An efficient, microwave-assisted, one-pot synthesis of novel 5,6,7,8-tetrahydroquinoline-3-carbonitriles

Article abstract of DOI:10.2298/JSC100702071D

An efficient, microwave-assisted synthesis of novel 2-alkoxy-5,6,7,8- -tetrahydroquinoline-3-carbonitriles, which have not hitherto been reported, via reactions of cyclohexanone and arylidene malononitriles in the corresponding alcohols in presence of sodium is described. All the newly synthesized compounds were characterized by the IR, ¹H-NMR, ¹³C-NMR and mass spectroscopic techniques and by elemental analyses. The newly synthesized compounds were evaluated for their antibacterial and antifungal activities.

Full text of DOI:10.2298/JSC100702071D

J. Serb. Chem. Soc. 76 (6) 823–830 (2011)
UDC 547.831+547.592+547.447:542.913
JSCS–4162
Original scientific paper
An efficient, microwave-assisted, one-pot synthesis of novel
5,6,7,8-tetrahydroquinoline-3-carbonitriles
*
DIPTI K. DODIYA , HARESH K. RAM, AMIT R. TRIVEDI and VIRESH H. SHAH
Department of Chemistry, Saurashtra University, Rajkot-360 005, Gujarat, India
(Received 2 July 2010, revised 9 January 2011)
Abstract: An efficient, microwave-assisted synthesis of novel 2-alkoxy-5,6,7,8-
-tetrahydroquinoline-3-carbonitriles, which have not hitherto been reported, via
reactions of cyclohexanone and arylidene malononitriles in the corresponding
alcohols in presence of sodium is described. All the newly synthesized com-
1
pounds were characterized by the IR, H-NMR, 13C-NMR and mass spectres-
copic techniques and by elemental analyses. The newly synthesized com-
pounds were evaluated for their antibacterial and antifungal activities.
Keywords: 2-alkoxytetrahydroquinoline-3-carbonitriles; quinoline-3-carbonit-
riles; one-pot synthesis; microwave-assisted synthesis; cyclohexanone; aryli-
dene malononitriles.
INTRODUCTION
As a privileged fragment, quinoline is a ubiquitous subunit in many quino-
line-containing natural products with remarkable biological activities. Members
of this family have wide applications in medicinal chemistry, being used as an-
timalarial, anti-inflammatory, antiasthmatic, antibacterial, antihypertensive and
1
tyrosine kinase inhibiting agents. Because of their importance as substructures
in a broad range of natural and designed products, significant efforts continue to
2
be directed into the development of new quinoline-based structures.
Among quinoline derivatives, tetrahydroquinolines are an important structu-
ral subunit of natural products and many tetrahydroquinoline derivatives exhibit
3
4,5
interesting biological and pharmaceutical activities, including anti-HIV, anti-
6
7
8
cancer, antimalarial, cholesteryl ester transfer protein inhibitors, anti-diabe-
tic, etc. Consequently, synthetic methodologies for preparing tetrahydroquino-
9
line derivatives have attracted considerable interest and several methods offering
good results have been reported. However, most of them describe the synthesis of
*Corresponding author. E-mail: diptid85@gmail.com
doi: 10.2298/JSC100702071D
823
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824
DODIYA et al.
10–12
the 1,2,3,4-tetrahydroquinoline nucleus,
and concise methods to access
13
usefully functionalized 5,6,7,8-tetrahydroquinolines are scarce in the literature.
Recently, 5,6,7,8-tetrahydroquinolines have drawn considerable attention
due to their interesting pharmacological applications as RET tyrosine kinase in-
14
15,16
17
18
hibitors, anti-HIV,
anti-fungal, anti-cancer and C5a receptor antago-
19
nists agents. The development of simple synthetic routes for widely used orga-
nic compounds from readily available reagents is one of the major tasks in or-
ganic synthesis. Application of microwave irradiation in achieving this task has
been the focus of considerable attention in recent years and is becoming an
20,21
increasingly popular technology.
In view of these observations and as part of a continuing effort in our labo-
ratory towards the development of new methods for the expeditious synthesis of
22
biologically relevant heterocyclic compounds, herein, a simple and efficient
microwave-assisted synthesis of functionalized novel 2-alkoxy-5,6,7,8-tetrahyd-
roquinoline-3-carbonitriles, which are also structurally relevant to recently
16–19
reported bioactive 5,6,7,8-tetrahydroquinolines,
is described (Fig. 1).
Fig. 1. Examples of some bioactive 5,6,7,8-tetrahydroquinolines structurally relevant to the
synthesized 5,6,7,8-tetrahydroquinoline-3-carbonitriles.
All the newly synthesized compounds 3a–j were evaluated for their anti-
bacterial and antifungal activity. The biological activity of the synthesized com-
pounds was compared with reference standard drugs.
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EFFICIENT SYNTHESIS OF 5,6,7,8-TETRAHYDROQUINOLINES
825
RESULTS AND DISCUSSION
Chemistry
There are very few reports in literature for the synthesis of 2-alkoxypyridine-
23,24
-3-carbonitriles,
but most of them do not describe the synthesis of fused
pyridines or benzo-fused pyridines, i.e., quinolines. During an extensive literature
survey, only one such synthesis of benzothiepino-fused 2-alkoxy-pyridine-3-car-
25
bonitriles was found. Herein, the efficient and rapid microwave-assisted one-
-pot synthesis of functionalized novel 2-alkoxy-5,6,7,8-tetrahydroquinoline-3-
-carbonitriles, which have not hitherto been reported, via the reactions of cyc-
lohexanone, arylidene malononitriles in the corresponding alcohol in the pre-
sence of sodium is reported.
The arylidene malononitriles 2 were prepared following a reported literature
26
procedure. Treatment of cyclohexanone 1 with arylidene malononitriles 2 in
the corresponding alcohol in the presence of sodium under microwave irradiation
at 300 W afforded the 2-alkoxy-5,6,7,8-tetrahydroquinoline-3-carbonitriles 3a–j
in excellent yields (88–95 %) in a very short time (Scheme 1) as compared to
conventional heating, which results in lower yields after longer reaction times (5–
–10 h).
Scheme 1. Microwave-assisted synthesis of 2-alkoxy-5,6,7,8-
-tetrahydroquinoline-3-carbonitriles.
27
The proposed mechanism involves Michael addition between 1 and 2 to
generate intermediate A, followed by nucleophilic alkoxide attack at one of the
nitrile groups of A with dehydration and subsequent dehydrogenation to give the
tetrahydroquinoline 3 (Scheme 2).
All the newly synthesized 2-methoxy- and 2-ethoxy-5,6,7,8-tetrahydroquino-
1
13
lines 3a–j were characterized by IR, H-NMR, C-NMR, mass spectroscopy
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826
DODIYA et al.
and elemental analyses (Supplementary material). The IR spectra of 3a–j re-
vealed the appearance of confirmatory bands characteristics of the stretching
–1
–1
vibrations of the –CN and C=N groups at 2231–2225 cm and 1635–1566 cm ,
respectively. Furthermore, asymmetric and symmetric stretching vibration bands
of the ether (C–O–C) linkage were also present in the IR spectra in the range of
–1
–1
1296–1263 cm and 1097–1018 cm , respectively, which confirmed the pre-
sence of 2-alkoxy groups. The H-NMR spectra of compounds 3a–e showed
1
confirmatory signals in the δ range 3.75–3.80 ppm as a singlet for (–OCH ),
3
1
while the H-NMR spectra of compounds 3f–j revealed signals at δ 1.27–1.44
ppm as a triplet for the (–CH –CH ) and signals at δ 4.15–4.51 ppm as a quartet
2
3
for (–CH –CH ) of the 2-ethoxy group in the title compounds, which confirmed
2
3
13
the structure. The C-NMR signals were also found to be in full agreement with
the proposed structure.
Scheme 2. Proposed mechanism for synthesis of 2-alkoxy-5,6,7,8-
-tetrahydroquinoline-3-carbonitriles
Biological screening
All the synthesized compounds 3a–j exhibited moderate to good antimic-
robial activity. Compound 3i showed the highest activity against all the bacteria
and fungi. Compounds 3c and 3g also showed quite good antibacterial activity as
compared to the standard drugs ampicillin and chloramphenicol. Compounds 3c,
3g and 3h exhibited good antifungal activity against A. niger as compared to the
standard griseofulvin.
EXPERIMENTAL
Chemistry
All chemicals were supplied either by E. Merck (Germany) or by S. D. Fine Chemicals
(India). Melting points were determined in open capillaries and are uncorrected. The IR spec-
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EFFICIENT SYNTHESIS OF 5,6,7,8-TETRAHYDROQUINOLINES
827
tra were recorded on a Shimadzu-FT-IR-8400 (Fourier transform infrared (FTIR)). The IR
1
spectra were taken using KBr pellets. The H- and 13C-NMR spectra were recorded on a
Bruker AMX-400 spectrometer at 400 MHz and 100 MHz, respectively, in DMSO-d6 with
TMS as an internal standard. Gas chromatography–mass spectrometry (GC–MS) was per-
formed on a Shimadzu-GCMS QP2010 series instrument and elemental analysis was per-
formed using a Heraus CHN rapid analyzer. The microwave-assisted reactions were realised
in a QPro-M microwave synthesizer.
General procedure for the synthesis of 2-alkoxy-5,6,7,8-tetrahydroquinoline-3-carbonitriles
(3a–j)
A mixture of cyclohexanone 1 (2.5 mmol), arylidene malononitrile 2 (2.5 mmol) in the
appropriate alcohol (15 mL) containing sodium (0.05 g) was irradiated under microwaves at
300 W for an appropriate time (Table I). The separated solid was collected, washed with water
and then with methanol affording the corresponding 3a–j in good purity. Further purification
of the product was not required by recrystallization or any other method.
TABLE I. Microwave assisted synthesis of 2-alkoxy-5,6,7,8-tetrahydroquinoline-3-carbonit-
riles
Compound
R
R’
Yield, %
Time, min
3a
3b
3c
3d
3e
3f
3g
3h
3i
Phenyl
–CH3
–CH3
–CH3
–CH3
–CH3
–C2H5
–C2H5
–C2H5
–C2H5
–C2H5
91
95
92
89
94
89
94
92
92
88
4
5
5
5
7
5
7
6
6
8
4-Chlorophenyl
4-Fluorophenyl
4-Methoxyphenyl
4-Hydroxyphenyl
Phenyl
4-Chlorophenyl
4-Fluorophenyl
4-Methoxyphenyl
4-Hydroxyphenyl
3j
Biological screening
Antimicrobial activity. All the title compounds 3a–j were evaluated for their in vitro an-
timicrobial activity against two Gram-positive bacterial strains, Streptococcus pyogenes,
Staphylococcus aureus and two Gram-negative bacterial strains Escherichia coli, Bacillus
subtilis and one fungal strain Aspergillus niger at a concentration of 50 µg mL-1 in N,N-dime-
thylformamide using the cup plate method.28 The solvent, N,N-dimethylformamide, showed
no zone of inhibition. After 24 h of incubation at 37 °C, the zones of inhibition were measured
in mm. The activities were compared with those of some known antibiotics, i.e., ampicillin,
chloramphenicol, ciprofloxacin, as well as griseofulvin at the same concentration. The results
are summarized in Table II.
TABLE II. Antimicrobial activity of 2-alkoxy-5,6,7,8-tetrahydroquinoline-3-carbonitriles 3a–j
Zone of inhibition, mm
No.
R
S. pyogenes S. aureus
E. coli
B. subtilis
A. niger
3a
3b
3c
Phenyl
4-Chlorophenyl
4-Fluorophenyl
13
11
14
13
14
13
10
12
12
09
12
11
11
12
14
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DODIYA et al.
TABLE II. Continued
Zone of inhibition, mm
No.
R
S. pyogenes S. aureus
E. coli
B. subtilis
A. niger
3d
3e
3f
3g
3h
3i
4-Methoxyphenyl
4-Hydroxyphenyl
Phenyl
4-Chlorophenyl
4-Fluorophenyl
4-Methoxyphenyl
4-Hydroxyphenyl
13
9
13
12
12
15
12
13
14
18
16
17
–
10
10
11
15
12
13
10
16
19
20
–
10
10
11
14
12
16
10
18
16
19
–
12
12
11
13
12
18
10
–
12
14
10
14
13
16
18
23
–
3j
Ampicillin
Chloramphenicol
Ciprofloxacin
Griseofulvin
–
–
20
CONCLUSIONS
A new and efficient microwave-assisted synthesis of novel functionalized
2-alkoxy-5,6,7,8-tetrahydroquinoline-3-carbonitriles from cyclohexanone and
arylidene malononitriles has been reported. Considering the availability of the
starting materials, the simple reaction procedure, simple work-up and robust na-
ture, this chemical process provides a very straightforward route to construct va-
rious highly functionalized tetrahydroquinoline-3-carbonitriles.
The newly synthesized heterocycles exhibited moderate to promising anti-
microbial activity against standard strains. These results make them interesting
lead molecules for further synthetic and biological evaluation. It can be con-
cluded that this class of compounds certainly hold great promise towards the pur-
suit of discovering novel classes of antimicrobial agents. Further studies to ac-
quire more information concerning structure–activity relationships are in progress.
SUPPLEMENTARY MATERIAL
1
The results of IR, H-NMR, 13C-NMR, mass spectroscopy and elemental analyses of
synthesized compounds are available electronically at http://www.shd.org.rs/JSCS/, or from
the corresponding author on request.
Acknowledgements. The authors are thankful to the Department of Chemistry, Saurashtra
University for providing the laboratory facilities. Ms. Dipti K. Dodiya is thankful to UGC,
New Delhi for providing financial support in the form of a Research Fellowship in Sciences
for Meritorious Students.
И З В О Д
ЕФИКАСНА СИНТЕЗА 5,6,7,8-ТЕТРАХИДРОХИНОЛИН-3-КАРБОНИТРИЛА
ОЗРАЧИВАЊЕМ МИКРОТАЛАСИМА
DIPTI K. DODIYA*, HARESH K. RAM, AMIT R. TRIVEDI и VIRESH H. SHAH
Department of Chemistry, Saurashtra University, Rajkot-360 005, Gujarat, India
Описана је ефикасна синтеза нових 2-алкокси-5,6,7,8-тетрахидрохинолин-3-карбонит-
рила, озрачивањем микроталасима, полазећи од циклохексанона и арилиден-малононитрила
Available online at www.shd.org.rs/JSCS/
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EFFICIENT SYNTHESIS OF 5,6,7,8-TETRAHYDROQUINOLINES
829
у одговарајућем алкохолу, у присуству натријума. Сва нова једињења окарактерисана су IC,
1H-NMR и 13C-NMR спектроскопијом и масеном спектрометријом и елементалном анали-
зом. Испитана је антибактеријска и антифунгална активност нових синтетисаних једињења.
(Примљено 2. јула 2010, ревидирано 9. јануара 2011)
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