Nitropyridines in the Synthesis of Bioactive Molecules

Abstract

Pyridines are one of the most important and promising classes of N-heterocycles actively studied in modern organic and medicinal chemistry; in particular, pyridine is a privileged structural motif in drug design. From a synthetic organic chemistry perspective, nitropyridines can be considered as convenient and readily available precursors for a wide range of mono- and polynuclear heterocyclic systems demonstrating diverse activities, such as antitumor, antiviral, anti-neurodegenerative, etc. This review is an analysis of the literature on the use of nitropyridines for the synthesis of biologically active compounds, covering the period from 2015 to the present.

1. Introduction

Pyridine-based ring systems represent a promising class of nitrogen heterocycles, which are actively studied in both medicinal and agricultural chemistry. In particular, pyridine serves as a privileged structural motif in drug design. As of 2021, approximately 14% of N-heterocyclic drugs approved by the FDA contain a pyridine moiety. Notable examples include isoniazid, omeprazole, enpiroline, piroxicam, tacrine, and many others [1]. Beyond their application in pharmacology [2], pyridine derivatives are in demand in material science [3,4]; they are used as dyes [5,6,7], in catalysis [8,9], and in other fields.

Among numerous pyridine derivatives, nitropyridines deserve particular attention. Over the past few decades, nitropyridines have been shown to exhibit diverse biological activities, such as the inhibition of cytosolic thioredoxin reductase 1, which serves as a mechanistic basis for anticancer therapy [10]. They also inhibit the human platelet Na⁺/H⁺ exchanger [11] and display antifungal activity [12]. Furthermore, nitropyridines have been utilized as precursors in the synthesis of various compounds, including azaindole hydroxamic acids, which are potent HIV-1 integrase inhibitors [13]; azaphenothiazines, which act as potent CNS-depressants [14]; and ligands for α-synuclein [15]. Remarkably, 3,5-dinitropyridine has been identified as a metabolite of Lactobacillus kunkeei [16].

Despite the availability and wide synthetic potential of nitropyridines, this class of compounds has not received much attention in the review literature. There is a review devoted to the synthesis of 3-nitropyridines from acyclic synthones and through the transformation of other heterocyclic rings, published as early as in 1991 [17]. Additionally, the synthesis and reactions of dinitropyridines were reviewed recently [18].

This review encompasses recent examples of the synthesis of bioactive nitropyridines, as well as their use as intermediates in the synthesis of promising biologically active compounds. For convenience, the considered methods for the preparation of target compounds are organized into separate subsections: the syntheses of nitropyridine-containing bioactive molecules, the formation of metal complexes with nitropyridine ligands, reactions involving nitro group(s) (where nitropyridines serve as intermediates), and the synthesis of radiolabeled compounds for the monitoring and treatment of pathological conditions.

2. Synthesis of Bioactive Nitropyridines

Many drugs have been created on the basis of aromatic nitro compounds [19]. In this section, syntheses of nitropyridine-containing bioactive molecules are considered.

A number of potent Janus kinase 2 (JAK2, a non-receptor tyrosine kinase) inhibitors were synthesized by Cho et al. [20]. 2-Chloro-5-methyl-3-nitropyridine 2, obtained from 2-amino-5-methylpyridine, was oxidized to carboxylic acid, followed by nucleophilic substitution of the activated chlorine atom with secondary amines (Scheme 1). The resulting carboxylic acids 3 were coupled with aromatic amines in the presence of N,N′-dicyclohexylcarbodiimide (DCC), thus affording target compounds 4 in moderate yields. Similarly, starting with isomeric 2-amino-3-methylpyridine 5, a series of sulfamides 6 were synthesized. The most potent compounds inhibited JAK2 with IC₅₀ 8.5–12.2 µM.

Another application of nitropyridine derivatives was demonstrated by Bussiere et al. [21]. The authors synthesized a novel series of heterocyclic compounds that have been shown to inhibit glycogen synthase kinase-3 (GSK3), which mediates the addition of phosphate molecules onto serine and threonine amino acid residues. In 2,6-dichloro-3-nitropyridine 7, both chlorine atoms were successively substituted with aryl (Suzuki coupling) and aminoethylamine fragments (Scheme 2). Then, the nitro group was reduced to amine, which in turn was acylated with bromoacetyl chloride to give the corresponding amide 8. Further alkylation and intramolecular Mitsunobu reaction afforded intermediate 9, which, after Boc protection cleavage and reaction with 2-amino-6-chloro-3-nitropyridine, resulted in target compounds 10. The most active GSK3 inhibitor from this series contains 2,4-dichlorophenyl moiety (Ar = 2,4-Cl₂-C₆H₃); IC₅₀ 8 nM, EC₅₀ 0.13 μM.

2,6-Dichloro-3-nitropyridine 7 also served as a key intermediate in the synthesis of a potent kinase p70S6Kβ inhibitor [22]. The Buchwald–Hartwig arylamination of 3-chloroisoquinoline derivative 11 with benzophenonimine, followed by hydrolysis of the imine, produced 3-aminoisoquinoline intermediate 12, which was then reacted with 2,6-dichloro-3-nitropyridine 7 to yield target compound 13 as a product of the regioselective nucleophilic substitution of chlorine in position 2 of the pyridine nucleus (Scheme 3). Compound 13 did not demonstrate the expected inhibitory activity against kinase MPS1; however, it inhibited the kinase p70S6Kβ with IC₅₀ 444 nM. The authors considered this compound as a starting point for the development of p70S6Kβ inhibitors.

3-Nitropyridine fused with a furoxan ring (4-aza-6-nitrobenzofuroxan 16) was studied as a potential dual-action HIV-1 inhibitor [23]. Compound 16 inhibited two HIV-1-enzymes: integrase and the RNase H domain within reverse transcriptase. In particular, its activity levels (IC₅₀) against both catalytic activities of integrase (strand transfer reaction and 3′ processing) were 190 ± 30 μM and 60 ± 15 μM, respectively, while an IC₅₀ of 90 ± 20 μM was found for RNase H inhibition. This compound was synthesized by a two-step method, starting from 2-chloro-3,5-dinitropyridine 14 [24] (Scheme 4).

3-Nitropyridylpiperazine derivatives were synthesized and evaluated as potential urease inhibitors for the treatment of gastric diseases [25]. Synthesis was accomplished by the reaction of 2-chloro-3-nitropyridine 17 with piperazine and further N-alkylation with various aryl 2-chloroacetamides or 2-chloropropioamides (Scheme 5). The IC₅₀ values of the most promising compounds 19 are about 2.0–2.3 μM against jack bean urease, which is much lower than the IC₅₀ of thiourea (23.2 μM), a standard urease inhibitor.

New 5-aryl(hetaryl)-2,2-dimethyl-1,3-dioxane-4,6-dione derivatives 20 were was newly synthesized by the condensation of Meldrum’s acid (21), triethyl orthoformate, and substituted aromatic or heteroaromatic amines, and then evaluated for chymotrypsin and urease inhibition [26] (Scheme 6). Among the synthesized compounds, the 5-nitropyridin-2-yl derivative exhibited dual inhibition activities, with an IC₅₀ value of 8.67 ± 0.1 μM against chymotrypsin and 29.21 ± 0.98 μM against urease.

In addition, (5-nitropyridin-2-yl)imine 1,3,4-thiadiazole hybrids were designed as potent and selective factor IXa inhibitors and proven to be prospective candidates in the search for anticoagulant drugs [27]. These compounds have higher inhibition levels and lower IC₅₀ values with respect to similar compounds with no nitro group in the pyridine cycle.

The crystal structures of 4-phenylsulfanylbutyric acid and its cocrystal with 2-amino-5-nitropyridine have been studied [28]. The in vitro DNA cleaving and antimicrobial activity of the cocrystal against a number of Gram-positive (S. aureus, S. pneumoniae) and Gram-negative bacteria (E. coli, P. aeruginosa, P. vulgaris) were higher than those of 4-phenylsulfanylbutyric acid alone.

Silver nanoparticles (AgNPs) are recognized as effective agents against biofilms formed by P. aeruginosa. However, when combined with 4-nitropyridine-N-oxide (4-NPO), they demonstrate even more significant antibiofilm efficiency compared to AgNPs alone [29]. The primary advantage of the AgNP-4-NPO combination as an antimicrobial agent lies in its multi-level effects on bacterial physiological processes. This synergetic approach interferes with the quorum sensing system and inhibits the secretion of extracellular polymeric substances, which are crucial for P. aeruginosa to resist biofilm formation.

The effect of irreversible biofouling of the reverse osmosis membrane and the inhibitory effect of biofilm inhibitors on irreversible biofouling were investigated using P. aeruginosa as model bacteria [30]. The biofilm inhibitor 4-NPO, at concentrations of 30–50 μM, significantly inhibited the biofilm formation and reduced the irreversible biofouling resistance without affecting microbial growth in reverse osmosis processes.

The influence of bacterial quorum-sensing inhibitors on the formation of diatom biofilm has been studied [31]. 4-Nitropyridine-N-oxide demonstrated an inhibitory effect on the biofilm formation of Cylindrotheca sp. In addition, it effectively decreased the risk of human bacterial pathogens in soil [32].

The synthesis of antibacterial compounds based on nitropyridines was described very recently [33]. The authors studied the oxidative S_N^H alkylamination of nitroquinolines and nitroisoquinolines in an aqueous medium and found the process to be selective, depending solely on the position of the nitro group. For example, 3-nitroquinoline 22 was aminated in position 4 by the reactions with primary amines and K₃Fe(CN)₆ at room temperature (Scheme 7). However, the most active compounds were obtained from 5- and 8-nitroquinolines.

Novel 3(5)-nitropyridines, functionalized with azole or pyridazine moieties, were synthesized from 2-, 4-, and 6-chloro-3-nitropyridines (17, 24, and 25) [34] (Scheme 8). The authors observed moderate antibacterial activity of pyrazole and imidazole derivatives against S. aureus and E. coli, as well as an antiprotozoal effect against Colpoda steinii.

Kulakov and coworkers reported on the multicomponent synthesis and evaluation of the bacteriostatic properties of epoxybenzooxocino[4,3-b]pyridine derivatives [35]. Hantsch 1,4-dihydropyridine synthesis was carried out using 2-nitroacetophenone, paraformaldehyde, ammonium acetate, and substituted β-dicarbonyl compound as reactants (Scheme 9). The resulting compound 28 was oxidized with a solution of CrO₃ in acetic acid to yield 5-nitropyridine 29. Both compounds were active against Mycobacterium bovis 14, with MIC values of 12.5–50 μg/mL.

A number of articles have been published on the introduction of electron-deficient 3,5-dinitropyridine moiety into a fluorophore molecule (Scheme 10). The authors revealed an excellent electron sink of such compounds that allows for using it as novel fluorescent probes for the detection of biothiols [36,37,38]. Such molecules were prepared via nucleophilic substitution of chlorine in 2-chloro-3,5-dinitropyridine 14 under mild conditions.

Hybrid molecules containing chloroquine fragments linked with nitropyridyl fragments were reported to possess good antimalarial activity [39]. The target compounds 32 could be obtained by reacting chloroquine derivative 31 with the corresponding 3-R-2-chloro-5-nitropyridine (Scheme 11). The reactions proceeded in the presence of K₂CO₃ under reflux in MeCN. Some compounds demonstrated IC₅₀ values below 5 nM.

Nitropyridine linked 4-arylidenethiazolidin-4-ones, as potent anticancer agents, were synthesized from 2-amino-5-nitropyridine 33 [40]. It was reacted with chloroacetyl chloride, and then treated with ammonium thiocyanate to produce 5-nitropyridyliminothiazolidin-4-one 34, which was reacted with aromatic aldehyde to yield the corresponding arylidene derivatives 35 in 75–83% yields (Scheme 12). The target compounds possessed high selectivity against certain cancer types: 35a (R = OMe) was active against MCF-7 cells, with an IC₅₀ of 6.41 μM, while piperidine derivative 35d was active against HepG2 cells, with an IC₅₀ of 7.63 μM.

Novel imidazopyridines as inhibitors of necroptosis were described in patent [41]. Commercially available 2-chloro-3,5-dinitropyridine 14 was used for the synthesis of the target compounds (Scheme 13). Nucleophilic substitution of the chlorine atom with the corresponding amine afforded compounds 36 in high yields. Selective reduction of the 3-NO₂ group under the action of ammonium sulfide yielded compounds 37. Further transformation led to the desired compounds 38, which showed activity against TSQ-induced necroptosis at nanomolar concentrations.

The synthesis and herbicidal activity of novel nitropyridine-containing phenylaminoacetates and propionates were studied by Li and coworkers [42]. The chlorine atom in 2-chloro-3(5)-nitropyridines was substituted with 4-aminophenol, and the resulting aniline 39 was reacted with ethyl 2-chloroacetate or ethyl 2-chloropropionate to yield alkylation products 40 in moderate to high yields (Scheme 14). One of the synthesized compounds, namely ethyl 2-(4-(5-nitropyridin-2-yloxy)-phenylamino)propanoate, exhibited a high level of herbicidal activity on barnyard grass (IC₅₀ 27.7 mg/L).

Ye and coworkers synthesized and studied the herbicidal activity of some pyridyloxy-substituted acetophenone oxime ethers [43]. The reactions of 2-chloropyridines 41 with 4-hydroxyacetophenones 42 produced the corresponding 4-pyridyloxyacetophenones 43, which were further converted into oximes and finally coupled with furan- or thiophene-2-carboxylic acids (Scheme 15). The resulting compounds 44 were tested for protoporphyrinogen oxidase inhibitory activity, since this is an important target for the discovery of novel herbicides. The nitropyridine derivatives demonstrated moderate activity (IC₅₀ 3.11–4.18 μM).

2-Chloro-5-nitropyridine was used as the starting material in the synthesis of a new series of insecticides [44]. The method involves the nucleophilic substitution of a chlorine atom under the action of appropriate hydroxyl compounds. The two most promising derivatives, 45 and 46, active against M. separate, P. xylostella, and P. litura with a median lethal concentration (LD₅₀) of 4–12 mg/L, are presented below (Scheme 16).

3. Nitropyridines as Ligands in Bioactive Coordination Compounds

Mohanan and coworkers reported on the synthesis and biological evaluation of novel mononuclear Cu(II), Zn(II), and Ni(II) complexes with 2-methoxy-4-chromanones as ligands [45]. These bidentate ligands (compounds 47) were prepared from 2-amino-5-nitropyridine 33 or 2-aminopyridine 48 and 3-formyl chromene 49 (Scheme 17). The antifungal, antibacterial, antidiabetic, and DNA-binding abilities of the ligands and all complexes were tested. It was found that the nitropyridine-containing complexes 50a–c possessed antimicrobial activity against S. aureus, B. subtilis, P. aeruginosa, and E. coli, which is comparable to the activities of ciproflaxin (mean zone diameters of 9.1–17.9 mm and 22–26 mm, respectively) and Nystatin against C. albicans (21.9–25.3 and 19 mm, respectively).

The synthesis of Ru(arene) complexes, [(η6-p-cymene)Ru(dpb)(py-R)]²⁺ (dpb = 2,3-bis(2-pyridyl)benzoquinoxaline, py-R = 4-substituted pyridine, R = N(CH₃)₂, NH₂, OCH₃, H, COOCH₃, and NO₂) was also reported [46] (Scheme 18). The yields were not given. The complex with the 4-nitropyridine ligand demonstrated the highest DNA covalent binding capability under aerobic and anaerobic conditions, as well as moderate photocytotoxicity.

The synthesis and biological activity of Cu(II) and Zn(II) complexes 51, featuring (5-nitropyridin-2-yl)imine ligands, were investigated [47] (Scheme 19). The ability of both the complexes and the ligand to scavenge free radicals was evaluated; the Cu(II) complex exhibited a high level of antioxidant activity. Additionally, the ligand and its complexes were shown to effectively inhibit the α-glucosidase enzyme. Among them, the Cu(II) complex demonstrated the highest inhibitory activity, with an IC₅₀ of 108 μg/mL), while the ligand itself had the lowest inhibitory activity, with an IC₅₀ of 2.14 μg/mL.

In 2016, Jadhav et al. reported a method for the synthesis of a ruthenium complex based on the 3,5-dinitropyridine moiety [48]. It can be prepared by the reaction of 2-amino-3,5-dintropyridine 15, pyridine-2-carboxaldehyde 52, and ruthenium(II) dichloro-p-cymene dimer 53 (Scheme 20). Stirring in methanol for 24 h resulted in complex 54 in 94% yield. Compound 54 possessed significant activity against breast cancer (MCF7) and the cervical epithelioid carcinoma cell line (HeLa).

The synthesis and evaluation of the cytotoxicity of novel pyridine Pt(II) complexes was described by Kutlu and coworkers [49] (Scheme 21). The complexes were cytotoxic against DLD-1 colon cancer cell lines and A549 human lung cancer cells. It should be noted that the [PtCl₂L₂] complex 55, with the aminonitropyridine ligand, had a higher cytotoxic effect in comparison with the 3,4-dimethylpyridine ligand.

A series of novel photoactive Pt(IV) complexes with heterocyclic ligands was synthesized by Sadler and coworkers [50]. The synthetic route included the reaction of K₂PtCl₄ with pyridines 56 and sodium azide, followed by oxidation of the intermediate diazido complex with H₂O₂ (Scheme 22). The authors studied the photoactivation and phototoxicity of the resulting complexes 57 against bladder cancer cells. Nitropyridine complex (R¹ = NO₂) exhibited moderate non-light-induced cytotoxicity against the cell lines studied.

Nitrosyl iron complexes (NICs) were proven to be efficient inhibitors of enzymes in living organisms [51]. In particular, (N₂H₅)⁺[Fe(SR)₂(NO)₂]⁻ with R = 5-nitropyridin-2-yl was active against cyclic guanosine monophosphate phosphodiesterase (cGMP PDE) and sarcoplasmic reticulum (SR) Ca²⁺-ATPase. In addition, the NO-donating and hemolytic activity of similar iron complexes, with pyridine- and 5-nitropyridinethiolate ligands of the composition [Fe₂(SR)₂(NO)₄], were studied [52]. The compound with the 5-nitropyridine ligand was much more active than the corresponding complex with unsubstituted 2-mercaptopyridine.

Double-armed benzo 15-crown-5 compounds decorated with nitropyridine fragments were synthesized as ligands for the preparation of Na⁺, K⁺, and Ag⁺ complexes [53]. For this reason, 4′,5′-bis(bromomethyl)benzo-15-crown-5 58 was treated with the corresponding hydroxynitropyridine under basic conditions (Scheme 23). The resulting ligands 59 formed metal complexes of different types, depending on the metal used. Thus, reaction with sodium picrate yielded 1:1 complex 60, in which the crown fragment bound to Na⁺. In the case of potassium picrate, two cyclic ether parts coordinated to the potassium cation. Finally, reaction with silver nitrate produced complexes 62, in which the pyridine nitrogen atoms coordinated to Ag⁺. All synthesized compounds exhibited moderate to good antibacterial (E. coli, M. luteus, B. cereus, L. monocytogenes, S. Typhi) and antifungal (C. albicans) activities at levels of commercial antibiotics. In particular, Ag⁺ complexes 62 can be used as antimicrobial agents against pathogenic microorganisms. It was determined that the activities of the synthesized compounds in Gram-negative bacteria were more effective than in Gram-positive bacteria.

4. Nitropyridines as Precursors and Intermediates of Bioactive Compounds

An efficient method for the synthesis of highly selective DNA-dependent protein kinase inhibitor AZD7648 was reported recently [54]. Commercially available 2-amino-4-methyl-5-nitropyridine 63 was used as the starting compound, which reacted with DMF DMA, followed by hydroxylamine (Scheme 24). The resulting hydroxyimine derivative 64 was cyclized to triazolo[1,5-a]pyridine 65, and the nitro group was reduced with ammonium formate and Pd/C. Finally, Buchwald–Hartwig coupling with 2-chloropyrimidine derivative 66 produced the target AZD7648 in 60% yield. However, the authors achieved higher yields by replacing the chlorine atom of a pyrimidine intermediate with a methylsulfonyl group.

An efficient solid-phase synthesis of a library of substituted imidazo[4,5-b]pyridines with phosphodiesterase 4 and 7 inhibitory activity was described recently [55]. 2,4-Dichloro-3-nitropyridine 67 was used as a precursor, in which the chlorine atom in position 4 was more nucleofugal than Cl(2) and could be selectively substituted under the action of amines (free or immobilized to various types of resins) (Scheme 25). Next, the chlorine in position 2 was replaced by another amine, and the nitro group was converted into amine upon treatment with Na₂S₂O₄ to yield 2,3,4-triaminopyridines 68, the key intermediates in the synthesis of target imidazo[4,5-b]- and imidazo[4,5-c]pyridines 69. As a result, the study revealed new potential anti-inflammatory agents for the treatment of autoimmune diseases. The most active compounds were found to inhibit PDE4 with an IC₅₀ comparable to that of rolipram. They also exhibited a favorable pharmacokinetic profile, with a relatively long half-life in rats and good penetration to the brains of mice.

A synthetic route to the previously unexplored 3,7-disubstituted isothiazolo[4,3-b]pyridine, a potential inhibitor of cyclin G-associated kinase, was elaborated starting from 2,4-dichloro-3-nitropyridine 67 [56]. Selective Suzuki–Miyaura cross-coupling at C(4), followed by Pd-catalyzed aminocarbonylation, produced compound 70, which, after reduction of the nitro group and amide-thioamide interconversion, yielded intermediate 71 (Scheme 26). Next, the intramolecular cyclization of ortho-aminothioamide 71 to isothiazole 72 was carried out under the action of aqueous H₂O₂. Finally, debenzylation with DDQ and reaction with bis(2-bromoethyl)ether resulted in the formation of morpholine derivative 73. The authors found that the target compound was inactive as a GAK inhibitor, unlike its 5- and 6-(3,4-dimethoxyphenyl) isomers.

Substituted 3-nitropyridines were used as precursors for the synthesis of semicarbazide-sensitive amine oxidase (SSAO) inhibitors [57]. An example of the synthetic routes is given in Scheme 27. Vicarious nucleophilic substitution of hydrogen with methyl chloroacetate under strong basic conditions provided compound 74, which was then brominated and treated with morpholine to yield the corresponding morpholine derivative 75. Reduction of the nitro group proceeded in the quantitative yield, resulting in the key intermediate 76 for the synthesis of target compounds 77.

5-Nitropicolinic acid and 2-nitropyridine were reported to inhibit human NEU3—a key regulator of the beta1 integrin-recycling pathway, with IC₅₀ concentrations of 40–79 nM [58].

A series of functionalized thiazolo[5,4-b]pyridines 78 as potential MALT1 inhibitors in the treatment of autoimmune and inflammatory diseases and disorders were synthesized [59] (Scheme 28). In the first stage, the available 2-chloro-3,5-dinitropyridines 79 reacted with thioamides to yield 6-nitrothiazolo[5,4-b]pyridines 80. The nitro group in 80 was reduced, leading to amines 81, which then underwent a series of transformations to obtain target thiazolo[5,4-b]pyridines 78. For all the synthesized compounds, the extent of inhibition of MALT1 protease activity was determined. The authors found that the MALT1 inhibition IC₅₀ values of compounds 78 were in the 1–500 nM range.

A series of new 10-substituted 3,6-diazaphenothiazines were synthesized on the basis of nitropyridines and investigated for antitumor activity [60]. The reaction of sodium 3-amino-2-pyridinethiolate 82 and 4-chloro-3-nitropyridine 83 resulted in the formation of a sulfide through the nucleophilic substitution of a chlorine atom (Scheme 29). Heating this sulfide (compound 84) in DMF produced 3,6-diazaphenothiazine 85, presumably via a Smiles rearrangement. The target compounds 86 were obtained by the N-alkylation of compound 85 under the action of various alkyl- and hetaryl halides. 10H-3,6-diazaphenothiazines were found to be 10 times more active (IC₅₀ < 0.72 mg/mL) than cisplatin against the glioblastoma SNB-19, melanoma C-32, and breast cancer MCF-7 cell lines. Diazaphenothiazines containing 2-pyrimidinyl and dimethylaminopropyl moieties exhibited selective activity against MCF-7 and C-32 cells. All 3,6-diazaphenothiazines are non-toxic or almost non-toxic against the normal human fibroblast (HFF-1) cell line in comparison with toxic cisplatin.

6-(4-Phenylpiperazin-1-yl)pyridine-3-ylamine 87, which is known to exhibit anticancer activity, was used as the leading compound for the design of new Mannich bases active against prostate cancer cell lines PC3, LNCaP, and DU145 [61]. Lead compound 87 was synthesized by the reaction of 2-chloro-5-nitropyridine 25 with N-phenylpiperazine, followed by the reduction of the nitro group (Scheme 30). The results revealed that the Mannich bases 88 exhibited moderate cytotoxic activity against cancer cells tested compared with the lead compound.

The same group reported on the synthesis of another series of anticancer compounds on the basis of 6-(4-phenylpiperazin-1-yl)pyridine-3-ylamine by incorporating biologically relevant thiourea, isothiocyanate, and thiazolidinone moieties [62].

The antitumor effects of N-methyl pyridinium salts, namely 1-methyl-3-nitropyridine chloride (MNP, 89) and 3,3,6,6,10-pentamethyl-3,4,6,7-tetrahydro-[1,8(2H,5H)-dion]acridine chloride (MDION, 90), on sensitive leukemia HL60 cells and resistant topoisomerase II-defective HL60/MX2 cells were studied by Tarasiuk et al. [63] (Scheme 31). In the case of the HL60 cells, the IC₅₀ values for MNP and MDION were 24.3 and 80.5 μM, respectively, while for the HL60/MX2 cells, they were 20.5 and 95.5 μM.

An original and environmentally benign method for the synthesis of biologically active Shiff bases was reported in 2015 [64]. The reduction of 2-nitropyridine 91 with Fe/HCl in the presence of aromatic aldehydes provided azomethines 92 (Scheme 32). The authors performed antibacterial (S. aureus) and antifungal (A. niger) tests by the disc diffusion method; however, they did not provide certain MIC values.

The synthesis and antimicrobial activity of pyridoxazinone series were reported recently [65]. O-alkylation of 3-hydroxy-2-nitropyridine 93 with 2-bromoalkanoic esters and subsequent reductive cyclization resulted in N-hydroxy-pyridoxazinone derivatives 95 in 59–85% yields (Scheme 33). One of the synthesized compounds (R = n-Bu) showed a MIC of 62.5 μg/mL against C. albicans, C. glabrata, and C. tropicalis. In addition, this compound showed a high antibacterial activity against E. faecalis (MIC 7.8 μg/mL) and S. aureus (MIC 31.2 μg/mL). Remarkably, the ethyl group in compound 95 (R = Et) increased the activity against S. agalactiae (MIC 62.5 μg/mL), which is higher than that of chloramphenicol.

A series of new potential antimicrobial agents derived from (pyridin-2-yl)piperazine were synthesized by Boulebd’s group [66]. The reaction of 2-chloro-5-nitropyridine 25 with N-methylpiperazine and the subsequent reduction of the nitro group yielded amine 96, which was then alkylated by ethyl bromoacetate and converted into the corresponding hydrazide 97 (Scheme 34). The latter was reacted with aromatic aldehydes to obtain hydrazones 98. An antimicrobial activity study showed the resistance of S. aureus and S. typhimurium to all synthesized compounds. At the same time, a phenolic derivative (98, R = 2-OH) showed moderate antimicrobial activity against B. subtilis and C. krusei, with an MIC values of 62.5 μg/mL.

2-Chloro-3-nitropyridine 17 has been considered as a starting material in the synthesis of novel acyclic phosphonate nucleotide analogs incorporating azolopyridine fragments [67]. The synthetic scheme included the nucleophilic substitution of chlorine under the action of various ω-aminoalkylphosphonates and reduction of the nitro group (Scheme 35). Then, azole annulation was accomplished by reactions with HC(OEt)₃, diazotization, or CDI, followed by hydrolysis of the phosphonic ester. All synthesized compounds 101–103 were tested for antiviral activity against DNA and RNA viruses and cytotoxicity. Some of them demonstrated activity against cytomegalovirus (EC₅₀ = 76.47 μM; MCC > 100 μM) in human embryonic lung (HEL) cells and moderate activity against varicella zoster virus (VZV) (EC₅₀ = 52.53–61.70 μM MCC > 100 μM) in HEL cells. No compounds were cytotoxic at a concentration of 100 μM.

Imidazo[4,5-b]pyridines, as the salt-inducible kinase (SIK) modulators for the treatment of rheumatoid arthritis, were described in patent [68]. The authors proposed available 3,5-dinitropyridine derivatives as the starting materials for the synthesis of desired compounds (Scheme 36). In the first stage, 2-fluoro- or 2-chloro-3,5-dintropyridines 104 reacted with the corresponding amine 105, yielding compounds 106. Then, the nitro groups were reduced, followed by condensation to yield diamines 107, which were then converted into the target compounds 108.

5. Nitropyridines in the Synthesis of Radiolabeled Compounds

Positron-emission tomography (PET) is used in clinical oncology, neurology, cardiology, and other fields for the imaging of tumors, the search for metastases, and for the clinical diagnosis of certain diffuse brain diseases, such as those causing various types of dementias. The administration of radiolabeled drugs is considered essential in order to keep track of the drug molecules throughout the body and excreta even after their transformation into different metabolites [69]. Today, PET mainly uses positron-emitting isotopes of elements from the second period of the periodic table: ¹¹C, ¹⁸F, ¹⁵O, and ¹³N. Among them, ¹⁸F has optimal characteristics for use in PET, including the longest half-life and the lowest radiation energy. The relatively short half-life of ¹⁸F allows for obtaining high-contrast PET images with a low dose load on patients. The use of radiopharmaceuticals belonging to various classes of biologically active compounds makes PET a fairly universal tool in modern medicine. Therefore, the development of new radiopharmaceuticals and effective methods for synthesizing proven drugs is becoming a key stage in the development of the PET method [70].

The synthesis of the PET tau tracer [¹¹C]PBB3 compound for the imaging of Alzheimer’s disease was reported by Zheng and coworkers [71]. 5-Bromo-2-nitropyridine 109 was reacted with acroleine diethyl acetal under Heck reaction conditions, resulting in compound 110 in 30% yield (Scheme 37). Furthermore, the Wittig–Horner reaction with diethyl phosphonate 111 and reduction of the nitro group yielded amine 112, which was then O-demethylated with BBr₃. N-methylation with [¹¹C]CH₃OTf resulted in the target compound in 20–25% radiochemical yield.

Brugarolas and coworkers described the radiochemical synthesis of [¹⁸F]4-amino-3-fluoro-5-methylpyridine 114 based on 4-nitropyridine-N-oxide derivative 115 [72]. The authors used the ¹⁸F/¹⁹F isotope exchange method [73], followed by the Pd/C-mediated hydrogenation of both the nitro group and the N-oxide function (Scheme 38). The target compound was proven to effectively cross the blood–brain barrier in mice and was positioned as a promising candidate for neuroimaging applications, particularly for detecting demyelinated lesions in the brain.

The automated radiosynthesis of P2X7R imaging agent ¹⁸F-JNJ64413739 was elaborated [74]. The key step involved nucleophilic aromatic substitution of 3-NO₂ in substituted nitropyridine 117 under the action of a [¹⁸F]TEAF/TEAB mixture at 120 °C (Scheme 39). PET/CT imaging demonstrated higher radioactivity uptake in brain regions of the model rats with osteoporosis compared to the control group.

A new approach to human PET imaging of the Angiotensin II type 1 receptor was elaborated recently [75]. The authors accomplished the synthesis of [¹⁸F]fluoropyridine-losartan with high molar activity and high chemical and radiochemical purity through the introduction of ¹⁸F by nucleophilic substitution of the nitro group with [¹⁸F]KF. The fluoropyridine and losartan fragments were combined together by azide–alkyne cycloaddition (Scheme 40).

The influence of Lewis acid addition on the radiochemical yield of [¹⁸F]FMN3PU targeting leucine-rich repeat kinase 2 (LRRK2) was studied [76]. On addition of ferric nitrite, titanocene dichloride, or especially chromium(II) chloride, the radiochemical yield was increased to 41%, compared to 1.7% under conventional nucleophilic radiofluorination conditions (Scheme 41).

Finally, the synthesis of [¹⁸F]T807, a PET tau tracer for Alzheimer disease was reported by Zheng et al. [77] The authors proposed a reaction scheme starting with the Stille coupling of 5-bromo-2-nitropyridine 109 with 7-bromo-5H-pyrido[4,3-b]indole 124, followed by Boc protection (Scheme 42). In the final step, nucleophilic substitution of the nitro group under the action of radiolabeled potassium fluoride occurred, accompanied by deprotection. This approach can be used for the synthesis of other ¹⁸F-radiotracers for PET imaging.

6. Conclusions

This analysis of publications from 2015 to 2024 shows that nitropyridines represent valuable scaffolds for the development of bioactive molecules and have garnered significant interest from researchers. Over the past decade, numerous practical applications of nitropyridines have emerged, such as their use as radiolabeled compounds for positron-emission tomography, ligands for coordination compounds showcasing diverse bioactivity, and more. Additionally, nitropyridines may serve as convenient and readily available precursors for a wide variety of promising complex bioactive molecules, including antitumor, antibacterial, and antifungal compounds, as well as herbicides and insecticides. From a chemist’s perspective, nitropyridines offer vast synthetic potential. The nitro group facilitates reactions with nucleophilic reagents and can be a source of other nitrogen functions, such as amino groups, azo linkages, hydroxylamines, etc. These functionalities pave the way to the development of fused heterocyclic systems. Furthermore, as demonstrated in this review, nitropyridines themselves exhibit useful biological activities.

Nitropyridines have been an underexplored class of compounds, likely due to the limited methods available for their functionalization. However, recent studies have introduced simple synthetic routes to polyfunctional derivatives of pyridine. We believe that this progress will encourage more active exploration of their biological properties. The number of publications focused on the chemistry of nitropyridines is steadily increasing, suggesting that this area will continue to develop in the future. Consequently, the field related to the creation of biologically active molecules based on nitropyridines is expected to remain in demand and relevant.