Synthetic Imidazopyridine-Based Derivatives as Potential Inhibitors against Multi-Drug Resistant Bacterial Infections: A Review

Fused pyridines are reported to display various pharmacological activities, such as antipyretic, analgesic, antiprotozoal, antibacterial, antitumor, antifungal, anti-inflammatory, and antiapoptotic. They are widely used in the field of medicinal chemistry. Imidazopyridines (IZPs) are crucial classes of fused heterocycles that are expansively reported on in the literature. Evidence suggests that IZPs, as fused scaffolds, possess more diverse profiles than individual imidazole and pyridine moieties. Bacterial infections and antibacterial resistance are ever-growing risks in the 21st century. Only one IZP, i.e., rifaximin, is available on the market as an antibiotic. In this review, the authors highlight strategies for preparing other IZPs. A particular focus is on the antibacterial profile and structure–activity relationship (SAR) of various synthesized IZP derivatives. This research provides a foundation for the tuning of available compounds to create novel, potent antibacterial agents with fewer side effects.

Consequently, there is a continuous effort to conceive novel strategies to develop imidazopyridine with different substituents at various positions on the imidazole and

Strategies for the Synthesis of Imidazopyridines
Several approaches, e.g., multicomponent, condensation, oxidative coupling, aminooxygenation, tandem reaction, hydroamination, etc., have been put forward for synthesizing this privileged scaffold. These approaches have been consequently optimized to produce IZP and its derivatives with high yield and purity. The literature describes some important synthetic routes, which are summarized in Figure 2.

Strategies for the Synthesis of Imidazopyridines
Several approaches, e.g., multicomponent, condensation, oxidative coupling, aminooxygenation, tandem reaction, hydroamination, etc., have been put forward for synthesizing this privileged scaffold. These approaches have been consequently optimized to produce IZP and its derivatives with high yield and purity. The literature describes some important synthetic routes, which are summarized in Figure 2.
The (SR)-1a synthetic route is a traditional strategy involving a simple condensation reaction of 2-aminopyridines with α-haloketone in the presence of neutral alumina, as proposed by Sahu et al., 2005 [21]. Zhu et al., 2009 [22], later optimized this process. They reported the synthesis (SR-1b) of IZP with α-haloketone and 2-aminopyridine in the absence of a catalyst and in solvent-free conditions at 60 • C. Stasyuk et al., 2012 [23], reported the synthesis (SR-1c) of IZP from ketones via the in situ production of α-iodoketone. A library of compounds was synthesized via Ortoleva-King reaction followed by ring closure. The authors then studied the optoelectronic properties of these compounds. IZP with 2-hydroxyphenyl at position 2 established an excited-state intramolecular proton transfer (ESIPT), displaying strong emission bands in the blue region.
An oxidative C-H functionalization strategy was also utilized for the synthesis of IZPs from N-(alkylidene)-4H-1,2,4-triazole-4-amines and pyridines in the presence of a copper catalyst in DMF, as reported by Yu et al., 2013 (SR-4a) [41]. A later study by Huang et al., 2013 [42], synthesized IZPs through the oxidative cyclization of pyridines with ketone oxime esters in the presence of copper-iodide as a catalyst (SR-4b). Another methodology for the generation of 3-aroyl IZPs through oxidative coupling between 2-aminopyridines and chalcones in the presence of copper as catalyst (SR-4c) was reported by Monir et al., 2014 [43].
Another strategy, i.e., oxidative coupling, was proposed by Donohoe et al., 2012 (SR-7a) [48] for the synthesis of IZPs through the formation of in situ α-iodoketones which react with available alkenes and 2-aminopyridine. Further, Zeng et al., 2012 [49], reported a facile and robust synthesis method for IZPs through an oxidative coupling reaction of alkyne with aminopyridine in the presence of copper/iron catalysts (SR-7b). A study by Gao et al., 2013 [50], described a one-pot oxidative coupling methodology for the generation of 2-haloimidazo [1,2-a]pyridines from 2-aminopyridines and haloalkynes in the presence of copper triflate (SR-7c).
The oxidative coupling reaction uses 2-aminopyridine and nitroalkenes, which are good Michael acceptors, for the synthesis of 3-unsubstituted IZPs in the presence of Lewis acid. Yan et al., 2014 [51], developed a modified metal-free strategy employing TBHP as the oxidant and a TBAI catalyst in DMF (SR-7d). Bagdi et al., 2013 [52], established the synthesis of IZP and its derivatives using 2-aminopyridine and aryl ketones through C-H functionalization in the presence of a copper catalyst (SR-7e). At the same time, Chandra Mohan et al., 2013 [53], developed a synthesis method for IZP employing a CuI catalyst in a DMF solvent (SR-7f). Later, Cai et al., 2013 [54], demonstrated the synthesis of heteroaromatic IZPs in the presence of a copper iodide or boron trifluoride etherate catalyst (SR-7g). Another methodology was developed by Zhang et al., 2013 [55], for the synthesis of functionalized IZPs from aliphatic, aromatic, or unsaturated ketones in the presence of an In(OTf) 3 catalyst (SR-7h).
In addition to these synthetic strategies, eco-friendly and straightforward procedures, like photochemical methods, have been applied for the synthesis of IZPs to overcome challenges like environmental issues and dependence upon non-renewable sources. Photochemical methods offer many advantages over traditional heating strategies, notably using visible light and benign organic catalysts and solvents. Photocatalytic reactions have been applied with metallic or metal-free catalysts for the synthesis and functionalization of IZPs. Tran et al., 2022, reviewed the literature on recent advancements in the photochemical synthesis of IZPs [56].

Antibacterial Profile of Imidazopyridines
IZP is one of the most important scaffolds amongst various fused heterocyclic systems. It possess a broad range of pharmacological activities. Although significant research has been carried out against bacterial infections, only one drug, rifaximin, is available as an antibiotic on the market. There is considerable evidence to support the antibacterial activity of IZP. This may pave the way for the development of antibiotics to overcome resistance. An evaluation of the antibacterial qualities of IZPs suggested the involvement of various pathways in their mechanism of action. IZPs can target several enzymes associated with the synthesis of cell wall/peptidoglycan, protein, folic acid, DNA, or RNA, to eradicate infections. Investigations have exemplified various substituents on IZP rings which afford significant antibacterial activity. Once again, IZP is a multitargeted scaffold.
In 2021, Mishra et al. [57], reported that 2H-chromene-based IZP derivatives (1, Figure 3a,b) were potent peptide deformylase inhibitors. These compounds were synthesized using an eco-friendly, one-pot, three-component approach employing FeCl 3 as the catalyst under microwave irradiation at 60W, 100 • C in 15 min (Figure 3a). In addition, these compounds exhibited potent antibacterial activity against bacterial strains such as Klebsiella oxytoca, Streptococcus pyogenes, Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli). The most active compound is represented in Figure 3b, with its MIC and docking values.
Antibiotics 2022, 11, 1680 6 of 18 sized using an eco-friendly, one-pot, three-component approach employing FeCl3 as the catalyst under microwave irradiation at 60W, 100 °C in 15 min (Figure 3a). In addition, these compounds exhibited potent antibacterial activity against bacterial strains such as Klebsiella oxytoca, Streptococcus pyogenes, Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli). The most active compound is represented in Figure 3b, with its MIC and docking values.   Althagafi et al., 2021 [58], reported the synthesis of IZPs using substituted 2-APs and 3-chloroacetylacetone. The prepared IZPs reacted with substituted benzaldehyde and malononitrile or ethyl cyanoacetate in a three-component reaction. The synthetic schemes for the preparation of IZP and its derivatives are presented in Figure 3a. The IZP-based heterocycles (3a-d) were found to be more effective against E. coli, S. aureus, Bacillus subtilis (B. subtilis), Klebsiella pneumonia (K. pneumonia) than ampicillin and gentamicin. In summary, thiazole-based IZPs showed higher antibacterial activity than pyridine-or pyrazole-based IZPs against all tested organisms ( Figure 4b). Thus, thiazole-based IZPs (3a) provide an opportunity in medicinal chemistry to find novel antibacterial agents.
Thakur et al., 2020 [59], reported that IZP conjoined pyran bis-heterocyclic derivatives are potent antibacterial agents. The derivatives were synthesized by ultrasound-assisted synthetic reactions combining multicomponent reactions, an eco-friendly catalyst (gluconic acid aqueous solution), and a green solvent. The synthesis of an IZP-pyran heterocycle involved malononitrile, cyclochexanedione, and substituted IZPs. The synthetic reaction mechanism encompassed Knoevenagel condensation and Michael reaction, followed by cyclization and tautomerization (Figure 5a). Antibacterial studies showed significant results for the synthesized compounds against E. coli, S. aureus, and Salmonella typhi (S. typhi). Amongst the 11 synthesized derivatives, two molecules, i.e., 4a and 4b, exhibited particularly strong activity against S. aureus (4a: 7.8 µg/mL 4b: 31.25 µg/mL) (Figure 5b). Thakur et al. noted that the presence of fluorine atoms at the para position of the phenyl ring of IZP increased the activity of these compounds against S. aureus. Further, the derivatives also yielded a pronounced (<10%) hemolytic effect, indicating that the compounds were benign to erythrocytes, as evidenced by hemolysis tests.  Thakur et al., 2020 [59], reported that IZP conjoined pyran bis-heterocyclic derivatives are potent antibacterial agents. The derivatives were synthesized by ultrasound-assisted synthetic reactions combining multicomponent reactions, an eco-friendly catalyst (gluconic acid aqueous solution), and a green solvent. The synthesis of an IZP-pyran heterocycle involved malononitrile, cyclochexanedione, and substituted IZPs. The synthetic reaction mechanism encompassed Knoevenagel condensation and Michael reaction, followed by cyclization and tautomerization (Figure 5a). Antibacterial studies showed significant results for the synthesized compounds against E. coli, S. aureus, and Salmonella typhi (S. typhi). Amongst the 11 synthesized derivatives, two molecules, i.e., 4a and 4b, exhibited particularly strong activity against S. aureus (4a: 7.8 μg/mL 4b: 31.25 μg/mL) (Figure 5b). Thakur et al. noted that the presence of fluorine atoms at the para position of the phenyl ring of IZP increased the activity of these compounds against S. aureus. Further, the derivatives also yielded a pronounced (<10%) hemolytic effect, indicating that the compounds were benign to erythrocytes, as evidenced by hemolysis tests. In 2019, Ebenezer et al. [60] reported pyrazolo-IZP molecular conjugates as antibacterial agents targeting cell wall synthesis. These derivatives were synthesized using a onepot, three-component tandem reaction employing CuSO 4 /Na-ascorbate and Cs 2 CO 3 as a catalyst. Amongst the 12 synthesized derivatives, 5 ( Figure 6) exhibited significant bactericidal activity (zone of inhibition >9 mm) against Methicillin-Resistant Staphylococcus aureus Antibiotics 2022, 11, 1680 8 of 18 (MRSA), S. aureus, E. coli, S. typhi, K. pneumonia, Pseudomonas aeruginosa (P. aeruginosa) compared to ciprofloxacin. In addition, the conjugates were subjected to molecular docking analysis against glucosamine-6-phosphate (GlcN-6-P) synthase enzyme. From MBC studies, it was clear that compound 5a exhibited broad-spectrum inhibitory activity, with MBC values <2.50 µg/mL against a selected panel of bacteria. In contrast, compounds 5b and 5d showed significant activity only against Gram-negative strains (<1 µg/mL), while compound 5c (2-OH substituted) presented activity against both Gram-positive (S. aureus: 0.08 µg/mL; MRSA:19.53 µg/mL) and Gram-negative (S. typhi:0.63 µg/mL; K. pneumonia: 0.08 µg/mL; P. aeruginosa:0.63 µg/mL) strains. Compound 5e also showed significant bactericidal properties (<1 µg/mL) against all of the tested strains. Supporting the in vitro data, a docking analysis revealed the significant binding affinity with the key amino acid residue of the catalytic pocket of the GlcN-6-P synthase enzyme. In 2019, Ebenezer et al. [60] reported pyrazolo-IZP molecular conjugates as antibacterial agents targeting cell wall synthesis. These derivatives were synthesized using a onepot, three-component tandem reaction employing CuSO4/Na-ascorbate and Cs2CO3 as a catalyst. Amongst the 12 synthesized derivatives, 5 ( Figure 6) exhibited significant bactericidal activity (zone of inhibition >9 mm) against Methicillin-Resistant Staphylococcus aureus (MRSA), S. aureus, E. coli, S. typhi, K. pneumonia, Pseudomonas aeruginosa (P. aeruginosa) compared to ciprofloxacin. In addition, the conjugates were subjected to molecular dock-  40 .nH 2 O) as a catalyst. The synthetic mechanism comprised a double condensation reaction of maleimide with APs, which acts as a nucleophile (Michael addition) and as a basic reagent (Knoevenagel reaction) (Figure 7a). Out of the four synthesized IZPs, two (Figure 7b) showed significant broad-spectrum bactericidal activity. SAR suggested that substitutions (R) of either hydrogen or methyl groups on the dihydropyrrole moiety and side chain enhanced the activity of 6a and 6b. In contrast, increasing the carbon chain to ethyl or phenyl groups showed detrimental effects on the aforementioned activity.

Position 3 & 4 (diOCH 3 ) and 2 (OH);
Broad-spectrum activity on the dihydropyrrole moiety and side chain enhanced the activity of 6a and 6b. In contrast, increasing the carbon chain to ethyl or phenyl groups showed detrimental effects on the aforementioned activity. Malley et al., 2018 [62], reported on the use of IZPs (Figure 8) as direct targets of QcrB, a component of terminal cytochrome oxidase which is involved in the electron transport chain of Mycobacterium tuberculosis (Mtb). As IZPs show chemical resemblance with the clinical candidate drug Telacebec (Q203, antitubercular drug), proposed to affect QcrB directly, Malley predicted that IZPs may also target QcrB. In conclusion, compounds 7a and 7b exhibited significant bactericidal properties against Mtb by disrupting pH homeostasis and depleting intracellular ATP levels. on the dihydropyrrole moiety and side chain enhanced the activity of 6a and 6b. In contrast, increasing the carbon chain to ethyl or phenyl groups showed detrimental effects on the aforementioned activity. Malley et al., 2018 [62], reported on the use of IZPs (Figure 8) as direct targets of QcrB, a component of terminal cytochrome oxidase which is involved in the electron transport chain of Mycobacterium tuberculosis (Mtb). As IZPs show chemical resemblance with the clinical candidate drug Telacebec (Q203, antitubercular drug), proposed to affect QcrB directly, Malley predicted that IZPs may also target QcrB. In conclusion, compounds 7a and 7b exhibited significant bactericidal properties against Mtb by disrupting pH homeostasis and depleting intracellular ATP levels. Again in 2018, Kuthyala et al. [63] reported the synthesis and evaluated the antibacterial properties of trisubstituted IZPs. The derivatives were synthesized by engrossing oxadiazole in the presence of dichloromethane, hydrazine hydrate, ethanol, and POCl 3 in different steps (Figure 9a). The compounds exhibited pronounced bactericidal activity, as evidenced by their MIC and time-kill kinetics. Out of 9 synthesized compounds with varying substitutions, compound (8) (Figure 9b), containing methyl and nitro groups on the IZP and phenyl rings, exhibited significant antibacterial activity (S. aureus: 3.12 µg/mL) compared to the reference compound, ciprofloxacin. The results are summarized in Figure 9.
Again in 2018, Kuthyala et al. [63] reported the synthesis and evaluated the antiba terial properties of trisubstituted IZPs. The derivatives were synthesized by engrossi oxadiazole in the presence of dichloromethane, hydrazine hydrate, ethanol, and POCl3 different steps (Figure 9a). The compounds exhibited pronounced bactericidal activity, evidenced by their MIC and time-kill kinetics. Out of 9 synthesized compounds with var ing substitutions, compound (8) (Figure 9b), containing methyl and nitro groups on the IZ and phenyl rings, exhibited significant antibacterial activity (S. aureus: 3.12 μg/mL) com pared to the reference compound, ciprofloxacin. The results are summarized in Figure   ( Devi et al., 2017 [64], synthesized pyrazolopyridinone-fused IZPs using one-pot ta dem reactions through an In(OTf)-assisted Groebke-Blackburn Bienayme multi-comp nent strategy (Figure 10a). The compounds exhibited significant bactericidal activi against E. coli, P. aeruginosa, S. aureus, and Staphylococcus epidermis (S. epidermis) when com pared with ofloxacin. A few highly active compounds (9a-d) with different substitutio are represented in Figure 10. A preliminary SAR analysis (9, Figure 10b) revealed th substitution on the nitrogen of the pyrazole moiety with aryl or aryl substituted grou enhanced the antibacterial activity of the compounds. Devi et al., 2017 [64], synthesized pyrazolopyridinone-fused IZPs using one-pot tandem reactions through an In(OTf)-assisted Groebke-Blackburn Bienayme multi-component strategy (Figure 10a). The compounds exhibited significant bactericidal activity against E. coli, P. aeruginosa, S. aureus, and Staphylococcus epidermis (S. epidermis) when compared with ofloxacin. A few highly active compounds (9a-d) with different substitutions are represented in Figure 10. A preliminary SAR analysis (9, Figure 10b) revealed that substitution on the nitrogen of the pyrazole moiety with aryl or aryl substituted groups enhanced the antibacterial activity of the compounds. Further, Arora et al., 2014 [65], proposed that IZP (10) derivatives are potent antiMtb compounds that were shown to target respiratory bc1 complex in laboratory-adapted strains of Mtb such as K14B0DS, H37Rv, cydKO. The results (Figure 11) highlighted the promiscuity of the bc1 complex in Mtb and the risk of targeting ATP or energy metabolism with novel therapeutics. Further, Arora et al., 2014 [65], proposed that IZP (10) derivatives are potent antiMtb compounds that were shown to target respiratory bc1 complex in laboratory-adapted strains of Mtb such as K14B0DS, H37Rv, cydKO. The results (Figure 11) highlighted the promiscuity of the bc1 complex in Mtb and the risk of targeting ATP or energy metabolism with novel therapeutics.
Al-Tel et al., 2011 [66], proposed indole based-IZPs as antibacterial agents, synthesized using the Groebke-Blackburn three-component reaction and Ugi reaction [4+1] cycloaddition protocol involving amino-azole, aldehyde, and an isocyanide with TBTU, DIPEA, DMF, DIPEA as catalysts (Figure 12a). These derivatives exerted strong inhibition against S. aureus, E. faecalis, Bacillus megaterium (B. megaterium), E. coli, P. aeruginosa, and Enterococcus aerogenes (E. aerogenes), in the range of 0.11-23.45 µg/mL, compared to control antibiotics such as cefixime and amoxicillin. In summary, the nature of substituents such as armed aryl groups exemplified the extent of the activity of these compounds (11a-f, Figure 12b). Preliminary SAR studies revealed that bromo-fluoro substituents enhanced the antibacterial activity significantly, compared to other substituents. Al-Tel et al., 2011 [66], proposed indole based-IZPs as antibacterial agents, synthesized using the Groebke-Blackburn three-component reaction and Ugi reaction [4+1] cycloaddition protocol involving amino-azole, aldehyde, and an isocyanide with TBTU, DI-PEA, DMF, DIPEA as catalysts (Figure 12a). These derivatives exerted strong inhibition against S. aureus, E. faecalis, Bacillus megaterium (B. megaterium), E. coli, P. aeruginosa, and Enterococcus aerogenes (E. aerogenes), in the range of 0.11-23.45 μg/mL, compared to control antibiotics such as cefixime and amoxicillin. In summary, the nature of substituents such as armed aryl groups exemplified the extent of the activity of these compounds (11a-f, Figure 12b). Preliminary SAR studies revealed that bromo-fluoro substituents enhanced the antibacterial activity significantly, compared to other substituents.
(a) A study by Starr et al., 2009 [67], exemplified IZPs as antibacterial agents targeting, e.g., DNA gyrase and topoisomerase IV. The compounds were synthesized employing 2-amino-4-bromo-6-ethoxycarbonylpyridine as the starting material and using various reagents and catalysts. The reaction mechanism included the Suzuki coupling reaction and one-pot procedure to obtain greater yields (Figure 13a). The derivatives were found to be effective against S. aureus, MRSA, S. pyogenes, S. pneumonia, and Fluoroquinolone-Resistant Streptococcus pneumonia (FRSP), with significant MIC. In addition, a few derivatives exhibited significant enzyme inhibition activity against S. pneumonia GyrB (Spn GyrB) and ParE (Spn ParE) (Figure 13b). Further, Starr et al. also performed in vivo and pharmacokinetic studies for the most potent compounds. The results were stunning: pyridyl-substituted IZPs established excellent oral PD 50 and PK parameters in murine S. pyogenes sepsis and S. pneumonia lung models. Al-Tel et al., 2011 [66], proposed indole based-IZPs as antibacterial agents, synthesized using the Groebke-Blackburn three-component reaction and Ugi reaction [4+1] cycloaddition protocol involving amino-azole, aldehyde, and an isocyanide with TBTU, DI-PEA, DMF, DIPEA as catalysts (Figure 12a). These derivatives exerted strong inhibition against S. aureus, E. faecalis, Bacillus megaterium (B. megaterium), E. coli, P. aeruginosa, and Enterococcus aerogenes (E. aerogenes), in the range of 0.11-23.45 μg/mL, compared to control antibiotics such as cefixime and amoxicillin. In summary, the nature of substituents such as armed aryl groups exemplified the extent of the activity of these compounds (11a-f, Figure 12b). Preliminary SAR studies revealed that bromo-fluoro substituents enhanced the antibacterial activity significantly, compared to other substituents.

Conclusions
Imidazopyridine (IZP)-based derivatives have a broad range of pharmacological activities. Some IZP-based conjugates have been extensively utilized for the treatment of bacterial infections caused by wild or resistant bacteria. The intriguing scaffold of IZP plays a key role in the advancement of novel antibiotics. Because of its immense pharmacological significance, many synthetic approaches have been developed to apply diverse substitutions to this scaffold. In recent years, IZPs have been synthesized and screened for their in vitro and in vivo activity against Gram-negative, Gram-positive, and resistant strains of bacteria. This review highlights the synthetic strategies to develop IZPs and

Conclusions
Imidazopyridine (IZP)-based derivatives have a broad range of pharmacological activities. Some IZP-based conjugates have been extensively utilized for the treatment of bacterial infections caused by wild or resistant bacteria. The intriguing scaffold of IZP plays a key role in the advancement of novel antibiotics. Because of its immense pharmacological significance, many synthetic approaches have been developed to apply diverse substitutions to this scaffold. In recent years, IZPs have been synthesized and screened for their in vitro and in vivo activity against Gram-negative, Gram-positive, and resistant strains of bacteria. This review highlights the synthetic strategies to develop IZPs and ongoing advances in their potential antibacterial activity against wild and mutant strains of bacterial pathogens. This information might be used to develop new and diverse IZP derivatives to combat infections and resistance.