Electronically stabilized versions of the antimalarial acetal trioxanes artemether and artesunate

Article abstract of DOI:10.1016/j.bmc.2008.03.010

From 9-substituted DHA, several new artemisinin-derived C-10 acetal ethers and esters were prepared with either a 9-fluoro or a 9-sulfonyl substituent. The very strong inductive electron-withdrawing C-9 substituent is shown to retard considerably C-10 ionization (acid-promoted etherification) of 9-fluoro-DHA and 9-sulfonyl-DHA.

Full text of DOI:10.1016/j.bmc.2008.03.010

Available online at www.sciencedirect.com
Bioorganic & Medicinal Chemistry 16 (2008) 5247–5253
Electronically stabilized versions of the antimalarial acetal
trioxanes artemether and artesunate
Gary H. Posner,* William A. Maio and Alvin S. Kalinda
Department of Chemistry, The Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA
Received 25 January 2008; revised 27 February 2008; accepted 3 March 2008
Available online 6 March 2008
Dedicated to E.J. Corey with admiration and respect on the happy occasion of his 80th birthday in 2008.
Abstract—From 9-substituted DHA, several new artemisinin-derived C-10 acetal ethers and esters were prepared with either a 9-
fluoro or a 9-sulfonyl substituent. The very strong inductive electron-withdrawing C-9 substituent is shown to retard considerably
C-10 ionization (acid-promoted etherification) of 9-fluoro-DHA and 9-sulfonyl-DHA.
ꢀ 2008 Elsevier Ltd. All rights reserved.
H
1. Introduction
R1 = R2 = O,
Artemisinin (1)
O
O
H
R1=H, R2=OH, Dihydroartemisinin (2)
R1=OMe, R2=H, Artemether (3)
R1=H, R2=OC(O)CH2CH2C(O)ONa,
Sodium artesunate (4)
The effectiveness of quinoline-based antimalarials is
being compromised seriously by the widespread resis-
tance of malaria parasites to these drugs.1 A traditional
Chinese folk remedy for malaria has led to the isolation
and identification of the 1,2,4-trioxane artemisinin (1),
and subsequently to the synthesis of fast-acting semisyn-
thetic derivatives (Fig. 1) such as dihydroartemisinin
(DHA, 2), artemether (3), and sodium artesunate (4).2
The World Health Organization has recommended,
and most countries where malaria is endemic have
adopted, the use of artemisinin combination therapy
(ACT) for fast and reliable cure for malaria-infected
people.3 One practical shortcoming of C-10 acetals such
as 3 and 4 is the rapid but undesirable hydrolysis of their
C-10 acetal functionality into the corresponding parent
C-10 hemiacetal DHA which is reported to be toxic.4,5
O
9
O
10
R1 R2
Figure 1. Artemisinin and semisynthetic derivatives.
H
O
H
EWG = F,
SO2Me,
SO2Ph
O
O
9
O 10
EWG
OR
5
Figure 2. 9-Substituted artemisinin derivatives.
Such C-10 acetal to C-10 hemiacetal hydrolysis after
oral administration is likely due to the acidic environ-
ment of the human stomach. We thought that attaching
a strongly electron-withdrawing group (EWG) at C-9
would inductively disfavor carbocation formation at
C-10 (Fig. 2) and thus would electronically stabilize
any C-10 ether or ester derivative 5 thereby making it
a longer-lived and possibly better antimalarial. Related
work in this area has involved C-10 p-substituted phen-
oxy derivatives6 and C-10 sulfonamides,5 as well as the
incorporation of a trifluoromethyl group at C-10.7
Herein are the results of a new study using 9-fluoro
and 9-sulfonyl groups to retard acid-promoted etherifi-
cation via rate-determining ionization of the C-10 OH
group in hemiacetal DHA (2). Although C-9 hetero-
atom systems are known,8–10 9-fluoro and 9-sulfonyl
substituents have not been reported and were chosen
due to their very powerful inductive electron-withdraw-
ing ability.11
Keywords: Artemisinin; Artemether; Sodium artesunate; Antimalarial.
*
Corresponding author. Tel.: +1 410 516 4670; fax: +1 410 516
8420; e-mail: ghp@jhu.edu
0968-0896/$ - see front matter ꢀ 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmc.2008.03.010

5248
G. H. Posner et al. / Bioorg. Med. Chem. 16 (2008) 5247–5253
H
H
H
H
O
O
O
O
O
O
O
O
H
H
H
H
1. LDA
DIBAl-H
90%
O
O
O
O
O
1
9
9
9
9
2. NFSI, -78 oC
85%
O
O
10
10
O 10
O
10
F
F
2
3
O
OH
MeOH
OH
H
O
6
7a
TsOH
Scheme 1. 9-Fluoroartemisinin and 9-fluoro-DHA.
30 min, rt
O
O
H
2. Results and discussion
2.1. Fluoro derivatives
9
O 10
F
7a
7a
OH
Scheme 3. Competition experiment.
The synthesis of 9-fluoroartemisinin (6) was accom-
plished by reacting the lithium enolate of 1 with the elec-
trophilic
fluorinating
reagent
N-fluorobenzene-
OH
OH
OH
sulfonimide (NFSI) in 85% yield. The attachment of
the fluorine atom was exclusive to the b-face of artemis-
inin, as evident from X-ray crystal analysis. Attempts to
fluorinate 1 on the a-face using smaller reagents (Select-
flour or N-fluoropyridinium triflate) or by using NFSI at
elevated temperatures did not lead to the desired diaste-
reomer. Reduction of 6 using diisobutylaluminium hy-
dride (DIBAl-H) cleanly afforded 9-fluoro-DHA (7a)
in 90% yield (Scheme 1).
OH
R
HClO4
25 oC
HO
HO
2
2
1
1
O
O
R
2-
OPO3
OH
Relative Rates: R=H > R=OH > R=F
Scheme 4. Acid-catalyzed hydrolysis of sugar systems.
ration of the C-10-CF3 group, which acts to disfavor
carbocation formation at C-10.7
In an attempt to synthesize the fluoro analog of arteme-
ther (3), hemiacetal 7a was treated with camphorsulfonic
acid (CSA) in methanol. Unlike the rapid formation of
artemether (3) using the same procedure,12 the forma-
tion of 7b and 7c was sluggish (7b/7c = 2:1, 98% yield
at 60% conversion), reinforcing our hypothesis that the
9-fluorine atom disfavors carbocation formation at C-
10 (Scheme 2).
This result is also in agreement with work done on 2-
substituted a-D-glucopyranosyl phosphate systems.14–16
A series of analogs were prepared and subjected to
acid-catalyzed hydrolysis. It was found that the relative
rates followed the order 2-deoxy > 2-hydroxy > 2-
deoxy-2-fluoro (Scheme 4). The fluorine atom adjacent
to the anomeric center acts to inductively withdraw elec-
tron density, affecting both the equilibrium constant for
protonation as well as destabilizing a developing carbo-
cation at C-1.15
A competition etherification experiment was performed
at room temperature using 1 equivalent of DHA (2), 1
equivalent of 9-fluoro-DHA (7a) and 0.5 equivalent of
methanol in CDCl3 solvent in an NMR tube with a cat-
alytic amount of p-toluenesulfonic acid (Scheme 3).
After only 1 min, a small amount of artemether (3) (sin-
Wanting to synthesize the 9-fluoro derivative of sodium
artesunate (4) for biological testing, 7a was treated with
succinic anhydride in pyridine to afford, after aqueous
acidic work-up, 9-fluoroartesunic acid (7d) in 62% yield
(Scheme 5) as an inseparable mixture of C-10 diastereo-
mers (C-10 a/b = 14:1).
1
glet at d 3.43 ppm) was seen in the H NMR spectrum.
After 0.5 h, all the methanol was consumed, as deter-
1
mined by H NMR spectroscopy. The ratio of methyl
ether products 3:7b was at least 50:1 based on a 1H
NMR standard.13 This result confirmed that the fluorine
atom does in fact function to retard C-10 carbocation
formation. A similar observation was made during the
hydrolysis of C-10 trifluoromethyl artemether versus
the hydrolysis of artemether itself.7 The authors noted
a 60-fold lowering of the hydrolysis rate by the incorpo-
2.2. Sulfonyl derivatives
The synthesis of 9-methanesulfonylartemisinin (8) began
with the reaction of the lithium enolate of 1 with S-methyl
methanethiosulfonate affording a mixture of diastereo-
mers. The crude sulfide products were oxidized directly
in the presence of m-chloroperoxybenzoic acid (mCPBA)
H
O
H
O
H
O
H
O
O
O
O
CSA
O
O
7a
7a
9
pyridine
9
O
10
O
10
MeOH
F
O
F
62%
98%
O
O
OH
7b, C-10-α-OMe
7c, C-10-β-OMe
O
7d
Scheme 2. 9-Fluoroartemether.
Scheme 5. 9-Fluoroartesunic acid.

G. H. Posner et al. / Bioorg. Med. Chem. 16 (2008) 5247–5253
5249
O
H
H
H
H
1. LDA, -78 oC
Cl
Ar
55%
O
O
O
O
O
O
O
O
H
H
H
H
1
O
O
O
O
O
O
2. PhSSO2Ph
3. mCPBA
4. NaBH4
SO2Me
SO2Me
9a
( )4
9
9
O
10
9
9
O
10
O
10
O
10
SO2Ph
SO2Ph
10a
1. LDA
NaBH4
O 8a
+
H
OH
+
O
OH
10b
1
51%
log P = 8.1
2. MeSSO2Me
O
H
-78 o
C
O
O
O
O
H
H
3. mCPBA
Scheme 7. 9-Benzenesulfonyl p-pentylbenzoate ester.
77%
9
9
O
10
O
10
SO2Me
SO2Me
9b
OH
with p-pentylbenzoyl chloride to give analog 10b in 55%
yield and as a single diastereomer (Scheme 7).
8b
O
Scheme 6. 9-Methanesulfonyl systems.
3. Conclusion
to give 9-methanesulfonylartemesinin (9-SO2Me a/
b = 1:2) in 77% yield. The pair of diastereomers 8a and
8b was easily separable by column chromatography and
identified by X-ray crystallography. These lactones were
separately reduced with sodium borohydride (NaBH4)
to give a-9-SO2Me-DHA (9a) in 85% yield and b-9-
SO2Me-DHA (9b) in 87% yield (Scheme 6).
Several 9-substituted artemisinin analogs have been pre-
pared. The incorporation of the strongly inductive elec-
tron-withdrawing fluoro and sulfonyl groups led to
compounds in which the formation of the C-10 carbocat-
ion was retarded. A competition experiment between
DHA (2) and 9-fluoro-DHA (7a) solidified this claim, as
acid-promoted etherification was at least 50 times slower
for the formation of 9-fluoroartemether (7b and 7c) than
for the formation of artemether (3). When the fluoro sub-
stituent was replaced by a methanesulfonyl group, ether-
ification was not observed at all even under forcing
conditions presumably due to the instability of the C-10
carbocation. It was plausible to expect, therefore, that
the formation of the toxic 9-substituted-DHA via
in vivo hydrolysis of the corresponding orally-adminis-
tered ether and ester derivatives would be suppressed
due to the electron-withdrawing groups at C-9. Disap-
pointingly, however, the 9-fluoroartemethers 7b and 7c
and 9-fluoroartesunic acid (7d) as well as 9-sulfonyl ben-
zoates 9d and 10b were not strongly antimalarial upon
oral administration to rodents. As compared with our re-
cently reported antimalarials which upon oral adminis-
tration to mice at a 1 · 30 mg/kg dose, typically prolong
survival up to 11 days post infection21 these 9-fluoro
and 9-sulfonyl derivatives did not prolong mouse survival
past 7 days, which is the typical lifespan of malaria-in-
fected mice receiving no treatment at all.21 This lack of
antimalarial activity in our analogs, however, may be
due to changes in lipophilicity, transport, and/or metabo-
lism rather than to only the increased hydrolytic stability
of these new 9-substituted trioxanes. Alternatively, the
lack of activity may be due to the orientation of the C-9-
a-methyl group,22 which is on the non-natural a-face of
artemisinin. This orientation may inhibit one of the two
known radical pathways of artemisinin metabolism.23–25
Our attempts to synthesize 9-a-fluoroartemisinin have
not afforded promising results.
The reduction from 8a to 9a and from 8b to 9b pro-
ceeded stereospecifically; in each case only one lactol
product was formed. This result is most likely due to
the unzipping and rezipping of the lactol, under basic
conditions, to form the more thermodynamically stable
product.17 In an effort to synthesize the methanesulfonyl
analog of artemether (3), lactols 9a and 9b were each
treated with an equimolar amount of CSA in methanol
at room temperature. After 2 days, no reaction occurred
even when subsequently heated to reflux for 30 h. These
results suggest that carbocation formation at C-10 is
exceptionally retarded when a methanesulfonyl group
is present at C-9. Indeed, the inductive electron-with-
drawing ability of a sulfonyl group is known to be con-
siderably greater than that of a fluorine atom.11
To avoid the aforementioned toxicity associated with
DHA (2), a series of electronically stabilized C-10 esters
were synthesized for biological testing. Based on recent
reports that optimal antimalarial oral efficacy was
achieved with trioxanes of relatively high lipophilicity
(logP = 8)18,19 we prepared 10-stearoyl (9c) and 10-p-
decylbenzoyl (9d) esters (Fig. 3).
In order to maintain the high lipophilicity of these mol-
ecules while at the same time reducing the aliphatic
chain length, it was necessary to substitute the 9-meth-
anesulfonyl group with a 9-benzenesulfonyl group.20
The preparation of 10a followed a similar sequence to
that of 9b, affording a separable 12:1 mixture of diaste-
reomers in 51% yield. The major lactol 10a was treated
O
4. Experimental
4.1. General
H
9c, R=
9d, R=
O
O
( )16
log P = 7.8
H
O
O
9
O
10
SO2Me
log P = 8.0
( )9
All reactions were performed in oven dried glassware,
and stirred under an atmosphere of ultra high purity Ar-
gon. The tetrahydrofuran used was distilled from so-
dium/benzophenone, and the dichloromethane was
OR
Figure 3. 9-Methanesulfonyl 10-esters.

5250
G. H. Posner et al. / Bioorg. Med. Chem. 16 (2008) 5247–5253
distilled from calcium hydride. All other reagents were
used as received from commercial suppliers, unless
otherwise noted. Analytical TLC analysis was con-
ducted on precoated glass-backed silica gel plates
(Merck Kieselgel 60 F254, 250 l thickness) and visual-
ized with p-anisaldehyde or KMnO4 stains. Column
chromatography was preformed using flash silica gel
(particle size 230–400 mesh). Yields reported are for
pure products (>95% based on their chromatographic
and spectroscopic homogeneity), and are unoptimized.
NMR spectra were obtained on a Bruker-400 spectrom-
dried over MgSO4, and concentrated in vacuo. The crude
product was purified by silica gel chromatography (30%
ethyl acetate, 70% hexanes) to give 7a (0.136 g, 90%) as
an amorphous white solid and a 1:1 mixture of insepara-
21
D
3446, 2949, 2931, 2858, 1457, 1385, 1159, 1122, 1023,
ble isomers. ½aꢁ 69.8 (c 0.780, CDCl3); IR (neat) mmax
1
1005, 915, 869, 824 cmꢀ1; H NMR (400 MHz, CDCl3)
d 5.52 (s, 1H), 5.42–5.37 (m, 2H), 5.16–5.14 (m, 1H),
3.79–3.77 (m, 1H), 3.57–3.53 (m, 1H), 2.37–2.23 (m,
2H), 2.07–1.84 (m, 9H), 1.71–1.64 (m, 3H), 1.61–1.50
(m, 5H), 1.47–1.22 (m, 13H), 1.00–0.94 (m, 8H); 13C
NMR (100 MHz, CDCl3) d 104.5, 103.1, 94.7, 94.5,
93.9, 93.7, 92.9, 92.7, 91.5, 91.2, 89.1, 82.0, 81.9, 81.21,
81.17, 51.6, 51.47, 51.46, 50.44, 50.38, 50.22, 50.18,
37.23, 37.17, 36.3, 36.1, 33.59, 33.56, 33.45, 33.43, 26.0,
25.8, 25.6, 25.4, 24.3, 24.2, 23.9, 23.8, 23.0, 22.9, 20.1,
19.9, 19.8, 19.6; HRMS (FAB, M+1) calcd 325.14217
for C15H24FO5, found 325.14169.
1
eter, operating at 400 MHz for H, and 100 MHz for
13C. Chemical shifts are reported in ppm (d) and are ref-
erenced to CDCl3 (7.26 and 77.0 ppm). Mass spectra
were obtained using fast atom bombardment in the po-
sitive ion mode on a VG Instruments VG70S or electro-
spray ionization in the positive ion mode on a Bruker 12
Tesla APEX-Qe FTICR-MS with an Apollo II ion
source. IR spectra were carried out on a Bruker Vector
22 spectrometer from liquid films. [a]D values were re-
corded in CHCl3 on a Jasco P-1010 polarimeter. logP
values were calculated by using MarvinSketch and a cal-
culator plug-in by ChemAxon Kft.
4.4. Synthesis of 9-fluoroartemethers (7b and 7c)
A 5 mL roundbottom flask was charged with 7a (0.068 g,
0.22 mmol) and dissolved in methanol (1 mL). CSA
˚
(0.052 g, 0.22 mmol) and 6 4A mol sieves were added,
4.2. Synthesis of 9-fluoroartemesinin (6)
and the reaction was allowed to stir at room temperature
for 48 h, at which point the reaction did not appear to be
progressing any further by TLC analysis. The reaction
was quenched with H2O, washed with a saturated aque-
ous solution of NaHCO3, brine, dried over MgSO4, and
concentrated in vacuo. The crude products were purified
by silica gel chromatography (10% ethyl acetate, 90% hex-
n
A 25 mL roundbottom flash was charged with BuLi
(1.33 mL, 1.6 M, 2.1 mmol) and diluted with THF
(5 mL) before being cooled to ꢀ10 ꢁC. Diisopropyl-
amine (0.37 mL, 2.7 mmol) was added to the solution
dropwise, and allowed to stir for 10 min before cooling
to ꢀ78 ꢁC. Artemisinin (0.500 g, 1.8 mmol) dissolved
in THF (5 mL) was added via cannula, and allowed to
stir for 1 h before adding NFSI (1.68 g, 5.3 mmol). After
60 min, the reaction was quenched with H2O, washed
with a saturated aqueous solution of citric acid, fol-
lowed by a saturated aqueous solution of NaHCO3,
dried over MgSO4, and concentrated in vacuo. The
crude product was purified by silica gel chromatography
anes) to afford 7b (0.027 g, 38%), 7c (0.015 g, 21%), and
21
D
recovered 7a (0.027 g, 38%). Major (7b): ½aꢁ 22.3 (c
0.825, CDCl3); IR (neat) mmax 2946, 2888, 2856, 1463,
1379, 1183, 1127, 1025 cmꢀ1 1H NMR (400 MHz,
;
CDCl3) d 5.37 (m, 1H), 4.68 (d, J = 3.2 Hz, 1H), 3.58 (s,
3H), 2.39–2.31 (m, 1H), 2.09–1.99 (2H), 1.95–1.87 (m,
2H), 1.72–1.67 (m, 5H), 1.51–1.22 (m, 6H), 1.05–0.87
(m, 4H); 13C NMR (100 MHz, CDCl3) d 104.5, 98.7,
98.4, 94.2, 92.5, 91.4, 82.02, 81.96, 57.4, 51.76, 51.75,
50.5, 50.3, 37.3, 36.2, 33.5, 25.5, 24.3, 23.1, 23.0, 20.5,
(10% ethyl acetate, 90% hexanes) to give 6 (0.452 g,
25
D
85%) as an amorphous white solid. ½aꢁ 84.2 (c 0.005,
CDCl3); IR (neat) mmax 3000, 2973, 2964, 2928, 2883,
2856, 1755, 1457, 1376, 1232, 1205, 1124, 1033, 979,
20.3, 20.2; HRMS (ESI, M+Na) calcd 285.15021 for
21
D
C16H26FO5, found 285.15023. Minor (7c): ½aꢁ 111.2 (c
1
871 cmꢀ1; H NMR (400 MHz, CDCl3) d 5.91 (s, 1H),
0.525, CDCl3); IR (neat) mmax 2974, 2927, 2853, 1445,
1
2.43–2.36 (m, 1H), 2.16–1.96 (m, 4H), 1.873–1.871 (m,
1H), 1.810–1.808 (m, 2H), 1.78–1.77 (m, 1H), 1.49–
1.37 (m, 7H), 1.28–1.24 (m, 1H), 1.08–1.04 (m, 1H),
0.99 (d, J = 5.6 Hz, 3H); 13C NMR (100 MHz, CDCl3)
d 168.5, 168.3, 105.5, 94.3, 91.9, 90.1, 80.92, 80.86,
50.7, 50.6, 50.0, 49.8, 37.5, 35.9, 33.1, 33.08, 27.5, 27.3,
25.0, 24.6, 23.6, 23.5, 19.7; HRMS (FAB, M+1) calcd
301.14513 for C15H22FO5, found 301.14336.
1371, 1099, 1025 cmꢀ1; H NMR (400 MHz, CDCl3) d
5.46 (s, 1H), 4.75 (s, 1H), 3.53 (s, 3H), 2.34–2.29 (m,
1H), 2.06–1.87 (m, 5H), 1.71 (s, 1H), 1.67–1.62 (m, 3H),
1.58–1.20 (m, 6H), 0.96–0.90 (m, 4H); 13C NMR
(100 MHz, CDCl3) d 104.0, 103.1, 102.8, 92.6, 90.7,
87.7, 81.93, 81.88, 77.2, 56.4, 52.4, 49.7, 49.4, 37.3, 36.4,
33.8, 27.1, 26.9, 25.7, 24.2, 23.9, 23.8, 20.2; HRMS
(FAB, M+1) calcd 285.15021 for C16H26FO5, found
285.15016.
4.3. Synthesis of 9-fluoro-DHA (7a)
4.5. Synthesis of 9-fluoroartesunic acid (7d)
A 15 mL roundbottom flask was charged with 6 (0.150 g,
0.50 mmol), dissolved in CH2Cl2 (8 mL), and cooled to
ꢀ78 ꢁC. DIBAl-H (0.75 mL, 1.0 M, 0.75 mmol) was
added dropwise over 5 min. After 2 h, the reaction was
quenched with a saturated aqueous solution of Na2SO4,
and filtered over Celite. The crude mother liquor was
washed with a saturated aqueous solution of citric acid,
followed by a saturated aqueous solution of NaHCO3
A 10 mL roundbottom flask was charged with 7a
(0.025 g, 0.08 mmol) and dissolved in CH2Cl2
(2.5 mL). Pyridine (13 lL, 0.16 mmol) and dimethylami-
nopyridine (0.007 g, 0.6 mmol) were added neatly at
room temperature. After 60 min, the reaction was
quenched with H2O, washed with a saturated aqueous
solution of citric acid, followed by a saturated aqueous

G. H. Posner et al. / Bioorg. Med. Chem. 16 (2008) 5247–5253
5251
solution of NaHCO3, brine, dried over MgSO4, and
concentrated in vacuo. The crude products were purified
by silica gel chromatography (50% ethyl acetate, 50%
cooled to 0 ꢁC. NaBH4 (0.155 g, 4.11 mmol) was added
slowly over 5 min. After 1 h, the reaction was quenched
with a solution of 30% acetic acid in MeOH, and stirred
for 15 min. The resultant solution was concentrated in
vacuo, dissolved in CH2Cl2 (20 mL), and stirred with a
solution of saturated aqueous NaHCO3. The mixture
was extracted with CH2Cl2 dried over MgSO4, and con-
centrated in vacuo. The crude product was purified by
silica gel chromatography (70% ethyl acetate, 30% hex-
hexanes) to afford 7d (0.018 g, 62%) as an inseparable
21
D
14:1 mixture of isomers at C-10. Major: ½aꢁ 34.3 (c
0.110, CDCl3); IR (neat) mmax 3338, 2927, 1752, 1706,
1445, 1379, 1146, 1025 cmꢀ1 1H NMR (400 MHz,
;
CDCl3) d 6.06 (d, J = 4.0 Hz, 1H), 5.48 (m, 1H), 2.80–
2.69 (m, 4H), 2.38–2.30 (m, 1H), 2.09–1.86 (m, 4H),
1.75–1.67 (m, 4H), 1.55–1.23 (m, 8H), 1.06–0.83 (m,
5H); 13C NMR (100 MHz, CDCl3) d 177.1, 104.6,
93.0, 91.9, 91.2, 89.2, 88.9, 81.8, 81.7, 51.7, 50.6, 50.4,
37.2, 36.1, 33.4, 28.9, 25.5, 24.1, 23.0, 22.9, 21.0, 20.8,
20.6, 20.1; HRMS (ESI, M+Na) calcd 425.15822 for
C19H27FO8Na, found 425.15848.
anes) to give 9a (0.126 g, 85%) as an amorphous white
21
D
solid and a single diastereomer. ½aꢁ 217.2 (c 0.105,
CDCl3); IR (neat) mmax 3435, 2923, 2918, 2896, 1498,
1
1355, 1233, 1160, 1036, 996, 906, 886 cmꢀ1; H NMR
(400 MHz, CDCl3) d 6.20 (s, 1H), 5.38 (s, 1H), 4.44
(br s, 1H), 2.99 (s, 1H), 2.52–2.29 (m, 2H), 2.05–1.80
(m, 3H), 1.66–1.62 (m, 1H), 1.56–1.00 (m, 13H), 0.93
(d, J = 5.6 Hz, 3H); 13C NMR (100 MHz, CDCl3) d
102.8, 89.9, 89.8, 80.7, 68.6, 51.9, 41.7, 38.3, 36.6, 36.2,
33.7, 27.1, 25.5, 24.2, 20.9, 19.6; HRMS (ESI, M+Na)
calcd 385.12914 for C16H26O7S, found 385.12901.
4.6. Synthesis of 9-methanesulfonylartemisinin (8a and 8b)
n
A 25 mL roundbottom flash was charged with BuLi
(1.25 mL, 1.6 M, 1.9 mmol) and diluted with THF
(4 mL) before being cooled to ꢀ10 ꢁC. Diisopropyl-
amine (0.30 mL, 1.9 mmol) was added to the solution
dropwise, and allowed to stir for 10 min before cooling
to ꢀ78 ꢁC. Artemisinin (0.500 g, 1.8 mmol) dissolved
in THF (3 mL) was added via cannula, and allowed to
stir for 1 h before adding S-methyl methanethiosufate
(0.9 mL, 8.9 mmol). The crude sulfide product was oxi-
dized without purification with mCPBA (1.5 g,
8.8 mmol) in CH2Cl2 for 16 h. The reaction was
quenched with H2O, washed with a saturated aqueous
solution sodium bisulfite, followed by a saturated aque-
ous solution of NaHCO3, dried over MgSO4, and con-
centrated in vacuo. The crude product was purified by
silica gel chromatography (30% ethyl acetate, 70% hex-
anes) to give the minor diastereomer 8a and the major
4.8. Synthesis of 9-methanesulfonyl-DHA (9b)
Prepared in the same manner as 9a The crude product
was purified by silica gel chromatography (70% ethyl
acetate, 30% hexanes) to give 9b (0.09 g, 87%) as an
amorphous white solid and a single diastereomer. ½aꢁ
21
D
44.0 (c 0.173, CDCl3); IR (neat) mmax 3330, 2922, 2960,
1524, 1446, 1373, 1237, 1145, 1024, 1005, 920,
1
725 cmꢀ1; H NMR (400 MHz, CDCl3) d 5.52 (s, 1H),
5.42 (s, 1H), 4.61( br s, 1H), 3.06 (s, 1H), 2.52–2.47
(m, 1H), 2.38–2.30 (m, 1H), 2.21–2.15 (m, 1H), 2.02–
1.75 (m, 5H), 1.70–0.80 (m, 14H) includes 0.92 (d,
J = 5.6 Hz); 13C NMR (100 MHz, CDCl3) d 104.6,
91.2, 90.8, 81.2, 67.3, 52.1, 47.0, 41.9, 36.8, 36.1, 33.7,
25.7, 25.3, 24.5, 20.0, 15.0 HRMS (ESI, M+Na) calcd
385.12816 for C16H26O7S, found 385.12811.
diastereomer 8b in 77% yield (8a/8b = 1:2) as amorphous
20
D
white solids. Minor (8a): ½aꢁ 174.1 (c 0.290, CDCl3); IR
(neat) mmax 3019, 2986, 2956, 2944, 2863, 2846, 1759,
1350, 1246, 1165, 1045, 1001, 856 cmꢀ1
;
1H NMR
4.9. Synthesis of C-10-stearoyl ester 9c
(400 MHz, CDCl3) d 5.95 (s, 1H), 3.13 (s, 1H), 2.75–
2.71 (m, 1H), 2.39–2.31 (m, 1H), 2.13–2.05 (m, 1H),
1.97–1.85 (m, 1H), 1.78–1.72 (m, 3H), 1.71–1.41 (m,
6H), 1.39–1.24 (m, 4H), 1.16–1.01 (m, 3H), 0.99 (d,
J = 5.5 Hz, 3H); 13C NMR (100 MHz, CDCl3) d 168.1,
105.5, 94.7, 80.7, 68.1, 50.9, 42.3, 38.6, 36.9, 35.7, 33.6,
26.2, 25.4, 24.3, 19.6, 19.3; HRMS (FAB, M+1) calcd
A 25 mL roundbottom flask was charged with 9b
(0.102 g, 0.28 mmol), DMAP (0.03 g, 0.28 mmol), pyri-
dine (1 mL), dissolved in CH2Cl2 (1 mL), and cooled
to 0 ꢁC. To the mixture was added stearoyl chloride
(0.94 mL, 0.28 mmol) neat. After 2 h, the reaction was
quenched with a solution of 10% HCl (10 mL) and
CH2Cl2 (10 mL). The reaction mixture was washed with
an aqueous solution of 50% citric acid, followed by a
saturated aqueous solution of brine, dried over MgSO4,
and concentrated in vacuo. The crude product was puri-
fied by silica gel chromatography (20% ethyl acetate,
361.13210 for C16H24O7S, found 361.13088. Major
20
D
2986, 2956, 2944, 2863, 2846, 1759, 1350, 1246, 1165,
(8b): ½aꢁ 38.9 (c 1.030, CDCl3); IR (neat) mmax 3019,
1
1045, 1001, 856 cmꢀ1; H NMR (400 MHz, CDCl3) d
6.11 (s, 1H), 3.10 (s, 1H), 2.71–2.67 (m, 1H), 2.35–2.27
(m, 1H), 2.20–2.16 (m, 1H), 1.96–1.85 (m, 1H), 1.76–
1.68 (m, 3H), 1.67–1.40 (m, 6H), 1.39–1.24 (m, 4H),
1.16–1.01 (m, 2H), 0.99 (d, J = 5.6 Hz, 3H); 13C NMR
(100 MHz, CDCl3) d 167.1, 105.4, 94.4, 81.8, 68.4,
50.5, 47.8, 39.3, 37.2, 35.5, 33.8, 26.3, 24.96, 24.93,
24.7, 19.5; HRMS (FAB, M+1) calcd 361.13210 for
C16H24O7S, found 361.13088.
80% hexanes) to give 9c (0.08 g, 48%) as an oil and a sin-
21
D
gle diastereomer. ½aꢁ ꢀ74.5 (c 0.423, CDCl3); IR (neat)
mmax 2956, 2965, 2892, 1759, 1475, 1355, 1233, 1115,
1
1026, 996, 905, 876 cmꢀ1; H NMR (400 MHz, CDCl3)
d 6.26 (s, 1H), 5.50 (s, 1H), 2.94 (s, 1H), 2.60–2.40 (m,
2H), 2.38–2.33 (m, 3H), 2.30–2.15 (m, 1H), 2.10–0.80
(m, 51H); 13C NMR (100 MHz, CDCl3) d 170.3,
104.4, 91.6, 88.7, 81.1, 65.9, 52.1, 47.2, 41.7, 36.6, 36.0,
34.0, 33.5, 31.8, 29.59, 29.56, 29.55, 29.54, 29.50,
29.48, 29.32, 29.26, 29.1, 28.98, 28.86, 25.5, 25.2, 24.3,
24.2, 22.6, 19.9, 15.7, 14.0; HRMS (ESI, M+Na) calcd
651.39011 for C34H60O8SNa, found 651.38737.
4.7. Synthesis of 9-methanesulfonyl-DHA (9a)
A 100 mL roundbottom flask was charged with 8a
(0.142 g, 0.41 mmol), dissolved in MeOH (30 mL), and

5252
G. H. Posner et al. / Bioorg. Med. Chem. 16 (2008) 5247–5253
4.10. Synthesis of 10-p-decylbenzoyl ester 9d
7.57 (m, 1H), 7.56 (t, J = 4 Hz, 2H), 5.67 (s, 1H), 5.47 (s,
1H), 2.82–2.79 (m, 1H), 2.39–2.30 (m, 3H), 2.03–1.92
(m, 3H), 1.81–1.66 (m, 3H), 1.60–1.15 (m, 8H), 1.10–
0.80 (m, 3H); 13C NMR (100 MHz, CDCl3) d 139.1,
134.3, 133.3, 129.6, 104.5, 91.2, 81.5, 77.3, 69.0, 52.2,
47,9, 37.1, 36.1, 33.9, 25.9, 25.3, 24.6, 20.1, 16.3; HRMS
(ESI, M+Na) calcd 447.14479 for C21H28O7S, found
447.14371.
A 25 mL roundbottom flask was charged with 9b
(0.06 g, 0.16 mmol), DMAP (0.017 g, 0.28 mmol), pyri-
dine (1 mL), dissolved in CH2Cl2 (1 mL), and cooled
to 0 ꢁC. To the mixture was added p-decylbenzoyl chlo-
ride (0.10 g, 0.16 mmol). After 2 h, the reaction was
quenched with a solution of 10% HCl (10 mL) and
CH2Cl2 (10 mL). The reaction mixture was washed with
an aqueous solution of 50% citric acid followed by a sat-
urated aqueous solution of brine, dried over MgSO4,
and concentrated in vacuo. The crude product was puri-
fied by silica gel chromatography (30% ethyl acetate,
4.12. Synthesis of 10-p-pentylbenzoyl ester 10b
A 25 mL roundbottom flask was charged with 10a (0.06 g,
0.16 mmol), DMAP (0.017 g, 0.28 mmol), pyridine
(1 mL), dissolved in CH2Cl2 (1 mL), and cooled to 0 ꢁC.
To this mixture was added p-pentylbenzoyl chloride
(0.045 g, 0.16 mmol) neatly. After 12 h, the reaction was
quenched with an aqueous solution of 10% HCl (10 mL)
and CH2Cl2 (10 mL), washed with an aqueous solution
of 50% citric acid followed by saturated aqueous solution
of brine, dried over MgSO4, and concentrated in vacuo.
The crude product was purified by silica gel chromatogra-
phy (30% ethyl acetate, 70% hexanes) to give 10b (0.05 g,
70% hexanes) to give 9d (0.07 g, 70%) as an oil and a sin-
21
D
gle diastereomer. ½aꢁ 185.5 (c 0.425, CDCl3); IR (neat)
mmax 2948, 2926, 2878, 1761, 1625, 1510, 1471, 1355,
1233, 1125, 1036, 979, 905, 855 cmꢀ1
;
1H NMR
(400 MHz, CDCl3) d 7.98 (d, J = 8 Hz, 2H), 7.27 (d,
J = 8 Hz, 2H), 6.59 (s, 1H), 5.61 (s, 1H), 2.94 (s, 1H),
2.70–2.44 (m, 3H), 2.40–2.22 (m, 2H), 2.10–1.90 (m,
4H), 1.85–1.10 (m, 25H), 1.00–0.90 (m, 8H); 13C NMR
(100 MHz, CDCl3) d 163.2, 149.7, 130.0, 128.8, 126.3,
104.5, 91.7, 89.0, 81.3, 77.3, 66.4, 52.3, 47.5, 41.6, 36.8,
36.2, 36.1, 33.7, 31.9, 31.0, 29.6, 29.5, 29.4, 29.3, 28.2,
25.7, 25.3, 24.4, 22.6, 20.0, 14.0; HRMS (ESI, M+Na)
calcd 629.31186 for C34H60O8SNa, found 629.30985.
55%) as a white solid and a single diastereomer. ½aꢁ21 11.5
D
(c 0.435, CDCl3); IR (neat) mmax 2955, 2943, 2823, 1758,
1655, 1625, 1510, 1505, 1466, 1342, 1256, 1125, 1040,
981, 910, 855 cmꢀ1; 1H NMR (400 MHz, CDCl3) d 7.65
(d, J = 8 Hz, 2H), 7.56 (d, J = 8 Hz, 2H), 7.45 (t,
J = 7.6 Hz, 1H), 7.20–7.10 (m, 4H), 6.73 (s, 1H), 5.63 (s,
1H), 2.88–2.84 (m, 1H), 2.66 (t, J = 7.6 Hz, 2H), 2.42–
2.31 (m, 2H), 2.04–1.86 (m, 3H), 1.85–1.70 (m, 3H),
1.68–1.45 (m, 5H), 1.44–1.10 (m, 10H), 1.08–0.80 (m,
5H) includes 0.99 (d, J = 5.6 Hz) and 0.91 (t,
J = 7.2 Hz); 13C NMR (100 MHz, CDCl3) d 163.6,
149.4, 138.8, 133.0, 130.9, 130.6, 128.4, 128.2, 126.3,
104.5, 91.4, 89.0, 81.5, 77.3, 68.1, 52.3, 48.2, 37.0, 36.2,
35.9, 33.9, 31.3, 30.8, 26.0, 25.3, 24.6, 22.4, 20.1, 17.7,
14.0; HRMS (ESI, M+Na) calcd 621.24926 for
C34H60O8SNa, found 621.2492.
4.11. Synthesis of 9-benzenesulfonyl-DHA (10a)
n
A 25 mL roundbottom flash was charged with BuLi
(1.25 mL, 1.6 M, 1.9 mmol) and diluted with THF
(4 mL) before being cooled to ꢀ10 ꢁC. Diisopropylamine
(0.30 mL, 1.9 mmol) was added to the solution dropwise,
and allowed to stir for 10 min before cooling to ꢀ78 ꢁC.
Artemisinin (0.500 g, 1.8 mmol) dissolved in THF
(3 mL) was added via cannula, and allowed to stir for
1 h before adding S-phenyl benzenethiosulfate (1.27 g,
5.1 mmol). After 1 h, the reaction was worked up, and
the crude sulfide product was oxidized without purifica-
tion with mCPBA (1.7 g, 9.8 mmol) in CH2Cl2 for 16 h.
The reaction was quenched with H2O, washed with a sat-
urated aqueous solution sodium bisulfite, followed by a
saturated aqueous solution of NaHCO3, dried over
MgSO4, and concentrated in vacuo. The crude product
was purified by silica gel chromatography (30% ethyl ace-
tate, 70% hexanes) to give a pair of diastereomer products
9-phenylsulfoneartemisinin in ratio of 1:12. The major
Acknowledgments
This work was supported by the National Institutes of
Health (Grant AI 34885). We thank Dr. Amy Sarjeant
for X-ray crystal analysis and Dr. Phil Mortimer and
Mrs. Susan Hatcher for mass analysis as well as Dr.
Matthias Rottmann at the Swiss Tropical Institute for
oral antimalarial testing in rodents.
product,
b-9-penylsulfoneartemisinin
(0.150 g,
0.35 mmol), was placed in a 100 mL roundbottom flask,
dissolved in MeOH (20 mL), and cooled to 0 ꢁC. NaBH4
(0.135 g, 3.55 mmol) was added slowly over 5 min. After
1.5 h, the reaction was quenched with a solution of 30%
acetic acid in MeOH, and stirred for 15 min. The solution
was concentrated in vacuo, dissolved in 20 mL of CH2Cl2,
and stirred in a solution of saturated aqueous NaHCO3.
The mixture was extracted with CH2Cl2 dried over
MgSO4, and concentrated in vacuo. The crude product
was purified by silica gel chromatography (70% ethyl ace-
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D
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