Synthesis of oxadiazole-2-oxide analogues as potential antischistosomal agents

Article abstract of DOI:10.1016/j.tetlet.2009.01.120

The synthesis of several 1,2,5-oxadiazole-2-oxide (Furoxan) analogues is described herein. These compounds were prepared in an effort to probe the SAR around the phenyl substituent and oxadiazole core for our studies toward thioredoxin-glutathione reductase (TGR) inhibition and antischistosomal activity.

Full text of DOI:10.1016/j.tetlet.2009.01.120

Tetrahedron Letters 50 (2009) 1710–1713
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of oxadiazole-2-oxide analogues as potential antischistosomal agents
*
Ganesha Rai, Craig J. Thomas, William Leister, David J. Maloney
NIH Chemical Genomics Center, National Human Genome Research Institute, National Institutes of Health, 9800 Medical Center Drive, Rockville, MD 20850, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of several 1,2,5-oxadiazole-2-oxide (Furoxan) analogues is described herein. These com-
pounds were prepared in an effort to probe the SAR around the phenyl substituent and oxadiazole core
for our studies toward thioredoxin-glutathione reductase (TGR) inhibition and antischistosomal activity.
Published by Elsevier Ltd.
Received 29 October 2008
Revised 21 January 2009
Accepted 23 January 2009
Available online 27 January 2009
As part of our ongoing interest in neglected diseases of third
world countries, we recently reported the identification of several
novel chemotypes1,2 (Fig. 1) for the treatment of schistosomiasis.3,4
Schistosomiasis is caused by five species of the trematode flat-
worm, two of which are Schistosoma mansoni (S. mansoni) and
Schistosoma haetobium. It has been estimated that more than 200
million people are infected worldwide with an estimated
200,000–280,000 deaths occurring annually in sub-saharan Africa
alone as a result of this disease.5 Current treatment of schistosomi-
asis relies primarily on the use of praziquantel (PZQ) because of its
low cost and effectiveness against all schistosome species. How-
ever, given its widespread use, drug-resistant parasites may be-
come more prevalent and PZQ-resistant isolates have already
been identified.6,7
The majority of eukaryotes contain two native mechanisms by
which they detoxify reactive oxygen species, the tripeptide glu-
tathione (GSH) and the 12 kDa protein thioredoxin (Trx).8,9 These
systems also play a critical role in several other cellular functions
including cell proliferation, redox regulation of gene expression,
and xenobiotic metabolism to name a few.10 Interestingly, Wil-
liams and co-workers found that in S. mansoni both TrxR and GR
pathways are absent, and instead rely on a single multifunctional
selenocysteine-containing flavoenzyme, thioredoxin-glutathione
reductase (TGR).11 As a result, one may expect that inhibition of
TGR would present a viable target for the treatment of S. mansoni
parasites. Accordingly, we sought out and discovered numerous
small molecules with potent in vitro inhibition of TGR, including
the oxadiazole-2-oxide chemotype (Furoxan) (1).
tempts were met with limited success. Synthesis of amidoxime 3
did proceed as planned to haloglyoxime 4 using HBr and NaNO2,12
however this product proved unstable and thus could not be cy-
clized to halo-furoxan 5. Additionally, direct conversion of 4-ami-
no-3-cyano furoxan (2) using various conditions to compound 5
also failed. Alternatively, in a one-pot procedure, amidoxime 3
was treated with BF3ÁOEt2 to form the diazonium ion 6 in situ
which was then subjected to palladium-imidazolium carbene cat-
alyzed Suzuki–Miyaura coupling conditions.13 As shown in Scheme
1, this too failed to produce the desired product 7. In light of these
findings, we chose to focus our efforts on more conventional ap-
proaches to furoxan analogues.
Gasco and co-workers have studied furoxan and related ana-
logues for many years and have published pioneering work toward
their syntheses.14–21 The key step typically involves treatment of
the requisite cinnamyl alcohol 10 with sodium nitrate and acetic
acid at room temperature to form the corresponding 4-phenyl-3-
furoxanmethanol derivative 11 (Scheme 2). While Gasco and co-
workers often report the use of aqueous sodium nitrate in acetic
acid, we found that the use of glacial acetic acid with anhydrous
sodium nitrate worked best for the analogues reported herein.
The required cinnamyl alcohols were obtained either through
esterification of commercially available cinnamic acid derivatives
8 or via Heck coupling of the corresponding aryl bromides with
ethyl acrylate to give 9. Subsequent hydride reduction with DI-
BAL-H gave alcohols of type 10. Of note, the sodium nitrate-medi-
ated cyclization often gave a mixture of two products 11a and 11b,
always favoring the desired regioisomer 11a. The structural assign-
ments were based on known differences in 1H and 13C NMR reso-
Our initial interest was to explore modification of the phenyl
moiety of furoxan (1). Ideally, we hoped to develop a convergent
synthesis which would allow for late-stage installation of differen-
tially substituted phenyl groups providing rapid access to a library
of compounds. Unfortunately, as shown in Scheme 1, these at-
O
S
N
CN
P
HN
N+
* Corresponding author. Address: NIH Chemical Genomics Center, NHGRI,
National Institutes of Health, 9800 Medical Center Drive, Building B, Room 2055,
MSC: 3370, Bethesda, MD 20892-3370, USA. Tel.: +1 301 217 4381; fax: +1 301 217
5736.
N
O-
Furoxan (1)
R
O
Phosphinic amides
E-mail address: maloneyd@mail.nih.gov (D.J. Maloney).
Figure 1. Quantitative high-throughput screen (qHTS) hits.1
0040-4039/$ - see front matter Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2009.01.120

G. Rai et al. / Tetrahedron Letters 50 (2009) 1710–1713
H2N
1711
CN
N+
N
O-
O
2
various conditions
CN
NC
N
X
N
X
NC
N
NH2
N
Ar
CN
N+
Suzuki coupling
HO
N
N+
N
HO
O-
O-
O
O
HO
4
HO
3
3
5
NOH
NOH
CN
Ar
N2BF4
ArB(OH)2
BF3
Ar
NC
NC
N+
N
NOH
O-
Carbene ligand
Low temperature
NOH
7
O
6
In situ
Scheme 1. Unsuccessful attempts at a convergent synthesis of furoxan analogues.
R
Br
OEt
Pd(OAc)2, nBu4Br
O
NaHCO3, DMF, 90 ºC
DIBAL-H
PhMe
TMSCl
EtOH
OH
OEt
OH
R
R
-78 ºC-RT, 3 h
RT, 10-12 h
(90-97%)
O
R
O
8
9
10
(85-94%)
MnO2
NaNO2
AcOH
CH2OH
N+
CH2Cl2
CH2OH
R
R
CHO
R
RT, 6-40 h,
RT, 6-12 h
(12-80 %)
N+
N
N
O-
-O
N+
O-
(43-67 %)
O
N
O
O
11a
11b
12
SOCl2
DMF
NH2OH
OH
AcONa/EtOH
CN
N
R
R
reflux, 10-30 min
0
º
C-RT, 2-4 h
N+
O-
N+
O-
(25-91%)
N
(64-91%)
N
14-43
13
O
O
Scheme 2. General Scheme for the synthesis of furoxan analogues 14–43.
nances of the methylene group in the 3-position which results
from shielding by the N-oxide moiety.14,18 The chromatographic
separation of these isomers was typically complex and thus the
mixture was carried through and subsequently oxidized using
MnO2 to give the corresponding aldehydes 12 in yields ranging
from 12% to 80%. At this stage, the mixture of regioisomers was
easily separated using flash chromatography. Formation of oxime
13 was achieved using hydroxylamine hydrochloride in the pres-
ence of sodium acetate in ethanol. Dehydration was carried out
using SOCl2 in DMF to provide the requisite 4-phenyl-3-cyano
derivatives 14–43. Using this approach, 28 novel furoxan ana-
logues were synthesized in an effort to define SAR around the phe-
nyl moiety with respect to TGR inhibitory activity (Table 1).
Another primary interest was the importance of the N-oxide
moiety. As such, we sought to synthesize both 3-phenyl-4-cyano
CH2OH
CN
see scheme 2
P(OMe)3
N+
N+
N
N
-O
-O
O
O
4416
11b
CN
CN
N+
O-
N
N
N
O
O
4522
1
Scheme 3. Synthesis of furoxan analogues 44 and 45.

1712
G. Rai et al. / Tetrahedron Letters 50 (2009) 1710–1713
Table 1
through the remaining steps as shown in Scheme 2 gave 3-phe-
Phenyl ring analogues synthesized as depicted in Scheme 1
nyl-4-cyano-furoxan 44. Compound 45 was prepared by treatment
of furoxan 1 with neat P(OMe)3 as previously reported for a similar
system.22 The removal of the N-oxide moiety was confirmed by IR
in which the common absorbance for the N-oxide at 1590–
1620 cmÀ1 is absent.
Finally, we set out to synthesize the 4-(2-thienoyl)-3-cyano
furoxan derivative 55 (Scheme 4). We reported previously that
the bis-thienoyl furoxan 56 exhibits potent in vitro TGR inhibitory
R
CN
N+
N
O-
O
R = 4-F (14)
R = 3-OMe (28)
4-Cl (15)
4-Br (16)
3-OH (29)c
2-OMe (30)
activity as compared to 1 (20 nM and 8 lM, respectively) yet
lacked appreciable activity against ex vivo worms.1 Structurally
similar dibenzoyl furoxans have recently been shown by Nirode
and co-workers to be effective NO-donors through nucleophilic at-
tack of thiols.23 Thus, we reasoned 55 may retain some of the desir-
able potency while the presence of the nitrile may provide the
structural requirements necessary for ex vivo worm killing activity.
The synthesis commenced with condensation of commercially
available 2-acetylthiophene (46) with ethyl acetate in the presence
of sodium ethoxide to give 1,3-diketone 47 in 78% yield. Treatment
with sodium nitrate in HCl/AcOH provided 48 in 71% yield. Oxime
formation was achieved with hydroxyl amine hydrochloride to
give the desired bisoxime 49b in 53% yield along with a minor
amount of undesired 49a. Cyclization was accomplished with lead
dioxide to give the requisite 4-ketothiophene-3-methyl furoxan 50
in 34% yield. Allylic bromination followed by hydrolysis gave 4-
ketothiophene-3-furoxanmethanol 52 in 38% yield over two steps.
Interestingly, typical oxidation conditions that were utilized in
previous analogues (i.e., MnO2) failed to give the desired product.
Recently, Fukuyama and co-workers reported a mild two-step syn-
thesis of oximes from alcohols via Mitsunobu reaction with N-(tert-
butyldimethylsilyloxy)benzenesulfonamide followed by treatment
with Cesium fluoride.24 Gratifyingly, the reaction worked well on
our system providing oxime 54 in 42% yield over two steps. Subse-
4-CF3 (17)
4-NO2 (18)
4-OMe (19)
4-OH21 (20)a
4-Me (21)
4-Phe (22)
4-OCH2CCHb (23)
3-NO2 (24)
3-Cl (25)
4-F, 3-Br (31)
4-Cl, 3-NO2 (32)
3-CF3, 5-CF3 (33)
3,4,5-OMe (34)
3,4-OCH2O (35)
Naphthyl (36)
Furan (37)
Thiophene (38)
2-NO2-Furan (39)
3-Br (26)
3-CF3 (27)
3-Furoxan (i.e., m-bis-furoxan) (40)
4-Furoxan (i.e., p-bis-furoxan) (41)
1,4-Thiophene-bis-furoxan (42)
3-F,5-Furoxan (i.e., m-F, m-bis-furoxan) (43)
a
Was obtained via deprotection of 19 with AlCl3.
Was obtained from reaction of 20 with propargyl bromide (see Supplementary
b
data for details).
c
Was obtained via deprotection of 28.
furoxan 4416 and the 1,2,5-oxadiazole derivative 45 22 (Scheme 3).
As mentioned previously, cyclization of 10–11 gives a mixture of
isomers, with the minor being 3-phenyl-4-cyano-furoxan 11b
(see Scheme 2). However, Gasco showed that heating 11a in reflux-
ing toluene leads to partial isomerization to 11b,14 allowing access
to ample material of the minor isomer to be collected. Carrying
O
O
O
O
NaNO2
HCl/AcOH
AcOEt
NH2OH.HCl,
NaCO3/MeOH-H2O
O
S
S
NaOEt/THF
S
º
(78%)
0 C-RT, 4 h
N
12 h, RT
(53%)
HO
(71%)
46
47
48
O
O
HO
OH
N
N
NBS
CH2Br
PbO2
OH
N
AIBN/CCl4
AcOH/Ether
S
S
S
N+
N+
Reflux, 30 h
(57%)
N
N
RT, 12 h
(34%)
S
O-
O-
O
O
O
S
O
N
49a
OH
49b
50
51
O
O
6M H2SO4
Dioxane
OTBS
TBSONHTs, DEAD,
PPh3/THF-Toluene
CH2OH
N+
CsF
MeCN
N
S
Ts
Reflux, 15 h
(66%)
RT, 10 min
(47%)
N
º
0 C, 2 h
N+
O-
N
O
O-
52
53
(91%)
O
O
O
SOCl2
DMF
0 C-RT, 2 h
(79%)
CN
OH
O
O
N
S
S
º
N+
N
S
N+
N
S
O-
55
54
O
O
O-
N+
O-
N
ref. 1
56
O
IC50 = 22 nM; minimal activity
in ex vivo worms.
Scheme 4. Synthesis of 4-thienoyl-3-cyano furoxan (55).

G. Rai et al. / Tetrahedron Letters 50 (2009) 1710–1713
1713
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in 79% yield.
In summary, the preparation of numerous novel furoxan ana-
logues is described. While the convergent strategy was not suc-
cessful,
a combination of new and established chemistry
provided access to the desired analogues for biological testing
against a medicinally relevant target. Detailed evaluation of the
activity against both the TGR enzyme in vitro and ex vivo worms
(S. mansoni) is ongoing and will be reported in due course.
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Acknowledgments
The authors thank Jeremy Smith and Paul Shinn for assistance
with compound management. This research was supported by
the Molecular Libraries Initiative of the National Institutes of
Health Roadmap for Medical Research.
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Supplementary data
Supplementary data (experimental procedures and NMR data
for all final compounds) associated with this article can be found,
in the online version, at doi:10.1016/j.tetlet.2009.01.120.
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References and notes
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