1-Benzyloxy-4,5-dihydro-1H-imidazol-2-yl-amines, a novel class of NR1/2B subtype selective NMDA receptor antagonists

Article abstract of DOI:10.1016/S0960-894X(03)00713-3

Screening of the Roche compound depository led to the identification of (1-benzyloxy-4,5-dihydro-1H-imidazol-2-yl)-butyl amine 4, a structurally novel NR1/2B subtype selective NMDA receptor antagonist. The structure-activity relationships developed in this series resulted in the discovery of a novel class of potent and selective NMDA receptor blockers displaying activity in vivo.

Full text of DOI:10.1016/S0960-894X(03)00713-3

Bioorganic & Medicinal Chemistry Letters 13 (2003) 3155–3159
1-Benzyloxy-4,5-dihydro-1H-imidazol-2-yl-amines, a Novel Class
of NR1/2B Subtype Selective NMDA Receptor Antagonists
Alexander Alanine,a,* Anne Bourson,b Bernd Buttelmann,a Ramanjit Gill,b
Marie-Paule Heitz,a Vincent Mutel,b Emmanuel Pinard,a Gerhard Trubeb
and Rene Wylera
aPharma Division, Discovery Chemistry, F. Hoffmann-La Roche Ltd., CH-4070 Basel, Switzerland
bPharma Division, Preclinical CNS Research, F. Hoffmann-La Roche Ltd., CH-4070 Basel, Switzerland
Received 14 May 2003; revised 3 July 2003; accepted 4 July 2003
Abstract—Screening of the Roche compound depository led to the identification of (1-benzyloxy-4,5-dihydro-1H-imidazol-2-yl)-
butyl amine 4, a structurally novel NR1/2B subtype selective NMDA receptor antagonist. The structure–activity relationships
developed in this series resulted in the discovery of a novel class of potent and selective NMDA receptor blockers displaying activity
in vivo.
# 2003 Elsevier Ltd. All rights reserved.
Excessive activation of N-methyl-d-aspartate (NMDA)
receptors and resulting calcium overload of neurons is
thought to be a key contributor to neuronal cell death
following acute cerebral ischaemia. In this context
NMDA receptor antagonists have been demonstrated
to be potent neuroprotective agents in animal models of
stroke.1,2 Native NMDA receptors are present as com-
plexes containing an NR1 subunit together with one or
more of the four NR2 subunits (NR2A-D).3,4 During
the last decade, a number of NR1/2B subtype selective
blockers5 have been described such as ifenprodil 1,6 CP-
101,606 2,7 and others.8À13 These compounds display
neuroprotective effect in vivo without inducing the side
effects associated with non-selective receptor antago-
nists14À17 via a state-dependent block of NMDA recep-
tors.18 This makes NR1/2B subtype selective blockers
potentially attractive drugs for the treatment of neuro-
degenerative disorders such as stroke,2 brain trauma,2
pain17,19 and Parkinson’s disease.20
The majority of NR1/2B subtype selective NMDA
receptor antagonists described so far are structurally
related to ifenprodil 1. The pharmacophore for this ser-
ies consist of a basic nitrogen linked by spacers to two
aromatic groups, one bearing a phenolic-OH group or a
bioisostere.21 As part of our CNS discovery program,
we set out to develop potent, NR1/2B subtype selective
blockers structurally distinct from ifenprodil 1. To achieve
this, screening of the Roche compound depository was
performed employing tritiated Ro-25-6981 as radio-
ligand in a competition-binding assay.22 This screening
campaign led to the identification of a novel structural
class of NMDA antagonists exemplified by (1-benzyloxy-
4,5-dihydro-1H-imidazol-2-yl)-butyl-amine23,24 4 as the
most active representative with a Ki of 60 nM.
Chemistry
Compound 4 is related to the marketed hypotensive a2
adrenergic agonist clonidine25 3, interestingly however,
such 2-amino-alkyl imidazoline derivative were found to
be devoid of this undesired activity. Using 4 as a lead
compound we attempted to develop new, potent, sub-
*Corresponding author. Tel.: +41-61-688-1289; fax: +41-61-688-
6459; e-mail: alexander.alanine@roche.com
0960-894X/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0960-894X(03)00713-3

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A. Alanine et al. / Bioorg. Med. Chem. Lett. 13 (2003) 3155–3159
Scheme 1. (a) PPh3, DEAD, THF, 0–50 ꢂC, 45–85%; (b) N2H4.H2O, EtOH, 78 ꢂC, 80%; (c) DMF, NaHCO3, 80 ꢂC, 20–70%; (d) N2H4.H2O,
EtOH, 78 ꢂC 60–90%; (e) CS2, EtOAc, 60 ꢂC, 50–95%; (f) MeI, Me2CO, rt, 100%; (g) R2NH2 (3 equiv), EtOH, 78 ꢂC, 10–90%.
type selective NMDA receptor antagonists as alter-
natives to the known chemotypes, for detailed pharma-
cological evaluation. The main synthetic scheme for the
preparation of various 1-benzyloxy-4,5-dihydro-1H-
imadazol-2-yl-amines is outlined in Scheme 1. O-Ben-
zyl-hydroxylamines 5 were prepared by coupling benzyl
alcohols with N-hydroxyphthalimide under Mitsunobu
conditions26 in good yield, followed by release of the
phthalimide group with hydrazine monohydrate. Pri-
mary amines 7 were then obtained by N-alkylation of 5
with bromoethyl-phthalimide affording imides 6 which
were once again deprotected with hydrazine hydrate.
Amines 7 were then converted to the corresponding
cyclic thioureas with carbon disulfide and then activated
by S-alkylation using methyl iodide affording cyclic
isothiouronium salts 8. The final products 4, 9–40, 46–
47 were obtained in satisfactory yield and good purity
by heating salts 8 with an excess of amine under reflux
in ethanol for several h followed by extractive work-up,
chromatography and trituration of the free-base from
ether with ethanolic HCl resulting in isolation of the
corresponding hydrochloride salts. Although primary
amines reacted smoothly, secondary amines failed under
the same conditions.
dine congener 20 is significantly less active (Ki 380 nM).
Next, exploration around the benzyl substituent was
performed, by keeping the N-pentylamine substituent
fixed at C2 on the imidazoline ring (Table 2). Variations
at the ortho, meta and para positions were explored with
electron withdrawing, electron donating and small lipo-
philic residues to determine the optimal electronic and
steric requirements for activity.
In the ortho position small electron withdrawing lipo-
philic substituent such as fluorine 26 are preferred, (Ki
15 nM), whereas more bulky electron donating sub-
stituents such as methoxy are detrimental for binding
affinity 29 (Ki 280 nM). In the meta position the same
trend holds 30>31ꢀ32ꢁ33. Interestingly the nitro
group, although a strong sigma-acceptor and relatively
Table 1. NMDA affinities of compounds 4, 9–25, variation of R2
Ki [mM]a
Ki [mM]a
Compd
R2
NMDAb Compd
R2
NMDAb
9
1.5
5.6
17
18
0.26
21
Results and Discussion
10
11
12
13
4
Our first objective was to define the key structural ele-
ments required for activity (SAR). For synthetic reasons
the benzyloxy moiety was kept constant initially and the
2-amino substituent was varied (Table 1) in order to
identify the optimal aliphatic side-chain length. Since N-
aryl substitution is known to potentiate a-adrenergic
activity27 these were not explored further. Of the ali-
phatic derivatives the N-pentyl derivative 14 was rapidly
found to be optimal with a Ki of 26 nM. Longer deri-
vatives 15, shorter alkyl chains 4, 9, 12, 13 as well as
bulky derivatives 10 and those containing a polar group
11, 16 all displayed a sharp loss in affinity toward the
NMDA receptor. In particular, branched derivatives 17,
18 show a steep drop-off in affinity with increasing steric
bulk, this is also evident by comparing entries 10 and
17. In contrast, aryl-alkyl derivatives with varying
spacers displayed a relatively flat SAR. Polar atoms in
the linker 25 were found to be detrimental, whilst a
terminal phenolic moiety 21 slightly enhances affinity.
The most active analogue in this series is the benzyl
derivative 19 (Ki 130 nM), in contrast, the polar 3-pyri-
2.25
1.1
19
20
21
22
23
24
25
0.13
0.38
0.38
1.1
0.75
0.06
0.026
0.075
14
15
16
0.75
0.38
1.5
12.8
aBinding affinities are quoted as Ki values and are the geometric mean
of at least two experiments with <20% variance.
bDisplacement of [3H]-25-6981.22

A. Alanine et al. / Bioorg. Med. Chem. Lett. 13 (2003) 3155–3159
Table 2. Affinities of compounds 14, 26–40, variation of Aryl R1
3157
34 which stands in contrast to the ortho and meta posi-
tions. Accordingly the 4-nitro 38 and 4-phenyl 39 ana-
logues were as expected only found to be weakly active,
confirming that bulky groups are not accommodated in
this position. Finally, the electron-rich thiophene ana-
logue 40 is well tolerated, but slightly less active than
the phenyl counterpart 14.
Ki [mM]a
Ki [mM]a
Compd
R1
NMDAb Compd
R1
NMDAb
14
0.026
0.015
33
34
0.53
Surprisingly, in contrast to the results obtained from
exploration of space around the 4-position of the benzyl
group, extending the benzylether linker in 14 to the
26
27
0.10
0.17
phenyl-ethyl derivative 46 provides
a substantial
improvement in affinity, affording in the most potent
analogue identified in this study with Ki 7.5 nM. How-
ever, the chain extended phenyl-propyl ether analogue
47 suffers a substantially loss in activity (Ki 980 nM), in
accord with SAR expectations, indicating that a three-
carbon tether is clearly too long to be accommodated by
the NMDA receptor in this ligand binding site. Struc-
tural modifications were directed next toward the core
imidazoline ring system in order to determine the
importance of basicity, the hetero-atoms as well as to
investigate the steric and orientational requirements
(Table 3). Modest enlargement of the imidazoline ring
in 4 to the tetrahydropyrimidine homologue 41 causes
more than ten-fold loss in activity. This is likely due to
the larger steric demand of a 6-membered ring than the
small changes in substituent exit vector or enhanced
basicity. Similarly, the apparently conservative replace-
ment of the N1-oxygen in 14 by a methylene group in 45
significantly reduces the affinity by almost a log unit,
suggesting either conformation and/or basicity play an
important role. Since 42, 44, 45 are all isosteric to 14
and protonated a physiological pH, the reason for the
drop in activity of 45 compared to 14 is likely due to a
conformational change. Since by removal of the O-atom
acceptor an intramolecular hydrogen-bond from the
C2-NH to O which could fix the orientation of both the
N- and O-substituents is probably lost. In the case of
0.038
0.034
0.280
35
36
37
28
29
0.23
0.11
30
0.023
0.038
0.560
38
39
40
1.1
31
1.1
32
0.056
a,bSee Table 1.
small in size, also only possesses weak affinity suggest-
ing the benzyl moiety occupies a strict size limited
pocket. The para position is the most sensitive to sub-
stitution effects, even fluorine 34 has a detrimental
influence compared to hydrogen. Interestingly the 4-
methoxy group in 37 has identical activity to fluorine in
Table 3. Activities of imidazoline and tether variations 14, 41–47
Ki [mM]a
Ki [mM]a
c
c
Compd
Structure
NMDAb
pKa
Compd
Structure
NMDAb
pKa
>12
41
0.75
>12
45
0.23
42
43
44
6.0
8.5
14
46
47
0.026
0.0075
0.98
10.1
33
7.4
9.9
6.4
>11
10.0*
a,bSee Table 1; * calculated pKa value.
cpKa values determined using the potentiometric method.28

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A. Alanine et al. / Bioorg. Med. Chem. Lett. 13 (2003) 3155–3159
Table 4. selectivity profile of compounds 14 and 46
Compd
Ki [mM]a NMDAb
Ki [mM] a1c
Ki [mM] a2d
IC50 [mM] NR2Be
IC50 [mM] NR2Ae
K+ channel% effectf at 10mM
14
46
0.026
0.0075
5.5
4.9
30
10
0.21
<0.1
10
4.7
48
63
a,bSee Table 1.
c[3H]-prazosin displacement.30
d[3H]-RX821002 displacement.31
eElectro-physiology on Xenopus oocytes expressing recombinant rat NMDA receptor subunits, either NR2B or NR2A together with NR1C.32
Geometric mean values of 3–6 experiments.
fK+ current was evoked by depolarisation to 40 mV in whole-cell patch-clamp experiments on cultured rat cortical neurones. Arithmetic mean
values of 3–5 experiments.
entry 42 where the C2 NH group has been replaced by a
sulfur atom the loss in affinity is dramatic, less so the
change in basicity of 14 (pKa 10.1) to 42 (pKa 8.5) indi-
cating the requirement for an H-bond donor at C2.
Entry 44 also lends supports to this conclusion, as
replacement of the 3-nitrogen atom by a methylene
group removes the NH, resulting in loss of activity.
Here the pKa is very similar to that of 45 and 14 but
there is a 20-fold loss in affinity for the NMDA receptor
strongly suggesting a mandatory requirement for a
hydrogen bond donor at the C2-position. Attempts to
prepare the imidazole counterpart of the imidazoline
were unfortunately all unsuccessful. However, the ben-
zimidazole analogue 43 could be prepared and despite
adequate basicity for protonation at physiological pH
(pKa 7.4) NMDA receptor activity was lost completely,
probably as a result of the unfavorable additional steric
bulk of the annulated benzimidazole ring system.
NMDA receptor antagonists unrelated to the classical
ifenprodil chemotypes 1 and 2, justifying further pharm-
acological characterization, and with potential utility as
neuroprotective agents.
Acknowledgements
The skillful technical assistance of W. Vifian, P. Jurt,
and P. Zbinden is gratefully acknowledged. The authors
wish to thank W. Arnold, W. Meister for spectroscopic
characterization and B. Wagner for pKa determinations.
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