Synthesis and Anti-Staphylococcal Activity of 2,4-Disubstituted Diphenylamines

Infections caused by Staphylococcus aureus are ubiquitous and life threatening. Evolution of resistant strains has necessitated the need to continuously discover new drugs to combat such organisms. Diphenyl ethers, such as triclosan, have recently shown potential as antibacterial agents. In this study, a series of diphenyl amines were synthesized and evaluated for in vitro antibacterial activity against Gram-positive (Staphylococcus aureus, Bacillus subtilis, Staphylococcus epidermidis) and Gram-negative (Escherichia coli, Pseudomonas aeruginosa, Pseudomonas putida) bacteria. Preliminary results showed that six of the twelve synthesized molecules were active against Staphylococcus aureus. Most notable amongst them were compounds 2(2,4-dinitrophenylamino)phenol and 2(2-dinitrophenylamino)phenol having minimum inhibitory concentration (MIC) in the range of 7.8-15.6 µg mL-1 and 7.8-62.5 µg mL-1respectively for all the eight selected organisms. Five active compounds from the preliminary results were further screened against resistant S. aureus cultures where compounds 2(2,4-dinitrophenylamino)phenol, 2(2-dinitrophenylamino)phenol and 2-chloro-N-(2-(2,4-dichlorophenylamino)phenyl)acetamide gave encouraging results having MIC in the range 3.9-7.8 µg mL-1for most of the organisms. Results obtained above for the selected organisms and the resistant S. aureus strains conclude that hydroxyl group at 2-position of ring B potentiates the antibacterial activity and overcomes the antibiotic resistance.


Introduction
Infectious diseases remain the main cause of human premature deaths and account for 15 million cases per year worldwide. 1Among these, the infections caused by Staphylococcus aureus (Sau) are ubiquitous, life threatening and a serious public health issue. 2 The highly infectious pathogen is carried by thirty percent of healthy people, usually in the anterior nasal cavities and is the most common causative agents of nosocomial infections. 3It readily gets transferred to immune compromised patients causing post-surgical wound infections. 4One of the effective treatments in the past for the infections caused by Staphylococcus aureus was penicillin that later became resistant due to evolution of penicillinase by the organism.Celbenin, commonly known as methicillin, was introduced in 1959 to combat the penicillin resistant microbes.Within one year after its introduction methicillin resistant Staphylococcus aureus (MRSA) was reported. 5The spread of MRSA infection over the decades is treated by vancomycin. 6The isolation of vancomycin-resistant Staphylococcus aureus (VRSA)   strains in 2002 has once again necessitated the need to continuously discover new drugs to combat such organisms. 7echnological advances in genomics have enabled researchers to look more comprehensively at biological systems, resulting in the discovery of a number of novel drug targets. 8Bacterial fatty acid biosynthesis, which is type II fatty acid synthesis (FAS-II), is one of the essential pathways in which each reaction is performed by individual enzymes and is fundamentally distinct from the multienzyme complex, FAS-I (type I fatty acid synthesis), found in mammals.The enoyl-acyl carrier protein reductases (Fab I) of FAS II has been the target of a number of studies. 9Triclosan, a diphenyl ether, was initially thought to be a nonspecific antiseptic but was later found to target Fab I. 10 Triclosan resistant enzymes have been found in certain organisms (Fab K, Fab L, and Fab V in S. pneumoniae, B. subtilis and Vibrio cholerae). 11ever the less, enoyl reductases remain an excellent target for narrow spectrum drug discovery.
Efforts to improve the antibacterial activity of triclosan have involved synthesizing a series of diphenyl ethers with different substituent on both the rings by a number of groups including ours. 12,13In the present work, we have synthesized and evaluated the antibacterial effect of diphenyl amines which to the best of our knowledge first time shifts focus from diphenyl ethers to diphenyl amines.We contemplate, these compounds target enoyl reductases.Limited reports are available on the antimicrobial activity of diphenylamines.N-phenyanthranilic acid based diphenyl amines have been studied previously, 14 and recently, diarylamines have also been evaluated for antibacterial and anti fungal activities containing benzo [b] thiophene or pyridine as one of the moieties. 15,16The diphenyl amines synthesized here were initially screened for their antibacterial activity against both Gram-positive (Staphylococcus aureus, Bacillus subtilis, Staphylococcus epidermidis) and Gram-negative (Escherichia coli, Pseudomonas aeruginosa, Pseudomonas putida) strains.Most of the compounds displayed antibacterial activity against Staphylococcus aureus, and some even displayed activity against resistant strains.

Experimental
Purified reagents were procured from SD Fine Chemicals (Bombay, India).Solvents were distilled before use in column chromatography.Melting points are reported in o C and are uncorrected. 1H and 13 C NMR spectral analyses were performed on 400.1 and 100.6 MHz spectrometer (Avance-II (Bruker), Panjab University, Chandigarh, India) with tetramethylsilane and CDCl 3 as the internal standard (d, ppm) respectively.The following abbreviations were used to explain the multiplicities: s, singlet; d, doublet; t, triplet; dd, double doublet; m, multiplet, br, broad.Mass spectrometry analysis was done using mass spectrometer (Waters Micromass Q-Tof Micro, Panjab University, Chandigarh, India).Scheme 1 summarizes all the synthetic procedures.

Synthesis of 2,4-dinitro-N-phenylaniline (1a)
Although commercially available, we synthesized the compound using 1-fluoro-2,4-dinitrobenzene (186 mg, 1 mmol) and aniline (91 µL, 1 mmol) in dimethyl sulfoxide (DMSO) (1 mL) by heating in oil bath at 100 o C for 16 h under inert atmosphere.On cooling the reaction mixture was partitioned between ethyl acetate (15 mL) and water (15 mL) and extracted again with ethyl acetate (5 mL).The combined organic layers were dried over Na 2 SO 4 , filtered and evaporated in vacuo to yield crude product that was purified by silica gel column chromatography and solvent system (petroleum ether/ethyl acetate, 85:

Synthesis of compounds 1b, 1c, and 1d
To a solution of nitro fluoro benzene (7.5 mmol) in dimethylformamide (DMF) (3 mL) was added relevent aryl amine (5 mmol), potassium carbonate (10 mmol), cuprous iodide (0.5 mmol) and L-proline (1.0 mmol).The reaction mixture was heated at 100 o C for 24 h.After completion of the reaction (thyn layer chromatography, TLC, monitoring) it was allowed to cool.Work up was done by adding water (7 mL) and organic components extracted using ethyl acetate (3 × 10 mL).Combined organic solvent was washed with water till it gets neutral (pH paper) and dried over brine and Na 2 SO 4 .Evaporation of the organic solvent gave the crude product that was purified using silica column and solvent system (petroleum ether/ethyl acetate, 75:25, v/v).Evaporation of the solvent gave pure products that were characterized using 1 H, 13 C NMR spectroscopy and mass spectrometry.

Synthesis of compounds 2b and 2c
To the compound obtained above, 1b and 1c, (0.62 mmol) in acetic acid (10 mL) was added 48% aqueous HBr (1 mL) and heated under reflux for 2 h.After confirming completion of the reaction (TLC monitoring) it was cooled to room temperature and acetic acid evaporated in vacuo.Water (5 mL) was added to dissolve the contents and the aqueous layer extracted with ethyl acetate (3 × 10 mL).The combined organic layers were dried over Na 2 SO 4 .Evaporation of the organic solvent gave product which was purified using silica column chromatography and solvent system (pet ether/ethyl acetate, 65:35, v/v) to afford desired compounds.

Synthesis of compounds 2d, 2e and 2f
To a suspension of nitro diphenyl amines 1b, 1c and1d (10.6 mmol) in H 2 O (50 mL) was added Fe (106.2 mmol) and FeSO 4 .7H 2 O (10.4 mmol).The reaction mixture was refluxed for 8 h.After confirming the completion of reaction (TLC monitoring), it was cooled and filtered through celite.The organic components were extracted with ethyl acetate (2 × 75 mL).The combined organic layers were dried over Na 2 SO 4 .Evaporation of the organic solvent gave crude amino product that was purified using silica column and solvent system (pet ether/ethyl acetate, 80:20, v/v).Evaporation of the solvent gave pure product that was characterized using 1 H, 13 C NMR spectroscopy and mass spectrometry.

Synthesis of compounds 3f1, 3f2 and 3f3
The acid chloride was prepared from corresponding acid (0.5 mmol) and PCl 3 (0.5 mmol) by heating and stirring at 60-70 o C till the evolution of hydrochloric acid gas ceased.The acid chloride thus prepared was added to a stirred solution of compound 2f (0.5 mmol), K 2 CO 3 (0.3 mmol) in dicloromethane, DCM, (10 mL) at 0 o C. The reaction mixture was stirred for 2 h and quenched with water.The organic product was extracted with DCM (3 × 50 mL), dried over Na 2 SO 4 , and concentrated in vacuo.Crude product obtained was purified using silica column and solvent system (pet ether/ ethyl acetate, 70:30, v/v) to afford light brown solids.

In vitro microbroth dilution assay to determine minimal inhibitory concentration (MIC)
Stock solutions of the compounds were prepared and tested against a spectrum of standard strains and the clinical isolates of Staphylococcus aureus by in vitro microbroth dilution assay to ascertain antimicrobial potential of the compounds as discribed Clinical and Laboratory Standards Institute (CLSI) M7-A4. 17IC is considered to be the gold standard technique to evaluate the susceptibility of a microorganism to a particular antibiotic and performed as reported earlier. 18Briefly 50 µL of the 18 h old 0.5 McFarland adjusted bacterial suspension in physiological saline was dispensed in each well.125 µL of MH broth was added in the wells to achieve a final bacterial cell concentration of 10 6 cells in the well.Plates were incubated at 37 o C for 2.5 h.Next, 25 µL of the test compound at different concentrations (i.e. two fold serial dilutions in range between 62.5-0.48 µg mL -1 ) was added into the well and incubated for 24 h at 37 o C. Subsequently 20 µL of 0.02% of MTT, (3-(4,5-dimethylthiazol-2-yl)-2,5diphenyltetrazolium bromide), was added into each well and incubated at 37 o C for 1 h.The color change due to formation of formazan was visually observed.All the tests were performed in triplicates.

Chemistry
Synthesis of all the compounds has been illustrated in Scheme 1.All the initial compounds (1a to 1d) were synthesized by nucleophilic aromatic substitution of nitro fluorobenzene with corresponding anilines.For the synthesis of compound 1a there was no need of base, metal salt or the ligand.1][22] The demethylation of compound 1b and 1c was done by refluxing in 48% aqueous hydrobromic acid and acetic acid to give corresponding 2-hydroxy diphenyl amines 2b and 2c respectively.The disappearance of signals due to methoxy group in 1 H (3.8-3.9 ppm) and 13 C (55.7 ppm) NMR confirmed this.Earlier reports suggested synthesis of such compounds by Smiles rearrangement. 23he nitro groups on compounds 1b, 1c and 1d were reduced to corresponding amines 2d, 2e and 2f by refluxing with powdered iron and ferrous sulfate in aqueous media. 24ppearance of broad singlet in the range of 3.6-3.9ppm in 1 H NMR confirmed the presence of amine.The amine 2f obtained above was converted into novel compounds 3f1, 3f2 and 3f3 by reacting with corresponding acid chlorides in the presence of potassium carbonate and methylene chloride.This was confirmed by presence of signal in the range of 164-173 ppm in 13 C NMR due to carbonyl carbon of amide groups.The acid chlorides used were freshly prepared by treating acetic acid, proponic acid and chloroacetic acid with phosphorous trichloride.

Biological activity
Antibacterial screening was carried out against eight organisms comprising both Gram-positive and Gram-negative bacteria.Gram-positive microorganisms included two strains of Staphylococcus aureus and one strain each of Staphylococcus epidermidis and Bacillus subtilis while Gram-negative included two strains of Pseudomonas aeruginosa and one strain each of Escherichia coli and Pseudomonas putida.Preliminary results (Table S1, Supplementary Information) showed that 6 of the 13 compounds were active against Staphylococcus aureus that have been summarized in Table 1.Against the remaining strains three compounds (1a, 2b and 2c) gave encouraging results.Gram-negative, Pseudomonas putida was an exception to this as it was inhibited by all the synthesized compounds except compounds 2d and 2e, with MICs ranging from 7.8-62.5 µg mL -1 .
Most notable among all, were the compounds 2b and 2c as both were active against all the selected stains.Compound 2b having MIC in the range 7.8-15.6µg mL -1 was the most effective as it gave antibacterial activity comparable to that of control against all the organisms.Compound 2c, athough it displayed a higher MIC range, (7.8-62.5 µg mL -1 ) was also active against all the organisms.Triclosan and streptomycin, used as control compounds, had MICs in the range of 1.9-7.8µg mL -1 and 1.9-15.6µg mL -1 , respectively, in the preliminary studies.Whereas triclosan was active against all the strains Streptomycin did not give encouraging results against many stains including Staphylococcus aureus.A common feature of both the compounds 2b and 2c is the presence of hydroxyl group (OH) at ortho position of ring B.
Based on the results of initial screening against S. aureus (Table 1), five of seven active compounds 2b, 2c, 1d, 2f and 3f3, were tested against resistant strains of S. aureus for their antibacterial activity.

Structure activity relationship
Results in Table 1 and Table S1 indicate that the presence of hydroxyl group at 2-position of ring B is essential for the enhanced antibacterial activity.This is evident from the fact that two compounds 2b and 2c, having hydroxyl group at the said position, have shown good activity against all the Gram-positive and Gram-negative strains taken in this study.MIC range for compound 2b was 7.8-15.6µg mL -1 and for compound 2c was 7.8-62.5 µg mL -1 .Whereas compound 2b gave MIC value of 7.8 µg mL -1 against all the strains except Bacillus subtilis and S. epidermidis, compound 2c was comparatively less active having giving MIC values in the range of 7.8-62.5 µg mL -1 .Thus, the presence of two nitro groups at position 2 and 4 of ring A enhances the antibacterial activity of diphenyl amines as compared to only one at 2-position.All other compounds containing methoxy group at 2-position of ring B in diphenyl amines (1b, 1c, 2d, 2e) did not yield any antibacterial activity.Earlier reports have established that the 2-hydroxy group is essential for the inhibition of enoyl-ACP reductases (Fab I). 25 The study carried out with N-(2,4-dichlorophenyl)-2-methoxyaniline and N-(2,4-dichlorophenyl)-2-hydroxyaniline shows inhibition of enoyl-ACP reductases (FabI) of P. falciparum due to the formation of a ternary complex at the active site of the enzyme with the latter compound.Our study also corroborates with reported results and therefore it can be proposed that Fab I may be the possible target of compounds 2b and 2c.
Compound 1a, having neither methoxy nor hydroxyl substituent at 2-position of ring B, showed comparatively better results than compounds having methoxy group at the said position (compounds 1b, 1c, 2d and 2e).It showed antibacterial activity against Staphylococcus aureus, Staphylococcus epidermidis, Escherichia coli and Pseudomonas putida with MIC values in the range of 7.8-31.2µg mL -1 .This shows that methoxy group at 2-position of ring B is intolerable as compared to hydroxyl or hydrogen for the antibacterial activity.
Compounds 1d and 2f, having polar nitro and amino groups respectively at 2-position of diphenyl amine in the ring B and chloro groups at positions 2 and 4 of ring A, did not display broad spectrum inhibitory activity like compounds 2b and 2c.However, both demonstrated good antibacterial activity against Staphylococcus aureus.Compound 1d, having a nitro group at 2-position, demonstrated better MIC range of 7.8-15.6µg mL -1 as compared to amino containing compound 2f which had MICs in the range 3.9-31.2µg mL -1 .The former compound was also active against Escherichia coli and Pseudomonas aeruginosa, but at higher concentrations.Thus, although 2,4-dichloro diphenyl amines in compounds 1d and 2f have shown promise for antibacterial activity, further exploratory studies on the B ring are required to optimize for broad spectrum antibacterial activity.
Functionalization of amino group in compound 2f with acid chlorides gave the corresponding amides 3f1, 3f2 and 3f3.However, except for 3f3, these were found to be ineffective against all the strains other than Pseudomonas putida.Thus, amidation of amino group at 2-position of ring B further obliterates the antibacterial activity.However, the activity is regained by amidation with chloro acetic acid as in the case of compound 3f3.P. putida was the most sensitive among all the strains that were selected for the study.
Compounds 2d and 2e do not have any of the required features as discussed above and were ineffective against all the strains including sensitive P. putida.They have unfavorable features includeding the presence of methoxy group and the presence of amino group at ortho positions.

Antibacterial activity against resistant strains
As discussed above five compounds (1d, 2b, 2c, 2f and 3f3) were shortlisted for screening against resistant strains of S. aureus (Sau) with Sau NCTC 6571 and Sau MTCC 737 as reference strains and triclosan and cefepime as control drugs.Table 2 shows MIC of cefepime in the range of 40-160 µg mL -1 for the selected strains.The MIC range of cefepime against S. aureus (sensitive) and S. aureus (resistance) is between 0.007-8 µg mL -1 . 26The resistance profiles of the organisms are listed in Table 2.A total of seventeen (out of twenty) strains shown in Table 2 are resistant against methicillin, vancomycin, vancomycin intermediate or multi-antibiotic.
Antibacterial activity against the strains (Table 2) demonstrate that compound 2b having nitro groups at 2 and 4 position of ring A was the most effective having MIC value in the range of 3.9-7.8µg mL -1 for most of the resistant organisms.This was followed by slightly less potent compound 2c that has only one nitro group at 2-position of ring A, displaying MIC values in the range of 3.9-7.8µg mL -1 .A common feature of both these compounds, as mentioned above, is the presence of hydroxyl group at 2-position of ring B.
Compounds 1d and 2f were not as potent as compounds 2b and 2c, as discussed above.In both these compounds, the nitro group of ring A at 2 and 4 positions has been replaced by chloro group and the hydroxy group at 2-position of ring B has been replaced by nitro (1d) and amino (2f) groups, respectively.Compound 1d with the nitro group was the better of the two, showing MIC of 3.9-7.8µg mL -1 against eleven resistant strains and 15.6-31.2µg mL -1 against the remaining strains.However, compound 2f with amino group at same position showed MIC range of 3.9-7.8µg mL -1 against five resistant strains only.MIC value of 2f extended from 15.6-31.2µg mL -1 for rest of the fifteen strains.Compounds 1d and 2f again corroborated with our previous studies (Table 1), confirming that the presence of an amino group at 2-position of ring B reduces its potential as an antibacterial agent.
Compound 3f3 with a chloro amide group at 2-position of ring B displayed an MIC range of 7.8-15.6µg mL -1 against nineteen strains confirming good antibacterial potential.Table 3 summarizes antibacterial activity against different resistant strains.It is clear from the table that the most potent compound to least potent compounds can be written in the order, 2b > 2c > 1d > 3f3 > 2f.Thus, antibacterial response of the compounds was comparable to that of triclosan, but greatly improved than the other control, cefepime, having MIC in the range 40-160 µg mL -1 .

Table 1 .
MIC values of diphenylamines in µg mL -1 against reference strains of Staphylococcus aureus a Streptomycin and triclosan were used as reference compound.
a VISA = vancomycin intermediate S. Aureus; b