Is a nitrogen atom an important pharmacophoric element in sigma ligand binding?

Article abstract of DOI:10.1016/S0968-0896(00)00148-6

A lingering question in σ receptor ligand development is whether a nitrogen atom serves as an important pharmacophoric element in binding affinity. To address this question, we have synthesized several phenylalkylpiperidines and phenylalkylpiperazines and

Full text of DOI:10.1016/S0968-0896(00)00148-6

Bioorganic & Medicinal Chemistry 8 (2000) 2105±2111
Is a Nitrogen Atom an Important Pharmacophoric Element in
Sigma Ligand Binding?
Seth Y. Ablordeppey,a,* James B. Fischerb and Richard A. Glennonc
aCollege of Pharmacy & Pharmaceutical Sciences, Florida A&M University, Tallahassee, FL 32307, USA
bCambridge NeuroScience Inc., Cambridge, MA 02139, USA
cDept. of Medicinal Chemistry, School of Pharmacy, Medical College of Virginia,
Virginia Commonwealth University, Richmond, VA 23298, USA
Received 24 April 2000; accepted 18 May 2000
AbstractÐA lingering question in s receptor ligand development is whether a nitrogen atom serves as an important pharmaco-
phoric element in binding anity. To address this question, we have synthesized several phenylalkylpiperidines and phenylalkyl-
piperazines and demonstrated that removal of the N atom from a typical phenylalkylpiperidine led to little or no binding to s
receptors. In addition, where two N atoms occur in a compound, such as with phenylalkylpiperazines, the N atom on the longer
alkyl chain appears to be more important. Thus, based on this study, the N atom is an important pharmacophoric element in the
binding of phenylalkylpiperidines and phenylalkylpiperazines to s receptors. # 2000 Elsevier Science Ltd. All rights reserved.
Although the initial excitement about sigma ligands has
subsided somewhat, they continue to attract attention
because of the involvement of sigma receptors in several
physiological functions.1 8 For example, several recent
publications support the hypothesis that sigma ligands
may serve as atypical antipsychotic agents with minimal
extrapyramidal action.9 In addition, sigma receptors
appear to modulate K+ channels,10 and may serve as
targets for drug design against certain tumor cells.11
Unlike most neurotransmitter receptors, however,
sigma receptors seemingly accommodate a very wide
variety of structural classes.1,2 This situation has made it
dicult to obtain a unifying common pharmacophore
model for each of the known receptor subtypes.
Although several sigma receptor subtypes might remain
to be identi®ed, the presence of a nitrogen atom appears
to be a common pharmacophoric element for most sigma
ligands. Unfortunately, the role of the nitrogen atom as a
required pharmacophoric element is clouded by the ®nding
that steroid molecules lacking a basic nitrogen atom
bind to sigma receptors with moderate anity, and may
e€ect several physiological actions through sigma
receptors.12 17
used the N atom as a primary pharmacophore element
for alignment purposes.18,19 On a number of such occa-
sions, however, we were faced with the decision of
selecting one nitrogen atom, among several, as the pri-
mary pharmacophoric element. We have previously
shown that modi®cations around the nitrogen atom in
phenylalkylamines and phenylpiperidines often led to
changes in ligand binding anity.20,24,25 Therefore, we
have sought a better understanding of the role and con-
tributions of the nitrogen atom in sigma ligand binding
because it might lead to the design and synthesis of new
and more selective agents for sigma receptors. Thus, the
primary purpose of this study was to evaluate the con-
tributions of the N atom to sigma receptor binding
using phenylpiperazines, phenylpiperidines and pheny-
lalkylamines as the sca€old. It was also of interest to
determine if the resulting modi®cations lead to more
potent and selective s ligands; consequently, both s-1
and s-2 receptor binding data were obtained on most of
the compounds.
Chemistry
In our work,18 25 on developing structure±anity rela-
tionship models for sigma receptor ligands, we have
Standard synthetic methods were used to obtain most of
the compounds examined in this investigation. Compound
1 was available commercially from Aldrich Chemicals.
Both 3 and 4 were obtained by direct N-alkylation of 1
and 2, respectively. Using the appropriate phenylalkyl
*Corresponding author. Tel.:+1-850-599-3834; fax: 1-850-599-3934;
e-mail: sablordeppey@famu.edu
0968-0896/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0968-0896(00)00148-6

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S. Y. Ablordeppey et al. / Bioorg. Med. Chem. 8 (2000) 2105±2111
bromides generated from commercially available phenyl-
alkanols, compounds 5±9 were similarly synthesized.
The preparation of compound 10 (Scheme 1) required
the use of the Wittig reaction. Thus, triphenylphosphine
was alkylated with 5-phenylpentyl bromide24 to obtain
the phosphonium bromide 18, which was subsequently
reacted with sul®nyl carbanion generated from DMSO
and sodium hydride. The resulting carbanion was
reacted with N-methyl-4-piperidinone to form the
corresponding ole®n, 19, which was catalytically reduced
using palladium/carbon to yield the desired compound.
Compound 11 was resynthesized using a published pro-
cedure.24 To synthesize compound 12 (Scheme 2), the key
intermediate, 4-cyclohexylbutanal (21), was obtained by
oxidation of 4-cyclohexylbutanol (20) under Collins'
conditions. The aldehyde was added to a mixture of 4-
nitrobenzyltriphenylphosphonium bromide (22) and
methyl sul®nyl carbanion, generated from dimethyl-
sulfoxide and sodium hydride, to yield the required
nitrophenyl ole®n 23. Hydrogenation under Parr con-
ditions and in the presence of palladium and carbon
yielded the desired compound, 12.
Results and Discussion
Radioligand binding data are shown in Table 1. 1-Ben-
zylpiperazine (1) and 1-benzoylpiperazine (2) do not bind
to s receptors (i.e., Ki>10,000 nM).22 Alkylation of the
terminal N atom with a Ph-(CH2)4-X (where X=halide)
in each of these compounds, results in a dramatic
increase in the s-1 binding anity of the resulting pro-
ducts. However, while there is at least a 50,000-fold
increase from 1 to 3 (Ki=0.2 nM), there is a much
smaller increase from 2 to 4 (Ki=125 nM).22
At least two possible hypotheses can be expounded for
the signi®cant di€erence in the binding anities. A basic
nitrogen atom is required at N(a) and the introduction
of an amide N at that position therefore results in a lack
of binding anity. By implication, N(a) must be very
important for binding to s-1 receptors. A second hypoth-
esis for the di€erence in binding anity can be attributed
to the change in the position of the phenyl-A ring brought
about by the carbonyl group or by intolerance of the car-
bonyl oxygen itself. To further explore these issues, we
synthesized compound 5 in which N(a) is replaced by CH
in order to evaluate the importance of N(a) to binding.
Compound 5 binds with a Ki of 0.8 nM; only a 4-fold
di€erence with 3. Inserting an additional methylene
group in phenylalkyl derivatives 3 and 5 yielded similar
results (i.e., 6 Ki=0.4 nM, and 7 Ki=0.6 nM, respec-
tively). These results suggest that N(a) is not essential
for high anity binding to s-1 receptors.
Compound 13 was obtained by nitration of 11 to form
N-[5-(4-nitrophenyl)pentyl]piperidine and by the sub-
sequent hydrogenation using 10% palladium on carbon.
Compound 14 was obtained by the direct reduction of
the commercially available natural product, piperine,
while the synthesis of 15 (Scheme 3) was accomplished
by hydrogenation of piperine, followed by lithium alu-
minum hydride reduction of the resulting amide.
Finally, compound 16 was obtained by direct methyl-
ation of 7.
Interestingly, previous work in our laboratories has
demonstrated that the phenyl-A ring plays essentially
no role in the binding of phenylalkylamines or phenyl-
piperidines at sigma receptors. Thus, removal of the
Table 1. Sigma receptor ligand binding data
Scheme 1. Synthesis of compound 10.
Compound
Binding constants, Ki (nM)
s-1
s-2
Ratio of s-2/s-1
1a
2a
3
>10,000
>10,000
0.2 (0.1)c
125
0.8 (0.2)
0.4 (0.2)
0.6 (0.2)
1.4 (0.4)
0.07 (0.02)
1.3 (0.3)
0.48
>36,000
38 (8)
0.86 (0.24)
0.32 (0.10)
2.7 (0.9)
NTd
NT
NT
NT
3.1 (0.6)
NT
2.8 (0.8)
79 (13)
7 (2)
97 (18)
50 (12)
54,500
830 (132)
554 (180)
63 (12)
NT
4,a
5
4
6
7
8
9
10
11b
12
13
14
15
16b
5
64
100
75
Scheme 2. Synthesis of compound 12.
125
22
644
197
aPreviously reported as non-selective sigma receptor ligand; see ref 22.
bPreviously reported in ref 20.
cSEM in parenthesis.
Scheme 3. Synthesis of compound 15.
dNT=was not determined.

S. Y. Ablordeppey et al. / Bioorg. Med. Chem. 8 (2000) 2105±2111
2107
phenyl A ring from 6 and 7 led to compounds 8 (Ki=
1.4nM) and 9 (Ki=0.07nM) with anities consistent with
our previous ®ndings.
the para position of the phenyl ring based on previous
results indicating that substitution at this position might
be tolerated and anity may not be signi®cantly a€ec-
ted at s-1 receptors.25 We explored aromatic substitu-
tion to further evaluate this concept and found that
aromatic substitution is tolerated and can, depending
upon the substituents incorporated (e.g. 14 and 15),
actually enhance anity and/or selectivity; this will be
further discussed below. One of the compounds pre-
pared and examined was the amino-substituted deriva-
tive 12 (Ki>36,000 nM); compound 13 (Ki=38 nM) was
used as the positive control. Since compound 13 binds
with moderate anity, it can be concluded that replace-
ment of the piperidine nitrogen atom with CH reduces
anity by at least a 1000-fold and, hence, a nitrogen atom
is an essential pharmacophoric element in the binding of
these and related ligands to s-1 receptors. An alter-
native, but less satisfying, explanation is that compound
12 binds in an entirely di€erent manner to result in low
anity.
So far, the replacement of N(a) with CH in the pipera-
zines series appears to have no serious bearing on the
binding of these compounds to s-1 receptors. The ques-
tion then arises as to what role N(b) plays in the binding
of the piperazines. We reasoned that if N(a) does not
contribute signi®cantly to binding at s-1 receptors, and
if a nitrogen atom is required, N(b) must be the essential
N atom required for binding anity. We therefore syn-
thesized compound 10 in an attempt to answer this
question. Compound 10 (Ki=1.3 nM) binds as well as 8
although its binding anity is 20-fold lower than that of
9. We can deduce from these results that compounds in
which N(a) is present bind to s-1 receptors in essentially
the same way notwithstanding the presence of other
nitrogen atoms such as N(b). Furthermore, although N(b)
provides a more e€ective binding to sigma receptors, the
presence of other N atoms leads to additional interactions
which somehow detracts from maximum binding. This
observation is consistent with our previous suggestion that
phenylalkylamines, phenylalkylpiperidines and phenyl-
alkylpiperazines may utilize reverse modes of binding to
s-1 receptors26 (see Fig. 1 below).
In the course of exploring aromatic substitution, as
described above, we prepared several aryl substituted
compounds including 14 and15. The purpose of these
studies was to determine the in¯uence of aryl substituents
on s-1 binding and subsequently to determine if such
substitutions detract from binding. Compounds 14
(Ki=0.86 nM) and 15 (Ki=0.32 nM) were both found to
bind with high anity. Comparing compounds 11 (Ki=
0.48nM) and 15 (Ki=0.32 nM) also supports the sugges-
tion that aromatic substitution has little in¯uence on s-1
binding. Interestingly, compound 14 displayed the
highest s-2/s-1 selectivity among the ligands evaluated
in this study (14: s-1 Ki=0.86 nM; s-2 Ki=554 nM, for
a ratio of 664).
Compound 9 displays the highest anity of the com-
pounds examined. It is important, then, to examine the
role the N atom plays in the binding of such compounds.
We have previously reported that N-o-phenylpentylpiper-
idine (11), a demethylated analogue of 9, binds with high
anity at s-1 receptors (Ki=0.48nM). In order to evalu-
ate the necessity of the nitrogen atom for sigma binding,
we wished to replace it with a methine group. However,
this would likely result in a water-insoluble compound.
To overcome this problem, we needed to attach a water-
solubilizing group elsewhere in the molecule. We selected
Having established the importance of the N(b) atom for
maximum binding, it was also of interest to examine the
nature of the nitrogen atom as it binds the sigma receptors.
In this regard, we wanted to know if a lone pair of
electrons on the nitrogen was needed for binding or if
protonation of the nitrogen occurs on binding. A very
simple way to do this is to introduce a permanent positive
charge on the nitrogen such as occurs in quaternization.
We therefore prepared and examined the N-methylated
analogue of compound 7 (i.e. compound 16). Com-
pound 16 binds with a Ki of 2.7 nM; implying that the
lone pair of electrons on these compounds probably
becomes protonated at s-1 receptors and that a proto-
nated N atom may be required for s-1 receptor binding.
The binding anities of several of the present compounds
were also examined at s-2 receptors (Table 1). All of the
compounds investigated displayed (4- to>600-fold) lower
anity at s-2 than at s-1 receptors. Comparing com-
pounds 12 and 13, it would appear that N(b) might also
be an important feature for s-2 binding. It also seems that
Figure 1. Proposed receptor features important for s-1 binding18,20
and the modes of binding of phenylpiperazines (4 modes) and phe-
nylpiperidines (2 modes).

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S. Y. Ablordeppey et al. / Bioorg. Med. Chem. 8 (2000) 2105±2111
aromatic substitution (e.g., compare 11 with 13±15) is not
as well tolerated by s-2 receptors as it is by s-1 receptors.
4-Benzyl-1-(5-phenylpentyl)piperidine hydrochloride (7).
A stirred mixture of 4-benzylpiperidine (1.0 g, 5.7 mmol),
5-phenylpentyl bromide (1.95 g, 8.6 mmol), K2CO3
(1.57 g, 11.4 mmol), and KI (100 mg) in 1,2-dimethox-
yethane (DME) (10 mL) was heated under re¯ux over
the weekend (60 h) and allowed to cool to room tem-
perature. The mixture was partitioned between Et2O
(30 mL) and aqueous ammonia solution (15 mL). The
aqueous portion was re-extracted with Et2O (30 mL) and
the combined organic portions were pooled, washed with
H2O (20 mL) and dried (MgSO4). Solvent was removed
under reduced pressure and the residue (335 mg, 19%)
was converted to the hydrochloride salt using ethereal
HCl: mp 163±165 ꢀC. 1H NMR (300 MHz, DMSO-d6) d
7.42±7.10 (10 H, m), 3.76 (2H, t), 3.50 (2H, d, br), 3.28,
(2H, m), 2.62 (4H, m), 2.20, (1H, br), 1.88±1.32, m); 13C
NMR (75 MHz, DMSO-d6) d 23.43, 26.36, 28.84 (2C),
30.65, 35.54, 36.62, 41.95, 52.92 (2C), 57.52, 125.85,
126.36, 128.36 (4C), 128.49, (2C), 128.98 (2C), 139.12,
141.80. Anal. C23H32NCl, C, H, N.
Conclusion
In this study, it has been demonstrated that at least one
N atom in phenylpiperidines and phenylpiperazines is
important for binding at sigma receptors. In the case of
the piperazines, the N atom attached to the longer
chain, N(b), is more important than N(a). Furthermore,
several modes of binding are available to each molecule
at the binding site and therefore reversed modes of
binding are possible as well. Thus, we have now extended
our previously reported sigma ligand model to include the
various modes of binding for phenylpiperidines and phe-
nylpiperazines at the s-1 receptors (Fig. 1). If the N atom
is important in s binding, how can we explain the binding
of non-basic steroids to sigma receptors? At this point, it
can only be speculated that steroids probably bind in a
di€erent manner at the s receptors from the phenyl-
alkylamines, such as the ones discussed in this manuscript.
1-Methyl-4-(5-phenylpentyl)piperazine dihydrobromide (8).
A stirred mixture of 1-methylpiperazine (0.3g, 3.0 mmol),
5-phenylpentyl bromide (0.7 g, 3.1 mmol), K2CO3 (0.9 g,
6.2 mmol) and KI (100 mg) in 1,2-dimethoxyethane
(DME) (8 mL) was heated under re¯ux for 5 h and
allowed to cool to room temperature. The mixture was
partitioned between Et2O (30 mL) and 10% NaOH
solution (15 mL). The aqueous portion was reextracted
with Et2O (30 mL) and the combined organic portions
were pooled, washed with H2O (20 mL) and dried
(MgSO4). A saturated hydrogen bromide solution in
anhydrous Et2O was added to obtain a salt, which was
recrystallized from MeOH (0.51 g, 43%): mp 239 ꢀC
Experimental
Syntheses
Proton and carbon magnetic resonance spectra were
obtained on QE 300 (300 MHz/75 MHz) spectrometers
with tetramethylsilane as internal standard. The unit of
all J values shown in the experimentals is Hz, and all
spectra are consistent with the assigned structures. Melt-
ing points were determined on a Thomas Hoover appa-
ratus and are uncorrected. Elemental analyses were
performed by Atlantic Microlab and determined values
are within 0.4% of calculated values. No attempt was
made to optimize yields in any of the synthesis presented.
1
(Dec.). H NMR (300 MHz, DMSO-d6) d 7.22 (2H, d,
J=9 Hz), 7.17 (1H, d, J=7 Hz), 7.15 (2H, dd, J=9,
7 Hz), 3.73 (4H, s, br), 3.39 (4H, s, br), 3.18 (2H, s, br),
2.88 (3H, s), 2.54 (2H, t, J=7 Hz), 1.68 (2H, m), 1.56
(2H, m), 1.27 (2H, m); 13C NMR (75 MHz, DMSO-d6)
d 25.67, 30.58, 35.04, 38.96, 84.11, 125.94, 128.51,
142.17. Anal. C16H28N2Br2, C, H, N.
1-Benzyl-4-(5-phenylpentyl)piperazine (6) hydrochloride
(method A). A stirred mixture of 1-benzylpiperazine
(0.4 g, 2.3 mmol), 5-phenylpentyl bromide (0.5 g, 2.2
mmol), K2CO3 (0.6 g, 4.3 mmol), and KI (50 mg) in 1,2-
dimethoxyethane (DME) (8 mL) was heated under
re¯ux overnight (20 h) and allowed to cool to room
temperature. The mixture was partitioned between Et2O
(30 mL) and aqueous ammonia solution (15 mL). The
aqueous portion was reextracted with Et2O (30 mL) and
the combined organic portions were pooled, washed
with H2O (20 mL) and dried (MgSO4). Solvent was
removed under reduced pressure and the residue
chromatographed over silica gel using 50% EtOAc/hex-
ane to yield a yellowish oil, which was converted to the
hydrochloride salt using ethereal HCl (156 mg, 18%): mp
4-Methyl-1-(5-phenylpentyl)piperidine hydrogen oxalate
(9). Method A was used. Product (1.2 g, 75%), mp
138±139 ꢀC. Recrystallization solvent: MeOH/Et2O. H
1
NMR (300 MHz, DMSO-d6) d 7.23 (2H, d, J=9 Hz),
7.15 (2H, dd, J=9, 7 Hz), 7.12 (1H, d, J=7 Hz), 3.62
(2H, d, J=15 Hz), 2.92 (2H, dd, J=15, 9 Hz), 2.55 (2H,
t, J=7 Hz), 1.65 (11H, m), 1.29 (3H, d, J=6 Hz); 13C
NMR (75 MHz, DMSO-d6) d 20.82, 23.38, 26.06, 29.14,
30.48, 30.84, 35.31, 53.09, 57.29, 125.64, 128.16, 141.63,
163.08. Anal. C19H29NO4, C, H, N.
N-Methyl -4-(phenylpentyl)piperidine hydrogen oxalate
(10). A mixture of triphenylphosphine (3.8 g, 14.5 mmol)
and 5-phenylpentyl bromide (3.1g, 13.6mmol) in dry
benzene (20 mL) was allowed to re¯ux overnight (ꢁ24h).
After cooling to room temperature, anhydrous Et2O was
added, the solid was ®ltered and washed with several
portions of anhydrous Et2O and dried under reduced
pressure at 50ꢀC overnight (6.1 g, 91%); mp 143±145 ꢀC.
A solution of methyl sul®nyl carbanion was obtained by
stirring a mixture of dried DMSO (10 mL) and powdered
253 ꢀC (Dec.). H NMR (300 MHz, DMSO-d6) d 7.66
1
(2H, dd, J=4, 3), 7.47 (3H, m), 7.27 (2H, d, J=8), 7.18
(2H, dd, J=8, 6), 7.14 (1H, d, J=6 Hz), 4.34 (2H, s),
3.92 (4H, d, J=9 Hz), 3.57 (2H, d, J=9 Hz), 3.48 (2H,
d, J=9 Hz), 3.10 (2H, t, J=8 Hz), 2.62 (2H, t, J=6 Hz),
1.85 (2H, m), 1.66 (2H, m), 1.48 (2H, m); 13C NMR (75
MHz, DMSO-d6) d 23.48, 26.00, 30.54, 35.42, 47.84,
57.03, 60.64, 125.96, 128.38, 129.60, 130.65, 131.30,
.
141.61. Anal. C22H32N2Cl2 0.2H2O, C, H, N.

S. Y. Ablordeppey et al. / Bioorg. Med. Chem. 8 (2000) 2105±2111
2109
sodium hydride (0.2g) under N2 and heating at ꢁ50 ꢀC
for 15 min. The resulting solution was cooled to 0 ꢀC and
a solution of 5-phenylpentyltriphenylphosphonium bro-
mide (3 g, 6.1 mmol) in dry DMSO (10 mL) was added in
a dropwise manner over 5 min. After 15 min of stirring at
room temperature, a solution of 1-methyl-4-piperidone
(0.6 g, 5.3 mmol) in dry DMSO (10 mL) was added in a
dropwise manner over ꢁ5 min. The resulting reddish
yellow solution was allowed to stir at room temperature
for ꢁ20 h and then poured into H2O (80 mL) and
extracted with Et2O (3Â50 mL). The pooled Et2O solu-
tion was washed with H2O (2Â50 mL) and dried
(MgSO4). Solvent was removed under reduced pressure
and the residue was converted to the oxalate salt, which
was recrystallized from abs EtOH (0.31 g, 17%). 1H
NMR (300 MHz, DMSO-d6) d 7.19 (5H, m), 5.25 (1H, t,
J=9 Hz), 3.01 (4H, s, br), 2.65 (3H, s), 2.51(2H, t,
J=6 Hz), 2.32 (4H, m), 1.95 (2H, dt, J=9, 9 Hz), 1.51
(2H, m), 1.27 (2H, m). 13C NMR (75 MHz, DMSO-d6)
d 25.01, 26.85, 29.21, 30.87, 32.23, 35.38, 42.28, 54.16,
54.82, 125.73, 126.03, 128.63, 130.54, 142.56, 165.20.
was cooled to 0 ꢀC and a solution of 4-nitrobenzyl-
triphenylphosphonium bromide (5 g, 10 mmol) in dry
DMSO (10 mL) was added in a dropwise manner over
5 min. After 15 min of stirring at room temperature, a
solution of 4-cyclohexyl-1-butanal (0.9 g, 6 mmol) in dry
DMSO (5 mL) was added in a dropwise manner over
ꢁ5 min. The resulting deep red solution was allowed to
stir at room temperature for ꢁ6 h and then poured into
H2O (50 mL) and extracted with Et2O (3Â50 mL). The
pooled Et2O solution was washed with H2O (30 mL) and
dried (MgSO4). Solvent was removed under reduced
pressure and the residue was Kugelrohr distilled (1 mm,
240 ꢀC) to a€ord the desired compound as a yellowish
oil (1 g, 63%). A solution of this product (0.9 g) in MeOH
(40 mL) was hydrogenated under Parr conditions (psi 55)
for 7 h in the presence of Pd-C (10%) (0.2g). The mixture
was ®ltered and after the solvent was removed under
reducedpressure, the residue was redissolvedin Et2O, dried
(MgSO4) and a saturated oxalic acid solution was added to
form the salt. Recrystallization was achieved from MeOH/
anhydrous Et2O (0.6 g, 53%); mp 127±132 ꢀC. H NMR
1
(300 MHz, DMSO-d6) d 7.40 (3H, s, br), 6.87 (2H, d,
J=6 Hz), 6.61 (2H, d, J=6 Hz), 2.37 (2H, t, J=6Hz), 1.61
(6H, m), 1.43 (2H, m), 1.13 (9H, m , 0.87 (2H, m); 13C
NMR (75MHz, DMSO-d6) d 26.12, 26.37, 26.48, 29.18,
31.62, 33.16, 34.66, 37.24, 116.17, 129.02, 133.15, 144.16,
A solution of this product (0.25 g, 0.74 mmol) in abs
EtOH (40 mL) was hydrogenated under Parr conditions
(psi 50) for 3 h in the presence of Pd/C (10%) (0.1 g)
catalyst. The mixture was ®ltered, the solvent was
removed under reduced pressure and the residue was
recrystallized from MeOH/anhydrous Et2O/EtOAc
(0.19 g, 77%); mp 142±144 ꢀC. 1HNMR (300 MHz,
DMSO-d6) d 7.18 (5H, m), 3.25 (2H, d, J=12 Hz), 2.78
(2H, t, J=12 Hz), 2.62 (3H, s), 2.50 (2H, t, J=9 Hz),
1.72 (2H, d, J=15 Hz), 1.50 (2H, m), 1.23 (9H, m); 13C
NMR (75 MHz, DMSO-d6) d 26.19, 29.12, 31.33, 32.67,
35.33, 35.52, 42.87, 53.49, 125.99, 128.63, 142.65,
165.27. Anal. C19H29NO4, C, H, N.
.
162.07. Anal. C19H29NO4 0.5H2O, C, H, N.
N-[5-(4-aminophenyl)pentyl]piperidine dihydrogen oxalate
(13). An ice-cold solution of compound 11 (0.5 g,
1.6 mmol), concd H2SO4 (2 mL) and glacial AcOH (3mL)
were mixed, and a mixture of concd H2SO4 (1 mL) and
fuming HNO3 (1 mL) was added in a dropwise manner
to maintain the temperature at ꢁ0 ꢀC. Stirring was
continued at room temperature for 0.5 h. Thereafter, the
mixture was poured onto ice (50 g), basi®ed by the
addition of solid NaOH and extracted with Et2O
(2Â50 mL). The pooled organic portion was dried
(MgSO4) and the oxalate salt was prepared and recrys-
tallized from MeOH (205 mg, 37%), mp 159±161 ꢀC.
The resulting compound (0.38 g) in MeOH (50 mL) was
hydrogenated at a pressure of 50 psi in a Parr bottle
containing 10% Pd/C (0.2 g) until sucient H2 was
taken up (24 h). The catalyst was removed by ®ltration,
the solvent was removed under reduced pressure and the
oxalate salt was prepared. Recrystallization was achieved
from Abs EtOH/EtOAc (0.24 g, 51%), mp 99±103 ꢀC. 1H
NMR (300 MHz, DMSO-d6) d 6.87 (2H, d, J=12 Hz),
6.58 (2H, d, J=12 Hz), 3.31 (2H, s, br), 2.89 (2H, s, br),
2.72 (2H, s, br), 2.36 (2H, t, J=9Hz), 1.50 (10H, m), 1.19
(2H, s, br); 13C NMR (75 MHz, DMSO-d6) d 22.23, 23.27,
23.85, 26.51, 31.63, 34.99, 52.82, 56.69, 116.55, 129.70,
1-(5-Phenylpentyl)piperidine (11). A stirred mixture of
piperidine (1.1 g, 13 mmol), 5-phenylpentyl bromide24
(3.1 g, 14 mmol) KI (20 mg) and K2CO3 (2 g, 15 mmol)
in 1,2-dimethoxyethane (DME) (10 mL) was heated
under re¯ux for 12 h and allowed to cool to room tem-
perature. The mixture was partitioned between Et2O
(30 mL) and 10% NaOH solution (15 mL). The aqueous
portion was reextracted with Et2O (30 mL) and the
combined organic portions were pooled, washed with
H2O (20 mL) and dried (MgSO4). The oxalate salt was
prepared and recrystallized from MeOH (3.1 g, 74%),
mp 149±151 ꢀC (lit.24 149 ꢀC.)
5 - (4 - Aminophenyl)pentylcyclohexane hydrogen oxalate
(12). A solution of pyridine chlorochromate (16.9 g) in
CH2Cl2 (200mL) was added in a portionwise manner to a
stirred, ice-cold solution of 4-cyclohexyl-1-butanol (3.0 g,
19.2 mmol) in CH2Cl2 (50 mL). After addition was com-
plete, the mixture was allowed to stir at room temperature
for ꢁ15 min. The mixture was washed with H2O
(2Â50 mL), dried (MgSO4) and solvent was removed under
reduced pressure. Distillation (using Kulgerohl), under
reduced pressure, a€orded a colorless oil (1.1g, 38%).
.
.
164.03. Anal. C16H26N2 2.1C2H2O4 1.0H2O, C, H, N.
1-[1-(3,4-Methylenedioxyphenyl)-2,4-pentadienyl]piperi-
dine hydrochloride (14). A solution of piperine (1.0 g,
3.5 mmol) in Et2O (50 mL) was added to a stirred sus-
pension of LiAlH4 (0.93 g, 7 equiv) in Et2O (100 mL)
and the mixture allowed to re¯ux under an atmosphere of
N2 for 24h. The reaction was quenched by the addition of
drops of H2O (20 mL) and stirring was continued for an
additional 0.5 h. The ethereal portion was decanted and
10% NaOH (60 mL) was added to the residue. The
A solution of methyl sul®nyl carbanion, obtained by
stirring a mixture of dried DMSO (10 mL) and powdered
sodium hydride (0.25 g) under N2 at ꢁ50 ꢀC for 15 min,

2110
S. Y. Ablordeppey et al. / Bioorg. Med. Chem. 8 (2000) 2105±2111
resulting mixture was then ®ltered and the ®ltrate
extracted with Et2O (2Â50 mL). The total combined
organic phases was washed with H2O (20 mL), dried
(MgSO4) and ethereal HCl was added to obtain a pre-
cipitate of the salt. The precipitate was recrystallized
from EtOAc/MeOH to give yellow crystals, mp 174±
176 ꢀC. 1H NMR (300 MHz, CDCl3) d 12.10 (1H, s, br),
6.92 (1H, d, J=1.5 Hz), 6.82 (1H, d, J=1.5 Hz), 6.78
(1H, s), 6.65 (1H, dd, J=11, 16Hz), 6.54 (1H, d,
J=16 Hz), 6.48 (1H, dd, J=16, 12Hz), 6.03 (1H, dt,
J=16, 8 Hz), 5.98 (2H, s), 3.65 (2H, dd, J=4, 12Hz), 3.51
(2H, d, J=12 Hz), 2.63 (2H, dt, J=12, 4 Hz), 2.25 (2H,
m), 1.89 (2H, m), 1.42 (2H, m); 13C NMR (300MHz,
CDCl3) d 21.91, 22.41, 52.26, 59.13, 101.05, 105.42,
108.26, 118.80, 121.76, 124.65, 130.54, 135.46, 139.99,
membranes and ꢂ‡†-[3H]pentazocine (3±4 nM ®nal
concentration) in a ®nal volume of 500 mL of 50 mM
Tris±HCl bu€er (pH 8.0). For the standard equilibrium
essay, the mixtures were incubated for 4 h at 37 ꢀC, the
reactions quenched with 4 mL of ice-cold incubation
bu€er, and the mixtures rapidly ®ltered over Whatman
GF/B or Schleicher & Scheull no. 32 glass ®ber ®lters
followed by three 4 mL rinses with additional ice-cold
bu€er. The radioactivity on the ®lters was determined by
scintillation spectrometer at an eciency of about 50%.
Nonspeci®c binding was determined in the presence of
10mM haloperidol. IC50 values were determined from
competitive curves using nonlinear least-squares regres-
sion analysis and converted to Ki values with the Cheng±
Pruso€ transformation. Each Ki value was determined
from three to ®ve separate determinations.
.
147.79, 148.00. Anal. C17H22NO2Cl 0.15H2O, C, H, N.
1-[5-(3,4-Methylenedioxyphenyl)-pentyl]piperidine hydro-
gen oxalate (15). A mixture of piperine (1.0 g, 3.5
mmol), 10% Pd/C (0.24 g) and MeOH (50 mL) was
hydrogenated at 50 psi for 2 h. The mixture was ®ltered,
and solvent was removed in vacuo to a€ord an amide; IR
The s-2 selective binding assay27 was performed using
about 2 nM [3H]DTG as the radioligand in the presence
of 200 nM ꢂ‡†pentazocine to block the s-1 sites, with
400 mg of guinea pig brain membranes in a total volume
of 500 mL of 50 mM TRIS-HCl, 7.4. Nonspeci®c binding
was determined in the presence 10mM haloperidol. For
the standard equilibrium assay the mixtures were incu-
bated for 30 min at room temperature, ®ltered and the
radioactivity determined as described for s-1.
p
=1651 cm 1. Without further puri®cation, the amide
was dissolved in Et2O (50 mL) and added dropwise onto
a suspension of LiAlH4 (0.6 g, 0.01 mol) in Et2O
(50 mL). The mixture was allowed to re¯ux for 4 h after
which it was cooled in ice and quenched by the addition
of H2O (3 mL), 10% NaOH (3 mL) and H2O (10 mL).
The resulting mixture was allowed to stir for additional
0.5 h, the organic phase was decanted, and the aqueous
phase extracted with Et2O (2Â50 mL). The organic
phases were pooled, dried (MgSO4) and a saturated
solution of oxalic acid was added to precipitate the salt.
The salt was recrystallized from EtOAc/MeOH to give
Acknowledgements
We acknowledge the ®nancial support by the Cam-
bridge NeuroScience, Inc. This work is also supported
in part by a grant from the National Institute of Health,
Division of Research Resources, MBRS program,
Grant # GM 08111. We also acknowledge the valuable
help rendered by Dr. M. Dukat and Dr. Shouming Li
during the preparation of this manuscript.
white crystals (0.90 g, 70%), mp 122±123 ꢀC. H NMR
1
(300 MHz, DMSO-d6) d 6.72 (1H, d), 6.63 (1H, s), 6.58
(1H, d), 5.91 (2H, s), 3.62 (2H, d), 2.95 (2H, m), 2.60
(2H, m), 2.50 (2H, t), 2.08±1.66 (7H, m), 1.58 (2H, m),
1.45±1.22 (3H, m); 13C NMR (75 MHz, DMSO-d6) d
21.88, 22.51 (2C), 23.29, 25.90, 30.72, 35.01, 53.16 (2C),
57.17, 100.53, 107.91, 108.55, 120.88, 135.46, 163.08.
Anal. C19H27NO6, C, H, N.
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