Exploiting Multicomponent Reactions to Drive the Development of Novel Antitubercular Agents: An Overview

Abstract

Tuberculosis (TB) remains a global epidemic, and Mycobacterium tuberculosis (Mtb), its etiological agent, continues to be the deadliest single infectious agent worldwide. Our inability to eradicate TB is strongly related to Mtb's ability to alternate between an active and latent disease state, and a lack of pharmacological agents that effectively target the heterogeneity of TB, which leads downstream to the emergence of drug resistance. The ongoing quest to discover novel effective chemical agents to improve anti‐TB regimens remains a significant challenge. Multicomponent reactions (MCRs) have been shown to generate highly diverse chemical libraries in a time‐ and cost‐efficient, sustainable manner, and thus have emerged as powerful tools to accelerate drug discovery and development. This review provides an overview of the chemical libraries of anti‐TB agents developed by MCRs, highlighting the contributions of MCRs to advance anti‐TB drug discovery through a medicinal chemistry approach.

Multicomponent reactions (MCRs) have emerged as powerful tools to accelerate drug discovery. This review explores the chemical libraries of antitubercular (anti‐TB) agents developed by MCRs, emphasizing its importance to advance anti‐TB drug discovery.

1. Introduction

Despite being curable and preventable, with a treatment success of around 88%, tuberculosis (TB) remains a global epidemic. In 2024, 10.7 million people were estimated to have contracted TB and 1.23 million people to have died from it, making TB one of the top ten causes of death worldwide, and the leading cause of death from a single infectious agent. Progress in reducing the TB burden has been made after many setbacks during the COVID‐19 pandemic, but all indicators fell short of the 2025 targets defined by the WHO's “End TB Strategy” [1].

TB control is hampered by a combination of factors, with the nature of Mycobacterium tuberculosis (Mtb), TB's etiological agent, being a significant one. The Mtb cell wall has low permeability to chemotherapeutic agents, which, aligned with the large number of efflux pumps and its extensive metabolic arsenal to metabolize xenobiotics, causes a naturally high innate resistance and hampers the ability for drugs to enter the target cell. Furthermore, Mtb can shut down its metabolism to a latent state, with higher antibiotic tolerance due to reduced antibiotic uptake and lack of druggable targets [2, 3].

With the lack of anti‐TB drugs effective in latent bacteria, current therapeutic regimens are based on prolonged administration of traditional anti‐TB drugs to eliminate all populations of bacilli, leading to high adverse effects and low patient compliance, which facilitate the emergence and spread of drug resistance. Drug‐resistant (DR) strains pose a significant health and financial threat, with lower treatment success and higher lethality, with the emergence of disease forms not treatable by any currently available tool (total DR‐TB), that are programmatically incurable [4, 5].

Considering all, it is urgent to develop novel anti‐TB agents. The ideal anti‐TB drug should shorten and simplify treatment regimens, target latent and DR bacteria, whilst being safe and cheap. To achieve that, the drug should be potent, effectively target all the heterogeneous Mtb populations in the host, and act on novel pharmacological targets. For this, an increased understanding of druggable targets, disease models and validated biomarkers is also required, which can only be achieved by increasing our understanding of TB pathophysiology, and with increased investment in TB drug research.

Multicomponent reactions (MCRs) are chemical reactions where three or more reagents are combined in one step in a sequential manner to yield a single product that retains most of the atoms of the starting materials. MCRs are flexible reactions that provide a wide range of scaffolds with a large chemical diversity, particularly if combined with a subsequent secondary reaction. MCRs offer versatile applications, from drug discovery and combinatorial chemistry to natural product synthesis and material sciences and polymer chemistry [6].

Among the advantages of MCRs are their high efficiency and atom economy. Due to their highly convergent nature, MCRs reduce waste generation and shorten the number of synthetic steps. MCRs are regio‐ and chemo‐selective, and thus, the generation of side‐products is minimized and as by‐products are generally simple molecules, most commonly water, they are cost‐effective in both synthesis and purification regarding time, energy, and money. Furthermore, many MCRs are catalyst‐free or use recyclable catalysts, with moderate conditions and green solvents. For all these reasons, MCRs are aligned with the goal of making chemistry more sustainable and are a prime tool for green chemistry. MCRs allow the efficient discovery of small molecules, the majority of which are drug‐like according to Lipinski's rule of five (Ro5) and the Veber rules. Aligned with their ability to rapidly generate diverse libraries with often biological relevant scaffolds, MCRs are now recognized for their impact on drug discovery projects and strongly endorsed by industry and academia [7, 8].

The potential impact of MCRs in TB drug discovery is specifically valuable. Millions of patients still face barriers in accessing effective anti‐TB drugs due to regulatory delays, global supply chain breakdowns, and high costs, which severely impact patient adherence and ultimately drive the spread of DR‐TB [1]. MCRs are cost‐effective, minimize waste generation, and greatly simplify synthetic pathways, yielding cheaper drugs through more accessible pathways that facilitate the regionalization of pharmaceutical manufacturing. An anti‐TB drug must reach the highly enclosed, difficult‐to‐reach environments where Mtb bacilli often reside and penetrate the highly impermeable Mtb cell wall. Additionally, anti‐TB drugs should preferably be administered orally to improve treatment adherence, requiring good oral bioavailability and a carefully optimized pharmacokinetic profile. MCRs permit the efficient discovery of small molecules, the majority of which have drug‐like properties and good oral bioavailability. Furthermore, the rapid generation of structurally diverse libraries through MCRs is particularly relevant in TB drug discovery, given the urgency for novel anti‐TB agents fueled by the TB epidemic and the necessity to search for pharmacological agents that are active in heterogeneous populations to address multiple disease states in a single TB drug discovery campaign.

Some illustrative examples of MCRs are presented in Table 1, including the A³ coupling, Biginelli, Doebner, Groebke Blackburn, Bienaymé, Hantzsch, Kabachnik–Fields, Mannich, Nitro‐Mannich, Passerini, Povarov, and Ugi reactions. In this review, an overview of the literature concerning the applications of MCRs to the progress of drug design and development in the field of TB is presented, from a medicinal chemistry perspective.

TABLE 1.

Illustrative examples of MCRs.

Reaction	General reaction scheme
Hantzsch 3CR
Doebner 3CR
Povarov 3CR
Biginelli 3CR
Groebke Blackburn Bienaymé 3CR
Passerini 3CR
Ugi 4CR
Mannich 3CR
Nitro‐Mannich 3CR
Kabachnik–Fields 3CR
A3 coupling 3CR

2. Nitrogen‐Based Heterocycles

Nitrogen‐based heterocycles are among the most significant structural components of pharmaceuticals, with more than 75% of drugs available in the market containing a nitrogen‐based heterocycle. Frequently combined in five‐ and six‐membered rings with sulfur and oxygen, nitrogen‐based heterocycles span a myriad of biological activities and provide useful tools in the optimization of Absortion, distribution, metabolism, excretion (ADME) and toxicity properties of drug candidates through the modification of solubility, lipophilicity, polarity, and hydrogen‐bonding capacity [9, 10].

2.1. Pyridines Derivatives: The Hantzsch 3CR

The Hantzsch pyridine synthesis is a 3CR between an aldehyde, two equivalents of a β–ketoester, and a nitrogen donor, typically ammonia or ammonium acetate. First described by Arthur Rudolph Hantzsch in 1881 [11], the Hantzsch 3CR produces highly substituted dihydropyridines which can be oxidized to pyridines in a one‐pot synthesis. Current advances on the Hantzsch 3CR include the development of more sustainable methodologies and catalysts, including aqueous media, catalyst‐free and milder conditions, and alternative energy sources, such as microwave irradiation; the broadening of the reaction scope and development of asymmetric methodologies [12, 13].

Pyridine derivatives are the most common heterocyclic compounds, found in many natural products like vitamins, coenzymes, and alkaloids, and in many drugs and pesticides. This nucleus has a vital role in many bioimaging and biomedical applications, commonly used to enhance drug solubility and bioavailability. Unsurprisingly, pyridines are associated with a wide range of biological and pharmaceutical activities, such as antifungal, antibacterial, antiviral, analgesic, anti‐inflammatory, antitumor, antioxidant, and much more. The medical and synthetic perspectives of pyridines have been extensively reviewed [14, 15], and encourage continuous advances in the Hantzsch 3CR.

Several descriptions of Hantzsch 3CRs used to prepare libraries of anti‐TB pharmacological agents can be found in the literature, highlighting MCRs as a competitive alternative to traditional drug discovery. An example is the series of dihydropyridine derivatives bearing an azole nucleus (Scheme 1) described by Trivedi et al. and Desai et al. [16, 17], that designed a series of dual pharmacophores to also exploit the vast biological activities of azole derivatives, such as anti‐TB, antimicrobial, anti‐inflammatory, and antitumor. The reaction was suitable with either 2‐butyl‐4‐chloro‐1H‐imidazole‐5‐carbaldehyde, or a selection of 1,3‐diphenyl‐1H‐pyrazole‐4‐carbaldehydes, substituted with electron‐donating (EDG) and electron‐withdrawing groups (EWG) in the phenyl group, and various alkyl acetoacetates and substituted β‐keto amides, with EDGs and EWGs. The in vitro anti‐TB activity was tested against Mtb H37Rv, with five compounds showing very potent, nanomolar activity, with MIC values between 34 and 41 nM. (1a–b, representative, Figure 1). Compound cytotoxicity was assessed for selected compounds against VERO cells, where compounds showed moderate to low cytotoxicity; the most active compounds had great pharmaceutical windows (SI > 500).

SCHEME 1.

Hantzsch 3CR to produce dihydropyridine derivatives [16, 17].

FIGURE 1.

Most active compounds and SAR studies for libraries derived from MCRs to synthesize pyridine derivatives [16, 17, 18, 19, 20, 21, 22, 23, 24].

The development of asymmetric Hantzsch methodologies greatly enhanced the scope of pyridine derivatives accessible through MCRs, most relevantly to more structurally complex and chiral dihydropyridines. The library of polyhydroquinoline derivatives bearing a pyrazole nucleus (Scheme 2), reported by Jamale et al. [18], is an example of an asymmetric Hantzsch 4CR used to produce a library of anti‐TB agents. The reaction used two different 1,3‐dicarbonyl systems: dimedone and a selection of alkyl acetoacetates to obtain the asymmetric polyhydroquinolines and was suitable with a selection of 1,3‐diphenyl‐1H‐pyrazole‐4‐carbaldehydes, substituted with EDGs and EWGs in the phenyl group. The in vitro anti‐TB activity was screened against Mtb H37Rv, with three compounds showing potent inhibitory activity, the most active (2a, Figure 1) with a MIC of 3.2 μM.

SCHEME 2.

Asymmetric Hantzsch 4CR to produce pyrazole polyhydroquinoline derivatives [18].

Natural products provide access to a novel, diverse chemical space through unique and complex chemical structures, often not suitable to be obtained by synthetic chemistry. MCRs can help bridge the synthetic inaccessibility of natural products or simplify and enhance the efficiency of chemical derivatizations. The work reported by Stalinskaya et al. [19] is a prime example, with the use of an asymmetric Hantzsch 4CR to develop a pyridine derivative with an epoxybenzooxocine nucleus (Scheme 3), inspired by the structure of epicoccolide A and epicocconigron A, two fungal natural compounds with a wide range of biological properties, such as antimicrobial, antifungal, antitumor, antioxidant and antiviral. The compound showed moderate inhibitory activity against M. bovis, with a MIC of 25 μM (4, Figure 1).

SCHEME 3.

Asymmetric Hantzsch 4CR to obtain a pyridine derivative with an epoxybenzooxocine nucleus [19].

Besides the Hantzsch 3CR, multiple MCRs using different chemical strategies have been reported to synthesize pyridine derivatives. Kharade et al. [20] developed a series of chiral tetrahydropyridine derivatives (Scheme 4) through a 3CR between ethyl acetoacetate, two equivalents of p‐substituted anilines, and two equivalents of p‐substituted benzaldehydes, both with EDGs and EWGs. Compounds’ in vitro anti‐TB activity was evaluated against Mtb H37Rv, with three showing moderate activity, the most active (5a, Figure 1) with a MIC of 21 μM.

SCHEME 4.

3CR described by Kharade et al. [20] to synthesize tetrahydropyridine derivatives.

Through a similar 3CR between analogs of barbituric acid, aldehydes, and compounds with an enamine fragment, Satasia et al. [21] and Patil et al. [22] reported a series of fused pyridine derivatives (Scheme 5), specifically fused pyrazole pyrido pyrimidinones (6) and fused pyridodipyrimidines (7). The 3CR was suitable with either substituted amino pyrazoles or amino uracil, and with a wide range of aldehydes, including formylpyridines, formylcyclohexane, formylthiophene and a selection of benzaldehydes substituted with EDGs or EWGs. All tested compounds of both series showed weak or very weak activity in vitro against Mtb H37Rv, the most active of each series inhibiting growth by 88% at 240 μM (6a, Figure 1) and with a MIC of 121 μM (7a, Figure 1).

SCHEME 5.

3CR described by Satasia et al. [21] and Patil et al. [22] to synthesize fused pyridine derivatives. Reactional Conditions: (6): cellulose‐supported ionic liquid, ethanol, reflux 30–50′. (7): P₂O₅, ethanol, reflux, 1.3–2.5 h.

Manikannan et al. [23] described a 4CR between 2‐sulfanyl‐1‐aryl‐ethanones, malononitrile, benzaldehydes and alkyl alcohols to synthesize highly substituted pyridine derivatives (Scheme 6). The reaction was successful with both 2‐arylsulfanyl and 2‐cyclohexylsulfanyl‐1‐aryl‐ethanones, substituted with EDGs and EWGs in the aryl moieties, a variety of substituted benzaldehydes, also with EDGs and EWGs, and with both methanol and ethanol. All tested compounds showed potent activity in vitro against Mtb H37Rv, the most active (8a, Figure 1) with a MIC of 3.1 μM.

SCHEME 6.

4CR described by Manikannan et al. [23] to synthesize highly substituted pyridine derivatives.

Sangani et al. [24] combined cyanomethyl benzimidazoles, malononitrile, and 2‐aryloxy‐3‐formyl quinolines to produce pyrido benzimidazole derivatives containing a quinoline nucleus (Scheme 7). The 3CR allowed both 2‐cyanomethylbenzimidazole and 2‐cyanomethyl‐5‐methyl‐benzimidazole, and a range of 2‐aryloxy‐3‐formyl quinolines, with 7‐substitutions with EDGs and with p‐substitutions of both EDGs and EWGs in the 2‐aryloxy substituent. The in vitro anti‐TB activity was evaluated against Mtb H37Rv, with four compounds showing moderate activity, the most active (9a, Figure 1) with a MIC of 13 μM.

SCHEME 7.

3CR described by Sangani et al. [24] to produce pyrido‐benzimidazoles derivatives.

2.2. Quinoline Derivatives: Doebner 3CR and Povarov 3CR

Quinolines are an undisputed privileged scaffold in medicinal chemistry due to their wide spectrum of pharmacological activities, such as antimalarial, anti‐TB, antibacterial, antifungal, antiviral, anti‐inflammatory, antitumor, antioxidant, antiprotozoal, and more. The quinolone core exists in many naturally occurring pharmacologically active entities, and its applications extend beyond medicinal chemistry to the fields of organic and industrial chemistry. In recent years, as chemistry is expected to become greener, the development of synthetic routes based on MCRs has become more relevant. The synthetic perspectives and applications of quinoline derivatives have been extensively reviewed elsewhere [25, 26].

The Doebner 3CR, first described in 1887 [27], involves the condensation of an aniline, an aldehyde, and pyruvic acid to obtain quinoline‐4‐carboxylic acids. Interest in the Doebner 3CR is restricted to the efficient access to quinoline‐4‐carboxylic acids, used as key quinoline precursors, pharmacological agents and antioxidants in the chemical industry, through green and mild conditions [28].

An example of a library of anti‐TB agents obtained through the Doebner 3CR is reported in Muscia et al. [29], that synthesized a series of quinoline derivatives (Scheme 8) by coupling a variety of substituted anilines with EDGs and EWGs, a wide range of benzaldehydes and naphthaldehydes substituted with EDGs, and either pyruvic acid (Doebner 3CR), 3‐acetylindole, or substituted acetophenones (modified Beyer method). All tested compounds showed weak activity against Mtb H37Rv, the lead compound (10a, Figure 2) displaying an IC₅₀ of 29 μM. This compound was evaluated in a latency model (low‐oxygen recovery assay) in murine macrophages, and its bactericidal activity was determined, with weak activity in all assays (MIC = 136 μM, IC₅₀ = 82 μM, and MBC = 136 μM, respectively).

SCHEME 8.

Modified Doebner 3CR to produce quinoline derivatives [29].

FIGURE 2.

Most active compounds and SAR studies for libraries derived from MCRs to synthesize quinoline derivatives [29, 30, 31, 32, 33, 34, 35, 36].

The Povarov 3CR, first described in 1963 [37], transformed organic synthesis as an efficient method to synthesize quinolines or tetrahydroquinolines, depending on the reaction system used. The reaction is a formal cycloaddition between an electron‐rich alkene and an imine generated in situ through the condensation of aromatic amines and aldehydes. Interest in the Povarov 3CR arises from its efficiency, selectivity, versatility of substrates and catalysts, and potential for green chemistry conditions. Current research is focused on progressing sustainability and selectivity with novel methods, as new catalyst systems and eco‐friendly solvents [38, 39].

Several reports of the use of Povarov 3CRs to synthesize libraries of anti‐TB pharmacological agents can be found in the literature, showcasing its higher reaction scope and versatility. A representative illustration is reported by Kumar et al. [30], that synthesized a series of tetrahydroquinolines (11, Scheme 9) through the coupling of p‐substituted anilines with EDGs and EWGs, either formaldehyde or a diverse scope of benzaldehydes substituted with EDGs, and a range of activated alkenes, such as ethyl vinyl ether and a selection of cyclic alkenes. Interestingly, when salicylaldehyde was used, an alternate 3CR was observed to form benzopyran derivatives (12). The in vitro anti‐TB activity was determined against Mtb H37Rv, with three compounds showing moderate inhibitory activity, the most active (11a, Figure 2) with a MIC of 21 μM.

SCHEME 9.

Povarov 3CR to synthesize tetrahydroquinoline derivatives [30]. CellSA: cellulose sulfuric acid.

Since its discovery, many modifications of the Povarov 3CR have been described to expand its scope. Patel's group [31, 32, 33, 34] and Kalaria et al. [35] replaced the aromatic amine with β‐enaminone derivatives and cyanoacetate derivatives as the dienophile to obtain highly substituted quinolone derivatives (Scheme 10), specifically N‐substituted biquinoline derivatives (13) and dual pharmacophore imidazopyrazole polyhydroquinoline derivatives (14). The reaction allowed a wide range of N‐substituted hydrazinoketones and β‐enaminones, with either an isonicotinoyl or a substituted aryl group as the aromatic amine replacement, either a formyl imidazopyrazole or a wide range of substituted 3‐formyl quinolines or quinolones as the aldehyde, and either malononitrile, alkyl cyanoacetates, or cyanoacetamides as the dienophile. The in vitro anti‐TB activity was assessed against Mtb H37Rv, with seven compounds showing moderate activity, the most active (13a, Figure 2) with a MIC of 21 μM. All imidazopyrazole polyhydroquinoline derivatives showed very weak activity, the most active (14a, Figure 2) inhibiting growth by 94% at 188 μM. Brine shrimp lethality bioassay was done as a preliminary toxicity assessment for selected compounds, where they presented moderate to low cytotoxicity; the most active compound had a small pharmaceutical window.

SCHEME 10.

Modified Povarov 3CR to synthesize tetrahydroquinoline derivatives [31, 32, 33, 34, 35].

Another modification was reported by Jardosh et al. [36], which also replaced the aromatic amine with β‐enaminone derivatives but used Meldrum's acid as the dienophile to obtain a series of N‐substituted bicarbostyril derivatives (Scheme 11). The reaction was suitable with a wide range of N‐substituted hydrazinoketones or β‐enaminones with a phenyl, benzyl, benzoyl, and isonicotinoyl group, and a variety of 6‐substituted N‐allyl‐3‐formyl‐2‐quinolones, with EDGs and EWGs. The in vitro anti‐TB activity was tested against Mtb H37Rv, with two compounds showing moderate activity, the most active (15a, Figure 2) with a MIC of 26 μM.

SCHEME 11.

Modified Povarov 3CR to obtain N‐substituted bicarbostyril derivatives [36].

2.3. Dihydropyrimidinone Derivatives: Biginelli 3CR

The Biginelli 3CR, first described by Pietro Biginelli in 1981 [40, 41], is a widely studied MCR involving the condensation of an aldehyde, ethyl acetoacetate, and urea to obtain dihydropyrimidinones. Since its discovery, the Biginelli 3CR has seen significant advances to overcome several limitations, including some modifications and new methods to expand the scope of reagents, increase yield, reduce the complexity, greenify reaction conditions, and decrease reaction times. Currently, many methods using homogeneous and heterogeneous catalysts, microwave irradiation, ionic liquids, solid‐phase protocols, solvent free and green solvents have been reported, and reviewed elsewhere [42, 43]

Dihydropyrimidinones, the products of the Biginelli 3CR, are widely used in the pharmaceutical industry due to their versatile and diverse range of biological and pharmacological activities, such as antiviral, antibacterial, anti‐TB, antifungal, anti‐inflammatory, antitumor, and much more. The medicinal perspectives of dihydropyrimidinones have been extensively reviewed [44, 45], and inspired the scientific community to advance the Biginelli 3CR into a widely studied MCR with synthetic versatility and multiple applications in drug discovery.

Several studies describe the use of Biginelli 3CRs to generate libraries of anti‐TB agents, underscoring the appeal of MCRs, and the Biginelli 3CR in particular, for accelerating drug discovery. A representative example is the work of Akhaja et al. [46], who synthesized a series of indolone derivatives through a four‐step pathway initiated by a Biginelli 3CR (Scheme 12), further highlighting the capacity of MCRs to reduce synthetic routes. The reaction was suitable with urea and thiourea to obtain both dihydropyrimidinones and dihydropyrimidin(thio)ones, which were subsequently converted into indol‐2‐one derivatives with various 5‐substitutions with EWGs in the indole core. The in vitro anti‐TB activity was assessed against Mtb H37Rv, with all compounds showing weak inhibitory activity, the most active (17a, Figure 3) with a MIC of 55 μM.

SCHEME 12.

Biginelli 3CR as the first synthetic step to produce indol‐2‐one derivatives [46].

FIGURE 3.

Most active compounds and SAR studies for libraries derived from MCRs to synthesize dihydropyrimidine derivatives [46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61].

The Biginelli 3CR has had its scope significantly enhanced since its discovery through many reported modifications. Filatova et al. [47] and Fedorova et al. [48] used a modified Biginelli 3CR with different‐length podands with a 3‐oxobutanoate fragment as a replacement for the ethylacetoacetate to obtain a series of dihydropyrimidine derivatives with a polyether spacer (Scheme 13), aiming to increase compound efficiency due to facilitated cell penetration through the polyether fragment. The 3CR allowed both urea and thiourea, and benzaldehyde and 2‐thiophenecarboxaldehyde. The in vitro anti‐TB activity was assessed against Mtb H37Rv, with four compounds showing potent inhibitory activity, the most active (18a, Figure 3) with a MIC of 2.6 μM. Selected compounds were tested against a multidrug‐resistant (MDR) clinical isolate, where all tested compounds maintained their potency.

SCHEME 13.

Modified Biginelli 3CR to produce podands with a dihydropyrimidine fragment [47, 48].

The most common modification of the Biginelli 3CR found in the literature for the synthesis of anti‐TB agents was the replacement of ethyl acetoacetate by a variety of synthetic analogs, greatly expanding the substitution possibilities for the dihydropyrimidine products. Some examples are described by Yadlapalli et al. [49], Desai et al. [50], and Joshi et al. [51], all reporting series of functionalized dihydropyrimidine derivatives (Scheme 14) through similar 3CR despite having distinct rationales, showcasing the versatility offered by the reaction. Yadlapalli et al. [49] designed a series of dual pharmacophore dihydropyrimidine derivatives containing a diarylpyrazole moiety, with a lipophilic carbamoyl moiety for enhanced permeation (19), whilst Desai et al. [50] explored the biological properties of the imidazole scaffold in a series of dual pharmacophore imidazolyl dihydropyrimidine derivatives (20), and Joshi et al. [51] prepared a series of highly substituted dihydropyrimidine derivatives (21). The reaction displayed a broad scope, accepting acetoacetate as the 1,3‐dicarbonyl system, as well as 3‐oxobutanamides with both aromatic and aliphatic N‐substituents, most commonly acetoacetanilides, with both EDGs and EWGs. Both urea and thiourea were suitable for the reaction, as well as a wide range of aromatic aldehydes such as substituted formyl diarylpyrazoles, substituted formyl imidazoles, and most commonly benzaldehydes substituted with EDGs and EWGs. The libraries were tested for their in vitro antimycobacterial activity with distinct results, showcasing that despite having the same base scaffold, different substitutions and conjugation with other biologically active scaffolds can greatly modulate activity. The library of dihydropyrimidine derivatives prepared by Joshi et al. [51] was tested against M. bovis in both normalcy and dormancy conditions (nitrate reductase model), with nine compounds in normalcy conditions and five compounds in dormancy conditions moderately inhibiting growth. The most active compound in each condition (21a and 21b, Figure 3) showed an IC₅₀ of 8.5 and 15 μM, respectively. Finally, the libraries of dual pharmacophore dihydropyrimidine derivatives had two compounds each with potent, sub‐micromolar activities against Mtb H37Rv. The most active compounds (19a and 20a, Figure 3) displayed MIC values of 0.28 and 0.96 μM, respectively. Joshi et al. [51] progressed selected compounds to be evaluated against VERO cells, showing moderate to low cytotoxicity. The most active of them presented good pharmaceutical windows, with SI > 24. The most promising compounds were also tested against a TB‐infected macrophage model, where three compounds showed sub‐micromolar inhibitory activity, the most active (20b, Figure 3) with an EC₉₀ of 0.10 μM. Furthermore, an early in silico pharmacokinetic evaluation was performed for the lead compounds, which showed good predicted Caco‐2 permeability, good TPSA values, but low predicted aqueous solubility.

SCHEME 14.

Modified Biginelli 3CR to synthesize dihydropyrimidine derivatives [49, 50, 51].

Broadening the scope beyond 1,3‐dicarbonylic systems, Mohan et al. [52] replaced ethylacetoacetate with ethylcyanoactetate to produce a library of tetrahydropyrimidine carbonitrile derivatives (Scheme 15). The reaction was performed with both urea and thiourea, and a wide range of benzaldehydes substituted with EDGs and EWGs. The in vitro anti‐TB activity was assessed against Mtb H37Rv through the luciferase reporter phage assay, with all compounds showing very poor activity the most active (22a, Figure 3) showed a 76% reduction in relative light units (RLU) at 322 μM.

SCHEME 15.

Modified Biginelli 3CR to synthesize tetrahydropyrimidine carbonitrile derivatives [52].

Trivedi et al. [53, 54] and Siddiqui et al. [55] used a pyrazol‐3‐one as a replacement for the ethylacetoacetate to produce a series of fused pyrazolopyrimidine derivatives (Scheme 16), showcasing once again the versatility of the Biginelli 3CR. The series was designed as dual pharmacophore pyrazolopyrimidine derivatives containing a phenothiazine nucleus, to exploit the wide spread of biological activities of the phenothiazines, such as anti‐TB, antibacterial, antifungal, antiviral, and many others. The reaction allowed either urea, thiourea, or guanidine, and a variety of substituted benzaldehydes, with both EDGs and EWGs. The in vitro anti‐TB activity was determined against Mtb H37Rv, with eleven compounds showing potent inhibitory activity, the most active (23a, Figure 3) with nanomolar potency, with a MIC of 44 nM. A cytotoxicity evaluation against VERO cells was performed on selected compounds that showed moderate to low cytotoxicity, with the most active compound showing a great pharmaceutical window, with SI > 500.

SCHEME 16.

Modified Biginelli 3CR to synthesize pyrazolopyrimidine phenothiazine derivatives [53, 54, 55].

The other common modification of the Biginelli 3CR is the replacement or derivatisation of the urea moiety. An illustrative example for the use of substituted ureas is presented in Zalavadiya et al. [56], who reported a series of dihydropyrimidine derivatives (Scheme 17), obtained through the condensation of substituted phenyl urea, ethylacetoacetate or acetoacetanilides, and a wide variety of benzaldehydes substituted with EDGs and EWGs. The in vitro anti‐TB activity was determined against Mtb H37Rv, with all compounds showing poor activity; the most active (24a, Figure 3) showed a 74% growth inhibition at 15 μM.

SCHEME 17.

Modified Biginelli 3CR to synthesize dihydropyrimidine derivatives [56].

Warekar et al. [57], Bhoi et al. [58], Mokariya et al. [59], and Patel et al. [60] also reported the use of modified Biginelli 3CR to synthesize anti‐TB compound libraries, replacing the urea moiety by an amino‐azole or amino‐azine to synthesize heterofused dihydropyrimidine derivatives (Scheme 18), through similar 3CRs. The four reports share the rational of fusing the dihydropyrimidine moiety to different biologically active scaffolds, intending to enhance their activity profile. Warekar et al. [57] designed a series of benzimidazole dihydropyrimidine derivatives (25), Bhoi et al. [58] a series of benzothiazole dihydropyrimidine derivatives (26), Mokariya et al. [59] a series of benzimidazole and triazole dihydropyrimidine derivatives containing a benzyloxyphenyl group (27), and Patel et al. [60] a series of pyrazoloquinazoline derivatives (28). The reaction was suitable with either 2‐amino benzimidazole, substituted 2‐amino benzothiazoles, 3‐amino‐1,2,4‐triazine, or a substituted 5‐amino pyrazole as the urea replacement, a wide variety of aromatic aldehydes, including benzaldehydes substituted with EDGs and EWGs and formylpyridine, and a range of 1,3‐dicarbonyl systems, such as ethyl acetoacetate, ethyl benzoylacetate, substituted 1,3‐cyclohexanediones, and a scope of acetoacetanilides, substituted with both EDGs and EWGs. The libraries were tested for their in vitro anti‐TB activity, with moderate to very weak activity in the three libraries; the most active compound of each library presented MIC values of 39 μM (27a , Figure 3), 57 μM (25a, Figure 3), 65 μM (28a, Figure 3), and a 73% growth inhibition at 273 μM (26a, Figure 3). Patel et al. [60] also reported an in silico physicochemical and pharmacokinetic evaluation of their pyrazoloquinazoline derivatives, with compounds adhering to Lipinski's Ro5 and Veber's rules, and most of them demonstrating the ability to inhibit P‐glycoprotein, having no inhibition of cyt P450 enzymes, and negative predictions for the AMES test, mitochondrial toxicity, hERG blockade, and cytotoxicity against HepG‐2 cells.

SCHEME 18.

Modified Biginelli 3CR to synthesize heterofused azole and azine dihydropyrimidine derivatives [57, 58, 59, 60]. Reactional Conditions: (25): citric acid, ethanol, reflux, 1–1.1 h. (26): PdCl₂, neat, microwave irradiation, 3–5′. (27): TBAB, ethanol: water, 80 °C, 30–90′. (28): PEG‐400, 80 °C, 150 W, 10–15′.

In addition to the Biginelli 3CR, other MCRs using different strategies have been reported to enable access to pyrimidine derivatives. An example is the work of Muravyova et al. [61], who obtained tetrahydropyrimidine derivatives (Scheme 19) through a 3CR between ammonia, a wide variety of carbonyl compounds, including acetone, arylaldehydes and N‐substituted piperidones, and chalcones, substituted with both EDGs and EWGs. The in vitro anti‐TB activity was assessed against Mtb H37Rv, with one compound (29a, Figure 3) showing moderate activity, inhibiting growth by 98% at 13 μM.

SCHEME 19.

3CR reported by Muravyova et al. [61] to produce tetrahydropyrimidine derivatives. US: ultrasonic irradiation.

2.4. Imidazole Derivatives: Groebke–Blackburn–Bienaymé (GBB) 3CR

The GBB reaction is an isocyanide‐based 3CR described independently by three research groups in 1998 [62, 63, 64]. The GBB 3CR involves the reaction of 2‐aminoazines or azoles with aldehydes and isocyanides, generally with acid catalysis, to generate imidazole‐fused scaffolds. Current advances on the GBB 3CR focus on the development of more sustainable methodologies and catalysts, through green solvents or solvent‐free conditions, novel catalysts, and milder conditions, and on the broadening the reaction scope [65, 66].

Imidazoles are another class of heterocyclic compounds widely explored in medicinal chemistry, with a diverse range of biological applications, like anti‐TB, antitumor, antimicrobial, antifungal, antiviral, antioxidant, anti‐inflammatory, and antidiabetic. The therapeutical value of the imidazole scaffold is shown by the rapid expansion of imidazole‐based medicinal chemistry, and with the practical simplicity of the GBB 3CR in rapidly and efficiently generating diverse fused imidazole heterocyclic scaffolds, this reaction quickly became an important tool for medicinal chemists [67, 68].

An example of a library of anti‐TB agents obtained through the GBB 3CR is described in Sapegin et al. [69], who synthesized a series of fused imidazoles conjugated with a nitrofuran ring (Scheme 20), in a strategy to decrease the cytotoxicity of the nitrofuran moiety, while directing the potent antimicrobial activity. The GBB 3CR was suitable for a wide range of substituted 2‐amino‐azines and azoles, including pyridines, pyrimidines, thiazoles and thiadiazoles, and with either cyclohexyl or tert‐butyl isocyanide. The in vitro anti‐TB activity was evaluated against Mtb H37Rv, with the most active compound (30a, Scheme 20) showing moderate inhibitory activity with a MIC of 19 μM. An early in silico evaluation of compound molecular parameters was also performed, where all compounds showed no violations of Lipinski's Ro5.

SCHEME 20.

GBB 3CR to obtain imidazole‐fused derivatives, and the most active compound reported [69].

Some reports have described other MCRs with different synthetic strategies to produce imidazole derivatives, such as in Sahana et al. [70], where a series of trisubstituted imidazoles containing a quinoline moiety (Scheme 21) was obtained via a 3CR between substituted aryl diketones, with both EDGs and EWGs, formyl quinolines, and two equivalents of ammonium acetate. The in vitro anti‐TB activity was assessed against Mtb H37Rv, with three compounds showing moderate activity, the most active (31a, Scheme 21) with a MIC of 16 μM.

SCHEME 21.

3CR described by Sahana et al. [70] to synthesize imidazoles derivatives, the most active compound reported and corresponding SAR.

2.5. Spiropyrrolidine Derivatives

Spiroheterocyclic compounds, and particularly spiropyrrolidines, are structural motifs frequently found in natural products and recognized for the wide diversity of pharmaceutical and biological applications, such as anti‐TB, antitumor, antimicrobial, and antidiabetic. Guided by its medicinal perspectives, the synthetic methods for spiropyrrolidines are ever progressing to enhance efficiency, sustainability and selectivity, now encompassing conventional synthesis, but also new approaches such as liquid catalysis, green techniques, and microwave‐assisted synthesis [71, 72].

The literature shows multiple iterations of MCRs used to produce libraries of anti‐TB agents, generally based on the combination of isatin or an analog, an amino acid, and a dienophile. An illustrative example is the work described by Kumar et al. [73], to develop a series of functionalized dispiropyrrolidine derivatives (Scheme 22), through a 3CR between isatin, cyclic ketones and amino acids. The reaction was suitable with a variety of 5‐, and 7‐substituted isatins, with alkyl groups and halogens, either N‐substituted piperidinone or simple cyclic ketones, and either sarcosine or phenylglycine. Interestingly, the authors described that when using cyclic amino acids, the 3CR product was not obtained in favor of azomethine ylide dimers. The in vitro anti‐TB activity was screened against Mtb H37Rv, with nine compounds showing potent activity, the most active (32a, Figure 4) with a MIC of 2.0 μM.

SCHEME 22.

3CR described by Kumar et al. [73] to produce dispiropyrrolidine derivatives.

FIGURE 4.

Most active compounds and SAR studies for libraries derived from MCRs to synthesize spiropyrrolidine derivatives [73, 74, 75, 76, 77, 78, 79].

Dandia et al. [74], Allaka et al. [75], and Arumugam et al. [76] reported similar 3CRs combining substituted isatins, amino acids, and dienophiles (Scheme 23). Dandia et al. [74] synthesized trispiropyrrolidines (33) through a regioselective 3CR; Allaka et al. [75] designed spiropyrrolidine derivatives containing a quinazolinone scaffold (34), whilst Arumugam et al. [76] reported a series of spirooxindolopyrrolidines (35) through a highly stereo‐ and regioselective 3CR. The reaction allowed 5‐substituted isatin derivatives with both EDGs and EWGs, either sarcosine, proline, thioproline, or phenylalanine as the amino acid, and either substituted bis arylidene‐cyclic ketones with both EDGs and EWGs in the aryl groups, substituted quinazolinone chalcones bearing EDGs and EWGs, or ninhydrin alkyl acrylates as the dienophile. The in vitro anti‐TB activity of the three libraries was evaluated against Mtb H37Rv, with two compounds in each showing potent activity, the most active (33a to 35a, Figure 4), with MIC values of 5.2, 2.5, and 1.5 μM, respectively. A toxicity evaluation against a macrophage cell line (Raw 264.7) was performed for the libraries of spiropyrrolidine and the spirooxindolopyrrolidine derivatives, where compounds showed low cytotoxicity, with the most active compounds displaying a good pharmaceutical window.

SCHEME 23.

3CR reported by Dandia et al. [74], Allaka et al. [75], and Arumugam et al. [76] to synthesize spiropyrrolidines derivatives. Reactional Conditions: (33): trifluoroethanol, reflux, 30–40′. (34): acetonitrile, ultrasound irradiation, 35–60′. (35): ionic liquid, 100 °C, 30′.

Lavanya et al. [77] used isatin and condensation derivatives in a 3CR, expanding the substitutable positions in the spiropyrrolidine scaffold. A library of spiro‐pyrrolopyrrole derivatives (Scheme 24) was obtained through a 3CR between either isatin or condensation derivatives, N‐aryl maleimides, substituted with EDGs and EWGs in the aryl moiety, and phenylalanine. The in vitro anti‐TB activity was assessed against Mtb H37Rv, with three compounds showing moderate activity, the most active (36a, Figure 4) with a MIC of 23 μM.

SCHEME 24.

3CR described by Lavanya et al. [77] to obtain spiro‐pyrrolopyrrole derivatives.

Arumugam et al. [78, 79] enhanced the reaction components by replacing isatin with ninhydrin and adding o‐phenylenediamine to obtain an indenoquinoxaline scaffold. The authors developed a series of spiropyrrolidine derivatives tethered to an indenoquinoxaline scaffold (Scheme 25) via a highly stereo‐ and regioselective 4CR between ninhydrin, o‐phenylenediamine, nitrostyrenes substituted with both EDGs and EWGs, and either phenylalanine or tryptophan. The in vitro anti‐TB activity was screened against Mtb H37Rv, with four compounds showing potent activity, the most active (37a, Figure 4) with a MIC of 2.9 μM. Compound toxicity was evaluated against a macrophage cell line (Raw 264.7), where compounds showed low cytotoxicity, with the most active compound showing a good pharmaceutical window.

SCHEME 25.

4CR described by Arumugam et al. [78, 79] to produce spiropyrrolidine derivatives.

2.6. Other Nitrogen‐Based Heterocycles

The indolizine scaffold is an uncommon indole isomer with the nitrogen atom in the ring fusion position. This heterocycle has been associated with a wide range of pharmacological activities, such as analgesic, antitumor, antimicrobial, anti‐inflammatory, antioxidant, antiviral, and others. Considering its relevance in medicinal chemistry, Venugopala et al. [80] designed a series of indolizine derivatives (Scheme 26) via a 3CR between bromoacetophenones, p‐substituted with EWGs, ethyl alkynoates, and pyridines, p‐substituted with a methyl or formyl group. The in vitro anti‐TB activity was evaluated against Mtb H37Rv, with one compound (38a, Scheme 26) showing potent activity with a MIC of 9.7 μM. Compounds were also tested against an MDR strain, where all showed weak activity. Cytotoxicity was assessed for selected compounds against peripheral blood mononuclear cell lines, with no compound showing cytotoxicity in the micromolar range, showing a good pharmaceutical window.

SCHEME 26.

3CR reported by Venugopala et al. [80] to obtain indolizine derivatives, the most active compound reported and corresponding SAR.

Considering the vast pharmacological activities of the triazine ring, such as anti‐TB, antibacterial, antiviral, antimalarial, antioxidant, antifungal, and anti‐inflammatory, Patel et al. [81, 82] reported a series of dual pharmacophore triazole derivatives containing a triazine scaffold to combine the pharmaceutical potency of both scaffolds (Scheme 27). Compounds were prepared by a two‐step synthetic pathway, where the first step is a 3CR between either cyclohexanone or N‐methyl piperidone, thiosemicarbazide, and an amino triazine. All compounds showed potent activity against Mtb H37Rv, the most active (40a, Scheme 27) with a MIC of 1.1 μM. An early in silico ADME evaluation was also performed, where all compounds showed no violations of Lipinski's Ro5, and the Veber rules, thus having a good predicted oral bioavailability.

SCHEME 27.

3CR reported by Patel et al. [81, 82] to produce dual pharmacophore triazine‐containing triazole derivatives and the most active compound reported.

The thiazole scaffold can be found in natural products such as steroids, flavones, pigments, and alkaloids, and is relevant in medicinal chemistry due to its range of pharmacological activities such as analgesic, antiviral, and anti‐inflammatory. Considering its vast pharmacological activities, Kadam et al. [83] and Rupapara et al. [84] reported similar 3CRs combining aldehydes, a thiosemicarbazide, and a species with a bromoacetyl group (Scheme 28). Kadam et al. [83] designed a series of thiazole derivatives containing a pyrazole moiety (41), whilst Rupapara et al. [84] synthesized a library of dual pharmacophore fluorene‐thiazole derivatives (42) to exploit the wide range of biological activities of fluorene compounds, such as anti‐TB, antitumor, antibacterial, and antidepressant. The 3CR allowed a diphenyl pyrazole carbaldehyde, substituted with EDGs and EWGs, or formyl fluorene as the aldehyde, and either a highly substituted aryl with a bromoacetyl reactive group or a variety of 2‐bromophenyl ethanones, with both EDGs and EWGs. The in vitro anti‐TB activity of the two libraries was evaluated against Mtb H37Rv, with very distinct results observed. All thiazole derivatives containing a pyrazole moiety showed weak activity against Mtb H37Rv, the most active (41a , Scheme 28) with a MIC of 75 μM. Four fluorene‐thiazole derivatives showed potent activity, the most active (42a, Scheme 28) with a MIC of 1.8 μM. Selected derivatives were tested against MDR and extensively drug‐resistant (XDR) clinical strains, where compounds showed potent to moderate activity. Compound cytotoxicity was assessed against Vero cell lines, showing low cytotoxicity with a good pharmacological window for the most active compound, with an SI of 46.

SCHEME 28.

3CR described by Kadam et al. [83] and Rupapara et al. [84] to produce thiazole derivatives, the most active compounds reported and described SARs. Reactional Conditions: (41): H₃PO₄, acetonitrile, 50–60 °C, 0.5–1.5 h. (42): Sc(OTf)₃, methanol, r.t., 1.5 h.

Subhedar et al. [85] developed a series of thiazolidinone derivatives (Scheme 29) via a 3CR between p‐substituted benzaldehydes, with both EDGs and EWGs, thioglycolic acid, and p‐substituted anilines, with both EDGs and EWGs. All compounds showed weak activity against Mtb H37Ra, the most active (43a, Scheme 29) with an IC₉₀ of 74 μM, and presented no cytotoxicity against human cancer cell lines (MCF‐7, A549, HCT‐116), with the most active compound showing a small pharmaceutical window (SI > 4).

SCHEME 29.

3CR described by Subhedar et al. [85] to synthesize thiazolidinone derivatives and the most active compound reported.

Pyrazole derivatives are commonly explored by medicinal chemists for their broad spectrum of biological activities, such as anti‐TB, antibacterial, antifungal, antimalarial, antiviral, antitumor, anti‐inflammatory, and antidiabetic. Zala et al. [86, 87] developed a series of highly substituted pyrazolyl pyrazoline derivatives (Scheme 30) through a 3CR between substituted formyl pyrazole, with either amino triazole, amino tetrazole, or substituted thiophenol substituents; 2‐acetyl heterocycles, such as 2‐acetyl thiophene, 2‐acetyl thiazole, 2‐acetyl pyrrole and 2‐acetyl furan; and a wide range of substituted hydrazines, with benzimidazole, benzothiazole or acetylpyridine groups. The in vitro anti‐TB activity was assessed against Mtb H37Rv, with five compounds showing moderate activity, the most potent (44a, Scheme 30) with a MIC of 23 μM.

SCHEME 30.

3CR described by Zala et al. [86, 87] to produce pyrazolyl pyrazoline derivatives and the most active compound reported. US: ultrasound irradiation.

In a distinct strategy, Koli et al. [88] reported a series of isonicotinoyl pyrazoles (Scheme 31) by derivatising isoniazid in a 3CR with a wide range of substituted benzaldehydes, with both EDGs and EWGs, and malononitrile. The in vitro anti‐TB activity was tested against Mtb H37Rv, with six compounds showing potent, sub‐micromolar activity, the most active (45a, Scheme 31) with a MIC of 0.39 μM.

SCHEME 31.

3CR described by Koli et al. [88] to synthesize isonicotinoyl pyrazole derivatives and the most active compound reported.

Gunasekaran et al. [89] reported a series of pyrazolone derivatives (Scheme 32) through a 4CR between a wide range of substituted benzaldehydes, with both EDGs and EWGs, β‐naphthol, methyl acetoacetate, and p‐substituted phenylhydrazines with halogen atoms. The in vitro anti‐TB activity was evaluated against Mtb H37Rv, with seven of them showing potent inhibitory activity, the most active (46a, Scheme 32) with a MIC of 1.6 μM.

SCHEME 32.

4CR described by Gunasekaran et al. [89] to obtain pyrazolone derivatives and the most active compound reported, with the reported SAR.

3. Peptidomimetics: Passerini 3CR and Ugi 4CR

Discovered by Mario Passerini in 1921 [90], the Passerini 3CR was the first described isocyanide‐based MCR, combining an isocyanide, an aldehyde or ketone, and a carboxylic acid to obtain α‐acyloxy carboxamides. In 1959, Ivan Karl Ugi expanded isocyanide chemistry with the description of the Ugi 4CR between isocyanides, aldehydes or ketones, primary amines, and carboxylic acids to obtain aminoacyl amides [91]. These products are common building blocks present in the structures of a diverse array of compounds of interest, making the Passerini 3CR and the Ugi 4CR important resources not only in medicinal chemistry, to synthesize peptidomimetics or to shorten synthetic routes of more complex bioactive molecules, but also in combinatorial, green, and polymer chemistry. Historically, the Passerini 3CR was much less explored than its homologous, as esters are less attractive than amides in medicinal chemistry, but a renewal of interest was driven by the expansion of the reaction scope. Through reagent functionalisation or coupling with other reactions, Passerini and Ugi products evolved from simple pseudo‐peptides to a variety of different products, including heterocycles, spirocycles, polymers, macrocycles, polycyclic compounds, natural products, and amino acids. In recent years, advances on the Passerini and Ugi reactions are focused on expanding substrate scope and progressing reactional conditions to enantioselective conditions and greener methods and have been extensively reviewed elsewhere [92, 93, 94, 95].

Several descriptions of Ugi 4CRs can be found in the literature to prepare anti‐TB agents. Avilés et al. [96, 97] isolated a diterpenoid β‐lactam alkaloid from a Puerto Rican marine sponge Hymeniacidon sp. and synthesized a small library of compounds through a one‐step semisynthetic pathway based on the Ugi‐4CR. (Scheme 33), in another example of MCRs used to quickly and efficiently derivatise natural products. The reaction was suitable with both formaldehyde and benzaldehyde; either ammonia, alkylamines or aminoquinolines; and either formic acid, isonicotinic acid or β‐alanine. The in vitro anti‐TB activity was screened against Mtb H37Rv, with three compounds showing moderate activity, the most active (47a, Figure 5) with an MIC of 27 μM, less potent than the precursor natural product (MIC = 9.9 μM).

SCHEME 33.

Ugi 4CR to derivatise a marine natural product [96, 97].

FIGURE 5.

Most active compounds and SAR studies for libraries derived from MCRs to synthesize peptidomimetic derivatives [96, 97, 98, 99, 100, 101, 102, 103].

In a similar strategy, Kim et al. [104] derivatised tylosin, a macrolide bacteriostatic antibiotic, into a small library of compounds through a one‐step semisynthetic pathway based on either the Ugi 4CR or the Gewald 3CR between sulfur, aldehydes and cyanoacetates (Scheme 34). The Gewald 3CR was suitable for a wide variety of cyanoacetamides, with allyl, alkyl, and aryl N‐substituents. The in vitro anti‐TB activity was evaluated, with five compounds showing potent activity, the most active (48a, Scheme 34) with a MIC of 2.2 μM, more potent than the parent compound (MIC = 5.6 μM).

SCHEME 34.

Gewald 3CR and Ugi 4CR to derivatise a macrolide bactericidal antibiotic, and the most active compound reported, with the respective SAR [104].

Using the Ugi 4CR, Rezayan et al. [98] and Gondaliya et al. [99] synthesized a series of fluorene bisamide derivatives (52) and carboxamide derivatives (53) (Scheme 35). The scope of the 4CR encompassed 2‐(pyrimidin‐2‐ylthio)acetic acid or acetylene and phenyl acetylene carboxylic acid, aminofluorene or benzylamine, a wide variety of substituted benzaldehydes, with both EDGs and EWGs, and either cyclohexyl or tert‐butyl isocyanide. The in vitro anti‐TB activity of the two libraries was evaluated against Mtb H37Rv, with very distinct results observed. All carboxamide derivatives showed weak activity against Mtb H37Rv, the most active (53a, Figure 5) with a MIC of 54 μM. One fluorene bisamide derivative (52a, Figure 5) showed potent activity, with a MIC of 2.1 μM and maintained activity against DR strains. Compounds presented moderate to low cytotoxicity in a brine shrimp lethality bioassay with a large pharmaceutical window for the most active (SI = 103).

SCHEME 35.

Ugi 4CR to synthesize bisamide derivatives [98, 99].

Dömling et al. [100] designed a series of isoniazid‐ and pyrazinamide‐derived peptide analogs (Scheme 36) through a combinatorial approach based on the Ugi 4CR, which showcased the potential of MCRs to quickly generate libraries of compounds in high‐throughput methodology. The 4CR was suitable for both isonicotinic and pyrazinoic acid, a wide variety of alkyl, allyl, benzyl, and cyclic aldehydes or ketones, and a diverse range of alkyl, allyl, and aryl amines. Compounds were synthesized in 96‐well plates, where crudes were screened for their activity against Mtb H37Rv as a mixture of Ugi 4CR and Passerini 3CR products. The most promising compounds were then retested as pure samples, where three compounds showed potent activity, the most active (54a, Figure 5) with a MIC of 8.5 μM. Compounds showed moderate to low cytotoxicity against VERO cells, with the most active compound showing a good pharmaceutical window, with SI = 16.

SCHEME 36.

Ugi 4CR to produce isoniazid and pyrazinamide‐derived peptide analogs [100].

Roper et al. [101] reported a series of atropoisomeric peptide analogs (Scheme 37) through a stereodivergent Ugi 4CR, in an example of a regioselective MCR. The reaction was suitable with a diverse scope of aryl and alkyl carboxylic acids and N‐protected amino acids, a variety of aromatic amines, including substituted anilines and naphthylamines, a wide range of allyl, alkyl, cyclic, and aryl aldehydes, and a scope of alkyl and aryl isocyanides. The reaction was found to be stereodivergent, obtaining the syn product at high temperatures and the anti‐product at low temperatures. The in vitro anti‐TB activity was tested for selected compounds against Mtb mc² 7902 and three DR strains, with the most active compound (55a, Figure 5) showing moderate activity for the syn product with a range of MIC₉₀ between 32 and 38 μM in the tested strains, whilst the anti‐atropoisomer showed no activity.

SCHEME 37.

Ugi 4CR to synthesize atropoisomeric peptide derivatives [101]. TFEA: trifluoroethanol.

Lakontseva et al. [102] developed a series of isoniazid‐ and pyrazinamide‐derived dipeptide analogs (Scheme 38) through a two synthetic step synthesis, starting with a hydrazine‐Ugi 3CR between hydrazides, aldehydes, and isocyanides. The reaction was suitable with both isoniazid and pyrazinoic acid hydrazide, both cyclohexane carbaldehyde and butanal, and both benzyl and 2‐methoxyethyl isocyanide. When methanol was used as solvent, methyl ester derivatives (57, Scheme 38) were isolated, derived from methanolysis in the Mumm rearrangement. The in vitro anti‐TB activity was assessed against Mtb H37Rv, with six compounds showing moderate activity, the most active (57a, Figure 5) with a MIC of 12 μM. Compounds were found to have no detectable cytotoxicity against the HEK293 cell line, with the most active compound showing a good pharmaceutical window.

SCHEME 38.

Hydrazine‐Ugi 3CR to produce isoniazid‐ and pyrazinamide‐derived hydrazine dipeptide analogs [102].

Besides the Passerini 3CR and the Ugi 4CR, other MCRs using different chemical strategies have been reported to synthesize carboxylamides. An example is described by Rezayan et al. [103], who reported a series of coumarin derivatives (Scheme 39) through an isocyanide‐based 3CR and 4CR between substituted hydroxybenzaldehydes, with both EDGs and EWGs, Meldrum's acid, a variety of alkyl and aryl isocyanides, and a diverse range of alkyl and benzyl alcohols. The in vitro anti‐mycobacterial activity was assessed against M. bovis, with three compounds showing moderate inhibitory activity, the most active (59a, Figure 5) with a MIC of 32 μM.

SCHEME 39.

3CR and 4CR described by Rezayan et al. [103] to synthesize coumarin derivatives.

4. Pyran Derivatives

Pyrans are one of the most common structural subunits found in natural products, in scaffolds such as coumarins, chromenes, chromones, sugars, flavonoids, xanthones, naphthoquinones, and more. Unsurprisingly, pyrans are associated with a diverse scope of pharmacological properties, such as anti‐TB, antibacterial, antifungal, antiviral, antitumor, and anti‐inflammatory, among others. The medicinal perspectives of pyran‐fused analogs have been reviewed elsewhere [105], and inspired the advancement of several strategies to synthesize pyran ring derivatives via MCR using sustainable catalysts under green protocols [106].

A wide range of MCRs using different chemical strategies have been reported to synthesize libraries of anti‐TB agents. Patil et al. [107] reported a 3CR between 4‐hydroxycoumarin, aryl aldehydes and dimedone to obtain a series of fused coumarin‐chromene derivatives (Scheme 40). The 3CR, consisting of a Knoevenagel condensation, followed by a Michael addition and intramolecular cyclization with the elimination of water, was suitable for a wide range of aromatic aldehydes, such as naphthaldehyde, formylthiophene and substituted benzaldehydes, with both EDGs and EWGs. The in vitro anti‐TB activity was determined against Mtb H37Rv, with two compounds showing moderate inhibitory activity, with a MIC of 15 μM for the most active (61a, Figure 6). Compound drug likeness was also studied in silico, where all compounds showed no more than one violation of Lipinski's Ro5, with a good predicted oral bioavailability.

SCHEME 40.

3CR described by Patil et al. [107] to produce fused coumarin‐chromene derivatives.

FIGURE 6.

Most active compounds and SAR studies for libraries derived from MCRs to synthesize pyran derivatives [107, 108, 109, 110, 111, 112, 113, 114, 115].

A similar 3CR was described by Dige et al. [108], between 4‐hydroxycoumarin, a wide range of substituted salicylaldehydes, with both EDGs and EWGs, and barbituric acid, to obtain dichromeno pyrano pyrimidinone derivatives (Scheme 41). The in vitro anti‐TB activity was evaluated against Mtb H37Rv, with two compounds showing potent activity, the most active (62a, Figure 6) with a MIC of 3.1 μM.

SCHEME 41.

3CR described by Dige et al. [108] to produce dichromeno pyrano pyrimidinone derivatives.

Mungra et al. [109], Mohire et al. [110], Khare et al. [111], and Warekar et al. [112] all synthesized libraries of pyrano derivatives (Scheme 42) through a 3CR between aldehydes, malononitrile or analogs, and phenolic compounds. Mungra et al. [109] designed a series of dual pharmacophore pyranochromene derivatives containing a quinoline moiety (63), Mohire et al. [110] developed a series of pyrano pyran derivatives (64), Khare et al. [111] reported a series of dual pharmacophore triazole‐chromene derivatives (65), whilst Warekar et al. [112] prepared a series of benzochromene derivatives (66). The 3CR allowed a wide range of aryl aldehydes, such as benzaldehydes, triazolyl benzaldehydes, quinoline carboxaldehydes, tetralone carboxaldehydes, and thiophene carbaldehydes, substituted with both EDGs and EWGs, either malononitrile or p‐nitrophenylacetonitrile, and either hydroxycoumarins, hydroxypyranones, resorcinol or β‐naphthol. The in vitro anti‐TB activity of the four libraries was evaluated against Mtb H37Rv, with distinct results observed. All pyrano chromene derivatives containing a quinoline moiety showed very weak inhibitory activity, the most active (63a, Figure 6) with a MIC of 492 μM. Two triazole‐chromene derivatives showed moderate activity (65a, Figure 6), with a MIC of 33 μM for the most active. This library was evaluated in silico to predict compound drug likeness, where all compounds showed no violations of Lipinski's Ro5 and a good predicted oral bioavailability. Two compounds of each of the libraries of pyrano pyran derivatives and benzochromene derivatives showed potent activity, the most active (64a and 66a, Figure 6) with MIC values of 4.0 and 3.6 μM, respectively.

SCHEME 42.

3CR described by Mungra et al. [109], Mohire et al. [110], Khare et al. [111], and Warekar et al. [112] to obtain pyrano derivatives. Reactional Conditions: (63): piperidine, ethanol, reflux, 1.5 h. (64): ionic liquid, water, r.t., 1.5 h. (65): NaHCO₃, water, ultrasound irradiation, 30 °C, 1.5 h. (66): pTSA, ethanol, reflux, 1–2.3 h.

Chavan et al. [113] described the synthesis of dual pharmacophore spirochromene derivatives containing a triazole moiety (Scheme 43) to combine both pharmacophores' pharmacological activities. The compound series was synthesized through a 5CR between dimedone or 4‐hydroxy‐6‐methyl‐2‐pyrone, various halogen‐substituted N‐propargyl isatins, malononitrile, sodium azide, and various p‐substituted benzyl bromides with both EDGs and EWGs. The in vitro anti‐TB activity was determined against Mtb H37Ra, in a dormancy model and normalcy conditions, with two compounds (67a, b, Figure 6) showing MIC values of 9.6–9.9 μM in normalcy conditions and 8.3–8.6 μM in the dormancy model. Selected compounds were tested against human cancer cell lines (MCF‐7, A549, HCT‐116), and they were found to have low to no cytotoxicity against the MCF‐7 cell line and no cytotoxicity against the A549 and HCT‐116 cell lines. The most active compounds showed good pharmaceutical windows with SI > 13.

SCHEME 43.

5CR described by Chavan et al. [113] to obtain dual pharmacophore spirochromene derivatives. Cell‐CuI: cellulose‐supported cuprous iodide.

Vasava et al. [114] and Parikh et al. [115] reported a series of pyranopyrazole derivatives through a 4CR between hydrazines, ethyl acetoacetate, aldehydes, and malononitrile (68–69, Scheme 44). The 4CR was suitable with either isonicotinohydrazide or substituted phenylhydrazines with EWGs, either ethyl acetoacetate or ethyl trifluoroacetoacetate, and with a wide range of aromatic aldehydes, such as cinnamaldehyde, furaldehyde, formylpyridine and a variety of substituted benzaldehydes, with both EDGs and EWGs. All tested compounds of both libraries showed weak inhibitory activity in vitro against Mtb H37Rv, with the most active coumpounds (68a and 69a, Figure 6) with a MIC of 51 and 60 μM, respectively. Vasava et al. [114] also tested their pyranopyrazole library against an MDR isolate, but the most active compound showed a decrease in activity with a MIC of 103 μM. Cytotoxicity was assayed against a HeLa cell line, where compounds showed moderate to low cytotoxicity; the most active of them without a pharmaceutical window.

SCHEME 44.

4CR described by Vasava et al. [114] and Parikh et al. [115] to produce pyranopyrazole derivatives. Reactional Conditions: (68): SnCl₂, 180 W, 10–25′. (69): Zn(OTf)₂, microwave irradiation, 80–120 °C, 25′.

5. Mannich Bases: Mannich 3CR and Nitro‐Mannich 3CR

The Mannich 3CR, first described by Carl Mannich in 1912 [116], is a cornerstone of organic synthesis and drug design. The Mannich 3CR is a cost‐effective, versatile, flexible technique to synthesize β‐aminocarbonyl compounds through the amino alkylation of the α position of an aldehyde or ketone (substrate), with an amine and an aldehyde. The Mannich 3CR has been extensively explored for the synthesis and derivatisation of novel pharmacological agents, with Mannich bases having a wide scope of biological activities, such as anticonvulsant, antitumor, anti‐inflammatory, antibacterial, antiprotozoal, and anti‐TB. Furthermore, the Mannich 3CR is also used as a prodrug strategy, as Mannich bases generally have favorable pharmacokinetics, with enhanced aqueous solubility. Scientific research about the Mannich reaction and Mannich bases is prospering, with current efforts focused on green modifications for sustainability, such as exploring biodegradable and reusable catalysts or catalyst‐free conditions, the use of ionic liquids, solvent‐free or water‐based media, and the use of techniques like microwave and ultrasound irradiation, and have been reviewed elsewhere [117, 118].

An example of the Mannich 3CR used to develop a series of anti‐TB agents is described by Khudina et al. [119, 120, 121], that upon observation of the vast pharmacological activities of thiouracil derivatives, such as antiviral, antimicrobial, and antimalarial, synthesized series of pyrimidothiadiazines through a double modified Mannich 3CR (Scheme 45). The 3CR used 2‐thiouracils with different‐length polyfluoroalkyl substituents as the substrate for the amino alkylation. As the thiouracils have two nucleophilic centers, two amino alkylations are sustained. The reaction was suitable with a wide range of primary amines, including aliphatic and aromatic amines, amino acids, and diamines, where dimerization was observed. All tested compounds showed moderate activity in vitro against Mtb H37Rv, the most active (70a, Scheme 45) with a MIC of 16 μM.

SCHEME 45.

Modified double Mannich 3CR to produce pyrimidothiadiazine derivatives, and the most active compound described [119, 120, 121].

Louis Henry first described in 1896 the addition of nitroalkanes to an imine derived from the elimination of water from hemiaminal [122], the first description of a Nitro‐Mannich reaction, also known as the aza‐Henry reaction. The Nitro‐Mannich 3CR is a technique to synthesize β‐nitroamines through the amino alkylation of the α position of a nitroalkane (substrate) with an amine and an aldehyde or ketone. This reaction has a large similarity to the Mannich 3CR, with both involving the addition of an acidic carbon nucleophile to an imine generated in situ. Current interest in the Nitro‐Mannich 3CR stems mainly from the possibility of derivatisation of β‐nitroamines into 1,2‐diamines, and research is focused on asymmetric catalysis, green procedures, and expanding the reaction scope and catalytic agents [123, 124].

An example of a Nitro‐Mannich 3CR to develop anti‐TB pharmaceutical agents is the work of Fedorova et al. [125], that synthesized a series of chalcone podands containing azole rings through a modified Nitro‐Mannich 3CR (Scheme 46), aiming to increase efficiency due to compound‐facilitated penetration through the podand fragment. The 3CR used different‐length podands with an α, β‐unsaturated ketone, with an intramolecular cyclization after the condensation, releasing water and nitric oxide to form the pyrrole fragment. Additionally, a second MCR was also described between the same podands, hydrazine, and a variety of carboxylic acids, including formic, acetic, and propanoic acids, to synthesize a pyrazoline fragment. The in vitro anti‐TB activity was tested against Mtb H37Rv, with four compounds showing potent activity, with a MIC of 1.3 μM for the most active (72a, Scheme 46).

SCHEME 46.

MCRs described by Fedorova et al. [125] to obtain podand derivatives containing pyrrole or pyrazoline fragments and the most active compound reported.

6. Miscellaneous

The Kabachnik–Fields reaction, discovered independently by Martin Kabaschnik [126] and Ellis K. Fields [127] in 1952, is a technique to synthesize α‐aminophosphonates from primary or secondary amines, an aldehyde or ketone, and a dialkyl phosphonate. As bioisosteres of amino acids, α‐aminophosphonates represent a class of interest in medicinal chemistry and drug discovery, which have thrived with significant biological activities, from peptide mimics to enzyme inhibitors, antibiotics, and antivirals. Current efforts are focused on asymmetric catalysis, green procedures, and expanding the reaction scope and catalytical agents [128, 129].

An example of the Kabachnik–Fields 3CR used to synthesize anti‐TB agents is described in Bhagat et al. [130], that reported a series of α‐sulfonamidophosphonates, synthesized through a modified Kabachnik–Fields 3CR (Scheme 47), using p‐toluenesulfonamide or p‐chlorobenzenesulfonamide as replacement of the amine, a wide range of aromatic aldehydes, including naphthaldehyde, formylthiophene and substituted benzaldehydes and cinnamaldehyde, with both EDGs and EWGs, and either dimethyl or diethyl phosphite. The in vitro anti‐TB activity was evaluated against Mtb H37Rv, with six compounds showing potent activity, the most active (74a, Scheme 47) with a MIC of 3.5 μM. Selected compounds were assayed against RAW 264.7 (mouse leukemic monocyte macrophage) cells, and they were found to have low cytotoxicity, with a large pharmaceutical window for the most active compound. Compound drug likeness was also assessed in silico, where all compounds showed no more than one violation of Lipinski's Ro5, with a good predicted oral bioavailability.

SCHEME 47.

Modified Kabachnik–Fields 3CR to produce α‐sulfonamidophosphonates derivatives, and the most active compound described [130].

The A³ (aldehyde–alkyne–amine) coupling reaction is a widely used technique to synthesize propargylamines through the condensation of those three components, reported independently by three research groups in 2001–2002 [131, 132, 133], and coined by Chao‐Jun Li in 2002 [134]. Current interest in the A³ reaction derives from the appeal of chiral propargylamines as versatile synthetic precursors in the synthesis of many chiral nitrogen‐containing natural products or pharmaceutical agents, with important biological properties. The reaction allows access to propargylamines under very mild reaction conditions, with a wide scope of reagents and in the presence of a transition metal catalyst. With the use of chiral ligands and catalysts, propargylamines are obtained with high enantioselectivity. Current research is focused on enhancing sustainability, efficiency, and stereoselectivity through the development of novel methodologies with green and deep eutectic solvent systems, ionic liquids, microwave irradiation, sustainable nano catalysts, or metal‐free catalysts [135, 136].

Guided by the principle that the combination of two or more biologically active scaffolds in a singular molecular frame often enhances the biological profile, Tripathi et al. [137] designed a series of propargylamines containing both a chromene and a cyclic amine scaffold through an A³ coupling 3CR (Scheme 48). The reaction was suitable with phenylacetylene, 1‐heptyne or 1‐octyne, and with both piperidine and morpholine. The in vitro anti‐TB activity was assessed against Mtb H37Rv, with two compounds showing potent inhibitory activity, the most active (75a, Scheme 48) with a MIC of 4.2 μM.

SCHEME 48.

A³ coupling 3CR to obtain dual pharmacophore propargylamine derivatives, and the most active compound reported [137].

Rajeev et al. [138] reported a series of highly functionalized 3‐substituted indole derivatives through a MCR (Scheme 49), inspired by indole‐3‐propionic acid, a metabolite produced by the gut microbiota with anti‐TB activity. The reaction consists of an alkylation of the position 3 of the indole, with an aldehyde and Meldrum's acid, followed by a nucleophilic attack to lyse the Meldrum's acid. The reaction was suitable with a variety of substituted indoles, with both EDGs and EWGs; a diverse scope of aldehydes, including acetaldehyde, isobutyraldehyde, and a selection of substituted benzaldehydes with both EDGs and EWGs; and a diverse scope of nucleophiles, including water, ethanol, aniline, and a selection of alkylamines. The in vitro anti‐TB activity was screened against Mtb H37Rv, with six compounds showing good activity, the most active (76a, Scheme 49) with a MIC of 4.1 μM.

SCHEME 49.

MCR described by Rajeev et al. [138] to obtain highly functionalized three‐substituted indole derivatives, and the most active compound reported.

Considering the vast pharmacological activities of monocarbonylic curcumin analogs, such as antibacterial, anti‐TB, antiparasitic, anti‐inflammatory, and antitumor, Subhedar et al. [139, 140] developed a series of monocarbonyl curcumins (Scheme 50). The products were obtained through a 3CR between a wide range of aryl aldehydes, with both EDGs and EWGs, piperidinone, and alkyl chlorides, either chlorobenzimidazole or substituted chloro phenylacetamides, with both EDGs and EWGs. The in vitro anti‐TB activity was determined against Mtb H37Ra, with six compounds showing potent activity, the most active (77a, Scheme 50) with a MIC of 1.5 μM. Selected compounds were evaluated against human cancer cell lines (MCF‐7, A549, HCT‐116 and THP‐1), where compounds were found to be nontoxic, with the most active compound showing a great pharmaceutical window, with SI > 154.

SCHEME 50.

3CR described by Subhedar et al. [139, 140] to synthesize monocarbonyl curcumin derivatives and the most active compound reported.

7. Summary and Outlook

TB remains the deadliest disease caused by a single infectious agent and therefore continues to be a major global health priority. In 2015, the World Health Organization launched the End TB Strategy to create a world free of TB by 2035 by reducing TB deaths by 95% and TB incidence cases by 90%. Despite some success stories, the progress to end TB is limited, and to achieve the goal of ending the global TB epidemic, an intensification of efforts is urgently needed, with concerted political commitment and funding.

Novel pharmacological agents are urgently required to control TB, ideally targeting all disease forms including DR strains and latent populations, to reduce treatment durations, increase patient compliance and limit the emergence of DR strains. The ability of MCRs to quickly generate diverse chemical libraries makes them powerful tools to accelerate drug discovery that should be exploited by medicinal chemists.

This review discloses the contributions of MCRs to anti‐TB drug discovery in the last 20 years, from the pioneer reports of MCRs used to develop novel anti‐TB pharmaceutical agents until the present day. Through multiple examples, the synergy between MCRs and medicinal chemistry is shown to facilitate drug discovery and development, offering a highly competitive and attractive alternative to traditional drug discovery, whilst being generally more sustainable. However, although MCRs enable rapid and efficient library generation, the gap between library synthesis and the discovery of highly potent compounds persists because of biological barriers, physicochemical constraints, and high attrition rates. Table 2 summarizes the most potent compounds synthesized via MCRs reported to date.

TABLE 2.

Most potent reported anti‐TB compounds synthesized through MCRs.

Cpd	Structure	Scaffold (MCR)	Activity	Toxicity	References
1a		Dihydropyridine (Hantzsch 3CR)	MIC = 34 nM (H37Rv)	IC50 > 17 μM (VERO cells) SI > 500	[16]
1b		Dihydropyridine (Hantzsch 3CR)	MIC = 36 nM (H37Rv)	IC50 > 18 μM (VERO cells) SI > 500	[17]
19a		Dihydropyrimidin(thio)one (modified Biginelli 3CR)	MIC = 0.28 μM (H37Rv)	—	[49]
20a		Dihydropyrimidinone (modified Biginelli 3CR)	MIC = 0.96 μM (H37Rv) EC90 = 0.25 μM (infected macrophage)	IC50 = 23 μM (VERO cells) SI = 24	[50]
20b		Dihydropyrimidinone (modified Biginelli 3CR)	MIC = 1.0 μM (H37Rv) EC90 = 0.10 μM (infected macrophage)	IC50 > 26 μM (VERO cells) SI > 26	[50]
23a		Pyrazolopyrimidine (modified Biginelli 3CR)	MIC = 44 nM (H37Rv)	IC50 > 22 μM (VERO cells) SI > 500	[55]
45a		Pyrazole	MIC = 0.39 μM (H37Rv)	—	[88]

Throughout this review, examples of MCRs used to shorten synthetic pathways, derivatise natural products and approved drugs, and in combinatorial chemistry approaches showcased the versatility of the use of MCRs to enhance the drug discovery pipeline.

Furthermore, the structural diversity of the synthesized scaffolds and the breadth of chemical space explored highlight the versatility of MCRs as tools for accelerating drug discovery. Several studies have reported compound series generated through the same MCR or based on closely related scaffolds that exhibit markedly different biological activity profiles, demonstrating that MCRs do not constitute a universal or inherently productive approach for the discovery of anti‐TB agents. Rather, the most potent compounds identified to date have emerged from rational drug design strategies in which MCRs were leveraged as efficient synthetic tools. These findings suggest that the greatest value of MCRs lies in facilitating rapid access to rationally designed pharmacophores rather than in unbiased library generation alone. In this context, the integration of structure‐based drug design and AI‐driven molecular design with MCR‐enabled synthesis represents a promising strategy for bridging the gap between library generation and the discovery of potent anti‐TB compounds.

Interestingly, all seven of the most potent compounds identified share a high degree of aromaticity and feature prominent nitrogen‐containing aromatic heterocycles. While this observation may partly reflect the types of scaffolds most readily accessible through MCR chemistry, it is also consistent with established medicinal chemistry principles. Aromatic systems contribute favorable hydrophobic properties that can modulate lipophilicity and enhance membrane permeation, whereas nitrogen‐containing aromatic heterocycles are privileged scaffolds capable of engaging in diverse and productive drug–target interactions. Moreover, the beneficial effect of para‐electron‐withdrawing groups (p‐EWGs) was frequently reported and observed in six of the seven most potent compounds. Electron‐withdrawing substituents can enhance binding affinity through hydrogen‐bonding, hydrophobic, and halogen–bonding interactions, while also influencing pharmacokinetic properties. In the context of anti‐TB drug discovery, their positive effect may additionally stem from increased lipophilicity, which could facilitate penetration through the highly impermeable Mtb cell wall.

Funding

The study was supported by Fundação para a Ciência e a Tecnologia (https://doi.org/10.54499/UID/04138/2025, https://doi.org/10.54499/UID/PRR/04138/2025, https://doi.org/10.54499/UID/PRR2/04138/2025).

Conflicts of Interest

The authors declare no conflicts of interest.

Biographies

Diogo Nunes is a medicinal chemist whose research is focused on the discovery and development of novel therapeutic agents for the treatment of tuberculosis. He is currently a Ph.D. candidate in Pharmacy from the University of Lisbon, working at the interface of synthetic chemistry, drug design, mycobacteriology, and proteomics to design, develop, and characterize the mode of action of novel antitubercular chemical entities.

Rui Moreira earned his Ph.D. in Pharmaceutical Sciences from the University of Lisbon under the supervision of Eduarda Rosa. Following postdoctoral research with Jim Iley at the Open University (UK), he joined the Faculty of Pharmacy, University of Lisbon, as an independent investigator and was appointed full professor of Medicinal Chemistry in 2006. His research focuses on the development of chemical tools to elucidate molecular mechanisms underlying infectious diseases, cancer, and neurodegenerative disorders. Current interests include targeted drug delivery systems, targeted protein degradation and molecular glues, as well as the design of covalent inhibitors and chemical probes for activity‐based and photoaffinity‐based protein profiling.

Maria de Jesus Perry is an assistant professor and principal investigator in iMed‐UL (Research Institute for Medicines), in the Medicinal Chemistry Group. Her research work in Medicinal Chemistry is focused on drug design. Areas of active interest include organic synthesis and identification of biologically active small molecules, prodrug development, targeted drug delivery, drug metabolism, and drug stability. In recent years, her main involvement has been the development of hybrid drugs for resistant malignancies, namely cancer and tuberculosis. Has numerous publications in renowned journals in the field of medicinal chemistry. Science communication activities are another complement that also give her a special and personal reward.

Francisca Lopes earned her M.S. in Pharmaceutical Sciences and her Ph.D. in Pharmacy from the University of Lisbon. She joined the Faculty of Pharmacy of the University of Lisbon (FFUL) in 2001 as an assistant professor and later advanced to associate professor. In 2016, she obtained her Habilitation from FFUL. Over the years, she has established a successful research program in medicinal chemistry focused on tuberculosis and malaria. Her research interests center on the design and development of innovative chemical entities with multi‐target mechanisms of action for the treatment of malaria and tuberculosis.