent the MIC expressed in μg/mL. For both the antibacterial and antifungal assays the compounds were dissolved in DMSO. Further dilutions of the compounds and standard drugs in test medium were prepared at the required quantities of 800, 400, 200, 100, 50, 25, 12.5, 6.25, 3.13 and 1.63 μg/mL concentrations with Mueller-Hinton broth and Sabouroud dextrose broth [20]. Each experiment in the antimicrobial assays was replicated twice in order to define the MIC values ▶ Table 1. given in ●

Antimicrobial activity

Broth microdilution assay The cultures were obtained from Mueller-Hinton broth (Difco) for the bacterial strains after overnight incubation at 35 ± 1 °C. The yeasts were maintained in Sabouroud dextrose broth (Difco) after overnight incubation 35 ± 1 °C. The inocula of test microorganisms adjusted to match the turbidity of a Mac Farland 0.5 standard tube as determined with a spectrophotometer and the final inoculum size was 0.5–2.5 × 105 cfu/mL for antibacterial and antifungal assays. Testing was carried out in Mueller-Hinton broth and Sabouroud dextrose broth (Difco) at pH 7 and the 2-fold serial dilutions technique was applied. The last well on the microplates containing only inoculated broth was kept as controls and the last well with no growth of microorganism was

Results and Discussions

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Chemistry In the present work, 16 new quinoxaline compounds were ▶ Fig. 1). The reaction of synthesized in 3 reaction steps (● 2,3-bis(4-methoxyphenyl)-6-aminoquinoxaline or 2,3-bis(4methylphenyl)-6-aminoquinoxaline (2a, 2b) with benzoyl chlorides gave N-[2,3-bis(4-methoxy/methylphenyl)quinoxalin6-yl]-substituted benzamide derivatives (3a–3p). In the IR spectra, significant stretching bands due to C = O and N-H groups were observed at 1 665–1 674 cm − 1 and 3 285–3 296 cm − 1, respectively. In the 1H NMR spectra, the signal due to amide (-CONH2) protons appeared at 10.61–11.74 ppm, as singlet. The peaks belonging to aromatic protons were seen at the range of 6.88–8.72 ppm. The mass spectra (ES-MS) of compounds (3a–3p) showed [M + 1] peaks, in agreement with their molecular formula. All compounds gave satisfactory elemental analyses results.

Antimicrobial activity Due to microbiological importance of quinoxaline derivatives, 16 new compounds were tested for their antibacterial and antifungal activities. The antifungal activity of the compounds were found to ▶ Table 1). be higher than their antibacterial activity (● The synthesized compounds (3a–3p) displayed low antibacterial activity against gram positive bacteria. However, compounds 3a and 3d showed higher activity than the other compounds

Table 1 Antimicrobial activity of the compounds 3a–3p (μg/mL). Comp.

R

R’

A

B

C

D

E

F

G

H

I

J

K

L

3a 3b 3c 3d 3e 3f 3g 3h 3i 3j 3k 3l 3m 3n 3o 3p Ref-1 Ref-2

-OCH3 -OCH3 -OCH3 -OCH3 -OCH3 -OCH3 -OCH3 -OCH3 -OCH3 -CH3 -CH3 -CH3 -CH3 -CH3 -CH3 -CH3 – –

-H 4-Cl 4-F 4-OCH3 4-SCH3 4-CN 2,4-DiF 2,6-DiF 2-CF3-4-F 3,4-DiOCH3 3,4-DiCl 4-SCH3 4-CN 2,4-DiF 2,6-DiF 2-CF3-4-F – –

800 400 400 400 400 400 400 800 400 400 800 400 400 400 400 400 3.13 –

100 400 200 200 200 200 200 200 200 400 400 200 400 200 400 200 6.25 –

800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 200 –

400 400 400 100 400 400 400 800 200 200 200 200 200 200 200 200 12.5 –

400 400 400 200 400 400 400 800 200 400 400 200 400 200 400 400 12.5 –

800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 400 200 –

400 400 400 400 400 400 400 800 400 400 400 200 400 200 200 400 50 –

400 400 400 200 200 400 400 200 200 400 400 200 400 100 400 100 1.63 –

400 400 100 100 200 400 200 200 200 400 400 200 200 200 200 200 – 50

200 100 200 200 400 400 400 200 200 400 200 200 400 100 400 200 – 50

200 100 100 100 200 200 100 200 200 400 200 200 100 100 200 50 – 1.63

200 100 100 50 200 400 200 200 200 200 400 100 200 200 200 100 – 200

A: S. aureus (ATCC 25923), B: E. faecalis (ATCC 29212), C: E. faecalis (ATCC 51922), D: Listeria monocytogenes (ATCC-1911), E: K. pneumonia (ATCC 700603), F: P. aeruginosa (ATCC 27853), G: E. coli (ATCC 35218), H: E. coli (ATCC 25922), I: C. albicans (ATCC 90028), J: C. glabrata (ATCC 90030), K: C. krusei (ATCC 6258), L: C. parapsilosis (ATCC 7330). Ref-1: Chloramphenicol, Ref-2: Ketoconazole

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The study was designed to compare MICs obtained by the CLSI reference M7–A7 broth microdilution method [19]. MIC readings were performed twice for each chemical agent. Final products were tested for their in vitro growth inhibitory activity against human pathogenic as Gram-positive bacteria; Staphylococcus aureus (ATCC 25923), Enterococcus faecalis (ATCC 29212) and E. faecalis (ATCC 51922) as Gram-negative bacteria; Pseudomonas aeruginosa (ATCC 27853), Klebsiella pneumoniae (ATCC 700603), Escherichia coli (ATCC 35218), E. coli (ATCC 25922) and yeast as Candida albicans (ATCC 10231), Candida glabrata (ATCC 90030), Candida krusei (ATCC 6258) and Candida parapsilosis (ATCC 7330). Chloramphenicol and ketoconazole were used as control drugs.

Original Article

R

R O

NH2 + NH2

O2N

N

i

O

O2N

Fig. 1 Synthesis of the compounds (3a–p). Reagents: (i) CH3COOH, reflux, 3h; (ii) SnCl2.H2O, EtOH, reflux, 1 h; (iii) Substituted benzoyl chloride, TEA, THF, r.t, 6 h.

N

R

R

R =OCH3 1a CH3 1b R

1a, 1b

N

ii H2N

N R=OCH3 2a CH3 2b

R R N

O iii

N H R'

N

3a-p

R

against E. faecalis (ATCC-29212) and L. monocytogenes (ATCC1911), respectively. Although, antibacterial potency of the compounds (3a–3p) on gram negative bacteria were insignificant, compounds 3n and 3p exhibited the highest activity against E. coli (ATCC 25923). The Candida species were more sensitive to synthesized compounds than bacteria. Compounds 3a, 3e, 3g, 3 h, 3i, 3j, 3m, 3n, and 3o exhibited same efficiency with ketoconazole against C. parapsilosis (ATCC 7330). Furthermore, compounds 3b, 3c, 3l, and 3p were found twice as active as standard drug. The compound 3d exhibited the highest activity with a MIC value of 50 μg/mL, which is 4 fold higher than that of ketoconazole. Compound 3p showed higher activity than other compounds against C. krusei (ATCC6258). Against C. glabrata (ATCC90030), compounds 3b and 3n displayed moderate potency. Compound 3c and 3d showed higher activity than other compounds against C. albicans (ATCC90028). Among all Candida species, the most sensitive Candida strain was established as C. parapsilosis. The structures of the tested compounds based on a quinoxaline ring which contain 4-methyl or 4-methoxyphenyl ring at second and third positions. In antifungal activity evaluation, the compound 3d, which carries 2,3-bis(p-methoxyphenyl)quinoxaline and 4-methoxy benzamide substructures exhibited highest activity against C. parapsilosis. Besides, the compound 3p bearing 2,3-bis(p-methylphenyl)quinoxaline and 4-fluoro-2-trifluoromethyl benzamide substructures was the most active compound against C. krusei.

Conclusion

▼

The synthesis and preliminary in vitro antibacterial and antifungal screening results of novel quinoxaline derivatives were reported in the present study. Antibacterial potency of the compounds were not significant. On the other hand, some of the synthesized compounds showed good activity against Candida

species. Consequently, findings of this study may have an impact on medicinal chemists to achieve more effective antifungal compounds.

Declaration of Interest

▼

The authors report no conflicts of interest.

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