An Overview of 1,2,3-triazole-Containing Hybrids and Their Potential Anticholinesterase Activities

Acetylcholine (ACh) neurotransmitter of the cholinergic system in the brain is involved in learning, memory, stress responses, and cognitive functioning. It is hydrolyzed into choline and acetic acid by two key cholinesterase enzymes, viz., acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE). A loss or degeneration of cholinergic neurons that leads to a reduction in ACh levels is considered a significant contributing factor in the development of neurodegenerative diseases (NDs) such as Alzheimer’s disease (AD). Numerous studies have shown that cholinesterase inhibitors can raise the level of ACh and, therefore, enhance people’s quality of life, and, at the very least, it can temporarily lessen the symptoms of NDs. 1,2,3-triazole, a five-membered heterocyclic ring, is a privileged moiety, that is, a central scaffold, and is capable of interacting with a variety of receptors and enzymes to exhibit a broad range of important biological activities. Recently, it has been clubbed with other pharmacophoric fragments/molecules in hope of obtaining potent and selective AChE and/or BuChE inhibitors. The present updated review succinctly summarizes the different synthetic strategies used to synthesize the 1,2,3-triazole moiety. It also highlights the anticholinesterase potential of various 1,2,3-triazole di/trihybrids reported in the past seven years (2015–2022), including a rationale for hybridization and with an emphasis on their structural features for the development and optimization of cholinesterase inhibitors to treat NDs.


Introduction
Acetylcholine (ACh), a neurotransmitter produced by cholinergic neurons, is linked to signal transduction pathways involved in memory, motivation, behavioral adaptability, associative learning, sensory perception, and motor control [1]. A key step in the recovery of the cholinergic neuron is the hydrolysis of the neurotransmitter ACh into choline and acetic acid by a family of enzymes known as cholinesterases (ChE). The two kinds of ChE are acetylcholinesterase (AChE; EC 3.1.1.7) and butyrylcholinesterase (BuChE; EC 3.1.1.8). Both AChE and BuChE are serine esterases because they include a serine amino acid residue, which is required for catalytic activity [2].
AChE, also referred to as true/erythrocyte ChE, is one of the most efficient cholinergic neurotransmission enzymes. AChE is often produced or expressed by muscles, neurons, and some hematopoietic cells [1]. AChE found in the neuromuscular junction (NMJ) of skeletal muscle is produced by the muscle, not the nerve cell. Compared with AChE, BuChE is not as well known for its relevance. The ability of AChE to preferentially hydrolyze Ach over butyrylcholine distinguishes it from BuChE. AChE exists in a variety of molecular forms, all of which have comparable catalytic properties but differ in terms of how they oligomerize and how they adhere to the surface of cells [3]. BuChE, also known as pseudo/plasmatic ChE, is exported into the plasma after being generated in the Figure 1. A. Schematic illustration of a healthy brain and nervous system with a focus on cholinergic neurons and circuits in the general population. B. Alterations/changes in cholinergic function and related pathologies spurred by those changes in neurons and neuronal circuits. C. ACh's function at synapses: The primary precursor for the synthesis of Ach is choline. Choline and Acetyl-CoA are combined to create Ach by the action of choline acetyltransferase (ChAT). ACh controls cholinergic signaling in various parts of the brain and binds to nicotinic or muscarinic receptors on postsynaptic neurons/tissues, including muscles (NMJ). The NMJ's primary function is to translate the temporal sequence of motor neuron action potentials (APs) into muscular contractions. The activity of AChE causes extra ACh at synapses to be converted to choline and acetic acid. BuChE found in the glial cells is also involved in ACh metabolism. Transporters are then used to recycle choline back to the neuron and nerve terminal for further release.
ACh is known as a neuromodulator in the nervous system because it can produce activation or inhibition in a neuron's firing based on the sort of external stimuli or inputs that the target neuron receives [5]. Because the majority of the brain regions (cerebral neocortex, hippocampus, basal forebrain, and the nucleus basalis of Meynert) innervated by cholinergic neurons, these are involved in learning, memory, stress responses, and cognitive functioning. The degeneration of these cholinergic neurons is regarded as a significant contributing factor in neurodegenerative disorders. It has been demonstrated that the key indicators of cholinergic neuronal activity, ChAT and AChE levels, are impacted in The primary precursor for the synthesis of Ach is choline. Choline and Acetyl-CoA are combined to create Ach by the action of choline acetyltransferase (ChAT). ACh controls cholinergic signaling in various parts of the brain and binds to nicotinic or muscarinic receptors on postsynaptic neurons/tissues, including muscles (NMJ). The NMJ's primary function is to translate the temporal sequence of motor neuron action potentials (APs) into muscular contractions. The activity of AChE causes extra ACh at synapses to be converted to choline and acetic acid. BuChE found in the glial cells is also involved in ACh metabolism. Transporters are then used to recycle choline back to the neuron and nerve terminal for further release.
ACh is known as a neuromodulator in the nervous system because it can produce activation or inhibition in a neuron's firing based on the sort of external stimuli or inputs that the target neuron receives [5]. Because the majority of the brain regions (cerebral neocortex, hippocampus, basal forebrain, and the nucleus basalis of Meynert) innervated by cholinergic neurons, these are involved in learning, memory, stress responses, and cognitive functioning. The degeneration of these cholinergic neurons is regarded as a significant contributing factor in neurodegenerative disorders. It has been demonstrated that the key indicators of cholinergic neuronal activity, ChAT and AChE levels, are impacted in neurodegenerative disorders (NDs) [6]. It has become increasingly evident that the health of numerous interdependent neural circuits, notably striatal cholinergic interneurons (CINs), tightly controls mammalian brain processes, from executive and motor functioning to memory and emotional reactions. The cholinergic innervation of subcortical regions, which originates in the basal forebrain and brainstem, is crucial to orchestrating both cognitive and non-cognitive symptoms in Parkinson's disease (PD), Alzheimer's disease (AD), and other NDs [7,8]. There is also an established link between the cholinergic system and neuropsychiatric disorders [1]. ChAT activity was found to be reduced in the hippocampus, cortex, and amygdala of AD patients [8]. The peculiar pharmacology of AChE is seen in neurogenesis, cell adhesion, synaptogenesis, and the activation of dopamine neurons, as well as in the assembly of amyloid beta (Aβ) fibers and the control of glutamatemediated hippocampal activity. Its sudden blocking has catastrophic repercussions for AChE. Nerve agents and insecticides, which can irreversibly inhibit enzymes, belong to a family of very toxic substances frequently exhibit this effect. BuChE tangled in neurogenesis has a reclaiming impact on several xenobiotic substances and is crucial for cholinergic mediation. Pharmacologically, it is also believed that when AChE malfunctions, BuChE takes control [9]. According to studies, BuChE activity is greatly increased (41-80%) in the brains of people with advanced AD, particularly in areas affected by Aβ plaques and neurofibrillary tangles (NFT), in contrast to a 10-60% AChE deficit [10]. The fact that BuChE has a longer half-life than AChE and has more glycation sites, which are crucial for ChE stability and clearance, suggests that BuChE may be a possible target in the treatment of NDs, particularly AD. As an alternative to AChE, BuChE regulates ACh hydrolysis, and therapeutic drugs that inhibit both AChE and BuChE may be more advantageous for treating NDs than those that only inhibit AChE [11]. There is widespread consensus that reversible inhibitors that target ChE can enhance people's quality of life and, at the very least, temporarily lessen the symptoms of NDs [12]. Based on the abovementioned details, both ChEs are regarded as extremely pertinent targets in the drug discovery and development process for NDs. Novel ChE inhibitors have been the focus of numerous research teams. Recently, it was discovered that many families of heterocyclic compounds, such as 1,2,3-triazoles, indole, coumarin, quinolones, etc., potently inhibit both AChE and BuChE [13,14].
The 1,2,3-triazole heterocycle present in drugs exhibits antifungal, antibiotic, anticancer, antiviral, antimigraine, and anticonvulsant activities, as shown in Table 1. The 1,2,3-triazole ring produces anti-ChE activity by inhibiting both AChE and BuChE activities. The 1,2,3-triazole ring possesses low multidrug resistance, low toxicity, high bioavailability, and stability in both acidic and basic conditions. The nitrogen atom in 1,2,3-triazole ring is responsible for the enzyme-inhibitor interaction [15]. Despite the importance of 1,2,3triazole, with its wide range of biological activities, there is still a need for the development of novel, multitargeted inhibitors of ChE enzymes for ND treatment. This review covers an overview of the synthesis of 1,2,3-triazole derivatives with some in silico interaction, important for the anti-ChE activity reported recently by various researchers. Table 1. Therapeutic actions and chemical structures of triazole-based molecules.

Pharmacological Activities Chemical Structures
Antifungal Antibiotic Anticancer Antiviral Miscellaneous

Chemistry and the Importance of the Triazole Ring System
Triazoles are an important group of nitrogen-containing heterocyclic scaffolds containing two carbon atoms and three nitrogen atoms in an aromatic, five-membered ring framework [16]. Alternatively known as pyrrodiazoles, they are di-unsaturated rings composed of three nitrogen atoms within a heterocyclic core, with isomeric forms of 1,2,3triazoles and 1,2,4-triazoles [17,18]. In chemical terms, a triazole is one of two isomeric compounds ( Figure 2) with the molecular formula C2H3N3.

Pharmacological Activities Chemical Structures
Antifungal Antibiotic Anticancer Antiviral Miscellaneous

Chemistry and the Importance of the Triazole Ring System
Triazoles are an important group of nitrogen-containing heterocyclic scaffolds containing two carbon atoms and three nitrogen atoms in an aromatic, five-membered ring framework [16]. Alternatively known as pyrrodiazoles, they are di-unsaturated rings composed of three nitrogen atoms within a heterocyclic core, with isomeric forms of 1,2,3triazoles and 1,2,4-triazoles [17,18]. In chemical terms, a triazole is one of two isomeric compounds ( Figure 2) with the molecular formula C2H3N3.

Pharmacological Activities Chemical Structures
Antifungal Antibiotic Anticancer Antiviral Miscellaneous

Chemistry and the Importance of the Triazole Ring System
Triazoles are an important group of nitrogen-containing heterocyclic scaffolds containing two carbon atoms and three nitrogen atoms in an aromatic, five-membered ring framework [16]. Alternatively known as pyrrodiazoles, they are di-unsaturated rings composed of three nitrogen atoms within a heterocyclic core, with isomeric forms of 1,2,3triazoles and 1,2,4-triazoles [17,18]. In chemical terms, a triazole is one of two isomeric compounds ( Figure 2) with the molecular formula C2H3N3.

Pharmacological Activities Chemical Structures
Antifungal Antibiotic Anticancer Antiviral Miscellaneous

Chemistry and the Importance of the Triazole Ring System
Triazoles are an important group of nitrogen-containing heterocyclic scaffolds containing two carbon atoms and three nitrogen atoms in an aromatic, five-membered ring framework [16]. Alternatively known as pyrrodiazoles, they are di-unsaturated rings composed of three nitrogen atoms within a heterocyclic core, with isomeric forms of 1,2,3triazoles and 1,2,4-triazoles [17,18]. In chemical terms, a triazole is one of two isomeric compounds ( Figure 2) with the molecular formula C2H3N3.

Pharmacological Activities Chemical Structures
Antifungal Antibiotic Anticancer Antiviral Miscellaneous

Chemistry and the Importance of the Triazole Ring System
Triazoles are an important group of nitrogen-containing heterocyclic scaffolds containing two carbon atoms and three nitrogen atoms in an aromatic, five-membered ring framework [16]. Alternatively known as pyrrodiazoles, they are di-unsaturated rings composed of three nitrogen atoms within a heterocyclic core, with isomeric forms of 1,2,3triazoles and 1,2,4-triazoles [17,18]. In chemical terms, a triazole is one of two isomeric compounds ( Figure 2) with the molecular formula C2H3N3.

Chemistry and the Importance of the Triazole Ring System
Triazoles are an important group of nitrogen-containing heterocyclic scaffolds containing two carbon atoms and three nitrogen atoms in an aromatic, five-membered ring framework [16]. Alternatively known as pyrrodiazoles, they are di-unsaturated rings composed of three nitrogen atoms within a heterocyclic core, with isomeric forms of 1,2,3triazoles and 1,2,4-triazoles [17,18]. In chemical terms, a triazole is one of two isomeric compounds ( Figure 2) with the molecular formula C 2 H 3 N 3 .

Antibiotic
Anticancer Antiviral Miscellaneous

Chemistry and the Importance of the Triazole Ring System
Triazoles are an important group of nitrogen-containing heterocyc taining two carbon atoms and three nitrogen atoms in an aromatic, five framework [16]. Alternatively known as pyrrodiazoles, they are di-u composed of three nitrogen atoms within a heterocyclic core, with isome triazoles and 1,2,4-triazoles [17,18]. In chemical terms, a triazole is one compounds (Figure 2) with the molecular formula C2H3N3.
Several drugs and pharmaceutical agents contain triazoles as their core structure. In addition to being structurally and chemically diverse, triazole-based molecules, as central scaffolds, are capable of interacting with a variety of receptors and enzymes, showing broad biological activities [19,20]. As a result, a wide range of triazole-containing compounds has been frequently employed as clinical drugs or candidates, and they are vital for treating a wide variety of diseases, including antibacterial [21], antifungal [22], antitubercular [23], antiviral [24], anti-Alzheimer's [25], anticancer [26], antiparasitic [27], and antidiabetic treatments [28]. On the global market, triazole ring-containing drugs with medicinal value are actively exploited. Table 1 lists several pharmacological actions and chemical structures of triazole-based compounds.
Azide II, upon reaction with alkyne I, yields 1,2,3-triazole as a mixture of 1,4-adduct IIIa and 1,5-adduct IIIb after 18 h at 98 °C. In typical synthetic conditions, azides are preferred due to their relative stability and lack of side reactions. Another research group modified Huisgen cycloaddition into a more regioselective, copper-catalyzed stepwise reaction [34].

Copper-Catalyzed Azide-Alkyne Cycloaddition (Click Chemistry)
"Click chemistry" was coined by KB Sharpless in 2001 to describe reactions that are high-yielding and wide-ranging, contain only by-products that can be eliminated by chromatography, are stereospecific, are easy to perform, and can be conducted in solvents that are easily removed or benign [35]. Huisgen cycloadditions of organoazides and alkynes at high temperatures proceed slowly, resulting in 1,4-and 1,5-disubstituted 1,2,3-triazoles. Azide II, upon reaction with alkyne I, yields 1,2,3-triazole as a mixture of 1,4-adduct IIIa and 1,5-adduct IIIb after 18 h at 98 • C. In typical synthetic conditions, azides are preferred due to their relative stability and lack of side reactions. Another research group modified Huisgen cycloaddition into a more regioselective, copper-catalyzed stepwise reaction [34].
2.1.2. Copper-Catalyzed Azide-Alkyne Cycloaddition (Click Chemistry) "Click chemistry" was coined by KB Sharpless in 2001 to describe reactions that are high-yielding and wide-ranging, contain only by-products that can be eliminated by chromatography, are stereospecific, are easy to perform, and can be conducted in solvents that are easily removed or benign [35]. Huisgen cycloadditions of organoazides and alkynes Pharmaceuticals 2023, 16,179 6 of 43 at high temperatures proceed slowly, resulting in 1,4-and 1,5-disubstituted 1,2,3-triazoles. Copper-catalyzed azide-alkyne cycloadditions (CuAAC) were independently discovered by Meldal and Sharpless in 2002 [36]. The terminal alkyne substrate is attached to a hydrophilic tertiary amide-poly(ethylene glycol)-based resin using a peptide linker in Meldal's method ( Figure 4). The corresponding 1,2,3-triazole is produced when the azide is added in mild conditions using copper(I) salts as catalysts [37,38]. The 1,4-disubstituted 1,2,3-triazole is the only product of this reaction, which proceeds at room temperature in a variety of organic solvents with quantitative conversion [39].

Silver-Catalyzed Azide-Alkyne Cycloaddition (AgAAC)
In 2011, McNulty and coworkers pioneered the use of silver in the field of clic istry (primarily in AAC reactions) [42]. They described the creation of a novel acetate complex and located a well-defined species that catalyzed the first comm silver azide-alkyne cycloaddition (Ag-AAC) reaction ( Figure 3). Silver acetate was in a 1:1 ratio with the ligand 2-(di-tert-butylphosphino)-N,N-diisopropylbenzamid ate the chosen catalyst. Using only 2-2.5 mol% of this catalyst at 90 °C, phenylac and a slight excess of benzyl azide react to produce 1,4-triazole isomer (VII) in a than 99% yield [43]. On the other hand, the Sharpless group identified a solution-phase, copper-catalyzed azide-alkyne cycloaddition ( Figure 3). In their typical process, ascorbic acid or sodium ascorbate reduces the economically advantageous salt copper(II) sulfate pentahydrate in situ in a solvent mixture of water and alcohol ("Sharpless-Fokin conditions"). In contrast to Huisgen's uncatalyzed method, this strictly regioselective stepwise process only produces 1,4-disubstituted 1,2,3-triazole (IV) and speeds up the reaction by up to 107 times [40] ( Figure 3).

Silver-Catalyzed Azide-Alkyne Cycloaddition (AgAAC)
In 2011, McNulty and coworkers pioneered the use of silver in the field of click chemistry (primarily in AAC reactions) [42]. They described the creation of a novel silver(I) acetate complex and located a well-defined species that catalyzed the first common pure silver azide-alkyne cycloaddition (Ag-AAC) reaction ( Figure 3). Silver acetate was treated in a 1:1 ratio with the ligand 2-(di-tert-butylphosphino)-N,N-diisopropylbenzamide to create the chosen catalyst. Using only 2-2.5 mol% of this catalyst at 90 • C, phenylacetylene and a slight excess of benzyl azide react to produce 1,4-triazole isomer (VII) in a greater than 99% yield [43].  [44]. Rather than employing Cu(I) to activate the alkyne, the alkyne is placed into a strained difluorooctyne (DIFO), where the electron-withdrawing, propargylic, gem-fluorines work in conjunction with the ring strain to significantly destabilize the alkyne [45] (Figure 5).
Pharmaceuticals 2023, 16,179 activate the alkyne, the alkyne is placed into a strained difluorooctyne (DIFO), w electron-withdrawing, propargylic, gem-fluorines work in conjunction with t strain to significantly destabilize the alkyne [45] (Figure 5). With the incredible advancements in 1,2,3-triazole synthesis, the developm metal-and azide-free technique has emerged as a fairly significant problem from spectives of economics and green chemistry.
Synthesis of 1,2,3-triazole through α,α-dichlorotosylhydrazones A revolutionary work toward the metal-free, azide-free synthesis of 1,2,3 VIII was reported by Sakai et al. [46] as early as 1986 by condensing α-dichloro drazone IX with primary amine X under ambient conditions ( Figure 6). In 2012, va et al. [47] perfectly exploited the mechanism, scope, and limitations of the Sakai to demonstrate that this transformation is suitable as a scheme for metal-and az 1,2,3-triazole formation.  [48] modification of the Sakai reaction substituted α-chlorotosy zones XI in place of α,α-dichlorotosylhydrazones to produce 1,2,3-triazole. Via cloaddition of α-chlorotosylhydrazones with arylamines XII in a metal-free and With the incredible advancements in 1,2,3-triazole synthesis, the development of a metal-and azide-free technique has emerged as a fairly significant problem from the perspectives of economics and green chemistry.

Synthesis of 1,2,3-triazole through α,α-dichlorotosylhydrazones
A revolutionary work toward the metal-free, azide-free synthesis of 1,2,3-triazole VIII was reported by Sakai et al. [46] as early as 1986 by condensing α-dichlorotosylhydrazone IX with primary amine X under ambient conditions ( Figure 6). In 2012, van Berkel et al. [47] perfectly exploited the mechanism, scope, and limitations of the Sakai reaction to demonstrate that this transformation is suitable as a scheme for metal-and azide-free 1,2,3-triazole formation. activate the alkyne, the alkyne is placed into a strained difluorooctyne (DIFO), where the electron-withdrawing, propargylic, gem-fluorines work in conjunction with the ring strain to significantly destabilize the alkyne [45] ( Figure 5). With the incredible advancements in 1,2,3-triazole synthesis, the development of a metal-and azide-free technique has emerged as a fairly significant problem from the perspectives of economics and green chemistry.

Ionic Liquid-Catalyzed Synthesis
Ionic liquids (IL) have special qualities that make them effective solvents for a variety of organic, inorganic, and polymeric compounds. These properties include low vapor pressure, a large liquid range, great chemical stability, high thermal stability, and strong solvent power [50]. Choline chloride-CuCl, a straightforward bifunctional IL catalyst, was discovered to be extremely active for [3 + 2] Huisgen cycloaddition in H 2 O [51] (Figure 7).  To produce 1,4-disubstituted-1,2,3-triazoles, this singleoperation technique involves functionalizing the C(sp 3 )-H link, forming the N-N/C-N bonds, and cleaving the S-N bond.

Ionic Liquid-Catalyzed Synthesis
Ionic liquids (IL) have special qualities that make them effective solvents for a variety of organic, inorganic, and polymeric compounds. These properties include low vapor pressure, a large liquid range, great chemical stability, high thermal stability, and strong solvent power [50]. Choline chloride-CuCl, a straightforward bifunctional IL catalyst, was discovered to be extremely active for [3 + 2] Huisgen cycloaddition in H2O [51] (Figure 7).

Application of Triazoles in the Synthesis of Other Heterocyclic Compounds
The presence of three nitrogen atoms in triazole structures has provided possibilities for a wide variety of structural alterations, leading to development of novel, therapeutically promising medicines.
Currently, the most commonly prescribed antifungals in clinical therapy are triazole drugs (fluconazole, itraconazole, voriconazole, and posaconazole). Recent studies on triazoles have utilized the principles of conjugate chemistry to assemble active pharmacophores to produce hybrid molecules with the desired activities. Triazoles clubbed with other pharmacophores have been found to have a broader spectrum with antimicrobial [52], anti-inflammatory [53], antineoplastic [26], antiobesity [54], antidiabetic [28], immunomodulatory [55], anticholinesterase [56], and antiviral activities [24]. In addition to the available triazole drugs, researchers are interested in exploring and developing new triazole-based scaffolds with major applications in biomedical and biotechnology fields.

Application of Triazoles in the Synthesis of Other Heterocyclic Compounds
The presence of three nitrogen atoms in triazole structures has provided possibilities for a wide variety of structural alterations, leading to development of novel, therapeutically promising medicines.
Currently, the most commonly prescribed antifungals in clinical therapy are triazole drugs (fluconazole, itraconazole, voriconazole, and posaconazole). Recent studies on triazoles have utilized the principles of conjugate chemistry to assemble active pharmacophores to produce hybrid molecules with the desired activities. Triazoles clubbed with other pharmacophores have been found to have a broader spectrum with antimicrobial [52], anti-inflammatory [53], antineoplastic [26], antiobesity [54], antidiabetic [28], immunomodulatory [55], anticholinesterase [56], and antiviral activities [24]. In addition to the available triazole drugs, researchers are interested in exploring and developing new triazole-based scaffolds with major applications in biomedical and biotechnology fields.

Hesperetin-1,2,3-triazole Hybrids
Hesperetin, a 4 -methoxyflavanone, is one of the most important bioactive phytochemicals present in citrus fruits. It has been reported to exhibit a wide spectrum of pharmacological activities, including anti-ChE inhibitory activity [57]. To improve its anti-ChE, anti-neuroinflammatory, and neuroprotective repertoire, hesperetin was clubbed with a 1,2,3-triazole motif to obtain 39 new compounds belonging to 4 series of 7-O-1,2,3-triazole-hesperetin hybrids. Interestingly, all the synthesized hybrid compounds showed better AChE inhibitory activity in comparison with the parent flavanone, hesperetin (IC 50 = 3.04 ± 0.21 µM). Furthermore, 7-O-amide hesperetin derivatives displayed almost twofold greater BuChE inhibitory activity than 7-O-triazole derivatives. Among all the hybrids, four compounds, 1, 2, 3, and 4 (IC 50 = 3.08, 4.61, 6.37, and 5.51 µM, respectively), were found to be more potent than donepezil (IC 50 = 6.21 ± 0.52 µM) in BuChE inhibition ( Figure 8). Compound 1 (7-O-((1-(3-chlorobenzyl)-1H-1,2,3-triazol-4yl) methyl) hesperetin), the most potent anti-BuChE compound (IC 50 = 3.08 ± 0.29 µM), did not show any sign of neurotoxicity (Aβ-induced SH-SY5Y) in concentrations up to 50 µM and showed good blood-brain barrier (BBB) permeability along with notable neu-Pharmaceuticals 2023, 16, 179 9 of 43 roprotective and anti-neuro-inflammatory activities (IC 50 = 2.91 ± 0.47 µM) against NO production. Mechanistic studies revealed that the anti-inflammatory activity of compound 1 could be due to its ability to block the NF-κB signaling pathway via the inhibition of the phosphorylation of the P65 protein. Additionally, it was able to inhibit Aβ 1-42 aggregation, decreased the generation of ROS, and could selectively chelate biometals such as Cu 2+ , which all contribute to its neuroprotective activity. The administration of compound 1 to scopolamine-induced AD mice led to significant improvement in learning and memory impairment. Enzyme kinetic studies indicated compound 1 to be a mixed-type inhibitor of BuChE. The strong anti-BuChE activity of compound 1 was probed via molecular docking studies on BuChE (PDB code: 5NN0), which revealed strong binding to the active site on the target protein. The ketone of the pyrone ring interacted with Tyr128 to form one hydrogen bond, while ring A (benzene) of the flavanone formed hydrophobic interactions (π-sigma) with Trp82, Ala328, and His438 in the active pocket of the receptor [58]. A 1,2,3triazole-ring-bearing chlorobenzyl substituent formed a π-anion interaction with Asp70. Based on these results, compound 1 could be considered a potential MTDL candidate in the development of new anti-AD agents [59]. rmaceuticals 2023, 16, 179 9 o studies revealed that the anti-inflammatory activity of compound 1 could be due to ability to block the NF-κB signaling pathway via the inhibition of the phosphorylation the P65 protein. Additionally, it was able to inhibit Aβ1-42 aggregation, decreased the g eration of ROS, and could selectively chelate biometals such as Cu 2+ , which all contrib to its neuroprotective activity. The administration of compound 1 to scopolamine-indu AD mice led to significant improvement in learning and memory impairment. Enzy kinetic studies indicated compound 1 to be a mixed-type inhibitor of BuChE. The stro anti-BuChE activity of compound 1 was probed via molecular docking studies on BuC (PDB code: 5NN0), which revealed strong binding to the active site on the target prote The ketone of the pyrone ring interacted with Tyr128 to form one hydrogen bond, wh ring A (benzene) of the flavanone formed hydrophobic interactions (π-sigma) with Trp Ala328, and His438 in the active pocket of the receptor [58]. A 1,2,3-triazole-ring-bear chlorobenzyl substituent formed a π-anion interaction with Asp70. Based on these resu compound 1 could be considered a potential MTDL candidate in the development of n anti-AD agents [59].

Genipin-1,2,3-triazole Hybrids
Geniposide (an iridoid glycoside) is a major component of the fruit of Gardenia jasminoides Ellis, which, upon hydrolysis, yields biologically active genipin aglycone. Genipin shows potential neuroprotective activity by virtue of its ability to inhibit high-level lactate dehydrogenase (LDH) in the blood, thereby preventing amyloid-β (Aβ) peptide toxicity [60]. Encouraged by the anti-AChE and Aβ 1-42 aggregation inhibitory activity of piperazine-genipin hybrids [61], Silalai et al. prepared genipin-1,2,3-triazole hybrids in search of anti-AD agents. A total of 39 compounds were synthesized, which were evaluated for AChE and BuChE inhibitory activity in addition to neuroprotective action against hydrogen peroxide-induced neuronal toxicity. Intermediate genipin azides were reacted with different alkynes to undergo azide−alkyne Huisgen cycloaddition reactions using copper iodide to obtain C-10-substituted genipin-1,2,3-triazole derivatives in good-to-excellent yields. Substituted 1,2,3-triazoles with different carbon chain lengths, such as phenyl, benzyl ether, and benzylamine, as well as aliphatic, phthalimide, and alicyclic functionalities, were incorporated at the C-10 position in genipin. Synthesized compounds showed weaker eeAChE inhibitory activity (less than 50% inhibition), but they were observed to be strong inhibitors of eqBuChE. In fact, the anti-BuChE activity of the majority of synthesized compounds was found to be greater than geniposide and genipin, suggesting that the incorporation of a 1,2,3-triazole ring augments the anti-ChE activity of genipin. Among all the synthesized compounds, acetoxy and hemiacetal 1,2,3-triazole genipin analogs, viz., 10-[4 -(7 -bromoethyl)-1H-1,2,3-triazole-1-yl]-1-acetoxygenipin (5; yield 75%) and 10-[4 -(6 ,6 -diphenyl-6 -hydroxymethyl)-1H-1,2,3-triazole-1-yl] genipin (6; 64% yield), emerged as the most potent and selective BuChE inhibitors (IC 50 = 31.8 and 54.3 µM, respectively). Compound 5, with a bromoethyl-1,2,3-triazole scaffold, was found to be a better inhibitor than galantamine (IC 50 = 34.1 µM), while 6, containing a diphenylhydroxy group, displayed at par activity (IC 50 = 54.3 µM) to a positive control ( Figure 9). SAR studies of acetoxy genipin-1,2,3-triazole analogs revealed that the nature of the substituents at position 4 of the 1,2,3-triazole ring affect BuChE inhibition. The para substitution of a phenyl-1,2,3-triazole ring either with an electron-donating group (4-methoxy) or an electron-withdrawing group (4-fluoro) results in a decrease in activity. Similarly, replacing a phenyl ring of 1,2,3-triazole with di/triphenyl, benzyl ether, hydroxy-cyclic compounds, and benzylamine did not improve activity. However, 1,2,3-triazoles, upon replacing an aryl group with an alkyl chain, showed low-to-high inhibitory activity. In general, long-chain alkyl groups displayed lower inhibitory activity than the compound with a bromoethyl group (5). An improvement in activity was observed when long-chain alkyl groups are replaced with phthalimide moieties. Among the hemiacetal genipin-1,2,3-triazole derivatives, phenyl/diphenyl rings showed good-to-excellent inhibition against BuChE (67.51-99.85%). Inhibitory activity was found to decrease upon replacing the phenyl groups of 6 with phthalimide and hydroxy-cyclic moieties. Lineweaver−Burk plots of 5 and 6 confirmed them to be noncompetitive enzyme inhibitors with inhibition constants (KI, KIS), estimated to be 0.03 and 0.1 mM, respectively. Both the potent compounds protected the cells from H 2 O 2 -induced neurotoxicity. The BuChE inhibition mechanism of the most potent compounds, 5 and 6, was studied with the help of molecular docking. Compounds 5 and 6 showed binding energies of −9.77 and −9.74 kcal/mol within the active site of BuChE (PDB code: 4BDS). The ester unit at the C4 position of the iridoid moiety in compound 5 and 6 formed three H-bonds with the His438 (catalytic subsite) and Ser198 of the CAS. The acetoxy carbonyl group also interacted with Trp82, Trp430, and Tyr440 residues to form three H-bonds. Both the acetoxy and ester unit interact in the CAS region to inhibit BuChE, while the 1,2,3-triazole ring interacts with Tyr332 and Asp70 in the PAS region via the H-bond and ionic interactions. On the other hand, the diphenylhydroxy groups of 6 are oriented toward the PAS region, forming two π-π interactions with Tyr332, while the iridoid fragment forms an H-bond with Trp82 residue in the CAS pocket. be a better inhibitor than galantamine (IC50 = 34.1 μM), while 6, containing a diphenylhydroxy group, displayed at par activity (IC50 = 54.3 μM) to a positive control ( Figure 9). SAR studies of acetoxy genipin-1,2,3-triazole analogs revealed that the nature of the substituents at position 4 of the 1,2,3-triazole ring affect BuChE inhibition. The para substitution of a phenyl-1,2,3-triazole ring either with an electron-donating group (4-methoxy) or an electron-withdrawing group (4-fluoro) results in a decrease in activity. Similarly, replacing a phenyl ring of 1,2,3-triazole with di/triphenyl, benzyl ether, hydroxy-cyclic compounds, and benzylamine did not improve activity. However, 1,2,3-triazoles, upon replacing an aryl group with an alkyl chain, showed low-to-high inhibitory activity. In general, long-chain alkyl groups displayed lower inhibitory activity than the compound with a bromoethyl group (5). An improvement in activity was observed when long-chain alkyl groups are replaced with phthalimide moieties. Among the hemiacetal genipin-1,2,3-triazole derivatives, phenyl/diphenyl rings showed good-to-excellent inhibition against BuChE (67.51-99.85%). Inhibitory activity was found to decrease upon replacing the phenyl groups of 6 with phthalimide and hydroxy-cyclic moieties. Lineweaver−Burk plots of 5 and 6 confirmed them to be noncompetitive enzyme inhibitors with inhibition constants (KI, KIS), estimated to be 0.03 and 0.1 mM, respectively. Both the potent compounds protected the cells from H2O2-induced neurotoxicity. The BuChE inhibition mechanism of the most potent compounds, 5 and 6, was studied with the help of molecular docking. Compounds 5 and 6 showed binding energies of −9.77 and −9.74 kcal/mol within the active site of BuChE (PDB code: 4BDS). The ester unit at the C4 position of the iridoid moiety in compound 5 and 6 formed three H-bonds with the His438 (catalytic subsite) and Ser198 of the CAS. The acetoxy carbonyl group also interacted with Trp82, Trp430, and Tyr440 residues to form three H-bonds. Both the acetoxy and ester unit interact in the CAS region to inhibit BuChE, while the 1,2,3-triazole ring interacts with Tyr332 and Asp70 in the PAS region via the H-bond and ionic interactions. On the other hand, the diphenylhydroxy groups of 6 are oriented toward the PAS region, forming two π−π interactions with Tyr332, while the iridoid fragment forms an H-bond with Trp82 residue in the CAS pocket. It can be suggested that the introduction of substituted 1,2,3-triazoles to a genipin core structure improves their BuChE inhibitory potential, and, therefore, 1,2,3-triazolegenipin analogs such as 5 and 6 might serve as leading molecules in fighting AD [62].

Paeonol-1,2,3-Triazole Hybrids
It is well established that a low concentration of certain chemical neurotransmitters, including serotonin, epinephrine, norepinephrine, and dopamine, in the brain contributes to the development and progression of AD. The enzyme MAO-A is involved in the degradation of these neurotransmitters and causes catecholamine and 5-hydroxytryptamine (5-HT) inactivation, and, thus, MAO inhibitors can reduce the progression of AD. Paeonol is a phenolic derivative from the Paeonia genus herb with neuroprotective activity. 1,2,3triazole linked with tryptamine-paeonol derivatives were synthesized and evaluated for AChE, BuChE, MAO-A, and MAO-B inhibitory activities. Compound 7 showed the most potent BuChE inhibition (IC50 = 0.13 µM) with a selectivity index of more than 769 for It can be suggested that the introduction of substituted 1,2,3-triazoles to a genipin core structure improves their BuChE inhibitory potential, and, therefore, 1,2,3-triazole-genipin analogs such as 5 and 6 might serve as leading molecules in fighting AD [62].

Paeonol-1,2,3-triazole Hybrids
It is well established that a low concentration of certain chemical neurotransmitters, including serotonin, epinephrine, norepinephrine, and dopamine, in the brain contributes to the development and progression of AD. The enzyme MAO-A is involved in the degradation of these neurotransmitters and causes catecholamine and 5-hydroxytryptamine (5-HT) inactivation, and, thus, MAO inhibitors can reduce the progression of AD. Paeonol is a phenolic derivative from the Paeonia genus herb with neuroprotective activity. 1,2,3-triazole linked with tryptamine-paeonol derivatives were synthesized and evaluated for AChE, BuChE, MAO-A, and MAO-B inhibitory activities. Compound 7 showed the most potent BuChE inhibition (IC 50 = 0.13 µM) with a selectivity index of more than 769 for BuChE over AChE, whereas compound 8 showed selective MAO-B inhibition ( Figure 10). In comparison with other derivatives, the meta-CF 3 substituent increased the BuChE inhibitory activity, whereas the para-CF 3 increased MAO-B inhibition. Compound 7 was found to be a reversible non-competitive BuChE inhibitor, whereas 8 was found to be a reversible competitive MAO-B inhibitor. Comparing the in-silico binding energies, compound 7 showed higher binding energy than 8 against BuChE (−13.75 vs. −11.29 kcal/mol); however, compound 8 has higher binding energy against MAO-B than MAO-A (−11.31 vs. −6.72 kcal/mol). The derivatives were slightly cytotoxic against normal cells (MDCK) and human neuroblastoma cells (SH-SY5Y) [63]. In comparison with other derivatives, the meta-CF3 substituent increased the BuChE inhibitory activity, whereas the para-CF3 increased MAO-B inhibition. Compound 7 was found to be a reversible non-competitive BuChE inhibitor, whereas 8 was found to be a reversible competitive MAO-B inhibitor. Comparing the in-silico binding energies, compound 7 showed higher binding energy than 8 against BuChE (-13.75 vs -11.29 kcal/mol); however, compound 8 has higher binding energy against MAO-B than MAO-A (-11.31 vs. -6.72 kcal/mol). The derivatives were slightly cytotoxic against normal cells (MDCK) and human neuroblastoma cells (SH-SY5Y) [63].

Quinazoline-1,2,3-Triazole Hybrids
Quinazoline moiety is considered an important bioactive scaffold in designing therapeutic agents owing to its diverse array of biological properties [64,65]. This privileged structural motif has been shown to bind with both CAS and PAS on AChE, resulting in the significant inhibition of AChE [66]. Because of their powerful AChE inhibitory activities, quinazolines have been explored in designing anti-AD agents [66][67][68][69][70]. This was prompted by the results obtained by Rao and coworkers, who found that 2,4-disubstituted quinazolines bearing different primary amines at position C-4 of quinazoline exhibit promising anticholinesterase activities [70]. A 1,2,3-triazole nucleus containing a substituted aromatic ring at position 1 was linked through position C-6 to 4-amino-substituted quinazolines to obtain 4,6-disubstituted quinazoline-1,2,3-triazole hybrids, thus improving their AChE inhibitory activities. The 1,2,3-triazole nucleus was selected because it can form a hydrogen bond with the catalytic aspartate residue, and, secondly, various heterocyclic and or aromatic functionalities can be easily attached to it, helping in the optimization of physicochemical properties. The target quinazoline-1,2,3-triazole hybrids were prepared in a good yields (65-91%) by reacting intermediate 6-(prop-2-yn-1-yloxy) quinazolin-4-amines with different substituted arylazide derivatives using catalyst CuI in the presence of DIPEA in THF at an ambient temperature. All the twelve synthesized hybrid compounds inhibited the AChE enzyme (IC50 range = 0.2-83.9 μM), but it was observed that the inhibitory activity was influenced by the presence of the substituted amino group (3-nitrophenylamine < N-methylpiperazyl< benzylamine) at position C-4 of the quinazoline ring. Compound 10, bearing a benzylamine moiety at the fourth position of the quinazoline ring and 2-nitrophenyl attached to a 1,2,3-triazole nucleus, was identified as the most potent AChE inhibitor (IC50 = 0.23 μM), though it was less active than donepezil (IC50 = 0.12 μM). The authors argued that benzylamine-containing hybrids are more flexible than other derivatives, which facilitates favorable binding to the ChE enzyme. It was observed that the presence of nitrophenyl (NO2-Ph) functionality in a 1,2,3triazole ring imparts better inhibitory activity than the 3-trifluoromethyl-4-nitrile group. Furthermore, changing the position of the nitro group from ortho to meta in the phenyl

Quinazoline-1,2,3-triazole Hybrids
Quinazoline moiety is considered an important bioactive scaffold in designing therapeutic agents owing to its diverse array of biological properties [64,65]. This privileged structural motif has been shown to bind with both CAS and PAS on AChE, resulting in the significant inhibition of AChE [66]. Because of their powerful AChE inhibitory activities, quinazolines have been explored in designing anti-AD agents [66][67][68][69][70]. This was prompted by the results obtained by Rao and coworkers, who found that 2,4-disubstituted quinazolines bearing different primary amines at position C-4 of quinazoline exhibit promising anticholinesterase activities [70]. A 1,2,3-triazole nucleus containing a substituted aromatic ring at position 1 was linked through position C-6 to 4-amino-substituted quinazolines to obtain 4,6-disubstituted quinazoline-1,2,3-triazole hybrids, thus improving their AChE inhibitory activities. The 1,2,3-triazole nucleus was selected because it can form a hydrogen bond with the catalytic aspartate residue, and, secondly, various heterocyclic and or aromatic functionalities can be easily attached to it, helping in the optimization of physicochemical properties. The target quinazoline-1,2,3-triazole hybrids were prepared in a good yields (65-91%) by reacting intermediate 6-(prop-2-yn-1-yloxy) quinazolin-4-amines with different substituted arylazide derivatives using catalyst CuI in the presence of DIPEA in THF at an ambient temperature. All the twelve synthesized hybrid compounds inhibited the AChE enzyme (IC 50 range = 0.2-83.9 µM), but it was observed that the inhibitory activity was influenced by the presence of the substituted amino group (3-nitrophenylamine < N-methylpiperazyl < benzylamine) at position C-4 of the quinazoline ring. Compound 10, bearing a benzylamine moiety at the fourth position of the quinazoline ring and 2nitrophenyl attached to a 1,2,3-triazole nucleus, was identified as the most potent AChE inhibitor (IC 50 = 0.23 µM), though it was less active than donepezil (IC 50 = 0.12 µM). The authors argued that benzylamine-containing hybrids are more flexible than other derivatives, which facilitates favorable binding to the ChE enzyme. It was observed that the presence of nitrophenyl (NO 2 -Ph) functionality in a 1,2,3-triazole ring imparts better inhibitory activity than the 3-trifluoromethyl-4-nitrile group. Furthermore, changing the position of the nitro group from ortho to meta in the phenyl ring resulted in a reduction in activity (1.10 µM), while the para-substituted compound was almost inactive (IC 50 > 200 µM), suggesting that the electronic properties of substituents on the aromatic ring influence AChE inhibitory activity. Replacing the benzylamine group with the N-methylpiperazyl moiety on the C-4 position of quinazoline showed lower activity than corresponding compounds containing benzylamine, but, in contrast, m-nitrophenyl-1,2,3-triazole derivative 14 was found to be more active than o-nitrophenyl-1,2,3-triazole 13 by two times. Paranitro-substituted compound 15 in the series displayed no activity, similar to 12, suggesting that the 4-nitrophenyl group connected to the 1,2,3-triazole ring is detrimental to AChE inhibitory activity ( Figure 11). SAR analysis pointed out that the nature of the amino group present at the C-4 position of the quinazoline moiety and nitrophenyl 1,2,3-triazole are crucial in imparting anti-AChE activities to hybrid molecules. Molecular docking studies suggest that compounds 9-11 are dual-binding site inhibitors of AChE, as they have been found to bind to both CAS and PAS at the active site of the AChE enzyme. Compound 9 forms interaction with Trp86 residue in the choline-binding region; its quinazoline ring binds to Trp286 via a π-π interaction and, similar to donepezil, forms hydrogen bonds with Phe295 and/or Ser293 residues. 1,2,3-triazole and nitrophenyl rings interact with Tyr341 and Try337 residues, respectively. Donepezil and compound 9 have been found to have similar docking scores, confirming their high affinity for ChE enzymes [71]. ring resulted in a reduction in activity (1.10 μM), while the para-substituted compound was almost inactive (IC50 > 200 μM), suggesting that the electronic properties of substituents on the aromatic ring influence AChE inhibitory activity. Replacing the benzylamine group with the N-methylpiperazyl moiety on the C-4 position of quinazoline showed lower activity than corresponding compounds containing benzylamine, but, in contrast, m-nitrophenyl-1,2,3-triazole derivative 14 was found to be more active than o-nitrophenyl-1,2,3-triazole 13 by two times. Para-nitro-substituted compound 15 in the series displayed no activity, similar to 12, suggesting that the 4-nitrophenyl group connected to the 1,2,3triazole ring is detrimental to AChE inhibitory activity ( Figure 11). SAR analysis pointed out that the nature of the amino group present at the C-4 position of the quinazoline moiety and nitrophenyl 1,2,3-triazole are crucial in imparting anti-AChE activities to hybrid molecules. Molecular docking studies suggest that compounds 9-11 are dual-binding site inhibitors of AChE, as they have been found to bind to both CAS and PAS at the active site of the AChE enzyme. Compound 9 forms interaction with Trp86 residue in the choline-binding region; its quinazoline ring binds to Trp286 via a π-π interaction and, similar to donepezil, forms hydrogen bonds with Phe295 and/or Ser293 residues. 1,2,3-triazole and nitrophenyl rings interact with Tyr341 and Try337 residues, respectively. Donepezil and compound 9 have been found to have similar docking scores, confirming their high affinity for ChE enzymes [71].

Coumarin-1,2,3-triazole Hybrids
A coumarin scaffold is a privileged motif that, because of its potent anti-ChE activity, has been clubbed with various bioactive heterocyclic ring systems to obtain lead candidates. Coumarin-1,2,3-triazole hybrids from 2015-2020 have been reviewed [14]. The chemistry and neuroprotective actions of the most promising coumarin-1,2,3-triazole hybrids are presented in Table 2. Table 2. Chemical structure and mechanism of the neuroprotective activities of some selected coumarin-1,2,3-triazole derivatives.

S. No
Chemical Structure Neuroprotective Activity Structural Features Ref. A molecular docking analysis of compound 23 and donepezil on hAChE showed similar binding modes. Both formed interactions with the AChE catalytic triad residues Ser203, Glu334, and His447, as well as with Trp286 (π-π stacking with indanone) and Trp86 (π-π stacking with quinoline), located at the peripheral site and the catalytic cleft. The difference in activity between compound 23 and donepezil could be because of the formation of some additional interactions of 23 with Tyr124, Tyr337, and Tyr341 residues in the middle of the gorge. 1,2,3-triazole-quinoline scaffolds, because of their dual-binding mode to the active site of AChE, could be considered for further optimization to obtain potent ChE inhibitors [78].

Coumarin-1,2,3-Triazole Hybrids
A coumarin scaffold is a privileged motif that, because of its potent anti-ChE activity, has been clubbed with various bioactive heterocyclic ring systems to obtain lead candidates. Coumarin-1,2,3-triazole hybrids from 2015-2020 have been reviewed [14]. The chemistry and neuroprotective actions of the most promising coumarin-1,2,3-triazole hybrids are presented in Table 2. Table 2. Chemical structure and mechanism of the neuroprotective activities of some selected coumarin-1,2,3-triazole derivatives.

S. No
Chemical structure Neuroprotective activity Structural features Ref A molecular docking analysis of compound 23 and donepezil on hAChE showed similar binding modes. Both formed interactions with the AChE catalytic triad residues Ser203, Glu334, and His447, as well as with Trp286 (π-π stacking with indanone) and Trp86 (π-π stacking with quinoline), located at the peripheral site and the catalytic cleft. The difference in activity between compound 23 and donepezil could be because of the formation of some additional interactions of 23 with Tyr124, Tyr337, and Tyr341 residues in the middle of the gorge. 1,2,3-triazole-quinoline scaffolds, because of their dual-binding mode to the active site of AChE, could be considered for further optimization to obtain potent ChE inhibitors [78].

Coumarin-1,2,3-Triazole Hybrids
A coumarin scaffold is a privileged motif that, because of its potent anti-ChE activity, has been clubbed with various bioactive heterocyclic ring systems to obtain lead candidates. Coumarin-1,2,3-triazole hybrids from 2015-2020 have been reviewed [14]. The chemistry and neuroprotective actions of the most promising coumarin-1,2,3-triazole hybrids are presented in Table 2. Table 2. Chemical structure and mechanism of the neuroprotective activities of some selected coumarin-1,2,3-triazole derivatives.

S. No
Chemical structure Neuroprotective activity Structural features Ref Removal of 8-OCH3 group and increasing the number of chlorine atoms at 3-and 4-positions of the phenyl ring decreases AChE inhibitory activity. Unsubstituted coumarin and the phenyl ring of 1,2,3-triazole display very weak activity. [84] (IC 50 = 0.18 µM-hAChE).
2-Cl on phenyl ring significantly improves the activity, but changing its position to p or replacing it with p-Br/p-NO 2 greatly reduces anti-AChE activity (~40-250-times). [82] 3 A molecular docking analysis of compound 23 and donepezil on hAChE showed similar binding modes. Both formed interactions with the AChE catalytic triad residues Ser203, Glu334, and His447, as well as with Trp286 (π-π stacking with indanone) and Trp86 (π-π stacking with quinoline), located at the peripheral site and the catalytic cleft. The difference in activity between compound 23 and donepezil could be because of the formation of some additional interactions of 23 with Tyr124, Tyr337, and Tyr341 residues in the middle of the gorge. 1,2,3-triazole-quinoline scaffolds, because of their dual-binding mode to the active site of AChE, could be considered for further optimization to obtain potent ChE inhibitors [78].

Coumarin-1,2,3-Triazole Hybrids
A coumarin scaffold is a privileged motif that, because of its potent anti-ChE activity, has been clubbed with various bioactive heterocyclic ring systems to obtain lead candidates. Coumarin-1,2,3-triazole hybrids from 2015-2020 have been reviewed [14]. The chemistry and neuroprotective actions of the most promising coumarin-1,2,3-triazole hybrids are presented in Table 2. Table 2. Chemical structure and mechanism of the neuroprotective activities of some selected coumarin-1,2,3-triazole derivatives.

S. No
Chemical structure Neuroprotective activity Structural features Ref A molecular docking analysis of compound 23 and donepezil on hAChE showed similar binding modes. Both formed interactions with the AChE catalytic triad residues Ser203, Glu334, and His447, as well as with Trp286 (π-π stacking with indanone) and Trp86 (π-π stacking with quinoline), located at the peripheral site and the catalytic cleft. The difference in activity between compound 23 and donepezil could be because of the formation of some additional interactions of 23 with Tyr124, Tyr337, and Tyr341 residues in the middle of the gorge. 1,2,3-triazole-quinoline scaffolds, because of their dual-binding mode to the active site of AChE, could be considered for further optimization to obtain potent ChE inhibitors [78].

Coumarin-1,2,3-Triazole Hybrids
A coumarin scaffold is a privileged motif that, because of its potent anti-ChE activity, has been clubbed with various bioactive heterocyclic ring systems to obtain lead candidates. Coumarin-1,2,3-triazole hybrids from 2015-2020 have been reviewed [14]. The chemistry and neuroprotective actions of the most promising coumarin-1,2,3-triazole hybrids are presented in Table 2. Table 2. Chemical structure and mechanism of the neuroprotective activities of some selected coumarin-1,2,3-triazole derivatives.
It is a decursinol hybrid which itself is not active against BuChE. Increase in activity of hybrids could be attributed to the 1,2,3-triazole-lipoic acid scaffold. [89].
A 4-carbon chain-linker between 1,2,3-triazole and 3-hydroxycoumarin is optimal for inhibitory activity. 3,4-OCH3 phenyl at the C-3 position of a coumarin ring and linked via a three-carbon-long chain to 1,2,3-triazole is the most potent compound of the series. [91] 12 The potent anti-AChE activity of this compound is attributed to the presence of three -Cl atoms on the phenyl ring.
It is a decursinol hybrid which itself is not active against BuChE. Increase in activity of hybrids could be attributed to the 1,2,3-triazole-lipoic acid scaffold. [89].
It is a decursinol hybrid which itself is not active against BuChE. Increase in activity of hybrids could be attributed to the 1,2,3-triazole-lipoic acid scaffold. [89].
The potent anti-AChE activity of this compound is attributed to the presence of three -Cl atoms on the phenyl ring.
It is a decursinol hybrid which itself is not active against BuChE. Increase in activity of hybrids could be attributed to the 1,2,3-triazole-lipoic acid scaffold. [89].
It is a decursinol hybrid which itself is not active against BuChE. Increase in activity of hybrids could be attributed to the 1,2,3-triazole-lipoic acid scaffold. [89].
It is a decursinol hybrid which itself is not active against BuChE. Increase in activity of hybrids could be attributed to the 1,2,3-triazole-lipoic acid scaffold.
It is a decursinol hybrid which itself is not active against BuChE. Increase in activity of hybrids could be attributed to the 1,2,3-triazole-lipoic acid scaffold. [89].
The potent anti-AChE activity of this compound is attributed to the presence of three -Cl atoms on the phenyl ring. [92] Inhibits both AChE and BuChE (IC 50 of 7.3 and 68.6 µM).
It is a decursinol hybrid which itself is not active against BuChE. Increase in activity of hybrids could be attributed to the 1,2,3-triazole-lipoic acid scaffold. [89].
It is a decursinol hybrid which itself is not active against BuChE. Increase in activity of hybrids could be attributed to the 1,2,3-triazole-lipoic acid scaffold. [89].
The potent anti-AChE activity of this compound is attributed to the presence of three -Cl atoms on the phenyl ring.
(ii) The presence of a chlorine atom at the C-6 position of tacrine was also observed to decrease the inhibitory activity. Molecular docking studies revealed a similar binding interaction mode for 60 in the active sites of hAChE (PDB ID: 1B41) and hBuChE (1P0I), viz., the acridine ring system interacted with Trp86 (hAChE) and Trp82 (hBuChE) through a π-π interaction in the CAS region, while piperazine and 1,2,3-triazole interacted with Tyr124 and Trp286 residues in hAChE. In the PAS region of hBuChE, substituted quinolines present at the C-1 position of the 1,2,3-triazole ring were found to form π-π interactions with Ser287 and Pro285 residues. The authors suggested that the weaker anti-ChE activity of 60, in comparison with tacrine, could be due to the rigidity of the 1,2,3-triazole and piperazine rings in addition to the presence of many polar groups in its structure, which unfavorably contributes to the hydrophobicity of the active sites in the PAS region [106].

Tacrine-1,2,3-triazole-chalcone Hybrids
Tacrine is a low-molecular-weight compound and is an excellent ChE inhibitor. Despite its hepatotoxicity, tacrine is still used by medicinal chemists to develop MTDLs for AD [25,26]. Several previous studies tried to reduce its hepatotoxicity by conjugating or coupling the tacrine motif with antioxidant and hepatoprotective molecules [107]. Chalcones or benzyl acetophenones, or α, β-unsaturated ketones (1,3-diaryl-2-propen-1-ones), are part of natural products such as curcumin, butein, cardamonin, and isoliquiritigenin. Chalcones, owing to their useful anti-AChE, neuroprotective, antioxidant, anti-inflammatory, and vasodilator activities [108], are considered a versatile motif by medicinal chemists. This  ((1,2,3,4-tetrahydroacridin-9-yl)amino)methyl)-1H-1,2,3-triazol-1-yl)octyl)oxy)phenyl)prop-2-en-1-one 65 (IC 50 = 0.327 µM) -were more potent than the standard drug tacrine (IC 50 = 0.375 µM). These compounds did not show any acute toxicity and, thus, were further tested for anti-BuChE activity. A SAR analysis revealed that the nature of the substituents on the A ring of the chalcone, as well as the carbon chain length of the spacer between the chalcone and 1,2,3-triazole, influenced the AChE inhibitory activity. Conjugates having a 4-Cl group on ring A of the chalcone showed higher activity than other substituents (4-F, 4-OCH 3 , 2,3,4 tri -OCH 3 ). The optimal chain alkyl chain length was noted to be n = 3 or 4 because increasing the chain length resulted in a reduction in AChE inhibition. All five ferrocenyl conjugates showed lower activity than compound 63, and only 65 (n = 6) displayed at par AChE inhibition (IC 50 = 0.375 µM) to that of tacrine ( Figure 20). Furthermore, only compound 64 showed promising activity against BuChE (IC 50 = 5.328 µM), suggesting that these hybrid molecules are selective AChE inhibitors. The results of in vivo studies demonstrated that compounds 63 and 64 could reverse scopolamine-induced oxidative stress. In molecular docking studies performed using AChE (PDB id: 1H22), a docking score of 63 (−14.189 kcal/mol) was better than 64 (−13.459 kcal/mol), which clearly supported the results of the in vitro experiments. It was observed that the π-π, π-cationic, and salt bridge interactions of 63 influenced its binding mode with AChE. The benzene ring in the ethoxy benzene fragment, benzene ring, and pyridine ring of the tacrine of 63 showed π-π interactions with Tyr334 and Phe330, respectively. Two π-cationic interactions were seen between the NH group of the pyridine ring of 63 and Phe330 and Trp84 residues. The NH group of the pyridine ring also formed a salt bridge interaction with the carboxylate group of Asp72. Compound 64 showed a slightly different binding affinity than 63 and was involved in one H-bond (the C=O of the benzophenone and Ser286) and two π-cationic interactions between the tacrine moiety and Trp84. Both 63 and 64 fit snugly in the active site cavity of AChE, which is mostly forming hydrophobic interactions. Based on in vitro and molecular docking studies, it can be suggested that 1H-1,2,3-triazole-tethered tacrine-chalcone conjugates may prove to be a valuable scaffold for the treatment of neurodegenerative diseases such as AD [109].
(((1,2,3,4-tetrahydroacridin-9-yl)amino)methyl)-1H-1,2,3-triazol-1-yl)octyl)oxy)ph nyl)prop-2-en-1-one 65 (IC50 = 0.327 μM)-were more potent than the standard crine (IC50 = 0.375 μM). These compounds did not show any acute toxicity and, th further tested for anti-BuChE activity. A SAR analysis revealed that the nature of stituents on the A ring of the chalcone, as well as the carbon chain length of th between the chalcone and 1,2,3-triazole, influenced the AChE inhibitory activity gates having a 4-Cl group on ring A of the chalcone showed higher activity tha substituents (4-F, 4-OCH3, 2,3,4 tri -OCH3). The optimal chain alkyl chain length w to be n = 3 or 4 because increasing the chain length resulted in a reduction in AC bition. All five ferrocenyl conjugates showed lower activity than compound 63, a 65 (n = 6) displayed at par AChE inhibition (IC50 = 0.375 μM) to that of tacrine ( Fig  Furthermore, only compound 64 showed promising activity against BuChE (IC5 μM), suggesting that these hybrid molecules are selective AChE inhibitors. The r in vivo studies demonstrated that compounds 63 and 64 could reverse scopolam duced oxidative stress. In molecular docking studies performed using AChE ( 1H22), a docking score of 63 (−14.189 kcal/mol) was better than 64 (−13.459 kc which clearly supported the results of the in vitro experiments. It was observed π-π, π-cationic, and salt bridge interactions of 63 influenced its binding mode wit The benzene ring in the ethoxy benzene fragment, benzene ring, and pyridine rin tacrine of 63 showed π-π interactions with Tyr334 and Phe330, respectively. Tw onic interactions were seen between the NH group of the pyridine ring of 63 and and Trp84 residues. The NH group of the pyridine ring also formed a salt bridge tion with the carboxylate group of Asp72. Compound 64 showed a slightly differe ing affinity than 63 and was involved in one H-bond (the C=O of the benzophen Ser286) and two π-cationic interactions between the tacrine moiety and Trp84. and 64 fit snugly in the active site cavity of AChE, which is mostly forming hydr interactions. Based on in vitro and molecular docking studies, it can be suggested 1,2,3-triazole-tethered tacrine-chalcone conjugates may prove to be a valuable sca the treatment of neurodegenerative diseases such as AD [109].   (Figure 21). The SAR studies indicated that the presence of 4-CH 3 group in the coumarin ring and the -Cl group in the tacrine fragment increased AChE inhibitory activity. The length of the methylene linker also affects the anti-AChE activity because of its different lipophilicity and spatial hindrance [110].

Tacrine-1,2,3-Triazole Glycoconjugates
Some new tacrine-linked 1,2,3-triazole glycoconjugates were synthesized using Huisgen's [3 + 2] cycloaddition between anomeric azides and tacrine-containing terminal acetylenes. Because the drug tacrine is associated with hepatotoxicity, therefore, the compounds were evaluated for hepatotoxicity studies along with in vitro AChE inhibitory activity. The compounds were found to be nontoxic to HePG2 cell lines at 200 µM after 24 h of incubation. Compound 68 showed the most potent AChE inhibitory activity ( Figure  22) at IC50 of 0.4 µM and may be considered a versatiletemplate for the further development of drugs for AD [111].

Tacrine-1,2,3-triazole Glycoconjugates
Some new tacrine-linked 1,2,3-triazole glycoconjugates were synthesized using Huisgen's [3 + 2] cycloaddition between anomeric azides and tacrine-containing terminal acetylenes. Because the drug tacrine is associated with hepatotoxicity, therefore, the compounds were evaluated for hepatotoxicity studies along with in vitro AChE inhibitory activity. The compounds were found to be nontoxic to HePG2 cell lines at 200 µM after 24 h of incubation. Compound 68 showed the most potent AChE inhibitory activity ( Figure 22) at IC 50 of 0.4 µM and may be considered a versatiletemplate for the further development of drugs for AD [111].

Tacrine-1,2,3-Triazole Glycoconjugates
Some new tacrine-linked 1,2,3-triazole glycoconjugates were synthesized using Huisgen's [3 + 2] cycloaddition between anomeric azides and tacrine-containing terminal acetylenes. Because the drug tacrine is associated with hepatotoxicity, therefore, the compounds were evaluated for hepatotoxicity studies along with in vitro AChE inhibitory activity. The compounds were found to be nontoxic to HePG2 cell lines at 200 µM after 24 h of incubation. Compound 68 showed the most potent AChE inhibitory activity ( Figure  22) at IC50 of 0.4 µM and may be considered a versatiletemplate for the further development of drugs for AD [111].

Chalcone-1,2,3-Triazole Hybrids
A chalcone core with 1,2,3-triazole derivatives were synthesized, characterized and evaluated for their inhibitory activity against AChE and BuChE. The synthesized compounds showed good inhibitory activity against two ChE enzymes, AChE and BuChE, with Ki values in the range of 5.88-11.13 µM and 5.08-15.12 µM. The derivatives with benzothiophene showed the most potent inhibitory activities against AChE (IC50 = 7.92

Chalcone-1,2,3-Triazole Hybrids
A chalcone core with 1,2,3-triazole derivatives were synthesized, characterized and evaluated for their inhibitory activity against AChE and BuChE. The synthesized com pounds showed good inhibitory activity against two ChE enzymes, AChE and BuChE with Ki values in the range of 5.88-11.13 µM and 5.08-15.12 µM. The derivatives with benzothiophene showed the most potent inhibitory activities against AChE (IC50 = 7.92 µM) and BuChE (IC50 = 7.79 µM). The best inhibitory activities against AChE and BuChE were shown by compound 73 and compound 74 (Figure 25) [114].
It was further found that small halogen groups, such as fluorine, and electron-donating groups, such as methyl and methoxy, at the ortho or meta positions of the benzyl showed improvement in AChE inhibitory activity [116].

Oxadiazole-1,2,3-Triazole Hybrids
1,2,3-triazole-oxadiazole conjugates have been designed, synthesized, and evaluated against in vitro AChE. The level of oxidative stress biochemical markers, such as lipid peroxidation, superoxide dismutase, glutathione, and catalase induced by the scopolamine, were also evaluated in the presence of the compounds. Among them, derivatives 83-85 showed good activity against AChE (34.54-47.28% inhibition at 10 µM) ( Figure 29). The compounds also showed a promising role in decreasing oxidative stress (27.74-41.61% inhibition of DPPH at 10 µM). In molecular docking studies against recombinant hAChE, the compounds showed binding with amino acid residues at the active site of the receptors [118]. 3.13. Oxadiazole-1,2,3-triazole Hybrids 1,2,3-triazole-oxadiazole conjugates have been designed, synthesized, and evaluated against in vitro AChE. The level of oxidative stress biochemical markers, such as lipid peroxidation, superoxide dismutase, glutathione, and catalase induced by the scopolamine, were also evaluated in the presence of the compounds. Among them, derivatives 83-85 showed good activity against AChE (34.54-47.28% inhibition at 10 µM) ( Figure 29). The compounds also showed a promising role in decreasing oxidative stress (27.74-41.61% inhibition of DPPH at 10 µM). In molecular docking studies against recombinant hAChE, the compounds showed binding with amino acid residues at the active site of the receptors [118]. mine, were also evaluated in the presence of the compounds. Among them, derivatives 83-85 showed good activity against AChE (34.54-47.28% inhibition at 10 µM) ( Figure 29). The compounds also showed a promising role in decreasing oxidative stress (27.74-41.61% inhibition of DPPH at 10 µM). In molecular docking studies against recombinant hAChE, the compounds showed binding with amino acid residues at the active site of the receptors [118].

83-85
showed good activity against AChE (34.54-47.28% inhibition at 10 µM) ( Figure 29). The compounds also showed a promising role in decreasing oxidative stress (27.74-41.61% inhibition of DPPH at 10 µM). In molecular docking studies against recombinant hAChE, the compounds showed binding with amino acid residues at the active site of the receptors [118].
Pharmaceuticals 2023, 16,179 multiple interactions were observed with different active sites of the enzyme in the PAS [124].

Pyridazinone-1,2,3-Triazole Hybrids
Some novel derivatives of (p-tolyl)-3(2H)-pyridazinone with a 1,2,3-triazole have been synthesized as new agents against AD. The synthesis of the 1,2,3-triaz takes place via the cyclization of thiosemicarbazide derivatives in the presence of The most potent of the derivatives against AChE is a 4-trifluoromethoxy deriv ( Figure 37). The IC50 and the Ki constant for the inhibition against AChE for comp were 0.310 µM and 0.049 µM as compared with standard tacrine (IC50 = 0.519 an µM (Ki)). The molecular docking studies of the most potent compound showed a binding interaction as that of tacrine within the active sites [126].

Pyridazinone-1,2,3-triazole Hybrids
Some novel derivatives of (p-tolyl)-3(2H)-pyridazinone with a 1,2,3-triazole moiety have been synthesized as new agents against AD. The synthesis of the 1,2,3-triazole ring takes place via the cyclization of thiosemicarbazide derivatives in the presence of NaOH. The most potent of the derivatives against AChE is a 4-trifluoromethoxy derivative 95 ( Figure 37). The IC 50 and the Ki constant for the inhibition against AChE for compound 95 were 0.310 µM and 0.049 µM as compared with standard tacrine (IC 50 = 0.519 and 0.226 µM (Ki)). The molecular docking studies of the most potent compound showed a similar binding interaction as that of tacrine within the active sites [126].

Miscellaneous Hybrids of 1,2,3-triazoles
Nicotinic agonists are of interest to treat CNS diseases, particularly schizophrenia and AD. (R) quinuclidine-aryl-1,2,3-triazole derivatives were synthesized and tested for in vitro α7 nicotinic ACh receptor ligands. The aryl ring includes a phenyl ring with a small methoxy; fluorine, the fluoromethyl group, or furan; thiophenes; benzofuran; and benzothiophenes heterocycles. Two compounds, 98 and 99, showed an inhibition constant in a nanomolar range with Ki = 2.3 and 3 nM (Figure 39). The dose range for strict selectivity toward the α4β2 nicotinic receptor was 1 μM, and these interacted with 5HT3 receptors with Ki of 3 nM [129].

Miscellaneous Hybrids of 1,2,3-triazoles
Nicotinic agonists are of interest to treat CNS diseases, particularly schizophrenia and AD. (R) quinuclidine-aryl-1,2,3-triazole derivatives were synthesized and tested for in vitro α7 nicotinic ACh receptor ligands. The aryl ring includes a phenyl ring with a small methoxy; fluorine, the fluoromethyl group, or furan; thiophenes; benzofuran; and benzothiophenes heterocycles. Two compounds, 98 and 99, showed an inhibition constant in a nanomolar range with Ki = 2.3 and 3 nM (Figure 39). The dose range for strict selectivity toward the α4β2 nicotinic receptor was 1 µM, and these interacted with 5HT 3 receptors with Ki of 3 nM [129]. Earlier, the promising synthesis of 1,2,3-triazole araalkylamides and their interactions within both CAS and PAS as AChE inhibitors led Petrat et al. to design and synthesize new 1,2,3-triazole araalkylamide derivatives. The synthesis takes place via a 1,3-dipolar cycloaddition reaction of propargyl amide derivatives and azide derivatives. The most potent AChE inhibitor among them was compound 100, with an IC50 value of 15.01 µM ( Figure 40). It was also found that compound 100 non-competitively inhibit the AChE enzyme. Aromatic residue in the structure interacted with both the CAS and PAS sites, demonstrated by the docking studies, and the 1,2,3-triazole showed van der Waals and hydrogen bond interactions with the amino acid residue in the mid-gorge region of the enzymes [130].
Anil et al. synthesized 1H-1,2,3-triazole using the click chemistry approach. The synthesis started with a reaction of benzaldehyde and propargyl alcohol under basic conditions to obtain 1,4-dihydroxyalkyne derivatives. The primary alcohol was converted to mesyl chloride in the presence of triethylamine and then into 1,2,3-triazole derivatives after a reaction with sodium azide. The compounds were evaluated for inhibitory activity against AChE and were found to be potent inhibitors. The   Figure 40). It was also found that compound 100 non-competitively inhibit the AChE enzyme. Aromatic residue in the structure interacted with both the CAS and PAS sites, demonstrated by the docking studies, and the 1,2,3-triazole showed van der Waals and hydrogen bond interactions with the amino acid residue in the mid-gorge region of the enzymes [130]. Earlier, the promising synthesis of 1,2,3-triazole araalkylamides and their interactions within both CAS and PAS as AChE inhibitors led Petrat et al. to design and synthesize new 1,2,3-triazole araalkylamide derivatives. The synthesis takes place via a 1,3-dipolar cycloaddition reaction of propargyl amide derivatives and azide derivatives. The most potent AChE inhibitor among them was compound 100, with an IC50 value of 15.01 µM ( Figure 40). It was also found that compound 100 non-competitively inhibit the AChE enzyme. Aromatic residue in the structure interacted with both the CAS and PAS sites demonstrated by the docking studies, and the 1,2,3-triazole showed van der Waals and hydrogen bond interactions with the amino acid residue in the mid-gorge region of the enzymes [130].

Conclusions and Future Directions
Cholinesterase is one of the most conspicuous molecular targets for the development of pharmacotherapeutic agents to treat NDs such as AD, senile dementia, ataxia, myasthenia gravis (MG), and PD. AChE inhibitors such as donepezil, galantamine, tacrine, and rivastigmine are some of the most clinically useful anti-AD drugs. However, their usefulness is limited either due to toxicity or their inability to halt the progression of AD. Hence, medicinal chemists have used the hybridization strategy to synthesize potent dual ChE inhibitors and selective AChE or BuChE inhibitors to overcome their toxicity, improve their pharmacokinetic profiles, and develop more potent neuroprotective agents capable of acting on other molecular targets in addition to ChE inhibition. Over the past two decades, numerous hybrids of various heterocyclic moieties have been designed and synthesized as ChE inhibitors, but only one hybrid compound, ladostigil, could enter phase 3 clinical trials; however, these hybrid molecules have helped the scientific community advance the understanding of the complex pathogenesis of the disease. Furthermore, the safety, efficacy, and tolerability of ChE inhibitors entice researchers to design and develop new classes of ChE inhibitors.
Undoubtedly, cholinergic theory is the most widely accepted hypothesis in the pathogenesis of AD, and, therefore, ChE inhibitors are considered the primary therapy for the symptomatic treatment of AD. Despite uncertainty about the duration of the benefits of clinically used ChE inhibitors, the authors believe that developing potent and selective ChE inhibitors will certainly help in delaying the progression and alleviating the symptoms associated with AD and other NDs, which may lead to better cognitive performance and quality of life for individuals. In addition, these inhibitors have been shown to exhibit disease-modifying effects. However, the paradigm in recent times is slowly shifting to developing multitarget ligands capable of acting on other targets, including ChE, to combat AD. Available clinical evidence indicates that monotherapy with ChE inhibitors is not sufficient to halt the progression or treat AD; nevertheless, small molecules developed as potent ChE inhibitors can be used in combination (as an adjunct therapy) with recently approved US FDA drugs, viz., aducanumab and leqembi™ (lecanemab-irmb), to treat AD and improve the quality of life of patients. Some of the potent compounds that are hybrids of bioactive heterocyclic moieties (isatin, indanone, tacrine, coumarin) and 1,2,3-triazole mentioned in this paper could be used as lead templates for the further optimization and development of potential ChE inhibitors as anti-AD agents.