One-pot multi-component synthesis of polyhydroquinolines at ambient temperature

Article abstract of DOI:10.1016/j.crci.2010.09.009

An efficient and eco-friendly one-pot synthesis of polyhydroquinolines at ambient temperature from the multi-component reaction of aldehyde, 1,3-diketone, β-keto ester and ammonium acetate in ethanol medium is described. The probable mechanism of transformation is suggested. The features of this procedure are mild reaction conditions, no need of external catalyst, good yields and the operational simplicity at ambient temperature.

Full text of DOI:10.1016/j.crci.2010.09.009

C. R. Chimie 14 (2011) 511–515
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´
Full paper/Memoire
One-pot multi-component synthesis of polyhydroquinolines
at ambient temperature
*
K.A. Undale, T.S. Shaikh, D.S. Gaikwad, D.M. Pore
Department of Chemistry, Shivaji University, Kolhapur 416 004, India
A B S T R A C T
A R T I C L E I N F O
Article history:
An efficient and eco-friendly one-pot synthesis of polyhydroquinolines at ambient
temperature from the multi-component reaction of aldehyde, 1,3-diketone, -keto ester
Received 13 April 2010
b
Accepted after revision 24 September 2010
Available online 3 December 2010
and ammonium acetate in ethanol medium is described. The probable mechanism of
transformation is suggested. The features of this procedure are mild reaction conditions,
no need of external catalyst, good yields and the operational simplicity at ambient
temperature.
Keywords:
Multi-component
Polyhydroquinolines
Hantzsch condensation
One-pot synthesis
ß 2010 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1. Introduction
while polyhydroquinolines can be synthesized by the
utilization of cyclic 1,3-diketone by replacing one molecule
Heterocycles are of immense importance in the design
and discovery of new compounds for pharmaceutical
applications [1]. 1,4-dihydropyridine (DHP), which is
abitious nucleus of polyhydroquinolines is an important
structural motif in the preparation of the drugs for the
treatment of cardiovascular diseases, including hyper-
tension [2]. These also exhibit interesting biological
properties such as vasodilator, bronchodilator, anti-
atherosclerotic, antitumor, geroprotective, hepatoprotec-
tive and antidiabetic agent [3] and also behave as
neuroprotectants [4a], as well as chemosensitizers [4b]
in tumor therapy. Other than their biological importance,
DHPs have been meticulously used as reducing agents for
the direct reductive amination of aldehydes and ketones
[5] as well as their oxidation to pyridines being also
extensively studied [6].
of ethylacetoacetate in the Hantzsch reaction [8–18]. A
detailed literature survey about the synthesis of polyhy-
droquinoline compounds revealed that most of the
protocols employed for this reaction operate under reflux
conditions [8] i.e., under high thermal activation or they
need microwave irradiation [9]. There are a few protocols
operable at room temperature using triflates of Yb [10] and
Sc [11], I2 [12], bakers yeast [13], HY-zeolite [14], CAN [15],
ionic liquids [16], organo-catalyst [17a], p-TSA [17b], etc.
Recently, Mekheimer et al. [18] reported uncatalyzed
synthesis of polyhydroquinolines induced by solar heat.
However, many of these methodologies have been
associated with several shortcomings such as requirement
of long reaction times, use of expensive reagents and harsh
reaction conditions. Similarly, the difficulties such as low
product yields, occurrence of several by-products and
recovery of the catalyst are encountered. Therefore, it was
thought that there is room for improvement especially
towards developing a green protocol for synthesis of
polyhydroquinolines at ambient temperature. In continu-
ation with our research devoted to the development of
green organic chemistry [19a–e] we report herein an eco-
friendly, one-pot multi-component synthesis of polyhy-
droquinolines at ambient temperature (Scheme 1).
In 1882, Hantzsch and Jusius [7] reported the first
synthesis of symmetrical substituted 1,4-dihydropyridines
via multi-component condensation of aldehyde, ethylace-
toacetate and ammonia in acetic acid or refluxing alcohol,
* Corresponding author.
E-mail address: p_dattaprasad@rediffmail.com (D.M. Pore).
1631-0748/$ – see front matter ß 2010 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.crci.2010.09.009

[()TD$FIG]
512
K.A. Undale et al. / C. R. Chimie 14 (2011) 511–515
O
R
H
O
O
R
1
C2H5OH
O
OC2H5
R'
O
O
RT
N
H
+
R'
R'
OC2H5
O
R'
3
2
5
NH4OAc
4
Scheme 1.
2. Results and discussion
taken in 1:1 proportion [20]. Hence, there is no
likelihood of formation of product 5 in water. In the
case of a low dielectric constant medium, the enolization
is energetically unfavoured, leading to the low yield of
the product 5. However, these difficulties are easily
surmounted by using EtOH as a reaction medium due to
its medium dielectric constant as well as the compara-
tively less ability to form H-bonding with the enol form
Considering the view points of green chemistry,
initially the reaction of anisaldehyde, dimedone, ethyla-
cetoacetate and ammonium acetate was carried out
without using any external catalyst in aqueous medium
at room temperature. However, the reaction was sluggish
and corresponding polyhydroqinoline 5 was obtained in
15% (5 h). Since solvent properties play a crucial role in
organic synthesis, the effect of solvent was studied for
synthesis of polyhydroquinolines. The comparative
results for polyhydroquinolines obtained using different
solvents are summarized in Table 1 and it was found that
ethanol was a more economical and efficient solvent for
of the
b-keto ester. The requirement of protic solvents
for this reaction is also supported by observing very low
yield of the product in aprotic solvents where the small
increase in
% yield with dielectric constant of the
medium is observed in the studied solvents (Fig. 1).
The formation of polyhydroquinolines without use of
any external catalyst in ethanol medium at ambient
temperature is likely due to in situ formation of acetic acid
by hydrolysis of ammonium acetate, which acts as a
catalyst for the present transformation. This clearly implies
that there is no need of any external catalyst for the
synthesis of polyhydroquinolines. To improve the yield of
the product 5, an excess amount of ammonium acetate
could be used. The reason is twofold; firstly excess use of
ammonium acetate results in a large amount of in situ
the present transformation. The % yield as a function of
(dielectric constant) of solvent is exhibited in graphical
form in Fig. 1.
e
The observed yield of polyhydroquinoline
maximum in ethanol as a solvent having dielectric
constant ( ) of 24.3 (Fig. 1). This is an important
5 is
e
observation indicating that this transformation requires
the protic polar solvents of medium dielectric constant
for the enolization of active methylene compounds. In a
solvent with high dielectric constant viz H2O (
enol form of -ketoester may be strongly H-bonded in
aqueous medium becoming comparatively less reactive
for Knoevenagel condensation. literature survey
e = 78), the
[)(TD$FIG]
b
A
revealed that there was no formation of Knoevenagel
product in water when dimedone and aldehyde were
Table 1
Effect of solvent on the yield of polyhydroquinolines.
Entry
Solvent
Yield (%)a
1
2
3
4
5
6
7
8
H2O
15
54
78
54
42
32
15
26
CH3OH
CH3CH2OH
CH3CH2CH2OH
CH3CH2CH2CH2OH
CH3CN
THF
CH3COCH3
a
Reaction conditions: anisaldehyde (1 mmol), ethyl acetoacetate
(1 mmol), dimedone (1 mmol), ammonium acetate (1 mmol), solvent
(5 mL), time = 8 h, room temp.
Fig. 1. Effect of dielectric constant on the yield of 5.

K.A. Undale et al. / C. R. Chimie 14 (2011) 511–515
513
Table 2
Synthesis of polyhydroquinolines in ethanol medium at ambient temperature.
Entry
Product (5)
Time (h)
Yield (%)a,b
MP Obs. (lit. 8C)
O
R
O
OC2H5
[TD$INLINE]
R'
N
H
R'
a
b
c
d
e
f
R = C6H5; R’ = CH3
R = 4-CH3-C6H4; R’ = CH3
6.5
7
89
87
91
84
87
88
89
87
84
77
78
87
44
82
87
82
85
86
87
46
85
85
199-201(202-204) [8a]
258-260(260-261) [8a]
256-258(257-259) [8a]
244-246(245-246) [8a]
205-207(206-208) [8a]
180-182 (–)
R = 4-CH3 O - C6H4; R’ = CH3
R = 4-Cl- C6H4; R’ = CH3
R = 2-NO2 - C6H4; R’ = CH3
R = 4-isopropyl-C6H4; R’ = CH3
R = 2,5-(CH3)2- C6H3; R’ = CH3
R = 4-OH- C6H4; R’ = CH3
R = 4-OH-3-CH3O- C6H3; R’ = CH3
R = 2-furyl; R’ = CH3
8
6
7
6
g
h
i
6
244-246 (–)
6.5
7
233-234(232-234) [8a]
210-212(210-212) [8a]
242-245(246-248) [8a]
236-239(238-240) [8a]
261-263(263-264) [8a]
145-146(145-146) [8a]
193-195(194-195) [9a]
241-242(241-242) [9a]
233-235(234-235) [9a]
242-244(—)
j
6
k
l
R = 2-thienyl; R’ = CH3
6
R = 4-N(CH3)2- C6H4; R’ = CH3
R = CH3-CH2 ; R’ = CH3
7
m
n
o
p
q
r
7
R = 4-CH3 O – C6H4; R’ = H
R = 4-CH3- C6H4; R’ = H
7
7
R = 4-OH - C6H4; R’ = H
7
R = Piperinyl - C6H3; R’ = H
R = C6H5; R’ = H
6
6.5
6.5
7
239-241(240-241) [12]
196-199(198-200) [12]
179-181(180-182) [12]
192-194(—)
s
R = 3-NO2 - C6H4; R’ = H
R = (CH3)2- CH ; R’ = H
t
u
v
R = 3,4-(CH3 O)2- C6H3; R’ = H
R = 3,4,5-(CH3 O)3- C6H2; R’ = H
11
11
180-182(—)
a
All products showed satisfactory spectroscopic data (IR, 1H and 13C NMR, MS, elemental analysis).
Yields refer to pure, isolated products.
b
generation of acetic acid that acts as a catalyst and
(Table 2, entries 5m and 5t). Furthermore, the variation
of 1,3-diketone compound from dimedone to cyclohex-
ane-1,3-dione, which has been rarely used for present
transformation, worked equally well, proving the broad
scope of the present protocol.
Prediction of the mechanism of multi-component
reactions is a skillful task. In the mechanism of polyhy-
droquinoline there is ambiguity amongst the reactivity of
secondly unreacted ammonium acetate can be easily
removed from the reaction. This prompted us to use a
slightly excess amount of ammonium acetate rather than
using any external catalyst and led us to explore this
economical protocol. The present transformation was also
carried out in 100, 95, 90, 85 and 80% ethanol using
reaction conditions as specified in Table 1 with the
exception that amount of ammonium acetate (1.5 mmol)
and obtained 76, 91, 73, 68, 63% yield of polyhydroquino-
line, respectively.
Gratifyingly, polyhydroquinoline 5 of anisaldehyde,
dimedone, ethylacetoacetate and ammonium acetate
were obtained in 91% yield using 95% ethanol as a
solvent at ambient temperature. The scope and efficien-
cy of this approach was explored for the synthesis of a
active methylene compounds viz 1,3-diketones and b-keto
ester towards Knoevenagel condensation. To clarify this
(Table 3), Knoevenagel condensation was carried out using
1+2, 1+3 without any catalyst and yielded 6, 8 in 13 and 0%,
respectively. The influence of ammonium acetate on the
same reactions i.e. 1+2+4 yielded 6 in 82% while for 1+3+4
yielded mixture of 7 and 10 in 28 and 17%, respectively.
wide variety of polyhydroquinolines by treating
a
Table 3
diverse range of aldehydes possessing electron-donating
as well as electron-withdrawing groups, with dimedone,
ethyl acetoacetate, ammonium acetate and results are
summarized in Table 2. Therefore, it is amply clear that
in all the cases irrespective of the presence of electron-
donating or electron-withdrawing groups in an aldehyde
moiety, the desired products were obtained in good
yields. These reaction conditions were almost equally
efficient for heterocyclic aldehydes (Table 2, entries 5j,
5k). Although the heterocyclic and aromatic aldehydes
reacted efficiently under the similar conditions, aliphatic
aldehydes resulted the lower yield of the products.
Results of Knoevenagel condensation.
Entry
Reactants
Catalyst
Product
Yield (%)a
1
2
3
4
1+2
——
——
——
——
6
13
1+3
8
——
82
1+2+4
1+3+4
6
7
28
10
8
17
5
6
1+3
3+4
CH3COOH
——
——
96
7
a
Reaction conditions: anisaldehyde(1 mmol), ethyl acetoacetate/
dimedone (1 mmol), ethanol (5 mL), ammonium acetate (1.5 mmol),
acetic acid (1 mmol), time = 6 h, room temp.

[()TD$FIG]
514
K.A. Undale et al. / C. R. Chimie 14 (2011) 511–515
O
OEt
CH3
10
R
RCHO
1
N
O
O
OEt
R
+
H2N
CH3
H2O + AcOH
O
RCHO
6
7
1
Step 3
NH4OAc
Step 2
EtOH
EtOH
4
Step 1
O
O
R
O
O
OEt
OEt
O
CH3
3
N
H
O
5
2
EtOH
RCHO
1
NH4OAc
4
O
O
R
H2O + AcOH
OEt
+
NH2
O
CH3
9
6
8
O
R
O
N
H
11
Scheme 2. Proposed mechanism for the formation of 5.
Even though, addition of acetic acid for reaction of 1+3
failed to form 8. The reaction between 3+4 resulted in
exclusive formation of product 7.
cyclohexane-1,3-dione. The attractive features of this
method are simple experimental procedure worked well
at ambient temperature and easy isolation of product. The
beauty of protocol lies in mechanistic and green approach
in terms of absence of external catalyst, which makes the
process economical.
The above observations allow us to conclude that
Knoevenagel product 6 is formed by reaction of aldehyde
with 1,3-diketone and subsequent Michael addition of 7
yielding corresponding polyhydroquinoline 5 (Scheme 2).
The formation of acridinedione [21] 11 is possible when 9
reacts with 6, since this possibility is discarded due to non-
formation of intermediates 8 and 9. A literature survey also
complies with non-formation of acridinediones [8–18] as a
by-product during present transformation justifying the
clear mechanism for synthesis of polyhydroquinolines 5.
4. Experimental: Typical procedure
One-pot synthesis of polyhydroquinolines: A mixture of
an aldehyde (1 mmol), ethyl acetoacetate (1 mmol),
dimedone (1 mmol) and ammonium acetate (1.5 mmol)
in ethanol (5 mL) was stirred at room temperature, for an
appropriate time (Table 2). On completion of reaction
(TLC), the reaction mixture was poured into crushed ice
and the solid product separated was filtered and recrys-
tallized from ethanol to afford pure polyhydroquinoline,
which was characterized by physical constant and
spectroscopic methods.
3. Conclusion
The developed one-pot procedure for multi-compo-
nent synthesis of polyhydroquinolines was found to be
quite general for a variety of aryl and heterocyclic
aldehydes as well as for 1,3-diketones viz dimedone and

K.A. Undale et al. / C. R. Chimie 14 (2011) 511–515
515
References
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d 0.96 (s, 3H),
1.08(s, 3H), 1.17-1.2(m, 9H), 1.68-1.70(m, 2H), 2.19-
2.34(m, 5H), 2.79-2.82(m, 1H), 4.07 (q, J = 7.2 Hz, 2H),
5.02(s, 1H), 6.02(s, 1H), 7.03(d, J = 8 Hz, 2H), 7.19(d, J = 8 Hz,
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32.7, 33.6, 36.1, 41.1, 50.9, 59.6, 106.3, 112.1, 125.9, 127.5,
143.2, 144.5, 146.2, 148.5, 167.6, 195.5; EIMS: m/z 381
(M+); anal. calcd. for C24H31O3N: C, 75.56; H, 8.19; N, 3.67;
found: C, 75.61; H, 8.19; N, 3.69.
Entry 5g: mp. 244–246 8C; IR (KBr):3284, 3237, 1699,
1602 cmÀ1 1H NMR (400 MHz, CDCl3):
; d 0.94 (s, 3H),
1.08(s, 3H), 1.19 (t, J = 7.2 Hz, 3H), 1.61-1.64(m, 2H), 2.11-
2.31(m, 5H), 2.38(s, 3H), 2.65(s, 3H), 4.06 (m, 2H), 5.11(s,
1H), 5.67(s, 1H), 6.79(d, J = 8 Hz, 1H), 6.90(d, J = 8 Hz, 1H),
6.96(s, 1H); 13C NMR (100 MHz, CDCl3):
d 14.2, 19.1, 19.6,
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21.1, 27.2, 28.4, 29.2, 32.7, 33.1, 41.6, 50.9, 59.7, 107.7,
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195.1; EIMS: m/z 367 (M+); anal. calcd. for C23H29O3N: C,
75.17; H, 7.95; N, 3.81; found: C, 75.14; H, 7.94; N, 3.79.
Entry 5q: mp. 242–244 8C; IR (KBr): 3281, 1692, 1646,
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d 1.21 (t, J = 7.2 Hz,
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3H), 1.65-1.69(m, 3H), 1.91-2.02(m, 1H), 2.27-2.46(m, 5H),
4.08 (q, J = 7.2 Hz, 2H), 5.02(s, 1H), 5.87(s, 2H), 5.88(s, 1H),
6.65(d, J = 8 Hz, 1H), 6.78-6.80(m,2H); 13C NMR (100 MHz,
CDCl3):
d 14.2, 19.1, 21.1, 27.4, 36.2, 37.1, 59.8, 100.6, 106.4,
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2875 ;
107.7, 108.7, 113.5, 121, 141.5, 143, 145.7, 147.3, 149.6,
167.4, 195.6; EIMS: m/z 355 (M+); anal. calcd. for
C20H21O5N: C, 67.59; H, 5.96; N, 3.94; found: C, 67.63; H,
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Chem 8 (2006) 829 ;
5.97; N, 3.95.
Entry 5u: mp. 192–194 8C; IR (KBr):3307, 1697, 1647,
1607 cmÀ1; 1H NMR (400 MHz, CDCl3):
d 1.20 (t, J = 7.2 Hz,
3H), 1.68-1.71(m, 3H), 1.94-2.02 (m, 1H), 2.28-2.45(m,
5H), 3.80(s, 3H), 3.85(s, 3H), 4.07(q, J = 7.2 Hz, 2H), 5.05(s,
1H), 6.07(s, 1H), 6.71(d, J = 8.2 Hz, 1H), 6.76(d, J = 8.2 Hz,
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d 14.2, 19,
21.1, 27.3, 36, 37.1, 56, 59.7, 106.2, 111.5, 112.5, 113.3,
119.8, 140.4, 143.2, 147.5, 148.5, 150, 167.5, 196; EIMS: m/
z 371 (M+); anal. calcd. for C21H25O5N: C, 67.91; H, 6.78; N,
3.77 found: C, 67.86; H, 6.75; N, 3.79.
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592.
Entry 5v: mp. 180–182 8C; IR (KBr): 3275, 3192, 1695,
1607 cmÀ1; 1H NMR (400 MHz, CDCl3):
d1.21 (t, J = 7.2 Hz,
3H), 1.95-2.01(m, 2H), 2.25-2.40(m, 7H), 3.78(s, 3H),
3.80(s, 6H), 4.09 (q, J = 7.2 Hz, 2H), 5.07(s, 1H), 6.30(s,
1H), 6.5(s, 2H); 13C NMR (100 MHz, CDCl3):
d 14.2, 19.3,
[19] (a) D.M. Pore, M.S. Soudagar, U.V. Desai, T.S. Thopate, P.P. Wadagaon-
kar, Tetrahedron Lett 47 (2006) 9325 ;
20.3, 21.2, 27.7, 33, 36.5, 37.2, 56.3, 59.7, 60.8, 105.1, 106,
106.4, 113.5, 142.6, 142.8, 149, 153, 167.5, 196; EIMS: m/z
401 (M+); anal. calcd. for C22H27O6N: C, 65.82; H, 6.78; N,
3.49 found: C, 65.85; H, 6.78; N, 3.52.
(b) D.M. Pore, U.V. Desai, T.S. Thopate, P.P. Wadgaonkar, Aust. J. Chem
60 (2007) 435 ;
(c) D.M. Pore, U.V. Desai, T.S. Thopate, P.P. Wadgaonkar, Arkivoc xii
(2006) 75 ;
(d) D.M. Pore, T.S. Shaikh, N.G. Patil, S.B. Dongare, U.V. Desai, Synth.
Commun 40 (15) (2010) 2215 ;
(e) D.M. Pore, T.S. Shaikh, K.A. Undale, D.S. Gaikwad, C. R. Chimie
(2010) 015, doi:10.1016/j.crci.2010.06.
Acknowledgments
[20] Z. Ren, W. Cao, W. Tong, X. Jing, Synth. Commun 32 (13) (2002)
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[21] G.W. Wang, C.B. Miao, Green Chem 8 (2006) 1080.
Authors DMP and KAU thank UGC, New Delhi for
financial assistance [F.2-3/2007(Policy/SR)] and for the
research fellowship, respectively.