On the search of new I2-IBS aliphatic ligands: Bis-guanidino carbonyl derivatives

Article abstract of DOI:10.1016/j.bmcl.2007.07.093

Continuing with our search of aliphatic dicationic derivatives as I2-IBS ligands and looking at Amiloride, a known ligand of I2-IBS, we have incorporated the guanidinocarbonyl moiety into our aliphatic compounds with the intention of improving the binding to I2-IBS. Thus, we present the different approaches to the preparation and pharmacological evaluation (in human brain tissue) as I2-IBS ligands of a new series of aliphatic derivatives incorporating the guanidinocarbonyl group and with different chain length (n = 8-12, and 14 methylene groups).

Full text of DOI:10.1016/j.bmcl.2007.07.093

Bioorganic & Medicinal Chemistry Letters 17 (2007) 6009–6012
On the search of new I -IBS aliphatic ligands:
2
Bis-guanidino carbonyl derivatives
a
a
a,
*
Jonathan Corcoran, Fernando Rodriguez, Isabel Rozas,
b
b
J. Javier Meana and Luis F. Callado
a
Centre for Synthesis and Chemical Biology, School of Chemistry, Trinity College Dublin, Dublin 2, Ireland
Department of Pharmacology, Faculty of Medicine, University of the Basque Country, 48940-Leioa, Bizkaia, Spain
b
Received 19 June 2007; revised 12 July 2007; accepted 15 July 2007
Available online 23 August 2007
Abstract—Continuing with our search of aliphatic dicationic derivatives as I2-IBS ligands and looking at Amiloride, a known ligand
of I2-IBS, we have incorporated the guanidinocarbonyl moiety into our aliphatic compounds with the intention of improving the
binding to I2-IBS. Thus, we present the different approaches to the preparation and pharmacological evaluation (in human brain
tissue) as I2-IBS ligands of a new series of aliphatic derivatives incorporating the guanidinocarbonyl group and with different chain
length (n = 8–12, and 14 methylene groups).
Ó 2007 Elsevier Ltd. All rights reserved.
Imidazoline Binding Sites (IBS), only discovered two
1
Hence, we present here the preparation and biological
evaluation as I -IBS ligands of a new series of aliphatic
derivatives, similar to 1, incorporating the guanidinocar-
bonyl group and with different chain length (n = 8–12,
and 14 methylene groups).
decades ago, are involved in a large number of pharma-
cological functions and diseases. For example, there is
evidence of their over-expression in the brain of Alzhei-
2
2
3
mer’s and Parkinson’s disease patients as well as in
4
human glial tumours. On the other hand, it has been
described that the ligands of the I -IBS subtype can
To synthesise compounds 2a (n = 8) to 2f (n = 14), sev-
eral approaches could be considered such as those used
2
attenuate tolerance when using opioids as analgesic
5
9,10
11
therapy. To date, most I -IBS ligands prepared always
2
by Schmuck and Jansen to synthesise pyrrole based
receptors and analogues of Sorangicin A, respectively.
included a (hetero-)aromatic group structurally related
6
to clonidine as we recently showed. In fact, our group
was the first to test aliphatic dicationic derivatives
related to the endogenous IBS ligand, Agmatine, obtain-
ing compound 1 (Fig. 1) that showed good I -IBS affin-
Starting from the commercial dicarboxylic acids (3), the
corresponding methyl esters (4) were obtained in good
yield by reaction with thionyl chloride in methanol.
Then, following Schmuck method, reaction with guani-
dine, in the presence of sodium methoxide (Scheme 1),
2
7
9
ity and selectivity versus a -adrenoceptors (a -AR).
2
2
Thus, continuing with our search of aliphatic dicationic
derivatives as I -IBS ligands we have prepared a new
2
series of compounds incorporating a different cationic
moiety. Looking at Amiloride (Fig. 1), which shows a
good affinity for I -IBS, we have incorporated the gua-
2
O
NH
NH
NH
NH
Cl
N
N
2
H N
N
2
8
N
H
2
H
H
2
N
NH
2
nidinocarbonyl moiety into our aliphatic derivatives,
with the intention of improving the binding to I -IBS.
Agmatine
Amiloride
2
NH
NH
NH
NH
O
O
NH
NH2
H
2
N
N
N
2
2
H N
N
N
H
n H
H
n
H
Keywords: I2-IBS; Aliphatic; Amiloride; Guanidinocarbonyl.
*
1, n = 12
2a-f, n = 8-12, 14
Corresponding author. Tel.: +353
712826; e-mail: rozasi@tcd.ie
1 8963731; fax: +353 1
6
Figure 1.
0
960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.07.093

6010
J. Corcoran et al. / Bioorg. Med. Chem. Lett. 17 (2007) 6009–6012
NH
NH
NH
2
H N NHBoc
.
HCl
O
O
SOCl
MeOH
,4a-f n = 8-12, 14
2
O
O
NH
O
O
NH
NH
O
O
NH
O
O
NH
NHBoc
H
2
N
2
, BOP,
HO
OH
O
n
O
H
2
N
N
N
2
n
n
HO
OH
BocHN
NH
N
N
H
MeONa, MeOH
reflux
H
H
n
n
N-methylmorpholine
DMF, rt
H
3
4
2a-f, n = 8-12, 14
3
5a-f, n = 8-12, 14
3
a-f n = 8-12, 14
Scheme 1.
O
O
NH
NH
i) TFA, DCM, rt
.2HCl
5
a-f
H
2
N
N
N
n
2
-
H
H
yielded the corresponding bis-guanidinocarbonyl deriva-
ii) Amberlite (Cl form),
1
2
H
2
O, rt
2a-f, n = 8-12, 14
tives. Unfortunately, the yields obtained were disap-
pointing (highest yield obtained was 20% for 2a).
Probably these low yields are a consequence of the for-
mation of ketoesters through a Claisen condensation
reaction.
Scheme 3.
The pharmacological affinity of the prepared com-
pounds was evaluated through competition binding
1
1
Following Jansen’s procedure a second synthetic route
was attempted, which involved first the activation of the
carboxylic acids by means of CDI and then the use of
guanidine as guanidilating agent (Scheme 2). This
method provided better yields as presented in Table 1.
3
studies against the selective I -IBS radioligand [ H]-2-
2
17
[(2-benzofuranil)-2-imidazoline] (2-BFI). The studies
were performed in membranes from post-mortem
1
3
human frontal cortex, a brain area that shows an impor-
2,18
tant density of I -IBS.
The inhibition constants (Ki)
for each compound were obtained and are expressed
2
The hydrochloride salts of the neutral alkyl bis-guanid-
inylamides were obtained by dissolving the correspond-
ing compounds (1 mmol) in DCM (15 ml) and then
bubbling with HCl gas. The salts precipitated out of
the solution and were recrystallized from methanol.
as the corresponding pK in Table 2. Idazoxan, a com-
i
pound with well-established affinity for I -IBS, and the
2
alkyl bis-guanidine derivative 1, previously prepared
7
by us, were used as references.
Unfortunately, the affinity for the I -IBS of this new ser-
2
ies of compounds did not improve compared to the lead
compound 1. This was a disappointing result consider-
To improve the yields obtained, and based in another
method that had proved successful for Schmuck
1
0
et al. using PyBOP as coupling reagent, we prepared
the alkyl bis-guanidinylamide derivatives utilizing BOP
as the coupling reagent and the N-Boc-mono-protected
guanidine for the guanidilation step, as previously
ing the good I -IBS affinity showed by both Amiloride
2
8
(
pK = 6.8) and our lead compound. Moreover, in the
i
7
guanidinium series we found a good correlation
between the length of the aliphatic chain and the I2-
IBS affinity. However, in this series no relation at all
was found between the length of the linker chain and
14
15
reported by Zapf (Scheme 3).
The Boc groups were removed by treatment with TFA
in DCM at room temperature during 6 h and, then,
the HCl salts were obtained, in good yields (Table 1),
by treatment with an excess of basic anion exchange
resin (IRA-400, Chloride form) in water, at room tem-
perature during 12 h. Absence of the trifluoroacetate salt
the affinity for the I -IBS receptors.
2
It seems that the introduction of the carbonyl groups at
both cationic sides of the aliphatic chains could interfere
with the interaction between the guanidinium group and
the residues in the actual I -IBS binding pocket. Maybe
2
1
3
was checked by C NMR.
the intramolecular formation of a hydrogen bond (HB) be-
tween one of the H atoms of the guanidine and the O atom
of the carbonyl group is preventing the delocalisation
inherent to the guanidinium moiety that provides this par-
ticular functional cation with special chemical characteris-
tics to establish both ionic and HBs interactions.
O
O
NH
O
O
NH
NH2
i) CDI, DCM, rt
HO
OH
2
H N
N
N
n
n
NH
H
H
ii)
.HCl,
3
a-f n = 8-12, 14
H
2
N
NH
2
2a-f, n = 8-12, 14
drierite, DMF, rt
1
9
For that reason, we have performed DFT computations
of models of the guanidinylamides in both an aromatic
Scheme 2.
Table 1. Total yields obtained for the preparation of the HCl salts
isolated pure products) of the alkyl guanidinylamides 2 calculated
from the reactions shown in Schemes 1–3
2
Table 2. Affinity of the alkyl guanidinylamides prepared towards I -
(
i
IBS measured as pK values
3
Compound
i
pK ([ H]2-BFI)
Scheme 1 Scheme 2
% yield)
Scheme 3
(% yield)
(
(% yield)
Idazoxan
1
7.43
7.48
6.11
5.20
5.69
5.06
5.91
5.70
2
2
2
2
2
2
a (n = 8)
b (n = 9)
c (n = 10)
d (n = 11)
e (n = 12)
f (n = 14)
20
9
41
45
38
34
43
41
74
50
76
74
60
48
2a (n = 8)
2b (n = 9)
2c (n = 10)
2d (n = 11)
2e (n = 12)
2f (n = 14)
17
15
11
10
The full characterization of compound 2a is presented in Ref. 16.

J. Corcoran et al. / Bioorg. Med. Chem. Lett. 17 (2007) 6009–6012
6011
Figure 2. Optimised structures of N-(3-amino-pyrazine-2-carbonyl)-guanidinium (a), N-acetyl-guanidinium (b) and ethylguanidinium (c) optimised
*
at B3LYP/6-31+G level.
environment similar to that of Amiloride (N-(3-amino-
pyrazine-2-carbonyl)-guanidinium, A) and an aliphatic
environment such as that of the molecules presented in
this paper (N-acetyl-guanidinium, B), as well as the ethyl-
the I -IBS, and computational results to rationalize this
2
affinity values, concluding that the introduction of a
conjugated carbonyl group to the guanidinium moieties
does not improve the affinity towards I -IBS due to the
2
2
0
guanidinium (C) as a model of compound 1. All opti-
*
formation of an intramolecular HB.
mised structures at B3LYP/6-31+G level are shown in
Figure 2. The presence of HB has been assessed following
2
1
Acknowledgments
the Atoms in Molecules (AIM) theory.
This research was supported by a Cycle III HEA PRTLI
grant by means of the CSCB (J.C. and F.R.) and by Biz-
kaiko Foru Aldundia (Ekinberri 7/12/EK/2005/65 and
DIPE 06/04), and the Spanish Ministry of Health,
Instituto de Salud Carlos III, RETICS RD06/
In the aliphatic guanidinium C it is obvious that no intra-
molecular HB is established, the cation is free to rotate
and can freely interact with the binding pocket of the I2-
IBS. The characteristics of the guanidinium cation in
terms of planarity, acidity and electronic properties are
unaffected. However, in the aromatic guanidinylamidini-
um A (our model for Amiloride) and in the N-acetyl-guan-
idinium B the C@O is in the same plane and conjugated to
the guanidinium group, and in the aromatic case A, both
groups are rotated out-of-plane with respect to the pyra-
zine ring by 16°. This distortion does not affect the strong
conjugation between the guanidinium and the C@O
group, and in fact in both derivatives a similar and very
0011(REM-TAP Network). F.R. thanks the Consejeria
de Educacion Cultura y Deporte de la Comunidad
Autonoma de La Rioja for his grant.
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/
j.bmcl.2007.07.093.
.
. .
strong intramolecular N–H O HB is formed between
one of the NH of the guanidinium and the carbonyl oxy-
gen. This strength is reflected in the short HB distances:
˚
1
.86 A in both cases, high values of electron density at
References and notes
the bond critical point q(BCP): 0.0344 and 0.0343 a.u.
for A and B respectively, and a positive Laplacian of
1. Bousquet, P.; Feldman, J.; Schwartz, J. J. Pharmacol. Exp.
Ther. 1984, 230, 232.
2. Ru ´ı z, J.; Mart ´ı n, I.; Callado, L. F.; Meana, J. J.; Barturen,
F.; Garc ´ı a-Sevilla, J. A. Neurosci. Lett. 1993, 160, 109.
. Gargalidis-Moudanos, C.; Pizzinat, N.; Javoy-Agid, F.;
Remaury, A.; Parini, A. Neurochem. Int. 1997, 30, 31.
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Meana, J. J. J. Neurol. Neurosur. Psychiatry 2004, 75, 785.
2
q(BCP) in both cases ($ q(BCP) = 0.117 a.u. in both
derivatives). In the case of the amiloride-like derivative
2
1
A, an extra HB is found between the NH group in the
2
3
pyrazine ring (acting as an acceptor) and one of the NH
.
of the guanidinium moiety (d[N–H N] = 1.88 A,
. .
˚
2
q(BCP): 0.0379 a.u., $ q(BCP) = 0.103 a.u.).
5
. Boronat, M. A.; Olmos, G.; Garcia-Sevilla, J. A. Brit. J.
Pharmacol. 1998, 125, 175.
Hence, it seems that the optimal interaction between the
guanidinium derivatives and the I -IBS occurs in those
compounds where the guanidinium cation is not conju-
gated or interacting with any other functional group.
That will explain that the aliphatic bis-guanidinium
2
6. Dardonville, C.; Rozas, I. Med. Res. Rev. 2004, 24, 639.
7. Dardonville, C.; Rozas, I.; Callado, L. F.; Meana, J. J.
Bioorg. Med. Chem. 2002, 10, 1525.
8. Tesson, F.; Prip-Buus, C.; Lemoine, A.; Pogorier, J. P.;
Parini, A. J. Biol. Chem. 1991, 266, 155.
derivative 1 shows the best pK . Those compounds
i
9
. Schmuck, C. Chem.-Eur. J. 2000, 6, 709.
0. Schmuck, C.; Weinard, W. J. Am. Chem. Soc. 2003, 125,
52.
1. Jansen, R.; Schummer, D.; Holfe, G. Liebigs. Ann. Chem.
990, 975.
where the guanidinium cation is forming an intramolec-
ular HB with an adjacent carbonyl group showed poorer
pK values and the better performance of Amiloride
1
1
1
4
i
could be explained by other type of interactions in the
binding site such as p–p interactions or hydrogen bond-
ing through the 3- or 5-NH groups in the pyrazine ring.
2
1
2. Reagents, conditions and yields: (a) Corresponding car-
boxylic acid (1 equiv) and MeOH (20 ml), 0 °C; SOCl (2.2
equiv) added dropwise, rt, 6 h, 80–97%; (b) Guanidine
2
Summarizing, we have presented here different synthetic
approaches towards the preparation of aliphatic guanid-
inylamide derivatives, the results of the biological
evaluation of the affinity of these compounds towards
hydrochloride (10 equiv), NaMeO (10 equiv), MeOH,
reflux during 72 h.
3. Reagents and conditions: i—Corresponding carboxylic acid
1
(
1 equiv), CDI (2 equiv), DCM, rt, 1 h; ii—guanidine

6012
J. Corcoran et al. / Bioorg. Med. Chem. Lett. 17 (2007) 6009–6012
hydrochloride (10 equiv), drierite (2 equiv), DMF (2 ml), rt,
0 h.
4. Zapf, C. W.; Creighton, C. J.; Tomioka, M.; Goodman,
M. Org. Lett. 2001, 3, 1133.
5. Reagents and conditions: Corresponding carboxylic acid
19. The hybrid Density Functional Theory (DFT) method
*
2
B3LYP has been used with the 6-31+G basis set, and the
1
1
electron density has been analysed using the AIMPAC set
of programs. Both are incorporated in the Gaussian03 set
of programs. Complete Computational methods and
reference for Gaussian03 can be found in the Supplemen-
tal Information.
(
(
1 equiv), DMF (20 ml), Boc-guanidine (4 equiv), BOP
2.4 equiv), N-methylmorpholine (6 equiv), rt, 12 h.
1
6. Full characterization of compound 2a:Dihydrochloride salt
of 1,8-octane bisguanidinylamide: White solid; mp 241–
20. Details of the computational results could be supplied
upon request.
À1
1
2
43 °C; IR (neat) m 3418, 1730, 1600 cm
;
H NMR
21. According to the theory of atoms in molecules (AIM)
proposed by Bader [Bader, R. F. W. Atoms In Molecules.
A Quantum Theory; Oxford University: New York, 1990]
a bond or interaction between a pair of atoms can be
characterized by the properties of the electron density in
a particular point known as the Bond Critical Point
(BCP) which corresponds to a saddle point in the
electron density surface between these two atoms. Thus,
(D O) d 1.17–1.21 (m, 8H), 1.45–1.54 (m, 4H), 2.36 (t,
2
1
3
J = 6.5 Hz, 4H); C NMR (D O) d 23.8, 27.4, 27.5, 36.3,
2
+
54.1, 177.2; MS (ESI ) m/z 285.2039 [M+H] . Anal.
+
1
(
C H Cl N O Æ 0.5H O) Calcd C, 39.35; H, 7.43; N,
12
26
2
6
2
2
22.94. Found: C, 39.09; H, 7.26; N, 22.51.
3
1
1
7. Descriptions of the preparation of membranes, [ H]2-BFI
binding assay and analysis of binding data are provided in
Supplemental information.
8. Garcia-Sevilla, J. A.; Escriba, P. V.; Sastre, M.; Walzer,
C.; Busquets, X.; Jaquet, G.; Reis, D. J.; Guimon, X. J.
Arch. Gen. Psychiat. 1996, 53, 803.
À2
a electron density [q(BCP)] around 10 a.u. and a
2
positive Laplacian of the charge density [$ q(BCP)] at
the BCPs correspond to what is defined as ‘closed-shell’
interaction of the HB type.