Piperazine analogs of naphthyridine-3-carboxamides and indole-2-carboxamides: Novel 5-HT3 receptor antagonists with antidepressant-like activity

Article abstract of DOI:10.1002/ardp.201400293

Series of piperazine analogs of naphthyridine-3-carboxamides and indole-2-carboxamides were designed using a ligand-based approach with consideration of the pharmacophoric requirements for 5-HT₃ receptor antagonists. The title carboxamides were synthesized using appropriate synthetic routes. Initially, the 5-HT₃ receptor antagonistic activity of all the compounds was determined on isolated guinea pig ileum tissue against the 5-HT₃ agonist, 2-methyl-5-hydroxytryptamine, which was denoted in the form of pA₂ values. The structure-activity relationship regarding the influence of the aromatic part and basic moiety as features in the 5-HT₃ pharmacophore was derived. Among all the compounds screened, the piperazine derivatives of indole-2-carboxamide 13i and naphthyridine-3-carboxamide 8h exhibited prominent 5-HT₃ receptor antagonism with pA₂ values of 7.5 and 7.3, respectively. Subsequent investigation of the antidepressant activities of selected compounds in the mouse forced swim test (FST) led to the identification of the piperazine analogs of indole-2-carboxamide 13i and naphthyridine-3-carboxamide 8h as the most promising compounds. Both 13i and 8h demonstrated significant reduction in the duration of immobility as compared to the control. Importantly, none of the tested compounds affected the baseline locomotion of mice at the tested dose levels.

Full text of DOI:10.1002/ardp.201400293

RCHH HARM
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Arch. Pharm. Chem. Life Sci. 2015, 348, 34–45
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Full Paper
Piperazine Analogs of Naphthyridine-3-carboxamides and Indole-2-
carboxamides: Novel 5-HT3 Receptor Antagonists with
Antidepressant-Like Activity
Arghya K. Dhar1, Radhakrishnan Mahesh1, Ankur Jindal2, and Shvetank Bhatt1
1
Department of Pharmacy, FD-III, Birla Institute of Technology & Science, Pilani, Rajasthan, India
School of Medicine, University of New Mexico, Albuquerque, USA
2
Series of piperazine analogs of naphthyridine-3-carboxamides and indole-2-carboxamides were designed
using a ligand-based approach with consideration of the pharmacophoric requirements for 5-HT3 receptor
antagonists. The title carboxamides were synthesized using appropriate synthetic routes. Initially, the
5-HT3 receptor antagonistic activity of all the compounds was determined on isolated guinea pig ileum
tissue against the 5-HT3 agonist, 2-methyl-5-hydroxytryptamine, which was denoted in the form of pA2
values. The structure–activity relationship regarding the influence of the aromatic part and basic moiety as
features in the 5-HT3 pharmacophore was derived. Among all the compounds screened, the piperazine
derivatives of indole-2-carboxamide 13i and naphthyridine-3-carboxamide 8h exhibited prominent 5-HT3
receptor antagonism with pA2 values of 7.5 and 7.3, respectively. Subsequent investigation of the
antidepressant activities of selected compounds in the mouse forced swim test (FST) led to the
identification of the piperazine analogs of indole-2-carboxamide 13i and naphthyridine-3-carboxamide 8h
as the most promising compounds. Both 13i and 8h demonstrated significant reduction in the duration of
immobility as compared to the control. Importantly, none of the tested compounds affected the baseline
locomotion of mice at the tested dose levels.
Keywords: Antidepressant / Forced swim test / 5-HT3 receptor antagonist / Indole-2-carboxamide /
Naphthyridine-3-carboxamide
Received: July 29, 2014; Revised: October 5, 2014; Accepted: October 17, 2014
DOI 10.1002/ardp.201400293
still persists. In addition, a significant proportion of depressed
Introduction
patients do not respond to the current treatment, or show only
partial response. A common problem associated with the
current antidepressant therapies is the several side effects,
e.g., clinically useful antidepressant agents produce withdraw-
al syndromes, except agomelatine [6]. Classical antidepres-
sants like monoamine oxidase (MAO) inhibitors and tricyclic
antidepressants produce drug-drug/-food interactions, anti-
cholinergic, and cardiovascular side effects [7, 8]. Besides all
currently approved antidepressant drugs for example, selec-
tive serotonin reuptake inhibitors (SSRIs) have slow onset of
action, since there is a delay of about 4–6 weeks to alleviate
the symptoms [9]. Thus, the search for a novel and effective
antidepressant agent with less side effects has become a need.
The 5-HT3 receptor, which is a pentameric ligand-gated ion
channel (pLGIC), mediates neurotransmission and controls the
release of monoamine neurotransmitters in the central
nervous system (CNS) [10, 11] and is considered to be an
important therapeutic target. The 5-HT3 receptor antagonist
Depression is a psychiatric disorder associated with consider-
able impairment in a person’s quality of life and lack of
efficiency, and triggers a major public and economic health
problem affecting a large section of people, particularly in the
western world [1–3]. Typical symptoms of depression are
anhedonia or the loss of interest or pleasure in normal daily
activities and feelings of sadness [4]. World Health Organiza-
tion (WHO) has predicted that depression will become the
second leading cause of premature death or disability
worldwide by the year 2020 [5]. Although a number of
antidepressant agents are available, prevalence of the disease
Correspondence: Arghya K. Dhar, Department of Pharmacy, FD-III,
Birla Institute of Technology & Science, Pilani, Rajasthan 333031,
India.
E-mail: arghyakusumdhar@yahoo.in
Fax: þ91 1596 244183
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Antidepressant Activity of 5-HT3 Antagonists
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Figure 1. Proposed pharmacophore of 5-HT3 receptor antagonists; naphthyridine and indole piperazine carboxamides designed on
the basis of the pharmacophore.
ondansetron is in clinical use as antiemetic for the treatment of
cancer chemo-/radiotherapy induced nausea, vomiting [12].
Interestingly, 5-HT3 receptor antagonists have been exten-
sively evaluated for their neuro-psychopharmacological po-
tential [13, 14]. Moreover, numerous pre-clinical studies [15–
18] as well as previous studies by our group [19–22] have
identified 5-HT3 receptor antagonists as potential antidepres-
sant agents and recognized the role of 5-HT3 receptor in the
pathophysiological processes of depression [23]. Thus, it
seemed appropriate to design and develop novel 5-HT3
receptor antagonists as antidepressants. The key elements of
the three-component pharmacophoric model (Fig. 1) proposed
for the interaction of 5-HT3 receptor antagonists with the 5-HT3
receptor-binding site: consist of a heteroaromatic core, a
hydrogen-bond acceptor moiety, and a basic nitrogen located
at a specific distance [24–26]. In recent years, we have been
engaged in the preparation and screening of compounds
based on the above pharmacophoric pattern and our earlier
studies have indicated that nitrogen-containing fused aromatic
rings (heteroaromatic core) may serve as a suitable starting
point for the design of novel 5-HT3 receptor antagonists [27–
30]. In addition, compounds based on the indolyl aromatic core
were reported as being potential 5-HT3 receptor antago-
nists [31–33]. Thus, our attention turned toward the naph-
thyridine and indole core as aromatic part and substituent
effects on the basic moiety. In this work, the objective was to
study the influence of the aromatic part and basic moiety on 5-
HT3 receptor antagonistic and antidepressant activity. Keeping
similar hydrogen-bond acceptor moiety, variations were made
to the heteroaromatic core and distal nitrogen (N4) of the basic
moiety of the pharmacophore (Fig. 1) with the intention of
exploring the structure–activity relationship (SAR) associated
with such changes. Therefore, two different nitrogen-contain-
ing fused heterocyclic rings (aromatic part) were alternately
attached to various N4-substituted piperazines (basic moiety)
through a carbonyl group of carboxamide linkage (hydrogen-
bond acceptor), resulting in the construction of two new series
of piperazine analogs of naphthyridine-3-carboxamides and of
indole-2-carboxamides. The minimum energy conformation
(three least energy conformations for each compound) of each
designed molecule was generated by ACDLABS-10.0/3D Viewer
(CHARMM parameterization). The pharmacophoric distances
were measured from the centroid of the heteroaromatic ring
to the oxygen of the carbonyl group (naphthyridine carbox-
amides ꢀ3.71 Å, indole carboxamides ꢀ3.41 Å), the carbonyl
oxygen to N4 of piperazines (basic nitrogen) (naphthyridine
carboxamides ꢀ4.90 Å, indole carboxamides ꢀ4.75 Å) and the
centroid of the heteroaromatic ring to the basic nitrogen
(naphthyridine carboxamides ꢀ6.50 Å, indole carboxamides
ꢀ6.28 Å). The calculated distances between the pharmaco-
phoric elements of the designed compounds comply with the
proposed pharmacophore model (Fig. 1) for 5-HT3 receptor
antagonists. Lipophilicity is an important criterion to be
considered when designing compound to manifest drug-like
behavior, particularly CNS drugs. Hence, log p values (Tables 1
and 2) of all the compounds were calculated using JME
molecular editor (courtesy of Peter Ertl, Novartis). Synthesis of
various N4-substituted piperazine analogs of naphthyridine-3-
carboxamides and indole-2-carboxamides (Tables 1 and 2) was
accomplished using the procedure outlined in Schemes 1 and 2,
respectively.
Results and discussion
5-HT3 receptor antagonistic activity
The 5-HT3 receptor antagonistic activity data of all the
compounds are represented in Table 3.
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Table 1. Physical constants of (2-ethoxy-1,8-naphthyridin-3-yl)(4-substituted piperazin-1-yl)methanone derivatives.
Compound
R
MW
% Yielda
m.p. in °C
Log Pb
8a
8b
8c
8d
8e
8f
8g
8h
8i
C6H5-
362
376
405
396.5
396.5
396.5
392
392
376
376
300
314
328
53
56
62
67
54
80
76
77
89
63
65
62
76
126–128
118–120
196–198
134–136
138–140
145–147
130–132
112–114
90–92
146–148
114–116
120–122
135–137
3.41
2.95
3.30
3.58
3.58
3.58
3.32
3.32
3.63
3.63
1.24
1.66
2.02
C6H5-CH2-
p- NO2-C6H4-
o-Cl-C6H4-
m-Cl-C6H4-
p-Cl-C6H4-
o-OCH3-C6H4-
m-OCH3-C6H4-
o-CH3-C6H4-
p-CH3-C6H4-
CH3-
8j
8k
8l
CH3CH2-
CH3CH2CH2
8m
aYields refer to isolated pure compounds.
bLog p values are calculated by using JME molecular editor (courtesy of Peter Ertl, Novartis).
Table 2. Physical constants of (1-methyl-1H-indol-2-yl)(4-substituted piperazin-1-yl)methanone derivatives.
Compound
R
MW
% Yielda
m.p. in °C
Log Pb
13a
13b
13c
13d
13e
13f
13g
13h
13i
C6H5-
319
333
364
353.5
353.5
353.5
349
349
333
333
257
271
285
67
50
58
63
58
69
68
74
58
69
78
61
71
120–122
118–120
176–178
148–150
136–138
112–114
160–162
108–110
130–132
98–100
2.98
2.53
2.88
3.16
3.16
3.16
2.90
2.90
3.21
3.21
0.82
1.24
1.60
C6H5-CH2-
p- NO2-C6H4-
o-Cl-C6H4-
m-Cl-C6H4-
p-Cl-C6H4-
o-OCH3-C6H4-
m-OCH3-C6H4-
o-CH3-C6H4-
p-CH3-C6H4-
CH3-
13j
13k
13l
91–93
100–102
125–127
CH3CH2-
CH3CH2CH2
13m
aYields refer to isolated pure compounds.
bLog P values of all the synthesized indole derivatives were calculated by using JME molecular editor (courtesy of Peter Ertl,
Novartis).
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Antidepressant Activity of 5-HT3 Antagonists
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Scheme 1. Synthetic route of (2-ethoxy-1,8-naphthyridin-3-yl)(4-substituted piperazin-1-yl)methanone derivatives. Reagents and
conditions: (a) Ammonium sulfamate, neat, 200°C, 14 h, 64%; (b) 4 N HCl, 1 h, 38%; (c) diethyl malonate, EtOH, reflux, 6 h, 60%; (d)
POCl3, DMF, reflux, 1 h, 67%; (e) NaOCH2CH3, EtOH, rt, 30 min, 68%; (f) 10% aq. NaOH, rt, 1 h, dil. HCl, 70%; (g) EDCꢁHCl, HOBt, 0°C to
rt, piperazines, 1 h.
Scheme 2. Synthetic route of (1-methyl-1H-indol-2-yl)(4-substituted piperazin-1-yl)methanone derivatives. Reagents and conditions:
(a) Ethanol, cat H2SO4 reflux, 4–6 h, 80%; (b) CH3I, KOH pellets, DMSO, rt, 1 h, 88%; (c) 10% aq. KOH, reflux, 1 h, dil. HCl, 73%; (d)
EDCꢁHCl, HOBt, 0°C to rt, 1 h, piperazines, 1 h.
Naphthyridine-3-carboxamide (compound 8a, pA2: 5.6) and
indole-2-carboxamide (compound 13a, pA2: 6.25) with phenyl
ring at the distal (N4) nitrogen of piperazine displayed
moderate antagonistic activity.
Increasing the spacer length between the distal nitrogen of
piperazine and the phenyl from 0 to 1 methylene (naphthyr-
idine-3-carboxamide, compound 8b pA2: 5.0 and indole-2-
carboxamide, compound 13b pA2: 5.8) led to decrease in 5-HT3
receptor antagonism.
We also surveyed a variety of electron withdrawing as well as
electron donating substituents appended at different posi-
tions on the N4 phenyl ring of both naphthyridine and
indole (piperazino) carboxamides. However, the groups (e.g.,
o-methoxyphenyl, o-chlorophenyl, m-chlorophenyl, methyl,
ethyl, etc.) on the basic moiety (N4) of the piperazine ring
were kept similar in case of both the carboxamide series.
The presence of electron withdrawing p-NO2 group in the
N4 phenyl ring of both naphthyridine-3-carboxamide and
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Table 3. 5-HT3 receptor antagonistic activity of (2-methoxy-1,8-naphthyridin-3-yl)(4-substituted piperazin-1-yl)meth-
anones and (1-methyl-1H-indol-2-yl)(4-substituted piperazin-1-yl)methanones.
Compound
Antagonism to 2-Me-5-HT (pA2)a
Compound
Antagonism to 2-Me-5-HT (pA2)a
8a
8b
8c
8d
8e
8f
8g
8h
8i
5.60
5.00
5.00
5.90
6.20
5.30
6.60
7.30
6.25
5.20
3.36
3.50
3.67
13a
13b
13c
13d
13e
13f
13g
13h
13i
13j
13k
13l
13m
Ondansetron
6.25
5.80
5.20
5.60
6.60
5.40
6.60
6.80
7.50
6.40
4.50
4.80
4.20
6.60
8j
8k
8l
8m
apA2 values are the means of two separate experiments. SE was less than 10% of the mean.
indole-2-carboxamide decreased the antagonistic activity as
evidenced by compound 8c (pA2: 5.0) and compound 13c (pA2:
5.2).
Naphthyridine-3-carboxamide, compound 8d (pA2: 5.9) with
electron withdrawing o-Cl group showed better 5-HT3
antagonistic activity than compound 8f (pA2: 5.3) with -Cl at
p-position. However, m-Cl substitution in the N4 phenyl ring
appeared to be more favorable with compound 8e (pA2: 6.2)
showing higher antagonistic activity than both compounds 8d
and 8f.
Similar types of results were found in piperazine analogs of
indole-2-carboxamides with respect to chloro substitution on
the N4 phenyl ring wherein m-Cl derivative 13e (pA2: 6.6)
showed better 5-HT3 receptor antagonistic activity compared
to the o-Cl derivative 13d (pA2: 5.6) and p-Cl derivative 13f
(pA2: 5.4).
Introduction of o-OCH3 group in the N4 phenyl ring resulted
in moderate antagonistic activity as evident by naphthyridine-
3-carboxamide 8g (pA2: 6.0) and indole-2-carboxamide 13g
(pA2: 6.0). However, naphthyridine-3-carboxamide compound
8h (pA2: 7.5) and indole-2-carboxamide 13h (pA2: 6.8) with m-
OCH3 group in the phenyl ring showed appreciable 5-HT3
receptor antagonism.
Naphthyridine-3-carboxamide, compound 8i (pA2: 6.25)
bearing o-CH3 group in the N4 phenyl ring showed moderate
activity and the indole counterpart 13i (pA2: 7.3) displayed
substantial antagonistic activity. While, naphthyridine-3-car-
boxamide, compound 8j (pA2: 5.2) with p-CH3 group in the
phenyl ring showed less 5-HT3 receptor antagonistic activity
and the corresponding indole analog 13j (pA2: 6.20) showed
moderate activity.
Incorporation of aliphatic methyl group at the N4 pipera-
zine resulted in the lessening of 5-HT3 receptor antagonism, as
seen with naphthyridine-3-carboxamide 8k (pA2: 3.36), indole-
2-carboxamide 13k (pA2: 4.50).
Further incorporation of ethyl as well as propyl substituent at
the N4 of piperazine also was not tolerated, as illustrated by
the reduced antagonistic activities of naphthyridine-3-carbox-
amides compound 8l (pA2: 3.50), compound 8m (pA2: 3.67) and
indole-2-carboxamides compound 13l (pA2: 4.80), compound
13m (pA2: 4.20).
However, indole-2-carboxamides were found to be com-
paratively more tolerant of aliphatic substitutions at the N4
nitrogen than those of their naphthyridine counterparts.
It was observed that within both naphthyridine-3-carbox-
amide and indole-2-carboxamide series the 5-HT3 receptor
antagonistic activity was influenced by the nature of the N4
substituent of the piperazine. With respect to structure–
activity relationship, the results demonstrated that com-
pounds bearing methoxyphenyl group at the N4 piperazine
showed better activity than compounds with nitrophenyl
group and chlorophenyl group. Whereas, replacement of
phenyl ring and/or substituted phenyl ring with aliphatic
group was detrimental to the activity. It seemed, therefore,
the role of N4 substituent in these compounds for the
interaction with 5-HT3 receptor might be due to aromatic
interaction.
It was also observed that compounds 13a, 13e, 13i, and 13j
with indole core displayed relatively better activity than
compounds 8a, 8e, 8i, and 8j with naphthyridine core bearing
similar substituents in the N4 phenyl ring. However, naph-
thyridine carboxamide compound 8h showed better activity
in comparison to its indole counterpart 13h.
Few piperazine analogs of both naphthyridine and indole
carboxamides (compound 8a, compound 13a containing N4
phenyl substituent and compound 8c and 13c containing N4
benzyl substituent) were found to have better 5-HT3 antago-
nism compared to our earlier synthesized quinoxaline
carboxamide counterparts [30]. In case of compounds con-
taining m-chlorophenyl and o-methoxyphenyl substituents
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Antidepressant Activity of 5-HT3 Antagonists
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Table 5. Antidepressant activity of (2-ethoxy-1,8-naph-
thyridin-3-yl)(4-substituted piperazin-1-yl)methanones
and (1-methyl-1H-indol-2-yl)(4-phenylpiperazin-1-yl)methan-
ones in the forced swim test mice model.
at N4 piperazine, almost similar antagonistic activity was
observed among naphthyridine, indole, and quinoxaline
carboxamide derivatives. However, naphthyridine as well as
indole carboxamides with aliphatic substituents at N4
piperazine were found to have less 5-HT3 antagonistic
activity as compared to the corresponding quinoxaline
carboxamides [30].
Duration of immobility in
seconds (FST)a; dose, mg/kg (i.p.)
Compound
1
2
Antidepressant activity
8e
8g
8h
8i
13a
13e
13g
13h
128.33 ꢂ 12.80
98.33 ꢂ 07.12ꢃ
90.67 ꢂ 10.57ꢃ
120.33 ꢂ 11.45
145.33 ꢂ 04.83
166.33 ꢂ 07.56
100.43 ꢂ 03.91ꢃ
113.33 ꢂ 13.86
105.00 ꢂ 04.41ꢃ
80.00 ꢂ 07.76ꢃ
105.10 ꢂ 8.90ꢃ
174.67 ꢂ 5.21
109.00 ꢂ 05.18ꢃ
121.33 ꢂ 09.98
84.33 ꢂ 08.51ꢃ
64.33 ꢂ 16.36ꢃ
124.00 ꢂ 14.40
117.00 ꢂ 06.66ꢃ
88.33 ꢂ 06.53ꢃ
88.67 ꢂ 06.34ꢃ
85.67 ꢂ 05.06ꢃ
79.00 ꢂ 10.69ꢃ
98.50 ꢂ 05.50ꢃ
On the basis of 5-HT3 receptor antagonistic activity shown by
all the compounds, four naphthyridine-3-carboxamides (8e–8i)
and six indole-2-carboxamides (13a, 13e, 13g–13j) were
selected for antidepressant screening.
Initially to rule out the possible existence of any false
positive and (or) false negative result in the forced swim
test (FST) due to drug-induced stimulation/suppression in
the locomotor activity of mice, the compounds were subjected
to spontaneous locomotor activity study using actophotom-
eter [34]. Neither ondansetron, nor test compounds were
found to cause any significant changes in the locomotor
activity of mice when compared to the control as shown in
Table 4.
Subsequently, compounds were subjected to the mouse
FST [35, 36] in order to evaluate the antidepressant potential.
Based on the actophotometer study, compounds were
examined at dose of 1 mg/kg (i.p.) (lower dose level) and
2 mg/kg (i.p) (higher dose level) in the mouse FST. Antidepres-
sant activity was determined as mean immobility time in
13i
13j
Ondansetron
Control
aWater was used as a vehicle, the values are expressed as
mean, n ¼ 8 per group.
Data were analyzed by Graph Pad Prism (3) software through
one-way ANOVA followed by post hoc Dunnett’s test.
ꢃp < 0.05 as compared to vehicle treated group (control).
seconds. The antidepressant activity data from the FST is
shown as mean ꢂ S.E.M. in Table 5.
Among the compounds screened, three compounds (8h, 13i,
and 13j) showed prominent antidepressant-like activity as
indicated by significant (p < 0.05) reduction in immobility time
compared to the control, while two compounds (8g and 13g)
showed moderate antidepressant-like activity. Three com-
pounds (8h, 13i, and 13j) demonstrated a statistically signifi-
cant reduction in immobility time compared to control in a
dose-dependent manner. Compounds 8h, 13i, and 13j
emerged as most potential derivatives as they significantly
(p < 0.05) reduced the immobility time compared to the
control at both the tested dose levels (1 and 2 mg/kg).
Compounds 8e, 8i, 13e, and 13h showed significant antide-
pressant-like activity as compared to vehicle control only at
higher (2 mg/kg) dose level.
Table 4. Spontaneous locomotor scores of (2-ethoxy-1,8-
naphthyridin-3-yl)(4-substituted piperazin-1-yl)methan-
ones and (1-methyl-1H-indol-2-yl)(4-substituted pipera-
zin-1-yl)methanones.
Locomotor scoresa (10 min)
Dose, mg/kg (i.p.)
Compound
2
1
8e
8g
8h
8i
13a
13e
13g
13h
535.33 ꢂ 14.63
447.00 ꢂ 08.33
487.00 ꢂ 08.33
447.00 ꢂ 24.60
541.67 ꢂ 11.10
457.00 ꢂ 07.34
467.00 ꢂ 18.03
471.00 ꢂ 10.33
571.00 ꢂ 11.33
400.67 ꢂ 48.90
447.00 ꢂ 08.33
489.00 ꢂ 12.10
507.00 ꢂ 18.91
435.50 ꢂ 17.41
445.50 ꢂ 7.86
362.67 ꢂ 10.63
435.33 ꢂ 15.76
438.00 ꢂ 15.62
425.50 ꢂ 10.41
464.00 ꢂ 25.53
384.00 ꢂ 25.53
300.00 ꢂ 41.90
395.50 ꢂ 1.41
Conclusion
13i
13j
To summarize, the present study enabled us to extend the
structure–activity relationship studies of the carboxamides
with two different heteroaromatic cores in the proposed
pharmacophore. In the indole series, promising compounds
were obtained from methyl phenyl and methoxy phenyl
substitution at the N4 of piperazine. Similar results were
obtained with naphthyridine analogs bearing similar sub-
stituents. Supposedly, alteration in the heteroaromatic core
Ondansetron
control
aWater was used as a vehicle, the values are expressed as
mean, n ¼ 8 per group. Data were analyzed by Graph Pad
Prism 3 software through one-way ANOVA followed by post
hoc Dunnett’s test.
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did not elicit substantial changes in the activity of the
compounds. Nevertheless, few analogs from the indole series
were found to be comparatively more active than their
naphthyridine counterpart. Analysis of SAR study led to the
assumption that within both naphthyridine and indole
carboxamide series the activity is more sensitive to changes
in the N4-substituent of piperazine. Thus, it is likely that an
appropriate combination of heteroaromatic core and basic
centre (substitution at N4 nitrogen of piperazine) of the
pharmacophore produces potential 5-HT3 receptor antagonis-
tic activity. Studies in our laboratory are currently underway to
further understand the role of aromatic core and basic moiety
as 5-HT3 receptor pharmacophoric criteria within a molecule in
order to identify novel compounds with potential as
antidepressant agents.
thus precipitated out was filtered and washed thoroughly with
cold water.
Yield: 67%, melting point 170°–172°C. FT-IR (KBr, cmꢄ1):
3471, 3034, 2922, 2358, 2241, 1969, 1907, 1625, 1510, 1462,
1350, 1296, 1250, and 1090. 1H NMR, CDCl3, d (ppm): 8.78–8.76
(dd, 1H, J ¼ 4.8 Hz, 2 Hz, naphthyridine); 8.5 (s, 1H, naphthyr-
idine); 7.95–7.97 (m, 1H, naphthyridine); 7.20–7.17 (dd, 1H,
J ¼ 10 Hz, 7.2 Hz, naphthyridine); 4.38–4.35 (q, 2H, ester); 1.35–
1.33 (t, 3H, ester).
Synthesis of ethyl 2-ethoxy-1,8-naphthyridine-3-
carboxylate 6
To a solution of chloro derivative (5) (1 mmol) in a sufficient
amount of alcohol, sodium ethoxide (2 mmol) was added; the
reaction mixture was stirred at room temperature for 2 h. The
reaction was monitored by TLC. On completion of the reaction
the mixture was evaporated in vacuo, and the dark brown
thick liquid so obtained was dissolved in EtOAc (30 mL),
washed with saturated NaHCO3 solution, dil. HCl solution, and
brine. Organic portion was separated and evaporated to get
the ethoxy derivative, which was used further without
purification. Yield ¼ 70%.
Experimental
Chemistry
Melting points (m.p.) were determined in open capillary
tubes on a Buechi 530 melting point apparatus and are
uncorrected. Thin layer chromatography (TLC) was performed
to monitor progress of the reaction and assess purity of the
compounds; spots were detected by their absorption under
UV light. IR spectra were recorded with IR prestige-21 (FT-IR,
Shimadzu) and mass spectra were recorded using ’Hewlett-
Packard’ HP GS/MS 5890/5972. 1H NMR spectra were recorded
with Bruker DPX operating at 400 MHz in CDCl3 or DMSO-d6
solution, with tetramethylsilane (TMS) as an internal stan-
dard. Chemical shifts are shown as d values (ppm), the J values
are expressed in Hertz (Hz). Signals are represented as s
(singlet), d (doublet), t (triplet), q (quintet), or m (multiplet).
The substituted phenyl piperazine derivatives, and other
chemicals were commercially available (Aldrich or Fluka). The
synthetic route to the target carboxamides are illustrated
in the scheme.
Synthesis of 2-ethoxy-1,8-naphthyridine-3-carboxylic
acid 7
The ester 6 (1 mmol) was dissolved in sufficient amount of
ethanol, to this 10% aqueous NaOH (3 mmol) was added, and
the reaction mixture was stirred at room temperature for 1 h,
during which the reaction was monitored by TLC using EtOAc/
n-hexane (1:1) as solvent system. On completion of reaction,
the mixture was evaporated in vacuo to obtain a dark brown
thick liquid, to this crushed ice was added and acidified with
dilute HCl up to pH 3. Brown solid precipitated out which was
filtered and washed with ice cold water to obtain pure acid. 1H
NMR, DMSO-d6, d (ppm): 9.07–9.05 (m, 1H, naphthyridine),
8.90 (s, 1H, naphthyridine), 8.78–8.74 (m, 1H, naphthyridine),
7.70–7.67 (m, 1H, naphthyridine), 4.80–4.60 (q, 2H, -OCH2),
1.48–1.34 (t, 3H, -CH3 of ethoxy).
The piperazine analogs of naphthyridine-3-carboxamides
were prepared as depicted in Scheme 1. 2-Aminonicotinalde-
hyde was synthesized as reported in the literature via
sequence of reactions (Scheme 1) from the starting material
nicotinamide (1), which upon solvent free heating with
ammonium sulfamate followed by hydrolysis, gave 2-amino-
nicotinaldehyde 3 [37]. Condensation of 2-aminonicotinalde-
hyde with diethylmalonate in alcohol provided ethyl 2-oxo-
1,2-dihydro-1,8-naphthyridine-3-carboxylate 4 [38].
General procedure for the synthesis of compounds 8a–o
2-Ethoxy-1,8-naphthyridine-3-carboxylic acid (7) (1 mmol)
was dissolved in THF. To this EDCꢁHCl (1.2 mmol) and HOBt
(1.5 mmol) were added under nitrogen condition and stirred
at 0°C, for 5–10 min, followed by various substituted
piperazines were added in equimolar quantity. The reaction
mixture was further stirred for 1 h at room temperature.
On completion, the reaction mixture was evaporated and
washed with water; and compounds were purified by
recrystallization and (or) column chromatography. Physical
constants of the title compounds are represented in the
Table 1. Spectral data of the synthesized compounds are as
follow:
Synthesis of ethyl 2-chloro-1,8-naphthyridine-3-
carboxylate 5
To compound 4 (1 mmol), few drops of DMF and excess POCl3
were added. The reaction mixture was refluxed for 1–2 h.
After completion of the reaction (monitored by TLC using
ethyl acetate/hexane 7:3 by volume), the reaction mixture
was poured onto crushed ice kept in a large vessel. To this,
saturated NaHCO3 solution was added very carefully until
slightly basic pH was attained. The pale yellow solid compound
2-Ethoxy-1,8-naphthyridin-3-yl(4-phenylpiperazin-1-yl)metha-
none 8a: IR (KBr)/cm: 3125, 2954, 1620, 1500, 1485, 1232, 1157.
1H NMR, CDCl3, d (ppm): 9.06–8.96 (m, 1H, naphthyridine),
ß 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Antidepressant Activity of 5-HT3 Antagonists
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8.16–8.0 (m, 2H, naphthyridine), 7.45–7.35 (m, 1H, naphthyr-
idine), 7.31–7.26 (m, 2H, phenyl), 7.0–6.89 (m, 3H, phenyl),
4.82–4.57 (q, 2H, -OCH2), 4.13–3.90 (m, 2H, piperazine), 3.58–
3.40 (m, 2H, -piperazine), 3.34–3.29 (m, 2H, piperazine), 3.22–
3.05 (m, 2H, piperazine), 1.51–1.38 (t, 3H, -CH3 of ethoxy). ESI-
MS: m/z 363 [MþH] 100%.
(m, 2H, piperazine), 3.15–3.05 (m, 2H, piperazine), 1.50–1.37 (t,
3H, -CH3 of ethoxy). ESI-MS: m/z 397.1 [MþH] 100%.
(2-Ethoxy-1,8-naphthyridin-3-yl)[4-(2-methoxyphenyl)pipera-
zin-1-yl]methanone 8g: IR (KBr/cm): 3150, 3050, 1630, 1560,
1478, 1135, 1038. 1H NMR, CDCl3, d (ppm): 9.04–8.92 (m, 1H,
naphthyridine), 8.19–8.06 (m, 2H, naphthyridine), 7.45–7.33
(m, 1H, naphthyridine), 7.15–7.02 (m, 1H, phenyl), 6.99–6.85
(m, 3H, phenyl), 4.86–4.54 (q, 2H, -OCH2), 4.18–3.96 (m, 2H,
piperazine), 3.83 (s, 3H, -OCH3), 3.61–3.36 (m, 2H, -piperazine),
3.31–3.13 (m, 2H, piperazine), 3.11–2.93 (m, 2H, piperazine),
1.54–1.40 (t, 3H, -CH3 of ethoxy). ESI-MS: m/z 393.1 [MþH]
100%.
2-Ethoxy-1,8-naphthyridin-3-yl(4-benzylpiperazin-1-yl)metha-
none 8b: IR (KBr)/cm: 3110, 3050, 1620, 1554, 1483, 1034. 1H
NMR, CDCl3, d (ppm): 9.03–8.92 (m, 1H, naphthyridine), 8.17–
7.99 (m, 2H, naphthyridine), 7.47–7.35 (m, 2H, naphthyridine,
phenyl), 7.31–7.19 (m, 4H, phenyl), 4.81–4.54 (m, 2H, -OCH2),
3.96–3.76 (m, 2H, piperazine), 3.64–3.48 (m, 2H, -piperazine),
3.43–3.17 (m, 2H, piperazine), 2.69–2.48 (m, 2H, piperazine),
2.42–2.30 (m, 2H, piperazine), 1.52–1.37 (t, 3H, -CH3 of ethoxy).
ESI-MS: m/z 377.26 [MþH] 100%.
(2-Ethoxy-1,8-naphthyridin-3-yl)[4-(3-methoxyphenyl)pipera-
zin-1-yl]methanone 8h: IR (KBr/cm): 3150, 3050, 1630, 1560,
1470, 1142, 1038. 1H NMR, CDCl3, d (ppm): 8.98–8.92 (m, 1H,
naphthyridine), 8.19–8.06 (m, 2H, naphthyridine), 7.45–7.30
(m, 1H, naphthyridine), 7.20–7.10 (m, 1H, phenyl), 6.59–6.45
(m, 1H, phenyl), 6.48–6.35 (m, 2H, phenyl), 4.66–4.50 (q, 2H,
-OCH2), 4.18–3.98 (m, 2H, piperazine), 3.73 (s, 3H, OCH3), 3.48–
3.36 (m, 2H, -piperazine), 3.31–3.20 (m, 2H, piperazine), 3.15–
2.97 (m, 2H, piperazine), 1.48–1.40 (t, 3H, -CH3 of ethoxy). ESI-
MS: m/z 393.1 [MþH] 100%.
(2-Ethoxy-1,8-naphthyridin-3-yl)[4-(4-nitrophenyl)piperazin-1-
yl]methanone 8c: IR (KBr/cm): 3100, 3060, 1628, 1570, 1460,
1142, 1035. 1H NMR, CDCl3, d (ppm): 9.07–8.98 (m, 1H,
naphthyridine), 8.21–8.09 (m, 4H, naphthyridine, phenyl),
7.49–7.40 (m, 1H, naphthyridine), 6.90–6.78 (m, 2H, phenyl),
4.83–4.63 (q, 2H, -OCH2), 4.17–3.84 (m, 2H, piperazine),
3.65–3.58 (m, 2H, -piperazine), 3.49–3.34 (m, 4H, piperazine),
1.49–1.42 (t, 3H, -CH3 of ethoxy). ESI-MS: m/z 406 [MþH]
100%.
(2-Ethoxy-1,8-naphthyridin-3-yl)[4-(2-methylphenyl)pipera-
zin-1-yl]methanone 8i: IR (KBr)/cm: 3100, 3050, 1620, 1500,
1460, 1350. 1H NMR, CDCl3, d (ppm): 8.99–8.92 (m, 1H,
naphthyridine), 8.19–8.06 (m, 2H, naphthyridine), 7.41–7.36
(m, 1H, naphthyridine), 7.21–7.14 (m, 2H, phenyl), 7.04–6.96
(m, 2H, phenyl), 4.78–4.62 (q, 2H, -OCH2), 4.12–3.89 (m, 2H,
piperazine), 3.76–3.65 (m, 2H, piperazine), 3.58–3.32 (m, 2H,
-piperazine), 3.11–2.77 (m, 2H, piperazine), 2.33 (s, 3H, CH3),
1.52–1.46 (t, 3H, -CH3 of ethoxy). ESI-MS: m/z 377.1 [MþH]
100%.
(2-Ethoxy-1,8-naphthyridin-3-yl)[4-(2-chlorophenyl)piperazin-
1-yl]methanone 8d: IR (KBr/cm): 3120, 3060, 1628, 1570, 1460,
1137, 1035. 1H NMR, CDCl3, d (ppm): 9.01–8.85 (m, 1H,
naphthyridine), 8.17–7.93 (m, 2H, naphthyridine), 7.38–7.26
(m, 2H, naphthyridine, phenyl), 7.20–7.14 (m, 1H, phenyl),
7.05–6.86 (m, 2H, phenyl), 4.75–4.54 (q, 2H, -OCH2), 4.09–3.83
(m, 2H, piperazine), 3.49–3.28 (m, 2H, -piperazine), 3.15–3.07
(m, 2H, piperazine), 3.04–2.84 (m, 2H, piperazine), 1.48–1.34 (t,
3H, -CH3 of ethoxy). ESI-MS: m/z 397 [MþH] 100%.
(2-Ethoxy-1,8-naphthyridin-3-yl)[4-(4-methylphenyl)pipera-
zin-1-yl]methanone 8j: IR (KBr)/cm: 3100, 3060, 1620, 1550,
1430, 1340. 1H NMR, CDCl3, d (ppm): 9.09–8.93 (m, 1H,
naphthyridine), 8.23–8.03 (m, 2H, naphthyridine), 7.50–7.38
(m, 1H, naphthyridine), 7.21–7.02 (m, 2H, phenyl), 6.94–6.77
(m, 2H, phenyl), 4.82–4.57 (q, 2H, -OCH2), 4.13–3.92 (m, 2H,
piperazine), 3.56–3.37 (m, 2H, piperazine), 3.30–3.24 (m, 2H,
-piperazine), 3.17–2.98 (m, 2H, piperazine), 2.29 (s, 3H, CH3),
1.50–1.39 (t, 3H, -CH3 of ethoxy). ESI-MS: m/z 377.1 [MþH]
100%.
(2-Ethoxy-1,8-naphthyridin-3-yl)[4-(3-chlorophenyl)piperazin-
1-yl]methanone 8e: IR (KBr/cm): 3100, 3060 (aromatic C–H
–
–
–
str.), 1628 (C O str.of amide), 1570, 1460 (C C, C N ring str.),
–
–
–
1137, 1035 (C–O–C str. of aryl alkyl ether). 1H NMR, CDCl3, d
(ppm): 9.04–8.95 (m, 1H, naphthyridine), 8.18–8.08 (m, 2H,
naphthyridine), 7.47–7.36 (m, 1H, naphthyridine), 7.25–7.13
(m, 1H, phenyl), 6.94–6.84 (m, 2H, phenyl), 6.82–6.76 (m, 1H,
phenyl), 4.82–4.60 (q, 2H, -OCH2), 4.09–3.87 (m, 2H, pipera-
zine), 3.57–3.40 (m, 2H, -piperazine), 3.29–3.19 (m, 2H,
piperazine), 3.12–3.03 (m, 2H, piperazine), 1.50–1.41 (t, 3H,
-CH3 of ethoxy). ESI-MS: m/z 397.1 [MþH] 100%.
(2-Ethoxy-1,8-naphthyridin-3-yl)(4-methyl)piperazin-1-yl)-
methanone 8k: IR (KBr)/cm: 3100, 3070, 1620, 1550, 1435,
1340. 1H NMR, CDCl3, d (ppm): 9.06–8.98 (m, 1H, naphthyr-
idine), 8.25–8.18 (m, 1H, naphthyridine), 7.98–7.86 (m, 1H,
naphthyridine), 7.41–7.34 (m, 1H, naphthyridine), 4.75–4.57 (q,
2H, -OCH2), 4.10–3.95 (m, 2H, piperazine), 3.58–3.34 (m, 2H,
piperazine), 3.28–3.24 (m, 2H, -piperazine), 3.10–2.97 (m, 2H,
piperazine), 2.96 (s, 3H, CH3), 1.50–1.35 (t, 3H, -CH3 of ethoxy).
ESI-MS: m/z 301 [MþH] 100%.
(2-Ethoxy-1,8-naphthyridin-3-yl)[4-(4-chlorophenyl)piperazin-
1-yl]methanone 8f: IR (KBr/cm): 3100, 3060, 1628, 1570, 1460,
1142, 1035. 1H NMR, CDCl3, d (ppm): 9.03–8.96 (m, 1H,
naphthyridine), 8.16–8.0 (m, 1H, naphthyridine), 7.45–7.38 (m,
1H, naphthyridine), 7.35–7.26 (m, 3H, naphthyridine, phenyl),
6.90–6.69 (m, 2H, phenyl), 4.80–4.60 (q, 2H, -OCH2), 4.10–3.90
(m, 2H, piperazine), 3.52–3.40 (m, 2H, -piperazine), 3.30–3.20
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Archiv der Pharmazie
(2-Ethoxy-1,8-naphthyridin-3-yl)(4-ethylpiperazin-1-yl)metha-
none 8l: IR (KBr)/cm: 3010, 2960, 1620, 1520, 1430, 1290. 1H
NMR, CDCl3, d (ppm): 9.03–8.96 (m, 1H, naphthyridine), 8.15–
8.03 (m, 1H, naphthyridine), 7.98–7.86 (m, 1H, naphthyridine),
7.25–7.10 (m, 1H, naphthyridine), 4.53–4.25 (q, 2H, -OCH2),
3.98–3.93 (m, 2H, piperazine), 3.70–3.58 (m, 2H, piperazine),
3.42–3.33 (m, 2H, -piperazine), 3.25–3.10 (m, 2H, piperazine),
2.97–2.80 (m, 2H,CH2), 2.70–2.50 (m, 3H, CH3), 1.52–1.38 (t, 3H,
-CH3 of ethoxy). ESI-MS: m/z 315.1 [MþH] 100%.
7.26 (m, 1H, indole), 7.20–7.16 (m, 1H, indole), 6.6 (s, 1H,
indole).
General procedure for the synthesis of compounds 13a–m
N-substituted indole 2-carboxylic acid was dissolved in DCM, in
a round-bottomed flask, under nitrogen condition. To this 1.2
equiv EDCꢁHCl, followed by 1.5 equiv HOBt were added and
stirred at 0°C, for 5–10 min, followed by various commercially
available substituted piperazines (for compounds 13a–m)
were added in equimolar quantity. The reaction mixture
was further stirred for 1 h at room temperature. On
completion, the reaction mixture was evaporated and washed
with water; and compounds were purified by recrystalization
and (or) column chromatography. Physical constants of the
title compounds are represented in the Table 2. Spectral data
of the synthesized compounds are as follow.
(2-Ethoxy-1,8-naphthyridin-3-yl)(4-propylpiperazin-1-yl)meth-
anone 8m: IR (KBr)/cm: 3130, 3030, 1620, 1560, 1430, 1280. 1H
NMR, CDCl3, d (ppm): 9.0–8.93 (m, 1H, naphthyridine), 8.30–
8.20 (m, 1H, naphthyridine), 7.98–7.88 (m, 1H, naphthyridine),
7.30–7.20 (m, 1H, naphthyridine), 4.53–4.35 (q, 2H, -OCH2),
3.98–3.80 (m, 2H, piperazine), 3.70–3.58 (m, 2H, piperazine),
3.42–3.33 (m, 2H, -piperazine), 3.25–3.10 (m, 2H, piperazine),
2.94 (t, 2H, -CH2), 2.84–2.80 (m, 2H, -CH2), 2.76–2.70 (t, 3H,
-CH3), 1.73–1.52 (t, 3H, -CH3 of ethoxy). ESI-MS: m/z 329 [MþH]
100%.
(1-Methyl-1H-indol-2-yl)(4-phenylpiperazin-1-yl)methanone
13a: IR (KBr)/cm: 3100, 3080, 1628, 1540, 1230, 1135. 1H NMR,
CDCl3, d (ppm): 7.72–7.59 (m, 1H, indole ring), 7.44–7.36 (m, 1H,
indole, phenyl), 7.32–7.28 (m, 2H, indole), 7.23–7.14 (m, 1H,
phenyl), 7.10–7.04 (m, 3H, indole, phenyl), 6.98–6.90 (m, 2H,
phenyl), 6.64 (s, 1H, indole), 4.03–3.90 (m, 4H, piperazine), 3.86
(s, 3H, CH3), 3.35–3.10 (m, 4H, piperazine). ESI-MS: m/z 320
[Mþ1] 100%.
The synthetic route to piperazine analogs of indole-2-
carboxamides is illustrated in the Scheme 2. The compounds
were synthesized in a sequence of reactions from the starting
material indole-2-carboxylic acid 9.
Synthesis of ethyl 1H-indole-2-carboxylate 10
Indole-2-carboxylic acid (1 mmol) was dissolved in a sufficient
amount of ethanol in a round-bottomed flask. To this 2–3 mL
of conc. H2SO4 was added. The reaction mixture was refluxed
for 6 h. Upon completion of the reaction the reaction
mixture was poured onto crushed ice and neutralized using
sodium bicarbonate. The solid thus obtained was filtered and
washed with water and used for further reaction without
purification.
(1-Ethyl-1H-indol-2-yl)(4-benzylpiperazin-1-yl)methanone
13b: IR (KBr)/cm: 3000, 2063, 1620, 1523, 1240, 1155. 1H NMR,
CDCl3, d (ppm): 7.70–7.58 (m, 2H, indole ring), 7.50–7.37 (m, 2H,
indole), 7.45–7.36 (m, 3H, indole), 7.35–7.29 (m, 1H, phenyl),
7.23–7.09 (m, 1H, phenyl), 6.60 (s, 1H, indole), 4.28–4.26 (m,
2H), 4.22–3.96 (m, 4H, piperazine), 3.86 (s, 3H, CH3), 3.42–3.38
(m, 4H, piperazine), ESI-MS: m/z 334.1 [Mþ1] 100%.
(1-Methyl-1H-indol-2-yl)(4-nitrophenylperazin-1-yl)metha-
none 13c: IR (KBr)/cm: 3010, 2050, 1620, 1525, 1240, 1155. 1H
NMR, CDCl3, d (ppm): 8.14–8.07 (m, 2H, indole ring), 7.64–7.58
(m, 1H, indole, phenyl), 7.51–7.45 (m, 1H, indole), 7.36–7.24 (m,
1H, phenyl), 7.18–7.10 (m, 1H, phenyl), 6.97–6.95 (m, 2H,
phenyl), 6.71 (s, 1H, indole), 3.96–3.85 (m, 4H, piperazine), 3.82
(s, 3H, CH3 of N-CH3), 3.68–3.52 (m, 4H, piperazine). ESI-MS: m/z
365 [Mþ1] 100%.
Synthesis of ethyl 1-methyl-1H-indole-2-carboxylate
11 [39]
The ethyl ester (1 mmol) was dissolved in anhydrous DMSO, to
this KOH pellets (3 mmol) and methyl iodide (1.5 mmol) were
added and the reaction mixture was stirred at room
temperature for 1 h. During which the reaction was monitored
using TLC using EtOAc/n-hexane (3:7). Upon completion of the
reaction the reaction mixture was poured on crushed ice. The
solid thus obtained was washed with water and filtered.
(1-Methyl-1H-indol-2-yl)(2-chlorophenylpiperazin-1-yl)metha-
none 13d: IR (KBr)/cm: 3120, 3050, 1625, 1530, 1240, 1125. 1H
NMR, CDCl3, d (ppm): 7.54–7.52 (m, 1H, indole ring), 7.36–7.28
(m, 2H, indole, phenyl), 7.26–7.14 (m, 2H, indole), 7.13–7.04 (m,
1H, phenyl), 6.98–6.90 (m, 2H, phenyl), 6.60 (s, 1H, indole),
4.03–3.87 (m, 4H, piperazine), 3.76 (s, 3H, CH3), 3.14–2.84 (m,
4H, piperazine). ESI-MS: m/z 354.1 [Mþ1] 100%.
Synthesis of 1-methyl-1H-indole-2-carboxylic acid 12
The ester (2 g, 8.45 mmol) was dissolved in 10 mL ethanol; to
this aqueous NaOH, equimolar was added, and the reaction
mixture was stirred at room temperature for 1 h, during which
the reaction was monitored by TLC using EtOAc/n-hexane (1:1)
as solvent system. On completion of reaction the mixture was
evaporated in vacuo to obtain a dark brown thick liquid, to this
crushed ice was added and acidified with dilute HCl up to pH 3.
Brown solid precipitated out which was filtered and washed
with ice cold water to obtain pure acid. 1H NMR, CDCl3, d
(ppm): 7.77–7.60 (m, 1H, indole), 7.42–7.36 (s, 1H, indole), 7.30–
(1-Methyl-1H-indol-2-yl)(3-chlorophenylpiperazin-1-yl)metha-
none 13e: IR (KBr/cm): 3120, 3080, 1625, 1550, 1240, 1035. 1H
NMR, CDCl3, d (ppm): 7.72–7.56 (m, 1H, indole ring), 7.45–7.37
(m, 1H, indole, phenyl), 7.33–7.29 (m, 1H, indole), 7.26–7.22 (m,
3H, phenyl), 7.19–7.13 (m, 2H, phenyl), 6.64 (s, 1H, indole),
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4.02–3.88 (m, 4H, piperazine), 3.86 (s, 3H, CH3), 3.35–3.19 (m,
4H, piperazine). ESI-MS: m/z 354 [Mþ1] 100%.
CDCl3, d (ppm): 7.50–7.46 (m, 2H, indole ring), 7.38–7.36 (m, 1H,
indole), 7.28–7.18 (m, 1H, indole), 6.70 (s, 1H, indole), 3.96–3.85
(m, 4H, piperazine), 3.80 (s, 3H, -CH3 of N-CH3), 3.68–3.52 (m,
4H, piperazine), 2.90 (q, 2H, -CH2), 2.80 (t, 3H, -CH3). ESI-MS: m/
z 272 [Mþ1] 100%.
(1-Methyl-1H-indol-2-yl)(4-chlorophenylpiperazin-1-yl)metha-
1
none 13f: IR (KBr/cm): 3100, 3080, 1625, 1550, 1240, 1035. H
NMR, CDCl3, d (ppm): 7.68–7.59 (m, 1H, indole ring), 7.44–7.29
(m, 3H, indole, phenyl), 7.26–7.22 (m, 2H, indole), 7.20–7.10 (m,
1H, phenyl), 6.95–6.81 (m, 1H, phenyl), 6.63 (s, 1H, indole),
4.01–3.89 (m, 4H, piperazine), 3.85 (s, 3H, CH3), 3.34–3.08 (m,
4H, piperazine). ESI-MS: m/z 354 [Mþ1] 100%.
(1-Methyl-1H-indol-2-yl)(4-propylpiperazine-1-yl)methanone
13m: IR (KBr)/cm: 3130, 3050, 1625, 1580, 1244, 1149. 1H NMR,
CDCl3, d (ppm): 7.50–7.46 (m, 2H, indole ring), 7.38–7.36 (m, 1H,
indole), 7.28–7.18 (m, 1H, indole), 6.70 (s, 1H, indole), 3.96–3.85
(m, 4H, piperazine), 3.80 (s, 3H, CH3 of N-CH3), 3.68–3.52 (m,
4H, piperazine), 2.93–2.90 (t, 2H, -CH2), 2.86–2.83 (m, 2H, -CH2),
2.78–2.70 (t, 3H, -CH3). ESI-MS: m/z 286.1 [Mþ1] 100%.
(1-Methyl-1H-indol-2-yl)(2-methoxyphenylpiperazin-1-yl)-
methanone 13g: IR (KBr/cm): 3130, 3050, 1626, 1550, 1240,
1
1035. H NMR, CDCl3, d (ppm): 7.57–7.50 (m, 1H, indole ring),
Pharmacological evaluation
7.36–7.21 (m, 2H, indole, phenyl), 7.21–7.18 (m, 1H, indole),
7.06–7.04 (m, 1H, phenyl), 6.95–6.91 (m, 3H, indole, phenyl),
6.55 (s, 1H, indole), 4.08–3.83 (m, 4H, piperazine), 3.88 (s, 3H,
-CH3 of N-CH3), 3.79 (s, 3H, OCH3 of phenyl), 3.19–2.87 (m, 4H,
piperazine). ESI-MS: m/z 350 [Mþ1] 100%.
Animals
All the animals were obtained from Hissar Agricultural
University (Hissar, Haryana, India) and maintained under
standard light/dark 12.00–12.00 h, temperature (23 ꢂ 2°C), and
room humidity (60 ꢂ 10%) conditions. The rodents were
housed in standard polycarbonate cages and fed with
standard animal feed (standard pellet chow) and water ad
libitum. The animals were used only once for each experiment
and were acclimatized to the experimental room for 1 h before
testing. Experiments on animals were approved by the
Institutional Animal Ethics Committee of Birla Institute of
Technology & Science, Pilani, India (Protocol No. IAEC/RES/14/
04).
(1-Methyl-1H-indol-2-yl)(3-methoxyphenylpiperazin-1-yl)-
methanone 13h: IR (KBr/cm): 3130, 3050, 1628, 1550, 1230,
1
1035. H NMR, CDCl3, d (ppm): 7.67–7.60 (m, 1H, indole ring),
7.46–7.36 (m, 1H, indole, phenyl), 7.34–7.29 (m, 1H, indole),
7.14–7.11 (m, 2H, phenyl), 6.64 (s, 1H, indole), 6.61–6.55 (m, 1H,
phenyl), 6.49–6.45 (m, 2H, phenyl), 4.02–3.93 (m, 4H, pipera-
zine), 3.86 (s, 3H, CH3 of N-CH3), 3.80 (s, 3H, OCH3 of phenyl),
3.33–3.12 (m, 4H, piperazine). ESI-MS: m/z 350 [Mþ1] 100%.
(1-Methyl-1H-indol-2-yl)(2-methylphenylpiperazin-1-yl)meth-
anone 13i: IR (KBr)/cm: 3150, 3050, 1628, 1530, 1230, 1135. 1H
NMR, CDCl3, d (ppm): 7.69–7.55 (m, 1H, indole ring), 7.43–7.33
(m, 1H, indole, phenyl), 7.30–7.26 (m, 1H, indole), 7.25–7.13 (m,
3H, phenyl), 7.11–6.95 (m, 2H, phenyl), 6.64 (s, 1H, indole),
4.03–3.89 (m, 4H, piperazine), 3.87 (s, 3H, CH3), 3.14–2.79 (m,
4H, piperazine), 2.35 (s, 3H, CH3 of phenyl). ESI-MS: m/z 334
[Mþ1] 100%.
Determination of 5-HT3 receptor antagonism
The synthesized compounds were tested for 5-HT3 receptor
antagonistic activity in longitudinal muscle myenteric plexus
preparation from guinea pig ileum against 5-HT3 agonist, 2-
methyl 5-HT. Dunkin Hartley guinea pigs (250–300 g) were
sacrificed by cervical dislocation. The abdomen was cut open
and a length of ileum was excised about 2 cm from the ileo-
caecal junction. The longitudinal muscle-myenteric plexus
(LMMP), 3–4 cm in length, was prepared and mounted as
described by the literature method [40]. The tissue was kept
for equilibration for 30 min under a resting tension of 500 mg
and with constant aeration in a 40 mL organ bath containing
Tyrode solution (composition for 1 L NaCl 8 g, KCl 0.2 g, CaCl2
0.2 g, MgCl2 0.1 g, NaH2PO4 0.05 g, NaHCO3 1.00 g, glucose
1.0 g, pH 6.7) maintained at 25°C. Non-cumulative concen-
trations of the agonist 2-methyl-5-HT (Tocris, UK) were added
with a 15 min dosing cycle (to prevent desensitization) and left
in contact with the tissue until the maximal contraction had
developed. To assess the antagonistic effect of the synthesized
compounds on the response incited by the agonist, the
compounds were added to the organ bath and left in contact
with the tissue for at least 10 min prior to the addition of 2-
methyl-5-HT. The contractions were recorded using a T-305
force transducer coupled to a Student’s physiograph (Bio
Devices, Ambala, India). 5-HT3 receptor antagonism was
denoted in the form of pA2 values, which was determined
according to literature methods [40–43]. The pA2 values of the
(1-Methyl-1H-indol-2-yl)(4-methylphenylpiperazine-1-yl)-
methanone 13j: IR (KBr)/cm: 3150, 3050, 1625, 1530, 1230,
1
1135. H NMR, CDCl3, d (ppm): 8.14–8.07 (m, 2H, indole ring),
7.64–7.58 (m, 1H, indole), 7.51–7.45 (m, 1H, indole), 7.36–7.24
(m, 1H, phenyl), 7.18–7.10 (m, 1H, phenyl), 6.97–6.95 (m, 2H,
phenyl), 6.71 (s, 1H, indole), 3.96–3.85 (m, 4H, piperazine), 3.82
(s, 3H, CH3 of N-CH3), 3.68–3.52 (m, 4H, piperazine). ESI-MS: m/z
334.1 [Mþ1] 100%.
(1-Methyl-1H-indol-2-yl)(4-methylpiperazine-1-yl)methanone
13k: IR (KBr)/cm: 3130, 3010, 1620, 1580, 1244, 1150. 1H NMR,
CDCl3, d (ppm): 7.56–7.46 (m, 2H, indole ring), 7.38–7.34 (m, 1H,
indole), 7.28–7.18 (m, 1H, indole), 6.73 (s, 1H, indole), 3.96–3.85
(m, 4H, piperazine), 3.80 (s, 3H, CH3 of N-CH3), 3.68–3.52 (m, 4H,
piperazine), 2.60 (s, 3H, -CH3). ESI-MS: m/z 258.1 [Mþ1] 100%.
(1-Methyl-1H-indol-2-yl)(4-ethylpiperazine-1-yl)methanone
13l: IR (KBr)/cm: 3120, 3030, 1628, 1580, 1244, 1150. 1H NMR,
ß 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Archiv der Pharmazie
test compounds were compared with the standard antagonist
ondansetron (Natco Pharma, Hyderabad, India). The results
are represented in Table 5.
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