Discovery of (R)-1-[2-hydroxy-3-(4-hydroxy-phenyl)-propyl]-4-(4-methyl-benzyl)-piperidin- 4-ol: A novel NR1/2B subtype selective NMDA receptor antagonist

Article abstract of DOI:10.1016/S0960-894X(01)00392-4

Starting from Ro-25-6981 as a lead compound, highly potent and selective NR1/2B subtype selective NMDA receptor antagonists, with low activity at α₁ adrenergic receptors were developed.

Full text of DOI:10.1016/S0960-894X(01)00392-4

Bioorganic & Medicinal Chemistry Letters 11 (2001) 2173–2176
Discovery of (R)-1-[2-Hydroxy-3-(4-hydroxy-phenyl)-propyl]-
4-(4-methyl-benzyl)-piperidin-4-ol: ANovel NR1/2B Subtype
Selective NMDAReceptor Antagonist
Emmanuel Pinard,a,* Alexander Alanine,a Anne Bourson,b
Bernd Buttelmann,a Ramanjit Gill,b Marie-Paule Heitz,a Georg Jaeschke,a
Vincent Mutel,b 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 30 March2001; revised 17 May 2001; accepted 29 May 2001
Abstract—Starting from Ro-25-6981 as a lead compound, highly potent and selective NR1/2B subtype selective NMDA receptor
antagonists, withlow activity at a1 adrenergic receptors were developed. # 2001 Elsevier Science Ltd. All rights reserved.
Overactivation of NMDA receptors and the resulting
calcium overload of neurones is considered to be the
main contributor to neuronal cell deathfollowing acute
cerebral ischemia. NMDA receptor antagonists have,
thus, been shown to be potent neuroprotective agents in
animal models of focal cerebral ischemia.1,2
Native NMDA receptors exist as heteromeric assem-
blies containing NR1 subunits 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 the
structurally related CP-101,606 2,7 and Ro-25-6981 3.8
These compounds showed neuroprotective effect in
vivo, without inducing the side effects associated with
many non-selective NMDA receptor antagonists.9ꢀ12 In
addition, recent studies demonstrated that ifenprodil 1
produces a state-dependent block of NMDA recep-
tors.13 This makes NR1/2B subtype selective blockers
potentially attractive drugs for the treatment of neuro-
Unfortunately, Ro-25-6981, a compound structurally
related to ifenprodil elicits cardiovascular side-effects in
vivo. This liability was attributed to an antagonistic action
at a1 adrenergic receptors (Ki 0.18 mM; see Table 1).
Using Ro-25-6981 as a lead compound, potent NR1/2B
subtype selective antagonists withsignificantly reduced
activity at a1 adrenergic receptors were developed. This
work resulted in the discovery of (R)-1-[2-hydroxy-3-(4-
hydroxy-phenyl)-propyl]-4-(4-methyl-benzyl)-piperidine-
4-ol 26 as a novel, potent and highly selective NMDA
receptor antagonist.
2
degenerative diseases suchas stroke, brain trauma,2
pain12,14 and also Parkinson’s disease.15
Chemistry
Mannichcondensation of the commercially available 4-
hydroxyphenylketones with 4-benzylpiperidine in the
*Corresponding author. Tel.:+41-61-688-4388; fax:+41-61-688-8714;
e-mail: emmanuel.pinard@roche.com
0960-894X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(01)00392-4

2174
E. Pinard et al. / Bioorg. Med. Chem. Lett. 11 (2001) 2173–2176
presence of paraformaldehyde led to the b-aminoke-
tones 4,5 (Scheme 1). Reduction of 4,5 withsodium
borohydride gave rise to the benzylic alcohols 6,7 which
were transformed into the fully deoxygenated piper-
idines 8,9 using borane as a reducing agent. Racemic
epoxide 10, prepared in three standard steps from 4-
allylanisol, reacted smoothly with substituted-4-benzyl-
piperidines to provide the 2-propanol derivatives 11–15
(Scheme 2). O-Methylation or oxidation under Swern
conditions of the central hydroxyl group provided the
corresponding methylether or ketone derivatives,
respectively.
ꢁ
.
Scheme 1. (a) 4-Benzylpiperidine HCl salt, paraformaldehyde, DMF, 80 C, 60–70%; (b) NaBH4, EtOH, rt, 80–85%; (c) BH3 Me2S, THF, reflux,
60%.
Scheme 2. (a) (i) BBr3, CH2Cl2, ꢀ78 ꢁC to rt, 89%; (ii) MCPBA, NaHCO3, CH2Cl2, rt, 38%; (iii) TBDMSCl, Et3N, CH2Cl2, rt, 82%; (b) sub-
stituted-4-benzylpiperidine, MeOH, rt, 80–95%; (c) TBAF, THF, rt, 70–90%; (d) (i) NaH, MeI, THF, rt, 70%; (ii) TBAF, THF, rt, 70%; (e) (i)
(COCl)2, DMSO, Et3N, CH2Cl2, ꢀ40 to 0 ꢁC, 53%; (ii) TBAF, THF, rt, 65%.
Scheme 3. (a) (i) (DHQ)2AQN (1%), K3FeCN6, K2CO3, K2OsO2(OH)4, tBuOH, H2O, 0 ꢁC, 75%; (ii) recrystallisation from ether, 75%; (b) (i)
MeC(OMe)3, PPTS, CH2Cl2, rt; (ii) AcBr, CH2Cl2, rt; (iii) K2CO3, MeOH, rt, 67%; (c) (i) 4-(4-methyl-benzyl)-piperidin-4-ol, MeOH, rt, 94%; (ii)
BBr3, CH2Cl2, ꢀ78 ꢁC to rt, 74%; (d) (i) (DHQD)2AQN (1%), K3FeCN6, K2CO3, K2OsO2(OH)4, tBuOH, H2O, 0 ꢁC, 80%; (ii) recrystallisation
from ether 75%.

E. Pinard et al. / Bioorg. Med. Chem. Lett. 11 (2001) 2173–2176
2175
Cleavage of the silyl ethers led to the phenols 16–22.
Sharpless asymmetric catalytic dihydroxylation of 4-
allylanisol using (DHQ)2AQN16 as chiral ligand pro-
vided the crystalline diol 23 (ee 80%)17 (Scheme 3).
Enantiomerically pure material could be obtained after
a single recrystallisation from ether. Transformation of
23 into the S-configured terminal epoxide 24, [a]D20
+0.9 ꢁ (c 1, CHCl3),18 was achieved using a three-step
one pot procedure described by Sharpless.19 Epoxide
opening with4-(4-meythl-benzyl)-piperidin-4-ol fol-
lowed by cleavage of the methylether provided S-(+)-
25.20 The enantiomeric alcohol R-(ꢀ)-2620 was pro-
duced in the same way except that the pseudo-enantio-
meric ligand, (DHQD)2AQN,16 was used at the
dihydroxylation step (Scheme 3).
activity and/or to the undesired activity at a1 adrenergic
receptors.
As shown in Table 1, the benzylic hydroxyl group of 3
does not play a significant role withregard to NMDA
and a1 adrenergic activity since its removal led to com-
pound 9 witha similar in vitro profile to 3. Deletion of
the central methyl group of 3 yielded 6, a nearly equi-
potent compound which is, however, 5-fold less selective
versus a1 adrenergic receptors. Therefore, the linker
methyl group appears to play an important role in the
control of the selectivity profile of 3.
This observation led us to explore modifications at the
2-position of the central linker.
An increase of selectivity versus a1 adrenergic receptors
combined withretention of NMDA activity was
observed by replacing the methyl group with a hydroxyl
group (16). Furthermore, as shown with compounds 17
and 18, the potency was significantly reduced after oxi-
dation or methylation of 16. Therefore, a hydrogen
bond donating group suchas -OH, introduced at the 2-
position on the central linker, serves as a highly effective
receptor selectivity control element.
Results and Discussion
As withmost of the NR1/2B subtype selective NMDA
receptor antagonists reported in the literature, 3 incor-
porates a phenol and a 4-benzylpiperidine which are
connected via a central linker. Chemistry was initially
directed to identify the structural elements present in
this linker which contribute to the NMDA receptor
Interestingly, a further 15-fold drop of activity at a1
adrenergic receptors was observed by introducing an
additional hydroxyl group at the C-4 position of the
piperidine ring (19). Activity at the NMDA receptor
was also impaired but to a lower extent (6-fold). A
similar reduction of the a1 adrenergic activity has been
previously reported with 4-hydroxy substitution of the
piperidine ring in CP-101,606 2.7
Table 1. NMDA versus a1 adrenergic affinities of compounds 3, 6, 9,
16–22a
Ki (nM)
c
No.
A
X
Y
R
NMDAb
a1
Selectivityd
Chemical efforts were then directed towards modifica-
tion of the aromatic ring linked to the piperidine moi-
ety. As can be seen withcompounds 20, 21 and 22, a 2-
to 3-fold improvement of the NMDA activity was
observed by incorporating small lipophilic groups (Me,
Cl, Br) at the 4-position. In particular, the methyl sub-
stituted derivative 20 reached a very high level of activ-
ity (Ki 7.5 nM) and selectivity (ꢂ800-fold). Binding of
the non-phenolic aromatic group of structurally related
NMDA antagonists into a deep hydrophobic pocket
has been previously proposed.7,21 Therefore, the bene-
ficial effect observed after introduction of lipophilic
substituents, is most probably the result of improved
hydrophobic contacts within this pocket.
3
6
9
16
17
18
19
20
21
22
(R)-OH (S)-Me
H
H
H
H
H
H
H
H
H
H
H
H
5.6ꢃ0.6 180ꢃ8
3.8ꢃ0.9 24.4ꢃ4
32
6
27
80
27
15
200
789
374
216
OH
H
H
Me
3.4ꢃ1
91.5ꢃ11
H
H
H
H
H
H
H
OH
¼O
OMe
OH
OH
OH
OH
3.8ꢃ0.4 305ꢃ17
22.6ꢃ0.7 610ꢃ73
19.9ꢃ1.4 305ꢃ23
22.6ꢃ3.7 4512ꢃ563
H
OH
OH Me 7.5ꢃ1
5915ꢃ426
5610ꢃ313
OH Cl 15.0ꢃ1
OH Br 11.3ꢃ1.5 2439ꢃ456
aBinding affinities are quoted as Ki values and are the geometric mean
of at least two experiments.
bDisplacement of [3H]-25-6981.23
cDisplacement of [3H]-prazosin.
dRatio of the Ki (nM) values a1/NMDA.
Table 2. In vitro and in vivo profile of S-(+)-25 and R-(ꢀ)-26
In vitro
In vivo
Ki (nM)a
IC50 (nM)e
ED50 (mg/kg)f
NMDAb
a1
Selectivityd
1C/2A oocytes
1C/2B oocytes
Sound-induced seizures
c
No.
S-(+)-25
R-(ꢀ)-26
7.5ꢃ0.9
4.9ꢃ0.6
7300ꢃ1400
7300ꢃ1300
973
1490
n.d.
>10,000
n.d.
19
25
10
a,b,c,dSee Table 1.
eFrom voltage-clamp experiments on Xenopus oocytes expressing recombinant rat (NR1C/2A) and (NR1C/2B) NMDA receptor subtypes. Values
are geometric mean of three experiments covering 10 nM–10 mM range.
fCompounds were administered ip 30 min before testing in DBA/2 mice.

2176
E. Pinard et al. / Bioorg. Med. Chem. Lett. 11 (2001) 2173–2176
From the SAR so far developed, racemic compound 20
showed overall the best in vitro profile. Biological
results obtained for the corresponding enantiomers S-
(+)-25 and R-(ꢀ)-26 are presented in Table 2. R-(ꢀ)-26
showed a slightly higher activity at the NMDA receptor
than S-(+)-25. In addition, bothenantiomers were
found to be equipotent at a1 adrenergic receptors. A
very high selectivity ratio of ꢂ1500-fold was achieved
with R-(ꢀ)-26.
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In vivo activity was measured in mice after ip adminis-
tration using the standard sound-induced seizures
assay.24 As shown in Table 2, both compounds were
found to be in vivo active, indicating that they both
penetrate into the brain. However, R-(ꢀ)-26 (ED50
10 mg/kg) showed a better protective effect. Interest-
ingly, the difference of potency at the NMDA receptor
observed for the two enantiomers clearly translates into
a difference in in vivo activity.
In an in vitro functional assay performed in Xenopus
oocytes expressing recombinant rat (NR1C/2A) and
(NR1C/2B) subtypes,8 R-(ꢀ)-26 blocked the NR2B-
containing NMDA receptor withihghaffinity (IC
50
19 nM) (Table 2) and the selectivity for the NR2B versus
NR2A subunits was found to be greater than 500-fold.
13. Kew, J. N. C.; Trube, G.; Kemp, J. A. J. Phys. 1996, 497,
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Conclusion
Starting from Ro-25-6981 3 as a lead compound, highly
potent NR1/2B subtype selective NMDA antagonists
were prepared. The a1 adrenergic activity could be
separated from the NMDA receptor activity through
introduction of hydroxyl groups at the 2-position on the
central linker and at the 4-position on the piperidine
ring. From this series, R-(ꢀ)-26 was identified as the
most potent and selective derivative and was found to
be active in vivo.
16. Becker, H.; Sharpless, K. B. Angew. Chem., Int. Ed. Engl.
1996, 35, 448.
1
17. Enantiomeric excesses were determined by H NMR ana-
lysis of the corresponding bis-MPTA Mosher esters.
18. The preparation of the S-configured epoxide 24 has
already been described using (S)-epichlorohydrin as starting
material.22 The described optical rotation for 24 was [a]2D0
+0.8ꢁ (c 1, CHCl3).
19. Kolb, H. C.; Sharpless, K. B. Tetrahedron 1992, 48, 10515.
20. 25: [a]2D0 +11ꢁ (c 0.5, MeOH) and 26: [a]2D0 ꢀ10 ꢁ (c 0.5,
MeOH) were found to be enantiomerically pure using a chiral
phase HPLC (Chiracel OD-H, 250ꢄ4.6 mm, detection at
285 nm, flow: 1.5 mL/min, mobile phase: n-hexane/ethanol/
triethylamine, 99.40/0.56/0.04; 25: Rt=15.7 min and 26:
Rt=11.4 min).
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Acknowledgements
The skilful technical assistance of S. Burner, T. Muser,
and J. Padilla is gratefully acknowledged. The authors
wishto thank W. Arnold, V. Meduna and W. Meister
for spectroscopic characterisation.
References and Notes
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