Synthesis of Thymoquinone-Artemisinin Hybrids: New Potent Antileukemia, Antiviral, and Antimalarial Agents

Article abstract of DOI:10.1021/acsmedchemlett.7b00412

A series of hybrid compounds based on the natural products artemisinin and thymoquinone was synthesized and investigated for their biological activity against the malaria parasite Plasmodium falciparum 3D7 strain, human cytomegalovirus (HCMV), and two leukemia cell lines (drug-sensitive CCRF-CEM and multidrug-resistant subline CEM/ADR5000). An unprecedented one-pot method of selective formation of C-10α-acetate 14 starting from a 1:1 mixture of C-10α- to C-10β-dihydroartemisinin was developed. The key step of this facile method is a mild decarboxylative activation of malonic acid mediated by DCC/DMAP. Ether-linked thymoquinone-artemisinin hybrids 6a/b stood out as the most active compounds in all categories, while showing no toxic side effects toward healthy human foreskin fibroblasts and thus being selective. They exhibited EC₅₀ values of 0.2 μM against the doxorubicin-sensitive as well as the multidrug-resistant leukemia cells and therefore can be regarded as superior to doxorubicin. Moreover, they showed to be five times more active than the standard drug ganciclovir and nearly eight times more active than artesunic acid against HCMV. In addition, hybrids 6a/b possessed excellent antimalarial activity (EC₅₀ = 5.9/3.7 nM), which was better than that of artesunic acid (EC₅₀ = 8.2 nM) and chloroquine (EC₅₀ = 9.8 nM). Overall, most of the presented thymoquinone-artemisinin-based hybrids exhibit an excellent and broad variety of biological activities (anticancer, antimalarial, and antiviral) combined with a low toxicity/high selectivity profile.

Full text of DOI:10.1021/acsmedchemlett.7b00412

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Letter
Synthesis of Thymoquinone-Artemisinin Hybrids: New
Potent Antileukemia, Antiviral and Antimalarial Agents
Tony Fröhlich, Christoph Reiter, Mohamed E.M. Saeed, Corina Hutterer, Friedrich Hahn, Maria Leidenberger,
Oliver Friedrich, Barbara Kappes, Manfred Marschall, Thomas Efferth, and Svetlana B. Tsogoeva
ACS Med. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acsmedchemlett.7b00412 • Publication Date (Web): 21 Dec 2017
Downloaded from http://pubs.acs.org on December 22, 2017
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Synthesis of Thymoquinone-Artemisinin Hybrids: New Potent
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Tony Fröhlich,1 Christoph Reiter,1 Mohamed E. M. Saeed,2 Corina Hutterer,3 Friedrich Hahn,3 Maria Leidenberger,4 Oliver
Friedrich,4 Barbara Kappes,4 Manfred Marschall,3 Thomas Efferth2 and Svetlana B. Tsogoeva1*
1 Organic Chemistry Chair I and Interdisciplinary Center for Molecular Materials (ICMM), Friedrich-Alexander University of
Erlangen-Nürnberg, Henkestraße 42, 91054 Erlangen, Germany. 2 Department of Pharmaceutical Biology, Institute of Pharmacy
and Biochemistry, University of Mainz, Staudinger Weg 5, 55128 Mainz, Germany. 3 Institute for Clinical and Molecular Virology,
Friedrich-Alexander University of Erlangen-Nürnberg, Schlossgarten 4, 91054 Erlangen, Germany. 4 Institute of Medical
Biotechnology, Friedrich-Alexander University of Erlangen-Nürnberg, Paul-Gordon-Straße 3, 91052 Erlangen, Germany.
KEYWORDS Artemisinin, thymoquinone, natural product hybrid, antimalarial activity, antiviral activity, anticancer activity
ABSTRACT: A series of hybrid compounds based on the natural products artemisinin and thymoquinone was synthesized and
investigated for their biological activity against the malaria parasite Plasmodium falciparum 3D7 strain, human cytomegalovirus
(HCMV) and two leukemia cell lines (drug-sensitive CCRF-CEM and multidrug-resistant sub-line CEM/ADR5000). An
unprecedented one-pot method of selective formation of C-10α-acetate 14 starting from a 1:1 mixture of C-10α- to C-10β-
dihydroartemisinin (DHA), was developed. The key step of this facile method is a mild decarboxylative activation of malonic acid
mediated by DCC/DMAP. Ether-linked thymoquinone-artemisinin hybrids 6a/b stood out as the most active compounds in all
categories, while showing no toxic side effects towards healthy HFFs and thus being selective. They exhibited EC50 values of 0.2 µM
against the doxorubicin-sensitive as well as the multidrug-resistant leukemia cells and therefore can be regarded as superior to
doxorubicin. Moreover, they showed to be 5 times more active than the standard drug ganciclovir and nearly 8 times more active than
artesunic acid against HCMV. In addition, hybrids 6a/b possessed excellent antimalarial activity (EC50 =5.9/3.7 nM), which was
better than that of artesunic acid (EC50 = 8.2 nM) and chloroquine (EC50 = 9.8 nM). Overall, most of the presented thymoquinone-
artemisinin based hybrids exhibit an excellent and broad variety of biological activities (anticancer, antimalarial, antiviral) combined
with a low toxicity/high selectivity profile.
1. Introduction
Over the last decade, the hybridization of natural products
turned out to be a highly fruitful strategy for medicinal
chemistry and drug design.1-3 Not only does it keep synthetic
effort at a minimum and thus is relatively time and cost saving,
but it also can improve upon pharmacological properties of the
parent compounds such as increased biological efficacy,
reduced undesired side effects, a modified selectivity profile
(lower toxicity), better bioavailability and addition of
Figure 1. Structures of artemisinin (1), artesunic acid (2) and
thymoquinone (3).
completely new biological features that were absent in the
parent compounds.1,3-5 Artesmisinin (1), an enantiomerically
For that reason and also motivated by our previous studies
pure sesquiterpene containing a 1,2,4-trioxane ring, which was
applying the hybridization concept using artemisinin
extracted from the Chinese medicinal plant Artemisia annua L.
derivatives,13-15 we recently synthesized and published 7 novel
in 1972 by Youyou Tu (Nobel Prize 2015),6 and
thymoquinone-artemisinin hybrids 4-6 (Figure 2), which were
thymoquinone (3) (Figure 1),
a
naturally occurring
investigated for their anticancer potential and indeed some of
those compounds were highly active and selective towards
colon cancer outperforming their parent compounds.16
Following this study, we herein present the synthesis of 5
additional thymoquinone-artemisinin hybrids 7-10 (Figure 2)
and investigate for the first time all 12 compounds against
leukemia, malaria and HCMV.
phytochemical compound, which is the main constituent of the
volatile oil of Nigella sativa (black seed) and was first isolated
in 1963 by El-Dakhakhany,7 both exhibit great pharmacological
potential as anticancer agents.8,9 In addition, artemisinin also
possesses great antimalarial and antiviral properties just like its
semi-synthetic derivative artesunic acid (2) (Figure 1).10-12
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Figure 2. Hybrids 4-10 applied for biological tests against CCRF-CEM, CEM/ADR5000 cells, HCMV and P. falciparum 3D7
2. Results and Discussion
2.1 Chemistry
1:1 mixture of 10α- to 10β-isomer is formed.13 This is
remarkable, because DHA itself was applied in form of a 1:1
mixture of 10α- to 10β-DHA. This kind of chiral enrichment
under mild conditions might render itself as highly useful for
synthesis of different enantiomerically pure dihydroartemisinin
derivatives, as the reaction should also work in the same way
with 2,2-disubstituted malonic acids.
The synthesis of hybrids 4, 5 and 6 was already described in
our previous study (where the compounds were studied against
colon cancer)16 and the synthetic results are summarized in
Scheme 1. Notably, during the course of the current study we
found an unprecedented method of selective formation of
C-10α-acetate 14 (precursor for the synthesis of ether-linked
hybrids 6a/b) starting from a 1:1 mixture of C-10α- to C-10β-
dihydroartemisinin (DHA). Instead of using a standard
acetylation procedure with acetic anhydride, pyridine and
DMAP,17 we performed the reaction of DHA with malonic acid,
DCC and DMAP and obtained the desired product 14 in 85%
yield (Scheme 1). The key step of this novel method is a mild
decarboxylative activation of malonic acid (Scheme 2).
Notably, decarboxylative activation of malonic acid derivatives
usually proceed under harsh thermal conditions18,19 or by using
copper-catalyzed19 and enzyme-catalyzed20 methods. Routes
towards decarboxylative activation of malonic acid under mild
conditions are still few: one known method uses
N,N’-carbonyldiimidazole (CDI)21 and the second one applies
1-(methylsulfonyl)-1H-benzo[d][1,2,3]triazole in combination
with Et3N.22 The mechanism of the developed reaction might be
similar to the one proposed in literature for the decarboxylative
acylation of O-, N-nucleophiles using 2,2-disubstituted malonic
acids and 1-(methylsulfonyl)-1H-benzo[d][1,2,3]triazole.22 The
authors suggested that upon reaction of malonic acid with
benzotriazole derivative in the presence of Et3N an unstable
intermediate forms, which after release of CO2 can be attacked
The preparation of artesunic acid based hybrids 7a/b, in
which TQ acts as a linker, was achieved in 72%/65% yield by
performing a Steglich esterification reaction between two
molecules of artesunic acid and one molecule of TQ diol 13
(Scheme 1). DCC and DMAP were utilized as coupling agents.
The diol 13 was synthesized by applying the same method as
used for the monohydric alcohols of TQ 11 (Scheme 1): TQ and
different sodium hydroxyl carboxylate salts 18 (which were
obtained in quantitative yield by simple ester hydrolysis of
commercially available lactones 17) were treated with
(NH4)2S2O8 and catalytic amounts of AgNO3 in a mixture of
water and acetonitrile. This procedure was already reported in
literature in the context of synthesis of other TQ derivatives.23
In order to get the diol 13 in reasonable amounts, at least 2.0
equivalents of the carboxylate salts have to be applied. To be
able to compare the effect of the placement of two artemisinin
moieties around the TQ subunit on biological activity, we also
synthesized the artemisinin-derived dimer-TQ hybrid 8, where
both artemisinin subunits are connected to only one side of TQ.
Again Steglich esterification using TQ alcohol 11b and
artemisinin-derived dimer carboxylic acid 15 applying DCC
and DMAP as reagents afforded the desired product in 73%
yield. Artemisinin-derived dimer 15 was prepared according to
a published protocol and was chosen as starting material,24
because its core-structure was found to be highly beneficial for
antimalarial and anticancer activity leading to the synthesis and
extensive studies of a series of similar compounds, some of
which turned out to belong to the most active compounds
among artemisinin-derived dimers presented so far in
literature.25 This is supported by the positive results we were
able to obtain from our own research, when applying a similar
artemisinin-derived dimer for the synthesis of trimers, which
showed excellent anticytomegaloviral and antileukemia
efficacies.13
by
a
corresponding nucleophile.22 In our case, the
monocarbonyl activation of malonic acid is mediated by
DCC/DMAP, while DHA represents the nucleophile and can
react with reactive amide formed after the decarboxylation step
(Scheme 2). The formation of the 10α-isomer is favored, most
probably because the C-9 methyl group acts as steric hindrance,
which makes a reaction of 10β-DHA with activated malonic
acid less possible. The high yield (85%) for C-10α-acetate 14
can be explained by an C-10β  C-10α interconversion,
presumably through an aldehyde-alcohol intermediate in
solution (Scheme 2). The advantage of our novel
decarboxylative acetylation is that solely the C-10α-isomer is
generated, whereas with the literature known method, always a
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In order to investigate the radical scavenging effect of
fullerene on thymoquinone-artemisinin hybrids,
procedure, was obtained in 30% yield by reaction of malonyl
1
2
3
4
5
6
a
chloride, TQ alcohol 11a and artesunic acid derived alcohol 16,
which in turn was prepared by Steglich esterification of
artesunic acid and ethylene glycol in 91% yield. The
malonylation reaction was promoted by pyridine as base and
was conducted in CH2Cl2 as solvent.
Bingel-Hirsch reaction was performed between TQ-artesunic
acid hybrid 9, which contains a malonyl moiety, and fullerene
(C60), resulting in the desired product 10 with a yield of 30%
after 3 days. DBU was applied as base, CBr4 as reagent and
toluene as solvent. The starting compound 9, necessary for this
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Scheme 1. Synthesis route for hybrids 4-10 and thymoquinone precursors 11 and 13
Scheme 2. Proposed mechanism for the one-pot acetylation of dihydroartemisinin (DHA) via monocarbonyl activation of
malonic acid with DCC/DMAP followed by decarboxylation and nucleophilic substitution with DMAP as a leaving group.
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2.2 Biological activity of the hybrids
2.2.1 Cytotoxicity towards sensitive CCRF-CEM and
multidrug-resistant CEM/ADR500 leukemia cells
already shows some loss of activity due to multidrug resistance
(EC50(CCRF-CEM) = 0.009 µM; EC50(CEM/ADR5000) =
23.27 µM). Strikingly, dimer 7b, where one additional CH2
group is introduced on each side of the thymoquinone linker
subunit, is approximately 30 times less active than dimer 7a.
This result indicates that linker length plays a crucial role for
the antileukemia activity of artemisinin dimers, where
thymoquinone is applied as a spacer molecule. This effect was
not observed for the compounds containing only one
artemisinin subunit. No matter how long the spacer between the
two pharmacophores in hybrids 4 or 6 is, the EC50 values stay
almost the same. Interestingly, in our previously published
results, the linker length also was important for the anticancer
activity towards colon cancer cells of thymoquinone-
artemisinin hybrids with only one artemisinin moiety.16 Thus,
the influence of linker length seems to be specific to certain cell
lines or cancer types, which in turn may suggest that for each
cancer cell line different mechanisms or targets may be
involved. As expected, hybrid 10 containing a fullerene moiety
was not active at all, unlike its precursor 9, which clearly
showed a relatively strong antileukemia effect. This can be seen
as another proof that the biological activity of the artemisinin
moiety is mediated via radicals, as fullerene is well-known for
its radical scavenging and antioxidant properties.26,27
1
2
3
4
5
6
7
8
The cytotoxic potential against wild-type CCRF-CEM and
multidrug-resistant P-glycoprotein overexpressing CEM/
ADR5000 human leukemia cells was investigated for hybrids 4,
6-10 as well as their parent compounds artesunic
acid/artemisinin and thymoquinone (Table 1). Doxorubicin, a
clinically used drug, served as a fourth reference compound in
order to evaluate the effectiveness of the hybrids towards the
resistant cells. All hybrids except hybrid 10 show a moderate to
good antileukemic effect against both the doxorubicin-sensitive
and doxorubicin-resistant cell lines with EC50 values in the
micromolar to submicromolar range (EC50(CCRF-CEM) =
0.0027-6.071 µM; EC50(CEM/ADR5000) = 0.2-5.663 µM),
which in all cases is better compared to that of artemisinin
(EC50(CCRF-CEM) = 36.90 µM; EC50(CEM/ADR5000) =
26.90 µM). The best overall performance was observed for
ether-linked thymoquinone-artemisinin hybrids 6a/b and
artesunic acid based hybrid 7a, which showed the same high
efficacy towards the doxorubicin-resistant (EC50(CCRF-
CEM) = 0.2/0.3 µM) as against the doxorubicin-sensitive
(EC50(CEM/ADR5000) = 0.2 µM) leukemia cells and therefore
can be regarded as superior to doxorubicin, because this drug
9
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Table 1. EC50 values for hybrids 4-10 and selected reference compounds tested against sensitive wild-type CCRF-CEM as well as
multidrug resistant P-glycoprotein-overexpressing CEM/ADR5000 cells, HCMV and P. falciparum 3D7 parasites
Molecular
Weight (g/mol)
CCRF-CEM
EC50 (µM)
0.009[a]
CEM/ADR5000
EC50 (µM)
HCMV
EC50 (µM)
P. f. 3D7
EC50 (nM)
Compound
doxorubicin
579.98
579.98
319.87
282.14
284.35
384.42
164.20
588.69
602.72
574.67
530.66
544.69
502.65
488.62
1013.18
1041.24
925.17
718.79
1437.44
23.27[a]
-
-
ganciclovir
-
-
2.60 ± 0.5[b]
-
chloroquine
-
-
-
9.8 ± 2.8[d]
artemisinin (1)
36.90 ± 6.90[a]
26.90 ± 4.40[a]
> 10[b]
-
dihydroartemisinin
-
-
> 10[b]
5.41 ± 0.61[b]
2.4 ± 0.4[d]
8.2 ± 1.6
> 80 000
6.9 ± 2.5
7.7 ± 1.5
8.2 ± 1.2
54 ± 2.2
17.5 ± 1.4
5.9 ± 0.8
3.7 ± 0.8
4.3 ± 1.5
12.1 ± 0.8
12.2 ± 1.9
4.4 ± 1.4
≥ 341[e]
artesunic acid (2)
0.4 ± 0.1
0.3 ± 0.03
0.5 ± 0.1
1.4 ± 0.4
1.2 ± 0.4
-
0.1 ± 0.1
0.3 ± 0.01
1.3 ± 0.2
0.8 ± 0.3
1.0 ± 0.1
-
TQ (3)
4a
4b
4c
> 10
6.54 ± 1.33
5.36 ± 0.19
1.83 ± 0.39
0.80 ±0.02
2.12 ± 0.14
0.63 ± 0.02
0.56 ± 0.01
0.69 ± 0.01
ctx/nd[c]
5a
5b
6a
6b
7a
7b
8
-
-
0.2 ± 0.04
0.3 ± 0.2
0.2 ± 0.1
6.071 ± 0.247
0.0027 ± 0.001
0.08 ± 0.03
n.a.
0.2 ± 0.02
0.2 ± 0.02
0.2 ± 0.04
5.663 ± 0.190
7.872 ± 0.594
8.20 ± 4.43
n.a.
ctx/nd[c]
9
1.93 ± 0.17
> 10
10
[a] EC50 values for both leukemia cell lines have been previously reported for doxorubicin28 and artemisinin.14 [b] EC50 value against
HCMV has been previously reported for ganciclovir,29 artemisinin, dihydroartemisinin15 and artesunic acid.30 [c] Microscopically
detectable cytotoxicity approx. 50% at 1 µM, no antiviral activity detectable (ctx/nd). [d] EC50 values have been previously
reported.14 [e] Compound 10 precipitated in the used media. Therefore, the respective EC50 could not be determined unambiguously.
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2.2.2 In vitro inhibitory activity towards HCMV in
this one-pot process are: (1) monocarbonyl activation of
malonic acid with DCC/DMAP; (2) decarboxylation resulting
in N-acetyl-4-(dimethylamino)pyridine salt; (3) nucleophilic
substitution by DHA (DMAP serves as a leaving group,
Scheme 2).
1
2
3
4
5
6
7
8
primary human fibroblasts
The antiviral activity of hybrids 4-10 was determined for
HCMV, strain AD169-GFP. This recombinant virus expresses
the green fluorescent protein (GFP) that can be reliably utilized
to quantitate viral replication (Table 1). Infection experiments
were performed with cultures of primary human foreskin
fibroblasts (HFFs) and measurements of antiviral activity were
carried out according to a previously established protocol.31,32
Artesunic acid (2) and ganciclovir were applied as reference
compounds and both exerted strong anti-HCMV activity at
EC50 levels of 5.41 µM and 2.60 µM, respectively. The most
active compounds were again ether-linked hybrids 6a/b and
thymoquinone-bis-artesunic acid hybrid 7a with EC50 values of
0.63/0.56/0.69 µM thus showing substantially higher antiviral
activity in vitro than the reference drugs ganciclovir and
artesunic acid. Surprisingly, the thymoquinone-bis-artemisinin
hybrids 7b and 8 turned out to be partially cytotoxic towards
HFFs within the analyzed range of concentrations, so that in
these cases no EC50 values could be determined. Other hybrids
of this series 4-6/9, i.e. those exclusively containing one
artemisinin subunit, were selectively inhibitory towards HCMV
without showing cytotoxic side effects, which is an important
prerequisite for putative further antiviral development. Similar
to the findings obtained for antileukemia activity, the fullerene-
containing hybrid 10 was inactive against HCMV, while its
precursor 9 showed activity.
Among tested hybrids 4-10, ether-linked thymoquinone-
artemisinin hybrids 6a/b stood out as the most active
compounds in all categories, while showing no toxic side
effects towards healthy HFFs and thus being selective, which is
one of the most important aspects for drug development. They
exhibited an EC50 value of 0.2 µM against the
doxorubicin-sensitive as well as the multidrug-resistant
leukemia cells and therefore can be regarded as superior to
doxorubicin. Concerning the antiviral potential, several of the
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compounds (e.g. hybrids 5a, 6a/b and 7a) showed
a
substantially higher anti-HCMV activity than the reference
drugs ganciclovir and artesunic acid. In addition, hybrids 6a/b
possessed excellent antimalarial activity (EC50 =5.9/3.7 nM),
which was better than that of artesunic acid (EC50 = 8.2 nM) as
well as chloroquine (EC50 = 9.8 nM) and comparable to that of
dihydroartemisinin (EC50
= 2.4 nM). Another interesting
finding is that when a fullerene moiety was attached to hybrid 9
the compound lost completely its biological activity. This might
be another proof that the mechanism of action of artemisinin
compounds is mediated via radicals. The presented results
provide an insight into how linker length effects biological
activity. However, no general trend could be deduced. It
depends on many different factors such as the type of linkage,
the compound type (artemisinin dimer or monomer) and the
investigated cell lines/parasites/viruses or disease, whether
linker length has an effect or not and whether a shorter or longer
linker is more beneficial. In summary, it can be said that
thymoquinone-artemisinin hybrids 6a/b have the potential to
become drug candidates, as they exhibit an excellent and broad
variety of biological activities (anticancer, antimalaria,
antiviral) combined with a low toxicity/high selectivity profile.
2.2.3 Antiplasmodial activity
All synthesized hybrids 4-10 were investigated in vitro for
their potential antimalarial activity towards Plasmodium
falciparum 3D7 and compared to their parent compounds
artesunic acid (2)/dihydroartemisinin and thymoquinone (3) as
well as the standard drug chloroquine (Table 1). Overall, it can
be stated that all hybrids exhibit excellent antimalarial activities
with EC50 values within the nanomolar range (3.7-54 nM).
Compounds 4, 6, 7a and 9 were even more active than the
reference compounds artesunic acid (EC50 = 8.2 nM) and
chloroquine (EC50 = 9.8 nM) with ether-linked hybrid 6b being
once again the most effective one (EC50 = 3.7 nM). The
relationship between antimalarial activity and linker length
seems to be dependent on the type of linkage and compound
type (artemisinin monomer or dimer). For example, for
thymoquinone-artesunic acid hybrids 4 linker length had nearly
no effect on activity, while for compounds 5, which contain a
C-10 non-acetal linkage, a longer linker was able to improve
antimalarial activity by a factor of 3. In contrast, for artemisinin
dimers 7, a shorter linker was more beneficial. Moreover,
thymoquinone-artesunic acid hybrid 9 (EC50 = 4.4 nM) again
lost its activity, when fullerene was attached to the malonyl
moiety forming hybrid 10 (EC50 ≥ 341 nM).
ASSOCIATED CONTENT
Supporting Information
Experimental conditions and procedures as well as spectral data for
intermediates 11, 13, 14, 16, 18 and target compounds 4-10;
recorded spectra of target compounds; details of cell lines and
reagents as well as cell viability assay for biological evaluation.
The Supporting Information is available free of charge on the ACS
Publications website. (file type, PDF)
AUTHOR INFORMATION
Corresponding Author
* S.B. Tsogoeva, Fax: (+) 49 9131 85 22865;
E-mail: svetlana.tsogoeva@fau.de
3. Conclusion
In conclusion, we were able to expand upon our previously
published results of thymoquinone-artemisinin based
hybrids 4-6 by synthesizing five additional ones 7-10 and
investigating all hybrids for the first time for their in vitro
ACKNOWLEDGMENT
We gratefully acknowledge the financial support from Deutsche
Forschungsgemeinschaft (DFG) by grants TS 87/16-3, MM
1289/7-1/7-3 and from European Commission (Marie Skłodowska-
Curie Innovative Training Network, H2020-MSCA-ITN-
2016, project number: 722456 CORE). We also thank the
Interdisciplinary Center for Molecular Materials (ICMM), the
Graduate School Molecular Science (GSMS) for research support,
as well as Emerging Fields Initiative (EFI) “Chemistry in Live
Cells” supported by Friedrich-Alexander-Universität Erlangen-
Nürnberg for funding.
biological
activity
against
malaria
parasites
(P. falciparum 3D7), two leukemia cell lines (CCRF-CEM and
CEM/ADR5000) and HCMV. During the course of our study,
we developed an unprecedented one-pot method of exclusive
formation of C-10α-acetate 14 (needed as a precursor for the
synthesis of ether-linked hybrids) starting from a 1:1 mixture of
C-10α- to C-10β-DHA. Important steps in the mechanism of
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H.; Schneider-Stock, R.; Tsogoeva, S. B. Synthesis of Novel Hybrids
of Thymoquinone and Artemisinin with High Activity and Selectivity
Against Colon Cancer. Chem. Med. Chem. 2017, 12, 226-234.
DEDICATION
1
This paper is dedicated to Professor Andreas Seidel-Morgenstern.
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17. Höfle,
G.;
Steglich,
W.;
Vorbrüggen,
H.
4-Dialkylaminopyridines as Highly Active Acylation Catalysts. [New
synthetic method (25)]. Angew. Chem. Int. Ed. 1978, 17, 569-583.
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3294.
ABBREVIATIONS
DBU, 1,8-Diazabicyclo[5.4.0]undec-7-en; DCC, N,N’-dicyclo-
hexylcarbodiimide; DHA, dihydroartemisinin; DHU, 1,3-dicyclo-
hexylurea; DMAP, 4-(dimethylamino)pyridine; HCMV, human
cytomegalovirus; n.a., not active; TMSOTf, trimethylsilyl triflate;
TQ, thymoquinone.
9
19. Blanchet, J.; Baudoux, J.; Amere, M.; Lasne, M.-C.; Rouden,
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Chem. Int. Ed. 2009, 48, 7691-7694.
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Jackson, A. R.; Patel, K.; Steel, P. J.; Katritzky, A. R. One-Pot
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thymoquinone-fatty acid conjugates on cancer cells. Chem. Med.
Chem. 2009, 4, 761-8.
24. Posner, G. H.; Paik, I.-H.; Sur, S.; McRiner, A. J.; Borstnik,
K.; Xie, S.; Shapiro, T. A. Orally Active, Antimalarial, Anticancer,
Artemisinin-Derived Trioxane Dimers with High Stability and
Efficacy. J. Med. Chem. 2003, 46, 1060-1065.
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Preston, K. F. Radical reactions of C60. Science 1991, 254, 1183-5.
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Szabo, Z.; Huck, C. W.; Bonn, G. K. Medicinal applications of
fullerenes. Int. J. Nanomedicine 2007, 2, 639-649.
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Synthesis of Thymoquinone-Artemisinin Hybrids: New Potent Antileukemia, Antiviral and Antimalarial Agents
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Tony Fröhlich,1 Christoph Reiter,1 Mohamed E. M. Saeed,2 Corina Hutterer,3 Friedrich Hahn,3 Maria Leidenberger,4 Oliver
Friedrich,4 Barbara Kappes,4 Manfred Marschall,3 Thomas Efferth2 and Svetlana B. Tsogoeva1*
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