Synthesis, calcium-channel blocking activity, and conformational analysis of some novel 1,4-dihydropyridines: Application of PM3 and DFT computational methods

Article abstract of DOI:10.1007/s00044-011-9807-x

Novel 1,4-dihydropyridines were synthesized and subjected to calcium-channel blocking evaluation and conformational analysis using semi-empirical (PM3) and density functional theory (DFT) as computational methods. All molecules had a boat-like 1,4-dihydropyridine ring in both the methods. In PM3 method almost 54% of the molecules were deviated from planarity, but in DFT method all the molecules had perfect flattened-boat conformation. Using both the methods, the C-4 substituent was pseudoaxial with its phenylamino substitution in sp orientation in 82.14% of the molecules. Trans-trans and cis-cis conformation had the greatest and lowest proportion in the molecules, respectively. Trans-trans conformers which possessed sp conformation for the substituted group on the imidazole ring were active calcium-channel blocking agents.

Full text of DOI:10.1007/s00044-011-9807-x

MEDICINAL
CHEMISTRY
RESEARCH
Med Chem Res (2012) 21:2749–2761
DOI 10.1007/s00044-011-9807-x
ORIGINAL RESEARCH
Synthesis, calcium-channel blocking activity, and conformational
analysis of some novel 1,4-dihydropyridines: application of PM3
and DFT computational methods
•
Afshin Fassihi Karim Mahnam Behzad Moeinifard
•
•
•
Maryam Bahmanziari Hojjat Sadeghi Aliabadi Afshin Zarghi
•
•
•
Razieh Sabet Mona Salimi Mahboubeh Mansourian
•
Received: 4 December 2010 / Accepted: 6 October 2011 / Published online: 20 October 2011
Ó Springer Science+Business Media, LLC 2011
Abstract Novel 1,4-dihydropyridines were synthesized
and subjected to calcium-channel blocking evaluation and
conformational analysis using semi-empirical (PM3) and
density functional theory (DFT) as computational methods.
All molecules had a boat-like 1,4-dihydropyridine ring in
both the methods. In PM3 method almost 54% of the
molecules were deviated from planarity, but in DFT
method all the molecules had perfect flattened-boat con-
formation. Using both the methods, the C-4 substituent was
pseudoaxial with its phenylamino substitution in sp orien-
tation in 82.14% of the molecules. Trans–trans and cis–cis
conformation had the greatest and lowest proportion in the
molecules, respectively. Trans–trans conformers which
possessed sp conformation for the substituted group on the
imidazole ring were active calcium-channel blocking
agents.
Keywords 1,4-Dihydropyridines ꢀ
Conformational analysis ꢀ Semi-empirical ꢀ
Density functional theory
Introduction
It was in the beginning of 1880s when 4-aryl-1,4-dihydro-
pyridines (DHPs), one of the most studied classes of drugs,
were synthesized for the first time by Hantzsch (1882). This
class of drugs has been broadly studied for different phar-
macologic properties such as analgesic, anti-tumor, antiox-
idant, anti-inflammatory, antitubercular, and cardiovascular
activities (Shah et al., 2000; Klegeris et al., 2002; Matsubara
and Hasegawa, 2005; Fassihi et al., 2009). The most
important effect of 1,4-dihydropyridine is vascular smooth
muscle relaxation activity, preferentially in the arteries,
hence they display a negative inotropic effect on isolated
cardiac muscle (Fleckenstien, 1977). They exert these
effects by binding to a high-affinity binding site in L-type
voltage-dependent Ca2? channels (Goldmann and Stolte-
fuss, 1991). In therapy, this class of drugs is effective in the
treatment of hypertension, angina pectoris, and other car-
diovascular disorders (Triggle, 1992). Since the middle of
1970s, when the first DHP was introduced into clinic,
1,4-dihydropyridine analogues have been the subject of a
great deal of experimental and theoretical studies (Guadio
et al., 1994; Mahmoudian and Richard, 1986; Rovnyak et al.,
1988; Langs and Triggle, 1985; Gaggelli et al., 1987). In
experimental investigations, C-4 heteroaryl-substituted
1,4-dihydropyridines were also reported as potent Ca2?
channel antagonists and there has always been a great effort
to determine the calcium channel modulation structure–
activity relationships precisely (Amini et al., 2002; Baraldi
et al., 1993; Shafiee et al., 1996, 2002; Pourmoread et al.,
A. Fassihi (&) ꢀ H. S. Aliabadi ꢀ R. Sabet ꢀ M. Mansourian
Department of Medicinal Chemistry, School of Pharmacy,
Isfahan University of Medical Sciences, 81746-73461 Isfahan,
Iran
e-mail: fassihi@pharm.mui.ac.ir
K. Mahnam
Department of Biology, Faculty of Science,
Shahrekord University, 88186-34141 Shahrekord, Iran
B. Moeinifard ꢀ M. Bahmanziari
Department of Chemistry, Islamic Azad University,
Shahreza Branch, Shahreza, Iran
A. Zarghi
Department of Medicinal Chemistry, Faculty of Pharmacy,
Shaheed Beheshti University of Medical Sciences, Tehran, Iran
M. Salimi
Physiology and Pharmacology Department,
Pasteur Institute of Iran, Tehran, Iran
123

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Med Chem Res (2012) 21:2749–2761
1997; Zarghi et al., 2002, 2003a, b; Nguyen et al., 2005; Miri
et al., 1997; Fassihi et al., 2004). This class of compounds
has also attracted lots of attention from the theoretical point
of view (Bikker and Weaver, 1992; Liepina et al., 1997;
Hemmateenejhad et al., 2005, 2007; Ramusino and Vari,
1999; Memarian et al., 2007).
2-methylthio-1-phenylamino-5-imidazolyl as C-4 sub-
stituent. Most of the prepared compounds were as potent as
the reference drug, nifedipine, and two of them were more
active (Zarghi et al., 2003a, b). We now report the syn-
thesis, calcium-channel blocking evaluation, and confor-
mational analysis of novel C-3 and C-5 symmetrically and
asymmetrically substituted 1,4-dihydropyridines with the
same substituent at the C-4 position. Conformational fea-
tures of compounds previously reported by this group will
also be studied. We aim to study the structural features of
biologically active molecules in this series, and compare
them with these features in nifedipine, the prototype of
1,4-dihydropyridine calcium-channel blocker agents.
Many structure–activity relationship studies indicate that
there are some stereochemical and conformational require-
ments for the Ca2? channel modulation activity of this class
of compounds (Goldmann and Stoltefuss, 1991; Bikker and
Weaver, 1992; Liepina et al., 1997). Some of the most
important conformational properties of 1,4-dihydropyri-
dines which affect their biological activity are: (a) the extent
of the C-4 aryl group distortion from planarity, (b) cis or
trans orientation of the C-3 and C-5 carbonyl groups with
respect to their adjacent double bonds of the DHP ring,
(c) the orthogonal arrangement and the axial position of the
4-aryl substituent, and (d) in the case of the presence of any
substitution at ortho or meta positions of the aryl ring (or an
equal position of the C-4 heteroaryl ring) the orientation of
the substituent with respect to the C-4 hydrogen of the DHP
ring (Goldmann and Stoltefuss, 1991; Mahmoudian and
Richard, 1986; Triggle et al., 1980).
Results and discussion
Chemistry
The synthesis of the symmetric and asymmetric 1,4-dihy-
dropyridine derivatives 3a–e, 6a–o was achieved following
the steps outlined in Scheme 1. 1-Phenylamino-2-methyl-
thio-imidazole-5-carboxaldehyde 1 was prepared according
to the procedure described previously (Zarghi et al., 2003a, b).
The symmetrical 1,4-dihydropyridine derivatives 3a–e
were prepared (15–63% yield) by the classical Hantzch
In our previous studies, we have reported the synthesis
and calcium channel antagonist activity of some C-3 and
C-5 symmetrically substituted 1,4-dihydropyridines with
Scheme 1 Synthesis of
symmetric (3a–e) and
asymmetric 1,4-
dihydropyridines (6a–o)
SCH3
SCH3
N
N
N
N
N
N
H
1
1
H
CHO
CHO
R1OOC
COOR2
R1OOC
4a-c
COOR2
5a-o
2a-e
R1=R2
H3C
O
O
CH3
H3C
NH2
O
CH3
+
NH4 CH3COO-
CH3OH
CH3OH
SCH3
SCH3
N
N
N
N
N
H
N
H
R1OOC
COOR2
R1OOC
H3C
COOR2
6a-o
3a-e
R1=R2
H3C
N
H
CH3
N
H
CH3
123

Med Chem Res (2012) 21:2749–2761
2751
condensation in which the aldehyde 1 was reacted with
appropriate acetoacetate ester 2a–e and ammonium acetate
(Hantzsch, 1882). The asymmetrical compounds 6a–o were
synthesized by the modified method of the Hantzch con-
densation described by Meyer et al. 1981 in 16–38% yield.
Acetoacetate ester derivatives 2a–e, 5a–o were prepared
according to the method of Clemens by simple condensa-
tion of 2,2,6-trimethyl-1,3-dioxine-4-one with the appro-
priate alcohols (Clemens and Hyatt,1985). Methyl, ethyl,
and iso-propyl b-aminocrotonates 4a–c were prepared by
the condensation of corresponding acetoacetate esters and
ammonium acetate (Li et al., 1998).
rotate and provide different geometries for 1,4-dihydropyr-
idine ring in calcium-channel blocking agents. The C-4 aryl
group may adopt a pseudoaxial or equatorial position on this
ring. In the case of the presence of any substitution at ortho or
meta positions on the aryl ring, or equivalent positions on a
C-4 heteroaryl ring, it lays syn-periplanar (sp) or anti-
periplanar (ap) with respect to the C-4 hydrogen of the DHP
ring. Chemical structures and the numbering of the ring
system and substituents of the compounds used in confor-
mational studies are shown in Table 1.
The conformational features considered in this study
were: (i) the geometrical orientation of the ester carbonyl
groups bonded at C-3 and C-5 reflected by C2–C3–C7–O8
and C6–C5–C9–O10 dihedral angles (a1 and a2), (ii) relative
position of the substituted imidazole ring at C-4 position of
the 1,4-dihydropyridine ring with respect to the dihydro-
The structures of the title compounds were confirmed by
1
IR, H-NMR, EI mass spectrometry, and elemental analy-
sis. The purity of all products was determined by thin layer
chromatography using several solvent systems of different
polarities. All final products were pure and stable com-
pounds. Chemical structures of the prepared compounds
are summarized in Table 1.
0
pyridine ring measured by the C5 –C4–C3–C2 dihedral
angle (a3), (iii) orientation of the N-1 substitution of the
imidazole substitution of the 1,4-dihydropyridine ring
assigned by sp or ap definitions which is calculated by
0
Pharmacology
N–N1 –C4–H improper dihedral angle (a4) (N belongs to
0
phenylamino group on N1 and H is on the C-4 atom of
The calcium-channel blocking activities (IC50) of 3a–e and
6a–o are presented in Table 1. These values are determined
as the contraction needed to produce 50% relaxation of
contracted guinea pig ileal longitudinal smooth muscle. The
studied compounds caused dose-dependent inhibition of the
contractile responses induced by high K? concentration.
These effects were similar to those obtained with nifedipine,
and may be explained as being caused by inhibition of entry
of Ca2? through voltage-dependent calcium channels in the
muscle membrane. However, it should be taken into account
that a complex network of neuronal and muscular tissues
exists in the intestine and the possibility of different actions
of the synthesized compounds at several neuronal and
muscular sites on which they might act could not be ruled
out. Comparison of the activities of the synthesized com-
pounds indicated that in symmetrical esters series, as it was
previously shown by this research group, increasing the
length of methylene chain in C-3 and C-5 ester substituents
for more than two methylene units decreases the activity
(Zarghi et al., 2003a, b). The same explanation is applicable
for comparing the potencies of asymmetrical 1,4-dihydro-
pyridines prepared in this study. Finally, the results show
that five compounds 6a, 6b, 6e, 6i, and 6j were more active
than the reference drug nifedipine.
dihydropyridine ring), and finally (iv) deviation of the
1,4-dihydropyridine ring from planarity measured by the
C2–C3–C5–C6 improper dihedral angle (a5).
If a1 and a2 dihedral angles were between -90° and
?90°, the orientation of the ester groups was considered
cis. If these dihedral angles were (?90°)–(?180°) or
(-90°)–(-180°), the orientation was assumed trans. The
molecules were assigned as cis–cis, trans–trans, or cis–
trans with respect to the corresponding C=C bond of the
1,4-dihydropyridine ring. If improper dihedral angle a4 was
between 0° and ?90° or 0° and -90°, the orientation of the
N-1 substitution of the imidazole ring was assigned as sp.
A dihedral angle value of (?90°)–(?180°) or (-90°)–
(-180°) was represented as ap orientation. An improper
dihedral angle of between 1° and -1° for a5 was a repre-
sentative of a flattened-boat conformation for the 1,4-
dihydropyridine ring. When this angle was greater than 1°
or smaller than -1° the ring was considered twist-like.
Two different calculation methods, semi-empirical
(PM3) and the high level density functional theory (DFT)
B3LYP method with 6-31G* basis set, were used to obtain
the optimal 3D structures of the prepared compounds. The
conformational features together with the enthalpy of for-
mation, DH0f (kcal/mol), of the studied compounds are
summarized in Tables 2 and 3.
Computational study
Semi-empirical optimizations
1,4-Dihydropyridine ring in calcium-channel blockers of
nifedipine type has a flat boat conformation according to
X-ray crystallography as well as theoretical studies. The C-4
aryl ring and C-3 and C-5 ester groups are flexible enough to
Semi-empirical calculations were carried out by PM3 Ham-
iltonian which was previously reported in conformational
studies of other types of 1,4-dihydropyridines (Ramusino and
123

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Med Chem Res (2012) 21:2749–2761
Table 1 Chemical structures of the compounds prepared (3a–e, 6a–o) and used (3a–m, 6a–o) for conformational studies and numbering of the
1,4-dihydropyridine ring system
SCH3
3'
2'
N
N
N
10
O
4'
1'
H
5'
4
8
O
H
R1O
9
5
3
7
H3C
6
OR2
N
H
1
2
CH3
Compounds
R1
iso-Butyl
R2
ICa50
3a
3b
3c
iso-Butyl
6.71 0.37 9 10-9
1.79 0.21 9 10-8
4.25 0.16 9 10-7
4.65 0.31 9 10-7
6.21 0.65 9 10-7
3.54 0.85 9 10-9b
1.69 0.43 9 10-9b
2.29 0.79 9 10-9b
2.16 0.76 9 10-9b
1.21 0.27 9 10-7b
1.49 0.39 9 10-9b
2.79 0.69 9 10-9b
3.15 0.71 9 10-9b
1.66 0.24 9 10-9
2.40 0.33 9 10-9
6.35 0.15 9 10-9
9.65 0.44 9 10-9
2.21 0.35 9 10-9
3.05 0.26 9 10-9
3.22 0.21 9 10-9
2.91 0.16 9 10-9
1.86 0.36 9 10-9
2.54 0.54 9 10-9
6.82 0.33 9 10-8
5.34 0.65 9 10-8
7.24 0.22 9 10-9
6.22 0.46 9 10-9
0.56 0.28 9 10-7
2.60 0.25 9 10-9
Methoxyethyl
Cyclopentyl
Cyclohexyl
Methoxyethyl
Cyclopentyl
Cyclohexyl
Cyclohexylmethyl
Methyl
3d
3e
Cyclohexylmethyl
Methyl
3f
3g
3h
3i
Ethyl
Ethyl
n-Propyl
iso-Propyl
tert-Butyl
Benzyl
n-Propyl
iso-Propyl
tert-Butyl
Benzyl
3j
3k
3l
Phenethyl
Phenpropyl
Methyl
Phenethyl
Phenpropyl
Ethyl
3m
6a
6b
6c
Methyl
n-Propyl
Methyl
iso-Propyl
Methoxyethyl
Benzyl
6d
6e
Methyl
Methyl
6f
Methyl
Phenpropyl
iso-Propyl
Methoxyethyl
Benzyl
6g
6h
6i
Ethyl
Ethyl
Ethyl
6j
Ethyl
Phenpropyl
tert-Butyl
Methoxyethyl
Benzyl
6k
6l
iso-Propyl
iso-Propyl
iso-Propyl
iso-Propyl
iso-Propyl
6m
6n
6o
Nifedipine
a
Phenpropyl
Cyclohexylmethyl
The molar concentration of antagonist test compound causing a 50% in the tonic contractile response (IC50 SEM) in guinea-pig ileal
longitudinal smooth muscle by KCl (80 mmol/l) was determined graphically from dose–response curve (n = 6)
b
See Zarghi et al. (2003a)
123

Med Chem Res (2012) 21:2749–2761
2753
Vari, 1999; Memarian et al., 2007). The results of these cal-
culations in the gas phase are listed in Table 2.
(DFT) B3LYP with 6-31G* basis set was used to obtain the
best optimizations. The results of conformational features
of B3LYP/6-31G*-optimized molecules are summarized in
Table 3. As it can be seen from the data, all the molecules
had a perfect flattened-boat conformation with a pseudo-
axial substituent at C-4 position. The same conformation
for the 1,4-dihydropyridine ring and position for the C-4
substituent is reported in literature for nifedipine (Langs
and Triggle, 1985). 64.29% of the studied molecules were
trans–trans, 28.57% had cis–trans or trans–cis conforma-
tion, and only 7.14% were in cis–cis conformation. Here,
the same as PM3 optimizations, 82.14% of the molecules
had sp conformation. Cis–trans analogues (6a, 6c, and 6g)
in which the phenylamino substituent on the imidazole ring
had sp conformation were active and potent compounds,
but those with ap conformation (3c, 6i, and 6o) were
weak calcium-channel blockers. This is applicable for
explaining the activity of trans–cis compounds. Trans–
trans molecules were also active calcium-channel blocking
agents. All the active trans–trans conformers possessed sp
PM3 calculations resulted in a boat-like conformation. In
this type of semi-empirical calculations, 53.6% of the
molecules showed deviation from planarity. The dihedral
angle between the C=C bonds of the dihydropyridine ring
was 3.11 in 6c, which had the highest deviation. The C-4
substitution was in pseudoaxial position. Most of the stud-
ied molecules (67.86%) adopted a trans–trans conforma-
tion. 21.43% had cis–trans or trans–cis conformation
and only 10.71% were in cis–cis conformation. Most of
the molecules (82.14%) showed the sp conformation for the
phenylamino group of the imidazole ring with respect to the
C-4 hydrogen. The enthalpy of formation of the compounds
calculated by PM3 computations are reported in Table 2.
DFT Optimizations
As a more accurate method in comparison with semi-
empirical method, the high level density functional theory
Compounds C2–C3–C7–O8 C6–C5–C9–O10 C05–C4–C3–C2 N–N10 –C4–H C2–C3–C5–C6 DHf
Table 2 Dihedral angles (°)
and heat of formation, DHf0
(kcal/mol), of the semi-
empirical PM3-optimized
structures
(a1)
(a2)
(a3)
(a4)
(a5)
3a
3b
3c
3d
3e
3f
-140.24
98.88
-146.43
160.35
-82.89
167.37
10.27
82.81
83.48
-2.08
2.28
-79.93
-126.99
-80.44
-81.39
-86.97
-54.95
-58.76
-68.07
-68.69
-77.29
11.50
83.92
113.55
105.48
109.67
86.28
83.83
83.26
81.78
84.84
83.87
88.09
87.52
86.76
82.44
83.04
88.72
82.17
82.02
86.21
83.20
82.49
82.39
89.58
116.32
81.84
84.27
115.77
77.13
172.13
156.27
158.67
69.08
68.83
74.38
73.19
87.70
77.56
79.29
73.53
61.77
59.69
59.00
62.58
59.10
72.07
45.22
72.49
71.97
73.46
81.49
175.09
75.56
76.36
175.64
-64.32
165.84
-21.90
112.85
140.09
167.14
-176.20
-165.07
161.21
108.87
-168.98
-47.21
-156.03
-89.18
-21.81
-171.55
177.35
-91.06
166.40
172.34
171.49
117.54
-71.44
172.17
128.25
-75.37
0.02
-1.91
-1.32
1.83
138.78
174.10
176.90
171.48
-152.41
-170.52
149.32
-134.73
148.24
7.81
3g
3h
3i
2.03
0.92
0.51
3j
-1.32
0.34
3k
3l
2.00
-8.18
3m
6a
6b
6c
6d
6e
6f
-0.35
2.05
-15.35
-58.40
-59.47
-63.80
-92.16
-19.69
-29.36
-69.40
-91.21
-23.36
-33.26
-80.90
-108.74
-29.05
-42.76
-92.49
0.89
169.13
31.94
3.11
1.54
6.28
1.30
163.76
-164.77
176.00
176.09
-179.56
141.19
-103.08
171.17
160.51
-103.65
0.45
6g
6h
6i
2.09
0.93
0.66
6j
-0.29
1.69
6k
6l
-0.73
0.29
6m
6n
6o
2.21
-0.33
123

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Med Chem Res (2012) 21:2749–2761
Compounds C2–C3–C7–O8 C6–C5–C9–O10 C05–C4–C3–C2 N–N10 –C4–H C2–C3–C5–C6 DHf
Table 3 Dihedral angles (°)
and heat of formation, DHf0
(kcal/mol), of the B3LYP-
optimized structures
(a1)
(a2)
(a3)
(a4)
(a5)
3a
3b
3c
3d
3e
3f
179.02
164.33
83.83
72.64
-0.36
0.19
-62.71
-110.75
-53.93
-65.23
-64.48
-34.36
-44.60
-54.59
-54.65
-62.72
24.04
-178.84
-15.49
174.24
-7.13
178.67
178.86
177.89
177.98
178.88
177.58
179.47
158.52
-1.05
176.63
0.23
172.91
-158.25
-172.41
1.15
84.51
111.22
106.64
107.95
85.14
84.77
84.94
84.98
83.84
85.77
84.42
87.22
86.40
84.12
86.32
85.61
84.17
84.87
86.57
84.65
84.64
84.72
84.94
113.62
84.90
84.81
114.12
65.93
-159.32
-174.39
-177.05
66.11
0.55
0.15
-0.11
0.27
172.40
171.40
171.68
173.21
164.30
175.28
167.09
-112.73
172.51
-5.59
3g
3h
3i
67.25
0.24
66.92
0.21
65.99
0.28
3j
72.65
-0.35
0.26
3k
3l
63.39
66.68
-0.20
-0.39
0.63
12.12
3m
6a
6b
6c
6d
6e
6f
65.52
1.40
66.37
-40.77
-44.62
-45.59
-76.49
-5.38
69.59
-0.24
0.46
171.90
10.70
67.25
3.15
71.25
0.04
176.77
178.42
0.54
-4.30
67.09
-0.05
0.18
171.41
173.49
171.13
171.32
171.11
173.56
-157.37
173.78
173.04
156.16
67.08
-14.80
-50.82
-77.62
-10.24
-21.22
-58.47
-84.00
-15.30
-26.24
-66.37
6g
6h
6i
66.12
0.59
178.73
178.73
179.17
178.43
-23.99
178.66
178.97
-32.39
67.56
0.17
66.75
0.26
6j
67.13
0.26
6k
6l
65.49
0.27
-148.96
64.78
0.49
6m
6n
6o
0.21
65.40
0.28
-141.01
0.94
conformation for the C-3 and C-5 ester groups is different
from the conformation of ester groups in nifedipine which
is reported to be cis/trans (Langs and Triggle, 1985). The
enthalpy of formation of the compounds calculated by
B3LYP/6-31G* computations is compared with the
amounts of this energy obtained by PM3 optimizations in
Fig. 1. As it can be seen in Fig. 1, there is a very good
linear correlation between the results of B3LYP/6-31G*
and the semi-empirical PM3 method. This figure also
shows that PM3 method overestimates the heats of for-
mation for all molecules in comparison with B3LYP/6-
31G* method. The same conclusion was made by Me-
marian et al. (2007) for another set of 1,4-dihydropyridine
compounds.
40
20
0
-20
-40
-60
-80
-100
-120
-140
PM3
B3LYP/6-31G*
Compound
Fig. 1 Correlation between the values of DH0f (kcal/mol) for the
studied molecules optimized by PM3 and B3LYP/6-31G* methods
The calculated 3D geometry for two of the most potent
compounds, 3g and 3k, by PM3 and DFT methods is
provided in Fig. 2.
conformation for the phenylamino group. Hemmateenejad
et al. (2005) had also reported active trans–trans
conformers for 1,4-dihydropyridines containing substi-
tuted imidazole at the C-4 position of the ring. This
Overlaid structures of compounds 3g and 3k are shown
in Fig. 3 to compare the 3D structures obtained by PM3
and DFT methods.
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Med Chem Res (2012) 21:2749–2761
2755
3g, PM3
3k, PM3
3g, B3LYP/6-31G*
3k, B3LYP/6-31G*
Fig. 2 The optimized 3D structures of 3g and 3k obtained by PM3 and DFT methods
Conclusions
pseudoaxial with respect to the 1,4-dihydropyridine ring in
all molecules optimized by both the methods, as was
reported for nifedipine. The conformation for the phe-
nylamino group of the imidazole ring with respect to the
C-4 hydrogen was exactly the same in both methods,
82.14% of the molecules had sp conformation. In some of
the PM3 and DFT-optimized structures one of the carbonyl
groups was oriented cis and the other was oriented trans
with respect to the C=C bond of the 1,4-dihydropyridine
ring. Trans–trans and cis–cis conformations had the
greatest and the lowest proportions in the molecules,
respectively. Analysis of the biological and computational
methods implies that trans–trans molecules were also
capable to produce calcium-channel blocking activity. It
seems that for these molecules sp conformation for the
phenylamino group of the imidazole ring, and a flattened-
boat conformation are sufficient requirements for having
the same potency or a slightly more potent pharmacologic
Twenty symmetric and asymmetric 1,4-dihydropyridines
were synthesized and their molecular structures were con-
firmed by spectral data. Calcium-channel blocking activity
of the compounds was determined. The prepared com-
pounds as well as eight compounds which were reported
previously were subjected to conformational analysis.
The results of conformational studies showed that all
molecules had a boat-like conformation for the 1,4-dihy-
dropyridine ring both in PM3 and DFT optimizations. In
PM3 method almost 54% of the molecules were deviated
from planarity, but in DFT method all the molecules had
perfect flattened-boat conformation. The same conforma-
tion is observable for nifedipine, the prototype of
1,4-dihydropyridine calcium-channel blockers when it is
geometrically optimized by any of these computational
methods. The substituted imidazole at C-4 position was
123

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Med Chem Res (2012) 21:2749–2761
Electron-impact ionization was performed at an ionizing
energy of 70 eV. Elemental analyses were carried out with
a Perkin Elmer Model 240-C apparatus. Elemental
microanalyses were within 0.4% of the theoretical val-
ues for C, H, and N.
General procedure for the synthesis of symmetric dialkyl
1,4-dihydro-2,6 dimethyl-4-[2-methylthio-1-
(phenylamino)-1H-imidazole-5-yl]-3,5-
pyridinedicarboxylates (3a–e)
A mixture of aldehyde 1 (100 mg, 0.429 mmol), appro-
priate acetoacetate ester 2a–e (0.858 mmol) and ammo-
nium acetate (33 mg, 0.429 mmol) in 4 ml of methanol
was refluxed for 24 h. The solvent was evaporated and
residue was purified by preparative thin-layer chromatog-
raphy using chloroform as mobile phase. Crystallization
from acetone and petroleum ether gave pure compounds
3a–e (Scheme 1).
Di-iso-butyl-1,4-dihydro-2,6-dimethyl-4-[2-methylthio-1-
(phenylamino)-1H-imidazole-5-yl]-3,5-pyridinedicarboxyl-
ate (3a) Yield: 44%; m.p. = 176–177°C; IR (KBr): NH
3275, C=O 1693 cm-1 1HNMR (CDCl3): d 0.90 (d,
;
J = 6.6 Hz, 12H, 29 –CH2CH(CH3)2), 2.00 (m, 2H, 29
–CH2CH(CH3)2), 2.20 (s, 6H, C2,C6–CH3), 2.70 (s, 3H,
SCH3), 3.90 (d, J = 6.6 Hz, 4H, 29 –CH2CH(CH3)2), 5.45
(s, 1H, C4–H of dihydropyridine), 6.90 (bs, 1H: N1–H of
dihydropyridine), 7.10–7.50 (m, 6H, 1H: C4–H of imid-
azole, 5H: –NH–C6H5); MS: m/z 512.0 (M?). Anal. Calcd.
for C27H36N4O4S: C, 63.26; H, 7.08; N, 10.93. Found: C,
62.92; H, 7.28; N, 11.15.
Fig. 3 Overlaid structures of compounds 3g and 3k obtained by PM3
and DFT methods
activity than nifedipine. There was a very good linear
correlation between the values of DHf8 for the studied
molecules optimized by PM3 and DFT methods.
Dimethoxyethyl-1,4-dihydro-2,6-dimethyl-4-[2-methylthio-
1-(phenylamino)-1H-imidazole-5-yl]-3,5-pyridinedicar-
boxylate (3b) Yield: 63%; m.p. = 124–125°C; IR (KBr):
NH 3270, C=O 1684,1694 cm-1; 1HNMR (CDCl3): d 2.20
(s, 6H, C2,C6–CH3), 2.60 (s, 3H, SCH3), 3.25 (s, 6H, 29
–CH2CH2OCH3), 3.60 (t, J = 6.4 Hz, 4H, 29 –CH2CH2
OCH3), 4.30 (t, J = 6.4 Hz, 4H, 29 –CH2CH2OCH3), 5.45
(s, 1H, C4–H of dihydropyridine), 6.75 (bs, 1H, N1–H of
dihydropyridine), 7.10–7.70 (m, 6H, 1H: C4–H of imid-
azole, 5H: –NH–C6H5); MS: m/z 516.1 (M?). Anal. Calcd.
for C25H32N4O6S: C, 58.12; H, 6.24; N, 10.85. Found: C,
57.88; H, 6.45; N, 11.21.
Experimental
Chemistry
Chemicals from Merck Chemical Co (Hohenbrunn, Ger-
many). were used without further purification. Analytical
thin-layer chromatography (TLC) was performed on
Merck silica gel (60F254) plates. Melting points were
determined on a Mettler capillary melting point apparatus
and were uncorrected. The IR spectra were recorded with
a Perkin Elmer 1420 Ratio Recording IR spectrometer as
Dicyclopentyl-1,4-dihydro-2,6-dimethyl-4-[2-methylthio-1-
(phenylamino)-1H-imidazole-5-yl]-3,5-pyridinedicarboxyl-
ate (3c) Yield: 25%; m.p. = 222–223°C; IR (KBr): NH
3270, C=O 1691 cm-1; 1HNMR (CDCl3): d 1.75 (m, 16H,
29 cyclopentyl), 2.20 (s, 6H, C2,C6–CH3), 2.60 (s, 3H,
SCH3), 5.20 (m, 2H, 29 C1–H of cyclopentyl), 5.30 (s, 1H,
C4–H of dihydropyridine), 6.70 (bs, 1H, N1–H of
1
a KBr disc (c, cm-1). The H-NMR spectra (CDCl3) were
recorded on a Bruker 300 MHz spectrometer. Chemical
shifts (d) are reported in ppm downfield from the internal
standard tetramethylsilane (TMS). Mass spectra were
acquired with a Finnigan TSQ-70 mass spectrometer.
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Med Chem Res (2012) 21:2749–2761
2757
dihydropyridine), 7.10–7.55 (m, 6H, 1H: C4–H of imid-
azole, 5H: –NH–C6H5); MS: m/z 536.0 (M?). Anal. Calcd.
for C29H36N4O4S: C, 64.90; H, 6.76; N, 10.44. Found: C,
64.78; H, 6.81; N, 10.59.
Anal. Calcd. for C22H26N4O4S: C, 59.71; H, 5.92; N, 12.66.
Found: C, 59.88; H, 5.74; N, 12.85.
Methyl, n-propyl 1,4-dihydro-2,6-dimethyl-4-[2-methyl-
thio-1-(phenylamino)-1H-imidazole-5-yl]-3,5-pyridinedi-
carboxylate (6b) Yield: 35%; m.p. = 149–150°C; IR
(KBr): NH 3332, C=O 1694 cm-1; 1HNMR (CDCl3): d 0.85
(t, J = 6.5 Hz, 3H, –CH2CH2CH3), 1.60 (m, 2H,
CH2CH2CH3), 2.20 (s, 6H, C2,C6–CH3), 2.50 (s, 3H, SCH3),
3.70 (s, 3H, –COOCH3), 4.10 (t, J = 6.5 Hz, 2H,
–CH2CH2CH3), 5.35 (s, 1H, C4–H of dihydropyridine), 6.85
(bs, 1H, N1–H of dihydropyridine), 7.10–7.70 (m, 6H, 1H:
C4–H of imidazole, 5H: –NH–C6H5); MS: m/z 456.2 (M?).
Anal. Calcd. for C23H28N4O4S: C, 60.51; H, 6.18; N, 10.27.
Found: C, 60.45; H, 6.16; N, 10.36.
Dicyclohexyl-1,4-dihydro-2,6-dimethyl-4-[2-methylthio-1-
(phenylamino)-1H-imidazole-5-yl]-3,5-pyridinedicarboxyl-
ate (3d) Yield: 17%; m.p. = 221–223°C; IR (KBr): NH
3275, C=O 1694 cm-1; 1HNMR (CDCl3): d 1.60 (m, 20 H,
29 cyclohexyl), 2.30 (s, 6H, C2,C6–CH3), 2.60 (s, 3H,
SCH3), 4.90 (m, 2H, 29 C1-H of cyclohexyl), 5.45 (s, 1H,
C4–H of dihydropyridine), 6.70 (bs, 1H, N1–H of dihy-
dropyridine), 7.10–7.70 (m, 6H, 1H: C4–H of imidazole,
5H: –NH–C6H5); MS: m/z 564.1 (M?). Anal. Calcd. for
C31H40N4O4S: C, 65.93; H, 7.14; N, 9.92. Found: C, 65.75;
H, 6.97; N, 10.19.
Methyl, iso-propyl 1,4-dihydro-2,6-dimethyl-4-[2-methyl-
thio-1-(phenylamino)-1H-imidazole-5-yl]-3,5-pyridinedi-
carboxylate (6c) Yield: 18%; m.p. = 128–129°C; IR
(KBr): NH 3344, C=O 1698 cm-1; 1HNMR (CDCl3): d 1.25
(dd, J = 3.2 Hz, J = 1.6 Hz, 6H, –CH (CH3)2), 2.20 (s, 6H,
C2,C6–CH3), 2.65 (s, 3H, SCH3), 3.70 (s, 3H,–COOCH3),
5.10 (m, 1H, –CH(CH3)2), 5.35 (s, 1H, C4–H of dihydro-
pyridine), 6.55 (bs, 1H, N1–H of dihydropyridine),
7.15–7.70 (m, 6H, 1H: C4–H of imidazole, 5H: –NH–C6H5);
MS: m/z 456.1 (M?). Anal. Calcd. for C23H28N4O4S: C,
60.51; H, 6.18; N, 10.27. Found: C, 60.36; H, 6.25; N, 10.31.
Dicyclohexylmethyl-1,4-dihydro-2,6-dimethyl-4-[2-methyl-
thio-1-(phenylamino)-1H-imidazole-5-yl]-3,5-pyridinedi-
carboxylate (3e) Yield:15%; m.p. = 223–224°C; IR
(KBr): NH 3209, 3279, C=O 1645 cm-1
;
1HNMR
(CDCl3): d 0.70–2.00 (m, 22H, 29 –CH2C6H11), 2.25 (s,
6H, C2,C6–CH3), 2.60 (s, 3H, SCH3), 3.95 (d, J = 6.2 Hz,
4H, 29 –CH2C6H11), 5.40 (s, 1H, C4–H of dihydropyri-
dine), 6.85 (bs, 1H, N1–H of dihydropyridine), 7.20–7.75
(m, 6H, 1H: C4–H of imidazole, 5H: –NH–C6H5); MS: m/z
592.0 (M?). Anal. Calcd. for C33H44N4O4S: C, 66.86; H,
7.48; N, 9.45. Found: C, 66.78; H, 7.50; N, 9.56.
Methyl, methoxyethyl 1,4-dihydro-2,6-dimethyl-4-[2-methyl-
thio-1-(phenylamino)-1H-imidazole-5-yl]-3,5-pyridinedi-
carboxylate (6d) Yield: 28%; m.p. = 150–151°C; IR
(KBr): NH 3338, C=O 1694 cm-1; 1HNMR (CDCl3): d 2.15
(s, 6H, C2,C6–CH3), 2.60 (s, 3H, SCH3), 3.30 (s, 3H,
–CH2CH2OCH3), 3.60 (t, J = 6.4 Hz, 2H, –CH2CH2OCH3),
3.70 (s, 3H, –COOCH3), 4.25 (t, J = 6.4 Hz, 2H,
–CH2CH2OCH3), 5.40 (s, 1H, C4–H of dihydropyridine),
7.10 (bs, 1H, N1–H of dihydropyridine), 7.20–7.70 (m, 6H,
1H: C4–H of imidazole, 5H: –NH–C6H5); MS: m/z 472.0
(M?). Anal. Calcd. for C23H28N4O5S: C, 58.46; H, 5.97; N,
11.86. Found: C, 58.23; H, 6.05; N, 12.12.
General procedure for the synthesis of asymmetric methyl,
alkyl 1,4-dihydro-2,6 dimethyl-4-[2-methylthio-1-
(phenylamino)1H-imidazole-5-yl]-3,5-pyridine
dicarboxylates (6a–f)
A mixture of aldehyde 1 (100 mg, 0.429 mmol), appro-
priate acetoacetate ester 5a–f (0.429 mmol) and methyl
b-aminocrotonate 4a (0.429 mmol) in 4 ml of methanol
was refluxed for 24 h. The solvent was evaporated and
residue was purified by preparative thin-layer chromatog-
raphy using chloroform as mobile phase. Crystallization
from acetone and petroleum ether gave pure compounds
6a–f (Scheme 1).
Methyl, benzyl 1,4-dihydro-2,6-dimethyl-4-[2-methylthio-
1-(phenylamino)-1H-imidazole-5-yl]-3,5-pyridinedicarbo-
xylate (6e) Yield: 25%; m.p. = 181–182°C; IR (KBr): NH
Methyl, ethyl 1,4-dihydro-2,6-dimethyl-4-[2-methylthio-1-
(phenylamino)-1H-imidazole-5-yl]-3,5-pyridine dicarbox-
ylate (6a) Yield: 27%; m.p. = 151–152°C; IR (KBr): NH
1
3335, C=O 1695 cm-1; HNMR (CDCl3): d 2.10 (s, 6H,
3355, C=O 1699 cm-1
;
1HNMR (CDCl3): d 1.25 (t,
C2,C6–CH3), 2.50 (s, 3H, SCH3), 3.70 (s, 3H, –COOCH3),
5.20 (s, 2H, –CH2C6H5), 5.40 (s, 1H, C4–H of dihydropyri-
dine), 6.90–7.70 (m, 12H, 5H: –CH2C6H5, 1H: C4–H of
imidazole, 5H: –NH–C6H5, 1H: N1–H of dihydropyridine);
MS: m/z 504.1 (M?). Anal. Calcd. for C27H28N4O4S: C,
64.27; H, 5.59; N, 11.10. Found: C, 64.01; H, 5.64; N, 11.36.
J = 7.8 Hz, 3H, –CH2CH3), 2.25 (s, 6H, C2,C6–CH3), 2.60
(s, 3H, SCH3), 3.70 (s, 3H, –COOCH3), 4.20 (q, J = 7.8 Hz,
2H, –CH2CH3), 5.35 (s, 1H, C4–H of dihydropyridine), 7.00
(bs, 1H, N1–H of dihydropyridine), 7.15–7.65 (m, 6H, 1H:
C4–H of imidazole, 5H: –NH–C6H5); MS: m/z 442.0 (M?).
123

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Med Chem Res (2012) 21:2749–2761
Methyl, phenpropyl 1,4-dihydro-2,6-dimethyl-4-[2-methyl-
thio-1-(phenylamino)-1H-imidazole-5-yl]-3,5-pyridinedi-
carboxylate (6f) Yield: 38%; m.p. = 198–190°C; IR
(KBr): NH 3337, C=O 1694 cm-1; 1HNMR (CDCl3): d 1.90
(m, 2H, –CH2CH2CH2C6H5), 2.20 (s, 6H, C2,C6–CH3), 2.50
(s, 3H, SCH3), 2.60 (t, J = 6.5 Hz, 2H, –CH2CH2CH2C6H5),
3.70 (s, 3H, –COOCH3), 4.15 (t, J = 6.5 Hz, 2H,
–CH2CH2CH2C6H5), 5.45 (s, 1H, C4–H of dihydropyridine),
6.90–7.60 (m, 12H, 5H: –CH2CH2CH2C6H5 1H: C4–H of
imidazole, 5H: –NH–C6H5, 1H: N1–H of dihydropyridine);
MS: m/z 532.1 (M?). Anal. Calcd. for C29H32N4O4S: C,
65.39; H, 6.06; N, 10.52. Found: C, 65.45; H, 6.16; N, 10.82.
Ethyl, benzyl-1,4-dihydro-2,6-dimethyl-4-[2-methylthio-1-
(phenylamino)-1H-imidazole-5-yl]-3,5-pyridine dicarbox-
ylate (6i) Yield: 28%; m.p. = 184–185°C; IR: (KBr)
1
cm-1: NH 3337, C=O 1695; HNMR (CDCl3): d 1.25 (t,
J = 7.8 Hz, 3H, –CH2CH3), 2.20 (s, 6H, C2,C6–CH3), 2.55
(s, 3H, SCH3), 4.20 (q, J = 7.8 Hz, 2H, –CH2CH3), 5.20 (s,
2H, –CH2C6H5), 5.50 (s, 1H, C4–H of dihydropyridine),
7.00–7.60 (m, 12H, 5H: –CH2C6H5, 1H: C4–H of imidazole,
5H: –NH–C6H5, 1H: N1–H of dihydropyridine); MS: m/z
518.0 (M?). Anal. Calcd. for C28H30N4O4S: C, 64.84; H,
5.83; N, 10.80. Found: C, 65.02; H, 5.99; N, 11.03.
Ethyl, phenpropyl-1,4-dihydro-2,6-dimethyl-4-[2-methyl-
thio-1-(phenylamino)-1H-imidazole-5-yl]-3,5-pyridine dicar-
boxylate (6j) Yield: 20%; m.p. = 195–197°C; IR: (KBr)
General procedure for the synthesis of asymmetric ethyl,
alkyl 1,4-dihydro-2,6 dimethyl-4-[2-methylthio-1-
(phenylamino)-1H-imidazole-5-yl]-3,5-pyridine
dicarboxylates (6g–j)
1
cm-1: NH 3340, C=O 1694; HNMR (CDCl3): d 1.30 (t,
J = 7.8 Hz, 3H, –CH2CH3), 2.00 (m, 2H, CH2CH2
CH2C6H5), 2.25 (s, 6H, C2,C6–CH3), 2.40–2.85 (m, 5H, 2H:
–CH2CH2CH2C6H5, 3H: SCH3), 3.85–4.45 (m, 4H, 2H:
–CH2CH2CH2C6H5, 2H: CH2CH3), 5.45 (s, 1H, C4–H of
dihydropyridine), 6.90 (bs, 1H, N1–H of dihydropyridine),
7.00–7.65 (m, 11H, 5H: –CH2CH2CH2C6H5 1H: C4–H of
imidazole, 5H: –NH–C6H5); MS: m/z 552.0 (M?). Anal.
Calcd. for C30H34N4O4S: C, 65.91; H, 6.27; N, 10.25. Found:
C, 65.59; H, 6.19; N, 10.46.
A mixture of aldehyde 1 (100 mg, 0.429 mmol), appro-
priate acetoacetate ester 5 g–j (0.429 mmol) and ethyl
b-aminocrotonate 4b (0.429 mmol) in 4 ml of methanol
was refluxed for 24 h. The solvent was evaporated and
residue was purified by preparative thin-layer chromatog-
raphy using chloroform as mobile phase. Crystallization
from acetone and petroleum ether gave pure compounds
6g–j (Scheme 1).
General procedure for the synthesis of asymmetric iso-
propyl, alkyl 1,4-dihydro-2,6 dimethyl-4-[2-methylthio-1-
(phenylamino)1H-imidazole-5-yl]-3,5-pyridine
dicarboxylates (6k–o)
Ethyl, iso-propyl 1,4-dihydro-2,6-dimethyl-4-[2-methyl-
thio-1-(phenylamino)-1H-imidazole-5-yl]-3,5-pyridine
dicarboxylate (6g) Yield: 28%; m.p. = 155–157°C; IR:
(KBr) cm-1: NH 3205, 3270, C=O 1693; 1HNMR (CDCl3):
d 1.25 (m, 9H, 6H: –CH(CH3)2, 3H: –CH2CH3), 2.20 (s, 6H,
C2,C6–CH3), 2.60 (s, 3H, SCH3), 4.15 (q, J = 7.8 Hz, 2H,
–CH2CH3), 5.05 (m, 1H, –CH(CH3)2), 5.40 (s, 1H, C4–H of
dihydropyridine), 6.75 (bs, 1H, N1–H of dihydropyridine),
7.20–7.65 (m, 6H, 1H: C4–H of imidazole, 5H: –NH–C6H5);
MS: m/z 470.3 (M?). Anal. Calcd. for C24H30N4O4S: C,
61.26; H, 6.43; N, 11.91. Found: C, 60.95; H, 6.19; N, 12.03.
A mixture of aldehyde 1 (100 mg, 0.429 mmol), appro-
priate acetoacetate ester 5k–o (0.429 mmol) and iso-propyl
b-aminocrotonate 4c (0.429 mmol) in 4 ml of methanol
was refluxed for 24 h. The solvent was evaporated and
residue was purified by preparative thin-layer chromatog-
raphy using chloroform as mobile phase. Crystallization
from acetone and petroleum ether gave pure compounds
6k–o (Scheme 1).
Ethyl, methoxyethyl 1,4-dihydro-2,6-dimethyl-4-[2-methyl-
thio-1-(phenylamino)-1H-imidazole-5-yl]-3,5-pyridine dicar-
boxylate (6h) Yield: 29%; m.p. = 155–157°C; IR: (KBr)
Iso-propyl, tert-butyl-1,4-dihydro-2,6-dimethyl-4-[2-methyl-
thio-1-(phenylamino)-1H-imidazole-5-yl]-3,5-pyridinedicar-
boxylate (6k) Yield: 16%; m.p. = 175–177°C; IR: (KBr)
cm-1: NH 3330, C=O 1690; 1HNMR (CDCl3): d 1.25 (dd,
J = 3.2 Hz, J = 1.6 Hz, 6H, –CH(CH3)2), 1.50 (s, 9H,
–C(CH3)3), 2.25 (s, 6H, C2,C6–CH3), 2.65 (s, 3H, SCH3),
5.10 (m, 1H, –CH(CH3)2), 5.40 (s, 1H, C4–H of dihydro-
pyridine), 6.70 (bs, 1H, N1–H of dihydropyridine),
7.10–7.60 (m, 6H, 1H: C4–H of imidazole, 5H: –NH–C6H5);
MS: m/z 498.2 (M?). Anal. Calcd. for C26H34N4O4S: C,
62.63; H, 6.87; N, 11.24. Found: C, 62.55; H, 6.75; N, 11.56.
1
cm-1: NH 3339, C=O 1694; HNMR (CDCl3): d 1.25 (t,
J = 7.8 Hz, 3H, –CH2CH3), 2.20 (s, 6H, C2,C6–CH3), 2.60
(s, 3H, SCH3), 3.35 (s, 3H, –CH2CH2OCH3), 3.60 (t,
J = 6.4 Hz, 2H, –CH2CH2OCH3), 3.90–4.45 (m, 4H, 2H:
CH2CH2OCH3,2H: –CH2CH3), 5.40 (s, 1H, C4–H of dihy-
dropyridine), 6.95 (bs, 1H, N1–H of dihydropyridine),
7.10–7.70 (m, 6H, 1H: C4–H of imidazole, 5H: –NH–C6H5);
MS: m/z 486.0 (M?). Anal. Calcd. for C24H30N4O5S: C,
59.24; H, 6.21; N, 11.51. Found: C, 59.45; H, 6.19; N, 11.77.
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Med Chem Res (2012) 21:2749–2761
2759
Iso-propyl, methoxyethyl-1,4-dihydro-2,6-dimethyl-4-[2-
methylthio-1-(phenylamino)-1H-imidazole-5-yl]-3,5-pyr-
idinedicarboxylate (6l) Yield: 19%; m.p. = 160–162°C;
IR: (KBr) cm-1: NH 3334, C=O 1695; 1HNMR (CDCl3): d
1.25 (dd, J = 3.2 Hz, J = 1.6 Hz, 6H, –CH(CH3)2), 2.20
(s, 6H, C2,C6–CH3), 2.60 (s, 3H, SCH3), 3.35 (s, 3H, –CH2
CH2OCH3), 3.60 (t, J = 6.4 Hz, 2H, –CH2CH2OCH3),
4.25 (t, J = 6.4 Hz, 2H, –CH2CH2OCH3), 5.05 (m, 1H,
–CH(CH3)2), 5.40 (s, 1H, C4–H of dihydropyridine), 7.00
(bs, 1H, N1–H of dihydropyridine), 7.15–7.65 (m, 6H, 1H:
C4–H of imidazole, 5H: –NH–C6H5); MS: m/z 500.0 (M?).
Anal. Calcd. for C25H32N4O5S: C, 59.98; H, 6.44; N, 11.19.
Found: C, 59.75; H, 6.19; N, 11.33.
Pharmacology
Male albino guinea pigs (300–450 g) were killed by a blow
to the head. The intestine was removed above th eileo-
caecal junction and longitudinal smooth muscle segments
of 2-cm length were mounted under a resting tension of
400–500 mg. The segments were maintained at 37°C in a
20-ml jacketed organ bath containing oxygenated (100%
O2) physiological saline solution at the following milli-
molar composition: NaCl, 137; CaCl2, 1.8; KCl, 2.7;
MgSO4, 1.1; NaH2PO4, 0.4; NaHCO3, 12; and glucose, 5.
The muscles were equilibrated for 1 h with a solution
change for every 15 min. The concentrations were recor-
ded with a force displacement transducer (F-50) on a
NARCO physiograph. Test agents were dissolved in
dimethylsulfoxide and the same volume of the solvent was
used as a control in the absence of the test compounds. The
contractile response was taken as the 100% value for the
tonic (slow) component of the response. The contraction
was elicited with 80 mmol/l KCl. Test compounds were
cumulatively added and compound-induced relaxation of
contracted muscle was expressed as percent of control. The
IC50 of each compound was graphically determined from
the concentration–response curves (Bolger et al., 1983;
Rovnyak et al., 1992). Results are expressed as
mean s.e. Differences between the IC50 and response
percentage ratio values of compounds in each preparation
were compared by use of two-tailed Student’s t test. A
P value \0.05 was considered to be indicative of signifi-
cance. The calcium-channel antagonist activities (IC50) of
3a–e and 6a–o, determined as the contraction needed to
produce 50% relaxation of contracted guinea pig ileal
longitudinal smooth muscle, are presented in Table 1.
Iso-propyl, benzyl-1,4-dihydro-2,6-dimethyl-4-[2-methyl-
thio-1-(phenylamino)-1H-imidazole-5-yl]-3,5-pyridinedicar-
boxylate (6m) Yield: 34%; m.p. = 190–191°C; IR: (KBr)
cm-1: NH 3338, C=O 1693; 1HNMR (CDCl3): d 1.20 (dd,
J = 3.2 Hz, J = 1.6 Hz, 6H, –CH(CH3)2), 2.10 (s, 6H,
C2,C6–CH3), 2.55 (s, 3H, SCH3), 4.90–5.45 (m, 3H, 2H:
–CH2C6H5, 1H: –CH(CH3)2), 5.50 (s, 1H, C4–H of dihydro-
pyridine), 7.00–7.60 (m, 12H, 5H: –CH2C6H5, 1H: C4–H of
imidazole, 5H: –NH–C6H5, 1H: N1–H of dihydropyridine);
MS: m/z 532.0 (M?). Anal. Calcd. for C29H32N4O4S: C,
65.39; H, 6.06; N, 10.52. Found: C, 65.15; H, 6.15; N, 10.86.
Iso-propyl, phenpropyl-1,4-dihydro-2,6-dimethyl-4-[2-meth-
ylthio-1-(phenylamino)-1H-imidazole-5-yl]-3,5-pyridine-
dicarboxylate (6n) Yield: 23%; m.p. = 199–201°C; IR:
1
(KBr) cm-1: NH 3338, C=O 1695; HNMR (CDCl3): d
1.20 (dd, J = 3.2 Hz, J = 1.6 Hz, 6H, –CH(CH3)2), 1.70–
2.35 (m, 8H, 6H: C2,C6–CH3, 2H, –CH2CH2CH2C6H5),
2.40–2.90 (m, 5H, 3H: SCH3, 2H: –CH2CH2CH2C6H5),
4.10 (t, J = 6.5 Hz, 2H, –CH2CH2CH2C6H5), 5.05 (m, 1H,
–CH(CH3)2), 5.45 (s, 1H, C4–H of dihydropyridine),
6.90–7.65 (m, 12H, 5H: –CH2CH2CH2C6H5 1H: C4–H of
imidazole, 5H: –NH–C6H5, 1H: N1–H of dihydropyridine);
MS: m/z 560.0 (M?). Anal. Calcd. for C31H38N4O4S: C,
66.40; H, 6.47; N, 9.99. Found: C, 66.58; H, 6.29; N,
10.23.
Computational details
A Pentium IV personal computer (CPU at 2.54 GHz) with
windows XP operating system was used. The two-dimen-
sional structures of molecules were drawn using the
Hyperchem 7.0 software (Hypercube Inc.). All structures
were minimized with molecular mechanics/MM? calcula-
tions and the obtained geometries were used as starting
structures in semi-empirical PM3 optimizations in Hyper-
chem software. The molecular structures were optimized
using the Polak-Ribiere algorithm until the root mean
square gradient was 0.1 kcal mol-1 (HyperChem, Release
7.0 for Windows, Molecular Modeling System: Hyper-
Cube, 2002). The PM3-optimized structures were used as
the initial geometry for DFT optimizations. Gaussian 98
was used for B3LYP/6-31G* optimizations (Frisch et al.,
1998). Enthalpies of formation (DHf) of the studied com-
pounds were calculated in Hyperchem program after PM3
optimizations. B3LYP/6-31G*-optimized molecules were
Iso-propyl, cyclohexylmethyl-1,4-dihydro-2,6-dimethyl-4-
[2-methylthio-1-(phenylamino)-1H-imidazole-5-yl]-3,5-pyr-
idinedicarboxylate (6o) Yield: 21%; m.p. = 185–186°C;
1
IR: (KBr) cm-1: NH 3240, C=O 1694; HNMR (CDCl3):
d 0.7–2.00 (m, 17H, 6H: –CH(CH3)2, 11H: –CH2
C6H11), 2.15 (s, 6H, C2,C6–CH3), 2.60 (s, 3H, SCH3), 3.90
(d, J = 6.2 Hz, 2H, –CH2C6H11), 5.05 (m, 1H, –CH(CH3)2),
5.40 (s, 1H, C4–H of dihydropyridine), 7.0–7.70 (m, 7H, 1H:
C4–H of imidazole, 5H: –NH–C6H5, 1H: N1–H of dihydro-
pyridine); MS: m/z 538.7 (M?). Anal. Calcd. for
C29H38N4O4S: C, 64.66; H, 7.11; N, 10.40. Found: C, 64.75;
H, 7.25; N, 10.63.
123

2760
Med Chem Res (2012) 21:2749–2761
4,5-dichloroimidazolyl-1,4-DHP-based calcium channel block-
ers. J Iran Chem Soc 4:182–193
transferred to Hyperchem program to obtain the corre-
sponding values of DH0f .
Hemmateenejhad B, Miri R, Safarpour MA, Khoshneviszadeh M,
Edraki N (2005) Conformational analysis of some new deriv-
atives of 4-nitroimidazolyl-1,4-dihydropyridine-based calcium
channel blockers. J Mol Struct (Theohem) 717:139–152
HyperChem, Release 7.0 for Windows, Molecular Modeling System:
HyperCube, 2002. Visit Hyperchem official website at http://www.
hyperchem.com
Acknowledgment This study was supported by Isfahan Pharma-
ceutical Sciences Research Center.
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