Synthesis of Biologically Active Molecules through Multicomponent Reactions

Focusing on the literature progress since 2002, the present review explores the highly significant role that multicomponent reactions (MCRs) have played as a very important tool for expedite synthesis of a vast number of organic molecules, but also, highlights the fact that many of such molecules are biologically active or at least have been submitted to any biological screen. The selected papers covered in this review must meet two mandatory requirements: (1) the reported products should be obtained via a multicomponent reaction; (2) the reported products should be biologically actives or at least tested for any biological property. Given the diversity of synthetic approaches utilized in MCRs, the highly diverse nature of the biological activities evaluated for the synthesized compounds, and considering their huge structural variability, much of the reported data are organized into concise schemes and tables to facilitate comparison, and to underscore the key points of this review.


Introduction
Multicomponent reactions (MCRs) are a type of convergent organic reactions in which three or more precursors react in only one step to form a product that incorporates substantial portions of all components (i.e., atom economy) [1]. As a result, there is very little waste or unwanted by-product formation compared to sequential synthesis. The concomitant step economy, high convergence and structural diversity of the resulting products make this sustainable approach a powerful tool for the synthesis of biologically active molecules and optimization processes in the pharmaceutical industry [2]. Even though the MCR concept for the synthesis of diverse organic structures has been well known for over a century, it has only recently started gaining more attention and there is an increasing number of research articles emphasizing on the synthesis of biologically relevant organic molecules via this approach [3]. Such an attempt is presented in this review paper with an emphasis on organic molecules synthesized via MCRs and subjected to screening for biological activity. It was found that most of such screening was focused on anti-leihsmanial, anti-inflammatory, ROCK inhibitors, bromodomain inhibitors, antifibrotic agents, human toll-like receptor 8-active, neuroprotective agents,

Some Aspects of the Multicomponent Reactions
Reactions like Strecker (1850) [4], Hantzsch (1890) [5], Biginelli (1893) [6], Mannich (1912) [7], Passerini (1921) [8], Asinger (1956) [9] and Ugi (1959) [10], among others, are good classical examples of MCRs. In a MCR, the product is assembled according to a cascade of elementary two-component reactions. Thus, there is a network of reaction equilibria, which all finally flow into an irreversible step to afford the expected product. The use of MCRs in all areas of the applied chemistry are very popular because they offer a wealth of products, while requiring only a minimum of effort. As opposed to the classical way to synthesize complex molecules by sequential synthesis, MCRs allow the assembly of complex molecules in a one-pot manner. Unlike the usual stepwise formation of individual bonds in the target molecule via a multi-step synthetic approach, the defining attribute of MCRs is the inherent formation of several bonds in one operation without isolating the intermediates (referred to as the bond-forming efficiency, BFE) [1,11,12], nor changing the reaction conditions or adding further reagents.
Recently, chemists have renewed their interest in MCRs. This is driven in part, by the pharmaceutical industry due to the growing need to assemble libraries of small-molecules structurally complexes for evaluation as lead scaffolds in drug discovery and development programs. New libraries of such scaffolds are becoming more and more requested after as pathogens mutate to become resistant to current medications. In addition, anti-aging agents are needed for treatment of Alzheimer's, Parkinson's, diabetes and cancer, among other diseases [13]. Thus, MCRs represent an excellent tool for the generation of such libraries, which are indispensable for structure-activity relationship (SAR) studies in drug discovery programs.

Biologically Active Compounds Obtained from Multicomponent Approaches
As multicomponent reactions represent a powerful tool in the repertoire of sustainable organic synthesis its synergistic utilization with other green chemistry principles would bring organic chemists one-step closer to the ideal synthesis [14]. The rapid and efficient access to a plethora of heterocyclic building blocks through multicomponent reactions have been recognized by the synthetic community as a preferred strategy to design and synthesize biologically active compounds [15,16].
The wide variety of MCR procedures applied to the synthesis of relevant organic molecules clearly shows that MCR-based approaches are exceptionally useful for drug discovery and optimization processes in the pharmaceutical industry due to its high atom-economy, operational simplicity, time/cost efficiency and generation of structural diversity from multifunctional substrates [17,18]. In fact, the growing number of acyclic and heterocyclic building blocks on the market and in clinical evaluation discovered and synthesized by MCR approaches manifests their growing importance in medicinal chemistry and drug discovery programs. The corresponding biological activities displayed by the diverse organic compounds synthesized via MCRs approaches during the period 2002 to date will be discussed as follow.

Anti-Leihsmanial Activity
Leishmaniasis, a parasitic disease causes a major public health problem, which is prevalent in some tropical and sub-tropical areas of the world. One of its types, visceral leishmaniasis (VL), also known as kala-azar, is highly endemic in the Indian subcontinent and in East Africa. It is transmitted by the bite of infected female phlebotomine sandflies belonging to the genus Leishmania [19]. The existing chemotherapies are not effective enough as these have various drawbacks such as significant toxicity, variable efficacy, lack of oral bioavailability, and high cost involved during the treatment [20]. Thus, for the global health programs there has been a pressing need for the discovery of new lead compounds for the treatment of leishmaniasis [21]. In that direction, a series of structurally diverse α-aminophosphonates 4 were synthesized and evaluated for in vitro anti-leishmanial activity and cytotoxicity using the MTT assay (Scheme 1). Compounds 4 were prepared through a three-component reaction involving aldehydes/ketones 1, amines 2, and phosphites 3 via a Kabachnik-Fields type reaction under catalyst-and solvent-free reaction conditions at room temperature [22]. Several of the obtained compounds exhibited anti-leishmanial potency against the L. donovani promastigote with IC 50 values in the low micromolar range. The structure-activity relationships were quantitatively evaluated by a statistically reliable CoMFA model with high predictive abilities (r 2 pred = 0.87, r 2 ncv = 0.985) [23].

Anti-Inflammatory Activity
Bacterial infection and inflammation are strongly interrelated with each other. Pain and inflammation often arise due to bacterial infections [24]. Inflammation is nothing but a biological process that arises due to physical, chemical, biological and immunological stimuli to the human body [25], for that, inflammation is the significant indication in numerous pathological conditions such as Alzheimer's disease, osteoarthritis, rheumatoid arthritis and obesity-related diseases [26,27].
In this sense, a series of curcumin 3,4-dihydropyrimidinones/thiones/imines 7 were synthesized in 90-96% yield through a one-pot multicomponent cyclocondensation reaction between curcumin (5), substituted aromatic aldehydes 1 and urea/thiourea/guanidine 6 in the presence of chitosamine hydrochloride as a biodegradable and non-toxic catalyst under solvent-free microwave irradiation (MWI) (Scheme 2). All the synthesized curcumin derivatives 7 were screened for anti-inflammatory (but also for antioxidant) activities. The biological activity data of the synthesized compounds showed that most of them exhibited greater anti-inflammatory activity than curcumin [28].

ROCK Inhibitors
Rho-associated protein kinases (ROCKs) are ubiquitously expressed in most adult tissues, and are involved in modulating the cytoskeleton, protein synthesis and degradation pathways, synaptic function, and autophagy. Among the current limited number of ROCK inhibitors of clinical use, such as fasudil [31] and netarsudil [32], the synthesis and biological evaluation of a series of boronic acid-containing 3H-pyrazolo [4,3-f ]quinolones 17 as potential ROCK inhibitors have recently been reported [33]. The synthetic process involved a three-component Povarov type reaction between indazol-5-amines 15, methylene active ketones 16 and aldehydes 1 in the presence of catalytic amounts of HCl, affording the expected products 17 in 70-90% yield, as depicted in Scheme 5. After a SAR analysis of the obtained products 17, the biological trials indicated that compound labeled as HSD1590 (17p), resulted more potent than the reference drug netarsudil at binding to or inhibiting ROCK enzymatic activities. This compound exhibited single digit nanomolar binding to ROCK (Kds < 2 nM) and subnanomolar enzymatic inhibition profile (i.e., ROCK2 IC 50 was 0.5 nM for 17p while Netarsudil inhibited ROCK2 with IC 50 = 11 nM under similar conditions) [33].

Bromodomain Inhibitors
The inhibition of the bromodomain and extra-terminal (BET) domain subfamily of human bromodomains from chromatin [34], has contributed new insights into gene regulation and emerged as a promising therapeutic strategy in cancer. Structural analogy of early methyltriazolo BET inhibitors has prompted a need for structurally dissimilar ligands as bromodomain function probes. Using fluorous-tagged multicomponent reactions, a focused chemical library of bromodomain inhibitors 20 with micromolar biochemical IC 50 values was developed around a 3,5-dimethylisoxazole biasing element. Iterative synthesis and biochemical assessment allowed optimization of novel BET bromodomain inhibitors based on an imidazo[1,2-a]pyrazine scaffold. The synthesis of the target molecules 20 (in 11-58% yield), involved a three-component reaction between isocyanides 18, pyrazines/pyridines 2 and aldehydes 1 in the presence of Sc(OTf) 3 as catalyst, followed by a Suzuki-type coupling reaction with the boronic acid derivative 19 catalyzed by Pd(dppf)Cl 2 (Scheme 6). The lead compound 20c (R = tBu; R 1 = R 2 = H) binds BRD4 with a K d of 550 nM and 724 nM cellular potency in BRD4-dependent lines. Additionally, compound 20c showed potency against TAF1, a bromodomain-containing transcription factor previously unapproached by discovery chemistry [35]. Scheme 6. Elaboration of imidazopyridine scaffolds 20 as bromodomain inhibitors mediated by a three-component approach.

Antifibrotic Agents
Liver fibrosis is a critical wound healing response to chronic liver injury such as hepatitis C virus (HCV) infection. If persistent, liver fibrosis can lead to cirrhosis and hepatocellular carcinoma (HCC). The development of new therapies for preventing liver fibrosis and its progression to cancer associated with HCV infection remains a critical challenge [36]. Identification of novel anti-fibrotic compounds will provide opportunities for innovative therapeutic intervention of HCV-mediated liver fibrosis. In this sense, it was designed and synthesized a set of 5-arylthio-5H-chromenopyridines 22 as a new class of anti-fibrotic agents. Products 22 were synthesized in 16-45% yield through a pseudo-four-component reaction involving malonitrile (12, 2 mmol), thiophenols 21 (1 mmol) and 4-diethylaminosalicylaldehyde in the presence of triethylamine as catalyst (Scheme 7). Liver fibrosis assays demonstrated that compounds 22a (Ar = 4-FC 6 H 4 ) and 22c (Ar = 4-BrC 6 H 4 ) showed inhibitory activity towards human hepatic stellate cells (LX2) activation at 10 µM. The HCV NS3 and NS5A proteins in HCV subgenome-expressing cells were also significantly reduced in cells treated with 22a and 22c, suggesting the possible inhibitory role of the compounds in HCV translation/replication activities [37]. The reactivity of compounds 22 with medicinally-relevant metal compounds such as platinum and gold was also examined. The reactivity of these complexes with metals and during mass spectrometry suggested that C-S bond cleavage is relatively facile.

Human Toll-Like Receptor 8-Active Compounds
The innate immune system utilizes germline-encoded pattern recognition receptors (PRRs) to discern pathogen-associated molecular patterns (PAMPs) that are distinct to the pathogen [38]. The transmembrane PRRs include the toll-like receptors (TLRs) [39], which are expressed either on the plasma membrane or in the endolysosomal compartments [38]. At least ten functional TLRs are encoded in the human genome, each with an extracellular domain having leucine-rich repeats and a cytosolic domain called the toll/IL-1 receptor domain [40]. The discovery of TLRs has not only served to greatly accelerate the understanding of the interplay between the innate and adaptive immune systems, but is also catalyzing novel approaches to vaccine design and development. The ligands for these receptors are highly conserved microbial molecules [40,41]. For instance, it was proposed that imidazo[1,2-a]pyridine/pyrazines 23 (Scheme 8), could work as TLR7/8 ligands. Compounds 23, were obtained (in 13-86% yield) via a Groebke-Blackburn-Bienaymé type multicomponent reaction [42], along with the unplanned furo [2,3-c]pyridine/pyrazines 25 (in 10-84% yield), when pyridoxal (24) was used as aldehyde (Scheme 8) [43]. Both libraries of structures 23 and 25 were subjected to screening for TLR7/8 agonistic activities, as potential vaccine adjuvants. The biological assays showed that most of the obtained compounds 23 were inactive in NF-κB reporter gene assays specific for human TLR-3, -7, -8, and -9; however, most compounds 25 were found to specifically activate NF-κB signaling in TLR8-transfected HEK293 cells [43].

Neuroprotective Agents
Neurodegenerative disorders constitute a significant public health problem worldwide, and among these, cerebrovascular accidents represent one of the leading causes of death, neurological disability, and cognitive impairment [44]. During cerebral ischemia, oxygen and glucose deprivation induces a metabolic cascade that leads to neuronal death. One of the most significant consequences of these changes is the dysregulation of Ca 2+ homeostasis, leading to brain damage. In this context, a set of C 5 -unsubstituted-C 6 -aryl-1,4-dihydropyridines 28 were prepared by a CAN-catalyzed three-component reaction from chalcones 26, β-dicarbonyl compounds 27, and ammonium acetate in refluxing EtOH (Scheme 9). Compounds 28 were able to block Ca 2+ entry after a depolarizing stimulus and showed an improved Ca v 1.3/Ca v 1.2 selectivity in comparison with nifedipine. Furthermore, they were able to protect neuroblastoma cells against Ca 2+ overload and oxidative stress models. It is highlighted that the selectivity ratio of 28 makes them highly interesting for the treatment of neurological disorders where Ca 2+ dyshomeostasis and high levels of oxidative stress was demonstrated. Furthermore, their low potency toward the cardiovascular channel subtype makes them safer by reducing their probable side effects, in comparison to classical 1,4-dihydropyridines. Some of the obtained compounds 28 afforded good protective profile in a postincubation model that simulates the real clinical situation of ictus patients, offering a therapeutic window of opportunity of great interest for patient recovery after a brain ischemic episode. Good activities were also found in acute ischemia/reperfusion models of oxygen and glucose deprivation [45]. Scheme 9. Three-component synthesis of 5-unsubstituted dihydropyridines 28 with improved Ca v 1.3 selectivity as potential neuroprotective agents against ischemic injury.

Acetylcholinesterase Inhibitors
It is suggested that compounds that can inhibit cholinesterase enzyme may be considered as anti-Alzheimer, anti-Parkinson, and anti-autism drugs [46]. Due to the fact donepezil and other FDA approved drugs used for treatment of above diseases present some side effects [47], many efforts trying to find, develop, and explore more potent and permissive anti-Alzheimer drugs without any harmful side effects have been made so far. Among them, a borax-catalyzed protocol for the synthesis of a set of 4-aryl-substituted-4H-pyran derivatives fused to α-pyrone ring 30 in a one-pot procedure, as potential acetylcholinesterase inhibitors (AChEIs), was described. In this approach, products 30 were obtained in good to excellent yields, from a three-component reaction between aryl aldehydes 1, 4-hydroxy-6-methyl-2H-pyran-2-one (29) and malononitrile (12), in the presence of borax as catalyst and THF as solvent (Scheme 10). Subsequently, compounds 30 were evaluated in silico against acetylcholinesterase enzyme (AChE) and their Absorption, Distribution, Metabolism, Excretion and Toxicity (ADMETox) properties were also studied, to make the results more reliable and introduce them as remarkable potential candidates for inhibition of AChE, in the treatment of Alzheimer's, Parkinson's and autism diseases. Among the evaluated products, compound 30f (R = p-OCH 2 C 6 H 4 Br) showed the best activity against AChE [48].

Anti-HIV Activity
Reverse transcriptase (RT) is a key enzyme which plays an essential and multifunctional role in the replication of the human immunodeficiency virus (HIV) and thus represents an attractive target for the development of new drugs useful in AIDS therapy [49,50]. In view of the increasing incidence of resistance to current drug regimens and the frequency of adverse events, the development of novel, selective, potent, safe, inexpensive antiviral agents, that are also effective against mutant HIV strains, remains a high priority for medical research. In that direction, the design, synthesis, and the structure-activity relationship studies of a series of 2,3-diaryl-1,3-thiazolidin-4-ones 31 was performed. The synthesis of products 31 (in 8-87% yield), involved a three-component procedure reacting a suiTable 2,6-dihalo-substituted benzaldehydes 1 with an equimolar amount of a (hetero)aromatic amines 2 in the presence of an excess of mercaptoacetic acid 30 in refluxing toluene (Scheme 11). Some derivatives 31 proved to be highly effective in inhibiting HIV-1 replication at nanomolar concentrations with minimal cytotoxicity, thereby acting as nonnucleoside HIV-1 RT inhibitors (NNRTIs). Computational studies were used to delineate the ligand-RT interactions and to probe the binding of the ligands 31 to HIV-1 RT [51]. Scheme 11. Three-component synthesis of 2,3-diaryl-1,3-thiazolidin-4-ones 31 for structure-activity relationships, molecular modeling and studies as potent anti-HIV agents.

Antimicrobial Activity
An alarming increment in pathogenic resistance to existing drugs is a serious problem with antimicrobial therapy, indicating the necessity of continuing with the research for new classes and more effective of antimicrobials [52], possibly acting through mechanisms different from those of existing drugs [53]. In this context, it is very essential to successfully develop novel and efficient antimicrobial agents with clinically unexploited mode of action. As a contribution to this topic, Lakshmi et al., reported an InCl 3 -catalyzed three-component reaction for the synthesis of 3-pyranyl indole derivatives 33 as antimicrobial agents. The process was mediated by a tandem Knoevenagel-Michael reaction of 3-cyanoacetyl indole (32) diverse aromatic aldehydes 1 and malononitrile (12) catalyzed by InCl 3 in refluxing ethanol (Scheme 12). The antibacterial activity was screened by paper disc diffusion method against two Gram-positive bacteria (Staphylococcus aureus, Bacillus cereus), and two Gram-negative bacteria (Escherichia coli, Klebsiella pneumoniae) by using ciprofloxacin as reference compound. As shown in Scheme 12, compound 33e (R = 2-Cl) was found to exhibit the more potent in vitro antibacterial activity, with MIC values of 12.4, 16.4, 16.5, and 16.1 µM against S. aureus, B. cereus, E. coli, and K. pneumoniae, respectively. In addition, compounds 33a (R = H), 33d (R = 4-Cl), 33h (R = 4-F), and 33m (R = 4-OMe) exhibited significant antibacterial activity when compared to the standard drug ciprofloxacin [54]. A new class of pyrano[3,2-c]chromene derivatives 34 incorporating a validated molecular target was synthesized through a one-pot multicomponent cyclocondensation reaction between β-aryloxyquinoline-3-carbaldehydes 1, 4-hydroxycoumarins 10b and malononitrile (12) in ethanol containing a catalytic amount of piperidine, Scheme 13. Antibacterial activity was screened against three Gram-positive bacteria (Bacillus subtilis MTCC 441, Clostridium tetani, Streptococcus pneumoniae) and three Gram-negative bacteria (Escherichia coli, Salmonella typhi, Vibrio cholerae) by using ampicillin as a standard antibacterial drug. Remarkably, compounds 34f (R = Me, R 1 = Cl, R 2 = H), 34l (R = H, R 1 = Cl, R 2 = Me) and 34q (R = MeO, R 1 = Me, R 2 = Me) exhibited excellent in vitro antibacterial activity. The majority of compounds 34 were found to possess higher potency as compared to standard bactericidal ampicillin against Gram-positive bacteria B. subtilis [55]. containing substituted pyrazole moiety as potent antimicrobial, as well as, antioxidant agents [56]. The synthetic process involved a pseudo-four-component Hantzsch reaction between 3-aryl-1H-pyrazole-4-carbaldehydes 1, 1,3-dicarbonyl compounds 8 (ethyl acetoacetate and methyl acetoacetate) and ammonium acetate in ethanol under reflux conditions (Scheme 14). Antibacterial activity was screened against one Gram-positive bacteria (Staphylococcus aureus) and two Gram-negative bacteria (Escherichia coli, Pseudomonas aeruginosa) by using streptomycin as standard drug. The results indicated that among the tested compounds, 35c (R = Et, Ar = 4-MeSC 6 H 4 ) and 35f (R = Et, Ar = 4-ClC 6 H 4 ) showed excellent activity against all the tested microbial strains E. coli, S. aureus and P. aeruginosa at concentrations of 1, 0.5 and 0.25 mg/mL compared to the standard drug streptomycin. El-borai et al., reported an efficient protocol for the microwave-assisted synthesis of pyrazolo [3,4-b]pyridine derivatives 37 in good to excellent yields [57]. In this approach, products 37 were obtained from a three-component reaction between 5-amino-1-phenyl-3-(pyridin-3-yl)-1H-pyrazole type 2, pyruvic acid (36) and diverse aromatic aldehydes 1 using acetic acid as solvent under MWI at 160 • C for 20 min (Scheme 15). Subsequently, products 37 were screened for antibacterial, as well as, antifungal and antitumor activity. In particular, antibacterial activity was screened against one Gram-positive bacteria (Bacillus cereus) and three Gram-negative bacteria (Escherichia coli, Enterobacter cloaca and Serratia marcescens). Among the tested compounds, only 37a (R = MeO, R 1 = H), 37d (R = OH, R 1 = H), 37e (R = Br, R 1 = H) and 37f (R = R 1 = MeO) exhibited antibacterial activity of high order against all strains of the bacteria yeast tested. Shah et al., reported a small library of quinoline-pyridine hybrids 40 through a three-component reaction between a series of 2-chloro-3-formylquinolines 1, active methylene compounds 12/38 and 3-(pyridine-3-ylamino)cyclohex-2-enone (39) in the presence of catalytic amount of sodium hydroxide in ethanol under refluxing conditions (Scheme 16) [58]. This protocol afforded a time-efficient synthesis of the structurally diverse quinoline-pyridine hybrids 40 in good yields for antimicrobial, as well as, for antifungal and antitubercular screening. These products were screened for their antibacterial activity against three Gram-positive bacteria (Bacillus subtilis, Clostridium tetani, Streptococcus pneumoniae), and three Gram-negative bacteria (Escherichia coli, Salmonella typhi, Vibrio cholerae). Among the products tested only 40b (R = Me, R 1 = CN) and 40i (R = H, R 1 = CO 2 Me) showed better inhibitory effects for E. coli, and 40g (R = MeO, R 1 = CO 2 Et) showed better results for S. typhi compared to standard drugs such as ampicillin, chloramphenicol, ciprofloxacin and norfloxacin.

Scheme 16.
Three-component synthesis of quinoline-pyridine hybrids 40 as potential antibacterial agents.
Bhaskar et al., reported the synthesis of a series of spirooxindole derivatives 44 and 45 through a three-component 1,3-dipolar cycloaddition of an azomethine ylide generated in situ from sarcosine (41a) or L-proline (41b) and isatin 42 with the dipolarophile 1,4-naphthoquinone 43 followed by spontaneous air oxidation in atmospheric reflux conditions (Scheme 17) [59]. Subsequently, products 44 and 45 were screened for antibacterial (and for antifungal), activity against four Gram-positive bacteria (Staphylococcus aureus, methicillin-resistant Staphylococcus aureus, Enterobacter aerogenes, Micrococcus luteus), and four Gram-negative bacteria (Proteus vulgaris, Klebsiella pneumoniae, Salmonella typhimurium, and Salmonella paratyphi-B). Remarkably, compound 44n (R = COMe, R 1 = Me) was found to be more than 1.6 times active against methicillin-resistant S. aureus bacteria than streptomycin and ciprofloxacin. Also more than 6.4 times active against M. luteus and S. typhimurium bacteria than ciprofloxacin. In the course of a synthetic study toward other spiro-oxindole, Singh et al., reported an eco-friendly strategy for the synthesis of spiro-oxindole derivatives 46 in good yields and excellent stereoselectivities by a β-cyclodextrin-catalyzed one-pot multicomponent reaction from isatins 42, cyclic 1,3-diketones 10a and (thio)urea 6 in water under mild reaction conditions (Scheme 18) [60]. In this approach β-cyclodextrin not only formed an inclusion complex with isatin, but also was involved in intermolecular hydrogen bonding with the (thio)urea to promote the reaction. These products were screened for their antibacterial activity against one Gram-positive bacterium (Staphylococcus aureus), and one Gram-negative bacterium (Escherichia coli). Notably, compounds 46c (R = H, showed comparable antibacterial activity to the standard drug streptomycin. These results revealed that the presence of bromo-substituent and sulphur moiety in the synthesized compounds induced high potency, whereas the presence of methyl group decreased the effectiveness of the compounds. Contemporaneously, Darandale et al., proposed a green, practical and facile strategy for the synthesis of 1,2,3,6-tetrahydropyrimidine analogues as potent antimicrobial but also as antifungal agents [61]. Thus, the ZrOCl 2 -catalyzed pseudo-five-component reaction of substituted amines type 2, dialkyl acetylenedicarboxylates 47, and formaldehyde (1) in refluxing water furnished the target compounds 48 in good to excellent yields (Scheme 19). These compounds were screened for their antibacterial activity against two Gram-positive bacteria (Staphylococcus aureus, Bacillus subtilis), and one Gram-negative bacterium (Escherichia coli) by using ciprofloxacin and ampicillin as standard drugs. The results indicated that compounds 48a (R = H, X = C, R 1 = Et), 48b (R = 4-Cl, X = C, R 1 = Et), and 48e (R = H, X = N, R 1 = Et), all having the diethyl but-2-ynedioate functionality, were found to be most active and potent against the tested bacterial strains. They had MIC values (15-60 µM) compatible with standard drugs, except for bacterium S. aureus which showed MIC values between 60 and 100 µM. Lastly, to develop potent antibacterial agent, diethyl but-2-ynedioate was better choice than dimethyl but-2-ynedioate.
In 2017, an elegant microwave-assisted synthesis of pyrrolo [1,10]phenanthrolines 62 was achieved through a four-component reaction between 1,10-phenanthroline (61), aromatic aldehydes 1, malononitrile (12) and isocyanides 18, leading to the corresponding products 62 in excellent yields at 60 • C (Scheme 25) [68]. The benefits of MWI in terms of reaction times and efficiency were clearly demonstrated by a comparative study with thermal activation. Subsequently, compounds 62 were screened for antibacterial, as well as, for antitumoral and antifungal activity. These compounds were screened for their antibacterial activity against two Gram-positive bacteria (methicillin-resistant Staphylococcus aureus, Bacillus subtilis) by the agar diffusion method using erythromycin and oxacillin as standard drugs. Remarkably, compounds 62d (R = 4-Cl, R 1 = 4-MeOC 6 H 4 ) and 62l (R = 4-F, R 1 = 4-MeOC 6 H 4 ) exhibited the highest antibacterial activity with inhibition zones of 29 mm and 27 mm, respectively, against methicillin-resistant S. aureus bacteria.  (12), diverse cyclic CH-acids type 16 and ninhydrin (68) in refluxing ethanol to afford spiro-4H-pyran derivatives 69 in excellent yields (Scheme 28). The efficiency of this multicomponent reaction to access complex skeleton is once again remarkable [71]. Compounds 69 were screened for their antibacterial activity against one Gram-positive bacterium (Staphylococcus aureus) and one Gram-negative bacterium (Escherichia coli) by the disc diffusion method using tetracycline as standard antibiotic. Disc diffusion data showed inhibition zones of 4-15 mm for the synthesized compounds against S. aureus, except for 69a (from 16a), 69b (from 16b with R = CO 2 Me, R 1 = Ph) and 69c (from 16c), compared to standard tetracycline (30 mm). However, E. coli was resistant against all compounds tested. A one-pot three-component synthesis of 2,6-bis(1-coumarin-2-yl)-4-(4-substituted phenyl) pyridine derivatives 71 by a Chichibabin reaction was designed. These compounds were synthesized in 75-91% yield by the reaction of 3-acetyl coumarins 70 with substituted aromatic aldehydes 1 and ammonium acetate under acidic conditions (Scheme 29). The synthesized compounds 71 were evaluated for antimicrobial activity, as well as for DPPH free radical scavenging activity and ferrous ion-chelating ability. The mode of action of the active compounds was established by docking with the receptor GlcN6P synthase. The antimicrobial results revealed that compounds containing halogen or electron-withdrawing substituents either on the coumarin or phenyl rings exhibited potent antimicrobial and antioxidant activities. In the antibacterial studies, compounds bearing a bromo group in addition to a chloro group exhibited greater activity than those bearing only chloro groups. This observation suggested that di-substitution in the target compounds 71 by halogens enhanced the antimicrobial and antioxidant potential [72]. A convenient, one-pot, three-component protocol for the preparation of 2-(1-(2-oxo-2H-chromen-3-yl)ethylidene)hydrazinecarbothioamide derivatives 72 was achieved. Firstly, the synthesis of 3-acetyl-2H-chromen-2-one type 70 was carried out using starch sulfuric acid and cellulose sulfuric acid as biodegradable catalysts. Subsequently, the reaction of 70 with isothiocynates type 54 and hydrazine hydrate in the presence of catalytic amount of glacial acetic acid in refluxing ethanol afforded the target products 72 in 84-94% yield (Scheme 30). All synthesized compounds 72 were screened for antimicrobial activity. All compounds were found to show good to excellent activity against Escherichia coli MTCC 443 [73].

Antioxidant Activity
Compounds that exhibit antioxidant activity due to their chemical structure and redox properties have an important role in the uptake of singlet and triplet oxygen species, responsible for cardiovascular diseases, inflammatory bowel syndrome, cancer, aging, atherosclerosis and Alzheimer's disease [75]. Different areas converge on the need to synthesize antioxidant compounds as protection against these diseases. Thereby, Lakshmi et al., reported the three-component synthesis of the 3-pyranyl indoles 33 described previously in Scheme 12 [54]. The antioxidant activity of such compounds was determined using methods for antioxidant activity estimation such as DPPH and ABTS. Thus, compounds 33m (R = 4-MeO), 33n (R = 3,4-diMeO), 33o (R = 2-Br-4,5-diMeO) and 33p (R = 4-NMe 2 ) showed the most relevant radical scavenging activity in both methods due to the presence of electron-donating groups such as -OMe and -NMe 2 with an IC 50 < 50 µM in DPPH method. IC 50 values of 858.6 µM (33m), 867.2 µM (33n), 880.3 µM (33o) and 900.0 µM (33p) were observed using the ABTS method. Ascorbic acid was used as standard, giving an IC 50 < 50 µM in the DPPH method, and 650.0 µM in the ABTS method.
Very recently, a catalytic four-component synthesis of 1,3-cyclopentadiene derivatives was described by Ezzatzadeh et al. [78]. This unique and mild access to fully substituted 1,3-cyclopentadienes 84 involved terminal alkynes 82, sulfonyl azides 83, activated acetylenic compounds 47 and isocyanides 18 in the presence of zinc oxide nanoparticles (ZnO-NPs) and copper iodide as catalytic system in acetonitrile at room temperature (Scheme 35). This procedure offered several advantages such as being eco-friendly, ZnO-NPs could be re-used; the work-up was easy, non-toxic, and had a cleaner reaction profile. Antioxidant activity was investigated for the synthesized compounds 84a , and 84d (R = Pr, R 1 = Me, R 2 = Et, R 3 = tBu) using the DPPH radical trapping and comparing results with synthetic antioxidants (TBHQ and BHT). The DPPH scavenging power was achieved in the following order: TBHQ > BHT > 84a > 84c > 84d > 84b, respectively. The free radical scavenging power was enhanced from 200 to 1000 ppm concentrations. For example, concentration 1000 ppm of 84a had 30.2% inhibition, whereas 200 ppm of 84a showed 9.6% free radical inhibition.

Anti-Mycobacterial Activity
Emergence of resistance against new tuberculosis (TB) drugs is an alarming issue demanding new drug profiles. For that, designing drugs for the treatment of tuberculosis has been a challenging area in medicinal chemistry in view of the multi-drug resistance [79] and high mortality rate [80] associated with this disease. Among the approved anti-tubercular drugs, benzimidazole derivatives appear as privileged and promising structures in chemotherapy of tuberculosis [81][82][83]. As a contribution to this topic, Anand et al., reported a three-component reaction for the synthesis of 2-mercaptobenzimidazole linked coumarinyl triazoles 87 as anti-tubercular agents. The process is mediated by a Click type reaction between 2-propargylthiobenzimidazole (85), 4-bromomethyl coumarins/1-aza-coumarins 86 and sodium azide, in the presence of CuSO 4 /sodium ascorbate as catalytic system (Scheme 36). The anti-tubercular assays of the obtained compounds 87 against M. tuberculosis (H37Rv) coupled with in silico molecular docking studies indicated that dimethyl substituted products 87c (X = O, R = 5,7-diMe) and 87d (X = O, R = 7,8-diMe) showed promising activity with higher C-score values [84]. The products were evaluated against Mycobacterium tuberculosis H37Rv (MTB) and cytotoxicity. All the obtained compounds showed in vitro activity against MTB with MIC ranging from 0.12 to 41.2 µM. Thirteen compounds inhibited MTB with less than 1 µM. Six compounds were more potent compared than the standard first line drug INH (MIC of 0.36 µM) with MIC less than 0.36 µM and the compound N-(2-(4-(benzyloxy)phenyl)-4-oxo-1,3-thiazinan-3-yl)sonicotinamide inhibited MTB with MIC of 0.12 µM and was three times more potent than INH [85].
An efficient and green method was reported for the synthesis of fluorinated spiro-thiazine derivatives 92 and 93 via a three-component reaction, using 1-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF 6 ] as solvent and catalyst (Scheme 38). The synthesized compounds were subjected to antimycobacterial efficacy against Mycobacterium tuberculosis H37Rv strain and DNA cleavage activity. All compounds exhibited very poor antitubercular activities but DNA cleavage studies revealed that the tested compounds exhibited promising cleavage activity [86]. Twenty-four compounds were screened for in vitro anti-mycobacterial activity against Mycobacterium tuberculosis H37Rv. The compound dimethyl 1-(4-fluorophenyl)-4-(9-methyl-9H-carbazol-3-yl)-1H-pyrrole-2,3-dicarboxylate was the most active with MIC = 3.13 µM showing low cytotoxicity [88]. Moreover, derived of the 1,3-cyclohexadione type 10a containing methyl groups (i.e., R = Me) did not display activity. The molecular docking studies of compounds active against pantothenate synthetase revealed the favorable interactions with amino acid residues of such enzyme [89].

Quinoline Derivatives
Quinoline is one of the most important N-based heterocyclic compounds. Furthermore, quinoline derivatives are known to have a broad range of applications in medicinal, bioorganic, and industrial chemistry as well as in the field of synthetic organic chemistry [90,91]. Due to its importance, the synthesis of the quinoline scaffold has been studied for well over a century through the classical methods including the well-known Skraup, Doebner-Miller, Friedländer and Combes reactions, among others [92,93]. In 2006 Kouznetzov et al., reported the three-component cyclization of pyridinecarbaldehydes 1, anilines 2 and indene (100) in the presence of BF 3 ·OEt 2 as catalyst [94]. The process involved an imino Diels-Alder cycloaddition affording the corresponding indeno[2,1-c]quinolines 101 in 32−98% yield (Scheme 42). Nearly all of the obtained products 101 were active against breast (MCF-7), lung (H-460) and CNS (SF-268) human cancer cell lines. Regarding the results, the activity of products 101 seems not to be related to the electronic properties of the R substituent. In contrast, the position of N-atom in the pyridine ring appears to exert some influence on the activity. Thus, compounds possessing α-pyridine were less active than the βand γones. According to the reviewed literature, this report correspond to the first example in which is combined both a synthetic approach mediated by a multicomponent reaction, as well as, the biological evaluation of the obtained products. The synthesis of quinoline derivatives has also been carried out with homogeneous catalysts such as, piperidine (20 mol%) under mild reaction conditions. In 2014, Sangani et al., applied this chemistry to the synthesis of biquinoline-pyridine hybrids 104 [96]. The piperidine-catalyzed three-component reaction of tetrazolo Alternatively, the use of an asymmetric Povarov reaction allowed Alonso et al., to synthesize 1,2,3,4-tetrahydroquinolinylphosphanes 106 through a regio-and stereoselective aza-Diels-Alder reaction from aldehydes 1, styrenes 105 and phosphine oxide aniline 2a in the presence of BF 3 ·OEt 2 as Lewis acid and 4 Å molecular sieves in chloroform under reflux conditions (Scheme 45). The amount of BF 3 ·OEt 2 (3.0 equiv) and reaction times (48 h) are relatively high without any specific explanation as to why by the authors [97]. Furthermore, the multicomponent reaction also proceeded with an aniline containing the phosphine sulfide group type 2b to form 1,2,3,4-tetrahydro-quinolinylphosphine sulfides 107 in good yields using homogeneous acidic catalysis. The synthesized compounds were screened against three human cancer cell lines such as lung adenocarcinoma (A549), ovarian carcinoma (SKOV03) and embryonic kidney (HEK293). Notably, compound 106c (R = 4-FC 6 H 4 , R 1 = 4-F) with an IC 50 value of 0.08 µM showed excellent activity against the A549 cell line, while 1,2,3,4-tetrahydroquinolinylphosphine sulfide 107f (R = 4-FC 6 H 4 , R 1 = 4-F) with an IC 50 value of 0.03 µM was the most active against the A549 cell line.
In 2018, Castillo et al., described a catalyst-free method to construct diversely substituted 1,2,3,4-tetrahydroquinolines 110 through a Domino Mannich/Friedel-Crafts alkylation reaction of N-arylamines 108, paraformaldehyde (109) and electron-rich olefins 105 in ACN at room temperature [98]. This work showed that the choice of solvent was crucial and could greatly influence the reaction course: for example, γ-aminoethers were observed when methanol, ethanol, n-propanol or n-butanol were used as solvent [99]. These conditions enabled the construction of a broad library of 1,2,3,4-tetrahydroquinolines 110 in good to excellent yields via the formation of N-aryl-N-alkylmethyleneiminium ions as the key intermediates (Scheme 46). Interestingly, nine of the synthesized compounds were evaluated by the U.S. National Cancer Institute (NCI), where compound 110f (R = 6-MeO, R 1 = 4-ClC 6 H 4 , X = pyrrolidin-2-onyl) presented a remarkable activity against 57 cancer cell lines, with the most important GI 50 values ranging from 1.46 to 8.28 µM. Very recently, an Ugi four-component synthesis of quinoline-coumarin hybrids 115 was described by Taheri et al. [101]. This efficient and simple access to quinoline-coumarin derivatives involved coumarin-3-carboxylic acid (114), diverse 2-chloroquinoline-3-carbaldehydes 1, aniline derivatives 2 and aliphatic isocyanides 18 in methanol at room temperature (Scheme 48).

Pyrazole Derivatives
Pyrazole is a heteroaromatic compound of 5-membered containing two adjacent nitrogen atoms. The wide range of biological and synthetic applications displayed by pyrazoles, as well as, by their fused heterocyclic systems has been well documented in several comprehensive reviews [102][103][104][105][106]. In particular, formylpyrazoles occupy a noticeable place in the field of organic and medicinal chemistry despite somewhat being less popular than the amino derivatives, since such heteroaryl aldehydes are key intermediates for obtaining a wide range of biologically active compounds. Nikalje's group developed an efficient and green synthetic protocol to prepare 6-amino-4-substituted-3-methyl-2,4-dihydropyrano [2,3- 4 ] as a green reaction medium and also as a catalyst (Scheme 50). The fused heterocyclic products were isolated in high yields after short reaction times [108]. It is noteworthy that the catalyst was reused up to 4 times without much loss of catalytic activity. The in vitro anticancer activity of the synthesized compounds was carried out by the sulforhodamine B (

Curcumin Derivatives
Curcumin (diferuloylmethane or 1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione) is an edible natural pigment extracted from Curcuma longa [109]. The therapeutic activity of curcumin has been widely investigated over the last few decades and reports suggest the role of curcumin in innumerable biological activities, particularly its prominent in vitro and in vivo anticancer activity [109,110]. However, multiple structural-pharmacokinetic challenges such as low solubility and poor bioavailability during oral administration greatly limit its clinical application. Albeit, several strategies have been developed to overcome these disadvantages, one of the most common is the synthesis of novel curcumin derivatives that have better therapeutic properties and bioavailability [111,112]. From the viewpoint of the organic chemistry, curcumin exists in solution as a tautomeric keto form in acidic and neutral solutions while enol form in the alkaline medium. Furthermore, it has both an electrophilic Michael acceptor and an active methylene Michael donor units. In that direction, a series of structurally diverse 4-dihydropyrimidin-2(H)-one/thione derivatives of curcumin 7, as shown in Scheme 2, were synthesized through a three-component reaction involving curcumin (5), substituted aromatic aldehydes 1, and urea/thiourea 6 via a Biginelli type reaction in ethanol and concentrated sulphuric acid under reflux conditions [113]. Compounds 7b (Ar  (5), substituted aromatic aldehydes 1 and malononitrile/ethyl 2-cyanoacetate (12)/ (38) in ethanol at room temperature to afford functionalized curcumin derivatives 118 in 80−92% yield, Scheme 51. The construction of the cyclohexene ring proceeded through a Knoevenagel/Michael/cyclization sequence catalyzed by DABCO (10 mol%) [114]. Compounds 118j (R = 4-Me, R 1 = CO 2 Et) and 118k (R = 2-NO 2 , R 1 = CO 2 Et) were screened for their in vitro antitumor activity against human breast cancer cells (MCF-7) using the MTT assay. Compound 118j showed excellent activity with IC 50 value of 10.0 µM against human breast cancer cells (MCF-7). In addition, molecular docking studies allowed rationalizing the anti-apoptotic Bcl-2 binding of all synthesized compounds and revealed that the docking of compounds 118j and 118k with Bcl-2 was more potent compared to curcumin 5.

Pyrrole Derivatives
Pyrrole is one of the most relevant N-heterocyclic unit because of its presence in diverse natural and synthetic compounds with a broad range of applications in medicinal chemistry, drug discovery and materials science [115][116][117]. Although, several synthetic approaches for this scaffold have been developed, there remains a great need to find simpler and atom-economical approaches for the construction of functionalized pyrrole-fused derivatives, which have been considered of interest in view of their pharmacological importance [118][119][120]. In this sense, Magedov et al., employed a series of N-(aryland alkylsulfonamido)acetophenones 119 in combination with diverse aromatic aldehydes 1 and malonitrile (12) in a TEA-catalyzed three-component procedure in ethanol under reflux conditions to afford highly functionalized 2-pyrrolines 120 in good yields, high regioselectivity and short reaction times (Scheme 52) [121]. However, the poor diastereoselectivity generated a mixture of cisand trans-2-pyrrolines in 1:1.2 to 1:2 ratio. Rationalization of the low diastereoselectivity was not discussed. The synthesized compounds 120 were screened against two human cancer cell lines such as cervical adenocarcinoma (HeLa) and breast cancer (MCF-7). Because of the labor-intensive separation of the stereoisomeric pyrroline mixtures, the tests were performed with these mixtures. Remarkably, compound 120m (R = Ph, R 1 = 4-MeOC 6 H 5 , R 2 = Ph) showed the most potent inhibitory activity with GI 50 values of 36.0 µM and 50.0 µM against the HeLa and MCF-7 cell lines, respectively.
As originally described by Pagadala [122], highly functionalized pyrroles 122 are also available by means of a catalyst-free four-component reaction in aqueous medium. This green transformation involved an aromatic aldehyde 1, malononitrile (12), an isocyanide 18 and a cyclic secondary amine 121, allowing an efficient access to polysubstituted pyrroles 122 in good yields (Scheme 53). It is noteworthy that aromatic aldehydes 1 were successfully employed while aliphatic aldehydes such as acetaldehyde and n-butyraldehyde failed to provide the corresponding polysubstituted pyrrole. Easy accessibility of starting materials, short reaction times and catalyst-free reaction, as well as, the use of water as solvent are claimed as the key advantages of this procedure. The synthesized compounds 122 were screened for their antitumoral activity against breast (MCF-7) and colon (HT-29) human cancer cell lines using doxorubicin and cisplatin as reference standards. Notably, compound 122g (R = 2,4-diMeOC 6 H 3 , R 1 = 3,4-diClC 6

Chromone and Chromene Derivatives
The oxygen-containing heterocycles are an important class of compounds in organic chemistry. The fusion of an aromatic ring to an oxygen-containing heterocycle will alter the electron density; thereby, the physical, chemical and biological properties will change. In particular, chromone is an oxygen-containing heterocyclic system with a benzoannelated γ-pyrone ring being chromone (4H-chromen-4-one, 4H-1-benzopyran-4-one) the parent compound [123,124]. Furthermore, bicyclic oxygen-containing heterocycles resulting from fusion of benzene ring with 5,6-positions of either 2Hor 4H-pyran ring system are designated as 2H-chromene and 4H-chromene, respectively. Hence, the structural importance of the chromone and chromene moiety has elicited a great deal of interest in the field of organic synthesis, medicinal chemistry and drug discovery to develop novel and improved synthesis of these molecular skeletons [125][126][127]. In this sense, Huang et al., described a catalyst-free access to dithiocarbamate substituted chromones 124 by means of a three-component reaction (Scheme 54) [128]. A mixture of 3-chloromethyl chromone derivatives 123, cyclic secondary amines 121 and an excess of carbon disulfide was stirred at room temperature in DMF affording compounds 124 in good yields. The synthesized compounds were screened for their in vitro antiproliferative activity against six cancer cell lines, including HCCLM-7 (hepatocellular carcinoma cell), He-La (cervical carcinoma cell), MDA-MB-435S (mammary adenocarcinoma cell), SW-480 (colon carcinoma cell), Hep-2 (laryngocarcinoma cell) and MCF-7 (mammary adenocarcinoma cell) by using the MTT method. It should be noted that compound 124u (R = 6-Cl, R 1 = piperidinyl) was identified as the most promising candidate due to their high potency against all cancer cell lines with IC 50 values ranging from 0.24 to 0.85 µM. Further flow-activated cell sorting analysis revealed that compound 124u arrest the cell cycle of MDA-MB-435S and SW-480 both in G 2 /M phase with dose-dependent effect and might display apoptosis-inducing effect on these tumor cell lines.  [129]. Great functional diversity was achieved with this thoughtful strategy that accommodated aromatic, heteroaromatic, and aliphatic aldehydes. The advantages of this method include the use of a recyclable catalyst, short reaction times, simple work-up procedure and easy isolation. From a mechanistic standpoint, a sequence initiated by the formation of an ortho-quinonemethide through the nucleophilic addition of 2-naphthol to the aldehyde catalyzed by CAN. Then, Michael addition of malononitrile or ethyl cyanoacetate onto the ortho-quinonemethide, followed by intramolecular 6-exo-dig cyclization/imine-enamine tautomerization sequence generated the benzochromene ring. The synthesized compounds 125 were screened for their antiproliferative activity in prostate cancer (DU-145), breast cancer (MCF-7), cervical carcinoma (C-33A) and lung carcinoma (A-549) human cell lines. It is noteworthy that compound 125b (R = indol-3-yl, R 1 = CN) exhibited the highest activity against all cancer cell lines with IC 50 values ranging from 5.4 to 12.2 µM. An alternative access to chromenyl phosphonates from dialkylphosphites was reported in 2014 [131]. Thus, an ethanolic solution of a substituted salicylaldehyde type 1, a dialkylphosphite type 3 and malononitrile (12) was stirred at room temperature in the presence of a catalytic amount of dibutylamine, afforded the 2-amino-3-cyano-4H-chromen-4-ylphosphonates 127 in very good yields, (Scheme 57). 3.13.6. Pyridone Derivatives 2-Pyridone, the tautomer of 2-hydroxypyridine, is one of the privileged heteroaromatic rings in natural products, bioactive molecules and pharmaceutical agents [133,134]. Furthermore, pyridone fused with a benzene or heterocyclic ring gives rise to diverse heterocyclic systems with innumerable pharmacological properties. Thus, the selective synthesis of functionalized 2-pyridone derivatives and fused-pyridone heterocycles has been one of the important longstanding subjects in organic synthetic chemistry [135][136][137]. In that direction, a series of highly functionalized pyrano[3,2-c]pyridones 130 were synthesized through a TEA-catalyzed three-component reaction of 4-hydroxy-1,6-dimethylpyridin-2(1H)-one (129), an aromatic aldehyde 1 and malononitrile (12) in ethanol under reflux conditions (Scheme 59) [138]. Annexin-V staining and DNA laddering assays.

Thiazole Derivatives
S-Heterocycles have maintained their status as an important nucleus with high medicinal value and low toxicity profile, in comparison to previous N-heterocycles. Particularly, thiazole has been widely found in diverse pharmacologically active substances and some naturally-occurring compounds [141]. For that reason, thiazole is a versatile building-block for the preparation of thiazole-based compounds with innumerable biological properties [142]. Thus, thiazole derivatives possessing anticancer activity will reignite the interest of the scientific community in the usefulness of these S-heterocycles in the medicinal chemistry and drug discovery [143,144]. In this context, tetronic acid type 27 was used as the 1,3-dicarbonyl parent in a straightforward construction of the 4-aza-podophyllotoxin skeleton embedded with a thiazole unit [145]. Thus, tetronic acid (27)  Combining two potentially bioactive moieties to form heterocyclic scaffolds is a known process in drug discovery. In 2012, Gu et al., reported an impressive three-component reaction of 3-nitro-2-bromopyridine 136, primary 1-aminophosphonates 3 and carbon disulfide in the presence of copper(II) chloride (1 equiv.), tin(II) chloride (4 equiv.) and potassium carbonate (3 equiv.) at 100 • C in DMF, leading to α-aminophosphonates 137 containing thiazole [5,4-b]pyridine moiety with yields up to 85% (Scheme 63) [146]. According to the authors, SnCl 2 ·2H 2 O enabled reduction of the nitro group to amine, while CuCl 2 ·2H 2 O promoted C-S bond cross-coupling reaction between dithiocarbamate salts and 3-amino-2-bromopyridine. This unique transformation featured multiple bonds breaking and forming events in a single, atom-economic process. The antitumor activity of the obtained compounds 137 was determined by the MTT assay against three human cancer cell lines such as PC-3, Bcap-37 and H460. Notably, compound 137f (R = Et, Ar = 4-FC 6  Alternatively, an eco-friendly access to thiadiazolo[3,2-a]pyrimidine-6-carbonitriles from a 1,3,4-thiadiazol-2-amine was reported in 2016 [148]. An ethanolic solution of 5-(4-chlorophenyl)-1,3,4-thiadiazol-2-amine 2, an aromatic aldehyde 1 and malononitrile (12) was sonicated at 80 • C in the presence of catalytic amounts of NaOH, affording the thiadiazolo In 2017, Semenov's group developed similar reactions using Meldrum's acid (133) as the 1,3-dicarbonyl partner [149]. Since 3-arylisothiazol-4-aminium chloride 2·HCl has been rarely exploited as a nucleophile in multicomponent processes, this substrate was reacted by refluxing glacial acetic acid with an aromatic aldehyde 1 and Meldrum's acid (133)  Indoleamine 2,3-dioxygenase 1 (IDO1) has emerged as an attractive target for cancer immunotherapy. In this context, the Passerini reaction was employed to assemble a small library of imidazothiazoles 143 that target IDO1 [151]. The reaction of isocyanides 18 with aqueous formaldehyde (1) and functionalized phenylacetic acids 114 led to imidazothiazoles 143 containing a α-acyloxyamide moiety in the side chain, in moderate to good yields (Scheme 68). Notably, compound 143d (R = 4-hydroxybenzyl) showed an IC 50 value of 0.20 µM in the IDO1-based assay, a full biocompatibility at 10 µM, together with a modest inhibitory activity in A375 cells. Furthermore, molecular docking studies showed that 143d displayed a unique binding mode in the indoleamine 2,3-dioxygenase 1 (IDO1) active site, with the side-chain protruding in an additional pocket C, where a crucial hydrogen bond is formed with Lys238.

Indole-Based Anticancer Heterocyclic Systems
Interest in synthesizing new indole derivatives continue due to their biological properties displayed [152]. In particular, substituted indole derivatives play a key role in the synthesis of biologically active compounds especially with anticancer, antitumor, and anti-inflammatory activities [153][154][155]. They are also reported as potent inducers of apoptosis through a cell-based HTS caspase assay [156]. Based on previous molecular hybridization techniques [158], Gupta et al., designed a series of indole-chalcone based benzopyran hybrid compounds 145 in order to identify molecules that can inhibit DNA ligation and cell proliferation, and may serve as drug-like molecules. Products 145 were obtained in 70-78% yield, via a three-component reaction between substituted 2-methylindoles 32, chalcones 26 and malononitrile (12) in the presence of L-proline as catalyst and acetonitrile as solvent, Scheme 70. The synthetic molecules 145 were tested for their antiligase and antiproliferative activities in cancer cells. A detailed study of the most active compound 145a (R = H, R 1 = tBu, R 2 = C 4 H 3 S) was carried out in order to verify the mode of action and cytotoxicity in in vitro and 3D tumor models [159].

Carbazole (Benzo[b]Indole)-Substituted Heterocyclic Derivatives
Carbazole is a benzo[b]indole in which the benzene ring is fused with the 2,3-position of the indole ring. This motif is a privileged pharmacophore scaffold found in many biologically active compounds of diverse origins, from natural products to synthetic sources. Widespread interests of chemists have been attracted to these structures due to their biological activities and potential applications as pharmacological agents [160].
Thus Synthesis of highly functionalized pyrrolo[3,2-a]carbazoles 151 via ring contraction through an aerobic metal free rearrangement and intramolecular Michael addition reaction using one-pot three-component reaction was reported.
Thus, a mixture of (tetrahydro-carbazol-1'-ylidene)-propanedinitriles type 146, ninhydrin (150) and o-phenylenediamines 111 were subjected to reflux in methanol as the solvent and catalytic amount of TEA to afford the target products 151 in 67-90% yield (Scheme 73). The obtained compounds were studied for colorectal cancer activity, as well as, free radical scavenging in vitro via MTT and DPPH assays, respectively. Further, the structure-activity relationships were also carried out [163].

Pyrimidine-Based Anticancer Heterocyclic Systems
The great interest in pyrimidine and dihydropyrimidine (DHPM) derivatives lies in the fact that these classes of compounds has, in principle, pronounced biological activity [164,165]. Consequently, diverse methodologies have been developed to improve the synthesis of this attractive family of compounds [166].

Biginelli-Mediated Synthesis of Dihydropyrimidines
The Biginelli reaction, is an MCR that involves the cyclocondensation of acetoacetic esters, aromatic aldehydes and (thio)urea [167]. The products of this three-component synthesis are identified as 3,4-dihydropyrimidin-2(1H)-ones (DHPMs). Thus, a library of Biginelli adducts 152 was synthesized in 31-92% yield, as described in Table 1, and evaluated as potential inhibitors of in vitro cancer cells proliferation, but also, as scavengers of reactive nitrogen and oxygen species (RNS and ROS, respectively). The capacity of all compounds 152 to inhibit cancer cells growth was dependent on the histological origin of cells, except for 152p (X = S, R = MeSC 6 H 4 ), which was highly active against all cell lines. Compounds 152k (X = O, R = 4-HO-3-MeOC 6 H 3 ), and 152x (X = S, R = C 6 H 11 ), were as potent as the reference drug doxorubicin against adriamycin-resistant ovarian and prostate cancer cells, respectively [168].
Yadlapalli et al., reported the synthesis of a series of dihydropyrimidine derivatives 153 in 46-93% yield via a Biginelli reaction, as described in Table 1. The obtained compounds were evaluated for their in vitro anticancer activity against MCF-7 human breast cancer (HBC) cell line using sulforhodamine B (SRB) assay, but also, for their antitubercular activity against Mycobacterium tuberculosis (MTB) H37Rv using the Microplate Alamar Blue Assay (MABA). Interestingly, compounds 153p (X = S, R = OEt, R 1 = Cl) and 153t (X = S, R = 4-MeC 6 H 4 NH, R 1 = Cl) exhibited 70.6% and 63.7% of HBC cell growth inhibition, respectively, at 10 µM concentration. Compound 153p was also found to be the most potent in the series against MTB H37Rv with MIC value of 0.125 µM [169].
The synthesis, characterization, and application of a reusable ion-tagged iron catalyst was described. The catalyst was employed in the Biginelli reaction with impressive performance. High yields (42-99%) of DHPMs 154 were achieved when the reaction was carried out in imidazolium-based ionic liquids (ILs) (BMI·PF 6 , BMI·NTf 2 , and BMI·BF 4 ) ( Table 1), thus showing that the IL effects play a role in the reaction. Further, the cytotoxicity of the obtained compounds 154 was evaluated against MCF-7 cancer cell linages with encouraging results of some derivatives, which were virtually non-toxic against healthy cell linages (fibroblasts) [170].  On the other hand, two coordination polymers (CPs) were synthesized, characterized and successfully applied as robust heterogeneous catalysts for the Biginelli multicomponent reaction to obtain 3,4-dihydropyrimidin-2(1H)-one/thione (DHPMs) derivatives 155 in 80-99% yield ( Table 1). The reaction was initially developed using both CPs and the Zn-based material showed much better catalytic activity. After the reaction optimization under batch conditions, a continuous flow protocol was developed and applied with impressive results. The mechanism of the transformation was also investigated by electrospray (tandem) mass spectrometry (ESI-MS(/MS)) analyses. Nine of the obtained DHPMs 155 had their antitumoral activities evaluated against MCF-7 (human breast cancer cells), A549 (human alveolar basal epithelial cells) and Caco-2 (human epithelial colorectal cells) cancer cell linages.

Fibroblasts (healthy cells)
were not affected by the tested DHPMs showing an excellent selectivity for tumor cells. Three DHPMs returned impressive results, being capable of inhibiting tumor cell proliferation in 72 h [171].
An additional series of DHPMs 156 was synthesized in 32-93% yield through a bismuth(III) catalyzed Biginelli reaction, Table 1, and their in vitro antiproliferative activity was evaluated in different human cell lines. A quantitative structure-activity relationship (QSAR) analysis was performed using Bayesian regularized artificial neural networks to model the relationships between in silico molecular descriptors and the observed antiproliferative activity of molecules across the tested cell lines. Among the compounds prepared, the molecules containing chloro atoms in their structure demonstrated a relevant potency and a selective antiproliferative activity against a Hepatic cancer cell line (HepaRG) without exhibiting noticeable cytotoxicity in normal dermal cells (NHDF). In prostatic (LNCaP), colon (Caco-2) and Breast (T47D and MCF-7) cancer cell lines generally compounds 156 did not exhibit relevant cytoxicity. A statistically valid QSAR model was obtained (internal validation Q 2 = 0.663, RMSE CV = 0.071, 10-fold cross-validation procedure, and external validation R 2 pred = 0.740, RMSE = 0.077), which allowed the analysis of the involved relationships between molecular descriptors and the reliable prediction of the antiproliferative activity for hypothetical related compounds in the studied cell lines. Flow cytometry analysis showed that in HepaRG and MCF-7 cell lines, compound 156p (R = MeO, Ar = 2,4-diCl 2 C 6 H 3 ) did not decrease cell viability but, led to an accumulation of cells in G 0 /G 1 phase of the cell cycle [172].

Dihydropyridine-Based Anticancer Heterocyclic Systems
Dihydropyridine derivatives (DHPs) are recognized because of their potential antioxidant, antituberculosis, analgesic, antimicrobial, and antitumor activity [173,174]. Particularly, there is much interest in the anticancer activity of these compounds owing to different types of biological targets they might interfere with for this effect to occur (e.g., PDE3, PIM1 Kinase, and Survivin protein) [175].
On the other hand, a series of symmetrical DHPs 159 and 160 were synthesized in 87-95% and 38-55% yield, respectively, through a rapid, four-component MWI-based protocol ( Table 2). Compounds 159/160 were evaluated for their tumor cell cytotoxicity against HL-60 tumor cells. A 3D-QSAR study using CoMFA and CoMSIA was carried out to decipher the factors governing MDR reversing ability in cancer. The resulting contour maps derived by the best 3D-QSAR models provided a good insight into the molecular features relevant to the biological activity in this series of analogs. 3D contour maps as a result of 3D-QSAR were utilized to identify some novel features that can be incorporated into the DHP framework to enhance the activity [177].
A sequence of DHP analogues 161 was also synthesized in 79-93% yield by Kumari et al., through a tetracomponent green synthetic method mediated by Montmorillonite-K10 (Table 2). Besides, promoter reusability, easy handling of the chemical reagent, simple reaction process, time minimization, ethanol-water solvent compatibility, and cost reduction reagent were key tools for this fruitful path. In addition, all compounds 161 were evaluated for their cytotoxic activities against three human cancer cell lines and mouse melanoma and figured out the most active compounds. Thus, these examinations recommended that DHPs and their derivatives are motivating moieties for the discovery of new anticancer drugs [178].

Fused Dihydroquinoline-Based Anticancer Heterocyclic Systems
Dihydroquinolines (DHQs) and their fused analogues are heterocyclic scaffolds that are ubiquitous in natural products, therapeutics, fluorophores and dyes [179]. They are structures of great versatility, and their physical and chemical properties can be finely tuned using synthetic chemistry [180].
Thus  Table 3). The method not only provided a valuable tool in design and synthesis of the fused systems 162 but also had the advantages of atom-economy, environmental-friendliness, good yields and operational simplicity. Additionally, the preliminary evaluation on the cytotoxic activity of this type of compounds resulted in the finding of several structures with potent and efficacious cytotoxicity to three carcinoma cell lines M14, MCF7 and SW1116 [145]. Another series of para-naphthoquinone 163 embodied 4-aza-podophyllotoxin hybrids, designed via molecular hybridization approach, were synthesized in 7-99% yield using a one-pot three-component condensation of 2-hydroxy-1,4-naphthoquinone (94), aldehydes 1 and 3,4-methylene-dioxyaniline (2) in the presence of L-proline as catalyst ( Table 3). The synthetic derivatives 163 were evaluated for their antitumor activity on human hepatoma cells (HepG2) and Henrietta Lacks strain of cancer cells (Hela). Among the eighteen compounds screened, 163o (R = 3,4,5-triMeOC 6 H 2 ) has pronounced activity. The results demonstrated potential importance of molecular hybridization in the development of 163o as potential antitumor agent [181].
Disruption of F-actin cytoskeleton structure and cell migration inhibition in DU145 cells indicate that the tumor progression and metastasis are affected by compound 164l. Cell cycle analysis revealed that it arrests the cells in G2/M phase. Acridine orange/ethidium bromide (AO/EB) staining, Hoechst staining and annexin-V binding assays showed that cell proliferation is inhibited through induction of apoptosis [182].
Additionally, an expeditious microwave-assisted one-pot three-component synthesis of new cytotoxic phenanthrene fused-tetrahydrodibenzo-acridinones 165 was successfully accomplished in 89-95% yield. This protocol offers wide substrate scope, catalyst-free synthesis, atom-economy, simple recrystallization, high yields, and ethanol was used as green solvent ( Table 3). The obtained compounds 165 were tested for their in vitro cytotoxicity against cervical (HeLa), prostate (PC-3), fibrosarcoma (HT-1080), ovarian (SKOV-3) cancer cells, and were safer to normal (Hek-293T) kidney cell line. All the compounds displayed significant cytotoxicity profile, among them 165m (R = H, Ar = 3-CF 3 C 6 H 4 ) being the most potent compound with an IC 50 0.24 µM against SKOV-3 ovarian cancer cells. Flow cytometry analysis revealed that cells were blocked at the G2/M phase of the cell cycle. The effect of 165m on F-actin polymerisation was also studied. Hoechst staining showed the decreased number of viable cells and indicated apoptosis progression. Compound 165m caused the collapse of mitochondrial membrane potential as observed via JC-1 staining and also enhanced the generation of reactive oxygen species. The increase of caspase-3 activation by 3.7 folds supported the strong apoptosis induction. In addition, an in vitro 3D-spheroid progression assay was performed with 165m that significantly suppressed the tumor cells [183].

Purine-Like Pyrrolo[2,3-d]Pyrimidine Systems of Anticancer Interest
Several purine-derivatives have been considered as important and effective drugs used in cancer chemotherapy, for immunosuppression in kidney or heart transplantation and autoimmune diseases [184,185]. Various pharmacological effects of such compounds including antiviral, antibacterial, antitumor, and antifungal activity were developed for treatment of patients suffering different illness [186]. For this reason, many thiopurine derivatives and analogs have been synthesized for evaluation of their biological activities and reduced toxicity.
In that direction a synthetic approach to access the marine alkaloid rigidins and over forty synthetic analogues based on the 7-deazaadenine (166), 7-deazapurine (167) and 7-deaza-hypoxanthine skeletons 168 and 169 was developed (Table 4). Analogues based on the 7-deaza-hypoxanthine skeleton 169 exhibited nanomolar potencies against cell lines representing cancers with dismal prognoses, tumor metastases and multidrug resistant cells. Studies aimed at elucidating the modes of action of compounds 169 in cancer cells revealed that they inhibited in vitro tubulin polymerization and disorganized microtubules in live HeLa cells. Experiments evaluating the effects of compounds 169 on the binding of [ 3 H]colchicine to tubulin identified the colchicine site on tubulin as the most likely target for these compounds in cancer cells [187].

Coumarin Derivatives
Coumarins are one of the largest classes of naturally occurring compounds and are an essential component in the pharmaceutical, cosmetics and perfumes industry. Moreover, coumarins are considered as a privileged scaffold in the design of compounds with several biological targets. Coumarin derivatives displayed a wide range of pharmacological activities like anti-cancer [188], among others [189][190][191][192][193].
A simple and efficient one-pot three-component method for the synthesis of a series thiazolyl-coumarin hybrids 175 was reported by Kavitha et al. [197]. The  3.13.14. Spiro Derivatives Spiro compounds are considered privileged structures and often show interesting biological activity. They are found in a number of natural and synthetic compounds exhibiting wide range of activities against a variety of disease areas, such as anticancer [202], among others [203][204][205][206][207]. Multicomponent reaction (MCR) is a powerful tool for get new spiro-derivates with potential biological activity. In this sense, Arun et al., synthesized a series of dispirooxindole-pyrrolidine derivatives 182 and 184 through a one-pot tandem/domino approach via a 1,3-dipolar cycloaddition reaction. This reaction was performed by heating an equimolar mixture of substituted isatins 42, sarcosine (41a), 3-(1H-indol-3-yl)-3-oxo-2-(2-oxoindolin-3-ylidene)propanenitrile (181)   Hui et al., published a direct route to prepare biologically relevant spirooxindole-pyrrolidine, pyrrolizidines and pyrrolothiazoles 188 by a one-pot, multicomponent 1,3-dipolar cycloaddition reaction via azomethine ylides from isatin type 42 and an amino acid derivative 114 (Scheme 78). The advantages of this protocol included, high yields, simple work-up procedure and regio-and diastereo-selectivities. Compounds 188 were evaluated for their antiproliferative activity against various cancer cell lines. Although all compounds showed good cytotoxicity activity against the tested cell lines, studies showed that compound 188j (R = R 1 = H, R 3 = Ph) displayed the highest inhibitory activity against HCT116 (colon cancer) and HepG2 (hepatocellular carcinoma) at a concentration of 10 µM [212].
In other study, a series of spirochromenocarbazoles 197 was synthesized via a click chemistry-based one-pot, five-component reaction between N-propargyl-isatins 42, alkyl/arylalkyl halides 196, 4-hydroxycarbazole (148), malononitrile (12) and sodium azide in a mixture DMF/H 2 O at 70 0 C using cellulose supported CuI nanoparticles (Cell-CuI NPs) as the heterogeneous catalyst, Scheme 80. In 2016, a three-component 1,3-dipolar cycloaddition reaction of pyrimidine-fused 3-alkenyloxindole 198 with azomethine ylides (thermally generated in situ from sarcosine (41a) and polyformaldehyde) for the synthesis of a series of pyrimidine-fused spiropyrrolidine oxindoles 199 was investigated by Liu et al. The products were obtained in high yields (up to 90% yield) with good diastereoselectivity (up to > 20:1) (Scheme 81). The anticancer activity was screened by MTT assays against lung cancer cells A549, human prostate cancer cells PC-3, and human leukemia cells K562 by using cisplatin as a positive control. Some spiropyrrolidine oxindoles 199 showed GI 50 ranging from 8.9 µM to 23.2 µM in vitro inhibitory activity against human leukemia cells K562, and exhibited equipotent or more potent activity than the positive control cisplatin (up to 3.0 times) [219]. The obtained compounds were screened as HDAC inhibitors allowing to identify some of these compounds with promising biological activity. Moreover, in order to rationalize the biological results, computational studies were also performed [221]. On the basis of structures of known topoisomerase II catalytic inhibitors and initial molecular docking studies, bicyclic N-fused aminoimidazoles 210-214 were predicted by Baviskar et al., as potential topoisomerase II inhibitors. These compounds were synthesized in high yields by a three-component reaction, as described in Scheme 84, and evaluated against human topoisomerase IIα (hTopoIIα) in decatenation, relaxation, cleavage complex, and DNA intercalation in vitro assays. Several of the obtained compounds exhibited potent inhibition of catalytic activity of hTopoIIα while not showing DNA intercalation. Molecular docking studies and molecular dynamics (MD) simulation analysis, ATPase-kinetics and ATP-dependent plasmid relaxation assay revealed the catalytic mode of inhibition of the obtained compounds plausibly by blocking the ATP-binding site. Some of the obtained compounds also showed potent anticancer activities in kidney and breast cancer cell lines, low toxicity to normal cells, relatively higher potency compared to etoposide and 5-fluorouracil in kidney cancer cell lines, and potent inhibition in cell migration. These compounds were found to exert apoptotic effect in G1/S phase [222]. A series of 2,3-tri-and tetrasubstituted γ-butyrolactone 217 analogous to paraconic acids were synthesized in 14-99% yield, through a one-step and straightforward three-component reaction between carbonyl compounds type 1/16, dimethyl itaconate (215)  This strategy led to a green and facile access to the above compounds with prominent features of high structural diversity, short reaction times, high yields and environmental friendliness. Further, the obtained compounds were subjected to the test of their in vitro cytotoxic, as well as, antioxidant activities, resulting in the finding that these compounds not only displayed significant antioxidant activity, but also exhibited remarkably selective cytotoxicity to carcinoma cell line HCT-116 [224].
The synthesized compounds 231 were found to be potential inhibitors to cathepsins B, H and L. The extent of inhibition varied with the substitution. Among the synthesized compounds, derivative 231d (Ar = 4-NO 2 C 6 H 4 ) was selected as most inhibitory to cathepsin H, however compound 231f (Ar = 4-FC 6 H 4 ) was the best inhibitor of cathepsin B and cathepsin L. In vitro inhibition studies correlated well when tested using MTT assay on HepG2 cells, a hepatocellular carcinoma cell line. The results validated by in silico studies performed with iGemDock predicted that among the synthesized compounds, 231d experienced the highest affinity for cathepsin B and H sites, whereas 231f had the highest affinity to cathepsin L [228].
In the current research scenario, an efficient synthesis of tetrazole scaffolds 232 in 93-96% yield was developed by a single step four-component reaction. The synthesis of the target compounds 232 was undertaken by the Ugi multicomponent approach with the condensation of various aryl amines 2, TMS-N 3 , cyclohexyl isocyanide 18 and aromatic aldehyde containing active pharmacophore 1, under catalyst-free reaction condition at room temperature (Scheme 91). The potency of the obtained tetrazoles 232 was checked at the NIH using sixty different cell-lines with respect to nine cancer panels, among which compounds 232a (Ar = 2,5-diMeC 6 H 3 ) and 232b (Ar = 4-FC 6 H 4 ) displayed the higher activity against different cell lines [229]. Scheme 91. Ugi-Mediated multicomponent synthesis of tetrazole scaffolds 232 to be evaluated by the NCI for their in vitro cytotoxicity against sixty different cancer cell lines.
A series of benzimidazole-isoquinolinone derivatives (BIDs) 233 was synthesized in 52-82% yield. The process was conducted by an Ugi four-component reaction (U-4CR) using an amine 2, carboxylic acid 114, isonitrile 18, and the methyl 2-formylbenzoate type 1 in methanol at room temperature (Scheme 92). Subsequently, the solvent was removed under a stream of nitrogen, and the intermediate crude Ugi product obtained was deprotected and cyclized in situ under MWI using 10% TFA/DCE at 150 • C. Further, the target products 233 were screened to identify novel scaffolds for Colorectal cancer CRC. Among the compounds evaluated, 233g (R = iBu, R 1 = 3-Py) exhibited the most promising anti-cancer properties. Employing two CRC cell lines, SW620 and HT29, 233g was found to suppress growth and proliferation of the cell lines at a concentration of ∼20 µM. Treatment followed an increase in G2/M cell cycle arrest, which was attributed to cyclin B1 and cyclin-dependent kinase 1 (CDK1) signaling deficiencies with simultaneous enhancement in p21 and p53 activity. In addition, mitochondrial mediated apoptosis was induced in CRC cells. Interestingly, 233g decreased phosphorylated AKT, mTOR and 4E-BP1 levels, while promoting the expression/stability of PTEN [230]. Scheme 92. Ugi-Mediated multicomponent synthesis of benzimidazole-isoquinolinone derivatives (BIDs) 233 for screening to identify novel scaffolds for Colorectal cancer CRC.
Series of diverse thiophene, pyran, azole and azine derivatives 234-242 in 63-89% yield were synthesized from cyclopentanone type 16 as common starting material, which reacted with malononitrile and ethyl cyanoacetate (12)/(38), among other reagents (Tables 5 and 6). The biological evaluation of the obtained compounds 234-242 was tested on three different tumor cell lines such as breast adenocarcinoma, CNS cancer, and non-small lung cancer and was compared to the inhibitory effect of doxorubicin. The results revealed that, among the heterocyclic products, furan derivatives 234e (Ar = 4-MeOC 6 H 4 , Y = CONH 2 , X = NH 2 ) and 234f (Ar = 2-furyl, Y = CN, X = OH) had the highest inhibitory effect [231].  Finally, in the search for new drug-like selective G-quadruplex binders, a bioinspired design focused on the use of nucleobases as synthons in a multicomponent reaction was provided by Pelliccia et al. [232]. Thus a series of multifunctionalized imidazo[2,1-i]purine derivatives 246 (57% yield), 247 (28% yield) and 248 (22-84% yield) were synthesized via a convergent Groebke-Blackburn-Bienaymé three-component reaction (GBB-3CR) of amino-aza-heterocycles 243-245, benzaldehydes 1, and isocyanide 18, followed by a S N 2 with aminoalkyl chlorides type 216 (Scheme 93). Biophysical studies over products 246-248 allowed for the identification of the first dual BCL2/c-MYC gene promoter G-quadruplex ligand, which involved circular dichroism melting experiments, microscale thermophoresis measurements, NMR titrations, and computational docking calculations, as well as biological investigations including cytotoxicity and apoptotic assays, and quantitative polymerase chain reaction and Western blot analyses. Resulst permitted to assess the potency and to characterize the binding mode of the newly identified lead compound. The absence of toxicity toward normal cells, together with the small molecular weight ( 500 Da), the water solubility, the ease of functionalization, and the selectivity profile, showed to be promising and desirable features to develop G-quadruplex binders as safe and effective anticancer agents.

Conclusions
Through this review it has been underscored the tremendous potential of MCRs as a very important tool for the synthesis of a vast number of organic acyclic and heterocyclic molecules, coupled to the prospect that many of them are biologically active or at least have been submitted to any biological screen. After a trip through the most relevant literature dealt with libraries of organic molecules synthesized via MCRs and subjected to screening for biological activity, it was found that most of such screening were addressed to anticancer, antimicrobial, anti-leihsmanial, anti-inflammatory, ROCK inhibitor, antioxidant, antimycobacterial, bromodomain inhibitor, antifibrotic agents, human receptor 8-active, neuroprotective agents, acetylcholinesterase inhibitor and anti-HIV activities. Interestingly, more than 60% of the found literature was oriented to anticancer activity evaluation. Thus, after the searching for appropriated reports to support the subject of this review we could establish that MCRs still represent an excellent synthetic strategy for the generation of a vast number of organic scaffolds, suitable for structure-activity relationship (SAR) studies in Medicinal Chemistry and drug discovery programs.