Microwave-assisted Doebner synthesis of 2-phenylquinoline-4-carboxylic acids and their antiparasitic activities

Article abstract of DOI:10.1002/jhet.5570450251

(Chemical Equation Presented) A series of twelve substituted 2-phenylquinoline-4-carboxylic acids analogous to antimalarial and antileishmanial natural products was developed via the Doebner reaction employing microwave irradiation (MW). The products were obtained in moderate yields in 0.5-3 minutes and nine of them were evaluated in vitro against the parasites responsible for malaria, leishmaniasis and trypanosomiasis diseases (WHO, Switzerland). Four compounds exhibited activity against Trypanosoma cruzi and another two resulted active against Plasmodium falciparum and Leishmania infantum, respectively.

Full text of DOI:10.1002/jhet.5570450251

Mar-Apr 2008
611
Microwave-assisted Döbner Synthesis of 2-Phenylquinoline-4-
carboxylic Acids and their Antiparasitic Activities.
Gisela C. Muscia, Juan P. Carnevale, Mariela Bollini and Silvia E. Asís*
Departamento de Química Orgánica, Facultad de Farmacia y Bioquímica, Universidad de Buenos Aires.
Junín 956 (1113) Buenos Aires, Argentina
Received July 16, 2007
COOH
R1
N
R2
A series of twelve substituted 2-phenylquinoline-4-carboxylic acids analogous to antimalarial and
antileishmanial natural products was developed via the Döbner reaction employing microwave irradiation
(MW). The products were obtained in moderate yields in 0.5-3 minutes and nine of them were evaluated in
vitro against the parasites responsible for malaria, leishmaniasis and trypanosomiasis diseases (WHO,
Switzerland). Four compounds exhibited activity against Trypanosoma cruzi and another two resulted
active against Plasmodium falciparum and Leishmania infantum, respectively.
J. Heterocyclic Chem., 45, 611 (2008).
INTRODUCTION
synthesis of a large family of substituted 2-phenyl-4-
quinolinemethanols from cincophens as possible
antimalarials, was reported. Seven of the appropriate
cincophens were made through the corresponding isatins
by the Pfitzinger method and the other twelve were made
by the Döbner-Miller synthesis. Of the two methods, the
Pfitzinger gave by far the best yields but it was preferred
when the materials were readily available [7].
Ten years ago, a series of eight 2-arylquinoline-4-
carboxamides was achieved in good yields employing
solid phase synthesis as well as a multi-component
condensation approach based on Döbner reaction [6].
More recently, the preparation of quinaldines and
lepidines by Döbner-Miller reaction under thermal and
microwave irradiation (MW) conditions using phospho-
tungstic acid was reported [8].
Quinoline derivatives represent a major class of
heterocycles, and a number of preparations have been
known since the late 1800s. The quinoline ring system
occurs in various natural products, especially in alkaloids.
In 1820, quinine was isolated as the active ingredient
from the bark of Cinchona trees and replaced the crude
bark for the treatment of malaria and its synthetic
derivative chloroquine (CQ) was the prime therapy for
nearly half a century [1]. Plasmodium falciparum, the
cause of most deadly variety of malaria, is now CQ-
resistant in nearly all regions of the globe. However, in
the search of new candidates, this drug remains the one to
emulate [2].
Chimanine alkaloids, simple 2-substituted quinolines
isolated from the bark of Galipea longiflora trees of the
Rutaceae family, are effective against the parasites
Leishmania sp., which are the agents of leishmaniasis, a
protozoan disease of the tropical areas in South America.
Two of these compounds had the same efficacy as the
reference antimonial drug currently administered by the
parenteral route. On the basis of the results obtained in
studies with 2-substituted quinolines, the World Health
Organization (WHO) has established as a priority their
ability to provide oral treatment for cutaneous
leishmaniasis [3,4].
The microwave-assisted organic synthesis (MAOS)
without the use of catalyst or solvent is particularly eco-
friendly and has the advantages of short reaction time and
high yields [9]. Moreover, the utility of multi-component
reactions (MCRs) in preparing libraries to screen is well
appreciated [10].
RESULTS AND DISCUSSION
Herein, the synthesis of twelve 2-arylquinoline-4-
carboxylic acids (4a-l) prepared via Döbner reaction [11]
under MW is described (Scheme 1). The one-pot reactions
of pyruvic acid (1) and substituted anilines (2) and
benzaldehydes (3) were carried out in a MW domestic
oven adapted for the use of a reflux condenser, at constant
power (400 W). All the reactions proceed to completion
between 0.5 and 3 min (Table 1), instead of the 2 h with
conventional heating and a longer time did not increase
the yields.
On the other hand, despite the progress made in
Trypanosoma cruzi biochemistry and physiology, the
chemotherapy of Chagas´ disease (trypanosomiasis) is
deficient and no medication is adequate in the chronic
phase [5].
2-Phenylquinoline-4-carboxylic acid also known as
cincophen acid (or atophan) and its derivatives have
shown a variety of biological effects [6]. In 1946, the

612
G. C. Muscia, J. P. Carnevale, M. Bollini and S. E. Asís
Vol 45
Scheme 1
COOH
O
O
OH
MW
R1
R2
R1
+
+
400 W
NH2
OHC
N
R2
1
2
3
4a-l
Table 1
COOH
R1
N
R2
Entry
R1
R2
Product
4a
4b
4c
4d
4e
4f
4g
4h
Time (s)
240
60
30
120
30
30
30
30
60
Yield (%)
mp (Lit.) (ºC)
1
2
3
4
5
6
7
8
9
H
H
H
H
H
H
8-Cl
7-Cl
6-Cl
H
12
55
11
24
10
10
18
63
37
208-210 (209-210)12a
195-196
4-OCH3
4-OH
4-Cl
2-OH
2-NO2
H
191-192
142-144
37-40
176-178 (253-255)12b
85-87
H
H
191-192 (244-246)12c
147-149 (243)12d
4i
10
11
12
6-F
7-CF3
7-CH3
H
H
H
4j
4k
4l
18
60
90
23
13
12
134-136
127-129
217-218
Compounds 4a, 4f, 4h and 4i have been known for many
years and the reaction times for their preparation were long
and the yields were usually low [12]. However, they were
made without application to the malaria problem.
antimalarial and antileishmanial activities but 4k resulted
moderately active against L. infantum (Table 3). In
addition, compounds 4d, 4h and 4l were evaluated against
T. cruzi and they exhibited some activity (Table 4) [15].
However, all their IC50 values are quite far from the
reference drugs and the named compounds were not
selected for further screening.
As an approach for establishing structure-activity
relationships, electron-donating groups at position 4 of the
2-phenyl moiety improved antimalarial activity and the
OCH3 was the best. The activity against leishmaniasis was
better when a CF3 group was attached at position 7 of the
quinoline ring, meanwhile for Chagas´ disease the same
position could be substituted either with electron-
attracting or donating groups. These results suggest that
compound 4k could also be active against T. cruzi.
In a previous paper [13], we have reported the synthesis
and antiparasitic activity of a closely related quinoline
series. Four of these compounds exhibited some activities:
two of them against leishmaniasis, one against malaria
and another one against both malaria and Chagas´disease.
These results prompted us to synthesize this new
quinoline series, possessing also a chloro atom and a
phenyl ring as substituents in the quinoline skeleton. The
replacement of chloro for fluor, methyl or trifluoromethyl
moieties could improve the activity, taking in account
recognized antimalarial agents, such as mefloquine and
amodiaquine. The substitution in position
resemble Chimanine alkaloids.
2 could
In conclusion,
microwave-assisted
solvent-free
Nine of the synthesized products were selected for a
primary in vitro screening at Tropical Disease Research
(TDR) Program, World Health Organization (WHO,
Switzerland). Compounds 4e, 4f (with a nitro group) and
4i were refused. Compound 4a resulted moderately active
against T. cruzi meanwhile 4b exhibited moderate activity
against P. falciparum (Table 2) [14]. The remaining
compounds were tested at University of Antwerp (Tables
3 and 4). Compounds 4c, 4d, 4g, 4h, 4j and 4l lacked of
reactions were employed to develop a series of 2-phenyl-
quinoline-4-carboxylic acids. Although the yields were
not high, the starting materials are available and
inexpensive and the method offers quite short reaction
times. The promising biological evaluation encouraged us
to enlarge this series in order to modulate the antichagasic
activity and diminish their toxicity. Furthermore, the
synthesis of new derivatives of compound 4k could be
interesting against leishmaniasis.

Mar-Apr 2008
Microwave-assisted Döbner Synthesis of 2-Phenylquinoline-4-carboxylic Acids
613
Table 2
Antiprotozoal activity of compounds 4a and 4b (IC50 values are given in µg/mL)a
Compound
P. falciparum
chloroquine 0.04
L. donovani
miltefosine 0.2
T. cruzi
benznidazole 0.25
T. b. rhodesiense
melarsoprol 0.005
cytotoxicity L6 cell
line
podophyllotoxin 0.007
4a
4b
> 5
3.1
> 30
22.4
13.8
> 30
> 90
35.25
35.6
75.6
a IC50 = 50 % growth inhibition.
Table 3
Antiprotozoal activity of compounds 4c, 4g, 4j and 4k
(IC50 values are given in µg/mL)a
Compound
P. falciparum
L. infantum
T. cruzi
T. b. rhodesiense cytotoxicity L6 cell
chloroquine 0.026
pentostam 2.4
Benznidazole
suramin 0.13
line
tamoxifen 4.9
> 17
> 18.2
> 18.8
> 20.3
4c
4g
4j
> 17
> 17
nt
nt
nt
nt
> 17
> 18.1
> 18.1
> 20.3
> 18.1
> 18.1
8.6
> 18.1
> 18.1
2.2
4k
a IC50 = 50 % growth inhibition; nt: not tested.
Table 4
Antiprotozoal activity of compounds 4d, 4h and 4l (IC50 values are given in µg/mL)a
Compound
P. falciparum
L. infantum
T. cruzi
T. b. rhodesiense cytotoxicity
MRC-5
artemether 0.038
pentostam 2.4
nifurtimox 0.57
suramin 0.061
cell line
niclosamide 0.87
> 18.2
> 19.9
4d
4h
4l
> 18.16
> 19.95
> 16.85
>18.2
> 19.9
> 16.9
> 18.2
> 19.9
> 16.9
13.3
> 19.9
> 16.9
> 16.9
a IC50 = 50 % growth inhibition; nt: not tested.
EXPERIMENTAL
6.07-8.15 (m, 10 H, arom.) ppm. 13C NMR (DMSO-d6): δ
109.74, 116.63, 120.18, 121.43, 124.36, 126.70, 127.66, 128.61,
128.67, 128.92, 131.72, 137.09, 137.91, 141.94, 166.46 ppm.
Anal. calcd. for C16H11NO2: C, 77.10; H, 4.45; N, 5.62. Found:
C, 76.90; H, 4.56; N, 5.50.
Melting points were determined in a capillary Electrothermal
9100 SERIES-Digital apparatus and are given uncorrected. 1H and
13C NMR spectra were recorded at r.t. using a Bruker 200 MHz
spectrometer with TMS as the internal standard. The chemical
shifts (δ) are given in ppm. Infrared spectra were recorded on a FT
Perkin Elmer Spectrum One from KBr discs. Analytical TLC were
performed on DC-Alufolien Kiesegel 60 F254 Merck. Microwave-
assisted reactions were carried out in a household MW oven
BGH-QUICK Chef 15240. This apparatus was modified for
laboratory applications adapting an external Liebig condenser.
General procedure for compounds 4a-4l. A mixture of
aniline (4.1 mmoles), benzaldehyde (3.9 mmoles) and pyruvic
acid (4.3 mmoles) placed in a 50 mL round-bottomed flask was
subjected to MW irradiation (400 W). After completion of the
reaction (TLC), the reaction mixture was allowed to cool and the
residual semisolid crystallized. When this did not occur, the
reaction mixture was diluted with CH2Cl2 (15 mL) and washed
with water, 5 % HCl (10 mL) and brine (10 mL). This was then
dried (Na2SO4) and concentrated in vacuo to give a solid product
which was triturated with EtOH.
2-(4-Methoxyphenyl)-quinoline-4-carboxylic acid (4b).
This compound was obtained as white powder, mp 195-196 ºC;
IR: 3313 (OH), 3061 (CH), 1678 (CO), 1649 (CN), 1034, 1251
1
(COC), 778, 748 (CH arom.) cm-1; H NMR (DCCl3): δ 3.75 (s,
3 H, OCH3), 5.64 (d, 1 H, arom.), 6.07 (d, 1 H, arom.), 6.68-7.54
(m, 7 H, arom.) ppm. Anal. calcd. for C17H13NO3: C, 73.11; H,
4.69; N, 5.02. Found: C, 73.28; H, 4.38; N, 5.40.
2-(4-Hydroxyphenyl)-quinoline-4-carboxylic acid (4c).
This compound was obtained as dark yellow powder, mp 191-
192 ºC; IR: 3435 (OH), 2916 (CH), 1603 (CO), 1286 (CO), 841
1
(CH arom.) cm-1; H NMR (DMSO-d6): δ 6.46-8.61 (m, 9 H,
arom.), 9.77 (s, 1 H, OH) ppm. Anal. calcd. for C16H11NO3: C,
72.45; H, 4.18; N, 5.28. Found: C, 72.59; H, 3.90; N, 5.10.
2-(4-Chlorophenyl)-quinoline-4-carboxylic acid (4d). This
compound was obtained as pale yellow powder, mp 142-144 ºC;
IR: 3346 (OH), 3060 (CH), 1742 (CO), 1656 (CN), 1121 (CO),
1
754, 696 (CH arom.) cm-1; H NMR (DCCl3): δ 6.00 (d, 2 H,
2-Phenylquinoline-4-carboxylic acid (4a). This compound
was obtained as white needles, mp 208-210 ºC; IR: 3311 (OH),
Ph), 6.05 (d, 2 H, Ph), 6.99-7.51 (m, 5 H, arom.) ppm. Anal.
calcd. for C16H10ClNO2: C, 67.74; H, 3.55; N, 4.94. Found: C,
68.03; H, 3.37; N, 5.00.
1
1678 (CO), 746, 694 (CH arom.) cm-1; H NMR (DMSO-d6): δ

614
G. C. Muscia, J. P. Carnevale, M. Bollini and S. E. Asís
Vol 45
2-(2-Hydroxyphenyl)-quinoline-4-carboxylic acid (4e).
REFERENCES AND NOTES
Corresponding author. Tel.: +54-11-49648251; fax: +54-11-
4508-3645; E-mail: elizabet@ffyb.uba.ar
[1] Kouznetsov, V. V.; Vargas Méndez, L. V.; Meléndez Gómez,
C. M. Current Org. Chem. 2005, 9, 141.
[2] Ncokazi, K. K.; Egan, T. J. Anal. Biochem. 2005, 338, 306.
[3] a) Richomme, P.; Bruneton, J.; Hocquemiller, R.; Roblot, F.;
Cavé, A.; Fournet, A. J. Nat. Prod. 1993, 56, 1547. b) Franck, X.;
Fournet, A.; Prina, E.; Mahieux, R.; Hocquemiller, R.; Figadère, B.
Bioorg. Med. Chem. Lett. 2004, 14, 3635.
[4] Fournet, A.; Ferreira, M. E.; Rojas de Arias, A.; Torres de
Ortiz, S.; Fuentes, S.; Nakayama, H.; Schinini, A.; Hocquemiller, R.
Antimicrob. Agents Chemother. 1996, 2447.
This compound was obtained as orange powder, mp 37-40 ºC;
IR: 3336 (OH), 2911 (CH), 1616 (CO), 1273 (CO), 753 (CH)
cm-1; 1H NMR (DMSO-d6): δ 6.95-7.67 (m, 9 H, arom.), 8.96 (s,
1 H, OH), 13.10 (s, 1 H, COOH) ppm. Anal. calcd. for
C16H11NO3: C, 72.45; H, 4.18; N, 5.28. Found: C, 72.15; H,
3.92; N, 4.93.
*
2-(2-Nitrophenyl)-quinoline-4-carboxylic acid (4f). This
compound was obtained as white powder, mp 176-178 ºC; IR:
3314 (OH), 3056 (CH), 1692 (CO), 1656 (CN), 1523, 1346
1
(NO2), 746 (CH) cm-1; H NMR (DCCl3): δ 6.30-8.03 (m, 9 H,
arom.) ppm. Anal. calcd. for C16H10N2O4: C, 65.31; H, 3.43; N,
9.52. Found: C, 65.48; H, 3.51; N, 9.88.
8-Chloro-2-phenylquinoline-4-carboxylic acid (4g). This
compound was obtained as yellow needles, mp 85-87 ºC; IR:
3467 (OH), 3067 (CH), 1705 (CO), 1269 (CO), 760, 698 (CH)
[5] Vega, M. C.; Montero Torres, A.; Marrero-Ponce, Y.; Rolón,
M.; Gómez-Barrio, A.; Escario, J. A.; Aran, B. J.; Nogal, J. J.; Meneses-
Marcel, A.; Torrens, F. Bioorg. Med. Chem. Lett. 2006, 16, 1898.
[6] Gopalsamy, A.; Pallai, P. V. Tetrahedron Lett. 1997, 38, 907.
[7] Lutz, R. E.; Bailey, P. S.; Clark, M. T.; Codington, J. F.;
Deinet, A. J.; Freek, J. A.; Harnest, G. H.; Leake, N. H.; Martin, T. A.;
Rowlett, R. J.; Salsbury, J. M.; Shearer, N. H.; Smith, J. D.; Wilson, J.
W. J. Am. Chem. Soc. 1946, 68, 1813.
1
cm-1; H NMR (DCCl3): δ 7.52-7.60 (m, 4 H), 7.92-7.94 (m, 1
H), 8.33-8.36 (m, 2 H), 8.64 (s, 1 H), 8.78-8.82 (m, 1 H) ppm.
Anal. calcd. for C16H10ClNO2: C, 67.74; H, 3.55; N, 4.94.
Found: C, 67.44; H, 3.26; N, 4.67.
7-Chloro-2-phenylquinoline-4-carboxylic acid (4h). This
compound was obtained as pale yellow powder, mp 191-192 ºC;
IR: 3319 (OH), 3025 (CH), 1678 (CO), 1582 (CN), 1264 (CO),
780, 679 (CH) cm-1; 1H NMR (DCCl3): δ 5.66 (d, 1 H), 6.11 (d,
1 H), 6.69-7.39 (m, 6 H), 7.70 (t, 1 H) ppm. Anal. calcd. for
C16H10ClNO2: C, 67.74; H, 3.55; N, 4.94. Found: C, 67.61; H,
3.39; N, 5.12.
[8] Sivaprasad, G.; Rajesh, R.; Perumal, P. T. Tetrahedron Lett.
2006, 47, 1783.
[9] Perreux, l.; Loupy, A. Tetrahedron 2001, 57, 9199.
[10] Tu, S.; Zhu, X.; Zhang, J.; Xu, J.; Zhang, Y.; Wang, Q.; Jia,
R.; Jiang, B.; Zhang, J.; Yao, C. Bioorg. Med. Chem. Lett. 2006, 16,
2925.
[11] Döbner, O. Ann. 1887, 242, 265.
[12] a) Beil., XXII, 518. b) Ibid. 104, c) Ibid. 105, d) Ibid. 520.
[13] Muscia, G. C.; Bollini, M.; Carnevale, J. P.; Bruno, A. M.;
Asís, S. E. Tetrahedron Lett. 2006, 47, 8811.
6-Chloro-2-phenylquinoline-4-carboxylic acid (4i). This
compound was obtained as pale yellow powder, mp 147-149 ºC;
IR: 3328 (OH), 3066 (CH), 1760 (CO), 1670 (CN), 822, 696
(CH) cm-1; 1H NMR (DCCl3): δ 5.65 (d, 1 H), 6.05 (d, 1 H), 6.38
(d, 1 H), 7.17-7.49 (m, 6 H) ppm. Anal. calcd. for C16H10ClNO2:
C, 67.74; H, 3.55; N, 4.94. Found: C, 68.03; H, 3.75; N, 4.65.
6-Fluoro-2-phenylquinoline-4-carboxylic acid (4j). This
compound was obtained as pale yellow powder, mp 134-136 ºC;
IR: 3348 (OH), 3067 (CH), 1760 (CO), 1658 (CN), 1218 (CO),
782, 697 (CH) cm-1; 1H NMR (DCCl3): δ 6.02 (d, 1 H), 6.32 (d,
1 H), 7.02-7.40 (m, 7 H) ppm. Anal. calcd. for C16H10FNO2: C,
71.91; H, 3.77; N, 5.24. Found: C, 72.18; H, 3.49; N, 5.06.
2-phenyl-7-trifluoromethylquinoline-4-carboxylic acid
(4k). This compound was obtained as white needles, mp 127-
129 ºC; IR: 3330 (OH), 3086 (CH), 1680 (CO), 1655 (CN),
[14] The in vitro protocols and activity criteria can be found at
website
WHO
(www.who.int/tdr/grants/workplans/pdf).
For
antimalarial activity (K1 strain is used), if the IC50 is >5 µg/mL, the
compound is classified as inactive. If the IC50 is 0.5–5 µg/mL, the
compound is classified as moderately active. If the IC50 is <0.5 µg/mL,
the compound is classified as active and is further evaluated using two
strains, K1 and NF54. For Chagas disease, if the IC50 is >30 µg/mL, the
compound is classified as inactive. If the IC50 is between 2 and 30
µg/mL, the compound is classified as moderately active. If the IC50 is <2
µg/mL, the compound is classified as active and is further evaluated in
an in vivo screen. Regarding cytotoxicity, if the IC50 is > 90 µg/mL, the
compound is classified as not cytotoxic. If the IC50 is between 2 and 89
µg/mL, the compound is classified as moderately cytotoxic and if the
IC50 is < 2 µg/mL, the compound is classified as cytotoxic.
[15] Antwerp University, LMPH, standard procedures used for the
TDR in vitro screening. For antimalarial activity GHA strain is used and
for antileishmanial activity MHOM/MA(BE)/67 strain is used. The
compound is classified as inactive when the IC50 is higher than 16
µg/mL. When IC50 lies between 16 and 1 µg/mL, the compound is
regarded as being moderately active. When the IC50 is lower than 1
µg/mL, the compound is classified as active and is evaluated in a
1
1329, 1164, 1133 (CF3), 794, 691 (CH) cm-1; H NMR (DCCl3):
δ 5.73 (d, 1 H), 6.15 (d, 1 H), 6.82 (s, 1 H), 7.20-7.43 (m, 4 H),
7.71 (d, 1 H), 7.92 (s, 1 H) ppm. Anal. calcd. for C17H10F3NO2:
C, 64.36; H, 3.18; N, 4.41. Found: C, 64.52; H, 3.40; N, 4.38.
7-Methyl-2-phenylquinoline-4-carboxylic acid (4l). This
compound was obtained as white powder, mp 217-218 ºC; IR:
3346 (OH), 3030 (CH), 2915, 2851 (CH3), 1706 (CO), 1599
1
(CN), 1224 (CO), 749, 702 (CH) cm-1; H NMR (DMSO): δ
secondary screening. For Chagas disease, Trypanosoma cruzi
β
galactosidase strain is used. The compound is classified as inactive when
the IC50 is higher than 16 µg/mL. When IC50 lies between 16 and 1
µg/mL, the compound is regarded as being moderate active. When the
IC50 is lower than 1 µg/mL, the compound is classified as highly active
and is further evaluated in a secondary screening. For toxicity, the
compound is classified as non-toxic when the IC50 is higher than 32
µg/mL or µM. Between 16 and 4 µg/mL, the compound is regarded as
moderately toxic. When the IC50 is lower than 4 µg/mL, the compound is
classified as highly toxic.
2.56 (s, 3 H, CH3), 7.53-7.60 (m, 4 H), 7.96 (s, 1 H), 8.26-8.29
(m, 2 H), 8.39 (s, 1 H), 8.56 (d, 1 H) ppm. Anal. calcd. for
C17H13NO2: C, 77.55; H, 4.98; N, 5.32. Found: C, 77.27; H,
5.00; N, 4.99.
Acknowledgement. We wish to thank Dr. Foluke Fakorede,
TDR, WHO (Switzerland). We are grateful for the financial
support from Universidad de Buenos Aires (UBACyT B-020)
and Mr. F. Cuello for technical help.