β-Cyclodextrin-propyl sulfonic acid catalysed one-pot synthesis of 1,2,4,5-tetrasubstituted imidazoles as local anesthetic agents

Article abstract of DOI:10.3390/molecules201119696

Some functionalized 1,2,4,5-tetrasubstituted imidazole derivatives were synthesized using a one-pot, four component reaction involving 1,2-diketones, aryl aldehydes, ammonium acetate and substituted aromatic amines. the synthesis has been efficiently carried out in a solvent free medium using β-cyclodextrin-propyl sulfonic acid as a catalyst to afford the target compounds in excellent yields. the local anesthetic effect of these derivatives was assessed in comparison to lidocaine as a standard using a rabbit corneal and mouse tail anesthesia model. the three most potent promising compounds were subjected to a rat sciatic nerve block assay where they showed considerable local anesthetic activity, along with minimal toxicity. among the tested analogues, 4-(1-benzyl-4,5-diphenyl-1h-imidazol-2-yl)-n,n-dimethylaniline (5g) was identified as most potent analogue with minimal toxicity. it was further characterized by a more favourable therapeutic index than the standard.

Full text of DOI:10.3390/molecules201119696

Article
β–Cyclodextrin–Propyl Sulfonic Acid Catalysed
One-Pot Synthesis of 1,2,4,5-Tetrasubstituted
Imidazoles as Local Anesthetic Agents
Yan Ran 1, , Ming Li 2, and Zong-Ze Zhang 1,
*
Received: 5 September 2015 ; Accepted: 7 October 2015 ; Published: 12 November 2015
Academic Editors: Derek J. McPhee and Maria Emília de Sousa
1
Zhongnan Hospital of Wuhan University, Department of Anesthesiology, Wuhan University,
Wuhan 430071, China; ranyanqwe@sina.com
Department of Doppler Ultrasound, PLA421 Hospital, Guangzhou 510318, China; liming8782@sina.com
2
*
Correspondence: zhongzongze@hotmail.com; Tel./Fax: +86-27-67812988
These authors contributed equally to this work.
Abstract: Some functionalized 1,2,4,5-tetrasubstituted imidazole derivatives were synthesized using
a one-pot, four component reaction involving 1,2-diketones, aryl aldehydes, ammonium acetate
and substituted aromatic amines. The synthesis has been efficiently carried out in a solvent free
medium using β-cyclodextrin-propyl sulfonic acid as a catalyst to afford the target compounds
in excellent yields. The local anesthetic effect of these derivatives was assessed in comparison to
lidocaine as a standard using a rabbit corneal and mouse tail anesthesia model. The three most
potent promising compounds were subjected to a rat sciatic nerve block assay where they showed
considerable local anesthetic activity, along with minimal toxicity. Among the tested analogues,
4-(1-benzyl-4,5-diphenyl-1H-imidazol-2-yl)-N,N-dimethylaniline (5g) was identified as most potent
analogue with minimal toxicity. It was further characterized by a more favourable therapeutic index
than the standard.
Keywords: synthesis; imidazoles; local anesthetic agent
1. Introduction
Due to the ease of modification, diversity and effective synthetic options, heterocyclic
compounds were considered as the preferred choice for the discovery of novel medications [1]. Of
the many varieties available, compounds containing imidazole moieties have attracted considerable
attention from medicinal chemists due to their wide array of medicinal properties [2], as it presents
diverse pharmacological activities such as antimalarial [3], antibacterial [4], antihistaminic [5],
anti-inflammatory [6], analgesic [7], antitubercular [8], antiprotozoal [9] and anthelmintic effects [10].
On the other hand, it has also found a special place in green chemistry as an ionic liquid
component [11].
Therefore, extensive work has been carried on the development of efficient ways to prepare
imidazoles. Particularly, four component syntheses involving condensation of a 1,2-diketone,
a hydroxyketone or a ketomonoxime with an aldehyde, primary amine and ammonium acetate
using HY zeolite [12], silica gel/NaHSO4 [13], HClO4–SiO2 [14], molecular iodine [15], BF3–SiO2 [16],
InCl3 3H2O [17] or potassium dodecatugstocobaltatetrihydrate (K5CoW12O40 3H2O) [18] as catalysts
afforded 1,2,4,5-tetrasubstituted imidazoles in excellent yield. More recently, Ziarani et al., have
reported the synthesis of 1,2,4,5-tetrasubstituted imidazoles in excellent yield under solvent free
conditions using sulfonic acid functionalized silica (SiO2-Pr-SO3H) as a catalyst [19]. However, the
existing methods are not effective and are associated with several drawbacks, which include long
Molecules 2015, 20, 20286–20296; doi:10.3390/molecules201119696
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Molecules 2015, 20, 20286–20296
reaction times, harsh reaction conditions, low yields of the products, tedious work-up procedures,
and the use of toxic reagents.
Herein, we provide a green and economical one-pot synthesis of some 1,2,4,5-tetrasubstituted
imidazoles catalysed by β-cyclodextrin–propyl sulfonic acid (β-CD-PSA) as catalyst, as depicted in
Scheme 1. The products were also tested for local anesthetic activity using the rat tail flick and corneal
hyperflexion methods.
Scheme 1. One pot synthesis of 1,2,4,5-tetrasubstituted imidazoles catalysed by β-CD-PSA. Where
n = 0 means no spacer methylene fragment, i.e., aniline (5a–5d); n = 1 means having the methylene
spacer, i.e., benzylamine (5e–5j).
2. Results and Discussion
2.1. Chemistry
The synthesis of β-CD-PSA is shown in Scheme 2. Initially, the commercially available β-CD
was allowed to react with 1,3-propanesultone in NaOH solution to furnish sulfopropyl ether
β-cyclodextrin (SPE-β-CD). This was further treated with an acidic resin to afford β-CD-PSA. The
identification of β-CD-PSA was performed via spectroscopic methods and elemental analysis. The
1
average degree of substitution in β-CD-PSA was estimated from H-NMR spectroscopy, and it was
about 4.8. Moreover, the loading efficiency of propylsulfonic acid per g catalyst was also determined
by elemental analysis as 2.45 mmol/g.
Scheme 2. The synthesis of β-CD-PSA.
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Next, to optimise the reaction conditions, initially the condensation reaction was examined using
benzil (1 mmol), 4-chlorobenzaldehyde (1 mmol), ammonium acetate (3 mmol) and the catalyst
β-CD-PSA to afford compound 5a (Scheme 1). As evident from Table 1, in the absence of catalyst at
room temperature, the reaction does not proceed. Further, no change in reaction behaviour was noted
on increasing the temperature. However, on introduction of the catalyst, the reaction proceeds well
at room temperature to yield compound 5a. Significant improvement in product yield was reported
on increasing the temperature up to 100 C, keeping the catalytic load constant, i.e., at 50 C, 80 C,
and 100 C, the yield of product obtained is 55%, 85% and 96%, respectively. This result suggests the
role of catalyst and temperature for the completion of the reaction. In order to further confirm the
role of β-CD-PSA, the next set of reactions (entries 7 and 8) was carried out at higher and lower
catalyst concentration than 2 mol %. To our surprise, a significant decline in product yield was
observed in both the cases, along with longer time being required for completion of the reaction.
No further improvement in product yield was seen in water as well as in other solvents. It has thus
been corroborated that the maximum yield of the product was obtained under solvent free conditions
at 80 C with a catalytic load of 2 mol % of β-CD-PSA.
Table 1. Optimisation of reaction conditions for the synthesis of 5a a.
Yield (%) b
Entry
Solvents
Temp ( C)
Amount (mol %) Time (min)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
No Solvent
No Solvent
No Solvent
No Solvent
No Solvent
No Solvent
No Solvent
No Solvent
Water
CH3CH2OH
ClCH2CH2Cl
EtOAc
THF
CH3CN
rt.
80
rt.
50
80
100
80
80
–
–
2
2
2
2
1
3
2
2
2
2
2
2
2
150
150
150
40
30
20
70
50
30
40
No reaction
No reaction
28
55
85
96
74
92
63
78
45
70
56
64
72
80
reflux
reflux
reflux
reflux
100
100
40
40
40
40
DMF
40
a
Reaction conditions: benzyl (1 mmol), aromatic aldehyde (1 mmol), ammonium acetate (3 mmol), catalyst
β-CD-PSA, solvent free or with solvent (5 mL); b Isolated yield.
The next part of the study was aimed at comparing the efficiency of β-CD-PSA with other
reported catalysts for the synthesis of 1,2,4,5-tetrasubstituted imidazoles. The results are shown in
Table 2. It was found that the presence of catalyst together with high heat leads to the completion
of the reactions to afford the target derivatives in considerable yield, accompanied by variations in
the reaction time. Thus, it could be suggested that β-CD-PSA acts as a better catalyst in respect of
reaction times and yields of the products than other previously used catalysts.
After successful optimisation of the reaction conditions, the next part of the work was focused
on examining the versatility and limitations of this method using diverse substituents (Table 3). In
the majority of the cases the reaction proceeds smoothly, for instance, compound 5a was furnished
in the highest yield among all the derivatives. In the case of compound 5b, the time required to
complete the reaction was increased significantly, along with a minor reduction in yield. In general
the reaction time was further increased in the case of compounds bearing electron withdrawing
substituents i.e., entries 3 and 4. It is noteworthy to mention that, the yield of the product was
decreased significantly along with extended reaction times with the introduction of a methylene
bridge to connect phenyl at R2. Thus, it has been inferred that derivatives containing phenyl without
the spacer were well suited to this condensation reaction to afford 1,2,4,5-tetrasubstituted imidazoles
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with varying reaction times and yields. On further comparing the results of these derivatives with
the methods reported earlier (cf. Table 2), it has been found that a variety of catalysts were used for
the synthesis of the target compounds. The data presented in Table 2 suggests that, in the majority
of cases, the reaction was carried out at higher temeperatures together with use of polar solvents
such as ethanol, methanol, acetic acid, DMF and DMSO, leading to complex isolation and recovery
procedures. Additionally, these processes generate waste containing catalyst and solvent, which have
to be recovered, treated and disposed off. Moreover, the use of microwave irradiation prevents the
synthesis of these compounds in resource-poor laboratories because of its expensive instrumentation.
In this regard, the present methodology offers advantages over these other methods and furnisheds
the desired compounds using greener aspects of efficient conditions without extensive purification.
1
The structures of all newly compounds were ascertained on the basis of IR, H-NMR, 13C-NMR
analysis, mass and elemental analysis and correlated with established M.P. reported in previous
literatures as given in Table 3 (melting point).
Table 2.
The comparison of efficiency of various catalysts for the synthesis of 1,2,4,5-
tetrasubstituted imidazoles a.
Yield (in %) b
Entry
Catalyst
Solvent
Condition
Time
Ref.
1
2
3
4
5
6
7
8
10
11
12
13
14
15
16
17
18
19
20
Zeolite HY
Silica gel
Silica gel/NaHSO4
HClO4–SiO2
I2
K5CoW12O40 3H2O
BF3 SiO2
InCl3 3H2O
[(CH2)4SO3HMIM]
MCM-41
-
-
MW
MW
reflux
42–85
60–90
85–90
60–94
97–99
15–95
80–96
47–84
85–95
74–82
75–85
75–82
73–83
82–93
82–93
87–98
0
6 min
6 min
2.5 h
15–20 min
15–20 min
2.5 h
2 h
6–9 h
2–2.5 h
1.92–2.25 h
25–35 min
1.92–2.17 h
13–23 min
1.5–5 h
3–8 min
15–55 min
3 h
[12]
[12]
[13]
[14]
[15]
[8]
[16]
[17]
[20]
[21]
[21]
[21]
[21]
[22]
[22]
[23]
[20]
DCM
-
Ethanol
DCM
140
C
75 C
140
140
C
C
-
MeOH
-
-
AcOH
-
EtOH
r.t.
140
140
C
C
MCM-41
p-TsOH
p-TsOH
reflux
140
C
reflux
-
-
1-Butyl-3-methylimidazolium bromide
1-Butyl-3-methylimidazolium bromide
140
C
MW
P2O5/SiO2
-
SiO2-Pr-SO3H
β-CD-Pr-SO3H
-
-
-
-
100
140
140
100
C
C
C
C
85–98
68–96
10 min–3 h
15 min–4 h
[19]
(This work)
a
Reaction condition: benzil (1 mmol), aromatic aldehyde (1 mmol), ammonium acetate (3 mmol), catalyst
β-CD-PSA (0.02 mmol), 100 C. b Isolated yield.
Table 3. Synthesis of various 1,2,4,5-tetrasubstituted imidazoles catalysed by β-CD-PSA under solvent
free conditions a [20–24].
Yield (in %) b
Entry
R1
R2
Product Time
M.P. in C (Observed)
M.P. in C (in Literature)
1
2
3
4
5
6
4-Cl
3-NO2
4-OCH3
4-CH3
4-Cl
C6H5
C6H5
C6H5
C6H5
-CH2C6H5
-CH2C6H5
5a
5b
5c
5d
5e
5f
30 min
15 min
3 h
2.5 h
3 h
96
93
90
90
89
80
158–160
247–249
180–182
188–190
166–168
132–134
157–159
249–250
181–183
189–190
163–165
131–132
4-OH
3 h
7
-CH2C6H5
5g
3.5 h
73
154–155
155–157
8
9
10
4-CH3
3-OCH3
3,4-OCH3 -CH2C6H5
-CH2C6H5
-CH2C6H5
5h
5i
5j
3.5 h
4 h
4 h
75
70
68
160–162
115–117
189–191
163–165
128–129
188–190
a
b
without use of solvent, neat. Isolated yield.
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2.2. Reusability of the Catalyst
Based on the above results and following the reaction optimization, we decided to explore the
recyclability of β-CD-PSA taking the synthesis of 5a as a model reaction (Table 4). The catalyst was
easily recovered upon filtration of the reaction mixture after addition of water. The filtrates were
dried under vacuum and the resulting catalyst was reused directly for the next run. It has been found
that the catalyst can be efficiently recycled for four consecutive runs with slight loss of activity. It was
predicted that the catalyst recovery was efficient with a gradual decline in yield of product.
Table 4. Reusability of β-CD-PSA as catalyst for 5a.
No. of Runs
Percentage Yield
1
2
3
4
96
94
90
90
2.3. Pharmacology
Local Anesthetic Activity
The target compounds were subjected to evaluation of local anesthetic activity on rabbit corneal
and mouse tail anesthetic models which mimic surface and infiltration anesthesia. As shown in
Table 5, except for compounds 5g and 5i, none of the derivatives showed any significant activity
in both the models. Compound 5g found to be more active than lidocaine used as a standard. In
surface anesthesia, more than half the maximal response was observed in the case of compound 5h.
We can summarized that the presence of a benzylamine group has significant impact on the anesthetic
potential of the compounds, while its replacement by aniline makes the compounds inactive.
Table 5. Local anesthetic evaluation of the target compounds in the rabbit corneal and mouse tail
anesthetic models.
Mouse Tail Anesthesia b
Compound
Corneal Anesthesia a
2
5a
5b
5c
5d
5e
5f
5g
5h
20.3 8.6
12.4 9.3
15.5 12.3
10.32 0.12
21.65 4.7
5.5 3.5
121.23 12.3
65.65 5.8
95.4 13.2
20.3 10.3
100
11.3 ( 0.11) 10
5.7 ( 0.42) 10
6.5 ( 0.23) 10
4.3 ( 0.54) 10
3.4 ( 0.32) 10
5.4 ( 0.14) 10
1.6 ( 0.26) 10
9.2 ( 0.16) 10
2
2
2
2
2
2
2
2
5i
5j
7.4 ( 0.18) 10
4.1 ( 0.45) 10
2.1 ( 0.25) 10
2
Lidocaine HCl c
2
a
All compounds were in aqueous solution at 2% concentration. The values, expressed as percentage of the
b
anesthetic activity of lidocaine (=100), are means
SE of three determinations. IC50 values expressed as
mol/L. c Lidocaine hydrochloride was used as standard agent for comparison.
Owing to the excellent anesthetic activity, these compounds were further subjected to a rat sciatic
nerve block assay for the determination of the duration of anesthetic activity. The result is shown
Figure 1. It has been found that compound 5g exhibits a similar anesthetic profile to that of the
reference drug lidocaine.
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Molecules 2015, 20, 20286–20296
Figure 1. In vivo duration of local anesthetic activity (rat sciatic nerve block) of lidocaine and
compounds 5g, 5i and 5h. Each rat received 0.2 mL of the 2% anesthetic solution. The values are
means standard deviation of three independent experiments.
The compound 5g was further subjected for the determination of the acute toxicity and
therapeutic index in mice. As shown in Figure 2 (LD50) and Figure 3 (LD50/IC50), it is characterised
by the most favourable ratio between toxicity and anesthetic activity. Moreover, the therapeutic index
of the above compound was indicated as significantly higher than that of the lidocaine standard.
Figure 2. Acute toxicity of (ip) of lidocaine and compound 5g in male mice.
Figure 3. Therapeutic index of lidocaine and compound 5g evaluated as ratio between LD50 and IC50
(mouse tail test expressed in mg/kg).
3. Experimental Section
3.1. General Information
All chemicals were of analytical grade and used directly. Melting points were determined
in open capillary tubes with an Electrothermal melting point apparatus and are uncorrected. The
completion of reaction was checked by thin layer chromatography (TLC) using silica gel-G coated
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Molecules 2015, 20, 20286–20296
Al-plates (0.5 mm thickness; Merck, Darmstadt, Germany) and the plates were illuminated under
UV (254 nm) and evaluated in iodine vapour. FT-IR (in 2.0 cm 1, flat, smooth, abex, KBr) spectra
were recorded on a Spectrum RX-I spectrometer (Perkin Elmer Instruments, Buckinghamshire, UK).
Elemental analysis were determined on a Vario EL III CHNOS elemental analyzer (Elementar, Hanau,
Germany). 1H-NMR spectra were recorded on a Bruker Avance II 400 NMR Spectrometer and the
13C-NMR spectra on a Bruker Avance II 100 NMR spectrometer (Bruker Corporation, Billerica, MA,
USA) in D2O or DMSO-d6 using TMS as the internal standard. The chemical shifts are reported in
parts per million (ppm, d), and signals are described as s (singlet), d (doublet), t (triplet), q (quartet),
and m (multiplet). Mass spectra were obtained on a VG-AUTOSPEC spectrometer (VG Analytical,
Manchester, UK).
3.1.1. Synthesis of β-CD-PSA
Initially, the β-CD (1 g) was dissolved in NaOH solution (5 M, 10 mL) at 75 C. To the above
solution, 1,3-propane sultone (1.1 g) was added in small amounts and the mixture was stirred for
4 h. The reaction solution was then cooled to room temperature and adjusted to neutral using
HCl solution (3 M). The mixture was poured in ethanol to afford sulfopropyl ether β-cyclodextrin
(SPE-β-CD). In the next step, the acidic resin was activated in a saturated aqueous solution of NaCl
for 1 day. This was followed by treatment with 2.5 wt % NaOH aqueous solution for 80 min, and then
washing with distilled water until the pH reaches neutral. It was treated further with 5.0 wt % HCl
aqueous solution for 12 h and afterwards, the resin was transferred to a column and washed with
deionized water until the eluent become neutral.
Finally, the sodium salt of SPE-β-CD (0.5 g) was dissolved in water (100 mL), and the solution
was allowed to flow through the acidic resin column at a speed of 20 drops per minute. The acidic
eluent was collected and then freeze-dried for 12 h to obtain the β-CD-PSA product. FT-IR (ν
cm
;
max
1
KBr): 3354, 2930, 2365, 1653, 1361, 1153, 1030, 700. 1H-NMR (400 MHz, CDCl3) δ, ppm:
5.01–5.11 (m, C1–H), 3.49–3.81 (m, –OCH2CH2CH2SO3H, CH), 2.85 (s, –OCH2 CH2CH2SO3H), 2.28
(s, –OCH2CH2CH2SO3H); 13C-NMR (100 MHz, D2O): δ, ppm: 80.9, 72.7, 71.6, 69.4, 67.8, 53.2, 28.2,
27.8, 21.1, 19.9; Anal. Found: C 33.52, S 7.834, H 6.67.
3.1.2. General Procedure for the Synthesis of 1,2,4,5-Tetrasubstituted Imidazoles 5(a–j)
The catalyst β-CD-PSA (0.02 mol), an aromatic aldehyde (1 mmol), benzil (1 mmol), aniline or
benzylamine (1 mmol) and ammonium acetate (3 mmol) were placed in a flask and stirred at 100 C
under solvent free conditions for a suitable time (Table 3). The progress of the reaction was monitored
by TLC. After completion of the reaction, ethyl acetate (5 mL) was added to the reaction mixture,
which was filtered to remove the insoluble catalyst. The crystals of 1,2,4,5-tetrasubstituted imidazoles
appeared after gradual evaporation of solvent at room temperature. The solution of β-CD-PSA was
dried under vacuum for recycling the catalyst in the next run, and can be used several times without
much loss of catalytic activity.
1
2-(4-Chlorophenyl)-1,4,5-triphenyl-1H-imidazole (5a). MW: 406.91; Rf : 0.75; FT-IR (νmax; cm
KBr): 3038, 2979 (C-Hstretch), 1648 (C=Nstretch), 1215 (C-Nstretch), 891 (aromatic-Cl) cm 1; H-NMR
1
(400 MHz, CDCl3) δ, ppm: 8.09–8.04 (d, 2H J = 8.4 Hz, Ar-H), 7.79–7.76 (d, 2H J = 8.1 Hz, Ar-H),
7.58–7.41 (m, 15H, Ar-H); 13C-NMR (100 MHz, CDCl3) δ, ppm: 144.2, 141.3, 138.8, 138.2, 134.3, 133.1,
131.8, 129.7, 129.3, 129.2, 128.9, 128.7, 128.2, 127.5, 122.1; Mass spectrum: 407.89 (M + 1); Elemental
analysis for C27H19ClN2: Calculated: C, 79.70; H, 4.71; N, 6.88; Found: C, 79.68; H, 4.71; N, 6.89.
2-(3-Nitrophenyl)-1,4,5-triphenyl-1H-imidazole (5b). MW: 417.46; Rf : 0.63; FT-IR (νmax; cm 1 KBr): 3045,
1
2972 (C-Hstretch), 1651 (C=Nstretch), 1213 (C-Nstretch), 878 (aromatic-NO2) cm 1; H-NMR (400 MHz,
CDCl3) δ, ppm: 8.58–8.53 (d, 2H J = 8.2 Hz, Ar-H), 7.94–7.91 (d, 2H J = 7.6 Hz, Ar-H), 7.73–7.42 (m,
15H, Ar-H); 13C-NMR (100 MHz, CDCl3) δ, ppm: 148.5, 144.4, 138.7, 138.2, 133.7, 133.1, 131.7, 131.4,
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Molecules 2015, 20, 20286–20296
130.2, 129.6, 129.2, 128.7, 128.2, 127.5, 123.9, 122.8, 122.1; Mass spectrum: 418.48 (M + 1); Elemental
analysis for C27H19N3O2: Calculated: C, 77.68; H, 4.59; N, 10.07; Found: C, 77.69; H, 4.61; N, 10.08.
1
2-(4-Methoxyphenyl)-1,4,5-triphenyl-1H-imidazole (5c). MW: 402.49; Rf : 0.78; FT-IR (νmax; cm KBr):
1
3052, 2971 (C-Hstretch), 1653 (C=Nstretch), 1218 (C-Nstretch), 892 cm 1; H-NMR (400 MHz, CDCl3) δ,
ppm: 7.98–7.94 (d, 2H J = 8.3 Hz, Ar-H), 7.79–7.41 (m, 15H, Ar-H), 7.04–7.01 (d, 2H J = 6.3 Hz, Ar-H),
3.87 (s, 3H, OCH3); 13C-NMR (100 MHz, CDCl3) δ, ppm: 160.6, 144.4, 138.7, 138.2, 135.5, 133.1, 131.7,
130.4, 129.7, 129.2, 128.7, 128.1, 127.5, 122.2, 114.8, 55.9; Mass spectrum: 403.49 (M + 1); Elemental
analysis for C28H22N2O: Calculated: C, 83.56; H, 5.51; N, 6.96; Found: C, 83.58; H, 5.52; N, 6.97.
1
1,4,5-Triphenyl-2-(p-tolyl)-1H-imidazole (5d). MW: 386.49; Rf : 0.65; FT-IR (νmax; cm
KBr): 3059,
2968 (C-Hstretch), 1657 (C=Nstretch), 1214 (C-Nstretch), 899 cm 1; H-NMR (400 MHz, CDCl3) δ, ppm:
8.58–8.56 (d, 2H J = 8.4 Hz, Ar-H), 7.78–7.43 (m, 15H, Ar-H), 7.28–7.24 (d, 2H J = 6.7 Hz, Ar-H), 2.35
(s, 3H, CH3); 13C-NMR (100 MHz, CDCl3) δ, ppm: 144.5, 140.2, 138.7, 138.2, 133.1, 131.7, 129.7, 129.4,
129.1, 128.7, 128.1, 127.5, 125.7, 122.7, 21.3; Mass spectrum: 387.51 (M + 1); Elemental analysis for
C28H22N2: Calculated: C, 87.01; H, 5.74; N, 7.25; Found: C, 87.03; H, 5.74; N, 7.26.
1
1
1-Benzyl-2-(4-chlorophenyl)-4,5-diphenyl-1H-imidazole (5e). MW: 420.93; Rf : 0.64; FT-IR (νmax; cm
KBr): 3063, 2969 (C-Hstretch), 1654 (C=Nstretch), 1211 (C-Nstretch), 893 (Cl) cm 1; H-NMR (400 MHz,
1
CDCl3) δ, ppm: 8.12–8.09 (d, 2H J = 8.2 Hz, Ar-H), 7.78–7.23 (m, 15H, Ar-H), 7.48–7.44 (d, 2H
J = 6.2 Hz, Ar-H), 5.59 (s, 2H, CH2); 13C-NMR (100 MHz, CDCl3) δ, ppm: 153.7, 141.4, 138.2, 137.3,
134.3, 133.1, 129.6, 129.4, 129.1, 128.9, 128.4, 128.2, 127.6, 127.5, 125.7, 47.5; Mass: 421.94 (M + 1);
Elemental analysis for C28H21ClN2: Calculated: C, 79.89; H, 5.03; N, 6.66; Found: C, 79.91; H, 5.03;
N, 6.66.
1
4-(1-Benzyl-4,5-diphenyl-1H-imidazol-2-yl)phenol (5f). MW: 402.49; Rf : 0.51; FT-IR (νmax; cm KBr):
1
3068, 2962 (C-Hstretch), 1657 (C=Nstretch), 1218 (C-Nstretch), 897 cm 1; H-NMR (400 MHz, CDCl3) δ,
ppm: 7.91–7.89 (d, 2H J = 8.4 Hz, Ar-H), 7.79–7.23 (m, 15H, Ar-H), 6.89–6.84 (d, 2H J = 6.7 Hz, Ar-H),
5.58 (s, 2H, CH2), 5.28 (s, 1H, OH); 13C-NMR (100 MHz, CDCl3) δ, ppm: 158.5, 153.7, 141.3, 138.2,
137.3, 133.1, 130.7, 129.6, 129.1, 128.8, 128.4, 127.6, 127.3, 125.7, 123.2, 116.4, 47.5; Mass spectrum:
403.51 (M + 1); Elemental analysis for C28H22N2O: Calculated: C, 83.56; H, 5.51; N, 6.96; Found:
C, 83.58; H, 5.52; N, 6.96.
4-(1-Benzyl-4,5-diphenyl-1H-imidazol-2-yl)-N,N-dimethylaniline (5g). MW: 429.56; R : 0.61; FT-IR (ν
;
max
f
cm 1 KBr): 3069, 2957 (C-Hstretch), 1652 (C=Nstretch), 1215 (C-Nstretch), 891 cm 1; 1H-NMR (400 MHz,
CDCl3) δ, ppm: 7.97–7.95 (d, 2H J = 8.2 Hz, Ar-H), 7.79–7.24 (m, 15H, Ar-H), 6.82–6.79 (d, 2H
J = 6.4 Hz, Ar-H), 5.58 (s, 2H, CH2), 3.06 (s, 6H, CH3
2); 13C-NMR (100 MHz, CDCl3) δ, ppm:
155.3, 153.7, 141.3, 138.2, 137.3, 133.1, 129.6, 129.2, 128.7, 128.4, 128.1, 127.6, 127.2, 125.7, 120.2, 112.8,
47.5, 41.3; Mass spectrum: 430.58 (M + 1); Elemental analysis for C30H27N3: Calculated: C, 83.88;
H, 6.34; N, 9.78; Found: C, 83.91; H, 6.33; N, 9.78.
1-Benzyl-4,5-diphenyl-2-(p-tolyl)-1H-imidazole (5h). MW: 400.51; Rf : 0.53; FT-IR (νmax; cm 1 KBr): 3069,
1
2956 (C-Hstretch), 1659 (C=Nstretch), 1218 (C-Nstretch), 887 cm 1; H-NMR (400 MHz, CDCl3) δ, ppm:
8.48–8.46 (d, 2H J = 8.4 Hz, Ar-H), 7.79–7.22 (m, 15H, Ar-H), 7.28–7.24 (d, 2H J = 6.1 Hz, Ar-H), 5.58
(s, 2H, CH2), 2.28 (s, 3H, CH3); 13C-NMR (100 MHz, CDCl3) δ, ppm: 153.7, 141.3, 138.2, 137.3, 133.1,
131.7, 129.8, 129.5, 129.1, 128.7, 128.3, 127.6, 127.2, 125.7, 47.5, 21.3; Mass spectrum: 401.48 (M + 1);
Elemental analysis for C29H24N2: Calculated: C, 86.97; H, 6.04; N, 6.99; Found: C, 86.98; H, 6.04;
N, 6.98.
1
1-Benzyl-2-(4-methoxyphenyl)-4,5-diphenyl-1H-imidazole (5i). MW: 416.51; Rf : 0.65; FT-IR (νmax; cm
KBr): 3069, 2959 (C-Hstretch), 1655 (C=Nstretch), 1216 (C-Nstretch), 884 cm
;
1H-NMR (400 MHz,
1
CDCl3, TMS) δ, ppm: 7.97–7.96 (d, 2H J = 8.3 Hz, Ar-H), 7.79–7.23 (m, 15H, Ar-H), 7.05–7.02 (d,
2H J = 6.4 Hz, Ar-H), 5.58 (s, 2H, CH2), 3.82 (s, 3H, OCH3); 13C-NMR (100 MHz, CDCl3) δ, ppm:
160.6, 153.7, 141.2, 138.3, 137.3, 133.1, 130.2, 129.7, 129.2, 128.8, 128.3, 127.8, 127.4, 125.6, 122.8, 114.8,
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Molecules 2015, 20, 20286–20296
55.7, 47.5; Mass spectrum: 417.51 (M + 1); Elemental analysis for C29H24N2O: Calculated: C, 83.63;
H, 5.81; N, 6.73; Found: C, 83.64; H, 5.80; N, 6.73.
1-Benzyl-2-(3,4-dimethoxyphenyl)-4,5-diphenyl-1H-imidazole (5j). MW: 446.54; R : 0.69; FT-IR (ν
;
max
f
cm 1 KBr): 3073, 2955 (C-Hstretch), 1659 (C=Nstretch), 1218 (C-Nstretch), 881 cm 1; 1H-NMR (400 MHz,
CDCl3) δ, ppm: 7.53–7.39 (d, 2H J = 8.7 Hz, Ar-H), 7.79–7.24 (m, 15H, Ar-H), 6.94 (s, 1H, Ar-H), 5.57
(s, 2H, CH2), 3.83 (s, 6H, OCH3 2); 13C-NMR (100 MHz, CDCl3) δ, ppm: 153.7, 150.3, 149.8, 141.3,
138.2, 137.3, 133.1, 129.6, 129.1, 128.8, 128.4, 127.6, 127.3, 125.7, 123.9, 122.3, 112.3, 111.4, 56.2, 47.6;
Mass spectrum: 447.56 (M + 1); Elemental analysis for C30H26N2O2: Calculated: C, 80.69; H, 5.87;
N, 6.27; Found: C, 80.71; H, 5.88; N, 6.27.
3.2. Pharmacology
3.2.1. Corneal Anesthesia
Male New Zealand rabbits were used for the determination of local anesthetic activity of the
synthesized compounds. Local surface anesthesia was evaluated by determining the number of
stimuli to the cornea every 3 min. [25] This was affected rhythmically with a Frey’s horse-hair, in
order to produce the blink reflex. If the reflex did not occur after 100 stimulations, anesthesia was
considered total. At the beginning of the experiment, special care was taken to ascertain that this
reflex was normal in both eyes of the rabbits used. The aqueous solutions (2%) of the compounds
studied were dropped onto the conjunctival sac so that the space between the eyelids contained
a clearly visible film of solution for the set time of 3 min. Lidocaine solution (2%) was used as standard
for comparison.
3.2.2. Mouse Tail Anesthesia
Male Swiss mice (weighing 18–20 g) were used. The test was performed according to the method
of Bianchi in which the aqueous anesthetic solution (0.1 mL) is injected SC about 1 cm from the root
of the tail [26]. Fifteen minutes after injection, the pain reflex of all the injected animals was tested
applying a small artery clip to the zone where the compound was injected. The proportion of animals
which do not show the usual pain reflex within 30 s was noted for each dose. Lidocaine solutions
were used for comparison. IC50 values were calculated for each compound by probit analysis using
a computer program [27].
3.2.3. Rat Sciatic Nerve Block
Triplicate sets of three groups of three male Wistar rats (weighing 180–200 g) were used,
according to the method as outlined by Al-Saadi and Sneader [28]. Each rat received an injection
(0.2 mL) of the aqueous anesthetic solution (2%) into the posterior aspect of the femur head. A positive
effect of the drug resulted in a complete loss of motor control of the injected limb. In order to assess the
duration of the effect, the animals were observed from the time of onset of the motor paralysis at 5 min
intervals for the first 30 min, and at 15 min intervals after that up to the first sign of motor activity.
3.2.4. Acute Toxicity
The intra paritoneal acute toxicity of the most active compound 5g was determined in male Swiss
mice weighing 18–20 g 7 days after treatment. LD50 values were calculated for each compound by
probit analysis using a computer program [27].
4. Conclusions
In summary, we have presented a novel, highly rapid and efficient protocol for the synthesis
of 1,2,4,5-tetrasubstituted imidazoles as local anesthetic agents via a one-pot condensation reaction.
The developed methodology offers an environmentally benign and safe protocol which includes
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Molecules 2015, 20, 20286–20296
a simple reaction setup not requiring specialized equipment, and all the reactions peform well at
100 C to afford the desired compounds in excellent yield without use of the solvent. Moreover,
the catalyst used in present method can be recovered and recycled making the procedure potentially
useful for commercial applications. In bioactivity screening using various methods, some of these
compounds were identified as excellent potential local anesthetic agents. Thus, together with efficient
methodology of synthesis and pronounced local anesthetic activity, these compounds may serve as
a prospective leads for further drug development.
Acknowledgments: Authors are thankful to the authorities for Zhongnan Hospital of Wuhan University,
Wuhan University, China for providing necessary facilities for this project. Authors are also thankful to Custom
Synthesis Corporation, Anderson, CA, USA for extending technical support for the synthesized compounds
reported herein.
Author Contributions: Y.R. and M.L. designed, synthesized and characterized the compounds. Z.-Z.Z.
performed local anesthetic and toxicity activity assays.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds 5a–5j are available from the authors.
© 2015 by the authors; licensee MDPI, Basel, Switzerland. This article is an open
access article distributed under the terms and conditions of the Creative Commons by
Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).
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