Application of multicomponent reactions to antimalarial drug discovery. Part 3: Discovery of aminoxazole 4-aminoquinolines with potent antiplasmodial activity in vitro

Article abstract of DOI:10.1016/j.bmcl.2007.06.070

The synthesis and antimalarial activity of a novel series of first generation 4-aminoquinoline-containing 2,4,5-trisubstituted aminoxazoles against two strains of the Plasmodium falciparum parasite in vitro is described. A number of compounds significantly more potent than the standard drug chloroquine were identified.

Full text of DOI:10.1016/j.bmcl.2007.06.070

Bioorganic & Medicinal Chemistry Letters 17 (2007) 4733–4736
Application of multicomponent reactions to antimalarial
drug discovery. Part 3: Discovery of aminoxazole
4-aminoquinolines with potent antiplasmodial activity in vitro
Chitalu C. Musonda,a Susan Little,b Vanessa Yardleyb and Kelly Chibalea,c,
*
aDepartment of Chemistry, University of Cape Town, Rondebosch 7701, South Africa
bDepartment of Infectious & Tropical Diseases, London School of Hygiene and Tropical Medicine,
Keppel Street, London WC1E 7HT, UK
cInstitute of Infectious Disease and Molecular Medicine, University of Cape Town, Rondebosch 7701, South Africa
Received 5 June 2007; accepted 21 June 2007
Available online 27 June 2007
Abstract—The synthesis and antimalarial activity of a novel series of first generation 4-aminoquinoline-containing 2,4,5-trisubsti-
tuted aminoxazoles against two strains of the Plasmodium falciparum parasite in vitro is described. A number of compounds signif-
icantly more potent than the standard drug chloroquine were identified.
Ó 2007 Elsevier Ltd. All rights reserved.
The problem of endemic malaria continues unabated
globally. Malaria affects 40% of the global population,
NEt2
HO
HN
N
causing an estimated annual mortality of 1.5–2.7 million
people.1,2 The World Health Organisation (WHO) esti-
mates that 90% of these deaths occur in sub-Saharan
Africa among infants under the age of five. There are
four species of Plasmodium that cause malaria in hu-
mans, namely P. falciparum, P. malariae, P. ovale and
P. vivax. Of these P. falciparum is the most virulent
and causes the majority of deaths. While a vaccine
against malaria continues to be elusive, chemotherapy
remains the most viable alternative towards treatment
of the disease. With the malaria parasite having devel-
oped multiple drug resistance to clinically established
drugs, there is a compelling need to introduce new chem-
ical entities that can overcome the resistance (Fig. 1).
N
MeO
Cl
N
2
1
Figure 1. Chemical structures of chloroquine (1) and quinine (2).
pharmacophore found present in a number of antimalar-
ial drugs including chloroquine (CQ) 1 and quinine 2.3,4
In this communication, we wish to report on the synthe-
sis and antiplasmodial activity of a novel class of
4-aminoquinolines that contain a heterocyclic 2,4,5-trisub-
stituted aminoxazole unit in the lateral side chain. The
synthesis was based on the 3-component condensation
domino reaction reported by Zhu et al.,5,6 wherein an
amine, aldehyde and isocyanoacetamide react to afford
2,4,5-trisubstituted aminoxazoles. We were attracted to
this reaction owing to its simplicity and amenability to
parallel synthesis and rapid library generation.
In the context of our ongoing research towards the dis-
covery of new antimalarial compounds, we have been
successfully employing the isocyanide-based multicom-
ponent reaction (MCR) strategy to synthesize new
compounds in which diversity is introduced in a single
step. To this end, we have synthesized varying classes of
compounds based on the quinoline substructure, a
Chemistry. We intended to incorporate the 4-amino-
quinoline substructural motif into the amine inputs
required for the MCR. As such, the requisite amines
9–12 were synthesized according to Scheme 1,7 whereby
Keywords: 4-Aminoquinolines; Aminoxazoles; Malaria; P. falciparum;
Antiplasmodial activity.
*
Corresponding author. E-mail: Chibale@uct.ac.za
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.06.070

4734
C. C. Musonda et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4733–4736
NHCH2(CH2)nNH2
gave the formamide 17 in 94% yield. Finally, dehydra-
tion of 17 to the isocyanoacetamide 18 was achieved in
98% yield by treatment with POCl3 in DCM at
À25 °C.10 Finally, the multicomponent reactions were
conducted in parallel array format in MeOH at 60 °C
according to Scheme 3.
Cl
N
NH2CH2(CH2)nNH2
4 - 7,
9, n = 1, 100%
10, n = 2, 96%
4, n = 1; 5, n = 2
6, n = 3; 7, n = 5
+
80-135 Cl
˚C, 4 h
N
11, n = 3, 98%
12, n = 5, 96%
Cl
3
Cl
8
13, 88%
HN
NH
N
N
NH
Following pre-treatment of the amines 9–13 with either
para-formaldehyde or acetaldehyde for 30 min, iso-
cyanoacetamide 18 was added to each of the reaction
mixtures. The initial choice of para-formaldehyde and
acetaldehyde as the aldehyde inputs was governed by
our desire to obtain low molecular weight compounds.11
Isolation of the compounds was achieved by means of
chromatography on silica (SiO2) gel. Table 1 shows
the isolated yields and purities of the compounds
obtained.
K2CO3, Et3N, NMP,
135 ˚C, 4 h
Scheme 1. Synthesis of 4-aminoquinoline amines 9–13.
Bn
HN
O
O
Bn
HCO2H, Ac2O
16
H2N
CO2H rt, 24 h, 78%
EDC, HOBt, rt
12 h, 94%
H
N
CO2H
14
H
15
Bn
O
Biological results. Compounds 19–28 were evaluated
against two different strains of P. falciparum, namely a
chloroquine-sensitive (CQS) 3D7 strain and a chloro-
quine-resistant
O
Bn
O
POCl3,-25 ˚C,
2.5 h, 98%
N
N
NC
H
N
18
O
H
O
17
Scheme 2. Synthesis of isocyanoacetamide 18.
(CQR) K1 strain. Results of these assays are shown in
Table 2. From the results in Table 2, compound 26
was the most active in the chloroquine-sensitive 3D7
strain which, with an IC50 = 0.0038 lM, was 5 times
more efficacious than chloroquine (IC50 = 0.02 lM).
Also of note was 19 (IC50 = 0.0084 lM) which was also
active in the low nanomolar range. On the other hand,
20 exhibited slightly weaker potency at IC50 = 0.041 lM
than chloroquine although not significantly so, while 25
commercially available 4,7-dichloroquinoline 3 was
treated with an excess of the dialkyl amines 4–7, initially
at 80 °C and then at 135 °C for 3 h. On the other hand,
treatment of 4,7-dichloroquinoline with excess pipera-
zine7 afforded the monosubstituted amine 13, Scheme
1.8 The five amines 9–13 were obtained in excellent
yields (88–100%), the results of which are shown in
Scheme 1.
(IC50 = 0.079 lM), 22 (IC50 = 0.15 lM), 23 (IC50
=
0.52 lM) and 28 (IC50 = 0.14 lM) exhibited weakened
antiplasmodial activities in comparison to chloroquine.
Interestingly, the introduction of a methyl substituent
into the side chains of compounds 19 and 20 caused
reductions in the activities of the corresponding deriva-
tives 24 and 25, respectively. In contrast, the introduc-
tion of a methyl substituent into the side chains of 21
and 22 resulted in significant improvements in the effica-
cies of the resulting compounds 26 and 27. With IC50s of
0.0038 and 0.018 lM, respectively, 26 and 27 exhibited
the best potencies, showing 21-fold and eightfold
improvements in efficacy against the chloroquine-sensi-
tive strain.
The next step was to generate the isocyanoacetamide re-
quired for the multicomponent reaction, and this was
realised from L-phenylalanine 14 according to Scheme
2 under standard conditions.9
Treatment of L-phenylalanine with a mixture of formic
acid (HCOOH) and Ac2O at sub-zero temperature for
20 min, followed by stirring at room temperature, affor-
ded the formamide 15 in 78% yield.
Subsequent coupling of 15 with morpholine 16, medi-
ated by EDC and HOBt in DCM at room temperature,
Bn
N
H
N
( )n
NH2
N
HN
N
( )n
O
HN
N
O
R
Bn
O
19 - 22 and
24 - 27
N
Cl
Cl
NC
Cl
MeOH, 60 ˚C,
O
R
or
+
8 - 12 h, 39 - 93%
Cl
N
Bn
N
RCHO
O
N
N
N
23, R = H
28, R = CH3
N
N
NH
O
Scheme 3. Multicomponent synthesis of compounds 19–28.

C. C. Musonda et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4733–4736
Table 1. Isolated yields and purities of aminoxazoles 19–28
4735
the unsubstituted or methyl-substituted derivatives. In
the unsubstituted compounds 19–22, the two com-
pounds with the 2- and 3-carbon spacers were the
more active, while this activity was lost in the 4- and
6-carbon-spaced compounds. On the contrary, the
methyl-substituted compound 24 with the 2-carbon
spacer was not as active as the 3-carbon spaced com-
pound 25. The lowest activity was observed with the
piperazinyl derivative 28.
Entry
R
Product
n
%Yield
%Puritya
1
2
H
19
20
21
22
23
24
22
26
27
28
1
70
68
62
39
93
68
51
67
43
87
93
95
97
98
99
96
97
96
95
98
H
2
3
H
3
4
H
5
5
H
—
1
6
CH3
CH3
CH3
CH3
CH3
7
2
8
3
9
10
5
—
The observed improvement in the activities of these no-
vel 4-aminoquinoline 2,4,5-trisubstituted aminoxazoles
may be speculated to result from several factors. The
4-amino-7-chloroquinoline subunit is an antimalarial
pharmacophore that inhibits haem dimerization into
nontoxic haemozoin.12 Therefore, the primary antiplas-
modial activities associated with these compounds may
be arising from inhibition of haem dimerization causing
a build up of the toxic haem, resulting in parasite death.
It is also possible that the increase in basicity caused by
the side chain NH on N and the oxazole N may be
improving the accumulation of the compounds in the
acidic food vacuole of the parasite via pH trapping.13
The presence of the heterocycle in the lateral chain
may also be resulting in secondary interactions with
other targets apart from haem, resulting in the observed
antiplasmodial activities.
a HPLC purity, conditions: flow rate: 1.0 ml/min, UV 220 and 254 nm,
30% CH3CN: 70% 25 mM phosphate buffer.
Table 2. Antiplasmodial activity of 19–28 against CQSa 3D7 and
CQRb K1
Compound
n
R
IC50 (lM)
3D7
K1
CQ
19
20
21
22
23
24
25
26
27
28
—
1
—
H
0.02
0.25c/1.03d
0.0084
0.041
0.079
0.15
0.23c
0.14c
0.099c
0.11d
0.36d
0.25d
0.16d
0.019d
0.037d
0.97d
2
H
3
H
5
H
—
1
H
0.52
0.12
CH3
CH3
CH3
CH3
CH3
2
0.079
0.0038
0.018
0.14
3
In conclusion, we have shown the potential of 4-amino-
quinoline-containing 2,4,5-trisubstituted aminoxazoles
as antiplasmodial agents in vitro and thereby demon-
strated a simple approach to the synthesis of analogues
of existing antimalarial drugs. The efficiency of this ap-
proach is manifested in the preclusion of large-sized li-
braries as the SAR libraries would already be enriched
in antimalarial pharmacophores. Rationally, such a
combination of antimalarial pharmacophores and other
functionalities offers many attractive features for accel-
erating antimalarial drug discovery.
5
—
a CQS, chloroquine-sensitive.
b CQR, chloroquine-resistant.
c Average CQ IC50 was determined to be 0.25 lM.
d In this experiment the average CQ IC50 was determined to be
1.03 lM.
The results also reveal that all compounds were more ac-
tive than chloroquine in the resistant (K1) strain. In the
first experiment, chloroquine had an IC50 of 0.25 lM,
whereas the 2 out of the 3 compounds tested along with
the control drug, namely 20 and 21, were both more po-
tent than chloroquine with respective IC50 values of 0.14
and 0.099 lM. In the second experiment, compounds
22, 23 and 24–27 were several orders of magnitude more
active than chloroquine. In fact, 25, the least active
among the three (IC50 = 0.16 lM), was 6 times more po-
tent than chloroquine (IC50 = 1.03 lM), while 26
(IC50 = 0.019 lM) was 54 times more efficacious than
chloroquine and exhibited the greatest potency.
Acknowledgments
This work was supported by the South African National
Research Foundation under Grant No. 2053362 (K.C.).
The investigation also received support from UNDP/
World Bank/WHO Special Programme for Research
and Training in Tropical Diseases (TDR) (V.Y. and S.L.).
Supplementary data
The results suggest that the introduction of a methyl
=
Experimental details for the synthesis of all compounds
and biological evaluation. Supplementary data associ-
ated with this article can be found, in the online version,
at doi:10.1016/j.bmcl.2007.06.070.
substituent in 20 (IC50 = 0.14 lM) to yield 25 (IC50
0.16 lM) had no effect on the antiplasmodial activity,
whereas a similar change for 21 (IC50 = 0.099 lM)
resulted in significantly improved efficacy for compound
26 (IC50 = 0.019 lM). A similar increase in potency was
seen in the analogous compounds 22 (IC50 = 0.11 lM)
and 27 (IC50 = 0.037 lM).
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