High-speed microwave assisted synthesis of SEA0400 - A selective inhibitor of the Na+/Ca2+ exchanger

Article abstract of DOI:10.1016/j.tetlet.2012.04.120

An efficient and rapid five-step synthesis of SEA0400, a selective Na ⁺/Ca²⁺ exchanger (NCX) inhibitor, using controlled microwave heating is reported. In addition, a novel three-step route omitting the key Baeyer-Villiger oxidation/

Full text of DOI:10.1016/j.tetlet.2012.04.120

Tetrahedron Letters 53 (2012) 3731–3734
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High-speed microwave assisted synthesis of SEA0400—a selective inhibitor
of the Na+/Ca2+ exchanger
Gemma Guedes de la Cruz a, Klaus Groschner b, C. Oliver Kappe a, , Toma N. Glasnov a,
⇑
⇑
a Christian Doppler Laboratory for Microwave Chemistry and Institute of Chemistry, Karl-Franzens-University Graz, Heinrichstrasse 28, A-8010 Graz, Austria
b Institute of Biophysics, Medical University of Graz, Harrachgasse 21/IV, 8010 Graz, Austria
a r t i c l e i n f o
a b s t r a c t
An efficient and rapid five-step synthesis of SEA0400, a selective Na+/Ca2+ exchanger (NCX) inhibitor,
using controlled microwave heating is reported. In addition, a novel three-step route omitting the key
Baeyer–Villiger oxidation/hydrolysis sequence has been developed.
Article history:
Received 15 March 2012
Revised 25 April 2012
Accepted 25 April 2012
Available online 2 May 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Baeyer–Villiger oxidation
Williamson ether synthesis
Microwave-assisted synthesis
Continuous-flow hydrogenation
Process intensification
SEA0400
The Na+/Ca2+ exchanger (NCX) is an electrogenic cation trans-
porter that has a vital function in the cardiac myocytes where it
is of essential importance for the Ca2+ expulsion during the excita-
tion–contraction cycle. Thus, disturbances in NCX function are
associated with compromised cardiac Ca2+ handling and may
underlie cardiac dysfunction in a variety of pathophysiological sit-
uations such as ischemia/reperfusion injury, arrhythmias, hyper-
trophy, and heart failure.1a–c Suppression of NCX has been
proposed as a potential strategy to prevent pathophysiological car-
diac remodeling1d as well as arrhythmias1e and inhibitors of NCX
have therefore attracted significant attention in view of their ther-
apeutic potential.
for the development of novel drugs for the treatment of a number
of cardiovascular diseases such as ischemic heart disease, arrhyth-
mias, heart failure, or stroke.
Herein we disclose an improved synthetic protocol that allows
the rapid synthesis of SEA0400 by applying controlled microwave
heating as enabling technology.4 Compared to the patented five
step method requiring ꢀ6 days overall reaction time (five reaction
steps, ꢀ37% overall yield, Scheme 1) our operationally intensified
protocol allows the generation of SEA0400 within less than 2 h
reaction time. In an attempt to improve the efficiency of this pro-
tocol, we aimed at performing all of the synthetic steps using
sealed vessel controlled microwave heating, as well as to substi-
tute some of the originally chosen synthetic methods with more
reliable protocols in terms of microwave and/or continuous-flow
chemistry.
Recently, Matsuda and co-workers have described a 4,40-
dihydroxydiphenyl ether-based compound—SEA0400, as an inhib-
itor of NCX with appreciable selectivity for the transporter (1,
Fig. 1).2 The IC50 value of SEA0400 for both forward (Ca2+ extru-
sion) and reverse (Ca2+ uptake) mode NCX operation in guinea
pig and canine cardiomyocytes is in the range of 20–300 nM.3
For the initial Williamson ether synthesis using commercially
available 2,5-difluorobenzyl bromide (2) and 4-hydroxyacetophe-
none 3 leading to 4-(2,5-difluorobenzyloxy)acetophenone (4) we
Furthermore, concentrations up to 1 lM are inhibiting NCX with
70–80% efficiency while lacking significant effects on other ion
transport mechanisms.2,3 The NCX-selective inhibitor SEA0400,
therefore, appears both as a useful tool to study the in vivo func-
tion of the Na+/Ca2+ exchanger and as a valuable lead structure
⇑
Corresponding authors. Tel.: +43 316 380 5352; fax: +43 316 380 9840 (C.O.K.);
tel.: +43 316 380 5348; fax: +43 316 380 9840 (T.N.G.).
E-mail addresses: oliver.kappe@uni-graz.at (C. Oliver Kappe), toma.glasnov@
uni-graz.at (T.N. Glasnov).
Figure 1. Structure of the NCX-inhibitor SEA0400 (1).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.04.120

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G. G. de la Cruz et al. / Tetrahedron Letters 53 (2012) 3731–3734
Scheme 1. Patented five step synthesis of NCX inhibitor SEA0400 (1).5
modified the original reaction conditions in a way that the reaction
temperature was increased from room temperature to 150 °C
employing sealed-vessel microwave technology. Among the differ-
ent solvent/base combinations evaluated for this nucleophilic sub-
stitution the use of acetonitrile (MeCN) and K2CO3 (1.05 equiv)
provided very satisfactory results. Eventually, full conversion could
be achieved at 150 °C (5–6 bar) within 5 min leading to a 98% iso-
lated product yield of ether 4 (Scheme 2). The generation of the
next intermediate in the synthesis—4-(2,5-difluorobenzyloxy)phe-
nol 5, seemed somewhat more demanding since it required the
longest reaction time in the sequence—ꢀ4 days, combining a Bae-
yer–Villiger oxidation6 step employing m-chloroperbenzoic acid
(m-CPBA) as oxidant, and hydrolysis of the formed acetate 4a into
the desired phenol 5 (Table 1, Scheme 1).5 High-temperature Bae-
yer–Villiger oxidations are rather rare and underexplored; never-
theless several microwave protocols exist in the literature.7 After
an extensive optimization regime we have found that high conver-
sions in this process can indeed be obtained in an elevated temper-
ature regime using m-CPBA as oxidant in combination with
p-toluenesulfonic acid (p-TSA) as a catalyst. Employing dichloro-
methane (CH2Cl2) as a solvent >90% conversion levels could readily
be achieved at 100–130 °C (5–9 bar) within 20 min. Fine-tuning of
reaction temperature and m-CPBA/p-TSA stoichiometry ultimately
led to optimized reaction conditions (2.5 equiv m-CPBA, 0.2 equiv
p-TSA, 130 °C, 20 min) that provided full conversion and a high
selectivity for the desired acetate 4a, along with minor quantities
of the hydrolysis by-product 5 (Table 1, entry 9). Isolation of ace-
tate 4a by flash chromatography furnished a 72% product yield, vir-
tually identical to the value given in the patent literature.5
Hydrolysis of acetate 4a with 1 equiv of K2CO3 in MeOH at
130 °C (5 bar) for 1 min provided 4-(2,5-difluorobenzyloxy)phenol
5 in quantitative yield. Since the hydrolysis by-product in the Bae-
yer–Villiger oxidation is the eventually required phenol 5, the oxi-
dation/hydrolysis step (Scheme 2) was ultimately performed
without chromatographic isolation of the acetate intermediate
4a. Thus, after the Baeyer–Villiger oxidation of acetophenone 4
was completed and the microwave vial had cooled down to
50 °C, the resulting solution was subjected to an extractive work-
up with a mixture of Na2S2O3/NaHCO3 to remove excess m-CPBA.
After solvent evaporation the crude residue was taken-up in meth-
anol, transferred into a second microwave vial containing K2CO3/
MeOH and subjected to microwave heating for 1 min at 130 °C.
Scheme 2. Microwave-assisted synthesis of NCX inhibitor SEA0400 (1).

G. G. de la Cruz et al. / Tetrahedron Letters 53 (2012) 3731–3734
3733
Table 1
to be of sufficient reactivity, not requiring the use of more reactive
(and expensive) 1,4-cyclohexadiene or 1-methyl-1-cyclohexene,
which have recently been also used in microwave-assisted cata-
lytic transfer hydrogenations.12 In the event, clean and complete
reduction of the nitro group was obtained employing 7 mol % of
a standard 5% (w/w) Pt/C hydrogenation catalyst, 10 equiv of cyclo-
hexene as donor and ethanol as the solvent at 160 °C (14 bar) with-
in 30 min, providing a 75% isolated yield of target structure 1
(SEA0400). Lower reaction temperatures and/or catalyst/donor
stoichiometries, or the use of Pd/C as a catalyst led to a significant
decrease in conversion or selectivity.
In a further attempt to improve the synthetic protocol, we addi-
tionally performed the hydrogenation 7?1 under continuous flow
conditions using the H-Cube reactor.13,14 The flow hydrogenation
7?1 was initially optimized using catalyst cartridges filled with
Pd/C, RaNi or Pt/C and ethanol as a reaction solvent. Although
Pd/C as well as Pt/C gave promising results, Ra/Ni proved to be
the ultimate catalyst of choice, providing high selectivity for the ni-
tro-group reduction at 55 °C and 30 bar of H2-gas. Applying a 1 mL/
min flow rate and 10 mg/mL substrate concentration prevented the
formation of side products and provided SEA0400 in quantitative
isolated yield, thus improving the overall yield of the three-step
synthesis up to ꢀ70%.
Optimization of Baeyer–Villiger oxidation 4?4aa
Entry
A/B(equiv)
T (°C)
Conversionb (%)
4a/5b (%)
1
2
3
4
5
6
7
8
9
2/0.1
2/0.1
2/0.1
2/0.2
2/0.2
2/0.1
2/—
70
100
120
120
130
130
130
130
130
72
96
97
98
98
94
6
91
72/—
94/2
93/4
95/3
88/10
90/4
6/—
1.5/0.2
2.5/0.2
89/2
92/8
100 (72)c
a
Reaction conditions: 0.1 mmol of acetophenone 4, 1.5–2.5 equiv of m-CPBA and
0–0.2 equiv of p-TSA in CH2Cl2 (2 mL). Single-mode microwave heating (Biotage
Initiator 2.5) with magnetic stirring and IR temperature control.
b
Based on HPLC–UV analysis (215 nm).
Isolated yield.
c
Chromatographic work-up provided 71% isolated yield of 4-(2,5-
difluorobenzyloxy)phenol 5, matching the yield presented in the
patent for this two step synthesis. However, instead of the required
4 days at room temperature (Scheme 1),5 the combined reaction
time under microwave conditions is only ꢀ25 min (Scheme 2).
At this stage, we evaluated the possibility to obtain the key phe-
nol intermediate 5 directly from 2,5-difluorobenzyl bromide 2 and
hydroquinone in a one-step procedure. Work by Feringa and
coworkers has indicated that mono-benzylated hydroquinones
can be obtained in satisfactory selectivity and yield by simply heat-
ing a benzyl chloride with a large excess (4 equiv) of hydroquinone
in acetone/K2CO3 under reflux conditions for 12 h.8 Gratifyingly,
this method could be successfully translated to benzyl bromide 2
and high-temperature microwave conditions (160 °C, 12 bar), pro-
viding 4-(2,5-difluorobenzyloxy)phenol 5 in 74% isolated yield
after 20 min, with only 7% of the dibenzylated hydroquinone being
formed as a byproduct. Although the yield of phenol 5 using this
one-step method may not appear attractive at first glance, the
operational simplicity avoiding the often difficult to scale Bae-
yer–Villiger oxidation step, combined with the low cost of hydro-
quinone, makes this direct route to phenol 5 very appealing.
The next step in the synthesis of SEA0400 was an SNAr-substi-
tution reaction to prepare 1-(4-(2,5-difluorobenzyloxy)phenoxy)-
4-ethoxy-2-nitrobenzene (7).5 Using phenol 5 and 4-ethoxy-1-flu-
oro-2-nitrobenzene (6a)—readily prepared from commercially
available 4-fluoro-3-nitrophenol (6) via O-alkylation with diethyl
sulfate (MeCN, 120 °C, 10 min)—as starting materials optimization
experiments quickly demonstrated that the original DMF reflux
conditions (150 °C) requiring 5 h for completion could be improved
to 40 min in a sealed microwave vial at 180 °C (2 bar). Clean and
complete SNAr-substitution was obtained using K2CO3 instead of
the originally reported t-BuOK as a base providing a 94% isolated
product yield after flash column chromatography.
In conclusion, we have presented an intensified synthetic proto-
col for the generation of SEA0400, a selective Na+/Ca2+ exchanger
(NCX) inhibitor, using controlled microwave heating and continu-
ous-flow hydrogenation technology. Employing automated sealed-
vessel microwave reactors, optimum reaction conditions were
readily obtained, resulting in a drastic reduction of the required
reaction and overall processing times compared to the protocol
described in the patent literature. Replacing the original Baeyer–
Villiger/hydrolysis sequence with a simple and direct nucleophilic
substitution step using inexpensive hydroquinone as a starting
material additionally simplifies the synthetic process. We believe
that this new high-speed method will be of importance for efficient
generation of SEA0400 analogs for pharmacological testing.
Acknowledgments
This work was supported by a grant for the Christian Doppler
Research Foundation (CDG). G.G. de la Cruz thanks the Gobierno
de Canarias for the provided financial support.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.04.
120.
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