Synthesis of substituted indole derivatives as a new class of antimalarial agents

Article abstract of DOI:10.1016/j.bmcl.2005.04.011

A series of substituted indole derivatives were synthesized and evaluated for their in vitro antimalarial activity against P. falciparum. Out of the 24 compounds synthesized six compounds have shown MIC of 1 μg/mL. These compounds are in vitro several fol

Full text of DOI:10.1016/j.bmcl.2005.04.011

Bioorganic & Medicinal Chemistry Letters 15 (2005) 3133–3136
Synthesis of substituted indole derivatives as a
new class of antimalarial agents
Anu Agarwal,a Kumkum Srivastava,b S. K. Purib and Prem M. S. Chauhana,*
aDivision of Medicinal and Process Chemistry, Central Drug Research Institute, Lucknow 226001, India
bDivision of Parasitology, Central Drug Research Institute, Lucknow 226001, India
Received 21 February 2005; revised 6 April 2005; accepted 8 April 2005
Available online 4 May 2005
Abstract—A series of substituted indole derivatives were synthesized and evaluated for their in vitro antimalarial activity against
P. falciparum. Out of the 24 compounds synthesized six compounds have shown MIC of 1 lg/mL. These compounds are in vitro
several folds more active than pyrimethamine.
Ó 2005 Elsevier Ltd. All rights reserved.
1. Introduction
Indole and its derivatives represent one of the most active
class of compounds possessing a wide spectrum of anti-
parasitic activities. Indole alkaloids as apicidin (2) have
shown potent antimalarial activity against P. falciparum.
Apicidin is known to be a potent inhibitor of HDAC (his-
tone deacetylase), a nuclear enzyme that regulates gene
transcription and the assembly of newly synthesized chro-
matin. Apicidin reversibly induces histone hyperacetyl-
ation, causing altered transcriptional regulation and
ultimately cell death. Studies on apicidin have suggested
that the indole region is a key constituent of enzyme bind-
ing and HDAC activity.5 Compounds, which act on more
than one target sites are more liable to be active. Keeping
in view all of the above facts we have synthesized hybrid
derivatives containing both indole (HDAC inhibitor)
and pyrimidine moiety (DHFR inhibitor).
Malaria remains the most serious and refractory health
hazard amongst the parasitic diseases, affecting around
500 million people and causing around 2 million deaths
annually.1 According to WHO estimates 40% of the
worldÕs population presently lives under malarial threat.
Due to the fact that the parasites are developing resis-
tance to conventional antimalarial drugs, the develop-
ment of drugs attacking crucial targets in the
metabolism of the malaria pathogen is imperative.2
Development of active and selective chemotherapeutic
agents could be achieved by rational drug design taking
into consideration the biochemical machinery of the par-
asite. One of the targets for drugs against malaria is the
enzyme dihydrofolate reductase (DHFR). Pyrimeth-
amine (1) is a specific inhibitor of the plasmodial DHFR.
The role of DHFR is to catalyze the NADPH dependent
reduction of dihydrofolate to give tetrahydrofolate, a
central component in the single carbon metabolic path-
way. The tetrahydrofolate is methylated to methylene
tetrahydrofolate, which is directly involved in thymidine
synthesis (assisting the methylation of deoxyuridine
monophosphate to give thymidine monophosphate)
and indirectly implicated in the metabolism of amino
acids and purine nucleotide. Inhibition of DHFR thus
prevents biosynthesis of DNA leading to cell death.3,4
O
C2H5
O
N
H
N
N
N
Cl
NH2
N
NH
OMe
H
N
O
H2N
1
O
2
O
Antimalarial drugs as quinine, mefloquine have piper-
idine nucleus and amopyroquine, cycloquine have pyr-
rolidine moiety. A large number of compounds having
piperidine, pyrrolidine, piperazine and morpholine
Keywords: Dihydrofolate reductase; Antimalarial; Pyrimidine.
*
Corresponding author. Tel.: +91 5222212411x4332; fax: +91
5222623405; e-mail addresses: prem_chauhan_2000@yahoo.com;
premsc58@hotmail.com
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.04.011

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A. Agarwal et al. / Bioorg. Med. Chem. Lett. 15 (2005) 3133–3136
moiety have shown potent antimalarial activity.6 These
results prompted us to synthesize compounds having in-
dole moiety along with pyrimidine and these cyclic
amines at second position of pyrimidine ring.
Table 1. Antimalarial in vitro activity against P. falciparum
S. No
R
MIC (lg/mL)
3(a–e) 4(a–e)
X = NMe X = CH2 X = nil X = O
2(a–e)
5(a–e)
a
b
c
d
e
f
H
4-Me
1
1
1
1
1
1
10
10
10
10
10
10
10
10
10
10
10
10
50
50
50
50
50
50
As part of our ongoing programme devoted to the syn-
thesis of diverse heterocycles as antiinfective agents,7 we
had previously reported antimalarial activity in substi-
tuted triazines, pyrimidines and quinolines.8 This com-
munication describes the synthesis of substituted
indoles 2–5(a–f) as antimalarial agents.
4-OCH3
3,4-Di-OCH3
2,5-Di-OCH3
4-Cl
MIC = Minimum inhibiting concentration for the development of ring
stage parasite into the schizont stage during 40 h incubation.
Standard drug, pyrimethamine, MIC 10 lg/mL.
2. Chemistry
The chalcones 1(a–f) were synthesized by reacting 3-form-
yl indole with different substituted acetophenones (a–f)
by refluxing them in methanol in the presence of piper-
idine.9 Cyclic carboxamidine hydrochlorides were syn-
thesized by refluxing the corresponding cyclic amine
with S-methyl-isothiourea sulfate in water according to
a reported procedure.10 The chalcones 1(a–f) were fur-
ther cyclized with imidine hydrochlorides in the presence
of sodium isopropoxide (synthesized in situ by adding
sodium metal in isopropanol) to afford the substituted
indole derivatives 2–5(a–f) as shown in Scheme 1. All
the synthesized compounds were well characterized by
spectroscopic data as IR, mass, NMR and elemental
analysis.14
rum was synchronized after 5% D-Sorbitol treatment to
obtain parasitized cells harbouring only the ring stage.13
For carrying out the assay, an initial ring stage parasita-
emia of ꢀ1% at 3% haematocrit in total volume of
200 lL of medium RPMI-1640 was uniformly main-
tained. The test compound in 20 lL volume at required
concentration (ranging between 0.25 lg and 50 lg/mL)
in duplicate wells, were incubated with parasitized cell
preparation at 37 °C in candle jar. After 36–40 h incuba-
tion, the blood smears from each well were prepared and
stained with giemsa stain. The slides were microscopi-
cally observed to record maturation of ring stage para-
sites into trophozoites and schizonts in presence of
different concentrations of compounds. The test concen-
tration, which inhibits the complete maturation into
schizonts, was recorded as the minimum inhibitory con-
centration (MIC). Pyrimethamine was used as the stan-
dard reference drug. Activity of all the tested
compounds is shown in Table 1.
3. Biological activity
The in vitro antimalarial assay was carried out in 96-well
microtitre plates according to the micro assay of Rieck-
mann et al.11 The culture of P. falciparum NF-54 strain
is routinely being maintained in medium RPMI-1640
supplemented with 25 mM HEPES, 1% D-glucose,
0.23% sodium bicarbonate and 10% heat inactivated
human serum.12 The asynchronous parasite of P. falcipa-
4. Results and discussion
Among all the 24 compounds 2–5(a–f) tested, six com-
pounds showed MIC of 1 lg/mL, while 12 compounds
O
CHO
R
NH
.HCl
NH2
R
a
X
N
+
COCH3
(a-f)
+
N
N
H
H
1(a-f)
NH
R
MeS
.H2SO4
2
c
b
NH2
N
N
N
X
N H
N
H
X= N-Me, CH2, Nil, O
X
2(a-f) X = N-Me
3(a-f) X = CH2
4(a-f) X = Nil
5(a-f) X = O
Scheme 1. Reagents and conditions: (a) Different acetophenone (a–f), piperidine, methanol, reflux, 8 h; (b) (i) different secondary amines,
S-methylisothiourea sulfate, water, reflux, 15 min; (ii) barium chloride, reflux, 15 min; (c) different imidines, HCl, sodium isopropoxide, isopropanol,
reflux, 8 h.

A. Agarwal et al. / Bioorg. Med. Chem. Lett. 15 (2005) 3133–3136
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have shown MIC of 10 lg/mL. All the compounds,
which had N-methyl piperazine group at 2-position of
the pyrimidine ring showed a MIC of 1 lg/mL. Varying
the substituents on the phenyl ring at the sixth position
of the pyrimidine ring have no effect on the activity,
while substitution at the second position plays a crucial
role in exerting antimalarial activity. On replacing the
piperazine ring with pyrrolidine and piperidine moiety
the activity dropped to 10 lg/mL. On further substitu-
ting it with morpholine moiety the activity reduced fur-
ther having a MIC of 50 lg/mL. These results emphasize
the better efficacy of N-methyl piperazine group over
pyrrolidine, piperidine and morpholine group in antima-
larial activity.
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Bhaduri, A. P.; Singh, S. N.; Fatima, N.; Chatterjee, R.
K.; Bose, C. Bioorg. Med. Chem. Lett. 1996, 6, 2623; (b)
Srivastava, S. K.; Agarwal, A.; Chauhan, P. M. S.;
Agarwal, S. K.; Bhaduri, A. P.; Singh, S. N.; Fatima, N.;
Chatterjee, R. K. J. Med. Chem. 1999, 42, 1667; (c)
Srivastava, S. K.; Agarwal, A.; Chauhan, P. M. S.;
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5. Conclusion
Twenty-four substituted indole 2–5(a–f) derivatives were
synthesized as pyrimethamine analogues. Out of the
synthesized compounds six compounds showed MIC
of 1 lg/mL. These compounds are 10 times more potent
than pyrimethamine. The present study suggested that
the newly synthesized indole derivatives are new lead
in antimalarial chemotherapy. These molecules can be
very useful for further optimization work in malarial
chemotherapy.
8. (a) Agarwal, A.; Srivastava, K.; Puri, S. K.; Chauhan, P.
M. S. Bioorg. Med. Chem. Lett. 2005, 15, 531; (b)
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M. S. Bioorg. Med. Chem. Lett. 2005, 15, 1881–1883; (c)
Shrivastava, S.; Tiwari, S.; Chauhan, P. M. S.; Puri, S. K.;
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Acknowledgements
A.A. thanks the Council of Scientific and Industrial Re-
search (India) for the award of Senior Research Fellow-
ship. We are also thankful to SAIF Division, CDRI,
Lucknow for providing spectroscopic data. CDRI com-
munication no. 6737.
9. Manna, F.; Chimenti, F.; Bolasco, A.; Bizzarri, B.;
Filippelli, W.; Filippelli, A.; Gagliardi, L. Eur. J. Med.
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14. Spectroscopic data for 2c. MS: 400 (M+1); mp 130–
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Salzberg, S. L.; Nene, V.; Shoaibi, A., et al. Nature
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Med. Res. Rev. 2002, 22, 465–491.
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(NIAID). Global Health Research Plan for HIV/AIDS,
Malaria and Tuberculosis; World Health Organization:
Geneva, 2001, p 18; (b) Kumar, A.; Katiyar, S. B.; Agarwal,
A.; Chauhan, P. M. S. Curr. Med. Chem. 2003, 10, 1137; (c)
Shrivastava, S. K.; Chauhan, P. M. S. Curr. Med. Chem.
2001, 8, 1535; (d) Kumar, A.; Katiyar, S. B.; Agarwal, A.;
Chauhan, P. M. S. Drugs Future 2003, 28, 243.
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Saesaengseerung, N.; Jarprung, D.; Kamchonwongpaisan,
S.; Sirawarapornc, W.; Yuthavong, Y. Bioorg. Med. Chem.
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pornpisut, P., et al. Bioorg. Med. Chem. 2000, 8, 1117.
5. (a) Singh, S. B.; Zink, D. L.; Polishook, J. D.; Dombrow-
ski, A. W.; Darkin-Rattray, S. J.; Schmatz, D. M.; Goetz,
M. A. Tetrahedron Lett. 1996, 37, 8077; (b) Meinke, P. T.;
Colletti, S. L.; Ayer, M. B.; Darkin-Rattray, S. J.; Myers,
132 °C; IR (KBr) 2932, 1645, 1574, 1484, 1319, 1280 cmÀ1
;
1H NMR (CDCl3, 200 MHz): d (ppm) 8.70 (s, 1H, NH),
8.43 (d, 1H, J = 8.6 Hz), 8.09 (d, 2H, J = 8.8 Hz), 7.90 (s,
1H), 7.42 (d, 1H, J = 8.2 Hz), 7.29–7.26 (m, 2H), 7.27 (s,
1H), 7.01 (d, 2H, J = 8.8 Hz), 4.08 (t, 4H, J = 4.9 Hz), 2.58
(t, 4H, J = 4.9 Hz), 2.38 (s, 3H, NMe); 13C (CDCl3,
50 MHz): d 168.3, 167.5, 166.5, 166.2, 142.6, 135.9, 133.5,
132.4, 130.7, 127.4, 126.7, 125.9, 120.5, 119.1, 117.2, 106.2,
55.8, 55.5, 46.7, 44.2. Anal. Calcd for C24H25N5O: Calcd
C, 72.16; H, 6.31; N, 17.53. Found: C, 72.52; H, 6.54; N,
17.71. Spectroscopic data for 2f. MS : 404 (M+1); mp 204–
206 °C; IR (KBr) 2956, 1654, 1568, 1478, 1325, 1275 cmÀ1
;
1H NMR (CDCl3, 200 MHz): d (ppm) 8.83 (s, 1H, NH),
8.42 (d, 1H, J = 8.1 Hz), 8.05 (d, 2H, J = 8.5 Hz), 7.90 (s,
1H), 7.48 (d, 2H, J = 8.5 Hz), 7.41 (d, 1H, J = 8.3 Hz),
7.30 (s, 1H), 7.28–7.25 (m, 2H), 4.10 (t, 4H, J = 5.4 Hz),
2.57 (t, 4H, J = 5.4 Hz), 2.38 (s, 3H); 13C (CDCl3,
50 MHz): 165.8, 163.6, 162.5, 162.3, 137.8, 135.9, 128.8,
128.7, 127.5, 125.9, 122.6, 122.0, 121.2, 115.9, 112.3, 104.5,

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A. Agarwal et al. / Bioorg. Med. Chem. Lett. 15 (2005) 3133–3136
55.5, 46.7, 44.2. Anal. Calcd for C23H22ClN5: Calcd C,
7.17 (m, 2H), 3.75 (t, 4H, J = 5.2 Hz), 2.04 (t, 4H,
J = 5.2 Hz); 13C (CDCl3, 50 MHz): 168.3, 167.5, 166.5,
142.6, 135.9, 133.5, 132.4, 130.2, 129.4, 128.2, 126.3, 126.7,
125.9, 120.5, 119.1, 106.2, 43.8, 25.4. Anal. Calcd for
C22H20N4: Calcd C, 77.62; H, 5.92; N, 16.46. Found: C,
77.46; H, 5.77; N, 16.31. Spectroscopic data for 5d. MS
417 (M+1); mp 220–222 °C; IR (KBr) 2948, 1636, 1584,
68.39; H, 5.49; N, 17.34. Found: C, 68.58; H, 5.68; N,
17.61. Spectroscopic data for 3b. MS: 369 (M+1); mp
decomposes at 240 °C; IR (KBr) 2924, 1648, 1576, 1486,
1328, 1284 cmÀ1 1H NMR (CDCl3, 200 MHz): d (ppm)
;
8.9 (s, 1H, NH), 8.48 (d, 1H, J = 8.2 Hz), 8.03 (d, 2H,
J = 8.1 Hz), 7.99 (s, 1H), 7.47 (d, 1H, J = 8.3 Hz), 7.34 (d,
2H, J = 8.1 Hz), 7.27 (s, 1H), 7.25–7.21 (m, 3H), 4.00 (t,
4H, J = 4.2 Hz), 1.70–1.65 (m, 6H); 13C NMR (CDCl3,
50 MHz): d 167.4, 167.1, 166.1, 142.5, 137.3, 135.6, 133.2,
132.6, 133.3, 131.7, 129.8, 126.7, 125.6, 120.5, 118.7, 103.9,
45.5, 26.1, 25.6, 18.9. Anal. Calcd for C24H24N4: Calcd C,
78.23; H, 6.57; N, 15.21. Found C, 78.46; H, 6.72; N,
15.39. Spectroscopic data for 3f. MS 389 (M+1); mp 228–
1486, 1325, 1265 cmÀ1 1H NMR (CDCl3, 200 MHz): d
;
(ppm) 8.62 (s, 1H, NH), 8.44 (d, 1H, J = 8.2 Hz), 7.95 (s,
1H), 7.73 (d, 1H, J = 7.9 Hz), 7.67 (s, 1H), 7.45 (d, 1H,
J = 8.4 Hz), 7.32 (s, 1H), 7.26–7.21 (m, 2H), 6.97 (d, 1H,
J = 8.4 Hz), 4.05 (t, 4H, J = 4.2 Hz), 4.00 (s, 3H, OMe),
3.97 (s, 3H, OMe), 3.87 (t, 4H, J = 4.2 Hz); 13C NMR
(CDCl3, 50 MHz): 164.1, 163.2, 162.8, 151.4, 149.5, 137.4,
131.7, 126.7, 125.9, 123.3, 122.2, 121.7, 120.5, 116.9, 111.9,
111.5, 110.6, 102.2, 67.5, 56.5, 56.4, 45.1. Anal. Calcd for
C24H24N4O3: Calcd C, 69.21; H, 5.81; N, 13.45. Found: C,
69.47; H, 5.74; N, 13.29. Spectroscopic data for 5e. MS
417 (M+1); mp 236–238 °C; IR (KBr) 2936, 1642, 1578,
230 °C; IR (KBr) 2926, 1638, 1580, 1480, 1325, 1272 cmÀ1
;
1H NMR (CDCl3, 200 MHz): d (ppm) 8.60 (s, 1H, NH),
8.47 (d, 1H, J = 8.2 Hz), 8.05 (d, 2H, J = 8.6 Hz), 7.89 (s,
1H), 7.46 (d, 2H, J = 8.6 Hz), 7.39 (d, 1H, J = 8.3 Hz),
7.30 (s, 1H), 7.28–7.24 (m, 2H), 4.00 (t, 4H, J = 4.2 Hz),
1.71–1.65 (m, 6H); 13C NMR (CDCl3, 50 MHz): d 165.9,
163.7, 162.7, 162.6, 137.7, 135.9, 128.9, 128.6, 127.5, 125.9,
122.6, 122.0, 121.2, 115.9, 112.3, 100.9, 45.4, 26.0, 25.4.
Anal. Calcd for C23H21ClN4: Calcd C, 71.03; H, 5.44; N,
14.41. Found: C, 71.34; H, 5.65; N, 14.58. Spectroscopic
data for 4a. MS: 341 (M+1); mp 232–234 °C; IR (KBr)
1488, 1324, 1284 cmÀ1 1H NMR (CDCl3, 200 MHz): d
;
(ppm) 8.68 (s, 1H, NH), 8.40 (d, 1H, J = 8.1 Hz), 7.94 (s,
1H), 7.68 (s, 1H), 7.43 (d, 1H, J = 8.2 Hz), 7.33 (s, 1H),
7.24 (m, 2H), 6.96 (d, 2H, J = 7.8 Hz), 4.00 (t, 4H,
J = 4.6 Hz), 3.88 (s, 3H, OMe), 3.84 (s, 3H, OMe), 3.80 (t,
4H, J = 4.6 Hz); 13C (CDCl3, 50 MHz): d 164.3, 163.3,
162.7, 151.5, 149.6, 137.5, 131.7, 126.7, 125.9, 123.3, 122.2,
121.7, 120.5, 116.8, 111.9, 111.6, 110.6, 102.2, 67.6, 56.6,
56.4, 45.2. Anal. Calcd for C24H24N4O3: Calcd C, 69.21;
H, 5.81; N, 13.45. Found: C, 69.38; H, 5.68; N, 13.62.
2932, 1645, 1574, 1484, 1319, 1280 cmÀ1 1H NMR
;
(CDCl3, 200 MHz): d (ppm) 8.72 (s, 1H, NH), 8.58 (d,
1H, J = 8.1 Hz), 8.16 (d, 2H, J = 8.4 Hz), 8.05 (s, 1H), 7.54
(d, 1H, J = 7.8 Hz), 7.49–7.45 (m, 3H), 7.33 (s, 1H), 7.21–