Synthesis of novel artesunate-benzothiophene and artemisinin-benzothiophene derivatives

Article abstract of DOI:10.1080/14786419.2021.1928116

Natural products are used for the treatment of a variety of diseases for many years. Last decades, design and synthesis of novel biologically active hybrid molecules including natural product is gained big importance due to their unique and new biological

Full text of DOI:10.1080/14786419.2021.1928116

Natural Product Research
Formerly Natural Product Letters
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/gnpl20
Synthesis of novel artesunate-benzothiophene
and artemisinin-benzothiophene derivatives
Omruye Ozok, Emrah Kavak & Arif Kivrak
To cite this article: Omruye Ozok, Emrah Kavak & Arif Kivrak (2021): Synthesis of novel
artesunate-benzothiophene and artemisinin-benzothiophene derivatives, Natural Product Research,
DOI: 10.1080/14786419.2021.1928116
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NATURAL PRODUCT RESEARCH
https://doi.org/10.1080/14786419.2021.1928116
Synthesis of novel artesunate-benzothiophene and
artemisinin-benzothiophene derivatives
a,b
a
a,c
Omruye Ozok
, Emrah Kavak and Arif Kivrak
a
Department of Chemistry, Faculty of Sciences, Van Y u€ z €u nc u€ Yil University, Van, Turkey;
b
Department of Molecular Biology and Genetics, Faculty of Sciences, Van Y €u z u€ nc u€ Yil University,
c
Van, Turkey; Faculty of Sciences and Arts, Department of Chemistry, Eskisehir Osmangazi University,
Eski s¸ ehir, Turkey
ABSTRACT
ARTICLE HISTORY
Natural products are used for the treatment of a variety of dis-
eases for many years. Last decades, design and synthesis of novel
biologically active hybrid molecules including natural product is
gained big importance due to their unique and new biological
properties. In the present study, novel artemisinin-benzothio-
phene derivatives (12 A-F) are synthesised. Initially, benzothio-
phene derivatives (4 A-4F) are prepared via the Pd-catalyzed
coupling reactions and iodocyclisation reactions. Then, Suzuki-
Miyaura coupling reactions were used for the formation of inter-
mediates 6 A-6F (between 64% and 91% yields). Finally, the
Steglich esterification reaction between intermediate 6 and arte-
sunate formed the artemisinin-benzothiophene hybrids (9 A-9F) in
moderate to excellent yields under very mild reaction conditions.
When intermediate 6 was reacted with dihydroartemisinin, prod-
uct 12 A-12F was also obtained with high yields.
Received 8 February 2021
Accepted 4 May 2021
KEYWORDS
Artemisinin; artesunate;
benzothiophene; natural
products; drug design;
wormwood plant
CONTACT Arif Kivrak
arifkivrak@yahoo.com
Supplemental data for this article can be accessed online at https://doi.org/10.1080/14786419.2021.1928116.
ß 2021 Informa UK Limited, trading as Taylor & Francis Group

2
O. OZOK ET AL.
1. Introduction
Natural product based medical therapy has been gained big importance for the treat-
ment of diseases due to their amazing biological activities (Tan et al. 2011). Therefore,
scientists are trying to synthesis or isolate new natural products. Artemisinin (ART), iso-
lated from wormwood plant (Artemisia absinthium), was discovered by Chinese scien-
tist Youyou Tu at the end of 1960s (Miller and Su 2011; Kong and Tan 2015) as an
antimalarial. According to the World Health Organization (WHO), more than half a mil-
lion people have died of the malaria (Price et al. 1996; Su and Miller 2015). ART dis-
played extreme activities than other antivirals, so it is known the most effective
miracle drug for the treatments of malaria (Klayman 1985; Kong and Tan 2015; Guo
2
016). Moreover, recent in vivo and in vitro studies showed that ART have higher activ-
ities against different cancer cells (Meshnick et al. 1996; Efferth et al. 2001; Chen et al.
020; Li et al., 2020; Zhang 2020). Moreover, different biological properties have been
2
reported after discovery of ART (Kong and Tan 2015). Semi-synthetic derivatives
including artesunate (Efferth et al. 2008), artermer and dihydroartemisinin were also
synthesised by starting from ART (Liu et al. 2013) (Figure 1). For example, artesunate
was approved as a new anti-viral drug for the treatment of malaria by FDA in 2020. In
addition, artesunate showed inhibitory effects against breast, melanoma, leukemia,
ovarian, colon, prostate and renal cancer cells (Efferth et al. 2001; Firestone and
Sundar 2009). Other semi synthetic derivative of ART is dihydroartemisinin which was
also found as anti-cancer agents against osteosarcoma, pancreatic, leukemic, and lung
cancer cells (Singh and Lai 2001; Chaturvedi et al. 2010).

NATURAL PRODUCT RESEARCH
3
Figure 1. Artemisinin and semi-synthetic derivatives.
In recent years, new hybrid molecules consisting of artemisinin and heterocyclic
compounds were designed and synthesized (Ha et al. 2016; Pepe et al. 2020).
Interestingly, ART based hybrid molecules created new biological activities, or increase
the known biological properties than artemisinin (Dhingra et al. 2000; Yang et al.
2020). Artemisinin-chloroquine hybrid structures displayed the higher activity for mal-
aria as a new anti-malarial drug candidates (Pepe et al. 2020). In the lights of the pre-
vious studies, the novel hybrid molecules could be new drug candidates with
unknown pharmacological activities against different kinds of cancer types and other
diseases. It was known that COVID-19 has infected more than 65 million individuals
worldwide, more than 1.6 million patients have died because of the infection (Gibson
et al. 2020; Wu and McGoogan 2020). Anti-viral drugs like hydroxychloroquine, quin-
ine, favipiravir etc. have been used to treatments of this viral pneumonia of SARS-CoV-
2
(Nittari et al. 2020). Artemisinin hybrid molecules may be the biggest candidates for
the treatment of SARS-CoV-2 as an anti-covid (Gendrot et al. 2020; Zhang 2020).
Benzothiophene derivatives are very important heteroaromatic compounds, and are
used as; analgesic, antipyretic, antibacterial, antiparasitic, antitumor and anticancer
agents (Keri et al. 2017; Algso et al. 2018; Algso and Kivrak 2019). Commercially avail-
able Raloxifene, Zileton, and Sertaconazole are the best examples of benzothiophene
motifs. Herein, novel hybrid molecules consisting of artemisinin and biologically
important benzothiophene were designed and synthesised by using Sonogashira cou-
pling reaction, iodocyclisation reaction, Suzuki-Miyaura coupling reaction and Steglich
esterification reaction.
2. Result and discussion
Initially, 3-iodobenzothiophene derivatives were prepared by starting from the 2-iodo-
thioanisole. Sonogashira coupling reactions and iodocyclisation reaction was used to
preparation of desired 3-iodobenzothiophene structures (Table S2). In literature, Pd-
catalyzed coupling reaction like Sonogashira (Chinchilla and Najera 2007), Suzuki-
Miyaura (Karabiyikoglu and Zora 2016) and Heck (Peris and Crabtree 2004) reactions
are the most popular reactions for the formation of new carbon-carbon bonds. These
reactions need mild reaction conditions, catalytic amounts of Pd-catalyst, and give the
higher yields in a short time. Therefore, Sonogashira coupling reaction and Suzuki-
Miyura coupling reaction were chosen in the present study. If the reaction between 2-
iodothianisole and terminal alkyne was carried out in the presence of PdCl (PPh ) in
2
3 2

4
O. OZOK ET AL.
triethylamine under inert atmosphere, corresponding methyl(2-(aryl/alkyl-ethynyl)phe-
nyl)sulfane derivatives (3 A-F) were obtained. The yields changed between 72% and
9
3
4% yields (Table S2). Then, iodocyclisation reactions were performed to synthesis of
-iodobenzothiophene derivatives. Iodocyclisation reactions have played critical roles
for the formation of heteroaromatic such as; indoles, pyrazoles, benzothiophenes, ben-
zofurans etc. (Yue and Larock 2004; Cho et al. 2009; Zora and Kivrak 2011; Zora et al.
2011). 3-Iodo-benzothiophene derivatives 4 were regioselectively synthesised by using
iodocyclisation cyclisation reaction. When compounds 3 was allowed to react with
molecular iodide in dichloromethane, 3-iodo-benzothiophenes were isolated between
75% and 95% yields (Table S2). After iodocyclisation reaction, we focused on the syn-
thesis of corresponding aldehydes 6. Recently, we reported a new modified Suzuki-
Miyaura coupling reaction for the affording benzothiophene derivatives including alde-
hyde functional groups. When 4 A was reacted with 4-formylphenylboronic acid in the
presence of Pd-catalyst and Li CO in methyl alcohol, aldehyde 6 A was obtained in
2
3
76% yields (Table S1). By using this reaction condition, aliphatic, aromatic and poly-
aromatic substituted benzothiophene derivatives 6B, 6 C, 6 D, 4E and 4 F were syn-
thesised in moderate to high yields (Table S2).
Next, we examined the esterification reaction between artesunate and alcohols 7.
Actually, there are a variety of esterification reactions such as; Steglich Esterification,
Yamaguchi Esterification, Fischer Esterification etc. in literature. The Steglich esterifica-
tion reaction is the more efficient methods, and it needs mild reaction conditions with
shorter reaction time. This reaction also most convenient methods for the synthesis of
some natural products. Therefore, Steglich esterification reaction was chosen for the
synthesis of our final products. Firstly, we needed to prepare the corresponding alco-
hols from the aldehydes via reduction reaction. When aldehydes 6 was reacted with
NaBH in Chloroform/MeOH (4:1), the desired alcohols were formed. These alcohols
4
were not stable, so we used alcohols without any purification. The reaction between
artesunate and 7 A in dichloromethane with DCC/DMAP catalyst systems, 98% yield of
desired product 9 A was obtained. By using same methodology, corresponding 9B,
9
C, 9 D, 9E and 9 F were isolated in good yields (Table S3).
The H-NMR spectra of the artesunate-benzothiophene 9 gave aromatic peaks
1
between 7–8 ppm belonging to benzothiophene. The characteristic peaks of artesu-
nate can be seen around 5.4 ppm as a doublet and 5.7 ppm as a singlet due to elec-
tronegative effect of the oxygen atoms on the ring. There is a singlet at 2.30 ppm
came from the CH3 where neighbors peroxide bridge. As seen in H-NMR spectra,
Artesunate’s remain CH peaks are detected around 0.96 ppm and 0.84 ppm. In add-
3
13
ition, there is a singlet at 5 ppm belonging to alcohol’s CH . The C-NMR spectra are
2
important to determination of carbonyl (C ¼ O) peaks of hybrid molecule. There are
two ester functional groups on the hybrid molecule, so carbonyl’s peaks are shifted
13
around 170 ppm in C-NMR spectra. Moreover, there are four artesunate’s peaks
between 110 and 80 ppm (Figure S1).
The plausible reaction pathway is shown in Figure 2. Firstly, carboxyl group was
activated with a carbodiimide reagent to form the O-acylisourea. This intermediate
takes a proton from the alcohol to give the carbocation intermediate. Then, DMAP
reagent attacked this carbocation for the formation of second carbocation

NATURAL PRODUCT RESEARCH
5
Figure 2. Proposed reaction mechanism for the formation of artesunate-benzothiophenes.
intermediates. Finally, the alcohol reacts with the activated carboxylic acid to form the
desired product and dicyclohexylurea as a side product.
A series of artemisinin-benzothiophene hybrid derivatives 12 were also synthesized
by using esterification reactions (Table S4). The aldehyde functional group of 6 was
selectively oxidised, converting the aldehyde group to carboxylic acid 10. Then, car-
boxylic acid derivatives 10 were underwent to esterification reaction without add-
itional purification. When dihydroartemisinin was allowed to react with carboxylic acid
1
0 A in the presence of EDCI/DMAP as catalyst system in dichloromethane, we isolated
in 62% yield of 12 A. When pentyl substituted carboxylic acid 10B was used for esteri-
fication, the corresponding product 12B was obtained in 50% yield. As seen in Table
S4, a variety of artemisinin-benzothiophene hybrids including naphthyl, p-MePh, p-
MeOPh groups were synthesized in moderate to high yields.
3. Conclusion
In the present study, novel artesunate-benzothiophene and artemisinin-benzothio-
phene hybrids were synthesized. Initially, 3-iodobenzothiophene derivatives were pre-
pared by using Sonogashira Coupling reaction and electrophilic cyclization reactions.
Then, Pd-catalyzed Suzuki-Miyaura coupling reactions were applied for the formation
of aldehyde functionalised benzothiophenes (6 A-6F). Then, we focused on the synthe-
sis of artesunate-benzothiophenes by using the Steglich esterification reaction. The
corresponding alcohols 7 was prepared via reduction reaction, then underwent to
esterification reaction for the synthesis of artesunate-benzothiophene hybrids 9. We
obtained 9 A-9F in good yields. In the last part, novel artemisinin-benzothiophene
1
2 A-F hybrids were synthesized from the reaction between carboxylic acid 10 and
dihydroartemisinin in moderate to high yields. As a result, novel artemisinin based
hybrid molecules were synthesized and characterized, and plausible reaction mechan-
ism was proposed. Our new hybrid molecules may be new generation anti-viral candi-
dates for the treatments of viral diseases.
Acknowledgment
The authors thank to The Scientific and Technological Research Council of Turkey (TUBITAK) pro-
ject 218Z028 for chemicals and solvents. O. Ozok thanks to the council of higher education

6
O. OZOK ET AL.
(YOK) for 100/2000 scholarships. In addition, E. Kavak and O. Ozok thank to TUBITAK STAR
scholarships.The author (A. Kivrak) would like to acknowledge networking contribution by the
COST Action CA17104 ‘New diagnostic and therapeutic tools against multidrug resist-
ant tumours’.
Disclosure statement
No potential conflict of interest was reported by the authors.
ORCID
Omruye Ozok
http://orcid.org/0000-0002-4164-8650
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