Ultrasound-promoted synthesis of bi-, tri- and tetrapodal polyhydroquinolines, 1,4-dihydropyridines and the corresponding pyridines

Article abstract of DOI:10.1039/c3ra45138k

Bi-, tri- and tetrapodal polyhydroquinolines and 1,4-dihydropyridines were synthesised by the reaction of various alkylating agents with polyhydroquinolines and 1,4-dihydropyridines under sonication conditions. These were in turn converted to the corresponding pyridines also using sonication. Sonication produced higher yields in a faster reaction time. All the synthesized compounds were characterized using spectral data.

Full text of DOI:10.1039/c3ra45138k

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Ultrasound-promoted synthesis of bi-, tri- and
tetrapodal polyhydroquinolines, 1,4-
Cite this: RSC Adv., 2014, 4, 39
dihydropyridines and the corresponding pyridines†
*
G. L. Balaji, K. Rajesh, M. Venkatesh, S. Sarveswari and V. Vijayakumar
Bi-, tri- and tetrapodal polyhydroquinolines and 1,4-dihydropyridines were synthesised by the reaction of
various alkylating agents with polyhydroquinolines and 1,4-dihydropyridines under sonication conditions.
These were in turn converted to the corresponding pyridines also using sonication. Sonication produced
higher yields in a faster reaction time. All the synthesized compounds were characterized using spectral
data.
Received 15th September 2013
Accepted 2nd October 2013
DOI: 10.1039/c3ra45138k
www.rsc.org/advances
benzene derivatives as catalysts,7 as well as in the eld of
analytical chemistry.8 Some of the bi-, tri- and tetrapodal
1. Introduction
moieties like bis(indolyl)methanes are found in cruciferous
plants and are known to promote benecial estrogen metabo-
lism and induce apoptosis in human cancer cells.9
There are some reports on the synthesis of tripodal 1,4-
DHPs, but these are associated with serious limitations such as
the use of costly reagents (e.g. tripodal aldehydes) and longer
reaction times (6 hours).10
In recent years, a great deal of consideration has been given to
the synthesis of polyhydroquinoline compounds, which is
attributed to their diverse therapeutic and pharmacological
signicance. Quinolines with a 1,4-dihydropyridine (1,4-DHP)
nucleus are of high pharmacological importance. Members of
the quinoline family have been widely used as anti-asthmatic,
antibacterial, anti-inammatory, antimalarial and tyrosine
kinase inhibiting agents.1 1,4-Dihydropyridine derivatives have
been used as drugs for treating cardiovascular diseases.2
Cardiovascular agents such as amlodipine, nicardipine, nifed-
ipine and other similar derivatives are 1,4-DHPs, and have been
found to be efficient in the treatment of hypertension.3 Exten-
sive studies on these compounds have revealed various thera-
peutic applications, including their use as cerebral anti-
ischemic agents, chemosensitizers, platelet anti-aggregators
and neuroprotectants.4 These examples demonstrate the
outstanding potential of 1,4-DHPs as signicant drug candi-
dates. In addition, oxidation of these compounds into pyridines
has also been extensively studied.5 Hence, the synthesis of this
particular heterocyclic nucleus is also of great interest.
Ultrasonic-assisted organic synthesis has been considered a
powerful technique to accelerate reactions using a green
chemistry approach, since it affords maximum yield by mini-
mizing the amount of waste by-products.11 Although this tech-
nique has been employed in many organic reactions, this is the
rst ever attempt to synthesize tri- and tetrapodal poly-
hydroquinolines and 1,4-DHPs under ultrasonic irradiation.
The ultrasonic irradiation technique has been employed to
synthesize the bi-, tri-, and tetrapodal 1,4-DHPs and their cor-
responding pyridines with the aim to reduce reaction times. The
desired products have been synthesised using cheaper reagents
like K2CO3 and 20% HNO3, and as the reaction was accelerated
due to ultrasonication, we can say that we succeeded in our
endeavor. Due to the technical efficiency of reagent activation
under ultrasonic irradiation, this technique has also been
adopted to oxidize the synthesized bi-, tri- and tetrapodal
polyhydroquinolines and 1,4-DHPs to their corresponding
pyridines.12 Compared to conventional thermal heating, ultra-
sound-assisted reactions require less time to complete, due to
the reduction of particle size13 under high frequency sound
waves (more than 20 kHz), which leads to an acceleration of the
reaction.
Thus far, this interest has oen manifested in the synthesis
of monopodal polyhydroquinoline derivatives, and highly
functional ones are infrequently examined. The tripodal deriv-
ative 1,3,5-tris(N-alkylaminomethyl)benzene has drawn much
attention as an efficient building block for the synthesis of
functional moieties such as molecular receptors, since its aryl
ring acts as a tiny inexible platform for receptor synthesis.6
Recent investigations revealed the utility of 1,3,5-tris methyl
Centre for Organic and Medicinal Chemistry, School of Advanced Sciences, VIT
University, Vellore 632 014, Tamil Nadu, India. E-mail: kvpsvijayakumar@gmail.
com; Tel: +91-416 220 2332
2. Results and discussions
In a continuation of our earlier reports on the synthesis of poly-
hydroquinolines, 1,4-DHPs14,15 and heterocyclic compounds
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ra45138k
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bearing bi-, tri- and tetrapodal moieties,16 we synthesized These cavities burst, resulting in the liberation of high amounts
various derivatives of polyhydroquinoline and 1,4-DHPs using of energy. In addition, due to some intermediate conditions,
hydroxy benzaldehydes, ammonium acetate and b-ketoesters. intermolecular reactions are facilitated outside the cavity. These
The obtained products were then treated with the alkylating events act to accelerate the reaction rate, and hence, the reac-
agents bis-, tris- and tetra-bromomethyl benzene to afford bi-, tri- tion could be completed within 20 minutes using K2CO3 as a
and tetrapodal polyhydroquinolines, respectively (Scheme 1) in mild base, affording a good yield (Table 1). It has also been
the presence of K2CO3 as a mild base (in order to control the suggested that ultrasonication accelerates the rate of reaction
reaction). Initially, the reaction was attempted with the by minimizing the particle size of the catalyst. The results here
strong base NaH, but its strong alkaline nature afforded both were found to be in agreement with earlier reports that ultra-
N-alkylated and O-alkylated products, and hence, a low yield was sonication reduces the particle size of the catalyst into the
observed for the desired product. Consequently, the reaction was nanometric range.13 Aer completion of the reaction, a green
attempted with K2CO3,
a mild base which afforded the work up was chosen to extract the product using an aqueous
desired product in a high yield since it only favoured O-alkylation. medium. The described molecular structures have been
Aer monitoring the reaction with various solvents like characterized and also conrmed using the corresponding
acetonitrile, DMF, DMSO and THF, an optimum yield was spectral data.
observed using DMF and hence, DMF was considered as a suit-
Bipodal polyhydroquinolines were aromatized to their cor-
able solvent. The reaction was carried out at a controlled responding pyridines (Scheme 2) quantitatively under mild
temperature of 60 ꢀC to yield the corresponding bi-/tri-/tetrapodal and heterogeneous conditions. In this regard, 20% HNO3 was
polyhydroquinolines.
used as an oxidizing agent. Furthermore, it is an excellent
The conventional methodology used required 48 hours for monoprotic acid source and is established as a highly effective
the completion of the reaction due to high steric hindrance, reagent for the oxidation of bipodal polyhydroquinolines.
especially for the tri- and tetrapodal derivatives. Hence, the However, it failed to convert tri- and tetrapodal poly-
methodology was changed to an ultrasonic-assisted reaction hydroquinolines. This prompted us to attempt the aromatiza-
since sonochemical-associated enhancement of the reaction tion of bi-, tri- and tetrapodal 1,4-DHPs to the corresponding
rate is a well known phenomenon in organic reactions. During pyridines, which was successfully carried out (Schemes 3
the ultrasonic irradiation process, formation of cavitation and 4). Hence, this synthetic method also proved to be an
bubbles with a high temperature and pressure takes place. efficient and economically cheap method for oxidizing bi-, tri-
and tetrapodal 1,4-DHPs. Moreover, conducting the reaction
under ultrasonic irradiation was sufficient to produce a >99%
product yield within 3 minutes (Table 2). The spectral data
conrmed the formation of the desired novel bi-, tri- and tet-
rapodal pyridines.
The 1H NMR spectrum for compound 9 exhibits singlets at d
0.93 ppm and d 1.06 ppm, which correspond to the methyl
groups at C-15, 150 and C-16, 160, respectively. A triplet at d 1.21
ppm corresponds to the protons at C-14, 140. Multiplets at d 2.17
ppm and at d 2.24 ppm correspond to the protons at C-8, 80 and
C-6, 60 respectively. Singlets at d 2.31 ppm and d 2.33 ppm
correspond to the protons at C-11 and C-110, respectively
and the quartet at d 4.08 ppm corresponds to the protons at
C-13, 130.
Singlets at d 5.04 and d 5.07 ppm correspond to the protons
at C-23, 230 and C-4, 40, respectively. A broad singlet at d 6.18
ppm corresponds to the protons of nitrogen and another at d
6.76 ppm corresponds to the aromatic protons at C-20, 200.
Doublets at d 6.81 ppm and d 6.84 ppm correspond to the
protons at C-22 and C-220, respectively, and the triplet at d 6.99
ppm corresponds to the protons at C-21, 210. A multiplet at d
7.14 ppm corresponds to the protons at C-18, 180. A singlet at d
7.41 ppm corresponds to the protons at C-25, 250 and C-26, 260.
The m/z value observed at the 812.4785 (M + 1) peak in the
HRMS spectrum also conrms the formation of the target
molecule. Furthermore, during the conversion of 9 to its cor-
responding pyridine 28, a slight change in chemical shi
values has been observed, along with the absence of two
signals; one is the broad singlet due to the –NH proton and the
Scheme 1 Synthesis of bi-, tri- and tetrapodal polyhydroquinolines.
40 | RSC Adv., 2014, 4, 39–46
other is a singlet for a proton at C-4, 40. This clearly revealed
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Table 1 Reaction of 1,4-DHPs with alkylating agents in the presence of K2CO3 using a conventional method and under ultrasonic irradiation
Time
Yield (%)
Conv.a
Substrate (1,4-DHP)
Alkylating agent
Product
Conv.a (hours)
U.S.b (minutes)
U.S.b
1
1
1
1
1
1
2
2
2
2
2
2
2
3
3
3
3
3
3
3
4
9
48
48
48
48
48
48
48
48
48
48
48
48
48
48
48
48
48
48
48
48
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
65
70
72
75
52
45
67
19
67
10
67
50
49
72
64
59
52
69
63
54
81
75
82
89
73
65
79
45
85
40
75
68
77
74
81
74
60
76
70
62
5a
5b
6
7
8
10a
10b
11
12
13
14
16
17
20
22
23
26
15
18
19
21
24
25
27
4
5a
5b
6
7a
7b
8
4
5a
5b
6
7a
7b
8
a
Reaction was carried out by a conventional method. b Reaction was carried out by an ultrasound irradiation method.
Scheme 2 Oxidation of bipodal polyhydroquinolines.
that the compound 9 had been oxidized to 28. The m/z
value observed at the 808 (M + 1) peak in the LC-MS spectrum
also conrmed the formation of the desired target with high
Scheme 3 Synthesis of bi-, tri- and tetrapodal 1,4-DHPs.
purity.
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plates of silica gel (SD Fine). Visualization was carried out using
an iodine chamber. Column chromatography was performed
using silica gel (60–120 mesh).
3.2 Ultrasound set-up
The ultrasound system for sonochemical synthesis was gener-
ated using an ultrasonic instrument set-up (horn type). The
specication and details of the experimental set-up are as
follows: Make: E-Chrom Tech Co. Ltd., Taiwan. Operating
frequency: 22 kHz. Rated output power: 800 W. Diameter of
titanium tip of horn: 1.3 ꢁ 10ꢂ2 m. Surface area of ultrasound
irradiating face: 1.32 ꢁ 10ꢂ4 m2.
3.3 Representative procedure for the synthesis of bi-, tri- and
tetrapodal 1,4-DHPs
3.3.1 Conventional method. To a suspension of 1.05 equiv.
of ethyl 4-(3-hydroxyphenyl)-2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-
hexahydroquinoline-3-carboxylate, diethyl or dimethyl 4-(4-
hydroxyphenyl)-2,6-dimethyl-1,4-dihydropyridine-3,5-dicarboxy-
late (1 or 2 or 3) in 10 volumes of dry DMF, 3.0 equiv. of
powdered K2CO3 was added, and the mixture was stirred well for
20 minutes at 40 ꢀC. An alkylating agent dissolved in the
minimum amount of DMF was added dropwise to the mixture
for an hour (with stirring), at 50 ꢀC. Aer complete addition, the
reaction mixture was maintained at 60 ꢀC and stirred for 48
hours. The completion of the reaction was monitored by TLC.
The reaction mixture was then allowed to cool at room
temperature, ltered to remove the insoluble solids and the
lter cake was washed with DMF. Excess solvent was removed
under reduced pressure and then quenched with ice-cold water,
while the obtained crude products were puried by column
chromatography (60–120 mesh) using petroleum ether–EtOAc.
They were then recrystallized using the same mixture of
solvents, which resulted in the products being obtained in good
to moderate yields (Table 1).
Scheme 4 Oxidation of bi-, tri- and tetrapodal 1,4-DHPs.
Table 2 Conversion of bi-, tri- and tetrapodal 1,4-DHPs to the cor-
responding pyridines under an ultrasonic irradiation method
Substrate
Product
Time (minutes)
Yielda
9
28
29
30
33
34
35
36
37
38
39
40
3
3
3
3
3
3
3
3
3
3
3
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
10a
11
15
16
17
22
23
24
26
27
3.3.2 Ultrasonic irradiation method. To a suspension of
1.05 equiv. of ethyl 4-(3-hydroxyphenyl)-2,7,7-trimethyl-5-oxo-
a
Isolated yields.
1,4,5,6,7,8-hexahydroquinoline-3-carboxylate,
diethyl
or
dimethyl 4-(4-hydroxyphenyl)-2,6-dimethyl-1,4-dihydropyridine-
3,5-dicarboxylate (1 or 2 or 3) in 10 volumes of dry DMF, 3.0
equiv. of powdered K2CO3 was added and the mixture was sub-
jected to sonication using an ultrasonic horn (E-Chrom ultra-
sonic horn, 22 kHz frequency) at 50% amplitude with a pulse of
3. Experimental section
3.1 General
Solvents and reagents were commercially sourced and used 30 s ON and 30 s OFF cycle for 5 minutes. The reaction mixture
without further purication. Melting points were recorded on was then exposed to acoustic cavitation (E-Chrom ultrasonic
an Elchem Microprocessor-based DT apparatus in open capil- horn) at 50% amplitude for 20 minutes with 5 intervals of 4
lary tubes and corrected with benzoic acid. NMR spectra were minutes each, at 60 ꢀC. The reaction mixture was then allowed to
recorded either on Bruker 400 or 500 MHz spectrometers using cool at room temperature, ltered to remove the insoluble solids
TMS as an internal standard (chemical shis d in ppm). Mass and the lter cake was washed with DMF. Excess solvent was
spectra were recorded either via HRMS or LC-MS by Agilent 1200 removed under reduced pressure and then quenched with ice-
series LC and Micromass ZQ spectrometers. HRMS spectra were cold water, while the obtained crude products were puried by
recorded using a 1260 Innity Nano HPLC with Chipcube 6550 column chromatography (60–120 mesh) using petroleum ether–
and iFunnel Q-TOF system. Positive and negative modes of EtOAc. These were then recrystallized using the same mixture of
ionization were recorded using electrospray ionization. Thin- solvents, which resulted in the products being obtained in good
layered chromatography (TLC) was performed on preparative to moderate yields (Table 1).
42 | RSC Adv., 2014, 4, 39–46
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220), 121.42 (C-25, 23), 128.87 (C-22, 220), 137.59 (C-10, 100),
143.75 (C-17, 170), 148.39 (C-26), 148.79 (C-2, 20), 157.01 (C-24,
240), 158.05 (C-19, 190), 167.43 (C-12, 120), 196.01 (C-5, 50). HRMS
(m/z): calcd 813.1614 (M+). Found: 813.1427 (M+).
3.4 Diethyl 4,40-(3,30-(1,4-phenylenebis(methylene)bis(oxy)-
bis(3,1-phenylene))bis(2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-
hexahydroquinoline-3-carboxylate)) (9)
Yellow solid. Mp 130–132 ꢀC. 1H NMR (500 MHz, CDCl3) dH: 0.93
(s, 6H, C-15, 150), 1.06 (s, 6H, C-16, 160), 1.21 (t, J ¼ 7.25 Hz, 6H,
C-14, 140), 2.17 (m, 4H, C-8, 80), 2.24 (m, 4H, C-6, 60), 2.31 (s, 3H,
C-11), 2.33 (s, 3H, C-110), 4.08 (q, J ¼ 7.24 Hz, 4H, C-13, 130), 5.04
(s, 4H, C-23, 230), 5.07 (s, 2H, C-4, 40), 6.18 (bs, 2H), 6.76 (m, 2H,
C-20, 200), 6.81 (d, J ¼ 8 Hz, 1H, C-22), 6.84 (d, J ¼ 8 Hz, 1H,
C-220), 6.99 (t, J ¼ 8, 2H, C-21, 210), 7.14 (m, 2H, C-18, 180), 7.41
(s, 4H, C-25, 250, 26, 260). 13C NMR (125 MHz, CDCl3) dc: 14.26
(C-14, 140), 19.35 (C-11, 110), 27.18 (C-15, 150), 29.43 (C-16, 160),
32.66 (C-7, 70), 36.36 (C-4, 40), 40.98 (C-8, 80), 50.74 (C-6, 60), 59.81
(C-13, 130), 70.19 (C-23, 230), 105.73 (C-3, 30), 111.99 (C-20, 200),
112.77 (C-9, 90), 114.64 (C-18, 180), 121.28 (C-22, 220), 127.86
(C-25, 250), 128.79 (C-26, 260), 137.19 (C-21, 210), 143.65 (C-24,
240), 143.76 (C-17, 170), 148.15 (C-2, 20), 148.63 (C-10, 100), 158.63
(C-19, 190), 167.41 (C-12, 120), 195.44 (C-5, 50). HRMS (m/z): calcd
812.4359 (M+). Found: 812.4785 (M+).
3.7 Diethyl 4,40-(((1,2-phenylenebis(methylene))bis(oxy))-
bis(3,1-phenylene))bis(2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-
hexahydroquinoline-3-carboxylate) (11)
White solid. Mp 86–88 ꢀC. 1H NMR (500 MHz, CDCl3) dH: 0.87 (s,
3H, C-15), 0.88 (s, 3H, C-150), 0.97 (s, 3H, C-16), 1.01 (s, 3H, C-160),
1.17 (t, J ¼ 7 Hz, 6H, C-14, 140), 2.05 (m, 4H, C-6, 60), 2.17 (s, 6H, C-
11, 110), 2.29 (m, 4H, C-8, 80), 4.05 (q, J ¼ 7.25 Hz, 4H, C-13, 130),
5.03 (s, 4H, C-23, 230), 5.07 (s, 2H, C-4, 40), 6.74 (bs, 2H), 6.94 (m,
4H, C-20, 200, 22, 220), 7.11 (m, 4H, C-25, 250, 26, 260), 7.30 (s, 1H,
C-18), 7.45 (s, 1H, C-180), 7.68 (t, J ¼ 6.4 Hz, 2H, C-21, 210). 13C
NMR (125 MHz, CDCl3) dC: 14.31 (C-14, 140), 19.08 (C-11, 110),
27.16 (C-15, 150), 29.48 (C-16, 160), 32.64 (C-7, 70), 36.61 (C-4, 40),
40.60 (C-8, 80), 50.83 (C-6, 60), 59.84 (C-13, 130), 67.43 (C-23, 230),
105.80 (C-3, 30), 111.28 (C-20, 200), 112.28 (C-9, 90), 113.91 (C-18,
180), 121.08 (C-22, 220), 128.93 (C-25, 250), 130.79 (C-26, 260), 135.05
(C-21, 210), 144.34 (C-24, 240), 144.34 (C-17, 170), 148.93 (C-2, 20),
149.79 (C-10, 100), 158.52 (C-19, 190), 167.58 (C-12, 120), 196.01 (C-
5, 50). HRMS (m/z): calcd 812.4359 (M+). Found: 812.4785 (M+).
3.5 Diethyl 4,40-(3,30-(1,3-phenylenebis(methylene)bis(oxy)-
bis(3,1-phenylene))bis(2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-
hexahydroquinoline-3-carboxylate)) (10a)
White solid. Mp 102–104 ꢀC. 1H NMR (500 MHz, CDCl3) dH: 0.89
(s, 3H, C-15), 0.91 (s, 3H, C-150), 1.03 (s, 3H, C-16), 1.05 (s, 3H, C-
160), 1.21 (t, J ¼ 7.25 Hz, 6H, C-14, 140), 2.14 (m, 4H, C-8, 80), 2.25
(m, 4H, C-6, 60), 2.31 (s, 6H, C-11, 110), 4.07 (q, J ¼ 7.16 Hz, 4H, C-
13, 130), 5.04 (s, 2H, C-4, 40), 5.05 (s, 4H, C-23, 230), 6.78 (m, 4H,
C-20, 200, 22, 220), 6.89 (bs, 2H), 6.99 (t, J ¼ 8 Hz, 2H, C-21, 210),
7.15 (m, 2H, C-18, 180), 7.366 (s, 3H, C-25, 250, 260), 7.437 (s, 1H,
C-26). 13C NMR (125 MHz, CDCl3) dC: 14.27(C-14, 140), 19.16 (C-
11, 110), 27.16 (C-15, 150), 29.41 (C-160, 160), 32.60 (C-7, 70), 36.34
(C-8, 80), 40.79 (C-4, 40), 50.76 (C-6, 60), 59.81 (C-13, 130), 70.03
(C-23, 230), 105.64 (C-3, 30), 111.76 (C-20, 200), 112.37 (C-9, 90),
112.53 (C-18, 180), 114.52 (C-22, 220), 121.14 (C-24, 240), 126.47
(C-22, 220), 127.14 (C-10, 100), 128.81 (C-17, 170), 129.03 (C-260),
137.37 (19, 190), 144.09 (C-26), 148.81 (C-2, 20), 158.50 (C-24, 240),
167.46 (C-12, 120), 196.01 (C-5, 50). HRMS (m/z): calcd 812.5142
(M+). Found: m/z 812.4785 (M+).
3.8 Triethyl 4,40,400-(3,30,300-(2,4,6-trimethylbenzene-1,3,5-triyl)
tris(methylene)tris(oxy)tris(benzene-3,1-diyl))tris(2,7,7-trimethyl-
5-oxo-1,4,5,6,7,8-hexahydroquinoline-3-carboxylate) (12)
White solid. Mp 124–126 ꢀC. 1H NMR (500 MHz, CDCl3) dH: 0.92
(s, 9H, C-15, 150, 1500), 1.05 (s, 9H, C-16, 160, 1600), 1.24 (t, J ¼ 7
Hz, 9H, C-14, 140, 1400), 2.19 (m, 9H, C-26, 260, 2600), 2.32 (m, 21H,
C-6, 60, 600, 8, 80, 800, 11, 110, 1100), 4.09 (q, J ¼ 6.25 Hz, 6H, C-13,
130, 1300, 13000), 5.02 (s, 3H, C-4, 40, 400), 5.06 (s, 6H, C-23, 230, 2300),
6.65 (bs, 3H), 6.80 (d, J ¼ 8 Hz, 3H, C-20, 200, 2000), 6.90 (s, 3H, C-
18, 180, 1800), 7.00 (d, J ¼ 7.5 Hz, 3H, C-22, 220, 2200), 7.15 (t, J ¼
6.4 Hz, 3H, C-21, 210, 2100). 13C NMR (125 MHz, CDCl3) dC: 14.28
(C-14, 140, 1400), 19.08 (C-11, 110, 1100), 26.14 (C-26, 260, 2600),
27.09 (C-15, 150, 1500), 29.40 (C-16, 160, 1600), 32.66 (C-7, 70, 700),
36.39 (C-4, 40, 400), 40.90 (C-8, 80, 800), 50.75 (C-6, 60, 600), 59.81
(C-13, 130, 1300), 65.28 (C-23, 230, 2300), 105.77 (C-3, 30, 300), 111.89
(C-20, 200, 2000), 112.01 (C-9, 90, 900), 115.14 (C-18, 180, 1800),
121.23 (C-22, 220, 2200), 128.71 (C-25, 250, 2500), 138.78 (C-21, 210,
2100), 139.05 (C-24, 240, 2400), 143.86 (C-17, 170, 1700), 148.58 (C-2,
20, 200), 149.12 (C-10, 100, 1000), 159.02 (C-19, 190, 1900), 167.49
(C-12, 120, 1200), 195.71 (C-5, 50, 500). HRMS (m/z): calcd 1221.1178
(M+). Found: 1221.1200 (M+).
3.6 Diethyl 4,40-(3,30-(pyridine-2,6-diylbis(methylene)bis(oxy)-
bis(3,1-phenylene))bis(2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-
hexahydroquinoline-3-carboxylate)) (10b)
Yellow solid. Mp 114–116 ꢀC. 1H NMR (500 MHz, CDCl3) dH: 0.87
(s, 3H, C-15), 0.89 (s, 3H, C-150), 0.97 (s, 3H, C-16), 1.01 (s, 3H, C-
160), 1.17 (t, J ¼ 12 Hz, 6H, C-14, 140), 1.78 (m, 4H, C-8, 80), 2.17
(m, 4H, C-6, 60), 2.27 (s, 6H, C-11, 110), 4.06 (q, J ¼ 7 Hz, 4H, C-13,
130), 5.03 (s, 2H, C-4, 40), 5.12 (s, 4H, C-23, 230), 6.47 (bs, 2H), 6.77
(d, J ¼ 5.2 Hz, 2H, C-22, 220), 6.83 (d, J ¼ 8 Hz, 2H, C-20, 200), 6.98
3.9 Tetraethyl 4,40,400,4000-(((benzene-1,2,4,5-tetrayltetrakis
(methylene))tetrakis(oxy))tetrakis(benzene-3,1-diyl))-
tetrakis(2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-hexahydroquinoline-
3-carboxylate) (13)
(t, J ¼ 7.2 Hz, 2H, C-21, 210), 7.13 (m, 2H, C-18, 180), 7.39 (d, J ¼ 8 White solid. Mp 100–102 ꢀC. 1H NMR (500 MHz, CDCl3) dH: 0.93
Hz, 2H, C-25, 250), 7.67 (t, J ¼ 8.2, 1H, C-26). 13C NMR (125 MHz, (s, 6H, C-15, 150), 0.96 (s, 6H, C-1500, 15000), 1.00 (s, 6H, C-16, 160),
CDCl3) dC: 14.31 (C-14, 140), 19.08 (C-11, 110), 27.16 (C-15, 150), 1.04 (s, 6H, C-1600, 16000), 1.19 (t, J ¼ 7 Hz, 12H, C-14, 140, 1400,
29.48 (C-16, 160), 32.64 (C-7, 70), 36.61 (C-8, 80), 40.66 (C-4, 40), 14000), 2.14 (s, 12H, C-11, 110, 1100, 11000), 2.32 (s, 8H, C-6, 60, 600,
50.74 (C-6, 60), 59.80 (C-13, 130), 70.09 (C-23, 230), 105.66 (C-3, 30), 6000), 2.61 (s, 8H, C-8, 80, 800, 8000), 4.04 (q, J ¼ 7.25 Hz, 8H, C-13,
111.82 (C-20, 200), 112.96 (C-9, 90), 113.92 (C-18, 180), 120.74 (C-22, 130, 1300, 13000), 5.02 (s, 8H, C-23, 230, 2300, 23000), 5.09 (s, 4H, C-4,
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40, 400, 4000), 6.77 (bs, 2H), 6.82 (bs, 2H), 6.93 (d, J ¼ 8 Hz, 4H, C-
20, 200, 2000, 20000), 7.11 (d, J ¼ 8 Hz, 4H, C-22, 220, 2200, 22000), 7.33
(m, 6H, C-21, 210, 2100, 21000, 25, 250), 7.51 (s, 4H, C-18, 180, 1800,
18000). 13C NMR (125 MHz, CDCl3) dC: 14.28 (C-14, 140, 1400, 14000),
19.12 (C-11, 110, 1100, 11000), 27.22 (C-15, 150, 1500, 15000), 28.41 (C-
16, 160, 1600, 16000), 32.69 (C-7, 70, 700, 7000), 36.73 (C-4, 40, 400, 4000),
40.78 (C-8, 80, 800, 8000), 50.85 (C-6, 60, 600, 6000), 59.79 (C-13, 130,
1300, 13000), 67.44 (C-23, 230, 2300, 23000), 105.16 (C-3, 30, 300, 3000),
111.63 (C-20, 200, 2000, 20000), 113.19 (C-9, 90, 900, 9000), 113.62 (C-
18, 180, 1800, 18000), 120.89 (C-22, 220, 2200, 22000), 128.45 (C-25, 250,
2500, 25000), 135.14 (C-21, 210, 2100, 21000), 139.05 (C-24, 240, 2400,
24000), 143.98 (C-17, 170, 1700, 17000), 144.21 (C-2, 20, 200, 2000), 149.52
(C-10, 100, 1000, 10000), 158.50 (C-19, 190, 1900, 19000), 167.55 (C-12,
120, 1200, 12000), 195.85 (C-5, 50, 500, 5000). HRMS (m/z): calcd
1547.4263 (M+). Found: 1547.4030 (M+).
3.13 Tetramethyl 4,40-(((1,4-phenylenebis(methylene))-
bis(oxy))bis(4,1-phenylene))bis(2,6-dimethylpyridine-3,5-
dicarboxylate) (33)
White solid. Mp 150–152 ꢀC. 1H NMR (500 MHz, CDCl3) dH: 1.60
(s, 12 H-7, 70), 2.60 (s, 6 H-9), 3.59 (s, 6 H-90), 5.11 (s, 4 H-15, 150),
7.00 (d, J ¼ 7.5 Hz, 4 arom. H-13, 130), 7.20 (d, J ¼ 7.5 Hz, 4 arom.
H-12, 120), 7.49 (s, 4 arom. H-17, 170). 13C NMR (125 MHz,
CDCl3) dC: 22.30 (C-7, 70), 52.39 (C-9, 90), 69.70 (C-15, 150), 104.17
(C-3, 5), 114.76 (C-13, 130), 127.51 (C-17, 170), 127.84 (C-12, 120),
128.60 (C-16, 160), 136.52 (C-11), 155.08 (C-2, 6), 159.02 (C-4, 14),
168.10 (C-8, 80). HRMS (m/z): calcd 730.6514 (M+). Found:
730.6116 (M+).
3.14 Tetramethyl 4,40-(((1,3-phenylenebis(methylene))-
bis(oxy))bis(4,1-phenylene))bis(2,6-dimethylpyridine-3,5-
dicarboxylate) (34a)
White solid. Mp 100–102 ꢀC. 1H NMR (500 MHz, CDCl3) dH: 2.60
(s, 12H, C-7, 70), 3.59 (s, 12H, C-9, 90), 4.93 (s, 2H, C-4), 5.12 (s,
4H, C-15, 150), 7.01 (d, J ¼ 6.9 Hz, 4H, C-13, 130), 7.20 (d, J ¼ 8.4
Hz, 4H, C-12, 120), 7.44 (s, 3H, C-16, 18, 180), 7.56 (s, 1H, C-19).
HRMS (m/z): calcd 730.0176 (M+). Found: 730.0000 (M+).
3.10 Oxidation of bi-, tri- and tetrapodal 1,4-DHPs into the
corresponding bi-, tri- and tetrapodal pyridines
A suspension of 1.0 equiv. of bi-, tri- and tetrapodal 1,4-DHPs in
4 equiv. volumes of 20% HNO3 were kept under ultrasonic
irradiation for 3 minutes at 60 ꢀC. The progress of the reaction
was monitored by TLC. Aer completion of the reaction, the
mixture was ltered and poured into 500 ml of water, neutral-
ized with Na2CO3, then dried over anhydrous Na2SO4 (50 mg),
which was added to the ltrate. Bi-, tri- and tetrapodal pyridines
were obtained through simple ltration and evaporation of the
solvent.
3.15 Tetramethyl 4,40-(((pyridine-2,6-diylbis(methylene))-
bis(oxy))bis(4,1-phenylene))bis(2,6-dimethylpyridine-3,5-
dicarboxylate) (34b)
White solid. Mp 106–108 ꢀC. 1H NMR (500 MHz, CDCl3) dH: 2.31
(s, 12H, C-7, 70), 3.63 (s, 12H, C-9, 90), 5.13 (s, 4H, C-15, 150), 6.81
(d, J ¼ 8.4 Hz, 4H, C-13, 130), 7.16 (d, J ¼ 8.1 Hz, 4H, C-12, 120),
7.42 (d, J ¼ 7.8 Hz, 2H, C-18, 180), 7.70 (t, J ¼ 7.5 Hz, 1H, C-19).
13C NMR (125 MHz, CDCl3) dC: 22.93 (C-7, 70), 52.24 (C-9, 90),
70.58 (C-15, 150), 114.63 (C-3, 5), 120.51 (C-13, 130), 126.95 (C-18,
180), 129.20 (C-12), 129.29 (C-120), 137.65 (C-11, 110), 145.69 (C-
19), 155.44 (C-2, 6), 156.49 (C-17, 170), 158.52 (C-4, 14), 168.58 (C-
8, 80). HRMS (m/z): calcd 731.8956 (M+). Found: m/z 731.9000
(M+).
3.11 Diethyl 4,40-(((1,4-phenylenebis(methylene))bis(oxy))-
bis(3,1-phenylene))bis(2,7,7-trimethyl-5-oxo-5,6,7,8-
tetrahydroquinoline-3-carboxylate) (28)
White solid. Mp 134–136 ꢀC. 1H NMR (500 MHz, CDCl3) dH: 0.96
(t, J ¼ 9 Hz, 6H, C-14, 140), 1.14 (s, 12H, C-15, 150, 16, 160), 2.51 (s,
4H, C-6, 60), 2.71 (s, 6H, C-11, 110), 3.22 (s, 4H, C-8, 80), 4.02 (q,
J ¼ 9.0 Hz, 4H, C-13, 130), 5.06 (s, 4H, C-23, 230), 6.76 (m, 4H,
C-20, 200, 22, 220), 7.02 (d, J ¼ 9.4 Hz, 2H, C-18, 180), 7.31 (d, J ¼
9.4 Hz, 2H, C-21, 210), 7.45 (s, 4H, C-25, 250, 26, 260). LCMS: m/z
808 (M + 1); HRMS (m/z): calcd 808.1291 (M+). Found: m/z
808.1300 (M+).
3.16 Tetramethyl 4,40-(((1,2-phenylenebis(methylene))-
bis(oxy))bis(4,1-phenylene))bis(2,6-dimethylpyridine-3,5-
dicarboxylate) (35)
Yellow solid. Mp 88–90 ꢀC. 1H NMR (500 MHz, CDCl3) dH: 2.53
(s, 12H, C-7, 70), 3.24 (s, 12H, C-9, 90), 5.38 (s, 4H, C-15, 150), 6.81
(d, J ¼ 7.4 Hz, 4H, C-13, 130), 7.17 (d, J ¼ 8.0 Hz, 4H, C-12, 120),
7.34–7.49 (m, 4H, C-17, 170, 18, 180). 13C NMR (125 MHz, CDCl3)
dC: 22.35 (C-7, 70), 52.38 (C-9, 90), 67.91 (C-15, 150), 114.74 (C-3,
5), 127.46 (C-13, 130), 128.57 (C-18, 180), 128.77 (C-12, 120),
129.00 (C-17, 170), 129.24 (C-16, 160), 134.70 (C-11, 110), 155.09
(C-2, 6), 158.86 (C-4, 14), 168.08 (C-8, 80). HRMS (m/z): calcd
730.1498 (M+). Found: m/z 730.1442 (M+).
3.12 Diethyl 4,40-(((pyridine-2,6-diylbis(methylene))bis(oxy))-
bis(3,1-phenylene))bis(2,7,7-trimethyl-5-oxo-5,6,7,8-
tetrahydroquinoline-3-carboxylate) (29)
Yellow solid. Mp 126–128 ꢀC. 1H NMR (500 MHz, CDCl3) dH: 0.96
(t, J ¼ 9 Hz, 6H, C-14, 140), 1.12 (s, 6H, C-15, 150), 2.19 (s, 6H,
C-16, 160), 2.47 (s, 4H, C-6, 60), 2.62 (s, 6H, C-11, 110), 3.10 (s, 4H,
C-8, 80), 4.01 (q, J ¼ 9.0 Hz, 4H, C-13, 130), 5.19 (s, 4H, C-23, 230),
6.77 (m, 4H, C-20, 200, 22, 220), 6.99 (m, 2H, C-20, 200), 7.31 (d,
3.17 Hexamethyl 4,40,400-(4,40,400-(2,4,6-trimethylbenzene-
1,3,5-triyl)tris(methylene)tris(oxy)tris(benzene-4,1-diyl))-
tris(2,6-dimethyl-1,4-dihydropyridine-3,5-dicarboxylate) (36)
J ¼ 9.0 Hz, 2H, C-18, 180), 7.46 (d, J ¼ 8.6 Hz, 2H, C-25, 250), 7.76 White solid. Mp 94–96 ꢀC. 1H NMR (400 MHz, CDCl3) dH: 2.46 (s,
(t, J ¼ 8.4 Hz, 4H, C-26). LCMS (m/z): calcd 808 (M + 1). Found: 9H, C-18, 180, 1800), 2.57 (s, 18H, C-7, 70), 3.62 (s, 18H, C-9, 90),
808 (M + 1).
5.12 (s, 6H, C-15, 150, 1500), 7.02 (d, J ¼ 8.2 Hz, 6H, C-13, 130), 7.22
44 | RSC Adv., 2014, 4, 39–46
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(d, J ¼ 8 Hz, 6H, C-12, 120). 13C NMR (100 MHz, CDCl3) dC: 16.17 15000), 6.98 (d, J ¼ 8 Hz, 8H, C-13, 130), 7.19 (d, J ¼ 8 Hz, 8H, C-12,
(C-18, 180, 1800), 23.07 (C-7, 70), 52.43 (C-9, 90), 65.13 (C-15, 150, 120), 7.76 (s, 2H, C-17, 170). HRMS (m/z): calcd 1602.1456 (M+).
1500), 114.59 (C-3, 5), 127.16 (C-13, 130), 128.98 (C-12, 120), 129.42 Found: m/z 1602.1463 (M+).
(C-17, 170, 1700), 131.77 (C-16, 160, 1600), 139.59 (C-11), 145.86 (C-
4), 155.49 (C-2, 6), 159.42 (C-14), 168.81 (C-8, 80). HRMS (m/z):
calcd 1101.1521 (M+). Found: m/z 1101.1537 (M+).
4. Conclusion
In conclusion, polyhydroquinolines and 1,4-DHPs with bi-, tri-
3.18 Hexaethyl 4,40,400-(((2,4,6-trimethylbenzene-1,3,5-
triyltris(methylene))tris(oxy))tris(benzene-4,1-diyl))tris(2,6-
dimethylpyridine-3,5-dicarboxylate) (37)
and tetrapodal moieties, and the corresponding pyridines, have
been synthesized using cheap reagents such as K2CO3 and 20%
HNO3, under ultrasonic irradiation, with a faster reaction time.
This protocol is an economically attractive method to synthesize
similar types of bi-, tri- and tetrapodal compounds with other
heterocycles.
Yellow solid. Mp 106–108 ꢀC. 1H NMR (500 MHz, CDCl3) dH: 1.03
(t, J ¼ 7.25 Hz, 18H, C-10, 100), 2.46 (s, 9H, C-18, 180, 1800), 2.61 (s,
18H, C-7, 70), 4.10 (q, 16H, C-9, 90), 5.13 (s, 6H, C-15, 150, 1500),
7.03 (d, J ¼ 8.5 Hz, 6H, C-13, 130), 7.25 (d, J ¼ 8.5 Hz, 6H, C-12,
13
120). C NMR (125 MHz, CDCl3) dC: 15.62 (C-10, 100), 15.99 (C-
Acknowledgements
18, 180, 1800), 21.69 (C-7, 70), 60.25 (C-9, 90), 69.29 (C-15, 150, 1500),
104.13 (C-3, 5), 114.39 (C-13, 130), 129.78 (C-12, 120), 134.65 (C-
17, 170, 1700), 137.28 (C-16, 160, 1600), 141.32 (C-11), 145.60 (C-2,
6), 159.77 (C-4, 14), 168.29 (C-8, 80). HRMS (m/z): calcd
1183.0061 (M+). Found: m/z 1183.0000 (M+).
The authors are thankful to VIT University administration in
Vellore, India for providing facilities to carry out research work,
and also thankful to SAIF, IIT-Madras and VIT-TBI for providing
NMR, Mass and IR spectral facilities, respectively. The author G.
L. Balaji is thankful to VIT University for providing a Research
Associateship.
3.19 Hexamethyl 4,40,400-(((benzene-1,3,5-triyltris(methylene))
tris(oxy))tris(benzene-4,1-diyl))tris(2,6-dimethylpyridine-3,5-
dicarboxylate) (38)
References
Yellow solid. Mp 100–102 ꢀC. 1H-NMR (500 MHz, CDCl3) dH:
2.60 (s, 18H, C-7, 70), 3.59 (s, 18H, C-9, 90), 5.19 (s, 6H, C-15, 150,
1500), 7.02 (d, J ¼ 8.1 Hz, 6H, C-13, 130), 7.22 (d, J ¼ 8.1 Hz, 6H, C-
12, 120), 7.54 (s, 3H, C-17, 170, 1700). 13C NMR (125 MHz, CDCl3)
dC: 22.93 (C-7, 70), 52.24 (C-9, 90), 69.65 (C-15, 150, 1500), 114.59
(C-3, 5), 126.37 (C-13, 130), 126.97 (C-17, 170, 1700), 129.10 (C-12,
120), 129.29 (C-11), 137.63 (C-16, 160, 1600), 145.69 (C-2, 6), 155.43
(C-4), 158.79 (C-14), 168.60 (C-8, 80). HRMS (m/z): calcd
1059.2498 (M+). Found: m/z 1059.2540 (M+).
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3.20 Octaethyl 4,40,400,4000-(4,40,400,4000-(benzene-1,2,4,5-
tetrayltetrakis(methylene))tetrakis(oxy)tetrakis(benzene-4,1-
diyl))tetrakis(2,6-dimethyl-1,4-dihydropyridine-3,5-
dicarboxylate) (39)
White solid. Mp 252–254 ꢀC. 1H NMR (500 MHz, CDCl3) dH: 0.98
(t, J ¼ 6 Hz, 24H, C-10, 100), 2.61 (s, 24H, C-7, 70), 4.07 (q, 16H, C-
9, 90), 5.07 (s, 8H, C-15, 150, 1500, 15000), 6.79 (d, J ¼ 6 Hz, 8H, C-13,
130), 7.24 (d, J ¼ 7.2 Hz, 8H, C-12, 120), 7.75 (s, 2H, C-17, 170). 13C
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61.32 (C-9, 90), 67.52 (C-15, 150, 1500, 15000), 104.14 (C-3, 5), 114.35
(C-13, 130), 127.24 (C-17, 170), 129.67 (C-12, 120), 136.92 (C-11),
140.68 (C-16, 160, 1600, 16000), 143.81 (C-2, 6), 155.21 (C-4, 14),
167.92 (C-8, 80). HRMS (m/z): calcd 1490.2116 (M+). Found: m/z
1490.2146 (M+).
3.21 Octamethyl 4,40,400,4000-(4,40,400,4000-(benzene-1,2,4,5-
tetrayltetrakis(methylene))tetrakis(oxy)tetrakis(benzene-4,1-
diyl))tetrakis(2,6-dimethyl-1,4-dihydropyridine-3,5-
dicarboxylate) (40)
White solid. Mp 234–236 ꢀC. 1H NMR (500 MHz, CDCl3) dH: 2.56
(s, 24H, C-7, 70), 3.55 (s, 24H, C-9, 90), 5.20 (s, 8H, C-15, 150, 1500,
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