Synthesis, Antimicrobial Evaluation and Molecular Docking Studies of Novel Oxazolidinone-Thiophene Chalcone Hybrid Derivatives

 

Naresh Panigrahi^1,2*, Swastika Ganguly², Jagadeesh Panda³

¹GITAM Institute of Pharmacy, GITAM (Deemed to be University), Rushikonda, Visakhapatnam-530 045

²Department of Pharmaceutical Sciences and Technology, Birla Institute of Technology, Mesra, Ranchi-835 215

³Raghu College of Pharmacy, Dakamarri, Bheemunipatnam, Visakhapatnam, 531 162, India

 

ABSTRACT:

In the present study, in order to synthesize broad-spectrum antibacterial agents, we coupled both the oxazolidinone and novel thiophene chalcone derivatives by using Buchwald's protocol under ultrasound irradiation. All the newly synthesized 20 oxazolidinone-thiophene chalcone hybrid compounds were characterized by IR, ¹HNMR and Mass spectral analysis and evaluated for in vitro antibacterial and antifungal activities. Antibacterial and antifungal activities were tested using the serial dilution method. The test compounds 7e to 7g exhibited very high activities at 3.125 µg/ml when compared to the standard drug linezolid and were as other compounds showed moderate to weak activities. In case of antifungal activities, the test compounds 7e to 7g, 7i ,7j, and  7m to 7o exhibited moderate activities compared to the standard fluconazole, whereas the rest of the compounds showed weak activities against tested fungal strains. The binding mode analysis of the test compounds 7e to 7g were investigated by the docking studies using Glide 6.6., inside the active site of peptidyl transferase center of E.coli ribosome (PDB ID: 2AW4), revealed that the experimental results were comparable to that of co‑crystallized ligand linezolid.

 

 

 

INTRODUCTION:

Oxazolidinones (1) are a novel and promising class of synthetic antibiotics that have recently emerged as important therapeutic agents. They have been associated with various activities, which include antimicrobial^1-4, antitubercular^5-7, anticancer⁸, anticonvulsant⁹, antiprotozoal¹⁰, antihiv¹¹, anticoagulant¹², antidepressant¹³, MAO- inhibitors¹⁴ and other pharmacological activities.

 

Figure-1: Structural information of oxazolidinone.

 

Chemically chalcones, are 1, 3-diaryl-2-propene-1-one, in which two aromatic rings are linked by a three carbon α, β-unsaturated carbonyl system (-CH_β=CH_α-C=O).  Replacing one of the aryl ring in chalcones with thiophene heterocyclic system enabled to obtain interesting antibacterial and antifungal activities¹⁵.

 

Great attention has been paid in recent years to develop hybrid compounds with broad spectrum of activity along with improved potency. Combining two cores into a single entity is a changeling task in organic chemistry. To date only a few number of synthetic methods have been established towards the synthesis of these hybrid compounds¹⁶. However, few examples were found where, C–N bonds are formed by copper mediated Ullmann and Goldberg type reactions with the aid of amine or phosphane ligands.¹⁷. Hartwig, Buchwald and Shakespeare have reported that arylation of amines, amides, and C-N bond forming cross-coupling reactions of NH-containing substrates in presence of copper metal has emerged as a powerful methodology.¹⁸ This discovery has had a significant impact on the synthesis of natural products and pharmaceuticals.¹⁹

 

Increasing attention is now being focused to develop N-arylation of nitrogen-containing heterocycles with aryl halides in the presence diamine ligands under green chemical methods, which are not only economical but also low cost and more environmentally friendly.^{18, 20} Traditional synthetic methods of N-arylation of copper-mediated coupling reactions has various limitations, such as high-temperature reaction conditions, moderate yields, use of expensive ligands, and stoichiometric amounts of the catalyst.¹⁸ Hence, efforts have been put towards the development of new, ecofriendly, high yielding, inexpensive and green chemistry approach for coupling of aryl halides (bromine containing thiophene chalcones) with oxazolidinone.

 

Recently Selvakumar et al., ²¹ synthesized a series of hybrid compounds possessing both chalcone and oxazolidinone moieties that were tested for antibacterial activity against gram-positive organisms and these hybrid molecules were found to be active. Based on our analysis of the limited data available on published oxazolidinone-chalcone hybrids till date, we decided that the most prudent approach would be to synthesize a novel type of analogs, which embody both the characteristic oxazolidinone core and a thiophene chalcone fragment.

 

In view of the above and in continuation of our research work, we took the advantages of sonochemical (green chemistry approach) method for coupling of 20 bromine containing thiophene chalcones with 5-chloromethyl-2-oxazolidinone. As far as we are aware, there are no reports on any reaction between these above-mentioned chemical entities.

 

MATERIALS AND METHODS:

Materials:

Chemicals and reagents were purchased from Sigma Aldrich India, Merck, SRL (Sisco Research Laboratories Pvt. Ltd.) and Molychem India. All the solvents and reagents used were of analytical grade. Melting point was checked using capillary method and was uncorrected. The IR spectra of synthesized compounds were taken on Bruker FTIR spectrophotometer. ¹H NMR spectra were recorded on Bruker Avance DRX400 (400 MHz, FTNMR) in DMSO and CDCl₃ solvents. Chemical shifts were reported as δ (ppm); TMS was taken as internal standard. Coupling constants J are expressed in Hertz. Mass spectra was recorded on Agilent 6410 Triple Quadrupole LC/MS. Reaction progress was monitored by TLC, reaction spots were visualized  under UV radiation chamber and iodine chamber. Purification of the synthesized compounds were carried out by column chromatography using silica gel (60–120 mesh). Sonication was performed by locating the reaction flask in the maximum energy area in ultrasonicator (with a frequency of 50 Hz and a nominal power 170 W).

 

Experimental:

The synthesis of novel oxazolidinone-thiophene hybrid consists of the three major steps as follows (represented in scheme-1).

 

Step-1: General procedure for synthesis of thiophene chalcones.¹⁵

A mixture of ketone 2-acetyl-5-bromo-thiophene (2) (1 g, 3.63 mmol), and corresponding aryl aldehydes (3a-3t) (shown in Table.No-1) (3.63 mmol) in ethanol (25 mL) was treated with lithium hydroxide monohydrate (LiOH·H₂O) (15 mg, 0.363 mmol). The mixture was irradiated in water bath of an ultrasonicator at room temperature until a precipitate was formed. This reaction mixture was then poured over crushed ice and acidified with dilute HCl. The solid thus obtained was filtered, washed with water and dried over anhydrous sodium sulfate (Na₂SO₄). Further, the residue obtained was purified on column chromatography to afford pure crystals of (4a-4t) (as shown in Scheme-1, step-1).  Physicochemical properties and spectral data of thiophene chalcones were reported in our earlier article.¹⁵

 

 

 

Scheme 1: General procedure for synthesis of oxazolidinone-thiophene chalcone hybrid derivatives (7a-7t).

Table No.1: Corresponding aryl aldehydes used during synthesis of thiophene chalcones.

 

(3a-3t).

 

 

Step-2: General procedure for Synthesis of 5-(chloromethyl)-oxazolidin-2-one (6).

To a stirred solution of epichlorohydrin (5)  (5.0 g, 0.054 mol) in water (50 mL), potassium cyanate (8.76 g, 0.108 mol) and magnesium sulfate (13.0 g, 0.108 mol) were added at ambient temperature. The temperature of the reaction mixture was raised to 100 °C and refluxed for 15 hrs. The reaction mixture was filtered to remove solids and the resulted filtrate was extracted with ethyl acetate (2×25 mL). The combined organic layer was washed with saturated sodium chloride solution (25 mL), dried over anhydrous sodium sulfate and the solvent was removed by evaporation under reduced pressure. The solid obtained was purified by column chromatography with n-hexane: ethylacetate (7:3) to afford 5-(chloromethyl)-oxazolidin-2-one (6)²² as a white solid (as shown in scheme-1, step-2).

 

Step-3: General procedure for coupling of 5-(chloromethyl)-oxazolidin-2-one (6) with thiophene chalcones (4a-4t)²³.

A mixture of corresponding thiophene chalcones (4a-4t) (shown in Table.No-2) (1 gm, 2.6 mmol), 5-(chloromethyl)-oxazolidin-2-one (6) (358 mg, 2.6 mmol), copper iodide (49 mg, 0.26 mmol), potassium carbonate (718 mg, 5.2 mmol), (±)-trans-1,2-diaminocyclohexane (29 mg, 0.26 mmol) and 2 mL of dry 1,4-dioxane were added using a syringe at room temperature to 25 mL of volumetric flask. The cap of the flask was fitted with a rubber septum and was evacuated and back filled with nitrogen gas by using a balloon and this sequence was repeated twice. The mixture was irradiated in the water bath of an ultrasonic cleaner at the room temperature for 2 to 3hrs. The reaction was quenched by adding ethyl acetate and the resulting solution was filtered through a pad of celite. The filtrate was concentrated and the residue was purified by column chromatography on silica gel (60-120 mesh) using n-hexane: ethylacetate (7:3) as solvent to afford (7a-7t) (as shown in scheme-1, step-3).

 

Antimicrobial screening:

The in vitro antibacterial and antifungal screening of newly synthesized 20 novel oxazolidinone-thiophene chalcone hybrid derivatives were determined using serial dilution method ^{31, 32}.

 

Antibacterial screening:

The test compounds 7a to 7t were screened for in vitro antibacterial activity against three gram-positive bacteria [Staphylococcus aureus NCIM 5257, Bacillus subtilis NCIM 2097 and Streptococcus faecalis NCIM 2080] and three gram-negative bacteria [Escherichia coli NCIM 2065, Pseudomonas aeruginosa NCIM 5210, Klebsiella pneumoniae NCIM 5289] by serial  dilution method. The above bacterial strains were obtained from NCIM -National Collection of Industrial Microorganisms, Pune, India.  Linezolid was used as the standard drugs. The medium used was double strength nutrient broth. Cultures of test organisms were maintained on nutrient agar slants and were sub cultured in Petri dishes prior to activity. The nutrient agar and nutrient broth used for in vitro antibacterial studies were procured from Himedia Laboratories, Mumbai.

 

Stock solutions of the synthesized compounds of different concentrations were prepared in the range of 1000 µg/ml to 1.56 µg/ml, using dimethyl sulfoxide (DMSO) as solvent for antibacterial activity. Water was used as solvent to dissolve the standard drug linezolid.

To verify that the solvent had no effect on bacterial growth, a control test was performed with test medium employing DMSO at the same dilution as used in the experiments. The minimum inhibitory concentration (MICs) values were the lowest concentration of compounds, which resulted in no visible growth or turbidity in the culture media after 24 h of incubation at 37 °C and are listed in Table No.4.

 

Antifungal screening:

The test compounds 7a to 7t were screened for in vitro antifungal activity against two standard organisms [Candida albicans NCIM 3628 and Aspergillus niger NCIM 1317] by two fold serial broth dilution method. Fluconazole was used as the standard. The medium used was double strength malt yeast extract broth. Test compounds and standard drug (Fluconazole) were dissolved in dimethyl sulfoxide (DMSO) to give a concentration range of 1000 µg/ml to 1.56 µg/ml. Minimum Inhibitory Concentration (MIC) of the synthesized compounds was determined. The MIC is the lowest concentration of tested compounds that completely inhibited the growth of the test organisms after 48 h of incubation at 25-27 °C and are listed in Table No.5.

 

Molecular docking studies:

A perusal of the literature revealed that oxazolidinones exert their antimicrobial effects by blocking protein synthesis at peptidyl transferase center (PTC) on the bacterial ribosome.²⁴ Considering the well obtained in-vitro results, a profound docking study was carried out to consider the possible binding mode of the highest active compounds 7e-7g inside the active site of peptidyl transferase center of E.coli ribosome in complex with linezolid (PDB ID: 2AW4) using Glide 6.6.²⁵

 

Protein preparation:

The three‑dimensional structure of the ribosome for this study was downloaded from the PDB (code; PDB ID: 2AW4, a model of linezolid binding to peptidyl transferase center of E.coli ribosome).²⁶ Protein preparation of the ribosome structure was carried out using Protein preparation Wizard tool of Schrodinger Suite 2015.²⁷ Initially the ribosome was imported to the workspace and then the structure was preprocessed by assigning bond orders, by adding hydrogens, treating metals, checking disulfide bonds, missing atoms, bonds, loops, and contacts using prime. Water molecules were deleted manually. After assigning charge and protonation state, finally, energy minimization was done using OPLS 2005 force field.

 

The Glide grid generation wizard generated a grid area around the binding site of the ribosome, by manually picking up the co‑crystallized ligand (linezolid), which determines the position and size of the active site automatically and set up Glide constraints for docking the ligands.

 

Ligand structure preparation:

The test compounds 7e-7g were initially drawn on 2D wizard of ChemOffice 16.0 version (PerkinElmer, United States), energy minimization was carried using MM2 force field and saved in MDL mole format. Using the LigPrep (ligand preparation) utility of Glide, these structures were geometry optimized using the Optimized Potentials for Liquid Simulations‑2005 (OPLS‑2005) force field with the steepest descent followed by truncated Newton conjugate gradient protocol.

Validation of docking protocol:

The most suitable method of evaluating the accuracy of a docking procedure is to determine how closely the lowest energy pose predicted by the scoring function resembles an experimental binding mode as determined by X-ray crystallography.^{28, 29}

 

To validate the docking protocol, the ligand present in the respective target (cocrystallized linezolid structure) were extracted previously from their receptor complex and docking was performed at their respective binding site. The lowest RMSD (root mean square deviation) value relative to the crystallographic pose determines the reliability and reproducibility of the docking protocol^{28, 29}. Linezolid was successfully redocked with a RMSD of 0.6 Å. This indicates the reliability of the docking method in reproducing the experimentally observed binding mode for 2AW4.

 

Docking ligands:

Based on the results obtained from the antibacterial activities, docking studies were carried out on the highest active compounds 7e-7g inside the active site of peptidyl transferase center of E.coli ribosome in complex with linezolid (PDB ID: 2AW4) using Glide 6.6. Version.

 

The linezolid binding site (or active site) of E. coli 50S ribosome (PDB ID: 2AW4)  was found to lie within the 20 nucleic acid residues G2053, A2054, C2055, G2056, G2057, A2058, A2059, G2061, A2062, C2452, A2451, A2503, U2504, G2505, U2506, G2576, A2577, U2585, C2611, C2612, etc.³⁰

 

RESULTS AND DISCUSSION:

Chemistry:

As part of our research to synthesize molecules with enhanced broad spectrum of activity we combined the two antibacterial cores (oxazolidinones with thiophene chalcones) into a single entity to achieve novel hybrid scaffolds of potent antibacterial agents. Herewith we attempted to couple both thiophene chalcone and oxazolidinone cores by using buchwald's protocol under ultrasound irradiation. In this paper, we report the synthesis and antimicrobial activities of 20 novel oxazolidinone-thiophene chalcone hybrid derivatives (7a-7t). IR, ¹H NMR and Mass spectral data was collected for the synthesized compounds and the data was found consistent to the assigned structure.

 

The synthesis of the compounds were carried out in three steps. The first step involves the synthesis of thiophene chalcones by treating the aryl ketone 2-acetyl-5-bromo-thiophene (2) with corresponding aryl aldehydes (3a-3t) in ethanol with lithium hydroxide monohydrate (LiOH·H₂O) as base catalyst under ultrasound irradiation¹⁵. The second step involves synthesis of compound 5-(chloromethyl)-oxazolidin-2-one²² (6) by refluxing epichlorohydrin, potassium cyanate and magnesium sulfate in water at 100 ⁰C for 15 hrs until a white solid is formed. The third step involves coupling of corresponding thiophene chalcones (4a-4t) with 5-(chloromethyl)-oxazolidin-2-one (6), in presence of copper iodide, potassium carbonate (base), (±)-trans-1,2-diaminocyclohexane (ligand) under nitrogen gas environment²³. The mixture was irradiated in the water bath of an ultrasonicator at the room temperature for a period mentioned in the Table No 3.

 

Initially the reaction was carried out in air to know the effect of ultrasonication on percentage yield and time of product formation, but we found that the reactions proceeded very slowly with poor yields (less than 40 percentage). Reactions were thus carried out under nitrogen gas environment (Figure-2), which provided good yields (above 65 percentage) with shorter period of time that is less than 5 hrs at normal room temperature when compared to the conventional buchwald’s protocol (5-20 hrs at 110 ⁰C).

 

 

Figure-2: Ultrasonication setup to carry out the coupling reaction between 5-(chloromethyl)-oxazolidin-2-one and thiophene chalcones under nitrogen gas environment.

 

The FT-IR spectra of the synthesized oxazolidinone-thiophene chalcone hybrids showed the vibration bands at a range of ν 3000-3200 cm^-1 and  bands at 1500-1610 cm^-1 are assigned to the vibrations of the aromatic =C-H and aromatic –C=C- stretch respectively. The vibration bands at a range of ν 1650-1786 cm^-1 assigned to C=O strech of α, β unsaturated ketone nucleus which confirms the formation of chalcones. The ¹H NMR of the novel oxazolidinone-thiophene chalcones suggested two doublets, one at a range of δ 7.15–8.23 ppm (for Hα) and another at a range of δ 7.45–8.07 ppm (for H_β) for vinylic protons nearer the carbonyl group                          (-CH_β=CH_α-C=O). Interestingly for all compounds we found the coupling constants value (J) in the range of J_{Hα –Hβ} = 15-16 Hz, that confirms the trans configuration of the vinylic system. Further, the aromatic protons of thiophene ring were observed at a range of δ 6.9–7.5 ppm. The coupling of oxazolidinone ring system with thiophene chalcones were confirmed by absence of singlet at  δ 5.2-6.5 which was assigned to –NH of oxazolidinone ring. ¹H NMR spectrum of oxazolidinone ring showed a multiplet of two protons assigned to the methylenic protons of -CH₂-Cl ranging from δ 3.50-3.60, while a methynic proton of oxazolidinone -CH-CH₂- showed a multiplet of one proton ranging from δ 4.80-5.80, the methylenic protons of oxazolidinone -CH-CH₂- showed multiplet of two protons ranging from δ 3.63-4.07. Finally, the molecular ion peaks observed for all the synthesized oxazolidinone-thiophene chalcone hybrid molecules (7a-7t) by ESI-MS strongly reveals their predicted molecular weights.

 

Spectral data of novel oxazolidinone-thiophene chalcone hybrids:

Spectral data of thiophene chalcones were already reported in our earlier article.¹⁵

 

5-(chloromethyl)-oxazolidin-2-one (6)

Anal: IR (KBr) (cm-1): 3257.86 and 3444.02 (-N-H stretch), 1749.88 (-C=O), 714.46(-C-Cl stretch), ¹H NMR (400 MHz, Chloroform-d) δ: 3.536–3.572 (m, 1H), 3.663–3.787 (m, 3H), 4.823–4.887 (m, 1H), 5.60 (s, NH of oxazolidinone ring), ¹³C NMR (100 MHz, Chloroform-d) δ: 43.39, 43.59, 74.70, 159.12, MS (ESI m/z): 136.12 [M+H]⁺.

 

5-(chloromethyl)-3-(5-(3-(thiophen-2-yl)acryloyl)thiophen-2-yl)oxazolidin-2-one (7a)

IR (KBr) (cm^-1): 3088.69 (aromatic =C-H), 1691.68 (C=O), 1597.94 (aromatic –C=C-), 783.34 (C-Cl), ¹H NMR (400 MHz, Chloroform-d) δ  7.94–7.98 (d, 1H), 7.58–7.59 (d, 1H), 7.44–7.46 (d, 1H), 7.38–7.39 (d, 1H), 7.15–7.16 (d, 1H), 7.09–7.13 (d, 2H), 4.83–4.89 (m, 1H, oxazolidinone), 3.74–3.79 (m, 1H), 3.67–3.72 (dd, 2H), 3.58–3.56 (m, 1H), MS (ESI m/z): 300.10 [M+H]⁺.

 

5-(chloromethyl)-3-(5-(3-(p-tolyl)acryloyl)thiophen-2-yl)oxazolidin-2-one  (7b)

IR (KBr) (cm^-1): 3063.79 (aromatic =C-H), 1672.78 (C=O), 1588.20 (aromatic –C=C-), 776.43 (C-Cl), ¹H NMR (400 MHz, DMSO-d) δ 8.21–8.22 (d, 1H), 7.76–7.82 (m, 3H), 7.62–7.66 (d, 1H), 7.27–7.29 (d, 2H), 6.78–6.79 (d, 1H), 5.15–5.18 (m, 1H), 4.26–4.30 (m, 1H), 4.00–4.09 (m, 2H), 3.90-3.93 (m, 1H), 2.50 (s, 3H), MS (ESI m/z): 308.00 [M+H]⁺.

5-(chloromethyl)-3-(5-(3-(2-hydroxyphenyl)acryloyl)thiophen-2-yl)oxazolid in-2-one (7c)

IR (KBr) (cm^-1): 3018.97 (aromatic =C-H), 3444.45 (-OH stretch), 1747.20 (C=O), 1487.33 (aromatic –C=C-), 762.78 (-C-Cl), ¹H NMR (400 MHz, Chloroform-d) δ 7.69 (s, 1H), 7.42–7.43 (d, 4H), 7.09–7.10 (dd, 4H), 4.82-4.88 (m, 1H), 3.66-3.78 (m, 3H), 3.53–3.56 (m, 1H), MS (ESI m/z): 310.48 [M+H]⁺.

 

5-(chloromethyl)-3-(5-(3-(4-hydroxyphenyl)acryloyl)thiophen-2-yl)oxazolid in-2-one (7d)

IR (KBr) (cm^-1): 3125.60 (aromatic =C-H), 3434.01 (-OH stretch), 1673.21 (C=O), 1523.54 (aromatic –C=C-), 753.50 (-C-Cl), ¹H NMR (400 MHz, Chloroform-d) δ 7.60–7.63 (d, 2H), 7.59 (s, 1H), 7.51–7.53 (d, 2H), 7.29–7.31 (d, 1H), 7.12–7.13 (d, 1H), 6.79–6.81 (d, 2H), 5.03-5.09 (m, 1H), 4.00-4.04 (m, 1H), 3.75–3.82 (m, 3H), MS (ESI m/z): 310.48 [M+H]⁺.

 

5-(chloromethyl)-3-(5-(3-(2-fluorophenyl)acryloyl)thiophen-2-yl)oxazolidin-2-one (7e)

IR (KBr) (cm^-1): 3158.40 (aromatic =C-H), 1675.36 (C=O), 1597.60 (aromatic –C=C-), 790.96 (-C-Cl), 1035.81 (-C-F), ¹H NMR (400 MHz, Chloroform-d) δ 7.90–7.94 (d, 1H), 7.61–7.65 (m, 2H), 7.38–7.48 (d, 2H), 7.02–7.26 (m, 3H),  4.84–4.88 (m, 1H), 3.70–3.79 (m, 3H), 3.53–3.57 (m, 1H),  MS (ESI m/z): 312.20 [M+H]⁺.

 

5-(chloromethyl)-3-(5-(3-(3-fluorophenyl)acryloyl)thiophen-2-yl)oxazolidin-2-one (7f)

IR (KBr) (cm^-1): 3570.76 (aromatic =C-H), 1654.94 (C=O), 1590.72 (aromatic –C=C-), 750.01 (-C-Cl), 1024.56 (-C-F), ¹H NMR (400 MHz, Chloroform-d) δ 7.75–7.79 (d, 1H), 7.62–7.64 (d, 2H), 7.49-7.51 (d, J = 15.3 Hz, 1H), 7.39–7.42 (m, 2H), 7.28–7.30 (d, 1H), 7.18–7.19 (d, 1H), 4.83–4.90 (m, 1H), 3.75–3.80 (m, 1H), 3.67–3.73 (m, 2H), 3.54–3.58 (m, 1H),  MS (ESI m/z): 312.20 [M+H]⁺.

 

5-(chloromethyl)-3-(5-(3-(4-fluorophenyl)acryloyl)thiophen-2-yl)oxazolidin-2-one (7g)

IR (KBr) (cm^-1): 3570.76 (aromatic =C-H), 1654.94 (C=O), 1590.72 (aromatic –C=C-), 750.01 (-C-Cl), 1024.56 (-C-F), ¹H NMR (400 MHz, Chloroform-d) δ 8.81–8.82 (d, 1H), 7.53–7.92 (m, 3H), 7.54–7.57 (d, J = 15.0 Hz, 1H), 7.08–7.16 (d, 2H), 4.84–4.91 (m, 1H), 3.68–3.81 (m, 3H), 3.55–3.59 (m, 1H), MS (ESI m/z): 312.20 [M+H]⁺.

5-(chloromethyl)-3-(5-(3-(4-methoxyphenyl)acryloyl)thiophen-2-yl)oxazolid in-2-one (7h)

IR (KBr) (cm^-1): 3081.28 (aromatic =C-H), 1697.69 (C=O), 1575.44 (aromatic –C=C-), 786.32 (-C-Cl), ¹H NMR (400 MHz, Chloroform-d) δ 7.79–7.83 (d, 1H), 7.58–7.61 (m, 3H), 7.18–7.22 (d, J = 15.0 Hz, 1H), 7.14–7.15 (d, 1H), 6.93–6.95 (d, 2H), 4.81–4.88 (m, 1H), 3.86 (s, 3H), 3.73–3.78 (m, 1H), 3.66-3.713.52–3.73 (m, 2H), MS (ESI m/z): 324.18 [M+H]⁺.

 

5-(chloromethyl)-3-(5-(3-(2-chlorophenyl)acryloyl)thiophen-2-yl)oxazolidin-2-one (7i)

IR (KBr) (cm^-1): 3020.02 (aromatic =C-H), 1749.78 (C=O), 1597.60 (aromatic –C=C-), 762.78 (-C-Cl), ¹H NMR (400 MHz, Chloroform-d) δ 7.75–7.79 (d, 1H), 7.63–7.64 (d, 2H), 7.49–7.51 (d, 1H), 7.39–7.42 (m, 2H), 7.30–7.37 (d, 1H), 7.17–7.18 (d, J = 15.3 Hz, 1H), 4.83–4.89 (m, 1H), 3.67 – 3.79 (m, 3H), 3.54 – 3.57 (m, 1H), MS (ESI m/z): 328.20 [M+H]⁺.

 

5-(chloromethyl)-3-(5-(3-(4-chlorophenyl)acryloyl)thiophen-2-yl)oxazolidin-2-one (7j)

IR (KBr) (cm^-1): 3020.02 (aromatic =C-H), 1749.78 (C=O), 1597.60 (aromatic –C=C-), 762.78 (-C-Cl), ¹H NMR (400 MHz, Chloroform-d) δ 7.78–7.81 (d, 2H), 7.57–7.61 (m, 2H), 7.40–7.42 (d, 2H), 7.17–7.18 (d, 2H), 4.83–4.89 (m, 1H), 3.67–3.80 (m, 3H), 3.54–3.58 (m, 1H), MS (ESI m/z): 328.20 [M+H]⁺.

 

3-(5-(3-(1H-indol-2-yl)acryloyl)thiophen-2-yl)-5-(chloromethyl)oxazolidin- 2-one (7k)

IR (KBr) (cm^-1): 3020.03 (aromatic =C-H), 3232.39 (N-H stretch), 1646.99 (C=O), 1522.18 (aromatic –C=C-), 762.67 (-C-Cl), ¹H NMR (400 MHz, Chloroform-d) δ 10.08 (s, 1H), 8.74–8.78 (d, 1H), 8.59–8.61 (d, 2H), 8.27–8.29 (d, 1H), 7.60–7.67 (m, 1H), 7.56–7.57 (m, 2H), 7.28–7.32 (d, 1H), 7.16–7.17 (d, 1H), 4.84–4.88 (m, 1H), 3.70–3.79 (m, 3H), 3.53–3.57 (m, 1H), MS (ESI m/z): 333.90 [M+H]⁺.

 

5-(chloromethyl)-3-(5-(3-(4-(dimethylamino)phenyl)acryloyl)thiophen-2-yl)oxazolidin-2-one  (7l)

IR (KBr) (cm^-1): 3020.03 (aromatic =C-H), 1643.25 (C=O), 1522.16 (aromatic –C=C-), 760.75    (-C-Cl), ¹H NMR (400 MHz, Chloroform-d) δ  7.43–7.44 (d, 4H), 7.10–7.11 (d, 4H), 4.84–4.87 (m, 1H), 3.69–3.79 (m, 3H), 3.53–3.57 (m, 1H),  2.51 (s, 6H), MS (ESI m/z): 337.12 [M+H]⁺.

 

 

5-(chloromethyl)-3-(5-(3-(2-nitrophenyl)acryloyl)thiophen-2-yl)oxazolidin-2-one (7m)

IR (KBr) (cm^-1): 3069.50 (aromatic =C-H), 1639.77 (C=O), 1577.9 (aromatic –C=C-),1340.3 (Ar-NO₂), 786.85 (-C-Cl), ¹H NMR (400 MHz, Chloroform-d) δ 8.19–8.23 (d, 1H), 8.09–8.11 (d, 1H), 7.71–7.75 (m, 2H), 7.58–7.64 (m, 2H), 7.14–7.19 (d, 2H), 4.83–4.90 (m, 1H), 3.75–3.80 (m, 1H), 3.68-3.73 (m, 2H), 3.54–3.58 (m, 1H), MS (ESI m/z): 339.11 [M+H]⁺.

 

5-(chloromethyl)-3-(5-(3-(3-nitrophenyl)acryloyl)thiophen-2-yl)oxazolidin-2-one (7n)

IR (KBr) (cm^-1): 3096.24 (aromatic =C-H), 1643.83 (C=O), 1583.28 (aromatic –C=C-),1331.74 (Ar-NO₂), 798.84 (-C-Cl), ¹H NMR (400 MHz, Chloroform-d) δ 8.52–8.53 (d, 1H), 8.27–8.29 (d, 1H), 7.85–7.93 (m, 2H), 7.62–7.68 (m, 2H), 7.47-7.48 (d, 1H), 7.20–7.21 (d, 1H), 4.83–4.89 (m, 1H), 3.67–3.80 (m, 3H), 3.56–3.58 (m, 1H), MS (ESI m/z): 339.11 [M+H]⁺.

 

5-(chloromethyl)-3-(5-(3-(4-nitrophenyl)acryloyl)thiophen-2-yl)oxazolidin-2-one (7o)

IR (KBr) (cm^-1): 3502.10 (aromatic =C-H), 1694.53 (C=O), 1551.63 (aromatic –C=C-),1339.32 (Ar-NO₂), 778.14 (-C-Cl), ¹H NMR (400 MHz, Chloroform-d) δ 8.52–8.53 (d, 1H), 8.27–8.29 (d, 1H), 7.85–7.93 (m, 2H), 7.62–7.68 (m, 2H), 7.47-7.48 (d, 1H), 7.20–7.21 (d, 1H), 4.83–4.89 (m, 1H), 3.67–3.80 (m, 3H), 3.56–3.58 (m, 1H), MS (ESI m/z): 339.11 [M+H]⁺.

 

5-(chloromethyl)-3-(5-(3-(4-hydroxy-3-methoxyphenyl)acryloyl)thiophen-2-yl)oxazolidin-2-one (7p)

IR (KBr) (cm^-1): 3664.97 (aromatic =C-H), 3407.81 (-OH stretch), 1641.26 (C=O), 1580.80 (aromatic –C=C-), 793.28 (-C-Cl), ¹H NMR (400 MHz, Chloroform-d) δ 8.20 (s, 1H), 7.89–7.81 (d, 1H), 7.61–7.63 (d, 1H), 7.30–7.33 (d, 1H), 7.12–7.15 (m, 2H), 6.99–7.01 (d, 1H), 6.83–6.85 (d, J = 1.5 Hz, 1H),  5.03–5.09 (m, 1H), 4.03-4.06 (m, 1H) 3.95–4.00 (m, 1H), 3.84 (s, 3H), 3.75–3.79 (m, 3H), MS (ESI m/z): 340.15 [M+H]⁺.

 

5-(chloromethyl)-3-(5-(3-(3,4-dimethoxyphenyl)acryloyl)thiophen-2-yl)oxazolidin-2-one. (7q)

IR (KBr) (cm^-1): 3074.57 (aromatic =C-H), 1656.65 (C=O), 1583.65 (aromatic –C=C-), 720.81 (-C-Cl), ¹H NMR (400 MHz, Chloroform-d) δ 7.74–7.78 (d, 1H), 7.58–7.59 (d, J = 15.0 Hz, 1H), 7.19–7.22 (m, 1H), 7.15 (s, 1H), 7.12–7.13 (d, 2H),  6.87–6.89 (d, 2H), 4.75–4.84 (m, 1H), 3.90 (s, 3H), 3.92 (s,3H), 3.67–3.74 (m, 3H), 3.48–3.51 (m, 1H), MS (ESI m/z): 354.05 [M+H]⁺.

3-(5-(3-(4-bromothiophen-2-yl)acryloyl)thiophen-2-yl)-5-(chloro methyl)-oxazolidin-2-one

IR (KBr) (cm^-1): 3088.71 (aromatic =C-H), 1655.33 (C=O), 1546.33 (aromatic –C=C-), 680.50    (-C-Br), ¹H NMR (400 MHz, Chloroform-d) δ 7.82–7.86 (d, 1H), 7.58–7.59 (d, 1H), 7.29–7.33 (d, 2H), 7.16–7.17 (d, 1H), 7.10–7.13 (d, J = 1.5 Hz, 1H), 7.12–7.13 (d, 1H), 7.07–7.08 (d, 1H), 4.83–4.90 (m, 1H), 3.74–3.80 (m, 1H), 3.68–3.73 (m, 2H), 3.54–3.58 (m,1H),  MS (ESI m/z): 379.32 [M+H]⁺.

 

5-(chloromethyl)-3-(5-(3-(2,3,4-trimethoxyphenyl)acryloyl)thiophen-2-yl)oxazolidin-2-one (7s)

IR (KBr) (cm^-1): 3069.50 (aromatic =C-H), 1639.77 (C=O), 1525.51 (aromatic –C=C-), 786.85 (-C-Cl), ¹H NMR (400 MHz, Chloroform-d) δ 7.97–8.01 (d, 1H), 7.57–7.58 (d, 1H), 7.35–7.39 (d, 2H), 7.14–7.15 (d, 1H),  6.71–6.73 (d, 1H), 4.82–4.88 (m, 1H), 3.89 (s, 3H), 3.92 (s, 3H), 3.95 (s, 3H), 3.66–3.78 (m, 3H), 3.52–3.56 (m, 1H), MS (ESI m/z): 384.06 [M+H]⁺.

 

5-(chloromethyl)-3-(5-(3-(3,4,5-trimethoxyphenyl)acryloyl)thiophen-2-yl)oxazolidin-2-one (7t)

IR (KBr) (cm^-1): 3561.55 (aromatic =C-H), 1691.29 (C=O), 1554.33 (aromatic –C=C-), 739.10 (-C-Cl), ¹H NMR (400 MHz, Chloroform-d) δ 7.96–8.00 (d, 1H), 7.56–7.57 (d, 1H), 7.34–7.38 (d, 2H), 7.12–7.13 (d, 1H),  6.70–6.72 (d, 1H), 4.80–4.86 (m, 1H), 3.88 (s, 3H), 3.90 (s, 3H), 3.94 (s, 3H), 3.72–3.76 (m, 1H), 3.68–3.70 (m, 2H), 3.50–3.54 (m, 1H), MS (ESI m/z): 384.06 [M+H]⁺.

 

Different substitutions on the aromatic ring (R) and physicochemical properties of thiophene chalcones (4a-4t) along with newly synthesized novel oxazolidinone-thiophene chalcone hybrid compounds (7a-7t) were shown in Table.No.2 and Table.No.3 respectively.

 

 

Table No.2: Substitutions at (R) and physicochemical properties of thiophene chalcones (4a-4t).

 

 

 

 

 

 

 

 

 

 

 

 

 

Table No.3: Substitutions at (R₁) and Physicochemical properties of newly synthesized oxazolidinone-thiophene chalcone hybrid derivatives (7a-7t).

 

 

 

Antimicrobial activity:

All the newly synthesized oxazolidinone-chalcone hybrid compounds (7a-7t) were screened for their in vitro antibacterial and antifungal activities. The results are produced in Table No.4 and Table No.5 respectively.

 

Results of antibacterial screening:

The results of antibacterial screening revealed that all the tested compounds showed moderate to good bacterial inhibition against gram positive and gram-negative bacteria [six bacterial strains, three gram positive  S. aureus, B. subtilis, S. faecalis and three gram negative bacteria, E. coli, P. aeruginosa, K. pneumonia.] compared to standard compound linezolid. Compounds 7e-7g were found to be active at 3.125 µg/ml against gram positive strains were as same compounds showed activity at 6.25 µg/ml against gram negative strains compared to the standard drug linezolid.

 

Test compounds 7c-7d, 7h-7j and 7p exhibited significant activities at 3.125 to 6.25 µg/ml against gram-positive bacteria but showed moderate activities at 25-50 µg/ml against gram-negative strains compared to standard linezolid. The other test compounds exhibited varied degree of antibacterial activity against the tested strains of bacteria.

 

 

Table No.4: Minimum inhibitory concentration MIC (μg/ mL) of newly synthesized oxazolidinone-thiophene chalcone hybrid derivatives (7a-7t).

 

Results of antifungal screening:

The results of antifungal screening revealed that the tested compounds 7e-7g, 7i-7j and 7m-7o showed moderate antifungal inhibition at 12.5 µg/ml against Candida albicans and Aspergillus niger compared to the standard fluconazole whereas all other compounds showed weak activities against both the fungal strains. It is evident from the literature and from our studies that the oxazolidinones have better antibacterial activity rather than antifungal activity.

 

Table No.5: Minimum Inhibitory Concentration (MIC μg/ mL) of Test Compounds (7a to 7t) against Candida albicans and Aspergillus niger.

 

Structure activity relationships (SAR):

The synthesized compounds contains both the combination of electron with drawing (EWG) and electron donating (EDG) groups at different positions on the ring R₁. Structure activity relationship (SAR) of the screened compounds results suggest that the effect of substitutions on the ring R₁ on antibacterial activity is mostly steric, with only small or electronegative substituents (F < Cl < Br < NO₂ and OH < CH₃ < OCH₃) tolerates the ribosomal binding site (oxazolidinones inhibits protein synthesis),²⁴ whereas increase in the bulk of substitution are not tolerated. Substitution with electron with drawing (EWG) groups like fluorine and chlorine derivatives are more active amongst all and are even better than that of hydroxyl derivatives against each strain, that may be attributed to higher lipophilicity of these groups.

 

Molecular docking results:

Based on the results obtained from the antibacterial activities, docking studies were carried out on the highest active compounds 7e-7g inside the active site of peptidyl transferase center of E.coli ribosome in complex with linezolid (PDB ID: 2AW4) using the “Extra Precision” (XP) mode of Glide 6.6.Version.

 

The docking protocol was validated by redocking the cocrystallized linezolid structure in the binding site to determine the lowest RMSD (root mean square deviation) relative to the crystallographic pose. Linezolid was successfully redocked with a RMSD of 0.6 Å. The docking pose and the binding modes of the co-crystalized ligand (linezolid) and the redocked conformer of linezolid in the active site of ribosome is shown in Fig. 2a and 2b respectively.

 

Fig.2 (a). Validation of docking by Glide.

 

Fig.2 (b).  

Superimposed structures of Cocrystallized conformation of linezolid (red colour) (tubular model) and that predicted by Glide (green) (ball and stick model) showing interactions in the active site of peptidyl transferase center of E.coli ribosome (PDB ID: 2AW4), with the nucleic acid residue G2505. Active site nucleic acid residues are represented as sticks. Hydrogen bond interactions are represented by yellow and pink‑dotted line. Fig.2 (b). 2D Ligand plot diagram of redocked conformer linezolid predicted by Glide showing interaction in the active site of peptidyl transferase center of E.coli ribosome (PDB ID: 2AW4), with the nucleic acid residue G2505. Active site nucleic acid residues were represented in blue, hydrogen bond interactions are represented by pink‑dotted line.

 

The binding modes of highest active test compound 7e-7g into the active site of peptidyl transferase center of E.coli ribosome in complex with linezolid (PDB ID: 2AW4) is shown in the Fig. 3a , Fig. 3b and 3c respectively. A summary of docking results (hydrogen bond interactions, Glide XP scores and interactions with nucleic acid residues) were depicted in the Table-No-6.

 

Table-No-6: A summary of docking results into the active site of peptidyl transferase center of E.coli ribosome (PDB ID: 2AW4).

Test Compd.	Glide XP scores	No Of Hydrogen Bonds (HB)	Hydrogen bond distance between interacting residues (A0)	Nucleic Acid interactions in the active site
5	-4.93	1HB	2.20	A2062
6	-5.12	1HB	1.92	A2062
7	-4.64	0HB	------	No interactions
Linezolid	-5.0	1HB	2.46	G2505

 

It was interesting to note that the C=O (ketone) of α,β-unsaturated carbonyl group of test compounds 7e  (C=O _{α,β-unsaturated carbonyl group} ….. NH_{2 A2062} = 2.20 A⁰) and 7f  (C=O _{α,β-unsaturated carbonyl group} …… NH_{2 A2062} = 1.92 A⁰) formed  a hydrogen bond with the  -NH₂  group of A2062 residue, while the test compound 7g showed hydrophobic interaction with the residues in the active site. Thus, it is evident that these interactions and hydrogen bonds may be responsible for the similar docking scores -4.93, -5.12 and -4.64 of test compounds 7e, 7f and 7g respectively with that of the co-crystallized ligand linezolid (-5.0). Finally, the binding mode analysis of the test compounds inside the active site of peptidyl transferase center of E.coli ribosome (PDB ID: 2AW4), revealed that the experimental results were comparable to that of co‑crystallized ligand linezolid.

 

 

 

Fig.3 (a)

 

 

Fig.3 (b)

 

Fig.3 (c)

 

Fig.3 (a) and 3 (b). 2D Ligand plot diagram of XP Glide-predicted pose of highly active molecule 7e and 7f showing hydrogen bond interaction in the active site of peptidyl transferase center of E.coli ribosome (PDB ID: 2AW4), with the nucleic acid residue A2062. Fig.3 (c) 2D Ligand plot diagram of XP Glide-predicted pose of highly active molecule 7g  showing hydrophobic interactions in the active site of peptidyl transferase center of E.coli ribosome (PDB ID: 2AW4). Active site nucleic acid residues were represented in blue, hydrogen bond interactions are represented by pink‑arrows.

 

CONCLUSION:

We have successfully coupled two antibacterial scaffolds in to single entity using the Buchwald protocol under ultrasound irradiation with a simple set up at room temperature. This approach would be a worthwhile in the development of green chemistry protocols for CuI-mediated N-arylation of oxazolidinones. The method is experimentally simple, high yielding, short time and efficient process. Antimicrobial activities of the newly synthesized hybrid molecules (7a-7t) were evaluated against selected strains of bacterial and fungal strains. Among them compounds, 7e to 7g have shown significant antibacterial and moderate antifungal activities when compared to the standard drug linezolid and fluconazole against bacterial and fungal strains respectively. A systematic SAR study on screened compounds reveals that the effect of substitutions on the ring R₁ on antibacterial activity is mostly steric, with only small or electronegative substituents (F < Cl < Br < NO₂ and OH < CH₃ < OCH₃) tolerates the ribosomal binding site, whereas increase in the bulk of substitution are not tolerated. The binding mode analysis of the test compounds 7e to 7g were investigated by the docking studies inside the active site of peptidyl transferase center of E.coli ribosome (PDB ID: 2AW4), revealed that the experimental results were comparable to that of co‑crystallized ligand linezolid. Finally, we hope that our current results will stimulate researchers to further modify the oxazolidinone-thiophene chalcone hybrid nucleus for the development of board spectrum chemotherapeutic agents.

 

ACKNOWLEDGEMENT:

Authors are deeply grateful for M/s GITAM (Deemed to be University) for providing necessary facilities to carry out this research work. The authors also thank to Dr. S. Murugesan, BITS Pilani as well as Dr. M. Murali Krishna form Andhra University for spectral data collection.

CONFLICT OF INTEREST:

The authors confirm that this article content has no conflict of interest.

 

REFERENCES:

1.       Rosa G et al. New potential antibacterials: A synthetic route to N-aryloxazolidinone/3-aryltetrahydroisoquinoline hybrids. Bioorganic and Medicinal Chemistry. 2006; 16: 529–531.

2.       Lee CS et al. Carbon–Carbon-linked (Pyrazolylphenyl) oxazolidinones with antibacterial activity against multiple drug resistant gram-positive and fastidious gram-negative bacteria. Bioorganic and Medicinal Chemistry.  2001; 9: 3243–3253.

3.       Weidner-Wells M A. et al. Novel piperidinyloxy oxazolidinone antimicrobial agents. Bioorganic and Medicinal Chemistry. 2001; 11: 1829–1832.

4.       Yeong WJ et al. Synthesis and antibacterial activity of oxazolidinones containing pyridine substituted with heteroaromatic ring. Bioorganic and Medicinal Chemistry. 2004; 12: 5909–5915.

5.       Ashtekar DR et al. Oxazolidinones, a new class of synthetic antituberculosis agent in vitro and in vivo activities of DuP-721 against Mycobacterium tuberculosis. Diagnostic Microbiology and Infectious Disease. 1991; 14: 465-471.

6.       Barbachyn, MR et al. Identification of a novel oxazolidinone (U-100480) with potent antimycobacterial activity. Journal of Medicinal Chemistry. 1996; 39: 680–685.

7.       Balasubramanian, V et al. Bactericidal activity and mechanism of action of azd5847, a novel oxazolidinone for treatment of tuberculosis. Antimicrobial Agents and Chemotherapy. 2014; 58: 495–502.

8.       Sarbjit S et al. Structure–activity relationship study of a series of novel oxazolidinone derivatives as IL-6 signaling blockers. Bioorganic and Medicinal Chemistry. 2016; 26:1282–1286.

9.       Samuel BK et al. Novel actions of oxazolidinones: in vitro screening of a triazolyloxazolidinone for anticonvulsant activity. Medical Principles and Practice. 2013;22:340–345.

10.     Rufer C et al. Chemotherapeutic Nitroheterocycles. 6.  Substituted 5-aiminomethyl-3-(5-nitro-2-imidazolylmethyleneamino)-2-oxazolidinones.  Journal of Medicinal Chemistry. 1971; 14: 94-96.

11.     Kiran Kumar GSR et al. Design and synthesis of HIV-1 protease inhibitors incorporating oxazolidinones as P2/P2' ligands in pseudosymmetric dipeptide isosteres. Journal of Medicinal Chemistry. 2007; 50: 4316-4328.

12.     Susanne R et al. Discovery of the novel antithrombotic agent 5-Chloro-N-({(5S)-2-oxo-3-[4-(3-oxomorpholin-4-yl)phenyl]-1,3-oxazolidin-5-yl}methyl)thiophene-2-carboxamide (BAY 59-7939): An oral, direct Factor Xa Inhibitor. Journal of Medicinal Chemistry.2005; 48: 5900-5908.

13.     Radhakrishnan M. et al. Evaluation of anti-depressant-like activity of linezolid, an oxazolidinone class derivative – An investigation using behavioral tests battery of depression. Biochemical and Biophysical Research Communications. 2011; 409: 723-726.

14.     Lamanna C et al. Synthesis and biological evaluation of 3-alkyloxazolidin-2-ones as reversible MAO inhibitors. ARKIVOC. 2004; V: 118-130.

15.     Naresh P et al. Ultrasound assisted synthesis and antimicrobial evaluation of novel thiophene chalcone derivatives. Chemical Science Transactions. 2014; 3(3): 1163-1171.

16.     Mahy W, Plucinski PK and Frost CG. Copper-catalyzed one pot synthesis of N-aryl oxazolidinones from amino alcohol carbamates, Organic Letters. 2014; 16 (19): 5020–5023.

17.     Yang F, Zheng Y and Wang Z. Direct base-assisted C–N bond formation between aryl halides and aliphatic tertiary amines under transition-metal-free conditions, European Journal of  Organic Chemistry.2012;1495–1498.

18.     Swapna K, Murthy SN and Nageswar, YVD. Copper iodide as a recyclable catalyst for Buchwald N-arylation, European Journal of  Organic Chemistry. 2010; 6678–6684.

19.     David SS and Buchwald SL. Diamine ligands in copper-catalyzed reactions. Chemical Science. 2010; 1: 13–31.

20.     Allen CL and Williams JMJ. Metal-catalysed approaches to amide bond formation. Chemical Society Reviews. 2011; 40: 3405–3415.

21.     Selvakumar, N et al. Synthesis, SAR and antibacterial studies on novel chalcone oxazolidinone hybrids. European Journal of Medicinal Chemistry. 2007; 42: 538-543.

22.     Rajesh T. et al. Design and enantiopure synthesis of (R)-2-((2-Oxooxazolidin-5-yl) methyl)isoindoline-1, 3-dione: A key precursor to build 2-Oxazolidinone class of antibacterial agents, E-Journal of Chemistry, 2011; 8(3): 1417-1423.

23.     Mallesham B. et al. Highly efficient CuI-Catalyzed coupling of aryl bromides with oxazolidinones using Buchwald’s protocol: A short route to linezolid and toloxatone. Organic Letters. 2003; 5 (7): 963-965.

24.     Wilson DN. et al.  The oxazolidinone antibiotics perturb the ribosomal peptidyl-transferase center and effect tRNA positioning. Proceedings of the National Academy of Sciences of the United States of America.  2008 Sep 9; 105(36): 13339–13344.

25.     Friesner R A. et al.   Extra precision Glide: Docking and scoring incorporating a model of hydrophobic enclosure for protein-ligand complexes. Journal of Medicinal Chemistry. 2006; 49: 6177-6196.

26.     Schuwirth BS. et al. Structures of the bacterial ribosome at 3.5 Å resolution. Science. 2005;310: 827-834.

27.     Glide 6.6 User Manual, Schrödinger, Inc. New York, U.S.A , 2015.

28.     Ganguly S and Naresh P. Docking studies of some Novel 1-{2-(diarylmethoxy)ethyl]-2-methyl-5-Nitroimidazole (DAMNI) analogs. International Journal of ChemTech Research. 2009; 1(4): 974-984.

29.     Romeo R. et al. Synthesis and biological evaluation of pyrimidine-oxazolidin-2-arylimino hybrid molecules as antibacterial agents. Molecules. 2018; 23: 1754-1767.

30.     Vandana K. et al. Mode of action of ranbezolid against staphylococci and structural modeling studies of its interaction with ribosomes. Antimicrobial Agents and Chemotherapy. 2009; 1427–1433.

31.      Patel JB and Miller LA.  NCCLS:  Methods for dilution antimicrobial susceptibility tests for bacteria that grow aerobically, 10^th ed: The Clinical & Laboratory Standards Institute (CLSI). 2015.

32.     Canan K,  Fatma S and Nurten A. Synthesis and antimicrobial activity of some novel 2-[4-(substituted-piperazin-/piperidin-1-ylcarbonyl)phenyl]-1H-benzimidazole derivatives. ArchPharm Chemistry in Life Sciences. 2009; 342: 54–60.

 

 

 

Accepted on 24.12.2018          © RJPT All right reserved

Research J. Pharm. and Tech 2018; 11(12): 5611-5622.