Room Temperature Synthesis and Antibacterial Activity of New Sulfonamides Containing N,N-Diethyl-Substituted Amido Moieties

Sulfonamide drugs which have brought about an antibiotic revolution in medicine are associated with a wide range of biological activities. We have synthesized a series of α-tolylsulfonamide, 1–11 and their substituted N,N-diethyl-2-(phenylmethylsulfonamido) alkanamide derivatives, 12–22 in improved and excellent yields in aqueous medium at room temperature through highly economical synthetic routes. The chemical structures of the synthesized compounds 1–22 were confirmed by analytical and spectral data such as IR, 1H- and 13C-NMR, and mass spectra. The in vitro antibacterial activity of these compounds along with standard clinical reference, streptomycin, was investigated on two key targeted organisms. It was observed that 1-(benzylsulfonyl)pyrrolidine-2-carboxylic acid, 2 emerged as the most active compound against Staphylococcus aureus at MIC value of 1.8 μg/mL while 4-(3-(diethylamino)-3-oxo-2-(phenylmethylsulfonamido) propyl)phenyl phenylmethanesulfonate, 22 was the most active sulfonamide scaffold on Escherichia coli at MIC value of 12.5 μg/mL.


Introduction
The development of sulfonamides is a fascinating and informative area in medicinal chemistry [1][2][3]. Its functional group has a long and rich history in organic chemistry and drug discovery [4,5]. The p-toluenesulfonamide and benzenesulfonamides have been widely explored in synthetic chemistry [4,5]; however, few work has been done on the αtolylsulfonamide. For instance, synthesis of benzenesulfonamide derivative of pipecolic acid [6] and that of glycine [7] had been reported. In addition, benzenesulfonamide of alanine was achieved as a result of synthetic usage of such amino acid as a linker to 6H-1,3,4-thiadiazine scaffold [8] while naphthylsulfonamide was prepared as antagonist of chemokine receptor [9]. Domagk's discovery of antibacterial activity for the azo dye prontosil led to the first effective chemotherapeutic agent, sulfanilamide [10]. A retrospective look at sulfonamides leaves no doubt that besides providing the first effective treatment of bacterial infections [10,11], they also unleashed an antibiotic revolution in medicine [12][13][14][15] to rationally design new therapeutic agents [16,17]. These compounds provided an excellent lead for structural modification and ushered in the modern era of chemotherapy and drug design. Sulfonamides inhibit the multiplication of bacteria by acting as competitive inhibitors of p-aminobenzoic acid (PABA) in the folic acid metabolism cycle [18,19]. In fact, the discovery that sulfonamides act through folate inhibition resulted in the development of dihydrofolate reductase inhibitors such as trimethoprim [20,21].
In addition, the choice of S. aureus and E. coli as the targeted organisms in this study was due to wide range of infectious diseases and life threatening conditions associated with such isolates. S. aureus, which produces heat stable toxin, is among the invasive gram positive known as pyogenic cocci implicated in several diseases of human [58,59]. From the literatures, S. aureus had shown to be very resistant to a wide variety of antibiotics [60]. Infections caused by methicillin-resistant S. aureus (MRSA) and vancomycinresistant S. aureus are associated with high morbidity and mortality, high treatment cost, and long stays in hospitals [60]. E. coli, a facultative anaerobe of wide distribution in the environment, has been implicated in the cause of urinary tract infections, meningitis, sepsis, wound infections, nosocomial pneumonia, and arthritis. A subgroup enterohemorrhagic E. coli (EHEC) can cause severe potentially fatal illness known as hemorrhagic colitis with symptoms of blood diarrhea and severe abdominal pain [61].
Based on the numerous applications of sulfonamides and challenges associated with drug usage and multidrug resistance microorganisms aforementioned among others, there is a continuous need for the synthesis of new organic compounds as potential antimicrobial agents for the replacement of the old existing ones currently available in the market or to enhance the potency of the former ones. Thus, it is conceivable to develop a series of functionalized sulfonamides in order to investigate the in vitro antibacterial activities of such scaffolds on the targeted organisms, namely, S. aureus and E. coli.

Results and Discussion
2.1. Chemistry. In the continuation of our effort on the discovery of rationally designed therapeutic agents [62,63], we have herein reported the synthesis of a series of new alkylarylsulfonamide with potent antibacterial activity.
We started with the reaction of equimolar proportion of readily available amino acid, L-pipecolic acid with αtoluene sulfonylchloride (α-TsCl) via a continuous magnetic stirring at room temperature in the presence of aqueous sodium carbonate for 48 h according to a known procedure [64]. After the completion of the reaction which was TLC monitored, the excess unreacted α-TsCl was recovered with dichloromethane (DCM) using separatory funnel while the aqueous layer was worked up by acidifying to a pH of 2.2 to get clear solution which upon freeze drying and column purification afforded 1-(benzylsulfonyl)piperidine-2-carboxylic acid 1 in excellent yield (98%). Being motivated by this encouraging discovery, we then proceeded on the coupling of nine other amino acids with α-TsCl under similar condition to obtain α-tolylsulfonamides 2-10 in good to excellent yields (Scheme 1).
In contrast to monosulfonylation that occurred in formation of 1-10, the synthesis of 11 was achieved in higher yield only when disulfonylation of the required amino acid (tyrosine) was utilized. This was noticed when monosulfonylation of tyrosine afforded 11 in lower yield (45%) when compared with other sulfonamide products 1-10 with their yields ranging from 87.5% to 98.8%. This was because under the monosulfonylation of tyrosine, the reaction terminated when the α-TsCl, being the limiting reagent, has been exhausted living excess of the tyrosine in the aqueous medium. This was as a result of two nucleophilic attacking sites available on the L-tyrosine moiety. This means that the phenolate anion was competing with the amino group in terms of the nucleophilic potential on the α-TsCl precursor, thereby resulting in an unusually lower yield (45%). Based on this development, the stoichiometric ratio was revisited in the production of 11 to give allowance for selective disulfonylation of tyrosine. Hence, the reaction for the production of 11 was reconducted in such a way that the molar equivalent of α-TsCl was doubled that of tyrosine in order to accommodate nucleophilicity from both NH 2 and PhO − of the latter, then the yield for 11 increased drastically to 89.6% (Scheme 2). Structures of all eleven αtolylsulfonamides were confirmed by spectroscopic means which include IR, mass spectra, 1 H, and 13 C NMR as well as the elemental analytical data. Thus, the detail structural elucidation was here-in confirmed using compound 1 as the representative of the α-tolylsulfonamide templates. The infrared spectrum of compound 1 gave rise to the absorption bands of OH and carbonyl of carboxylic acid at 3422 and 1736 cm −1 , respectively. Also, the CH stretching of both aliphatic and aromatic occurred at the vibrational frequency of 2974 and 2822 cm −1 , respectively. The two IR bands for SO 2 unit were observed at 1159 and 1238 cm −1 as expected, whereas the band accountable for C=C of aromatic was observed at 1603 cm −1 . The only easily accountable band resulting from a bending vibrational mode at the finger print region was Ar-H which appeared at 700 cm −1 .
In the 1 H-NMR of compound 1, all the five aromatic protons resonated downfield as a singlet at δ 7.52 ppm while the CH 2 of benzylic group appeared as a two-proton singlet at δ 4.28 ppm. Considering the upfield region, it was discovered that CH proton adjacent to COOH resonated The second stage of this experiment involved the chemical transformation of the carboxylic acid side chain moieties of the sulfonamides 1-11 into N,N-diethyl alkanamides 12-22 using highly efficient nonconventional method in its slightly modified version [65]. The reaction optimization for this aspect was carried out using amination of 1 with diethyl amine under triethyl amine basified condition as a representative. Thus, having established the C-amidoalkylation of the sulfonamide 1 to achieve the alkanamide 12 in higher yield as a bountiful success, we were motivated to extend our investigation to the reaction of other α-tolylsulfonamides 2-11 with diethylamine under the same condition (Scheme 3). These attempts also afforded the N,N-diethyl-substituted products 13-22 in good to excellent yields as envisaged. The spectroscopic assignment was consistent with that of proposed structures for the obtained N,N-diethyl alkanamides 12-22. So, it is necessary to consider the spectral data of 12 as a typical representative of the N,N-diethyl alkanamides in order to authenticate such compounds. The IR spectrum of 12 had no absorption band above 3028 cm −1 indicating the absence of -OH bond. This in turn confirmed the effective amidation of COOH of compound 1. The bands at 3028 and 2951 cm −1 depicted CH of aromatic and aliphatic, respectively. Also, the C=O and C=C frequency appeared at 1720 and 1593 cm −1 , respectively, while the two bands of SO 2 were observed at 1188 and 1148 cm −1 . Furthermore, the chemical shifts and multiplicity patterns of 1 H-and 13 C-NMR correlated well with that of the proposed alkylated sulfonamides 12-22. The 1 H-NMR spectrum of 12 in D 2 O showed five aromatic protons as singlet at δ 7.54 and CH 2 of benzylic group as singlet at δ 4.30. One triplet at δ 1.40-1.37 (6H) was assigned to two methyl of diethyl group that were chemically equivalent while one quartet at δ 3.21-3.16 (4H) was assigned to two methylene protons adjacent to the methyl protons with a coupling constant of 7.20 Hz. The 13 C-NMR of 12 in D 2 O in the presence of a drop of dioxane revealed the presence of seventeen different carbon atoms with C=O having the highest signal at δ 173.5 ppm while the CH 3 carbon atoms appeared to have the least signals at δ 11.4 ppm.

Antibacterial Activity.
The general antibacterial sensitivity testing (inhibition zone, mm) of all the series of twenty-two synthesized sulfonamides alongside with that of streptomycin clinical standard was assayed on test organisms (E. coli and S. aureus) using agar diffusion technique [66]. The choice of E. coli as the gram −ve organism is because it is easily transmissible through food, water, soil, animal, and man [60]. E. coli is a normal flora of human body which causes a lot of vancomycin-resistant Enterococci and methicillin-resistant Staphylococcus aureus (MRSA) [51]. Based on our previous report [62], the choice of streptomycin as clinical standards is due to the fact that, at low concentrations, streptomycin only inhibits the growth of the bacteria through induction of prokaryotic ribosomes to misread mRNA [67] and it also possesses broad spectrum of antibacterial activity. There were reported cases of E. coli and Staphylococcus aureus being susceptible to streptomycin [4,68]. The biological relevance of the synthesized sulfonamides here-in was authenticated by screening them in vitro against Staphylococcus aureus ATCC 6538 (S. aureus) and Escherichia coli ATCC 25922 (E. coli) with the reported selectivity index (SI) duly calculated from zones of inhibition (ZOI). Hence, it should be noted afterwards that the abbreviated forms were given in the bracket for the sake of brevity and conciseness.
From the result of sensitivity testing, probable activities of α-tolylsulfonamide family on the test organisms were categorized based on the size of zone of inhibition (Table 1). Interestingly, it was observed that some of the compounds exhibited probable significant activities based on the large zone of inhibition reported. For instance, compounds 6, 7, 12, 14, and 22 were highly active on E. coli while compounds 1, 2, 4, 5, 10, 11, 17, and 20 exhibited moderate activities on the same organism. All other compounds showed low activities on E. coli except 3 and 16 which showed no activity at all on the E. coli even at 1000 μg/mL. The scenario of comparative study of effect of the sulfonamides and streptomycin on E. coli could be vividly understood by observing the selectivity index (SI). All the sulfonamides have selectivity indices ranging from 0.29 for compound 18 to 0.96 for compound 22 (i.e., less than 1). This implies that streptomycin (SI = 1) was probably more active than any of the sulfonamide scaffolds as regarding the inhibition of E. coli growth. In the same vein, looking through the effect on S. aureus, compounds 2, 5, 17, 21, and 22 were highly active; 4, 6, 8, 10, 11, 12, 14, and 19 were moderately active; 1, 3, 7, 9, 13, 15, 18, and 20 exhibited low activity while 16 showed no activity on S. aureus ( Table 1). The comparative study of α-toluenesulfonamides to streptomycin on S. aureus growth inhibition is worthy, of commendation. From the SI values, compounds 9, 12, 18, and 20 competed favourably with streptomycin while 2, 4, 5, 6, 8, 13, 14, 17, 19, 21, and 22 (SI = 1.08-2.31) showed even a better activity than streptomycin on S. aureus. All other compounds exhibited lesser activity than streptomycin (SI = 0.46-0.92) on S. aureus except 16 which showed no activity.
Due to high zones of inhibition obtained during general sensitivity testing, the minimum inhibitory concentration (MIC) was conducted, first at 100 μg/mL using Russell and Furr method [69]. However, those compounds that could not affect the inhibition of microbial growth at this concentration were further repeated for MIC test at 1000 μg/mL. The result of the MIC of this class of compounds on E. coli and S. aureus was as shown in Table 2. Interestingly, all the sulfonamides tested showed a concentration-dependent inhibitory effect on the in vitro microbial growth assays [70]. Considering the MIC testing on the gram negative organism (E. coli), it was observed that compounds 1, 6, 7, 11, 12, 14, and 22 inhibited the microbial growth at varying values less than or equal to 100 μg/mL, whereas, all other compounds were active on E. coli at higher concentration (between 125 and 1000 μg/mL) except 3 and 16 which had no activity even at 1000 μg/mL. Specifically, MIC values of the synthesized compounds on E. coli were reported to be 50 μg/mL for 12 and 14; 100 μg/mL for 1 and 11; 125 μg/mL for 2, 4, 5, 10, 17, and 20; 250 μg/mL for 8, 9, 13, 15, 19, and 21.
Although, the most active sulfonamides on E. coli were 6, 7, and 22 with MIC values of 25, 25, and 12.5 μg/mL, respectively, none of them could compete with streptomycin (MIC value of 6.25 μg/mL) in terms of activity. The two rings presence in 22 and their π character might be responsible for it being the most active as deduced from the finding of Aissaoui and coworkers [71]. In addition, eight sulfonamides (

12-13
Scheme 3: Synthesis of N,N-diethylamide substituted α-tolylsulfonamides, 12-22. from 125 μg/mL to 1000 μg/mL except 16 which had no activity even at 1000 μg/mL. The significant antibacterial activity of the synthesized compounds may be explained by the ability of its sulfonamide binding site to mimic paminobenzoic acid (PABA) which is an essential growth factor in the targeted organisms as earlier documented in the literatures [14][15][16]. The explanation for this encouraging activity of the synthesized sulfonamides could be traceable to the mode of action of sulfonamide drugs. This is based on the inhibition of DNA synthesis [72] by interfering with para-aminobenzoic acid (PABA) in biosynthesis of folic acid [73]. Furthermore, from the structure activity relationship (SAR) study, it was observed that the nature of side chains (R 1 and R 2 -CH-R 3 ) of the α-tolylsulfonamides and the presence of N,N-diethylated amido moieties {(CH 3 CH 2 ) 2 N-C=O} of the amide contributed immensely toward synergistic or antagonistic effect on the reported in vitro antibacterial activity. For instance, the antibacterial activity of structurally related α-tolylsulfonamides containing R 1 = (CH 2 ) 2 SCH 3 , 6; R 2 = R 3 = CH 3 , 7, on E. coli were very high with MIC value of 25 ppm whereas the activity reduced to moderate when R 1 = CH 3 , 4; R 1 = CH 2 SH, 5 with MIC value of 125 ppm. The synergistic effect due to the presence of N, N-diethylamido moieties was noticed in some synthetic conversions such as 1 → 12, 3 → 14 and 11 → 22. Thus, α-tolylsulfonamides 1 and 11 had moderate activity on E. coli while their corresponding diethylamido 12 and 22 had better and higher activity on E. coli. In fact, α-tolylsulfonamide 3 had no activity on E. coli whereas its N, N-diethyl amidated counterpart, 14 had very high activity with MIC value of 50 ppm. The synergistic effect noticed herein might be as a result of electron donating nature of the R 1 side chain. On the contrary, antagonistic effect was observed in the synthetic modification of 5 → 16 and 6 → 17 as far as presence of N, N-diethyl-substituted amido moieties was concerned. This reverse order of activity on E. coli might be as a result of the presence of sulfur atom in 5 and 6 which might have altered the electron donating prowess. Considering the structural relationship effect on the activity of the synthesized compounds against S. aureus, it was discovered that N,N-diethyl-substituted amides 14, 17, 21, and 22 had better activity than their α-tolylsulfonamide precursors 3, 6, 10, and 11.

Conclusion
It was discovered that aqueous medium approach at ambient temperature used here-in was a highly efficient procedure for the preparation of various α-tolylsulfonamide derivatives 1-11 in good to excellent yield. The synthetic modification of these derivatives as precursors to furnish disubstituted amide bearing sulfonamides 12-22 was also successfully achieved. The antibacterial evaluation study showed 1-(benzylsulfonyl) pyrrolidine-2-carboxylic acid, 2 to be the most a ctive sulfonamide on S. aureus while 4-(3-(diethylamino)-3oxo-2-(phenylmethylsulfonamido)propyl) phenylphenylmethane sulfonate, 22 emerged as the most active on the growth inhibition of E. coli. Thus, this work will be very useful for further studies in terms of toxicity effect and structural activity relationship (SAR) study by using various types of In addition, the pH was monitored and confirmed during acidification by using Portable pH Meter Model PHB4. All drying was conducted at a reduced pressure with DHG-9023A Vacuum Oven. The reaction progress was monitored with TLC using CHCl 3 /CH 3 OH solvent system and the developed plates were visualized under UV lamp and/or in iodine tank where necessary. Column chromatographic purifications were carried out on Merck silica gel F (Mesh 200-300). Organic solutions were dried over anhydrous Na 2 SO 4 and concentrated with a Buchi Rotary Evaporator at reduced pressure. At all stages of the experiments, the synthetic protocols were effected in bone dried solvents under nitrogen atmosphere in dried glassware which were wiped with stream flow of nitrogen gas prior to use and SOCl 2 was freshly distilled prior to use. Other reagents were used directly after ascertaining the purity condition. Derivatives 1-11. To a solution of L-amino acid (5 mmol) in H 2 O (6 mL) was added Na 2 CO 3 (1.113 g, 10.5 mmol) with a continuous stirring until all the solutes had dissolved. The clear solution was cooled to −10 • C and α-toluenesulfonyl chloride, α-TsCl (1.144 g, 6 mmol) was added in three batches over a period of 1 h. It was warmed up to 0 • C and stirred there for 1 h. Finally, the reacting mixture was then warmed up to room temperature and allowed to stir there for 48 h. The reaction was quenched by the addition of DCM (10 mL) and transferred into separatory funnel where the excess of α-TsCl was removed by extraction. The aqueous layer was then worked up to give a clear solution by the addition of 2N HCl until the pH 2.2 was attained. The clear liquid was then lyophilized at −52 • C under reduced pressure for 12 h to obtain the crude solid product which was purified by column chromatography (CHCl 3 /CH 3 OH, 3 : 1) to afford α-toluene sulfonamides 1-11 in excellent yields. ppm. 13

General Procedure of N,N-Diethyl-2-(phenylmethylsulfonamido)propanamide,12-22.
To a solution of α-tolylsulfonamide derivatives 1-11 (2.96 mmol) in H 2 O (10 mL) in a streaming flow of nitrogen gas was added oxalyl chloride (0.34 mL, 3.85 mmol, 1.30 eq.) via dropping pipette followed by carefully controlled addition of 1 drop of DMF. The resulting mixture was stirred at room temperature for 2 h to get crude acid chloride which was kept air-tighted prior to use. In a separate 250 mL three-necked round bottom flask, equipped with a magnetic stirring bar, was added Na 2 CO 3 (0.628 g, 5.92 mmol, 2 equiv.) to H 2 O (10 mL) followed by diethyl amine DEA (0.4 mL, 3.85 mmol, 1.3 equiv.) in a continuous stirring and cooled to −15 • C. Then, earlier kept acid chloride was added in such a way to maintain the internal temperature of the reaction mixture at around −10 • C. The reacting mixture was then stirred at −10 • C for 1 h; at 0 • C for 1 h; finally at room temperature for 1 h. The reaction was terminated, worked up by acidifying with 2 N HCl, and concentrated in a rotary evaporator. The clear solution obtained was freeze-dried to get crude solid product which was purified by column chromatography (CHCl 3 /CH 3 OH, 3 : 1) to afford N,N-dimethylacetamide of α-tolylsulfonamide derivatives 12-22.

Antibacterial Activity Assays.
The antimicrobial properties of the sulfonamides were investigated in the form of the general sensitivity testing and minimum inhibitory concentration (MIC) with respect to freshly cultured targeted organisms. The two organisms of interest in this present study are one gram positive (Staphylococcus aureus ATCC 6538) and one gram negative (Escherichia coli ATCC 25922) organisms which are associated with the gastrointestinal tract damage in man and animal.

Preparation of the Inoculum.
The standard strains of S. aureus and E. coli used were obtained from Test Center of Antimicrobial Materials, TIPC, Beijing. No clinically isolated organism was used based on inavailability of such as at the time of this study. The strains were propagated on nutrient agar plates and maintained on the plate at 4 • C. The isolates were subcultured in nutrient broth at 37 • C for 8 h prior to antibacterial testing.

Antibacterial Sensitivity Testing of the Synthesized Compounds.
Agar well diffusion technique as described by Adeniyi and coworkers was used to determine the antibacterial activity of the synthesized compounds [66]. Sensitivity test agar plates were seeded with 0.1 mL of an overnight culture of each bacterial strain (equivalent to 10 7 -10 8 CFU mL −1 ). The seeded plates were allowed to set and a standard cork borer of 8 mm diameter was used to cut uniform wells on the surface of the agar. The wells were then filled with 0.3 mL of each sulfonamide solution in an appropriate solvent at a concentration of 1000 μg/mL (0.02 g of sulfonamide dissolved in 20 mL distilled water). All the plates were incubated at 37 • C for 24 h. The assay was conducted at regular intervals of 24 h until a marked decline in the potency of the sulfonamide solution to inhibit the growth of the test organisms was noticed. Zones of clearance round each well means inhibition and the diameter of such zones were measured. The procedure was repeated for the streptomycin (standard). (MIC). Agar well dilution method as described by Russell and Furr was used to determine the minimum inhibitory concentration (MIC) of the sulfonamides and streptomycin [69]. Different dilutions of the sulfonamides were prepared first at ≤100 μg/mL to give final concentrations in the range of 100, 50, 25, 12.5, 6.25, and 1.8 μg/mL. The different dilutions of sulfonamide derivatives that could not inhibit the microbial growth at ≤100 μg/mL were later prepared at ≤1000 μg/mL to give final concentrations in the range of 1000, 500, 250, 125, and 62.5 μg/mL. Two milliliter (2 mL) of each dilution was mixed with 18 mL of Mueller Hinton agar (MHA, Difco, France) and poured into Petri dishes and allowed to set. The agar was streaked with an overnight broth culture of the bacterial strains and incubated overnight. The plates were then examined for the presence or absence of growth. The minimum concentration that completely inhibited macroscopic growth was regarded as the minimum inhibitory concentration of the respective sulfonamide. The procedure was repeated for streptomycin (standard). Selectivity index (SI) is the ratio of the zone of the inhibition of compound to that of the streptomycin.