Design and synthesis of 1,2-dithiolane derivatives and evaluation of their neuroprotective activity

Article abstract of DOI:10.1016/j.bmcl.2007.05.036

We designed and synthesized new analogues containing 1,2-dithiolane-3-alkyl and protected or free catechol moieties connected through heteroaromatic rings such as triazole, 1,2,4-oxadiazole, 1,3,4-oxadiazole, tetrazole or thiazole in order to explore the influence of the bioisosteric replacement of the amide group on the neuroprotective activity of the lipoic acid/dopamine conjugate. Evaluation of the activity of the new compounds, using glutamate-challenged hippocampal HT22 cells, showed that incorporation of heteroaromatic rings in the alkyl-1,2-dithiolane moieties in conjunction with another antioxidant, in this case catechol, may result in strong neuroprotective activity.

Full text of DOI:10.1016/j.bmcl.2007.05.036

Bioorganic & Medicinal Chemistry Letters 17 (2007) 4223–4227
Design and synthesis of 1,2-dithiolane derivatives and evaluation
of their neuroprotective activity
Maria Koufaki,a,* Christina Kiziridi,a Faidra Nikoloudakib and Michael N. Alexisb
aNational Hellenic Research Foundation, Institute of Organic and Pharmaceutical Chemistry, 48 Vas. Constantinou Avenue,
116 35 Athens, Greece
bInstitute of Biological Research and Biotechnology, 48 Vas. Constantinou Avenue, 116 35 Athens, Greece
Received 3 April 2007; revised 9 May 2007; accepted 11 May 2007
Available online 17 May 2007
Abstract—We designed and synthesized new analogues containing 1,2-dithiolane-3-alkyl and protected or free catechol moieties
connected through heteroaromatic rings such as triazole, 1,2,4-oxadiazole, 1,3,4-oxadiazole, tetrazole or thiazole in order to explore
the influence of the bioisosteric replacement of the amide group on the neuroprotective activity of the lipoic acid/dopamine conju-
gate. Evaluation of the activity of the new compounds, using glutamate-challenged hippocampal HT22 cells, showed that incorpo-
ration of heteroaromatic rings in the alkyl-1,2-dithiolane moieties in conjunction with another antioxidant, in this case catechol,
may result in strong neuroprotective activity.
Ó 2007 Elsevier Ltd. All rights reserved.
Oxidative stress is a ubiquitously observed hallmark of
neurodegenerative disorders.1 Neuronal cell dysfunction
and cell death due to oxidative stress may causally con-
tribute to the pathogenesis of progressive neurodegener-
ative disorders, as well as of acute syndromes of
neurodegeneration.2–5 The pathways to nerve cell death
induced by a diverse array of neurotoxins, including
peptides, excitatory amino acids, and cytokines, com-
monly share oxidative downstream processes which
can cause oxidative destruction of key regulators of cell
survival and/or activation of secondary events leading to
apoptosis.6–8
dence demonstrating that antioxidant compounds can
act as a means to prevent and/or treat neurodegenerative
disease is still relatively scarce.9
a-Lipoic acid (LA)10–12 was first isolated in 1951 and is
known by a diversity of names including 1,2-dithio-
lane-3 pentanoic acid, 1,2-dithiolane-3 valeric acid, and
thioctic acid. The naturally occurring R-enantiomer of
LA is an essential cofactor in a-ketoacid dehydrogenase
complexes and is known to be covalently attached to the
amino group of a lysine residue. Exogenously supplied
LA is transported to tissues and reduced to dihydrolipoic
acid (DHLA). LA and DHLA were found to be highly
reactive against a variety of ROS in vitro. The antioxi-
dant activity of LA is also attributed to its capacity to
regenerate intracellular GSH, vitamin C, and vitamin
E.13–15 Numerous studies have reported that LA is
beneficial in a number of oxidative stress models of cell
death pertinent to ischemia–reperfusion injury, neurode-
generation, diabetes, inflammation, and radiation injury.
Thus, LA possesses the potential to intervene in various
therapeutically interesting pathways.16–20 However, it
should be noted that LA reportedly exerts most of its
effects at high micromolar concentrations; that amides
of LA exhibit higher biological activity than the parent
compound;21,22 and that molecular combinations
obtained by coupling LA with an amino-substituted
bioactive moiety possess multifunctional activity.23–31
Elevated levels of the excitatory amino acid glutamate
are thought to be responsible for CNS disorders through
various mechanisms causing oxidative stress via deple-
tion of intracellular glutathione (GSH). Maintenance
of normal GSH levels within the brain may hold an
important key to the prevention and therapy for neuro-
degenerative diseases. In this respect, neuroprotective
antioxidants are considered a promising approach to
slowing the progression and limiting the extent of neuro-
nal cell loss in these disorders. However, clinical evi-
Keywords: Lipoic acid; Heteroaromatic; Oxidative stress;
Neurodegeneration.
*
Corresponding author. Tel.: +30 210 7273818; fax: +30 210
7273831; e-mail: mkoufa@eie.gr
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.05.036

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M. Koufaki et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4223–4227
The present work involves replacement of the amide
functionality by heterocyclic five-membered rings.
Organic compounds containing five-membered aromatic
heterocyclic rings are widely distributed in nature and
often play an important role in various biochemical pro-
cesses. Moreover, heteroaromatic rings serve as bioisos-
teres of several substituents such as amides, esters,
phenyl rings and can contribute substantially higher
pharmacological activity to the resulting com-
pounds.32–34 In the light of this, they are often incorpo-
rated into new chemical entities by medicinal chemists.
was converted to N-hydroxy-amidine 1 by treatment
with hydroxylamine hydrochloride. Reaction of 1 with
LA in the presence of DCC afforded the acyl amidoxime
2. Intramolecular cyclization in the presence of tetrabu-
tylammonium fluoride37 gave the oxadiazole analogue 3
which was deprotected using BF3(SMe)2. Reduction of
the 1,2-dithiolane ring of 3 using NaBH4 afforded
dithiol 5.
1,3,4-Oxadiazole containing analogues were synthesized
as shown in Scheme 2. Hydrazides 6 and 7 were pre-
pared from the corresponding esters and then reacted
with N-hydroxysuccinimide-activated LA to give 8 and
We designed and synthesized new LA analogues contain-
ing 1,2-dithiolane-3-alkyl and protected or free catechol
moieties connected through heteroaromatic rings such
as triazole, 1,2,4-oxadiazole, 1,3,4-oxadiazole, tetrazole
or thiazole. The aim of this work is to compare the neuro-
protective activity of the new compounds with that of LA
as well as its amide with dopamine. The neuroprotective
activity of the new derivatives was evaluated using gluta-
mate-challenged HT22 mouse hippocampal cells.35,36
38
9. Cyclodehydration in boiling POCl3 afforded 10
and 11.
Triazoles (Scheme 3) were synthesized by cycloaddi-
tion39 of lipoylazide and 3,4-dimethoxyphenylacetylene.
Azide 14 was prepared by reduction of LA with catechol
borane, conversion of the resulting alcohol to mesylester
13 and then to azide 14 by treatment with sodium azide.
The synthesis of 1,2,4-oxadiazole containing derivatives
is illustrated in Scheme 1. 3,4-Dimethoxybenzonitrile
The synthesis of tetrazole derivatives is depicted in
Scheme 4. N-Hydroxysuccinimide-activated LA reacted
O
N
NOH
S
S
MeO
MeO
MeO
CN
MeO
MeO
O
a
NH2
b
c
NH2
MeO
2
1
O
O
N
N
RO
RO
MeO
N
N
SH
d
SH
S
S
MeO
3
4
R= Me
R= H
5
e
Scheme 1. Reagents: (a) NH2OHÆHCl, Et3N; (b) lipoic acid, DCC, THF; (c) TBAF, THF; (d) NaBH4, EtOH; (e) BF3ÆS(Me)2, CH2Cl2.
O
H
MeO
MeO
MeO
MeO
MeO
MeO
N
COOH
CONHNH2
a,b
c
N
H
n
n
n
O
S
S
8
9
n= 0
n= 2
6
7
n= 0
n= 2
n= 0
n= 2
RO
RO
O
d
S
S
n
N
N
n=0 R= Me
10
11 n=2 R= Me
12
e
n=2 R= H
Scheme 2. Reagents and condition: (a) dimethyl sulfate, K2CO3, TBAI, acetone; (b) NH2NH2ÆH2O, MeOH; (c) N-hydroxysuccinimide-activated LA,
THF; (d) POCl3, reflux; (e) BF3ÆS(Me)2, CH2Cl2.

M. Koufaki et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4223–4227
COOH
4225
c
a, b
OSO2Me
N3
S
S
S
S
S
d
S
14
13
N
N
SH
SH
N
MeO
MeO
RO
RO
e
S
S
N
N
N
17
15
16
R = Me
R = H
f
Scheme 3. Reagents: (a) catechol borane, CH2Cl2; (b) CH3SO2Cl, CH2Cl2; (c) NaN3, DMF; (d) 3,4-dimethoxyphenylacetylene, CuSO4Æ5H2O,
sodium ascorbate; (e) NaBH4, EtOH; (f) BF3ÆS(Me)2, CH2Cl2.
H
MeO
MeO
N
SH
SH
O
d
H
N
20
RO
RO
S
a, b
COOH
S
e
O
H
N
S
S
MeO
MeO
S
S
S
18
R = Me
c
19 R = H
21
f
N
RO
RO
d
N
MeO
MeO
S
SH
S
N
N
SH
N
N
N
N
R = Me
R = H
22
23
c
24
Scheme 4. Reagents and condition: (a) NHS, DCC, dioxane; (b) 3,4-dimethoxyphenethylamine, CH2Cl2; (c) BF3ÆS(Me)2, CH2Cl2; (d) NaBH4,
EtOH; (e) Lawesson’s reagent, THF, reflux; (f) DIAD, TMSA, Ph3P, THF.
with 3,4-dimethoxyphenethylamine to afford amide 18
which in turn converted to thioamide 21 by treatment
with Lawesson’s reagent. Tetrazole 22 was obtained by
reaction of 21 with trimethylsilyl azide (TMSA), in the
presence of triphenylphosphine and diisopropylazodi-
carboxylate (DIAD).40 Deprotection of the catechol moi-
ety, by treatment with BF3ÆS(Me)2, gave analogue 23.
HT22 cells suffer cell death within 24 h following expo-
sure to 1–5 mM glutamate. The cells lack ionotropic glu-
tamate receptors (that could mediate excitotoxicity) and
are known to succumb to acute oxidative stress due to
glutamate inhibition of cystine uptake and the ensuing
depletion of intracellular GSH.35,41,42
The new compounds were tested at concentrations 0.01–
10 lM. As expected, LA was inactive at these low lM
concentrations. The presence of catechol group (4, 12,
Thiazole 27 was prepared from lipoamide and 3,4-
dimethoxyacetophenone as depicted in Scheme 5.
S
O
a
NH2
Br
NH2
S
S S
S
S
25
MeO
c
N
O
S
S
O
MeO
MeO
MeO
MeO
MeO
b
27
26
Scheme 5. Reagents and condition : (a) Lawesson’s reagent, THF; (b) pyridinium tribromide, CHCl3, EtOH; (c) EtOH, reflux.

4226
M. Koufaki et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4223–4227
Table 1. Neuroprotective activity of the new compounds, using
glutamate-challenged hippocampal HT22 cells43
In conclusion, analogues possessing strong neuroprotec-
tive activity can be obtained by inserting heteroaromat-
ics in the alkyl-1,2-dithiolane moieties in conjunction
with the incorporation of another antioxidant entity,
in this case the catechol moiety.
a
Compound
EC50 (lM)
(means SEM)
Compound
EC50 (lM)
(means SEM)
3
4
>10
17
18
19
20
22
23
24
27
>10
>10
3.63 0.33
>10
>10
5
>10
>10
10
11
12
15
16
Acknowledgments
>10
>10
4.21 0.40
>10
6.23 0.97
2.99 0.14
>10
>10
This work was supported in part by the Center for Drug
Discovery, Northeastern University, Boston, USA, and
by ‘EURODESY’ MEST-CT2005-020575.
a EC50 values are compound concentrations required to secure a via-
bility in the glutamate-exposed cells equal to 50% of that of the non-
exposed cells calculated as described in the Experimental section.
Values are means SEM of at least three independent experiments.
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/
j.bmcl.2007.05.036.
16, 23) was requisite for neuroprotective activity in all
the LA derivatives we tested, except in the case of the
dopamine conjugate 19, which did not show any activity
at the concentrations tested (Table 1). Interestingly,
however, its analogue 23, in which the amide functional-
ity was replaced by the tetrazole ring, ranked as the
strongest neuroprotectant, while the 1,3,4-oxadiazole
derivative 12 was somewhat less potent. Thus, it appears
that the replacement of the amide functionality by the
aromatic heterocycles conveys greater neuroprotective
activity to the resulting compounds. As regards com-
pounds in which the heterocyclic ring is directly attached
to the catechol moiety, 1,2,4-oxadiazole analogue 4 is
almost two times more potent than the triazole deriva-
tive 16. From the present data one can assume that
the presence of an alkyl chain between catechol and
the heteroaromatic ring has little influence on the activ-
ity, since oxadiazoles 4 and 12 have very similar activity.
By comparison, the nature of heterocycle seems to have
a somewhat higher effect on the activity (tetrazole deriv-
ative 23 is more active than the oxadiazole derivative 12
and the oxadiazole derivative 4 is more potent than the
triazole analogue 16).
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