Biological evaluation of some novel 1,3-bis substituted-2-isopropylamidines by in silico molecular dynamics and simulation studies

Pradeep Kumar P. S.a, Jeevan Chakravarthy A. S.*b, Shriraksha Ac and Sunil K*a
aDepartment of Chemistry, Sri Siddhartha Institute of Technology, SSAHE, Tumakuru 572105, India. E-mail: sunilk999@gmail.com
bDepartment of Chemistry, BMSIT&M affiliated to VTU, Avalahalli, Yelahanka, Bangalore 560064, India. E-mail: jeechakravarthy@gmail.com
cDepartment of Biochemistry, Bengaluru City University, Bengaluru 560001, India

Received 29th July 2024 , Accepted 12th August 2024

First published on 12th August 2024


Abstract

A tandem one-pot synthesis of some novel 1,3-bis substituted-2-isopropylamidines is reported. The conversion involves the base-mediated tandem acylation of benzylamines with isobutyryl chloride in DCM, and the subsequent amidation of the amide intermediate generated in situ in POCl3 in a nitrogen atmosphere. In-silico ADME and drug-likeness prediction showed good pharmacokinetic properties. Molecular docking was performed to assess the binding affinities of the purified 1,3-bis substituted-2-isopropylamidines with a wide range of protein targets of pathological significance. Among the synthesized molecules, compound 5b showed the highest binding affinity (−10.5 kcal mol−1) with the cdk2 receptor, an important cancer progression receptor. The best pose of the 5b-cdk2 complex was further used in simulation studies of the molecular dynamics. The stability of the protein–ligand complex was theoretically assessed in the simulations of the molecular dynamics using RMSD, RMSF, and analysis of hydrogen bonds.


Introduction

Amidines are a versatile type of organic compound that are structural analogues of carboxylic acids and esters.1 Their profound biological importance is evident in the presence of several naturally occurring biological cores (Scheme 1).2 A wide spectrum of properties, such as anti-inflammatory, diuretic, and anti-diabetic properties, exhibited by this class is attributed to their direct structural relationship with biomolecules.3 Amidines are extensively used as potential precursors in the synthesis of a wide class of biologically important heterocycles, including imidazoles,4 1,2,4-triazoles,5 1,2,3-triazoles,6 isoquinolines, quinolines, quinazolines, 1,2,4-oxadiazoles, thiazoles, and thiadiazoles.7
image file: d4nj03384a-s1.tif
Scheme 1 Some naturally occurring amidines.

In recent years, several synthetic routes have been developed for the synthesis of amidines due to their wide applicability in many fields. Recent synthetic developments include metal-free multicomponent synthesis of sulfonyl amidines by the reaction of enamines generated in situ with azides.8 They are also synthesized from N-arylbromodifluoroacetimidoyl chloride or N-aryltrifluoroacetimidoyl chloride under mild reaction conditions with amines,9 one-pot reactions of alicyclic amines, cyclic ketones and 4-azidoquinolin-2(1H)-ones by tandem conversion of nitriles,10 and the selective conversion of lithium amides generated in situ (Scheme 2a–c).11 In recent decades, extensive work has been carried out on the theoretical and practical applications of heterocycles and their synthesis and bio-evaluation.12


image file: d4nj03384a-s2.tif
Scheme 2 Reported routes for the synthesis of amidines.

The broad scope of heterocycles encouraged us to develop a novel path for their synthesis and the evaluation of their pharmacological properties. Hence, in this study, the following problems were addressed.

Cancer stands second with respect to high mortality rates worldwide;13 more than 10 million deaths were reported due to cancer in 2020. Past decades have seen rapid technological and conceptual progress in various fields to fight cancer, with efforts ranging from massive next-generation sequencing, high-resolution microscopy, molecular immunology, flow cytometry, analysis and sequencing of individual cells, new cell culture techniques, and the development of animal models to the synthesis of novel small molecules with anticancer potency.14 Although there have been consistent efforts in developing analytical techniques and new drugs to target cancer, chemoresistance poses one of the major setbacks to effective cancer therapy, creating demand for new drugs.

Similarly, HIV1-protease, a vital enzyme in HIV replication, is involved in the generation of mature virions that infect CD4+ cells by cleavage of Gag and Pol polyproteins; therefore, it is a potent target to tackle AIDS. Although HIV does not cause any diseases directly, the weakened immune system exposes the infected to the risk of various types of diseases, such as cancer and bacterial and viral infections.15

A constant search for vaccines against the RNA virus, especially the virus behind the global pandemic of 2020, i.e., SARS-CoV-2, is in high demand due to the mutagenic nature of the virus. The viral strains continue to infect people of all ages and genders. Scientific investigations have revealed that the coronaviral genome encodes a main protease and a papain-like protease, both of which are involved in the generation of mature virions. Thus, targeting one of these cysteine-like proteases can help in the reduction of SARS-CoV-2 infections considerably.16

Acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) play vital roles in cholinergic signaling. BChE has been associated with the hallmarks of Alzheimer's, such as amyloidogenesis; in addition, it has been reported to compensate for AChE when its levels go down. Despite numerous strategies for the treatment of Alzheimer's disease, there has been no cure for it, leaving palliative care as the only option.17,18

Methicillin-resistant Staphylococcus aureus is a global cause of concern due to its resistance to the majority of treatment options available. Most antibiotics targeting S. aureus contain beta-lactams that inhibit the four types of penicillin-binding proteins (PBPs) found in MRSA. The fifth type, i.e., PBP2a, is not inhibited by beta-lactams, allowing the synthesis of bacterial cell walls.19 We aim to analyze the ability of our ligands to bind to PBP2a, and thus, inhibit bacterial growth.

Alzheimer's disease is a neurodegenerative disease that has a disruptive cholinergic system that results in the release of low levels of acetylcholine. Reports have shown that the breakdown of acetylcholine can be prevented by targeting the enzyme that causes neurotransmitter hydrolysis, which can then improve the conditions of patients diagnosed with Alzheimer's disease.20

The use of amidines as chemical cores has progressed over several years in biomedical research. This inspired us to synthesize amidines following a new approach. In this present work, we report the synthesis of a set of novel amidines by the tandem conversion of amines to amidines. The presence of a pre-tested pharmaceutically active core in the synthesized molecules led us to conduct a biological evaluation based on their interactions with different proteins of pathological significance using a bioinformatics approach.

Results and discussion

In recent years, the use of amidines as reactants, intermediates, and application products has been widely reported. Langer et al. reported the use of amidines as an intermediate in the total synthesis of dabigatran, a direct thrombin inhibitor.21 Glomb et al. identified and synthesized a novel amidine cross-link structure, N1,N2- bis-(5-amino-5-carboxypentyl)-2-hydroxy-acetamidine (glyoxal lysine amidine, GLA), that was formed from glyoxal through the isomerization cascade; it is used to quantify AGE in processed food proteins.22 Yudin et al. incorporated the amidine functionality into cyclic peptides to serve as a pH-sensitive conformational probe for them and to study the effect of hydrogen bonding on the macrocyclic conformation of proteins.23 Ramachary et al. used amidines, along with ynone moieties, which are Michael acceptors in formal [4+2]-cycloaddition reactions, in the Ca(OTf)2-catalyzed regioselective synthesis of tricyclic azepines.24 Hong et al. reported the synthesis of novel 10π-electron cyclic amidines with excellent fluorescence properties and great potential for biological imaging. The reported amidines were successfully used as fluorescent probes for the localization of the NK-1 receptor in living systems in fluorescence–structure relationship studies.25 An amidine-based ionic liquid was reported by Lucio et al. that could be used as an electrolyte in Al batteries.26 Yang et al. reported the synthesis of a set of amidine derivatives of avibactam and their use as antibacterial agents by in vitro inhibition of β-lactamase, in combination with an antibiotic.27

Given the wide use of the amidine functionality, in the present work, a simple and convenient methodology for the synthesis of novel amidines is proposed via two different approaches, i.e., a step-wise and a tandem one-pot route. (Scheme 3)


image file: d4nj03384a-s3.tif
Scheme 3 Synthesis of amidines 5a–e from amine 1.

Method 1 involves two steps, including (step a) the synthesis and isolation of N- arylisobutyryl amides 3 from the reaction of corresponding amines 1 and isobutyrylchloride 2 in DCM using triethylamine as a base and (step b) the synthesis of 1,3-bis(het)aryl-2- isopropylamidines 5 by amidation of in situ-generated iminylchloride that was formed by the reaction of amides 3 with POCl3 using amine 4. All reactions were monitored by TLC aliquots. Workup and column chromatography purification produced bis(het)aryl-2-isopropylamidines 5a–e in more than 80% yields.

Method 2 involves the conversion of amide 3 formed by the reaction of amine 1 and isobutyryl chloride 2 via sequential conversion to iminylchloride by the addition of POCl3. This reaction was followed by the addition of arylamine 4 in a tandem one-pot synthesis process to obtain the novel 1,3-bis(het)aryl-2-isopropylamidine 5.

Both the methods yielded the novel amidine 5 in greater than 80% yields, however, better yields under minimal reaction conditions were obtained from the one-pot method. The yields of amidines by both methods are furnished in Table 1. Each reaction was carried out for a minimum of three trials and optimum isolated yields were reported.

Table 1 Relative yields of 1,3-bis(het)arylisopropylamidines with two methods
Sl. no. 1,3-Bis(het)arylisopropylamidines % yield in stepwise method % yield in tandem one-pot method
1. image file: d4nj03384a-u1.tif 82% 89%
2. image file: d4nj03384a-u2.tif 84% 91%
3. image file: d4nj03384a-u3.tif 81% 87%
4. image file: d4nj03384a-u4.tif 84% 87%
5. image file: d4nj03384a-u5.tif 81% 85%


The synthesized novel amidine 5 was further evaluated for its physiological profile using molecular docking and simulation of molecular dynamics.

Biological activity prediction

The probable molecular targets of compounds 5a–e were predicted using online PASS Targets.28 The top three targets for each compound were chosen for further analysis. Values of Pa above 0.3 were considered for molecular docking with targets specific to the disease, and the results are represented in Fig. 1. Compounds 5a, 5b, and 5c showed Pa values of 0.336, 0.337, and 0.333 respectively, for the treatment of preneoplastic and neoplastic conditions. It was observed that compound 5b also showed Pa > 0.3 for the treatment of neurodegenerative diseases; it was also anticipated to be effective for interactions with the P-glycoprotein. Compounds 5d and 5e had almost equal Pa values (0.30–0.39) for neurodegenerative diseases and P-glycoprotein. Except for compound 5c, all compounds were predicted to have antiviral activity against SARS coronavirus and retroviruses.
image file: d4nj03384a-f1.tif
Fig. 1 Molecular targets prediction.

ADMET studies

The drug likeliness of all five synthesized compounds 5a–e was analyzed for ADMET properties in silico using online tools, swiss-ADME and pkCSM.28,29 Results are summarized in Table 2. All the synthesized compounds, except 5c, showed high gastrointestinal absorptivity, which obeyed Lipinski's rule of five. Furthermore, compounds, 5d and 5e, could permeate the blood–brain barrier. Fig. 2 shows the boiled egg and radar map of compounds 5a–e, where the pink area defines the oral bioavailability of the query compound. All the molecules were found to have the same oral bioavailability value of 0.55. The toxicity profiles of all five compounds are shown in Table 3.
Table 2 Drug likeliness and pharmacokinetics properties of the synthetic compounds
Ligands Drug likeliness Bioavailability score Pharmacokinetics
Lipinski Ghose GI absorption BBB permeant
5a Yes No; 1 0.55 High No
violation: WLOGP > 5.6
5b Yes; 1 violation: MLOGP > 4.15 No; 1 0.55 High No
violation: WLOGP > 5.6
5c Yes; 1 violation: MLOGP > 4.15 No; 1 0.55 Low No
violation: WLOGP > 5.6
5d Yes Yes 0.55 High Yes
5e Yes Yes 0.55 High Yes



image file: d4nj03384a-f2.tif
Fig. 2 Boiled egg diagram and bioavailability radar of synthetic compounds 5a–e. (a) Boiled egg map of 5a–e, (b) ligand 5a, (c) ligand 5c, (d) ligand 5b, (e) ligand 5d, and (f) ligand 5e.
Table 3 Toxicity profile of the synthetic compounds
Toxicity parameters ↓ 5a 5b 5c 5d 5e
AMES mutagenesis Safe Toxic Safe Toxic Safe
Bioconcentration factor (log10 L kg−1) 0.10 0.81 2.37 1.05 0.48
Biodegradation Safe Safe Safe Safe Safe
Carcinogenesis Safe Toxic Safe Toxic Safe
Liver injury I Safe Safe Safe SAFE Safe
Max. tolerated dose (log mg kg−1 day−1) 0.37 0.26 0.1 0.53 0.73
Skin sensitization Safe Safe Toxic Toxic Safe


Validation of the docking model

The molecular docking models were validated by extracting and re-docking the co-crystallized ligand into the docking site with the same grid parameters using Autodock Vina. This approach was adopted to ensure that the ligand of interest (i.e., the inhibitor) binds to the active site without deviation from the co-crystallized complex. Fig. 3 describes the superimposed ligands with their RMSD values.
image file: d4nj03384a-f3.tif
Fig. 3 Superimposed images of native ligand (red) and re-docked ligand (colored). (a) 1B6M, RMSD value = 0.604, (b) 3PXQ, RMSD value = 0.425, (c) 3VHE, RMSD value = 0.413, (d) 4CJN, RMSD value = 1.57, (e) 6YJC, RMSD value = 0.914, (f) 7BGC, RMSD value = 1.478, and (g) 7XN1, RMSD value = 0.648.

Molecular docking

For a better understanding of the interaction of compounds with proteins of interest, molecular docking was performed using Autodock Vina and the results are summarized in Table 4. Cancer is a class of diseases, which involves the uncontrolled proliferation of cells. There have been many chemotherapeutic strategies to inhibit cell differentiation and proliferation. However, the cells develop resistance to drugs causing them to have little to no effect on the target cells. Thus, the development of new drugs against the same target is a challenge. The novel amidines 5a–e synthesized here were tested as ligands for their anti-cancer properties through molecular docking against the 3D structures of p38 α, cdk2, and vegfr-2 as targets.
Table 4 The docking score results of the novel amidines 5a–e against selected proteins
  Binding affinity (kcal mol−1)
P38 α (6YJC) cdk2 (3PXQ) vegfr-2 (3VHE) HIV1 Protease (1B6M) PBP2a (4CJN) SARS- COV2 main protease (6WTT) Acetyl cholinesterase (7XN1) Butyryl cholinesterase (7BGC)
5a (C16H18F3N3S) −8.9 −9.7 −9.4 −7.1 −7.3 −7.2 −4.9 −9.1
5b C20H22F3N3O2 −9.2 −10.5 −8.3 −7.5 −8.5 −7.5 −5.1 −9.4
5c (C20H22ClF3N2) −9.7 −10.2 −9.9 −7.4 −7.2 −6.5 −5.5 −10
5d (C18H21N3O2) −7.2 −9.0 −7.3 −7.0 −6.7 −7.3 −4.0 −9.6
5e (C18H19FN2O2) −7.9 −8.6 −7.6 −7.0 −6.8 −6.1 −3.8 −9.7


Various stress and non-stress stimuli can activate p38 MAPK signaling that regulates numerous cellular processes. Reports suggest many pro-oncogenic mechanisms by p38 in various cancers, bringing into light the prospects of developing drugs targeting p38.30 Docking studies for all five ligands 5a–e were carried out for p38 alpha inhibitors at a resolution of 1.74 Å. Docking of the ligands with p38 alpha showed binding affinities ranging from −7.2 kcal mol−1 to −9.7 kcal mol−1. The ligand 5c showed the least docking score of −9.5 kcal mol−1. The molecular docking of ligand 5c with 6YJC revealed the active site amino acids to have more hydrophobic interactions with the ligand as shown in Fig. 4a.


image file: d4nj03384a-f4.tif
Fig. 4 PyMol images of the docking results. (a) P38α with ligand 5c, (b) cdk2 with ligand 5b, (c) vegfr-2 with ligand 5c, (d) HIV1 protease with ligand 5b, (e) PBP2a with ligand 5b, (f) SARS-COV2 main protease with ligand 5b, (g) acetylcholinesterase with ligand 5c, (h) butyryl cholinesterase with ligand 5c.

The amino acid residues, Glu(71), Asp(168), and Thr(106), could interact hydrophobically with the ligand 5c. A recent study exploring the dynamic back pocket of p38α revealed a type II inhibitor of the same type to bind to the abovementioned pockets, thereby inhibiting the activity of p38α.31

Targeting CDK can help accentuate other modalities of cancer treatment, while decreasing cell proliferation because it is involved in many oncogenic pathways. Different ligand-binding modes were observed for the cdk2 protein (3PXQ) with a resolution of 1.90 Å. Ligand 5b had the lowest docking score of −10.5 kcal mol−1. The 3D orientation of the best conformation is shown in Fig. 4b. Molecular docking with cdk2 revealed that most of the interacting residues in the active site are hydrophobic in nature. Although all five ligands have promising inhibitory interactions with the receptor, ligand 5b displays the highest binding energy of −10.0 kcal mol−1 and is thus likely to be a potent inhibitor of cdk2 as shown in Table 4. Inhibitory small molecules were studied to bind at the allosteric pockets mentioned above via hydrophobic interactions with Tyr(15), Ile(35), Leu (55), and Lys(56) with a predicted docking score of −10.2 kcal mol−1 (Fig. 4c).32,33

Despite advances in the development of the inhibitors of vegfr, their potential toxicity and low clinical efficacy limit their use. Thus, there is a need for the development of new vegfr inhibitors. Reports show that blocking vegfr2 could effectively suppress tumor growth and metastasis.34,35 A docking study was performed on all the synthesized ligands 5 for vegfr-2 (3VHE–1.55 Å resolutions). Among the ligands tested for docking analysis, (E)-N′-(2-chlorobenzyl)-N-[4-(trifluoromethyl) phenylisobutyrimidamide] (ligand 5c) had the least binding energy of −9.2 kcal mol−1. The docking scores for different ligands with various receptors are summarized in Table 4. In this study, all the ligands 5 were docked deeply into the binding pocket of 3VHE with hydrophobic interactions. The ligand 5c showed hydrophobic interactions with the following amino acid residues: Val(848), Glu(885), Ile(888), Leu(889), Ile(892), Val(899), His(1026), and Asp(1046), as depicted in Fig. 4c.

HIV-affected individuals can be at a higher risk of contracting SARS-CoV-2 or any other infection primarily due to their weakened immune systems. Reports suggest that the use of antivirals that target proteases might alter the risk of infection.36 Docking with HIV1 protease and SARS CoV-2 main protease revealed ligand 5b to have the lowest docking score of −7.5 and −8.5 kcal mol−1, respectively. PyMol images showed that ligand 5b interacts hydrophobically with the active site residues of SARS CoV-2 main protease as shown in Fig. 4f. The ligand 5b shows high binding affinity with HIV1 protease as well with a docking score of −7.5 kcal mol−1. Hydrogen bonding, as shown in Fig. 4d, is seen with Ile(50), and is surrounded by Ile(84), Asp(25), Gly(49), and Phe(53) between HIV1 protease and ligand 5b.

MRSA is resistant to most penicillin-based antibiotics due to structural modifications in the PBPs. The presence of the mecA gene in PBP2a renders the organism resistant to β-lactams.37 Based on the molecular docking scores summarized in Table 4, ligand 5b has the lowest docking score of −8.5 kcal mol−1. This ligand displayed hydrogen bond interaction (2.5 Å) with only the Lys(148) residue. Interactions with His(293), Glu(294), Val (256), Val(277), and Arg(241) were mostly hydrophobic as seen in Fig. 4e.

Acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) play a vital role in cholinergic signalling. BChE is associated with the hallmarks of Alzheimer's, such as amyloidogenesis, and has been shown to compensate for AChE when its levels decrease. Despite having numerous strategies for the treatment of Alzheimer's disease, there is no cure, and only palliative care is available.38,39

Among the esterase receptors, BChE showed the lowest docking score of −10.0 kcal mol−1 with ligand 5c, whereas AChE shows an average docking score of −5.5 kcal mol−1 with the same ligand as indicated by Table 4. No hydrogen bond interactions were observed between ligands and AChE or BChE receptors as shown in Fig. 4g and h. Docking results indicated that BChE modulation was more favorable with ligand 5c.

Simulations of molecular dynamics

Molecular dynamic simulations are a strong technique that is similar to physical experiments. Based on the docking results, the ligand2-cdk2 complex was considered for molecular dynamic simulations. Group properties, such as RMSD, RMSF, and MM-PBSA free energy calculations, were performed using trajectory files. The free binding energy of the docked complex was estimated using molecular mechanics Poisson–Boltzmann surface area (gmx-MMPBSA). Results are presented in Table 5. The total energy of the complex (ΔG) was found to be −6.48 kJ mol−1, indicating stable conformation of the ligand with cdk2.
Table 5 Binding free energy from MM-PBSA analysis. Energies are represented in kJ mol−1
Energy component Average SD (Prop.) SD SEM (Prop.) SEM
Delta (complex–receptor–ligand)
ΔBOND −0.00 0.13 0.00 0.07 0.00
ΔANGLE −0.00 1.94 0.00 0.97 0.00
ΔDIHED −0.00 0.39 0.00 0.19 0.00
ΔUB 0.00 0.00 0.00 0.00 0.00
ΔIMP −0.00 0.06 0.00 0.03 0.00
ΔCMAP 0.00 0.00 0.00 0.00 0.00
ΔVDWAALS 43.71 0.26 0.95 0.13 0.48
ΔEEL 94.73 6.26 7.74 3.13 3.87
Δ1–4 VDW 0.00 1.56 0.00 0.78 0.00
Δ1–4 EEL −0.00 1.33 0.00 0.67 0.00
ΔEPB −52.60 0.95 4.99 0.47 2.50
ΔENPOLAR −4.89 0.06 0.19 0.03 0.09
ΔEDISPER 0.000.00 0.000.000 0.000.00 0.00 0.000.00
ΔGGAS 51.01 6.27 6.86 3.13 3.43
ΔGSOLV −57.49 0.95 5.05 0.47 2.53
ΔTOTAL −6.48 6.34 8.65 3.17 4.32


Root mean square deviation (RMSD) was determined to illustrate the dynamic stability of proteins when they were bound to the ligand of interest. The RMSD of proteins fluctuated for 12 ns as observed in Fig. 5. The RMSD tends to increase along the simulation but becomes stable toward the end of the 100-ns-long simulation.


image file: d4nj03384a-f5.tif
Fig. 5 RMSD obtained for the protein and ligands. Ligand is shown in yellow; protein RMSD is shown in violet.

RMSF analysis showed residual fluctuations during the simulation time. The computed RMSF showed major fluctuations between the amino acid residues number 150 and 160, which can be attributed to the presence of loops in the protein structure as depicted in Fig. 6. However, no major fluctuations were observed near the active site pockets. Most of the residues were found to have RMSF values less than 0.2 nm indicating them to exist in rigid complex states.


image file: d4nj03384a-f6.tif
Fig. 6 RMSF of the protein.

Hydrogen bond analysis provides insight into the interaction power for the stabilization of protein and ligand. Fig. 7 shows that the hydrogen bond of the simulated complex was observed to have a stable trend. Fewer hydrogen bonds were seen at the start of the simulations, however, strong hydrogen bonding was observed after 20 ns until the end of the simulation, indicating stable binding.


image file: d4nj03384a-f7.tif
Fig. 7 Hydrogen bonds between cdk2 and the ligand 5b.

The presence of non-binding interactions also accounts for the stability of the protein structure. One such parameter is hydrophobic interaction, which can be described by the SASA (solvent-accessible surface region). The SASA of the protein–ligand complex during the 100-ns-long simulation is depicted in Fig. 8. The plot of the SASA showed stabilization at an average of 160 nm2, confirming the compactness of the complex.


image file: d4nj03384a-f8.tif
Fig. 8 Solvent accessible surface (SASA) region of the cdk2-ligand 5b complex.

The radius of gyration (Rg) provides insights into the compactness of protein–ligand interaction. The average Rg value of the system was found to be 2.04 nm as shown in Fig. 9. The low Rg value indicates that the protein–ligand interaction showed maximum compactness throughout the simulation.


image file: d4nj03384a-f9.tif
Fig. 9 Radius of gyration (Rg) of cdk2-ligand 5b complex.

The obtained results indicate that all the ligands bind to the active site pockets of the receptors in a similar position and orientation. In all the receptor–ligand interactions discussed above, hydrophobic interactions were crucial in stabilizing the ligands at the binding site. A high number of hydrophobic interactions at the active site of the ligand–receptor interface tend to increase the biological activity of the ligand. The existing relationship between the free binding energy and the affinity of small organic molecules contributes to the prediction and interpretation of the activity of the small molecule toward the target receptors.40 The overall free energy of binding of the ligands studied here indicated a good affinity of the ligands for the given receptors. ADME studies and molecular dynamics simulation of compound 5b with that of cdk2 receptor suggested that the compound could be a potential anti-cancer agent. In this study, we considered the molecule with the best docking score for further studies. However, reports have suggested molecules with low binding affinity to work efficiently with minimal side effects. Further studies using in vivo techniques are required to further develop the application of the proposed compounds in cancer therapeutics.

General procedure

Method 1

(a) Synthesis of N-aryl isobutyrylamides 3 from amine 1. One gram of TEA (1.0 equiv.) was added to a stirred solution of amines 1a or 1b (1 g, 1.0 equiv.) in dry DCM (10 mL) at 0 °C under a N2 atmosphere. After stirring for 5–10 min, a solution of isobutyroyl chloride 2 (1 mmol, 1.0 equiv.) in DCM (5 mL) was added dropwise and stirred at room temperature for 15 minutes. The reaction was periodically monitored by TLC analysis of small aliquots, followed by a micro-workup. After the complete consumption of reactants as indicated by the TLC analysis, after an hour, the reaction mixture was quenched with saturated aqueous NH4Cl solution (50 mL) and extracted with EtOAc (5 × 25 mL). The combined organic layer was washed with water (5 × 25 mL) and brine (100 mL), dried over anhydrous Na2SO4, and evaporated under vacuum to give the corresponding amides, 3a and 3b, in 84 and 92% yields, respectively, which were further purified by column chromatography using hexane/ethyl acetate as eluent and 60–120 silica gel as the stationary phase. The pure amides obtained after the column chromatography were used for the synthesis of amidines.
(b) Synthesis of 1,3-bis(het)aryl-2-isopropylamidines 5 from N-aryl isobutyrylamides 3. A solution of amides, 3a or 3b (0.5 g, 1.0 equiv.), was added to dry POCl3 (10 mL) and stirred at 80 °C for 6 hours under a N2 atmosphere. After the complete consumption of the amide, which was confirmed by TLC analysis, the reaction temperature was reduced to 0 °C by using an ice bath and solutions of amines 4a–e (1 mmol, 1.0 equiv.) in THF (2 mL) were added for 5–10 min. Stirring was continued at room temperature for 15 minutes. The progress of the reaction was periodically monitored by TLC analysis of small aliquots of the mixture, followed by a micro-workup. After the complete consumption of the reactants as indicated by TLC analysis, the reaction mixture was quenched with saturated aqueous NH4Cl solution (50 mL) and extracted with EtOAc (5 × 25 mL). The combined organic layer was washed with water (5 × 25 mL) and brine (100 mL), dried over anhydrous Na2SO4, and evaporated under vacuum to give the corresponding amidines 4a–e in greater than 80% yields. The product was further purified by column chromatography using hexane/ethyl acetate as eluent and 60–120 silica gel as the stationary phase.

Method 2

One-pot synthesis of 1,3-bis(het)aryl-2-isopropylamidines 5 from amines 1. One gram of TEA (1.0 equiv.) was added to a stirred solution of amines, 1a or 1b (1 g, 1.0 equiv.), in dry DCM (10 mL) at 0 °C under a N2 atmosphere. After stirring for 5–10 min, a solution of isobutyroyl chloride 2 (1 mmol, 1.0 equiv.) in DCM (5 mL) was added dropwise and continuously stirred at room temperature for 15 minutes. The reaction was periodically monitored by TLC analysis of small aliquots, followed by a micro-workup. After the complete consumption of reactants as indicated by the TLC analysis, 10 mL of dry POCl3 was added to the reaction mixture at 0 °C. The reaction was heated at 80 °C for 6 hours under a N2 atmosphere. After the complete consumption of the amide, which was confirmed by TLC analysis, the reaction temperature was reduced to 0 °C by using an ice bath, and solutions of amines 4a–e (1 mmol, 1.0 equiv.) in THF (2 mL) were added over a period of 5–10 min. Stirring was continued at room temperature for 15 minutes. The progress of the reaction was periodically monitored by TLC analysis of small aliquots, followed by a micro-workup. After the complete consumption of reactants as indicated by the TLC analysis, the reaction mixture was quenched with saturated aqueous NH4Cl solution (50 mL) and extracted with EtOAc (5 × 25 mL). The combined organic layer was washed with water (5 × 25 mL) and brine (100 mL), dried over anhydrous Na2SO4, and evaporated under vacuum to give the corresponding amidines 5a–e in greater than 85% yields. The products were further purified by column chromatography using hexane/ethyl acetate as eluent and 60–120 silica gel as the stationary phase.

Conclusion

In the present work, we report a simple and straightforward method for the synthesis of some biologically important amidines. We also evaluated them as potent candidates for therapeutic work. The result of the work carried out here indicates that compound 5b is a promising candidate and acts as a good modulator of proteins that play a vital role in cancer cell differentiation and proliferation. The present work forms a solid theoretical platform for the wet lab evaluation of compound 5b in future studies.

Data availability

The data underlying this study are available in the published article and its ESI.

Conflicts of interest

There are no conflicts to declare.

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Footnote

Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4nj03384a

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