Synthesis of antitrypanosomal 1,2-dioxane derivatives based on a natural product scaffold

Article abstract of DOI:10.1016/j.bmcl.2011.06.059

A short practical synthesis of a new natural product based scaffold (6), based on antitrypanosomal and antimalarial compounds isolated from different Plakortis species is described. The scaffold contains a peroxide unit that is surprisingly stable to chem

Full text of DOI:10.1016/j.bmcl.2011.06.059

Bioorganic & Medicinal Chemistry Letters 21 (2011) 4793–4797
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis of antitrypanosomal 1,2-dioxane derivatives based on a natural
product scaffold
Harish Holla a, Mehdi Labaied b, Ngoc Pham a, Ian D. Jenkins a, Kenneth Stuart b, Ronald J. Quinn a,
⇑
a Eskitis Institute, Griffith University, Brisbane QLD4111, Australia
b Seattle BioMed, 307 Westlake Ave. N, Seattle, WA 98109-5219, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A short practical synthesis of a new natural product based scaffold (6), based on antitrypanosomal and
antimalarial compounds isolated from different Plakortis species is described. The scaffold contains a per-
oxide unit that is surprisingly stable to chemical manipulation elsewhere in the molecule, enabling it to
be elaborated into a small library of derivatives. It is stable to ozonolysis, reductive work-up with dimeth-
ylsulfide and the Wittig reaction with stabilized phosphorus ylides. The scaffold along with its Wittig
Received 28 April 2011
Revised 9 June 2011
Accepted 13 June 2011
Available online 13 July 2011
analogues has displayed low to sub-micro molar (0.2–3.3
l
M) antitrypanosomal activity.
Keywords:
Ó 2011 Elsevier Ltd. All rights reserved.
Natural product scaffold
Ene reaction
1,2-Dioxane
Antitrypanosomal
HAT
Human African Trypanosomiasis (HAT) and Malaria are two of
the world’s deadliest diseases, with more than one million deaths
worldwide in 2008, most of them in sub-Saharan Africa and Asia.1
Owing to the complexity of these diseases and to the emergence of
parasites that are resistant to currently available anti-HAT and
antimalarial drugs, there is an urgent need for new, effective and
affordable drugs.2
generating cyclic peroxides, but importantly, the 1,2-dioxane sys-
tem is not stable to many of the reaction conditions required for
further functional group manipulations. For example, the peroxide
moiety is not stable to metal-mediated reactions, most reducing
agents and strong bases.10a,b Thus the synthesis of compounds such
as 1–3, and analogues required for structure–activity relationship
(SAR) studies, presents a significant challenge, often involving
During the course of our on-going drug discovery program
aimed at new antitrypanosomal compounds from natural sources,
we found that 11,12-didehydro-13-oxo-plakortide Q (1), isolated
from the Australian marine sponge Plakortis species, exhibited
antitrypanosomal activity at an IC50 of 49 nM against Trypanosoma
brucei brucei (one of the parasites responsible for HAT).3 Interest-
ingly, the methyl ester of peroxyplakortic acid B3 (2) from the
Okinawan marine sponge Plakortis sp.4 and plakortin5 (3), along
with a number of analogues from Plakortis simplex, have shown
sub-micromolar to low micromolar in vitro antimalarial activity.6
A comparison of 1–3 with the known antimalarial artemisnin7
(4) and yingzhaosu A8 (5) suggested that the substituted 1,2-diox-
ane (endoperoxide) moiety might be the core skeleton responsible
for the antimalarial and antitrypanosomal activity (Fig. 1).
Although a number of cyclic peroxides with antimalarial, anti-
HAT and cytotoxic profiles have been reported, effective methods
for their synthesis or for generating analogues are limited.9a,b This
is not only because of the limited number of methods available for
OCH3
O
O
O
6
O
O
3
OCH3
O
O
OH
O
2
1
H
O
H
O
O
O
O
O
4
O
OH
O
O
OCH3
OH
3
5
O
O
O
O
6
⇑
Corresponding author. Tel.: +61 7 3735 6000; fax: +61 7 3735 6001.
E-mail address: r.quinn@griffith.edu.au (R.J. Quinn).
Figure 1.
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.06.059

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H. Holla et al. / Bioorg. Med. Chem. Lett. 21 (2011) 4793–4797
synthesis of the dihydro analogue of 3 requires 18 steps.11a Such
O
a lengthy synthesis would generally preclude the development of
an affordable drug to treat HAT. An alternative approach to drug
discovery, and one that we are exploring, is to prepare a natural
product-based scaffold or building block15 (<6-step synthesis)
and to convert this into a small focused library of analogues. An
example of this approach is outlined below.
O
OBn
O
O
O
OBn
OH
6
Our initial target was the scaffold 6, which contains both the
1,2-dioxane ring and carboxylate moieties present in 1–3, as well
as a vinyl ‘handle’ with potential for further synthetic elaboration.
The benzyl ester was chosen for convenience (UV chromophore),
and we used established chemistry to develop a short synthesis
of 6.
O
OBn
Scheme 1. Retrosynthesis of scaffold 6.
A retrosynthetic analysis (Scheme 1), suggested that the 1,2–
dioxane could be generated by intramolecular Michael addition
of a hydroperoxy enoate, which in turn could be obtained by a sin-
glet oxygen ene-reaction (Schenck reaction) of the corresponding
2,6-dienoate.16
O
Swern oxidation
OH
OBn
Ph3PCHCOOBn
CH2Cl2, r.t., 84%
7
8
Swern oxidation of (E)-hex-4-en-1-ol (7) followed by Wittig
reaction with benzyl triphenylphosphoranylidene acetate in a
one-pot procedure gave the desired (2E,6E)-benzyl octa-2,6-dieno-
ate (8) in 84% isolated yield (Scheme 2).17 The singlet oxygen (1O2)
ene-reaction on 8 was achieved by photo oxidation using two
300 W flood lamps in CDCl3 at 5–10 °C for 6 h with a catalytic
amount of tetraphenylporphine as sensitizer. This resulted in the
formation of the allylic hydroperoxides 9 and 10 as a regioisomeric
mixture (50:50). Changing the solvent (CD2Cl2, CD3COCD3), tem-
perature (À78 °C to room temperature), or sensitizer (methylene
blue, rose bengal) did not improve the yield of the desired regio-
isomer, (E)-benzyl 6-hydroperoxyocta-2,7-dienoate (10). At-
tempted separation of 9 and 10 by flash chromatography was
unsuccessful, however it was found that the presence of the unde-
sired regioisomer 9 in the reaction mixture did not affect the sub-
sequent intramolecular Michael addition of hydroperoxide to the
Tetraphenyl porphine
CDCl3
O2, 2 x 300W, 6h
100%
Conversion
HO
O
O
OBn
9
Et2NH (0.2 eq),
CF3CH2OH, 2h
O
+
O
O
O
OBn
41-46%
OBn
6
O
10
OH
(CH3)3SnOH,
1, 2-dichloroethane,
800C, 20h
60%
50:50
O
a
,b-unsaturated benzyl ester in 10 (Scheme 2).
The Michael addition to give the scaffold 6 was carried out using
O
O
OH
the protocol employed by Kobayashi and co-workers.13a Small
changes in the reaction conditions resulted in significant changes
in the yield of 6 (Scheme 3).
11
Scheme 2. Synthesis of scaffold 6.
A side reaction observed when more than 0.5 equiv of Et2NH
were employed, was the formation of the epoxy dialkenoate 12,
presumably formed by nucleophilic attack by the 5,6-double bond
rather lengthy syntheses with introduction of the peroxide func-
tionality late in the synthesis.11–14 For example, the most recent
Et2NH (0.2-1.0 eq),
MeOH:CF3CH2OH (1:1),
0oC,12h
O
O
O
OBn
OBn
O
+
O
OBn
OBn
12
6
44%
HO
O
O
Et2NH (0.1-1.2 eq),
CF3CH2OH,4-8h
O
OBn
O
O
9
O
+
+
O
O
23-40%
OBn
10
O
OH
Et2NH (0.2 eq),
CF3CH2OH,2h
O
O
O
OBn
41-46%
O
O
O
OBn
O
+
Et2NH (0.2-.1.0 eq),
CH2Cl2:CF3CH2OH(1:2),
0oC, 12h
O
OBn
30%
Scheme 3. Standardization for intramolecular Michael addition of allylic hydroperoxide to scaffold 6.

H. Holla et al. / Bioorg. Med. Chem. Lett. 21 (2011) 4793–4797
4795
Table 1
Reaction conditions for one pot ozonolysis and Wittig reaction on (6)
H
H
S. No.
Condition
Heat for 12 h
Et3N (2.0 equiv)
4-Methoxymorpholine N-oxide (3.0 equiv)
K2CO3 (2.0 equiv)
Yield (%)
O
3
1
30–50
Nil
Nil
Nil
30
O
2
3
4
5
6
O
H
O
6
H
Me2S (6.0 equiv) for 2 h
Me2S (10.0 equiv) for 12 h
60–70
Figure 2. Key NOESY correlations of (6).
following ozonolysis according to the procedure of Hon et al.19
The reaction was continued at room temperature for 12 h to obtain
a 30–50% combined yield of E and Z products. In order to improve
the yield, we also tried various bases, as shown in Table 1.20 How-
ever, use of either triethylamine or 4-methoxymorpholine-N-
oxide, led to cleavage of the O–O bond. Best results were obtained
using dimethylsulfide for reductive work-up of the ozonide. Inter-
estingly, dimethylsulfide appears to selectively react with the
ozonide, and not the peroxide under these conditions (room tem-
perature for 12 h).
on the hydroperoxide 9 with loss of hydroxide and (possibly con-
certed) removal of a proton from the 4-position to give the conju-
gated diene and the 6,7-epoxide ring. Use of 0.2 equiv of Et2NH in
CF3CH2OH at room temperature for 2 h gave a reasonable yield
(41–46%) of scaffold 6, with negligible amounts of the epoxy dial-
kenoate. Interestingly, only one diastereomer of the scaffold 6 was
obtained. By employing 2D NOESY analysis in deuterated pyridine,
the relative configuration of the two substituents on the scaffold
was found to be trans, with the benzyl ester and alkene side chains
at the 3 and 6 positions preferring equatorial orientations as ex-
pected (Fig. 2).
The hydrolysis of the benzyl ester group using LiOH led to cleav-
age of the peroxide bond. However, Nicolaou’s mild method of
hydrolysis employing trimethyltin hydroxide at 80 °C in 1,2-
dichloroethane was found to be effective in removal of the benzyl
group to give peroxy acid 11 (the free acid form of scaffold 6).18
With the synthesis of the scaffold accomplished, generating dif-
ferent analogues using the vinyl side chain at C6 as a handle was
the next task. Initially we attempted to extend the side chain via
a cross-metathesis reaction employing 4-methylstyrene and either
Grubbs I or II as catalyst. Unfortunately, the peroxide linkage was
incompatible with the Grubbs catalyst and although we observed
some cross metathesis products, none contained the peroxide moi-
ety. We then turned our attention to a protocol involving oxidative
cleavage of the vinyl group to an aldehyde followed by elaboration
with a Wittig reagent (Scheme 4). The scaffold 6 was treated with
ozone in anhydrous dichloromethane at À78 °C until the color of
the reaction mixture turned to blue (an indication of excess ozone
and complete conversion of the scaffold to its ozonide 13; the reac-
tion was also followed by TLC and NMR). Excess dimethylsulfide
was then added and the mixture stirred at room temperature for
12 h to ensure decomposition of the ozonide. This was followed
by addition of a range of stabilized phosphorus ylides (Table 2)
to obtain the corresponding chain-extended products as mixtures
of E- and Z- isomers that could be separated in all cases except
the nitrile 19.21
The synthesized peroxy ester derivatives along with the scaffold
6 were screened for antitrypanosomal activities against T. brucei
brucei BF427 (Table 3).22 Interestingly, the scaffold 6 itself displayed
significant (IC50 3.03 lM) activity, providing support for our hypoth-
esis that the substituted 1,2-dioxane moiety is the core skeleton
responsible for the antimalarial and antitrypanosomal activity.
Further, elaboration of the scaffold to give the analogues 14–19
generally resulted in improved potency, with analogues 15b and
16a, for example, displaying a 10-fold increase in potency (IC50 0.2
and 0.24 lM, respectively). This provides some support for our
strategy of natural product scaffold-based drug discovery.
The fact that neither the size of the substituents nor the stereo-
chemistry of the analogues 14–18 had a major effect on the activity
suggests that the binding pocket at this site might be fairly large or
open. The in silico physicochemical profiling of the compounds
carried out with Instant J-Chem 2.5.2 software (Table 3), showed
that all of the synthesized compounds obey Lipinski’s rule of five
for drug likeness.23 All compounds have molecular weight (MW)
less than 500 Da and partition coefficient (cLogP) along with
hydrogen bond accepter count (HBA) less than 5. The percent polar
surface area (%PSA) was also found to be less than 140 Å2 which
satisfies the parameter for oral bioavailability of these
compounds.24
In conclusion, we have developed a short and practical route
for synthesizing a new 1,2-dioxane scaffold based on the struc-
ture of several bioactive natural products. The scaffold contains
a vinyl group at C6 that can be readily chain-extended while
keeping intact the peroxide bond. The scaffold along with a
Initially the one-pot reaction was carried out by adding the
phosphorus ylide directly to the reaction mixture immediately
O
O
i. O3, CH2Cl2,
O
ii. Me2S (10 eq)
rt, 12h
iii. Ph3PCHCOOC2H5
rt, 6h
O
- 78oC, 2 min
O
O
Ar Flush
O
O
O
O
O
14a
O
O
O
O
O
O
+
O
6
O
O
13
O
O
O
14b
Scheme 4. One-pot ozonolysis followed by Wittig reaction on (6).

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H. Holla et al. / Bioorg. Med. Chem. Lett. 21 (2011) 4793–4797
Table 2
Wittig derivatives of scaffold 6
S. No.
Phosphorus Ylide
Product
Yield (E + Z) (%)
O
O
O
O
+
+
Z
1
Ph3P@COOC2H5
Ph3P@COOC(CH3)3
Ph3P@COOCH2C6H5
Ph3P@COCH3
(47 + 27)
O
O
O
O
O
14a
14b
O
O
Z
2
3
4
(40 + 23)
(45 + 22)
(16 + 5)
O
15a
15b
O
O
O
O
+
Z
O
O
O
16a
16b
O
O
Z
+
O
O
17a
17b
O
O
+
Z
5
6
Ph3P@COC6H5
Ph3P@CHCN
(45 + 14)
O
O
O
18a
18b
NC
O
O
(61)
O
O
19
E-trans isomer; Z-cis isomer.
Table 3
Antitrypanosomal activities of scaffold (6) and its Wittig analogues
Compound
Physicochemical parametera
HBA
IC50 SD (lM) T. b. brucei BF427
MW
c log P
HBD
% PSA
6
14a
14b
15a
15b
16a
16b
17a
17b
18a
18b
19
262.30
334.36
334.36
362.42
363.42
396.43
396.43
304.34
304.34
366.15
366.15
287.31
3.15
3.31
3.31
4.01
4.01
4.68
4.68
2.91
2.91
4.33
4.33
2.80
3
4
4
4
4
4
4
4
4
4
4
4
0
0
0
0
0
0
0
0
0
0
0
0
11.3
14.1
14.1
12.5
12.5
12.3
12.3
13.6
13.6
11.7
11.7
16.6
3.03 0.78
1.35 0.05
0.62 0.12
0.37 0.09
0.2 0.025
0.24 0.017
0.64 0.09
1.88 0.44
0.9 0.032
0.58 0.11
0.51 0.07
3.14 0.42
0.00086 0.007
Pentamidine isethionate salt
HBA-hydrogen bond donor; HBD-hydrogen bond acceptor.
a
In silico calculations by Instant J-Chem 2.5.2 software.
small library of analogues displayed low to sub-micro molar
(0.2–3.3 M) antitrypanosomal activity. Synthesis of a range of
analogues of this and related scaffolds is being pursued for com-
ance Program is acknowledged. We thank the Australian Research
Council (ARC) for support towards NMR and MS equipment (LE
0668477 and LE 0237908). We also thank Dr. Sue Boyd for 2D
NMR analysis, Dr. Hoan T. Vu for HRMS measurements, and Dr.
Graeme Stevenson for useful discussions.
l
prehensive SAR studies.
Acknowledgements
Supplementary data
Financial support from Drugs for Neglected Disease initiative
(DNDi), Trypanosome Drug Development Consortium (Grant No.
5U01AI075641-04) and Queensland state government smart state
innovation projects fund–National and International Research Alli-
Supplementary data (detailed procedure and NMR data) associ-
ated with this article can be found, in the online version, at
doi:10.1016/j.bmcl.2011.06.059.

H. Holla et al. / Bioorg. Med. Chem. Lett. 21 (2011) 4793–4797
4797
21. General procedure for one pot ozonolysis followed by Wittig reaction on scaffold 6:
A solution of scaffold (6) (1 equiv) in 5 mL dry CH2Cl2 at À78 °C was treated
with stream of O3/O2 for approximately 2 min until the solution turned steady
blue in color. Excess ozone was removed from the solution by flushing with
Argon. Dimethyl sulfide (10 equiv) was added to this solution. The solution was
allowed to stir for 12 h at room temperature followed by addition of respective
stabilized triphenylphosphorane (2 equiv) and further stirred for 4 h. Then, the
reaction mixture was quenched with water and extracted with excess CH2Cl2
and brine. The organic phase was separated, dried over MgSO4 and evaporated
under reduced pressure. The crude mass was subjected to column
chromatography. The E and Z products were separated using hexane/ethyl
acetate as eluent. The spectral data for selected products. Compound 14a: (Z)-
ethyl 3-[(3S,6S)-6-(2-(benzyloxy)-2-oxoethyl)-1,2-dioxan-3-yl]acrylate; pale
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a
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