Synthesis of novel triazole-linked mefloquine derivatives: Biological evaluation against Plasmodium falciparum

Article abstract of DOI:10.1016/j.bmcl.2014.10.015

Using 2,8-bis(trifluoromethyl)quinoline, the pharmacophore of mefloquine, as scaffold, eleven novel triazole-linked compounds have been synthesised by the application of CuAAC chemistry. The in vitro biological activity of the compounds on the Plasmodium falciparum chloroquine-sensitive strain NF54 was then determined. The compounds all showed IC₅₀s in the lower μM range with (1R,3S,5R)-N-{[1-(2,8-bis(trifluoromethyl)quinoline-4-yl)-1H-1,2,3-triazol-4-yl]methyl}adamantan-2-amine (29) exhibiting the best activity of 1.00 μM.

Full text of DOI:10.1016/j.bmcl.2014.10.015

Accepted Manuscript
Synthesis of novel triazole-linked mefloquine derivatives: Biological evaluation
against Plasmodium falciparum
Anton R. Hamann, Carmen de Kock, Peter J. Smith, Willem A.L. van Otterlo,
Margaret A.L. Blackie
PII:
S0960-894X(14)01060-9
DOI:
http://dx.doi.org/10.1016/j.bmcl.2014.10.015
BMCL 22070
Reference:
Bioorganic & Medicinal Chemistry Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
2 September 2014
30 September 2014
2 October 2014
Please cite this article as: Hamann, A.R., de Kock, C., Smith, P.J., van Otterlo, W.A.L., Blackie, M.A.L., Synthesis
of novel triazole-linked mefloquine derivatives: Biological evaluation against Plasmodium falciparum, Bioorganic
& Medicinal Chemistry Letters (2014), doi: http://dx.doi.org/10.1016/j.bmcl.2014.10.015
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com
Synthesis of novel triazole-linked mefloquine derivatives: Biological evaluation
against Plasmodium falciparum
Anton R. Hamann,a Carmen de Kock,b Peter J. Smith,b Willem A.L. van Otterloa and Margaret A.L.
Blackiea,*
aDepartment of Chemistry and Polymer Science, University of Stellenbosch, Private Bag X1, Matieland, 7602, South Africa
bDepartment of Pharmacology, University of Cape Town, Groote Schuur Hospital, South Africa
ARTICLE INFO
ABSTRACT
Article history:
Received
Revised
Using 2,8-bis(trifluoromethyl)quinoline, the pharmacophore of mefloquine, as scaffold, eleven
novel triazole-linked compounds have been synthesised by the application of CuAAC chemistry.
The in vitro biological activity of the compounds on the Plasmodium falciparum chloroquine-
sensitive strain NF54 was then determined. The compounds all showed IC50s in the lower μM
Accepted
Available online
range
with
(1R,3S,5R)-N-{[1-(2,8-bis(trifluoromethyl)quinoline-4-yl)-1H-1,2,3-triazol-4-
yl]methyl}adamantan-2-amine (29) exhibiting the best activity of 1.00 μM.
Keywords:
2,8-Bis(trifluoromethyl)quinoline
1,2,3-Triazole
2009 Elsevier Ltd. All rights reserved.
P. falciparum
CuAAC chemistry
Since the 1940s, the development of synthetic antimalarial
drugs has played a vital role in the control and treatment of
malaria. However, the widespread use of antimalarials has caused
the spread of resistant Plasmodium falciparum parasites which
has resulted in an increased malaria morbidity and mortality.1 To
highlight the extent of the problem, between 200 to 500 million
malaria related cases occur annually, with an estimated 1.7 to 3
million deaths the vast majority of whom are children.2 For
decades, the general treatment of malaria depended on
chloroquine (1), a 4-aminoquinoline analogue that is known for
its rapid efficacy, ready availability, low toxicity and
affordability.3 After the discovery of chloroquine, a number of
quinoline derivatives such as amodiaquine (2), primaquine (3)
and mefloquine (4) emerged (Fig.1).
The understanding of the mode of action of quinoline-based
antimalarials has advanced in the recent years, but remains
incomplete. The quinolines are known to inhibit heme
aggregation and prevent the formation of hemozoin. This leads to
intraparasitic accumulation of free heme that is toxic to the
parasite.4
Mefloquine has a high inhibitory activity against chloroquine-
resistant P. falciparum and is currently one of the drugs used to
treat chloroquine-resistant malaria. Unfortunately, the use of
mefloquine is usually associated with neuropsychiatric side
effects.5 The development of novel bis(trifluoromethyl)quinoline
analogues may thus provide a fruitful avenue for further
exploration.6
Figure 1: Structures of the early antimalarials
One of the attractive options to use in medicinal chemistry is the
use of 1,2,3-triazole functional group as a structural feature.7-10
This functional group can be incorporated by means of well-
defined copper-catalyzed azide-alkyne cyclization (CuAAC)
12
methods,11,
and facilitates the introduction of structural

diversity into the target molecules with relative ease. Triazoles
are, in general, stable under acidic and basic environments, as
well as under oxidative and reductive conditions.13 This
The synthesis of the piperazyl-alkyne series, shown in Scheme
2, started with the monoalkylation21 of piperazine (7) with
propargyl bromide to form the crude compound 8. Compound 8
was the starting material for a range of different reactions to form
the series of substituted piperazyl-alkyne components (9 - 14).
The six alkyne analogues (9 - 14) are tabulated in Table 1, along
with the reaction conditions used to form the corresponding
products.
heterocycle, with a moderate dipole moment can also form
14
hydrogen bonds with biomolecular targets.8,
Thus, triazoles
provide a moiety which is useful to employ from the perspective
of synthetic strategy, and can also be advantageous biologically.
In continuation of our pursuit of the synthesis of novel
biologically potent functional entities to combat malaria,7, 15-17 we
present herein the synthesis and evaluation of novel triazole-
linked
2,8-bis(trifluoromethyl)quinoline
derivatives.
We
envisaged that combining the mefloquine pharmacophore with
the triazole moiety18 could result in the development of potent
antimalarials. Encouraged by the results that Chibale and co-
workers9 obtained for the triazole moiety attached directly on the
chloroquine pharmacophore, we attempted to follow a similar
route and develop a series of novel compounds with the
mefloquine pharmacophore containing the triazole moiety (Fig.
2). In general, antimalarial drugs which target the inhibition of
hemazoin formation require the ability to accumulate within the
digestive vacuole of the parasite via a weak base trapping
mechanism that allows the drug to concentrate inside the
vacuole.19 Therefore, we aimed to insert moieties such as
piperazine and other amines into the side chains to provide the
necessary basic properties. In addition to standard alkyl and aryl
derivatives, we also chose to include a ferrocenyl moiety. The
inclusion of ferrocene in antimalarial compounds is known,
ferroquine being the most notable example.20 We have found
previously that there was no significant difference in activity
between phenyl and ferrocenyl derivatives.16
Scheme 2: Synthesis of the piperazyl-alkynes. Reagents and
conditions: (a) Propargyl bromide, DCM, N2, 0 °C - rt, overnight,
74 % (crude).
With the crude 8 in hand, method 1, involved the coupling
reaction of benzyl bromide and 4-(bromomethyl)pyridine, to
offer the corresponding alkynes (9, 10) in moderate yield. The
reactions were slightly adapted from the literature23 by carrying
out the reactions under reflux, instead of room temperature, so as
to speed up the reactions. Method 2 was a reductive amination24
of the crude
8 with cyclohexanone and adamantanone
respectively, using NaBH(OAc)3 as the reducing agent. Again the
corresponding alkynes (11, 12) were isolated in moderate yield.
A different reductive amination (method 3) was used in the
reaction with ferrocene carboxaldehyde to incorporate
a
metallocene. The reaction was initially carried out using the
procedure of method 2 but failed to form any product and starting
materials were recovered. However, by changing the reducing
agent to NaBH3CN,19 the aldehyde was fully consumed, and the
product (13) was isolated in moderate yield. Lastly, method 4
involved the Mannich reaction using a procedure described by
Atalay,25 with slight adaptation regarding the solvent used for this
reaction. The substituted piperazine 8 was poorly soluble in a
mixture of ethanol and water; therefore, the crude 8 was allowed
to react with formaldehyde and indole in a mixture of EtOH:i-
PrOH to facilitate the solubility of 8, and the corresponding
alkyne 14 was formed in good yield in this manner.
Figure 2: Structures of the novel compounds based on the
mefloquine pharmacophore
The reaction sequence employed for synthesis of the scaffold, 4-
azido-2,8-bis(trifluoromethyl)quinoline (8) is shown in Scheme
1.
Table 1: The reactions with crude compound 8 to form the
corresponding alkyne analogues (9 - 14).
Entry
Alkyne analogue
Yield
Method 1
55 %
DIPEA, CHCl3, reflux, 24 h,
Scheme 1: Synthesis of the scaffold 6. Reagents and conditions:
(a) NaN3, DMF, 50 °C, 2 h, 90 %.
The synthesis of chloroquinoline derivative 5 was done using
the procedure as described by Thomas and co-workers,21 which
then underwent a nucleophilic substitution with sodium azide9 to
afford the scaffold 6 in good yield.
72 %

products. In this regard, CuI was used as the catalyst to provide
the necessary CuI source. The scaffold 8 was used in slight
excess (1.1 eq.) to ensure that all of the alkyne was consumed, as
many of the final products had very similar polarities to the
alkyne starting material. It was determined that using the catalyst
Method 2
NaBH(OAc)3, CH3CO2H,
DCM, rt, 24 h,
49 %
50 %
in catalytic amounts ( 0.1 eq.) resulted in the reaction being

sluggish. Increasing the amount to 0.25 eq. ensured the
completion of the reaction within 24 hours. In addition, most of
the products were isolated in moderate to good yields (Table 2).
The adamantyl and diisopropyl amines were the notable
exceptions, giving rather poor yields. However, sufficient
material was obtained for biological testing, so the reactions were
not further optimised.
Method 3
NaBH3CN, CH3CO2H, i-
PrOH, rt, overnight,
50 %
Method 4
indole, formaldehyde,
CH3CO2H, EtOH:i-PrOH, rt,
overnight
65 %
Scheme 4: Synthesis of the triazoles 20-30 from scaffold 6 and
the corresponding alkynes 9-19. Reagents and conditions: (a) 1.2
eq. DIPEA, 0.25 eq. CuI, CH3CN, rt, 24 h, 36-76 % (See Table 2
for structures involved and yields for the CuAAC reaction).
The synthesis of shorter chain, bulky alkynes was achieved by
the alkylation of five different amines with propargyl bromide
shown in Scheme 3. The alkylation of three secondary amines
with propargyl bromide was carried out at -78 °C using NaH in
THF, using an adaptation of the procedure described by Li and
co-workers,26 followed by stirring overnight at room temperature.
The eleven synthesised triazoles were subjected to in vitro
testing using the modified method of Trager30 and Makler31 on
the sensitive P. falciparum NF54 strain. A full dose-response was
performed on all compounds to determine the concentration
inhibiting 50 % of parasite growth (IC50) and the values are
shown in Table 2.
For the primary amines, the reactions were carried out at room
temperature using two similar procedures.27,
The alkyne
28
products (15 - 19) were all isolated in good yields.
Table 2: Products obtained from the CuAAC click reactions of
scaffold 6 and the 11 alkynes (20-30) using the procedure in
Scheme 4 and their activity against the sensitive strain of P.
falciparum NF54.
Yields
for
IC50
IC50
(NF54)
(µM)
#
R
(NF54)
(µg/mL)
CuAAC
20
76 %
1.55±0.14
2.46
21
22
64 %
74 %
13.5±1.43
5.52±1.40
25.9
10.6
Scheme 3: Synthesis of the alkynes 15 - 19. Reagents and
conditions: (a) Propargyl bromide, NaH, THF, -78 °C - rt,
overnight, (15) 70 %, (16) 65 %, (17) 94 %; (b) Propargyl
bromide, THF:DCM, N2, rt, 3 d, 82 %; (c) propargyl bromide,
DCM, N2, rt, 24 h, 78 %
23
24
47 %
63 %
0.708±0.11
16.3±6.32
1.30
31.8
With the alkynes in hand, the next step was the click reaction29
with scaffold 8 (Scheme 4) to form the corresponding triazole

Medical Research Council (CdK, PS). E. Malherbe is
acknowledged for her work with the NMR spectroscopy.
25
36 %
7.96±2.50
14.1
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Graphical Abstract
Novel triazole-linked mefloquine analogues:
Synthesis and biological evaluation against
Plasmodium falciparum
Leave this area blank for abstract info.
Anton R. Hamann, Carmen de Kock, Peter J. Smith, Willem A.L. van Otterlo, Margaret A.L. Blackie*