Research, Volume: 16( 5)
Synthesis and Potential Antibacterial Activity of Hydrazone Derivatives with Imidazo[1,2-a]pyridine support against Escherichia Coli.
- Souleymane Coulibaly and Drissa Sissouma Laboratoire de Constitution et Réaction de la Matière, UFR Sciences des Structures de la Matière et Technologie, Université Félix Houphouët-Boigny de Cocody,E-mail: [email protected] ,[email protected]
Received: September-05-2022; Accepted:September-07-2022; Published:September-5-2022
Citation:Adingra KF, Coulibaly S, Etienne CT. Synthesis and Potential Antibacterial Activity of Hydrazone Derivatives with Imidazo[1,2-a] pyridine support against Escherichia Coli. Org Chem.: Indian J. 2022;16(9):210. ©2022 Trade Science Inc.
This work presents the synthesis and investigated the antibacterial activity of seventeen (17) hydrazone derivatives with imidazo[1,2-a] pyridine support (5a-q). These compounds were obtained by condensation between 2-hydrazino-3-nitroimidazo[1,2-a] pyridine and aldehyde derivatives. The synthesized compounds were characterized by spectroscopic analyses (1H, 13C NMR), and High-Resolution Mass Spectrometry (HRMS). A preliminary antibacterial activity of the 5a-q compounds was determined on an E. coli strain by the disc diffusion method. The results showed that among the 17, 12-imidazo[1,2-a] pyridinehydrazone derivatives were potent with inhibition diameters between 8 mm and 11 mm. Compound 5a was more active with a diameter of 11 mm.
Imidazo[1,2-a]pyridine, Hydrazone, Antibacterial activity, Inhibition diameter.
Imidazopyridine drugs exhibit a wide range of biological activities as a result of changes in the groups on the core structure, as shown in Figure 1 [1-3]. There's a lot of interest in that heterocycle, particularly in terms of pharmacology. It’s a reference pharmacophore for its presence in the structures of many marketed drugs such as Zolimidine , a drug used in the treatment of duodenal gastroulcer while Zolpidem is used for insomnia . Molecules such as Alpidem , Nicopidem, and Saripidem are used as an anti-anxiolytic agents . Finally, GSK812397 is used in the treatment of HIV infections . In addition to these commercial molecules, several research studies have largely revealed the different biological activities of imidazo[1,2-a]pyridine derivatives such as antibacterial, antifungal, anthelminthic, and antimalarial [8-11]. This heterocycle can be used in the research of new active drugs against infectious germs. Infectious diseases are indeed one of the leading causes of death in the world and especially in developing countries. As seen with the Covid-19 pandemic in 2019, more than 100 million people were infected with more than two million deaths . One of the leading causes of death was bacterial infections in addition to the coronavirus [13–15]. However, antibiotics used in the treatment of these bacterial infections face resistance . Antimicrobial resistance is one of the most pressing health hazards of our time. WHO estimates that drug-resistant infections contribute to nearly 5 million deaths every year in the world .
Figure 1: Imidazo[1,2-a]pyridine-containing drugs marketed
In this context, the emergence of multiresistant bacterial strains poses important public health problems. Germs such as E. coli are bacteria of the family Enterobacteriaceae that reside in the digestive tract of humans and animals . The majority of E. coli strains are harmless, and only a few are pathogenic to humans. But in this context, all those kinds of pathogens became dangerous.
This is the case with strains of Enterohemorrhagic E. coli (ECEH) responsible for urinary tract infections, diarrhea, and meningitis in newborns [19-23]. E. coli infections are generally transmitted through the consumption of undercooked or raw animal (meat or dairy) products . Thus, the need for innovative molecules to circumvent drug resistance problems becomes paramount. The use of chemical grounds with anti-infective potentials, such as imidazo[1,2-a]pyridine derivatives, and hydrazone derivatives [25– 27], may be of interest. The purpose of this study is to synthesize imidazo[1,2-a]arylhydrazone derivatives under the principle of the juxtaposition of biologically active entities and to evaluate the antibacterial activity of these compounds.
Materials and Method
All reagents and solvents were purchased at the highest commercial quality and used without further purification unless otherwise noted. All anhydrous solvents, reagent grade solvents for chromatography, and starting materials were purchased at the highest commercial quality from either Aldrich Chemical or Fisher Scientific. The reactions were monitored by TLC on precoated Merck 60 F254 silica gel plates and visualized using UV-Lamp (6 W, 254 nm, and/or 365 nm) or KMnO4 solution followed by heating. Unless otherwise indicated, 1H and 13C NMR spectra were recorded either on a Bruker Advance at 300, 400, 500, and 75, 101, or 126 MHz. The spectra were internally referenced to the residual proton solvent signal. Residual solvent peaks were taken as reference (CDCl3: 7.26 ppm, Acetone-d6: 2.05 ppm, DMSO-d6: 2.50 ppm) at room temperature. For 1H NMR assignments, the chemical shifts are given in ppm on the δ scale. Multiplicities are described as s (singlet), d (doublet), dd (doublet of doublets), t (triplet), q (quartet), m (multiplet), and further qualified as app (apparent), BS (broad signal) coupling constants, J are reported in Hz. HRMS were measured in the Electrospray (ESI) mode on an LC-MSD TOF mass analyzer. Solid compound melting points were measured using a Köfler bench.
The antibacterial activity assessment of imidazo[1,2-a]pyridinehydrazone derivatives was conducted on an E. coli 1289 strain.This strain was provided by the Laboratoire de Microbiologie du Centre National de Floristique (CNF) of the Université Felix Houphouët Boigny de Cocody. To evaluate this activity, the disk diffusion method was used .
Methods of synthesis
A round bottom flask immersed in an ice bath containing 15 mL of H2SO4, 1 eq (1.5 g, 9.83mmol) of 2-chloro-H-imidazo[1,2-a] pyridine 1, and 3.5 eq (1.6 mL, 34.40 mmol) of HNO3 was added. The reaction mixture was stirred at room temperature for 3 h and followed by TLC analysis. The reaction mixture was extracted with DCM and the organic layer was dried over Na2SO4. The organic phase was evaporated under vacuum, dried, and without any further purification to yield 1.76 g (91%) compound 5 as yellow crystals, m.p: 166°C-168°C. 1H NMR (400 MHz, Acetone-d6) δ 9.42 (dt, J=7.0, 1.1 Hz, 1H; HAr), 7.92–7.79 (m, 2H; HAr), 7.52 (td, J=7.0, 1.5 Hz, 1H; HAr). 13C NMR (400 MHz, Acetone-d6) δ 132.08, 117.33. HRMS (ESI): Calc for C7H5ClN3O2 [M+H] +=198.8974 Found =198.8977
Synthesis methods of 2-hydrazino-3-nitroimidazo[1,2-a]pyridine 3: In a flask containing 5 mL of ethanol were added compound 2 (1 mmol), and hydrated hydrazide (20 eq, 20 mmol) were added dropwise. The mixture was stirred at 60°C -70°C and then monitored by TLC for 30 minutes. The precipitate formed was filtered, washed with 2 mL of ethanol, and recrystallized in ethanol yielded to 2- hydrazino-3-nitroimidazo[1,2-a]pyridine. Yellow powder, m.p=198°C-200°C, yield=78%. 1H NMR (300 MHz, CDCl3) δ 9.42 (d, J =6.8 Hz, 1H, HAr), 8.23 (s, 1H, NH), 7.65 (dd, J=11.7, 4.4 Hz, 1H, HAr), 7.52 (d, J=8.8 Hz, 1H, HAr), 7.13 (t, J=6.5 Hz, 1H, HAr), 4.25 (s, 2H, NH2). 13C NMR (75 MHz, CDCl3) δ 133.48, 128.62, 117.29, 115.44, 114.39. HRMS (ESI): Calc for C7H5ClN2O2 [M+H] +=194.0832 Found=194.0834 General procedure for the synthesis of 1-(3-nitroimidazo[1,2-a]pyridinyl)-3-phenylhydrazone derivatives 5a-q The compound 3 (1 mmol) and aromatic aldehydes 4 (1 eq, 1 mmol) were dissolved in 5 mL of methanol. Then two drops of acetic acid were added to the mixture medium. The reaction mixture was refluxed for 30 min to 1 h. After cooling to room temperature, the precipitate formed was filtered, dried, and then purified by recrystallization in ethanol to give compounds 5a-q with yields between 49 and 95%.
1-(3-nitroimidazo[1,2-a] pyridinyl)-3-phenylhydrazone 5a: Yellow powder, m.p=258°C-260°C; yield=80%. 1H NMR (300 MHz, DMSO-d6) δ 11.25 (s, 1H, NH), 9.34 (d, J=6.8 Hz, 1H, HAr), 8.67 (s, 1H, CH=N), 7.89–7.65 (m, 4H, HAr), 7.56–7.41 (m, 3H, HAr), 7.29 (td, J=7.0, 1.2 Hz, 1H, HAr). 13C NMR (75 MHz, DMSO-d6) δ 150.57, 148.37, 146.75, 134.85, 134.55, 130.40, 129.34, 129.03, 127.47, 116.01, 115.37. HRMS (ESI) Calc. for C14H12N5O2 [M+H] +=282.1881 Found=282.1883
1-(3-nitroimidazo[1,2-a]pyridinyl)-3-(4-methoxyphenyl) hydrazone 5b: Yellow powder, m.p=251-253°C, yield=89%. 1H NMR (300 MHz, DMSO-d6) δ 11.12 (s, 1H, NH), 9.32 (d, J=6.8 Hz, 1H, HAr), 8.58 (s, 1H, CH=N), 7.87–7.74 (m, 1H, HAr), 7.67 (d, J=8.8 Hz, 3H, HAr), 7.27 (td, J=7.0, 1.0 Hz, 1H, HAr), 7.03 (d, J=8.8 Hz, 2H, HAr), 3.82 (s, 3H, OCH3). 13C NMR (75 MHz, DMSOd6) δ 161.15, 150.60, 148.39, 146.90, 134.59, 129.11, 129.04, 127.37, 117.59, 115.83, 115.22, 114.83, 55.79. HRMS (ESI) C15H14N5O3 [M+H] +=312.2571 Found=312.2573
1-(3-nitroimidazo[1,2-a]pyridinyl)-3-(4-fluorophenyl) hydrazone 5c: Yellow powder, m.p=260°C-262°C, yield=88%. 1H NMR (300 MHz, DMSO-d6) δ 11.27 (s, 1H, NH), 9.42–9.30 (m, 1H, HAr), 8.66 (s, 1H, CH=N), 7.85–7.76 (m, 3H, HAr), 7.70 (d, J=8.8 Hz, 1H, HAr), 7.37–7.25 (m, 3H, HAr). 13C NMR (75 MHz, DMSO-d6) δ 150.56, 147.18, 146.73, 134.54, 131.49, 131.45, 129.65, 129.53, 129.03, 116.58, 116.29, 116.00, 115.39. HRMS (ESI) Calc. for C14H11FN5O2 [M+H]+=300.1752 Found=300.1756
1-(3-nitroimidazo[1,2-a]pyridinyl)-3-(2-methylphenyl) hydrazone 5d: Yellow powder, m.p=234-236°C, yield=94%. 1H NMR (300 MHz, CDCl3 ) δ 10.53 (s, 1H, NH), 9.41 (dt, J=6.8, 1.1 Hz, 1H, HAr), 8.44 (s, 1H, CH=N), 8.11 (dd, J=7.6, 1.5 Hz, 1H, HAr), 7.69–7.63 (m, 2H, HAr), 7.33–7.27 (m, 1H, HAr), 7.25–7.10 (m, 3H, HAr), 2.51 (s, 3H, CH3). 13C NMR (75 MHz, CDCl3) δ 150.47, 147.11, 146.24, 137.09, 133.69, 131.41, 130.78, 130.37, 128.52, 127.09, 126.31, 116.32, 114.74, 19.45. HRMS (ESI) Calc. for C15H14N5O2 [M+H] +=296.1147 Found=296.1150
1-(3-nitroimidazo[1,2-a]pyridinyl)-3-(2-hydroxyphenyl) hydrazone 5e: Yellow powder, m.p=n.d (>266°C), yield=95%. 1H NMR (300 MHz, DMSO-d6) δ 11.55 (s, 1H, OH), 11.29 (s, 1H, NH), 9.33 (d, J=6.8 Hz, 1H, HAr), 8.85 (s, 1H, CH=N), 7.81 (ddd, J=8.5, 7.1, 1.2 Hz, 1H, HAr), 7.68 (d, J=8.8 Hz, 1H, HAr), 7.47 (dd, J=8.0, 1.6 Hz, 1H, HAr), 7.36–7.23(m, 2H, HAr), 6.92 (dd, J=10.3, 4.5 Hz, 2H, HAr). 13C NMR (75 MHz, DMSO-d6) δ 157.80, 150.02, 148.89, 146.51, 134.48, 131.62, 130.05, 128.99, 119.82, 119.24, 116.92, 116.01, 115.47. HRMS (ESI) Calc. for C14H11N5O3Na [M+Na]+=320.0538 Found=320.0543
1-(3-nitroimidazo[1,2-a]pyridinyl)-3-(4-chlorophenyl) hydrazone 5f: Yellow powder, m.p=264-266°C, yield=90%. 1H NMR (300 MHz, DMSO-d6) δ 11.31 (s, 1H, NH), 9.34 (d, J=6.8 Hz, 1H, HAr), 8.65 (s, 1H, CH=N), 7.87–7.65 (m, 4H, HAr), 7.54 (d, J=8.5 Hz, 2H, HAr), 7.29 (td, J=7.0, 1.2 Hz, 1H, HAr). 13C NMR (75 MHz, DMSO-d6) δ 150.45, 146.87, 146.65, 134.75, 134.51, 133.80, 129.44, 129.02, 116.04, 115.44. HRMS (ESI) Calc. for C14H10ClN5O2Na [M+Na]+=338.0381 Found=338.0384
1-(3-nitroimidazo[1,2-a]pyridinyl)-3-(3-cyanophenyl) hydrazone 5g: Yellow powder, m.p=n.d (>266°C), yield=91%. 1H NMR (300 MHz, DMSO-d6) δ 11.44 (s, 1H, NH), 9.33 (d, J=6.7 Hz, 1H, HAr), 8.68 (s, 1H, CH=N), 8.13–8.02 (m, 2H, HAr), 7.92–7.77 (m, 2H, HAr), 7.69 (dd, J=15.1, 7.9 Hz, 2H, HAr), 7.30 (t, J=6.9 Hz, 1H, HAr). 13C NMR (75 MHz, DMSO d6) δ 150.36, 146.52, 145.74, 136.22, 134.50, 133.45, 131.52, 130.70, 129.02, 116.12, 115.58, 112.52. HRMS (ESI) Calc. for C15H10N6O2Na [M+Na]+=329.0487 Found=329.0489
1-(3-nitroimidazo[1,2-a]pyridinyl)-3-(4-hydroxyphenyl) hydrazone 5h: Orange powder, m.p=n.d (>266°C), yield=90%, 1H NMR (500 MHz, DMSO-d6) δ 11.07 (s, 1H, NH), 9.93 (s, 1H, OH), 9.42–9.26 (m, 1H, HAr), 8.54 (s, 1H, CH=N), 7.80 (ddd, J=8.6, 7.1, 1.3 Hz, 1H, HAr), 7.67 (d, J=8.8 Hz, 1H, HAr), 7.60–7.56 (m, 2H, HAr), 7.27 (td, J=7.0, 1.2 Hz, 1H, HAr), 6.88(m, 2H, HAr)–6.83 (m, 2H, HAr). 13C NMR (126 MHz, DMSO-d6) δ 159.37, 150.16, 148.40, 146.49, 134.11, 128.81, 128.56, 125.33, 115.74, 115.37, 114.65. HRMS (ESI) Calc. for C14H12N5O3 [M+H] +=298.0733 Found =298.0736
1-(3-nitroimidazo[1,2-a]pyridinyl)-3-(2-nitrophenyl) hydrazone 5i: Yellow powder, m.p=260°C-262°C, yield=73%. 1H NMR (300 MHz, DMSO-d6) δ 11.71 (s, 1H, NH), 9.37–9.31 (m, 1H, HAr), 9.09 (s, 1H, CH=N), 8.17 (dd, J=7.9, 1.2 Hz, 1H, HAr), 8.07 (dd, J=8.2, 1.1 Hz, 1H, HAr), 7.82 (ddd, J=8.4, 6.9, 2.7 Hz, 2H, HAr), 7.74 -7.62 (m, 2H, HAr), 7.31 (td, J=7.0, 1.3 Hz, 1H, HAr). 13C NMR (75 MHz, DMSO-d6) δ 150.27, 148.63, 146.35, 142.68, 134.35, 134.07, 130.88, 129.18, 128.98, 128.19, 125.08, 116.15, 115.60. HRMS (ESI) Calc. for C14H11N6O4 [M+H] +=327.0664 Found =327.0668
1-(3-nitroimidazo[1,2-a]pyridinyl)-3-(4-methyl phenyl) hydrazone 5j: Yellow powder, m.p=n.d (>266°C), yield=71%. 1H NMR (300 MHz, DMSO-d6) δ 11.20 (s, 1H, NH), 9.34 (d, J=6.8 Hz, 1H, HAr), 8.63 (s, 1H, N=CH), 7.82 (dd, J=8.5, 7.1, 1.3 Hz, 1H, HAr), 7.67 (dd, J=18.0, 8.4 Hz, 3H, HAr), 7.28 (dd, J=9.8, 4.3 Hz, 3H, HAr), 2.36 (s, 3H, CH3). 13C NMR (75 MHz, DMSO-d6) δ 150.61, 148.47, 146.82, 140.26, 134.59, 132.15, 129.96, 129.06, 127.49, 115.97, 115.32, 21.54. HRMS (ESI) Calc. for C15H13N5O2Na [M+Na]+=318.1835 Found=318.1837
1-(3-nitroimidazo[1,2-a]pyridinyl)-3-(2,4-chlorophenyl) hydrazone 5k: Yellow powder, m.p=n.d (>266°C), yield=66%. 1H NMR (300 MHz, DMSO-d6) δ 11.70 (s, 1H, NH), 9.34 (d, J=6.8 Hz, 1H, HAr), 9.06 (s, 1H, CH=N), 8.06 (d, J=8.6 Hz, 1H, HAr), 7.89–7.65 (m, 4H, HAr), 7.55 (dd, J=8.6, 2.0 Hz, 1H, HAr), 7.31 (td, J=7.0, 1.2 Hz, 1H, HAr). 13C NMR (75 MHz, DMSO-d6) δ 150.28, 146.42, 142.83, 135.19, 134.39, 134.15, 131.54, 129.86, 128.99, 128.44, 116.12, 115.56. HRMS (ESI) Calc. for C14H10Cl2N5O2 [M+H] +=351.1725 Found =351.1727
1-(3-nitroimidazo[1,2-a]pyridinyl)-3-(3-nitrophenyl) hydrazone 5l: Yellow powder, m.p=n.d (>266°C), yield=91%. 1H NMR (500 MHz, DMSO-d6) δ 11.45 (s, 1H, NH), 9.34 (dt, J=6.8, 1.1 Hz, 1H, HAr), 8.78 (s, 1H, CH=N), 8.55 – 8.51 (m, 1H, HAr), 8.26 (ddd, J=8.2, 2.4, 1.0 Hz, 1H, HAr), 8.14–8.09 (m, 1H, HAr), 7.85–7.81 (m, 1H, HAr), 7.79(m, 2H, HAr)–7.74 (m, 2H, HAr), 7.31 (td, J=6.9, 1.3 Hz, 1H, HAr). 13C NMR (126 MHz, DMSO-d6) δ 149.82, 148.25, 146.02, 145.20, 136.20, 133.99, 133.31, 130.52, 128.50, 123.99, 120.44, 115.69, 115.12. HRMS (ESI) Calc. for C14H11N6O4 [M+H] +=327.0562 Found =327.0567
1-(3-nitroimidazo[1,2-a]pyridinyl)-3-(3-bromophenyl) hydrazone 5m: Yellow powder, m.p =260-262°C, yield=65%. 1H NMR (300 MHz, DMSO-d6) δ 11.37 (s, 1H, NH), 9.34 (d, J=6.8 Hz, 1H, HAr), 8.63 (s, 1H, CH=N), 7.93 (t, J=1.6 Hz, 1H, HAr), 7.88–7.79 (m, 1H, HAr), 7.71 (dd, J=8.2, 7.2 Hz, 2H, HAr), 7.63 (ddd, J=7.9, 1.9, 0.9 Hz, 1H, HAr), 7.44 (t, J=7.8 Hz, 1H, HAr), 7.31 (td, J=6.9, 1.3 Hz, 1H, HAr). 13C NMR (75 MHz, DMSO-d6) δ 150.43, 146.60, 146.36, 137.33, 134.54, 132.85, 131.58, 129.18, 129.03, 126.77, 122.68, 116.09, 115.53. HRMS (ESI) Calc. for C14H11BrN5O2 [M+H] +=361.0991 Found =361.0995
1-(3-nitroimidazo[1,2-a]pyridinyl)-3-(4-nitrophenyl) hydrazone 5n: Yellow powder, m.p=264-266°C, yield=69%. 1H NMR (300 MHz, DMSO-d6) δ 11,54 (s, 1H, HAr), 9,35 (s, 1H, CH=N), 8.82 (d, J=6,3 Hz, 1H, HAr), 8,35 (d, J=7,3 Hz, 2H, HAr), 8,07 (d, J=7,2 Hz, 3H, HAr), 7,78 (dd, J=7,7, 7,9 Hz, 2H, HAr), 7,32 (td, J=6,8, 1,3 Hz, 1H, HAr). 13C NMR (75 MHz, DMSO d6) δ 150.05, 148.44, 146.19, 143.94, 135.26, 134.03, 132.09, 129.85, 128.74, 126.09, 122.76, 115.92, 115.36. HRMS (ESI) Calc. for C14H11N6O4 [M+H] +=327.0268 Found=327.0271
1-(3-nitroimidazo[1,2-a]pyridinyl)-3-(4-hydroxy-3-methoxyphenyl) hydrazone 5o: Orange powder, m.p=n.d (>266°C), yield=79%. 1H NMR (300 MHz, DMSO-d6) δ 11.07 (s, 1H, NH), 9.59 (s, 1H, OH), 9.34 (d, J=6.7 Hz, 1H, HAr) , 8.53 (s, 1H, CH=N), 7.81 (ddd, J=8.5, 7.1, 1.2 Hz, 1H, HAr), 7.67 (d, J=8.8 Hz, 1H, HAr), 7.28 (ddd, J=13.9, 6.4, 1.4 Hz, 3H, HAr), 7.11 (dd, J=8.2, 1.8 Hz, 1H, HAr), 6.87 (d, J=8.1 Hz, 1H, HAr), 3.86 (s, 3H, OCH3). 13C NMR (75 MHz, DMSO-d6) δ 150.57, 149.50, 149.25, 148.52, 146.97, 134.66, 129.08, 126.17, 122.59, 117.55, 116.02, 115.87, 115.18, 109.62, 56.11. HRMS (ESI) Calc. for C15H14N5O4 [M+H] +=328.1522 Found=328.1525
1-(3-nitroimidazo[1,2-a]pyridinyl)-3-(2,4-dimethoxyphenyl) hydrazone 5p: Orange powder, m.p=262-264°C, yield=76%. 1H NMR (300 MHz, DMSO-d6) 11.23 (s, 1H, NH), 9.33 (d, J=6.8 Hz, 1H, HAr), 8.85 (s, 1H, CH=N), 7.93–7.78 (m, 2H, HAr), 7.66 (d, J=8.8 Hz, 1H, HAr), 7.26 (td, J=7.0, 1.2 Hz, 1H, HAr), 6.74 –6.59 (m, 2H, HAr), 3.87 (s, 3H, OCH3), 3.83 (s, 3H, OCH3). 13C NMR (75 MHz, DMSO-d6) δ 162.86, 159.57, 150.61, 146.90, 144.11, 134.52, 129.04, 127.25, 117.45, 115.83, 115.10, 106.89, 98.75, 56.28, 55.93. HRMS (ESI) Calc. for C16H16N5O [M+H] +=342.2956 Found=342.2960
1-(3-nitroimidazo[1,2-a]pyridinyl)-3-pyridinylhydrazone 5q: Yellow powder, m.p=n.d (>266°C), yield=81%. 1H NMR (300 MHz, DMSO-d6) δ 11.52 (s, 1H, NH), 9.35 (d, J=6.8 Hz, 1H, HAr), 8.67 (s, 3H, HAr, CH=N), 7.88–7.78 (m, 1H, HAr), 7.74 (d, J=8.7 Hz, 1H, HAr), 7.66 (d, J=5.9 Hz, 2H, HAr), 7.33 (td, J=6.9, 1.3 Hz, 1H, HAr). 13C NMR (75 MHz, DMSO-d6) δ 150.66, 150.23, 146.41, 145.51, 141.99, 134.48, 129.02, 121.30, 117.89, 116.22, 115.70. HRMS (ESI) Calc. for C13H11N6O2 [M+H]+=283.0745 Found=283.0747
Preparation of chemical compounds
A 1000 μg/mL stock solution was prepared by dissolving 1 mg of substance in 1 mL of a 50/50 DMSO/distilled water mixture. Then, this solution was put in a water bath for 10 minutes at 45°C. After homogenization by a vortex, it was finally left for 24 hours at room temperature. From this solution, a concentration of 500 μg/mL has been prepared.
Preparation of bacterial inoculum
The bacteria to be tested were transplanted to chromogenic E. coli agar for E. coli strains and then incubated at 37°C for 24 hours to obtain young, well-isolated colonies. After incubation, 1-2 well-isolated and perfectly identical bacterial colonies were collected using a platinum loop and then emulsified in a tube containing 2 mL of physiological water and stirred in the vortex. The inoculum density was adjusted to 0.5 Mac Farland using DENSIMAT.
Seeding and deposition of disks
0.1 mL of the bacterial inoculum was inoculated on the surface of a Muller Hinton agar and spread evenly. Disks of 6 mm diameter sterile blotting paper were impregnated with a volume of 20 μL of the chemical substance supplemented with 10% DMSO of varying concentrations . Two controls were performed, negative control with 20 μL of sterile distilled water in the presence of 10% DMSO and an antibiotic disc as a positive control. These discs were then deposited on the surface of the Muller Hinton agar.
The boxes were left for one hour at room temperature and incubated at 37°C for 18 hours to 24 hours . After incubation, the inhibition diameter was measured in millimetres (disc included) using a caliper.
Results and Discussion
The synthesis of compounds 5a-r was performed in three steps starting from the intermediate 2-chloroimidazo[1,2-a] pyridine 1. This compound 1 was obtained in two steps following the work described by Brad et al. . The 2-chloro-3-nitroimidazo[1,2-a]pyridine 2 was obtained by the nitration of the position-3 of compound 1. Then, compound 3 was synthesized via a nucleophilic substitution reaction between 2-chloro-3-nitroimidazo[1,2-a]pyridine 2 and hydrazine-hydrate at around 60°C-70°C in ethanol for 20 minutes. Compound 3 was obtained as a yellow solid with a 78% yield. The new 1-(3-nitroimidazole[1,2-a] pyridinyl)-3phenylhydrazone derivatives (5a-q) synthesis was carried out by condensation of 2-hydrazino-3-nitroimidazo[1,2-a]pyridine 3 with 4a-q aromatic aldehydes (Figure 2).
These derivatives 5a-q were obtained by heating the mixture of compound 3 and the aromatic aldehydes 4a-q in the presence of two drops of acetic acid for one hour under reflux of methanol. A precipitate was formed, filtered off while hot, and then washed with methanol. The compounds 5a-q were isolated and purified by recrystallization in ethanol. The 1H NMR spectrum of compound 3 revealed the presence of peaks corresponding to the protons of the different nitrogen. We note the presence of two new singlets, one at 4.25 ppm integrating for two protons (NH2 proton) and the other one at 8.23 ppm integrating for one proton (NH proton). The NMR spectra of compounds 5a-q obtained show, besides the presence of new peaks in the aromatic zone, the disappearance of the singlet at 4.25 ppm corresponding to the protons of the NH
The antibacterial activity of the seventeen (17) imidazo[1,2-a]pyridinylhydrazone derivatives synthesized was achieved by the E. coli disk diffusion method. The inhibition diameters of the 5a-q compounds have been described in Table 1.
Table 1. Antibacterial activity of imidazo[1,2-a]pyridinylhydrazone derivatives 5a-q.
|Compounds||General structure||R||X||Inhibition diameter (mm)|
According to Ponce et al. , the compound could be designed as non-sensitive when its inhibition diameter is less than 8 mm. When this diameter is between 9 mm and 14 mm, the compound is called sensitive. In addition, when the diameter is between 15 mm and 19 mm, the compound is considered very sensitive. Beyond 20 mm, the compound is described as extremely sensitive. The determination of inhibition diameters allows an estimation of the sensitivity of the bacterial strain against the tested compounds. Thus, twelve (12) imidazo[1,2-a]pyridinylhydrazone derivatives generated activity against E. coli. These compounds were sensitive to E. coli at a concentration of 500 µg/mL with inhibition diameters ranging from 8 mm to 11 mm. However, at the same concentration, five derivatives were found to be non-sensitive on the same strain. If in the absence of a substituent, compound 5a showed the best activity with a diameter of 11 mm, the various substitutions made on the phenyl core did not improve the antibacterial activity. The introduction of a mesomeric donor group (OCH3, F, and Cl) in position-4 did not improve the activity of compound 5a. These different compounds each produced a diameter of 10 mm almost superimposed on that of compound 5a. On the other hand, when a mesomeric donor group such as bromine in position-3 (compound 5m) and a hydroxy in position-2 (compound 5e) is introduced, there is a considerable decrease in activity with respective diameters of 9 mm and 8 mm. This same hydroxy group in position-4 (compound 5h) leads to a loss of activity. In addition, the replacement of the groupings (OCH3, F, and Cl) by an inductive donor group such as the 4-position methyl (compound 5j) did not improve activity. The same applies when methyl is in the 2-position (compound 5d). The 5j and 5d compounds produced diameters of 10 mm. Other attempts to improve the activity did not yield better results. The addition of an EWG group by a mesomeric effect such as nitro in position-4 or -3 (compounds 5n or 5l) resulted in nearly superposable diameters equal to 10 mm and 9 mm respectively. This same grouping in position-2 leads to a loss of activity (compound 5i). In addition, the presence of the cyano group, which is a mesomeric effect attractor in position-3 (compound 5g) has generated an activity with a diameter of 9 mm. Similarly, double substitutions did not improve the activity of the compounds 5k, 5o, and 5p, leading to a loss of activity. The replacement of phenyl by aromatic nuclei having a heteroatom like nitrogen did not allow to have better activity. Indeed, the pyridinic compound 5q showed an activity of 10 mm comparable to that of compound 5a. These various pharmacomodulations allow us to say that the improvement of the antibacterial activity of the imidazo[1,2-a]pyridinylhydrazone model is not only related to variations in phenyl and compounds with heteroatoms.
This work resulted in the development of seventeen imidazo[1,2-a]pyridine hydrazone derivatives. The structure of the synthesized compounds was characterized by 1H, 13C NMR spectroscopy, and HRMS. The antibacterial activity of the 5a-q compounds showed that twelve compounds were active on E. Coli. Compound 5a had the best potential antibacterial activity with a diameter of 11mm.
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