Green and Regioselective Approach for the Synthesis of 3-Substituted Indole Based 1,2-Dihydropyridine and Azaxanthone Derivatives as a Potential Lead for SARS-CoV-2 and Delta Plus Mutant Virus: DFT and Docking Studies

Great attempts have been done for the development of novel antiviral compounds against SAR-CoV-2 to end this pandemic situation and save human society. Herewith, we have synthesized 3-substituted indole/2-substituted pyrrole 1,2-dihydropyridine and azaxanthone scaffolds using simple, commercially available starting materials in a one-pot, green, and regioselective manner. Further, the regioselectivity of product formation was confirmed by various studies such as controlled experiments, density functional theory (DFT), Mulliken atomic charge, and electrostatic potential (ESP) surface. In addition, 3-substituted indole 1,2-dihydropyridine was successfully converted into a biologically enriched pharmacophore scaffold, viz., indolylimidazopyridinylbenzofuran scaffold, in excellent yield. Moreover, the synthesized 3-substituted indole 1,2-dihydropyridine/2-substituted pyrroles were analyzed in docking studies for anti-SARS-CoV-2 properties against their main protease (Mpro) and anti-Delta plus properties against their protein of the Delta plus K417N mutant. Further, the drug-likeness prediction was analyzed by the Lipinski rule and other pharmacokinetic properties like absorption, distribution, metabolism, excretion, and toxicity using preADMET prediction. Interestingly, the docking results show that out of 20 synthesized compounds, 5 of them for Mpro of SAR-CoV-2 and 9 of them for 7NX7 spike glycoprotein’s A chain of Delta plus K417N show greater binding affinity when compared with remdesivir that is the first to receive FDA approval and is currently used as a potent drug for the treatment of COVID-19. These results suggest that indole/pyrrole substituted 1,2-dihydropyridine derivatives are capable of combating SARS-CoV-2 and its Delta plus mutant.


■ INTRODUCTION
Severe acute respiratory syndrome coronavirus (SARS-CoV) and Middle East respiratory syndrome coronavirus (MERS-CoV) mainly cause zoonotic infections originating from the bat/civet and dromedary. 1 In 2019, a new type of coronavirus was first identified in Wuhan, a city in China, which spreads quickly by human-to-human transmission worldwide. 2 Hence, a global emergency was made on 11 Feb 2020 by the International Virus Classification Commission (ICTV) to classify the new coronavirus as ″2019-nCoV″. In the meantime, the World Health Organization also named the 2019-nCoV ″COVID-19″. The pandemic waves have been causing severe acute respiratory syndrome and other chronic illnesses to affected individuals regardless of age. Later, COVID-19 was renamed as ″severe acute respiratory syndrome coronavirus-2″ (SARS-CoV-2). An outbreak of the SARS-CoV-2 seemed inevitable due to the rapid transmission rate by infected or asymptomatic persons' aerosols and the improper social distance in affected countries . 3 Typically, symptoms of the SARS-CoV-2 include cough, fever, breathing difficulty, sore throat, diarrhea, headache, nausea, congestion, and loss of taste or smell, leading to severe respiratory problems and finally death. Therefore, SARS-CoV-2 is called a ″novel coronavirus″. The World Health Organization (WHO) statistically reports that the global pandemic distribution rate is 572,239,451 confirmed cases of COVID-19, including 6,390,401 confirmed deaths, as of 29 July 2022 in 223 countries overall. Noteworthily, the SARS-CoV-2 spike receptor protease plays a critical role in facilitating the virus invasion on the human cell receptor (angiotensin converting enzyme-2, ACE2) and the host cell receptor (transmembrane protease serine-2, TMPRSS2). 4 Later, the pandemic rapidly moved to the second wave due to the sudden mutant of the SARS-CoV-2 spike proteins known as Delta variant B.1.617. This Delta variant seems to be around 60% more transmissible than the other variants such as Alpha (B.1.1.7), Beta (B.1.351), and Gamma (P.1) across the world; even vaccine-induced antibodies are less potent in reducing the effect of these variants because of their two E484Q and L452R mutations. 5 Very recently, the B.1.617.2 variant was identified as another new variant called Delta plus; this additional K417N mutation is expected to have a greater affinity toward lung tissues as compared to previous strains. 6 At present, no selective drugs are available for effective treatment; hence, the development of potential drugs against SARS-CoV-2 variants is essential to safeguard our society. Due to the synthetic complications of new drug discovery and their unknown mechanistic pathways against SARS-CoV-2, most research scientists and drug discovery research centers have suggested the repurposing of some known existing potent small-molecule-based drugs such as remdesivir, hydroxychloroquine, favipiravir, pirfenidone, baricitinib camostat, and lopinavir/ritonavir based on their mechanistic actions. 7 Unfortunately, none of the above drugs could effectively cure SARS-CoV-2 except remdesivir. Regrettably, the synthesis of remdesivir involves multiple steps 8 and increased costs, and it also causes several side effects on post-COVID-19 treatment. Hence the development of superior drugs against this SARS-CoV-2 by an efficient and straightforward approach is highly desirable.
Therefore, exploring structurally diverse active drug molecules via multicomponent reactions (MCRs) 9 plays a vital role in drug discovery progress due to its large-scale production, fewer synthesis steps, and process efficiency for societal benefits. MCR approaches are widely used as a superior method to overcome traditional stepwise synthetic complications because of their simple operation, shorter reaction time, high atom economy, reduced toxic byproducts, easy isolation process, avoidance of the tedious column chromatography purification process, etc. Hence, MCRs have been utilized as an excellent tool for synthesizing biologically significant nitrogen-based heterocyclic compounds via diversity-oriented synthesis (DOS).
Nitrogen-containing heterocycles, especially indole scaffolds, exhibit various biological activities, i.e., antiviral, antiinflammatory, anticancer, anti-HIV, antioxidant, antimicrobial, anti-tubercular, antidiabetic, antimalarial, anticholinesterase activities, etc., which makes researchers more intent on them for the development of indole-based drugs. 10 Recently, indolebased organic compounds have been identified as potential drug candidates for SARS-CoV-2, as shown in Figure 1. Specifically, in 2020, Dai et al. developed an indole-based antiviral drug for targeting the SARS-CoV-2 main protease. 11 In 2021, Hattori et al. synthesized a small molecule with an indole-benzothiazole hybrid that targets the main protease of SARS-Cov-2 and blocks viral replication. 12 Similarly, Ghosh et al. developed indole-chloropyridinyl derivatives that exhibited potential activity against SARS-CoV-2. 13 Recently, Pfizer developed an indole-based antiviral drug as an inhibitor of the coronavirus 3CL protease for the potential treatment of COVID-19. 14 Although these indole compounds were shown to be very much effective against SARS-CoV-2, unfortunately, they have several drawbacks such as the multiple steps, requirement of prefunctionalized starting materials, toxic solvent used, lesser yield, poor atom economy, and tedious separation process. To overcome these drawbacks, effective and green pathways are required to develop antiviral drugs for SARS-CoV-2. Recently, our research group reported a wide range of indole-chalcones using in silico pharmacokinetics and molecular docking studies against essential proteins of SARS-CoV-2. 15 Similarly, other classes of N-fused heterocycles such as imidazopyridine and pyridopyrimidine have shown a wide range of biological activities such as antiviral, antifungal, anticancer, anti-inflammatory, anti-ulcer, anxiolytic, antidepressant, gastrointestinal, neurotropic, stress protecting agents, etc. 16 For our drug discovery program (DDP), the existence of the potential biological profile of indole-based heterocyclic compounds, our intent is to design and develop 3-substituted indoles appended N-fused heterocycles of imidazopyridine and pyridopyrimidine via a one-pot MCR approach to acquire remarkable biological applications in COVID-19 disease using target-oriented synthesis (TOS).

■ RESULTS AND DISCUSSION
In our DDP, we utilize a nitroketene S,S-acetal (NKA) as an  efficient synthon or versatile intermediate with multiple  reactive sites to synthesize several attractive biologically active  heterocyclic scaffolds via a simple one-pot and greener  approach (see Supporting Information S2). Herein, first, we report a novel regioselective 3-substituted indole/2-substituted pyrrole based 1,2-dihydropyridine and azaxanthone scaffold using In(OTf) 3 as a catalyst in ethanol solvent under a reflux condition. Further, the synthesized compounds were examined against SARS-CoV-2 by docking studies. The current regioselective approach involves NKA, various nitrogen-based dinucleophiles, 3-formylchromones, and indoles in the presence of a reusable catalyst in an eco-friendly solvent under a reflux condition with a shorter reaction time, as shown in Scheme 1.
The reaction was completed within 0.5 to 2 h, and the resultant reaction mixture was purified by a simple filtration technique and gave the corresponding compounds with excellent yields. In this work, our aim was to develop biologically significant heterocycles through a novel methodology 17 and explore the chemistry of the NKA system 18 for synthesizing interesting heterocycles via a one-pot multicomponent reaction in a simple and eco-friendly approach.
Initially, the MCR of NKA 1, ethylenediamine 2a, 3formylchromone 3a, and indole 4a led to the formation of 3substituted indole 6a that was taken as a model reaction to investigate the feasibility to optimize the reaction conditions as illustrated in Table 1. When the reaction was performed without a catalyst in ethanol as a solvent, no product formation was observed even after 24 h at ambient temperature. The same reaction was carried out in various solvent polarities such as water, methanol, acetonitrile, and toluene, including neat conditions under reflux conditions. Again, no product formation was observed even after 24 h ( Table 1, entries 1  and 2). Further, the reaction was tested with an organic base, viz., piperidine, and surprisingly, the product was formed at a moderate yield of 40% in ethanol used as a solvent. With the interesting result obtained from piperidine, next we tested other organic bases such as TEA, DBU, and DIPEA under the same reaction condition as well as solvent polarity, and the Scheme 1. Regioselective Synthesis of 3-Substituted Indole Derivatives (6) obtained results are summarized in Table 1 (entries 3−6). To further improve the product yield, we used certain Lewis acids such as p-TSA, AlCl 3 , InCl 3 , In(OTf) 3 , Cu(OTf) 3 , Sc(OTf) 3 , and Yb(OTf) 3 on the same reaction condition (Table 1, entries 7, 8, 9, 10,14, 16, and 17). Interestingly, when InCl 3 was used as a catalyst (5 mol %), it enhanced the reactivity with a good yield of 75% (Table 1, entry 9). Notably, when In(OTf) 3 was used as a catalyst (5 mol %), a notable improvement of product 6a was observed with a 90% yield (Table 1, entry 10). Interestingly, the reaction gave a maximum yield of up to 90% when In(OTf) 3 2 mol % was used (Table 1, entry 12), and further reduction of the amount of the catalyst decreased the yield (Table 1, entry13). The 2 mol % of indium triflate was sufficient to afford product 6a in 90% yield within 0.5 h in the ethanol solvent under reflux conditions. The completion of the reaction was monitored by TLC, and the formation of product 4a was initially confirmed by NMR spectroscopy (see Supporting Information S3). Hence, it was found as the optimized reaction condition and was used to synthesize indole substituted 1,2-dihydropyridine derivatives efficiently. Using this optimized condition, the scope of the reaction was further extended with different diamines, substituted 3-formylchromones, and indoles. To our delight, all the substituted compounds worked well and afforded diverse 3-substituted indole based 1,2-dihydropyridine derivatives 6a−l in an excellent yield from 85 to 90% in a shorter reaction time with high regioselectivity ( Table 2). For further diversity, pyrrole was used as an interesting candidate in place of indole to understand the present scope of the reaction. Interestingly, the reaction occurred very well, and the pyrrole appended products 6m−o were obtained in good yields (82−86%), as shown in Table 2. Based on the product formation, a plausible reaction mechanism was proposed to synthesize 6a, as shown in Scheme 2.
In the first step, NKA 1 reacts with ethylenediamine 2 to    To understand these reaction pathways and product formation, we attempted to separate all the intermediates; unfortunately, the intermediates could not be isolated except A and 7, and it was revealed that intermediates A and 7 might be formed concomitantly and reacted instantly to give 3substituted 1,2-dihydropyridine 6 in a straightforward approach. As shown in the proposed reaction pathway in Scheme 2, we found that the reaction goes via the formation of an interesting heterocycle, viz., azaxanthone 7, and acts as a key intermediate to synthesize the entire final product 6.
Further, azaxanthone scaffolds are another class of nitrogenbased heterocyclic compounds that shows significant biological activities for the treatment of tumors and allergies and as bronchodilators. 19 Significantly, the azaxanthone-based amlexanox drug was approved by the Food and Drug Administration (FDA) in 1996 for the treatment of aphthous ulcers and allergies and as inflammatory immunomodulators. 20 Based on the interesting biological profile of azaxanthone scaffolds, we have a plan to develop azaxanthone alone by performing a three-component reaction with the same starting materials in the absence of indole such as NKA 1, ethylenediamine 2, and 3-formylchromone 3 in the presence of In(OTf) 3 as the catalyst in an ethanol solvent under reflux conditions. After completion of the reaction, the product was separated and confirmed by NMR spectroscopy (see Supporting Information S3). To our delight, all the azaxanthone derivatives (7a−e) were obtained in good yields (83−87%) in the same optimized reaction condition as shown in Scheme 3 and Table 3.
Based on the fruitful results and to understand the regioselectivity of the product formation, we performed controlled experiments with compounds 6 and 4, as shown in Scheme 4. Here, the reaction was performed with azaxanthone 7 and indole 4 using the optimized reaction  condition to get any possibility of the other regioisomer of product 5. Unfortunately, no product formation of 5 was observed as we expected, but product 6 was only formed regioselectively. Further, the same reaction was monitored by increasing the reaction time up to 12 h. Regrettably, no significant product formation of 5 and product 6 was observed again even at room temperature. Similarly, the reaction was performed with product 6 and indole 4 under the same optimized reaction condition to get any possibility of the formation of product 5. The reaction did not proceed again, and starting materials remained unchanged. A similar fashion continued when the reaction was performed for 12 h under reflux and at room temperature individually. The observed controlled experiments strongly supported that the formation of product 6 follows only on path B through pyridinium ion intermediate F over the other pathways of the A or C mechanism.
To demonstrate further diversity and develop a multisubstituted dihydropyridine framework, we next performed the reaction with interesting and highly reactive heterocyclic nucleophiles such as pyrazolone, imidazole 4-hydroxycoumarin, and 1,4-dihydroxynaphthoquinone in the place of indole under similar optimized reaction conditions. When we used pyrazolone as a nucleophile with azaxanthone, the expected product was either pyrazole-appended 1,2-dihydropyridine or 1,4-dihydropyridine in the optimized reaction conditions. Unfortunately, the reaction did not proceed, and the starting material remained unchanged. Further, the optimized reaction condition was altered by increasing catalyst loading up to 10 mol % and reaction time up to 24 h. Unfortunately, we could not obtain any appreciable results. Similarly, the same trend was seen for all the heterocyclic nucleophiles except indole, as shown in Scheme 5.
These results reveal that the indole nucleophile reacts rapidly and selectively opens the ring of azaxanthone in a regioselective fashion compared to other nucleophiles such as pyrazolone, imidazole, 4-hydroxycoumarin, and 1,4-dihydroxynaphthoquinone.
Moreover, the current highlight of the MCR protocol reveals that In(OTf) 3 acts as an interesting Lewis acid catalyst, 21 which was quickly recovered and reused in the subsequent reactions up to the sixth run (see Supporting Information S4). The catalyst yielded compound 6a with 90% yield in the first and second runs. In the third run, we observed only a 2% loss in the yield. Even in the fourth and fifth runs, a 3% loss was observed. In the sixth run, a 5% loss was obtained. The obtained results revealed that the catalyst is highly efficient and recyclable with negligible loss in product formation. The significant highlight of the synthetic protocol is that almost all the criteria of green chemistry and sustainable principles are fulfilled. All the reactions were carried out in a less alcoholic and hazardous solvent, viz., ethanol, with high atom economy with low E-factor (see Supporting Information 109) and a reusable catalyst, and mainly avoided the chromatographic separation (waste generated by separation process) for ecological concerns.
To study the scalability of the present synthetic strategy, the reaction of 1, 2a, 3a, and 4a in the presence of indium triflate was performed on a gram scale to afford the desired product 6a in a good yield (88%, 3.664 g) after isolation as shown in Scheme 6. This easy, scalable process is beneficial in pharmaceutical industries for affordable bulk scale synthesis and commercialization of significant drug candidates.
To the best of our knowledge, this is the first example of the regioselective synthesis of 3-substituted indole and 2substituted pyrrole-based imidazopyridine, pyridopyrimidine, and benzoimidazopyridine derivatives from nitroketene-S,Sacetal, diamines, 3-formylchromones, and indoles in the presence of In(OTf) 3 as a reusable catalyst in the domino one-pot reaction sequence. Remarkably, azaxanthones were also synthesized with the same starting materials without indole in optimized reaction conditions. Thus, the novel domino approach delivers a straightforward method for constructing 3-substituted indole/2-substituted pyrrole-based N-fused nitrogen heterocyclic compounds via a one-pot multicomponent reaction with a greener pathway.
In addition, the presence of the o-hydroxy ketone group at the 5-position of synthesized compounds 6a−o makes these compounds excellent candidates as precursors for further synthetic manipulations to meet the need for diverse and useful purposes. To demonstrate the synthetic utilization of this reaction, we performed the synthetic transformations of product 6, as highlighted in Scheme 7.
The product could be readily transformed into indolylimidazopyridinylbenzofuran derivatives (8a−c) through the nucleophilic substitution of substituted phenacyl bromide with the o-hydroxyketone group present in compound 6b followed by an intramolecular Knoevenagel condensation reaction to obtain excellent yields (up to 93−95%) in a simple approach as shown in Table 4.
To the best of our knowledge, this is the first example of the synthesis of an indole substituted imidazole framework from S,S-acetals, diamines, 3-formylchromones, and indoles adopting interesting and unusual reaction pathways, viz., condensation−Henry−intramolecular Michael addition−ring opening−nucleophilic addition−rearrangement sequence reaction, using reusable In(OTf) 3 as a catalyst in a simple and greener approach. Further, the synthesized indole/pyrrole substituted imidazole analogues are easily converted into indolylimidazopyridinylbenzofuran scaffolds containing biologically significant pharmacophores, viz., imidazopyridineindole-benzofuran, with multiple functional groups in a single molecule. Further, this novel approach exhibits a very good Efactor index and high atom economy, indicating that this approach completely adopted the greener chemistry principle; hence, it is a greener approach (see Supporting Information S109).
All the synthesized compounds were characterized by various techniques such as NMR and high-resolution mass spectroscopy, and all the spectra of the synthesized compounds were in good agreement with the proposed structures (see Supporting Information S3). Further, the synthesized 6h compound structure was confirmed by single-crystal X-ray analysis, as shown in Figure 2 Figure 3b. The azaxanthone molecule 7 has more electron-rich regions on the top due to the high electron cloud of carbonyl and nitro groups to prevent the Michael addition of incoming electronrich indole by electrostatic repulsive force. This result favors the formation of 1,2-dihydropyridine 6 through chromone ring opening followed by nucleophilic addition via a path B mechanism over the Michael addition product of 1,4dihydropyridine 5. The observed results strongly confirm that product 6 favored the nucleophilic addition reaction over the Michael addition reaction. The presence of nitro groups at the 3-position and carbonyl groups at the 5-position on the azaxanthone moiety may drive the regioselectivity to afford the products 6. Based on the interesting result and to explore our DDP, the efficacy of synthesized compounds, viz., 3substituted indole/2-substituted pyrrole-based 1,2-dihydropyridine and azaxanthone derivatives, was successfully investigated against SARS-CoV-2 and the Delta plus mutant via in silico studies. Further, the obtained results were compared to the FDA-approved antiviral remdesivir to know the structure− property relationships and the possibilities of using effective leads against COVID-19.
Structure of SARS-CoV-2. SARS-CoV-2 belongs to the spherical β-coronaviruses and is pleomorphic in nature. 23 The overall coronavirus structure is made up of the following structural fragments: (i) spike proteins (S), (ii) membrane proteins (M), (iii) envelope proteins (E), (iv) nucleocapsid (N), and (v) hemagglutinin-esterase dimer (HE) glycoproteins, along with RNA as the genetic material. 24 In an electron microscope, the ″S″ protein peripheral surface resembles a ″crown″ as shown in Figure 4.
The SARS-CoV-2 nucleotide analysis indicates an 80, 55, and 50% homology to SARS-CoV-1, MERS, and common cold CoV, respectively. Further, SARS-CoV-2 has a 30 kb positivesense (single-stranded) RNA genome with 14 open reading frames (ORFs) lying under conserved gene classifications. In the conserved gene, ORF1a and ORF1ab codes located in S, M, E, and N proteins (encoded at the 3′ end) are essential for viral protein integrity. In ORF1ab, the sequences of an amino acid domain show 94.4% similarity with those of SARS-CoV. 25 The nonstructural proteins (Nsp) encoded by the pp1ab polypeptide are needed to form the replicate mechanism in Nsp1−16. Viral enzymes such as the chymotrypsin-like protease (3CL Pro ) and main protease (M Pro ) process the polypeptides to produce nonstructural proteins (Nsp1−16), which encode for endoribonuclease activity and thus play an essential role in viral replication. 26 However, 14 unique   The SARS-CoV-2 Spike Protease Mechanistic Action against the Human Cell. A brief mechanistic action of the spike protease against humans is illustrated by the graphical representation in Figure 5. The SARS-CoV-2 spike protein cross-species transmission is facilitated by the ACE2 biomolecule, which acts as a human entry receptor. According to sequence alignment results, the spike-receptor binding domain (RBD) sequences of SARS-CoV-2 and SARS-CoV are 76% similar. Hence, SARS-CoV-2 could easily bind to the ACE2 receptor. 27 Furthermore, the spike-RBD sequence has a 95% homology between the SARS-CoV-2 and the Bat-CoV RaTG13. In the RaTG13 and the SARS-CoV-2 spike protein series, four of the five most significant amino acids (L465, L495, Y502, D510, and H514) bind to the ACE2 in the Bat-CoV RaTG13, which differ from the SARS-CoV-2. The pathogen causing the SARS-CoV-2 virus can be prevented by several pathways: (i) the target viral enzymes or functional proteins act as a blocker for virus RNA synthesis and replication; (ii) the target viral structural proteins are preventing viral entry to human cell ACE2 receptors or preventing virus self-assembly; (iii) some of the virulence factors are generated to restore the innate host immunity; and (iv) specific receptors or enzymes present in the host may prevent the virus entry into the host cells. Here, Nsp1, Nsp3 (Nsp3b, Nsp3c, PL Pro , and Nsp3e), Nsp7 Nsp8 complex, Nsp9-Nsp10, Nsp14-Nsp16, 3CLPro, E-channel (E protein), ORF7a, spike, ACE2, helicase, RdRp, and TMPRSSS2 are the target proteins.
The papain-like protease (PL Pro ) enzyme is responsible for the cleavage of replicate polyprotein at the N-terminus to release Nsp1, Nsp2, and Nsp3, which are needed for virus replication. 26a The PLPro has been considered a common target for coronavirus inhibitors because of the replication enzymatic mechanism and infection of the host. For example, the antiviral drug ribavirin is expected to bind PLPro with low binding energy. Hence, the SARS-PLPro ribavirin inhibitor is bounded in the active site of the enzyme. Until now, no potential inhibitor for PLPro has been approved by the FDA for commercialization to treat the SARS-CoV-2 illness. The 3CLPro is also known as Nsp5, which cleaved automatically in the polypeptides to generate mature enzymes. Further, it cleaves the downstream of Nsps at 11 sites to release Nsp4-Nsp16. 28 The observed result revealed that the 3CLPro plays a vital role in the maturation of Nsps and is essential for the virus life cycle. The structure and enzymatic mechanism of 3CLPro have been thoroughly investigated to discover promising target drug molecules for SARS-CoV-2. Several peptide and smallmolecule inhibitors are used as common inhibitors for communicable diseases like SARS-CoV 3CLPro. 29 The spike functional protein exists as a trimer on the virus surface in a unique corolla-like structure. The TMPRSS2 initiates virus invasion on the tissue. Afterward, the host cell protease TMPRSS2 cleaves spike into S1 and S2 subunits. 30 The resultant S1 subunit is found in the region (residues 14−685) of the S-N-terminal protein that contains both the receptorbinding domain (RBD) and receptor binding motif (RBM) (residues 25 and 28−32). When the S1 subunit is bound to the host cell receptor, another S2 subunit initiates the virus cell and cell membrane fusion. Both the invasion and virulence of viruses depend on the spike protein structural stability and its cleavage. 31 Hence, antiviral drugs are essential to prevent coronavirus entry via the host surface of the spike protein. All the fusion peptides (FPs), heptad repeat 1 (HR1), heptad repeat 2 (HR2), transmembrane domain (TM), and a cytoplasmic domain (CP) are located in the S2 subunit of the spike protein. 32 The spike protein binds to the ACE2 receptor, and the metastable prefusion occurs via a significant structural reorientation in the confirmation. 33 The cryo-EM analysis revealed that the ACE2 host cell binds via five hydrogen bonds with the RBD of the S-protein. The resultant ACE2-RBD complex of SARS-CoV-2 has a binding affinity of 15 nM, and it is 10 to 20 times higher than that of SARS-CoV-1. 34 Further, the entire RBD of the S-protein is recognized in the extracellular peptidase domain of ACE2 via polar residues. 35 Recently, a unique RRAR Furin cleavage boundary site between the S1 and the S2 subunits was discovered in the SARS-CoV-2, which facilitates the Furin cleavage of the Sprotein of S1 and S2 subunits. 36 This cleavage was thought to be one of the main reasons for the SARS-CoV-2 being superior in cell fusion compared to SARS-CoV-1.
The neuropilin-1 (NRP1) is abundantly expressed in the respiratory and olfactory epithelium, which binds to the cleavage of the S-Furin S1 subunit. This binding may increase the viral infection in humans. 37 In addition, stimulation of the S2 subunit by the transmembrane protease serine 2 (TMPRSS2) is another contributing factor to the SARS-CoV-2 cell invasion capacity of disconnection from the S1

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http://pubs.acs.org/journal/acsodf Article subunit. 38 The priming S2 subunit leads to the reversible folding of a postfusion configuration in which the heptad repeat 1 (HR1) and the heptad repeat 2 (HR2) domains may bind closely to each other. 39 Both of these factors might have been responsible for the SARS-CoV-2 in human-to-human transmission. To suppress the interactions between the virus and host cells, ACE2 binding site antiviral drugs must be designed simply and efficiently to stop spike protein physicochemical activities. Otherwise, the furin protease, TMPRSS2 protease, NRP1, and interfaces of HR1 and HR2 domains might be critical.

6LU7 Main Protease Binding Interactions against the Synthesized 1,2-Dihydropyridines and Azaxanthones.
In modern structure-based drug discovery, molecular docking is used as a powerful preliminary screenshot technique that offers a reliable way to design efficient drug molecules based on the binding affinity of noncovalent interactions of proteins (receptor) and a small molecule (drug). Herein, the SARS-CoV-2 6LU7 M Pro crystal structure is obtained and downloaded from the RCSB Protein Data Bank (PDB). 40 At first, co-crystallized ligands, water molecules, and co-factors are removed before preparing protein for docking study. Now, all molecular docking calculations are demonstrated on the Auto Dock-Vina software against a series of the optimized structure (see Supporting Information S54) of all synthesized small molecules and compared with that of the FDA-approved antiviral remdesivir drug molecule to understand the structure−property relationship. 41 To understand the 6LU7 M Pro binding interaction, we have docked with the synthesized regioselective 3-substituted indole or 2-substituted pyrrole-based 1,2-dihydropyridines and azaxanthones. Interestingly, out of 20 molecules, five of them, viz., 6a, 6c, 6h, 6l, and 7a, have shown strong binding affinity (lower docking score) compared to the FDA-approved standard drug remdesivir as shown in Table 5.
To further visualize effective binding affinity, the 6LU7 M pro dimer, monomer, and receptor-drug binding sites (remdesivir, 6a, 6c, 6h, 6l, and 7a) are illustrated in Figure 6. The sequential amino acid residues of the dimer are classified into three domains: IA or IA′, IIA or IIA′, and IIIA or IIIA′. This The obtained results reveal that the nature of H-bonds, other noncovalent interactions, number of interactions, molecular geometry, and a specific active site of amino acid residual interactions play a vital role in effective binding affinity to facilitate physicochemical properties of the cellular function. Additionally, the inhibition constants for the standard antiviral drug remdesivir and the synthesized lead molecules, viz., 6a, 6c, 6h, 6l, and 7a, were calculated by using Ki = exp(ΔG/RT) equation, where ΔG, R, and T are the docking binding affinity, gas constant (1.9872036 × 10 −3 kcal mol −1 ), and room temperature (298.15 K), respectively. The inhibition constant (Ki) measures the overall binding affinity of lead molecules (Table 5).
At the same time, if Ki is smaller, a lesser amount of medication is needed to inhibit the cellular activity. The calculated inhibition constant (Ki) values for standard antiviral drug remdesivir and the synthesized lead molecules, viz., 6a, 6c, 6h, 6l, and 7a, are 0.496768633, 1.916718917, 1.367594556, 1.619041184, 1.367594556, and 2.269127829 μM. Here, 6a, 6c, 6h, 6l, and 7a compounds exhibit lower Ki values than the standard antiviral drug remdesivir. Hence, these five organic molecules could have many possibilities to act as a potential lead against SARS-CoV-2. Further druglikeness predictions are essential before practical usage.
After the severe outbreak of the COVID-19 first wave, the virus had certain specific mutations in the spike protein, viz., Alpha (B.  1. 617, India). The Delta strains such as E484Q, L452R, and P614R caused an extremely deadly second wave of infections during last summer in India.
The Delta variant spikes hook onto the ACE2 receptor of the human cell, unlock, and then spread infection faster via replicating the virus genetic code. Unfortunately, more adults are affected by this Delta variant even though they have very strong immunity. Recently, the Delta plus of the K417N mutant was first identified in India and spread before completely ending the second wave. This Delta plus variant spreads more quickly and strongly and binds more to lung cells than the previous outbreaks of SARS-CoV-2 versions.  Figure 6. Illustration of 6LU7 M Pro dimer, monomer, and receptordrug binding site (remdesivir and lead molecules of 6a, 6c, 6h, 6l, and 7a). Currently, the Delta plus variant has been found in nine countries, viz., USA, UK, Portugal, Switzerland, Japan, Poland, Nepal, Russia, and China, and it is spreading worldwide. It is expected to cause a severe third wave of the pandemic. Hence, the recent outbreak of the Delta plus variant can be controlled only by the development of effective novel drugs by a simple and greener approach, which is highly desirable. Significantly, the synthesized 3-substituted indole-based 1,2-dihydropyridines and azaxanthones showed good results against 6LU7 Main Protease of SARS-CoV-2. Next, we attempted to find the efficiency of our synthesized compounds against the 7NX7 spike glycoprotein of delta plus K417N mutant. The crystal structure of the 7NX7 spike glycoprotein was downloaded from the Protein Data Bank 42 (PDB code: 7NX7) and docked with all synthesized compounds.
Excitingly, out of 20 compounds, 9 compounds, viz., 6a, 6b, 6c, 6e, 6h, 6l, 6p, 7a, 7c, and 7e, exhibit strong binding energy with a lower bind score compared to the FDA-approved standard drug remdesivir as shown in Table 6. To visualize an effective binding affinity score, the 7NX7 spike glycoprotein's A chain is selectively chosen from hetero-5-mer (chains of A, B, E, H, and L) because of the larger amino acid sequence length as shown in Figure 9 (see Supporting Information S97).
For analysis, all kinds of intermolecular interactions of 2D representation between 7NX7 spike glycoprotein and developed lead molecules (6a, 6b, 6c, 6e, 6h, 6l, 7a, 7c, and 7e) with the standard antiviral remdesivir drug are illustrated in Figure 8. This figure shows that our synthesized compounds were exactly interacting with the protein, which is similar to the remdesivir. In addition, the corresponding hydrogen bond 3D receptor-side surface interaction, noncovalent 3D receptorside surface interaction, and 2D representation of the 7NX7 spike glycoprotein's A chain are shown in Figures 10 and 11 (for other compound interactions, see Supporting Information S103).
against the upcoming third wave of the Delta plus variant. Further, the drug-likeness predictions and ADMET properties were addressed to find the physicochemical properties of lead molecules to be considered effective drug candidates.

Analysis of Drug-Likeness Prediction and ADMET Properties for the Standard Antiviral Drug Remdesivir and the Synthesized Lead Molecules.
The drug-likeness forecast was analyzed by the Molinspiration server with the Lipinski rule, which states that an orally active drug must comply with five laid down criteria viz., molecular mass, mol LogP, hydrogen bond donor, hydrogen bond acceptor, and total polar surface area, for the synthesized nine lead compounds as shown in Table 7. 43 In addition, over 90% of a drug can be absorbed if the TPSA value is <60 Å 2 . It is a significant hydrogen bonding correlation of a molecule for the bioavailability of a drug molecule. The obtained results suggest that all the developed lead compounds 6a, 6b, 6c, 6e, 6l, 6h, 6p, 7a, 7c, and 7e did not violate Lipinski's rule. Unfortunately, the standard antiviral drug remdesivir violated Lipinski's rule; viz., it had more hydrogen bond acceptors (14) and higher TSPA (203.57 Å 2 ). Similarly, other drug-likeness rules such as CMC (Comprehensive Medicinal Chemistry), WDI (World Drug Index), and MDDR (Modern Drug Data Report) were measured using the preADMET server, 44 as shown in Table S8 (see Supporting information S106).
If the developed lead molecules are found to be suitable and qualified and to have a drug-like/mild structure and 90% cutoff value, they should be considered potential lead molecules for real applications. The obtained results suggest that the predominant synthesized compound has a better chance of being used as a potential lead for all SARS-CoV-2.
Caco-2 cell permeability supports the use of drug lead molecules for real-time applications.
The blood−brain barrier (BBB) is a highly selective semipermeable border of endothelial cells that prevents solutes in the circulating blood from nonselective crossing into the extracellular fluid of the central nervous system. If a compound is CNS-active, it causes unintended side effects. If a compound is inactive, it does not pass and avoids the CNS side effects. The 6a, 6c, 6l, and 7a compounds were CNS-active, and 6b, 6e, 6h, 7c, and 7e were inactive, including the standard antiviral drug remdesivir. In plasma protein binding (PPB), the nature of the chemically weak or unbound drug (less than 90%) is highly desirable for diffusion across the cell membrane, including disposition and efficacy. The following compounds, such as remdesivir, 6a, 7c, and 7e, exhibit less than 90% PPB value, and these compounds were weakly bounded across the plasma protein-membrane for diffusion.
The rest of the molecules, such as 6b, 6c, 6e, 6h, 6l, and 7a drugs, show chemically strong bound against PPB. The percentage of human intestinal absorption (HIA) value was calculated by three main methods such as bioavailability, percentage of urinary excretion of drug material following oral administration, and the ratio of cumulative urinary excretion of drug material following oral and intravenous administration. Interestingly, the lead molecules, such as 6a, 6b, 6c, 6e, 6h, 6l, 7a, 7c, and 7e compounds, show more than 70% of the HIA value. But unfortunately, the standard antiviral remdesivir shows a value of less than 70%, which suggests that our synthesized compounds have good absorption properties and could be easily absorbed by the human intestine. Further, the Caco-2 and MDCK cell models are also reliable for in vitro prediction of oral drug absorption. Notably, the synthesized compounds 6a, 6b, 6c, 6e, 6h, 6l, and 7a show moderate permeability within the range of 4−70 nm s −1 , while 7c, 7e and the standard antiviral remdesivir exhibit low permeability with less than 4 nm s −1 in the Caco-2 cell model.
In addition, the two animal models, viz., mouse and rat models, have been used in the carcinogenicity test. In the mouse model, compounds 6a, 6b, 6c, 6e, 6 h, 6l, and 7c showed negative tests. In contrast, compounds 7a and 7e and remdesivir exhibited positive tests. Similarly, in the rat model, compounds 6e, 6h, 7c, and 7e and remdesivir exhibited negative carcinogenicity tests. Unfortunately, the rest of the five compounds, viz., 6a, 6b, 6c, 6l, and 7a, showed a positive test. Finally, the derived in silico and molecular docking studies concluded that the nine compounds, viz., 6a, 6b, 6c, 6e, 6h, 6l, 7a, 7c, and 7e, are expected to act as a potential anti-SARS-CoV-2 drug even for the mutant Delta plus variant.

■ CONCLUSIONS
In summary, we have developed an eco-friendly and regioselective approach for the synthesis of 3-substituted indole/2-substituted pyrrole-based 1,2-dihydropyridine and azaxanthone derivatives from nitroketene S,S-acetal (NKA), diamines, 3-formylchromone, and indole/pyrrole using indium triflate as a green and reusable catalyst in ethanol through a domino condensation−Henry reaction−intramolecular Michael addition−cyclization−ring opening−nucleophilic addition sequence in a one-pot multicomponent reaction. Further, the controlled experiments, selective ring-opening experiments, density function theory, Mulliken atomic charge, and electrostatic potential (ESP) surface studies strongly support the regioselectivity of the product formation of 3-substituted indole based 1,2-dihydropyridine. Moreover, the synthesized dihydropyridine was successfully converted into biologically significant motifs, viz., indolylimidazopyridinylbenzofuran derivatives, in facile synthetic transformation. Further, the synthesized compounds of indole/pyrrole substituted 1,2dihydropyridine derivatives were examined in silico for their pharmacokinetic properties via docking studies Lipinski and ADMET prediction and were confirmed as having a drug-like nature. The docking studies reveal that out of 20 compounds, 5 of them, viz., 6a, 6c, 6h, 6l, and 7a, for the main protease (M pro ) of SARS-CoV-2 and 9 of them, viz., 6a, 6b, 6c, 6e, 6h, 6l, 7a, 7c, and 7e, for 7NX7 spike glycoprotein's A chain of the Delta plus K417N mutant exhibit greater binding ability when compared to the antiviral drug remdesivir that is an FDAapproved drug for the treatment of COVID-19. Further, the

■ GENERAL INFORMATION
Commercially available starting materials were purchased from Sigma-Aldrich and used without further purification. 1 H, 13 C{ 1 H}, and DEPT 135 NMR spectra were recorded in DMSO-d 6 using TMS as an internal standard on a Bruker Avance spectrometer at 400 and 100 MHz, respectively. Chemical shifts (δ) are given in parts per million (ppm) with reference to TMS. J values are given in hertz. The standard Bruker software was used throughout the experiment. X-ray single-crystal diffraction data were collected on X8 Kappa APEXII (Bruker). Mass spectra were recorded on HRMSESI (Q-TOF). The molecular docking Auto Dock-Vina software indole/pyrrole 4 (1 mmol) in ethanol (3 mL), and indium triflate (2 mol %) was added to the reaction mixture and heated in an oil bath at reflux conditions for the stipulated period. The consumption of the starting material was monitored by TLC. The precipitated solid was filtered and washed with ethanol (2 to 3 mL) and dried under a vacuum to obtain the pure products (6a−o) in excellent yields (85−90%) without any further purification. The identities of all of the synthesized products were confirmed by NMR and HRMS (ESI), which were in good agreement with those of the assigned structures. The further represented compound 6h was confirmed by single-crystal X-ray analysis.