Aluminized polyborate-catalysed green and efficient synthesis of polyhydroquinolines under solvent-free conditions

Article abstract of DOI:10.1007/s11164-020-04158-z

Abstract: The present work reports a catalytic application of aluminized polyborate for the solvent-free one pot four-component green, efficient and cost-effective synthesis of polydroquinoline derivatives. The aluminized polyborate was synthesized from r

Full text of DOI:10.1007/s11164-020-04158-z

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-020-04158-z
Aluminized polyborate‑catalysed green and efcient
synthesis of polyhydroquinolines under solvent‑free
conditions
Dilip Aute1,2 · Akshay Kshirsagar1 · Bhagwat Uphade1 · Anil Gadhave1
Received: 24 February 2020 / Accepted: 15 April 2020
© Springer Nature B.V. 2020
Abstract
The present work reports a catalytic application of aluminized polyborate for the
solvent-free one pot four-component green, efcient and cost-efective synthesis
of polydroquinoline derivatives. The aluminized polyborate was synthesized from
readily available non-toxic, inexpensive boric acid and aluminium trichloride and
characterized by IR, XRD, SEM and EDAX techniques. The polyhydroquinolines
were successfully synthesized by well-known Hantzsch condensation reaction of dif-
ferently substituted aldehydes, dimedone, ethylacetoacetate and NH4OAc at 100 °C
under solvent-free condition. The catalyst is stable and easy to use with some sig-
nifcant features such as good acidic catalytic activity, inexpensive and eco-friendly
nature. Along with this, excellent product yields, short reaction time, clean reaction,
operational simplicity, solvent-free condition and non-column purifcation are some
more additional aspects of this protocol.
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s1116
4-020-04158-z) contains supplementary material, which is available to authorized users.
*
Anil Gadhave
anilgadhave@gmail.com
1
2
Department of Chemistry and Research Centre, Padmashri Vikhe Patil College (Afliated
to Savitribai Phule Pune University, Pune), Pravaranagar, Ahmednagar, MS 413713, India
Department of Chemistry, Arts, Commerce and Science College, Satral, India
Vol.:(0123456789)
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D. Aute et al.
Graphic abstract
Keywords Aluminized polyborate · Polyhydroquinoline · Solvent free · Hantzsch
condensation · Multicomponent reaction
Introduction
In recent years, the catalytic applications of boric acid have attracted the attention
of synthetic chemists due to its easy commercial availability, solubility in water,
inexpensive and non-hazardous nature. It is used to catalyse the organic transfor-
mations such as Biginelli reaction, thia-Michael addition, transamidation reaction
and protocol for synthesis of amido- alkyl-naphthols, etc. [1–4]. Along with these
merits, in the literature it is reported that, boric acid undergoes dehydration above
100 °C and gets converted into polymeric form. This polymeric borate is pre-
sumed as the active species which catalyses the reactions [5, 6]. The formation of
polymeric borate leads to liberation of water which may hamper the rate of some
reactions.
This leads to a need of more investigation in the development of supported poly-
meric boric acid catalyst such as sulphated polyborate and aluminized polyborate.
The enhanced catalytic activity of sulphated polyborate and aluminized polyborate
fnds their application in green and efcient multicomponent protocols for the syn-
thesis of biologically important heterocycles [7–9]. In the aluminized polyborate,
the aluminium is supported on polymeric borate. The Lewis acidic property of both
boron and aluminium increases the acidic catalytic efcacy of the aluminized polyb-
orate and makes it more useful in green synthesis [10].
1 3

Aluminized polyborate‑catalysed green and efcient synthesis…
The multicomponent reactions have attracted the attention of the chemist to
achieve the synthesis of target molecules in one step with minimum eforts. This
approach boost the conversion of two or more substrate components into pharma-
cologically important products with high molecular diversity, decrease in reaction
time, use of non-toxic chemicals and green solvents, with high atom economy and
selectivity [11–13]. In addition to this, the green chemistry approach for the synthe-
sis of important heterocycles is very much essential to protect the human health and
environment from the chemical pollution [14]. Therefore, there is always an increas-
ing demand to develop green methods for multicomponent synthesis of heterocycles
[15].
Polyhydroquinoline is an important class of nitrogen heterocycle which have
attracted attention due their broad spectrum biological as well as pharmacological
properties. They are synthesized by well-known Hantzsch pseudo-four-component
reaction reported by Arthur Rudolf Hantzsch in 1881. In polyhydroquinoline, the
1,4-dihydropyridine is an important pharmacophore found in well-known Ca2+
channel blockers drugs such as nicardipine, amlodipine, nifedipine, lacidipine and
felodipine [16–18] (Fig. 1). These are used for treatment of hypertension and angina
[19, 20]. They also exhibit antioxidant, anti-infammatory, antitubercular and antivi-
ral activities [21–23].
In general, one-pot four-component reaction of aldehyde, cyclic 1,3-dicarbonyls,
ethylacetoacetate and NH4OAc gives polyhydroquinolines which are catalysed by
2−
variety of systems such as Fe3O4/SiO2-OSO3H [24], Fe3O4@SiO2@PPh3@Cr2O7
[25], nicotinic acid [26], amine functionalized graphene oxide nanosheets [27], cop-
per complex supported on MCM-41 [28], ZnO-[DABCO(C2COOH)2]2+[Br−]2 [29],
KH2PO4 [30], TEDETA@BNPs [31], nano Fe3O4@Ni [32], SBA-15@Glycine [33],
PMO-ICS-PrSO3H [34], Fe3O4@SiO2/ZnCl2 [35], CdS nanowires [36], H5BW12O40
[37], Fe3O4@Schifs-Base-Cu [38], Fe3O4@FSM-16-SO3H [39], [Msim]Cl [40],
[MSAIM]HSO4 [41], nano V-TiO2 [42], [Pyridine-SO3H]Cl [43], Cafeine-H3PO4
−
[44], ascorbic acid [45], Cu(II)-DCC-CMK-3 [46], spinel FeAl2O4 [47], ClO4 /Zr-
MCM-41 nanoparticles [48], RuIII@CMC/Fe3O4 nanoparticles [49], Ni-dithizone@
boehmite [50], Fe3O4@SiO2PPh3@[CrO3Br] [51], γ-Fe2O3/Cu@Cellulose [52],
Fe3O4@PEO-SO3H [53]. All these literature reported methods along with some
important features may also possess one or more drawbacks such as use of expen-
sive catalyst, drastic reaction conditions, low product yields, tedious workup proce-
dure, need of column purifcation, etc.
Cl
NO2
O2N
O
MeO2C
CO2Et
MeO2C
CO2Me MeO2C
N
O
O
N
H
NH2
N
H
N
H
amlodipine
nifedipine
nicardipine
Fig. 1 Some drugs containing 1,4-dihdyopyridine nucleus which is a core of polyhydroquinolines
1 3

D. Aute et al.
Therefore, there is still a demand to develop newer protocols for the synthesis of
polyhydroquinolines using green chemistry approach. With this background and in
continuation to our research work [54–57] herein, we wish to report a use of alumin-
ized polyborate as a green catalyst for environmentally benign solvent-free synthesis
of polyhydroquinolines by using variously substituted aldehydes, dimedone, ethyl-
acetoacetate and NH4OAc at 100 °C.
Experimental section
All the required chemicals were purchased commercially and used without further
purifcation. Silica G60 F254 plates (Merck) TLC plates were used to monitor the
progress of reaction. The melting points were recorded in open capillary tube and are
uncorrected. Aluminized polyborate was characterized by Fourier transform infra-
red (FT-IR) spectroscopy (Perkin-Elmer, 400FT-IR spectrometer), X-ray difraction
(XRD) analysis (PANalyticals X’Pert Pro, X-ray difractometer) and scanning elec-
tron microscope analysis (SEM) (JSM6100, JEOL scanning electron microscope).
The polyhydroquinolines were characterized by FT-IR, nuclear magnetic resonance
spectroscopy (1H-NMR), (Bruker Avance 500 MHz NMR spectrometer) and Mass
spectrum (Waters Q-ToF Micromass spectrometer) techniques.
Preparation of aluminized polyborate catalyst
The aluminized polyborate was synthesized by previously reported method [10]
(Scheme 1). It is synthesized form very easily available and inexpensive boric
acid and aluminium trichloride. The catalyst possesses good Lewis acidic catalytic
activity.
OH
2
OH
2
O
O
O
O
OH
OH
HO
dehydration
200oC, -H2O
AlCl3
-HCl
B
B
B
B
Al
Cl
HO B
OH OH
OH OH
Polymeric borate
Boric acid
Aluminized polyborate
Scheme 1 Representation for the preparation of aluminized polyborate
1 3

Aluminized polyborate‑catalysed green and efcient synthesis…
General procedure for the synthesis of polyhydroquinoline derivatives (5a‑p)
Diferently substituted aromatic aldehydes (1a–p, 1 mmol), dimedone (2, 1 mmol),
ethyl acetoacetate (3, 1 mmol) and NH4OAc (4, 1 mmol) was taken in a round bottom
fask. To this, 0.075 g of aluminized polyborate catalyst was added. The round bot-
tom fask was placed in an oil bath, and reaction mixture was heated under solvent-
free condition at 100 °C for appropriate time as indicated in Table 3. The TLC (ethyl
acetate/n-hexane, 3:7) was used to monitor the progress of reaction. After comple-
tion of reaction, hot ethanol was added to reaction mixture and the hot solution was
fltered to remove any insoluble material. The solid appeared in mother liquor was
fltered at room temperature to aford pure solid polyhydroquinolines (5a–p).
1
The comparison of physical and spectral data (IR, H-NMR and MS) with liter-
ature-reported authentic samples confrms the formation of synthesized compounds
(Scheme 2).
R
O
O
CHO
O
OEt
aluminized polyborate
o
O
NH4OAc
+
+
R
+
solvent free
100 C,
O
O
N
H
O
4
2
3
1a-p
5a-p
Scheme 2 Synthesis of polyhydroquinoline derivatives
1 3

D. Aute et al.
Spectral data of some polyhydroquinoline derivatives
Ethyl 2,7,7‑trimethyl‑5‑oxo‑4‑phenyl‑1,4,5,6,7,8‑hexahydroquinoline‑3‑carboxylate
5a IR (υ max,cm−1): 3289, 3081, 2960, 1699, 1643, 1611, 1483, 1308 1279; 1H NMR
(500 MHz, CDCl3, δ ppm): 7.29–7.30 (m, 2H), 7.17–7.20 (m, 2H), 7.09–7.10 (m, 1H),
6.46 (s, 1H), 5.04 (s, 1H), 4.03 (q, J=7 Hz, 2H), 2.34 (s, 3H), 2.12–2.32 (m, 4H), 1.18
(t, J=7 Hz, 3H), 1.06 (s, 3H), 0.92 (s, 3H); MS for C21H25NO3 (m/z): 340 [M+H]+.
Ethyl 2,7,7‑trimethyl‑5‑oxo‑4‑(p‑tolyl)‑1,4,5,6,7,8‑hexahydroquinoline‑3‑carboxylate
1
5b IR (υ max, cm−1): 3245, 3078, 2960, 1701, 1647, 1605, 1492, 1281; H NMR
(CDCl3, 500 MHz, δ ppm): 7.17 (d, J=8 Hz, 2H), 6.98 (d, J=8 Hz, 2H), 6.24 (s,
1H), 5.01 (s, 1H), 4.03 (q, J=7 Hz, 2H), 2.33 (s, 3H), 2.28–2.31 (m, 1H), 2.24 (s,
3H), 2.12–2.23 (m, 3H), 1.19 (t, J=7 Hz, 3H), 1.06 (s, 3H), 0.94 (s, 3H); MS for
C22H27NO3 (m/z): 354 [M+H]+.
Ethyl 2,7,7‑trimethyl‑4–(4‑nitrophenyl)–5‑oxo‑1,4,5,6,7,8‑hexahydroquinoline‑3‑car‑
boxylate 5d IR (υ max, cm−1): 3275, 3074, 2966, 1703, 1648, 1607, 1517, 1493,
1
1379, 1215; H NMR (CDCl3, 500 MHz, δ ppm): 8.06 (d, J=8.5 Hz, 2H), 7.46 (d,
J=8.5 Hz, 2H), 6.05 (s, 1H), 5.16 (s, 1H), 4.02 (q, J=7 Hz, 2H), 2.41 (s, 3H), 2.12–
2.39 (m, 4H), 1.16 (t, J=7 Hz, 3H), 1.08 (s, 3H), 0.91 (s, 3H); MS for C21H24N2O5
(m/z): 385 [M+H]+.
Ethyl 4–(4‑methoxyphenyl)–2,7,7‑trimethyl‑oxo‑1,4,5,6,7,8‑hexahydroquinoline‑3‑car‑
boxylate 5 g IR (υ max, cm−1): 3273, 3077, 2958, 1701, 1647, 1606, 1492, 1308, 1216;
1H NMR (CDCl3, 500 MHz, δ ppm): 7.20 (d, J=8.5 Hz, 2H), 6.72 (d, J=8.5 Hz, 2H),
6.29 (s, 1H), 4.99 (s, 1H), 4.04 (q, J=7 Hz, 2H), 3.72 (s, 3H), 2.35 (s, 3H), 2.13–2.30
(m, 4H), 1.19 (t, J=7 Hz, 3H), 1.06 (s, 3H), 0.93 (s, 3H); MS for C22H27NO4 (m/z):
370 [M+H]+.
Ethyl 4‑(4‑bromophenyl)‑2,7,7‑trimethyl‑5‑oxo‑1,4,5,6,7,8‑hexahydroquinoline‑3‑car‑
boxylate 5 h IR (υ max, cm−1): 3274, 3077, 2959, 1703, 1648, 1604, 1488, 1309,
1
1280; H NMR (CDCl3, 500 MHz, δ ppm): 7.29 (d, J=8.5 Hz, 2H), 7.17 (d,
J=8.5 Hz, 2H), 6.21 (s, 1H), 5.00 (s, 1H), 4.03 (q, J=7.1 Hz, 2H), 2.36 (s, 3H), 2.13–
2.33 (m, 4H), 1.17 (t, J=7.1 Hz, 3H), 1.06 (s, 3H), 0.92 (s, 3H); MS for C21H24BrNO3
(m/z): 418 [M+H]+.
Ethyl 2,7,7‑trimethyl‑5‑oxo‑4‑(thiophen‑2‑yl)‑1,4,5,6,7,8‑hexahydroquinoline‑3‑car‑
boxyla1te 5p IR (υ max, cm−1): 3285, 3079, 2964, 1696, 1642, 1611, 1485, 1281,
1210; H NMR (CDCl3, 500 MHz, δ ppm): 7.01 (m, 1H), 6.81–6.83 (m, 2H), 6.06
(s, 1H), 5.41 (s, 1H), 4.12 (q, J=7 Hz, 2H), 2.37 (s, 3H), 2.22–2.33 (m, 4H), 1.23 (t,
J=7 Hz, 3H), 1.09 (s, 3H), 1.02 (s, 3H); MS for C19H23NO3S (m/z): 346 [M+H]+.
1 3

Aluminized polyborate‑catalysed green and efcient synthesis…
Results and discussion
Preparation and characterization of aluminized polyborate
The aluminized polyborate was synthesized by recently reported method [10]. The
FT-IR, XRD, SEM and EDAX spectral techniques were used to characterize the catalyst.
FT‑IR analysis
The FT-IR spectra of aluminized polyborate is shown in (Fig. 2). The spectrum
shows strong band around 3400 cm−1due to the Al–O–H stretching vibrations
while the B–O–H stretching band appears at 3227 cm−1. The bending vibra-
tions of Al–OH appear at 1607 cm−1, while the peak at 1459 cm−1 is due to B–O
stretching. The peak at 642 cm−1 is due to B–O–B stretching [58, 59].
XRD analysis
The formation of aluminized polyborate was also confrmed by powder X-ray dif-
fraction pattern (Fig. 3). The peak in XRD at 2θ = 28° is due to the B–O bonds in
the aluminized polyborate [60]. The XRD pattern (JCPDS 6-0297) of crystalline
cubic boron particles and for aluminium (JCPDS Card No: 00-004-0787) matches
well with the XRD of aluminized polyborate. The (310) plane of the cubic phase
is assigned to the major peak at 2θ = 28° [61, 62]. This indicates the crystalline
cubic nature of aluminized polyborate and also supports its formation.
Fig. 2 FT-IR spectrum of aluminized polyborate
1 3

D. Aute et al.
Z1-AG
4000
3000
2000
1000
0
10
20
30
40
50
60
70
Position [°2θ] (Copper (Cu))
Fig. 3 XRD analysis of aluminized polyborate
SEM analysis
The SEM image of aluminized polyborate indicates crystalline nature of the catalyst
with diferent shapes and sizes of the particles (Fig. 4).
EDAX analysis
The EDAX analysis of aluminized polyborate shows the presence of oxygen/alu-
minium/chlorine which confrm its formation. As the boron was used as core for the
synthesis of aluminized polyborate, it was not detected on the surface (Fig. 5).
Study of catalytic activity of aluminized polyborate for the synthesis
of polyhydroquinoline derivatives
The surface morphology of the catalyst from SEM images clearly highlights the
crystalline solid nature. In addition to this, catalyst also exhibits good Lewis acidic
character. Both of this features prompted our interest to check its catalytic activity.
Fig. 4 SEM image of aluminized polyborate
1 3

Aluminized polyborate‑catalysed green and efcient synthesis…
Fig. 5 EDAX analysis of aluminized polyborate
The catalytic application of aluminized polyborate was studied in the multicompo-
nent synthesis of polyhydroquinolines (Scheme 2). For the initial study, a model
reaction of reaction 4-chloro benzaldehyde (1c, 1 mmol), dimedone (2, 1 mmol),
ethylacetoacetate (3, 1 mmol) and NH4OAc (4, 1 mmol) was selected. The model
reaction was used to study the infuence of diferent solvents and temperature, suit-
ability and loading of catalyst on the reaction time and yield of polyhydroquinolines.
Initially the reaction was carried out in solvents like H2O, EtOH, DMF, THF and
CH3CN as well as under solvent-free condition using the aluminized polyborate cat-
alyst at various temperatures as shown in (Table 1). The result indicates that, the
reaction works smoothly under solvent-free conditions at 100 °C with short reaction
time, good to excellent product yields and no side reaction (Scheme 3).
After optimizing the suitable condition, we have investigated the efect of amount
of catalyst on the yield and time required for completion of reaction.
A model reaction under the above optimized condition was carried out in the
absence of catalyst and by using diferent amount of catalyst like 0.025 g, 0.050 g,
0.075 g and 0.100 g (Table 2). The observation indicates that 0.075 g of catalyst is
sufcient to complete the reaction in short time with excellent yield. There is no
efect on yield and rate of reaction by further addition of catalyst. In the absence of
catalyst, the product is obtained formed in minor amount (10%).
After studying various reaction parameters, we checked the compatibility of
present protocol for the synthesis of variety of polyhydroquinolines using aro-
matic aldehydes having –Me, –Cl, –Br, –F, –OH, –OMe and –NO2 groups
(Table 3).
The study reveals that the reaction worked smoothly with excellent yields for aro-
matic aldehydes having wide range of groups such as nitro, chloro, bromo, furo,
methyl, methoxy and hydroxyl. The sterically hindered aldehydes such as ortho-
chloro (89%) and ortho-nitro (91%) also gives good product yield indicating the
suitability of present protocol for diverse substrates. The reaction of heterocyclic
1 3

D. Aute et al.
Table 1 Optimization of
solvent and temperature for the
synthesis 5c
Entry Solvent
Temperature (°C) Time (min) Yielda (%)
1
H2O
r.t.
60
60
60
60
60
60
60
60
60
30
15
15
–
2
3
4
EtOH
H2O:EtOH
H2O
r.t.
–
r.t.
–
Refux
Refux
80 °C
80 °C
80 °C
35
80
45
40
55
40
85
94
94
5
6
EtOH
DMF
7
THF
8
CH3CN
9
Solvent-free r.t.
10
11
12
Solvent-free 80 °C
Solvent-free 100 °C
Solvent-free 110 °C
Reaction conditions: 4-chloro benzaldehyde (1 mmol), dimedone
(1 mmol), ethylacetoacetate (1 mmol) and ammonium acetate
(1 mmol) in various solvents and solvent-free conditions at various
temperatures using aluminized polyborate catalyst (0.075 g)
aIsolated yield of product
O
O
CHO
O
OEt
aluminized polyborate
o
O
NH4OAc
+
+
+
solvent free
100 C,
O
O
N
H
O
4
2
3
Cl
1c
5c
Scheme 3 Model reaction for the synthesis of polyhydroquinolines
Table 2 Optimization of amount
Entry
Catalyst amount
(g)
Time (min)
Yield (%)
of catalyst
1
2
3
4
5
0.000
0.025
0.050
0.075
0.100
30
30
30
15
15
10
70
82
94
94
1 3

Aluminized polyborate‑catalysed green and efcient synthesis…
Table 3 Synthesis of
Entry Product Time (min) Yield (%)a M.P. (°C)
polyhydroquinolines (5a–p)
Observed Reported
1
20
92
217–219 218–220 [31]
O
O
O
N
H
5a
2
20
93
260–262 259–262 [36]
O
O
O
N
H
5b
3
15
94
245–246 244–246 [36]
Cl
O
O
O
N
H
5c
4
10
93
240–242 240–242 [36]
NO2
O
O
O
N
H
5d
1 3

D. Aute et al.
Table 3 (continued)
Entry Product Time (min) Yield (%)a M.P. (°C)
Observed Reported
5
6
7
8
25
89
92
93
91
231–232 232–234 [36]
185–186 184–186 [31]
258–260 257–260 [36]
254–256 256–257 [36]
OH
O
O
O
O
O
O
N
H
5e
15
F
O
O
N
H
5f
20
OCH3
O
O
N
H
5g
15
Br
O
O
N
H
5h
1 3

Aluminized polyborate‑catalysed green and efcient synthesis…
Table 3 (continued)
Entry Product Time (min) Yield (%)a M.P. (°C)
Observed Reported
9
15
NO2
90
91
89
90
182–184 182–184 [30]
205–206 205–207 [30]
198–200 199–201 [30]
196–197 196–198 [42]
O
O
O
O
O
N
H
5i
10
11
12
15
Cl
O
O
N
H
5j
20
OCH3
O
O
N
H
5k
25
OCH3
OCH3
O
O
O
N
H
5l
1 3

D. Aute et al.
Table 3 (continued)
Entry Product Time (min) Yield (%)a M.P. (°C)
Observed Reported
206–208 207–209 [36]
13
20
Cl
89
O
O
O
N
H
5m
14
15
91
206–207 206–207 [42]
NO2
O
O
O
O
O
N
H
5n
15
16
15
90
92
247–248 248–250 [36]
O
O
O
N
H
5o
20
240–241 240–241 [36]
S
O
O
N
H
5p
aIsolated yield of product
1 3

Aluminized polyborate‑catalysed green and efcient synthesis…
aldehydes like furfural (90%) and thiophene-2-carbaldehyde (92%) also gives cor-
responding products with good yields.
The catalytic competence of aluminized polyborate for the preparation of poly-
hydroquinolines was also compared with literature reported methods (Table 4).
It was observed that the non-toxic and inexpensive aluminized polyborate shows
good catalytic activity in terms of operational simplicity and applicability for
variety of aldehydes (Table 3).
Plausible reaction mechanism
A plausible reaction mechanism was proposed for four-component synthesis
of polyhydroquinolines as depicted in Scheme 4. The aldehyde, dimedone and
ethylacetoacetate would be activated by the aluminized polyborate catalyst due
to its Lewis acid nature. In the presence of catalyst, the dimedone enolises and
undergoes Knoevenagel condensation to give intermediate. The activated ethyl-
acetoacetate reacts with NH4OAc to give enamine. The ensuing Michael addition
of enamine to intermediate followed by sequential cyclization and dehydration
reactions gives polyhydroquinolines.
Table 4 Comparison of results with reported method for the synthesis of polyhydroquinolines
Entry Catalyst and solvent
Temperature (°C)
Time (min) Yield (%) References
1
2
AFGO nanosheets in EtOH
r.t.
80 °C
2–4 h
170
87–91
96
27
28
MCM-41@serine@Cu(II) in
EtOH
3
4
5
6
KH2PO4 in EtOH:H2O
TEDETA@BNPs in EtOH
SBA-15@Glycine-M in EtOH 60 °C
100 °C
80 °C
5 h
60
110
30
91
95
92
92
30
31
33
35
Fe3O4@SiO2/ZnCl2 under
110 °C
solvent-free
7
8
CdS nanowires in EtOH
Presence of visible light 30
92
70–99
36
45
Ascorbic acid under solvent-
80 °C
1.5–5 h
free
9
Cu(II)-DCC-CMK-3 under
solvent-free
80 °C
25
98
46
10
11
spinel FeAl2O4 in EtOH
80 °C
100 °C
30
15
96
94
47
Aluminized polyborate under
Present work
solvent-free
1 3

D. Aute et al.
O
O
Ar
H
O
Cl
Al
O
Cl
Al
OH
HO
O
Ar
O
CO2Et
Cl
Al
Ar
H
Cl
Al
N
H
O
HO
OH
Polyhydroquinolines
-H2O
NH3
O
Ar
OEt
H
CO2Et
O
O
O
NH4OAC
AcOH
C
NH
OH
Cl
Al
O
Ar
H
CO2Et
O
+
NH2
Scheme 4 Plausible reaction mechanism
Conclusions
In conclusion, herein we report the successful application of green and inexpensive
aluminized polyborate as a catalyst for the multicomponent synthesis of polyhyd-
roquinolines using diferently substituted aldehydes, dimedone, ethylacetoacetate
and NH4OAc at 100 °C under solvent-free conditions. This environmentally benign
protocol ofers green chemistry features such solvent-free conditions, short reaction
time, non-toxic catalyst, non-column purifcation and excellent product yields.
Acknowledgements The authors are grateful to Principal (P.V.P. College, Pravaranagar & A.S.C. Col-
lege, Satral) for providing necessary facilities and constant encouragement. We express our gratitude
towards Principal Dr. B. K. Karale for valuable guidance. We also thank to SAIF, Punjab University,
Chandigarh, for spectral analysis.
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