Selenosartans: Novel selenophene analogues of milfasartan and eprosartan

Article abstract of DOI:10.1016/j.bmcl.2007.11.136

A series of selenophene analogues of the thiophene-containing antihypertensives milfasartan and eprosartan were prepared and tested for AT₁ receptor antagonist properties. All four selenophene compounds proved to be potent AT₁ recept

Full text of DOI:10.1016/j.bmcl.2007.11.136

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Bioorganic & Medicinal Chemistry Letters 18 (2008) 1241–1244
Selenosartans: Novel selenophene analogues
of milfasartan and eprosartan
Rebecca L. Grange,a,b James Ziogas,c Andrea J. North,a,b
James A. Angusc and Carl H. Schiessera,b,
*
aSchool of Chemistry, The University of Melbourne, Vic. 3010, Australia
bBio21 Molecular Science and Biotechnology Institute, The University of Melbourne, Vic. 3010, Australia
cDepartment of Pharmacology, The University of Melbourne, Vic. 3010, Australia
Received 21 September 2007; revised 13 November 2007; accepted 15 November 2007
Available online 1 January 2008
Abstract—A series of selenophene analogues of the thiophene-containing antihypertensives milfasartan and eprosartan were pre-
pared and tested for AT1 receptor antagonist properties. All four selenophene compounds proved to be potent AT1 receptor antag-
onists, with pKB estimates indicating that these selenides are at least as effective as the thiophene parent compounds at blocking AT1
receptor mediated responses. These results reveal that replacement of sulfur with selenium in thiophene-containing sartans does not
interfere with sartan activity.
Ó 2007 Elsevier Ltd. All rights reserved.
R2
N
Work in our laboratories has been directed towards the
synthesis of selenium-containing molecules of potential
therapeutic value.1 In this regard, we recently reported
that selenium-containing analogues of the antihyperten-
sive compound, fonsartan (1), retained AT1 receptor
blocking activity.2 We also recently showed that a sele-
nium-containing allosteric enhancer of adenosine A1
receptor binding was significantly more potent than its
thiophene-containing parent,3 and that a selenium ana-
logue of the A1 AR agonist, tecadenoson, proved to
have similar activity to the parent compound.4
CO2H
R1
Se
O
N
N
N
CO2H
Se
N
N
N
N
H
4
5: R1 = R2 = H
6: R1 = H, R2 = CO2Me
7: R1 = CO2Me, R2 = H
CO2Me
CO2H
S
Milfasartan5 (2) and eprosartan6–9 (3) are thiophene-con-
taining selective AT1 receptor antagonists (sartans).10
Milfasartan (2) reached Phase I clinical trials, while epro-
sartan (3) is on the market in many countries.6,11,12 Given
the outcomes of previous work, we were curious about the
effect that sulfur/selenium exchange may have on milfa-
sartan and eprosartan potency. Described herein is the
synthesis and preliminary pharmacological testing of sel-
enoeprosartan (4) and several selenophene analogues of
milfasartan, including the nor-ester (5), selenomilfasartan
(6) and its regioisomer (7).
MeS
N
O
CO2H
N
N
N
N
N
CO2H
N
N
O
O
O
N
S
N
S
N
H
N
H
H
1
2
3
Keyword: AT1 receptor antagonist sartan.
*
Selenoeprosartan (4) was prepared following the general
methodology developed by Keenan for the parent
Corresponding author. Tel.: +61 3 8344 2432; fax: +61 3 9347
8189; e-mail: director@freeradical.org.au
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.11.136

1242
R. L. Grange et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1241–1244
The route to selenomilfasartan (6) called for the novel
CHO
a – c
Se
selenophene (11) and we envisioned that selenophene
11 could be obtained from selenophene-3-carboxylic
acid (12). While 12 has been prepared previously via
pathways involving regioselective dehalogenation of
tri- or tetrahalides of selenophene,15 none of these pro-
cedures were reproducible in our hands. We instead re-
sorted to the free-radical methods developed in our
laboratories (Scheme 2).16
CO2Me
CHO
CO2H
CO2Et
N
N
Se
9
10
d,e
To that end benzyl acrylate was reacted with parafor-
maldehyde in a Baylis–Hillman reaction to afford alco-
hol (13) in 61% yield. Epoxidation with m-CPBA
followed by Swern oxidation provided aldehyde (15).
Wittig methodology using (iodomethylene)triphenyl-
phosphorane gave iodide (16) in 34% overall yield.
Our previously established methodology16 was then ap-
plied to iodide (16): treatment with sodium benzylsele-
noate to afford 1717 and free-radical mediated ring
closure (presumably involving intermediate 18) followed
by ester hydrolysis afforded selenophene-3-carboxylic
acid (12) in useful quantities. Acid (12) is a synthetic
intermediate in the preparation of the antitumour agent
selenophenfurin18 and the procedure described above
represents a novel synthesis of this useful compound.
Selenophene-3-carboxylic acid (12) was smoothly con-
verted to the requisite bromide 11 over several routine
steps.
4
Scheme 1. Reagents and conditions: (a) diethyl malonate, piperidine,
benzoic acid, cyclohexane, reflux; (b) NaBH4, EtOH, rt, 49% over two
steps; (c) KOH, EtOH, rt, 81%; (d) 10, Piperidine, benzoic acid,
cyclohexane/toluene 9:1, reflux, 76%; (e) NaOH, EtOH, H2O, rt, 68%.
compound 3 (Scheme 1).9 Accordingly, 2-selenophene-
carboxaldehyde13 was condensed with diethyl malonate.
The resulting alkene was not purified but was rather re-
duced directly with sodium borohydride to provide the
saturated diester (8) in 49% yield. Saponification of 8
furnished the required half-acid 9 in 81% yield. Conden-
sation of formylimidazole8 10 with three equivalents of
half-acid 9 followed by saponification provided the tar-
get compound (4) in 68% yield.
Chinese hamster ovary cells stably expressing the rat
AT1a receptor were used to assess the ability of com-
pounds 3 and 4 to inhibit angiotensin II mediated in-
creases in intracellular calcium.14 Figure 1 shows the
rightward shift of angiotensin concentration–response
curves by selenoeprosartan at 1, 10 and 100 nM. Similar
data were obtained with eprosartan (not shown). The
calculated pKB values were 8.3 for selenoeprosartan
compared with 8.8 for eprosartan.
The route to selenomilfasartan analogues (5, 7) required
the preparation of 2-(bromomethyl)selenophene (19)19
and methyl 5-(chloromethyl)selenophene-2-carboxylate
CO2Bn
a
CO2Bn
O
b
CO2Bn
R
HO
13, 61%
14 R = CH2OH
15 R = CHO
c
d
CO2R'
OH
H
25000
H
control
CO2Bn
e
H
1 nM
H
O
20000
I
SeBn
I
10 nM
16
17
CO2R'
H
15000
10000
5000
0
100 nM
n=6
R' = Et / Bn
OH
f, g
SeBn
18
h – k
CO2H
CO2Me
Br
Se
Se
-14 -13 -12 -11 -10 -9 -8 -7 -6 -5
log [Angiotensin II]
11
12
Figure 1. Selenoeprosartan (4) inhibits angiotensin II-evoked increases
in intracellular calcium in CHO cells stably expressing the AT1a
receptor. CHO cells were loaded with Fluo4-AM and then incubated
Scheme 2. Reagents and conditions: (a) (CHO)n, NMe3 (aq), H2O,
60 °C, 61%; (b) m-CPBA, 3-tert-butyl-4-hydroxy-5-methylphenyl sul-
fide, CCl4, reflux, 73%; (c) (COCl)2, DMSO, NEt3, CH2Cl2, ꢀ78 °C;
(d) (Ph3PCH2I)+I, NaHMDS, HMPA, THF, ꢀ78 °C ! rt, 34%; (e)
(SeBn)2, NaBH4, EtOH, rt; (f) TTMSS, AIBN, benzene, reflux; (g)
LiOH, THF, H2O, 26% from 16; (h) sBuLi, ꢀ78 °C, THF; (i) MeI, rt,
90%; (j) K2CO3, MeI, acetone, rt, 87%; (k) NBS, AIBN, CCl4, reflux,
45% (NMR yield).
with 1, 10 or 100 nM selenoeprosartan. Changes in intracellular [Ca2+
]
evoked by increasing concentrations of angiotensin II in the absence
and presence of the antagonist were measured over a 3 min period
using the Flexstation II (Molecular Devices). RFU, relative fluores-
cence units.

R. L. Grange et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1241–1244
1243
trary to expectation,5 regioisomer (7) was a potent
AT1 receptor antagonist (Fig. 2). The rightward shift
of the angiotensin concentration–response curve and
the depression of the maximum response are character-
istic of many of the high affinity competitive AT1 antag-
onists.21 We therefore chose to synthesize the analogous
thiophene (22) for direct comparison with 7; 22 was not
tested by Salimbeni but was expected to be less effective
than 2 as an AT1 receptor antagonist on the basis of
substituent effect studies.5 The thiophene (22) was
prepared from methyl 5-(bromomethyl)thiophene-2-car-
boxylate, itself obtained from methyl 5-methylthioph-
ene-2-carboxylate22 by NBS bromination, in an
identical manner to that described for 7.
a, b
OH
HO
CHO
Br
OHC
Se
Se
O
O
c, d
Se
Cl
MeO
Se
19
20
Scheme 3. Reagents and conditions: (a) NaBH4, EtOH, rt, 84%; (b)
AgNO3, NaOH, H2O, 0 °C, 80%; (c) SOCl2, benzene, reflux; (d)
MeOH, rt, 85%.
(20). The former compound was readily prepared from
2-selenophenecarboxaldehyde13 through
Figure 3a compares the ability of the selenomilfasartans
(compounds 5–7) each at a concentration of 30 nM to
inhibit the angiotensin II-evoked increases in intracellu-
lar calcium in CHO cells stably expressing the AT1a
receptor. The estimated pKB values were 10.1, 9.8 and
9.9 for compounds 5, 6 and 7. Figure 3b shows the abil-
ity of the corresponding sulfur-containing sartans (com-
a sequence
involving reduction and bromination, while 20 was pre-
pared from 2,5-selenophenedicarboxaldehyde20 via a se-
quence consisting of selective reduction, oxidation,
double chlorination and esterification (Scheme 3). The
precursors (11, 19, 20) were coupled to the common tri-
tyl-protected biphenyl core structure (21) following the
procedure of Salimbeni.5 Subsequent deprotection affor-
ded the selenosartans (5–7) in acceptable quantities
(Scheme 4).
During the course of the biological testing of these sele-
nomilfasartan analogues, it became apparent that con-
R2
H
N
O
R1
Se
N
N
O
b
a
N
5 – 7
N
N
N
N
N
N
N
Tr
N
Tr
21
Scheme 4. Reagents and conditions: (a) 11, 19 or 20, NaH, LiBr,
DMF, 0 °C ! rt; (b) MeOH, reflux. Overall yields: 40–50%.
25000
control
1 nM
20000
10 nM
15000
10000
5000
0
100 nM
n=8
Figure 3. Inhibition of angiotensin II-evoked increases in intracellular
calcium in CHO cells stably expressing the AT1a receptor by (a)
selenomilfasartans 5–7; and (b) the parent sulfur-containing milfasar-
tans 2, 22 and 23. A similar shift in the angiotensin concentration–
response curve was elicited by 30 nM of each of the compounds.
Methods as described in Figure 1. n = 3–4 RFU, relative fluorescence
units.
-14 -13 -12 -11 -10 -9 -8 -7 -6 -5
log [Angiotensin II]
Figure 2. Inhibition of angiotensin II-evoked increases in intracellular
calcium in CHO cells stably expressing the AT1a receptor by
selenomilfasartan regioisomer (7). Methods as described in Figure 1.
RFU, relative fluorescence units.

1244
R. L. Grange et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1241–1244
pounds 2,5 22, 235) to inhibit AT1a receptor mediated re-
sponses in the CHO cell assay. The estimated pKB val-
ues for the corresponding sulfidemilfasartan analogues
were 10.0, 9.6 and 10.1. These data suggest that the sel-
enomilfasartans are each as effective as the correspond-
ing sulfur-containing parent and suggest that replacing
the thiophene sulfur with selenium does not interfere
with the AT1 receptor antagonist capabilities of the
sartans.
7. Weinstock, J.; Keenan, R. M.; Samanen, J.; Hempel, J.;
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R
S
N
O
N
22: R = CO2Me
23: R = H
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N
N
N
N
H
These findings are consistent with our earlier report that
replacing the sulfur with selenium in the non-thiophene
fonsartan also does not interfere with AT1 receptor
antagonist potency.2 The finding that the regioisomers
(7, 22) of selenomilfasartan and milfasartan are effective
antagonists of AT1 receptor mediated responses con-
trasts with the substituent effect studies of Salimbeni
et al. and warrants further investigation.
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˘
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We thank the Australian Research Council through the
Centres of Excellence program for financial support. We
thank Dr. Walter Thomas, Baker Heart Research Insti-
tute, Melbourne, Australia, for providing the CHO cells
expressing the AT1a receptor. We thank Anja Mast and
Mark Ross-Smith for conducting the intracellular cal-
cium measurements.
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