3
Methods of Synthesis of Benzimidazole Derivatives
Many approaches have been developed and confirmed for the synthesis of benzimidazoles.
One of these methods is the direct method for the preparation of benzimidazole through
a direct condensation reaction between o-phenylenediamine and formic acid (Scheme [1]).[29] There are also many ways to prepare benzimidazole derivatives. Some of these methods
face some challenges, such as low yield, harsh reaction conditions, or long reaction
times, and other methods are more suitable for preparation, as summarized in the following
methods.
Scheme 1 Synthesis of benzimidazole
3.1
Condensation of 2-Aminoaniline (o-Phenylenediamine)
3.1.1
With Aldehyde
In this method, the benzimidazole derivatives are prepared from the condensation reaction
between various aldehydes and 2-aminoaniline (o-phenylenediamine) (Scheme [2]). This reaction occurs in an oxidation process with either air or several other
oxidizing agents.[30]
Scheme 2 General condensation of o-phenylenediamines with aldehyde to synthesize benzimidazoles
It was also reported that o-phenylenediamine can be used with alkyl, aryl, and heterocyclic aldehydes in the
presence of palladium or copper catalysts. Good product and purity were obtained under
these conditions (Scheme [3]).[31]
[32]
Scheme 3 Synthesis of benzimidazole using copper acetate as catalyst
The benzaldehyde derivative shown in Scheme [4] was prepared by the reaction of 2,4-dichlorobenzaldehyde with o-phenylenediamine in the presence of sodium pyrosulfite Na2S2O5 (sodium metabisulfite).[33]
Scheme 4 Synthesis of benzimidazole using sodium metabisulfite catalyst
Furthermore, 2-substituted benzimidazoles were prepared by Rushi et al. as shown in
Scheme [5], by condensing aldehydes with o-phenylenediamine under solvent-free conditions using [In(OTf)3] as catalyst at room temperature. This method gave good and dependable yield.[34]
Scheme 5 Use of solvent-free conditions for the synthesis of benzimidazoles
Benzimidazoles have also been prepared in the presence of gold nanocomposites in different
form such as Au/ZnO, Au/TiO2, Au/Al2O3, which is a very effective method. This procedure was reported by L. Hao et al. (Scheme
[6]).[35]
Scheme 6 Synthesis of benzimidazole catalyzed by Au/TiO2 nanoparticles
3.1.2
With Ketones
Another method that is used to prepare benzimidazole derivatives is through the condensation
reaction between ketones with o-phenylenediamine; however, this procedure includes some limitations that make this
reaction not as widely used to prepare benzimidazole derivatives, which will be discussed
later. The general method includes a condensation reaction between diamine and ketones
at temperatures that differ from one reaction to another according to the reaction
conditions (Scheme [7]).[30]
Scheme 7 Synthesis of benzimidazoles using condensation reaction between ketones and o-phenylenediamines
Alaqeel reported in 2017 that it is possible to prepare benzimidazole derivatives
in this way and that the stability of the prepared products depend on the alkyl group
substituent on the ketones. In the same way, Ladenbrug and Rugheimer documented the
possibility of synthesizing 2-phenyl-5(or 6)-methylbenzimidazole compounds by conducting
a condensation reaction between acetophenone and 3,4- diaminotoluene by heating at
180 °C (Scheme [8]).[36]
Scheme 8 Using the reaction of o-phenylenediamine with acetophenone to synthesize benzimidazole derivatives
The reaction suffered from some difficulties such as the formation of undesirable
products, long reaction time, as well as some challenging working conditions, which
led to the development of better conditions for this type of reaction, such as the
possibility of completing the reaction under solvent-free conditions or using other
catalysts. Despite these improvements in the method, this method could not be used
to produce benzimidazoles, with other heterocyclic compounds being produced instead.
For example, as presented by Radatz et al., when the reaction of acetophenone or other
ketones with o-phenylenediamine was conducted at moderate temperatures using glycerin as a recyclable,
economical, and environmentally friendly solvent, the resulting product was a low
yield of benzodiazepine instead of benzimidazole. A similar product was formed when
the temperature was increased, but with higher yields (Scheme [9]).[37]
Scheme 9 Schematic reaction between the ketones and o-phenylenediamine.
Digwal et al. confirmed that quinoxaline derivatives )2,3-diphenyl quinoxaline derivatives)
are formed instead of benzimidazoles when dibenzyl ketones react with o-phenylenediamines using VOSO4 as a catalyst (Scheme [10]).[38]
Scheme 10 Reaction of 1,2-diphenylethane-1,2-dione with o-phenylenediamine using VOSO4 as catalyst
In summary, the reaction of o-phenylenediamine or its derivatives with ketones did not give the expected benzimidazoles;
on the contrary, other heterocyclic organic compounds were sometimes obtained as discussed
above.
3.1.3
With Carboxylic Acids
Many studies and research centers have presented research over a period of years indicating
that it is possible to prepare benzimidazole and its derivatives through a condensation
reaction between carboxylic acids and o-phenylenediamine or its derivatives in the presence of hydrochloric acid as a catalyst
(Scheme [11]).[36]
Scheme 11 General condensation of o-phenylenediamines with carboxylic acids to synthesize benzimidazole derivatives
Alam et al. documented the preparation of new benzimidazoles under reflux in xylene
and polyphosphoric acid with equimolar amounts of o-phenylenediamine with p-aminobenzoic acid. The reaction took place within 6 hours and the yield was good
and reliable (Scheme [12]).[39]
Scheme 12 Condensation of o-phenylenediamines with aromatic carboxylic acids to synthesize benzimidazole derivatives
Rithe et al. reported the possibility of preparing new benzimidazole derivatives in
good yield by using a one-step condensation reaction with equimolar amounts of aromatic
acid and o-phenylenediamine at 80–90 °C, with ammonium chloride as catalyst. This method gives
good yield and high purity as well as being an economical and environmentally friendly
approach (Scheme [13]).[40]
In 2015, A. Saberi documented the possibility of preparing new benzimidazole derivatives
using alumina, silica gel, or zeolite as catalyst under solvent-free conditions under
microwave irradiation. The reaction was performed by mixing equal moles of aromatic,
aliphatic, and heterocyclic carboxylic acid with o-phenylenediamine with 50 g of catalyst, grinding the mixture well in a mortar, and
then irradiating the ground mixture in a microwave for 5–9 minutes (Scheme [14]).[41]
Scheme 13 Synthesis of benzimidazole derivatives using ammonium chloride as catalyst
Scheme 14 Synthesis of benzimidazole derivatives using Al2O3, SiO2, or zeolite as catalyst
Huynh et al., recently used the same microwave irradiation method to prepare new benzimidazole
derivatives through condensation of monocarboxylic acids with o-phenylenediamine under catalyst-free conditions (Scheme [15]).[42] This method is considered very important and is one of the most economical and environmentally
friendly methods. It gave a good yields for the benzimidazole derivatives targeted
in the preparation.
Scheme 15 Synthesis of benzimidazole derivatives using microwave under catalyst-free conditions
Benzimidazole derivatives can also be prepared by using a condensation reaction between
o-phenylenediamines with different classes of organic compounds such as carboxylic
acid derivatives (anhydrides, esters, amides, acid chlorides), nitriles, as well as
urea. These methods can give benzimidazole derivatives with different yield ratios
and purity, depending on the presence of by-products.[36]
3.2
Via Rearrangement
Benzimidazole derivatives are synthesized by condensation reactions as discussed above.
In addition, some of the most effective methods that are used to prepare benzimidazole
derivatives have been based on rearranging heterocyclic compounds.
Scheme 16 General method for the synthesis of benzimidazolone from benzodiazepinones via a
rearrangement process
Scheme 17 Synthesis of benzimidazolone from benzodiazepinones via rearrangement
Scheme 18 Suggested mechanism for the preparation of benzimidazolone from quinoxaline-1,4-dioxides
Heating of a mixture of ethyl acetoacetate with o-phenylenediamine in xylene solvent to reflux gives a derivative of benzodiazepinone
(4,7-dihydro-5-methyl-1H-2,3-benzo-1,4-diazepin-7-one). But when this reaction is carried out under the same
conditions and then potassium hydroxide is added to this mixture, benzimidazole derivatives
are formed (Scheme [16]).[43]
It was also mentioned that dicarbonyl compounds can be used in this reaction, making
it readily applicable.[30] Eleftheriadis et al. showed that the reaction of dimethylacetone dicarboxylate compounds
with o-phenylenediamine could give a benzimidazolone through the formation of a diazepine
derivative, with subsequent action of mercaptoacetic acid to afford the benzimidazolone
(Scheme [17]).[44]
Moreover, in 2017, the rearrangement of quinoxaline-1,4-dioxides was documented by
which different benzimidazole derivatives were prepared. These studies revealed that
the rearrangement process depends on the composition of the compound quinoxaline-2-one
as an intermediate material. This study is distinguished because it depends on the
preparation of benzimidazole derivatives that contain a wider range of functional
groups (Scheme [18]).[43]
This method was also used to prepare a series of benzimidazole derivatives that are
difficult to prepare by other methods. For example, the polynuclear benzimidazole
derivatives shown in Scheme [19] were prepared through a reaction between ethyl acetoacetate and 3-(2-aminophenyl)-quinoxalin-2(1H)-one, with acetic acid as a solvent.[45]
Scheme 19 Using rearrangement for the synthesis of polynuclear benzimidazole derivatives from
quinoxalinone derivatives
On the other hand, 1,2,3-triazole derivatives were used as basic rearranged materials
to prepare benzimidazolone derivatives. In 2017, the use of 5-amino-4-carbamoyl-1-(2-nitrophenyl)-1H-1,2,3-triazol was reported as a base compound for the preparation of benzimidazole
derivatives. When the 1,2,3-triazole derivative was heated to reflux for 4 hours,
the corresponding benzimidazole derivative was generated, as shown in Scheme [20].[30]
Scheme 20 Rearrangement of triazole derivative to synthesize a benzimidazole derivative
3.3
Green Synthesis of Benzimidazole
Green chemistry is the invention, design, and application of products and processes
that reduce the use and synthesis of substances that are hazardous to the environment.
Green chemistry begins with the process of researching the type of product or type
of process that reduces the use of hazardous substances. The principle of green chemistry
has been focused on since the beginning of the 1990s, as the world began to pay attention
to this principle as the only way to ameliorate the danger of chemicals and life-threatening
processes. The preparation of organic compounds is one of the processes that leads
to environmental pollution due to the gases and vapors emitted during the production
process, as well as the waste that is generated as by-products. Therefore, it is necessary
to devise or use alternative means to prepare organic materials that preserves the
natural balance of the environment.[46]
[47]
[48] Currently, there are many technologies that are used to prepare organic compounds
that are considered green technologies because they improve the quality of the prepared
organic compounds, reduce waste emitted and save energy. Such technologies include
microwave-mediated processes, photochemical reactions, reactions that occur in the
presence of water as a solvent, organic synthesis using ultrasound methods, reactions
that occur in the presence of catalysts (catalytic reactions), and other new technologies
that are used to produce chemicals.[48]
Benzimidazole and its derivatives are organic compounds that have been prepared using
green techniques instead of the traditional industrial or chemical methods. Microwave
technology (MW) was used to prepare some benzimidazole derivatives by condensing aldehydes
with o-phenylenediamine in the presence of specific amounts of acetic acid (Scheme [21]). Advantages of this method are that the reaction times are short, they use non-toxic
solvents, and give high yields of the products.[49]
Scheme 21 Synthesis of benzimidazole using a microwave-assisted method
Some benzimidazole derivatives have been prepared with various transition-metal nanocatalysts
by condensing aldehydes with o-phenylenediamine (Scheme [22]). The approach gave good yields of 76–93% when new benzimidazole derivatives were
prepared by Inamdar et al. in the presence of nanomaterials (CuOnp-SiO2 10%) as a catalyst. The yield reached 88–97% when the new benzimidazole derivatives
were prepared in the presence of the nanocatalyst ((Cu(II)-TD@nSiO2)) when it was reported by Nasr-Esfahani et al.[50]
Scheme 22 Use of a copper-based nanocatalyst for the synthesis of benzimidazole derivatives
Benzimidazole derivatives were also prepared from the reaction between formic acid
and substituted o-phenylenediamine with zinc oxide nanoparticles as a catalyst at 70 °C (Scheme [23]). The advantages of this method are the use of solvent-free conditions, excellent
yields, the use of an inexpensive catalyst, as well as the moderate conditions under
which the reaction takes place.[51]
Scheme 23 Synthesis of benzimidazole from o-phenylenediamine derivatives and formic acid using ZnO NPs as catalyst
Recently, Bharathi et al. reported that new benzimidazole derivatives were prepared
with Cu-phen-MCM-41 and Cu-complex-bipy-MCM-41 as catalysts under ultrasonic irradiation
for 90 minutes at 60 °C. The process was carried out in the presence of small amounts
of a mixture of ethanol/methanol as solvent (Scheme [24]). After completing the preparation of purified benzimidazole derivatives, the catalyst
was re-used three times without any significant loss in its catalytic activity. Advantages
of this method include the use of environmentally friendly techniques, the catalyst
can be reused, and high yields of up to 95% can be achieved.[52]
Scheme 24 Synthesis of benzimidazole derivatives using Cu-complex-(phen or bipy)-MCM-41 as
catalysts with ultrasonic irradiation
Sapkal et al. developed a highly efficient green protocol for the preparation of substituted
1H-benzimidazole derivatives. The reaction between phenylenediamine with aldehydes was
carried out using water as solvent under the influence of ultrasound, and nano-BiOCl
was used as a catalyst. It was found that this nanocatalyst could be used seven more
times in the activation process without significant loss in activity. The important
advantages of this approach, which depends on the use of ultrasound, are the low temperature
required to complete the reaction (35 °C), the reduced time required to complete the
process, the possibility of reusing the catalyst, and the use of water as a solvent,
which is a natural friend of the environment. The productivity reached 94% in some
cases (Scheme [25]).[53]
Scheme 25 Synthesis of 2-substituted benzimidazole using BiOCl NPs as catalysts with ultrasound
In 2014, Kumar et al. were able to synthesize 2-aryl-1-arylmethyl-1H-benzimidazole derivatives from aromatic aldehydes with o-phenylenediamine by sonication in the presence of silica-gel-supported trichloroacetic
acid. The process was carried out at 50 °C and gave high yields. This reaction is
considered to have been achieved by green technologies, because it represents an excellent
modified method that proceeds in a short time, does not cause pollution to the environment,
and uses an inexpensive catalyst (Scheme [26]).[54]
Scheme 26 Synthesis of 1,2-disubstituted benzimidazole derivatives from aromatic aldehydes
with o-phenylenediamine under sonication
Skolia et al. in 2021 reported a new approach to the synthesis of benzimidazole derivatives
using a photochemical protocol. In this process, CFL lamps were used as a light source
with 2,2-dimethoxy-2-phenylacetophenone as a photoinitiator. The method relied on
mixing substituted o-phenylenediamine with aldehydes at room temperature, which led to the cyclization
of diamine with aldehydes and the synthesis of benzimidazole derivatives in good to
high yields. This method is considered a green approach because it does not produce
compounds that pollute the environment, is performed at room temperature in a short
time in the presence of visible light, and is economical (Scheme [27]).[55]
Scheme 27 Synthesis of benzimidazole derivatives from o-phenylenediamine and aldehydes under visible light
Finally, we note the focus of researchers over the past years on the use of different
methods to synthesize substituted benzimidazole derivatives with different groups
as presented above. This reflects the importance of these compounds and their significant
role in various aspects of life, especially in biological applications, as will be
discussed in the next section.
4
Biological Activity of Benzimidazole Derivatives
Benzimidazole derivatives have been associated since their first synthesis to this
day with biological applications, due to the high efficacy of this class of heterocyclic
compounds in medicine, which has been effectively proven over the past years. Benzimidazole
derivatives have been used in many applications, including as antitumors, antimicrobials,
anti-inflammatories, and anticonvulsants.[56]
[57]
[58]
[59]
[60]
[61]
[62]
[63] The following are the most important biological applications that use benzimidazole
derivatives as basic compounds.
4.1
Anticancer Activity
The second leading cause of death worldwide is cancer. Therefore, researchers have
recently been directed towards preparing compounds that have high efficacy as anticancer
drugs. Effective anticancer drugs must have a high selectivity, so that they are toxic
to cancer cells and do not affect normal cells. Previously prepared anticancer drugs
showed a high toxic activity against cancer cells, but they also had a significant
effect on normal cells. Because of these side effects and the toxicity of anticancer
drugs, there is a strong tendency to halt chemotherapy for cancer patients. Among
all this, heterocyclic compounds have been repeatedly discovered as anticancer drugs
that have high selectivity and low side effects. These advantages in anticancer drugs
have presented a critical challenge for researchers and an urgent need for humanity.
The benzimidazole derivatives have been shown to be highly effective in this regard.[64]
Wanga et al.[65] studied the preparation of chrysin benzimidazole derivatives and their anticancer
activity. The strongest antiproliferative activity of MFC tumor cells appeared in
compound 1, which gave IC50 values of 25.72 ± 3.95 μM (Figure [2]). The results showed that it is the dosing regimen that increases the death of MFC
cells. These compounds have also been studied for their anticancer activities in mice,
for which compound 1 also inhibited tumor growth.
Figure 2 Antiproliferative agent 1
Morais et al.[66] also prepared a series of compounds and studied their anticancer activity. The prepared
compounds consisted of benzimidazole derivatives containing hydroxylated or fluorinated
alkyl substituents. Compound 2 showed a distinct anticancer activity (Figure [3]).
Figure 3 Anticancer agent 2
Onnis et al.[67] studied the synthesis of a series of benzimidazole derivatives and then tested their
antiproliferative activity. The growth of all tested cell lines was were inhibited
by hydrazone derivatives 3a and 3b (Figure [4]). Table [1] shows the IC50 values and the cell lines used. The researchers also noted that a
new type of derivative showed a clear antiproliferative activity when the pharmaceutical
drug hydrazone was combined with benzimidazole derivatives.
Figure 4 Antiproliferative derivatives 3a and 3b
Cevik et al.[68] tested the anticancer activity of the prepared benzimidazole derivatives. The results
showed that toxic activity against diseased cells was shown by compounds 4a and 4b (Figure [5]). The standard anticancer drug cisplatin was also compared with the derivative 4c, which showed significant efficacy compared to the standard compound. Table [2] summarizes the IC50 values with the cell line used.
Table 1 Growth Inhibition by 3a and 3b Against Tested Cell Line
|
Cell line used
|
IC50 (μM)
|
|
3a
|
3b
|
|
Murine leukemia (L1210)
|
1.6±0.9
|
2.9±1.3
|
|
Human T-lymphoblastic leukemia (GEM)
|
0.98±0.02
|
1.0±0.01
|
|
Human cervix carcinoma (Hela)
|
4.0±0.4
|
2.5±1.4
|
|
Human pancreas carcinoma (Mia paca-2)
|
6.3±3.2
|
7.9±0.3
|
Figure 5 Cytotoxic compounds 4a–c
Figure 6 Potential anti-AML compounds 5a–p
Shaldam et al.[69] developed new compounds 5a–p, derived from pyrimido[1,2-a]benzimidazole, with potential anti-AML properties (Figure [6]). The compounds were tested for their ability to inhibit tumor growth in vitro,
and compound 5h showed effective activity against various human cancer cell lines, particularly in
leukemia. Another set of compounds (5e–l) was tested specifically on human acute leukemia cell lines, and 5e–h demonstrated promising activity. The compounds were initially screened against leukemia-associated
mutant FLT3-ITD, ABL, CDK2, and GSK3 kinases, but they showed no significant activity.
Subsequently, a broader kinase profiling revealed that compounds 5e and 5h effectively inhibited BMX kinase. The researchers also investigated the impact of
the compounds on cell cycle, and caspase 3/7 activity in HL60 and MV4-11 cells.
Table 2 Cytotoxic Activity of Compounds 4a–c Against Tested Cell Line
|
Cell line used
|
IC50 (μM)
|
Cisplatin
|
|
4a
|
4b
|
4b
|
|
|
Human lung adenocarcinoma (A549)
|
0.0316
|
0.0316
|
0.06
|
0.045
|
|
Human breast adenocarcinoma (MCF-7)
|
0.0316
|
0.0316
|
0.03
|
0.052
|
Othman et al.[70] developed two new series of benzimidazole-triazole hybrids 6a–h and 7a–g as multi-target inhibitors of EGFR, VEGFR-2, and Topo II (Figure [7]). The compounds were tested for their anticancer activity and compounds 5a and 6g showed the highest potency against four cancer cell lines: HepG-2, HCT-116, MCF-7,
and HeLa. These compounds were further evaluated for their inhibitory effects on EGFR,
VEGFR-2, and Topo II. Compound 5a was a particularly good inhibitor of EGFR, a moderate inhibitor for VEGFR-2, and
a stronger inhibitor for Topo II compared to reference drugs. Compound 6g exhibited moderate inhibition of EGFR and VEGFR-2, and weaker inhibition of Topo
II.
Figure 7 Benzimidazole-triazole hybrids 6a–h and 7a–g as multi-target inhibitors of EGFR, VEGFR-2, and Topo II
Gali et al.[71] developed a series of new compounds called 3-(1H-benzo[d]imidazol-2-yl)-3,4-dihydro-2H-benzo[e][1,3]oxazines. The compounds were synthesized using a two-step process and their
structures were confirmed with a range of analytical techniques (Figure [8]). The anticancer activity of compounds 8a–k was evaluated against two breast cancer cell lines (MCF 7 and MDA-MB-231), compared
to the standard drug Doxorubicin. Compound 4e showed the highest activity against both cell lines, with IC50 values of 8.60±0.75
μM and 6.30±0.54 μM, respectively. Compound 8i also displayed good activity with an IC50 value of 9.85±0.69 μM against MCF-7 cells.
Compound 8g exhibited the best activity with an IC50 value of 8.52±0.62 μM against the MDA-MB-231
cell line. Docking studies against the target EGFR revealed that all compounds exhibited
strong binding affinities. The compounds demonstrated favorable drug-like properties,
indicating their potential as therapeutic agents.
Figure 8 Potential anticancer compounds 8a–k
4.2
Anti-inflammatory Activity
Anti-inflammatories are a term used for substances that have properties that reduce
the effect of various inflammations in the body. Many analgesic drugs have multiple
properties, especially anti-inflammatory drugs; these analgesic drugs reduce inflammation
and thus stop the pain. Benzimidazole derivatives are organic compounds possessing
a heterogeneous aromatic ring combined with a benzene ring, and this gives them a
distinct structure in medicinal chemistry that has recently emerged as the preferred
pharmaceutical material from which to design analgesic and anti-inflammatory agents
that act on different clinically approved targets for pain and inflammation.[72]
Gaba et al. investigated the ability of some substituted benzimidazole derivatives
such as 9 to inhibit the selective COX-2 enzyme in the kidneys and stomach using celecoxib
as a standard drug (Figure [9]). Most of the prepared derivatives showed a characteristic activity equivalent to
that of the standard drug.[73]
Figure 9 Anti-inflammatory compound 9
Sharma et al.[74] tested a series of prepared compounds in a sample of carrageenan-induced rat paw
edema. The substances derived from 5-methanesulphonamido benzimidazole were tested
as anti-inflammatory compounds. Rofecoxib and Indomethacin were also used as standard
drugs for testing. Derivatives 10a, 10b, and 10c showed the maximum activities (92.73, 95.64, and 97.62%, respectively) in edema at
the tested doses (Figure [10]). This indicates the active role played by benzimidazole derivatives in this field.
Figure 10 Potential anti-inflammatory compounds 10a–c
Saha et al.[75] tested a number of substituted benzimidazole derivatives as anti-inflammatory agents
by inhibiting carrageenan-induced rat claw edema (Figure [11]). Benzimidazole derivatives 11b, 11c, and 11d showed prominent anti-inflammatory activity at a dose of 100 mg/kg body weight (%
inhibition of claw edema 81.75, 79.09, and 86.69%, respectively), which is comparable
to standard aceclofenac (87.83%).
Figure 11 anti-inflammatory agents 11b–d
Figure 12 Compound 12 as an anti-inflammatory and analgesic compound
Kankala et al.[76] prepared a series of substituted benzimidazole derivatives and tested their efficacy
as analgesic and anti-inflammatory. The notable prepared compound 12 consisted of isoxazole-mercapto-benzimidazole derivatives (Figure [12]). The researchers found that substituted derivatives with electron-donating groups
had higher activity than those with electron-donating groups, and it was found that
these derivatives substituted with electron-withdrawing groups were more effective
than diclofenac for anti-inflammatory activity and pentazocine for analgesic, which
were used as standard drugs.
Moharana et al.[77] developed new benzimidazole compounds with anti-inflammatory properties. The compounds
13, 14, and 15 were synthesized, purified, and characterized (Figure [13]). The compounds were found to be non-toxic both in vitro (at a concentration of
100 μM for 24 hours on 3000 Vero cells/well) and in vivo (at a dose of 100 mg/kg in
female Wistar rats, with a 48 h observation period for mortality). Compounds 13 and 14 exhibited significant anti-inflammatory properties. In silico analysis indicated
that both compounds are drug-like and have good oral bioavailability. These findings
suggest that further research should be conducted to explore the potential of these
benzimidazoles as anti-inflammatory agents for various diseases.
Figure 13 Anti-inflammatory compounds 13–15
Nagesh et al.[78] focused on developing safer non-steroidal anti-inflammatory drugs (NSAIDs) for inflammation
therapy. They aimed to achieve dual inhibition of COX/5-LOX, which improves efficacy
and reduces side effects. To this end, they designed, synthesized, and characterized
a series of compounds 16a–j for their anti-inflammatory, analgesic, and ulcerogenic properties (Figure [14]). The investigation revealed that these compounds exhibited significant anti-inflammatory
activity. The compounds showing potential analgesic activity were further evaluated
for their analgesic, anti-inflammatory, and ulcerogenic effects. In vitro analyses
were conducted for COX-1, COX-2, and 5-LOX. Compound 16c, which had a para-fluoro substitution on the benzoyl ring and two ortho-chloro groups on the phenyl ring of benzophenone, demonstrated good inhibitory potency.
Additionally, in silico docking studies were performed using AutoDock tools docking
software.
Figure 14 Potential non-steroidal anti-inflammatory drugs 16a–f
There are also many studies showing the use of benzimidazole derivatives as anti-inflammatories,
and most of them were shown to have great efficacy equal to or exceeding the effectiveness
of the drugs used as standard substances.[79]
[80]
[81]
[82]
[83]
[84]
[85]
4.3
Antioxidant Activity
An antioxidant is a molecule capable of slowing down or preventing the oxidation of
free radicals that cause a destructive danger to the cells of an organism’s body.
The formation of free radicals in air-breathing organisms is inevitable, because of
the dependence on the aerobic oxidation process, which is necessary for life. These
emerging free radicals are highly toxic and are likely to cause cell malfunction,
malignant transformation, or death. Oxidative stress can cause damage to proteins,
lipids, and DNA, and the latter may lead to a change in the arrangement of chromosomes
or the occurrence of genetic mutations that lead to cancer tumors, diabetes, neurodegeneration,
and cardiovascular diseases.[86] Therefore, it has become necessary to remove these free radicals from the body before
exposing the body to diseases, and this is done by taking antioxidants. Antioxidants
may be present in fruit in the form of vitamins, such as vitamins A, C, and E, or
antioxidants may be synthesized from some organic compounds.[87] The benzimidazole derivatives have a notable record in this field because they consist
of two aromatic rings (benzene and imidazole), so the free radical enters the state
of resonance with the electrons of the aromatic rings, and its effect decreases or
ends.[88]
[89]
Archie et al.[90] evaluated the antioxidant activity of some prepared benzimidazole derivatives 17 (Figure [15]). All the derivatives showed good antioxidant activity, with IC50 values in the
region 3.17 to 7.59 μg/mL, while 18.42 μg/mL was found for butylated hydroxytoluene
(BHT). It was also found that the best compound used as an antioxidant in this study
was compound 17c (IC50 = 3.17 μg/mL).
Figure 15 Anitoxidant compounds 17a–c
Abd et al.[91] synthesized substituted benzimidazoles by including chalcone, oxirane, pyrimidines,
oxazoline, and pyrazolines derivatives. The DPPH method was used to evaluate the antioxidant
activity of these prepared compounds. The researchers from this study found that compounds
18 and 19 showed antioxidant properties comparable to that of ascorbic acid when using the
DPPH method at a concentration of 100 μM (Figure [16]). Moreover, the results of the study showed that the pyrazoline ring system, as
in compound 18, is not responsible for this inhibitory activity; rather, the presence of the pyrimidine
thione in compound 19 and the epoxide ring in compound 20 are responsible for this activity.
Figure 16 Antioxidant compounds 18–20
Bellam et al.[92] reported the preparation of some N-substituted pyrazole-containing benzimidazole
derivatives (Figure [17]). The research team studied the effectiveness of all these prepared compounds as
antioxidants to evaluate their ability to scavenge free radicals versus against DPPH
and H2O2. Compounds 21a, 21b, and 21c, which all contained a benzyl group as a substitute for the benzimidazole nitrogen,
exhibited distinct activity in both methods.
Patagar et al.[93] synthesized a series of benzimidazole-coumarin-3-carboxamide analogues 22a–j and evaluated their antioxidant potential using DPPH and ABTS assays (Figure [18]). The authors also assessed the α-amylase inhibition activity of the compounds using
the DNSA method. Compound 22j, with a picoline substitution on the carboxamide linker, demonstrated the highest
potency for DPPH radical scavenging, with an IC50 value of 89.57 μM, compared to the
standard ascorbic acid (IC50 value of 92.67 μM). Compound 22f, with a 3,5-dimethoxyphenyl substituted carboxamide linker, exhibited the strongest
scavenging activity against ABTS radical, with an IC50 value of 93.45 μM, surpassing
the standard drug Trolox (IC50 value of 96.24 μM). However, all compounds in the series
exhibited moderate α-amylase inhibition, and compound 22f emerged as the most effective antidiabetic analogue, with an IC50 value of 67.52
μM, compared to the standard metformin (IC50 value of 54.13 μM). Molecular docking
studies with the α-amylase receptor protein (PDB ID 4x9y) using Autodock Vina software
provided further insights into the obtained results.
Figure 17 Antioxidant compounds 21a, 21b, and 21c
Figure 18 Antioxidants 22a–j
Küçükoğlu et al.[94] synthesized 11 new benzimidazole-1,3,4-oxadiazole derivatives 23a–k and evaluated their inhibitory activities against human carbonic anhydrase isoforms
(hCA I, hCA II) and antioxidant activity (Figure [19]). The compounds were characterized using 1H NMR, 13C NMR, and HRMS. Compound 23a showed the highest potency against hCA I, with an IC50 value of 1.322 μM, whereas
compound 23d exhibited an IC50 value of 1.989 μM. Among all the compounds, 23a, 23d, and 23g were the most active against hCA II. Compound 23a, containing a 4-bromophenyl structure, demonstrated efficacy against both hCA I and
hCA II, indicating its potential as a promising candidate for CA inhibition. Antioxidant
activity was evaluated using the TAS assay, and cytotoxic activity was assessed using
the MTT assay on the L929 cell line. Finally, molecular docking studies were conducted
to compare the biological activities of the most active compounds against hCA I and
hCA II enzymes.
Figure 19 Antioxidants 23a–k
There are also a large number of studies regarding the use of benzimidazole derivatives
as antioxidants because of their great role in capturing free radicals formed in the
body. This is because they contain electrons of the π-bonds present in the aromatic
rings, which are always in a state of delocalization, and the entry of free radicals
in this delocalization state reduces or halts the harmfull effects of the radicals.[82]
,
[94]
[95]
[96]
[97]
[98]
[99]
[100]
4.4
Anticonvulsant Activity
Anticonvulsants are a diverse group of drugs that are currently used to treat patients
with recurrent epileptic seizures. Many anticonvulsants are also used for the treatment
of neuropathic pain and as mood stabilizers. Anticonvulsants also prevent the spread
of the seizure within the brain.[101]
[102]
[103] Benzimidazole derivatives have been examined in a number of studies as anticonvulsant
substances, and these studies have established their success in use.
Shingalapur et al.[104] investigated the anticonvulsant activity of some benzimidazole derivatives that
consist of 2-mercapto benzimidazole containing 4-thiazolidinones 24a–j (Figure [20]). The prepared derivatives were studied with respect to their efficacy as in vivo
anticonvulsants using the Maximal Electroshock (MES) model. Compounds 24c, 24d, 24g, and 24i showed strong anticonvulsant results. Pharmacophores derived from active molecules
suggested that the presence of an OH group was a common feature in all active compounds.
Figure 20 Anticonvulsants 24a–j
Figure 21 Anticonvulsants 25a–j
Shaharyar et al.[105] examined the anticonvulsant activities of the benzimidazole derivatives prepared
in their study. In this study, the researchers synthesized a number of compounds consisting
of 2-[2-(phenoxymethyl)-1H-benzimidazol-1-yl]acetohydrazide derivatives (Figure [21]). The results obtained from the study of anticonvulsants were compared with the
standard drugs phenytoin and ethosuximide. The study proved that compounds 25g and 25j gave more effective anticonvulsant activity than the other prepared compounds as
well as the standard drugs used. The protection index of these compounds was also
studied, and it was found that the two compounds 25g and 25j possess indices of 40.5 and 24.7, respectively, which is much higher than this of
the standard drugs. The neurotoxicity values TD50 for compounds 25g and 25j, which are very high, indicate that these compounds have much lower neurotoxicity
than standard drugs. From this study it was found that the effect of substitution
plays a key role in the biological activity of the compound. Compounds substituted
with electron-withdrawing groups (such as nitro and fluorine) had higher biological
activity than others. Compounds with strong electron-withdrawing groups were more
effective than those with weak electron-withdrawing groups. On the other hand, it
was found that the substitution of electron-donating groups on the basic structure
has no effect on the biological activities studied.
Siddiqui et al.[106] biologically evaluated the anticonvulsant activity of new benzimidazole derivatives
26a–p, consisting of 2-[(1-(2-substituted-benzyl)-1H-benzo[d]imidazole-2-yl)methyl]-N-substituted-phenylhydrazinecarbo-thioamides derivatives (Figure [22]). For albino rats, the two most well-known models are maximal electroshock seizure
(MES)- and subcutaneous pentylenetetrazole (scPTZ)-induced seizures. Compound 26p is the most biologically active and promising anticonvulsant in MES and scPTZ screens.
Moreover, many other studies during the past years have focused on studying the medicinal
activity of benzimidazole derivatives as anticonvulsants, which gave distinctive results
that, in some of these compounds, were superior to standard drugs. This indicates
the great efficacy of benzimidazole derivatives, which could for the basis of future
drugs in the field of anticonvulsants.[107]
[108]
[109]
[110]
[111]
[112]
[113]
Figure 22 Anticonvulsants 26a–p
4.5
Anticoagulant Activity
Anticoagulants, also known as blood thinners, are potent chemicals designed to reduce
or prevent blood clots in the body.[114] Anticoagulants are naturally present in some animals that feed on blood, such as
mosquitoes and leeches, which help prevent the formation of blood clots in the bite
area for a period of time in order to absorb blood.[115]
[116] Anticoagulants are used in many cases, including stroke, heart attack, atrial fibrillation,
deep venous thrombosis, pulmonary embolism, and others. Some anticoagulants are also
used in medical equipment, such as blood transfusion bags, heart-lung machines, dialysis
machines, and sample tubes.[117–119] Benzimidazole derivatives have been prepared and tested for their efficacy as anticoagulants.
Yang et al.[120] synthesized a series of substituted benzimidazole derivatives and evaluated their
potential as anticoagulants (Figure [23]). In this study, the researchers investigated the synthesis of new 1,2,5-trisubstituted
benzimidazole fluorinated derivatives, and these compounds were used initially to
verify their inhibitory activity against thrombin. Compounds 27a, 27b, and 27c, with IC50 values of 2.26, 1.54 and 3.35 nmol/L, respectively, showed higher anticoagulant
activity than standard argatroban used as a reference, for which the IC50 value was
9.88 nmol/L. It was also observed from this study that a methyl group substituent
at the ortho-position of the benzene ring on the main structure give useful results as anticoagulants.
Figure 23 Anticoagulants 27a, 27b, and 27c
Wang and Ren[121] documented the preparation of benzimidazole derivatives consisting of fluorinated
derivatives of substituted 1-ethyl-1H-benzimidazole, then biologically evaluated these compounds to study their antithrombin
activity. The researchers in this study discovered that all the tested compounds 28a–f had better anticoagulant activity against thrombin than argatroban, which was used
as a standard drug (Figure [24]). Moreover, compound 28f had a higher potency as a thrombin inhibitor IC50 = 3.21±0.57 nM than the standard
drug IC50 = 9.88±2.26 nM.
Figure 24 Antithrombin compounds 28a–f
Several other studies during the past years have documented the synthesis of a large
number of new substituted benzimidazole derivatives and their biological evaluation
for their use as anticoagulants. The results obtained from these studies were very
positive and could be applied for the development of thrombin inhibitors in the future.[122]
[123]
[124]
[125]
[126]
[127]
4.6
Antimicrobial Activity
Antimicrobial materials are a class of substances that are used to kill microorganisms
(fungi and harmful bacteria) or stop their growth. Antimicrobial drugs can be divided
according to the microorganisms that they can work against. For example, antibiotics
are used against bacteria, and antifungals are used against fungi. The class can also
be subdivided according to function. Compounds that kill microbes are called microbicides,
while compounds that stop microbes from growing are called biostatic.[128]
[129] Benzimidazoles inhibit protein synthesis in microbes because the structure of benzimidazole
mimics the structure of purine. Commonly, 2-substituted benzimidazoles show stronger
pharmacological activity.[72,130]
El-Gohary et al.[131] tested the antimicrobial activity for new synthesized benzimidazole derivatives
(Figure [25]). These new derivatives were assessed for antimicrobial efficacy toward Staphylococcus aureus (S. aureus), Bacillus cereus (B. cereus), Escherichia coli (E. coli), Candida albicans (C. albicans), and Aspergillus fumigatus 293 (A. fumigatus 293). Among the studded compounds, 29 and 30 displayed good activity against S. aureus, with a minimum inhibitory concentration (MIC) value 0.524 μg/mL and 0.684 μg/mL,
respectively, whereas compound 31 showed good activity toward B. cereus, with an MIC value of 0.489 μμg/mL. In addition, compound 30 exhibited the good results toward A. fumigatus 293, with an MIC value of 1.37 μg/mL.
Figure 25 Antimicrobial compounds 29–31
Ajani et al.[132] reported the antimicrobial activity for some 2-substituted benzimidazoles (Figure
[26]). The activity of these compounds was biologically evaluated with gram-positive
bacteria (P. vulgaris, S. faecalis, and S. aureus) and gram-negative bacterial strains (Pseudomonas aeruginosa, E. coli and K. pneumoniae) by using the zone of inhibition system. Of the studied compounds, 32–35 presented higher zones of inhibition toward all the above organisms as compared with
the gentamicin as a standard drug. Specifically, compound 35 provided the highest inhibition area against Klebsiella pneumonia, which was 42±0.10 mm.
Figure 26 Antimicrobial compounds 32–35
Ersan et al.[133] synthesized new 2-phenyl substituted benzimidazole derivatives and evaluated their
biological activity as anticancer and antimicrobial agents (Figure [27]). In the antimicrobial study, all compounds were tested to confirm their antimicrobial
properties against gram-positive bacteria, gram-negative bacteria and fungi. All compounds
presented moderate activity against all tested bacteria and fungi. However, some phenoxy
methyl derivatives 5-chloro-2-((4-chlorophenoxy) methyl)-1H-benzo[d]imidazole (36) and 5,6-dichloro-2-((4-chlorophenoxy) methyl)-1H-benzo[d]imidazole (37) showed the highest activity against Candida, which gave the same MIC value (< 3.90 μM) as the reference compound uconazole possess
against Candida albicans and Candida parapsilosis. In general, the benzimidazole derivatives that showed the highest biological effect
on bacteria and fungi were those containing Cl-substitutes.
Figure 27 Antimicrobial compounds 36 and 37
Shinde et al.[134] reported new 2-(3-arylureido)benzimidazole derivatives via sequential oxidative
cyclization of 4-nitrobenzaldehyde and 2-amino aniline (o-phenelynediamine), then evaluated their antifungal and antibacterial activity at
10 μM concentration (Figure [28]). The compounds 38b, 38f, and 38g showed effective antifungal and antibacterial biological activity. The compounds
38f and 38g provided effective activity against most of the microbes used in the test, 1.5–2.5
times higher than the control drugs Micanazole and Fluconazole.
Figure 28 Antimicrobial compounds 38a–i
Jasim et al.[135] conducted a study on a series of benzimidazole compounds 39–46 containing a fluoro-benzene moiety (Figure [29]). The compounds were tested for their antimicrobial activity against gram-positive
bacteria, gram-negative bacteria, and fungal strains. The results indicated that the
fluoro-substituted compounds 40–42 and 44–46 exhibited better antibacterial and antifungal properties compared to the unsubstituted
parent compounds 39 and 43. Compound 45, which contained a fluorine atom in the meta-position of the phenyl ring side chain, showed high activity against gram-negative
bacteria, with a MIC value of 31.25 μg/mL. Additionally, derivatives 41 and 45, with a 2-(m-fluorophenyl)benzimidazole structure, demonstrated good activity against B. subtilis, with a MIC value of 7.81 μg/mL. Structure-activity relationship (SAR) analysis suggested
that the presence of a methyl substitution group at the 5-position of benzimidazole
is beneficial for significant antifungal activity against C. parapsilosis. The strong potency of compound 45 suggests its potential as a starting point for the development of novel antimicrobial
agents.
Figure 29 Antimicrobial compounds 39–46
Rajagopal et al.[136] synthesized bis-benzimidazole derivatives 47a–h using simple and environmentally friendly reaction conditions and crystallization
procedures (Figure [30]). The structures of the compounds were determined using 1H NMR, 13C NMR, and mass spectrometry. Compound 47c was crystalline and its structure was confirmed through single-crystal X-ray analysis.
The synthesized bis-benzimidazole compounds were tested for antimicrobial activity
against gram-positive bacteria (Bacillus subtilis, Staphylococcus aureus), gram-negative bacteria (Escherichia coli, Serratia sp.), and the fungal pathogen Candida albicans. Compound 47e exhibited the strongest activity, with a minimum inhibitory concentration of 16 μg/mL
against C. albicans. The purified compound showed no hemolysis except at a concentration of 100 μg/mL.
Molecular docking of compound 47g was performed with fungal protein from C. albicans (dihydrofolate reductase PDB ID: 1IA4), and pose 1 exhibited the best binding energy
of –6.23 kJ mol–1.
Myeloperoxidase (MPO) is an enzyme involved in the human antimicrobial system, but
its release outside phagocytes can cause tissue damage. It has been implicated in
various inflammatory diseases. Saylam et al. designed isomeric derivatives of 1,3-dihydro-2H-benzo[d]imidazole-2-thione with amide compound 48C1–17
, hydrazide compound 48C19
, and hydroxamic acid groups compound 48C20
on nitrogen or sulfur atoms (Figure [31]). The inhibitory activity of these derivatives on the chlorination and peroxidation
cycles of MPO was assessed. Compound 48C19
, specifically 2-(2-thioxo- 2,3-dihydro-1H-benzo[d]imidazole-1-yl)acetohydrazide, showed the highest inhibitory activity on both cycles.[137]
Figure 30 Antimicrobial compounds 47a–h
Figure 31 Inhibitors of the chlorination and peroxidation cycles of MPO
At the present time, there are very many studies on substituted benzimidazole derivatives,
which show a distinctive activity to kill microbes or stop their growth. Many of the
prepared derivatives have efficacy comparable or superior to the standard drugs used
for the same purpose.[138]
[139]
[140]
[141]
[142]
[143]
[144]
[145]
[146]
[147]
[148]
[149]
[150]
[151]
[152]
[153]
4.7
Other Biological Applications
In addition to the above medical applications, benzimidazole derivatives have been
widely used as an essential part of biological applications including: antihypertensive
activity,[154]
[155]
[156]
[157]
[158] antiprotozoal activity,[159–163] antiviral activity,[164–167] anthelmintic activity,[168]
[169]
[170]
[171]
[172] antidiabetic agents,[173]
[174]
[175]
[176]
[177] analgesic activity,[78]
,
[178]
[179]
[180]
[181] antispasmodic activity,[182]
[183] antidepressant activity,[184]
[185]
[186]
[187]
[188]
[189] sphingosine kinase 1 inhibitor,[190]
[191] anti-Alzheimer’s agents,[192]
[193]
[194] angiotensin II-AT1 receptor antagonists,[195]
[196]
[197]
[198]
[199]
[200] antiulcer activity,[201]
[202]
[203]
[204] anti-tubercular activity,[205]
[206]
[207]
[208]
[209]
[210]
[211] anti-HIV agents,[212]
[213]
[214]
[215]
[216]
[217]
[218] and anti-secretory activity.[219]
[220]
[221]
In the end, we can conclude that benzimidazole derivatives are considered one of the
most important classes of synthetic heterocyclic compounds, which have shown great
biological activity over the past years. The pharmacological efficacy of these compounds
may be comparable to those of the standard drugs at the present time, as was shown
above when their efficacy was compared with the drugs used for most studies. This
is clear evidence of the high efficacy of benzimidazoles in these applications. The
large number of studies directed previously and currently towards the development
of benzimidazole derivatives as antibiotics for various diseases also indicates the
importance of this class of organic compounds. This is the main reason for writing
this review on the synthetic and biological chemistry of benzimidazoles to serve researchers
working in this field.
