4-hydroxy-1-[2-(4-hydroxyphenoxy)ethyl]-4-(4-methylbenzyl)piperidine: A novel, potent, and selective NR1/2B NMDA receptor antagonist

Article abstract of DOI:10.1021/jm990246i

A structure-based search and screen of our compound library identified N-(2-phenoxyethyl)4-benzylpiperidine (8) as a novel N-methyl-D-aspartate (NMDA) receptor antagonist that has high selectivity for the NR1/2B subunit combination (IC₅₀ = 0.63 μM). We report on the optimization of this lead compound in terms of potency, side effect liability, and in vivo activity. Potency was assayed by electrical recordings in Xenopus oocytes expressing cloned rat NMDA receptors. Side effect liability was assessed by measuring affinity for α₁-adrenergic receptors and inhibition of neuronal K⁺ channels. Central bioavailability was gauged indirectly by determining anticonvulsant activity in a mouse maximal electroshock (MES) assay. Making progressive modifications to 8, a hydroxyl substituent on the phenyl ring para to the oxyethyl tether (10a) resulted in a ~25-fold increase in NR1A/2B potency (IC₅₀ = 0.025 μM). p-Methyl substitution on the benzyl ring (10b) produced a ~3-fold increase in MES activity (ED₅₀ = 0.7 mg/kg iv). Introduction of a second hydroxyl group into the C-4 position on the piperidine ring (10e) resulted in a substantial decrease in affinity for α₁ receptors and reduction in inhibition of K⁺ channels with only a modest decrease in NR1A/2B and MES potencies. Among the compounds described, 10e (4- hydroxy-N-[2-(4-hydroxyphenoxy)ethyl]-4-(4-methylbenzyl)piperidine, Co 101244/PD 174494) had the optimum pharmacological profile and was selected for further biological evaluation.

Full text of DOI:10.1021/jm990246i

J . Med. Chem. 1999, 42, 2993-3000
2993
4-Hyd r oxy-1-[2-(4-h yd r oxyp h en oxy)eth yl]-4-(4-m eth ylben zyl)p ip er id in e: A
Novel, P oten t, a n d Selective NR1/2B NMDA Recep tor An ta gon ist
Zhang-Lin Zhou,*,† Sui Xiong Cai,† Edward R. Whittemore,† Christopher S. Konkoy,† Stephen A. Espitia,†
Minhtam Tran,† David M. Rock,§ Linda L. Coughenour,§ J on E. Hawkinson,† Peter A. Boxer,§
Christopher F. Bigge,§ Lawrence D. Wise,§ Eckard Weber,† Richard M. Woodward,† and J ohn F. W. Keana‡
CoCensys, Inc., 213 Technology Drive, Irvine, California 92618, Department of Chemistry, Parke-Davis Pharmaceutical
Research, Division of Warner-Lambert Company, Ann Arbor, Michigan 48105, and Department of Chemistry, University of
Oregon, Eugene, Oregon 97403
Received May 18, 1999
A structure-based search and screen of our compound library identified N-(2-phenoxyethyl)-
4-benzylpiperidine (8) as a novel N-methyl-D-aspartate (NMDA) receptor antagonist that has
high selectivity for the NR1/2B subunit combination (IC50 ) 0.63 µM). We report on the
optimization of this lead compound in terms of potency, side effect liability, and in vivo activity.
Potency was assayed by electrical recordings in Xenopus oocytes expressing cloned rat NMDA
receptors. Side effect liability was assessed by measuring affinity for R1-adrenergic receptors
and inhibition of neuronal K+ channels. Central bioavailability was gauged indirectly by
determining anticonvulsant activity in a mouse maximal electroshock (MES) assay. Making
progressive modifications to 8, a hydroxyl substituent on the phenyl ring para to the oxyethyl
tether (10a ) resulted in a ∼25-fold increase in NR1A/2B potency (IC50 ) 0.025 µM). p-Methyl
substitution on the benzyl ring (10b) produced a ∼3-fold increase in MES activity (ED50 ) 0.7
mg/kg iv). Introduction of a second hydroxyl group into the C-4 position on the piperidine ring
(10e) resulted in a substantial decrease in affinity for R1 receptors and reduction in inhibition
of K+ channels with only a modest decrease in NR1A/2B and MES potencies. Among the
compounds described, 10e (4-hydroxy-N-[2-(4-hydroxyphenoxy)ethyl]-4-(4-methylbenzyl)pip-
eridine, Co 101244/PD 174494) had the optimum pharmacological profile and was selected for
further biological evaluation.
Ch a r t 1
In tr od u ction
Overactivation of N-methyl-D-aspartate (NMDA)
receptors by glutamate is believed to play a role in
numerous acute and chronic neurodegenerative dis-
orders.1,2 In the search for clinically effective neuro-
protectants,
a
large variety of NMDA receptor
antagonists have been identified and characterized.3
Thus far, however, many NMDA antagonists tested
clinically have been compromised by dose-limiting car-
diovascular, behavioral, or neurotoxic side effects.4
Native NMDA receptors are heterooligomeric as-
semblies composed of two different types of subunits
termed NMDA receptor 1 (NR1) and NR2.5 NR1 sub-
units have eight different isoforms generated by alter-
native RNA splicing, and NR2 subunits have four
distinct subtypes.5 NMDA receptor subunits are differ-
entially distributed across brain regions.6 This raises
the possibility that subtype-selective NMDA receptor
antagonists could have therapeutic potential as neuro-
protectants without the cardiovascular, behavioral, and
neurotoxic side effects often associated with broad-
spectrum antagonists.
pounds have subsequently been identified as NR1/2B-
selective NMDA receptor antagonists. Exemplary com-
pounds (Chart 1) include eliprodil (2),7b,8 haloperidol
(3),7b,9 CP-101,606 (4),10 and Ro 25-6981 (5).11 Parallel
experiments have investigated the complex mechanism
of allosteric inhibition and the actions of this class of
antagonists on neuronal NMDA currents in situ.11a,12,13
Herein we report on a novel series of NR2B-selective
antagonists. Structure-activity relationship (SAR) stud-
ies reveal how three simple structural modifications can
Ifenprodil (1) was the first NMDA receptor antagonist
shown to have pronounced subtype selectivity. Selectiv-
ity for this class of antagonist is directed toward the
NR2B subunit.7 A number of structurally related com-
† CoCensys, Inc.
§ Parke-Davis Pharmaceutical Research.
‡ University of Oregon.
10.1021/jm990246i CCC: $18.00 © 1999 American Chemical Society
Published on Web 07/02/1999

2994 J ournal of Medicinal Chemistry, 1999, Vol. 42, No. 15
Zhou et al.
Sch em e 1a
a
(i) K2CO3/CH3CN/reflux; (ii) 1 M HCl/MeOH; (iii) 10% Pd/C or 20% Pd(OH)2/MeOH/H2.
Sch em e 2a
Sch em e 3a
a
(i) Mg/THF, then 1-benzyl-4-piperidone/-78 °C to rt; (ii) 1 M
HCl/MeOH; (iii) 10% Pd/C, 95% EtOH/H2.
4-methylbenzyl chloride (15) and magnesium, with
1-benzyl-4-piperidone in THF afforded the correspond-
ing alcohol 16 in 93% yield. Debenzylation of compound
16 by hydrogenation in the presence of 10% Pd/C gave
the desired piperidine 6e in excellent yield.
a
(i) Ph3P/ether/rt; (ii) NaH/DMSO/80 °C, then 1-benzyl-4-
piperidone; (iii) PtO2/MeOH/H2, then 1 M HCl/MeOH; (iv) 10% Pd/
C, 95% EtOH/H2.
Unsubstituted 2-phenoxyethyl bromide (7a ) and 2-(4-
benzyloxyphenoxy)ethyl bromide (7b) were synthesized
from the reaction of the corresponding phenol with 1,2-
dibromoethane.
transform the screening lead into a potent and specific
inhibitor.14
Ch em istr y
P h a r m a cology
Compound 8 was prepared by N-alkylation of 4-ben-
zylpiperidine (6b ) with 2-phenoxyethyl bromide (7a ).
Compounds 10a -10e were prepared by N-alkylation of
the requisite piperidines 6a -6e with bromide 7b fol-
lowed by debenzylation of the benzyloxy analogues 9a -
9e as shown in Scheme 1.
Those substituted 4-benzylpiperidines that were not
commercially available were prepared from the ap-
propriately substituted benzyl bromides through a
Wittig reaction.15 Briefly, reaction of substituted benzyl
bromide 11 with triphenylphosphine afforded the cor-
responding phosphonium salt 12. Treatment of 12 with
sodium methylsulfinyl carbanion in dimethyl sulfoxide
at 80 °C gave the ylide which reacted with 1-benzyl-4-
piperidone to form the olefin 13. Reduction of 13 with
H2 in the presence of PtO2 gave compound 14. Further
debenzylation of 14 by hydrogenation in the presence
of 10% Pd/C gave the desired piperidines 6c and 6d in
quantitative yield (Scheme 2).
Compounds were tested for antagonism of cloned
NMDA receptors expressed in Xenopus oocytes using
standard two-electrode voltage clamp techniques.16,17
IC50 values for inhibition of NMDA responses were
determined by curve fitting.18 Sample data for com-
pound 10e illustrating selectivity for NR1A/2B are given
in Figure 1. Affinity for R1-adrengeric receptors was
deterimed by [3H]prazocin binding assays.19 Inhibition
of voltage-gated K+ currents was measured by whole-
cell voltage clamp recordings from dissociated rat
superior cervical ganglion neurons.20 Mouse MES assays
were done as reported previously.21,22
Resu lts a n d Discu ssion
SARs of N-(2-phenoxyethyl)-4-benzylpiperidine (8)
and related analogues are given in Figure 2 and Table
1. The screening lead 8 had an IC50 value of 0.63 µM at
NR1A/2B receptors: roughly comparable in potency to
the prototype compound eliprodil and 6-fold less active
than ifenprodil. Like eliprodil and ifenprodil, 8 was
>100-fold selective for NR1A/2B as compared to NR1A/
4-Hydroxy-4-(4-methylbenzyl)piperidine (6e) was pre-
pared according to Scheme 3. Reaction of 4-methylben-
zylmagnesium chloride, which was made in situ from

Novel, Potent, Selective NMDA Receptor Antagonist
J ournal of Medicinal Chemistry, 1999, Vol. 42, No. 15 2995
inhibited neuronal K+ channels by 67% at 10 µM). Like
eliprodil, 8 was a potent anticonvulsant when admin-
istered intravenously (iv) to mice (ED50 ) 2 mg/kg). Our
goal was to modify 8 to improve NR2B potency while
at the same time reducing R1 affinity and K+ channel
inhibition while retaining in vivo activity.
Introduction of a hydroxyl group on the 4-position of
the phenyl ring, which is attached to the tether con-
necting to the nitrogen atom of the piperidine ring, led
to compound 10a and increased NR2B potency by 25
times. As described in other series of compounds,10,18,23
this emphasizes the importance of having a hydrogen-
bond-donating group at this position for interaction with
the NMDA receptor. Though NR2B potency was in-
creased in 10a , the R1 activity remained less than 1 µM.
Removing the methylene group from the spacer
connecting the C-4 phenyl group to the piperidine ring
gave compound 10d and unexpectedly resulted in a 200-
fold drop in potency. In this instance, the SAR did not
follow that described previously for compounds such as
CP-101,606 where phenylpiperidines are optimal.10
F igu r e 1. Inhibition of inward currents mediated by cloned
NMDA receptors expressed in Xenopus oocytes by 10e il-
lustrating the effect of varying the NR2 subunit. Inhibition is
plotted as a fraction (FR) of control responses elicited by
saturating, or near-saturating, concentrations of agonists:
NR1A/2B and NR1A/2C, glutamate (100 µM) and glycine (1
µM); NR1A/2A, glutamate (100 µM) and glycine (10 µM). Curve
fitting is as described in the Experimental Section. Data are
taken from three individual oocytes for NR1A/2B and two
oocytes for NR1A/2A and NR1A/2C.
2A or NR1A/2C. In terms of potential side effects, 8 had
submicromolar affinity for R1-adrenergic receptors (IC50
) 0.82 µM), presenting a liability to produce hypoten-
sion. In addition, 8 inhibited neuronal voltage-gated K+
channels by 65% at 10 µM, suggesting a potential for
prolongation of cardiac QT interval, a dose-limiting
clinical side effect for eliprodil (in our assay eliprodil
Simple substitution on the benzyl ring of 10a , such
as 4-methyl (10b) and 4-fluoro (10c), had little effect
on NR1A/2B potency or on the potential side effects
profile. However, the in vivo potency of 10b was
increased roughly 3 times (MES ED50 ) 0.7 mg/kg)
compared to that of 10a , while the in vivo potency of
10c was slightly reduced (MES ED50 ) 3 mg/kg). This
F igu r e 2. SAR of the substituted benzylpiperidines at the NR1A/2B subtype.

2996 J ournal of Medicinal Chemistry, 1999, Vol. 42, No. 15
Zhou et al.
Ta ble 1. In Vitro and in Vivo Profiles of Substituted Benzylpiperidinesa
cloned NMDA receptors expressed in oocytes: IC50 (µM)
K
+ channels:
MES:
no.
NR1A/2A
NR1A/2B
NR1A/2C
R1: IC50 (µM)
% inhib at 10 µM
ED50 (mg/kg)
1
2
8
10a
10b
10c
10d
10e
20 ( 6
100.0
0.11 ( 0.01
1.40
>100
100.0
>100
>100
>100
>100
>100
>100
0.22
3.30
0.82
0.47
1.80
1.60
nd
54 ( 7.7
67 ( 6.2
65 ( 4.3
46 ( 2.9
67 ( 5.2
53 ( 8.9
nd
7.0
1.0
2.0
2.0
0.7
3.0
nd
>100
0.63 ( 0.10
0.025 ( 0.009
0.028 ( 0.004
0.05 ( 0.017
7.7 ( 1.5
>100
>100
21 ( 4
110 ( 19
>100
0.043 ( 0.004
27
23 ( 5.6
1.5
a
IC50 values for inhibition of NMDA responses at cloned NMDA receptors expressed in Xenopus oocytes, as illustrated in Figure 1.
Data are presented as mean ( SEM. Values were obtained from at least three oocytes for NR1A/2B and at least two oocytes for the other
subunits. nd, not determined.
mL) and extracted with ether. The extract was dried over
MgSO4 and treated with Norite. After filtration, the ether was
removed, and the product was vacuum-distilled (45.6 g, bp )
159-162 °C at 0.1 mmHg, 75% yield). The hydrochloride salt
was formed in 2-propanol/HCl and recrystallized from ethyl
acetate to give 8 as colorless crystals: mp 170-172 °C; 1H
NMR (300 MHz, CDCl3) δ 1.62 (m, 3 H), 1.82 (m, 2 H), 2.10
(m, 2 H), 2.80 (m, 2 H), 3.39 (m, 2 H), 3.65 (m, 2 H), 4.54 (m,
2 H), 6.85-7.31 (m, 10 H), 12.50 (brs, 1 H). Anal. (C20H26ClNO)
C, H, N.
2-(4-Ben zyloxyp h en oxy)eth yl Br om id e (7b). A mixture
of 4-benzyloxyphenol (10 g, 0.05 mol) and potassium carbonate
(17.3 g, 0.125 mol) in 50 mL of acetonitrile and 21.6 mL of
1,2-dibromoethane was refluxed with stirring for 24 h. The
inorganic salt was removed by filtration through a short
column of silica gel which was washed with ethyl acetate (3 ×
25 mL). The combined filtrate was evaporated in vacuo, and
the residue was further purified by flash chromatography (5%
EtOAc in hexane) to give 12 g (79%) of the title product as a
white solid: mp 75-77 °C; 1H NMR (300 MHz, CDCl3) δ 3.61
(t, J ) 6.2 Hz, 2 H), 4.24 (t, J ) 6.2 Hz, 2 H), 5.02 (s, 2 H),
6.87 (m, 4 H), 7.38 (m, 5 H).
4-Ben zyl-1-[2-(4-b en zyloxyp h en yl)et h oxy]p ip er id in e
Hyd r och lor id e (9a ). A mixture of 2-(4-benzyloxyphenoxy)-
ethyl bromide (7b) (1.44 g, 4.7 mmol), 4-benzylpiperidine (0.876
g, 5.0 mmol), potassium carbonate (1.725 g, 12.5 mmol), and
50 mL of acetonitrile was refluxed for 24 h. The inorganic salt
was removed by filtration through a short column of silica gel
which was washed with ethyl acetate (3 × 25 mL). The
combined filtrate was evaporated in vacuo, and the residue
was further purified by flash chromatography (50% EtOAc in
hexane), giving 1.62 g (86%) of the free amine. The amine was
dissolved in 50 mL of methanol and treated with 1 N HCl in
methanol (6 mL). The solution was evaporated in vacuo and
titratuted with ether (100 mL). The white solid was collected
by filtration and dried in vacuo, giving the title HCl salt in
100% yield: mp 164-166 °C; 1H NMR (CDCl3, 300 MHz) δ
1.51 (m, 1 H), 1.68 (d, J ) 12.3 Hz, 2 H), 2.46 (m, 4 H), 2.94
(m, 2 H), 3.35 (m, 2 H), 3.45 (d, J ) 11.7 Hz, 2 H), 4.26 (s, 2
H), 5.01 (s, 2 H), 6.89 (m, 4 H), 7.18-7.40 (m, 10 H), 10.2 (brs,
2 H); EIMS m/e 311 (M+, 5), 202 (10), 188 (100), 91 (30).
suggests that substitutions on the benzyl ring can have
significant effects on in vivo potency.
Introduction of a hydroxyl group into the C-4 position
of the piperidine ring of 10b led to 10e and resulted in
a modest reduction in NR2B potency (IC50 ) 0.043 µM).
However, this substitution greatly improved the side
effects profile: R1 activity was reduced by 15-fold (IC50
) 27 µM), and K+ channel inhibition was reduced to
only 23% at 10 µM. A similar reduction in R1 activity
has been reported with 4-hydroxy substitution into
phenylpiperidines such as CP-101,606.10 The in vivo
activity of 10e was not greatly compromised (MES ED50
) 1.5 mg/kg) giving a molecule with the desired phar-
macological properties.
In conclusion, 10e (4-hydroxy-N-[2-(4-hydroxyphe-
noxy)ethyl]-4-(4-methylbenzyl)piperidine, Co 101244/PD
174494) has the best overall profile of the compounds
described, one that rivals or exceeds that of previously
reported NR2B antagonists. In addition, 10e has the
advantage of achieving the desired profile without
introduction of stereocenters. The SAR leading to 10e
illustrates some of the key features necessary for
designing potent, highly specific NR2B antagonists.
Compound 10e is currently undergoing pharmacological
evaluation for a variety of therapeutic applications
including stroke and Parkinson’s disease.24
Exp er im en ta l Section
Melting points were determined in open capillary tubes on
a Mel-Temp apparatus and are uncorrected. The 1H NMR
spectra were recorded at 300 MHz. Chemical shifts are
reported in ppm (δ), and J coupling constants are reported in
Hz. Elemental analyses were performed by Desert Analytics,
Tucson, AZ. Mass spectra (MS) were obtained with a VG 12-
250 or VG ZAB-2FHF mass spectrometer. Reagent grade
solvents were used without further purification unless other-
wise specified. TLC was performed using TLC plates GF254
.
4-Ben zyl-1-[2-(4-h yd r oxyph en yl)eth oxy]p ip er id in e Hy-
d r och lor id e (10a ). To a solution of 4-benzyl-1-[2-(4-benzyl-
oxyphenyl)ethoxy]piperidine (9a ) (401 mg, 1.0 mmol) in 25 mL
of ethanol were added 1.0 mL of 1 M HCl in methanol and
100 mg of 10% Pd/C. The resulting mixture was hydrogenated
at 30 psi of H2 for 2 h. The catalyst was removed by filtration
through a short column of Celite (5 g) and washed with
methanol (3 × 15 mL). The combined filtrate was evaporated
in vacuo to give an oil, and then ether (30 mL) was added to
the residue. The resulting mixture was stirred at room
temperature overnight. The white solid was collected by
filtration and dried in vacuo, giving 330 mg (100%) of the title
Column chromatography was performed on silica gel (230-
400 mesh). Reverse-phase HPLC analyses were obtained at
254 nm on a 4.6- × 250-mm microsorb-MV C18 column, using
0.1% trifluoroacetic acid in water (A) and 0.1% trifluoroacetic
acid in acetonitrile (B) as solvents. The linear gradient was
20% B in A to 95% B in A with a flow rate of 1 mL/min.
4-Ben zyl-1-(2-p h en oxyeth yl)p ip er id in e Hyd r och lor id e
(8). A mixture of 4-benzylpiperidine (35 g, 0.20 mol), 1-bromo-
2-phenoxyethane (42 g, 0.21 mol), and potassium carbonate
(111 g, 0.80 mol) in 400 mL of methyl ethyl ketone was heated
at reflux with vigorous stirring for 12 h. The reaction mixture
was filtered, and the filtrate was evaporated. The residue was
extracted with water/ether, and the ether layer was washed
with 2 N hydrochloric acid. The amine hydrochloride salt
separated from the aqueous layer was collected by filtration.
The salt was treated with 2 N aqueous NaOH solution (150
compound: mp 212-215 °C; 1H NMR (300 MHz, CDCl3
+
DMSO-d6) δ 1.66 (m, 3 H), 1.83 (m, 2 H), 2.43 (s, 2 H), 2.63
(m, 2 H), 3.19 (m, 2 H), 3.40 (brs, 1 H), 4.25 (s, 2 H), 6.55 (m,
4 H), 6.94-7.09 (m, 5 H), 12.0 (brs, 1 H); EIMS m/e 311 (M+,
5), 202 (10), 188 (100), 91 (30). Anal. (C20H26ClNO2) C, H, N.

Novel, Potent, Selective NMDA Receptor Antagonist
J ournal of Medicinal Chemistry, 1999, Vol. 42, No. 15 2997
4-Meth ylben zyltr ip h en ylp h osp h on iu m Br om id e (12a ).
To a solution of triphenylphosphine (35.45 g, 0.135 mol) in 100
mL of ether was added 4-methylbenzyl bromide (11a ) (25 g,
0.135 mol). The resulting solution was stirred at room tem-
perature overnight. The white solid was collected by filtration
and dried to give 60.0 g (98%) of the title product as a white
2 H), 6.99 (d, J ) 7.8 Hz, 2 H), 7.08 (d, J ) 7.8 Hz, 2 H), 7.32-
7.39 (m, 5 H), 12.51 (brs, 1 H).
1-[2-(4-H yd r oxyp h e n yl)e t h oxy]-4-(4-m e t h ylb e n zyl)-
p ip er id in e Hyd r och lor id e (10b). To a solution of 1-[2-(4-
benzyloxyphenyl)ethoxy]-4-(3-methylbenzyl)piperidine hydro-
chloride (9b) (250 mg, 0.55 mmol) in 25 mL of ethanol was
added 60 mg of 20% Pd(OH)2. The resulting mixture was
hydrogenated at 30 psi of hydrogen for 2 h. The catalyst was
removed through a short column of Celite (5 g) and washed
with methanol (3 × 15 mL). The combined filtrate was
evaporated in vacuo and triturated with 30 mL of ether. The
white solid was collected by filtration and dried in vacuo, giving
1
solid: mp 256-258 °C; H NMR (CDCl3, 300 MHz) δ 2.38 (s,
3 H), 5.27 (d, J ) 14.4 Hz, 2 H), 6.93 (m, 4 H), 7.59-7.76 (m,
15 H).
1-Ben zyl-4-(4-m eth ylben zylid en e)p ip er id in e (13a ). A
suspension of sodium hydride (1.20 g, 0.03 mol, 60% in mineral
oil) in 20 mL of dry DMSO was heated at 80 °C under N2 for
1 h. The resulting solution was cooled in an ice-water bath
and treated with a suspension of 4-methylbenzyltriphenylphos-
phonium bromide (12a ) (13.41 g, 0.03 mol) in 120 mL of warm
DMSO. The resulting solution was stirred at 0 °C for 10 min
and then at room temperature for 15 min, to which 1-benzyl-
4-piperidone (5.67 g, 0.03 mol) was then added dropwise under
N2. The resulting mixture was stirred at 80 °C overnight, then
poured over ice (400 g), and extracted with ether (3 × 200 mL).
The combined extracts were dried over sodium sulfate. The
solvent was evaporated in vacuo and further purified by flash
chromatography (eluent 5% EtOAc in hexanes), giving 7.0 g
(84%) of the title compound as a pale-yellow oil: 1H NMR
(CDCl3, 300 MHz) δ 2.26 (s, 3 H), 2.32 (m, 4 H), 2.43 (m, 4 H),
3.45 (s, 2 H), 6.16 (s, 1 H), 7.03 (m, 4 H), 7.18-7.26 (m, 5 H).
1-Ben zyl-4-(4-m eth ylben zyl)p ip er id in e Hyd r och lor id e
(14a ). To a solution of 1-benzyl-4-(4-methylbenzylidene)-
piperidine (13a ) (4.16 g, 15 mmol) in 100 mL of methanol was
added 200 mg of PtO2. The resulting mixture was hydroge-
nated at 40 psi for 8 h. The catalyst was removed by filtration
through a short column of Celite (10 g) and washed with
methanol (3 × 20 mL). The filtrate was evaporated in vacuo,
redissolved into 20 mL of methanol, and treated with 30 mL
of 1 M solution of HCl in methanol. The resulting solution was
evaporated in vacuo and triturated with 60 mL of ether. An
off-white solid was collected by filtration and dried to give 4.6
g (97%) of the title product: mp 190-192 °C; 1H NMR (CDCl3,
300 MHz) δ 1.60 (m, 1 H), 1.75 (m, 2 H), 2.07 (m, 2 H), 2.29 (s,
3 H), 2.54 (m, 4 H), 3.40 (d, J ) 10.5 Hz, 2 H), 4.09 (d, J ) 4.8
Hz, 2 H), 6.99 (d, J ) 7.8 Hz, 2 H), 7.05 (d, J ) 7.8 Hz, 2 H),
7.42 (m, 3 H), 7.60 (m, 2 H), 12.40 (brs, 1 H).
1
140 mg (88%) of the title product: mp 198-200 °C; H NMR
(300 MHz, CD3OD) δ 1.60 (m, 2 H), 1.88-1.92 (m, 3 H), 2.29
(s, 3 H), 2.57 (d, J ) 6.6 Hz, 2 H), 3.06 (m, 2 H), 3.47 (m, 2 H),
3.61 (m, 2 H), 4.24 (t, J ) 5.1 Hz, 2 H), 6.71 (dd, J 1 ) 2.4 Hz,
J 2 ) 6.6 Hz, 2 H), 6.83 (dd, J 1 ) 2.4 Hz, J 2 ) 6.6 Hz, 2 H),
7.70 (m, 4 H). Anal. (C21H28ClNO2) C, H, N.
4-F lu or oben zyltr ip h en ylp h osp h on iu m Br om id e (12b).
To a solution of triphenylphosphine (26.2 g, 0.1 mol) in 100
mL of ether was added 4-fluorobenzyl bromide (12b) (18.9 g,
0.1 mol). The resulting solution was stirred at room temper-
ature overnight. The white solid was collected by filtration and
dried to give 37.0 g (82%) of the product as a white solid: mp
1
280-282 °C; H NMR (CDCl3, 300 MHz) δ 5.49 (d, J ) 14.4
Hz, 2 H), 6.77 (d, J ) 8.7 Hz, 2 H), 7.12 (m, 2 H), 7.60 (m, 6
H), 7.75 (m, 9 H).
1-Ben zyl-4-(4-flu or oben zylid en e)p ip er id in e (13b). A
suspension of sodium hydride (1.44 g, 0.036 mol, 60% in
mineral oil) in 20 mL of dry DMSO was heated at 80 °C under
N2 for 1 h. The resulting solution was cooled in an ice-water
bath and treated with a suspension of 4-fluorobenzyltriph-
enylphosphonium bromide (12b) (16.2 g, 0.036 mol) in 120 mL
of warm DMSO. The resulting solution was stirred at 0 °C for
10 min and then at room temperature for 15 min, to which
1-benzyl-4-piperidone (5.67 g, 0.03 mol) was then added
dropwise under N2. The resulting mixture was stirred at 80
°C overnight and then poured over ice (400 g) and extracted
with ether (3 × 200 mL). The combined extracts were dried
over sodium sulfate. The solvent was evaporated in vacuo and
further purified by flash chromatography (eluent 5% EtOAc
in hexanes), giving 7.0 g (83%) of the title compound as a pale-
yellow oil: 1H NMR (CDCl3, 300 MHz) δ 2.36-2.54 (m, 8 H),
3.53 (s, 2 H), 6.22 (s, 1 H), 6.98 (m, 2 H), 7.14 (m, 2 H), 7.26-
7.34 (m, 5 H).
4-(4-Meth ylben zyl)p ip er id in e Hyd r och lor id e (6c). A
mixture of 1-benzyl-4-(4-methylbenzyl)piperidine hydrochlo-
ride (14a ) (4.1 g, 13 mmol) and 1.02 g of 10% Pd/C in 100 mL
of 95% ethanol was hydrogenated at 60 psi for 12 h. The
catalyst was removed by filtation through a short column of
Celite (10 g) and washed with methanol (3 × 20 mL). The
filtrate was evaporated in vacuo and triturated with ether (50
mL). A white solid was collected by filtration and dried in
vacuo, giving 2.8 g (98%) of the title product: mp 209-211
°C; 1H NMR (CDCl3, 300 MHz) δ 1.70 (m, 3 H), 1.82 (m, 2 H),
2.32 (s, 3 H), 2.55 (m, 2 H), 2.78 (m, 2 H), 3.44 (d, J ) 8.7 Hz,
2 H), 6.99 (d, J ) 7.8 Hz, 2 H), 7.05 (d, J ) 7.8 Hz, 2 H), 9.30
(brs, 1 H), 9.60 (brs, 1 H).
1-Ben zyl-4-(4-flu or oben zyl)p ip er id in e Hyd r och lor id e
(14b). To a solution of 1-benzyl-4-(4-fluorobenzylidene)piperi-
dine (13b) (4.22 g, 15 mmol) in 100 mL of methanol was added
200 mg of PtO2. The resulting mixture was hydrogenated at
40 psi for 8 h. The catalyst was removed through a short
column of Celite (10 g) and washed with methanol (3 × 20
mL). The filtrate was treated with 30 mL of 1 M HCl in
methanol. The resulting solution was evaporated in vacuo and
triturated with 100 mL of ether. An off-white solid was
collected by filtration and dried to give 4.6 g (96%) of the title
1
product: mp 168-170 °C; H NMR (CDCl3, 300 MHz) δ 1.63
1-[2-(4-Ben zyloxyp h en yl)et h oxy]-4-(4-m et h ylb en zyl)-
p ip er id in e Hyd r och lor id e (9b). A mixture of 2-(4-benzyl-
oxyphenoxy)ethyl bromide (7b) (0.61 g, 2.0 mmol), 4-(4-
methylbenzyl)piperidine hydrochloride (6c) (0.45 g, 2.0 mmol),
and potassium carbonate (0.69 g, 5.0 mmol) in 20 mL of
acetonitrile was refluxed for 24 h. The inorganic salt was
removed through a short column of silica gel and washed with
ethyl acetate (3 × 25 mL). The combined filtrate was evapo-
rated in vacuo to give a crude mixture, which was purified by
flash chromatography (20% methanol in ethyl acetate), giving
a pale-yellow oil. A solution of this oil in 10 mL of methanol
was treated with 3 mL of 1 M HCl in methanol. The resulting
solution was evaporated in vacuo and triturated with 50 mL
of ether. A white solid was collected by filtration and dried in
vacuo, giving 650 mg (72%) of the title product: mp 194-196
°C; 1H NMR (300 MHz, CDCl3) δ 1.70 (m, 1 H), 1.80 (m, 2 H),
2.02 (m, 2 H), 2.31 (s, 3 H), 2.57 (d, J ) 6.9 Hz, 2 H), 2.72 (m,
2 H), 3.34 (m, 2 H), 3.63 (d, J ) 11.4 Hz, 2 H), 4.49 (s, 2 H),
5.01 (s, 2 H), 6.81 (d, J ) 10.8 Hz, 2 H), 6.88 (d, J ) 10.8 Hz,
(m, 2 H), 1.73 (m, 2 H), 2.09 (q, J ) 12.3 Hz, 2 H), 2.56 (m, 3
H), 3.41 (d, J ) 11.1 Hz, 2 H), 4.10 (d, J ) 5.1 Hz, 2 H), 6.94
(m, 2 H), 7.05 (m, 2 H), 7.43 (m, 3 H), 7.61 (m, 2 H), 12.41 (s,
1 H).
4-(4-F lu or oben zyl)p ip er id in e Hyd r och lor id e (6d ). A
mixture of 1-benzyl-4-(4-fluorobenzyl)piperidine hydrochloride
(14b) (4.5 g, 14 mmol) and 1.93 g of 10% Pd/C in 100 mL of
95% ethanol was hydrogenated at 60 psi for 12 h. The catalyst
was removed through a short column of Celite (10 g) and
washed with methanol (3 × 20 mL). The filtrate was evapo-
rated in vacuo and triturated with 80 mL of ether. A white
solid was collected by filtration and dried in vacuo, giving 3.2
g (98%) of the title product: mp 158-160 °C; 1H NMR (CDCl3,
300 MHz) δ 1.70-1.81 (m, 5 H), 2.57 (m, 2 H), 2.79 (m, 2 H),
3.45 (m, 2 H), 6.98 (m, 2 H), 7.05 (m, 2 H), 9.45 (brs, 2 H).
1-[2-(4-Ben zyloxyp h en yl)et h oxy]-4-(4-flu or ob en zyl)-
p ip er id in e (9c). A mixture of 2-(4-benzyloxyphenoxy)ethyl
bromide (7b ) (3.50 g, 11.4 mmol), 4-fluorobenzylpiperidine

2998 J ournal of Medicinal Chemistry, 1999, Vol. 42, No. 15
Zhou et al.
hydrochloride (6c) (2.6 g, 11.4 mmol), and potassium carbonate
(3.91 g, 28 mmol) in 60 mL of acetonitrile was refluxed for 12
h. The inorganic salt was removed by filtration through a short
column of silica gel and washed with ethyl acetate (3 × 25
mL). The combined filtrate was evaporated in vacuo to give a
crude mixture, which was purified by flash chromatography
(20% methanol in ethyl acetate), giving 4.0 g (84%) of the title
After the addition was complete, the reaction mixture was
stirred at room temperature for 3 h and stood overnight. The
reaction mixture was treated with 100 mL of saturated
aqueous NH4Cl solution at 0 °C and then extracted with
dichloromethane (2 × 50 mL). The combined organic phases
were evaporated in vacuo to give an oil, which was redissolved
in 200 mL of dichloromethane, washed with saturated aqueous
NH4Cl solution (2 × 30 mL) and brine (50 mL), and then dried
over sodium sulfate. Evaporation of solvent followed by flash
chromatography (EtOAc, Rf ) 0.25) gave 7.5 g (96%) of the
product as a pale-yellow oil: 1H NMR (300 MHz, CDCl3) δ 1.48
(m, 2 H), 1.73 (m, 2 H), 2.05 (s, 1 H), 2.32 (m, 5 H), 2.61 (m, 2
H), 2.71 (s, 2 H), 3.51 (s, 2 H), 7.09 (m, 4 H), 7.30 (m, 5 H).
1
product as a pale-yellow solid: mp 73-75 °C; H NMR (300
MHz, CDCl3) δ 1.26 (m, 3 H), 1.62 (m, 1 H), 2.01 (m, 2 H),
2.47 (d, J ) 6.9 Hz, 2 H), 2.73 (t, J ) 6.0 Hz, 2 H), 2.93 (m, 2
H), 4.02 (t, J ) 5.7 Hz, 2 H), 4.99 (s, 2 H), 6.78-6.96 (m, 7 H),
7.07 (m, 2 H), 7.29-7.41 (m, 4 H).
4-(4-F lu or ob e n zyl)-1-[2-(4-h yd r oxyp h e n yl)e t h oxy]-
p ip er id in e Hyd r och lor id e (10c). To a solution of 1-[2-(4-
benzyloxyphenyl)ethoxy]-4-(4-fluorobenzyl)piperidine (9c) (4.0
g, 9.5 mmol) in 100 mL of methanol was added 1.0 g of 5%
Pd/C. The resulting mixture was hydrogenated at 35 psi of
hydrogen for 4 h. The catalyst was removed through a short
column of Celite (5 g) and washed with methanol (3 × 15 mL).
The combined filtrate was treated with 15 mL of 1 M HCl in
methanol. The solution was evaporated in vacuo and triturated
with 100 mL of ether. An off-white solid was collected by
filtration and dried in vacuo, giving 3.2 g (95%) of the title
product: mp 196-198 °C; 1H NMR (300 MHz, CD3OD) δ 1.58
(m, 2 H), 1.89 (m, 3 H), 2.60 (d, J ) 6.3 Hz, 2 H), 3.08 (m, 2
H), 3.49 (t, J ) 5.1 Hz, 2 H), 3.62 (m, 2 H), 4.25 (t, J ) 5.1 Hz,
2 H), 6.77 (d, J ) 9.3 Hz, 2 H), 7.02 (m, 2 H), 7.20 (m, 2 H).
Anal. (C20H25ClFNO2) C, H, N.
1-[2-(4-Ben zyloxyp h en yl)et h oxy]-4-p h en ylp ip er id in e
Hyd r och lor id e (9d ). A mixture of 2-(4-benzyloxyphenoxy)-
ethyl bromide (7b) (0.377 g, 1.23 mmol), 4-phenylpiperidine
hydrochloride (6a ) (0.20 g, 1.23 mmol), and potassium carbon-
ate (0.42 g, 3.07 mmol) in 20 mL of acetonitrile was refluxed
for 12 h. The inorganic salt was removed through a short
column of silica gel and washed with ethyl acetate (3 × 25
mL). The combined filtrate was evaporated in vacuo to give a
crude mixture, which was purified by flash chromatography
(20% methanol in ethyl acetate) to give a residue, a solution
of which in 10 mL of methanol was treated with 3 mL of 1 M
HCl in methanol. The resulting solution was evaporated in
vacuo and triturated with 50 mL of ether. A white solid was
collected by filtration and dried in vacuo, giving 455 mg (88%)
of the title product: mp 163-165 °C; 1H NMR (300 MHz,
CDCl3) δ 2.01 (d, J ) 13.5 Hz, 3 H), 2.62 (m, 3 H), 2.98 (m, 2
H), 3.44 (m, 2 H), 3.79 (m, 2 H), 4.54 (m, 2 H), 5.02 (s, 2 H),
6.83 (d, J ) 8.7 Hz, 2 H), 6.90 (d, J ) 8.7 Hz, 2 H), 7.32 (m, 10
H), 12.80 (brs, 1 H).
1-[2-(4-Hydr oxyph en yl)eth oxy]-4-ph en ylpiper idin e Hy-
d r och lor id e (10d ). To a solution of 1-[2-(4-benzyloxyphenyl)-
ethoxy]-4-phenylpiperidine hydrochloride (9d ) (254 mg, 0.60
mmol) in 25 mL of methanol was added 85 mg of 20% Pd-
(OH)2. The resulting mixture was hydrogenated at 20 psi of
hydrogen for 2 h. The catalyst was removed through a short
column of Celite (5 g) and washed with methanol (3 × 15 mL).
The combined filtrate was evaporated in vacuo and triturated
with 30 mL of ether. The white solid was collected by filtration
and dried in vacuo, giving 180 mg (90%) of the title product:
mp 198-200 °C; 1H NMR (300 MHz, CD3OD) δ 2.11 (m, 4 H),
2.85 (m, 1 H), 3.30 (m, 2 H), 3.59 (m, 2 H), 3.77 (d, J ) 10.5
Hz, 2 H), 4.31 (d, J ) 5.1 Hz, 2 H), 6.73 (d, J ) 8.7 Hz, 2 H),
6.87 (d, J ) 8.7 Hz, 2 H), 7.29(m, 5 H). Anal. (C19H24ClNO2‚
0.3H2O) C, H, N.
1-Ben zyl-4-h yd r oxy-4-(4-m eth ylben zyl)p ip er id in e (16).
To a suspension of 2.31 g (0.095 mol) of Mg turnings in 15 mL
of anhydrous THF was added a solution of 1,2-dibromoethane
(0.489 g, 2.65 mmol) in 5 mL of THF dropwise at room
temperature under N2. After the addition, the THF was
removed and the residue was rinsed with THF (2 × 5 mL). To
this residue was added a solution of 4-methylbenzyl chloride
(15) (13.0 g, 92.6 mmol) in 50 mL of THF dropwise at 0 °C.
After the addition, the solution was stirred at room temper-
ature for 2 h and another 50 mL of THF was added. After the
mixture cooled to -35 to -40 °C, a solution of 4-benzylpiperi-
done (5.0 g, 26.5 mmol) in 20 mL of THF was added dropwise.
4-Hyd r oxy-4-(4-m eth ylben zyl)p ip er id in e Hyd r och lo-
r id e (6e). A mixture of 1-benzyl-4-hydroxy-4-(4-methylbenzyl)-
piperidine (16) (2.8 g, 9.5 mmol) and 700 mg of 10% Pd/C in
100 mL of 95% ethanol was hydrogenated at 50 psi for
overnight. The catalyst was removed by filtration through a
short column of Celite (10 g) and washed with methanol (3 ×
15 mL). The filtrate was treated with 12 mL of 1 M HCl in
methanol. The resulting solution was evaporated in vacuo and
triturated with 30 mL of ether. A white solid was collected by
filtration, giving 2.1 g (92%) of the title product: mp 183-
1
185 °C; H NMR (300 MHz, CDCl3) δ 1.68 (m, 2 H), 2.10 (m,
2 H), 2.34 (s, 3 H), 2.78 (s, 2 H), 3.24 (m, 5 H), 7.05 (d, J ) 7.5
Hz, 2 H), 7.14 (d, J ) 7.5 Hz, 2 H), 9.30 (brs, 1 H), 9.52 (brs,
1 H).
1-[2-(4-Ben zyloxyp h en oxy)eth yl]-4-h yd r oxy-4-(4-m eth -
ylben zyl)p ip er id in e Hyd r och lor id e (9e). A mixture of 2-(4-
benzyloxyphenoxy)ethyl bromide (7b) (368 mg, 1.2 mmol),
4-hydroxy-4-(4-methylbenzyl)piperidine hydrochloride (6e) (290
mg, 1.2 mmol), and potassium carbonate (414 mg, 3 mmol) in
30 mL of acetonitrile was refluxed for 12 h. The inorganic salt
was removed through a short column of silica gel and washed
with ethyl acetate (3 × 25 mL). The combined filtrate was
evaporated in vacuo to give a crude mixture, which was
purified by flash chromatography (5% methanol in ethyl
acetate), giving a pale-yellow oil, a solution of which in 10 mL
of methanol was treated with 4 mL of 1 M HCl in methanol.
The resulting solution was evaporated in vacuo and triturated
with 50 mL of ether. A white solid was collected by filtration
and dried in vacuo, giving 420 mg (75%) of the title product:
1
mp 179-181 °C; H NMR (300 MHz, CDCl3) δ 1.61 (s, 2 H),
1.73 (d, J ) 14.1 Hz, 2 H), 2.33 (s, 3 H), 2.45 (m, 2 H), 2.81 (s,
2 H), 3.22 (m, 2 H), 3.36 (s, 1 H), 3.46 (d, J ) 8.4 Hz, 2 H),
4.49 (s, 2 H), 5.01 (s, 2 H), 6.82 (d, J ) 9.0 Hz, 2 H), 6.90 (d,
J ) 9.0 Hz, 2 H), 7.08 (d, J ) 7.5 Hz, 2 H), 7.17 (d, J ) 7.5 Hz,
2 H), 7.38 (m, 5 H), 12.40 (brs, 1 H).
4-Hyd r oxy-1-[2-(4-h yd r oxyp h en oxy)eth yl]-4-(4-m eth yl-
ben zyl)p ip er id in e Hyd r och lor id e (10e). To a solution of
1-[2-(4-benzyloxyphenoxy)ethyl]-4-hydroxy-4-(4-methylbenzyl)-
piperidine (9e) (0.25 g, 0.53 mmol) in 30 mL of methanol was
added 62.5 mg of 20% Pd(OH)2. The resulting mixture was
hydrogenated at 20 psi of hydrogen for 3 h. The catalyst was
removed through a short column of Celite (5 g) and washed
with methanol (3 × 15 mL). The filtrate was evaporated in
vacuo and triturated with 30 mL of ether. A white solid was
collected by filtration and dried in vacuo, giving 200 mg (100%)
of the title product: mp 133-135 °C; 1H NMR (300 MHz, CD3-
OD) δ 1.58 (m, 2 H), 1.75 (m, 2 H), 2.12 (s, 3 H), 2.62 (s, 2 H),
3.20-3.30 (m, 6 H), 4.06 (m, 2 H), 6.53 (d, J ) 9.0 Hz, 2 H),
6.65 (d, J ) 9.0 Hz, 2 H), 6.94 (s, 4 H). Anal. (C21H28ClNO3) C,
H, N.
Oocyte Electr op h ysiology. Oocytes were obtained from
mature female Xenopus laevis and were prepared as described
previously.16 Individual oocytes were microinjected with a
mixture of NMDA receptor-encoding cRNAs, provided by Dr.
P. H. Seeburg (Heidelberg University, Heidelberg, Germany).5d
NR1A and NR2A were injected at a 1:4 ratio; all other binary
subunit combinations were injected 1:1 (1-10 ng of each
subunit). Oocytes were stored in Barth’s medium containing
(in mM): NaCl, 88; KCl, 1; CaCl2, 0.41; Ca(NO3)2, 0.33; MgSO4,
0.82; NaHCO3, 2.4; Hepes, 5; pH 7.4, with 0.1 mg/mL genta-
mycin sulfate. Standard voltage clamp recordings were made

Novel, Potent, Selective NMDA Receptor Antagonist
J ournal of Medicinal Chemistry, 1999, Vol. 42, No. 15 2999
at -70 mV in nominally Ca2+-free Ringer solution (in mM):
NaCl, 115; KCl, 2; BaCl2, 1.8; Hepes, 5; pH 7.4.17 Drugs were
diluted in Ca2+-free Ringer solution and applied by bath
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a conventional flow-through
chamber (volume ∼0.2 mL). Test drugs were dissolved in
DMSO and diluted into Ringer just prior to application (final
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I/Icontrol ) {(1 - min)/{1 + ([antagonist]/IC50)n}} + min
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mV, and 30-ms steps to +50 mV elicited K+ channel currents.
Control responses were obtained before application of drug to
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inhibition of sustained K+ current.
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