Synthesis of 3-(5-bromo-2,3-dimethoxy-phenyl)-[1, 2, 4] oxadiazole analogues and their evaluation as anti-Parkinson's agents

Article abstract of DOI:10.1007/s00044-007-9074-z

A series of 3-(5-bromo-2,3-dimethoxy-phenyl)-[1, 2, 4] oxadiazole derivatives was prepared and their evaluation for anti-Parkinson's activity was measured in vivo using albino rats. The result of the biological activity studies indicated that some of the

Full text of DOI:10.1007/s00044-007-9074-z

MEDICINAL
CHEMISTRY
RESEARCH
Med Chem Res (2008) 17:386–398
DOI 10.1007/s00044-007-9074-z
ORIGINAL RESEARCH
Synthesis of 3-(5-bromo-2,3-dimethoxy-phenyl)-[1, 2, 4]
oxadiazole analogues and their evaluation as
anti-Parkinson0s agents
Shashi B. Tiwari Æ D. V. Kohli
Received: 21 November 2007 / Accepted: 22 November 2007 / Published online: 28 March 2008
¨
Ó Birkhauser Boston 2008
Abstract A series of 3-(5-bromo-2,3-dimethoxy-phenyl)-[1, 2, 4] oxadiazole
derivatives was prepared and their evaluation for anti-Parkinson’s activity was
measured in vivo using albino rats. The result of the biological activity studies
indicated that some of the synthesized compounds have good agonistic activity on
the dopamine receptors and a few of them were also found to be free from
neurotoxicity. Thus these compounds might be useful ligands for studying the
functional role of dopamine receptors in vivo. The high log P value of the com-
pounds indicates that they should easily cross the blood-brain barrier (log P [ 2.6).
O
OCH3
N
H3CO
R
N
Br
Keywords Dopamine receptor antagonist ꢀ Antiparkinson ꢀ Oxadiazole
Introduction
Parkinson’s disease (PD) and Alzheimer disease (AD) are the most common
neurodegenerative disorders. They affect at least 5% of the population above the age
of 65 years (McDowell, 2001). The current drug therapy used for PD consists
mainly of L-dopa and/or dopamine (DA) agonist, monoamine oxidase B inhibitors
such as rasagiline, selegline, catechol-O-methyl transferase inhibitor (COMT), and
S. B. Tiwari ꢀ D. V. Kohli (&)
Pharmaceutical Chemistry and Drug Design Research Laboratory, Department of Pharmaceutical
Sciences, Dr. H. S. Gour University, Sagar, MP 470003, India
e-mail: drdvkohali@rediffmail.com

Med Chem Res (2008) 17:386–398
387
entacapone. These drugs can improve clinical symptoms but cannot mitigate
progression of the disease process underlying PD (Mandel et al., 2003). Dopamine
receptors are G-protein coupled receptors and share a characteristic seven
transmembrane domain. The five-dopamine receptor subtypes are classified into
two categories (Drukarch and maiswinkel, 2000). The first one is known as the D1
family, containing D1- and D5-subtype receptors; they are characterized by
activation of adenyl cyclase mediated by G5 protein consequently effecting higher
concentrations of the secondary messenger cyclic adenosine 3,5-monophosphate
(cAMP). These receptors lack introns in their coding gene (Levant, 1997). The
second category, the D2 family, consists of the D2, D3, and D4 receptors subtypes,
which couple to G1 proteins and can inhibit adenyl cyclase. In the gene of dopamine
D2-like receptors introns can be found (Civelli et al., 1991; Civelli et al., 1993).
Within each of dopamine-containing systems dopamine receptors are located either
postsynaptically, terminal of afferent fiber projection on the cell bodies, dendrites of
neurons or presynaptically on the dopamine nerve terminal (Broughton et al., 1975;
Waddington, 1993; Hodgetts et al., 2001; Cohen et al., 1990).
The third intracellular loop exhibits the largest sequence dissimilarities among the
different DA receptors. The D1 and D5 receptors have relatively short third
intracellular loops which are coupled to Gs proteins and have long C-terminal tails.
These two receptors stimulate the activity of adenyl cyclase and the pharmacological
functions of known ligands are more or less identical. The D2-like receptors, i.e., D2,
D3, and D4 receptors on the other hand, all have long third intracellular loops with short
C-terminal tails which might couple to Gi proteins (or G0 proteins) and inhibit adenyl
cyclase. Interesting, two forms of the DA D2 receptors have been found, differing in 29
amino acids in the third intracellular loop; they seem to have identical pharmacology
but their presence in various cerebral tissues differ. Hence, a difference in
functionality is likely still to be found. The long and the short forms of the D2
receptors (D2L and D2S, respectively) consist of 443 and 414 amino acid residues,
respectively. The genes of the dopamine receptors superfamily can be divided into two
different categories: (1) intronless genes that code for the D1-like receptors and (2)
genes with their coding sequences in discontinuous DNA segments (axons) separated
by sequences (introns) that do not form a part of the mature mRNA. The latter category
of genes are found in the D2-like family of dopamine receptors, which explains the
occurrence of a long and a short form of the DA D2 receptor. In the biosynthesis of
mRNA a mechanism called alternative splicing, in which a given axon in the pre-
mRNA is present or absent in the final mRNA, results in two different proteins coded
by the same gene (Fuller et al., 1968; John, 2002; Strange, 1993).
Furthermore, both DA neurons and receptors are markedly reduced by normal
aging and Parkinson’s disease and have been implicated in a variety of other
disorders, including schizophrenia and drug abuse. Dopaminergic neurotransmis-
sion and dopamine receptors underlie manifold neurological and psychiatric
disorders for e.g. Parkinson’s disease, Huntington disease, Schizophrenia, Tourette
syndrome, Tardive dyskinesia and Drug abuse. (Sedvall and Farade, 1995)
Parkinson’s disease is characterized by tremor (uncontrolled shaking), rigidity
(muscular rigidity), and akinesia (inability to walk properly). There is no curative
treatment for Parkinson’s disease till date and the present treatment involves only

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Med Chem Res (2008) 17:386–398
eliminating or reducing exposure to the toxic substances followed by symptomatic
and supportive therapy (Whitehouse, 1962; Langridge et al., 1981; Schoenborn,
1969). This is because there is no drug available for selective dopamine D2 and D3
agonism. In view of this and in continuation of our research work towards a cure for
Parkinson’s disease we synthesized a series of 3-(5-bromo-2,3-dimethoxy-phenyl)-
[1, 2, 4] oxadiazole derivatives. The structural elucidation was carried out by
various spectroscopic methods. The compounds were screened for anti-Parkinson’s
activity as well as neurotoxicity using albino rats in vivo. The result of the
biological activity studies indicated that some of the synthesized compounds have
good agonistic activity on the dopamine receptors and a few of them were also
found to be free from neurotoxicity. Showing an agonistic effect on the dopamine
receptors, these synthesized compounds may be very useful for the treatment of
Parkinson’s disease (Dandiya and Bhargava, 1968).
Chemistry
The synthesis of 3-(5-bromo-2,3-dimethoxy-phenyl)-[1, 2, 4] oxadiazole was
accomplished via the sequence reaction outlined in Table 1. 2,3-Dihydroxy benzoic
acid (1) on bromination afforded 5-bromo-2,3-dihydroxy benzoic acid (2), which on
methylation gave 5-bromo-2,3-dimethoxy benzoic acid (3). 5-Bromo-2,3-dimethoxy
benzoic acid (3) on reaction with thionyl chloride followed by treatment with tert-
butyl amine afforded N-tert-butyl-5-bromo-2,3-dimethoxy-benzamide (5). The
synthesis of 5-bromo-2,3-dimethoxybenzonitrile (6) was accomplished via the
treatment of N-tert-butyl-5-bromo-2,3-dimethoxy-benzamide (5) with phosphorous
oxychloride (Litchfield and Wilcoxan, 1949). The treatment of 5-bromo-2,3-
dimethoxybenzonitrile (6) with hydroxyl amine in the presence of base afforded 2,3-
dimethoxy-5-bromo benzamide oxime (7), which on treatment with various alkyl
methyl esters gave alkyl 3-(5-bromo-2,3-dimethoxy-phenyl)-[1, 2, 4] oxadiazole
derivatives (8a-h).
In Vivo Evaluation
The in vivo performance of the compounds was carried out by assessing the
reduction in the degree of drug-induced catatonia (chlorpromazine) in albino rats
and compared with dopamine, plain levodopa, and the combination of levodopa and
carbidopa. The purpose of this type of study was to demonstrate the anti-Parkinson’s
activity of the synthesized compounds and its comparison with dopamine, plain
levodopa, and the combination of levodopa and carbidopa. The reduction in the
degree of drug-induced catatonia (rigidity and akinesia) was assessed by the method
described by Kulkarni et al. in which the drug is administered intraperitoneally and
after 30 min chlorpromazine is administered to induce extrapyramidal effects
(tremor, rigidity, and akinesia) (Kulkarni, 1980; Kulkarni et al., 1980). (Table 1,
Scheme 1).

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389
Table 1 Substituents in the oxadiazole ring
Compound (8a-h)
Compound (8a-h)
R
R
CH3
N
CH2Ph
N
8a
8e
H3COOC
CH3
CH2Ph
8b
8c
8f
N
N
CH2Ph
N
CH3
N
8g
8d
8h
N
CH2Ph
N
CH3
OCH3
OH
Br
OCH3
OH
H3CO
HO
H3CO
COOH
COOH
COCl
HO
COOH
b
c
a
Br
1
Br
4
2
3
OCH3
OCH3
OCH3
O
NOH
NH2
H3CO
CN
H3CO
H3CO
d
C(CH3)3
e
f
N
H
Br
Br
6
Br
7
5
OCH3
N
H3CO
O
R
g
N
Br
8a-h
Scheme 1 (a) Br2, AcOH, rt, 88%; (b) (CH3)2SO4, acetone, K2CO3, reflux, 86%; (c) SOCI2, benzene,
reflux, 95%; (d) t-BuNH2, CH2CI2, rt, 96%; (e) POCI3, C6H6, reflux, 88%; (f) NH2OH.HCI, K2CO3,
EtOH, reflux, 88%; (g) NaH, RCOOCH3, HCOOH, alumina, reflux
Seventy-two albino rats were selected, weighed, and divided into 12 groups of six
animals each. The animals in the first group were kept as control (without drug
administration) while plane dopamine solution (dose 10 mg/kg), plain levodopa

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Med Chem Res (2008) 17:386–398
solution (dose 10 mg/kg), and the marketed preparation syndopa (levodopa + car-
bidopa, dose 2.5 mg/kg), were injected intraperitoneally to the animals in the
second, third, and fourth groups respectively. The animals in the fifth to twelve
groups were injected with the synthesized compounds 8a-h intraperitoneally
(100 mg/kg). Thirty minutes after administrating the above drugs and synthesized
compounds, chlorpromazine (5 mg/kg) was injected intraperitoneally to all the
animals to induce catatonia. All the animals were fasted for 12 hours before
injection of the drugs. The reduction in the degree of drug-induced catatonia
(difficulty to move and change the posture) was assessed by placing the front paw of
the rat on a block of 3 and 9 cm height alternatively. The observations are recorded
into in Table 2. The comparative responses of drug-induced catatonia are shown in
Fig. 1 (time versus mean score of catatonic activity (MSCA) for different
molecules). Table 3
Experimental Protocol for Neurotoxicity
Drug interference activity with motor coordination was checked by using the
rotorod test (D’Amour and Smith, 1941; Litchfield and Wilcoxan, 1949) in
which rats (20–25 g) were trained to stay on a knurled plastic rod (3.2 cm
diameter) that was rotated at 10 rpm; normal rats can maintain equilibrium on
such a rotating rod for a longer time. The test compounds were injected
intraperitoneally at a dose of 200 mg/kg and tested at the time of peak drug
effect. The neurotoxicity of compounds was indicated by inability of the animals
to maintain equilibrium on the rod for at least 1 min in each of three trials. Rats
lost balance due to the skeletal muscle relaxation effect of the drugs. The dose at
which 50% of the animals were unable to balance and fell off the rotating rod
was determined.
Table 2 Catatonic activity scores in rats
S. Animal
no. group
Compound
MSCA (min)
15 30
45
60
90
120
150 180
01
02
03
04
05
06
07
08
09
10
11
02
03
04
05
06
07
08
09
10
11
12
Dopamine
Levodopa
0.0 0.84 1.33
2.08
2.667 3.333 3.500 3.500
0.0 0.68 1.1667 1.9167 2.5833 3.1667 3.500 3.500
Levodopa + carbidopa 0.0 0.0
0.0
0.0
0.083 0.833 1.167 3.0833
8a
8b
8c
8d
8e
8f
0.0 0.16 0.66
0.0 0.25 0.75
0.0 0.25 0.75
0.0 0.16 0.66
0.0 0.083 0.66
0.0 0.33 0.75
1.00
1.16
1.16
1.08
1.33
1.5
1.7
2.83 3.333
3.00 3.417
3.08 3.417
2.92 3.333
3.25 3.333
3.167 3.417
3.167 3.417
2.926 3.333
2.00
2.08
1.75
2.08
2.33
2.16
1.58
1.58
1.083 1.42
1.167 1.67
8g
8h
0.0 0.3
0.66
1.08
1.55
0.0 0.08 0.58
0.927 1.334 1.92

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391
5
4.5
4
3.5
3
2.5
2
888
1.5
1
0.5
0
0
30
60
90
120
8c
150
180
210
8h
Time
plain DA
levodopa
C+L
8a
8b
8d
8e
8f
8g
Fig. 1 Comparative response of drug-induced catatonia (time versus MSCA)
Table 3 Mean score of
catatonic activity value of the
compounds after 180 min,
S.
no.
Compound
Mean Score of
Catatonic Activity testing
Neurotoxicity Log P
After 180 Min.
(MSCA)
(30 min)
neurotoxicity and log P values
of the synthesized compounds
01
02
03
04
05
06
07
08
09
8a
8b
8c
8d
8e
8f
3.333
3.417
3.417
3.333
3.333
3.417
3.417
3.333
3.5
+
+
+
+
–
3.7901
4.0798
4.2074
3.8568
5.5227
5.8122
5.8122
5.5894
–
Dose: 200 mg/kg i.p.;
neurotoxicity were measured
after 30 min
8g
8h
–
–
(+) Indicates 50% or more
passed the neurotoxicity test,
i.e., not showing neurological
disorder
Plain
+
dopamine
10
11
Plain
3.5
+
+
levodopa
(–) Indicates 50% or more failed
the neurotoxicity test, i.e.,
showing neurological disorder
(Levodopa
3.083
+ carbidopa)
Discussion
Mach et al. (1999) reported a series of benzamide analogues possessing a high
affinity for dopamine D2 and D3 receptors. Molecular modeling studies revealed

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Med Chem Res (2008) 17:386–398
differences in the stereoelectronic properties of the oxadiazole binding region of the
D2 and D3 receptors. These subtle differences in the electrostatic properties of this
class of compounds suggested that isosteric replacement of the amide group with the
heterocylic ring could produce a shift in affinity of these compounds for D2 and D3
receptors. These observations led to the synthesis and evaluation of a series of
oxadiazole analogues as potential dopamine-receptor-selective ligands. Lipophilic-
ity is thought to be a major determinant in the permeability of a drug. In order to
correlate the permeability of the synthesized compounds to their lipophilicity, we
performed the partitioning and log P studies, which confirmed that the compounds
are more lipophilic in nature and should cross the blood–brain barrier (log P [ 2.6)
(Chu et al., 2005). Robert et al. (2003) suggested that the pyrole ring, which is a
p-excessive heteroaromatic ring, represents a better isoteric substitution for the
benzamide moiety than an imidazole ring, which p-deficient. Therefore we decided
to test this hypothesis with another p-excessive heteroaromatic ring system, i.e.,
oxadiazole. The first goal of the current studies was to synthesize a number of
oxadiazole analogues and evaluate them for dopamine receptors agonistic activity.
The results of the above studies revealed that the oxadiazole analogues with affinity
for the dopamine receptors act as agonists to reduce the degree of drug-induced
catatonia. The second goal of the study was to determine to the effect on the
location of the aromatic ring in the tertiary amine moiety on D2-like dopamine
receptors binding. This type of compounds are also capable of forming intermo-
lecular hydrogen bonds between the ortho and para methoxy groups with the
receptors amino acid sequences and proton donor. Previous studies have suggested
that the dopamine receptors have three aromatic binding pockets, named AR1, AR2,
and AR3, as well as the site that recognizes basic nitrogen atoms. The benzamide
aromatic ring containing compounds such as clebopride and sulpride are thought to
bind to the AR1 aromatic pocket (primary aromatic binding site) while the benzylic
aromatic group of MABN, IABN, and related compounds (e.g., clebopride and
synthesized compounds) bind to either AR2 or AR3 regions (Ludtke et al., 2000).
Therefore these oxadiazole derivatives showed lower activity profile than the
combination of levodopa and carbidopa but greater activity profile than plain
dopamine and levodopa. The in vivo studies revealed that none of the synthesized
compounds caused any considerable reduction in the drug-induced catatonia
compared to the combination of levodopa and carbidopa, but that all the compounds
showed a greater reduction in drug-induced catatonia than dopamine or levodopa
alone (Fig. 1). The results can be summarized as:
levodopa + carbidopa (3.083) = greater reduction in drug-induced catatonia.
8a, 8d, 8e, 8h (3.33) = less reduction in drug-induced catatonia than levo-
dopa + carbidopa but more than 8b, 8c, 8f, 8g.
8b, 8c, 8f, 8g (3.417) = less reduction in drug-induced catatonia than the
compounds above but more than dopamine or levodopa alone.
Dopamine and levodopa alone did not show any reduction in drug-induced
catatonia.
Compounds 8e, 8f, 8g, and 8h showed neurotoxicity.
In future these studies can help in the design and optimization of ligands with
even greater potency/selectivity.

Med Chem Res (2008) 17:386–398
393
Experimental
Chemistry
Melting points were determined by open-capillary melting-point apparatus and were
uncorrected. Elemental analysis of the synthesized compounds was carried out on a
Carlo ERBA-1108 analyzer (Sophisticated Analytical Instrument Facility, Central
Drug Research Institute, Lucknow) and the values were found to be within 4% of the
1
theoretical values. H nuclear magnetic resonance (NMR) spectral analysis of the
synthesized compounds was recorded on a Brucker Advance DPX at 200 and
300 MHz (IICT Hyderabad and CDRI Lucknow) using CDCl3 as a solvent and tetra
methyl silane (TMS) as an internal reference unless stated otherwise; chemical shift
values are expressed in d parts per million (ppm). Fast atom bombardment/parts per
million (FAB/ppm) mass spectra of the synthesized compounds were recorded on a
JEOL (Tokyo, Japan) SX 102/DA-6000 mass spectrometer using argon/xenon (6 kV,
10 MA) as the FAB gas. Thin-layer chromatography (TLC) of the compounds was
performed on precoated silica gel-G at a 0.2 mm thickness on aluminum sheet using a
different solvent system to ascertain the purity of the synthesized compounds. Flash
column chromatography on silica gel 230–400 mesh was used to purify the
compounds. Iodine vapor and an ultraviolet (UV) lamp were used for detection.
All the starting materials and solvents were purchased from Aldrich, Fisher or
Lancaster and were used without purification.
Synthesis of 2,3-dihydroxy-5-bromobenzoic acid (2)
To a solution of 2,3-dihydroxy benzoic acid (1, 25.0 g, 0.16 mol) in acetic acid
(200 mL), bromine (25.9 g, 0.16 mol) was added dropwise. The reaction mixture
was stirred at room temperature overnight. Then the reaction, the reaction mixture
was concentrated in vacuo to give a solid which was recrystallized from ethyl
acetate/hexane to give 2,3-dihydroxy-5-bromobenzoic acid (2, 33.2 g, 88%), m.p.
202–204°C, lit. m. p. 204°C; anal. (C7H5O4Br).
Synthesis of 2,3-dimethoxy-5-bromobenzoic acid (3)
A mixture of 2,3-dihydroxy-5-bromobenzoic acid (2, 25.0 g, 0.11 mol), dimethyl-
sulfate (48.71 g, 0.38 mol), and potassium carbonate (53.4 g, 0.38 mol) in acetone
(200 mL) was refluxed overnight. Then the reaction mixture was filtered, the filtrate
was concentrated, and then dissolved in methanol (200 mL). To this solution, NaOH
(40%, 12 mL) was added and the reaction mixture was refluxed for 2 h, the reaction
mixture was concentrated and the residue was dissolved in water (200 mL). The pH
of the aqueous phase was adjusted to d 2 and then extracted with ethyl acetate
(3 9 60 mL), dried over Na2SO4, and concentrated under reduced pressure. The
residue was further recrystallized from ethyl acetate/hexane to give 2,3-dimethoxy-
5-bromobenzoic acid (3, 24.2 g, 86%), m.p. 199–200°C, lit. m. p. 200°C; analysis
(C9H9O4Br). 1H NMR (300 MHz, CDCl3): 7.84–7.85 (d, J = 3 Hz, 1 H), 7.25–7.26
(d, J = 3 Hz, 1 H), 4.07 (s, 3 H), 3.93 (s, 3 H).

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Med Chem Res (2008) 17:386–398
Synthesis of 2,3-dimethoxy-5-bromobenzoyl chloride (4)
A solution of 2,3-dimethoxy-5-bromobenzoic acid (3, 20.0 g, 0.08 mol) and thionyl
chloride (18.2 g, 0.15 mol) in benzene (200 mL) was refluxed over night. The
reaction mixture was then concentrated and the residue was recrystallized from
ethyl acetate/hexane to give 2,3-dimethoxy-5-bromobenzoyl chloride (4, 20.3 g,
1
95%), m.p. 65–66°C, analysis (C9H8O3BrCl). H NMR (300 MHz, CDCl3): 7.61–
7.62 (d, J = 3 Hz, 1 H), 7.23–7.24 (d, J = 3 Hz, 1 H), 3.90 (s, 6 H).
Synthesis of N-t-butyl-2,3-dimethoxy-5-bromo benzamide (5)
To a solution of (4, 20.0 g, 0.07 mol) in dry dichloromethane (200 mL) and
t-butylamine (7.3 g, 0.1 mol), triethyl amine (5 mL) was added dropwise. The
reaction mixture was stirred overnight at room temperature. Then the reaction
mixture was concentrated in vacuo and the residue was dissolved in dichlorometh-
ane (100 mL). The organic layer was washed with 0.5 N NaOH in water and dried
over Na2SO4, concentrated under reduced pressure. Purification of the crude
reaction product by column chromatography on silica gel using hexane-EtOAc (4:1)
as the eluant furnished the N-t-butyl-2,3-dimethoxy-5-bromo benzamide (5, 22.6 g,
96%) as a yellow syrup; analysis (C13H18BrNO3).1H NMR (300 MHz, CDCl3):
7.78–7.77 (d, J = 2.4 Hz, 1 H), 7.11–7.10 (d, J = 2.5 Hz, 1 H), 3.86, 3.88 (2 s, 6
H), 1.45 (s, 9 H).
Synthesis of 2,3-dimethoxy-5-bromo benzonitrile (6)
To a solution of N-tert-butyl-2,3-dimethoxy-5-bromobenzamide (8, 22.6 g,
0.07 mol) in benzene (100 mL), phosphorus oxychloride (109 g, 0.7 mol) was
added portionwise and the reaction mixture was refluxed for 10 h. Then the reaction
mixture was concentrated in vacuo. The residue was dissolved in CH2Cl2 (200 mL)
and washed with water, dried over Na2SO4, and concentrated under the reduced
pressure. The residue was recrystallized from ethylacetate/pentane to give 2,3-
1
dimethoxy-5-bromobenzonitrile (6, 15.3 g, 88%), analysis (C9H8NO2Br). H NMR
(300 MHz, CDCl3): 7.25–7.76 (d, J = 3 Hz, 1 H), 7.19–7.20 (d, J = 3 Hz, 1 H),
4.10 (s, 3 H), 3.89 (s, 3 H).
Synthesis of 2,3-dimethoxy-5-bromo benzamide oxime (7)
To the solution of 2,3-dimethoxy-5-bromobenzonitrile (9, 6.53 g, 0.04 mol) in
ethanol (100 mL), hydroxylamine hydrochloride (8.34 g, 0.12 mol) and potassium
carbonate (11.06 g 0.08 mol) were added and the reaction mixture was refluxed for
16 h. Then the reaction mixture was poured into cold water and 2 N NaOH was
added to the solution to pH greater than 12. The product was extracted with CH2Cl2,
washed with water, dried over Na2SO4, and concentrated under reduced pressure.
The residue was recrystallized from ethylacetate/hexane to give 2,3-dimethoxy-5-
bromo benzamide oxime (7, 5.38 g, 68%), m.p. 148–150°C, analysis

Med Chem Res (2008) 17:386–398
1
395
(C9H11N2O3Br). H NMR (300 MHz, CDCl3): 7.02–7.03 (d, 1 H), 7.19–7.20 (d, 1
H), 3.76 (s, 3 H), 3.89 (s, 3H), 2.0 (s, 2 H).
Typical procedure for the synthesis of compounds 8a-h: 3-(5-bromo-2,
3-dimethoxy-phenyl)-5-(1-methyl-pyrrolidin-2-yl)-[1, 2, 4] oxadiazole (8a)
To a solution of 2,3-dimethoxy-5-bromo benzamide oxime (7, 1 g, 0.36 mmol) in
tetrahydrofuran, NaH (0.09 g, 0.36 mmol) was added at 5°C. The reaction mixture
was stirred at room temperature for 30 min. and then refluxed for one hour. After
cooling the reaction mixture 1-methyl-pyrrolidine-2-carboxylic acid methyl ester,
alumina, and HCOOH were added and the reaction mixture was refluxed over night.
After completion of the reaction, the reaction mixture was poured into ice-cold
water and extracted with CH2Cl2 (3 9 50 mL), washed with water, dried over
Na2SO4, and concentrated under reduced pressure. Purification of the crude reaction
product by column chromatography on silica gel using ethyl acetate/ethanol (3:2)
solvent afforded the 3-(5-bromo-2,3-dimethoxyphenyl)-5-(1-methyl-pyrrolidin-2-
yl)-[1, 2, 4] oxadiazole as a yellow syrup, which was converted into corresponding
oxalic salt and recrystallized from ethyl acetate/ethanol; yield 62%; analysis
C15H18BrN3O3, m.p. 63–65°C, found: C, 48.90; H, 5.24; N, 11.31; 1H NMR
(300 MHz, CDCl3):TM = 7.10 (s, 1 H,), 6.69 (s, 1 H,), 3.73 (s, 6 H, OCH3), 3.52,
2.52 (m, 7 H,), 2.27 (s, 3 H, NCH3).
Synthesis of 3-[3-(5-bromo-2,3-dimethoxyphenyl)-[1, 2, 4] oxadiazol-5-yl]-1-
methyl-piperidine (8b)
The derivative 8b was synthesized using 2,3-dimethoxy-5-bromo benzamide oxime
(7, 1 g, 0.36 mmol), NaH (0.09 g, mol), 1-methyl-piperidine-2-carboxylic acid
methyl ester, alumina, and HCOOH in tetrahydrofuran following the same
procedure as discussed for compound 8a. Yield (61%), analysis C16H20BrN3O3,
1
m.p. 63–65°C, found: C, 50.17; H, 5.62; N, 10.89; H NMR (300 MHz, CDCl3):
= 7.10 (s, 1 H), 6.69 (s, 1 H), 3.73 (s, 6 H, OCH3), 2.78–2.32 (m, 9 H), 2.27
(s, CH3).
TM
Synthesis of 2-[3-(5-bromo-2,3-dimethoxy-phenyl)-[1, 2, 4] oxadiazol-5-yl]-1-
methyl-piperidine (8c)
The derivative (8c) was synthesized using 2,3-dimethoxy-5-bromo benzamide
oxime (7, 1 g, 0.36 mmol), NaH (0.09 g, 0.36 mmol), 1-methyl-piperidine-1-
carboxylic acid methyl ester, alumina, and HCOOH in tetrahydrofuran following
the same procedure as discussed for compound 8a; yield 52%, analysis
1
C16H20BrN3O3, m.p. 139–141°C, found: C, 50.20; H, 5.47; N, 10.89; H NMR
(300 MHz, CDCl3): 7.10 (s, 1 H), 6.69 (s, 1 H), 3.73 (s, 6 H, OCH3), 3.02–2.32
(m, 9 H), 2.27 (s, CH3).

396
Med Chem Res (2008) 17:386–398
Synthesis of 4-[3-(5-bromo-2,3-dimethoxy-phenyl)-[1, 2, 4] oxadiazol-5-yl]-1-
methyl-piperidine (8d)
The derivative 8d was synthesized using 2,3-dimethoxy-5-bromo benzamide oxime
(7, 1 g, 0.36 mmol), NaH (0.09 g, 0.36 mmol), 1-methyl-piperidine-3-carboxylic
acid methyl ester, alumina, and HCOOH in tetrahydrofuran following the same
procedure as discussed for compound 8a; yield 53%, analysis C16H20BrN3O3, m.p.
68–70°C, found: C, 50.17; H, 5.52; N, 10.85; 1H NMR (300 MHz, CDCl3): 7.10 (s,
1 H), 6.69 (s, 1 H ), 3.73 (s, 6 H, OCH3), 2.74–2.32 (m, 9 H ), 2.27 (s, CH3).
Synthesis of 5-(1-benzyl-pyrrolidin-2-yl)-3-(5-bromo-2,3-dimethoxy-phenyl)-
[1, 2, 4] oxadiazole (8e)
The derivative 8e was synthesized using 2,3-dimethoxy-5-bromo benzamide oxime
(7, 1 g, 0.36 mmol), NaH (0.09 g, 0.36 mmol), 1-benzyl-pyrrolidine-2-carboxylic
acid methyl ester, alumina, and HCOOH in tetrahydrofuran following the same
procedure as discussed for compound 8a; yield 60%, analysis C21H22BrN3O3, m.p.
1
63–65°C, found: C, 56.67; H, 5.22; N, 9.36; H NMR (300 MHz, CDCl3): 7.26–
7.14 (m, 5 H), 7.10 (s, 1 H), 6.69 (s, 1 H), 3.73 (s, 6 H, OCH3), 3.62 (s, CH2), 3.53–
2.50 (m, 7 H).
Synthesis of 1-benzyl-3-[3-(5-bromo-2,3-dimethoxy-phenyl)-[1, 2, 4]
oxadiazol-5-yl]-piperidine (8f)
The derivative 8f was synthesized using 2,3-dimethoxy-5-bromo benzamide oxime
(7, 1 g, 0.36 mmol), NaH (0.09 g, 0.36 mmol), 1-benzyl-piperidine-3-carboxylic
acid methyl ester, alumina, and HCOOH in tetrahydrofuran following the same
procedure as discussed for compound 8a; yield (58%), analysis C22H24BrN3O3, m.p.
1
64–66°C, found: C, 57.60; H, 5.50; N, 9.07; H NMR (300 MHz, CDCl3): 7.26–
7.14 (m, 5 H), 7.10 (s, 1 H), 6.69 (s, 1 H), 3.73 (s, 6 H, OCH3), 3.62 (s, CH2), 2.78–
1.79 (m, 9 H).
Synthesis of 1-benzyl-3-[3-(5-bromo-2,3-dimethoxy-phenyl)-[1, 2, 4] oxadiazol-
5-yl]-piperidine (8g)
The derivative 8g was synthesized using 2,3-dimethoxy-5-bromo benzamide oxime
(7, 1 g, 0.36 mmol), NaH (0.09 g, 0.36 mmol), 1-benzyl-piperidine-2-carboxylic
acid methyl ester, alumina, and HCOOH in tetrahydrofuran following the same
procedure as discussed for compound 8a; yield 51%, analysis C22H24BrN3O3, m.p.
138–140°C, found: C, 57.60; H, 5.45; N, 9.07; 1H NMR (300 MHz, CDCl3): 7.26–
7.14 (m, 5 H), 7.10 (s, 1 H), 6.69 (s, 1 H), 3.73 (s, 6 H, OCH3), 3.62 (s, CH2), 3.02–
1.50 (m, 9 H).

Med Chem Res (2008) 17:386–398
397
Synthesis of 1-benzyl-4-[3-(5-bromo-2,3-dimethoxy-phenyl)-[1, 2, 4] oxadiazol-
5-yl]-piperidine (8h)
The derivative 8h was synthesized using 2,3-dimethoxy-5-bromo benzamide oxime
(7, 1 g, 0.36 mmol), NaH (0.09 g, 0.36 mmol), 1-benzyl-piperidine-4-carboxylic
acid methyl ester, alumina, and HCOOH in tetrahydrofuran following the same
procedure as discussed for compound 8a; yield (54%), analysis C22H24BrN3O3, m.p.
1
70–71°C, found: C, 57.61; H, 5.45; N, 9.11; H NMR (300 MHz, CDCl3): 7.26–
7.14 (m, 5 H), 7.10 (s, 1 H), 6.69 (s, 1 H), 3.73 (s, 6 H, OCH3), 3.62 (s, CH2), 2.74–
1.50 (m, 9 H).
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