A Review on the Physicochemical and Biological Properties of Benzoxazole and Thiazole Derivatives

Heterocyclic compounds play a vital role in pharmaceutical research due to their unique structural diversity and broad spectrum of biological activities. Among these, benzoxazole and thiazole derivatives have emerged as important scaffolds in medicinal chemistry, exhibiting notable chemical, physical, and medicinal properties. Through structural modifications and substitution patterns, the benzoxazole core—a fused benzene and oxazole ring—contributes to diverse pharmacological actions, including antibacterial, antifungal, anticancer, anti-inflammatory, and central nervous system activities.^[1]

BENZOXAZOLE

Benzoxazole- First created in 1947, oxazole is an unsaturated five-membered heterocycle with oxygen (O) and nitrogen (N) heteroatoms at positions 1 and 3, separated by a carbon (C) atom (Fig. 1a).^[2] A fused bicyclic aromatic planar heterocycle with a 1,3-oxazole ring structure and benzene is called benzoxazole (Fig. 1b). The benzene ring is planar and composed of six carbon atoms, while the 1,3-oxazole ring contains one atom each of oxygen (O) and nitrogen (N). The fusion of these rings creates a bridging structure, resulting in a fused bicyclic system with aromatic properties. ^[3,4] The composition of the benzoxazole scaffold's structure enables effective communication with biological targets. Planar benzene rings can produce π-cation or π-π stacking interactions. engagement with the host molecule, whereas the 1-oxygen and the oxazole moiety's third nitrogen form a hydrogen bond. Therefore, non-covalent interactions occur between acceptors. Additionally, the lipophilic properties of benzoxazoles enable interactions with hydrophobic target proteins.^[5] Benzoxazoles are also considered the bio isosteres of guanine and adenine, two nucleic bases. Benzoxazoles and biopolymers can interact favorably in living systems due to their structural similarity. The wide range of biological activity seen in them could also be attributed to this.^[6]

[a] 1,3-oxazole                             [b] benzo [d] oxazole

Fig 1. Structural formulae of [a] oxazole and [b] benzo[d] oxazole

2. Reaction with Aldehydes:

2-Arylbenzoxazoles were produced directly from substituted 2-aminophenols and aldehydes in xylene with activated carbon in an oxygenated environment. For the production of 2-arylbenzoxazole compounds of potential medicinal interest, a straightforward and effective technique has been designed. The process entails the reaction of 2-aminophenol with substituted aromatic aldehydes in acetonitrile, with anhydrous bismuth trichloride acting as a catalyst. Some appealing aspects of this technology include the use of environmentally benign bismuth trichloride, generally gentle reaction conditions, and moderate to good product yield.^[11]

3. Reaction with alcohols:

2-substituted benzoxazole is produced in good yield when O-amino phenol and alcohol react in the presence of a catalytic quantity of a ruthenium complex.^[12]

4. Reaction with ester:

Trialkyl Ortho ester, o-aminophenol, o-phenylenediamine, or 2-amino-3-hydroxypyridine, and silica sulfuric acid were added together and stirred for the proper amount of time at either room temperature or 85°C. TLC (eluent: n-hexane: ethyl acetate,2:1) was used to monitor the reaction's progress. The mixture was filtered once the reaction was finished and diluted with 10 cc of CHCl3. After being cleaned with CHCl3, the solid material was dried at 60° C. The residue was purified by either column chromatography on neutral alumina or recrystallization in n-hexane after the filtrate was evaporated.^[13]

THIAZOLE

It was first described by Hantzsch and Weber in 1887.^[14] The thiazole ring is a naturally occurring substance that is predominantly found in marine and microbial sources. Natural compounds that include peptides ^[15], vitamins (such as thiamine), alkaloids, epothilone, and chlorophyll ^[16,17]. Thiazoles have relatively low toxicity toward mammals. It has shown various biological. activities like antioxidant, analgesic, antibacterial, anticancer, antiallergic, anti-inflammatory, antifungal, and antimalarial. ^[18,19,20] It has also shown hypnotic,^[21] diuretic and neuroprotective activities. ^[22,23] The thiazole ring, characterized by a five-membered heterocycle containing nitrogen and sulfur, serves as a key structural motif in numerous bioactive compounds. Its physicochemical features, such as the presence of heteroatoms and aromaticity, significantly influence pharmacokinetic behavior and molecular interactions with biological targets.^[24

Fig 2. Structure of thiazole

Chemistry and Structural Features of Thiazole:

Thiazole is a five-membered heterocyclic compound containing one nitrogen and one sulfur atom in its ring. Due to conjugation, structure is stable and flattened. Thiazole contains unique electronic properties due to the presence of sulfur and nitrogen atoms, making it important in medicinal chemistry and drug development.^[25]                                 

Ring Structure and Aromaticity: Thiazole is an aromatic compound that follows Huckel’s rule (4n + 2 π electrons), having a total of 6 π-electrons in the ring. Sulfur helps in electron delocalization, while nitrogen's lone pair helps the aromatic system. Its aromaticity influences its chemical behavior and provides it stability.^[26]

Substitution Pattern and Reactivity of Thiazole: C-5 position in thiazole has a higher electron density, electrophilic substitution usually occurs here. Since nitrogen and sulfur atoms exhibit an electron-withdrawing property, the C-2 position is more reactive to nucleophilic substitution. Thiazole is versatile for chemical modifications because it shows both electrophilic and nucleophilic reactivity.^[27]

Functionalization at Different Positions of Thiazole: Thiazole can be easily modified at specific position by using a number of methods. Because the C-5 position is more reactive toward electrophiles, methods including nitration, halogenation, and sulfonation are usually used here. Different groups are added at the C-2 position through coupling reactions like Suzuki, Heck, as well as nucleophilic substitution. Selective substitution is further helped by metalation methods such as lithiation. These reactions make it easy to prepare a variety of useful thiazole derivatives.^[28]

Fig3. Resonance of the thiazole ring

It can successfully interact with biological targets due to its adaptability, particularly through van der Waals forces and hydrogen bonds that connect molecules with amino acid residues in the receptor protein.^[30] This structure has both an electron-donating group (–S–) and an electron-withdrawing group (–C = N–). The system stability and reactivity patterns are directly influenced by the electronic distribution within the ring, which is strengthened by the electronegativity of nitrogen and the involvement of sulfur's orbitals. While nucleophiles often target the C-2 position, electrophilic substitution processes preferentially take place at the C-5 and C-4 Position. ^[29–31] In addition, the type of substituents linked to the ring can alter the thiazole's activity profile. The structure's basicity and nucleophilicity are increased by electron-donating groups, enhancing its use as a bioactive scaffold in the development of new drugs.^[29] 

A number of compounds, like thioamides, thiourea, ammonium thiocarbamate or Di thiocarbamate, and their derivatives, may act as nucleophilic reagents in this reaction. In 1887, Hantzsch created the basic thiazole nucleus.^[32] This synthesis approach involves cyclization and condensation of haloketones with thioamide, and it is considered the most widely popular process for the synthesis of thiazole moiety.^[33]

Synthesis from α-halo carbonyl compounds (Hantzsch’s synthesis)

1. Reactions with thioamides: Several thiazoles with alkyl, aryl, arylalkyl, or heteroaryl functional groups at positions 2, 4, or 5 were produced through an interaction with thioamides and various α-halo carbonyl compounds. (Synthesis of 2-, 4-, 5-trisubstituted thiazole.) ^[34]

2. Reactions with N-substituted thiourea: Halo carbonyl compounds reacted with N-substituted thiourea compounds to produce monosubstituted or disubstituted aminothiazoles.^[35] (Synthesis of substituted aminothiazoles.)

3. Reaction with esters of Thio carbamic acid: 2-hydroxythiazole derivatives were produced by condensing α-halo carbonyl compounds with thiocarbamates.^[36] (Synthesis of 2-hydroxythiazole derivatives.)

Table 1. Physiochemical properties of benzoxazole and Thiazole ^{[37,38,39,40,41]}

MEDICINAL PROPERTIES OF BENZOXAZOLE DERIVATIVES ^[42]

Fig 4. Medicinal properties of benzoxazole

1. Anticancer Activity-

Balasubrahmanian N et al. (2020) reported the synthesis of 2-(2-((benzoxazol-2-ylthio) methyl)-1H-benzimidazol-1-yl) acetyl hydrazide derivatives. The human colorectal carcinoma [HCT116 (ATCC (American Type Culture Collection) CCL247)] cancer cell line was used in the 2- (3-diethyl-amino-6-diethylazaniumylidene-xanthen 9-yl)-5-sulfobenzene-sulfonate (SRB) assay to evaluate the anticancer activity of the synthesized derivatives. Previously, Balasubrahmanian reported the synthesis of benzoxazole derivatives. The human colorectal cancer cell line HCT 116 (ATCC CCL-247) was used to assess the antiproliferative characteristics of the benzazole derivatives. The standard drug is 5-fuorouracil (IC50=12.2 μM).^[43]

2. Antioxidant Activity-

Using the amount of copper as an oxidizing agent, the authors et al. (2025) synthesized 2(3H)-benzoxazolone derivatives and evaluated their in-vitro antioxidant activity at a dose of 10 μM to stop human LDL copper-induced oxidation. Compound 3 demonstrated more antioxidant activity and inhibited the beginning and growth of copper-mediated LDL oxidation in a dose- and time-dependent manner.^[44]

3. Anti-inflammatory Activity-

The inhibition of cyclooxygenase (COX) is largely responsible for the therapeutic and adverse effects of non-steroidal anti-inflammatory medications (NSAIDs), which are a standard in the treatment of inflammation.^[45] One of the biggest challenges in the design and synthesis of these medications has been isolating the therapeutic effects from the negative effects. The identification of a second isoform of COX, COX-2, has led to new research based on the hypothesis that the constitutive isoform COX-1 produces physiological prostaglandins, whereas the inducible isoform COX-2 produces pathological prostaglandins.^[46] FT-IR, 1H NMR, MS, and elemental analysis were used to characterize a series of N-(acridin-9-yl)-4-(benzo[d]imidazol/oxazol-2-yl) benzamides who were produced by combining 9-amino acridine derivatives with benzoxazole derivatives. It was discovered that these chemical compounds had noticeable anti-inflammatory properties.^[47]

4. Anticonvulsant Activity

PTZ (Pentylenetetrazol) -induced convulsions in albino mice were used to test their in vivo anticonvulsant efficacy of 2-mercapto benzoxazole and 2-mercapto benzimidazole. A large number of the substances indicated the capacity to prevent convulsions produced by the chemical pentylenetetrazol. If compared to a normal medication, several molecules showed the highest level of action. Biologically active Benzoxazole.^[48]

5. Antimicrobial Activity

Elnima EI et al. (2017) examined the antifungal and antimicrobial properties of six benzimidazole and benzoxazole derivatives in vitro. They have been compared to reference strains and 59 clinical isolates. Out of the six compounds, only two (both benzoxazoles) exhibited any action. Following treatment with these medications, all common strains—including fungus and gram-positive and gram-negative bacteria—showed significant growth reduction. Fifty-nine clinical isolates of Pseudomonas aeruginosa, Escherichia coli, and Staphylococcus aureus were tested for their susceptibility to the two drugs. The most susceptible were the Staphylococcus aureus isolates. Although compound (6) showed slightly more activity that compound (7), both drugs were equally effective against every isolate. Either of its respective minimal the bacterium S. aureus was 90% inhibited at inhibitory doses of 25 and 50 μg/ml. The gram-negative bacteria needed just 200 μg/ml of inhibitory concentration to produce a comparable level of inhibition because they were immune against both compounds.^[49]

The authors Sener E. et al. (2010) synthesized derivatives of 5-amino-2-(p-substituted-phenyl) benzoxazole. They were made by heating 2, 4-diaminophenol with the appropriate carboxylic acids in the presence of polyphosphoric acid. 2-substituted benzoxazoles have been extensively investigated since position 5 predominates in terms of activity intensity, leading to the conclusion that biological activity is determined by this position. Benzoxazoprofen and zoxazolamin are examples of benzoxazole compounds that are substituted at positions two and five.^[50]

6. Antifungal Activity-

1. New antifungal drugs with a 1,3-benzoxazole-4-carbonitrile skeleton and a novel mode of action that inhibits the synthesis of β-1,6-glucan, which is known to be important for the cellular growth and proliferation of Candida spp.11—were discovered, according to Jun-Ichi Kuroyanagi et al. (2007) Compound (9) showed strong antifungal activity against Candida spp. among those 1,3-benzoxazole analogs.^[51]

2. Beom Joon Kim et al. (2010) developed and synthesized benzoxazole amides and assessed their antifungal efficacy against Malassezia furfur based on the chemical structure of Malassezin-reported benzoxazole derivatives. Twelve amides of benzoxazole derivatives (4a-h and 8a-d) were prepared with substitution shown in Table 2 and 3. Compounds 4a, 8b, 8c and 8d showed antifungal activity with clear zone of 15, 8, 10 and 12 mm, respectively.^[52]

Table 2. Different substituted compounds 4(a-h).

Table 3. Different substituted compounds 8 (a-d).

7. Analgesic Activity-

Praveen et al. (2013) created a benzoxazole derivative using a cyclocondensation reaction and used the tail immersion method to test its analgesic properties. According to the study's findings, 12, 13, 14,15, and 16 had the lowest analgesic potency (50.6%, 50.9%, 51.3%, 51.3%, and 50%). While compounds 17, 18, and 19 showed greater analgesic potency (73.5%, 76.4%, and 74%), compounds 20 and 21 showed intermediate analgesic potential (54.5% and 59.6%, respectively). In this experiment, pentazocine served as a positive control.^[53]

8. Antitubercular Activity-

1. A series of 2-benzylsulfanyl derivatives of benzoxazole were synthesized by Klimesova V et al. (2012) and their antimycobacterial activity against Mycobacterium tuberculosis, nontuberculous mycobacteria, and multidrug-resistant M. tuberculosis was assessed in vitro. Benzoxazole's dinitrobenzylsulfanyl derivative is one of the most promising synthesized small-molecule antimycobacterial.^[54]

2. Vinsova Jarmila et al. synthesized 3, 5-di-tert-butyl-1, 2-benzoquinone, which reacts with amino acids and dipeptides containing N-terminal glycine to produce a series of lipophilic 2-substituted 5, 7-di-tertbutylbenzoxazoles. Oxidative deamination occurs in dipeptides with different N-terminal amino acids. 5,7-Di-tert-butylbenzoxazoles (23) shown efficacy against certain nontuberculous strains of Mycobacterium Tuberculosis, while isoniazid has not showed any effect.^[55]

MEDICINAL PROPERTIES OF THIAZOLE DERIVATIVES

1. Anti-inflammatory Activity-

The anti-inflammatory and lipoxygenase and cyclooxygenase inhibitory properties of a number of adamantane derivatives of thiazolyl-N-substituted amides were investigated. (24) of the substances that were evaluated had strong activity.^[56]

2. Anticonvulsant Activity-

4-fluoro-N-[4-[6-(isopropyl amino)-pyrimidin-4-yl] was discovered by Satoh et al. (2009) [1,3-thiazol-2-yl] N-methyl benzamide (25) is a strong mGluR1 antagonist that could be used as a PET tracer to better understand how mGluR1 functions in humans. A series of N4- (naphtha[1,2-d] thiazol-2-yl) semi carbazide (26) was developed and synthesized by Azam.^[57] and assessed for their neurotoxicity and anticonvulsant properties. A molecule with p-methoxyphenyl (27) substitution at C-2 of the thiazolidinone ring was discovered to be beneficial for activity in a series of thiazolidinonyl 2-oxo/Thio barbituric acid derivatives and shown to have more potent anticonvulsant activity compared to the standard drug sodium phenytoin.^[58]

3. Antitubercular Activity-

Due to poverty and the HIV/AIDS pandemic, Mycobacterium tuberculosis infections are becoming more common these days. Therefore, the primary issues are the efficacy of traditional antitubercular medications and the development of multidrug-resistant strains of M. tuberculosis.^[56] Certain compounds containing thiazoles showed promising antitubercular effects among antitubercular agents. A number of 2-(2-hydrazinyl) thiazole derivatives were created and tested against Mycobacterium tuberculosis in a study by Makam, PhD, and colleagues. Ethyl-4-methyl-2-[(E)-2-[1-(pyridin-2-yl) ethylidene] hydrazin1-yl] -1.3-Thiazole-5-carboxylate (28) was one of the produced compounds that showed significant inhibitory efficacy against M. tuberculosis.^[57]

4. Antiviral Agents-

Viral infection is one of the most common and fatal diseases, taking several lives annually. There is an urgent need to discover more powerful and efficient agents, even with recent developments in the field of antiviral medications. Many thiazole-based compounds were produced and assessed in this field; some of these are reported below. Mayhoub et al. produced and evaluated a novel generation of methyl 4-dibromomethyl 2-(4-chlorophenyl) (29), the most effective derivative among the primary compounds, against the yellow fever virus using a cell-based assay.^[59]

5. Anticancer activity-

Examined the synthesis and activity of many 4-thiazolyl-substituted analogs of new pyrrolocarbazole as poly (ADP-ribose) polymerase-1 (PARP-1) inhibitors. (30) of these substances have been shown to be stronger than others.^[60]

For the purpose of isolating aryl-substituted benzothiazoles, the researchers S et al. (2014) refluxed aminophenol’s with substituted benzoic acid in the presence of polyphosphoric acid (31) at a higher temperature. They subsequently evaluated these compounds as treatments against human cervical cancer cell lines.^[61]

The synthesis of 2-(5-((5-(4-chloro-phenyl) furan-2-yl) methylene)-4-oxo-2-thioxothiazolidin-3-yl) acetic acid derivatives and an evaluation of their cytotoxic activities have been reported by Chandrappa et al. (2007), Compounds with electron-donating groups at the C-terminal of the phenyl ring were found to improve activity by inducing cell death, while compounds with electron-withdrawing groups (CN, F, and CF?) showed lower activity. Due to its electron-donating methoxy group, compound (31) was found to be the most potent since it showed the greatest cytotoxicity. In addition, compound (32) shows chromosomal DNA fragmentation at 50 μM.^[62]

6. Antifungal activity-

By synthesizing 4-bromoacetyl-3-methyl-5-oxo-1-phenyl-2-pyrazoline with thiourea, Mohamed Salah K. Youssef et al. (2009) synthesized a series of 4-(2-Aminothiazol-4-yl)-3-methyl-5-oxo-1-phenyl-2-pyrazoline. They evaluated the antibacterial efficacy of these substances against various fungal strains, including Aspergillus flavus, Candida albicans, Geotrichum candidum, and Scopulariopsis brevicaulis. Aspergillus rubrum and Aspergillus niger. 3- (4,5-dihydro -3-methyl-5-oxo-1-phenyl-1H-pyrazol-4- [3,2-a] yl)-6H-thiazolo with low inhibitory concentrations (MIC=5-50 mg/cm³), pyrimidine-5,7-dione (33) shows a broad range of antifungal activity but a small spectrum of antibacterial activity.^[63]

Applying the serial plate dilution method 24, thiazole derivatives have been evaluated for their antifungal activity against Trichophyton and Geotrichum (recultured) in DMSO. The diameter of the fungal colonies was used to measure the antifungal activity. At concentrations of 20 mg/ml and 25 mg/ml, compound (34) proved to be the most effective against Trichophyton and Geotrichum.

7. Antitumor activity-

A number of new ethyl 2-substituted aminothiazole-4-carboxylate analogs have been synthesized, and the National Cancer Institute (NCI) tested the compounds for their potential anticancer activity against 60 human tumor cell lines in vitro. 2-[3-(diethylamino)-prop ethyl-thiazole-4-carboxylate (35) shows outstanding activity against the

leukemia cell line RPMI-8226 with a GI value of 0.08 µM and a broad-spectrum activity against each tumor cell line used, with a GI (MG-MID) value of 38.3 µM.^[64]

8. Antimicrobial activity-

Six 3-methyl -1-[(5-substituted-1H-indol-2-yl) carbonyl] -4-{[4-(substitutedthiazol-2-yl) iminoethyl) phenyl] hydrazono} Derivatives of 2-pyrazolin-5-one were created using both traditional and microwave methods. The antimicrobial capacity of the resulting compounds has been evaluated against three fungal strains and six bacterial strains. While most of the other compounds displayed diverse antibacterial activity, compound (36) showed a wide range of activity against bacteria, and compound (37) has shown excellent antimicrobial activity.^[65]

The C-2 position in benzoxazole is the main substitution site and is important for controlling biological activity. The molecule's general physicochemical properties, steric effects, and electronic characteristics are able to be modified via substitutions at the C-5 and C-6 positions. The C-2 position acts as the main pharmacophoric site in thiazole derivatives, but modifications at the C-4 and C-5 positions influence molecular stability, biological activity, and selectivity. Designing new derivatives with better therapeutic qualities needs an understanding of these substitution patterns. ^[66,67]