Design, synthesis, biological evaluation, and computational insights of 2-(Aryl)benzo[d]imidazo[2,1-b]thiazole-7-sulfonamide derivatives as potent antitubercular and antibacterial agents

Abstract

A series of 2-(aryl)benzo[d]imidazo[2,1-b]thiazole-7-sulfonamide derivatives were synthesized and evaluated for their antitubercular and antibacterial activities, molecular docking, and DFT studies. The compounds were synthesized through a multi-step reactions, with yields varying based on the electronic and steric effects of the substituents. Among the derivatives, 5b, 5d, and 5h exhibited potent antitubercular activity against Mycobacterium tuberculosis (H37Rv strain) with minimum inhibitory concentrations (MICs) of 1.6 µg/mL, comparable to some standard drugs like isoniazid. Antibacterial testing revealed that 2-(2,4-dichlorophenyl)benzo[d]imidazo[2,1-b]thiazole-7-sulfonamide displayed significant activity against Gram-positive bacteria, with MICs as low as 6.25 µg/mL for Staphylococcus aureus and Bacillus subtilis. The molecular docking and DFT analyses provided insights into the binding interactions and electronic structures of these compounds, with halogen substitutions enhancing biological activity due to increased electron-withdrawing effects. MESP studies showed a distinct electron density distribution, supporting the observed bioactivity. For antitubercular activity, compounds 5b, 5d, and 5h demonstrated potent binding affinities (−6.2 to −5.9 kcal/mol) against the DprE1 enzyme. Compound 5f showed remarkable antibacterial efficacy, with a docking score of −7.9 kcal/mol against 2,2-dialkylglycine decarboxylase The DFT analysis revealed that 5a, with a methoxy substituent, had the highest reactivity (ΔE = 3.86 eV), while halogenated derivatives, such as 5f, exhibited increased chemical stability (ΔE = 4.24 eV). ADME studies showed that 5f having favorable lipophilicity and enzyme inhibition. These findings suggested that these derivatives could serve as potential candidates for further drug development against bacterial and mycobacterial infections.

Graphical Abstract

Introduction

In recent years, benzimidazole and thiazole-based heterocyclic compounds have gained significant attention in the field of medicinal chemistry due to their wide range of biological activities, including antibacterial [1, 2], antitubercular [3, 4], antifungal [5, 6], and anticancer [7, 8] properties. Among these, the imidazo[2,1-b]thiazole scaffold has shown promising results in drug discovery for the development of new therapeutic agents [9,10,11,12,13]. The incorporation of sulfonamide functionality within these frameworks is particularly intriguing due to the well-established pharmacophoric nature of sulfonamides, which exhibit potent antimicrobial activities [14, 15]. Given the growing concern over antibiotic resistance and the need for novel therapeutic agents, these chemical structures offer a promising approach to addressing microbial infections, including tuberculosis.

Despite the diverse biological potential of imidazo[2,1-b]thiazole derivatives, there remains a gap in exploring their antibacterial and antitubercular properties. Previous studies have largely focused on either the antibacterial or antifungal activities of related compounds, but comprehensive studies that also evaluate their antitubercular efficacy are limited [16,17,18,19]. Moreover, while sulfonamide-containing heterocycles have been widely reported, their synergistic combination with benzimidazo-thiazole scaffolds remains underexplored. Additionally, the incorporation of aryl groups into these systems could further enhance their biological activity, making it necessary to synthesize and evaluate new derivatives with diverse aryl substitutions.

Several studies have reported the antibacterial activities of benzimidazole derivatives. For instance, some analogs have been found to exhibit potent activity against Gram-positive and Gram-negative bacteria by targeting specific bacterial enzymes [20]. Similarly, thiazole-based compounds have demonstrated considerable antibacterial and antitubercular activity through inhibition of bacterial protein synthesis [21]. However, limited studies have comprehensively combined the benzimidazole and thiazole cores with sulfonamide groups to assess their dual antibacterial and antitubercular potential, leaving a gap in the current literature. Figure 1 highlights the some bioactive compounds containing benzothiazole and imidazole structures.

Fig. 1

Some bioactive compounds containing benzothiazole and imidazole structures

The advent of computational techniques such as molecular docking and density functional theory (DFT) has revolutionized the field of drug discovery [22,23,24]. These methods provide insights into the interaction mechanisms between bioactive compounds and their target biomolecules, facilitating the rational design of new therapeutic agents. Molecular docking studies allow researchers to predict the binding affinity and orientation of compounds within the active sites of target proteins, while DFT analysis offers detailed electronic structure information, enabling a deeper understanding of the compounds' reactivity and stability [25,26,27,28].

This study is crucial because the rapid emergence of drug-resistant bacterial strains, especially Mycobacterium tuberculosis, necessitates the development of new drugs with novel mechanisms of action. The World Health Organization (WHO) has emphasized the urgent need for new antitubercular agents [29]. By investigating the potential of 2-(aryl)benzo[d]imidazo[2,1-b]thiazole-7-sulfonamide derivatives, this research addresses a critical need for novel antimicrobial agents that target both bacterial and tubercular infections. In this work, we aim to synthesize a series of novel 2-(aryl)benzo[d]imidazo[2,1-b]thiazole-7-sulfonamide derivatives and evaluate their antibacterial and antitubercular activities. Furthermore, molecular docking studies are conducted to assess the potential interactions of these compounds with key bacterial and tubercular enzymes. Targeting important enzymes involved in the formation of the mycobacterial cell wall offers a promising approach in the continuous hunt for new antitubercular and antibacterial medicines. One such target is the enzyme Decaprenylphosphoryl-β-D-ribose oxidase (DprE1), which is essential for the production of components of mycobacterial cell walls. DprE1 was chosen as the study's target because to its crucial function in cell wall production and its proven druggability, which is evidenced by the discovery of strong inhibitors that specifically target this enzyme. DFT analysis is performed to explore the electronic properties of the synthesized compounds. This multifaceted approach will provide valuable insights into the structure–activity relationships (SAR) of these derivatives and guide the development of new therapeutic agents.

Materials and methods

Chemicals and instrumentations

The chemicals (Make: Merck, Sigma Aldrich, and Avra synthesis) with high purity were purchased from Virion Enterprizes, Mumbai. The chemicals were used as received without any further purification. Melting points were determined in open capillary and uncorrected. NMR spectra were recorded with a Bruker using DMSO–d₆ as the solvent, FT-IR spectra were obtained with potassium bromide pellets. Reactions were monitored by thin-layer chromatography using aluminium sheets with silica gel 60 F254 (Merck).

Procedure for the synthesis of 4-thioureidobenzenesulfonamide (Scheme 1)

Scheme 1

Synthesis of 4-thioureidobenzenesulfonamide. Reaction conditions: (i) Concentrated HCl (3.5 mL), water (30 mL), Ammonium thiocyanate (0.075 mol), 95–100 °C

In a round-bottom flask, 4-aminobenzenesulfonamide (1, 3.5 g, 0.0375 mol) was dissolved in water (30 mL). Concentrated hydrochloric acid (3.5 mL) was added dropwise to the mixture, and the resulting solution was heated to 100 °C. Ammonium thiocyanate (5.71 g, 0.075 mol) was then added portion wise while maintaining the reaction temperature at 95–100 °C. The progress of the reaction was monitored by TLC. Upon completion, the reaction mixture was cooled to 10–15 °C. The precipitated solid was collected by filtration, washed with cold water, and dried to afford the desired product, 2-aminobenzo[d]thiazole-6-sulfonamide (2).

Procedure for the synthesis of 2-aminobenzo[d]thiazole-6-sulfonamide (Scheme 2)

Scheme 2

Synthesis of 2-aminobenzo[d]thiazole-6-sulfonamide. Reaction conditions: (ii) CHCl₃ (13.75 mL), rt, Br₂ (0.0143 mol in 15 mL CHCl₃), 50–55 °C

A round-bottom flask was charged with 4-thioureidobenzenesulfonamide (2, 2.75 g, 0.01 mol) and chloroform (13.75 mL) at room temperature. Bromine (2.286 g, 0.0143 mol) was diluted in chloroform (15 mL) at room temperature. The bromine solution was then added dropwise to the 4-thioureidobenzenesulfonamide solution while maintaining the temperature at room temperature. During the addition, the temperature of the reaction mixture rose to 40 °C. After the complete addition of the bromine solution, the reaction mixture was stirred at room temperature for 12 h. Subsequently, the reaction mass was heated to 50–55 °C and maintained at this temperature for an additional 12 h. The progress of the reaction was continuously monitored using TLC. Upon completion of the reaction, the solid product was filtered and washed with chloroform. The crude solid was acidified with HCl up to pH 1 and then after filteration the filtrate was basified with ammonia up to pH 12 to offer 2-aminobenzo[d]thiazole-6-sulfonamide (3) as a pure product.

Procedure for the synthesis of 2-(aryl)benzo[d]imidazo[2,1-b]thiazole-7-sulfonamide derivatives (Scheme 3)

Scheme 3

Synthesis of 2-(aryl)benzo[d]imidazo[2,1-b]thiazole-7-sulfonamide derivatives. Reaction conditions: (iii) PEG-400 (10 mL), reflux

The starting material, 2-aminobenzo[d]thiazole-6-sulfonamide (3), was reacted with various substituted 2-bromoacetophenones (4a–4h) to yield the corresponding compounds (5a–h). In a typical reaction, 2-aminobenzo[d]thiazole-6-sulfonamide (3, 0.01 mol) was dissolved in a PEG-400 (10 mL) at room temperature. To this solution, the substituted 2-bromoacetophenone derivatives (4a–4h, 0.01 mol) were added for the total eight reactions. The reaction mixture was stirred and heated under reflux for a specific time depending on the reactivity of the 2-bromoacetophenone derivatives. The progress of the reaction was monitored by thin layer chromatography (TLC). Upon completion, the precipitate was filtered and dried. Then the crude product was purified by recrystallization from methanol solvent to afford the desired products (5a–5h) in good to excellent yields. The resulting compounds (5a–5h) were then characterized by spectroscopic techniques, including FT-IR, ¹H NMR, and ¹³C NMR to confirm their structure. The physicochemical data is given in Table 1. The spectral images (FT-IR, ¹H NMR, and ¹³C NMR) are given in supplementary material.

Table 1 Synthesis of 2-(aryl)benzo[d]imidazo[2,1-b]thiazole-7-sulfonamide derivatives (5a-5h)^a

Spectral results

2-(4-methoxyphenyl)benzo[d]imidazo[2,1-b]thiazole-7-sulfonamide (5a)

FT-IR (cm⁻¹): 3356, 2959, 1685, 1591, 1545, 1493,1410, 1247, 1159, 1073, 1022, 897, 819, 738, 700; ¹H NMR (500 MHz, DMSO-d₆) δ 8.70 (s, 1H), 8.55 (d, J = 1.8 Hz, 1H), 8.12 (d, J = 8.5 Hz, 1H), 8.01 (dd, J = 8.5, 1.8 Hz, 1H), 7.81 (d, J = 8.8 Hz, 2H), 7.50 (s, 2H), 7.02 (d, J = 8.8 Hz, 2H), 3.80 (s, 3H): ¹³C NMR (126 MHz, DMSO-d₆) δ 159.31, 148.24, 147.33, 140.97, 134.23, 130.23, 126.68, 126.61, 124.97, 123.49, 114.71, 113.83, 108.60, 55.62.

2-(4-bromophenyl)benzo[d]imidazo[2,1-b]thiazole-7-sulfonamide (5b)

FT-IR (cm⁻¹): 3362, 3263, 2982, 1574, 1539, 1495, 1409, 1321, 1207, 1255, 1070, 1004, 953, 879, 811, 692; ¹H NMR (500 MHz, DMSO-d₆) δ 8.89 (s, 1H), 8.57 (d, J = 1.7 Hz, 1H), 8.13 (d, J = 8.5 Hz, 1H), 8.02 (dd, J = 8.5, 1.7 Hz, 1H), 7.83 (d, J = 8.5 Hz, 2H), 7.64 (d, J = 8.5 Hz, 2H), 7.52 (s, 2H); ¹³C NMR (126 MHz, DMSO-d₆) δ 148.74, 146.09, 141.27, 134.10, 133.30, 132.23, 130.40, 127.21, 125.04, 123.56, 120.84, 114.06, 110.37.

2-(4-chloroophenyl)benzo[d]imidazo[2,1-b]thiazole-7-sulfonamide (5c)

FT-IR (cm⁻¹): 3337, 3272, 3133, 1575, 1495, 1401, 1324, 1161, 1011, 910, 816, 732, 696; ¹H NMR (500 MHz, DMSO-d₆) δ 8.87 (s, 1H), 8.57 (d, J = 1.7 Hz, 1H), 8.14 (d, J = 8.5 Hz, 1H), 8.02 (dd, J = 8.5, 1.7 Hz, 1H), 7.89 (d, J = 8.6 Hz, 2H), 7.53–7.50 (m, 4H); ¹³C NMR (126 MHz, DMSO-d₆) δ 148.72, 146.06, 141.26, 134.11, 132.94, 132.30, 130.40, 129.32, 126.90, 125.03, 123.56, 114.05, 110.32.

2-(4-fluorophenyl)benzo[d]imidazo[2,1-b]thiazole-7-sulfonamide (5d)

FT-IR (cm⁻¹): 3588, 3319, 3151, 3036, 1593, 1494, 1412, 1323, 1225, 1159, 1103, 921, 841, 802, 760, 718; ¹H NMR (500 MHz, DMSO-d₆) δ 8.81 (s, 1H), 8.56 (d, J = 1.7 Hz, 1H), 8.13 (d, J = 8.5 Hz, 1H), 8.02 (dd, J = 8.5, 1.7 Hz, 1H), 7.91 (dd, J = 8.8, 5.5 Hz, 2H), 7.52 (s, 2H), 7.29 (t, J = 8.9 Hz, 2H); ¹³C NMR (126 MHz, DMSO-d₆) δ 162.08 (d, J = 244.2 Hz), 148.57, 146.35, 141.18, 134.16, 130.61, 130.59, 130.34, 127.20 (d, J = 8.1 Hz), 125.02, 123.54, 116.19 (d, J = 21.6 Hz), 113.98, 109.70.

2-phenylbenzo[d]imidazo[2,1-b]thiazole-7-sulfonomide (5e)

FT-IR (cm⁻¹): 3347, 3152, 3034, 1557, 1500, 1326, 1211, 1152, 1102, 1055, 901, 796, 711; ¹H NMR (500 MHz, DMSO-d₆) δ 8.84 (s, 1H), 8.56 (d, J = 1.7 Hz, 1H), 8.15 (d, J = 8.5 Hz, 1H), 8.02 (dd, J = 8.5, 1.7 Hz, 1H), 7.89 (m, 2H), 7.51 (s, 2H), 7.46 (m, 2H), 7.33–7.30 (m, 1H); ¹³C NMR (126 MHz, DMSO-d₆) δ 148.50, 147.28, 141.16, 134.19, 134.03, 130.35, 129.28, 127.98, 125.25, 124.99, 123.53, 114.00, 109.88.

2-(2,4-dichlorophenyl)benzo[d]imidazo[2,1-b]thiazole-7-sulfonomide (5f)

FT-IR (cm⁻¹): 3492, 3319, 3162, 3094, 1575, 1533, 1497, 1322, 1205, 1155,1098, 1032,905, 877, 852, 797, 702; ¹H NMR (500 MHz, DMSO-d₆) δ 9.05–9.02 (m, 1H), 8.57 (d, J = 1.7 Hz, 1H), 8.40–8.38 (m, 1H), 8.22 (d, J = 8.6 Hz, 1H), 8.01 (dd, J = 8.6, 1.8 Hz, 1H), 7.71 (d, J = 2.2 Hz, 1H), 7.54–7.51 (m, 3H); ¹³C NMR (126 MHz, DMSO-d₆) δ 148.19, 142.15, 141.47, 134.21, 132.68, 131.54, 131.12, 131.09, 130.54, 130.20, 128.09, 124.95, 123.48, 114.66, 113.99.

2-(2-chlorophenyl)benz[d]imidazo[2,1-b]thiazole-7-sulfonomide (5g)

FT-IR (cm⁻¹): 3319, 3234, 3088, 1595, 1501, 1411, 1320, 899, 785, 695; ¹HNMR(500 MHz, DMSO-d₆) δ 8.62 (d, J = 1.8 Hz, 1H), 8.56 (d, J = 8.7 Hz, 1H), 8.04 (dd, J = 8.7, 1.8 Hz, 1H), 8.01 (t, J = 1.8 Hz, 1H), 7.99–7.97 (m, 1H), 7.58 (s, 2H), 7.52 (d, J = 7.8 Hz, 1H), 7.46 (ddd, J = 8.0, 2.1, 1.0 Hz, 1H); ¹³C NMR (126 MHz, DMSO-d₆) δ 149.52, 142.29, 141.65, 134.87, 134.67, 133.81, 131.04, 130.83, 128.29, 126.55, 125.53, 124.73, 123.62, 114.04, 94.14.

2-(p-tolyl)benzo[d]imidazo[2,1-b]thiazole-7-sulfonamide (5h)

FT-IR (cm⁻¹): 3314, 3157, 3011, 1572, 1487, 1409, 1324, 1165, 1104, 931, 914, 816, 718; ¹H NMR (500 MHz, DMSO-d₆) δ 8.60 (d, J = 1.8 Hz, 1H), 8.56 (d, J = 8.7 Hz, 1H), 8.03 (dd, J = 8.7, 1.8 Hz, 1H), 7.89 (d, J = 8.2 Hz, 2H), 7.55 (s, 2H), 7.31 (d, J = 8.2 Hz, 2H), 2.36 (s, 3H); ¹³C NMR (126 MHz, DMSO-d₆) δ 149.13, 144.16, 141.42, 137.99, 134.78, 130.69, 130.02, 129.63, 127.12, 124.69, 123.59, 113.86, 92.70, 21.35.

Antitubercular screening

The antitubercular activity of the synthesized compounds was assessed using the microplate Alamar Blue assay (MABA) against Mycobacterium tuberculosis H37Rv strain (ATCC No. 27294). This method, which is non-toxic and employs a thermally stable reagent, provides a good correlation with the proportional and BACTEC radiometric methods. The procedure involved adding 200 µL of sterile deionized water to the outer perimeter wells of a sterile 96-well microplate to minimize evaporation. Subsequently, 100 µL of Middlebrook 7H9 broth was added to the inner wells, and serial dilutions of the test compounds were prepared directly on the plate, with final concentrations ranging from 100 to 0.2 µg/mL. Plates were sealed with parafilm and incubated at 37 °C for 5 days. After this period, 25 µL of a freshly prepared 1:1 mixture of Alamar Blue reagent and 10% Tween 80 were added to each well, followed by 24-h incubation. The results were determined by the color change in the wells: blue indicated no bacterial growth (i.e., effective inhibition by the compound), while pink indicated bacterial growth. The minimum inhibitory concentration (MIC) was defined as the lowest concentration that prevented the color change from blue to pink. Standard antitubercular drugs were used to validate the assay, with MIC values of 1.6 µg/mL for isoniazid and ethambutol, 3.125 µg/mL for pyrazinamide, and 0.8 µg/ml for rifampicin and streptomycin.

Antibacterial screening

The serial dilutions of each drug were prepared using brain heart infusion (BHI) broth. Initially, 20 µL of the drug was added to 380 µL of BHI broth in a tube. For the subsequent dilutions, 200 µL of BHI broth was added to each of nine separate tubes. From the initial tube, 200 µL of the drug mixture was transferred to the first tube containing 200 µL of BHI, resulting in a 10^–1 dilution. This process was repeated sequentially by transferring 200 µL from each preceding tube into the next, up to a 10^-9 dilution. A 5 µL aliquot of maintained stock cultures of the test organisms was added to 2 mL of BHI broth, and 200 µL of this culture suspension was introduced into each of the serially diluted drug tubes. The tubes were incubated for 24 h and examined for turbidity. For facultative anaerobes, the tubes were incubated at 37 °C for 48–72 h in a CO₂ jar, while for strict anaerobes, incubation was performed in anaerobic jars for 48–72 h. The BHI broth used was prepared from HIMEDIA M210-500G, containing 200 g/L of calf brain infusion and 250 g/L of beef heart infusion.

DFT methodology

All the DFT computations for the synthesized benzothiazoles (5a–5h) were performed in gas phase using Gaussian 03 program [30]. The calculations have employed Becke’s three parameter hybrid exchange [31] with Lee–Yang–Parr correlation functional (B3LYP) [32] with 6-311G(d,p) basis set. The visualization was done using the Gauss View 4.1.2 molecular visualization program [33].

Molecular docking

Protein and ligand preparation, grid generation, and docking were performed using AutoDockTools 1.5.6 [34]. Ligands were optimized in their stable conformation using B3LYP 6-311G(d) with Gaussian 03. 3D crystal structures of proteins were retrieved from the RCSB PDB (IDs: 1D7U, 6G83) based on prior literature [12, 33,34,35,36,37]. Target proteins were pre-processed to remove ligands and water, followed by the addition of Kollman charges and hydrogen bond optimization. Ligands were prepared by adding hydrogen atoms and Gasteiger charges, merging non-polar hydrogens. Docking utilized the Lamarckian Genetic Algorithm, and the best poses were analyzed for ligand-receptor interactions using PyMOL [38]. software (https://www.pymol.org/).

ADME

The ADME (Absorption, Distribution, Metabolism, and Excretion) study is essential for comprehending a drug's pharmacokinetic profile in the field of drug development. A set of metrics and predictive models helpful in assessing the pharmacokinetics, drug-likeness, and medicinal chemistry friendliness of small compounds can be accessed for free via the web-based program SwissADME [39]. SwissADME, a computational tool developed by the Molecular Modeling Group of the Swiss Institute of Bioinformatics (SIB), uses molecular structure to predict drug-likeness and ADME features. In order to predict these features early in the drug development process, ADME studies make use of sophisticated computer models and in silico tools. This method not only boosts the efficiency of the drug development process but also increases the chances of clinical success by ensuring that only the most promising compounds progress to later testing stages. SwissADME calculates essential metrics such as hydrogen bond donors and acceptors, rotatable bonds, topological polar surface area (TPSA), and solubility. It evaluates lipophilicity using various logP calculations, including iLOGP, XLOGP3, WLOGP, MLOGP, and SILICOS-IT, which are vital for understanding a molecule’s capacity to cross cell membranes. Additionally, it predicts the solubility of compounds in water, critical for oral bioavailability. ADME parameters like gastrointestinal absorption, blood–brain barrier penetration, P-glycoprotein substrate status, and interactions with cytochrome P450 enzymes are also predicted, which are crucial for metabolism and potential drug-drug interactions. According to Lipinski's "rule of five," compounds should not have more than 10 hydrogen bond acceptors (HBAs) or 5 hydrogen bond donors (HBDs), and all the compounds' structures should follow these guidelines. Additionally, the topological polar surface area (tPSA), which evaluates the impact of polar fragments on the surface of a molecule, should be no greater than 140 Ų. A higher tPSA may hinder the compound's ability to cross the blood–brain barrier and decrease its membrane permeability.

Results and discussion

Chemistry

The synthesis of 2-aminobenzo[d]thiazole-6-sulfonamide (2) was achieved through a multi-step process starting with the reaction of 4-aminobenzenesulfonamide (1) with ammonium thiocyanate under acidic conditions, conducted in water at 95–100 °C. This reaction facilitated the formation of the thiazole ring, confirmed by the precipitation of the product upon cooling. The isolated product was further brominated by adding bromine in chloroform. The reaction was carefully controlled, with the temperature rising to 40 °C during bromine addition, followed by a 24-h reflux at 50–55 °C to ensure complete transformation yielding 2-aminobenzo[d]thiazole-6-sulfonamide (3). The final step involved reacting compound 3 with various substituted 2-bromoacetophenones (4a–4h) in PEG-400, an environmentally friendly solvent. The yields of the final products (5a–5h) varied depending on the electronic and steric effects of the substituents on the acetophenone derivatives. The synthesized compounds were characterized using FT-IR, ¹H NMR, and ¹³C NMR spectroscopy, confirming their structures and purity. The synthetic route was efficient and reproducible, highlighting the potential of PEG-400 as a sustainable solvent in heterocyclic chemistry. The 2-aminobenzo[d]thiazole-6-sulfonamide derivatives synthesized in this study offer promising prospects for further biological evaluation and potential therapeutic applications.

The FT-IR spectra for the compounds 5a–5h show distinct peaks corresponding to various functional groups. For 5a, the broad absorption at 3356 cm⁻¹ is assigned to the sulfonamide (–SO₂NH₂) stretching, while the sharp peaks at 2959 and 1574 cm⁻¹ are attributed to aromatic C–H and C = N stretches, respectively. Similarly, 5b displays a sulfonamide stretching at 3362 cm⁻¹, with additional C = N and C = C vibrations at 1574 cm⁻¹ and 1539 cm⁻¹, respectively. In 5c, the peak at 3337 cm⁻¹ confirms the sulfonamide presence, along with C = N stretches around 1575 cm⁻¹. Compound 5d exhibits a similar sulfonamide peak at 3588 cm⁻¹, while 5e shows a slight shift of this stretch to 3347 cm⁻¹. For 5f, the FT-IR spectrum highlights a sulfonamide stretch at 3492 cm⁻¹, and in 5g and 5h, characteristic sulfonamide stretches are noted at 3319 cm⁻¹ and 3314 cm⁻¹, respectively, with supporting aromatic C = C stretches around 1409–1595 cm⁻¹ across all compounds. The ¹H NMR spectra reveal significant insights into the chemical environment of protons in these benzimidazole-thiazole derivatives. In 5a, the singlet at δ 8.70 corresponds to the proton on the benzimidazole ring, while the doublet at δ 8.55 (J = 1.8 Hz) indicates the coupling between aromatic protons. The methoxy group’s protons resonate as a singlet at δ 3.80, highlighting the electron-donating nature of the methoxy substituent. In 5b, the downfield shift of the benzimidazole proton to δ 8.89 reflects the electron-withdrawing effect of the bromine substituent. The coupling constant (J = 1.7 Hz) between protons at δ 8.57 and δ 8.02 reflects ortho-coupling, while the doublet at δ 7.83 is indicative of the para-substituted bromophenyl group. In 5c, the similar chemical shifts for the chlorophenyl moiety at δ 7.89 and δ 7.50 suggest a deshielding effect due to chlorine's inductive withdrawing property. The coupling constant (J = 1.7 Hz) remains similar across these derivatives, indicative of weak ortho-coupling. In 5d, the fluorine atom causes notable changes in the coupling constants (J = 8.8, 5.5 Hz), with the aromatic protons displaying splitting due to fluorine’s electronegativity. The multiplet at δ 7.29 suggests meta- and para-coupling effects within the fluorophenyl ring. Compound 5e presents a slightly different profile with the phenyl group, with protons appearing as a multiplet around δ 7.89–7.30, reflecting the overlapping signals of the aromatic protons. In 5f, the presence of two chlorine atoms in the 2,4-positions leads to a downfield shift for the protons at δ 9.05–9.02, further accentuating the electron-withdrawing nature of these groups. The aromatic proton at δ 7.54 in 5g is also highly deshielded due to chlorine’s inductive effects, similar to 5f. Lastly, in 5h, the methyl group at the para position leads to a singlet at δ 2.36, indicative of a shielded environment due to the electron-donating methyl group. The ¹³C NMR spectra complement the proton data with further structural confirmation. For 5a, the methoxy carbon resonates at δ 55.62, confirming its attachment to the aromatic ring, while the benzimidazole carbons resonate between δ 159.31 and δ 108.60. In 5b, the bromophenyl carbons appear at δ 120.84 and δ 110.37, showing the electron-withdrawing nature of bromine. In 5c, the chlorophenyl carbons appear at δ 148.72 and δ 132.94, reflecting chlorine’s deshielding effect. In 5d, the signal for the carbon directly attached to fluorine is significantly downfield at δ 162.08 (J = 244.2 Hz), due to the strong C–F coupling. For 5e, the phenyl carbons resonate between δ 129.28 and δ 109.88, while in 5f, the dichlorophenyl carbons shift between δ 148.19 and δ 113.99, showing further deshielding. In 5g, the signals for the chlorophenyl carbons are consistent with similar patterns of electron withdrawal, while in 5h, the methyl carbon appears shielded at δ 21.35, reinforcing the electron-donating effects observed in the ¹H NMR spectrum. Overall, the FT-IR, ¹H NMR, and ¹³C NMR spectra provide comprehensive structural insights into these benzimidazole-thiazole derivatives, showcasing the effects of various substituents on chemical shifts and coupling patterns.

Antitubercular study

The anti-mycobacterial activity of thiazole derivatives was evaluated against Mycobacterium tuberculosis using the Microplate Alamar Blue Assay (MABA). The minimum inhibitory concentrations (MICs) of the tested benzo[d]imidazo[2,1-b]thiazole-7-sulfonamide derivatives ranged from 1.6 to 12.5 µg/mL, revealing significant variation in activity depending on the substituents on the phenyl ring (Table 2). Among the compounds, 5b, 5d and 5h exhibited the most potent inhibition with MIC values of 1.6 µg/mL, equivalent to standard antitubercular drugs such as isoniazid and ethambutol. These halogen-substituted derivatives demonstrated strong potential as anti-mycobacterial agents, likely due to the electron-withdrawing nature of bromine and fluorine, which may enhance their interaction with key bacterial enzymes. The 5h compound also showed significant activity with an MIC of 1.6 µg/mL, comparable to isoniazid, ethambutol, and pyrazinamide. The presence of a methyl group in the para position may play a role in enhancing the compound’s bioactivity by increasing lipophilicity and potentially facilitating better penetration into the bacterial cell. The 5f derivative displayed moderate activity, with an MIC of 3.125 µg/mL, a value matching pyrazinamide, a commonly used first-line antitubercular drug. The two chlorine atoms likely enhance the electron-withdrawing effects, contributing to moderate inhibitory action. In contrast, compounds such as 5a, 5c, 5g, and 5e demonstrated weaker activity, with MICs of 6.25–12.5 µg/mL. The methoxy group is an electron-donating substituent, which may reduce the inhibitory effect, while the single chlorine atom, in contrast to the dichloro derivative, exhibited reduced activity, likely due to a weaker electron-withdrawing effect. The structure–activity relationship (SAR) analysis of these derivatives indicates that halogen substitutions, particularly bromine and fluorine in the para position, significantly enhance anti-mycobacterial activity, likely through increased electron-withdrawing capacity, which improves the compounds' binding affinity to bacterial targets. Conversely, bulkier or electron-donating groups, such as methoxy, resulted in weaker inhibition. Additionally, the presence of two chlorine atoms, as in the 5f derivative, contributed to moderate inhibition, while single chlorine substitution reduced activity. These findings suggest that specific structural modifications, such as halogenation, play a critical role in enhancing the antimycobacterial potential of thiazole derivatives. The compounds containing 4-bromo, 4-fluoro, and 4-methyl derivatives, with MIC values of 1.6 µg/mL, exhibit strong potential as lead compounds for future development. These results highlight the importance of chemical modifications in the development of novel antitubercular agents, with halogen-substituted thiazole derivatives offering a promising avenue for further exploration.

Table 2 The antitubercular activity of the 2-(aryl)benzo[d]imidazo[2,1-b]thiazole-7-sulfonamide derivatives (5a-5h) against M. tuberculosis (H37 RV strain) using microplate Alamar Blue assay

Antibacterial study

The present study focuses on the comparative antibacterial activity of eight benzo[d]imidazo[2,1-b]thiazole-7-sulfonamide derivatives against both Gram-negative (Escherichia coli, Klebsiella pneumoniae) and Gram-positive (Staphylococcus aureus, and Bacillus subtilis) bacterial strains (Table 3). Gram-negative bacteria, characterized by a more complex outer membrane and efflux pumps, generally present greater resistance to antibacterial agents. In contrast, Gram-positive bacteria have a simpler cell wall structure that may allow for easier penetration by antibacterial compounds, making them more susceptible to many drugs. Understanding the distinction between Gram-negative and Gram-positive bacteria is crucial, as it helps interpret the structure–activity relationships of the tested compounds. All compounds were tested using the broth dilution method, and ciprofloxacin was employed as a standard for comparison. Ciprofloxacin, a well-known broad-spectrum antibiotic, exhibited the highest potency against all strains with a minimum inhibitory concentration (MIC) of 1.25 μg/mL. Among the benzo[d]imidazo[2,1-b]thiazole-7-sulfonamide derivatives, 2-(2,4-dichlorophenyl)benzo[d]imidazo[2,1-b]thiazole-7-sulfonamide displayed the most significant antibacterial activity, particularly against Gram-positive bacteria, with MIC values as low as 6.25 μg/mL for Staphylococcus aureus and Bacillus subtilis. The improved activity of this compound suggests that the presence of two chlorine atoms on the phenyl ring may enhance lipophilicity and contribute to better cell wall penetration and interaction with bacterial enzymes. The electron-withdrawing effect of the halogen substituents likely plays a key role in this heightened activity, as seen in the enhanced efficacy of 2-(2,4-dichlorophenyl)benzo[d]imidazo[2,1-b]thiazole-7-sulfonamide. For Gram-negative bacteria like E. coli and K. pneumoniae, most derivatives exhibited moderate activity with MIC values of 25 μg/mL, except for the dichlorophenyl derivative, which showed an improved MIC of 12.5 μg/mL. This suggests that structural modifications to increase the permeability of the compounds could further enhance activity against Gram-negative strains. In contrast, several compounds, including 2-(4-methoxyphenyl)benzo[d]imidazo[2,1-b]thiazole-7-sulfonamide and 2-phenylbenzo[d]imidazo[2,1-b]thiazole-7-sulfonamide, exhibited good antibacterial activity against Gram-positive S. aureus and Bacillus subtilis, with MIC values of 12.5 μg/mL. The introduction of electron-donating groups like methoxy (-OCH₃) and methyl (-CH₃) at the phenyl ring slightly enhanced activity against Gram-positive strains, suggesting that these substituents may improve the binding affinity of the compounds to bacterial targets.

Table 3 The antibacterial activity of the 2-(aryl)benzo[d]imidazo[2,1-b]thiazole-7-sulfonamide derivatives (5a-5h)

Interestingly, despite being structurally similar, compounds with 4-bromo, 4-chloro, and 4-fluorophenyl substitutions exhibited uniform activity across both Gram-positive and Gram-negative strains, with MIC values of 25 μg/mL for all tested bacteria. This uniformity in activity suggests that these halogen substitutions might not be as effective in modulating antibacterial properties compared to dichloro substitution.

The study highlights the importance of understanding the structure–activity relationship (SAR) in designing potent antibacterial agents. It is clear that electron-withdrawing halogens, particularly in the dichloro configuration, enhance the antibacterial potency of these compounds, while simple halogen or alkyl substitutions exhibit more generalized activity. The antibacterial study study identifies 2-(2,4-dichlorophenyl)benzo[d]imidazo[2,1-b]thiazole-7-sulfonamide as the most promising lead compound, particularly for targeting Gram-positive pathogens. Further structural modifications, especially aimed at enhancing activity against Gram-negative bacteria, may lead to the development of more effective antibacterial agents. However, the study also emphasizes that none of the synthesized derivatives were as potent as ciprofloxacin, indicating the need for further optimization of these benzo[d]imidazo[2,1-b]thiazole-based scaffolds.

Molecular structure and MESP study

The optimized geometries of the benzo[d]imidazo[2,1-b]thiazole-based compounds (5a–5h) given in Table 4 shows a planar benzimidazole-thiazole core, promoting extended π-conjugation across the fused rings, which enhances stability. The sulfonamide group attached at position 1 introduces a slight deviation from planarity due to its tetrahedral geometry around the sulfur atom, while the electron-withdrawing nature of this group pulls electron density away from the core. Conversely, the phenyl group attached at position 12, with its electron-donating methoxy substituent at position 16, aligns closely to the plane of the central structure, allowing efficient conjugation and resonance effects. The Molecular Electrostatic Potential (MESP) diagrams for the benzo[d]imidazo[2,1-b]thiazole-based compounds (5a–5h) given in Table 4 highlights the distribution of electrostatic potential over the molecular surface, with color gradients representing areas of electron density. The red regions around the sulfonamide group, particularly the oxygen atoms, indicate zones of high negative potential, suggesting strong electron density and potential hydrogen-bond acceptor sites. In contrast, the blue regions near the hydrogen atoms of the sulfonamide group represent areas of positive electrostatic potential, making them suitable for nucleophilic interactions. The relatively neutral green and yellow regions across the aromatic rings, including the benzimidazole and phenyl groups, demonstrate areas of moderate electron density, where electron-withdrawing and electron-donating groups influence the overall electron distribution. This distribution highlights the compound's duality, with electron-rich and electron-poor regions influencing its reactivity, binding affinities, and interactions in potential molecular docking or biochemical environments.

Table 4 Optimized molecular structures and Molecular electrostatic surface potential of the 2-(aryl)benzo[d]imidazo[2,1-b]thiazole-7-sulfonamide derivatives (5a-5h)

Electronic and global descriptors' analysis

The results of the frontier molecular orbital (FMO) analysis and global reactivity descriptors (Table 5) for the benzo[d]imidazo[2,1-b]thiazole-7-sulfonamide derivatives (5a-5h) reveal significant insights into their electronic properties. These compounds have shown variations in key parameters such as the HOMO (E_HOMO), LUMO (E_LUMO) energies, energy gap (ΔE), chemical hardness (η), electronegativity (χ), and electrophilicity index (ω), all of which are crucial for understanding their reactivity and stability. The electron-donating methoxy group in 5a leads to a relatively higher E_HOMO value (-5.75 eV), indicating that this compound is more prone to donating electrons compared to other derivatives. The smaller energy gap (ΔE = 3.86 eV) and higher softness (S = 0.52 eV⁻¹) suggest that 5a is more reactive, with higher polarizability and less chemical stability. This reflects the influence of the methoxy group, which enhances electron density and reduces the energy gap, favoring reactivity. In contrast, compounds with halogen substituents like 5b (bromo), 5c (chloro), 5d (fluoro), and 5f (dichloro) exhibit lower E_HOMO values, ranging from −6.36 eV (5f) to −6.18 eV (5d), indicating stronger resistance to electron donation and enhanced stability. The halogen atoms, particularly the dichloro group in 5f, lead to the largest energy gap (ΔE = 4.24 eV) and the highest chemical hardness (η = 2.12 eV), indicating that this compound is less reactive but more chemically stable. The electrophilicity index (ω) for 5f is also the highest at 4.25 eV, suggesting its stronger electrophilic nature, which is favorable for reactions with nucleophiles. The unsubstituted phenyl derivative 5e presents an intermediate behavior with an E_HOMO of −6.12 eV and an energy gap of 4.13 eV. Its properties, such as chemical hardness (2.07 eV) and electrophilicity (3.97 eV), are moderate, showing that 5e balances between stability and reactivity, lacking the strong polarizing effects of electron-withdrawing or donating groups. Compound 5 g, with a 2-chlorophenyl substituent, has a similar reactivity profile to 5f but with a slightly smaller energy gap (4.27 eV). Interestingly, 5h, containing amethyl group, demonstrates a relatively lower energy gap (4.04 eV), slightly higher softness (S = 0.49 eV⁻¹), and a higher E_HOMO (−5.98 eV) compared to the halogenated derivatives, reflecting the mild electron-donating effect of the methyl group. Overall, the introduction of electron-withdrawing groups, such as halogens, increases chemical stability, as evidenced by lower E_HOMO values and higher energy gaps, while electron-donating groups like methoxy and methyl enhance reactivity by raising E_HOMO and lowering ΔE. This study illustrates the clear impact of substituents on the electronic properties and reactivity of the benzo[d]imidazo[2,1-b]thiazole-7-sulfonamide derivatives, providing insights into their potential applications in chemical and pharmaceutical fields.

Table 5 Electronic and global descriptors of the of the 2-(aryl)benzo[d]imidazo[2,1-b]thiazole-7-sulfonamide derivatives (5a-5h)

Molecular docking and ADME study

For antitubercular target

The DprE enzyme, specifically Decaprenylphosphoryl-β-d-ribose 2′-epimerase (DprE1), plays a crucial role in mycobacterial cell wall synthesis [35]. It catalyzes the conversion of Decaprenylphosphoryl-d-ribose (DPR) to decaprenylphosphoryl-2-ketoribose (DPX), which is then further reduced to Decaprenylphosphoryl arabinose [35] (DPA) by DprE2. DPA serves as a key substrate for arabinosyltransferases involved in the synthesis of essential cell wall components, such as arabinogalactan and lipoarabinomannan [35]. Targeting DprE1 offers a promising strategy for developing new anti-TB drugs, especially against drug-resistant strains of Mycobacterium tuberculosis [35]. From our in-vitro analysis, we moved forward for molecular docking analysis to check the probable binding modes of the compounds, 5b [MIC value: 1.6 µg/mL], 5d [MIC value: 1.6 µg/mL], and 5h [MIC value: 1.6 µg/mL]. The selection of the pdb id (PDB ID: 6G83, Resolution: 2.40 Å, R-Value Work: 0.198) was based on earlier literature report [12] and same was downloaded from protein database bank [https://www.rcsb.org/structure/6g83]. The compound 5b had interactions with amino acid residues such as Lys134, Pro116, Val365, Thr118, Ile386, etc. and obtained as best docked candidate with docking score of −6.2 kcal/mol (Fig. 2). Compounds, 5d and 5h showed binding site residues as Lys134, Pro116, Val365, Thr118, Ile386, etc. with binding scores as −6.1 kcal/mol and −5.9 kcal/mol. In all three compounds, we noted the H-bonding interaction with Lys134 amino acid. The co-crystal ligand, EQ8 was also chosen to have comparison of docking affinities and found with score of −5.7 kcal/mol. The EQ8 had interactions with amino acid residues Lys134, Pro116, and Thr118, etc. Our docking validation also resulted the RMSD value of 1.23 Å indicating the correctness of docking protocol followed.

Fig. 2

Molecular docking interaction diagrams for the best active candidates, a 5b [Green], b 5d [Pink], c 5h [Yellow] and d EQ8 [Maroon], respectively against the binding site of Mycobacterium tuberculosis Decaprenylphosphoryl-β-d-Ribose Oxidase (DprE1) enzyme (PDB ID: 6G83)

For common bacterial target

For antibacterial activity of best in-vitro candidate 5f, we chosen common antibacterial target as 2,2-dialkylglycine decarboxylase (PDB ID: 1D7U, Resolution: 1.95 Å) [https://www.rcsb.org/structure/1D7U] [40]. 2,2-Dialkylglycine decarboxylase (DGD) is an enzyme found in some bacteria that catalyzes the decarboxylation of 2,2-dialkylglycines to produce dialkylamines and carbon dioxide [40]. This reaction is part of bacterial amino acid metabolism, playing a crucial role in the utilization of specific amino acids as nitrogen sources. By facilitating the breakdown of 2,2-dialkylglycines, DGD helps bacteria adapt to diverse environments, enhancing their metabolic flexibility [40]. Additionally, the production of dialkylamines may be involved in various physiological functions, including signalling and osmoregulation. Overall, DGD contributes to bacterial survival and growth under varying conditions [40]. The compound 5f had best docking score throughout the series as −7.9 kcal/mol, compared to the standard Ciprofloxacin (docking score: −6.83 kcal/mol) (Fig. 3a). Compound 5f had interactions with amino acid residues such as Ser214, Trp138 (hydrophobic interactions), Gly111 (H-bonding), Lys272 (van der Waals interactions). Standard Ciprofloxacin also retained interactions similar to 5f with His 139 (H-bonding), Trp138, Ala112, Arg251, Lys272, etc. (Fig. 3b).

Fig. 3

Molecular docking interaction diagrams for the best active candidate, a 5f and b Ciprofloxacin, respectively against the binding site of 2,2-dialkylglycine decarboxylase enzyme (PDB ID: 1D7U)

ADME study

The ADME and drug-likeness profiles of compounds 5b, 5d, 5f, and 5h were evaluated, revealing distinct patterns (Table 6). Each compound had 23 to 24 heavy atoms, with no significant variation in the fraction of sp³ hybridized carbons, except for compound 5h, which showed a slight increase (0.03). All compounds exhibited two rotatable bonds, suggesting moderate molecular flexibility, while the number of hydrogen bond acceptors varied slightly, with compound 5d having five and the others four. Each compound possessed one hydrogen bond donor. The molar refractivity values ranged from 86.96 (5d) to 97.03 (5f), reflecting minor variations in their electronic polarizability. All compounds had the same topological polar surface area (TPSA) of 114.08 Ų, which indicates that they have comparable polar regions, important for solubility and permeability. The consensus Log Po/w values varied from 2.91 to 3.76, suggesting that compound 5f is more lipophilic compared to the others. The solubility (Log S, ESOL) values were lowest for 5f (−5.74), indicating poor solubility, while 5d and 5h showed relatively better solubility (−4.72 and −4.86, respectively). Gastrointestinal (GI) absorption was high for all compounds except 5f, which exhibited low absorption. None of the compounds permeated the blood–brain barrier (BBB), and they were not substrates for P-glycoprotein (P-gp), indicating limited efflux. All four compounds inhibited CYP1A2, CYP2C19, and CYP2C9 enzymes, but none inhibited CYP2D6. Compound 5h was the only inhibitor of CYP3A4, indicating potential drug-drug interaction risks for this enzyme. Drug-likeness evaluations based on Lipinski, Ghose, and Veber rules were favorable for all compounds, confirming their potential oral bioavailability. However, Egan and Muegge criteria were only met by 5f and 5h, which may limit the drug-likeness of 5b and 5d. Despite these differences, the bioavailability score was consistent across all compounds (0.55), suggesting moderate bioavailability potential for this series of benzo[d]imidazo[2,1-b]thiazole-7-sulfonamide derivatives. The compound 5f stood out for its higher lipophilicity and distinct enzyme inhibition profile, while 5h presented slightly improved solubility and drug-likeness.

Table 6 ADME and drug likeness of the compounds 5b, 5d, 5f and 5h

The graphical abstract, illustrated in Fig. 4, provides a concise and visually engaging summary of the key findings and methodologies of the research work. It serves as a quick reference for readers to understand the core concepts, objectives, and outcomes of the study, highlighting its significance and potential applications in an easily interpretable format.

Fig. 4

Graphical abstract

Conclusion

The present study highlights the successful synthesis of a series of 2-(aryl)benzo[d]imidazo[2,1-b]thiazole-7-sulfonamide derivatives (5a–5h), which exhibited significant antibacterial and antitubercular activity. Among the derivatives, compound 5f showed the highest antibacterial potency with a docking score of −7.9 kcal/mol against 2,2-dialkylglycine decarboxylase, surpassing the standard Ciprofloxacin. Similarly, compounds 5b, 5d, and 5h exhibited notable antitubercular activity, with binding affinities ranging from −6.2 to −5.9 kcal/mol against the DprE1 enzyme, a critical target in Mycobacterium tuberculosis. The DFT analysis provided key insights into the electronic properties of these derivatives, revealing that the presence of electron-donating and withdrawing groups significantly influences their reactivity and stability. Compound 5a, with a methoxy group, showed the highest reactivity with a lower energy gap (ΔE = 3.86 eV), while the halogenated derivative 5f exhibited the highest chemical stability (ΔE = 4.24 eV), reflecting the impact of substituents on molecular properties. ADME studies revealed that compound 5f stood out with higher lipophilicity and an optimal enzyme inhibition profile, while compound 5h showed improved solubility and drug-likeness, making both promising candidates for further drug development. The results from molecular docking, DFT, and ADME analyses underline the potential of these derivatives as antibacterial and antitubercular agents. Future work should focus on optimizing these compounds to enhance their bioavailability and efficacy against drug-resistant pathogens.