Mallotojaponins B and C: Total Synthesis, Antiparasitic Evaluation, and Preliminary SAR Studies

Article abstract of DOI:10.1021/acs.orglett.5b03676

(Chemical Equation Presented). The first total syntheses of mallotojaponin B and C as well as several analogues have been achieved. Biological evaluation of the synthesized compounds against Plasmodium falciparum and Trypanosoma brucei have also been carried out.

Full text of DOI:10.1021/acs.orglett.5b03676

Letter
pubs.acs.org/OrgLett
Mallotojaponins B and C: Total Synthesis, Antiparasitic Evaluation,
and Preliminary SAR Studies
Tatyana D. Grayfer,† Philippe Grellier,‡ Elisabeth Mouray,‡ Robert H. Dodd,† Joelle Dubois,*,†
̈
and Kevin Cariou*,†
†Institut de Chimie des Substances Naturelles, CNRS UPR 2301, Universite
Gif-sur-Yvette, France
́
Paris-Sud, Univ. Paris-Saclay, 1, av. de la Terrasse, 91198
‡UMR 7245 CNRS, Dep
́
́
artement RDDM, Museum National d’Histoire Naturelle CP52, 57 Rue Cuvier, 75005 Paris, France
S
* Supporting Information
ABSTRACT: The first total syntheses of mallotojaponin B
and C as well as several analogues have been achieved.
Biological evaluation of the synthesized compounds against
Plasmodium falciparum and Trypanosoma brucei have also been
carried out.
further investigation. Following an ongoing interest in the
alaria is a tropical and subtropical disease caused by
Mparasites of the Plasmodium genus carried by Anopheles development of novel antiparasitic compounds,3 a program was
initiated aimed at the synthesis of these two compounds and at
the further evaluation of their biological activities. In particular,
based on the difference of activity between mallotojaponins B
and C, assessing the influence of the number of prenyl units
was considered. Since the target compounds are dimeric, two
strategies could be envisioned: functionalization followed by
dimerization or vice versa. Based on our previous experience
with the dimerization of phloroglucinol derivatives,4 the latter
route was initially deemed more convergent, especially toward
homodimeric adducts. Thus, phloroacetophenone 3 was first
treated with trimethylsilyldiazomethane5 leading predominantly
to the desired para-O-methylated product 4a (77% yield based
on recovered starting material−brsm), along with minor
amounts of o-O- and O,O′-dimethylated compounds 4b and
4c, respectively (Scheme 1).
mosquitoes, among which the most lethal form is caused by
Plasmodium falciparum. Global estimations show that between
300 and 600 million people are infected and that about a
million die every year from this infection. The discovery of new
drugs for fighting this epidemic is essential, since resistances
have emerged to most of the currently employed treatments.
Natural products have constituted a seemingly endless source
of inspiration for the development of drugs, as showcased by
artemisinin, which is now the World Health Organization’s
recommended first-line treatment against malaria and for which
discovery the Nobel Prize of Medicine was awarded in 2015.
However, resistance to artemisinin has recently been reported
and is spreading,1 thus increasing the urgent need for
developing alternative treatments. Mallotojaponins B (1) and
C (2) are phloroglucinol dimers that have recently been
isolated from a Madagascar euphorbiacea, Mallotus oppositifo-
lius, that display potent antiplasmodial activities (Figure 1).2
In particular, mallotojaponin C displays both cytocidal and
gametocidal activities (IC50 = 0.14 μM and 3.6 μM,
respectively) on P. falciparum, making it a good candidate for
The first set of mallotojaponin analogues was obtained by
subjecting all three acetophenones 4a−c to methylenation
conditions using MOMCl (generated in situ from dimethoxy-
methane) followed by heating with hydrochloric acid (Scheme
2).4 The corresponding bridged dimers 5a−c were all obtained
Scheme 1. Methylation of Phloroacetophenone 3
Figure 1. Mallotojaponins B and C: structure and activities against P.
falciparum Dd2 (chloroquine/mefloquine-resistant strain).
Received: December 27, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b03676
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
considered, as it was briefly reported by Minassi and
Appendino to be a suitable promoter for this kind of
transformation in the synthesis of arzanol.16 Reacting
prenylated monomer 6 with 0.5 equiv of the methylene salt
allowed the synthesis of mallotojaponin C (2) in 35% yield
(Scheme 4). In order to improve the efficiency of this final step,
Scheme 2. Methylenation of Acetophenones 4a-c
Scheme 4. Synthesis of Mallotojaponin C
with excellent yields; however, further elaboration of these
substrates to mallotojaponins, especially C-prenylation, proved
extremely laborious.6
At this stage, the strategy was reconsidered, and the
functionalized monomers were targeted first before carrying
out the dimerization, keeping in mind that the ideal protocol
should grant access to both homo- and heterodimeric
compounds. Although well documented,7 selective C-prenyla-
tion of compound 4a was found to be the major hurdle to
overcome in this endeavor. Many multistep strategies such as
selective TBS-monoprotection/O-prenylation8 or selective O-
prenylation followed by Claisen rearrangement and Pd-
catalyzed O-3,3-dimethylallylation/sigmatropy9 were evaluated
to little or no avail. Finally, direct C-prenylation with prenyl
the reagent-to-starting material ratio was increased. Interest-
ingly, when 3 equiv of reagents was used, exclusive formation of
the dimethylammonium salt took place, giving 9 in 95% yield.
This platform could in turn be reacted with 1 equiv of the
starting monomer 6 to give the desired natural product in
quantitative yield.17
bromide,10 in the presence of Hunig’s base in dichloromethane,
̈
offered the best compromise (Scheme 3).11 Indeed, a
Scheme 3. Prenylation, Geranylation, and Methylation of
Acetophenone 4a
This encouraging result prompted us to synthesize the two
remaining dimethylammonium salts 10 and 11 from the
corresponding geranylated and methylated phenols 7 and 8,
respectively. This was achieved by reacting each monomer with
the Eschenmoser’s salt in chloroform at room temperature
(Scheme 5).
Scheme 5. Synthesis of Dimethylammonium Salts 10 and 11
reasonable 53% (brsm) yield of prenylated phenol 6 could be
attained, and these conditions also enabled the isolation of the
corresponding geranyl adduct 7 (51% brsm). The synthesis of
methylated monomer 8 has previously been reported by a
three-step sequence starting with 2,4,6-trihydroxybenzalde-
hyde.12 Nevertheless, direct access from readily available 4a
was thought to be more practical. This was achieved by a two-
step sequence involving formylation with dichloro(methoxy)-
methane13 followed by reduction using sodium cyanoborohy-
dride under acidic conditions.14
With these three functionalized monomers in hand, the
dimerization reaction was attempted, which was unsuccessful
using the MOMCl protocol.15 After various formaldehyde
equivalents were screened, Eschenmoser’s salt was eventually
With both the phenols and the ammonium salts now
available an efficient access to either homo- or heterodimers
was within reach. First, digeranyl and dimethyl adducts 12 and
13 could indeed be obtained with excellent yields by reacting
together the similarly substituted phenols and salts (Table 1,
entries 1 and 2). The latter compound 13 is actually
mallotophenone, another naturally occurring phloroglucinol
dimer, that was also isolated from Mallotus species.2,18
Heterodimers could in turn be obtained with good yields
(Table 1, entries 3−5) by reacting together phenols and salts
bearing different substituents such as geranyl/prenyl 14,
B
DOI: 10.1021/acs.orglett.5b03676
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Organic Letters
Letter
a
4−6) display at best modest activity against P. falciparum. Only
5a, which structurally most resembles 13, displays an activity
below 50 μM (Table 2, entry 4). Micromolar activities were
restored with geranyl-substituted dimers 12, 14, and 15 (Table
2, entries 7−9). The most active compound of this latter series
is monogeranylated 15 (IC50 = 1.0 μM) (Table 2, entry 9). In
order to further probe the influence of the prenyl units,
diisopentyl dimer 16 and diallyl dimer 17 were also synthesized
and evaluated.19 Both analogues were found to be more active
(Table 2, entries 10 and 11) than mallotojaponin B (1), yet less
so than mallotojaponin C (2), thus demonstrating the
activating influence of the prenyl units. Finally, several
monomers and salts were also tested for their anti-plasmodium
activity (Table 2, entries 12−16). All monomers except
methylated phenol 8 exhibited some inhibitory potency, with
the most active of these (7, IC50 = 4.4 μM) bearing a geranyl
unit (Table 2, entry 13). Both dimethylammonium salts 9 and
10 were found to be active (Table 2, entries 15 and 16),
although this could be mostly due to their intrinsic reactive
nature. The scope of this study was further extended by
screening all compounds against the bloodstream forms of T.
brucei, the parasite responsible for trypanosomiasis in humans
and cattle (Table 2, column 4). Interestingly, an activity profile
that almost mirrors that found against P. falciparum could be
evidenced with the three most active compounds (Table 2,
entries 2, 9, and 10) also being mallotojaponin C (2), methyl/
geranyl dimer 15, and saturated dimer 16 (IC50 = 0.45, 0.53,
and 0.77 μM, respectively). Finally, considering the apparent
importance of the prenyl units for activity, the substrates were
also tested as potential inhibitors of T. brucei protein farnesyl
transferase (Table 2, column 5). A moderate correlation could
thus be observed as mallotojaponin C (2) and methyl/geranyl
dimer 15 were found to be good inhibitors of the enzyme in the
micromolar range (IC50 = 2.0 and 5.2 μM, respectively).
However, in this regard, saturated analogue 16 still appears as a
relatively potent compound (IC50 = 4.4 μM) (Table 2, entry
10).
Table 1. Synthesis of Dimers 12−15 and 1
entry
R
R′
time (min)
dimer, yield (%)
1
2
3
4
5
7, geranyl
8,methyl
7, geranyl
8, methyl
8, methyl
10, geranyl
11, methyl
9, prenyl
90
50
75
60
45
12, 98
13, 94
14, 83
15, 68
1, 62
10, geranyl
9, prenyl
a
Isolated yields.
geranyl/methyl 15 and prenyl/methyl (i.e., mallotojaponin B,
1).
Following our initial objective, the antimalarial activity
(Table 2, column 3) of the various synthesized compounds
Table 2. Evaluation of Antiparasitic and Enzyme Inhibitory
a
Activities
b
c
d
entry
compd
P. falciparum
T. brucei
farnesyl transferase
1
1
3.4 0.2
1.0 1.0
>30
2
2
0.75 0.11
12.9 4.6
17.6 4.3
>50
0.45 0.01
7.9 0.4
10 1.1
>50
2.0 0.3
>30
3
13
5a
5b
5c
12
14
15
16
17
6
4
25.5 2.9
>30
5
6
>50
>50
>30
7
6.0 0.4
2.1 1.3
1.0 0.8
2.0 1.3
4.1 0.9
7.4 1.1
4.4 1.3
>50
3.7 0.2
1.1 0.04
0.53 0.03
0.77 0.08
2.0 0.2
27 0.8
8.5 0.2
>50
>30
8
14.4 2.4
5.2 0.5
4.4 0.7
22.8 6.9
>30
Overall, these synthetic efforts culminated in the first total
syntheses of three natural products that were recently isolated
from M. oppositifolius: mallotojaponins B and C as well as
mallotophenone.2 Biological evaluation of these compounds
and of various analogues confirmed their promising antimalarial
activity and uncovered an interesting trypanocidal activity.
Preliminary SAR studies showed the importance of the two
prenyl units and of the dimeric structure. Further synthetic and
biological studies are currently underway to explore the full
therapeutic potential of this family of compounds and will be
reported in due course.
9
10
11
12
13
14
15
16
7
>30
8
>30
9
3.5 0.5
3.7 0.1
4.1 0.1
8.4 0.6
>30
10
>30
a
b
IC50 (μM). Chloroquine-resistant P. falciparum FcB1/Colombia
strain; chloroquine was used as antimalarial drug control (IC50 = 0.072
0.0074 μM). T. brucei brucei strain 93; pentamidine was used as
antitrypanosomal drug control (IC50 = 0.011 0.0017 μM). T. brucei
brucei protein farnesyl transferase; under these conditions, the
commercially available inhibitor FTI-276 displayed an IC50 = 0.010
0.002 μM.
c
d
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.5b03676.
was first evaluated on the intraerythrocytic stage of P.
falciparum, starting with natural products 1, 2, and 13 (Table
2, entries 1−3). While the measured IC50’s are slightly higher
than the ones reported2 (on a different strain), the same order
of potency prevails, with diprenylated mallotojaponin C (Table
2, entry 2) being the most active, followed by monoprenylated
mallotojaponin B (Table 2, entry 1) and nonprenylated
mallotophenone (Table 2, entry 3). Unsubstituted dimers
with different methoxylation patterns 5a−c (Table 2, entries
Synthetic and biological protocols; copies of NMR
spectra (PDF)
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: joelle.dubois@cnrs.fr.
̈
*E-mail: kevin.cariou@cnrs.fr.
C
DOI: 10.1021/acs.orglett.5b03676
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(19) Diisopentyl compound 16 was obtained by hydrogenation on
Pd/C of mallotojaponin C, and diallyl compound 17 was prepared in
three steps from 4a; see the Supporting Information for details.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the CNRS for financial support and the Institut de
Chimie des Substances Naturelles for financial support and a
fellowship for T.D.G.
REFERENCES
■
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D
DOI: 10.1021/acs.orglett.5b03676
Org. Lett. XXXX, XXX, XXX−XXX