Synthesis, receptor potency, and selectivity of halogenated diphenylpiperidines as serotonin 5-HT2a ligands for PET or SPECT brain imaging

Article abstract of DOI:10.1021/jm0200411

A series of 4′-substituted phenyl-4-piperidinylmethanol and benzoyl-4-piperidine derivatives was synthesized as potential novel serotonin 5-HT_2A receptor ligands that can be radiolabeled for in vivo brain imaging. Compounds were prepared by alk

Full text of DOI:10.1021/jm0200411

J . Med. Chem. 2002, 45, 2319-2324
2319
Syn th esis, Recep tor P oten cy, a n d Selectivity of Ha logen a ted
Dip h en ylp ip er id in es a s Ser oton in 5-HT2A Liga n d s for P ET or SP ECT Br a in
Im a gin g
Xing Fu,† Ping-Zhong Tan,†,‡ Nora S. Kula,§ Ross Baldessarini,§ Gilles Tamagnan,† Robert B. Innis,† and
Ronald M. Baldwin*,†
Department of Psychiatry, Yale University School of Medicine and VA Connecticut Health Care Service,
West Haven, Connecticut 06516, and Department of Psychiatry and Neuroscience Program, Harvard Medical School, and
Laboratories for Psychiatric Research, McLean Research Center, McLean Division of Massachusetts General Hospital,
Belmont, Massachusetts 02478
Received J anuary 28, 2002
A series of 4′-substituted phenyl-4-piperidinylmethanol and benzoyl-4-piperidine derivatives
was synthesized as potential novel serotonin 5-HT2A receptor ligands that can be radiolabeled
for in vivo brain imaging. Compounds were prepared by alkylation of 4-substituted benzoyl-
4-piperidine with an iodo- or fluoro-substituted phenylalkyl halide followed by reduction with
sodium borohydride. Potency of novel compounds was determined by in vitro radioreceptor
affinity assays selective for serotonin 5-HT2A receptors. Potent compounds were further
evaluated for selectivity at serotonin-2A versus 2C, 6, and 7, as well as dopamine D2 and
adrenergic R1 and R2 receptors. The novel compound (4-fluorophenyl)-(1-[2-(4-fluorophenyl)-
ethyl]piperidin-4-yl])methanol was particularly promising with high 5-HT2A potency (Ki ) 1.63
nM) and >300-fold selectivity over other 5-HT receptor types.
In tr od u ction
Cerebral receptors for serotonin (5-hydroxytryptamine,
5-HT) have been implicated in the pathogenesis of major
mood and psychotic disorders such as schizophrenia,1,2
and 5-HT2A receptors probably contribute to the actions
of psychotropic agents used in the treatment of these
common disorders.3,4 Nevertheless, knowledge of the
localization and functional roles of the large number of
5-HT receptors, including 5-HT2A receptors, remains
incomplete. A powerful method of contributing to such
knowledge is the use of clinical anatomical and func-
tional radionuclide brain imaging with PET (positron
emission tomography) and SPECT (single photon emis-
sion computed tomography). These techniques provide
methods of high sensitivity for measuring in vivo
neurochemical and pharmacological effects at specific
target-receptor proteins. Imaging of the 5-HT2A receptor
in vivo can be used to assess receptor occupancy, guide
dosing, examine cerebral and plasma pharmacokinetics,
and perhaps predict the efficacy of the treatment with
antipsychotic and mood-altering drugs that interact
with targeted sites.5 Development of improved ra-
diotracers for PET (labeled with 18F or 11C) and SPECT
F igu r e 1. 5-HT2A selective ligands. For altanserin, 5-HT2A
Ki ) 0.30 nM, and selectivity for serotonin 2A over 2C receptors
was 20-fold. For MDL-100 907, 5-HT2A Ki ) 0.36 nM, and
selectivity was 300-fold.
and selectivity for the 5-HT2A receptor compared to
other 5-HT receptors,7,8 and [18F]altanserin and [11C]-
MDL-100 907 have been developed for in vivo imag-
ing.9,10 However, neither is fully satisfactory, owing to
limitations of potency, selectivity, or biological half-life.
Notably, the selectivity of altanserin for 5-HT2A over
5-HT2C is about 20-fold. Although MDL-100 907 has a
high affinity and selectivity for the 5-HT2A receptor, its
radiolabeling with short-lived 11C (half-life of 20 min)
limits its suitability to short-term studies. A more ideal
radioligand for clinical in vivo imaging of 5-HT2A recep-
tors would have high 5-HT2A potency (Ki e 1 nM) and
selectivity (>100-fold) toward other, particularly 5-HT,
receptors and be radiolabeled with 18F (half-life of 110
min) or 123I (half-life of 13.2 h).
(
123I) with high specific uptake and selectivity for 5-HT2A
receptors would, therefore, be very useful.
Among the reported ligands for the 5-HT2A receptor,6
the two structurally related compounds altanserin and
MDL-100 907 (Figure 1) have relatively high affinity
In our program for the design, synthesis, and neu-
ropharmacological assessment of potent and selective
5-HT2A receptor ligands for PET or SPECT imaging, we
developed novel compounds based on common structural
features of altanserin and MDL-100 907 to provide a
new series of 4′-substituted phenyl-4-piperidinylmetha-
nol and benzoyl-4-piperidine derivatives. We now report
on their synthesis and binding affinities to 5-HT2A and
other potentially competing receptors.
* To whom correspondence should be addressed. Telephone: (203)
932-5711 ext 3590. Fax: (203) 937-3897. E-mail: ronald.baldwin@
yale.edu.
† Yale University School of Medicine and VA Connecticut Health
Care Service.
‡ Present address: Pharmaceutical R&D, Honeywell, 20 Peabody
Street, Buffalo, NY 14210.
§ Harvard Medical School and McLean Division of Massachusetts
General Hospital.
10.1021/jm0200411 CCC: $22.00 © 2002 American Chemical Society
Published on Web 04/30/2002

2320 J ournal of Medicinal Chemistry, 2002, Vol. 45, No. 11
Sch em e 1. Synthesis of New Ligands
Fu et al.
Resu lts a n d Discu ssion
Ta ble 1. Serotonin Radioreceptor Affinity Assay Conditions
Ch em istr y. Iodo-substituted phenylethyl bromides
were synthesized by the borane reduction of the corre-
sponding substituted phenylacetic acids, followed by a
reaction with dibromotriphenylphosphorane in dichlo-
romethane to give alkyl bromides (Scheme 1). 3-Io-
dophenylacetic acid was prepared by hydrolysis of
3-iodophenylacetonitrile. The target compounds were
synthesized by N-alkylation of (4-nitrophenyl)piperidin-
4-ylmethanone (1a ) or (4-fluorophenyl)piperidin-4-yl-
methanone (1b) with the corresponding chloride, bro-
mide, or iodide, followed by the reduction of the ketone
moiety with sodium borohydride. The known compound
4e and intermediates, such as (4-nitrophenyl)piperidin-
4-ylmethanone (1a ) and 1-(2′-bromoethyl)-4-fluoroben-
zene, were synthesized according to published meth-
ods.8,11
receptor
type
membrane
source
[3H]ligand
(nM)
blank (µM)
Serotonin
rat forebraina ketanserin (1.2)
5-HT2r
5-HT2Ah GF-62 cells
5-HT2Ch PO-1C cells
5-HT6h
5-HT7h
cinanserin (10)
altanserin (10)
clozapine (10)
clozapine (10)
clozapine (10)
ketanserin (1.2)
mesulergine (1.0)
LSD (1.0)b
HD-6 cells
HD-7 cells
LSD (1.0)b
Dopamine
nemonapride (0.075) haloperidol (10)
D2
rat caudatea
Adrenergic
prazosin (0.20)
prazosin (1.0)
R1
R2
rat braina
rat braina
phentolamine (2)
phentolamine (10)
a
Rat brain tissue was used for initial screening assays, followed
by assays with transfected cell membranes expressing human (-h)
5-HT2A receptors when Ki < 500 nM. Rat (-r) brain tissue was
forebrain (transected anterior to optic chiasm) for 5-HT2, caudate-
putamen for D2, and whole brain minus cerebellum for R-adren-
b
Neu r op h a r m a cology. Novel compounds were evalu-
ated in vitro for their potency (expressed as an inhibition
constant, Ki, nanomolar) in competitive serotonin recep-
tor radioaffinity assays (Table 1). 5-HT2A receptor
affinity was initially assessed in homogenates of the rat
forebrain with [3H]ketanserin as a radioligand and
altanserin as a blank agent to define nonspecific bind-
ing. Test compounds with Ki < 500 nM in this 5-HT2A
assay were further evaluated in a more specific assay
with membranes prepared from genetically transfected
cells selectively expressing human 5-HT2A receptor
genes and proteins and the same radioligand and blank
agent. The two assay methods for 5-HT2A potency agreed
very closely (nonparametric rank rs ) 0.982, p < 0.0001).
For compounds with relatively high 5-HT2A receptor
potency (Ki < 10 nM), additional tests of selectivity for
5-HT2A receptors were carried out with similar assay
methods using transfected cells selectively expressing
human 5-HT2C, 5-HT6, and 5-HT7 receptors, as well as
rat brain tissue to assay for potency at dopamine (D2)
and adrenergic (R1, R2) receptors by way of the assay
methods summarized in Table 1.
ergic receptor assays. LSD ) lysergic acid diethylamide.
the alkyl link of the piperidine ring to a phenyl moiety.
Generally, compounds with an ethylene bridge (n ) 2)
to the second phenyl moiety had higher 5-HT2A potency
than those with a methylene bridge (n ) 1). This
contrast can be appreciated by comparing ethylene-
bridge compounds (4a -c, 5a -c) with those having a
methylene bridge (2a -c, 3a -c). Some of the ethylene-
bridge compounds (notably, 4b, 5a ,d ) showed high
5-HT2A potency (Ki ) 1.15 nM) but only limited selectiv-
ity (ca. 30-fold), thus, limiting their candidacy for brain-
imaging radioligand development. The nature of the
linking moiety to a second phenyl ring (carbonyl or
methanol) or the p-substituent on that ring (F or NO2)
all yielded some compounds with relatively high 5-HT2A
affinity (e.g., compare 5a vs 5d , and 4a vs 5a ). Also, in
general, for the other phenyl ring, p-substituted iodi-
nated derivatives were less potent at the 5-HT2A recep-
tors than the corresponding o- and m-substituted posi-
tional isomers. Nevertheless, compound 5e showed high
affinity for the 5-HT2A receptor (Ki ) 1.63 nM). This
novel compound was approximately 300-, 825-, and
>6000-fold less potent at the 5-HT2C, 5-HT6, and 5-HT7
Among the compounds reported here, 5-HT2A potency
and selectivity depended significantly on the length of

Selectivity of Halogenated Diphenylpiperidines
J ournal of Medicinal Chemistry, 2002, Vol. 45, No. 11 2321
Ta ble 2. Structure and Receptor Potency (Ki, nM) of Novel Compounds
receptor affinity (Ki, nM)
a
compound
X
n
Y
Z
5-HT2A
5-HT2C
5-HT6
5-HT7
ratio 2C/2A
D2
R1
R2
2a
2b
2c
3a
3b
3c
4a
4b
4c
5a
5b
5c
5d
4d
5e
4e
O
O
O
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
2-I
3-I
4-I
2-I
3-I
4-I
2-I
3-I
4-I
2-I
3-I
4-I
H
F
F
F
F
F
F
F
F
F
F
F
F
191
12.7
227
-
-
-
-
-
-
-
-
-
-
-
-
-
-
1.3
51.8
176
-
-
-
-
-
-
-
-
-
-
H, OH
H, OH
H, OH
O
O
O
H, OH
H, OH
H, OH
H, OH
O
H, OH
O
>10000
>10000
>10000
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
1.95
35.5
16.6
144
28.6
-
138
3.68
8.76
7.32
101
-
18.0
33.0
40.0
31.0
-
24.8
140
244
381
813
-
0.51
3.62
0.91
13.1
32.1
1.15
9.34
1.63
9.54
48.9
4.92
26.1
108
300
430
-
>10000
-
-
-
-
-
-
-
-
-
-
NO2
NO2
F
-
-
-
-
-
-
H
4-F
4-F
-
-
-
-
-
-
-
503
493
>10000
3000
1345
63.0
308
52.0
>10000
453
303
1032
614
F
33.6
a
5-HT2A data were obtained with the membranes of cells transfected with human 5-HT2A genes. Values obtained in 5-HT2A screening
assays for all 16 compounds with rat forebrain tissue agreed closely with the results shown above (Spearman rank rs ) 0.982, p < 0.0001).
Infrared (IR) spectra were recorded on a Perkin-Elmer FT 1600
IR spectrometer, and H-NMR spectra were obtained using a
Bruker AM 500 MHz spectrometer. Mass spectra were ob-
receptors, respectively, and also had a low potency at
dopamine D2 and R-adrenergic receptors (Table 2).
Of the compounds reported here, 5e seems to satisfy
the most important criteria for a radioligand for in vivo
imaging of the 5-HT2A receptor with high 5-HT2A
potency and high selectivity over serotonin 2C, 6, and
7 receptors. Compared to altanserin (20-fold 2A vs 2C
selectivity), compound 5e has much higher selectivity
(300-fold, Table 2). Furthermore, compound 5e lends
itself to 18F labeling by nitro exchange.
All of the alcohol compounds in the present series are
racemic by virtue of the asymmetric center introduced
in their structure by NaBH4 reduction. One enantiomer
may be more potent, as suggested by the previous report
that (R)-(+)MDL-100 907 had 30-times greater selectiv-
ity to 5-HT2A over 5-HT2C receptors than the (S)-(-)-
enantiomer.12 We resolved the previously reported
analogue 4-fluorophenyl(1-phenethylpiperidin-4-yl)meth-
anol6 by chiral HPLC and found that the enantiomer
eluting earlier (Ki ) 3.07 nM) was 30-times more potent
than the later-eluting enantiomer (Ki ) 94.0 nM). The
absolute configuration of these enantiomers is not
known, but it is reasonable to assign the more potent
isomer to the (R) configuration, based on the analogy
to MDL-100 907. Finally, we also resolved compound 5e
by chiral HPLC and found in preliminary experiments
that one enantiomer was 30-times more potent in 5-HT2
radioreceptor assays in the rat brain.
1
tained with
a Micromass Q-Tof spectrometer. Elemental
analyses were performed by Atlantic Microlab Inc. (Knoxville,
TN), and values within 0.4% of the theoretical values were
accepted as valid. Flash chromatography12 was performed with
40 µm mesh silica gel 60 (J . T. Baker) using eluents as
indicated. Chiral HPLC was carried out on a Chiralcel OD
column (Daicell) using hexane/2-propanol/diethylamine (85:
15:0.3, v/v/v) at 1.3 mL/min and 254 nm UV detection. Starting
materials for syntheses were purchased either from Sigma-
Aldrich Chemicals (St. Louis, MO) or from Acros Corp.
(Pittsburgh, PA). Chemicals and drug substances used for
binding assays were purchased from Sigma-Research Bio-
chemicals International (Sigma-RBI, Natick, MA) or Sigma
Corp. (St. Louis, MO), unless stated otherwise. [3H]Ketanserin
(80 Ci/mmol), [3H]LSD (70 Ci/mmol), [3H]MK-912 (70 Ci/
mmol), [3H]nemonapride (90 Ci/mmol), and [3H]prazosin (80
Ci/mmol) were obtained from NEN Corp. (Boston, MA); [3H]-
mesulergine (80 Ci/mmol) was purchased from Amersham
(Arlington Heights, IL). Cell membranes from genetically
transfected cells selectively expressing 5-HT2A, 5-HT2C, 5-HT6,
or 5-HT7 receptor proteins were obtained from the NIMH
Screening Program through Bryan Roth, Ph.D., Case Western
Reserve University, Cleveland, OH.13
(4-Flu or oph en yl)-[1-(2-iodoben zyl)piper idin -4-yl]m eth -
a n on e (2a ). To a solution of 200 mg (0.97 mmol) of (4-
fluorophenyl)-piperidin-4-ylmethanone (1a) in 10 mL of dry
4-methylpentan-2-one under a nitrogen atmosphere was added
244 mg (0.97 mmol) of o-iodobenzyl chloride, followed by 199
mg (1.45 mmol) of K2CO3 and 20 mg (0.12 mmol) of KI. The
mixture was heated at reflux for 22 h. After being cooled to
room temperature, the insoluble material was removed by
filtration. The filtrate was evaporated to dryness on a rotary
evaporator, and the residue was purified by flash chromatog-
raphy with 10:50 EtOAc/hexane containing 10% Et3N to give
346 mg (85% yield) of 2a as a white solid. Mp: 89.5-91 ×b0C.
1H-NMR (CDCl3, δ): 7.97 (q, 2H, F-ArH), 7.83 (dd, 1H,
I-ArH, J ) 0.9 Hz, J ) 7.8 Hz), 7.44 (dd, 1H, I-ArH, J ) 1.3
Hz, J ) 7.7 Hz), 7.31 (dt, 1H, J ) 7.6 Hz), 7.13 (t, 2H, F-ArH),
6.94 (dt, 1H, I-ArH, J ) 1.6 Hz, J ) 7.6 Hz), 3.54 (s, 2H,
CH2Ar), 3.23 (m, 1H), 3.00 (dt, 2H), 2.24 (dt, 2H), 1.83 (m, 4H).
m/z (ES+): 424 (m + 1). Anal. (C19H19FINO) C, H, N.
In conclusion, we developed a series of iodo- and
fluoro-substituted diphenylpiperidine ligands. In this
series of compounds, we found that the potency and
selectivity for the 5-HT2A receptors depended on the
length of the carbon bridge between the phenyl and
piperidinyl ring. The in vitro pharmacological profiles
of compound 5e suggest that this ligand may be a
particularly attractive potential candidate for receptor
imaging because of the high potency and selectivity for
5-HT2A sites.
4-(Flu or oph en yl)-[1-(3-iodoben zyl)piper idin -4-yl]m eth -
a n on e Hyd r och lor id e (2b). Compound 2b was prepared as
described for 2a using 200 mg (0.97 mmol) of 1a in 10 mL of
dry 4-methylpentan-2-one, 244 mg (0.97 mmol) of m-iodobenzyl
Exp er im en ta l Section
Gen er a l. Melting points were determined on a Thomas-
Hoover capillary melting point apparatus and are uncorrected.

2322 J ournal of Medicinal Chemistry, 2002, Vol. 45, No. 11
Fu et al.
bromide, 199 mg (1.45 mmol) of K2CO3, and 20 mg (0.12 mmol)
of KI. Flash chromatography (10:30 EtOAc/hexane, 10% Et3N)
afforded 366 mg (90%) of 2b. The HCl salt was made from
HCl in ether. Mp: 202-204 °C. 1H-NMR (CD3OD, δ): 8.09 (q,
2H, F-ArH), 7.97 (m, 1H, I-ArH), 7.89 (mm, 1H, I-ArH), 7.54
(mm, 1H, I-ArH), 7.25 (m, 3H, ArH), 4.33 (s, 2H, CH2Ar), 3.72
(tt, 1H), 3.59 (d, 2H), 3.19 (t, 2H), 2.15 (d, 2H), 1.94 (dq, 4H).
m/z (ES+): 424 (m + 1). Anal. (C19H20ClFINO) C, H, N.
(4-Flu or oph en yl)-[1-(4-iodoben zyl)piper idin -4-yl]m eth -
a n on e (2c). Compound 2c was prepared as described for 2a
above using 70 mg (0.34 mmol) of 1a in 10 mL of dry
4-methylpentan-2-one, 100 mg (0.34 mmol) of 1-bromomethyl-
4-iodobenzene, 70 mg (0.55 mmol) of K2CO3, and 7 mg of KI.
Flash chromatography (50:50 EtOAc/hexane, 10% Et3N) af-
forded 120 mg (84%) of 2c. Mp: 118.5-120.5 °C. 1H-NMR
(CDCl3, δ): 7.96 (q, 2H, F-ArH), 7.65 (d, 2H, I-ArH), 7.13 (t,
2H, F-ArH), 7.09 (d, 2H, I-ArH), 3.47 (s, 2H, CH2Ar), 3.19
(m, 1H), 2.93 (dt, 2H), 2.12 (dt, 2H), 1.83 (m, 4H). m/z (ES+):
424 (m + 1). Anal. (C19H19FINO) C, H, N.
(4-Flu or oph en yl)-[1-(2-iodoben zyl)piper idin -4-yl]m eth -
a n ol Hyd r och lor id e (3a ). To a solution of 150 mg (0.355
mmol) of 4-fluorophenyl[1-(2-iodobenzyl)piperidin-4-yl]metha-
none (2a ) in 25 mL MeOH was added NaBH4 (40 mg, 1.1
mmol) at 0 °C. The mixture was stirred at room temperature
overnight. Excess NaBH4 was decomposed with 0.6 N HCl; the
acidic solution was then neutralized with NH4OH (28%) to pH
>10, and the solution was extracted with ether (5 × 20 mL).
The pooled ether extract was dried with K2CO3, filtered, and
evaporated on a rotary evaporator. Flash chromatography
(10:30 EtOAc/hexane, 5% Et3N) gave 66 mg (44%) of 4-fluoro-
phenyl[1-(3-iodobenzyl)piperidin-4-yl]methanol (3a), which was
converted to the HCl salt by a reaction with HCl in ether to
give 72 mg. Mp: 123 °C dec. 1H-NMR (CD3OD, δ): 8.03 (d,
1H, I-ArH), 7.63 (m, 1H, I-ArH), 7.51 (t, 1H, I-ArH), 7.34
(q, 2H, F-ArH), 7.22 (m, 1H, I-ArH), 7.07 (t, 2H, F-ArH),
4.45 (s, 2H, CH2Ar), 4.42 (d, 1H, CHOH), 3.62 (d, 1H), 3.54 (d,
1H), 3.16 (q, 2H), 2.13 (d, 1H), 1.93 (m, 1H), 1.62 (m, 3H). m/z
(ES+): 426 (m + 1). Anal. (C19H22ClFNO‚0.5H2O) C, H, N.
or F-ArH), 7.04 (m, 1H, I-ArH), 3.73 (t, 1H), 3.67 (d, 2H),
3.20-3.16 (m, 6H), 2.04-1.95 (m, 4H). m/z (ES+): 438 (m +
1). Anal. (C20H22ClFINO) C, H, N.
(4-F lu or op h en yl)-[1-[2-(3-iod op h en yl)eth yl]p ip er id in -
4-yl]m eth a n on e (4b). Compound 4b was prepared as de-
scribed for 2a above using 132 mg (0.643 mmol) of 1a in 10
mL of dry 4-methylpentan-2-one, 200 mg (0.643 mmol) of 1-(2′-
bromoethyl)-3-iodobenzene, 133 mg (0.964 mmol) of K2CO3, 13
mg of KI, and flash chromatography (20:10 EtOAc/hexane, 5%
Et3N) to give 90 mg (32%) of 4b. Mp: 94-95 °C. 1H-NMR
(CDCl3, δ): 7.97 (q, 2H, F-ArH), 7.57 (s, 1H, I-ArH), 7.54
(dt, 1H, I-ArH), 7.18 (d, 1H, I-ArH), 7.14 (t, 2H, F-ArH),
7.02 (t, 1H, I-ArH), 3.19 (m, 1H), 3.07 (dt, 2H), 2.76 (t, 2H),
2.59 (t, 2H), 2.17 (m, 2H), 1.86 (m, 4H). m/z (ES+): 438 (m +
1). Anal. (C20H21FINO) C, H, N.
(4-F lu or op h en yl)-[1-[2-(4-iod op h en yl)eth yl]p ip er id in -
4-yl]m eth a n on e (4c). Compound 4c was prepared as de-
scribed for 2a above using 250 mg (1.21 mmol) of 1a in 10 mL
of dry 4-methylpentan-2-one, 276 mg (1.21 mmol) of 1-(2′-
bromoethyl)-4-iodobenzene, 250 mg (2.21 mmol) of K2CO3, 25
mg of KI, and flash chromatography (20:20 EtOAc/hexane, 10%
1
Et3N) to give 240 mg (45%) of 4c. Mp: 280-282 °C. H-NMR
(CDCl3, δ): 7.97 (q, 2H, F-ArH), 7.61 (d, 2H, I-ArH), 7.14 (t,
2H, F-ArH), 6.97 (d, 2H, I-ArH), 3.21 (m, 1H), 3.06 (dt, 2H),
2.76 (t, 2H), 2.58 (t, 2H), 2.17 (m, 2H), 1.86 (m, 4H). m/z
(ES+): 438 (m + 1). Anal. (C20H21FINO) C, H, N.
(4-Nit r op h e n yl)-(1-p h e n e t h ylp ip e r id in -4-yl)m e t h a -
n on e (4d ). Compound 4d was prepared as described for 2a
above using 100 mg (0.427 mmol) of 1b in 5 mL of dry
4-methylpentan-2-one, 93 mg (0.70 mmol) of 2-bromoethyl-
benzene, 100 mg (0.70 mmol) of K2CO3, 10 mg of KI, and flash
chromatography (50:50 EtOAc/hexane, 2% Et3N) to give 37 mg
(22%) of 4d as a yellow powder. Mp: 155.5-156.5 °C. IR (KBr,
1
cm-1): 1678. H-NMR (CDCl3, δ): 8.32 (d, 2H, ArH, J ) 8.8
Hz), 8.08 (d, 2H, ArH, J ) 8.8 Hz), 7.31-7.21 (m, 5H, ArH),
3.25 (tt, 1H, J ) 5.0, 5.0 Hz), 3.09 (d, 2H, J ) 11.8 Hz), 2.83
(d, 2H, J ) 8.2 Hz, CH2N), 2.64 (d, 2H, J ) 8.2 Hz, CH2Ar),
2.05 (dt, 2H, J ) 3.8, 10.9 Hz), 1.90 (m, 4H). m/z (ES+): 339
(m + 1). Anal. (C20H22N2O3) C, H, N.
(4-Flu or oph en yl)-[1-(3-iodoben zyl)piper idin -4-yl]m eth -
a n ol Hyd r och lor id e (3b). Compound 3b was prepared as
described for 3a above using 150 mg (0.355 mmol) of 4-fluo-
rophenyl[1-(3-iodobenzyl)piperidin-4-yl]methanone (2b) in 25
mL of MeOH, 40 mg (1.1 mmol) of NaBH4, flash chromatog-
raphy (10:20 ethyl acetate/hexane, 5% Et3N), and HCl in ether
(4-Flu or oph en yl)-[1-[2-(4-flu or oph en yl)eth yl]piper idin -
4-yl]m eth a n on e (4e). Compound 4e was prepared as de-
scribed for 2a using 273 mg (1.32 mmol) of 1a in 10 mL of dry
4-methylpentan-2-one, 268 mg (1.32 mmol) of 1-(2′-bromo-
ethyl)-4-fluorobenzene, 273 mg (1.98 mmol) of K2CO3, 27 mg
of KI, and flash chromatography (50:20 EtOAc/hexane, 5%
Et3N) to give 260 mg (60%) of 4e as a white powder. Mp: 106-
108 °C. 1H-NMR (DMSO-d6, δ): 8.11 (q, 2H, ArH), 7.39 (t, 2H,
ArH), 7.34 (q, 2H, ArH), 7.18 (t, 2H, ArH), 3.72 (t, 1H), 3.64
(d, 2H), 3.29 (m, 2H), 3.10-3.05 (m, 4H), 2.03-1.88 (m, 4H).
m/z (ES+): 330 (m + 1). Anal. (C20H21F2NOH2O‚0.25H2O) C,
H, N.
1
to give 46 mg of 3b (28%). Mp: 125 °C dec. H-NMR (DMSO-
d6, δ): 8.00 (s, 1H, I-ArH), 7.78 (d, 1H, I-ArH), 7.62 (d, 1H,
I-ArH), 7.29 (q, 2H, F-ArH), 7.21 (t, 1H, I-ArH), 7.12 (t, 2H,
F-ArH), 4.27 (d, 1H, CHOH), 4.17 (s, 2H, CH2Ar), 3.28 (d,
1H), 3.22 (d, 1H), 2.81 (m, 2H), 1.89 (d, 1H), 1.64-1.58 (m,
2H), 1.37 (m, 1H). m/z (ES+): 426 (m + 1). Anal. (C19H22
ClFNO‚0.5H2O) C, H, N.
-
(4-Flu or oph en yl)-[1-(4-iodoben zyl)piper idin -4-yl]m eth -
a n ol Hyd r och lor id e (3c). Compound 3c was prepared as
described for 3a above using 70 mg (0.16 mmol) of 4-fluorophe-
nyl[1-(4-iodobenzyl)piperidin-4-yl]methanone (2c) in 20 mL of
MeOH, 20 mg (0.5 mmol) of NaBH4, flash chromatography
with 50:50 EtOAc/hexane (10% Et3N), and HCl in ether to give
40 mg of 3c (52%). Mp: 175 °C dec. 1H-NMR (DMSO-d6, δ):
7.81 (d, 2H, I-ArH), 7.35 (d, 2H, I-ArH), 7.30 (q, 2H, F-ArH),
7.13 (t, 2H, F-ArH), 4.29 (d, 1H, CHOH), 4.16 (s, 2H, CH2-
Ar), 3.26 (m, 2H), 2.80 (m, 2H), 1.87 (d, 1H), 1.67 (m, 1H), 1.54
(4-F lu or op h en yl)-[1-[2-(2-iod op h en yl)eth yl]p ip er id in -
4-yl]m eth a n ol Hyd r och lor id e (5a ). Compound 5a was
prepared as described for 3a above using 60 mg (0.13 mmol)
of 4-fluorophenyl[1-[2-(2-iodophenyl)ethyl]piperidin-4-yl]meth-
anone hydrochloride 4a in 10 mL of MeOH, 15 mg (0.38 mmol)
of NaBH4, flash chromatography (EtOAc, 3.5% Et3N), and HCl
1
in ether to give 25 mg (41%) of 5a . Mp: 174 °C dec. H-NMR
(DMSO-d6, δ): 7.86 (d, 1H, I-ArH), 7.40-7.32 (m, 4H, I-ArH
or F-ArH), 7.16 (t, 3H, F-ArH), 7.06 (m, 1H, I-ArH), 4.34
(t, 1H, CHOH), 3.56 (d, 1H), 3.50 (d, 1H), 2.91 (q, 2H), 1.89 (d,
(m, 2H), 1.42 (m, 1H). m/z (ES+): 426 (m + 1). Anal. (C19H22
ClFNO‚0.5H2O) C, H, N.
-
1H), 1.75-1.48 (m, 4H). m/z (ES+): 440 (m + 1). Anal. (C20H24
ClFINO) C, H, N.
-
(4-F lu or op h en yl)-[1-[2-(2-iod op h en yl)eth yl]p ip er id in -
4-yl]m eth a n on e Hyd r och lor id e (4a ). Compound 4a was
prepared as described for 2a above using 79 mg (0.39 mmol)
of 1a in 5 mL of dry 4-methylpentan-2-one, 120 mg (0.385
mmol) of 1-(2′-bromoethyl)-2-iodobenzene, 79 mg (0.58 mmol)
of K2CO3, and 8 mg of KI. Flash chromatography with 50:50
EtOAc/hexane (5% Et3N) afforded 110 mg (65% yield) of (4-
fluorophenyl)-[1-[2-(2-iodophenyl)ethyl]piperidin-4-yl]metha-
none 4a . The HCl salt was made from HCl in ether. Mp: 203-
205 °C. IR (KBr, cm-1): 1676. 1H-NMR (DMSO-d6, δ): 8.11
(q, 2H, F-ArH), 7.88 (d, 1H, I-ArH), 7.41-7.37 (m, 4H, I-ArH
(4-F lu or op h en yl)-[1-[2-(3-iod op h en yl)eth yl]p ip er id in -
4-yl]m eth a n ol Hyd r och lor id e (5b). Compound 5b was
prepared as described for 3a above using 60 mg (0.14 mmol)
of (4-fluorophenyl)-[1-[2-(3-iodophenyl)ethyl]piperidin-4-yl]-
methanone (4b) in 10 mL of MeOH, 15 mg (0.38 mmol) of
NaBH4, flash chromatography (EtOAc, 3.5%Et3N), and HCl
1
in ether to give 63 mg (96% yield) of 5b. Mp: 228 °C dec. H-
NMR (DMSO-d6, δ): 7.66 (s, 1H, I-ArH), 7.62 (d, 1H, I-ArH),
7.33 (q, 2H, F-ArH), 7.27 (d, 1H, I-ArH), 7.15 (t, 2H, F-ArH),
7.13 (t, 1H, I-ArH), 4.34 (d, 1H, CHOH), 3.54 (dd, 2H), 3.19

Selectivity of Halogenated Diphenylpiperidines
J ournal of Medicinal Chemistry, 2002, Vol. 45, No. 11 2323
(t, 2H), 2.99 (t, 2H), 2.84 (q, 2H), 1.88 (d, 1H), 1.71-1.43 (m,
4H). m/z (ES+): 440 (m + 1). Anal. (C20H24ClFINO) C, H, N.
receptor assays were used to test agents with relatively high
5-HT2A affinity for receptor selectivity, thus, to determine Ki
values as already described.
Ser oton in 5-HT2C. Transfected cell membranes selectively
expressing human 5-HT2C receptors were assayed with 0.80
nM [3H]mesulergine at room temperature for 60 min using
10 µM clozapine as the blank agent.
Ser oton in 5-HT6 a n d 5-HT7. Cell membranes (1-10 µg of
protein) from transfected cells selectively expressing human
5-HT6 or 5-HT7 receptors were incubated with 1 nM [3H]LSD
at room temperature for 90 min in the dark using 10 µM
clozapine as the blank agent.18
Dop a m in e D2. Homogenates of rat caudate-putamen tissue
were incubated with 75 pM [3H]nemonapride at 30 °C for 90
min, using 10 µM haloperidol as the blank agent.19
Alp h a -1 (R1) Adrenergic. Homogenates of whole rat brain
were incubated with 0.20 nM [3H]prazosin at 30 °C for 30 min,
using 2 µM phentolamine as the blank agent.20
(4-F lu or op h en yl)-[1-[2-(4-iod op h en yl)-eth yl]p ip er id in -
4-yl]m eth a n ol (5c). Compound 5c was prepared as described
for 3a using 80 mg (0.18 mmol) of (4-fluorophenyl)-[1-[2-(4-
iodophenyl)ethyl]piperidin-4-yl]methanone (4c) in 10 mL of
MeOH, 20 mg (0.55 mmol) of NaBH4, and flash chromatogra-
phy (EtOAc, 3.5% Et3N) which gave 50 mg (64%) of 5c. Mp:
117-119 °C. 1H-NMR (CDCl3, δ): 7.58 (d, 2H, I-ArH), 7.26
(q, 2H, F-ArH), 7.02 (t, 2H, F-ArH), 6.93 (d, 2H, I-ArH),
4.36 (d, 1H, CHOH), 3.01 (dd, 2H), 2.71 (t, 2H), 2.49 (t, 2H),
1.98 (m, 2H), 1.94 (m, 1H), 1.58 (m, 1H), 1.42 (m, 1H), 1.25
(m, 2H). m/z (ES+): 440 (m + 1). Anal. (C20H23FINO) C, H,
N.
(4-Nit r op h e n yl)-(1-p h e n e t h ylp ip e r id in -4-yl)m e t h a -
n ol (5d ). Compound 5d was prepared as described for 3a
above using 130 mg (0.385 mmol) of (4-nitrophenyl)-(1-phen-
ethylpiperidin-4-yl)methanone (4d ) in 20 mL THF, 30 mg (0.77
mmol) of NaBH4, and flash chromatography (EtOAc, 4% Et3N)
to give 60 mg (46%) of 5d as a white solid powder. Mp: 154-
155 °C. IR (KBr, cm-1): 3420, 3083, 2929, 2813, 1602, 1515,
1340, 852. 1H-NMR (CD2Cl2, δ): 8.19 (d, 2H, ArH, J ) 9.0 Hz),
7.51 (d, 2H, ArH, J ) 9 Hz), 7.27-7.17 (m, 5H, ArH), 4.58 (d,
1H, J ) 6.6 Hz, CHOH), 2.99 (m, 2H), 2.75 (t, 2H, CH2N), 2.54
(t, 2H, CH2Ar), 1.96 (m, 4H), 1.80 (m, 2H), 1.41 (m, 1H). m/z
(ES+): 341 (m + 1). Anal. (C20H24N2O3) C, H, N.
Alp h a -2 (r2) Ad r en er gic. Homogenates of whole rat brain
were incubated with 1.0 nM [3H]MK-912 for 80 min at room
temperature, using 10 µM phentolamine as the blank agent.20
Ack n ow led gm en t. This work was sponsored in part
by grants from the U.S. Department of Veterans Affairs
(VA Connecticut Schizophrenia and REAP Depression
Research Centers) and the U.S. Public Health Service
(NIH MH-58620, MH-34006, and MH-47370) as well as
an award from the Bruce J . Anderson Foundation and
the McLean Private Donors Neuropharmacology Re-
search Fund.
(4-Flu or oph en yl)-[1-[2-(4-flu or oph en yl)eth yl]piper idin -
4-yl]m eth a n ol (5e). Compound 5e was prepared as described
for 3a using 90 mg (0.27 mmol) of (4-fluorophenyl)-[1-[2-(4-
fluorophenyl)ethyl]piperidin-4-yl]methanone (4e) in 20 mL of
THF, 33 mg (0.82 mmol) of NaBH4, and flash chromatography
(EtOAc, 3.5% Et3N) to give 45 mg (50%) of 5e white powder.
Refer en ces
1
Mp: 109-110 °C. H-NMR (DMSO-d6, δ): 7.33 (q, 2H, ArH),
(1) Sandler, M.; Coppen, A.; Harnett, S. 5-Hydroxytryptamine in
Psychiatry; Oxford University Press: Oxford, U.K., 1991.
(2) Mann, J . J .; Underwood, M. D.; Baird, F.; McBride, P. A.
Neurochemical correlates of suicidal behavior: involvement of
serotonergic and nonserotonergic systems. Pharmacol. Toxicol.
1990, 66, 37-60.
7.28 (q, 2H, ArH), 7.15 (m, 4H, ArH), 4.34 (d, 1H, CHOH),
3.46 (q, 2H), 3.12 (t, 2H, CH2N), 2.93 (t, 2H, CH2Ar), 2.79 (m,
2H), 1.90 (d, 1H), 1.69 (m, 1H), 1.53-1.40 (m, 3H). m/z (ES+):
332 (m + 1). Anal. (C20H23F2NO) C, H, N.
Ra d ior ecep tor Assa ys. Ser oton in 5-HT2A. The affinity
and selectivity were tested at 5-HT2 receptors initially using
rat brain homogenates incubated with 0.4 nM [3H]ketanserin
for 15 min at 37 °C and using 10 µM altanserin as a blank
agent with which to determine nonspecific binding. Com-
pounds were further tested on genetically transfected COS-7
cell membranes selectively expressing human 5-HT2A recep-
tors. Cell membranes and test drugs were incubated with 1.0
nM [3H]ketanserin at room temperature for 60 min, again
using 10 µM altanserin as the blank. Cell membranes were
harvested on glass fiber filter sheets (ISC Bioexpress; Kaysville,
UT) in a Brandel cell harvester (Gaithersburg, MD), and
tritium radioactivity was counted in mini-vials containing
Emulsifier-Safe (Packard Instruments; Meriden, CT) in a
Beckman-Coulter â-scintillation spectrometer (Fullerton, CA)
at 50% efficiency. Similar competition assay, harvesting, and
radioactivity-counting procedures were used in the other
radioreceptor assays described below.
(3) Baldessarini, R. J . Drugs and the treatment of psychiatric
disorders: Antidepressant and antianxiety agents. Goodman
and Gilman’s The Pharmacological Basis of Therapeutics, 10th
ed.; McGraw-Hill Press: New York, 2001; Chapter 19.
(4) Baldessarini, R. J .; Tarazi, F. I. Drugs and the treatment of
psychiatric disorders: Antipsychotic and antimanic agents.
Goodman and Gilman’s The Pharmacological Basis of Thera-
peutics, 10th ed.; McGraw-Hill Press: New York, 2001; Chapter
20.
(5) van Dyck, C. H.; Tan, P.-Z.; Baldwin, R. M.; Amici, L. A.; Garg,
P. K.; et al. PET quantification of 5-HT2A receptors in the human
brain: a constant infusion paradigm with [18F]altanserin. J .
Nucl. Med. 2000, 41, 234-241.
(6) Ismaiel, A. M.; Arruda, K.; Teitler, M.; Glennon, R. A. Ketanserin
analogues: Effect of structural modification on 5-HT2 serotonin
receptor binding. J . Med. Chem. 1995, 38, 1196-1202.
(7) J anssen, P. A. Pharmacology of potent and selective S2-seroton-
ergic antagonists. J . Cardiovasc. Pharmacol. 1985, 7, S2-S11.
(8) Lundkvist, C.; Halldin, C.; Ginovart, N.; Nyberg, S.; Swahn, C.
G.; et al. [11C]MDL 100 907, a radioligand for selective imaging
of 5-HT2A receptors with positron emission tomography. Life Sci.
1996, 58, 187-192.
Typically, each test agent was initially screened at 1 and
10 µM. If inhibition of radioligand binding was g30% at 1 µM,
at least 6 concentrations were tested further to bracket the
approximate IC50. Resulting concentration-inhibition data
were used to compute IC50 ( SE with the ALLFIT program14
adapted for the Macintosh microcomputer, following the initial
determination of the affinity (apparent Kd) of each radioligand
in each assay system using the LIGAND program.15 Both
programs were generously provided by Drs. P. J . Munson and
D. Rodbard of the NIH Biostatistical and Computing Center.
Values of IC50 were then converted to Ki by the Chang and
Prusoff relationship,16 Ki ) IC50/(1 + radioligand concentration/
radioligand Kd), all as described in detail previously.17
Selected compounds showing relatively high affinity (Ki )
10 nM) at 5-HT2A receptors in the initial screening with
transfected cell membranes were tested further for receptor
selectivity by comparing their affinities at other 5-HT receptor
types. Selectivity is defined as the ratio of Ki values for each
test site versus the 5-HT2A receptor. The following radio-
(9) Lemaire, C.; Cantineau, R.; Guillaume, M.; Plenevaux, A.;
Christiaens, L. Fluorine-18 altanserin: a radioligand for the
study of serotonin receptors with PET: radiolabeling and in vivo
biologic behavior in rats. J . Nucl. Med. 1991, 32, 2266-2272.
(10) Mathis, C. A.; Huang, Y.; Simpson, N. R.; Mahmood, K.; Gerdes,
J . M.; et al. [11C]MDL 100 907: a potent and selective antagonist
of 5-HT2A receptors labeled with 11C at two different positions.
J . Labelled Compd. Radiopharm. 1996, 37, 316-317.
(11) Sugimaoto, H. N.; Takahiro; Ueno, M.; Yamatsu, K. Preparation
of N-containing heterocycles for treatment of cerebral disorders.
PCT Int. Appl. J P WO8802365, 1988.
(12) Still, W. C.; Kahn, M.; Mitra, A. Rapid chromatographic tech-
nique for preparative separations with moderate resolution. J .
Org. Chem. 1978, 43, 2923-2925.
(13) Tan, P.-Z.; Baldwin, R. M.; Fu, T.; Charney, D. S.; Innis, R. B.
Rapid synthesis of 18F and 2H dual-labeled altanserin, a meta-
bolically resistant PET ligand for 5-HT2A receptors. J . Labelled
Compd. Radiopharm. 1999, 42, 457-467.
(14) DeLean, A. M.; Rodbard, D. Simultaneous analysis of families
of sigmoidal curves: application to bioassay, radioligand assay,
and physiological dose-response curves. Am. J . Physiol. 1978,
235, 97-102.

2324 J ournal of Medicinal Chemistry, 2002, Vol. 45, No. 11
Fu et al.
(15) Munson, P.; Rodbard, D. LIGAND: A versatile computerized
approach for characterization of ligand binding systems. Anal.
Biochem. 1980, 107, 220-239.
(16) Cheng, Y.; Prusoff, W. Relationship between the inhibition
constant (Ki) and the concentration of inhibitor which causes
50% inhibition (IC50) of an enzymatic reaction. Biochem. Phar-
macol. 1973, 22, 3099-3108.
(17) Baldessarini, R. J .; Marsh, E. R.; Kula, N. S. Interactions of
fluoxetine with metabolism of dopamine and serotonin in rat
brain regions. Brain Res. Bull. 1992, 579, 152-156.
(18) Roth, B. L.; Craigo, S. C.; Choudhary, M. S.; Uluer, A.; Monsma,
F. J ., J r.; et al. Binding of typical and atypical antipsychotic
agents to 5-hydroxytryptamine-6 and 5-hydroxytryptamine-7
receptors. J . Pharmacol. Exp. Ther. 1994, 268, 1403-1410.
(19) Baldessarini, R. J .; Kula, N. S.; Campbell, A.; Bakthavachalam,
V.; Yuan, J .; et al. Prolonged D2 antidopaminergic activity of
alkylating and nonalkylating derivatives of spiperone in rat
brain. Mol. Pharmacol. 1992, 42, 856-863.
(20) Kula, N. S.; Baldessarini, R. J .; Kebabian, J . W.; Bakthavacha-
lam, V.; Xu, L. X. RBI-257-a potent dopamine D4 receptor-
selective ligand. Eur. J . Pharmacol. 1997, 331, 333-336.
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