Novel 2-methyl-6-arylpyridines carrying active pharmacophore 4,5-dihydro 2-pyrazolines: synthesis, antidepressant, and anti-tuberculosis evaluation

Article abstract of DOI:10.1007/s11164-017-3083-4

As part of the effort to develop new hybrid molecules for the treatment of depression and tuberculosis, a new series of novel 6-aryl-2-methyl-3-(5-aryl-4,5-dihydro-1H-pyrazol-3yl)pyridines 5a–l were synthesized. These compounds were screened for in vivo a

Full text of DOI:10.1007/s11164-017-3083-4

Res Chem Intermed
DOI 10.1007/s11164-017-3083-4
Novel 2-methyl-6-arylpyridines carrying active
pharmacophore 4,5-dihydro 2-pyrazolines: synthesis,
antidepressant, and anti-tuberculosis evaluation
P. V. Sowmya1 Boja Poojary1 B. C. Revanasiddappa2
•
•
•
M. Vijayakumar2 P. Nikil1 Vasantha Kumar3
•
•
Received: 31 March 2017 / Accepted: 12 July 2017
Ó Springer Science+Business Media B.V. 2017
Abstract As part of the effort to develop new hybrid molecules for the treatment of
depression and tuberculosis, a new series of novel 6-aryl-2-methyl-3-(5-aryl-4,5-
dihydro-1H-pyrazol-3yl)pyridines 5a–l were synthesized. These compounds were
screened for in vivo antidepressant activity following both modified forced swim-
ming test (FST) and tail suspension test (TST) at a test dose of 100 mg kg-1
.
Preliminary antidepressant screening of 5a–l revealed that compound 5b showed
significant decrease in the immobility time when compared to normal control group
in both FST and TST and was found to be more active than the standard imipramine
(10 mg kg-1, ip). Compounds 5f, 5h, 5j, and 5l displayed moderate to good
antidepressant activity. All synthesized compounds were also screened for their
in vitro anti-tuberculosis activity against Mycobacterium tuberculosis H37Rv strain.
Compounds 5a, 5c, and 5i showed highest anti-tubercular activity with MIC value
12.5 lg mL-1 comparable to the standard drugs.
Keywords Aryl pyridine Á 2-Pyrazoline Á Antidepressant activity Á Tail
suspension method Á Forced swim method Á Anti-TB activity
Electronic supplementary material The online version of this article (doi:10.1007/s11164-017-3083-4)
contains supplementary material, which is available to authorized users.
&
Boja Poojary
bojapoojary@gmail.com
1
2
Department of Chemistry, Mangalore University, Mangalagangothri, Karnataka 574199, India
Department of Pharmaceutical Chemistry, NGSM Institute of Pharmaceutical Sciences Paneer,
Deralakatte, Mangalore, Karnataka 575018, India
3
Department of Chemistry, SDM College(Autonomous), Ujire, Karnataka 574240, India
123

P. V. Sowmya et al.
Introduction
The discovery of new effective drugs for the treatment of depression remains a top
priority as depression affects approximately 21% of the world population. The
World Health Organization calculates that depression will become the second
leading cause of premature death or disability worldwide by the year 2020 [1].
Depression is a common chronic recurrent syndrome, characterised by loss of
energy, apathy, slow thinking and activity, as well as profound feelings of
gloominess, despair, and suicidal ideation [2]. At present, many antidepressant
agents [3] involved in medical therapy, such as tricyclic antidepressants (TCAs),
monoamine oxidase inhibitors (MAOIs), selective serotonin reuptake inhibitors
(SSRIs), and specific serotonin–noradrenaline reuptake inhibitors (SNRIs). These
drugs have proven to be effective in reducing depression, at the expense of some
undesirable side effects defeating their therapeutic efficacy. These clinical
limitations and adverse effects of currently used antidepressants, along with the
sharp rise of depressive cases in today’s scenario, necessitate continuous develop-
ment of novel, efficient, and safe drugs for the treatment of depression.
In recent years, there has been an increased interest in the treatment of one of the
most life-threatening diseases tuberculosis (TB), caused by Mycobacterium
tuberculosis (Mtb). The current frontline therapy for tuberculosis is associated
with severe side effects including hepatotoxicity, thrombocytopenia, itching, rashes,
fever, drug induced hepatitis [4, 5]. Additionally, the evolution of new virulent
forms such as multidrug resistant (MDR), extensively drug resistant (XDR), and the
recent cases of total drug resistant (TDR) strains increase the challenges to eliminate
TB worldwide [6]. Because of this, synthetic compounds with potential anti-TB
activity are receiving increased attention in biological research, medicine, and
pharmacology.
In this regard, considerable interest has been focused on the pyrazoline structure, as
it has shown to possess a broad spectrum of biological activities such as anti-
inflammatory, analgesic, anti-pyretic, psychoanaleptic, monoamine oxidase hypoten-
sive, anti-arrhythmic, muscle relaxant, tranquillizing, anticonvulsant, antibacterial,
and antidiabetic activities [7]. Prodrug-based monoamine oxidase (MAO) inhibitors
with hydrazide, hydrazine, and amine moiety, such as isocarboxazide [8], phenelzine
[9], and meclobemide [10, 11], show prominent antidepressant activity in clinical
trials. Moreover, many of the substituted hydrazines and hydrazides have been studied
as MAO inhibitors [12, 13]. Pyrazolines are also well known for their antidepressant
activityand anti-TB activity. Earlier studies bySoni et al. and Parmar etal. showed that
some substituted pyrazolines, such as 1,3,5-triphenyl-2-pyrazolines, which bear a
cyclic hydrazine moiety, have monoamine oxidase (MAO) inhibitory properties in the
rat brain [14, 15]. Later, Gokhan et al. [16] demonstrated that the antidepressant
activity of 1-N-substituted thiocarbamoyl-3-phenyl-5-thienyl-2-pyrazolines was
similar to that of MAO inhibitors. Moreover, from the literature it was found that
1-thiocarbamoyl-3,5-diphenyl-2-pyrazolines, 3,5-difenil-2-pyrazolines, 1,3,5-triph-
enyl-2-pyrazolines, bicyclic pyrazoline 8-thiocarbamoyl-7,8 diazabicyclo[4.3.0]non-
6-ene derivatives have antidepressant activity [7, 17–19]. Similarly, numerous
123

Novel 2-methyl-6-arylpyridines carrying active…
2-pyrazoline derivatives are reported to exhibit potent anti-tuberculosis activity
[20–23]. Investigating the chemical structure of the aforementioned compounds, the
2-pyrazolines can be considered as an important heterocyclic scaffold for the
development of potential antidepressant and anti-TB activity.
Like 2-pyrazolines, the role of pyridine in medicinal chemistry cannot be ignored
due to its diverse biological activities. The pyridine substructure is one of the most
prevalent heterocycle found in natural products, pharmaceuticals and functional
materials [24–26]. Additionally, non fused pyridines found in a large variety of
naturally occurring and synthetic compounds exhibit diverse bioactivities such as
insecticidal [27], antitubercular [28], antimicrobial, anticancer [29] and anti-
inflammatory [30] activity. On the other hand, several studies have demonstrated
that pyridines and pyrazoles conjugated with a range of heterocyclic moieties
[18, 31–35] are significant pharmacophores for antidepressant and anti-TB activity
(Fig. 1). However, during literature survey, it was found that an aryl pyridine ring
has not appeared to be linked with 4,5-dihydro-2-pyrazolines so far. It was
hypothesized that if these two pharmacophores were linked, it would generate a new
molecular model, which is a prerequisite for antidepressant and anti-TB activity and
may prove to be a breakthrough for the development of lead molecule for future
antidepressant and anti-TB research. In view of these findings, it was considered of
interest to undertake the synthesis of pyridyl pyrazoline derivatives, hoping that
these compounds might possess certain antidepressant and anti-TB activity.
Materials and methods
All the chemicals and reagents used for this study were obtained from Merck, S.D.
Fine, Sigma-Aldrich Company. The chemicals purchased were of analytical grade
and used directly. Melting point of the synthesized compounds was determined in
open glass capillaries and is uncorrected. FT (ATR)-IR absorption spectra were
recorded on Thermo Nicolet, Avatar 370 in range 4000–400 cm-1. 1H NMR spectra
and 13C NMR spectra were recorded on Bruker Avance III, 400 MHz and chemical
shift values were expressed in parts per million (ppm) downfield from the internal
standard, tetramethylsilane. Mass spectra were recorded on Water, synapt G2 high
detection mass spectrometry and are uncorrected. Elemental analysis was carried
out using CHNS Elementar Vario EL III. All reactions were monitored by TLC
analyses and were performed on silica plates; spots were visualised by UV light,
with exposure to iodine vapours. Hexane:ethyl acetate (3:1) and hex-
ane:dichloromethane (2:1) were the adopted solvent systems. Compounds were
prepared according to reported procedures.
General synthetic procedure for compounds, 3a–b
Acetyl acetone (1.1 mmol) and ammonium acetate (6 mmol) were added to the
solution of appropriate enaminone 2a–b (1 mmol) in glacial acetic acid (5 mmol).
The reaction mixture was heated under reflux for 3 h. Upon cooling, the solution
123

P. V. Sowmya et al.
O
O
R
O
Cl
O
N
N
O
N
N
N
F
N
N
Antidepressant activity
NH2
J.Med.Chem., 58, 7485 (2015)
S
Antidepressant activity
Arch. Pharm. Chem. Life Sci., 341, 209(2008)
O
H
R1
N
R3
R2
N
O
N
O
R4
CN
N
O
N
Antidepressant activity
Eur.J.Med.Chem., 44, 3480 (2009)
Anti-TB activity
Bioorg. Med. Chem. Lett., 20, 3169 (2010)
N
N
Cl
R
S
O
O
O
N
N
N
N
O
Anti-TB activity
H
Bioorg.Med.Chem.Lett.,26, 7065 (2016)
Antidepressant activity
Eur.J.Med.Chem., 36, 539-543 (2001)
Simplification
Combination
R
R1
N
N
N
H
Fig. 1 Literature reports for active pyridine, pyrazole based antidepressant and anti-TB derivatives and
strategy employed for designing pyridine based 2-pyrazolines
was poured into ice-cold water, the residue obtained was filtered, and washed with
hexane followed by water, recrystallized from ethanol [36].
1-(6-(3,5-Difluorophenyl)-2-methylpyridine-3-yl)ethanone (3a)
Brown solid (yield 92%), m.p. 50–52 °C; IR (ATR) (cm-1): 1680 (C=O), 1572
1
(C=N) and 1118 (C–F); H NMR (400 MHz, CDCl3, d ppm): 2.62 (s, 3H, –CO–
CH3), 2.83 (s, 3H, CH3), 6.88 (t, 1H, J = 8.4 Hz, C4-H of 3,5-F2C6H3), 7.62 (d, 3H,
J = 8.4 Hz, C2 and C6-H of 3,5-F2C6H3 and C4-H of pyridine), 8.07 (d, 1H,
J = 8 Hz, C5-H of pyridine); 13C NMR (100 MHz, CDCl3, d ppm): 199.74
1
([C=O), 164.6 and 162.2 (1C, d, JC–F = 247.1 Hz), 158.6, 155.7, 141.6, 138,
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Novel 2-methyl-6-arylpyridines carrying active…
2
131.8, 117.2, 110.2 and 109.9 (2C, d, JC–F = 26.1 Hz), 104.8 (1C, t,
2JC–F = 25.3 Hz), 29.3 (–CO–CH3), 25.1 (–CH3); EI-MS: m/z: (M ? 1, 248.2);
Anal. calcd for C14H11NF2O: C, 68.01; H, 4.48; N, 5.67. Found: C, 68.02; H, 4.47;
N, 5.66.
1-(6-(4-Bromophenyl)-2-methylpyridine-3-yl)ethanone (3b)
Off-white solid (yield 95%), m.p. 67–68 °C; IR (ATR) (cm-1): 1667 (C=O), 1569
(C=N) and 797 (C–Br); 1H NMR(400 MHz, CDCl3, d ppm): 2.52 (s, 3H, CO–CH3),
2.78 (s, 3H, CH3), 7.52 (d, 2H, J = 8.7 Hz, C3 and C5-H of 4-BrC6H4), 7.71 (d, 2H,
J = 8.6 Hz, C2 and C6-H of 4-BrC6H4), 7.91 (d, 1H, J = 7.1 Hz, C4-H of pyridine),
8.01 (d, 1H, J = 7.2 Hz, C5-H of pyridine); 13C NMR (100 MHz, CDCl3, d ppm):
199.8 ([C=O), 158.2, 157.7 138.2, 137.2, 135.3 131.7, 131.0, 124.4, 119, 29.23
(–CO–CH3), 23.75 (–CH3); EI-MS: m/z (M ? 1, 290.2; M ? 2, 292.06); Anal.
Calcd for C14H12NBrO: C, 57.95; H, 4.17; N, 4.83. Found: C, 57.93; H, 4.18; N,
4.81.
General synthetic procedure for chalcones (4a–l)
A mixture of 3a–b (0.06 mol), appropriate aromatic aldehyde (0.06 mol), and
sodium methoxide (0.12 mol) in methanol (50 mL) was stirred at room temperature
for about 12 h. It was then poured into ice cold water and neutralized with dil. HCl.
The precipitate obtained was collected by filtration, washed with petroleum ether
followed by water, dried and recrystallized from suitable solvents to afford 4a–l in
70–90% yield.
1-(6-(3,5-Difluorophenyl)-2-methylpyridin-3-yl)-3-(3,4-dimethoxyphenyl)prop-2-
en-1-one (4a)
Pale yellow powder (yield 82%), m.p. 124–125 °C; IR (ATR) (cm-1): 1658 (C=O),
1
1587 (C=C), 1437 (C=N), and 1116 (C–F); H NMR (400 MHz, CDCl3, d ppm):
1
2
2.84 (s, 3H, –CH3), 3.87 (s, 6H, –OCH3), 6.79 (tt, 1H, J = 8.8 Hz, J = 2.4 Hz,
C4-H of 3,5-F2C6H3), 6.98 (d, 1H, J = 8 Hz, C4-H of 3,4-(OCH3)2C6H3), 7.12
(d, 1H, J = 16 Hz, Hb of HbC=CHa), 7.24 (d, 1H, J = 2 Hz C2-H of 3,4-(OCH3)2
1
2
C6H3), 7.39–7.42 (dd, 1H, J = 8.2 Hz, J = 2 Hz, C6-H of 3,4-(OCH3)2C6H3),
7.46 (d, 1H, J = 8.2 Hz, C4-H of pyridine), 7.56 (dd, 2H, 1J = 8.2 Hz and
2J = 2.4 Hz, C2 and C6-H of 3,5-F2C6H3), 7.82–7.88 (overlapped dd, 2H,
J = 16 Hz and 8 Hz, Ha of HbC=CHa and C5-H of pyridine); 13C NMR
(100 MHz, CDCl3,
1JC–F = 246.2 Hz), 157.7, 155.3, 149.4, 149.1, 142.3, 141.8, 137, 133.5, 132.4,
125.8, 122.4, 117.1, 112.3, 110.4, 109.6 and 109.3 (2C, d, 2JC–F = 26.6 Hz), 104.7
d ppm): 193.9 (C=O), 164.6 and 162.2 (1C, d,
2
(1C, t, JC–F = 25.2 Hz), 56.2 (–OCH3), 56.1 (–OCH3), 24.2 (–CH3); EI-MS:
m/z (M ? 1, 396.16); Anal. calcd for C23H19NF2O3: C, 69.87; H, 4.84; N, 3.54.
Found: C, 69.74; H, 4.82; N, 3.49.
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P. V. Sowmya et al.
1-(6-(3,5-Difluorophenyl)-2-methylpyridin-3-yl)-3-(3,4,5-trimethoxyphenyl)prop-2-
en-1-one (4b)
Yellow solid (yield 94%), m.p. 141–142 °C; IR (ATR) (cm-1): 1657 (C=O), 1580
1
(C=C), 1425 (C=N), and 1114 (C–F); H NMR (400 MHz, CDCl3, d ppm): 2.81
(s, 3H, –CH3), 3.76 (s, 3H, 4-(OCH3), 3.79 (s, 6H, 3,5-(OCH3)2, 6.77 (tt, 1H,
1J = 8.8 Hz, 2J = 2.4 Hz, C4-H of 3,5-F2C6H3), 6.86 (s, 2H, C2 and C6-H of 3,4,5-
(OCH3)3C6H2), 7.14 (d, 1H, J = 16 Hz, Hb of HbC=CHa), 7.43 (d, 1H, J = 8.2 Hz,
C4-H of pyridine), 7.54 (dd, 2H, J = 8.4 Hz and 2.2 Hz, C2 and C6-H of
3,5-F2C6H3), 7.81–7.86 (overlapped dd, 2H, J = 16 Hz and 8 Hz Ha of HbC=CHa
and C5-H of pyridine); 13C NMR (100 MHz, CDCl3, d ppm): 193.2 (–C=O), 164.8
1
and 162.4 (1C, d, JC–F = 246.9 Hz), 157.7, 155.6, 153.6, 153.4, 149.6, 142.2,
141.6, 137.2, 132.6, 132.1, 125.9, 117.2, 110.1 and 109.8 (2C, d, 2JC–F = 25.9 Hz),
2
104.9 (1C, t, JC–F = 25 Hz), 103.2, 64.2 (–OCH3), 58.2 (–OCH3), 24.1(–CH3);
EI-MS: m/z (M ? 1, 425.53); Anal. calcd for C24H21NF2O4: C, 67.76; H, 4.98; N,
3.29. Found: C, 67.74; H, 4.99; N, 3.26.
3-(2-Chloro-6-fluorophenyl)-1-(6-(3,5-difluorophenyl)-2-methylpyridin-3-yl)prop-
2-en-1-one (4c)
Off white solid (yield 89%), m.p. 133–134 °C; IR (ATR) (cm-1): 1648 (C=O),
1592 (C=C), 1415 (C=N), 1110 (C–F) and 874 (C–Cl); 1H NMR (400 MHz, CDCl3,
1
2
d ppm): 2.65 (s, 3H, –CH3), 6.84 (tt, 1H, J = 8.4 Hz, J = 2.4 Hz, C4-H of 3,5-
F2C6H3), 7.11 (d, 1H, J = 15.8 Hz, Hb of HbC=CHa), 7.16 (m, 1H, C5-H of 2-Cl-6-
F-C6H3), 7.25–7.31 (m, 3H, C2, C6-H of 3,5-F2C6H3 and C3-H of 2-Cl-6-F-C6H3),
7.51 (dd, 1H, 1J = 8.1 Hz, 2J = 7.3 Hz, C4-H of 2-Cl-6-F-C6H3) 7.64–7.95 (m, 3H,
Ha of HbC=CHa, C4 and C5-H of pyridine); 13C NMR (100 MHz, CDCl3, d ppm):
1
192.9 (–C=O), 164.7 and 162.3 (1C, d, JC–F = 240 Hz), 161.9 and 159.5 (1C, d,
1JC–F = 240.4 Hz), 157.2, 155.2, 142.4, 140.9, 137.7, 134.5, 132.6, 132.2, 127.7,
125.9, 125.6, 117.2, 115.4, 110.5 and 110.1 (1C, d, 2JC–F = 25.3 Hz), 104.8 (1C, t,
2JC–F = 25.3 Hz), 23.9 (–CH3); EI-MS: m/z (M ? 1, 387.16; M ? 2, 389.16);
Anal. calcd for C21H13NClF3O: C, 65.04; H, 3.38; N, 3.61. Found: C, 65.10; H,
3.32; N, 3.51.
3-(2,3-Dichlorophenyl)-1-(6-(3,5-difluorophenyl)-2-methylpyridin-3-yl)prop-2-en-
1-one (4d)
Off-white solid (yield 96%), m.p. 101–102 °C; IR (ATR) (cm-1): 1641 (C=O),
1590 (C=C), 1415 (C=N), 1114 (C–F) and 841 (C–Cl); 1H NMR (400 MHz, CDCl3,
d ppm): 2.76 (s, 3H, –CH3), 7.12 (d, 1H, J = 16 Hz, Hb of HbC=CHa), 7.91–9.95 (t,
2H, J = 16 Hz and 8 Hz Ha of HbC=CHa, and C5-H of pyridine), 6.89 (tt, 1H,
2
1
1J = 8.8 Hz, J = 2.4 Hz, C4-H of 3,5-F2C6H3), 7.29 (d, 1H, J = 8 Hz, C6-H of
2,3-Cl2C6H3), 7.54 (dd, 1H, 1J = 8 Hz, 2J = 1.6 Hz, C5-H of 2,3-Cl2C6H3),
7.59–7.64 (m, 4H, C2, C6-H of 3,5-F2C6H3, C4-H of pyridine and C4-H of 2,3-
Cl2C6H3); 13C NMR (100 MHz, CDCl3, d ppm): 193.9 (–C=O), 164.7 and 162.2
1
(2C, d, JC–F = 246.8 Hz), 157.6, 155.6, 142.1, 141.8, 137.1, 134.9, 134.2, 133.5,
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Novel 2-methyl-6-arylpyridines carrying active…
2
132.8, 132, 129.2, 127.5, 125.9, 117.1, 110.2 and 109.9 (2C, d, JC–F = 26.3 Hz),
2
104.8 (1C, t, JC–F = 25.4 Hz), 24.0 (–CH3); EI-MS: m/z (M ? 1, 404.13; M ? 2,
406.12). Anal. calcd for C21H13NCl2F2O: C, 62.40; H, 3.24; N, 3.46. Found: C,
62.21; H, 3.32; N, 3.44.
3-(4-Bromophenyl)-1-(6-(3,5-difluorophenyl)-2-methylpyridin-3-yl)prop-2-en-1-one
(4e)
Off-white solid (yield 95%), m.p. 142–143 °C; IR (ATR) (cm-1): 1656 (C=O),
1
1560 (C=C), 1435 (C=N), 1467 (C=C), 1110 (C–F) and 741 (C–Br); H NMR
(400 MHz, CDCl3, d ppm): 2.67 (s, 3H, –CH3), 6.89 (tt, 1H, 1J = 8.2 Hz,
2J = 2.2 Hz, C4-H of 3,5-F2C6H3), 7.14 (d, 1H, J = 16 Hz, Hb of HbC=CHa),
7.2–7.50 (m, 4H, C2 and C6-H of 3,5-F2C6H3 and C3 and C5-H of 4-BrC6H4)
7.68–7.96 (m, 5H, Ha of HbC=CHa and C4, C5-H of pyridine and C2, C6-H of
4-BrC6H4); 13C NMR (100 MHz, CDCl3, d ppm): 193.4 (–C=O), 164.4 and 162.2
1
(1C, d, JC–F = 246.2 Hz), 157.5, 155.4, 142.2, 141.6, 137.3, 132.8, 132.1, 131.9.
128.4, 125.7, 124.1, 117.2, 109.9 and 109.7 (2C, d, 2JC–F = 25.3 Hz), 104.2 (1C, t,
2JC–F = 24.8 Hz), 24.0 (–CH3); EI-MS: m/z (M ? 1, 414.22; M ? 2, 415.12).
Anal. calcd for C21H14NBrF2O: C, 60.89; H, 3.41; N, 3.38. Found: C, 60.78; H,
3.39; N, 3.48.
1-(6-(3,5-Difluorophenyl)-2-methylpyridin-3-yl)-3-(4-fluorophenyl)prop-2-en-1-one
(4f)
Brown solid (yield 80%), m.p. 93–94 °C; IR (ATR) (cm-1): 1664 (C=O), 1548
1
(C=C), 1453 (C=N), 1462 (C=C) and 1116 (C–F); H NMR (400 MHz, CDCl3, d
1
2
ppm): 2.72 (s, 3H, –CH3), 6.88 (tt, 1H, J = 8.8 Hz, J = 2.4 Hz, C4-H of 3,5-
F2C6H3), 7.11 (d, 1H, J = 16 Hz, Hb of HbC=CHa), 7.26–7.64 (m, 5H, C2 and C6-H
of 3,5-F2C6H3, C3 and C5-H of 4-FC6H4 and C4-H of pyridine) 7.72–7.98 (m, 4H,
Ha of HbC=CHa and C5-H of pyridine and C2, C6-H of 4-FC6H4); 13C NMR
(100 MHz, CDCl3,
d ppm): 193.2 (–C=O), 164.9 and 162.5 (1C, d,
1
1JC–F = 246.9 Hz), 164.5 and 162.1 (1C, d, JC–F = 247.1 Hz), 157.8, 155.4,
142.4, 141.2, 137.7, 132.8, 130.9, 130.1, 125.6, 117.4, 116.2 and 116.0 (2C, d,
2JC–F = 25.3 Hz), 110.3 and 110.0 (2C, d, JC–F = 26.3 Hz), 104.9 (1C, t,
2JC–F = 25.1 Hz), 23.8 (–CH3); EI-MS: m/z (M ? 1, 353.34); Anal. calcd for
C21H14NF3O: C, 71.38; H, 3.99; N, 3.96. Found: C, 71.33; H, 3.89; N, 3.89.
2
1-(6-(4-Bromophenyl)-2-methylpyridin-3-yl)-3-(3,4-dimethoxyphenyl)prop-2-en-1-
one (4g)
Pale yellow solid (yield 90%), m.p. 120–122 °C; IR (ATR) (cm-1): 1673 (C=O),
1578 (C=C), 1452 (C=N), 1463 (C=C) and 776 (C–Br); 1H NMR (400 MHz,
CDCl3, d ppm): 2.87 (s, 3H, –CH3), 3.89(s, 6H, –OCH3), 6.79 (d, 1H, J = 16 Hz,
Hb of HbC=CHa), 7.53–7.70 (m, 6H, C4-H of pyridine, C3 and C5-H of 4-BrC6H4
and 3,4-(OCH3)2C6H3), 7.64–7.97(m, 4H, Ha of HbC=CHa and C2, C6-H of
4-BrC6H4 and C5-H of pyridine); 13C NMR (100 MHz, CDCl3, d ppm): 193.9
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P. V. Sowmya et al.
(–C=O), 157.7, 157.2, 149.4, 148.9, 142.0, 137.3, 137.0, 132.2, 128.5, 128.4, 126.8,
125.7, 124.3, 118.6, 116.4, 111.3, 109.8, 56.1 and 56.0 (–OCH3), 26.5 (–CH3); EI-
MS: m/z (M ? 1, 438.21; M ? 2, 439.21); Anal. calcd for C23H20NBrO3: C, 63.02;
H, 4.60; N, 3.20. Found: C, 63.12; H, 4.54; N, 3.24.
1-(6-(4-Bromophenyl)-2-methylpyridin-3-yl)-3-(3,4,5-trimethoxyphenyl)prop-2-en-
1-one (4h)
Yellow solid (yield 93%), m.p. 80–82 °C; IR (ATR) (cm-1): 1672 (C=O), 1585
(C=C), 1462 (C=N), 1483 (C=C) and 782 (C–Br); 1H NMR (400 MHz, CDCl3-d, d
ppm): 2.82 (s, 3H, –CH3), 3.76 (s, 3H, 4-OCH3), 3.79 (s, 6H, 3.5-OCH3), 6.86 (s,
2H, C2 and C6-H of 3,4,5-(OCH3)3C6H2), 7.12 (d, 1H, J = 16 Hz, Hb of HbC=CHa),
7.50–7.70 (m, 3H, C4-H of pyridine and C3, C5-H of 4-BrC6H4), 7.86–7.97 (m, 4H,
Ha of HbC=CHa and C2, C6-H of 4-BrC6H4 and C5-H of pyridine); 13C NMR
(100 MHz, CDCl3, d ppm): 193.8 (–C=O), 157.7, 157.4, 153, 152.9, 149.6, 142.2,
137.2, 137, 132.4, 131, 130.2, 129.1, 125.2, 124.2, 116.8, 106.3, 60.5 (–OCH3), 56.2
(–OCH3), 24.3 (–CH3); EI-MS: m/z (M ? 1, 467.06; M ? 2, 469.08); Anal. calcd
for C24H22NBrO4: C, 61.55; H, 4.73; N, 2.99. Found: C, 61.49; H, 4.71; N, 2.93.
1-(6-(4-Bromophenyl)-2-methylpyridin-3-yl)-3-(2-chloro-6-fluorophenyl)prop-2-en-
1-one (4i)
Brown solid (yield 82%), m.p. 108–109 °C; IR (ATR) (cm-1): 1654 (C=O), 1575
(C=C), 1458 (C=N), 1473 (C=C), 948 (C–Cl) and 776 (C–Br); 1H NMR (400 MHz,
CDCl3-d, d ppm): 2.79 (s, 3H, –CH3) 7.14 (d, 1H, J = 16 Hz, Hb of HbC=CHa),
7.18–7.50 (m, 5H, C3, C5-H of 4-BrC6H4 and C3, C4 and C5-H of 3-Cl-6-FC6H3),
7.69–7.98 (m, 5H, Ha of HbC=CHa and C4, C5-H of pyridine and C2, C6-H of
4-BrC6H4); 13C NMR (100 MHz, CDCl3, d ppm): 193.9 (–C=O), 164.2 and 162.2
1
(1C, d, JC–F = 244.9 Hz), 157.7, 157.2, 141.2, 137.4, 137.1, 134.2, 131.9, 131,
2
130.9, 129.4, 126.1, 125.0, 124.1, 117, 116.4 and 116.1 (1C, d, JC–F = 25.7 Hz),
26.1 (–CH3); MS-EI: m/z (M ? 1, 428.06; M ? 2, 430.09; M ? 4, 432.05); Anal.
calcd for C21H14NBrClFO: C, 58.56; H, 3.28; N, 3.25. Found: C, 58.62; H, 3.27; N,
3.30.
1-(6-(4-Bromophenyl)-2-methylpyridin-3-yl)-3-(2,3-dichlorophenyl)prop-2-en-1-
one (4j)
Off-white solid (yield 92%), m.p. 152–153 °C; IR (ATR) (cm-1): 1666 (C=O),
1585 (C=C), 1442 (C=N), 1453 (C=C), 813 (C–Cl) and 778 (C–Br); H NMR
1
(400 MHz, CDCl3, d ppm): 2.76 (s, 3H, –CH3) 7.12 (d, 1H, J = 16 Hz, Hb of
HbC=CHa), 7.53–7.70 (m, 6H, C4-H of pyridine, C3, C5-H of 4-BrC6H4 and 2,3-
Cl2C6H3) 7.89–7.97 (m, 4H, Ha of HbC=CHa and C2, C6-H of 4-BrC6H4 and C5-H
of pyridine); 13C NMR (100 MHz, CDCl3, d ppm): 193.9 (–C=O), 157.7, 157.1,
141, 137.1, 137, 135.0, 134.2, 133.4, 132, 131, 129, 128.8, 128, 127, 125, 124.2,
116, 24.1 (–CH3); EI-MS: m/z (M ? 1, 446.06; M ? 2, 448.06; M ? 4, 450.05).
123

Novel 2-methyl-6-arylpyridines carrying active…
Anal. calcd for C21H14NCl2O: C, 56.41; H, 3.16; N, 3.13. Found: C, 56.42; H, 3.12;
N, 3.16.
3-(4-Bromophenyl)-1-(6-(4-bromophenyl)-2-methylpyridin-3-yl)prop-2-en-1-one
(4k)
Off-white solid (yield 84%), m.p. 132–133 °C; IR (ATR) (cm-1): 1656 (C=O),
1552 (C=C), 1439 (C=N), 1453 (C=C), and 847 (C–Br); 1H NMR (400 MHz,
CDCl3-d, d ppm): 2.87 (s, 3H, –CH3) 7.14 (d, 1H, J = 16 Hz, Hb of HbC=CHa),
7.32–7.48 (m, 4H, C3 and C5-H of 4-BrC6H4), 7.53–7.72 (m, 5H, C2, C6-H of
4-BrC6H4 and C4-H of pyridine), 7.89–7.95 (d (overlapped), 2H, Ha of HbC=CHa
and C5-H of pyridine); 13C NMR (100 MHz, CDCl3, d ppm): 193.2 (–C=O), 157.9,
156.9, 141.6, 137.9, 137.7, 137.1, 131.8, 131.5, 131.4, 129.6, 128.9, 124.3, 124.2,
123.2, 116.9, 24.3 (–CH3); EI-MS: m/z (M ? 1, 454.36; M ? 2, 456.36; M ? 4
458.35); Anal. calcd for C21H15NBr2O: C, 55.17; H, 3.31; N, 3.06. Found: C, 55.27;
H, 3.32; N, 3.04.
1-(6-(4-Bromophenyl)-2-methylpyridin-3-yl)-3-(4-fluorophenyl)prop-2-en-1-one
(4l)
Brown powder (yield 77%), m.p. 139–140 °C; IR (ATR) (cm-1): 1647 (C=O), 1568
(C=C), 1442 (C=N), 1463 (C=C), 1108 (C–F) and 867 (C–Br); 1H NMR (400 MHz,
CDCl3, d ppm): 2.76 (s, 3H, –CH3) 7.15 (d, 1H, J = 16 Hz, Hb of HbC=CHa), 7.23
(m, 2H, C3 and C5-H of 4-FC6H4), 7.47–7.78 (m, 5H, 4H’s of 4-BrC6H4 and C4-H of
pyridine), 7.80–7.98 (m, 4H, Ha of HbC=CHa and C2, C6-H of 4-FC6H4 and C5-H of
pyridine); 13C NMR (100 MHz, CDCl3, d ppm): 193.9 (–C=O), 164.4 and 162.2
(1C, d, C–F,1J = 248.2 Hz), 157.2, 156.2, 142.2, 137.7, 137.2, 131.8, 131.4, 130.9,
129.9, 128.9, 124.6, 123.3, 116.7, 114.8 and 114.6 (2C, d, C–F,2J = 25.2 Hz), 23.9
(–CH3); EI-MS: m/z (M ? 1, 396.21; M ? 2, 397.21); Anal.calcd for C21H15
NBrFO: C, 63.65; H, 3.82; N, 3.53. Found: C, 63.67; H, 3.79; N, 3.51.
General synthetic procedure for 2-pyrazolines (5a–l)
Hydrazine hydrate (98%) (2 mL) was added to a suspension of appropriate chalcone
(4a–l) (5 mmol) in ethanol and a catalytic amount of dil. HCl. The reaction mixture
was heated under reflux condition for 3 h. Completion of the reaction was
monitored using TLC. After completion of the reaction, reaction mixture was
concentrated to small volume and left at room temperature for 24 h to give solid
compound. It was then filtered and recrystallized with ethanol to afford a
corresponding
3yl)pyridines (5a–l) in 57–93%.
yield
6-(aryl)-2-methyl-3-(5-(aryl)-4,5-dihydro-1H-pyrazol-
123

P. V. Sowmya et al.
6-(3,5-Difluorophenyl)-2-methyl-3-(5-(3,4-dimethoxyphenyl)-4,5-dihydro-1H-
pyrazol-3yl)pyridine (5a)
Off-white solid (yield 92%), m.p. 117–119 °C; IR (ATR) (cm-1): 32,760
(NH-sharp), 1585 (C=N), 1457 (C=C) and 1121 (C–F); 1H NMR (400 MHz,
CDCl3, d ppm): 2.89 (s, 3H, CH3), 3.08 (dd, 1H, HB, HBA = 16 Hz, HBX = 9.2
Hz), 3.48 (dd, 1H, HA, HAB = 16.2 Hz, HAX = 10.6 Hz), 3.88 (s, 6H, 3,4-
(OCH3)2), 4.90 (t, 1H, HX, J = 9.6 Hz), 6.83–6.86 (m, 2H, C2-H of 3,4-(OCH3)2
1
2
C6H2 and C4-H of 3,5-F2C6H3), 6.90–6.92 (dd, 1H, J = 8.4 Hz, J = 2 Hz, C6-H
of 3,4-(OCH3)2C6H2), 6.96 (d, 1H, J = 8.2 Hz, C5-H of 3,4-(OCH3)2C6H2) 7.55 (d,
1H, J = 8 Hz, C4-H of pyridine), 7.60 (m, 2H, C2 and C6-H of 3,5-F2C6H3), 7.70 (d,
1H, J = 8.4 Hz, C5-H of pyridine); 13C NMR (100 MHz, CDCl3, d ppm): 164.6 and
1
162.2 (2C, d, JC–F = 246.2 Hz), 157.2, 152.4, 149.6, 149.3, 148.8, 142.3, 136.2,
134.6, 127.1, 118.6, 117.3, 111.3, 109.7 and 109.4 (2C, d, 2JC–F = 26.2 Hz), 109.2,
2
104 (1C, t, JC–F = 25.5 Hz) 64.1, 55.9, 43.4, 26.3 (CH3); EI-MS: m/z (M ? 1,
409.53); Anal.calcd. for C23H21N3F2O2: C, 67.47; H, 5.17; N, 10.26. Found: C,
67.42; H, 5.07; N, 10.28.
6-(3,5-difluorophenyl)-2-methyl-3-(5-(3,4,5-trimethoxyphenyl)-4,5-dihydro-1H-
pyrazol-3-yl) pyridine (5b)
Off-white solid (yield 90%), m.p. 124–126 °C; IR (ATR) (cm-1): 3290 (NH-sharp),
1
1587 (C=N), 1453 (C=C) and 1123 (C–F); H NMR (400 MHz, CDCl3, d ppm):
2.82 (s, 3H, CH3), 3.02 (dd, 1H, HB, HBA = 16.2 Hz, HBX = 9.8 Hz), 3.45 (dd, 1H,
HA, HAB = 16 Hz, HAX = 10.4 Hz), 3.77 (s, 3H, 4-OCH3), 3.79 (s, 6H, 3,5-
(OCH3)2, 4.81 (t, 1H, HX, J = 10.2 Hz), 6.56 (s, 2H, C2 and C6-H of 3,4,5-
(OCH3)3C6H2), 6.75 (tt, 1H, J = 8.4 Hz and 2.4 Hz, C4-H of 3,5-F2C6H3), 7.47 (d,
1H, J = 8 Hz, C4-H of pyridine), 7.52 (m, 2H, C2 and C6-H of 3,5-F2C6H3); 13C
NMR (100 MHz, CDCl3, d ppm): 164.6 and 162.2 (2C, d, 1JC–F = 246 Hz), 157.2,
153.6, 152.4, 149.6, 142.3, 137.7, 136.9 136.2, 127.0, 117.3, 109.7 and 109.4 (2C, d,
2
2JC–F = 26.2 Hz), 104.1 (1C, t, JC–F = 25.2 Hz), 103.2, 64.6, 60.8, 56.1, 43.6,
26.3 (–CH3); EI-MS: m/z (M ? 1, 440.27); Anal. calcd for C24H23N3F2O3: C,
65.59; H, 5.28; N, 9.56. Found: C, 65.53; H, 5.22; N, 9.59.
3-(5-(2-chloro-6-fluorophenyl)-4,5-dihydro-1H-pyrazol-3-yl)-6-(3,5-difluorophenyl)-
2-methylpyridine (5c)
Brown crystals (yield 69%), m.p. 136–138 °C; IR (ATR) (cm-1): 3293 (NH-sharp),
1581 (C=N), 1453 (C=C), 1123 (C–F) and 743 (C–Cl); 1H NMR (400 MHz, CDCl3,
d ppm): 2.84 (s, 3H, CH3), 3.04 (dd, 1H, HB, HBA = 16 Hz, HBX = 9.6 Hz), 3.42
(dd, 1H, HA, HAB = 16.2 Hz, HAX = 10.6 Hz), 4.84 (t, 1H, HX, J = 10.4 Hz), 6.73
(tt, 1H, J = 8.2 Hz and 2.4 Hz, C4-H of 3,5-F2C6H3), 6.92 (m, 1H, C5-H of 2-Cl-6-
F-C6H3), 7.13 (dd, 1H, J = 8.2 Hz and 1.4 Hz, C3-H of 2-Cl-6-F-C6H3), 7.33–7.37
(m, 3H, C2 and C6-H of 3,5-F2C6H3 and C4-H of 2-Cl-6-F-C6H3), 7.47 (d, 1H,
J = 8 Hz, C4-H of pyridine), 7.68 (d, 1H, J = 8.4 Hz, C5-H of pyridine); 13C NMR
1
(100 MHz, CDCl3, d ppm): 164.6 and 162.2 (2C, d, JC–F = 246 Hz), 161.8 and
123

Novel 2-methyl-6-arylpyridines carrying active…
1
158.1 (1C, d, JC–F = 240 Hz) 157.3, 152.6, 149.2, 136.4, 134.6, 133.4, 129.8,
129.2, 127.1, 126.3, 117.3, 114.2 and 113.9 (1C, d, 2JC–F = 25.5 Hz), 109.7, 109.4
2
2
and 109.1 (2C, d, JC–F = 24.6 Hz), 104.1 (1C, t, JC–F = 25.2 Hz), 64.2, 43.6,
26.2 (–CH3); MS-EI: m/z (M ? 1, 401.91; M ? 2, 403.91); Anal. calcd for
C21H15N3ClF3: C, 62.77; H, 3.76; N, 10.46. Found: C, 62.57; H, 3.66; N, 10.56.
3-(5-(2,3-Dichlorophenyl)-4,5-dihydro-1H-pyrazol-3-yl)-6-(3,5-difluorophenyl)-2-
methylpyridine (5d)
Off-white crystals (yield 93%), m.p. 125–127 °C; IR (ATR) (cm-1): 3308 (NH-
sharp), 1585 (C=N), 1443 (C=C), 1112 (C–F) and 811 (C–Cl); 1H NMR (400 MHz,
CDCl3, d ppm): 2.87 (s, 3H, –CH3), 2.95 (dd, 1H, HB, HBA = 16.2 Hz,
HBX = 9.6 Hz), 3.74 (dd, 1H, HA, HAB = 16.4 Hz, HAX = 10.4 Hz), 5.37 (t, 1H,
1
2
HX, J = 10.2 Hz), 6.83 (tt, 1H, J = 8.8 Hz, J = 2.4 Hz, C4-H of 3,5-F2C6H3),
7.26 (t, 1H, J = 4 Hz C6-H of 2,3,-(Cl2C6H3), 7.43 (dd, 1H,1 J = 8 Hz,
2J = 1.2 Hz, C5-H of 2,3-Cl2C6H3) 7.51 (d, 1H, J = 8.4 Hz, C4-H of pyridine),
7.56–7.59 (m, 3H, C2 and C6-H of 3,5-F2C6H3 and C4-H of 2,3-Cl2C6H3), 7.68 (d,
1H, J = 8.4 Hz, C5-H of pyridine); 13C NMR (100 MHz, CDCl3, d ppm): 164.6 and
1
162.2 (2C, d, JC–F = 240 Hz), 157.3, 152.5, 149.3, 141.9, 137.6, 136.3, 133.4,
130.9, 129.6, 127.8, 126.8, 125.1, 117.3, 109.8 and 109.5 (2C, d, 2JC–F = 26.3 Hz),
109.7 104.1 (1C, t, 2JC–F = 25.3 Hz), 61.4, 41.7, 26.2 (–CH3); EI-MS: m/z (M ? 1
418.16; M ? 2 420.16; M ? 4 422.16); Anal. calcd for C21H15N3Cl2F2: C, 60.30;
H, 3.61; N, 10.05. Found: C, 60.29; H, 3.63; N, 10.10.
3-(5-(4-bromophenyl)-4,5-dihydro-1H-pyrazol-3-yl)-6-(3,5-difluorophenyl)-2-
methylpyridine (5e)
Off-white crystals (yield 74%), m.p. 90–92 °C; IR (ATR) (cm-1): 3301 (NH-sharp),
1587 (C=N), 1453 (C=C), 1123 (C–F) and 767 (C–Br); 1H NMR (400 MHz, CDCl3,
d ppm): 2.89 (s, 3H, –CH3), 3.10 (dd, 1H, HB, HBA = 16 Hz, HBX = 9.8 Hz), 3.72
(dd, 1H, HA, HAB = 16.2 Hz, HAX = 10.2 Hz), 5.36 (t, 1H, HX, J = 10.2 Hz), 6.86
(tt, 1H, J = 8.6 Hz and 2.4 Hz, C4-H of 3,5-F2C6H3), 7.35–7.42 (m, 6H, C2 and
C6-H of 3,5-F2C6H3 and 4-BrC6H4) 7.52 (d, 1H, J = 8.2 Hz, C4-H of pyridine),
7.69 (d, 1H, J = 8 Hz, C5-H of pyridine); 13C NMR (100 MHz, CDCl3, d ppm):
1
164.6 and 162.2 (2C, d, JC–F = 244 Hz), 157.2, 154, 149.6, 139.4, 137.7, 136.2,
133.4, 128.8, 127.7, 123.4, 117.4, 109.6 and 109.3 (2C, d, 2JC–F = 24.6 Hz), 104.2
2
(1C, t, JC–F = 25.6 Hz), 62.1, 43.2, 26.2 (CH3); EI-MS: m/z (M ? 1, 427.10;
M ? 2, 429.10); Anal. calcd for C21H16N3BrF2: C, 58.89; H, 3.77; N, 9.81. Found:
C, 58.86; H, 3.65; N, 9.80.
6-(3,5-difluorophenyl)-2-methyl-3-(5-(4-fluorophenyl)-4,5-dihydro-1H-pyrazol-3-
yl)pyridine (5f)
Off-white crystals (yield 93%), m.p. 108–112 °C; IR (ATR) (cm-1): 3299 (NH-
1
sharp), 1578 (C=N), 1453 (C=C) and 1112 (C–F); H NMR (400 MHz, CDCl3, d
ppm): 2.36 (s, 3H, –CH3), 3.02 (dd, 1H, HB, HBA = 16.2 Hz, HBX = 9.6 Hz), 3.23
123

P. V. Sowmya et al.
(dd, 1H, HA, HAB = 16.2 Hz, HAX = 10.6 Hz), 5.32 (t, 1H, HX, J = 10.6 Hz), 6.79
(tt, 1H, 1J = 8.8 Hz, 2J = 2.2 Hz, C4-H of 3,5-F2C6H3), 6.98 (sextet, 2H,
2
1
1J = 8.8 Hz, J = 1.2 Hz, C3 and C5-H of 4-FC6H4), 7.30 (t, 2H, J = 8 Hz, C2
and C6-H of 4-FC6H4), 7.47 (m, 2H, C2 and C6-H of 3,5-F2C6H3), 7.68 (d, 1H,
J = 8.4 Hz, C4-H of pyridine), 7.98 (d, 1H, J = 8.2 Hz, C5-H of pyridine); 13C
1
NMR (100 MHz, CDCl3, d ppm): 164.6 and 162.2 (2C, d, JC–F = 240 Hz), 162.8
and 160.4 (1C, d, 1JC–F = 240 Hz), 157.2, 153.1, 149.3, 140.1, 136.6, 132.2, 128.6,
2
127.2, 117.4, 114.6 and 114.4 (1C, d, JC–F = 26.2 Hz), 109.8 and 109.5 (2C, d,
2JC–F = 25.8 Hz), 104.2 (1C, t, JC–F = 25.3 Hz), 63.9, 43, 26.3 (–CH3); EI-MS:
m/z (M ? 1, 367.57); Anal. calcd for C21H16N3F3: C, 68.66; H, 4.39; N, 11.44.
Found: C, 68.56; H, 4.42; N, 11.41.
2
6-(4-Bromophenyl)-2-methyl-3-(5-(3,4-dimethoxyphenyl)-4,5-dihydro-1H-pyrazol-
3-yl)pyridine (5g)
Off-white solid (yield 87%), m.p. 116–119 °C; IR (ATR) (cm-1): 3321 (NH-sharp),
1585 (C=N), 1458 (C=C), 1255 (C-O) and 750 (C–Br); 1H NMR (400 MHz,
CDCl3, d ppm): 2.89 (s, 3H, –CH3), 3.09 (dd, 1H, HB, HBA = 16 Hz,
HBX = 9.6 Hz), 3.50 (dd, 1H, HA, HAB = 16.2 Hz, HAX = 10.4 Hz), 3.87 (s, 6H,
–OCH3), 4.88 (t, 1H, HX, J = 10 Hz), 6.84 (d, 1H, J = 8 Hz, C5-H of 3,4-
1
2
(OCH3)2C6H3) 6.89–6.91 (dd, 1H, J = 8 Hz, J = 2 Hz, C6-H of 3,4-(OCH3)2-
C6H3), 6.96 (d, 1H, J = 2 Hz, C2-H of 3,4-(OCH3)2C6H3), 7.53–7.66 (m, 4H, 4H’s
of 4-BrC6H4), 7.93 (dd, 1H, J = 6.8 Hz, C4 and C5-H of pyridine); 13C NMR
(100 MHz, CDCl3, d ppm): 157.1, 153.9, 149.9, 149.3, 148.7, 137.8, 136.2, 134.7,
131.84, 128.4, 126.3, 123.4, 118.6, 117.0, 111.3, 109.2, 64.1, 56.0, 55.9, 43.5, 26.3
(–CH3); EI-MS: m/z (M ? 1, 452.19; M ? 2, 453.18); Anal. calcd for C23H22N3-
BrO2: C, 61.07; H, 4. 90; N, 9.29. Found: C, 61.12; H, 4. 87; N, 9.31.
6-(4-Bromophenyl)-2-methyl-3-(5-(3,4,5-trimethoxyphenyl)-4,5-dihydro-1H-pyrazol-
3-yl)pyridine (5h)
Off-white crystals (yield 86%), m.p. 134–136 °C; IR (ATR) (cm-1): 3294 (NH-
sharp), 1586 (C=N), 1457 (C=C), 1239 (C-O) and 687 (C–Br); 1H NMR (400 MHz,
CDCl3, d ppm): 2.87 (s, 3H, –CH3), 2.95 (dd, 1H, HB, HBA = 16.4 Hz,
HBX = 10.2 Hz), 3.42 (dd, 1H, HA, HAB = 16.2 Hz, HAX = 10.4 Hz), 3.76 (s,
3H, 4-OCH3), 3.78 (s, 6H, 3,5-(OCH3)2, 4.91 (t, 1H, HX, J = 10.3 Hz), 6.53 (s, 2H,
C2 and C6-H of 3,4,5-(OCH3)3C6H2), 7.52–7.58 (m, 3H, C3 and C5-H of 4-BrC6H4
1
2
and C4-H of pyridine), 7.60 (dd, 2H, J = 8.5 Hz, J = 1.4 Hz, C2 and C6-H of
4-BrC6H4 ), 7.96 (d, 1H, J = 6 Hz, C5-H of pyridine); 13C NMR (100 MHz, CDCl3,
d ppm): 161.0, 159.8, 152.6, 152.4, 151.1, 138.5, 137.8, 135.3, 132.2, 131.0, 127.8,
123.4, 125.2, 117.2, 64.6, 61.4, 56.2, 42.8, 26.3 (–CH3); EI-MS: m/z (M ? 1,
482.40; M ? 2, 483.16); Anal. calcd for C24H24N3BrO3: C, 59.76; H, 5.01; N, 8.71.
Found: C, 59.56; H, 5.10; N, 8.61.
123

Novel 2-methyl-6-arylpyridines carrying active…
6-(4-Bromophenyl)-2-methyl-3-(5-(2-chloro-6-fluorophenyl)-4,5-dihydro-1H-pyrazol-
3-yl)pyridine (5i)
Off-white solid (yield 79%), m.p. 140–142 °C; IR(ATR) (cm-1): 3298 (NH-sharp),
1586 (C=N), 1443 (C=C), 1123 (C–F), 997 (C–Cl) and 799 (C–Br); H NMR
1
(400 MHz, CDCl3, d ppm): 2.36 (s, 3H, –CH3), 2.98 (dd, 1H, HB, HBA = 16.4 Hz,
HBX = 10.4 Hz), 3.28 (dd, 1H, HA, HAB = 16.2 Hz, HAX = 10.2 Hz), 5.32 (t, 1H,
HX, J = 10.3 Hz), 6.92 (dd, 1H, 1J = 8.3 Hz, 2J = 1.2 Hz, C5-H of 2-Cl-6-FC6H3),
7.01 (dd, 1H, 1J = 8.6 Hz, 2J = 1.2 Hz, C3-H of 2-Cl-6-FC6H3) 7.32 (t, 1H,
1J = 8.3 Hz, C4-H of 2-Cl-6-FC6H3), 7.52 (d, 2H, J = 8.5 Hz, C3 and C5-H of
4-BrC6H4), 7.62–7.67 (m, 3H, C2 and C6-H of 4-BrC6H4 and C4-H of pyridine), 7.96
(d, 1H, J = 5.7 Hz, C5-H of pyridine); 13C NMR (100 MHz, CDCl3, d ppm): 164.1,
162.3, 158.8, 151.6, 137.3, 132.6, 129.8, 127.6, 127.4, 127.0, 124.3, 124.02, 122.5,
121.3, 114.2, 60.0, 43.08, 23.5 (–CH3); EI-MS: m/z (M ? 1, 443.03; M ? 2,
445.03; M ? 4, 447.03); Anal. calcd for C21H16N3BrClF: C, 56.71; H, 3.63; N,
9.45. Found: C, 56.70; H, 3.58; N, 9.30.
6-(4-Bromophenyl)-2-methyl-3-(5-(2,3-dichlorophenyl)-4,5-dihydro-1H-pyrazol-3-
yl)pyridine (5j)
Off-white solid (yield 80%), m.p. 96–99 °C; IR (ATR) (cm-1): 3279 (NH-sharp),
1576 (C=N), 1453 (C=C), 799 (C–Cl) and 583 (C–Br); 1H NMR (400 MHz, CDCl3,
d ppm): 2.88 (s, 3H, –CH3), 2.94 (dd, 1H, HB, HBA = 16.4 Hz, HBX = 10.4 Hz),
3.73 (dd, 1H, HA, HAB = 16.4 Hz, HAX = 10.8 Hz), 5.37 (t, 1H, HX, J = 10.0 Hz),
1
2
7.22–7.24 (dd, 1H, J = 7.8 Hz, J = 1.2 Hz, C6-H of 2,3-Cl2C6H3), 7.40–7.43
(dd, 1H, 1J = 8 Hz, 2J = 1.6 Hz, C5-H of 2,3-Cl2C6H3) 7.52–7.65 (m, 5H, 4H’s of
1
2
4-BrC6H4 and C4-H of 2,3-Cl2C6H3), 7.92 (dd, 2H, J = 6.8 Hz, J = 1.6 Hz, C4
and C5-H of pyridine), 13C NMR (100 MHz, CDCl3, d ppm): 157.2, 154.0, 149.6,
142.6, 137.7, 136.2, 133.4, 131.8, 130.9, 129.5, 128.4, 127.7, 125.9, 125.2, 123.4,
117.0, 61.3, 41.7, 26.3 (–CH3); EI-MS: m/z (M ? 1, 460.04; M ? 2, 462.04;
M ? 4, 464.04); Anal. calcd for C21H16N3BrCl2: C, 54. 69; H, 3.50; N, 9.11. Found:
C, 54. 62; H, 3.51; N, 9.10.
6-(4-Bromophenyl)-2-methyl-3-(5-(4-bromophenyl)-4,5-dihydro-1H-pyrazol-3-
yl)pyridine (5k)
Off-white solid (yield 79%), m.p. 106–108 °C; IR (ATR) (cm-1): 3309 (NH-sharp),
1569 (C=N), 1443 (C=C) and 687 (C–Br); 1H NMR(400 MHz, CDCl3, d ppm): 2.36
(s, 3H, –CH3), 2.98 (dd, 1H, HB, HBA = 16 Hz, HBX = 10.2 Hz), 3.15 (dd, 1H, HA,
HAB = 16.2 Hz, HAX = 10.6 Hz), 5.27 (t, 1H, HX, J = 10.0 Hz), 7.32 (d, 2H,
0
0
J = 7.8 Hz, C2 and C6-H of 4-BrC6H4), 7.40–7.46 (m, 4H, C3, C5-H and C3 , C5 -H
of 4-BrC6H4) 7.50–7.62 (m, 3H, C2, C6-H of 4-BrC6H4 and C4-H of pyridine), 7.98
(d, 1H, J = 8.4 Hz, C5-H of pyridine); 13C NMR (100 MHz, CDCl3, d ppm): 157.4,
154, 149.4, 139.2, 137.6, 136.4, 133.3, 131, 129.9, 128.4, 127.6, 123.3, 123.1,
117.1, 61.4, 43.2, 26.4 (–CH3); EI-MS: m/z (M?, 468.48; M? 2, 470.98; M ? 4,
123

P. V. Sowmya et al.
472.8); Anal. calcd for C21H17N3Br2: C, 53.53; H, 3. 64; N, 8.92. Found: C, 53.49;
H, 3. 60; N, 8.90.
6-(4-Bromophenyl)-2-methyl-3-(5-(4-fluorophenyl)-4,5-dihydro-1H-pyrazol-3-
yl)pyridine (5l)
Off-white solid (yield 57%), m.p. 143–145 °C; IR (cm-1): 3289 (NH-sharp), 1587
1
(C=N), 1453 (C=C), 1123 (C–F) and 691 (C–Br); H NMR (400 MHz, CDCl3, d
ppm): 2.50 (s, 3H, –CH3), 2.98 (dd, 1H, HB, HBA = 16.2 Hz, HBX = 10.6 Hz), 3.20
(dd, 1H, HA, HAB = 16.2 Hz, HAX = 10 Hz), 5.32 (t, 1H, HX, J = 10.3 Hz), 6.92
(m, 2H, C3 and C5-H of 4-FC6H4), 7.10 (m, 2H, C2 and C6-H of 4-FC6H4), 7.40 (d,
2H, J = 8.5 Hz, C3 and C5-H of 4-BrC6H4) 7.52–7.60 (m, 3H, C2, C6-H of
4-BrC6H4 and C4-H of pyridine), 7.96 (d, 1H, J = 8.2 Hz, C5-H of pyridine); 13C
NMR(100 MHz, CDCl3, d ppm): 164.2 and 162.6 (1C, d, 1JC–F = 246.2 Hz), 157.3,
154.1, 149.2, 142.2, 136.7, 136.2, 131.8, 128.4, 128.2, 127.7, 117.4, 114.4 and 114.2
2
(2C, d, JC–F = 26.2 Hz), 63.6, 43.2, 26.2 (–CH3); EI-MS: m/z (M ? , 409.6;
M ? 2, 411.08); Anal. calcd for C21H17N3BrF: C, 61.48; H, 4. 18; N, 10.24. Found:
C, 61.52; H, 4. 16; N, 10.29.
Pharmacological screening
Animals
Male Swiss albino mice weighing 25–30 g were used for the present study. They
were maintained under standard environmental conditions of 22 3 °C and
65 55% relative humidity, and during the whole experiment, they were exposed
to a 12-h light and dark cycle. Animals were given standard food pellets, and water
was supplied ad libitum.
Forced swimming test (FST)
The behavioral despair test (forced swimming test) was used to evaluate the anti-
depressant activity of newly synthesized 4,5-dihydropyrazoline derivatives. The
tested compounds and reference drug (imipramine) were suspended in 2% aqueous
solution of Tween 80. On the testing day, mice were assigned into different groups
(n = 6 for each group). All the tested compounds (5a–l) (100 mg kg-1) and
standard drug Imipramine (10 mg kg-1) were administered orally to mice at a
volume of 0.5 mL/body weight. Control animals were similarly treated with 2%
aqueous solution of Tween 80. After 1 h, the mice were dropped at a time into flexi
glass cylinder (25 cm height), 30 cm diameter containing water to a height of 20 cm
at 21–23 °C and left for 6 min. At the end of the first 2 min, the animal showing
initial vigorous struggling was considered immobile. The duration of immobility
was recorded during the last 4 min to total a 6-min test.
123

Novel 2-methyl-6-arylpyridines carrying active…
Tail suspension test (TST)
The effectiveness of synthesized compounds was re-evaluated in a tail suspension
test (TST). After 1 h of drug application, mice were suspended on the edge of the
table, 50 cm above the floor with the help of adhesive tape placed 1 cm from the tip
of the tail, for a period of 6 min. Immobility time was measured during the 6-min
period. Mice were considered immobile when they did not show any body
movement while hanging passively and completely motionless. The percentage
decrease in immobility duration (%DID) is considered the indicator of the anti-
depressant-like activity [37]. Standard drug used was imipramine at a dose level of
10 mg kg-1
.
Statistical analysis
The data obtained by pharmacological studies were analyzed by one-way analysis of
variance (ANOVA) followed by Dunnet’s test and used to evaluate the results by
GraphPad Prism software version 5.0. The experimental results were expressed as
mean SEM. n represents the number of animals. A p value of less than 0.05 was
considered statistically significant.
Percentage decrease in immobility duration (% DID) for test and standard drugs
was calculated using following relation:
% DID ¼ ðA À BÞA Â 100;
where A is the duration of immobility (s) in the control group, and B is the duration
of immobility (s) in the test group.
Ethics
All procedures involving animals were carried out as per OECD guidelines, and the
animal treatment protocol was approved by Reg. No. SCSCP/IEEC/09/2016-17.
Anti-TB assay
The inhibition effect of the target compounds against M. tuberculosis H37Rv was
tested using a Microplate Alamar Blue Assay (MABA) technique according to the
reported method [38]. This methodology shows good correlation with the BACTEC
radiometric method. This technique is nontoxic and uses a thermally stable reagent.
Here, Middlebrook 7H9 broth base with Middlebrook OADC growth supplement
was used as medium. Two hundred microliters of sterile deionized water was added
to all outer perimeter wells of a sterile 96-well plate to minimize evaporation of
medium in the test wells during incubation. The 96-well plate received 100 lL of
the Middlebrook 7H9 broth. One milligram of each compound was dissolved in
1 mL of DMSO separately, and to make stock solutions of 100 lg mL-1 and
10 lg mL-1, it was further diluted with DMSO. Using the stock solution, serial
dilutions of 50, 25, 12.5, 6.25, 3.12, 1.6, 0.8, 0.4, and 0.2 lg mL-1 were made with
123

P. V. Sowmya et al.
DMSO directly on the 96-well plate. We further inoculated the test organism with a
medium containing ciprofloxacin, streptomycin, and isoniazid drugs used as
positive control, and a medium containing only DMSO (50 lL) used as negative
control. These wells were inoculated with M. tuberculosis H37Rv strain. Plates were
covered and sealed with Parafilm and incubated at 37 °C for 5 days. After this time,
25 lL of a freshly prepared 1:1 mixture of Alamar Blue reagent and 10% Tween 80
were added to the plate and incubated for 24 h. A blue color in the well was
interpreted as no bacterial growth, and pink was scored as growth. This colour
change of the Alamar Blue from a fully oxidized, non-fluorescent blue to a fully
reduced, fluorescent pink is due to the cellular metabolism. MIC was defined as
lowest drug concentration, which prevented the color change from blue to pink.
Results and discussion
Chemistry
The reaction sequence involving the synthesis of desired 6-arylpyridine based 4,5-
dihydro-2-pyrazlines is depicted in Scheme 1. We initiated our synthetic protocol
O
O
O
O
CH3
DMF-DMA
CH3
H3C
CH3
N
90 oC
CH3
NH4OAc , AcOH
R
R
1(a,b)
2(a,b)
O
R1
O
ArCHO
MeOH, NaOMe
CH3
N
CH3
N
CH3
4a-4l
R
R
3(a,b)
EtOH
NH2NH2.H2O
R1
N
N
CH3
H
N
5a-5l
R
Where R = 3,4-F2, 4-Br and R1 = 3,4-(OMe)2, 3,4,5-(OMe)3, 2-Cl-6-F, 2,3-Cl2, 4-Br, 4-F
Scheme 1 Synthesis of 6-aryl-2-methyl-3-(5-aryl-4,5-dihydro-1H-pyrazol-3-yl)pyridines (5a–l)
123

Novel 2-methyl-6-arylpyridines carrying active…
with commercially available 3,5-difluroacetophenone and 4-bromoacetophenone
1a–b refluxing with dimethyl formamide dimethyl acetal in solvent-free condition,
which furnished corresponding enaminones 2a–b. 1-(2-Methyl-6-arylpyridin-3-
yl)ethanones 3a–b were prepared in excellent yields by one-pot, three-component
heterocylocondensation of enaminones 2a–b, diketone and ammonium acetate in
glacial acetic acid media. The intermediates, 1-(2-Methyl-6-arylpyridin-3-yl)-3-
arylprop-2-en-1-one (4a–l) have been synthesized by conventional Claisen-Schmidt
condensation reaction of 1-(2-methyl-6-arylpyridin-3-yl)ethanones and appropriate
benzaldehydes in presence of base. Finally, preparation of 4,5-dihydro-2-pyrazo-
lines (5a–l) was achieved via the reaction of the appropriate chalcone derivatives
(4a–l) with excess of hydrazine hydrate in refluxed ethanol in the presence of a
catalytic amount of dil.HCl. The formation of 6-aryl-2-methyl-3-(5-aryl-4,5-
dihydro-1H-pyrazol-3-yl)pyridines (5a–l) (Table 1) are evidenced by their elemen-
tal analysis and spectral data.
The formation of intermediates (4a–l) from corresponding aromatic aldehydes
and ketones were confirmed by the appearance of characteristic stretching vibrations
for C=O and C=C groups in FT-IR. The compound 4d, showed the carbonyl
stretching vibration of keto-enol moiety as a strong and sharp absorption band at
1641 cm-1. Another sharp and strong absorbtion band at 1590 cm-1 was assigned
1
to C=C of a, b-unsaturated system. The H NMR spectrum showed trans olefinic
protons Ha and Hb as two ortho-coupled doublets at 7.12 (J = 16 Hz) and 7.93
(J = 16 Hz) respectively. The downfield resonance of the Hb as compared to Ha
could be attributed to the electron deficient environment of the b carbon in the
Table 1 Physical data of 1-(4-aryl-thiazol-2-yl)-2-((1-(fluoroaryl)-2,5-dimethyl-1H-pyrrol-3-yl)pyridi-
nes (5a–l)
R1
R2
N
N
N
H
Compound
R1
R2
Mol. formula
Mol. weight
M.p. (°C)
5a
5b
5c
5d
5e
5f
3,5-F2
3,5-F2
3,5-F2
3,5-F2
3,5-F2
3,5-F2
4-Br
3,4-(OCH3)2
3,4,5-(OCH3)3
2-Cl-6-F
2,3-Cl2
C23H21F2N3O2
C24H23F2N3O3
C21H15ClF3N3
C21H15Cl2F2N3
C21H16BrF2N3
C21H16F3N3
409.43
439.45
401.81
418.27
428.27
367.37
452.09
482.37
443.02
461.18
471.19
410.28
117–119
124–126
136–138
125–127
90–92
4-Br
4-F
108–112
116–119
134–136
140–142
96–97
5g
5h
5i
3,4-(OCH3)2
3,4,5-(OCH3)3
2-Cl-6-F
2,3-Cl2
C23H22BrN3O2
C24H24BrN3O3
4-Br
4-Br
C
21H16BrClFN3
C21H16BrCl2N3
5j
4-Br
5k
5l
4-Br
4-Br
C
21H17Br2N3
21H17BrFN3
106–107
143–145
4-Br
4-F
C
123

P. V. Sowmya et al.
enone moiety. The coupling constant value (J) for these olefinic protons was found
to be 16 Hz. Similarly, for all other chalcones (4a–l), the ‘‘J’’ values were in the
range of 15.8–16 Hz, indicating that they are stereoselective and attained trans
(E) configuration, whereas the proton of the pyridine unit methyl group appeared as
a sharp singlet at 2.76 ppm. The protons belonging to the aromatic ring were
observed with expected chemical shifts and integral values. The 13C NMR spectrum
of compound 4d showed signal at 193.9 ppm corresponding to the C=O group of the
a-b unsaturated fragment. The signal at 24.0 ppm corresponding to the methyl
carbon of the pyridine moiety evidences the formation of compound 4d. Further
confirmation of the structure 4d was given by mass spectrum that showed M? at
404.1 m/z and isotopic peak M??2 at 406.1 m/z. The spectral data of other
compounds followed similar pattern.
Structures of the synthesized 6-aryl-2-methyl-3-(5-aryl-4,5-dihydro-1H-pyrazol-
1
3-yl)pyridines (5a–l) were elucidated by IR, H NMR, 13C NMR, mass spectral
data, and elemental analyses. In FT-IR spectra (5a–l), the band corresponding to
carbonyl functionality of a, b-unsaturated carbonyl disappeared. Also, new
characteristic peaks corresponding to C=N, N–N stretching of the ring and NH
stretching vibrations were observed, indicating the formation of cyclized pyrazoline
analogues. Additionally, in 1H NMR spectra the signals corresponding to the
olefinic protons Ha and Hb of intermediates (4a–l) were found to be missing, which
clearly demonstrated the involvement of enone moiety during cyclization. In
addition to this, the major feature of these spectra was the signals of the pyrazoline
ring protons Hx, Ha , and Hb. In all cases, the two methylene protons Ha and Hb of
the prochiral carbon atom C-4 and the sterogenic proton Hx in C-5 generates an
1
ABX spin pattern. For the prototype compound 5b, H NMR spectrum reveals that
the signals of the pyrazoline ring protons which were found in the region Hx at
4.81 ppm (1H, t, J = 10.2 Hz) and Ha, Hb displayed as double-doublets centred at
3.42 ppm (Jab = 16 Hz, Jax = 10.4 Hz), 2.99 ppm (Jba = 16.2 Hz, Jbx = 9.8 Hz),
respectively, which evidenced the presence of two unequivalent geminal protons of
the methylene group coupled to each other and in turn with the vicinal methine
proton. It has been also noticed that the vicinal coupling constant range is
9.8–10.4 Hz, indicating the presence of trans configuration, in other words, the
chiral proton is cis to the aryl group attached to the chiral carbon of the pyrazoline
ring. Furthermore, the upfield shifting of prochiral Ha and Hb protons compared with
the Hx proton of the pyrazoline ring can be assumed due to its structure. All the
other aromatic and aliphatic protons were observed at expected regions. Addition-
ally, the structures of all target compounds were further established by 13C NMR,
and the spectrum of 5b confirmed the formation of a pyrazoline ring by displaying
signals at 43.6 ppm and 60.8 ppm due to the sp3 carbons of C-4 and C-5,
respectively. The downfield resonance of C-5 as compared to C-4 can be assumed
due to its benzylic nature and proximity to nitrogen that provide an additional
support for the formation of the target compound. All of the synthetic compounds
gave satisfactory mass spectroscopic data, which were in accordance with their
depicted structure. Characteristic M ? 2, M ? 4 isotope peaks were observed for
the compounds having chloro and bromo substituents. The mass spectrum of 5b
123

Novel 2-methyl-6-arylpyridines carrying active…
exhibited a molecular ion peak at 440.2 m/z in accordance with its formula and
evidences the formation of the compound.
Pharmacological activity
Antidepressant activity
The synthesized compounds (5a–I) were evaluated for antidepressant activity via
tail suspension test (TST) and forced swim test (FST) [39, 40]. We used a well-
known MAO-inhibitor imipramine as standard. Results are summarized in Tables 2
and 3 and expressed as immobility time in seconds and percentage decrease in
immobility duration (%DID). The ability of tested compounds to decrease
immobility time in both FST and TST was taken as a measure of its antidepres-
sant-like activity. As for activity of the tested compounds showed reduced
immobility times when compared to the normal control group at 100 mg kg-1 dose
level.
In the FST method, the treatment with doses of 100 mg kg-1 of 5b with 3,5-
difluorophenyl moiety attached to the pyridine ring and 3,4,5-trimethoxyphenyl ring
attached to the other extreme end of the pyrazoline ring was emerged as most active
antidepressant compound by exhibiting significantly decreased immobility time
66.67 7.15 s, when compared to normal control group 198.5 4.03 s and was
more effective than the standard-treated group 75.50 3.35 s at a test dose of
10 mg kg-1. Meanwhile, replacing the 3,5-difluorophenyl group by 4-bromophenyl
Table 2 Antidepressant activity of the title compounds (5a–l) by forced swimming test method
Sl. no.
Compoundsa
Duration of immobilityb (s)
% decrease in immobility
duration (%DID)
1
5a
109.3 15.55
66.67 – 7.15
130.7 13.46
156.0 13.68
160.3 15.17
79.17 – 4.50
109.2 16.66
94.33 – 14.96
151.3 14.22
128.2 11.42
137.7 13.22
81.67 – 4.40
75.50 3.35
198.5 4.03
45.05
66.58
34.24
21.46
19.29
60.01
44.84
52.47
23.58
35.25
30.45
58.75
62.12
–
2
5b
3
5c
4
5d
5
5e
6
5f
7
5g
8
5h
9
5i
10
11
12
13
14
5j
5k
5l
Imipramine
Control
Values represent the mean SEM; n = 6
a
Tested compounds and imipramine were tested at 100 and 10 mg/kg dose level, i.p. respectively
Bold font indicates a significant activity of the compounds
b
123

P. V. Sowmya et al.
Table 3 Antidepressant activity of the title compounds (5a–l) by tail suspension test method
Sl. no.
Compoundsa
Duration of immobilityb (s)
% decrease in immobility
duration (%DID)
1
5a
154.7 25.78
70.33 – 13.09
136.0 36.72
178.3 22.58
184.7 21.77
115.3 23.32
161.5 50.0
88.50 – 22.81
156.2 21.20
89.83 – 12.84
110.3 24.88
97.33 – 14.96
75.50 3.35
198.5 4.03
22.06
64.56
31.48
10.17
6.95
2
5b
3
5c
4
5d
5
5e
6
5f
41.91
18.64
55.41
21.31
54.74
44.43
50.96
61.96
–
7
5g
8
5h
9
5i
10
11
12
13
14
5j
5k
5l
Imipramine
Control
Values represent the mean SEM; n = 6
a
Tested compounds and imipramine were tested at 100 and 10 mg/kg dose level, i.p. respectively
Bold font indicates a significant activity of the compounds
b
in 5b led to compound 5h, whose immobility time was decreased to
94.33 14.96 s, indicating that the 4-bromophenyl substituent was less efficient
when compared to the 3,5-difluorophenyl group. Acute treatment with the
compound 5f having 3,5-difluorophenyl substitution at the pyridine ring and
4-flourophenyl substitution at the other end of the pyrazoline ring showed good
antidepressant-like activity by exhibiting decreased immobility time
79.17 4.50 s, and this effect was significant (p \ 0.001) when compared to
standard-treated group. Further, replacing 3,5-difluorophenyl substituent in the
pyridine ring of 5f with 4-bromophenyl substituent resulted in compound 5l, which
promoted a decrease in the immobility time, their activity being nearly equal to
92.44% that of the imipramine and emerged as good antidepressant active
compound, but when compared to 5f, the activity was slightly less, which clearly
indicates the efficacy of 3,5-difluoro substituent on the antidepressant activity. The
results clearly indicated that introducing 4-bromophenyl substituent on the pyridine
ring failed to increase significantly the antidepressant activity when compared to
3,5-difluoro phenyl substituent. The other treated compounds 5a, 5c, 5d, 5e, 5g, 5i,
5j, and 5k, were least active, that is, the immobility time did not statistically differ
from the control-treated group.
On the other hand, compounds exhibit good results in antidepressant screening by
the TST method. The screening results are reported in Table 3. Some of the
compounds, especially 5b, 5h, 5j, and 5l, exhibit good antidepressant activity at a
tested dose of 100 mg kg-1. Particularly, 5b with 3,5-difluorophenyl substitution at
the pyridine ring and 3,4,5-trimethoxyphenyl substitution at the other end of the
123

Novel 2-methyl-6-arylpyridines carrying active…
pyrazoline ring emerged as the most potent antidepressant agent by exhibiting
reduction in the immobility time to 70.33 13.09 s from 198.5 4.03 s of the
control group. Additionally, the animals treated with the compound 5j having
4-bromophenyl substitution at the pyridine ring and 2-chloro-6-fluorophenyl
substitution at the other end of the pyrazoline moiety, which was inactive in FST,
showed reduction in immobility time, their activity being nearly equal to 85.31% of
that in the standard treated group. In the TST method also, a fluoro substituent on
the aryl ring displayed good results by a reduction in immobility time when
compared to the normal-treated group, indicating their antidepressant efficacy.
When we correlate the structures of the test samples and their activity, it appears
that the halogen substituents on the phenyl ring in either one or the other position of
the pyrazoline moiety imparted superior activity when compared to the other
substituents. This is because of the lipophilic nature of the halogen atoms especially
fluorine atom can increase the rate of cell penetration and transportation of
molecules to the active site to create the desired level of biological activity [41],
which is reflected on the antidepressant activity of 5b, being the most potent of all.
Further, derivatives with an electron donating group, particularly the methoxy group
on the phenyl ring at the 5th position of pyrazoline, considerably enhanced the
antidepressant activity when compared to the pyrazolines with other substituents on
the phenyl ring.
Anti-TB activity
Antimycobacterial potency of the all newly synthesized compounds (5a–l) was
checked by performing in vitro antimycobacterial activity by Microplate Alamar
Blue Assay (MABA) method against Mycobacterium tuberculosis H37Rv using
Table 4 Antimycobacterial
No.
Compound
MIC (lg mL-1
)
activity of selected target
molecules against
M. tuberculosis H37Rv
1
5a
12.5
25
2
5b
3
5c
12.5
25
4
5d
5
5e
25
6
5f
25
7
5g
25
8
5h
50
9
5i
12.5
25
10
11
12
13
14
15
5j
5k
50
5l
50
Isoniazid
Streptomycin
Ciprofloxacin
6.25
6.25
3.125
Bold font indicates a significant
activity of compounds against
M. tuberculosis H37Rv
123

P. V. Sowmya et al.
ciprofloxacin, streptomycin, and isoniazid as standards. The results are reported in
Table 4. From the results, it is clear that the tested compounds displayed moderate
to good inhibitory activity with MIC value ranging from 12.5 to 50 lg mL-1
.
Among the series, compound 5a (R1 = 3,5-F2 and R2 = 3,4-(OCH3)2), 5c
(R1 = 3,5-F2 and R2 = 2-Cl-6-F) and 5i (R1 = 4-Br and R2 = 2-Cl-6-F) showed
highest inhibition with MIC value 12.5 lg mL-1. Meanwhile, introducing the
additional methoxy group in compound 5a led to compound 5b (R1 = 3,5-F2 and
R2 = 3,4,5-((OCH3)3), whose activity was reduced with MIC value 25 lg mL-1
.
Again, replacement of 3,5-difluoro group by 4-Bromo in 5b, that is compound 5h
(R1 = 4-Br and R2 = 3,4,5-((OCH3)3), led to significant loss of activity with MIC
value 50 lg mL-1. Further, tested compounds 5d, 5e, 5f, 5g, and 5j exhibited
moderate activity (25 lg mL-1) against Mycobacterium tuberculosis H37Rv strain.
Further, isosteric replacement of 2,4-difluoro group at phenyl ring attached to the
pyridine ring in 5e and 5f with a 4-Br group led to compounds 5k and 5l, which
displayed reduction in inhibition activity with MIC value 50 lg mLo-1, thus it was
clear that introduction of a bromo group did not improve their mycobacterial
activity. In addition to this, comparison of the activity results of the target
compounds demonstrated that the fluoro and chloro substituents in the aryl part are
essential for the pronounced activity.
Conclusions
In conclusion, a series of 6-aryl-2-methyl-3-(5-aryl-4,5-dihydro-1H-pyrazol-
3yl)pyridines (5a–l) were prepared in good yield and their antidepressant and
anti-TB activity were investigated. Evaluations of the antidepressant activity of the
derivatives were carried out by following FST and TST methodologies in mice.
Compound 5b induced amazing antidepressant activity by exhibiting the highest %
DID of 66.58 and 64.56 in the FST and TST, respectively, whereas, the reference
drug imipramine displayed % DID of 62.12 and 61.91 in the FST and TST,
respectively. Therefore, 5b can be used as lead molecule for further optimization.
Further, compounds 5f, 5h, 5j, and 5l furnished good antidepressant activity
compared to the standard drug in both FST and TST model. Antidepressant activity
results demonstrated that halogen atoms, particularly fluorine and chlorine on the
aryl ring, have made good contribution to potency. In addition to this, in vitro anti-
TB activity results reveals that compounds 5a, 5c, 5i displayed good growth
inhibitory activity against Mycobacterium tuberculosis H37Rv strain with MIC
value 12.5 lg mL-1. Consequently, such compounds would signify the fruitful
matrix for the development of new class of anti-TB and antidepressant agents.
Acknowledgements The authors are thankful to DST-PURSE Mangalore University, MIT Manipal and
SAIF Cochin for providing spectral data. We are grateful to the Head, Dept. of Pharmaceutical
Chemistry, NGSM Institute of Pharmaceutical Sciences, Paneer, Deralakatte for their help in carrying out
the antidepressant study. We also gratefully acknowledge the Head, Dept. of Microbiology, MM’sNG.
Halgekar Institute of Dental Sciences and Research centre, Belgaum for their help in carrying out the
antimycobacterial studies.
123

Novel 2-methyl-6-arylpyridines carrying active…
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