Synthesis and biological investigation of (+)-3-hydroxymethylartemisinin

Article abstract of DOI:10.3762/bjoc.15.51

Herein, we describe a biomimetic entry to (+)-3-hydroxymethylartemisinin (2) as well as to the artemisinin derivatives (+)-3-hydroxymethyl-9-desmethylartemisinin (16) and (+)-3-hydroxymethyl-9-epi-artemisinin (18), starting from the known and readily available chiral aldehyde 3 and alkyne 4. Subsequently, the synthesized compounds have been evaluated for their antimalarial activity against the drug-sensitive P. falciparum NF54 strain. All of them were inactive. In addition, they did not show any toxicity against L6 cells (a primary cell line derived from rat skeletal myoblasts). These results contribute to a better understanding of artemisinins mechanism of action.

Full text of DOI:10.3762/bjoc.15.51

Synthesis and biological investigation of
+)-3-hydroxymethylartemisinin
(
Toni Smeilus1, Farnoush Mousavizadeh1, Johannes Krieger1, Xingzhao Tu1,
Marcel Kaiser2,3 and Athanassios Giannis*1
Full Research Paper
Open Access
Address:
Faculty of Chemistry and Mineralogy, Institute of Organic Chemistry,
University of Leipzig, Johannisallee 29, 04301 Leipzig, Germany,
Dr. M. Kaiser Swiss Tropical and Public Health Institute,
Beilstein J. Org. Chem. 2019, 15, 567–570.
1
doi:10.3762/bjoc.15.51
2
Received: 09 January 2019
Accepted: 20 February 2019
Published: 27 February 2019
Socinstrasse 57, 4051 Basel, Switzerland and 3University of Basel,
Petersplatz 1, 4003 Basel, Switzerland
Email:
Associate Editor: K. N. Allen
Athanassios Giannis* - giannis@uni-leipzig.de
©
2019 Smeilus et al.; licensee Beilstein-Institut.
*
Corresponding author
License and terms: see end of document.
Keywords:
artemisinin; biomimetic synthesis; Diels–Alder reaction; malaria;
peroxides
Abstract
Herein, we describe a biomimetic entry to (+)-3-hydroxymethylartemisinin (2) as well as to the artemisinin derivatives
(
+)-3-hydroxymethyl-9-desmethylartemisinin (16) and (+)-3-hydroxymethyl-9-epi-artemisinin (18), starting from the known and
readily available chiral aldehyde 3 and alkyne 4. Subsequently, the synthesized compounds have been evaluated for their antimalar-
ial activity against the drug-sensitive P. falciparum NF54 strain. All of them were inactive. In addition, they did not show any tox-
icity against L6 cells (a primary cell line derived from rat skeletal myoblasts). These results contribute to a better understanding of
artemisinins mechanism of action.
Introduction
The isolation of artemisinin (1; qinghaosu, Figure 1) from used in combination with other drugs for the treatment of
Artemisia annua L. and the discovery of its antimalarial proper- malaria [4]. However, the exact mechanism of action of this
ties in 1971 represent one of the greatest medical break- endoperoxide sesquiterpene lactone is still unknown [5].
throughs of the 20th century [1,2]. For these achievements
Youyou Tu was awarded the Nobel Prize in Physiology or Whereas artemisinins are almost non-toxic to normal cells,
Medicine in 2015 [3]. As artemisinin has poor solubility and several studies have confirmed their potent antitumor activity
bioavailability, several derivatives of this natural product like [6,7]. In addition, they have been reported to possess immuno-
artesunate and artemether were developed and are currently suppressive, anti-inflammatory, antiviral, antifungal and
567

Beilstein J. Org. Chem. 2019, 15, 567–570.
Figure 1: Structures of the natural (+)-artemisinin (1) and the synthe-
sized (+)-3-hydroxymethylartemisinin (2).
antiparasitic activities [8-10]. Recently, it was shown that
artemisinin interacts with the mammalian protein gephyrin and
by stabilizing it, it enhances GABAA receptor signaling result-
ing in in vivo conversion of pancreatic α-cells into functional
β-like cells [11]. Therefore, this sesquiterpene lactone may also
find an application in the treatment of diabetes mellitus. These
facts indicate the clinical potential of artemisinins and “it is
likely that artemisinin drugs will become a major armamen-
tarium combating a variety of human diseases beyond malaria”
Scheme 1: Synthesis of the Diels–Alder precursor 8 over four steps in
71% yield, starting from aldehyde 3 and alkyne 4. Reaction conditions:
a) alkyne 4, n-BuLi, THF, −78 °C to rt, 16 h, 86%; b) Red-Al, THF,
0
°C, 10 min, 96%; c) BAIB, TEMPO, DCM, rt, 16 h, 92%;
d) BrCH2CO2Et, Zn, toluene, reflux, 30 min, 93%. Red-Al = sodium
bis(2-methoxyethoxy)aluminum dihydride, BAIB = (diacetoxy-
iodo)benzene, TEMPO = (2,2,6,6-tetramethylpiperidin-1-yl)oxyl.
[
9].
The structure of all these isomers was confirmed by NOE ex-
periments (see Supporting Information File 1 for full experi-
In the past, several semi-synthetic artemisinin derivatives were mental details). Both derivatives 9 and 10 (Scheme 2) yielded
prepared for experimental therapy of pathologies like the ones α,β-unsaturated esters 12 and 13 after treatment with Martin
mentioned before [12,13]. However, the variety of these deriva- sulfurane [16]. Reduction of compound 12 using NiCl2/NaBH4
tives is limited as all of them are produced modifying the in methanol as solvent yielded derivative 14 with excellent dia-
artemisinin skeleton at the same positions due to intrinsic syn- stereomeric ratio (dr = 1:0.03). On the other hand, derivative 13
thetic challenges. Recently, we reported a biomimetic was reduced under Birch conditions (Li/NH3) to afford ester 15
artemisinin synthesis that addresses these challenges and pave in 61% yield (dr = 1:0.4). Gratifyingly, both artemisinin precur-
the way for derivatization of the (+)-artemisinin skeleton at po- sors 14 and 15 possess the desired stereoconfiguration at car-
sitions not accessible using current methodology [14]. Based on bon center 8a.
our approach we report here on the synthesis and biological ac-
tivity of the title compound (+)-3-hydroxymethylartemisinin (2, Subsequently, a solution of 14 and 15 in dichloromethane con-
Figure 1).
taining methylene blue as photosensitizer was exposed to
sunlight and oxygen. The treatment of the resulting intermedi-
ate hydroperoxide with a small amount of trifluoroacetic acid as
Results and Discussion
Our synthesis (Scheme 1) started from the known and readily previously described [17,18], afforded in the frame of a Hock
available aldehyde 3 [14] which was treated with the organo- cleavage (+)-3-hydroxymethyl-9-desmethylartemisinin (16) in
lithium species obtained from derivative 4 [15] and n-BuLi to 24% and 16% yield, respectively. Protection of the free hydroxy
yield derivative 5. The latter afforded allylic alcohol 6 after group of 16 as a silyl ether and methylation of the obtained
reduction of the propargylic moiety with Red-Al. BAIB/ lactone in α-position (LDA/MeI/HMPA) afforded derivative 17.
TEMPO oxidation of this alcohol gave ketone 7. By a Refor- After removal of the TES protecting group (+)-3-hydroxy-
matsky reaction of 7 using Zn/ethyl bromoacetate derivative 8 methyl-9-epi-artemisinin (18, Scheme 3) was obtained, where-
was obtained, which was subjected to a thermal (190 °C, tolu- as the desired (+)-3-hydroxymethylartemisinin (2) was pro-
ene) intramolecular Diels–Alder reaction resulting in the forma- duced from 17 in two steps including epimerization and
tion of the β-hydroxy esters 9, 10 and 11 (dr = 1.74:0:38:0.24) cleavage of the TES group in excellent yield (Scheme 3).
in a total yield of 84% (Scheme 2). Gratifyingly, the percentage
of derivatives 9 and 10 that show the desired stereochemistry at Finally, we evaluated the antimalarial activity of (+)-3-hydroxy-
carbon center 5a is around 90%.
methylartemisinin (2) as well as of the derivatives 16 and 18
568

Beilstein J. Org. Chem. 2019, 15, 567–570.
Scheme 2: Synthesis of (+)-3-hydroxymethyl-9-desmethylartemisinin (16), starting from Diels–Alder derivatives 9 and 10. Reaction conditions: a) tolu-
ene, 190 °C, 24 h, 84% (dr 9:10:11 = 1.74:0.38:0.24); b) Martin sulfurane, DCM, 0 °C, 10 min, 97% [(E)/(Z) = 1:1]; c) Martin sulfurane, DCM, 0 °C,
1
0 min, 95%; d) NiCl2, NaBH4, −60 °C to −40 °C, 1 h, 96% (dr = 1:0.03); e) Li, EtOH, NH3, −70 °C, 10 min, 61% (dr = 1:0.4); f) i. O2, methylene blue,
light, DCM, −30 °C, 30 h; ii. then O2, cat. TFA, DCM, rt, 2 d, 24%; g) i. O2, methylene blue, light, DCM, −30 °C, 30 h; ii. then O2, cat. TFA, DCM, rt,
d, 16% [13].
2
against the drug-sensitive P. falciparum NF54 strain as de-
scribed previously [14]. We found them to be inactive. In addi-
tion, 2 did not show any toxicity against L6 cells (a primary cell
line derived from rat skeletal myoblasts). In both assays the
highest concentration used was 100 μg/mL.
Conclusion
In the past it has been postulated that artemisinins kill
intraerythrocytic parasites such as P. falciparum in the presence
of Fe2+ forming reactive oxygen species (ROS) [4]. As
artemisinin and similar derivatives are destroyed by Fe2+
in vitro (Fenton reaction), it is difficult for us to explain their
inactivity against Plasmodium. Furthermore, in cellular systems
free ferrous iron [19] as well as haem [20,21] have been
proposed to be the main iron sources for artemisinin activation.
However, the results on that matter remain controversial [4,22].
Our results challenge the radical hypothesis of artemisinins
action and also indicate that the methyl group at the C-3
position of artemisinin is possibly involved in lipophilic
interactions with putative proteins essential for survival of the
Plasmodium parasite. Derivative 2 will be used to attach further
substituents at position 14 of artemisinin to prove this hypoth-
esis.
Scheme 3: Synthesis of (+)-3-hydroxymethyl-9-epi-artemisinin (18)
and (+)-3-hydroxymethylartemisinin (2). Reaction conditions:
a) i. TESCl, NEt3, 0 °C, 16 h; ii. LDA, MeI, HMPA, THF, −78 °C to
−50 °C, 1 h, 73% (over two steps); b) TBAF, THF, 0 °C, 10 min, 95%;
c) i. DBU, DCM, rt, 1 d; ii. TBAF, THF, 0 °C, 10 min, 87% (over two
steps). LDA = lithium diisopropylamide, HMPA = hexamethylphosphor-
amide, DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene, TESCl = triethylsilyl
chloride.
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Experimental details, NMR spectra and other physical data are
shown in Supporting Information File 1.
doi:10.1002/9781118219683.ch10
1
4.Krieger, J.; Smeilus, T.; Kaiser, M.; Seo, E.-J.; Efferth, T.; Giannis, A.
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Angew. Chem. 2018, 130, 8425–8428. doi:10.1002/ange.201802015
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