Recent Advances in Pyridine Scaffold: Focus on Chemistry, Synthesis, and Antibacterial Activities

Multidrug-resistant (MDR) pathogens have created a fatal problem for human health and antimicrobial treatment. Among the currently available antibiotics, many are inactive against MDR pathogens. In this context, heterocyclic compounds/drugs play a vital role. Thus, it is very much essential to explore new research to combat the issue. Of the available nitrogen-bearing heterocyclic compounds/drugs, pyridine derivatives are of special interest due to their solubility. Encouragingly, some of the newly synthesized pyridine compounds/drugs are found to inhibit multidrug-resistant S. aureus (MRSA). Pyridine scaffold bearing poor basicity generally improves water solubility in pharmaceutically potential molecules and has led to the discovery of numerous broad-spectrum therapeutic agents. Keeping these in mind, we have reviewed the chemistry, recent synthetic techniques, and bacterial preventative activity of pyridine derivatives since 2015. This will facilitate the development of pyridine-based novel antibiotic/drug design in the near future as a versatile scaffold with limited side effects for the next-generation therapeutics.


Introduction
Most of the nitrogen-bearing heterocyclic compounds are biologically potential [1][2][3] and play a vital role in progressive drug design and discovery [4]. Among them, the sixmembered heteroaromatic pyridine nucleus is ubiquitous and found in natural sources such as alkaloids (nicotine), vitamins (niacin and pyridoxine), and coenzymes [5]. Although pyridine is a very common solvent in organic laboratories, its derivatives have diverse applications in functional nanomaterials, as important ligands for organometallic compounds, and in asymmetric catalysis [6,7]. In organic chemistry, pyridine and its derivatives play vital roles [8] and are the most extensively applied scaffolds for drug design and synthesis. In fact, pyridine scaffold-containing compounds have received significant interest in multiple research fields. This is mainly related to their (i) unique heteroaromatic functional role in organic chemistry, (ii) easy conversion into different functional derivatives, (iii) profound effect on pharmacological activity, and (iv) application as pharmacophores in medicinal chemistry [9]. These properties led to the discovery of many broad-spectrum therapeutic agents [10] and agrochemical products [11].
Privileged with the pyridine scaffold, many drugs have been synthesized/discovered and several of them are on the market as therapeutic drugs. Hundreds of compounds with pyridine scaffolds are listed as drugs (https://go.drugbank .com/categories/DBCAT000227) including FDA [5]. For example, the attachment of this auspice nucleus to sulfanilamide produced an antibacterial antibiotic named sulfapyridine ( Figure 1). The most important pyridine-based therapeutic drugs are isoniazid (brand name: Nydrazid; an antibiotic for tuberculosis), esomeprazole (brand name: Nexium), lansoprazole (brand name: Takepron), montelukast (brand name: Singulair), pioglitazone (brand name: Actos), etc. (Figure 1). In addition, low molecular weight antibacterial drugs like ozenoxacin and ethionamide are notable [12,13].
Insertion of a simple COOH group at the C-3 position of pyridine (as in nicotinic acid or niacin, the oldest drug for dyslipidemia) enhances pyridine's bioactivity, and niacin was reported as a precursor for many other significant bioactive molecules (such as NAD + and NADP + [18]). In general, the incorporation/fusion of other rings, especially heterocyclic ring(s) with the pyridine nucleus, enhances its bioactivity and intensifies its antimicrobial properties [4,19]. Additional attachment of several functional groups (amino, hydroxy, methoxy, sulfamide, hydrazide, etc.) enhances the compound's bioactivity further [20].
Thus, pyridine scaffold compounds and materials are valued for their biological, medicinal, optical, chemical, and physical properties among nitrogen-based heterocycles. This review is aimed at focusing on chemistry and the reported synthetic methods of pyridine scaffolds since 2015, emphasizing the antibacterial potential.

Chemistry of Pyridine Compounds
Mononitrogen containing a six-membered heteroaromatic compound structurally similar to benzene with the molecular formula C 5 H 5 N is named pyridine (Py, Figure 2). It is also known as monoazabenzene, azaarene, azine, etc. and is the parent compound of the class pyridines. This simplest and most common compound is water-miscible, flammable, and colorless to yellow liquid (bp 115.5°C and mp -41.6°C). Due to water miscibility, it is used to dissolve other substances. However, pyridine bears an unpleasant/foul smell and some hazardous properties. Cyclic pyridine is planar with a sp 2 hybridized N atom and five C atoms and has a delocalized pi-molecular orbital that fulfills the Hückel criteria (ð4n + 2Þπ electrons) and thus confirms its aromaticity. Structurally, pyridine is isoelectronic with benzene and exhibits unusual isotopic polymorphism properties [21]. However, its physical and chemical properties are quite different from benzene. Unlike neutral benzene, pyridine is a weak base and behaves as a tertiary amine in many respects [22]. The presence of the nonconjugated lone pair electrons on the sp 2 hybridized nitrogen atom governs pyridine's basicity. In fact, its basicity is greater than N,N-dimethylaniline and weaker than aliphatic 3°amines. It can form hydrogen bonding and utilizes lone pair n(N) or π-electrons. This property governs its interaction with other substances, enzymes, etc. and is a viable factor for its regioselective and catalytic C-H functionalization [23][24][25].
Due to pyridine's basic nature, it can form stable salts when treated with stronger acids or alkyl halides (the Menshutkin reaction). In addition, it can be used to neutralize some acids produced in chemical reactions. Like other aromatic compounds, pyridine is more prone to substitution reactions. However, it prefers nucleophilic substitution (at the C-2 and C-4 positions) to electrophilic substitution (at the C-3 position under drastic reaction conditions) because of the -I effect of the ring nitrogen ( Figure 2). This -I effect enables the nitrogen atom to become electron-rich and the aromatic ring to become electron-deficient, i.e., charge separation in the ring.

Synthetic Approach for Biologically Significant Pyridine Compounds
Because of its properties, including basicity, water solubility, stability, the capacity to establish hydrogen bonds, and its tiny molecular size, pyridine moieties are frequently utilized in pharmaceuticals [26][27][28]. Hence, several innovative synthetic methods have been developed in the last couple of years for both (a) substituted pyridines and (b) ring-fused pyridines. The significant strategies are discussed here.
Several more synthetic methods are reported for the diversely substituted pyridine preparation. Methods related to biological/pharmaceutical potential are presented below.
Magnetic nanocatalysts, i.e., magnetic nanoparticles (MNPs), have also been applied in multicomponent reactions (MCRs) for pyridine synthesis. This is mainly due to their high surface-to-volume ratios and magnetically recoverable properties. Some common catalysts employed for the pyridine synthesis are Similarly, MNPs 13 [34] or CoFe 2 O 4 @SiO 2 MNPs (14) [35] catalysts were successfully employed for the synthesis of 17 via MCRs. In each case, 17 was obtained with good to high yields in a short reaction time. Encouragingly, ionic MNPs such as Fe 3 O 4 @O 2 PO 2 (CH 2 ) 2 NH 3 + CF 3 CO 2 -(18) were used for easy access to terpyridines 19 (an important precursor for several antimicrobial agents [36]) using MCRs [37].
Uredi et al. [39] showed a simple, metal-free, and mild strategy for the construction of multisubstituted pyridines with excellent yields (Scheme 6). Thus, condensation between α,β-unsaturated aldehydes 27 and propargylamine 28 catalyzed by a very cheap NaHCO 3 furnished 29 and byproduct water only. The condensation proceeds

Green Synthesis.
A facile green protocol for the preparation of substituted pyridines has been reported [41]. In this protocol, a novel, facile, and green conversion of ketoxime acetates 35 and benzaldehyde was conducted using FeCl 3 as a catalyst, and chemoselective 2,4,6-trisub-stituted symmetrical pyridines 36 were constructed (Scheme 8). Encouragingly, the reaction was completed greenly without any additives. Possible mechanisms of Fe-catalyzed cyclization of ketoxime acetates and aldehydes are also discussed.  El-Sayed et al. [42] synthesized three pyridine-based sulfa-drugs from o-hydroxy cyanopyridine derivative 39 (Scheme 10) for antimicrobial tests. Sulfurization of 39 followed by alkylation gave 40, which on separate treatment with sulfacetamide, sulfadiazine, and sulfadimidine (sulfamethazine), furnished corresponding sulfonamides 41, 42, and 43, respectively. Compounds 41-43 exhibited significant broad antimicrobial activities against four bacterial and two fungal pathogens.
Considering biological interest, a simple strategy for the preparation of pyridine-based chitosan thiosemicarbazide 45a, b was described [43]. Treatment of chitosan 44 with ammonia followed by carbon disulfide produced ammo-nium dithiocarbamate chitosan (ADC, 45). This ADC, upon stirring with sodium chloroacetate, formed sodium carbethoxy dithiocarbamate chitosan, which was refluxed with pyridine-based carboxaldehyde and produced chitosan pyridine-2-thiosemicarbazones 46a and chitosan 2-acetyl pyridine-2-thiosemicarbazones 46b (Scheme 11). For efficient preparation, the overall process was conducted in one pot without the isolation of the intermediates. Compound 46a, b was found biologically potential after further conversion and tests.
Recently, a lengthy linear method was reported for the synthesis of a number of 3-(pyridine-3-yl)-2-oxazolidinone derivatives (Scheme 12) [44]. Starting from the readily accessible 3-fluoro-2-hydroxypyridine (47), the target oxazolidinone products (48; six examples) were prepared in eight steps. The antibacterial efficacy of these compounds 48a-d was found to be comparable to the first oxazolidinone   : Ar = Scheme 9: Synthesis of pyridine-3-thiazole hydrazides 38a-l. 6 BioMed Research International antibacterial agent, linezolid, and exerted strong inhibition against Gram-positive bacteria. In fact, 48d showed a stable and longer resistance (15 days) on S. pneumoniae (ATCC 49619), considerably longer than that of linezolid antibiotic. In addition, a novel series of N-sulfonyl aminopyridines containing either a benzothiazole or benzimidazole ring was developed by Azzam et al. [45]. Of the twelve synthesized novel compounds, compounds 49 and 51 ( Figure 3) showed excellent antimicrobial potential. The antimicrobial testing of the novel compounds also revealed that 49 and 50 displayed a greater inhibition zone against Klebsiella pneumo-nia than sulfadiazine and gentamicin. Moreover, compound 51 showed a higher inhibition zone compared to ampicillin against Staphylococcus aureus.   [46].

Ring-Fused
The incorporation of functional moieties in the pyridine nucleus is essential to enhance its biological properties. In this regard, Fu et al. [47] reported a palladium and tri(2furyl) phosphine-catalyzed alkylation of iodo-substituted quinoline with moderate to good yields (Scheme 14). The reaction proceeds with the Catellani reaction among iodoquinoline 55, iodoalkane 56, and α,β-unsaturated ester 57 and produced 2,3,4-trisubstituted-quinolines 58.
Later, Şendil et al. [50] employed a strategy that was nearly identical to the above one to access pyridine-fused aromatic molecules.  In the same year, Khansole [53] conducted the green synthesis of 78 using activated fly ash. The condensation of 2-aminopyridine (76) with substituted phenacyl bromides (77) in the presence of reusable activated fly ash afforded pyridines (78) (Scheme 20).

Antibacterial Properties of Pyridine-Based Compounds
As antibiotic resistance becomes an increasingly serious threat to public health, scientists are always on the lookout for new bacterial inhibitors. Most of the antibiotics now in use are becoming ineffective against bacterial infections 10 BioMed Research International [59,60]. Therefore, the development of new, more potent antibacterial drug candidates is an urgent medical priority. The incorporation of a pyridine motif into a pharmaceutical product can raise that product's biochemical potency and metabolic stability [61], as well as its permeability and difficulty in forming protein-binding interactions [62,63]. There is a wide variety of medications in hospital use (Figure 1), and several drugs have been approved by the FDA since 2015 for cancer/HIV therapy, such as fostemsavir (2020), ivosidenib (2019), lorlatinib (2018), apalutamide (2018), and abemaciclib (2015). Encouragingly, the FDA has also approved several antibiotics stemming from the pyridine motif, including delafloxacin (Baxdela™; 2017), ceftazidime (Fortaz™; 2015), tedizolid (Sivextro™; 2014), and ceftaroline fosamil (Teflaro™; 2010) [5]. Encouraged by these results, many synthetic pyridines have been tested for their biological functionality since 2015. Some significant results are discussed herein.
After synthesizing pyridine derivatives containing oxazolidinone, Jo et al. [67] found that they were very active against bacteria. Two antibiotic-resistant bacterial strains and a number of other Gram-negative and Gram-positive bacterial strains were tested for antibacterial activity with 54a-c in vitro and in vivo [38] (Figure 4). Although the pyridine moiety may tolerate a wide variety of substituted (hetero)aromatic rings, the presence or orientation of methyl groups on the (hetero)aromatic rings significantly affected bacterial activity [68]. Another line of inquiry resulted in   [4,5-b]pyridine derivatives. A methicillin-resistant strain of Staphylococcus aureus, the causative agent of many hospital-acquired infections, was particularly susceptible to the activity of these chemicals [69]. Oxazolo [4,5-b]pyridine analogs were shown to be more efficient against Grampositive bacteria than Gram-negative bacteria in subsequent studies. 2-Phenyloxazolo [4,5-b]pyridine was very efficient in killing methicillin-resistant S. aureus, with an activity of 1.56 to 3.12 μg/mL. The MICs for older antibiotics like ampicillin and streptomycin, on the other hand, ranged from 6.25 to 12.5 μg/mL. There is evidence that many different types of bacteria can be effectively combated with these chemicals [70,71]. As an added bonus, S. aureus may secrete a type A staphylococcal enterotoxin protein. Additional in vitro and computational studies have shown that 2-phenyloxazolo [4,5b]pyridine derivatives (89) are very active against bacteria, even when compared to conventional antibiotics like ampicillin and streptomycin ( Figure 4). The compounds were then tested for ligand-protein binding affinity using the S. aureus (MRSA) protein, with results showing a higher affinity for binding than that of currently used drugs [72].
Dihydropyridine-containing thiazole derivatives were first examined using in silico molecular docking simulations for their potential DNA gyrase inhibitory action. Testing the substances in question for their capacity to kill germs was done to back up the study done on a computer [70]. The results showed that N-aminothiazolyl-1,2-dihydropyridine with a substituent at the 4-position of thiazolyl group 90ac ( Figure 4) showed better antibacterial potentiality against the tested four bacteria (B. subtilis, E. coli, P. aeruginosa, and S. aureus) than the standard antibiotic ampicillin. The existence of the electron-withdrawing group (EWG) (as in 90b) in the phenyl ring attached to the thiazole part could be responsible for better inhibition and activity.

Conclusion
The pyridine skeleton generates a suite of flexibility, leading to the formation of libraries of compounds bearing a variety of functional groups. This is due to its characteristic solubility, basicity, and ability to form hydrogen bond-formation chemistry, which led to the bioisostere of amides, amines, and N-containing heterocycles. All these characteristics make this pyridine skeleton a significant unit in a plethora of drugs and pharmaceuticals. Thus, many improved/novel methods have been reported/developed for the synthesis of functionalized pyridines since 2015. The available antibacterial results concluded that (i) polysubstituted and ring-fused pyridines exhibited considerable antibacterial properties, including methicillin-resistant S. aureus (MRSA) and (ii) EWG in substituted pyridines and EDG group in ringfused pyridines were found to enhance antibacterial potentiality. In spite of the pyridine scaffold bearing overwhelming drug candidates, thoughtful research is essential to overcome drug resistance and side effects. Chemistry, synthesis, and antibacterial potential as highlighted in this minireview may promote better understanding and further effective research of the ever-expanding pyridine scaffold in medicinal chemistry.

Data Availability
All data are available on request.

Conflicts of Interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.