Design, synthesis, and structure-affinity relationships of regioisomeric N-benzyl alkyl ether piperazine derivatives as σ-1 receptor ligands

Article abstract of DOI:10.1021/jm100639f

A series of N-(benzofuran-2-ylmethyl)-N′-benzylpiperazines bearing alkyl or fluoroalkyl aryl ethers were synthesized and evaluated at various central nervous system receptors. Examination of in vitro σ₁ {[³H](+)-pentazocine} and σ₂ ([³H]DTG) receptor binding profiles of piperazines 11-13 and 25-36 revealed several highly potent and σ₁ selective ligands, notably, N-(benzofuran-2- ylmethyl)-N′-(4′-methoxybenzyl)piperazine (13, K_i = 2.7 nM, σ₂/σ₁ = 38) and N-(benzofuran-2-ylmethyl)- N′-(4′-(2″-fluoroethoxy)benzyl)piperazine (30, K_i = 2.6 nM, σ₂/σ₁ = 187). Structural features for optimal σ₁ receptor affinity and selectivity over the σ₂ receptor were identified. On the basis of its favorable log D value, 13 was selected as a candidate for the development of a σ₁ receptor positron emission tomography radiotracer. [ ¹¹C]13 showed high uptake in the brain and other σ receptor-rich organs of a Papio hamadryas baboon. The in vivo evaluation of [¹¹C]13 indicates that this radiotracer is a suitable candidate for imaging the σ₁ receptor in neurodegenerative processes.

Full text of DOI:10.1021/jm100639f

6228 J. Med. Chem. 2010, 53, 6228–6239
DOI: 10.1021/jm100639f
Design, Synthesis, and Structure-Affinity Relationships of Regioisomeric N-Benzyl Alkyl Ether
Piperazine Derivatives as σ-1 Receptor Ligands
Iman A. Moussa,† Samuel D. Banister,† Corinne Beinat,† Nicolas Giboureau,‡,§ Aaron J. Reynolds,† and Michael Kassiou*,†,‡,§
§
†School of Chemistry, and ‡Discipline of Medical Radiation Sciences, University of Sydney, NSW 2006 Australia, and Brain and Mind Research
Institute, Sydney, NSW 2050 Australia
Received May 26, 2010
A series of N-(benzofuran-2-ylmethyl)-N0-benzylpiperazines bearing alkyl or fluoroalkyl aryl ethers
were synthesized and evaluated at various central nervous system receptors. Examination of in vitro σ1
{[3H](þ)-pentazocine} and σ2 ([3H]DTG) receptor binding profiles of piperazines 11-13 and 25-36
revealed several highly potent and σ1 selective ligands, notably, N-(benzofuran-2-ylmethyl)-N0-
(40-methoxybenzyl)piperazine (13, Ki = 2.7 nM, σ2/σ1 = 38) and N-(benzofuran-2-ylmethyl)-N0-(40-
(200-fluoroethoxy)benzyl)piperazine (30, Ki = 2.6 nM, σ2/σ1 = 187). Structural features for optimal
σ1 receptor affinity and selectivity over the σ2 receptor were identified. On the basis of its favorable log D
value, 13 was selected as a candidate for the development of a σ1 receptor positron emission tomography
radiotracer. [11C]13 showed high uptake in the brain and other σ receptor-rich organs of a Papio
hamadryas baboon. The in vivo evaluation of [11C]13 indicates that this radiotracer is a suitable
candidate for imaging the σ1 receptor in neurodegenerative processes.
Introduction
about the nature of σ2 signaling. Like σ1 receptors,19 the
primary role of σ2 receptors is thought to involve the main-
tenance of Ca2þ homeostasis.20 Although σ receptors are
linked to a variety of signaling pathways, the precise trans-
duction mechanisms are complex and have not been fully
elucidated. In the CNS, σ receptors are known to modulate
several classical neurotransmitter systems, including those of
acetylcholine,21-23 dopamine,24-27 glutamate,28,29 5-hydroxy-
tryptamine,30,31 and norepinephrine,32-34 accounting for their
implication in a diverse spectrum of CNS disorders.35-37
Initial interest in σ receptors as therapeutic targets was
largely motivated by their high affinity for a range of clinically
used typical antipsychotics, such as haloperidol (1, Figure 1).38
The antipsychotic effect of functional antagonists of σ recep-
tors is well-known, and several σ ligands have entered clinical
trials for the treatment of schizophrenia.39 Similarly, the
finding that a number of structurally and pharmacologically
diverse antidepressants, including fluvoxamine (2) and imi-
pramine (3), also interact with σ receptors led to the investiga-
tion of σ receptors as a novel therapeutic target for the
treatment of affective disorders.40,41 Implication of σ receptors
in anxiety and depression is widely accepted and has been
comprehensively reviewed.42-44 Widely abused psychostimu-
lants, like cocaine (4) and methamphetamine (5), also interact
with σ receptors at physiologically relevant concentrations,
and it is believed that certain behavioral effects, addictive
properties, and the toxicity of these drugs involve their activity
at σ receptors.45-49 Dozens of functional σ receptor antago-
nists have been shown to attenuate 4-induced locomotor
stimulation, convulsion, and lethality, thereby identifying σ
receptors as an important target for the development of novel
anti-4 therapies.46,47,49
σ receptors were mistakenly identified as an opioid receptor
subtype over 30 years ago.1 The investigation of σ receptors
with nonselective ligands led to a decade of confusion as σ
receptors were erroneously reclassified as the phencyclidine/
N-methyl-D-aspartate receptor complex.2 Currently, σ recep-
tors are recognized as a distinct class of mammalian protein,
widely distributed in the central nervous system (CNS)a and
peripheral organs, comprised of two pharmacologically well-
defined subtypes: σ1 and σ2.3 The σ1 receptor has been cloned
from numerous sources, including human brain tissue, and
shows no similarity to any other mammalian protein.4,5 Very
recently, the σ2 receptor was identified as a member of the
histone protein family, but it has not been cloned.6,7 Crystal
structure data do not exist for either σ receptor subtype. An
endogenous ligand for σ receptors has not been definitively
identified; however, various neurosteroids, particularly proges-
terone, as well as the hallucinogenic trace amine, N,N-dimethyl-
tryptamine (DMT), have been proposed to fulfill this role.8-11
It is believed that σ1 receptors reside primarily at the interface
between the endoplasmic reticulum (ER) and the mitochon-
drion, the mitochondria-associated ER membrane (MAM),12
and modulate Ca2þ efflux from the ER by acting as mole-
cular chaperones of inositol (1,4,5)-triphosphate receptors.13,14
However, σ1 receptors are also known to translocate to the
plasma membrane where they regulate voltage-dependent
Ca2þ channels and Kþ channels.15-18 Much less is known
*To whom correspondence should be addressed. Tel: þ61-2-9351-
0849. Fax: þ61-2-9351-9146. E-mail: michael.kassiou@sydney.edu.au.
a Abbreviations: CNS, central nervous system; DMF, N,N-dimethyl-
formamide; EDC HCl, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide
3
Numerous human tumor cell lines overexpress σ recep-
tors,50-52 and many σ ligands possess antiproliferative acti-
vity.53-55 Indeed, σ2 agonists have been shown to activate
hydrochloride; ER, endoplasmic reticulum; HOBt, N-hydroxybenzotria-
zole; NMM, N-methylmorpholine; PET, positron emission tomography;
RT, room temperature; TBAH, tetra-n-butylammonium hydroxide.
r
pubs.acs.org/jmc
Published on Web 07/28/2010
2010 American Chemical Society

Article
Journal of Medicinal Chemistry, 2010, Vol. 53, No. 16 6229
Figure 1. Examples of psychoactive drugs known to interact with σ receptors.
Figure 2. Examples of σ-selective arylalkyl piperazine ligands.
apoptosis and potentiate the effects of other antineoplastic
agents.56 Despite a limited understanding of the precise mecha-
nistic role of σ receptors in cancer,57 there is already much
interest in σ receptors as cancer-associated biomarkers.58 The
design of selective σ ligands that are amenable to radiolabeling
with carbon-11 or fluorine-18 would provide cancer imaging
agents for use in the noninvasive technique, positron emission
tomography (PET).59
The relative paucity of truly selective σ ligands, especially
subtype-specific σ ligands, remains an impediment to σ re-
ceptor research. Consideration of the heterogeneity of known
σ ligands underscores the difficulty of establishing a compre-
hensive σ receptor pharmacophore to aid the rational design
of novel σ receptor probes. A model for σ1 binding has been
proposed; however, it is too simplistic to guide the rational
design of high-affinity, selective σ1 ligands.60-63 Recently,
several three-dimensional quantitative structure-activity re-
lationship (3D-QSAR) analyses of σ2 binding, each pertain-
ing to a particular structural class of σ2 ligands, were pub-
lished.64-66 Thebroadapplicability and utilityof thesemodels
to the rational design of selective σ ligands have not been
demonstrated.
Work in our laboratory has focused on the development of
increasingly selective σ ligands utilizing a polycarbocyclic
scaffold, to elucidate the requisite structural features of σ
receptor binding.67 Additionally, we aim to develop σ ligands
with potential therapeutic68 and diagnostic applications. We
report here the structure-affinity relationships of a series of
novel arylalkyl piperazines as selective σ ligands and the
development of a novel, carbon-11-labeled σ1 ligand as a σ
receptor imaging agent for use in PET.
The lead compound for this study was identified from a
series of disubstituted 1,4-piperazines, flanked by a chromene
ring and a benzyl group, reported by Baziard-Mouysset and
co-workers.69 The simplest member of this series, 6 (Figure 2),
containing an unsubstituted benzyl ring, displayed high affi-
nity for σ receptors (Ki = 3 nM) and negligible off-target
activity(5HT1A, Ki > 10μM;D2, Ki = 10μM).69 Substitution
of the benzyl ring was generally detrimental to σ binding, with
the exception of 4-chloro (7) or 4-methoxy substitution (8),
which furnished improved σ ligands (Ki = 1 and 0.6 nM,
respectively). By contrast, the chromene ring of 6 was shown
to have little effect on σ binding, and substitution with a large
number of alternate aromatic groups was well tolerated.70
In particular, substitution with a naphthylmethyl (9) or
2-benzofurylmethyl group (10) conferred improved σ affinity
(IC50 = 0.6 and 0.4 nM, respectively).70 Unfortunately, no
data are available regarding the subtype selectivity of the
above compounds.
From the previously reported data, 4-methoxy substitution
of the benzyl ring of 10 should be tolerated and could be
expected to enhance σ affinity. Utilizing 10 as a lead com-
pound, 2-, 3-, and 4-methoxybenzyl analogues were generated
to explore the effect of aromatic substitution on σ receptor
affinity and subtype selectivity. In addition to potential
therapeutic activity, methoxy-substituted σ ligands would
allow the development of 11C-radiolabeled analogues for use
as diagnostic probes in PET imaging in vivo.
To investigate the effect of steric augmentation of the
benzyl substituent on σ receptor affinity and selectivity within
this class, regioisomeric ethyl and propyl benzyl ethers were
synthesized. To probeelectronic factorsinfluencingσ binding,
a fluorine atom was introduced to the terminal position of the
alkyl chain in both the ethyl and the propyl benzyl ether series.
These compounds could also be labeled with fluorine-18 to
enable further in vivo pharmacological evaluation.
Chemistry
Piperazines 10-13 were prepared according to the synthetic
method shown in Scheme 1. Monoprotection of 1,4-piperazine
(14) with di-tert-butyl dicarbonate gave rise to N-Boc piper-
azine (15).71-73 Subjecting 15 to amide coupling with benzo-
furan-2-carboxylic acid gave 16. Cleavage of the Boc protecting
group under acidic conditions, to afford the hydrochloride salt
17, was followed by aluminum hydride reduction to furnish
amine 18 in 51% yield over four steps. Lead compound 10 and
substituted methoxybenzyl piperazines 11-13 (identified as
potential candidates for 11C radiolabeling) were successfully

6230 Journal of Medicinal Chemistry, 2010, Vol. 53, No. 16
Moussa et al.
Scheme 1. Synthesis of N-Benzyl Piperazine Derivative 10 and N-(Methoxybenzyl) Piperazine Derivatives 11-13a
a Reagents and conditions: (a) Boc2O, CH2Cl2, RT, 22 h, 81%. (b) Benzofuran-2-carboxylic acid, NMM, EDC HCl, HOBt, DMF, RT, 20 h, 84%.
3
(c) 4 M HCl in dioxane, RT, 0.5 h, quantitative. (d) AlH3, 0 °C, 2 h, 75%. (e) Appropriate benzyl halide, K2CO3, NaI, DMF, reflux, 20 h, 81-84%.
Scheme 2. Synthesis of N-(Hydroxybenzyl) Piperazine Deriva-
tives 19-21a
The σ receptor affinity and subtype selectivity of un-
substituted lead structure 10 served as a point of comparison
for the newly synthesized compounds, and several structure-
activity relationship (SAR) trends were identified. Like parent
compound 10, most analogues within this series showed a
distinct preference for the σ1 receptor. Compounds 13, 27, 30,
33, and 36, featuring substitution at the 40-position, all showed
high subtype selectivity for σ1 over σ2 receptors (37-187-fold)
and, with the exception of the 4-propyl benzyl ether derivative
33, high affinity for σ1, with Ki values in the low nanomolar
range (Table 1). The most potent compounds in this series
were the 40-methyl benzyl ether (13) and 40-(200-fluoroethyl)
benzyl ether (30) derivatives, with σ1 Ki values of 2.7 and
2.6 nM, respectively. Compound 30 displayed greater σ1 selec-
tivity(σ2/σ1 =187, the greatestσ subtype selectivitywithin the
series) than 13, making it an ideal candidate as an 18F-labeled
radiotracer in PET imaging studies.
As compared to their 40-substituted analogues, compounds
12, 26, 29, 32, and 35, featuring substitution at the 30-position,
all showed substantially reduced σ1 affinity but less pro-
nounced attenuation of σ2 affinity (Table 2). Consequently,
all 30-substituted analogues displayed moderate to poor levels
of σ1 versus σ2 selectivity.
Compounds 11 and 28, featuring substitution at the
20-position, showed essentially no subtype selectivity,
although the 20-ethyl benzyl ether 25 and 20-propyl benzyl
ether 31 derivatives retained moderate affinity and modest
selectivity for σ1 receptors (Table 3). Compounds 11 and 28
did, however, display the highest affinity at the 5-HT2B
receptor (Ki = 9.7 and 57 nM, respectively), making them
potential lead molecules for the development of selective
5-HT2B drugs. The 20-(300-fluoropropyl) benzyl ether deriva-
tive 34 had a high affinity for σ1 (Ki = 9.1 nM) and good
subtype selectivity (σ2/σ1 = 60), a binding profile comparable
to the 40-substituted analogues within the series.
a Reagents and conditions: (a) Appropriate hydroxybenzaldehyde,
NaBH(OAc)3, ClCH2CH2Cl, RT, 4 h, 83-85%.
prepared from 18 by N-alkylation with the appropriate benzyl
halides.
The desired ethers 25-33 could be prepared by alkylation
of the appropriate regioisomeric phenolic precursors. Reduc-
tive amination of 18 with the appropriate hydroxybenzal-
dehyde in the presence of sodium triacetoxyborohydride pro-
vided the requisite phenols 19-21 in good yield and on a
multigram scale (Scheme 2).
Alkylation of the regioisomeric phenols 19-21 with tosy-
lates 22-24, derived from ethanol, 2-fluoroethanol, and
propanol, respectively, afforded the corresponding N-benzyl
alkyl ether derivatives 25-33 (Scheme 3).
Similarly, the 20-, 30-, and 40-(300-fluoropropyl) benzyl ether
derivatives 34-36 (Scheme 4) were prepared from the corre-
sponding phenols 19-21 by alkylation with 3-fluoropropyl
tosylate (37), accessible from commercially available 1,3-pro-
panediol via monotosylation and deoxyfluorination.
Results and Discussion
Newly synthesized compounds 10-13 and 25-36 were
subjected to in vitro binding assays against a comprehensive
CNS receptor panel. Ki determinations and receptor binding
profiles were provided by the National Institute of Mental
Health’s Psychoactive Drug Screening Program (NIMH
PDSP). Affinities at σ1 and σ2 receptors were determined
using a competitive modification of the protocol reported by
Kovacs and Larson.74 Competitive displacement of [3H](þ)-
pentazocine in a rat brain homogenate preparation was used
to determine σ1 receptor affinity for all compounds screened.
The affinity of test compounds for σ2 receptors was deter-
mined by competitive displacement of [3H]1,3-di-(2-tolyl)-
guanidine ([3H]DTG) from a PC12 cell preparation, a cell
line known to overexpress σ2 receptors, summarized in
Tables 1-3.
On the basis of these results, it is possible to make some
tentative predictions regarding the requirements for σ1 ligand
binding and subtype selectivity. Alkyl ethers up to two carbon
atoms long at the 40-position appear to be well tolerated at the
σ1 binding site. The 40-substituted derivatives containing a
terminal fluorine atom show greater σ1 affinity than their
nonfluorinated counterparts, suggesting that an electrostatic
interaction may be important. In general, substitution at
either the 20- or the 30-position of the N-benzyl piperazine
portion of molecule 10 led to poor subtype selectivity with the

Article
Journal of Medicinal Chemistry, 2010, Vol. 53, No. 16 6231
Scheme 3. Synthesis of Ethyl-, (200-Fluoroethyl)-, and Propyl Aryl Ether Derivatives 25-33a
a Reagents and conditions: (a) TsCl, pyridine, CH2Cl2, 0 °C to RT, 3 h, 76-95%. (b) Appropriate phenol (19-21), K2CO3, DMF, reflux, 16 h,
60-73%.
Scheme 4. Synthesis of (300-Fluoropropyl) Aryl Ether Derivatives 34-36a
a Reagents and conditions: (a) TsCl (1.1 equiv), pyridine (1.1 equiv), CH2Cl2, 0 °C to RT, 4 h, 65%. (b) Deoxo-Fluor, CH2Cl2, RT, 24 h, 60%.
(c) Appropriate phenol (19-21), K2CO3, DMF, reflux, 16 h, 61-63%.
Table 1. Affinities for 40-Substituted N-Benzyl Piperazine Derivatives
Ki ( SEM (nM)a
b
compd
R
5-HT1A
5-HT2B
D2
σ1
σ2
σ2/σ1
10
13
27
30
33
36
H
OCH3
OCH2CH3
833 ( 130
2192 ( 395
3339 ( 624
2439 ( 363
4722 ( 749
3300 ( 514
497 ( 51
15 ( 2
262 ( 37
96 ( 13
655 ( 66
232 ( 23
>10000
2700 ( 295
4868 ( 982
>10000
5.2 ( 0.8
2.7 ( 0.4
10.6 ( 2.4
2.6 ( 0.6
53.8 ( 7.7
14.7 ( 2.1
240 ( 47
103 ( 18
393 ( 93
486 ( 151
2401 ( 669
752 ( 175
46
38
37
OCH2CH2F
O(CH2)2CH3
O(CH2)2CH2F
187
45
>10000
>10000
51
a Affinities were determined in rat brain homogenates using [3H](þ)-pentazocine for σ1 and PC12 cells using [3H]DTG for σ2. The values in this table
represent the means ( SEMs from triplicate assays. b This value is derived from the Ki for σ2 binding affinity divided by the Ki for σ1 binding affinity.
Values >1 indicate selectivity for σ1 over σ2.
Table 2. Affinities for 30-Substituted N-Benzyl Piperazine Derivatives
Ki ( SEM (nM)a
b
compd
R
5-HT1A
5-HT2B
D2
σ1
σ2
σ2/σ1
10
12
26
29
32
35
H
OCH3
OCH2CH3
833 ( 130
373 ( 31
1720 ( 258
469 ( 61
774 ( 110
599 ( 35
497 ( 51
28 ( 3
302 ( 27
244 ( 25
259 ( 28
188 ( 20
>10000
5728 ( 905
4542 ( 773
2281 ( 387
2977 ( 425
2920 ( 259
5.2 ( 0.8
16.3 ( 2.4
80 ( 13
49.7 ( 7.8
177 ( 28
232 ( 42
240 ( 47
79.5 ( 11.4
238 ( 47
292 ( 67
1919 ( 545
2371 ( 642
46
4.9
3.0
5.9
11
OCH2CH2F
O(CH2)2CH3
O(CH2)2CH2F
10
a Affinities were determined in rat brain homogenates using [3H](þ)-pentazocine for σ1 and PC12 cells using [3H]DTG for σ2. The values in this table
represent the means ( SEMs from triplicate assays. b This value is derived from the Ki for σ2 binding affinity divided by the Ki for σ1 binding affinity.
Values >1 indicate selectivity for σ1 over σ2.
notable exception of the 20-(300-fluoropropyl) benzyl ether
derivative 34. As compared to the lead compound 10, the
40-(200-fluoroethyl) benzyl ether derivative 30 shows a 2-fold
increase in σ1 receptor affinity, a 4-fold increase in σ1 versus σ2
receptor selectivity, and a moderate affinity for the 5-HT2B
receptor subtype (Ki = 96 nM). Among the methyl benzyl

6232 Journal of Medicinal Chemistry, 2010, Vol. 53, No. 16
Table 3. Affinities for 20-Substituted N-Benzyl Piperazine Derivatives
Moussa et al.
Ki ( SEM (nM)a
b
compd
R
5-HT1A
5-HT2B
D2
σ1
σ2
σ2/σ1
10
11
25
28
31
34
H
OCH3
OCH2CH3
833 ( 130
806 ( 141
468 ( 50
3251 ( 605
2458 ( 215
3383 ( 468
497 ( 51
9.7 ( 1
72 ( 8
57 ( 4
116 ( 8
184 ( 20
>10000
5122 ( 1338
6005 ( 865
>10000
5.2 ( 0.8
198 ( 29
37.1 ( 7.8
488 ( 83
79 ( 12
240 ( 47
89 ( 12
295 ( 54
535 ( 101
778 ( 134
542 ( 168
46
0.45
8.0
1.1
10
OCH2CH2F
O(CH2)2CH3
O(CH2)2CH2F
>10000
>10000
9.1 ( 1.6
60
a Affinities were determined in rat brain homogenates using [3H](þ)-pentazocine for σ1 and PC12 cells using [3H]DTG for σ2. The values in this table
represent the means ( SEMs from triplicate assays. b This value is derived from the Ki for σ2 binding affinity divided by the Ki for σ1 binding affinity.
Values >1 indicate selectivity for σ1 over σ2.
Table 4. Summary of In Vitro σ1 and σ2 Binding Affinity and Lipophi-
licity Properties of 13
Scheme 5. Synthesis of Radiolabeled Ligand [11C]13 from the
Desmethyl Precursor 21a
Ki (nM)
σ1
σ2
σ2/σ1
log D
2.7
103
38
3.63
a Reagents and conditions: (a) [11C]CH3I, TBAH, DMF, RT for
ether analogues 11-13, the 40-methyl benzyl ether derivative
13 showed the greatest σ1 affinity (Ki = 2.7 nM, comparable
to 30, a lead compound for fluorine-18 radiolabeling) and
good subtype selectivity (σ2/σ1 = 38), indicating that it might
be a suitable candidate for carbon-11 radiolabeling at the
methyl ether position. This compound also showed high
affinity for the 5-HT2B receptor subtype (Ki = 15 nM). Prior
to attempting the synthesis of [11C]13 for subsequent mole-
cular imaging in a living system, it was necessary to examine
the lipophilicity of 13.
2 min, 80 °C for 5 min, followed by HPLC purification (XTerra C18).
activity of 73 GBq/μmol. No attempts were made to optimize
these conditions, as sufficient quantities of the radioligand
wereproduced to enable preliminary pharmacological evalua-
tion. Formulation of radiolabeled product for intravenous
injection was achieved following: (i) evaporation of the HPLC
mobile phase, (ii) reconstitution of the residue in water for
injections (5 mL), and (iii) sterile filtration through a 0.22 μm
filter. The final injectable solution was clear and colorless,
with a pH of 7.0. The preparation was free from labeling
precursor 21. Administration to the animal was performed
within 10 min following the end of synthesis.
Lipophilicity Studies
Compounds having log D values >4.0 generally display
a substantial nonspecific binding, reducing the target to
nontarget ratios.75 Using high-performance liquid chromato-
graphy (HPLC), a calibration curve correlating known log D
values with the retention times of several reference compounds
was obtained, and the log D value for piperazine 13 was
determined from its average retention time over three runs.76,77
Taking into consideration the in vitro binding data and the
lipophilicity estimations (Table 4), ligand 13 appears to be a
suitable candidate for the in vivo imaging of σ1 receptors.
Evaluation Using PET
PET studies were conducted with an anaesthetized Papio
hamadryas baboon to evaluate the in vivo regional distribu-
tion kinetics. Dynamic PET brain imaging commenced just
prior to intravenous administration of [11C]13 (100 MBq in
3 mL of saline; specific activity 73 GBq/μmol) and was
terminated 60 min postinjection. This was immediately fol-
lowed by a whole body acquisition for 2 min at each of the six
bed positions. The PET images confirmed the ability of
[11C]13 to traverse the blood-brain barrier with considerable
accumulation of radioactivity in the baboon brain (Figure 3).
Second, the in vivo specificity and selectivity of [11C]13 in the
baboon brain were assessed 3 weeks after initial PET evalua-
tion via a blocking study using 1 (1 mg/kg, iv).
Radiochemistry and PET Studies
Phenol 21 was labeled with carbon-11 (t1/2 = 20.4 min) to
give [11C]13 in 22% nondecay corrected radiochemical yield,
using [11C]methyl iodide ([11C]CH3I) by standard O-alkyla-
tion (Scheme 5). [11C]CH3I was synthesized from cyclotron-
produced [11C]carbon dioxide ([11C]CO2), trapped in a N,N-
dimethylformamide (DMF) solution containing 21 and tetra-
n-butylammonium hydroxide (TBAH). This mixture was
allowed to react at roomtemperature (RT) for 2 min, followed
by heating at 80 °C for 5 min. The reaction mixture was
purified by reverse phase semipreparative HPLC in an aver-
age synthesis time of 30 min (including HPLC purification
and formulation). Coinjection of nonradioactive 13 was
performed using analytical HPLC to confirm the identity of
the product. In the final product solution, both radiochemical
and chemical purities were greater than 98%, with a specific
Time-activity curves for the uptake of [11C]13 in the
baboon brain (Figure 4) showed homogeneous uptake of
[11C]13 in the cingulate cortex, frontal cortex, striatum,
thalamus, and cerebellum, which are responsible for motor
function and known to contain σ-receptors.78,79 Brain uptake
of [11C]13 reached maximal levels 5 min postinjection and
remained at a plateau to the end of the scan. Whole body
images of the baboon (Figure 6A), acquired at the conclusion
of dynamic screening, clearly show uptake of [11C]13 by the
brain, as well as organs outside the CNS. In particular, there

Article
Journal of Medicinal Chemistry, 2010, Vol. 53, No. 16 6233
Figure 6. PET images of whole P. hamadryas baboon brain during
60 min after intravenous injection of [11C] (baseline; A) and [11C]13
plus 1 (1 mg/kg; B).
Figure 3. P. hamadryas baboon PET summation images of selected
transaxial brain slices over 60 min.
of [11C]13 in the kidneys and bladder, indicative of metabo-
lism and excretion of the radioligand. Complete displacement
of [11C]13 binding in the aforementioned CNS and PNS
regions demonstrates the ease of displacing [11C]13 and
denotes specific σ1 binding outside the brain.
Conclusion
Wehave developeda seriesofpotentand subtype selectiveσ
receptor ligands, based on lead structure 10. Exploration of
benzylic substitution of 10 led us to synthesize regioisomeric
benzylic piperazines bearing alkyl and fluoroalkyl aryl ethers
(11-13 and 25-36). The σ receptor affinities of these regio-
isomers revealed that aryl ethers at the 40-position are generally
well tolerated at σ1, with comparable subtype selectivity
to that of 10. In the case of 40-substituted aryl alkyl ethers,
a terminal fluorine atom on the alkyl chain enhances both σ1
affinity and σ1 subtype selectivity, as compared to the respec-
tive nonfluorinated analogues. Within this series, the 40-(200-
fluoroethoxy) derivative 30 was identified as a high-affinity σ1
receptor ligand (Ki = 2.6 nM), with good subtype selectivity
(σ2/σ1 = 187), and negligible affinity for other CNS receptors
except the 5-HT2B receptor for which it shows moderate
affinity (Ki = 96 nM).
Figure 4. Time-activity curves depicting uptake of [11C]13 in
selected regions of the P. hamadryas baboon brain during a 60 min
PET scan.
The 40-methoxy analogue from this class (13), also a highly
selective σ1 ligand, was identified as a candidate for carbon 11
radiolabeling to produce a σ1 receptor PET imaging agent.
Preparation of [11C]13 was achieved in good radiochemical
yield, and thiscompound was showntocrossthe blood-brain
barrier and bind to σ1 receptors in baboon brain and periph-
eral organs in vivo. The pharmacological profile of [11C]13
makes it a useful probe for evaluating the roles of σ1 receptors
in neurodegenerative processes in vivo.
Figure 5. Time-activity curves depicting uptake of [11C]13 pre-
treated with 1 in selected regions of the P. hamadryas baboon brain
during a 60 min PET scan.
In conclusion, we have developed a series of N-benzyl alkyl
ether piperazine derivatives as high affinity and subtype
selective σ1 receptor ligands, which warrant further investiga-
tion into their potential therapeutic applications for the
treatment of CNS disorders. This class of compounds can
also serve as diagnostic probes when radiolabeled to not only
elucidate the roleof σ receptors in the onset and progression of
disease but also to monitor the efficacy of therapeutic regimes.
was substantial uptake of [11C]13 by the liver, which is known
to contain a high density of σ1 receptors.
Pretreatment of the baboon with 1 5 min prior to [11C]13
injection resulted in an initial increase in [11C]13 uptake 3 min
postinjection, followed by washout. There was a reduced
uptake of 80% of the radioligand in all regions of the brain,
which was maintained to the end of the scan (Figure 5).
Similar results have been reported for other 1 blocking
studies.80-83 Figure 6B shows the whole body images of the
baboon, which were acquired at the conclusion of dynamic
screening. These images demonstrate significant inhibition of
[11C]13 uptake in the brain and liver, as well as accumulation
Experimental Section
Chemistry. Melting point determinations were carried out on
an Optimelt Automated Melting Point System and are recorded
uncorrected. 1H NMR spectra were acquired at 300 K using a

6234 Journal of Medicinal Chemistry, 2010, Vol. 53, No. 16
Moussa et al.
Bruker Avance DP X300 (300 MHz) as solutions in D2O
or CDCl3, where residual H2O (δ 4.79 ppm) and CHCl3
(δ 7.26 ppm) were used as respective internal references. Data
are reported as chemical shifts (δ ppm), relative integral, multi-
plicity (s = singlet, br s = broad singlet, d = doublet, dd =
doublet of doublets, dt = double of triplets, t = triplet, m =
multiplet, and dm = doublet of multiplets), and coupling
constants (J, Hz). Solvent peaks for chloroform (δ 77.16) were
used as the internal reference for 13C NMR spectra. Residual
acid in deuterated chloroform was removed prior to use by
filtering through a pad of basic alumina. Low-resolution mass
spectra were recorded on a Finnigan LCQ mass spectrometer.
High-resolution mass spectra were recorded by the Mass Spec-
trometry Unit of the School of Chemistry, University of New
South Wales. Positive electrospray ionization (þESI) utilized
methanol and/or dichloromethane. The purity of all new com-
pounds was g95% based on C, H, and N analyses, conducted at
the Campbell Microanalytical Laboratory, Chemistry Depart-
ment at University of Otago, New Zealand. Analytical thin layer
chromatography was carried out using aluminum-backed silica
gel 60 F254 Merck KGaA sheets, and plates were visualized
under a UV lamp at λ = 254 nm. Flash chromatography
employed Merck Kieselgel 60 (230-400 mesh) silica gel. Solvents
for flash column chromatography were distilled prior to use.
Tetrahydrofuran was dried over sodium wire and distilled from
benzophenone. Dichloromethane was distilled from calcium hy-
dride. DMF was distilled from calcium hydride and stored over
80:20 f 75:25 f 70:30) afforded the title compound as a color-
less solid (6.82 g, 84%); mp 97-98 °C; Rf 0.19 (hexanes:EtOAc,
70:30). 1H NMR (300 MHz, CDCl3): δ 1.49 (9H, s), 3.55 (4H, t,
J = 8.6 Hz), 3.84 (4H, br s), 7.30 (1H, dd, J = 7.5, 7.5 Hz), 7.35
(1H, s), 7.41 (1H, dd, J = 7.6, 7.8 Hz), 7.53 (1H, d, J = 8.3 Hz),
7.66 (1H, d, J = 7.7 Hz). 13C NMR (75.5 MHz, CDCl3): δ 28.8,
30.1, 44.1, 80.8, 112.3, 112.9, 122.7, 124.1, 127.1, 127.3, 149.2,
155.0, 155.1, 160.3. HRMS (þESI) calcd for C18H22N2O4 m/z:
[M þ Na]þ, 353.1472; found, 353.1464.
Benzofuran-2-yl(piperazin-1-yl)methanone Hydrochloride (17).
tert-Butyl 4-(benzofuran-2-carbonyl)piperazine-1-carboxylate (16)
(6.82 g, 20.7 mmol) was added to a solution of HCl in dioxane
(4 M, 20 mL) and stirred at RT for 30 min. The reaction mixture
was concentrated in vacuo to give the title compound as a colorless
solid (5.52 g, quantitative); mp 234-235 °C. 1H NMR (300 MHz,
D2O): δ 3.42 (4H, t, J = 5.1 Hz), 4.11 (4H, br s), 7.40 (1H, dd, J =
7.3, 7.6 Hz), 7.45 (1H, s), 7.54 (1H, dd, J = 7.2, 8.2 Hz), 7.62 (1H, d,
J = 8.3 Hz), 7.78 (1H, d, J = 7.8 Hz). 13C NMR (75.5 MHz, D2O):
δ 40.5, 43.5, 112.2, 114.1, 123.2, 124.4, 126.8, 128.0, 146.6, 155.0,
161.7. HRMS (þESI) calcd for C13H14N2O2 m/z: [M þ H]þ,
231.1128; found, 231.1128.
N-((Benzofuran-2-yl)methyl)piperazine (18). To a suspension
of LiAlH4 (2.36 g, 66.9 mmol, 3.25 equiv) in anhydrous THF
(216 mL) at 0 °C was added, portionwise, AlCl3 (2.76 g, 20.6
mmol, 1.00 equiv). Stirring was continued at 0 °C for 15 min
prior to the portionwise addition of free base of 17 (4.77 g, 20.6
mmol, 1.00 equiv). The reaction mixture was stirred at 0 °C for
2 h after which time it was quenched by the successive addition of
H2O (2.4 mL), aqueous NaOH (4 M, 2.4 mL), and H2O (7.2 mL).
The resultant mixture was filtered, the filtrate was partitioned
with CH2Cl2 (300 mL), and the aqueous phase was isolated
and further extracted with CH2Cl2 (2 ꢀ 300 mL). The combined
organic fractions were washed with brine, dried (Na2SO4),
and concentrated under reduced pressure. Purification of the crude
residue by flash chromatography on silica gel (CH2Cl2:MeOH:
aqueous NH4OH, 90:10:0.1) afforded the title compound as
a pale yellow solid (3.36 g, 75%); mp 173-174 °C; Rf 0.20
(CH2Cl2:MeOH:aqueous NH4OH, 90:10:0.1). 1H NMR (300
MHz, CDCl3): δ 2.11 (1H, br s), 2.51 (4H, br s), 2.92 (4H, t,
J=9.6Hz), 3.66(2H, s), 6.58(1H, s),7.16-7.26 (2H, m), 7.45-7.52
(2H, m). 13C NMR (75.5 MHz, CDCl3): δ 45.9, 54.3, 56.0, 105.8,
111.3, 120.7, 122.7, 123.9, 128.3, 154.4, 155.1. HRMS (þESI) calcd
for C13H16N2O m/z: [M þ H]þ, 217.1335; found, 217.1331.
General Procedure for N-Alkylation. To a suspension of N-
((benzofuran-2-yl)methyl)piperazine 18 (216 mg, 1.00 mmol)
and anhydrous K2CO3 (552 mg, 4.00 mmol) in DMF (5 mL) at
RT were added NaI (15 mg, 0.1 mmol) and the appropriate
benzyl alkyl halide (1.1 equiv). The reaction mixture was heated
at reflux for 20 h before cooling to ambient temperature. H2O
(100 mL) was added, and the mixture was extracted with EtOAc
(3 ꢀ 60 mL). The combined organic fractions were washed with
brine, dried (Na2SO4), and concentrated under reduced pressure
to give a crude residue, which was purified by flash chromato-
graphy on silica gel (CH2Cl2:MeOH, 99:1).
˚
activated 4 A molecular sieves. Piperazine was recrystallized from
absolute ethanol. Anhydrous K2CO3 was ground and dried at
140 °C in the oven for at least 2 days prior to use. 4-Hydroxyben-
zaldehyde was recrystallized from distilled water. 1,2-Dichloro-
ethane was distilled from and stored over calcium hydride.
Ethanol, n-propanol, and pyridine were distilled from calcium
˚
hydride and stored over activated 3 A molecular sieves. Crude tosyl
chloride (∼10 g) was dissolved in chloroform (25 mL), filtered, and
diluted with hexanes (125 mL) to precipitate impurities. The
solution was then filtered, clarified with charcoal, and concentrated
under reduced pressure to give analytically pure material (∼7 g).
2-Fluoroethanol was purchased from Matrix Scientific and used
without further purification. Remaining reagents were purchased
from Sigma Aldrich Chemical Co. and used without further
purification.
tert-Butyl Piperazine-1-carboxylate (15).71-73 To a solution
of 1,4-piperazine (3.44 g, 39.9 mmol, 2.0 equiv) in CH2Cl2
(100 mL) at RT was added a solution of Boc2O (4.6 mL,
20 mmol, 1.0 equiv) in CH2Cl2 (50 mL) over a period of 3 h
after which the reaction mixture was stirred at RT for 22 h. The
solvent was evaporated, and the residue thus obtained was
treated with H2O (100 mL). The precipitated product was
collected by filtration, and the filtrate was further extracted
with CH2Cl2 (3 ꢀ 100 mL). The combined organic fractions
were dried (Na2SO4) and concentrated in vacuo to give the title
compound as a colorless solid (3.01 g combined yield, 81%); mp
46-47 °C (lit. mp 46-47 °C).71-73 1H NMR (300 MHz, CDCl3):
δ 1.46 (9H, s), 2.42 (1H, s), 2.82 (4H, t, J = 5.1 Hz), 3.40 (4H, t,
J = 5.2 Hz). The spectroscopic data matched that reported in
the literature.71-73
N-((Benzofuran-2-yl)methyl)-N0-benzylpiperazine (10). Treat-
ment of 18 with benzyl bromide according to the method
described above afforded the title compound as a pale yellow
1
tert-Butyl 4-(Benzofuran-2-carbonyl)piperazine-1-carboxylate
(16). To a suspension of 1-ethyl-3-(3-dimethylaminopropyl)-
carbodiimide hydrochloride (EDC HCl) (5.68 g, 29.6 mmol,
oil (251 mg, 82%); Rf 0.42 (CH2Cl2:MeOH, 99:1). H NMR
(300 MHz, CDCl3): δ 2.54 (8H, br s), 3.51 (2H, s), 3.69 (2H, s),
6.58 (1H, s), 7.16-7.37 (7H, m), 7.48 (2H, d, J = 7.9 Hz). 13
C
3
1.20 equiv) and N-methylmorpholine (NMM) (10.8 mL, 98.8
mmol, 4.00 equiv) in DMF (160 mL) were added successively
N-hydroxybenzotriazole (HOBt) (4.00 g, 29.6 mmol, 1.20 equiv),
benzofuran-2-carboxylic acid (4.00 g, 24.7 mmol, 1.00 equiv),
and 15 (4.60 g, 24.7 mmol). The reaction mixture was stirred at
RT for 20 h and then diluted with H2O (400 mL) and extracted
with EtOAc (3 ꢀ 400 mL). The combined organic fractions
were washed with brine, dried (Na2SO4), and concentra-
ted under reduced pressure. Purification of the crude resi-
due by flash chromatography on silica gel (hexanes:EtOAc,
NMR (75.5 MHz, CDCl3): δ 53.0, 53.2, 55.6, 63.1, 105.8, 111.4,
120.8, 122.8, 124.7, 127.2, 127.5, 128.3, 129.4, 139.0, 154.2,
154.6. HRMS (þESI) calcd for C20H22N2O m/z: [M þ H]þ,
307.1805; found, 306.1753.
N-((Benzofuran-2-yl)methyl)-N0-(20-methoxybenzyl)piperazine
(11). Treatment of 18 with 2-methoxybenzyl chloride according
to the method described above afforded the title compound as a
dark yellow oil (270 mg, 81%); Rf 0.39 (CH2Cl2:MeOH, 99:1).
1H NMR (300 MHz, CDCl3): δ 2.60 (8H, br s), 3.61 (2H, s), 3.69
(2H, s), 3.80 (3H, s), 6.58 (1H, s), 6.86 (1H, dd, J = 6.9, 8.2 Hz),

Article
Journal of Medicinal Chemistry, 2010, Vol. 53, No. 16 6235
1
6.92 (1H, d, J = 7.4 Hz), 7.16-7.26 (3H, m), 7.32 (1H, d, J =
7.4 Hz), 7.49 (2H, dd, J = 5.9, 8.8 Hz). 13C NMR (75.5 MHz,
CDCl3): δ 52.8, 53.2, 55.6, 56.0, 56.2, 105.8, 110.6, 111.4, 120.4,
120.8, 121.9, 122.7, 124.0, 128.3, 128.6, 130.8, 153.7, 154.5, 162.5.
HRMS (þESI) calcd for C21H24N2O2 m/z: [M þ H]þ, 337.1911;
found, 337.1909.
(CH2Cl2:MeOH, 95:5). H NMR (300 MHz, CDCl3): δ 2.59
(8H, br s), 3.46 (2H, s), 3.68 (2H, s), 6.57 (2H, br s), 6.60 (1H, s),
7.05 (2H, d, J = 8.4 Hz), 7.15-7.25 (2H, m), 7.41 (1H, d, J = 8.0
Hz), 7.49 (1H, d, J = 7.0 Hz). 13C NMR (75.5 MHz, CDCl3): δ
52.8, 53.2, 55.8, 62.8, 106.5, 111.9, 116.1, 121.3, 123.2, 124.6,
128.2, 128.8, 131.6, 154.5, 155.6, 156.2. HRMS (þESI) calcd for
C20H22N2O2 m/z: [M þ H]þ, 323.1754; found, 323.1765.
General Procedure for Tosylate Synthesis.84 To a solution of
the alcohol (4 mmol, 1 equiv) in CH2Cl2 (4 mL) at 0 °C were
added tosyl chloride (0.84 g, 4.4 mmol, 1.1 equiv) and pyridine
(0.36 mL, 4.4 mmol, 1.1 equiv). The reaction mixture was
warmed to RT and stirred for 3 h. H2O (20 mL) and saturated
aqueous NH4Cl (10 mL) were added, and the mixture was
extracted with CH2Cl2 (3 ꢀ 60 mL). The combined organic
fractions were washed with brine, dried (Na2SO4), and concen-
trated under reduced pressure to give the crude product, which
was purified by flash chromatography on silica gel (hexanes:
CH2Cl2, 50:50).
Ethyl Tosylate (22). The treatment of ethanol with tosyl
chloride according to the method described above afforded
the title compound as a colorless oil (0.62 g, 78%); Rf 0.41
(hexanes:CH2Cl2, 50:50). 1H NMR (300 MHz, CDCl3): δ 1.28
(3H, t, J = 7.2 Hz), 2.44 (3H, s), 4.11 (2H, q, J = 7.1 Hz) 7.35
(2H, d, J = 8.2 Hz), 7.80 (2H, d, J = 8.3 Hz). 13C NMR (75.5
MHz, CDCl3): δ 14.4, 21.3, 66.5, 127.6, 129.5, 133.0, 144.4.
HRMS (þESI) calcd for C9H12O3S m/z: [M þ Na]þ, 223.0399;
found, 223.0400.
2-Fluoroethyl Tosylate (23). The treatment of 2-fluoroethanol
with tosyl chloride according to the method described above
afforded the title compound as a colorless oil (0.74 g, 85%); Rf
0.34 (hexanes:CH2Cl2, 50:50). 1H NMR (400 MHz, CDCl3): δ
2.45 (3H, s, CH3), 4.21-4.30 (2H, dm, J = 27 Hz), 4.49-4.63
(2H, dm, J = 47 Hz), 7.35 (2H, d, J = 8.0 Hz), 7.80 (2H, d, J =
7.5 Hz). The spectroscopic data matched those reported in the
literature.85
n-Propyl Tosylate (24). The treatment of n-propanol with
tosyl chloride according to the method described above afforded
the title compound as a colorless oil (0.75 g, 88%); Rf 0.34
(hexanes:CH2Cl2, 50:50). 1H NMR (300 MHz, CDCl3): δ 0.90
(3H, t, J = 7.4 Hz), 1.63-1.73 (2H, m), 2.45 (3H, s), 3.99 (2H,
dd, J = 6.5, 6.5 Hz), 7.35 (2H, d, J = 8.0 Hz), 7.79 (2H, d, J =
8.1 Hz). 13C NMR (75.5 MHz, CDCl3): δ 10.5, 22.1, 22.8, 72.7,
128.4, 130.2, 133.8, 145.2. HRMS (þESI) calcd for C10H14O3S
m/z: [M þ Na]þ, 237.0556; found, 237.0556.
N-((Benzofuran-2-yl)methyl)-N0-(30-methoxybenzyl)piperazine
(12). Treatment of 18 with 3-methoxybenzyl bromide according
to the method described above afforded the title compound as a
pale yellow solid (279 mg, 83%); mp 83-84 °C; Rf 0.35 (CH2Cl2:
1
MeOH, 99:1). H NMR (300 MHz, CDCl3): δ 2.48 (8H, br s),
3.41 (2H, s), 3.62 (2H, s), 3.73 (3H, s), 6.58 (1H, s), 6.77-6.80
(1H, m), 6.87-6.90 (2H, m), 7.17-7.26 (3H, m), 7.45-7.53 (2H,
m). 13C NMR (75.5 MHz, CDCl3): δ 53.4, 53.6, 55.8, 56.0, 63.4,
105.7, 111.9, 113.1, 115.2, 121.2, 122.1, 123.1, 124.4, 128.9, 129.7,
140.3, 155.1, 155.7, 160.2. HRMS (þESI) calcd for C21H24N2O2
m/z: [M þ H]þ, 337.1911; found, 337.1913.
N-((Benzofuran-2-yl)methyl)-N0-(40-methoxybenzyl)piperazine
(13). Treatment of 18 with 4-methoxybenzyl chloride according
to the method described above afforded the title compound as a
pale yellow solid (282 mg, 84%); mp 93-94 °C; Rf 0.17 (CH2Cl2:
1
MeOH, 99:1). H NMR (300 MHz, CDCl3): δ 2.55 (8H, br s),
3.48 (2H, s), 3.70 (2H, s), 3.79 (3H, s), 6.58 (1H, s), 6.81-6.86
(2H, m), 7.18-7.27 (4H, m), 7.44-7.53 (2H, m). 13C NMR (75.5
MHz, CDCl3): δ 52.6, 52.9, 55.2, 55.4 (CH2), 62.2 (CH2), 105.7
(CH), 111.3 (CH), 113.6 (CH), 120.7 (CH), 122.6 (CH), 123.9
(CH, C), 128.3 (C), 130.5 (CH), 154.4 (C), 155.1 (C), 158.8 (C).
HRMS (þESI) calcd for C21H24N2O2 m/z: [M þ H]þ, 337.1911;
found, 337.1910.
General Procedure for Phenol Synthesis. The appropriately
substituted hydroxybenzaldehyde (1.1 equiv) was added to a
solution of N-((benzofuran-2-yl)methyl)piperazine 18 (3.07 g,
14.2 mmol) in DCE (71 mL) at RT, and stirring was continued
for 15 min prior to the portionwise addition of NaBH(OAc)3
(3.48 g, 22.7 mmol, 1.60 equiv). The reaction mixture was stirred
at RT for 4 h after which time it was quenched with saturated
aqueous NaHCO3 (150 mL). The aqueous phase was extracted
with CH2Cl2 (3 ꢀ 200 mL), and the combined organic extracts
were washed with brine, dried (Na2SO4), and concentrated
under reduced pressure to give the crude product, which was
purified by flash chromatography on silica gel.
N-((Benzofuran-2-yl)methyl)-N0-(20-hydroxybenzyl)piperazine
(19). Treatment of 18 with 2-hydroxybenzaldehyde according
to the method described above afforded the title compound as
a dark yellow solid (3.80 g, 83%); mp 126-127 °C; Rf 0.39
3-Fluoropropyl Tosylate (37). To a solution of 1,3-propane-
diol (0.30 mL, 4.0 mmol, 1.0 equiv) in CH2Cl2 (4 mL) at 0 °C
were added tosyl chloride (0.84 g, 4.4 mmol, 1.1 equiv) portion-
wise and pyridine (0.36 mL, 4.4 mmol, 1.1 equiv) dropwise. The
reaction mixture was warmed to RT and stirred for 4 h. H2O (20
mL) and saturated aqueous NH4Cl (10 mL) were added, and the
mixture was extracted with CH2Cl2 (3 ꢀ 60 mL). The combined
organic fractions were washed with brine, dried (Na2SO4), and
concentrated in vacuo. Purification of the crude material thus
obtained by flash chromatography on silica gel (hexanes:
EtOAc, 50:50) afforded 3-hydroxypropyl 4-methylbenzensulfo-
nate as a colorless oil (0.60 g, 65%); Rf 0.33 (hexanes:EtOAc,
50:50). 1H NMR (CDCl3, 300 MHz): δ 1.82-1.90 (2H, m), 2.44
(3H, s), 2.63 (1H, s), 3.65 (2H, t, J = 6.0 Hz), 4.07 (2H, t, J = 6.2
Hz), 7.35 (2H, d, J = 8.3 Hz), 7.75 (2H, d, J = 8.3 Hz). 13C
NMR (CDCl3, 75.5 MHz): δ 22.0, 32.0, 58.5, 68.1, 128.0,
130.5, 133.1, 145.4. HRMS (þ ESI) calcd for C10H14O4S m/z:
[M þ Na]þ, 253.0505; found, 253.0511.
1
(hexanes:EtOAc, 75:25). H NMR (300 MHz, CDCl3): δ 2.64
(8H, br s), 3.71 (4H, s), 4.88 (1H, s), 6.60 (1H, s), 6.75-6.91 (2H,
m), 6.97 (1H, d, J = 7.3 Hz), 7.17-7.28 (3H, m), 7.47 (1H, d, J =
7.7 Hz), 7.53 (1H, d, J = 7.3 Hz). 13C NMR (75.5 MHz, CDCl3):
δ 52.9, 53.4, 55.8, 61.9, 106.5, 111.9, 116.6, 117.1, 121.3, 121.7,
123.3, 124.6, 128.8, 129.2, 154.6, 155.7, 158.3. MS (þESI) m/z:
323.27 ([M þ H]þ, 100%). Anal. (C20H22N2O2) calcd: C, 74.51;
H, 6.88; N, 8.69. Found: C, 74.39; H, 6.71; N, 8.71.
N-((Benzofuran-2-yl)methyl)-N0-(30-hydroxybenzyl)piperazine
(20). Treatment of 18 with 3-hydroxybenzaldehyde according to
the method described above afforded the title compound as a
colorless solid (3.85 g, 84%); mp 124-125 °C; Rf 0.36 (CH2Cl2:
MeOH, 95:5). 1H NMR (300 MHz, CDCl3): δ 2.54 (8H, br s),
3.44 (2H, s), 3.68 (2H, s), 4.60 (1H, s), 6.56 (1H, s), 6.65-6.72
(2H, m), 6.79 (1H, d, J = 7.6 Hz), 7.09-7.16 (1H, m), 7.18-
7.23 (2H, m), 7.42 (1H, d, J = 7.9 Hz), 7.50 (1H, d, J = 6.7 Hz).
13C NMR (75.5 MHz, CDCl3): δ 53.2, 53.8, 63.3, 106.5,
111.9, 115.0, 117.0, 121.3, 122.0, 123.2, 124.5, 128.2, 128.8,
130.1, 139.5, 154.6, 155.6, 156.6. MS (þESI) m/z: 323.13
([M þ H]þ, 100%). Anal. (C20H22N2O2) calcd: C, 74.51; H,
6.88; N, 8.69. Found: C, 74.31; H, 6.81; N, 8.79.
To a solution of 3-hydroxypropyl 4-methylbenzensulfonate
(0.60 g, 2.6 mmol, 1.0 equiv) in CH2Cl2 (8 mL) at RT was added
bis(2-methoxyethyl)aminosulfur trifluoride (Deoxo Fluor, 0.52
mL, 2.86 mmol, 1.1 equiv). The reaction mixture was stirred at
RT for 24 h after which time saturated aqueous NaHCO3 was
added dropwise until the pH reached 8. The mixture was
extracted with CH2Cl2 (3 ꢀ 60 mL), washed with brine, dried
(Na2SO4), and concentrated in vacuo. Purification of the crude
N-((Benzofuran-2-yl)methyl)-N0-(40-hydroxybenzyl)piperazine
(21). Treatment of 18 with 4-hydroxybenzaldehyde according
to the method described above afforded the title compound
as a colorless solid (3.89 g, 85%); mp 148-149 °C; Rf 0.33

6236 Journal of Medicinal Chemistry, 2010, Vol. 53, No. 16
Moussa et al.
residue thus obtained by flash chromatography on silica gel
(hexanes:EtOAc, 50:50) afforded the title compound as a pale
yellow oil (0.56 g, 60%); Rf 0.78 (hexanes:EtOAc, 50:50). H
NMR (CDCl3, 400 MHz): δ 1.99-2.09 (2H, m), 2.45 (3H, s),
4.16 (2H, t, J = 6.2 Hz), 4.41-4.45 (2H, dt, J = 47 Hz), 7.36
(2H, d, J = 8.0 Hz), 7.80 (2H, d, J = 8.0 Hz). 13C NMR (CDCl3,
75.5 MHz): δ 22.2, 30.6, 66.6, 79.0, 81.2, 128.4, 130.6, 133.4,
145.5. HRMS (þ ESI) calcd for C10H13FO3S m/z: [M þ Na]þ,
255.0462; found, 255.0463.
General Procedure for O-Alkylation. To a suspension of the
appropriately substituted phenol (322 mg, 1.00 mmol) and
K2CO3 (522 mg, 4.00 mmol) in DMF (2.5 mL) at RT was added
a solution of the appropriate tosylate (2.00 mmol) in DMF (2.5
mL). The reaction mixture was heated at reflux for 15 h, cooled
to RT, treated with H2O (100 mL), and extracted with EtOAc (3
ꢀ 60 mL). The combined organic fractions were washed with
brine, dried (Na2SO4), and concentrated under reduced pressure
to give the crude product, which was purified by flash chromato-
graphy on silica gel.
N-((Benzofuran-2-yl)methyl)-N0-(20-ethoxybenzyl)piperazine
(25). The treatment of phenol 19 with ethyl tosylate (22)
according to the method described above afforded the title
compound as a yellow oil (217 mg, 62%); Rf 0.34 (hexanes:
EtOAc, 80:20). 1H NMR (300 MHz, CDCl3): δ 1.39 (3H, t, J =
6.9 Hz), 2.59 (8H, br s), 3.62 (2H, s), 3.70 (2H, s), 4.03 (2H, q,
J = 6.9 Hz), 6.59 (1H, s), 6.75-6.98 (2H, m), 7.14-7.33 (4H,
m), 7.45-7.54 (2H, m). 13C NMR (75.5 MHz, CDCl3): δ 15.3,
52.7, 53.0, 55.7, 56.2, 64.5, 106.0, 111.7, 120.0, 120.6, 121.0,
123.0, 124.2, 126.0, 127.9, 128.4, 129.2, 154.3, 154.8, 159.0.
HRMS (þESI) calcd for C22H26N2O2 m/z: [M þ H]þ,
351.2067; found, 351.2069.
N-(Benzofuran-2-ylmethyl)-N0-(30-ethoxybenzyl)piperazine
(26). The treatment of phenol 20 with ethyl tosylate (22) accord-
ing to the method described above afforded the title compound
as a golden oil (232 mg, 66%); Rf 0.29 (hexanes:EtOAc, 70:30).
1H NMR (300 MHz, CDCl3): δ 1.39 (3H, t, J = 7.0 Hz), 2.55
(8H, br s), 3.48 (2H, s), 3.69 (2H, s), 4.02 (2H, q, J = 7.0 Hz), 6.58
(1H, s), 6.77 (1H, d, J = 5.9 Hz), 6.86-6.88 (2H, m), 7.16-7.26
(3H, m), 7.49 (2H, dd, J = 7.1,7.7 Hz). 13C NMR (75.5 MHz,
CDCl3): δ 15.4, 53.4, 53.6, 56.0, 63.5, 63.9, 106.2, 111.9, 113.6,
115.8, 121.2, 121.9, 123.2, 124.4, 127.5, 129.7, 140.3, 155.1, 159.5.
HRMS (þESI) calcd for C22H26N2O2 m/z: [M þ H]þ, 351.2067;
found, 351.2066.
N-(Benzofuran-2-ylmethyl)-N0-(40-ethoxybenzyl)piperazine
(27). The treatment of phenol 21 with ethyl tosylate (22) accord-
ing to the method described above afforded the title compound
as a yellow solid (231 mg, 66%); mp 63-64 °C; Rf 0.28 (hexanes:
EtOAc, 50:50). 1H NMR (300 MHz, CDCl3): δ 1.36 (3H, t, J =
7.0 Hz), 2.54 (8H, br s), 3.44 (2H, s), 3.69 (2H, s), 4.04 (2H, q, J =
7.0 Hz), 6.58 (1H, s), 6.82 (2H, d, J = 8.4 Hz), 7.18-7.24 (4H,
m), 7.69 (2H, d, J = 7.3 Hz). 13C NMR (75.5 MHz, CDCl3): δ
15.0, 52.9, 53.1, 55.5, 62.5, 63.6, 105.8, 111.4, 114.3, 120.8, 122.8,
124.0, 127.9, 130.6, 132.2, 154.6, 158.3. MS (þESI) m/z: 351.27
([M þ H]þ, 100%). Anal. (C22H26N2O2) calcd: C, 75.40; H, 7.48;
N, 7.99. Found: C, 75.25; H, 7.49; N, 8.05.
tosylate (23) according to the method described above af-
forded the title compound as a golden oil (265 mg, 72%); Rf
0.19 (hexanes:EtOAc, 50:50). 1H NMR (300 MHz, CDCl3): δ
2.56 (8H, br s), 3.50 (2H, s), 3.70 (2H, s), 4.14-4.26 (2H, dt,
J = 4.1 Hz, J = 28 Hz,), 4.64-4.83 (2H, dt, J = 4.2 Hz, J = 47
Hz), 6.58 (1H, s), 6.81 (1H, d, J = 7.2 Hz), 6.90-6.91 (2H, m),
7.17-7.25 (3H, m), 7.49 (2H, dd, J = 7.2, 7.6 Hz). 13C NMR
(75.5 MHz, CDCl3): δ 52.9, 53.1, 55.5, 62.9, 67.0, 67.3, 80.9,
83.2, 105.9, 111.4, 113.4, 115.5, 120.8, 122.3, 122.8, 124.0,
128.4, 129.4, 139.9, 154.6, 158.6. HRMS (þESI) calcd for
C22H25FN2O2 m/z: [M þ H]þ, 369.1973; found, 369.1929.
N-(Benzofuran-2-ylmethyl)-N0-(40-(200-fluoroethoxy)benzyl)-
piperazine (30). The treatment of phenol 21 with 2-fluoroethyl
tosylate (23) according to the method described above afforded
the title compound as a yellow solid (230 mg, 60%); mp 76-
1
1
78 °C; Rf 0.25 (hexanes:EtOAc, 50:50). H NMR (300 MHz,
CDCl3): δ 2.54 (8H, br s), 3.45 (2H, s), 3.69 (2H, s), 4.10-4.28
(2H, dt, J = 4.1 Hz, J = 28 Hz,), 4.60-4.88 (2H, dt, J = 4.1 Hz,
J = 47Hz), 6.57 (1H, s), 6.86(2H, d, J = 8.5Hz), 7.19-7.25(4H,
m), 7.50 (2H, d, J = 7.3 Hz). 13C NMR (75.5 MHz, CDCl3): δ
52.4, 52.7, 55.1, 62.0, 72.9, 82.8, 105.4, 111.0, 114.1, 120.4, 122.3,
123.4, 127.3, 130.2, 132.8, 154.2, 154.8, 158.9. MS (þESI) m/z:
337.27 ([M þ H]þ, 100%). Anal. (C22H25FN2O2) calcd: C, 71.72;
H, 6.84; N, 7.60. Found: C, 71.49; H, 7.10; N, 7.60.
N-(Benzofuran-2-ylmethyl)-N0-(20-propoxybenzyl)piperazine
(31). The treatment of phenol 19 with propyl tosylate (24)
according to the method described above afforded the title
compound as a colorless oil (222 mg, 61%); Rf 0.26 (hexanes:
EtOAc, 80:20). 1H NMR (300 MHz, CDCl3): δ 1.03 (3H, t, J =
7.4 Hz), 1.79 (2H, q, J = 6.8 Hz), 2.59 (8H, br s), 3.61 (2H, s),
3.71 (2H, s), 3.90 (2H, t, J = 6.4 Hz), 6.59(1H, s), 6.74-6.98 (2H,
m), 7.13-7.32 (4H, m), 7.45-7.54 (2H, m). 13C NMR (75.5
MHz, CDCl3): δ 10.9, 22.9, 52.8, 53.3, 55.6, 61.5, 69.7, 105.8,
111.5, 116.2, 119.3, 120.2, 120.8, 122.8, 124.1, 128.2, 128.9, 130.9,
154.2, 154.8, 159.0. HRMS (þESI) calcd for C23H28N2O2 m/z:
[M þ H]þ, 365.2224; found, 365.2222.
N-(Benzofuran-2-ylmethyl)-N0-(30-propoxybenzyl)piperazine
(32). The treatment of phenol 20 with propyl tosylate (24) accord-
ing to the method described above afforded the title compound
as a golden oil (226 mg, 62%); Rf 0.32 (hexanes:EtOAc, 70:30).
1H NMR (300 MHz, CDCl3): δ 1.03 (3H, t, J = 7.4 Hz), 1.79 (2H,
q, J=7.0Hz), 2.56(8H, brs), 3.49(2H, s), 3.70(2H, s), 3.90(2H, t,
J = 6.8 Hz), 6.58 (1H, s), 6.78 (1H, d, J = 7.3 Hz), 6.86 (2H, br s),
7.16-7.26 (3H, m), 7.46 (1H, d, J = 7.5 Hz), 7.51 (1H, d, J = 7.8
Hz). 13C NMR (75.5 MHz, CDCl3): δ 10.7, 22.8, 53.0, 53.1, 55.5,
63.0, 69.6, 105.8, 111.4, 113.2, 115.5, 120.8, 121.5, 122.7, 124.0,
127.4, 129.2, 141.0, 154.6, 154.9, 157.0. HRMS (þESI) calcd for
C23H28N2O2 m/z: [M þ H]þ, 365.2224; found, 365.2223.
N-(Benzofuran-2-ylmethyl)-N0-(40-propoxybenzyl)piperazine
(33). The treatment of phenol 21 with n-propyl tosylate (24)
according to the method described above afforded the title com-
pound as a golden yellow oil (218 mg, 60%); Rf 0.29 (hexanes:
EtOAc, 70:30). 1H NMR (300 MHz, CDCl3): δ 1.03 (3H, t, J =
8.4 Hz), 1.79 (2H, q, J = 6.9 Hz), 2.67 (8H, br s), 3.60 (2H, s), 3.73
(2H, s), 3.90 (2H, t, J = 6.6 Hz), 6.60 (1H, s), 6.83 (2H, d, J = 8.5
Hz), 7.17-7.28 (4H, m), 7.45 (1H, d, J = 7.7 Hz), 7.51 (1H, d, J =
7.2 Hz), 7.49 (1H, d, J = 7.0 Hz). 13C NMR (75.5 MHz, CDCl3): δ
11.1, 23.1, 52.2, 55.4, 62.0, 70.1, 106.8, 111.9, 114.9, 121.4, 123.3,
124.7, 127.7, 130.2, 131.7, 154.3, 154.6, 158.1. HRMS (þESI) calcd
for C23H28N2O2 m/z: [M þ H]þ, 365.2224; found, 365.2223.
N-(Benzofuran-2-ylmethyl)-N0-(20-(300-fluoropropoxy)benzyl)-
piperazine (34). The treatment of phenol 19 with 3-fluoropropyl
tosylate (37) according to the method described above affor-
ded the title compound as a colorless oil (237 mg, 62%); Rf 0.26
(hexanes:EtOAc, 60:40). 1H NMR (300 MHz, CDCl3): δ
2.08-2.25 (2H, m), 2.59 (8H, br s), 3.59 (2H, s), 3.69 (2H, s),
4.08 (2H, t, J = 6.0 Hz), 4.55-4.75 (2H, dt, J = 5.8 Hz, J = 47
Hz), 6.60 (1H, s), 6.84-6.94 (2H, m), 7.16-7.27 (3H, m), 7.32
(1H, d, J = 7.4 Hz), 7.49 (2H, d, J = 7.5 Hz). 13C NMR (75.5
MHz, CDCl3): δ 30.9, 53.2, 53.6, 56.0, 56.4, 64.2, 80.3, 82.5,
N-((Benzofuran-2-yl)methyl)-N0-(20-(200-fluoroethoxy)benzyl)-
piperazine (28). The treatment of phenol 19 with 2-fluoroethyl
tosylate (23) according to the method described above afforded
the title compound as a pale yellow oil (269 mg, 73%); Rf 0.29
1
(hexanes:EtOAc, 60:40). H NMR (300 MHz, CDCl3): δ 2.59
(8H, br s), 3.64 (2H, s), 3.68 (2H, s), 4.13-4.25 (2H, dt, J = 4.1
Hz, J = 28 Hz,), 4.63-4.82 (2H, dt, J = 4.3 Hz, J = 47 Hz), 6.57
(1H, s), 6.83 (1H, d, J = 8.2 Hz), 6.94 (1H, dd, J = 7.4, 7.4 Hz),
7.15-7.25 (3H, m), 7.33 (1H, d, J = 7.4 Hz), 7.48 (2H, d, J = 7.5
Hz). 13C NMR (75.5 MHz, CDCl3): δ 52.7, 53.2, 55.6, 55.9, 67.7,
67.9, 105.8, 111.5, 112.1, 120.8, 121.2, 122.7, 124.0, 126.1, 127.9,
128.3, 131.3, 154.2, 154.9, 159.0. HRMS (þESI) calcd for
C22H25FN2O2 m/z: [M þ H]þ, 369.1973; found, 369.1973.
N-(Benzofuran-2-ylmethyl)-N0-(30-(200-fluoroethoxy)benzyl)-
piperazine (29). The treatment of phenol 20 with 2-fluoroethyl

Article
Journal of Medicinal Chemistry, 2010, Vol. 53, No. 16 6237
106.3, 119.6, 112.0, 121.1, 121.2, 123.2, 124.4, 126.5, 127.4, 128.8,
131.6, 154.2, 154.8, 159.0. HRMS (þESI) calcd for C23H27-
FN2O2 m/z: [M þ H]þ, 383.2057; found, 383.2132.
gas flow, synthesized [11C]CH3I was delivered to a 1 mL reac-
tion vessel containing 21 (phenol, 0.5 mg, 1.5 μmol) in DMF
(300 μL) and TBAH (1.8 μmol) and allowed to stand at RT for
2 min and then heated at 80 °C for 5 min. The reaction mix-
ture was diluted with 0.5 mL of a 0.1 M NaH2PO4-CH3CN
(70:30 v/v) solution and injected onto an HPLC XTerra RP C-18
(7.8 mm i.d. ꢀ 100 mm, 5 μm particle size) semipreparative
reverse phase column. Using a mobile phase of 0.1 M NaH2-
PO4-CH3CN (70:30 v/v) and a flow rate of 6.0 mL/min, the
retention time (tR) of [11C]13 was 5.10 min. The radioactive
fraction corresponding to [11C]13 was collected and evaporated
under vacuum. The residue was reconstituted in sterile saline
(3 mL) and filtered through a sterile Millipore GS 0.22 μm filter
into a sterile pyrogen-free evacuated vial.
N-(Benzofuran-2-ylmethyl)-N0-(30-(300-fluoropropoxy)benzyl)-
piperazine (35). The treatment of phenol 20 with 3-fluoropropyl
tosylate (37) according to the method described above afforded
the title compound as a golden oil (237 mg, 62%); Rf 0.29
(hexanes:EtOAc, 65:35). 1H NMR (300 MHz, CDCl3): δ
2.02-2.21 (2H, m), 2.55 (8H, br s), 3.46 (2H, s), 3.67 (2H, s),
4.05 (2H, t, J = 6.1 Hz), 4.51-4.71 (2H, dt, J = 5.8 Hz, J =
47 Hz), 6.56 (1H, s), 6.76 (1H, d, J = 7.3 Hz), 6.88 (2H, br s),
7.15-7.25 (3H, m), 7.45 (1H, d, J = 7.7 Hz), 7.49 (1H, d, J = 7.8
Hz). 13C NMR (75.5 MHz, CDCl3): δ 30.9, 52.8, 53.2, 55.4, 63.3,
63.8, 80.1, 82.3, 106.5, 111.9, 116.1, 121.3, 123.2, 124.6, 128.2,
128.8, 131.6, 154.5, 155.6, 156.2. HRMS (þESI) calcd for
C23H27FN2O2 m/z: [M þ H]þ, 383.2129; found, 383.2131.
N-(Benzofuran-2-ylmethyl)-N0-(40-(300-fluoropropoxy)benzyl)-
piperazine (36). The treatment of phenol 21 with 3-fluoropropyl
tosylate (37) according to the method described above afforded
the title compound as a colorless solid (241 mg, 63%); mp 59-
Quality Control for [11C]13. For the determination of specific
radioactivity and radiochemical purity, an aliquot of the final
solution of known volume and radioactivity was injected onto
an analytical reverse-phase HPLC column (Waters XTerra RP
C-18, 4.6 mm ꢀ 150 mm). A mobile phase of 0.1 M NaH2-
PO4-CH3CN (50:50 v/v) at a flow rate of 1.0 mL/min was used
to elute [11C]13 (tR = 5.10 min). The area of the UV absorbance
peak measured at 254 nm corresponding to the carrier product
was measured (integrated) on the HPLC chromatogram and
compared to a standard curve relating mass to UV absorbance.
PET Studies. Animals. A healthy male P. hamadryas baboon
that was 13 years old and weighed 26.5 kg was selected for PET
scanning. The baboon was maintained and handled in accor-
dance with the National Health and Medical Research Council
(NHMRC) code of practice for the care and use of nonhuman
primates for scientific purposes. Project application was ap-
proved by the Sydney South West Health Service Animal Ethics
Committee.
Baboon Carbon-11 PET Imaging. All PET data were acquired
using a Siemens Biography LSO PET-CT scanner in the Depart-
ment of PET and Nuclear Medicine at Royal Prince Alfred
Hospital. These dual modality devices have a fully 3D PET
scanner with 24 crystal rings and a dual slice CT scanner in the
same gantry. These scanners yield a reconstructed PET spatial
resolution of 6.3 mm FWDH (full width at half-maximum) in the
center of the field of view. A CT scan of the head was completed
prior to radioligand injection. The baboon was initially anaes-
thetized with ketamine (8 mg/kg, im) in addition to medetomi-
dine hydrochloride (Domitor, 30 μg/kg, im). Anaesthesia was
maintained with the use of an iv infusion of ketamine in saline at
a dose rate of 0.2 mg ketamine/kg/min. The baboon also received
MgSO4 (2 mL, iv) givenoverhalf an hour andatropine(1 mg, im)
plus metoclopramide (Maxolon; Mayne Pharma, 5 mg, im). The
head of the baboon was immobilized with plastic tape to mini-
mize motion artifacts. Acquisition of dynamic PET data (20 ꢀ
30 s, 30 ꢀ 60 s, and 4 ꢀ 300 frames) was commenced just prior to
radioligand injection and yielded a total of 54 frames over a
period of 60 min. The dynamic 3D PET sinograms were rebinned
using FORE (Fourier rebinning) and reconstructed into 47
transversal slices with filtered back projection and CT data-
based corrections for photon attenuation and scatter, each
comprising of 128 ꢀ 128 voxels. Reconstructed voxel values in
each frame are reported in units of Bq/mL, corrected for radio-
active decay to the time of injection, and voxel dimensions were
0.206 cm ꢀ 0.206 cm ꢀ 0.337 cm. Voxel values were then
converted to units of percentage injected dose per volume of
brain tissue (% dose/mL) and plotted against time. An auto-
mated 3D registration algorithm was used to coregister the two
reconstructed scans prior to region of interest definition. Decay-
corrected time-activity curves representing the variation on
radioligand concentration versus time were constructed from
selected slices of region of interest over the whole brain. After
dynamic acquisition, a whole body PET-CT scan was performed
to determine other sites of uptake of the radioligand.
1
60 °C; Rf 0.24 (hexanes:EtOAc, 50:50). H NMR (300 MHz,
CDCl3): δ 2.07-2.24 (2H, m), 2.55 (8H, br s), 3.45 (2H, s), 3.69
(2H, s), 4.25 (2H, t, J = 6.1 Hz), 4.54-4.74 (2H, dt, J = 5.8 Hz,
J = 47 Hz), 6.58(1H, s), 6.84(2H, d, J = 8.4Hz), 7.17-7.24(4H,
m), 7.49 (2H, d, J = 7.1 Hz). 13C NMR (75.5 MHz, CDCl3): δ
31.9, 52.9, 53.1, 55.6, 62.5, 63.7, 80.0, 105.8, 111.4, 114.3, 120.8,
122.7, 124.0, 127.3, 130.6, 132.2, 154.6, 158.2. MS (þESI) m/z:
383.33 ([M þ H]þ, 100%). Anal. (C23H27FN2O2) calcd: C, 72.23;
H, 7.12; N, 7.22. Found: C, 72.56; H, 7.22; N, 7.38.
Radioligand Binding Assay. Affinities for σ1 were determined
using rat brain homogenates and [3H](þ)-pentazocine, and affi-
nities for σ2 were determined using PC12 cells and [3H]DTG
using a modification of the protocol reported by Kovacs and
Larson.74 Binding to all receptors were performed by NIMH
PDSP, and conditions can be found in the PDSP handbook.
Binding data not shown in Tables 1-3 can be found in the
Supporting Information.
Lipophilicity Estimations. The lipophilicity of piperazine 13
was examined by determination of the log D value using a HPLC
method previously described.76,77 Phosphate buffer (0.05 M) was
prepared by dissolved weighed amounts of potassium dihydro-
gen phosphate in Alpha-Q water, and the pH was adjusted to 7.4
with sodium hydroxide solution (0.1 M). Samples were analyzed
using a MS C-18 column (Waters XTerra, 5 μm, 2.1 mm i.d. ꢀ
150 mm) and a mobile phase of methanol and phosphate buffer
(65:35 v/v, pH 7.47) with a flow rate of 0.3 mL/min, UV at
254 nm. The lipophilicity of piperazine 13 was estimated by a
comparison of its retention time to that of standards having
known log D values. The standards used were aniline, benzene,
toluene, cumene, triphenylamine, and hexachlorobenzene, pre-
pared as solutions in methanol. All sample injections were
performed in triplicate, and the results were averaged to provide
the final values. A calibration curve of log D versus retention
time was produced, which resulted in an exponential calibration
equation (y = 0.7981e0.7371x) with an r2 value 0.992. The expo-
nential equation of the trendline function from the calibration
graph and Excel allowed the log D values to be calculated.
Radiochemistry. Preparation of [11C]CH3I. The target gas
(14N þ 0.5% 16O2) was bombarded with protons using a 16
MeV cyclotron to produce [11C]CO2. The [11C]CO2 was trans-
ferred to a GE Microlab automated module and concentrated
onto molecular sieves. The sieves were heated to release [11C]CO2,
which was reduced by hydrogen on a nickel catalyst to form
[11C]CH4. The [11C]CH4 was released into the CH3I conversion
part of the module where it was recirculated through a quartz
column (packed with ascarite and iodine crystals) by helium
carrier gas. The [11C]CH3I was trapped in ascarite, while any
unconverted [11C]CH4 was transferred to waste.
Preparation and Formulation of 1-(Benzofuran-2-ylmethyl)-
4-(4-[11C]methoxybenzyl) Piperazine ([11C]13). Under a helium
Blocking Studies. Haloperidol (1 mg/kg) was dissolved in
saline with a small quantity of propylene glycol and acetic acid.

6238 Journal of Medicinal Chemistry, 2010, Vol. 53, No. 16
Moussa et al.
Haloperidol was injected iv 5 min prior to tracer injection into
the cephalic vein. Drug treatment was separated by at least
3 week intervals.
(18) Aydar, E.; Palmer, C. P.; Klyachko, V. A.; Jackson, M. B. The
sigma receptor as a ligand-regulated auxiliary potassium channel
subunit. Neuron 2002, 34, 399–410.
(19) Monnet, F. P. Sigma-1 receptor as regulator of neuronal intra-
cellular Ca2þ: clinical and therapeutic relevance. Biol. Cell 2005, 97,
873–883.
(20) Vilner, B. J.; Bowen, W. D. Modulation of cellular calcium by
sigma-2 receptors: release from intracellular stores in human SK-
N-SH neuroblastoma cells. J. Pharmacol. Exp. Ther. 2000, 292,
900–911.
(21) Matsuno, K.; Matsunaga, K.; Senda, T.; Mita, S. Increase in
extracellular acetylcholine level by sigma ligands in rat frontal
cortex. J. Pharmacol. Exp. Ther. 1993, 265, 851–859.
(22) Kobayashi, T.; Matsuno, K.; Nakata, K.; Mita, S. Enhancement of
acetylcholine release by SA4503, a novel sigma 1 receptor agonist,
in the rat brain. J. Pharmacol. Exp. Ther. 1996, 279, 106–113.
(23) Horan, B.; Gifford, A. N.; Matsuno, K.; Mita, S.; Ashby, C. R., Jr.
Effect of SA4503 on the electrically evoked release of 3H-acetylcho-
line from striatal and hippocampal rat brain slices. Synapse 2002,
46, 1–3.
Acknowledgment. Ki determinations and receptor binding
profiles were generously provided by the National Institute of
Mental Health’s Psychoactive Drug Screening Program, Con-
tract number NO1MH32004 (NIMH PDSP). The NIMH
PDSP is directed by Bryan L. Roth M.D., Ph.D., at the
University of North Carolina at Chapel Hill and Project
Officer Jamie Driscol at NIMH (Bethesda, MD).
Supporting Information Available: Off-target binding data for
compounds 10-13, 25, and 26. This material is available free of
charge via the Internet at http://pubs.acs.org.
(24) Gonzalez-Alvear, G. M.; Werling, L. L. Regulation of
[3H]dopamine release from rat striatal slices by sigma receptor
ligands. J. Pharmacol. Exp. Ther. 1994, 271, 212–219.
(25) Gonzalez-Alvear, G. M.; Werling, L. L. sigma1 Receptors in rat
striatum regulate NMDA-stimulated [3H]dopamine release via a
presynaptic mechanism. Eur. J. Pharmacol. 1995, 294, 713–719.
(26) Weatherspoon, J. K.; Gonzalez-Alvear, G. M.; Frank, A. R.;
Werling, L. L. Regulation of [3H] dopamine release from meso-
limbic and mesocortical areas of guinea pig brain by sigma
receptors. Schizophr. Res. 1996, 21, 51–62.
References
(1) Martin, W. R.; Eades, C. G.; Thompson, J. A.; Huppler, R. E.;
Gilbert, P. E. The effects of morphine- and nalorphine-like drugs in
the nondependent and morphine-dependent chronic spinal dog.
J. Pharmacol. Exp. Ther. 1976, 197, 517–532.
(2) Quirion, R.; Chicheportiche, R.; Contreras, P. C.; Johnson, K. M.;
Lodge, D.; Tam, S. W.; Woods, J. H.; Zukin, S. R. Classification
and nomenclature of phencyclidine and sigma receptor sites.
Trends Neurosci. (Pers. Ed.) 1987, 10, 444–446.
(3) Quirion, R.; Bowen, W. D.; Itzhak, Y.; Junien, J. L.; Musacchio,
J. M.; Rothman, R. B.; Su, T. P.; Tam, S. W.; Taylor, D. P. A
proposal for the classification of sigma binding sites. Trends
Pharmacol. Sci. 1992, 13, 85–86.
(4) Hanner, M.; Moebius, F. F.; Flandorfer, A.; Knaus, H. G.;
Striessnig, J.; Kempner, E.; Glossmann, H. Purification, molecular
cloning, and expression of the mammalian sigma1-binding site.
Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 8072–8077.
(5) Kekuda, R.; Prasad, P. D.; Fei, Y. J.; Leibach, F. H.; Ganapathy,
V. Cloning and functional expression of the human type 1 sigma
receptor (hSigmaR1). Biochem. Biophys. Res. Commun. 1996, 229,
553–558.
(6) Colabufo, N. A.; Berardi, F.; Abate, C.; Contino, M.; Niso, M.;
Perrone, R. Is the sigma2 receptor a histone binding protein?
J. Med. Chem. 2006, 49, 4153–4158.
(7) Abate, C.; Elenewski, J.; Niso, M.; Berardi, F.; Colabufo, N. A.;
Azzariti, A.; Perrone, R.; Glennon, R. A. Interaction of the sigma2
Receptor Ligand PB28 with the Human Nucleosome: Computa-
tional and Experimental Probes of Interaction with the H2A/H2B
Dimer. ChemMedChem 2010, 5, 268–273.
(8) Su, T. P.; London, E. D.; Jaffe, J. H. Steroid binding at sigma
receptors suggests a link between endocrine, nervous, and immune
systems. Science 1988, 240, 219–221.
(9) Monnet, F. P.; Maurice, T. The sigma1 protein as a target for the
non-genomic effects of neuro(active)steroids: molecular, physiolo-
gical, and behavioral aspects. J. Pharmacol. Sci. 2006, 100, 93–118.
(10) Fontanilla, D.; Johannessen, M.; Hajipour, A. R.; Cozzi, N. V.;
Jackson, M. B.; Ruoho, A. E. The Hallucinogen N,N-Dimethyl-
tryptamine (DMT) Is an Endogenous Sigma-1 Receptor Regula-
tor. Science 2009, 323, 934–937.
(11) Su, T.-P.; Hayashi, T.; Vaupel, D. B. When the endogenous
hallucinogenic trace amine N,N-dimethyltryptamine meets the
sigma-1 receptor. Sci. Signal. 2009, 2, pe12.
(12) Hayashi, T.; Rizzuto, R.; Hajnoczky, G.; Su, T.-P. MAM: More
than just a housekeeper. Trends Cell Biol. 2009, 19, 81–88.
(13) Hayashi, T.; Su, T.-P. Sigma-1 receptor chaperones at the ER-
mitochondrion interface regulate Ca2þ signaling and cell survival.
Cell 2007, 131, 596–610.
(27) Gonzalez, G. M.; Werling, L. L. Release of [3H]dopamine from
guinea pig striatal slices is modulated by sigma1 receptor agonists.
Naunyn-Schmiedeberg’s Arch. Pharmacol. 1997, 356, 455–461.
(28) Hayashi, T.; Kagaya, A.; Takebayashi, M.; Shimizu, M.; Uchitomi,
Y.; Motohashi, N.; Yamawaki, S. Modulation by sigma ligands of
intracellular free Caþþ mobilization by N-methyl-D-aspartate in
primary culture of rat frontal cortical neurons. J. Pharmacol. Exp.
Ther. 1995, 275, 207–214.
(29) Debonnel, G.; de Montigny, C. Modulation of NMDA and
dopaminergic neurotransmissions by sigma ligands: possible im-
plications for the treatment of psychiatric disorders. Life Sci. 1996,
58, 721–734.
(30) Bermack, J. E.; Debonnel, G. Modulation of serotonergic neuro-
transmission by short- and long-term treatments with sigma
ligands. Br. J. Pharmacol. 2001, 134, 691–699.
(31) Lucas, G.; Rymar, V. V.; Sadikot, A. F.; Debonnel, G.
Further evidence for an antidepressant potential of the selective
sigma1 agonist SA 4503: electrophysiological, morphological and
behavioural studies. Int. J. Neuropsychopharmacol. 2008, 11, 485–
495.
(32) Monnet, F. P.; Debonnel, G.; de Montigny, C. In vivo electro-
physiological evidence for a selective modulation of N-methyl-D-
aspartate-induced neuronal activation in rat CA3 dorsal hippo-
campus by sigma ligands. J. Pharmacol. Exp. Ther. 1992, 261,
123–130.
(33) Monnet, F. P.; Mahe, V.; Robel, P.; Baulieu, E. E. Neurosteroids,
via sigma receptors, modulate the [3H]norepinephrine release
evoked by N-methyl-D-aspartate in the rat hippocampus. Proc.
Natl. Acad. Sci. U.S.A. 1995, 92, 3774–3778.
(34) Monnet, F. P.; de Costa, B. R.; Bowen, W. D. Differentiation of
sigma ligand-activated receptor subtypes that modulate NMDA-
evoked [3H]-noradrenaline release in rat hippocampal slices. Br.
J. Pharmacol. 1996, 119, 65–72.
(35) Hashimoto, K.; Ishiwata, K. Sigma receptor ligands: Possible
application as therapeutic drugs and as radiopharmaceuticals.
Curr. Pharm. Des. 2006, 12, 3857–3876.
ꢀ
(36) Cobos, E. J.; Entrena, J. M.; Nieto, F. R.; Cendan, C. M.; Del
Pozo, E. Pharmacology and Therapeutic Potential of Sigma1
Receptor Ligands. Curr. Neuropharmacol. 2008, 6, 344–366.
(37) Maurice, T.; Su, T.-P. The pharmacology of sigma-1 receptors.
Pharmacol. Ther. 2009, 124, 195–206.
(38) Tam, S. W.; Cook, L. Sigma opiates and certain antipsychotic
drugs mutually inhibit (þ)-[3H] SKF 10,047 and [3H]haloperidol
binding in guinea pig brain membranes. Proc. Natl. Acad. Sci.
U.S.A. 1984, 81, 5618–5621.
(39) Frieboes, R.-M.; Steiger, A. The antipsychotic effects of sigma
drugs. In Neurotransmitter Receptors in Actions of Antipsychotic
Medications; Lidow, M. S., Ed.; CRC Press: Boca Raton, 2000; pp
137-152.
(14) Tsai, S.-Y.; Hayashi, T.; Mori, T.; Su, T.-P. Sigma-1 receptor
chaperones and diseases. Cent. Nerv. Syst. Agents Med. Chem.
2009, 9, 184–189.
(15) Hayashi, T.; Maurice, T.; Su, T. P. Ca2þ signaling via sigma1-
receptors: novel regulatory mechanism affecting intracellular Ca2þ
concentration. J. Pharmacol. Exp. Ther. 2000, 293, 788–798.
(16) Lupardus, P. J.; Wilke, R. A.; Aydar, E.; Palmer, C. P.; Chen, Y.;
Ruoho, A. E.; Jackson, M. B. Membrane-delimited coupling
between sigma receptors and Kþ channels in rat neurohypophysial
terminals requires neither G-protein nor ATP. J. Physiol. 2000,
526, 527–539.
(17) Zhang, H.; Cuevas, J. Sigma receptors inhibit high-voltage-
activated calcium channels in rat sympathetic and parasympathetic
neurons. J. Neurophysiol. 2002, 87, 2867–2879.
(40) Itzhak, Y.; Kassim, C. O. Clorgyline displays high affinity for
sigma binding sites in C57BL/6 mouse brain. Eur. J. Pharmacol.
1990, 176, 107–108.

Article
Journal of Medicinal Chemistry, 2010, Vol. 53, No. 16 6239
(41) Narita, N.; Hashimoto, K.; Tomitaka, S.; Minabe, Y. Interactions
of selective serotonin reuptake inhibitors with subtypes of sigma
receptors in rat brain. Eur. J. Pharmacol. 1996, 307, 117–119.
(42) Bermack, J. E.; Debonnel, G. The role of sigma receptors in
depression. J. Pharmacol. Sci. 2005, 97, 317–336.
(43) Hayashi, T.; Stahl, S. M. The sigma-1 (σ1) receptor and its role in
the treatment of mood disorders. Drugs Future 2009, 34, 137–146.
(44) Kulkarni, S. K.; Dhir, A. σ-1 receptors in major depression and
anxiety. Expert Rev. Neurother. 2009, 9, 1021–1034.
(45) Matsumoto, R. R.; McCracken, K. A.; Pouw, B.; Zhang, Y.;
Bowen, W. D. Involvement of sigma receptors in the beha-
vioral effects of cocaine: evidence from novel ligands and anti-
sense oligodeoxynucleotides. Neuropharmacology 2002, 42, 1043–
1055.
(46) Maurice, T.; Martin-Fardon, R.; Romieu, P.; Matsumoto, R. R.
Sigma(1) (σ1) receptor antagonists represent a new strategy against
cocaine addiction and toxicity. Neurosci. Biobehav. Rev. 2002, 26,
499–527.
(47) Matsumoto, R. R.; Liu, Y.; Lerner, M.; Howard, E. W.; Brackett,
D. J. Sigma receptors: Potential medications development target
for anti-cocaine agents. Eur. J. Pharmacol. 2003, 469, 1–12.
(48) Nguyen, E. C.; McCracken, K. A.; Liu, Y.; Pouw, B.; Matsumoto,
R. R. Involvement of sigma (σ) receptors in the acute actions
of methamphetamine: Receptor binding and behavioral studies.
Neuropharmacology 2005, 49, 638–645.
(49) Romieu, P.; Maurice, T. The sigma1 (s1) protein as a new target for
anti-cocaine therapy. In Cocaine and Heroin Abuse Research;
Pizzoli, L. A., Ed.; Nova Science Publishers: Hauppauge, NY, 2006;
pp 37-60.
(50) John, C. S.; Bowen, W. D.; Varma, V. M.; McAfee, J. G.; Moody,
T. W. Sigma receptors are expressed in human non-small cell lung
carcinoma. Life Sci. 1995, 56, 2385–2392.
(51) Vilner, B. J.; John, C. S.; Bowen, W. D. Sigma-1 and sigma-2
receptors are expressed in a wide variety of human and rodent
tumor cell lines. Cancer Res. 1995, 55, 408–413.
(52) Wang, B.; Rouzier, R.; Albarracin, C. T.; Sahin, A.; Wagner, P.;
Yang, Y.; Smith, T. L.; Bernstam, F. M.; Marcelo, A. C.; Hortobagyi,
G. N.; Pusztai, L. Expression of sigma 1 receptor in human breast
cancer. Breast Cancer Res. Treat. 2004, 87, 205–214.
(53) Vilner, B. J.; de Costa, B. R.; Bowen, W. D. Cytotoxic effects of
sigma ligands: sigma receptor-mediated alterations in cellular
morphology and viability. J. Neurosci. 1995, 15, 117–134.
(54) Moody, T. W.; Leyton, J.; John, C. Sigma ligands inhibit the
growth of small cell lung cancer cells. Life Sci. 2000, 66, 1979–1986.
(55) Berthois, Y.; Bourrie, B.; Galiegue, S.; Vidal, H.; Carayon, P.;
Martin, P. M.; Casellas, P. SR31747A is a sigma receptor ligand
exhibiting antitumoural activity both in vitro and in vivo. Br. J.
Cancer 2003, 88, 438–446.
model for σ2 receptors based on a series of substituted benzo-
[d]oxazol-2(3H)-one derivatives. Bioorg. Med. Chem. Lett. 2010, 20,
2954–2957.
(67) Banister, S. D.; Moussa, I. A.; Jordan, M. J. T.; Coster, M. J.;
Kassiou, M. Oxo-bridged isomers of aza-trishomocubane sigma
(σ) receptor ligands: Synthesis, in vitro binding, and molecular
modeling. Bioorg. Med. Chem. Lett. 2010, 20, 145–148.
(68) Liu, X.; Banister, S. D.; Christie, M. J.; Banati, R.; Meikle, S.;
Coster, M. J.; Kassiou, M. Trishomocubanes: Novel sigma ligands
modulate cocaine-induced behavioural effects. Eur. J. Pharmacol.
2007, 555, 37–42.
(69) Baziard-Mouysset, G.; Younes, S.; Labssita, Y.; Payard, M.;
Caignard, D.-H.; Rettori, M.-C.;Renard, P.; Pfeiffer, B.;Guardiola-
Lemaitre, B. Synthesis and structure-activity relationships of novel
2-amino alkyl chromones and related derivatives as s site-selective
ligands. Eur. J. Med. Chem. 1998, 33, 339–347.
(70) Younes, S.; Labssita, Y.; Baziard-Mouysset, G.; Payard, M.;
Rettori, M.; Renard, P.; Pfeiffer, B.; Caignard, D. Synthesis and
structure-activity relationships of novel arylalkyl 4-benzyl piper-
azine derivatives as sigma site selective ligands. Eur. J. Med. Chem.
2000, 35, 107–121.
€
(71) Lowik, D. W. P. M.; Lowe, C. R. Synthesis of Macrocyclic,
Triazine-Based Receptor Molecules. Eur. J. Org. Chem. 2001,
2001, 2825–2839.
(72) Carpino, L. A.; Mansour, E. M. E.; Cheng, C. H.; Williams, J. R.;
MacDonald, R.; Knapczyk, J.; Carman, M.; Lopusinski, A. Poly-
styrene-based deblocking-scavenging agents for the 9-fluorenyl-
methyloxycarbonyl amino-protecting group. J. Org. Chem. 1983,
48, 661–665.
(73) Tahtaoui, C.; Parrot, I.; Klotz, P.; Guillier, F.; Galzi, J.-L.; Hibert,
M.; Ilien, B. Fluorescent Pirenzepine Derivatives as Potential
Bitopic Ligands of the Human M1Muscarinic Receptor. J. Med.
Chem. 2004, 47, 4300–4315.
(74) Kovacs, K. J.; Larson, A. A. Up-regulation of [3H]DTG but not
[3H](þ)-pentazocine labeled sigma sites in mouse spinal cord by
chronic morphine treatment. Eur. J. Pharmacol. 1998, 350, 47–52.
(75) Eckleman, W. C. The Design of Site-Directed Radiopharmaceu-
ticals for Use in Drug Discovery. In Nuclear Imaging in Drug
Discovery, Development and Approval; Burns, D. H., Gibson, R. E.,
€
Dannals, R. F., Siegl, P. K., Eds.; Birkhauser: Boston, 1993; pp
113-134.
(76) Quinlivan, M.; Mattner, F.; Papazian, V.; Zhou, J.; Katsifis, A.;
Emond, P.; Chalon, S.; Kozikowski, A.; Guilloteau, D.; Kassiou,
M. Synthesis and evaluation of iodine-123 labelled tricyclic tro-
panes as radioligands for the serotonin transporter. Nucl. Med.
Biol. 2003, 30, 741–746.
(77) Waterhouse, R. N.; Mardon, K.; Giles, K. M.; Collier, T. L.;
O’Brien, J. C. Halogenated 4-(phenoxymethyl)piperidines as po-
tential radiolabeled probes for sigma-1 receptors: in vivo evalu-
ation of [123I]-1-(iodopropen-2-yl)-4-[(4-cyanophenoxy)methyl]-
piperidine. J. Med. Chem. 1997, 40, 1657–1667.
(56) Crawford, K. W.; Bowen, W. D. Sigma-2 receptor agonists activate
a novel apoptotic pathway and potentiate antineoplastic drugs in
breast tumor cell lines. Cancer Res. 2002, 62, 313–322.
(57) Aydar, E.; Palmer, C. P.; Djamgoz, M. B. Sigma receptors and
cancer: Possible involvement of ion channels. Cancer Res. 2004, 64,
5029–5035.
(58) Wheeler, K. T.; Wang, L. M.; Wallen, C. A.; Childers, S. R.; Cline,
J. M.; Keng, P. C.; Mach, R. H. Sigma-2 receptors as a biomarker
of proliferation in solid tumours. Br. J. Cancer 2000, 82, 1223–1232.
(59) Collier, T. L.; Waterhouse, R. N.; Kassiou, M. Imaging sigma
receptors: Applications in drug development. Curr. Pharm. Des.
2007, 13, 51–72.
(78) Walker, J. M.; Bowen, W. D.; Goldstein, S. R.; Roberts, A. H.;
Patrick, S. L.; Hohmann, A. G.; DeCosta, B. Autoradiographic
distribution of [3H](þ)-pentazocine and [3H]1,3-di-o-tolylguani-
dine (DTG) binding sites in guinea pig brain: a comparative study.
Brain Res. 1992, 581, 33–38.
(79) Bouchard, P.; Quirion, R. [3H]1,3-di(2-tolyl)guanidine and
[3H](þ)pentazocine binding sites in the rat brain: autoradiographic
visualization of the putative sigma1 and sigma2 receptor subtypes.
Neuroscience 1997, 76, 467–477.
(60) Glennon, R. A.; Ablordeppey, S. Y.; Ismaiel, A. M.; el-Ashmawy,
M. B.; Fischer, J. B.; Howie, K. B. Structuralfeatures important for
sigma 1 receptor binding. J. Med. Chem. 1994, 37, 1214–1219.
(61) Ablordeppey, S. Y.; Fischer, J. B.; Glennon, R. A. Is a nitrogen
atom an important pharmacophoric element in sigma ligand bind-
ing? Bioorg. Med. Chem. 2000, 8, 2105–2111.
(62) Ablordeppey, S. Y.; Fischer, J. B.; Law, H.; Glennon, R. A.
Probing the proposed phenyl-A region of the sigma-1 receptor.
Bioorg. Med. Chem. 2002, 10, 2759–2765.
(63) Glennon, R. A. Pharmacophore identification for sigma-1 (σ1)
receptorbinding: application ofthe “deconstruction-reconstruction-
elaboration” approach. Mini Rev. Med. Chem. 2005, 5, 927–940.
(64) Cratteri, P.; Romanelli, M. N.; Cruciani, G.; Bonaccini, C.;
Melani, F. GRIND-derived pharmacophore model for a series
of alpha-tropanyl derivative ligands of the sigma-2 receptor.
J. Comput.-Aided Mol. Des. 2004, 18, 361–374.
(80) Honer, M.; Bruhlmeier, M.; Missimer, J.; Schubiger, A. P.;
Ametamey, S. M. Dynamic Imaging of Striatal D2 Receptors
in Mice Using Quad-HIDAC PET. J. Nucl. Med. 2004, 45, 464–470.
(81) Kawamura, K.; Ishiwata, K.; Shimada, Y.; Kimura, Y.; Kobayashi,
T.; Matsuno, K.; Homma, Y.; Senda, M. Preclinical evaluation
of [11C]SA4503: Radiation dosimetry,in vivo selectivity and PET
imaging of sigma1 receptors in the cat brain. Ann. Nucl. Med. 2000,
14, 285–292.
(82) Yousef, K. A.; Fowler, J. S.; Volkow, N. D.; Dewey, S. L.; Shea, C.;
Schlyer, D. J.; Gatley, S. J.; Logan, J.; Wolf, A. P. [18F]haloperidol
binding in baboon brain in vivo. Nucl. Med. Biol. 1996, 23, 47–52.
(83) Yousef, K. A.; Volkow, N. D.; Schlyer, D. J.; Fowler, J. S.; Wolf,
A. P.; Wang, G.-J.; Smith, M. R.; Brodie, J. D.; Warner, D.
Haloperidol blocks the uptake of
[
18F]N-methylspiroperidol
by extrastriatal dopamine receptors in schizophrenic patients.
Synapse 1995, 19, 14–17.
(65) Abate, C.; Mosier, P. D.; Berardi, F.; Glennon, R. A. A Structure-
Affinity and Comparative Molecular Field Analysis of Sigma-2
(σ2) Receptor Ligands. Cent. Nerv. Syst. Agents Med. Chem. 2009,
9, 246–257.
(84) Kabalka, G. W.; Varma, M.; Varma, R. S.; Srivastava, P. C.;
Knapp, F. F. The tosylation of alcohols. J. Org. Chem. 1986, 51,
2386–2388.
(85) Parenty, A. D. C.; Smith, L. V.; Cronin, L. An unusual substitution
reaction directed by an intramolecular re-arrangement. Tetra-
hedron 2005, 61, 8410–8418.
(66) Laurini, E.; Zampieri, D.; Mamolo, M. G.; Vio, L.; Zanette, C.;
Florio, C.; Posocco, P.; Fermeglia, M.; Pricl, S. A 3D-pharmacophore