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

Most of the nitrogen-bearing heterocyclic compounds are biologically potential [1–3] and play a vital role in progressive drug design and discovery [4]. Among them, the six-membered 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].

Many naturally occurring compounds are reported to possess a pyridine nucleus. These are considered alkaloids, and notable examples are plumericidine (from Plumeria rubra L., used for cardiovascular treatment [14]), dictamnine-7-β-D-mannopyranoside (from Solidago canadensis [15]), epibatidine (from Epipedobates tricolor skin, an analgesic agent [16]), and pyrinadine A (a bis-pyridine alkaloid from Cribochalina sp., anticarcinogenic and antiparasitic [17]).

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.

Mononitrogen containing a six-membered heteroaromatic compound structurally similar to benzene with the molecular formula C₅H₅N 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 sp² hybridized N atom and five C atoms and has a delocalized pi-molecular orbital that fulfills the Hückel criteria ((4n + 2)π 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 sp² 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 [23–25].

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.

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 [26–28]. 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.

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 [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.

In 2016, newly prepared several pyridine-imidazo[2,1b]-1,3,4-thiadiazole compounds were tested against seven microbial pathogens and showed good antimicrobial activity (maximum 95.1% inhibition) [64]. Later on, another new type of pyridine compounds, namely, 3-chloro-1-(4-substituted phenyl)-4-(pyridin-3-yl)azetidin-2-one compounds, were prepared for antimicrobial interest [65] and found that such compounds are potent against S. aureus (MTCC-3160). Polysubstituted 2-amino-4-aryl-3,5-dicarbonitrile-6-thiopyridines 88a-k, synthesized by Koszelewski et al. [66], are found to inhibit Escherichia coli model strains K12 and R2–R4. MIC and MBC tests clearly demonstrated very low values of 0.2–1.3 μg/mL and 4-45 μg/mL, respectively. Compounds 88a, g, i, j with different electron-withdrawing groups (EWG: CN, Cl, Br, and NO₂) attached to the aromatic ring (at C-2 and C-4) enhance the E. coli inhibition process. They have shown that these compounds interact with bacterial cell walls irreversibly, leading to apoptosis. As a result, known antibiotics can be replaced with 2-amino-4-aryl-3,5-dicarbonitrile-6-thiopyridines.

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 the development of antibacterial drugs by the manufacture of oxazolo[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 Gram-positive 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[4,5-b]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 90a-c (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.

Among the ring-fused pyridines, synthetic novel 3H-imidazo[4,5-b]pyridine (11 compounds) and 1H-imidazo[4,5-b]pyridine (10 compounds) analogs were subjected to in vitro antitubercular activity against M. tuberculosis (H37RV) [73]. Compounds 91a-c, j (3.125 μg/mL) and 92a, c, f (3.125-6.25 μg/mL) were found highly active and comparable to pyrazinamide (MIC = 3.125 μg/mL) and streptomycin (MIC = 6.25 μg/mL). The electron-donating group(s) (EDG) attached to the imidazole ring probably reduced cytotoxicity and enhanced antibacterial functionality. Thus, these imidazo-fused pyridines might develop bacterial-related multidrug-resistant (MDR) infections caused by M. tuberculosis.

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 ring-fused 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.

We are thankful to the Research and Publication Cell of the University of Chittagong, Bangladesh (Grant Number 132/15 (2022)).

Md. Rezaur Rahman, Email: rmrezaur@unimas.my.

Mohammed Mahbubul Matin, Email: mahbubchem@cu.ac.bd.

All data are available on request.

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.

All the authors equally contributed to the manuscript preparation. MMM conceptualized and supervised the work. All authors approved the submitted version and the revised version.