Chemical and electro-chemical reduction of qinghaosu (artemisinin)

Article abstract of DOI:10.1039/b007056o

1,2,4-Trioxane is the essential segment of the new antimalarial agent qinghaosu 1, and hence its reduction is an important plausible process related to the bio-activity mode. An overview of its reduction and a careful examination of some reducing systems are presented herewith. Electrochemical reduction is a two-electron reduction, which is confirmed by the isolation of product deoxyqinghaosu. However, in the presence of a catalytic amount of Fe^II/III electrochemical reduction yields mainly the single-electron-reduction products, which are identified with the products from reduction of qinghaosu with one equivalent ferrous sulfate or a catalytical amount of Fe^II/III and excess of other reducing agents, such as ascorbic acid, or cysteine. These results mean that qinghaosu is reduced by ferrous ion and the resulting ferric ion is then reduced on the electrode to regenerate ferrous ion. In addition, qinghaosu could be reduced to deoxyqinghao by iodide, but not by bromide, and also could be reduced by the ascorbic acid-CuSO₄ system to give free-radical reaction products. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/b007056o

View Article Online / Journal Homepage / Table of Contents for this issue
Chemical and electro-chemical reduction of qinghaosu
(artemisinin)
Wen-Min Wu and Yu-Lin Wu*
1
State Key Laboratory of Bio-organic & Natural Products Chemistry, Shanghai Institute
of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Road, Shanghai 200032,
China. Email: ylwu@pub.sioc.ac.cn
Received (in Cambridge, UK) 30th August 2000, Accepted 27th October 2000
First published as an Advance Article on the web 30th November 2000
1,2,4-Trioxane is the essential segment of the new antimalarial agent qinghaosu 1, and hence its reduction is an
important plausible process related to the bio-activity mode. An overview of its reduction and a careful examination
of some reducing systems are presented herewith. Electrochemical reduction is a two-electron reduction, which is
confirmed by the isolation of product deoxyqinghaosu. However, in the presence of a catalytic amount of FeII/III
electrochemical reduction yields mainly the single-electron-reduction products, which are identified with the products
from reduction of qinghaosu with one equivalent ferrous sulfate or a catalytical amount of FeII/III and excess of other
reducing agents, such as ascorbic acid, or cysteine. These results mean that qinghaosu is reduced by ferrous ion and
the resulting ferric ion is then reduced on the electrode to regenerate ferrous ion. In addition, qinghaosu could be
reduced to deoxyqinghao by iodide, but not by bromide, and also could be reduced by the ascorbic acid–CuSO4
system to give free-radical reaction products.
Qinghaosu1 (artemisinin, 1, Fig. 1) isolated from Chinese herb
qinghao (Artemisia annua L.) belongs to a new generation of
antimalarial agents. Its derivatives, such as artemether2 (2) and
artesunnate3 (3), have been shown to be powerful weapons to
fight the malaria parasite, especially the multidrug-resistant
strains. Early structure–activity studies2 have already revealed
that the characteristic peroxy functionality in these compounds
is essential for their antimalarial activity. Therefore, exploring
the reduction of such peroxides could be an approach to
uncovering the antimalarial mechanism of this type of
compound. Since the period of structure determination in the
Fig. 1
1970s a series of studies on the chemical reduction have been
performed and then, in the early 1990s, the reduction of
qinghaosu and its derivatives or analogues with ferrous ion, a
process possibly occurring in malaria-infected red cells,
attracted attention from several laboratories. More recently
some reports about electrochemical reduction of qinghaosu
have also appeared in the literature, but no attempt was made to
clarify the details of this chemical process. Herewith we would
like to give an overview on all these reductions, and present
some new results, especially on the electrochemical reduction
and a comparison between both electrochemical and chemical
reduction.
the unmasked hemiketal and acetal to give the exhaustively
reduced product 8 and several partial reduction products.6,7
Recently we found that qinghaosu can also be reduced to
deoxyqinghaosu by iodide, a classic reagent for the qualitative
analysis of peroxide, but not by bromide. Triphenylphosphine
has been used as the reagent for the semiquantitative analysis of
peroxide during the structure determination of qinghaosu.1
Recent re-examination of this reduction showed that this was
a quite complicated reaction, but deoxyqinghaosu could still
be isolated among the product mixture in 23% yield.
Considering the high concentration of iron in the parasitized
erythrocytes (red blood cells), several laboratories have engaged
in a study of the reductive degradation of qinghaosu and its
derivatives with ferrous ion since the early 1990s.8 It was found
that qinghaosu could be degraded mainly to tetrahydrofuran
compound 9 and 3-hydroxydeoxyqinghaosu 10. A mechanism
via a primary and a secondary carbon-centred free radical was
proposed through careful isolation and identification of small
amounts of other reaction products, 11 and 12. The inter-
mediacy of both carbon-centred free radicals were also con-
firmed by an EPR experiment.8c,9 Thereafter we also found that,
in the presence of cysteine, qinghaosu could be degraded by even
catalytical amounts of ferrous or ferric ion.10 The isolation of
compound 13 and adduct 14, formed from the primary carbon-
centred radical by abstraction of a proton or coupling of
cysteine and then by a rearrangement, respectively, confirmed
convincingly the proposed free-radical mechanism (Scheme 2).
Results and discussion
Hydrogenation of qinghaosu in the presence of palladium on
carbon yields deoxyqinghaosu1 4, an inactive compound
containing one fewer oxygen atom (Scheme 1). This reduction
is proposed to proceed via a dihydroxy intermediate 5, which
may be obtained immediately after work-up and then cyclized
to compound 4 on standing. Reduction4 with zinc–acetic acid
also yielded this compound. Hydride reducing agents, on the
other hand, reduce preferentially the lactone in qinghaosu.
Thus, reduction of qinghaosu with sodium borohydride1 in
methanol at 0 ЊC gives dihydroqinhaosu 6, and reduction with
sodium borohydride–boron trifluoride–diethyl ether5 gives
deoxoqinghaosu 7. The more powerful reducing agent lithium
aluminium hydride may reduce the lactone, peroxide and then
DOI: 10.1039/b007056o
J. Chem. Soc., Perkin Trans. 1, 2000, 4279–4283
This journal is © The Royal Society of Chemistry 2000
4279

View Article Online
Table 1 Chemical and electrochemical reduction of QHSa
Total yield of products and their (%) proportions
1
Conversion
(%)
Run
(mmol)
Total yield
26
4
9
10
12
13
15
b
b
b
b
b
1-1
1-2
1
1
27
>98
100
23
2-1-1
2-1-2
2-2-1c
2-2-2
2-3
2-4
3-1
1
1
1
1
2.5
2.5
1
1
1
90
>98
20.5
35.5
98
67
7
98
15
90
98
20
35.5
98
67
5.6
98
≈10
1.2
3.8
2.4
16.7
0.8
12.3
100
6
0.5
41.3
26.6
7.1
19.7
63.3
9.8
31.5
45.3
26.1
11.9
3.5
1.3
14.9
13.8
47.4
37.5
2.2
b
10.3
16.6
13.9
30.4
0.5
b
b
b
b
b
b
20.7
36.8
26.7
4.2
b
b
b
b
3-2
59
3.6
7
29.3
6
19.5
22
47.2
3-3c
a 1-1, Reduction with NaI; 1–2, Reduction with Ph3P; 2-1-1, Reduction with Vc–FeCl3; 2-1-2, Reduction with Vc–0.1 equiv. FeCl3; 2-2-1, Reduction
with Vc–CuSO4ؒ5H2O; 2-2-2, Reduction with Vc–0.1 equiv. CuSO4ؒ5H2O; 2-3, Reduction with Vc–EDTA–FeIII; 2-4, Reduction with Vc–haemin.
3-1, Electrochemical reduction; 3-2, Electrochemical reduction in the presence of EDTA–FeIII; 3-3, Electrochemical reduction in the presence of
haemin. b Could not be separated. c The ratio of products was determined by HPLC.
Scheme 1
Along with a study of the chemical reaction mechanism there
is a very interesting observation that should be mentioned,
namely that the reactivity of these 1,2,4-trioxanes towards
ferrous ion is parallel to their in vivo antimalarial activity.11
In continuing our exploration of the Fe-catalyzed degrad-
ation of qinghaosu and comparing it with the electrochemical
reduction (vide infra), several other reduction systems were
recently examined. At first the Fe-catalyzed degradation was
performed with ferric chloride in the presence of excess of
ascorbic acid (Vc). This reaction proceeded smoothly as in the
reaction with stoicheiometrical amount of ferrous sulfate, and
compounds 9, 10, and 12 were obtained. Besides these
compounds traces of compound 13 and deoxyqinghaosu 4, as
well as compound 15, a new C-3 epimer of compound 12, were
also isolated and identified (Scheme 3). We have predicted the
formation of compound 15 in the reaction of qinghaosu and
FeSO4, but under that reaction condition we failed to isolate
it.8c In a further experiment it was shown that ascorbic acid–
Fe3ϩ–EDTA and ascorbic acid–haemin systems also reduced
qinghaosu to give predominantly these free-radical reduction
products. The difference was that the reduction was much more
rapid in the presence of the Fe3ϩ–EDTA complex than that in
the presence of haemin. In another experiment cupric sulfate,
instead of ferric chloride, was used for the reductive degrad-
ation. The Cu-catalyzed degradation took place to give all the
free-radical reaction products as in the case of Fe-catalyzed
degradation, but it was not so efficient as the latter. All
these experiments also showed that in the presence of other
reducing agents such as ascorbic acid a catalytical amount of
FeII/III or CuI/II was enough for the free-radical degradation
of qinghaosu.
Since 1992 a series of studies on the electrochemical reduc-
tion12 of qinghaosu and its derivatives on the electrode in the
presence of a ferric salt, ferric complex or in the absence of any
other reducing agents have been carried out. The reduction
potential of qinghaosu in 20% aq. acetonitrile with Ag/AgCl as
reference electrode was found to be Ϫ1.08 V and an irreversible
two-electron reducing process was detected. In the presence of
haemin or EDTA–Fe3ϩ 12c the reduction potential of qinghaosu
fell to Ϫ0.48 V or Ϫ0.49 V, respectively. However, no attempts
4280
J. Chem. Soc., Perkin Trans. 1, 2000, 4279–4283

View Article Online
Scheme 2
reduced rapidly to give predominantly compounds 9, 10, 12 and
15, the same as those obtained from chemical reduction of
qinghaosu by Vc–FeCl3. In the presence of haemin, qinghaosu
was reduced slowly and similar products, albeit in different
proportions, were detected. The new results are summarized in
Table 1.
The results listed in Table 1 show that the electrochemical
reduction of QHS in the absence of any additives proceeded
very slowly and only gave a two-electron-reduction product.
This situation is similar to the reduction with sodium iodide in
THF–acetic acid. In the presence of a catalytical amount of a
ferric compound the similar electrochemical reduction of QHS
proceeded much more quickly and gave predominantly the
single-electron reduction products, which means that in these
cases QHS is reduced by ferrous ion and the resultant ferric ion
is then reduced on the electrode. Similar results were also
obtained when ascorbic acid was used as the reducing agent for
the ferric ion instead of electricity. It is not clear yet whether
the single electron reduction takes place in solution or on the
surface of the electrode. However, the reduction of QHS in the
presence of EDTA–FeIII is faster than that in the presence of
haemin no matter whether electrode or ascorbic acid is used to
generate and regenerate FeII species. It suggests that whether
Scheme 3
have been made to isolate and characterize the products of the
electrochemical reduction to date.
To explore the electrochemical reduction and to compare it
with chemical reduction, reduction of qinghaosu was con-
ducted under different electrochemical reaction conditions in
our laboratory. Thus electrochemical reduction of qinghaosu
was performed in aq. acetonitrile–phosphate buffer. After
electrolyzing at 2 V for 4 h, only deoxyqinghaosu 4 was
obtained in 5% yield besides recovered qinghaosu. However, in
the presence of EDTA–Fe3ϩ qinghaosu was electrochemically
J. Chem. Soc., Perkin Trans. 1, 2000, 4279–4283
4281

View Article Online
Scheme 4
regeneration of FeII species occurs on electrode or in solution is
with saturated aq. Na2SO3, water and brine, and then dried
not a key point. Referring to our proposed unified mechanism
framework for the FeII-induced cleavage of QHS, all the reduc-
tions of QHS described above could be summarized as in
Scheme 4.
over Na2SO4. Removal of solvent gave a residue, which was
subjected to silica gel column chromatography to afford 70 mg
(26%) of deoxyqinghaosu 4 and 205 mg of recovered QHS.
1-2. Reduction with triphenylphosphine
A mixture of 282 mg (1 mmol) of QHS and 524 mg (2.0 mmol)
of triphenylphosphine in 10 mL of toluene was refluxed under
nitrogen for 6 h. To the cold reaction mixture was added 0.5 mL
of acetic acid and the stirring was continued for 2 h. TLC of the
reaction mixture showed a very complicated pattern, and only
deoxyqinghaosu 4 was detectable. After removal of toluene
under reduced pressure the obtained residue was diluted with
ethyl acetate, washed with water, and dried over MgSO4.
Removal of the solvent and chromatography on silica gel gave
60 mg (22.6%) of deoxyqinghaosu 4.
Experimental
Chemicals and general methods
A standard solution of haemin (from Shanghai Institute of
Biochemistry, Chinese Academy of Sciences) was prepared
immediately prior to each experiment by first dissolving haemin
in several drops of 0.1 M NaOH, followed by diluting this
solution with phosphate buffer (25 mM, pH 6.864) and finally
bringing it to pH 7.0 with 0.1 M HCl. The 0.05 mol LϪ1 EDTA–
FeIII was prepared by mixing 0.1 mol LϪ1 EDTA with 0.1 mol
LϪ1 FeCl3. In the electroreduction experiments, a three-
electrode cell system with a glass carbon or platinum wire as
working electrode, a platinum wire auxiliary electrode, and a
KCl-saturated Ag/AgCl reference electrode was employed.
HPLC: ALLMA C18 (4.6 × 250 mm, id 5 µm), Mobile phase:
CH3OH–water 80:20 (v/v), flow rate 1.0 mL minϪ1. Detection
wavelength: 210 nm.
Mps were measured on a ZMD-2 melting apparatus and
are uncorrected. Optical rotations were measured on a
Perkin-Elmer 241 MC autopol polarimeter and are given
in units of 10Ϫ1 deg cm2 gϪ1. IR spectra were obtained on a
Perkin-Elmer 983 spectrophotometer. H NMR spectra were
taken on an AMX-300 or Inova-600 instrument. Mass spectra
were measured on an HP 5989A spectrometer.
2-1-1. Reduction with Vc-FeCl3
To 282 mg (1 mmol) of QHS in 100 mL of 1:1 aq. acetonitrile–
water were added 1 mmol of ascorbic acid and 1 mmol of
FeCl3. The mixture was stirred at room temperature under a
nitrogen atmosphere for 4 h. Acetonitrile was removed under
reduced pressure (rotary evaporator), and the residue was
extracted with ethyl acetate (2 × 10 mL). The combined extracts
were washed with water and dried over MgSO4. After evapor-
ation of the solvent, the residue was chromatographed on a
silica gel column to give 28 mg of recovered QHS, 3 mg of
compound 4 (1.2%), 105 mg of compound 9 (41.3%), 25 mg
of compound 10 (9.8%), 80 mg of compound 12 (31.5%), 3 mg
of compound 138 (1.3%), and 38 mg of a new compound 15
(14.9%) in 90% total yield (weight-%).
1
1-1. Reduction with sodium iodide
Compound 15: mp 125–127 ЊC; [α]D20 Ϫ19.9 (c 0.8, CHCl3);
1
A mixture of 282 mg (1 mmol) of QHS, 375 mg (2.5 mmol)
of sodium iodide and 0.5 mL of acetic acid in 10 mL of THF
was stirred under nitrogen at room temperature overnight.
After removal of most of the THF under reduced pressure, the
reaction mixture was diluted with EtOAc, washed successively
IR (KBr) νmax 3509, 2981, 2877, 1746, 1706 cmϪ1; H NMR
(300 MHz; CDCl3) δ 0.99 (3H, d, J 6.1 Hz), 1.19 (3H, d, J 7.4
Hz), 2.32 (3H, s), 3.33 (1H, m), 4.44 (1H, d, J 7.9 Hz), 5.52
(1H, s); MS (m/z) 283 (M ϩ 1), 265, 207 (C15H22O5 requires C:
63.81, H: 7.85. Found: C: 63.69, H: 7.72%).
4282
J. Chem. Soc., Perkin Trans. 1, 2000, 4279–4283

View Article Online
1
All other known compounds were identified by H NMR
the solvent, the residue was chromatographed on silica gel or
(600 MHz) spectroscopy, MS, TLC, and HPLC.
analyzed by HPLC.
3-1. Electrochemical reduction of QHS without additive.
Besides 262 mg of unreduced QHS, 15 mg of deoxyqinghaosu
(4, yield 5.6%) was obtained, which was identified by 1H NMR
(300 or 600 MHz) spectroscopy, MS, TLC, and HPLC.
2-1-2. Reduction with Vc–0.1 equiv. FeCl3
The procedure was similar to 2-1-1, but 2 mmol of ascorbic acid
and 0.1 mmol of FeCl3 were used. Work-up gave no recovered
QHS, 10 mg of compound 4 (3.8%), 75 mg of compound 9
(26.6%), 29 mg of compound 10 (10.3%), 127 mg of compound
12 (45.3%), and 39 mg of compound 15 (13.8%) in 98% total
yield.
3-2. Electrochemical reduction of QHS in the presence of
EDTA–FeIII. <5 mg of unreduced QHS (<2%), 16 mg of
compound 4 (6%), 163 mg of compound 9 (59%), 19 mg
of compound 10 (7%), 17 mg of compound 12 (6%), and 61 mg
of compound 15 (22%) were separated by silica gel column
chromatography in 98% total yield (weight-%). Compounds
4, 9, 10 and 12 were identified with authoritative samples by
1H NMR (300 or 600 MHz) spectroscopy, MS, TLC, HPLC.
2-2-1. Reduction with Vc–CuSO4ؒ5H2O
The reduction was performed as 2-1-1 except for using Vc–
CuSO4ؒ5H2O instead of FeCl3. 224 mg of QHS was recovered
and the products were obtained in 20% total yield. The propor-
tions of these products were determined by RP-HPLC: 4 (2.4%),
9 (7.1%), 10 (16.6%), 12 (26.1%), 13 (0.5%), and 15 (47.4%).
3-3. Electrochemical reduction of QHS in the presence of
haemin. 240 mg of QHS was recovered. The proportions of
these obtained products were determined by RP-HPLC: 4
(0.5%), 9 (3.6%), 10 (29.3%), 12 (19.5%), and 15 (47.2%).
2-2-2. Reduction with Vc–0.1 equiv. CuSO4ؒ5H2O
The reduction was performed as 2-2-1 except for using 2 mmol
of Vc and 0.1 mmol of CuSO4ؒ5H2O instead of 1 mmol of
both reagents. 182 mg of QHS was recovered and the products
were obtained in 35.5% total yield. The proportions of these
products were determined by column chromatography: 16 mg
of compound 4 (16.7%), 20 mg of compound 9 (19.7%), 14 mg
of compound 10 (13.9%), 12 mg of compound 12 (11.9%), and
38 mg of compound 15 (37.5%).
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (29572075, 09561423, 29832020,
39870899), the Chinese Academy of Sciences (KJ951-A1-504),
and the Ministry of Science and Technology of China
(970211006-6). We thank Dr Yikang Wu for help improving the
manuscript.
2-3. Reduction with Vc–EDTA–FeIII
To 705 mg (2.5 mmol) of QHS in 100 mL of 1:1 aq.
acetonitrile–phosphate buffer (25 mM, pH 6.864) were added
5 mmol of ascorbic acid and 0.25 mmol of EDTA–FeIII. The
mixture was stirred at 25 ЊC under a nitrogen atmosphere for
4 h. Acetonitrile was removed under reduced pressure (rotary
evaporator). The residue was extracted with ethyl acetate
(3 × 10 mL). The combined extracts were washed with water
and dried over MgSO4. After evaporation of the solvent, the
residue was chromatographed on silica gel to give 11 mg of
recovered QHS and 5 mg of compound 4 (0.8%), 437 mg
of compound 9 (63.3%), 210 mg of compound 10 (30.4%),
24 mg of compound 12 (3.5%), and 15 mg of compound 15
(2.2%) in 98% total yield (weight-%). These products were
References
1 J.-M. Liu, M.-Y. Ni, J.-F. Fan, Y.-Y. Tu, Z.-H. Wu, Y.-L. Wu and
W.-S. Chou, Huaxue Xuebao, 1979, 37, 129 (Chem. Abstr., 1980, 92,
94594w).
2 Y. Li, P.-L. Yu, Y.-X. Chen, L.-Q. Li, Y.-Z. Gai, D.-S. Wang and
Y.-P. Zheng, Yaoxue Xuebao, 1981, 16, 429 (Chem. Abstr., 1983, 97,
92245n).
3 X. Liu, Yaoxue Tongbao, 1980, 15, 183.
4 Y. Li, P.-L. Yu, Y.-X. Chen, J.-L. Zhang and Y.-L. Wu, Kexue
Tongbao (Engl.), 1986, 31, 1038 (Chem. Abstr., 1987, 107, 23154v).
5 M. Jung, X. Li, D. A. Bostos, H. N. ElSohly and J. D. McChesney,
Tetrahedron Lett., 1989, 30, 973.
6 Y.-L. Wu and J.-L. Zhang, Youji Huaxue, 1986, 6, 153 (Chem. Abstr.,
1986, 105, 191426n).
7 L.-K. Sy, S.-M. Hui, K.-K. Cheung and G.-D. Brown, Tetrahedron,
1997, 53, 7493.
8 (a) J. N. Cumming, P. Polypradith and G. H. Posner, Adv.
Pharmacology, 1997, 37, 253; (b) G. H. Posner, S. B. Park,
L. Gonzalez, D.-S. Wang, J. N. Cumming, D. Klinedinst,
T. A. Shapiro and M. D. Balci, J. Am. Chem. Soc., 1996, 118, 3537;
(c) W.-M. Wu, Y.-K. Wu, Y.-L. Wu, Z.-J. Yao, C.-M. Zhou, Y. Li and
F. Shan, J. Am. Chem. Soc., 1998, 120, 3316 and references cited
therein; W.-M. Wu, Y.-K. Wu, Y.-L. Wu, Z.-J. Yao, C.-M. Zhou,
Y. Li and F. Shan, J. Am. Chem. Soc., 1998, 120, 13002 and
references cited therein.
9 A. R. Butler, B. C. Gilbert, P. Hulme, L. R. Irvine, L. Renton and
A. C. Whitwood, Free Radical Res., 1998, 28, 471.
10 Y.-K. Wu, Z.-Y. Yue and Y.-L. Wu, Angew. Chem., Int. Ed., 1999, 38,
2580.
11 D.-Y. Wang, Y.-K. Wu, Y.-L. Wu, Y. Li and F. Shan, J. Chem. Soc.,
Perkin Trans. 1, 1999, 1827.
1
identified with authoritative samples by H NMR (300 or 600
MHz) sprctroscopy MS, TLC, HPLC.
2-4. Reduction with Vc–haemin
The reduction was performed as 2-3 except for using Vc–
haemin instead of Vc–EDTA–FeIII. Together with 230 mg of
recovered QHS, 55 mg of compound 4 (12.3%), 98 mg of
compound 9 (20.7%), 174 mg of compound 10 (36.8%), 126 mg
of compound 12 (26.7%), and 20 mg of compound 15 (4.2%)
were obtained by column chromatography on silica gel in
67% total yield (weight-%). These products were identified
with authoritative samples by 1H NMR (300 or 600 MHz)
spectroscopy, MS, TLC, HPLC.
12 (a) Y. Chen, C.-X. He, S.-M. Zhu and H.-Y. Chen, J. Electrochem.
Soc., 1997, 144, 1891; (b) Y. Chen, S.-M. Zhu, H.-Y. Chen and Y. Li,
Huaxue Xuebao, 1997, 55, 921 (Chem. Abstr., 1997, 127, 311409n);
(c) Y. Chen, S.-M. Zhu and H.-Y. Chen, Huaxue Xuebao, 1998, 56,
925 (Chem. Abstr., 1998, 129, 316389g); (d) Y. Chen, J. M. Zheng,
S. M. Zhu and H. Y. Chen, Electrochim. Acta, 1999, 44, 2345;
(e) F. Zhang, D. K. Gosser Jr. and S. R. Meshnick, Biochem.
Pharmacol., 1992, 43, 1805; ( f ) H.-L. Jiang, K.-X. Chen, Y. Tang,
J.-Z. Chen, Y. Li, Q.-M. Wang, R.-Y. Ji and Q.-K. Zhuang, Indian
J. Chem. Sect. B, 1997, 36, 154; (g) Electrochemical reduction of
other bicyclic endoperoxides, ascaridole and dihydroascaridole, see:
M. S. Workentin and R. L. Donkers, J. Am. Chem. Soc., 1998, 120,
2664.
3. General procedure of electrochemical reduction
To 282 mg (1 mmol) of QHS in 100 mL of 1:1 aq. acetonitrile–
phosphate buffer (25 mM, pH = 6.864) was added haemin
(0.1 mmol) or 0.2 mL of 50 mM EDTA–FeIII (0.1 mmol), or
nothing for the control run. The mixtures were stirred at room
temperature under a nitrogen atmosphere and electrolyzed at
2.0 V for 4 h. Acetonitrile was removed under reduced pressure
(rotary evaporator). The residue was extracted with ethyl
acetate (2 × 10 mL). The combined extracts were washed
with water and dried over anhydrous MgSO4. After removal of
J. Chem. Soc., Perkin Trans. 1, 2000, 4279–4283
4283