Synthesis of ring-substituted 4-aminoquinolines and evaluation of their antimalarial activities

Article abstract of DOI:10.1016/j.bmcl.2004.12.037

A simple two-step synthesis method was used to make 51 B-ring-substituted 4-hydroxyquinolines allowing analysis of the effect of ring substitutions on inhibition of growth of chloroquine sensitive and resistant strains of Plasmodium falciparum, the dominant cause of malaria morbidity. Substituted quinoline rings other than the 7-chloroquinoline ring found in chloroquine were found to have significant activity against the drug-resistant strain of P. falciparum W2.

Full text of DOI:10.1016/j.bmcl.2004.12.037

Bioorganic & Medicinal Chemistry Letters 15 (2005) 1015–1018
Synthesis of ring-substituted 4-aminoquinolines and evaluation
of their antimalarial activities
Peter B. Madrid,a John Sherrill,a Ally P. Liou,b Jennifer L. Weisman,a,c
Joseph L. DeRisib and R. Kiplin Guya,c,
*
aDepartment of Pharmaceutical Chemistry, University of California, San Francisco, CA 94143-2280, USA
bDepartment of Biochemistry and Biophysics, University of California, San Francisco, CA 94143-2280, USA
cDepartment of Cellular and Molecular Pharmacology, University of California, San Francisco, CA 94143-2280, USA
Received 21 October 2004; revised 10 December 2004; accepted 14 December 2004
Available online 20 January 2005
Abstract—A simple two-step synthesis method was used to make 51 B-ring-substituted 4-hydroxyquinolines allowing analysis of the
effect of ring substitutions on inhibition of growth of chloroquine sensitive and resistant strains of Plasmodium falciparum, the domi-
nant cause of malaria morbidity. Substituted quinoline rings other than the 7-chloroquinoline ring found in chloroquine were found
to have significant activity against the drug-resistant strain of P. falciparum W2.
Ó 2004 Elsevier Ltd. All rights reserved.
Malaria is one of the most devastating infectious dis-
eases in the world, afflicting 200–500 million people
and killing 1–2 million annually.1 Of the four species
of the protozoa that cause malaria in humans,
P. falciparum is by far the most deadly. The worldwide
increase in drug-resistance to many of the affordable
chemotherapies, such as chloroquine, has created a
demand for the development of new malaria treatments.
Quinoline containing compounds have long been used
for treatment of malaria, beginning with quinine (Fig.
1).2 Systematic modification of quinine led to the potent
and inexpensive 4-aminoquinoline drug chloroquine.
After the worldwide development of drug-resistance to
chloroquine, focused chemistry and screening efforts
produced mefloquine, another quinoline containing
Figure 1. Quinoline antimalarial drugs with diverse substitutions
around the quinoline ring.
compound that was highly active against the chloro-
quine-resistant strains of P. falciparum.3 Since the devel-
opment of mefloquine, there have been several reports of
new potent quinoline compounds.4–7 Most of these con-
tain the 7-chloroquinoline nucleus of chloroquine and
vary in the length and nature of their basic amine side
chain. Currently, compounds such as amodiaquine and
AQ-13 are promising leads for the development of new
drugs.8
While it is known that modification of the basic amine
side chain can produce compounds active against
drug-resistant P. falciparum strains, it has generally been
assumed that changes to the quinoline nucleus itself will
not. Changes to the ring system affect the pKas of both
the quinoline ring nitrogen and the side-chain nitrogen
as well as other physical parameters such as lipophilicity,
Keyword: 4-Aminoquinoline malaria.
*
Corresponding author. Tel.: +1 415 502 7051; fax: +1 415 514
0689; e-mail: rguy@cgl.ucsf.edu
0960-894X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.12.037

1016
P. B. Madrid et al. / Bioorg. Med. Chem. Lett. 15 (2005) 1015–1018
Figure 2. (A) Generalized structure of compound library 1. (B)
Lettering of rings and numbering of positions around the quinoline
ring.
sterics, and electronegativity but have not significantly
correlated with activity in drug resistant strains.9 Recent
work on ring substitutions has focused almost exclu-
sively on the 7-position of the ring, so conclusions
drawn from these data are limited. In this study, we ex-
plore the effects of B ring substitutions (Fig. 2, positions
5, 6, 7, and 8) on antimalarial activity against drug-resis-
tant parasite strains.
Scheme 1. Reagents and conditions: (i) MeldrumÕs acid, CH(OCH3)3,
reflux, 1 h, then 2, DMF, reflux 2 h; (ii) diphenylether, 300 °C, 300 W,
5 min; (iii) POCl3 (neat), reflux, 3 h.
a small volume of phenyl ether as solvent and subjected
to microwave irradiation for 5 min at 300 °C.
Most of the current methods for synthesis of quinoline
rings are variations of the Skraup method in which an
aniline is heated with glycerol and an acid catalyst to
form a stable intermediate that undergoes cyclization
after a high-temperature Friedel–Crafts acylation.10
Much of the recent work on quinoline ring formation
reactions has been done using the method of Price and
Roberts, which involves condensation of an aniline with
ethoxymethylenemalonic ester.9,11 We employed a modi-
fication of this method that uses methoxymethylene
MeldrumÕs acid rather than ethoxymethylenemalonic
ester.12 This has the distinct advantage of giving the
same products in two steps rather than four steps. A
disadvantage to all of these reactions is they are usually
carried out at very high-temperatures (>300 °C) and are
notoriously messy. Another difficulty arises when using
meta substituted anilines—the two different reactive
ortho positions give regioisomeric products on the nucleo-
philic ring. The products favor the substituent being in
the 7-position over the 5-position, but in almost all cases
both products are formed. The separation of these iso-
mers can be quite difficult depending on the size and nat-
ure of the substituent.
The crude reaction mixtures were then directly purified
by silica chromatography to afford the desired pure 4-
hydroxyquinolines in 20–70% overall yield. In some
cases a mixture of isomers was formed. They could be
separated by HPLC and the ratios of isomers formed
is indicated in the Supplemental Materials (Table S1).
The hydroxyquinolines were dissolved in phosphorus
oxychloride and heated to reflux for 3 h to give the de-
sired 4-chloroquinolines, poised for nucleophilic addi-
tion of the side chain.
In order to better understand the structure–activity rela-
tionship (SAR) for relative effects of side chain structure
and substitutions on the quinoline ring, two different
amine side chains were attached to the four position of
the ring. One of the side chains was (N,N-diethyl)-1,4-
diaminopentane (X = 2) the side chain found in chloro-
quine, a series where there is substantial drug-resistance
among clinical isolates. The other side chain was (N,N-
diethyl)-1,3-diaminopropane (X = 1),
a
shortened
analog that is known to restore activity against drug-
resistant strains for the 7-chloroquinoline series.
This paper outlines a simple two-step method for the
synthesis of substituted quinoline rings and an evalua-
tion of how these substitutions effect the resistance pro-
file against a drug-sensitive and drug-resistant strain of
P. falciparum.
Both of these side chains were attached to the 4-chloro-
quinoline rings by nucleophilic substitution (Scheme 2).
The final products were obtained cleanly in 70–90%
yields after workup. Compounds with purity below
80% were purified by reversed-phase preparative HPLC
to give the final library 1{1–2,1–32} with an average
The first step in the sequence is the condensation of
an aniline with MeldrumÕs acid and trimethyl-ortho-
formate (Scheme 1). The MeldrumÕs acid is refluxed in
trimethyl-ortho-formate to form methoxymethylene
MeldrumÕs acid in situ. The aniline 2 is then added to
this solution where it enters into an addition–elimina-
tion reaction with the methoxymethylene moiety to af-
ford the ene–amine precursor for cyclization. Strongly
electron deficient anilines, such as nitro anilines, did
not quantitatively form the intermediate under the stan-
dard conditions. Adding an equal volume of DMF to
the reaction and increasing the reaction temperature
overcame this sluggish reactivity. The ene–amine inter-
mediates were then sealed into a glass reaction tube with
Scheme 2. Reagents and conditions: (i) X = 1: 3-diethylaminopropyl-
amine (neat), 135–155 °C, 2 h. or X = 2: 2-amino-5-diethylaminopen-
tane, 135–155 °C, 2 h.

P. B. Madrid et al. / Bioorg. Med. Chem. Lett. 15 (2005) 1015–1018
1017
Figure 3. Structures of quinoline rings synthesized for this study and chemset nomenclature for the resulting library. [aRegioisomers were inseparable
by reversed-phase HPLC. The 5-fluoro and 7-fluoro isomers were present in a ratio of 1:1.]
purity of 94%. Purity and identity was verified for all
samples by LC/MS; additionally, 1H NMR were ob-
tained for 10% of the library. Many of the reactions uti-
lizing side chain X = 2 failed due to substantial
impurities found in the commercially available amine
starting material.
After screening the activities of these compounds, the six
most active compounds were chosen for full dose–re-
sponse analysis. Along with this set, we also included
the 6-chloro-2-methoxyacridine ring system found in
quinacrine to compare a non-quinoline ring against
the set of quinolines (Table 1). The shorter side chain
variants consistently give higher potency than the
Compound library 1{1–2,1–32} (Fig. 3) was screened for
growth inhibition activity against P. falciparum using a
fluorescent-active cell sorting (FACS) assay (Fig. 4).13
We chose to screen at two concentrations (30 nM and
200 nM) against both a drug-sensitive strain, 3D7, and
a highly drug-resistant strain, W2. As expected, all com-
pounds were more active against 3D7 than W2 and com-
pounds with the shorter side chain were consistently
more potent, especially against the drug-resistant W2
strain. It should also be noted that the most active com-
pounds had substituents located at either the 6- or 7-po-
sition on the quinoline ring. In general, active
substitutions tended to be small electron withdrawing
groups with the notable exception of the 7-OPh com-
pound 1{1,19}, which proved to be one of the most ac-
tive compounds.
Table 1. Growth inhibitory activity, EC50 (nM)a, of selected com-
pounds against P. falciparum strains
Chemset number, substitution
3D7
W2
1{1,X} 1{2,X} 1{1,X} 1{2,X}
21, 7-Cl (CQ)
24, 7-CF3
19, 7-OPh
2
33
34
9
17
62
115
32
51
62
8
23
62
382
309
267
270
290
287
32
71
50
22, 7-Cl, 6-Me
18, 6-CF3
90
82
5
125
102
8
15, 6-OCF3
Quinacrineb
aValues are means of three experiments.
bAcridine.
Figure 4. P. falciparum growth inhibition results for quinoline ring substitution library at 30 nM concentration. Colorimetric scale represents the
percentage of growth of parasite relative to an untreated control population. White squares indicate a library member that could not be made or
purified. Data represents the average of three separate measurements with standard deviation <10% of value.

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P. B. Madrid et al. / Bioorg. Med. Chem. Lett. 15 (2005) 1015–1018
chloroquine side chain variants against both 3D7 and
W2. Interestingly, the pattern in EC50s for the propyl
side chain compounds on W2 mirrors that for 3D7 with
the EC50s being elevated by 1–2-fold, except in the case
of the compounds containing a 7-chloroquinoline sub-
structure. The 7-chloroquinoline and the 7-chloro-6-
methylquinoline rings had EC50s elevated by 11.5-fold
and 5.5-fold, respectively.
Acknowledgements
The Sandler Research Foundation and the NIH
(AI053862).
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.bmcl.
2004.12.037.
When comparing the activities of the compounds with
the chloroquine side chain against the two strains, it
can be seen that the EC50s increase, consistent with
the notion that drug resistance is primarily modulated
through the side chain identity. A feature worth noting
is that the only compound that is notably less potent
against W2 than the others is chloroquine (1{2,21}),
which contains the 7-chloroquinoline ring. This indi-
cates that perhaps the evolved resistance mechanism
does have some specificity for the 7-chloroquinoline ring
and therefore changes to the ring could be useful activity
modulators against drug-resistant parasite strains. An-
other interesting feature of these data is that quinacrine
retains its strong potency against the W2 strain despite
the fact that it contains the side chain of chloroquine.
The 7-OPh quinoline 1{1,19} also contains an addi-
tional third ring, but perhaps because of its flexible nat-
ure as opposed to the rigid three ring system of the
acridine, the drug-resistant parasite is able to survive
in the presence of this compound.
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In conclusion, we have used a simple two-step method
to synthesize quinoline rings with diverse substitutions
at the C-5, C-6, C-7, and C-8 positions. Modifications
to the substituents around the ring led to several new ac-
tive antimalarials, but their activity against the drug-
resistant W2 strain of P. falciparum was weak when they
contained the chloroquine side chain. This further sup-
ports the belief that the length and nature of the basic
side chain is the primary modulator of activity against
drug-resistant parasite strains. Despite this fact, modifi-
cations to the quinoline ring do somewhat improve the
activity of this series of compounds against the W2 par-
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with an acridine ring system has a profound affect on
the activity of these compounds against drug-resistant
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