Table of Contents  
ORIGINAL ARTICLE
Year : 2012  |  Volume : 11  |  Issue : 2  |  Page : 80-92

Synthesis, antioxidant, and antimicrobial activities of new 2-(1,5,6-trimethyl-1H-benzo[d]imidazole-2-carbonyl)-2,3-dihydro-1H-pyrazole-4-carbonitriles, (1,3,4-oxadiazol-2-yl)-1H-benzo[d]imidazol-5-yl)(phenyl)methanones, and (1,3,4-oxadiazol-2-yl)-1,5-dihydro-[1,2,4]triazolo[1,5-a]pyridine-8-carbonitriles ( QSAR and molecular docking analysis)


1 Department of Chemistry of Natural and Microbial Products; Pharmaceutical Research Group, Center of Excellence for Advanced Sciences, Cairo, Egypt
2 Department of Chemistry of Natural and Microbial Products, Centre of Excellence for Advanced Sciences, Cairo, Egypt
3 Pharmaceutical Research Group; Department of Chemistry of Medicinal Plants, Centre of Excellence for Advanced Sciences, Cairo, Egypt
4 Biomedical Informatics and Chemo Informatics Group, Department of Informatics and Systems, Centre of Excellence for Advanced Sciences, Cairo, Egypt
5 Department of Microbial Chemistry, National Research Centre, Dokki, Cairo, Egypt
6 Department of Analytical Chemistry, Stockholm University, Stockholm, Sweden

Date of Submission19-Feb-2012
Date of Acceptance06-Jun-2012
Date of Web Publication18-Jul-2014

Correspondence Address:
Fatma A. Bassyouni
PhD, Department Chemistry of Natural and Microbial Products and Pharmaceutical Research Group, Center of Excellence for Advanced Sciences, National Research Centre, 12311 Cairo
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.7123/01.EPJ.0000422113.69898.e0

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  Abstract 

Objectives

A new series of 2-(1,5,6-trimethyl-1H-benzo[d]imidazole-2-carbonyl)-2,3-dihydro-1H-pyrazole-4-carbonitrile (6a,b), (1,3,4-oxadiazol-2-yl)-1H-benzo[d]imidazol-5-yl)(phenyl) methanone (911), and (1,3,4-oxadiazol-2-yl)-1,5-dihydro-[1,2,4]triazolo[1,5-a]pyridine-8-carbonitrile (1416) derivatives were synthesized and evaluated for their antioxidant and antimicrobial activities; in addition, their quantitative structure–activity relationships and molecular docking were investigated.

Methods

The target compounds 6a,b were synthesized by the following method: reaction of 5,6-dimethyl-1H-benzoimidazole-2-carbohydrazide (2) with 4-(dimethyl amino)benzaldehyde or anthracene-9-carbaldehyde yielded Schiff’s bases 3a,b, which were reacted with ethyl cyanoacetate to yield 1H-pyrazole-4-carbonitriles 4a,b; N-methylation of 4a,b afforded 5a,b, which reacted with 4-aminoantipyrine to give 6a,b. In addition, 5-benzoyl-1H-benzo[d]imidazole-2-carbohydrazide (8) or 8-cyano-6-isocyano-5-oxo-7-phenyl-1,5-dihydro-[1,2,4]triazolo[1,5-a]pyridine-2-carbohydrazide (13) reacted with different carboxylic acids such as crotonic acid, 3,4-diaminobenzoic acid, and 6-hydroxy-4-methoxybenzofuran-5-carboxylic acid to form compounds 911 and 1416, respectively. The synthesized compounds were evaluated for their antioxidant activity using 2,2-diphenyl-1-picrylhydrazyl radical scavenging assay, and the diffusion plate method for antimicrobial activity.

Results and conclusion

Among other tested compounds, compounds 15, 11, and 10 possessed the highest antioxidant activity, whereas compounds 4a, 5b, 6b, 10, and 11 displayed high activity against Staphylococcus aureus, Salmonella typhimurium, and Candida albicans. The quantitative structure–activity relationships of the studied compounds 4a, 4b, 5b, 6b, 10, 11, 14, 15, and 16 indicated a high correlation (r 2=0.82) between the predicted and actual activities as obtained from molecular descriptors and the inhibitory activity of this set of tested molecules measured as antioxidant activity. Moreover, the three-dimensional (3D) pharmacophore was generated, and docking of the most active antibacterial compound 4a against the dihydropteroate synthase enzyme gave comparable scores for hydrogen bond interaction (−13.5 kcal/mol) and binding mode to the reference antibiotic sulfamethoxazole (−13.00 kcal/mol).

Keywords: antibacterial, antioxidant, benzimidazoles, molecular docking, quantitative structure–activity relationships, triazoles


How to cite this article:
Bassyouni FA, Tawfik HA, Hamed AR, Soltan MM, ElHefnawi M, ElRashedy AA, Moharam ME, Rehim MA. Synthesis, antioxidant, and antimicrobial activities of new 2-(1,5,6-trimethyl-1H-benzo[d]imidazole-2-carbonyl)-2,3-dihydro-1H-pyrazole-4-carbonitriles, (1,3,4-oxadiazol-2-yl)-1H-benzo[d]imidazol-5-yl)(phenyl)methanones, and (1,3,4-oxadiazol-2-yl)-1,5-dihydro-[1,2,4]triazolo[1,5-a]pyridine-8-carbonitriles ( QSAR and molecular docking analysis). Egypt Pharmaceut J 2012;11:80-92

How to cite this URL:
Bassyouni FA, Tawfik HA, Hamed AR, Soltan MM, ElHefnawi M, ElRashedy AA, Moharam ME, Rehim MA. Synthesis, antioxidant, and antimicrobial activities of new 2-(1,5,6-trimethyl-1H-benzo[d]imidazole-2-carbonyl)-2,3-dihydro-1H-pyrazole-4-carbonitriles, (1,3,4-oxadiazol-2-yl)-1H-benzo[d]imidazol-5-yl)(phenyl)methanones, and (1,3,4-oxadiazol-2-yl)-1,5-dihydro-[1,2,4]triazolo[1,5-a]pyridine-8-carbonitriles ( QSAR and molecular docking analysis). Egypt Pharmaceut J [serial online] 2012 [cited 2020 Aug 5];11:80-92. Available from: http://www.epj.eg.net/text.asp?2012/11/2/80/136955


  Introduction Top


Benzimidazoles have a wide range of biological activities such as anticancerous and fungicidal activities; they also serve as antioxidants and central nervous system depressants 1–6. A large number of benzotriazole derivatives are currently used in clinical applications 7–9. In addition, they are of great value as intermediates and final products in organic synthesis 10. Fused heterocyclic compounds containing 1, 2, 4-triazoles have biological potency such as central nervous system depressant 11, antifungal 12, antiviral, and antibacterial activities 13. Therefore, triazole derivatives have consistently attracted scientific and practical interest because of their widely varying chemical properties, synthetic versatility, and pharmacological activities 14,15.

Herein, we aim to prepare a new series of 2-(1, 5, 6-trimethyl-1H-benzo[d]imidazole-2-carbonyl)-2,3-dihydro-1H-pyrazole-4-carbonitrile, (1, 3, 4-oxadiazol-2-yl-1H-benzo[d]imidazol-5-yl)(phenyl) methanone, and (1, 3, 4-oxadiazol-2-yl)-1,5-dihydro- [1, 2, 4]triazolo [1,5-a] pyridine-8-carbonitrile derivatives and evaluate them for their antioxidant and antibacterial activities. In addition, the quantitative structure–activity relationships (QSAR) of this series will be investigated.


  Experimental Top


Chemistry

All melting points were determined using the Electro thermal capillary (Stuart, SMP10, UK) melting point apparatus and were uncorrected. 1H nuclear magnetic resonance (NMR) and 13C NMR spectra were measured in DMSO-d6 using a JEOL-500 spectrometer (Japan) with Me4Si as an internal standard. Mass spectra were obtained using a Finigan gas chromatography–mass spectrometry (USA) at 70 eV. The IR spectra (4000–400 cm−1) were recorded using KBr pellets in a Jasco FT/IR 300 E Fourier transform infrared spectrophotometer (USA) and in the 500–100 cm−1 region using polyethylene-sandwiched nujol mulls on a Perkin Elmer FT-IR 1650 spectrophotometer (Norwalk, USA). Elemental analyses were carried out at the Micro analytical Laboratory of the National Research Centre, Cairo, Egypt. Silica gel thin-layer chromatography cards were purchased from Merck (Darmstadt, Germany) (silica gel precoated aluminum cards with fluorescent indicator at 254 nm). Visualization was performed by illumination with a UV light source. Compounds 1, 7, and 12 were prepared according to the reported literature 16.

5,6-Dimethyl-ethyl-2-benzimidazole carboxylate (1)

Yield 9.1 g (84%), melting point (MP) 180–182°C. 1H NMR δ:1.30 (3H, t, CH3), 2.15 (6H, s, 2CH3), 4.10–4.30 (2H, q, CH2), 10.70 (1H, s, NH benzimidazole), 7.22–7.46 (2H, m, Ar-H). IR (KBr; cm–1): 3350 (NH), 1730 (C=O), 1640 (C=N). MS: m/z=218 [M+]. Anal. calcd for C12H14N2O2: C, 66.04; H, 6.47; N, 12.84. Found: C, 66.09; H, 6.40; N, 12.80.

Synthesis of 5,6-dimethyl-1,3-benzimidazole-2-carboxyhydrazide (2)

5,6-Dimethyl-ethyl-2-benzimidazole carboxylate (1; 0.025 mol, 5.40 g) was dissolved in absolute ethanol (30 ml). The hydrazine hydrate (0.050 mol, 1 ml) was added dropwise under stirring and refluxed for 6 h. The reaction mixture was cooled to room temperature and poured into ice-cold water. The solid formed was filtered, washed with water, dried, and then recrystallized from absolute ethanol to yield 4.15 g (80%), MP 230–232°C. 1H NMR δ: 2.15 (6H, s, 2CH3), 5.56 (2H, br, NH2, D2O– exchangeable), 9.56 (1H, s, NH), 10.70 (1H, s, NH benzimidazole), 7.22–7.46 (2H, m, Ar-H). IR (KBr; cm–1): 3340–3350 (NH), 3290 (NH2), 1725 (C=O), 1640 (C=N). MS: m/z=208 [M+]. Anal. calcd for C10H12N4O: C, 58.81; H, 5.92; N, 27.43. Found: C, 58.86; H, 5.87; N; 27.48.

General procedure for the synthesis of Nº-(4-(dimethylamino)benzylidene)-5,6-dimethyl-1H-benzo[d]imidazole-2-carbohydrazide (3a) and Nº-(anthracen-9-ylmethylene)-5,6-dimethyl-1H-benzo[d]imidazole-2-carbohydrazide (3b).

To a solution of 5,6-dimethyl-1,3-benzimidazole-2-carboxyhydrazide (2; 1 mmol, 0.208 g) in absolute ethanol (30 ml), the appropriate aromatic aldehydes, namely 4-dimethyl amino benzaldehyde (1 mmol, 0.133 g) and anthracene-9-carbaldehyde (1 mmol, 0.206 g), were added with a few drops of piperidine and refluxed under stirring for 7 h. After cooling, the product was poured into crushed ice; the formed solid was filtered, washed with water, and recrystallized from absolute ethanol to give 3a and 3b.

N º-(4-(Dimethylamino)benzylidene)-5,6-dimethyl-1H-benzo[d]imidazole-2-carbohydrazide (3a)

Yield 0.22 g (86%), MP 166–168°C. 1H NMR δ: 2.15 (6H, s, 2CH3), 3.17 (6H, s, N–CH3), 6.88–7.10 (4H, m, Ar-H), 7.22–7.46 (2H, m, Ar-H), 8.10 (1H, s, N=CH), 9.60 (1H, s, NH), 10.70 (1H, s, NH benzimidazole). C13 NMR: 19.80 (CH3), 40.50 (N–CH3), 110.30, 115.50, 123.50, 125.30, 130.50, 135.90 (Ar-CH), 145.80 (C=N). IR (KBr; cm–1): 3350–3360 (NH), 1720 (C=O), 1640 (C=N). Anal. calcd for C19H21N5O: C, 68.04; H, 6.31; N, 20.88. Found: C, 68.09; H, 6.38; N, 20.84.

Nº-(Anthracen-9-ylmethylene)-5,6-dimethyl-1H-benzo[d]imidazole-2-carbohydrazide (3b)

Yield 0.32 g (84%), MP 190–192°C. 1H NMR δ: 2.15 (6H, s, 2CH3), 7.20–7.46 (2H, m, Ar-H), 7.50–7.95 (9H, m, Ar-H), 8.10 (1H, s, N=CH), 9.60 (1H, s, NH), 10.70 (1H, s, NH benzimidazole), C13 NMR: 19.80 (CH3), 115.50, 110.30, 123.50, 125.30, 128.20, 130.50, 131.80, 135.90 (Ar-CH), 145.80 (N=CH), 142.30 (C=N), 155.50 (C = O). IR (KBr; cm–1): 3350–3365 (NH), 1720 (C=O), 1640 (C=N). Anal. calcd for C25H20N4O: C, 76.51; H, 5.14; N, 14.28. Found: C, 76.56; H, 5.18; N, 14.33.

General procedure for the synthesis of 2-(5,6-dimethyl-1H-benzo[d]imidazole-2-carbonyl)-5-(4-(dimethylamino)phenyl)-3-oxo-2,3-dihydro-1H-pyrazole-4-carbonitrile (4a) and 5-(anthracen-9-yl)-2-(5,6-dimethyl-1H-benzo[d]imidazole-2-carbonyl)-3-oxo-2,3-dihydro-1H-pyrazole-4-carbonitrile (4b)

Compound 3a (1 mmol, 0.335 g) or 3b (1 mmol, 0.392 g) was dissolved in dry benzene (25 ml) and ethyl cyanoacetate (1 mmol, 0.15 ml), and a few drops of TEA were added. The reaction mixture was heated under reflux with stirring for 5 h. It was cooled to room temperature and poured into ice-cold water. The precipitate was filtered off and purified by crystallization from chloroform to form compounds 4a or 4b.

2-(5,6-Dimethyl-1H-benzo[d]imidazole-2-carbonyl)-5-(4-(dimethylamino)phenyl)-3-oxo-2,3-dihydro-1H-pyrazole-4-carbonitrile (4a)

Yield 0.3 g (84%), MP 242–244°C. 1H NMR δ:2.15 (6H, s, 2CH3), 6.85–7.10 (4H, m, Ar-H), 7.20–7.46 (2H, m, Ar-H), 10.70 (1H, s, NH benzimidazole), 11.20 (1H, s, NH pyrazole). C13 NMR: 19.80 (CH3), 40.50 N(CH3)2, 115.90 (CN), 110.50, 123.50, 125.30, 130.50, 131.80, 135.90 (Ar-CH), 142.30 (C=N), 150.50 (C=O), 165.20 (C=O). IR (KBr; cm–1): 3370–3380 (NH), 1710–1720 (C=O), 1640 (C=N), 2150 (CN). MS: m / z=401[M++1]. Anal. calcd for C22H20N6O2: C, 65.99; H, 5.03; N, 20.99. Found: C, 65.95; H, 5.07; N, 20.96. MS: m/z=401[M++1].

5-(Anthracen-9-yl)-2-(5,6-dimethyl-1H-benzo[d]imidazole-2-carbonyl)-3-oxo-2,3-dihydro-1H-pyrazole-4-carbonitrile (4b)

Yield 0.35 g (80%), MP 210–212°C. 1H NMR δ: 2.15 (6H, s, 2CH3), 3.17–3.30 [6H, s, N–(CH3)2], 7.20–7.46 (2H, m, Ar-H), 7.50–7.95 (9H, m, Ar-H), 10.70 (1H, s, NH), 11.20 (1H, s, NH pyrazole). C13 NMR: 19.80 (CH3), 32.40 (CH3), 115.90 (CN), 110.50, 123.50, 125.30, 130.50, 131.80, 135.90 (Ar-CH), 142.30 (C=N), 150.50 (C=O), 165.20 (C=O). IR (KBr; cm–1): 3354–3360 (NH), 1710–1720 (C=O), 1640 (C=N), 2165 (CN). MS: m/z=458 [M++1]. Anal. calcd for C28H19N5O2: C, 73.51; H, 4.19; N, 15.31. Found: C, 73.56; H, 4.24; N, 15.36.

General procedure for the synthesis of 5-(4-(dimethylamino)phenyl)-3-oxo-2-(1, 5, 6-trimethyl-1H-benzo[d]imidazole-2-carbonyl)-2,3-dihydro-1H-pyrazole-4-carbonitrile (5a) and 5-(anthracen-9-yl)-3-oxo-2-(1, 5, 6-trimethyl-1H-benzo[d]imidazole-2-carbonyl)-2,3-dihydro-1H-pyrazole-4-carbonitrile (5b)

Method A: A solution of methyl iodide (2 ml) in n-hexane (2 ml; 1/1, v/v) was added to the reaction mixture of compound 4a (1 mmol, 0.40 g) or 4b (1 mmol, 0.45 g) and sodium hydride/DMF (0.1 g sodium hydride/1.0 ml DMF) and stirred at room temperature. The reaction was stopped by the careful addition of a few drops of water followed by 20 ml of water under stirring at room temperature for 8 h. The product was extracted with 30 ml of n-hexane, dried with anhydrous sodium sulfate, filtered, and the solvent was evaporated under vacuum to afford 5a or 5b in yields up to 78–80%, respectively.

Method B: A mixture of compound 4a (1 mmol, 0.40 g) or 4b (1 mmol, 0.45 g), anhydrous K2CO3 (0.01 mol, 1.0 g), and dimethyl carbonate (DMC; 0.03 mol, 2.5 ml) in DMF (10 ml) was refluxed for 3 h. The reaction mixture was cooled to room temperature, following which ice water was added; the precipitated solid was filtered, dried, and crystallized from ethanol to afford 5a or 5b in yields up to 85–88%, respectively.

5-(4-(Dimethylamino)phenyl)-3-oxo-2-(1, 5, 6-trimethyl-1H-benzo[d]imidazole-2-carbonyl)-2,3-dihydro-1H-pyrazole-4-carbonitrile (5a)

Yield 0.36 g (88%) by method B, MP 234–236°C. 1H NMR δ: 2.15 (6H, s, 2CH3), 4.00 (3H, s, CH3), 6.88–7.10 (4H, m, Ar-H), 7.20–7.46 (2H, m, Ar-H), 10.70 (1H, s, NH), 11.20 (1H, s, NH pyrazole). IR (KBr; cm–1): 3320 (NH), 1710–1720 (C=O), 1640 (C=N). MS: m/z=413 [M+−1]. Anal. calcd for C23H22N6O2: C, 66.65; H, 5.35; N, 20.28. Found: C, 66.70; H, 5.40; N; 20.25. MS: m/z =414 [M+].

5-(Anthracen-9-yl)-3-oxo-2-(1, 5, 6-trimethyl-1H-benzo[d]imidazole-2-carbonyl)-2,3-dihydro-1H-pyrazole-4-carbonitrile (5b)

Yield 0.40 g (85%) by method B, MP 205–207°C. 1H NMR δ: 2.15 (6H, s, 2CH3), 4.00 (3H, s, CH3), 7.20–7.46 (2H, m, Ar-H), 7.50–7.95 (9H, m, Ar-H), 10.70 (1H, s, NH), 11.20 (1H, s, NH pyrazole). IR (KBr; cm–1): 3345 (NH), 1710–1720 (C=O), 1640 (C=N). MS: m/z=471 [M+]. Anal. calcd for C29H21N5O2: C, 73.87; H, 4.49; N, 14.85. Found: C, 73.84; H, 4.55; N, 14.90.

General procedure for the synthesis of 3-(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-ylimino)-5-(4-(dimethylamino)phenyl)-2-(1, 5, 6-trimethyl-1H-benzo[d]imidazole-2-carbonyl)-2,3-dihydro-1H-pyrazole-4-carbonitrile (6a) and 5-(anthracen-9-yl)-3-(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-ylimino)-2-(1, 5, 6-trimethyl-1H-benzo[d]imidazole-2-carbonyl)-2,3-dihydro-1H-pyrazole-4-carbonitrile (6b)

Compound 5a (1 mmol, 0.41 g) or 5b (1 mmol, 0.47 g) was dissolved in absolute ethanol (25 ml), followed by the addition of 4-aminoantipyrine (1 mmol, 0.20 g) in the presence of acetic acid (2 ml). The reaction mixture was heated under reflux for 8 h. After cooling, the solvent was evaporated under vacuum and the precipitated product was crystallized from ethanol to afford 6a or 6b.

3-(1,5-Dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-ylimino)-5-(4-(dimethylamino)phenyl)-2-(1, 5, 6-trimethyl-1H-benzo[d]imidazole-2-carbonyl)-2,3-dihydro-1H-pyrazole-4-carbonitrile (6a)

Yield 0.5 g (85%), MP 168–170°C. 1H NMR δ: 2.10 (6H, s, 2CH3), 4.00 (3H, s, CH3), 6.85–7.10 (4H, m, Ar-H), 7.20–7.46 (2H, m, Ar-H), 10.70 (1H, s, NH). IR (KBr; cm–1): 3358 (NH), 1640–1645 (C=N), 1690, 1685 (C=O), 2100 (CN). MS: m/z=598 [M+−1]. Anal. calcd for C34H33N9O2: C, 68.10; H, 5.55; N, 21.02. Found: C, 68.02; H, 5.50; N, 20.96.

5-(Anthracen-9-yl)-3-(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-ylimino)-2-(1, 5, 6-trimethyl-1H-benzo[d]imidazole-2-carbonyl)-2,3-dihydro-1H-pyrazole-4-carbonitrile (6b)

Yield 0.54 g (83%) MP 177–179°C. 1H NMR δ: 2.10 (6H, s, 2CH3), 4.00 (3H, s, CH3), 7.20–7.46 (2H, m, Ar-H), 7.50–7.95 (9H, m, Ar-H), 10.90 (1H, s, NH). IR (KBr; cm–1): 3369 (NH), 1640–1665 (C=N), 1690, 1680 (C=O), 2120 (CN). Anal. calcd for. C40H32N8O2: C, 73.15; H, 4.91; N, 17.06. Found: C, 73.11; H, 4.87; N, 16.96.

Synthesis of compounds 7 and 12

Compounds 7 and 12 were synthesized using the same procedure as that described for the synthesis of compound 1 and were obtained in 77 and 74% yields, respectively.

Ethyl 5-benzoyl-1H-benzo[d]imidazole-2-carboxylate (7): Yield (77%), MP 158–160 °C. 1H NMR δ: 2.25 (3H, s, CH3), 3.80 (2H, q, CH2), 7.4 0–7.60 (2H, m, Ar-H), 7.80–7.90 (4H, m, Ar-H), 11.80 (1H, s, NH). IR (KBr; cm–1): 3320 (NH), 1700 (C=O), 1660 (C=N). Anal. calcd for C16H14N2O3: C, 68.00, H, 6.90, N, 9.90. Found: C, 68.15; H, 6.82; N, 9.81.

Synthesis of compounds 8 and 13

Compounds 8 and 13 were synthesized by adopting the general procedure used for the preparation of compound 2 and were obtained in 75 and 78% yields, respectively.

5-Benzoyl-1H-benzo[d]imidazole-2-carbohydrazide (8): Yield (75%), MP 182–184°C. 1H NMR δ: 5.60 (2H, s, NH2), 7.4 0–7.60 (2H, m, Ar-H), 7.80–7.90 (4H, m, Ar-H), 11.80 (1H, s, NH). IR (KBr; cm–1): 3320 (NH), 3250 (NH2), 1700 (C=O), 1660 (C=N). Anal. calcd for C15H12N4O: C, 68.18; H, 5.30; N, 10.60. Found: C, 68.1; H, 5.23; N, 10.61.

General procedure for the synthesis of compounds 911

To a solution of compound 8 (3 mmol, 0.792 g) in dry DMF (5 ml), POCl3 was added dropwise (6 mmol, 1 ml), followed by 3 mmol of each acid, namely crotonic acid (3 mmol, 0.30 g), 3,4-diaminobenzoic acid (3 mmol, 0.75 g), and 6-hydroxy-4-methoxybenzofuran-5-carboxylic acid (3 mmol, 0.63 g). The reaction mixture was stirred at room temperature for 15 min, thereafter at 80–90°C for 5 h. After cooling, the reaction mixture was poured into crushed ice and neutralized by NaHCO3 (20%). The precipitated product 9, 10, or 11 was filtered, washed with ice water, dried, and crystallized from ethanol.

Phenyl(2-(5-(prop-1-enyl)-1, 3, 4-oxadiazol-2-yl)-1H-benzo[d]imidazol-5-yl)methanone (9): Yield 0.25 g (75%), MP 223–225°C. 1H NMR δ: 2.20 (3H, d, CH3), 5.80 (1H, d, CH), 5.95 (1H, m, CH), 7.4 0–7.60 (2H, m, Ar-H), 7.80–7.90 (4H, m, Ar-H), 11.80 (1H, s, NH). IR (KBr) cm–1: 3320 (NH), 1700 (C=O), 1640–1645 (C=N). MS: m/z=330 [M+]. Anal. calcd for C19H14N4O2: C, 69.08; H, 4.27; N, 16.96. Found: C, 69.12; H, 4.20; N, 16.90.

(2-(5-(3,4-Diaminophenyl)-1, 3, 4-oxadiazol-2-yl)-1H-benzo[d]imidazol-5-yl)(Phenyl) methanone (10): Yield 0.308 g (78%), MP 188–190 °C. 1H NMR δ: 5.60 (2H, m, NH2), 6.80–7.10 (3H, m, Ar-H), 7.40–7.60 (2H, m, Ar-H), 7.80–7.90 (4H, m, Ar-H), 11.80 (1H, s, NH). IR (KBr; cm–1): 3320 (NH), 1700 (C=O), 1640–1650 (C=N). MS: m/z=396 [M+]. Anal. calcd. for C22H16N6O2: C, 66.66; H, 4.07; N, 21.20. Found: C, 66.71; H, 21.25; N, 8.12.

(2-(5-(6-Hydroxy-4-methoxybenzofuran-5-yl)-1, 3, 4-oxadiazol-2-yl)-1H-benzo[d]imidazol-5-yl)(phenyl)methanone (11): Yield 0.346 g (77%), MP 198–200°C. 1H NMR δ: 2.35 (3H, s, CH3), 3.60 (3H, s, OCH3), 6.40–6.60 (2H, m, CH furan), 6.70 (1H, m, CH furan), 6.90 (1H, m, CH aromatic), 7.40–7.60 (4H, m, Ar-H), 7.80–7.90 (2H, m, Ar-H), 11.80 (1H, s, NH). IR (KBr; cm–1): 3320 (NH), 1700 (C=O), 1640–1655 (C=N). Anal. calcd for C25H16N4O5: C, 66.30; H, 3.54; N, 12.39. Found: C, 66.39; H, 3.59; N, 12.45.

Ethyl 8-cyano-6-isocyano-5-oxo-7-phenyl-1,5-dihydro- [1, 2, 4]triazolo [1,5-a]pyridine-2-carboxylate (12): Yield 0.215 g (74%), MP 236–238°C. 1H NMR δ: 2.20 (3H, s, CH3), 3.80 (2H, q, CH2), 7.30–7.75 (4H, m, Ar-H), 10.90 (1H, s, NH). IR (KBr; cm–1): 3350 (NH), 1720 (C=O), 2120–2127 (CN). Anal. calcd f or C16H11N5O3: C, 63.90; H, 3.78; N, 24.05. Found: C, 63.82; H, 3.70; N, 24.10.

8-Cyano-6-isocyano-5-oxo-7-phenyl-1,5-dihydro- [1, 2, 4]triazolo [1,5-a]pyridine-2-carbohydrazide (13): Yield 0.25 g(78%), MP193–195°C. 1H NMR δ: 5.80 (1H, m, CH), 5.90 (1H, d, CH), 6.50 (2H, s, NH2), 7.30–7.75 (4H, m, Ar-H), 9.80 (1H, s, NH), 10.90 (1H, s, NH). IR (KBr; cm–1): 3320 (NH), 3210 (NH2), 1710 (C=O), 2150–2155 (CN). Anal. calcd for C14H9N7O2: C, 52.33; H, 2.80; N, 34.89. Found: C, 52.28; H, 2.72; N, 34.82.

General procedure for the synthesis of compounds 1416

To a solution of compound 13 (1 mmol, 0.321 g) in dry DMF, POCl3 (5 ml) was added dropwise (3 mmol, 0.50 ml), followed by the addition of 1 mmol of each of the following carboxylic acids: crotonic acid, 3,4-diaminobenzoic acid, or 6-hydroxy-4-methoxybenzofuran-5-carboxylic acid. The reaction mixture was stirred at room temperature for 15 min and then at 80–90°C for 6 h. After cooling, the reaction mixture was poured into crushed ice and neutralized by NaHCO3 (20%); the precipitated products 14, 15, or 16 were filtered, washed with ice water, dried and, crystallized from methanol.

6-Isocyano-5-oxo-7-phenyl-2-(5-(prop-1-enyl)-1, 3, 4-oxadiazol-2-yl)-1,5-dihydro- [1, 2, 4]triazolo [1,5-a]pyridine-8-carbonitrile (14): Yield 0.30 g (82%), MP 239–241°C. 1H NMR δ: 2.28 (3H, d, CH3), 5.80 (1H, m, CH), 5.90 (1H, d, CH), 7.30–7.75 (4H, m, Ar-H), 10.90 (1H, s, NH). IR (KBr; cm–1): 3320 (NH), 1710 (C=O), 2150–2156 (CN). MS: m/z=369 [M+]. Anal. calcd for C19H11N7O2: C, 61.79; H, 3.00; N, 26.55. Found: C, 61.84; H, 3.06; N, 26.59.

2-(5-(3,4-Diaminophenyl)-1, 3, 4-oxadiazol-2-yl)-6-isocyano-5-oxo-7-phenyl-1,5-dihydro- [1, 2, 4]triazolo [1,5-a]pyridine-8-carbonitril (15): Yield 0.34 g (80%), MP 220–222°C. 1H NMR δ: 5.60 (2H, m, NH2), 6.90–7.10 (3H, m, Ar-H), 7.30–7.75 (5H, m, Ar-H), 10.90 (1H, s, NH). IR (KBr; cm–1): 3320 (NH), 1710 (C=O), 2140–2150 (CN). Anal. calcd for C22H13N9O2: C, 60.69; H, 3.01; N, 28.95. Found: C, 60.75; H, 3.06; N, 29.05.

1,5-Dihydro-2-(5-(6-hydroxy-4-methoxybenzofuran-5-yl)-1, 3, 4-oxadiazol-2-yl)-6-isocyano-5-oxo-7-phenyl- [1, 2, 4]triazolo [1,5-a]pyridine-8-carbonitrile (16): Yield 0.40 g (81%), MP 230–232°C. 1H NMR δ: 2.35 (3H, s, CH3), 3.60 (3H, s, OCH3), 6.40–6.60 (2H, m, CH furan), 6.70 (1H, m, CH furan), 6.90 (1H, m, CH aromatic), 7.30–7.75 (5H, m, Ar-H), 10.90 (1H, s, NH). IR (KBr; cm−1): 3320 (NH), 1710 (C=O), 2150–2156 (CN). Anal. calcd forC25H13N7O5: C, 61.09; H, 2.64; N, 19.96. Found: C, 61.15; H, 2.70; N, 20.09.


  Results and discussion Top


Compound 5,6-dimethyl-1H-benzo[d]imidazole-2-carbohydrazide (2) was prepared by the condensation reaction of 5,6-dimethyl-ethyl-2-benzimidazole carboxylate (1) with hydrazine hydrate in the presence of absolute ethanol. Schiff’s bases Nº-(4-(dimethylamino)benzylidene)-5,6-dimethyl-1H-benzo[d]imidazole-2-carbohydrazide (3a) and Nº-(anthracen-9-ylmethylene)-5,6-dimethyl-1H-benzo[d]imidazole-2-carbohydrazide (3b) were obtained by the reaction of 2 with 4-(dimethylamino) benzaldehyde and anthracene-9-carbaldehyde in ethanol, respectively, in the presence of a few drops of piperidine. The target compounds 2-(5,6-dimethyl-1H-benzo[d]imidazole-2-carbonyl)-5-(4-(dimethylamino)phenyl)-3-oxo-2,3-dihydro-1H-pyrazole-4-carbonitrile (4a) and 5-(anthracen-9-yl)-2-(5,6-dimethyl-1H-benzo[d]imidazole-2-carbonyl)-3-oxo-2,3-dihydro-1H-pyrazole-4-carbonitrile (4b) were synthesized by the reaction of 3a and 3b with ethyl cyanoacetate in ethanol in the presence of triethylamine, respectively. Methylation of 4a and 4b was achieved by their reaction with methyl iodide or DMC that yielded 5-(4-(dimethylamino)phenyl)-3-oxo-2-(1, 5, 6-trimethyl-1H-benzo[d]imidazole-2-carbonyl)-2,3-dihydro-1H-pyrazole-4-carbonitrile (5a) and 5-(anthracen-9-yl)-3-oxo-2-(1, 5, 6-trimethyl-1H-benzo[d]imidazole-2-carbonyl)-2,3-dihydro-1H-pyrazole-4-carbonitrile (5b), respectively [Scheme 1]. [Additional file 1] Their structures were established on the basis of elemental analysis and spectral data.

Furthermore, compounds 6a or 6b were synthesized by the following reaction: 5a and 5b reacted with 4-aminoantypyrine in ethanol and in the presence of catalytic amounts of acetic acid to give 3-(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-ylimino)-5-(4-(dimethylamino)phenyl)-2-(1, 5, 6-trimethyl-1H-benzo[d]imidazole-2-carbonyl)-2,3-dihydro-1H-pyrazole-4-carbonitrile (6a) and -5-(anthracen-9-yl)-3-(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-ylimino)-2-(1, 5, 6-trimethyl-1H-benzo[d]imidazole-2-carbonyl)-2,3-dihydro-1H-pyrazole-4-carbonitrile (6b), respectively [Scheme 2]. [Additional file 2]

In Scheme 3, [Additional file 3] ethyl 5-benzoyl-1H-benzo[d]imidazole-2-carboxylate (7) and 5-benzoyl-1H-benzo[d]imidazole-2-carbohydrazide (8) were prepared according to the method used for the synthesis of compounds 1 and 2. Compounds 911 were formed by intermolecular cyclization of the hydrazide derivative 8 through a condensation reaction with crotonic acid, 3,4-diaminobenzoic acid, and 6-hydroxy-4-methoxybenzofuran-5-carboxylic acid in the presence of POCl3 and DMF that yielded the corresponding substituted oxadiazol derivatives identified as phenyl(2-(5-(prop-1-enyl)-1, 3, 4-oxadiazol-2-yl)-1H-benzo[d]imidazol-5-yl)methanone (9), 2-(5-(3,4-diaminophenyl)-1, 3, 4-oxadiazol-2-yl)-1H-benzo[d]imidazol-5-yl)(phenyl)methanone (10), and (2-(5-(6-hydroxy-4-methoxybenzofuran-5-yl)-1, 3, 4-oxadiazol-2-yl)-1H-benzo[d]imidazol-5-yl)(phenyl)methanone (11), respectively. The structures of compounds 911 were confirmed on the basis of their elemental analysis and spectral data (cf. Experimental data).

Moreover, the condensation reaction of compound 12 with hydrazine hydrate yielded 8-cyano-6-isocyano-5-oxo-7-phenyl-1,5-dihydro- [1, 2, 4]triazolo [1,5-a]pyridine-2-carbohydrazide (13), which was reacted with crotonic acid, 3,4-diaminobenzoic acid, or 6-hydroxy-4-methoxybenzofuran-5-carboxylic acid in the presence of POCl3 in DMF to produce the corresponding derivatives 6-isocyano-5-oxo-7-phenyl-2-(5-(prop-1-enyl)-1, 3, 4-oxadiazol-2-yl)-1,5-dihydro- [1, 2, 4]triazolo [1,5-a]pyridine-8-carbonitrile (14), 2-(5-(3,4-diaminophenyl)-1, 3, 4-oxadiazol-2-yl)-6-isocyano-5-oxo-7-phenyl-1,5-dihydro- [1, 2, 4]triazolo [1,5-a]pyridine-8-carbonitrile (15), or 1,5-dihydro-2-(5-(6-hydroxy-4-methoxybenzofuran-5-yl)-1, 3, 4-oxadiazol-2-yl)-6-isocyano-5-oxo-7-phenyl- [1, 2, 4]triazolo [1,5-a]pyridine-8-carbonitrile (16), respectively [Scheme 3]. The structure of compounds 1416 was assigned on the basis of their elemental analyses and spectral data.

Biological screening

Antioxidant activity

2,2-Diphenyl-1-picrylhydrazyl radical (DPPH) scavenging assay

Compounds 4a, 4b, 5a, 5b, 6b, 10, 11, 14, 15, and 16 were prepared in DMSO as 10× stocks from each of the tested concentrations (0, 10−3, 10−4, 10−5, 10−6 and 10−7 mol/l) and briefly sonicated when necessary in an ultrasonic water bath. The compounds were submitted for testing to determine the effective concentration of the compound producing 50% scavenging of the DPPH (EC50). Two reference radical scavengers, quercetin, and gallic acid were tested in the assay as positive controls. The method used in the present study was based on previously published methods in literature. The compound stock solutions (15 µl/well) were pipetted in duplicates into 96-well plates. The assay was started with the addition of DPPH reagent (0.004% w/v) in methanol (135 µl/well). Appropriate negative controls were simultaneously run using methanol as a correction for the optical density of colored compounds at 540 nm. The plate was immediately shaken for 30 s and incubated in the dark for 30 min at room temperature. The remaining DPPH was measured in a microplate reader (BMG Fluostar Optima, Ortenberg, Germany) at 540 nm. The percentage of antioxidant activity (%AA) was calculated using the following equation:



Statistical analysis

Regression analysis was used to determine the EC50 values for each compound using the concentration–%AA relationship. All data were represented as the mean value of the duplicate absorbance measurement.

[Table 1] illustrates the antioxidant effects of the tested compounds 4a, 4b, 5a, 5b, 6b, 10, 11, 14, 15, and 16, represented as EC50 values. The compounds showed antioxidant activities against the DPPH radical in the following order of higher activity (lower EC50): 15>11>10>16>14>4b>5a>6b>5b>4a. As shown in [Table 1], compounds 2-(5-(3,4-diaminophenyl)-1, 3, 4-oxadiazol-2-yl)-6-isocyano-5-oxo-7-phenyl-1,5-dihydro- [1, 2, 4]triazolo [1,5-a]pyridine-8-carbonitrile (15), (2-(5-(6-hydroxy-4-methoxybenzofuran-5-yl)-1, 3, 4-oxadiazol-2-yl)-1H-benzo[d]imidazol-5-yl)(phenyl)methanone (11), and 2-(5-(3,4-diaminophenyl)-1, 3, 4-oxadiazol-2-yl)-1H-benzo[d]imidazol-5-yl)(phenyl)methanone (10) showed the highest antioxidant activity compared with other tested compounds and their activities were comparable with the activities of the reference antioxidants quercetin and gallic acid.
Table 1: 2,2-Diphenyl-1-picrylhydrazyl radical scavenging activity (cell-free system) of compounds 4a, 4b, 5a, 5b, 6b, 10, 11, 14, 15, and 16 represented as EC50 values

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Antimicrobial activity

The antibacterial and antifungal activities of the tested compounds 4a, 4b, 5b, 6a, 6b, 10, 11, 14, 15, and 16 were estimated using the diffusion plate method. A sterilized filter paper disc saturated with a measured quantity (25 µl) of the sample (1 mg/ml) was placed on a plate (9 cm in diameter) containing a solid bacterial medium (nutrient agar) or a fungal medium (potato dextrose agar) that had been seeded with the spore suspension of the test organism. After incubation of the bacterial culture at 37°C for 24 h (in the case of fungi, 25°C for 72 h), the diameter of the clear zone of inhibition surrounding the sample is taken as a measure of the inhibitory power of the sample against the particular test organism (% inhibition=sample inhibition zone (cm)/plate diameter×100). All measurements were taken with methanol as a solvent, which has zero inhibitory activity 22–26.

The antimicrobial activity of the tested compounds 4a, 4b, 5b, 6a, 6b, 10, 11, 14, 15, and 16 was examined with gram-positive bacteria Bacillus subtilis, Bacillus cereus, and Staphylococcus aureus, gram-negative bacteria Escherichia coli, Pseudomonas aeruginosa, and Salmonella typhimurium, and the fungus Candida albicans. The obtained results were compared with that of the reference antibiotic amoxicillin that was purchased from Pfizer (Cairo, Egypt).

[Table 2] shows that compounds 2-(5,6-dimethyl-1H-benzo[d]imidazole-2-carbonyl)-5-(4-(dimethylamino)phenyl)-3-oxo-2,3-dihydro-1H-pyrazole-4-carbonitrile (4a) and (2-(5-(6-hydroxy-4-methoxybenzofuran-5-yl)-1, 3, 4-oxadiazol-2-yl)-1H-benzo[d]imidazol-5-yl)(phenyl)methanone (11) were the most active compounds against S. typhimurium, whereas compounds 5-(anthracen-9-yl)-3-oxo-2-(1, 5, 6-trimethyl-1H-benzo[d]imidazole-2-carbonyl)-2,3-dihydro-1H-pyrazole-4-carbonitrile (5b) and 5-(anthracen-9-yl)-3-(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-ylimino)-2-(1, 5, 6-trimethyl-1H-benzo[d]imidazole-2-carbonyl)-2,3-dihydro-1H-pyrazole-4-carbonitrile (6b) exhibited high antimicrobial activity against S. aureus, reaching an 18 mm clear zone; other compounds showed variable antibacterial activity with different extents. With respect to C. albicans, compounds 2-(5-(3,4-diaminophenyl)-1, 3, 4-oxadiazol-2-yl)-1H-benzo[d]imidazol-5-yl)(phenyl)methanone (10) and (2-(5-(6-hydroxy-4-methoxybenzofuran-5-yl)-1, 3, 4-oxadiazol-2-yl)-1H-benzo[d]imidazol-5-yl)(phenyl)methanone (11) exhibited significantly high activity against the species.
Table 2: The antibacterial and antifungal activities of the tested compounds 4a, 4b, 5b, 6a, 6b, 10, 11, 14, 15, and 16

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Antioxidant quantitative structure–activity relationships study

The flexagen algorithm develops predictive pharmacophore models with both activity and structural data of the training set molecules 27. To guarantee the construction of robust models, it is crucial to generate a representative training set with sufficient coverage of both biological and chemical spaces occupied by the original data set. Therefore, diversity sampling of the original data set by considering both activity and structural information is the premise for rational selection of training sets. Furthermore, simultaneous inclusion of several similar compounds in the training sets should be avoided, as it may only provide redundant information and bias the resulting model toward those similar structures. Other important guidelines are in the literature 28. Pharmacophoric hypotheses are important tools in drug design and discovery as they provide excellent insights into ligand macromolecule recognition. However, their predictive value as three-dimensional (3D)-QSAR models is limited by steric clashes and bioactivity enhancing or reducing auxiliary groups. This point, combined with the fact that pharmacophore modeling of antioxidant activity furnished numerous binding hypotheses of comparable success criteria, prompted us to use classical QSAR analysis to search for the best combination of pharmacophore(s) and 2D descriptors capable of explaining bioactivity variation. Furthermore, 3D QSAR modeling was implemented in the current case as grounds of competition to select the best pharmacophore(s) that could explain bioactivity variation across the whole training list.

All computational work was carried out using the molecular operating environment (MOE) program (Chemical Computing Group Inc., Quebec, Canada; 2008). All compounds were drawn using Chem (Cambridge Soft Corporation, Cambridge, Massachusetts, USA). Draw 11, and energy was minimized by force-field MMFF94× optimization with a gradient of 0.0001 for determining the low-energy conformations with the most favorable (lowest energy) geometry.

The purpose of a QSAR descriptor is to calculate properties of molecules that serve as numerical descriptions or to characterize molecules that are correlated with activity. The descriptors computed using MOE could be classified into three classes: 2D descriptors based on the atom and connection information of the molecule, internal 3D descriptors (i3D) based on the 3D information on each molecule and variant to rotation and translation of the conformation, and external descriptors (x3D) based on the 3D information, fitting the antioxidant activity of the dependent variable pK i to that of the independent variables, namely the molecular descriptors.

Partial least squares

Partial least squares analysis was used to derive linear equations from the resulting matrices. Leave one out cross-validation was used to select the number of principal components and to calculate the cross-validated statistics. Regression analysis modules of statistical analysis tool were used to build the 3D QSAR models. Regression analysis was carried out using the pK i activity (antioxidant activity) as the dependent variable and the calculated descriptor as the predicted variable. As shown in the Blow correlation plot, a linear relationship exists between the actual and predicted activities; the correlation coefficient is 0.817603 [Figure 1].
Figure 1: Correlation plot showing a linear relationship between the actual and predicted activities of compounds 4a, 4b, 5a, 5b, 6b, 10, 11, 14, 15, and 16.

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Principal component analysis

The 2D molecular properties of 4a, 4b, 5a, 5b, 6b, 10, 11, 14, 15, and 16 were estimated using the ‘calculate molecular properties’ protocol implemented in the QSAR module. The PCA method was then applied using the ‘calculate principal components’ protocol in the ‘library analysis’ module to extract three principal components. The program initially generated more than 400 descriptors for each compound and it is predicted that some of the descriptors are highly correlated. Therefore, the PCA method was applied to reduce the dimensionality of the descriptor space and alleviate the correlations 29. Basically, the PCA method is a mathematical procedure that converts multiple sets of possibly correlated variables into a few orthogonal ‘principal components’ that are usually linear combinations of the correlated variables, each corresponding to an axis in multiple-dimensional space, as represented by the following equation:



where PC i is the principle component, C ij is the coefficient of the variable x j , and is the number of variables.

For clear graphical representation of the molecular diversity, we extracted three principal components that account for the variation in the descriptor space and plotted the molecules as discrete spots in a 3D coordinated system [Figure 2].
Figure 2: The three-dimensional scatter plot is a visual representation of the molecules as described by the three selected principal components (PCA1, PCA2, and PCA3). Each point corresponds to a molecule and is colored according to the molecule’s pK i value.

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Pharmacophore generation

The pharmacophore generation protocol used a flex align algorithm 30. The features of the hydrogen bond acceptor, hydrogen bond donor, hydrophilic or hydrophobic aromatic center, and the aromatic ring were predefined using a stochastic search conformation, and the parameter of ‘maximum excluded volumes’ (MEV) had a value of 5. The fundamental approach for pharmacophore elucidation described herein is to exhaustively search for all pharmacophore queries that induce good overlay of most of the active molecules [Figure 3]. Thus, the plausibility of the pharmacophore is measured by overlay of actives, and the relationship with activity is measured by classification accuracy and HipHop/HypoGen methods described in attempt to relate common feature geometries with activity or complexity 31.
Figure 3: The three-dimensional quantitative structure– activity relationships pharmacophore generated from compounds 4a, 4b, 5a, 5b, 6b, 10, 11, 14, 15, and 16 showed a hydrophobic center (green), two aromatic planar ring centers (orange), and a hydrogen bond acceptor (blue).

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Sensitivity and specificity

The sensitivity and specificity of the models should be established during validation to assess the propensity of the QSAR models for correct qualitative prediction of the dependent variable 32. Sensitivity can be calculated as follows:



Specificity can be calculated as follows:

TP−FN represents the number of corrected predictions, TP represents the number of true positives, and TN represents the number of true negatives.

To confirm the probability of identifying true selective molecules, sensitivity and specificity tests were performed on the observed versus predicted selectivity values for the dataset. The observed values for sensitivity and specificity were 90 and 93%, respectively. These results were in accordance with those expected from the earlier examination of probability of predicting the potency profile for the antioxidant activity of the tested compounds 4a, 4b, 5a, 5b, 6b, 10, 11, 14, 15, and 16.

Antibacterial molecular docking study

Molecular docking was performed and analyzed with the MOE program. Docking calculations were carried out using standard default variables for the MOE program. The binding affinity was evaluated from the binding free energies (S-score, kcal/mol), hydrogen bonds, and root mean square deviation values. Compound 4a docked into the same groove of the binding site of the native cocrystallize ligand. Scoring in MOE software was performed using the London dG scoring function and enhanced using two different refinement methods; the force-field and grid-min poses were updated to ensure that refined poses satisfy the specified conformations. Rotatable bonds were allowed; the best 10 poses were retained and analyzed for the binding pose’s best score. Energy was minimized through force-field MMFF94× optimization with a gradient of 0.0001 for determining low-energy conformations with the most favorable (lowest energy) geometry.

The antibiotic sulfamethaxazole was used as a reference for the docking study. It inhibits the dihydropteroate synthase enzyme, a key enzyme in the folate pathway.

Compound 4a was investigated for its binding affinity with the dihydropteroate synthase receptor (pdb 3TZF) 33 for the purpose of lead optimization and to study the interaction between compound 4a and the dihydropteroate synthase receptor [Figure 4], [Figure 5] and [Figure 6].
Figure 4: The ligand interaction and the binding mode of the native ligand sulfamethaxazole (O8D) showed one H-bond donor with HOH 333 with a distance of 2.76 (black color); it bonded with one H-bond acceptor with SER 222 at a distance of 2.9 (blue color) and one H-bond acceptor with HOH 289 (black color) depicted as hatched line. It gave a score of −13.0424.

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Figure 5: The ligand interaction and the binding mode of the compound 4a. It binds with one H-bond the acceptor with HOH 333 at a distance of 0.96. It gave a score of −13.5451 kcal/mol greater than that of the cocrystallized ligand.

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Figure 6: Superposition of compound 4a (blue color). The cocrystallized ligand sulfamethaxazole (O8D; gray color) showed binding with HOH 333 as a cocrystallized ligand.

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  Conclusion Top


QSAR between the molecular structure and the inhibitory antioxidant activity of the synthesized compounds 4a, 4b, 5a, 5b, 6b, 10, 11, 14, 15, and 16 were studied. Compounds 15, 11, and 10 displayed the highest antioxidant activity using the 2,2-diphenyl-1-picrylhydrazyl radical scavenging assay. The antimicrobial activity showed that compound 4a was the most active against S. typhimurium and its activity exceeded the activity of the reference antibiotic amoxicillin. Compounds 5b and 6b exhibited high antimicrobial activity against S. aureus, whereas compounds 10 and 11 showed significantly high activities against C. albicans. The molecular modeling study showed that compound 4a gave score of −13.5451 kcal/mol, which was greater than the score of the cocrystallized ligand sulfamethoxazole (−13.0 kcal/mol).


  Acknowledgements Top


The authors thank the National Research Center, Cairo, Egypt, and the Science and Technology Development fund (STDF Grant No.: 1169) for financial support.[33]



 
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    Figures

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