An efficient route into synthetically challenging bridged achiral 1,2,4,5-tetraoxanes with antimalarial activity

Article abstract of DOI:10.1016/j.bmcl.2008.01.053

Here we present an efficient route into synthetically challenging bridged 1,2,4,5-tetraoxanes. The key to the success of this route is the use of H₂O₂ and catalytic I₂ to form the gem-dihydroperoxide followed by a Ag₂

Full text of DOI:10.1016/j.bmcl.2008.01.053

Available online at www.sciencedirect.com
Bioorganic & Medicinal Chemistry Letters 18 (2008) 1720–1724
An efficient route into synthetically challenging bridged
achiral 1,2,4,5-tetraoxanes with antimalarial activity
Gemma L. Ellis,a Richard Amewu,a Charlotte Hall,b Karen Rimmer,b
Steven A. Wardb and Paul M. O’Neilla,*
aDepartment of Chemistry, University of Liverpool, Liverpool L69 7ZD, UK
bLiverpool School of Tropical Medicine, Pembroke Place, Liverpool L3 5QA, UK
Received 4 December 2007; revised 11 January 2008; accepted 12 January 2008
Available online 18 January 2008
Abstract—Here we present an efficient route into synthetically challenging bridged 1,2,4,5-tetraoxanes. The key to the success of this
route is the use of H2O2 and catalytic I2 to form the gem-dihydroperoxide followed by a Ag2O mediated alkylation using 1,3-diiodo-
propane. Using this methodology a range of bridged tetraoxanes which display good in vitro antimalarial activity were synthesized.
ꢀ 2008 Elsevier Ltd. All rights reserved.
Artemisinin is an extract of the Chinese wormwood
Artemisia annua and has been used since ancient times
to treat malaria. The active pharmacophore within this
drug is the endoperoxide bridge. Semi-synthetic artemis-
inin derivatives such as artesunate and artemether are
highly potent antimalarials and exhibit little or no cross
resistance with other antimalarials, however, they have a
limited availability, high cost, poor bioavailabilty, poor
pharmacokinetic properties and all derivatives are chiral
and are synthesized from the natural product artemisi-
nin (Fig. 1).1–4
ide pharmacophore into structurally simpler com-
pounds. Vennerstrom’s extensive work in this field has
moved from initial amine peroxides5 containing one per-
oxide bridge into 1,2,4,5-tetraoxanes6 and most recently
1,2,4-trioxolanes and the development of OZ277.7 Kim’s
elegant work in the 1990s included the synthesis of
spiro-1,2,4,5-tetraoxanes and yingzhaosu
A
ana-
logues.8,9 Solaja and co-workers have more recently
concentrated on the synthesis of steroidal 1,2,4,5-tetra-
oxanes.10 Fully synthetic 1,2,4-trioxanes and trioxolanes
have been successfully synthesized,7,11–19 and these com-
pounds are also found to be potent antimalarials but
have recently been found to be less stable than their
1,2,4,5-tetraoxane counterparts (see Fig. 2).
Some of the key developments within antimalarial drug
discovery have concentrated on incorporating the perox-
H
Advantages of the tetraoxane heterocycle include ease of
synthesis, potential achirality and a low cost of synthesis
from readily available reagents.
O
O
O
H
H
O
R
Artemisinin, R = =O
α
Artesunate, R = -OC(O)CH2CH2CO2Na
-OMe
Artemether, R =
β
Figure 1. Artemisinin and its semi-synthetic analogues artesunate and
artemether.
Keywords: Antimalarial; Artemisinin; 1,2,4-Trioxane; Tetraoxane;
Plasmodium falciparum.
*
Corresponding author. Fax: +44 151 794 3553; e-mail:
p.m.oneill01@liv.ac.uk
Figure 2. 1,2,4,5-Tetraoxanes 1, 2, 3 and 4 (RKA 216).
0960-894X/$ - see front matter ꢀ 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2008.01.053

G. L. Ellis et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1720–1724
1721
There are many methods available for the synthesis of
dispiro-1,2,4,5-tetraoxanes but these reactions are highly
dependant on substrate, temperature, concentration,
pH, addition mode, solvent and type of substrate.6,8,20–29
Bridged tetraoxanes are a particularly challenging syn-
thetic target with few literature methods available for
their synthesis. A recent publication by Kim et al. iden-
tified tetraoxane 1 as a highly active antimalarial both
in vitro (IC50 = 3 nM) and in vivo (ED50 = 15 mg/
kg).30 Our initial studies focused on the synthesis of tet-
raoxanes 2 and 3 as these compounds would have en-
hanced polarity. In addition, tetraoxane 2 was viewed
as a particularly attractive target as the ester group
can be converted into a range of polar solubilizing func-
tional groups. The presence of a morpholino group
within the tetraoxane template has previously been
found to greatly enhance activity as is exemplified by
tetraoxane 4 (RKA 216).28
Scheme 2. Synthetic route for synthesis of tetraoxane 3. Reagents and
conditions: (a) Et3N, ClCO2C2H5, DCM, 0 ꢁC; (b) morpholine, rt
À0 ꢁC; (c) formic acid, H2O2, 0 ꢁC; (d) ICH2CH2CH2l, Ag2O, EtOAc.
Tetraoxane 2 was synthesized in 63% yield from gem-
dihydroperoxide 528 by Ag2O mediated alkylation using
1,3-diiodopropane. In order to convert the ester 2 to the
amide 3 it was necessary to hydrolyze the ester to the
acid 7 (Scheme 1). This reaction was unsuccessful as a
‘Kornblum-de La Mare’ type rearrangement occurred
resulting in formation of ketone 8 as the major product.
An alternative synthetic route was required that in-
volved synthesis of the amide prior to formation of the
bridged tetraoxane. Ketone 8 was converted to the
amide 10 via a mixed anhydride intermediate 9. This
was then transformed into the gem-dihydroperoxide 11
using formic acid and hydrogen peroxide. Formation
of the tetraoxane 3 was achieved using the alkylation
reaction described previously (Scheme 2).31
Figure 3. New template for tetraoxanes 12.
The in vitro antimalarial activity of tetraoxanes 2 and 3
was measured versus the 3D7 strain of Plasmodium fal-
ciparum. Tetraoxane 2 had an IC50 of 117.5 nM and tet-
raoxane 3 an IC50 of 689.3 nM. It became evident that
there were several problems with this initial template,
as the ester was not amenable to hydrolysis and the
synthesis was longer than anticipated. Given the low
activity of these two derivatives the more lipophilic
template 12 became a new synthetic target (Fig. 3).
Varying the nature of the X group on the phenyl ring
would give a greater diversity as well as allow some
control over the lipophilicity and polarity of the tetraox-
anes (see Scheme 3).
Scheme 3. Proposed synthetic route for synthesis of tetraoxanes 12.
Reagents: (a) NaH, DMSO, X-PhCh2P+(Ph)3BrÀ; (b) H2, Pd/C; (c)
H2O2, tungstic acid; (d) I(CH2)3I, Ag2O.
Initially a four-step synthetic route to achieve the syn-
thesis of target tetraoxanes 12 was proposed. Ketal 13
underwent a Wittig reaction using the respective Wittig
reagent, NaH and DMSO, to give alkene 14. The alkene
functionality was then removed using catalytic hydroge-
nation to give the protected ketone 15. Direct treatment
Scheme 1. Synthetic route for synthesis of tetraoxane 7. Reagents and
conditions: (a) Ag2O, EtOAc; (b) KOH, MeOH, 70 ꢁC; (c) DCM,
H2O, HCl.

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G. L. Ellis et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1720–1724
of 15 with H2O2/THF (1:1) and tungstic acid gave the
gem-dihyroperoxide 16 which was then to be alkylated
using 1,3-diiodopropane and Ag2O to give the tetraox-
anes 12 (Scheme 1).28 Synthesis of the protected ketone
15 was achieved in high yields, however, conversion to
the gem-dihydroperoxide 16 gave a complex mixture of
products and the gem-dihydroperoxide could not be iso-
lated in a pure form. Use of the impure gem-dihydroper-
oxide in the alkylation step resulted in a further mixture
of complex products and although in one case some of
the tetraoxane was isolated the yield was less than 5%
and some impurities were still present.
using 10% aq HCl in acetone gave ketones 17a–f in
68–93% yield. gem-Dihydroperoxides 16a–f were synthe-
sized from ketones 17a–f using the H2O2, catalytic io-
dine system described above in yields of 67–87%. The
final alkylation step was initially carried out using 1,3-
diiodopropane and Ag2O in ethyl acetate at 0 ꢁC. How-
ever, yields on the first two reactions were less than 20%,
this number was significantly improved if the reaction
was carried out at room temperature. Tetraoxanes
12a–f were therefore synthesized in yields of 32–42%
(Scheme 4 and Table 1).
One further point of diversity explored was altering the
bridged alkyl group. In this case 2-methylene-1,3-
diiodopropane was used as the bridging alkyl reagent.
This first was synthesized from the dichloro compound
18 using NaI in acetone in 49% yield. The resulting diio-
do compound 19 was then used in the alkylation step as
described previously for 1,3-diiodopropane to give com-
pounds 20a–c in 21–24% yield (Scheme 5).34 The yield of
these reactions was generally lower than for 1,3-diiodo-
propane although again the best yields were achieved at
room temperature. This observation is perhaps unsur-
prising due to the more reactive nature of vinyl system
resulting in more side reactions.
It was apparent from the results of investigating this
method that it was possible to form these bridged tetra-
oxanes but that a better, higher yielding, cleaner meth-
odology was required for synthesis of the gem-
dihydroperoxides. A further review of the literature
ˇ
revealed a paper by Zimitek and co-workers using only
2 equivalents of H2O2 and catalytic iodine to form gem-
dihydroperoxides from ketones in excellent yields.32
This method would involve deprotection of the ketone
prior to conversion to the gem-dihydroperoxide but as
the deprotection step was believed to be high, yielding
this methodology was adopted.
Conversion of commercially available ketal 13 to al-
kenes 14a–f was achieved using a Wittig reaction as de-
scribed previously in 44–79% yield. Catalytic
hydrogenation of alkenes 14a–f gave near quantitative
conversion to 15a–f in all cases. Acetal deprotection
The average logP and tPSA of all the target compounds
synthesized can be seen below (Table 2). Interestingly
compounds 2 and 3 which exhibited poor antimalarial
activity have considerably lower average logP than the
phenyl tetraoxanes 12a–f and 20a–c (see Table 3).
O
O
O
a
X
X
15a-f
17a-f
OH OH
O
O
O
O
O
O
c
b
X
X
16a-f
12a-f
Scheme 4. Synthesis of propane bridged 1,2,4,5-tetraoxanes 12a–f.
Reagents: (a) 10% aq HCl, acetone; (b) H2O2, I2, MeCN; (c) I(CH2)3I,
Ag2O.
Scheme 5. Synthesis of bridged 1,2,4,5-tetraoxanes 20a–c. Reagents:
16b, X = F 16c, X = CF3 16e, X = CO2Me 20a, X = F, 21% 20b,
X = CF3, 23% 20c, X = CO2Me, 24%.
Table 1. Yields for the synthesis of tetraoxanes 12a–f and all intermediates
Compound
X
% Yield 14
% Yield 15
% Yield 17
% Yield 16
% Yield 12
a
b
c
d
e
f
H
74
79
44
52
54
48
96
97
95
95
97
95
93
68
84
83
79
81
79
68
67
87
71
73
3433a
42
F
CF3
CN
40
35
CO2Me
SO2Me
3433b
32

G. L. Ellis et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1720–1724
1723
Table 2. Average logP and tPSA of all target compounds
9. Tokuyasu, T.; Masuyama, A.; Nojima, M.; Kim, H. S.;
Wataya, Y. Tetrahedron Lett. 2000, 41, 3145.
Compound
X
Av logP35
tPSA36
10. Opsenica, R.; Angelovski, G.; Pocsfalvi, G.; Juranic, Z.;
Zizak, Z., et al. Bioorg. Med. Chem. 2003, 11, 2761.
11. Dong, Y. X.; Chollet, J.; Matile, H.; Charman, S. A.;
Chiu, F. C. K., et al. J. Med. Chem. 2005, 48, 4953.
12. Tang, Y. Q.; Dong, Y. X.; Wittlin, S.; Charman, S. A.;
Chollet, J., et al. Bioorg. Med. Chem. Lett. 2007, 17, 1260.
13. Griesbeck, A. G.; El-Idreesy, T. T. J. Chin. Chem. Soc.-
TAIP 2006, 53, 1469.
14. Griesbeck, A. G.; El-Idreesy, T. T.; Bartoschek, A. Pure
Appl. Chem. 2005, 77, 1059.
15. Benoit-Vical, F.; Lelievre, J.; Berry, A.; Deymier, C.;
Dechy-Cabaret, O., et al. Antimicrob. Agents. Chemother.
2007, 51, 1463.
2
—
—
2.20 1.12
1.16 0.91
3.93 1.10
4.01 1.17
4.79 1.16
3.46 1.14
3.75 1.11
2.65 1.35
4.20 1.24
4.95 1.24
4.05 1.30
63.24
66.48
36.94
36.94
36.94
60.73
63.24
71.08
36.94
36.94
63.24
3
12a
12b
12c
12d
12e
12f
20a
20b
20c
H
F
CF3
CN
CO2Me
SO2Me
F
CF3
CO2Me
16. Dong, Y. X.; Vennerstrom, J. L. Abstr. Pap. Am. Chem.
Soc. 2005, 230, U2617.
17. Jin, H. X.; Zhang, Q.; Kim, H. S.; Wataya, Y.; Liu, H. H.,
et al. Tetrahedron 2006, 62, 7699.
18. Tang, Y. Q.; Dong, Y. X.; Wang, X. F.; Sriraghavan, K.;
Wood, J. K., et al. J. Org. Chem. 2005, 70, 5103.
19. Vennerstrom, J. L.; Dong, Y. C.; Chollet, J.; Matile, H.
U.S. Patent 6,489,199, 2001.
20. Dong, Y. X.; Vennerstrom, J. L. J. Org. Chem. 1998, 63,
8582.
21. Dong, Y. X.; Matile, H.; Chollet, J.; Kaminsky, R.;
Wood, J. K., et al. J. Med. Chem. 1999, 42, 1477.
22. Tsuchiya, K.; Hamada, Y.; Masuyama, A.; Nojima, M.;
McCullough, K. J., et al. Tetrahedron Lett. 1999, 40,
4077.
Table 3. In vitro antimalarial activity of the bridged 1,2,4,5-tetraox-
anes against the 3D7 strain of Plasmodium falciparum
Compound
Mean IC50 (nM SD)
Artemether
Artemisinin
12a
3.20 1.97
9.20 1.97
51.85 22.84
68.25 25.24
52.35 18.17
99.65 2.19
93.50 27.86
43.40 1.41
42.80 3.68
116.18 40.52
12b
12c
12d
12e
20a
20b
20c
23. Vennerstrom, J. L.; Dong, Y. X.; Andersen, S. L.; Ager,
A. L.; Fu, H. N., et al. J. Med. Chem. 2000, 43, 2753.
24. Opsenica, D.; Pocsfalvi, G.; Milhous, W. K.; Solaja, B. A.
J. Serb. Chem. Soc. 2002, 67, 465.
25. Iskra, J.; Bonnet-Delpon, D.; Begue, J. P. Tetrahedron
Lett. 2003, 44, 6309.
26. Terent’ev, A. O.; Kutkin, A. V.; Starikova, Z. A.; Antipin,
M. Y.; Ogibin, Y. N., et al. Synthesis-Stuttgart 2004,
2356.
The in vitro antimalarial activity of these compounds
was measured versus the 3D7 strain of Plasmodium
falciparum.37 The majority of these 1,2,4,5-tetraoxanes
are active in the 40–100 nM IC50 range. Work is cur-
rently underway to further enhance potency within this
1,2,4,5-tetraoxane template.
27. Masuyama, A.; Wu, J. M.; Nojima, M.; Kim, H. S.;
Wataya, Y. Mini-Rev. Med. Chem. 2005, 5, 1035.
28. Amewu, R.; Stachulski, A. V.; Ward, S. A.; Berry, N. G.;
Bray, P. G., et al. Org. Biomol. Chem. 2006, 4, 4431.
29. Amewu, R.; Stachulski, A. V.; Ward, S. A.; Berry, N. G.;
Bray, P. G., et al. Org. Biomol. Chem. 2007, 5, 708.
30. Kim, H. S.; Nagai, Y.; Ono, K.; Begum, K.; Wataya, Y.,
et al. J. Med. Chem. 2001, 44, 2357.
To conclude, an efficient five-step synthesis of bridged
1,2,4,5-tetraoxanes has been achieved to generate a
range of tetraoxanes with good antimalarial activity.
The compounds synthesized have two points of poten-
tial diversity allowing several different functional groups
to be investigated in future research.
31. General procedure for the synthesis of bridged tetraoxanes:
gem-Dihydroperoxide (1.09 mmol) was dissolved in
EtOAc (7.25 ml) and Ag2O (2.18 mmol, 2.0 equiv) added.
A solution of diiodo alkane/alkene (1.09 mmol, 1.0 equiv)
in EtOAc (3.60 ml) was added dropwise over 15 min. The
reaction was then stirred at room temperature overnight
and filtered through a Celite pad. Ether (50 ml) was added
and the resulting solution was washed with 3% aq sodium
thiosulfate, NaHCO3 aq and brine. The organic layer was
dried over MgSO4, evaporated and purified by flash
column chromatography. Procedure for the synthesis of
compound 3. This product was prepared in 53% as a light
yellow powder according to the general procedure for the
preparation of bridged tetraoxanes. The product was
purified by flash column chromatography using ethyl
acetate/DCM (1:1, v/v, Rf = 0.17) as the eluent. Mp 90–
92 ꢁC tmax (CHCl3)/cmÀ1 1433.2, 1632.5, 2858.9, 2913.2,
3016.6 1H NMR (400 MHz, CDCl3) dH, 1.11–1.42 (m,
2 H, CH2), 1.45–1.80 (m, 4H, CH2), 1.85–1.89 (m, 1H,
CH), 2.08–2.31 (m, 4H, CH2), 2.22 (d, 2H, J = 6.7 Hz,
CH2), 3.48 (t, 2H, J = 4.8 Hz, NCH2), 3.55–3.72 (br s, 6H,
NCH2/CH2O), 3.98–4.18 (m, 2H, CH2O), 4.28–4.47 (m,
Acknowledgments
This work was supported by a grant from the EU (Anti-
mal, FP6 Malaria Drugs Initiative).
References and notes
1. Haynes, R. K.; Krishna, S. Microbes Infect. 2004, 6, 1339.
2. Haynes, R. K. Curr. Top. Med. Chem. 2006, 6, 509.
3. Haynes, R. K. Curr. Opin. Infect. Dis. 2001, 14, 719.
4. O’Neill, P. M.; Posner, G. H. J. Med. Chem. 2004, 47, 2945.
5. Vennerstrom, J. L. J. Med. Chem. 1989, 32, 64.
6. Vennerstrom, J. L.; Fu, H. N.; Ellis, W. Y.; Ager, A. L.;
Wood, J. K., et al. J. Med. Chem. 1992, 35, 3023.
7. Vennerstrom, J. L.; Arbe-Barnes, S.; Brun, R.; Charman,
S. A.; Chiu, F. C. K., et al. Nature 2004, 430, 900.
8. Nonami, Y.; Tokuyasu, T.; Masuyama, A.; Nojima, M.;
McCullough, K. J., et al. Tetrahedron Lett. 2000, 41,
4681.

1724
G. L. Ellis et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1720–1724
2H, CH2O) 13C NMR (100 MHz, CDCl3), dC 29.3, 31.1,
1.45–1.70 (m, 3H, CH2, CH), 2.05–2.25 (m, 4H,
2 · CH2), 2.57 (d, 2H, J = 6.7 Hz, CHCH2), 4.00–4.20
(m, 2H, 2 · CHHO), 4.29–4.51 (m, 2H, 2 · CHHO), 7.20
(m, 2H, aromatics), 7.95 (m, 2H, aromatics); 13C NMR
(100 MHz, CDCl3), dC 28.8, 29.2, 31.3, 39.0, 43.2, 52.3
74.4, 108.3, 128.3, 129.5, 130.0, 146.8, 167.5; MS (ES+),
359 [M+Na]+ (100), HRMS calculated for 359.1471
C18H24O6Na, found 359.1460.
34.2, 39.3, 42.4, 46.6, 67.2, 74.3, 108.1, 171.0 MS (ES+),
315.36 [M+Na]+ (100), 338.1, [2M+Na]+ 653.3 HRMS
calculated for 338.1580 C12H20O3N, found 338.1574.
Elemental analysis C: 56.99, H: 7.91, N: 4.39 (required
values C: 57.13, H: 7.99, N: 4.39).
32. Zmitek, K.; Zupan, M.; Stavber, S.; Iskra, J. Org. Lett.
2006, 8, 2491.
33. (a) Procedure for the preparation of compound 12a. This
product was prepared in 34% as a white powder according
to the general procedure for the preparation of bridged
tetraoxanes. This product was purified by flash column
chromatography gradient elution hexane to 3% hexane in
EtOAc. 1H NMR (400 MHz, CDCl3) dH, 1.11–1.40 (m,
4H, 2 · CH2), 1.45–1.70 (m, 3H, CH2, CH), 2.05–2.22 (m,
4H, 2 · CH2), 2.50 (d, 2H, J = 6.9 Hz, CHCH2), 4.00–4.20
(m, 2H, 2 · CHHO), 4.29–4.51 (m, 2H, 2 · CHHO), 7.10–
7.30 (m, 5H, aromatics); 13C NMR (100 MHz, CDCl3), dC
28.4, 29.2, 30.8, 39.1, 43.2, 74.4, 108.5, 126.2, 128.6, 129.5,
141.3; MS (CI+), 296 [M+NH3]+ (100), HRMS calculated
for 296.18619 C16H26O4N, found 296.18672.; (b) Proce-
dure for the preparation of compound 12e. This product
was prepared in 34% as a white powder according to the
general procedure for the preparation of bridged tetraox-
anes. Purification was achieved by flash column chroma-
34. Procedure for the preparation of compound 20b. This
product was prepared in 23% as a white powder according
to the general procedure for the preparation of bridged
tetraoxanes and was purified by flash column chromatography
gradient elution hexane to 3% hexane in EtOAc. 1H NMR
(400 MHz, CDCl3) dH, 1.15–1.40 (m, 4H, 2 · CH2), 1.45–
1.70 (m, 3H, CH2, CH), 2.00–2.25 (m, 2H, CH2), 2.55 (d,
2H, J = 6.7 Hz, CHCH2), 4.40 (m, 2H, 2 · CHHO), 4.80
(m, 2H, 2 · CHHO), 5.25 (s, 2H, @CH2), 7.20 (m, 2H,
aromatics), 7.52 (m, 2H, aromatics); 13C NMR (100 MHz,
CDCl3), dC 28.3, 29.1, 32.0, 43.0, 78.4, 106.9, 108.4, 115.2,
115.4, 119.3, 130.7, 136.8, 151.7; MS (CI+), 376
[M+NH3]+ (100), HRMS calculated for 376.17358
C18H25O4F3N, found 376.17470.
35. http://146.107.217.178/lab/alogps/.
36. http://www.molinspiration.com/cgi-bin/properties.
37. Smilkstein, M.; Sriwilaijaroen, N.; Kelly, J. X.; Wilairat,
P.; Riscoe, M. Antimicrob. Agents Chemother. 2004, 48,
1803.
1
tography using hexane to 5% hexane in EtOAc. H NMR
(400 MHz, CDCl3) dH, 1.15–1.40 (m, 4H, 2 · CH2),