Facile, microwave-assisted parallel syntheses of N-substituted 1,4-dihydropyridines of biological interest

Article abstract of DOI:10.1002/jhet.67

A solventless parallel synthesis of some novel, N-substituted dihydropyridines by the application of microwaves is reported. The synthetic methodology adopted to synthesize the titled compounds and their antioxidant activity is described.

Full text of DOI:10.1002/jhet.67

336
Facile, Microwave-Assisted Parallel Syntheses of N-Substituted
1,4-Dihydropyridines of Biological Interest
Vol 46
B. R. Prashantha Kumar,a* Pankaj Masih,a Chris Raju Lukose,a
Nimmy Abraham,a D. Priya,a Rinu Mary Xavier,a Kesiya Saji,a and Laxmi Adhikaryb
aDepartment of Pharmaceutical Chemistry, JSS College of Pharmacy, Rockland’s, Ooty 643001,
Tamilnadu, India
bBioanalytical Division, Biocon India Pvt. Ltd., Bangalore, India
*E-mail: brprashant@yahoo.com
Received April 28, 2008
DOI 10.1002/jhet.67
Published online 13 April 2009 in Wiley InterScience (www.interscience.wiley.com).
A solventless parallel synthesis of some novel, N-substituted dihydropyridines by the application of
microwaves is reported. The synthetic methodology adopted to synthesize the titled compounds and
their antioxidant activity is described.
J. Heterocyclic Chem., 46, 336 (2009).
INTRODUCTION
Industrial chemistry in this millennium is widely
adopting the concept of green chemistry to meet the
fundamental scientific challenges of protecting the
human health and environment while maintaining com-
mercial viability. The emerging area of green chemistry
envisages minimum hazard as the performance criteria
while designing new chemical process. One of the thirst
areas for achieving this target is to explore alternative
reaction conditions and reaction media to accomplish
the desired chemical transformations with minimized
by-products or waste as well as eliminating the use of
conventional organic solvents, if possible. Microwaves
(MWs) as an alternative energy source has become an
attractive tool in organic synthesis [13,14], resulting in a
significant decrease in reaction times, especially for
endothermic reactions.
Heterocyclic synthesis plays an important role in
medicinal chemistry by virtue of its application in the
production of large number of medicinal compounds.
Among many utility of heterocyclic syntheses one such
important use is in the production of drugs, either cur-
rently on the market or those yet to be released.
Dihydropyridine (DHP) chemistry began in 1882,
when Hantzsch for the first time published the syntheses
of DHPs by multicomponent reaction, which now bears
his name [1–3]. 1,4-DHPs such as nifedipine and nitren-
dipine are used even today as antihypertensive agents
[4–6].
1,4-DHPs are known to possess various biological
activities such as multidrug reversal (MDR) in tumor
cells [7], potential immunomodulating [8], as well as
antitubercular activities [9]. The 3D QSAR results show
the minimum structure requirement for DHPs to exhibit
antitubercular activity [10]. A further study of N-substi-
tution at the labile hydrogen of DHPs skeleton proved
to be important for MDR reversal in tumor cells apart
from analytical and synthetic interest [11]. These obser-
vations provoked us to work on N-substituted DHPs for
their possible antioxidant activity by incorporating phe-
nolic and carboxyl groups [12].
RESULTS AND DISCUSSION
Because of biological interest, we have synthesized
N-substituted 1,4-DHPs by parallel syntheses using p-
toluenesulphonic acid (PTSA) as a catalyst under MW
irradiation conditions as per Scheme 1. The PTSA was
found to be a better catalyst than AlCl3 and HCl in
C
V
2009 HeteroCorporation

March 2009
Facile, Microwave-Assisted Parallel Syntheses of N-Substituted 1,4-Dihydropyridines
of Biological Interest
337
Scheme 1. Microwave-assisted parallel syntheses of N-substituted-1,4-dihydropyridines; (a) Silica gel, MW, 700 W, 8 min; (b) PTSA, MW, 700
W, 12–15 min.
terms of yields. Also, it was convenient to perform the
parallel synthesis under solvent free conditions using
PTSA as a cyclizing agent and silica gel as adsorbent.
Initially, we made an attempt to synthesize N-aryl-
1,4-DHPs through conventional and MW methods. In
both the methods there was a formation of Schiff’s base,
but by the conventional method, we could not prepare
N-substituted 1,4-DHPs, successfully. We repeated the
experiment several times under conventional heating. In
each case, the thin layer chromatography (TLC) was
unclear and showed multiple spots. PTSA was devel-
oped as an efficient catalyst for the formation of final
products under MWs when compared with the other
reported catalysts such as HCl, AlCl3, etc. [2].
amine, aryl aldehyde with two equivalent amount of
ethyl acetoacetate under similar MW irradiation condi-
tions. However, only 10–20% yields of the final com-
pounds were obtained. In addition, the products were
found to be impure with many by-products. One of the
major reason why the single step syntheses was not suc-
cessful was found to be the formation of acetoacetani-
lide because of a reaction between arylamine and ethyl
acetoacetate. This was conformed by subjecting the
crude reaction mixture to high-resolution mass spec-
trometry (HRMS). The syntheses of the titled com-
pounds through the Schiff’s base preparation escaped
from the formation of acetoacetanilide. Another reason
for the success of the reaction was due to the possibility
of the correct steric alignment of the Schiff’s base with
a b-keto ester to react during its reaction mechanism as
shown in Scheme 2.
We had also made an attempt to prepare the titled
compounds by adopting one pot, one step, multicompo-
nent reaction involving equivalent amounts of aryl
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

338
B. R. P. Kumar, P. Masih, C. R. Lukose, N. Abraham, D. Priya, R. M. Xavier, K. Saji, and
L. Adhikary
Vol 46
Scheme 2. The possible reaction mechanism for the formation of N-substituted1,4-dihydropyridines.
ionization technique. Elemental analysis was carried out for
carbon, hydrogen, and nitrogen using an Elementar Vario (EL
III Carlo Erba11080). MW irradiation was done using, Whirl
Pool domestic MW oven as well as MW Synthesizer (2450
MHz, Catalyst Systems, India). All the reactions were moni-
tored using thin layer chromatogram. TLC was performed
using 4% methanol in chloroform as mobile phase on alumi-
num plates that are precoated with silica gel GF.
General procedure for the preparation of Schiff’s
bases. Equimolar amounts (0.01M) of aromatic amine and aro-
matic aldehyde were transferred to a clean and dry mortar, trit-
urate to become a uniform mixture. The reaction mixture was
then transferred to a 100 mL beaker containing 2 g of acti-
vated silica gel. Similarly, all other beakers containing differ-
ent reaction mixtures were kept inside the MW oven in a
circle and then MW irradiation was carried out at 700 W
power for about 8 min. Intermittent cooling was done after
every 60 s of MW irradiation. During intermittent cooling, the
reaction mixtures were thoroughly mixed. The reactions were
monitored through TLC. The completed reactions were taken
directly for the preparation N-substituted DHPs without any
work up, immediately.
General procedure for the preparation of N-substituted-
1,4-DHPs (1-6). This reaction was carried out in a parallel
synthetic way as shown in Scheme 1. Ethyl acetoacetate
(0.02M) was transferred in to a dry mortar containing the
Schiff’s base (ꢀ0.01M) obtained in the previous MW proce-
dure. Add a catalytic amount or pinch of PTSA and triturate to
become a uniform mixture. The reaction mixture was then
transferred to 100 mL beaker. Similarly, all other beakers con-
taining different reaction mixtures were kept inside the MW
oven in a circle and then MW irradiation was carried out at
700 W power for about 12–15 min. Intermittent cooling was
done after every 60 s of MW irradiation. During intermittent
cooling, the reaction mixtures were thoroughly mixed. The
reactions were monitored through TLC. The reaction mixtures
Final compounds were first analyzed by TLC and
melting point determination. Then, the synthesized com-
pounds were subjected to spectral analysis such as IR,
NMR, and MASS spectrometry to confirm the struc-
tures. All the analytical details showed satisfactory
results. For all compounds, a characteristic singlet peak
for symmetric CH was observed between 5.0 and 5.5
1
ppm in the H NMR spectrum.
The synthesized compounds were tested for in vitro
antioxidant activity using standard nitric oxide assay
method [15]. Among all the synthesized compounds,
compounds 4 and 6 exhibited potential antioxidant activ-
ity with IC50 value at 11 and 28 lg concentrations,
respectively. The in vitro antioxidant results reveal that
there is need for further investigation. Hence, the work
in this direction is in progress and it will be reported in
our next publication.
In conclusion, a simple protocol for the syntheses of
N-aryl-1,4-DHPs was developed under environmentally
benign, MW irradiation conditions using PTSA as a cat-
alyst. It was found that the MW method is selective and
efficient when compared with the conventional method.
EXPERIMENTAL
The IR was recorded on Perkin Elmer Infrared-283 FTIR by
KBr pellet technique. NMR was taken on Bruker ACF-300
MHz spectrometer apparatus using TMS as internal standard
and CDCl3 or DMSO-d6 as a solvent. The chemical shifts are
expressed in d ppm and following abbreviations are used: s,
singlet; d, doublet; t, triplet; q, quartet; m, multiplet. HRMS
analysis was done on Perkin-Elmer Sciex API-150-EX by ESI
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

March 2009
Facile, Microwave-Assisted Parallel Syntheses of N-Substituted 1,4-Dihydropyridines
of Biological Interest
339
were withdrawn from MW oven soon after the reaction is
completed based on TLC data at regular intervals. The reaction
mixtures were poured in to the beaker containing water and
extracted with ethyl acetate. The ethyl acetate layer was dried
over sodium sulfate and evaporated under vacuum to get crude
products. The obtained crude products were further purified by
recrystallization using ethyl acetate-hexane mixture (1:1) or by
column chromatography using 5% EtOAc in pet ether as a mo-
bile phase over silica gel.
C25 H27NO5: C, 71.24; H, 6.46; N, 3.32. Found: C, 70.87; H,
6.33; N, 3.41.
Diethyl-4-(furan-2-yl)-1,4-dihydro-1-(4-hydroxyphenyl)-2,6-
dimethylpyridine-3,5-dicarboxylate (6). Light brown colored
solid, Yield 72%, mp 67–69ꢁC; IR (KBr, cmꢂ1): 3410 (OAH),
1
3054 (Ar CAH), 2967 (Ali CAH), 1715 (C¼¼O). H NMR (d
ppm, CDCl3): 1.20 (t, 6H, 2CH3), 1.91 (s, 6H, 2CH3), 4.1 (q,
4H, 2CH2), 5.25 (s, 1H, CH), 5.9 (d, 1H, CH Fur), 6.1 (t, 1H,
CH Fur), 6.8 (d, 2H, ArH), 6.9 (d, 2H, ArH), 7.2 (d, 1H, CH
Fur), 9.83 (s, 1H, OH). MS (m/z): calcd. 411, found 411.32
(Mþ). MS: 397, 346, 251, 220, 121. Anal. Calcd. for
C23H25NO6: C, 67.14; H, 6.12; N, 3.40. Found: C, 67.49; H,
5.85; N, 3.32.
4-(3,5-Di(ethoxycarbonyl)-2,6-dimethyl-4-phenylpyridin-1
(4H)-yl)benzoic acid (1). Pale yellow colored solid, Yield 61%,
mp 135–137ꢁC; IR (KBr, cmꢂ1): 3471 (OAH), 3081 (Ar CAH),
1
2919 (Ali CAH), 1721 (C¼¼O), 1713 (C¼¼O), 1626 (C¼¼C). H
Antioxidant activity screening by nitric oxide radical in-
hibition assay. The reaction mixture (6 mL) containing so-
dium nitroprusside (10 mM, 4 mL), phosphate buffer saline (1
mL), and compound (1–64 lg/mL) was incubated at 25ꢁC for
150 min. After incubation, 0.5 mL of the reaction mixture was
removed, 1 mL of sulfanilic acid reagent (0.33% in 20% gla-
cial acetic acid) was mixed and allowed to stand for 5 min for
completion of diazotization reaction, and 1 mL of naphthyl
ethylene diamine dihydrochloride was added, mixed, and
allowed to stand for 30 min in diffused light. The absorbance
was measured at 540 nm against the blank in a 96-well micro-
titre plate using an ELISA reader.
NMR (d ppm, CDCl3): 1.19 (t, 6H, 2CH3), 2.10 (s, 6H, 2CH3),
4.04 (q, 4H, 2CH2), 5.31 (s, 1H, CH), 6.94–7.59 (m, 9H, ArH),
12.98 (s, 1H, OH). MS (m/z): calcd. 449, found 449.21 (Mþ).
MS: 434, 398, 311, 243, 121. Anal. Calcd. for C26H27NO6: C,
69.47; H, 6.05; N, 3.12. Found: C, 69.63; H, 5.94; N, 3.08.
4-(3,5-Di(ethoxycarbonyl)-4-(furan-2-yl)-2,6-dimethylpyridin-
1(4H)-yl)benzoic acid (2). Pink-colored solid, Yield 77 %, mp
93–95ꢁC; IR (KBr, cmꢂ1): 3428 (OAH), 3037 (Ar CAH),
2989 (Ali CAH), 1724 (C¼¼O), 1716 (C¼¼O). 1H NMR (d
ppm, CDCl3): 1.22 (t, 6H, 2CH3), 1.85 (s, 6H, 2CH3), 4.24 (q,
4H, 2CH2), 5.34 (s, 1H, CH), 6.01 (d, 1H, CH Fur), 6.23 (t,
1H, CH Fur), 6.88 (d, 2H, ArH), 7.02 (d, 2H, ArH), 7.21 (d,
1H, CH Fur), 12.70 (s, 1H, OH). MS (m/z): calcd.439, found
439.13 (Mþ). MS: 401, 329, 277, 211, 121. Anal. Calcd. for
C24H25NO7: C, 65.59; H, 5.73; N, 3.19. Found: C, 65.44; H,
5.87; N, 3.57.
REFERENCES AND NOTES
4-(3,5-Di(ethoxycarbonyl)-4-(4-methoxyphenyl)-2,6-dimethyl-
pyridin-1(4H)-yl)benzoic acid (3). Pale yellow solid, Yield 75
%, mp 99–101ꢁC; IR (KBr, cmꢂ1): 3446 (OAH), 3053 (Ar
CAH), 2939 (Ali CAH), 1717 (C¼¼O), 1709 (C¼¼O), 1623
[1] Hantzsch, A.; Liebigs, J. Ann Chem 1882, 1, 215.
[2] Eisner, U.; Kuthan, J. Chem Rev 1972, 72, 1.
[3] Stout, D. M.; Meyers, A. I. Chem Rev 1982, 82, 223.
[4] Loev, B.; Goodman, M. M.; Snader, K. M.; Tedeschi, R.;
Macko, E. J Med Chem 1974, 19, 956.
1
(C¼¼C). H NMR (d ppm, CDCl3): 1.21 (t, 6H, 2CH3), 2.19 (s,
6H, 2CH3), 3.71 (s, 3H, OCH3), 4.07 (q, 4H, 2CH2), 5.07 (s,
1H, CH), 6.91–7.53 (m, 8H, ArH), 12.48 (s, 1H, OH). MS (m/
z): calcd. 479, found 479.11 (Mþ). MS 464, 387, 301, 142.
Anal. Calcd. for C27 H29NO7: C, 67.63; H, 6.10; N, 2.92.
Found: C, 67.78; H, 5.89; N, 2.77.
[5] Mager, P. P.; Cobum, R. A.; Solo, A. J.; Triggle, D. J.;
Rothe, H. Drug Des. Discov 1992, 8, 273.
[6] Meyer, V. H.; Bossert, F.; Wehinger, K.; Stoepel, K.; Vater,
W. Arzneim-Forsch (Drug Res) 1981, 31, 407.
[7] Kawase, M.; Shah, A.; Gaveriya, H.; Motohashi, N.; Saka-
gami, H.; Varga, A.; Molnar, J. Bioorg Med Chem 2002, 10, 1051.
[8] Shah, A.; Gevariayar, H.; Motohashi, N.; Kawase, M.; Farkas,
S.; Gyorgyi, G.; Molnar, J. Int J Antimicrob Agents 2002, 20, 227.
[9] Desai, B.; Sureja, D.; Naliapara, Y.; Shah, A.; Saxena, K.
Bioorg Med Chem 2001, 9, 1993.
Diethyl-1,4-dihydro-1-(4-hydroxyphenyl)-4-(4-methoxyphenyl)-
2,6-dimethylpyridine-3,5-dicarboxylate (4). Yellowish brown
solid, Yield 81 %, mp 88–90ꢁC; IR (KBr, cmꢂ1): 3436
(OAH), 3048 (Ar CAH), 2927 (Ali CAH), 1719 (C¼¼O), 1638
1
(C¼¼C). H NMR (d ppm, CDCl3): 1.26 (t, 6H, 2CH3), 2.22 (s,
[10] Prashantha Kumar, B. R.; Yuvaraj, S.; Srivastava, A.; Cha-
turvedi, V.; Manju, Y. K.; Suresh, B.; Nanjan, M. J. Lett Drug Des
Discov 2008, 5, 7.
6H, 2CH3), 3.81 (s, 3H, OCH3), 4.02 (q, 4H, 2CH2), 5.19 (s,
1H, CH), 6.90–7.40 (m, 8H, ArH), 9.41 (s, 1H, OH). MS (m/
z): calcd. 451, found 451.48 (Mþ). MS: 436, 398, 312, 151.
Anal. Calcd. for C26 H29NO6: C, 69.16; H, 6.47; N, 3.10.
Found: C, 69.35; H, 6.39; N, 2.89.
[11] Mahendra, M.; Doreswamy, B. H.; Parecha, A. R.; Patel, J.
A.; Shah, A.; Sridhar, M. A.; Prasad, J. S. Anal Sci 2004, 20, 19.
[12] (a) Ramon, R.; Cleofina, B. Toxicol Pharmacol 2006, 142,
317; (b) Mojca, S. K.; Petra, K.; Majda, H.; Andreja, R. H.; Marjana,
S.; Zeljko, K. Food Chem 2005, 89, 191.
Diethyl-1,4-dihydro-1-(4-hydroxyphenyl)-2,6-dimethyl-4-
phenylpyridine-3,5-dicarboxylate(5). Yellow crystals, Yield 78
%, mp 89–91ꢁC; IR (KBr, cmꢂ1): 3448 (OAH), 3078 (Ar
CAH), 2912 (Ali CAH), 1719 (C¼¼O), 1712 (C¼¼O), 1611
[13] Varma, R. S. Green Chemistry: Challenging Perspectives;
Tundo, P.; Anastas, P. T., Eds. Oxford University Press: Oxford, 2000;
p 221.
1
(C¼¼C). H NMR (d ppm, CDCl3): 1.23 (t, 6H, 2CH3), 2.21 (s,
6H, 2CH3), 4.10 (q, 4H, 2CH2), 5.23 (s, 1H, CH), 6.84–7.47
(m, 9H, ArH), 9.90 (s, 1H, OH). MS (m/z): calcd. 421, found
421.17 (Mþ). MS: 406, 353, 331, 262, 121. Anal. Calcd. for
[14] Caddick, S. Tetrahedron 1995, 51, 10403.
[15] Marcocci, P. L.; Sckaki, A.; Albert G. M. Methods Enzy-
mol 1994, 234, 462.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet