Synthetic study of cnicin: Synthesis of the side chain and its esterification

Article abstract of DOI:10.1016/j.tetlet.2016.11.067

Cnicin is a germacranolide sesquiterpene lactone that possesses potent inhibitory activity against the protozoan parasite Trypanosoma brucei, which causes human African trypanosomiasis (HAT). Although cnicin has an interesting structure and attractive biological activity, synthetic studies of cnicin have not yet been reported. This report describes the synthesis of the protected side chain carboxylic acid moiety at C8 of cnicin via two routes starting from L-ascorbic acid. In addition, esterification between the synthetic side chain and salonitenolide derivative, which can be achieved via hydrolysis of cnicin and protection of the primary alcohol, was conducted. Thus, a semi-synthesis of cnicin was achieved.

Full text of DOI:10.1016/j.tetlet.2016.11.067

Accepted Manuscript
Synthetic study of cnicin: Synthesis of the side chain and its esterification
Manami Kurita, Miho Tanigawa, Shuhei Narita, Toyonobu Usuki
PII:
S0040-4039(16)31536-2
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.11.067
TETL 48353
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
21 October 2016
11 November 2016
16 November 2016
Please cite this article as: Kurita, M., Tanigawa, M., Narita, S., Usuki, T., Synthetic study of cnicin: Synthesis of
the side chain and its esterification, Tetrahedron Letters (2016), doi: http://dx.doi.org/10.1016/j.tetlet.2016.11.067
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Graphical Abstract
Synthetic study of cnicin: Synthesis of the
side chain and its esterification
Manami Kurita, Miho Tanigawa, Shuhei Narita, and Toyonobu Usuki*
O
8
HO
O
OH
O
HO
PGO
HO
O
O
O
O
OH
O
O
protected side chain
cnicin
salonitenolide derivative

H
O
O
8
1
1
11109276
3
1
Tetrahedron Letters
journal homepage: www.elsevier.com
Synthetic study of cnicin: Synthesis of the side chain and its esterification
Manami Kuritaa, Miho Tanigawaa, Shuhei Naritaa, and Toyonobu Usukia,*
aDepartment of Materials and Life Sciences, Faculty of Science and Technology, Sophia University, 7-1 Kioicho, Chiyoda-ku, Tokyo 102-8554, Japan
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Cnicin is a germacranolide sesquiterpene lactone that possesses potent inhibitory activity against
the protozoan parasite Trypanosoma brucei, which causes human African trypanosomiasis
(HAT). Although cnicin has an interesting structure and attractive biological activity, synthetic
studies of cnicin have not yet been reported. This report describes the synthesis of the protected
side chain carboxylic acid moiety at C8 of cnicin via two routes starting from L-ascorbic acid. In
addition, esterification between the synthetic side chain and salonitenolide derivative, which can
be achieved via hydrolysis of cnicin and protection of the primary alcohol, was conducted. Thus,
a semi-synthesis of cnicin was achieved.
Available online
Keywords:
Cnicin
Sesquiterpene lactone
L-Ascorbic acid
Isolation
2009 Elsevier Ltd. All rights reserved.
Esterification
1. Introduction
second-stage East African HAT, and is associated with
potentially lethal side effects.11
Cnicin (1, Figure 1) was first isolated from the leaves and
stalks of Cnicus benedictus by Šorm and co-workers in 1959.1 As
a germacranolide sesquiterpene lactone, 1 has a 10-membered
ring diene and a fused five-membered lactone with an ester side
chain at the C8 position. Cnicin 1 possesses a variety of
biological and pharmacological activities, such as allelopathy,2
cytotoxicity, anti-bacterial properties,3 anti-myeloma activity,4
and cytostatic activity.5 Despite the useful properties of 1,
synthetic studies have not yet been reported.
Although the antitrypanosomal activity of cnicin 1 is less
potent than that of melarsoprol (IC50 = 0.01 μM),6 the total
synthesis and a structure-activity relationship (SAR) study of 1
are necessary to aid the development of a more effective drug for
HAT. The present report describes the synthesis of the chiral side
chain moiety, which may serve as an advanced intermediate in
the subsequent total synthesis, and esterification of the 10-
membered ring main core and the synthetic side chain for the
semi-synthesis of 1.
2. Results and discussion
As illustrated in Scheme 1, cnicin
1
could be
retrosynthetically divided into two parts through esterification.
The main core 2 could be obtained via hydrolysis of 1, isolated
from Cnicus benedictus, followed by triisopropylsilyl (TIPS)
protection of the primary alcohol. The side chain moiety 3 with a
protecting group would be prepared via methyl ester 4, which can
be led from the starting material L-ascorbic acid 5.12
Figure 1. Structure of cnicin 1 (carbon numbering of 1 adapted from
Ref. 6).
In 2014, cnicin 1 was reported to exhibit inhibitory activity
against Trypanosoma brucei (IC50 = 0.4 μM),7,8 the parasite
responsible for human African trypanosomiasis (HAT).9 HAT,
also known as sleeping sickness, is transmitted by blood-feeding
tsetse flies in sub-Saharan Africa, and is fatal if left untreated.
The disease occurs in two stages: an initial hemolymphatic stage,
followed by an encephalitic second stage characterized by
invasion of the parasite into the central nervous system, which
causes breakdown of neurological functions.10 The arsenic-based
melarsoprol is the only drug available for the treatment of
Our synthesis commenced with the preparation of the methyl
ester 4 (Scheme 2). Protection of the alkyl diol in 5 with
acetonide was achieved by treatment with acetyl chloride (AcCl)
in acetone via acetonide formation to give 6 in 91% yield.13
Compound 6 was then converted into the methyl ester 4 in 67%
yield in one pot.14 In the first step, the lactone moiety of 6 was
hydrolyzed using 30% NaOH solution, followed by oxidative
cleavage using 30% hydrogen peroxide (H2O2) and sodium
hydrogen carbonate (NaHCO3) to afford carboxylate intermediate
6’. Subsequent methylation of 6’ using dimethyl sulfate
(Me2SO4), sodium sulfite (Na2SO3), and NaHCO3 gave alcohol 4.
∗Corresponding author. Tel.: +81 3 3238 3446; fax: +81 3 3238 3361; e-mail: t-usuki@sophia.ac.jp (T. Usuki)

2
Tetrahedron Letters
O
produce alcohol 11 in 84% yield. Compound 11 was then
8
HO
O
oxidized with manganese dioxide (MnO2) to give aldehyde 12.
OH
O
HO
TIPSO
Kraus-Pinnick oxidation16 of 12 was conducted using 1-methyl-
O
HO
O
O
OH
O
2-butene,
sodium
dihydrogenphosphate
dihydrate
O
O
3
(NaH2PO4·2H2O), and sodium perchlorate (NaClO4) in a 3:2
mixture of t-butanol (tBuOH) and H2O to give the desired
carboxylic acid 3 in 69% yield over two steps.17 Thus, a protected
side chain was prepared from L-ascorbic acid 5 over nine steps in
12% overall yield using this synthesis.
cnicin (1)
2
OH
HO
O
OH
H
MeO
O
O
OH
O
O
OH
L-ascorbic acid (5)
4
OH
OH
OH
a
b
MeO
HO
TBDPSO
O
O
O
O
O
O
O
4
7
8
Scheme 1. Retrosynthesis of 1.
O
TBDPSO
c
d
e
TBDPSO
O
O
O
O
9
10
HO
O
OH
H
HO
O
OH
OH
H
a
b
Na O
O
g
O
H
O
HO
HO
f
O
O
OH
O
O
O
O
O
O
OH
O
O
O
O
L-ascorbic acid (5)
6
6'
11
12
3
OH
Scheme 3. Synthesis of carboxylic acid 3. Reagents and conditions:
(a) LiAlH4, THF, 0 °C to rt, 1 h, 62%; (b) TBDPSCl, imidazole,
DMF, rt, 17 h, 83%; (c) AZADO, NaOCl, KBr, TBAB, CH2Cl2,
NaHCO3 aq, 0 °C, 30 min, 96%; (d) Ph3PCH3Br, nBuLi, THF, -78 °C
to 0 °C, 2 h, then rt, 2.5 h, 90%; (e) TBAF, THF, rt, 45 min, 84%; (f)
MnO2, CH2Cl2, rt, 6.5 h; (g) 2-methyl-2-butene, NaH2PO4·2H2O,
NaClO2, tBuOH/H2O (3:2), rt, 3 h, 69% (two steps).
c
MeO
O
O
O
4
Scheme 2. Synthesis of methyl ester 4. Reagents and conditions: (a)
acetone, AcCl, rt to 3.5 h, then 5 °C, 17 h, 91%; (b) 30% NaOH, 30%
H2O2, NaHCO3, H2O, rt, 1 h; (c) Me2SO4, Na2SO3, NaHCO3, 40 °C,
5 h, 67%.
While these results were satisfactory, obtaining the desired
carboxylic acid 3 in greater yield and fewer steps, starting from
methyl ester 4, was of interest. In this revised synthetic strategy,
direct oxidation of 4 was conducted using Swern oxidation (entry
1), tetrapropylammonium perruthenate (TPAP, entry 2), Dess-
Martin periodinane (DMP, entry 3), AZADO (entry 4), and
pyridinium chlorochromate (PCC, entry 5), as shown in Table 1.
However, a satisfactory yield of ketone 13 with satisfactory
purity could not be obtained in any of these cases. Therefore,
Wittig olefination of crude ketoester 13 was conducted to afford
the desired ester 14. In addition, oxidation of methyl ester 4 by
PCC, followed by Wittig reaction, gave 14 in 27% yield over two
steps. Despite the effort of the oxidation, the use of PCC gave the
best yield. The low yield of this oxidation was probably due to
the steric effects caused by the α,β-diol with both the acetonide
and methyl ester.
Scheme 3 shows the synthesis of 3 starting from methyl ester
4. Compound 4 was converted into diol 7 in 62% yield via
reduction with lithium aluminum hydride (LAH) in
tetrahydrofuran (THF). Subsequently, selective protection of the
primary alcohol of
(TBDPSCl) in N,N-dimethylformamide (DMF) gave alcohol 8 in
83% yield. Oxidation of with 2-azaadamantane-N-oxyl
7 with t-butyldiphenylsilyl chloride
8
(AZADO), sodium hypochlorite (NaOCl), potassium bromide
(KBr), and tetrabutylammonium bromide (TBAB) then afforded
ketone 9 in 96% yield.15 The resulting ketone 9 was then
converted into exo-olefin 10 in 90% yield via Wittig reaction
with methyltriphenylphosphonium bromide (Ph3PCH3Br) and n-
butyllithium (nBuLi). Removal of the TBDPS group of 10 was
achieved using tetrabutylammonium fluoride (TBAF) in THF to
Table 1. Synthesis of 3 via 13 and 14 from methyl ester 4. Reagents and conditions: (a) Ph3PCH3Br, nBuLi, THF, –78 °C to rt, 1.5 h,
then rt, 3 h; (b) LiOH, THF/H2O (1:1), rt, 24 h, 95%.
OH
O
Table 1
a
MeO
b
HO
MeO
MeO
O
O
O
O
O
O
O
O
O
O
O
O
13
14
3
4
Entry
Conditions
DMSO, (COCl)2, Et3N, –78 °C to 0 °C, 1.5 h
Yield from 4 to 14 (two steps, %)
1
2
3
4
0
TPAP, NMO, MS 4Å, CH2Cl2, 0 °C to rt, 18 h
0
DMP, CH2Cl2, rt, 5.5 h
8
AZADO, NaOCl, KBr, TBAB, CH2Cl2, NaHCO3 aq., 0 °C, 20
min
11
5
PCC, MS 4Å, CH2Cl2, rt, 16 h
27

3
The obtained 14 was then hydrolyzed with lithium hydroxide
(LiOH) to give the desired 3 in 95% yield. For this route, the
protected side chain was prepared from L-ascorbic acid over five
steps in 16% overall yield, which involved fewer steps and
slightly better yield than the previous synthetic route.
the semi-synthesis of cnicin 1’. This is the first reported
synthetic study of cnicin 1. The results are expected to aid further
efforts toward developing the first total synthesis of cnicin 1 for
SAR studies.
Acknowledgments
Next, esterification between the synthetic side chain moiety
and the salonitenolide derivative was investigated (Scheme 4).
For the preparation of the salonitenolide derivative, isolation of
cnicin 1 was carried out from Cnicus benedictus according to the
literature procedure.8a,18 Hydrolysis of 1 using 0.1 N sodium
carbonate (Na2CO3) was then conducted at room temperature for
16 h to afford salonitenolide19 15 in 51% yield.20 Then, treatment
of 15 with TIPSCl, imidazole, and DMAP at 0 °C for 2 h afforded
TIPS-protected salonitenolide derivative 2 in 63% yield.
We thank Dr. Michael Adams (University of Basel, currently
Bacoba AG, Switzerland) for valuable discussion.
References and notes
1. Suchý, M.; Benešová, V.; Herout, V.; Šorm, F. Tetrahedron Lett.
1959, 10, 5-9.
2. Kelsey, R. G.; Locken, L. J. J. Chem. Ecol. 1987, 13, 19-33.
3. Saroglou, V.; Karioti, A.; Demetzos, C.; Dimas, K.; Skaltsa, H. J.
Nat. Prod. 2005, 68, 1404-1407.
4. Johrer, K.; Obkircher, M.; Neurelter, D.; Partell, J.; Zelle-Rieser,
C.; Maizner, E.; Kern, J.; Hermann, M.; Hamacher, F.; Markel,
O.; Wacht, N.; Zidorn, C.; Scheideler, M.; Grell, R. J. Mol. Med.
2012, 90, 681-693.
O
8
O
OH
a
b
HO
HO
HO
O
O
OH
HO
O
O
cnicin (1)
salonitenolide (15)
5. Gonzalez, A. G.; Darias, V.; Alonso, G.; Boada, J. N.; Feria, M.
O
Planta Med. 1978, 33, 356-359.
6. Marco, J. A.; Sanz-Cervera, J. F.; Yuste, A.; Sancenón, F.; Carda,
M. Phytochemistry 2005, 66, 1644-1650.
O
O
3
O
7. Zimmermann, S.; Fouché, G.; Mieri, M, D.; Yoshimoto, Y.;
Usuki, T.; Nthambeleni, R.; Parkinson, C, J.; van der Westhuyzen,
C.; Kaiser, M.; Hamburger, M.; Adams, M. Molecules 2014, 19,
3523-3538.
c
OH
O
TIPSO
TIPSO
O
O
O
O
O
O
8. Related sesquiterpene lactone cynaropicrin has also
antitrypanosomal activity: (a) Zimmermann, S.; Kaiser, M.; Brun,
R.; Hamburger, M.; Adams, M. Planta Med. 2012, 78, 553-556.
(b) Usuki, T.; Sato, M.; Hara, S.; Yoshimoto, Y.; Kondo, R.;
Zimmermann, S.; Kaiser, M.; Brun, R.; Hamburger, M.; Adams,
M. Bioorg. Med. Chem. Lett. 2014, 24, 794-798.
2
16
O
d
O
HO
HO
O
OH
O
semi-synthetic cnicin (1')
9. (a) WHO Trypanosomiasis, human African (sleeping sickness)
http://www.who.int/mediacentre/factsheets/fs259/en/. (accessed
on October 7, 2016). (b) Brun, R.; Blum, J.; Chappuis, F.; Burri, C.
Lancet 2012, 375, 148-159.
Scheme 4. Synthesis of semi-synthetic cnicin (1’). Reagents and
conditions: (a) 0.1 N Na2CO3 aq, dioxane/H2O (2:1), rt, 16 h, 51%;
(b) TIPSCl, imidazole, DMAP, CH2Cl2, 0 °C, 2 h, 63%. (c) EDC,
DMAP, CH2Cl2, rt, 24 h, 10%; (d) 1N HCl, CH2Cl2, rt, 28 h, 33%.
10. Checchi, F.; Filipe, J. A. N.; Haydon, D. T.; Chandramohan, D.;
Chappuis, F. BMC Infect. Dis 2008, 8, 16.
11. Blum, J.; Nkunku, S.; Burri, C. Trop. Med. Int. Health 2001, 6,
390-400.
12. Cren, S.; Wilson, C.; Thomas, N. R. Org. Lett. 2005, 7, 3521-3523.
13. Gautam, D.; Rao, B. V. Tetrahedron Lett. 2010, 51, 4199-4201.
14. Cho, B. H.; Kim, J. H.; Jeon, H. B.; Kim, K. S. Tetrahedron 2005,
61, 4341-4346.
15. Shibuya, M.; Tomizawa, M.; Suzuki, I.; Iwabuchi, Y. J. Am. Chem.
Soc. 2006, 128, 8412-8413.
Segments for semi-synthesis of 1 were prepared successfully.
Esterification of TIPS-protected salonitenolide derivative 2 and
the
synthetic
side
chain
3
using
1-ethyl-3-(3-
dimethylaminopropyl) carbodiimide hydrochloride (EDC) and
N,N-dimethyl-4-aminopyridine (DMAP) was conducted at room
temperature for 24 h to afford 16 in 10% yield. The low yield
was probably due to an unexpected side reaction and/or steric
hindrance around C8. Removal of TIPS and acetonide protection
by 1 N HCl at room temperature for 28 h afforded semi-synthetic
cnicin 1’ in 33% yield. The optical rotation of 1’ {[α]D20 +159.9 (c
0.1, MeOH)} was good agreement with that of the isolated
natural product 1 ([α]D20 +169.6 (c 0.1, MeOH)).
16. (a) Kraus, G. A.; Roth, B. J. Org. Chem. 1980, 45, 4825-4830. (b)
Bal, B. S.; Childers Jr., W. E.; Pinnick, H. W. Tetrahedron 1981,
37, 2091-2096.
17. Tanaka, A.; Yamashita, K. Agric. Biol. Chem. 1980, 44, 199-202.
18. Adams, M.; Zimmermann, S.; Kaiser, M.; Brun, R.; Hamburger,
M. Nat. Prod. Commun. 2009, 4, 1377-1381.
19. Suchý, M.; Samek, Z.; Herout, V.; Šorm, F. Chem. Commun.
1967, 32, 2016-2019.
20. Sergio, R.; Antonella, M.; Rosa, A. R.; Maurizio, B. Eur. J. Org.
Chem. 2003, 14, 2690-2694.
21. Korte, F.; Beckmann, G. Die Naturwissenschaften 1958, 45, 390.
3. Conclusion
Supplementary Material
In summary, the protected side chain of antitrypanosomal
cnicin 1 was prepared via two routes starting from L-ascorbic
acid 5. In the initial route, synthesis of 3 was achieved in nine
steps in 12% overall yield. Subsequently, a more effective
synthesis eliminating placement of an unnecessary protecting
group was developed. The synthesis involved PCC oxidation and
Wittig reaction and yielded 3 in 16% overall yield in five steps.
Finally, esterification of synthetic side chain 3 and TIPS-
protected salonitenolide derivative 2 was conducted to achieve
Experimental procedures, NMR spectra. Supplementary data
associated with this article can be found in the online version, at
http://dx.doi.org/
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*Highlights



This is the first synthetic study of sesquiterpene lactone cnicin.
The side chain of cnicin was synthesized via two routes.
A semi-synthesis of cnicin was acheived.