Chemistry and pharmacology of the piperidine-based analogues of cocaine. Identification of potent DAT inhibitors lacking the tropane skeleton

Article abstract of DOI:10.1021/jm980028+

To discover agents that might be useful in the treatment of cocaine abuse, we have chosen to re-explore a class of molecules that was first reported by Clarke et al. in 1973 and that was and shown to lack locomotor stimulatory activity in mice. These compounds are piperidine-3-carboxylic acid esters bearing a 4-chlorophenyl group in position 4, and as such, these structures may be viewed as truncated versions of the WIN series compounds, i.e., they lack the two-carbon bridge of the tropanes. All members of this class were synthesized starting from arecoline hydrobromide and obtained in optically pure form through resolution methods using either (+)- or (-)- dibenzoyltartaric acid. Interestingly, we have found that these piperidines do, in fact, exhibit substantial affinity in both WIN 35,428 binding at the dopamine transporter and in the inhibition of [³H]dopamine uptake. Of all of the compounds synthesized, the 3-n-propyl derivative (-)-9 was found to be the most potent with a binding affinity of 3 nM. This simple piperidine is thus 33-fold more potent than cocaine in binding affinity and 29-fold more potent in its inhibition of dopamine uptake. Although no efforts have presently been made to 'optimize' binding affinity at the DAT, the substantive activity found for the n-propyl derivative (-)-9 is remarkable; the compound is only about 10-fold less active than the best of the high- affinity tropanes of the WIN series. As a further point of interest, it was found that the cis-disubstituted piperidine (-)-3 is only about 2-fold more potent than its trans isomer (+)-11. This result stands in sharp contrast to the data reported for the tropane series, for the epimerization of the substituent at C-2 from β to α has been reported to result in a lowering of activity by 30-200-fold. This smaller spread in binding affinities for the piperidines may reflect the smaller size of these molecules relative to the tropanes, which allows both the cis and the trans isomers to adjust themselves to the binding site on the DAT. Our present demonstration that these piperidine structures do, in fact, possess significant DAT activity, taken together with their reported lack of locomotor activity, provides a compelling argument for exploring this class of molecules further in animal behavioral experiments. The present work thus broadens the scope of structures that may be considered as lead structures in the search for cocaine abuse medications.

Full text of DOI:10.1021/jm980028+

1962
J . Med. Chem. 1998, 41, 1962-1969
Ch em istr y a n d P h a r m a cology of th e P ip er id in e-Ba sed An a logu es of Coca in e.
Id en tifica tion of P oten t DAT In h ibitor s La ck in g th e Tr op a n e Sk eleton
Alan P. Kozikowski,*,† Gian Luca Araldi,† J ohn Boja,‡ William M. Meil,‡ Kenneth M. J ohnson,§
J udith L. Flippen-Anderson,| Clifford George,| and Eddine Saiah
Drug Discovery Program, Institute of Cognitive and Computational Sciences, Georgetown University Medical Center,
3970 Reservoir Road, NW, Washington, D.C. 20007-2197, Department of Pharmacology, NEOUCOM, Rootstown, Ohio 44272,
Department of Pharmacology and Toxicology, University of Texas Medical Branch, Galveston, Texas 77555-1031, Laboratory
for the Structure of Matter, Code 6030, Naval Research Laboratory, 4555 Overlook SW, Washington, D.C. 20375-5000, and
Helios Pharmaceuticals, 9800 Bluegrass Parkway, Louisville, Kentucky 40299
Received J anuary 15, 1998
To discover agents that might be useful in the treatment of cocaine abuse, we have chosen to
re-explore a class of molecules that was first reported by Clarke et al. in 1973 and that was
and shown to lack locomotor stimulatory activity in mice. These compounds are piperidine-
3-carboxylic acid esters bearing a 4-chlorophenyl group in position 4, and as such, these
structures may be viewed as truncated versions of the WIN series compounds, i.e., they lack
the two-carbon bridge of the tropanes. All members of this class were synthesized starting
from arecoline hydrobromide and obtained in optically pure form through resolution methods
using either (+)- or (-)-dibenzoyltartaric acid. Interestingly, we have found that these
piperidines do, in fact, exhibit substantial affinity in both WIN 35,428 binding at the dopamine
transporter and in the inhibition of [3H]dopamine uptake. Of all of the compounds synthesized,
the 3-n-propyl derivative (-)-9 was found to be the most potent with a binding affinity of 3
nM. This simple piperidine is thus 33-fold more potent than cocaine in binding affinity and
29-fold more potent in its inhibition of dopamine uptake. Although no efforts have presently
been made to “optimize” binding affinity at the DAT, the substantive activity found for the
n-propyl derivative (-)-9 is remarkable; the compound is only about 10-fold less active than
the best of the high-affinity tropanes of the WIN series. As a further point of interest, it was
found that the cis-disubstituted piperidine (-)-3 is only about 2-fold more potent than its trans
isomer (+)-11. This result stands in sharp contrast to the data reported for the tropane series,
for the epimerization of the substituent at C-2 from â to R has been reported to result in a
lowering of activity by 30-200-fold. This smaller spread in binding affinities for the piperidines
may reflect the smaller size of these molecules relative to the tropanes, which allows both the
cis and the trans isomers to adjust themselves to the binding site on the DAT. Our present
demonstration that these piperidine structures do, in fact, possess significant DAT activity,
taken together with their reported lack of locomotor activity, provides a compelling argument
for exploring this class of molecules further in animal behavioral experiments. The present
work thus broadens the scope of structures that may be considered as lead structures in the
search for cocaine abuse medications.
In tr od u ction
viduals in treatment programs while slowly withdraw-
ing them from cocaine. Basically, what may be needed
for cocaine abuse treatment is the pharmacological
equivalent of methadone, a drug widely used in the
treatment of opiate abuse.2 In pursuit of a methadone
type of approach, we need to identify a partial agonist
of cocaine, a substance that elicits some of the same
effects in the user as cocaine itself, but without causing
the same degree of euphoria. This partial agonist
approach constitutes one possible direction to follow in
the discovery of a drug for maintenance of individuals
in treatment programs.
It is clear that immediate therapies are needed for
the treatment of cocaine abuse worldwide.1 It has been
estimated, for example, that there are at present over
7 million cocaine abusers in the US alone and that
approximately one-third of these users are consistent
or hard core users. Undoubtedly cocaine abuse has a
tremendous negative impact on our society, for the crime
costs associated with cocaine abuse alone amount to
nearly 50 billion dollars per year.
To develop agents that might find use in the treat-
ment of cocaine abuse, it will probably be best to identify
compounds that possess the ability to mimic partially
the effects of cocaine, thereby helping to maintain indi-
In experiments carried out by us we have shown that
N-ethylmaleimide was capable of inhibiting about 95%
of the specific binding of [3H]mazindol, and that the
effect of 10 mM N-ethylmaleimide was completely
prevented by 10 µM cocaine, while neither 300 µM
dopamine nor d-amphetamine afforded any significant
protection.3 Furthermore, a recent study of the struc-
† Georgetown University Medical Center.
‡ NEOUCOM.
§ University of Texas Medical Branch.
| Naval Research Laboratory.
Helios Pharmaceuticals.
S0022-2623(98)00028-4 CCC: $15.00 © 1998 American Chemical Society
Published on Web 04/28/1998

Piperidine-Based Analogue of Cocaine
J ournal of Medicinal Chemistry, 1998, Vol. 41, No. 11 1963
ture of the dopamine transporter revealed that aspar-
tate and serine residues lying within the first and
seventh hydrophobic putative membrane spanning re-
gions were critical for dopamine uptake, but less so for
[3H]CFT (WIN-35428) binding.4 For example, replace-
ment of the serine residues at positions 356 and 359 in
the seventh hydrophobic region by alanine or glycine
reduced [3H]DA uptake, whereas [3H]CFT (WIN-35428)
binding was less affected. More recent experiments
with DA and NE transporter chimeras show that trans-
membrane domains 6-8 determine cocaine binding
while domains 9-12 plus the carboxy tail are respon-
sible for DA binding affinity.5 Thus, these data support
the hypothesis that a significant portion of the cocaine
binding domain on the dopamine transporter is distinct
from that of either dopamine or amphetamine. This
distinction may be sufficient to allow properly designed
drugs to prevent cocaine binding without inhibiting
dopamine uptake.
In view of the above notion, we were led to investigate
the biological activity of compounds that can be consid-
ered to be truncated analogues of cocaine or more pre-
cisely truncated analogues of the WIN series of com-
pounds. These compounds, as diagrammed below, are
piperidines that lack cocaine’s two-carbon bridge. Prior
to our own work, several racemic piperidine analogues
were investigated by researchers at Sterling Winthrop
laboratories and were shown to lack locomotor stimulant
activity when tested in mice.6 Given the reduced mo-
lecular size of these piperidines relative to the tropanes
themselves, and the fact that they still embody cocaine’s
“pharmacophoric elements”, we were encouraged to
explore their biology. Below we detail the chemical syn-
thesis of these materials in optically pure form and
provide details of their in vitro pharmacology.
F igu r e 1. ORTEP drawing of (-)-3 which establishes its
absolute stereochemistry.
readily obtained by flash chromatography of the mother
liquor. We have been able to efficiently resolve the
racemic cis-configured ester by use of (+)- and (-)-
dibenzoyltartaric acid to provide the pure enantiomers
(-)-3 and (+)-4.8 An X-ray structure determination of
the salt formed from (-)-dibenzoyltartaric acid and 1
has been used to determine the absolute stereochemis-
try of (-)-3 which is depicted in Figure 1. As is
apparent, the absolute stereochemistry of the (-)-isomer
corresponds to that found in the WIN series of struc-
tures. The optically pure (+)- and (-)-cis esters have
been converted to their respective alcohols (-)-5 and
(+)-6 by LAH reduction and these in turn to their
acetate derivatives (-)-7 and (+)-8 by acetylation in the
presence of pyridine.
To explore the effect of having a less polar, nonhy-
drolyzable group at the C-3 position, we have in several
cases transformed the ester group of the piperidines to
an n-propyl substituent.9 In the case of (-)-3, this
chemistry was accomplished by converting ester to
aldehyde by way of the alcohol intermediate 5, perform-
ing a Wittig reaction, and last carrying out a catalytic
hydrogenation reaction to provide (-)-9. In one case,
to examine the contribution of the p-chloro substituent
to binding affinity, we have converted the cis piperidine
(-)-3 to the dechloro product (-)-10 by hydrogenolysis
over palladium on charcoal. Details of the reaction
methods can be found in Scheme 1.
In view of the fact that the biologically more interest-
ing compound is the trans isomer (()-2 (vide infra),
efforts have been made to resolve this compound as well.
Due to the fact that we could not obtain good crystals
from (()-2 and dibenzoyltartaric acid, we have instead
carried out a base-catalyzed epimerization reaction on
(-)-3 and (+)-4 to obtain the optically pure trans-
configured products (+)-11 and (-)-12 (Scheme 2). The
more active isomer (+)-11 was also converted, as
described for the cis isomer, to its alcohol (+)-13, acetate
(+)-14, and n-propyl derivative (+)-15. Details of the
chemistry are provided in Scheme 2.
Ch em ica l Syn th esis
The racemic piperidines 1 and 2 were prepared
starting from arecoline hydrobromide using chemistry
similar to that reported by Plati in his synthesis of the
unsubstituted phenyl bearing piperidine analogues.7
Thus, the hydrobromide salt of arecoline was converted
to its free base by sodium bicarbonate, and this inter-
mediate was subjected to a Grignard reaction using (p-
chlorophenyl)magnesium bromide. A mixture of the cis-
and trans-disubstituted piperidines 1 and 2 was pro-
duced in a 75/25 ratio. The cis derivative was obtained
by crystallization of the crude material using EtOAc/
hexane as solvent. The racemic trans piperidine was
In Vitr o Bioch em ica l P h a r m a cology
The racemic and optically pure piperidine derivatives
1-15 were tested for their ability to displace [3H]WIN-
35428 binding from rat striatal membranes and to
inhibit the high-affinity uptake of [3H]dopamine into rat
striatal nerve endings (synaptosomes) in accordance
with protocols previously described by Boja et al.10 The
results of these assays are provided in Table 1. As is

1964 J ournal of Medicinal Chemistry, 1998, Vol. 41, No. 11
Kozikowski et al.
Sch em e 1. Synthesis of the Cis-Disubstituted Piperidine Analogues
apparent from this table, the optically pure (-)-enan-
tiomer 3 is about 54-fold more potent than its (+)-isomer
4 in WIN-35428 binding and about 4-fold more potent
than cocaine. The racemic piperidine (()-1 is, as
expected, about 2-fold less potent than (-)-3. Reduction
of the ester group of 3 to alcohol causes a 3-fold drop in
potency, whereas the same transformation in the case
of (+)-4 leads to an improvement in binding affinity.
Acylation of the hydroxyl group of (-)-5 to yield (-)-7
leads to an increase in potency; the acetate is only 2-fold
less potent than the ester from which it is derived.
Curiously, acylation of (+)-6 to afford (+)-8 leads to a
2-fold decrease in binding affinity. Of all of the com-
pounds synthesized, the n-propyl derivative (-)-9 is the
most potent with a binding affinity of 3 nM. This simple
piperidine is thus 33-fold more potent than cocaine in
binding affinity and 29-fold more potent in the inhibition
of dopamine uptake. As to be expected based upon
previous work,14 the p-chloro substituent does add to
compound potency, as its removal, as in the case of (-)-
10, causes a 31-fold drop in potency.
measured binding affinity of (+)-11 is 57 nM, while its
activity in DA uptake inhibition is 35 nM. Its enan-
tiomer (-)-12 is considerably less potent with a binding
affinity of 653 nM. Reduction of (+)-11 to its alcohol
(+)-13 leads to a drop in binding affinity (∼4-fold), with
a further drop in potency being observed upon acylation
of the alcohol to give (+)-14. Again, as found in the cis
series, the transformation of the ester group to the more
hydrophobic n-propyl group results in a 3.8-fold increase
in binding affinity.
Discu ssion
As is apparent form the data presented, these pip-
eridine analogues of the WIN series structures are
potent compounds both in displacing WIN 35,428 bind-
ing and in inhibiting DA uptake at the dopamine
transporter. Although no efforts have presently been
made to “optimize” binding affinity at the DAT, the
substantive activity found for the n-propyl derivative
(-)-9 of 3 nM is remarkable; the compound is only about
10-fold less active than the best of the high-affinity
tropanes in the WIN series. As mentioned in the
Introduction, several structurally related piperidines
had first been reported by Clarke et al. in 1973 and
tested for their ability to increase locomotor activity in
Interestingly, the racemic trans isomer (()-2 is only
2-fold less potent than cocaine. The activity of its
eutomer (the more active isomer) should therefore be
essentially comparable to that of cocaine. Indeed, the

Piperidine-Based Analogue of Cocaine
J ournal of Medicinal Chemistry, 1998, Vol. 41, No. 11 1965
Sch em e 2. Synthesis of the Trans-Disubstituted Piperidine Analogues
Ta ble 1. IC50 Values and Hill Coefficients for the Piperidine Analogues in [3H]WIN 35,428 Binding and in the Inhibition of
[3H]Dopamine Uptake
IC50 (nM) [3H]WIN
35,428 binding
IC50 (nM) [3H]dopamine
uptake
compd
R
X
Hill coeff
Hill coeff
cocaine
(()-1
102 ( 9
53.7 ( 1.9
197 ( 8
0.87 ( 0.03
0.91 ( 0.01
0.91 ( 0.01
0.78 ( 0.04
0.81 ( 0.05
0.56 ( 0.05
0.66 ( 0.02
0.66 ( 0.04
0.89 ( 0.04
0.64 ( 0.09
1.02 ( 0.03
0.78 ( 0.02
0.96 ( 0.01
0.85 ( 0.01
0.76 ( 0.04
0.85 ( 0.01
239 ( 1
1.06 ( 0.04
1.04 ( 0.07
â-CO2Me
R-CO2Me
â-CO2Me
â-CO2Me
â-CH2OH
â-CH2OH
â-CH2OAc
â-CH2OAc
â-n-Pr
â-CO2Me
R-CO2Me
R-CO2Me
R-CH2OH
R-CH2OAc
R-n-Pr
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
H
Cl
Cl
Cl
Cl
Cl
37.8 ( 7.9
(()-2
(-)-3
24.8 ( 1.6
1360 ( 125
75.3 ( 6.2
442 ( 32
44.7 ( 10.5
928 ( 43
3.0 ( 0.5
769 ( 19
57.3 ( 8.1
653 ( 38
240 ( 18
461 ( 11
17.2 ( 0.5
85.2 ( 2.6
5090 ( 172
49.0 ( 3.0
0.94 ( 0.01
0.95 ( 0.11
1.04 ( 0.01
(+)-4
(-)-5
(+)-6
(-)-7
62.9 ( 2.7
2023 ( 82
8.3 ( 0.6
1.03 ( 0.01
1.07 ( 0.01
1.04 ( 0.03
(+)-8
(-)-9
(-)-10
(+)-11
(-)-12
(+)-13
(+)-14
(+)-15
34.6 ( 3.2
195 ( 8
683 ( 47
0.97 ( 0.03
1.08 ( 0.03
1.09 ( 0.07
23.2 ( 2.2
1.03 ( 0.01
mice. Clarke’s racemic version of our piperidine 10 was
found to have no stimulant activity when tested at 256
mg/kg po in mice. It was this very lack of locomotor
activity which caught our attention and led us to
question whether these molecules had any affinity for
the DAT. Our present demonstration that these pip-
eridine structures do, in fact, possess significant DAT
activity, taken together with their reported lack of
locomotor activity, provides a compelling argument for
exploring such compounds further as potential cocaine
antagonists. The lack of any observed locomotor activity
is unlikely to be due to the inability of the piperidines
to cross the blood-brain barrier, as arecoline itself has
shown some efficacy when administered intravenously
to Alzheimer’s patients.11 Moreover, in preliminary
animal studies, we have found some of these piperidines
to cause seizures when administered at high concentra-
tions. Extensive studies of locomotor activity, self-
administration, and drug discrimination are now un-
derway with several of the optically pure piperidines,
and this work will be reported in due course.
Furthermore, it is informative to take note of the fact
that the synthesis and activity of two more “cocaine-
like” piperidine analogues 16 and 17 have recently been
disclosed. These two compounds, whose structures are
shown below, exhibited IC50’s for displacement of WIN

1966 J ournal of Medicinal Chemistry, 1998, Vol. 41, No. 11
Kozikowski et al.
35428 binding of 11589 and 8064 nM, respectively.12
Accordingly, both compounds are less active than co-
caine or 4′-iodococaine (IC50’s reported for cocaine and
for 4′-iodococaine are 249 and 2522 nM, respectively).
The conclusion was accordingly reached that although
the interatomic distances between the nitrogen atom of
the piperidine ring and the iodine atom in compounds
16 and 17 are the same as that found in 4′-iodococaine,
the lack of conformational rigidity of the piperidines or
the adoption of the other chair conformations may lead
to poor binding affinity. Given the high affinity found
for our WIN series piperidines, we can conclude that in
fact neither the loss of structural rigidity due to the
absence of the two carbon bridge found in the tropanes
nor conformational flexibility are problematic to achiev-
ing high compound potency.
effect of halogen substitution on the in vitro activity of
the piperidines would offer evidence in support of their
binding to a similar site.
In conclusion, the potent DAT activity found for the
piperidines reported herein, taken together with an
earlier report of their lack of locomotor activity, suggests
a further novel direction to pursue in the search for a
cocaine abuse medication. The present work thus
broadens the scope of structures that may be considered
as lead structures in this search. Detailed biological
studies in vivo employing several members of this family
of piperidines will be reported in due course.
Exp er im en ta l Section
Gen er a l Meth od s. Starting materials were obtained from
Aldrich Chemicals or from other commercial suppliers. Di-
ethyl ether and cyclohexane were distilled from phosphorus
pentoxide; THF was freshly distilled under nitrogen from
sodium benzophenone.
IR spectra were recorded on an ATI Mattson Genesis
spectrometer. 1H and 13C NMR spectra were obtained with a
Varian Unity Inova instrument at 300 and 75.46 MHz, respec-
tively. 1H chemical shifts (δ) are reported in ppm downfield
from internal TMS. 13C chemical shifts are referred to CDCl3
(central peak, δ ) 77.0 ppm), benzene-d6 (central peak, δ )
128.0 ppm), or DMSO-d6 (central peak, δ ) 39.7 ppm). NMR
assignments were made with the help of COSY, DEPT, and
HETCOR experiments.
Melting points were determined in Pyrex capillaries with a
Thomas-Hoover Unimelt apparatus and are uncorrected.
Mass spectra were measured in the EI mode at an ionization
potential of 70 eV. TLC was performed on Merck silica gel
60F254 glass plates; column chromatography was performed
using Merck silica gel (60-200 mesh). The following ab-
breviations are used: DMSO ) dimethyl sulfoxide; ether )
diethyl ether; THF ) tetrahydrofuran; DCM ) dichlo-
romethane.
r a c-Meth yl 4-(4-Ch lor op h en yl)-1-m eth ylp ip er id in e-3-
ca r boxyla te (1, 2). To a solution of 4-chlorophenylmagnesium
bromide (166 mL, 1.0 M in ether) in ether (700 mL) was added
dropwise at -10 °C a solution of arecoline free base (12.9 g,
83 mmol, obtained from the hydrobromide by treatment with
sodium bicarbonate and extraction into methylene chloride)
in ether (300 mL). The mixture was stirred at -10 °C for 30
min, poured onto crushed ice, and treated slowly with 10%
HCl (200 mL). The aqueous layer was separated, washed with
ether (200 mL), and then treated while cooling in an ice bath
with a saturated solution of sodium bicarbonate (100 mL). The
mixture was extracted with ether (2 × 200 mL), and the
combined organic phases were washed with brine (200 mL),
dried, and concentrated under reduced pressure. The crude
mixture was crystallized from EtOAc/hexane to afford the
isomer 1 (5.0 g, 22%) as a white solid. Concentration of the
mother liquor gave a mixture of the 1 and 2 that was separated
by flash chromatography on silica gel using ether/Et3N, 9/1,
as eluent. The hydrochloride salts of these compounds were
prepared by dissolution of the free bases in a methanolic
solution of HCl(g), concentration, and final trituration of the
crude salts with ether.
An interesting point of difference between our pip-
eridines and the WIN series tropanes relates to the
effect of epimerization of the ester substituent. In the
case of WIN 35,065-2 which bears its ester and phenyl
substituents in the â orientation, conversion to WIN 35,-
140 which is its R-ester counterpart results in a 59-fold
drop in binding affinity.13 Thus in this case, the effect
of ester orientation has a substantial effect on binding.
In general, the epimerization of the substituent at C-2
from â to R in the tropane series has been reported to
result in a lowering of activity by 30-200-fold.14 In
sharp contrast to this result with the tropanes, the cis
isomer (-)-3 is only about 2-fold more potent than the
trans isomer (+)-11. This smaller spread in binding
affinities for the piperidines may reflect the smaller size
of these molecules relative to the tropanes, which allows
both the cis and the trans isomers to adjust themselves
to the binding site on the DAT. In the case of the
tropanes, the presence of the two-carbon bridge may
inhibit the C-2 R-isomer from achieving the optimal fit,
for enroute to this best fit the two carbon bridge may
encounter a steric barrier resulting from its interaction
with certain amino acids making up the transporter. A
small difference in binding affinites for R- and â-isomers
of the piperidine series is also observed with the
n-propyl derivatives (-)-9 and (+)-15. In this case, the
difference is about 5-fold.
As a further point of comparison, we draw attention
to the effect that the presence or absence of a p-chloro
substituent has on the binding of these piperidines. In
the WIN series, it is now well appreciated through the
work of Carroll that the introduction of a p-chloro or
p-iodo substituent to the aryl ring of 3â-phenyltropane-
2â-carboxylic acid methyl ester results in compounds
of higher potency in comparison to the unsubstituted
parent structure.15 The reported IC50’s of the p-chloro
and p-iodo compounds in WIN 35,428 binding are 1.17
and 1.26 nM, respectively. This compares to 23 nM for
the parent structure. Thus the halogen atom increases
binding affinity by 20-fold. In the piperidine family of
structures, the ratio of the IC50’s for the dechloro
compound (-)-10 relative to (-)-3 is 30. This parallel
Compound 1 (12.4 g, 56%): mp 98-99 °C; 1H NMR (CDCl3)
δ 1.74-1.86 (m, H5eq), 2.07 (dt, H6ax, J ) 3.0 and 11.4 Hz),
2.28 (s, 3H), 2.35 (dd, H2′, J ) 3.6 and 11.7 Hz), 2.66 (dq, H5ax
,
J ) 3.9 and 12.0 Hz), 2.78 (dt, H4, J ) 3.6 and 12.0 Hz), 2.9-
3.06 (m, H3 and H6eq), 3.18 (bd, H2′′, J ) 12.0 Hz), 3.52 (s, 3H),
6.2-6.35 (m, 4H); 13C NMR (CDCl3) δ 26.42 (C5), 41.27 (C4),
46.06 (C3), 46.53 (C7), 51.25 (C9), 55.88 (C6), 58.36 (C2), 128.08
(C11, C15), 128.95 (C12, C14), 131.79 (C13), 141.54 (C10), 172.47
(C8); MS m/z 267 (M+, 7), 208 (14), 128 (6), 70 (29), 44 (100).
1‚HCl: 1H NMR (methanol-d4) δ 2.05 (bd, 1H, J ) 4.0 Hz),
2.53 (bq, 1H, J ) 10.8 Hz), 2.94 (s, 3H), 3.14-3.5 (m, 4H), 3.45
(s, 3H), 3.6-3.7 (m, 1H), 3.78 (d, 1H, J ) 12.9 Hz), 7.22 (d,

Piperidine-Based Analogue of Cocaine
J ournal of Medicinal Chemistry, 1998, Vol. 41, No. 11 1967
2H, J ) 8.4 Hz), 7.35 (d, 2H, J ) 8.4 Hz). Anal. (C14H18
ClNO2‚HCl) C, H, N.
-
The hydrochloride salt was prepared by dissolution of the
free base in a methanolic solution of HCl(g), resulting in a
direct crystallization of the desired salt: [R]25 +55° (c 0.5,
Compound 2 (2.0 g, 18%): 1H NMR (benzene-d6) δ 1.4-1.5
(m, 1H), 1.62 (dq, 1H, J ) 3.9 and 12.6 Hz), 1.75 (dt, 1H, J )
2.7 and 12.0 Hz), 2.06 (s, 3H), 2.0-2.15 (m, 1H), 2.54-2.63
(m, 1H), 2.68 (dt, 1H, J ) 4.2 and 11.7 Hz), 2.86-3.0 (m, 2H),
3.08 (s, 3H), 6.87 (d, 2H, J ) 8.7 Hz), 7.07 (d, 2H, J ) 8.7 Hz);
13C NMR (CDCl3) δ 33.1, 44.0, 46.1, 49.1, 51.5, 55.7, 58.1, 128.6,
128.7, 132.3, 141.9, 173.4; MS m/z 267 (M+, 17), 208 (30), 128
(16), 114 (16), 43 (100).
D
EtOH); mp >230 °C. Anal. (C14H18ClNO2‚HCl) C, H, N.
(-)-Meth yl 1-Meth yl-4â-p h en ylp ip er id in e-3â-ca r boxy-
la te (10). A mixture of 3 (0.7 g, 2.61 mmol) and 10% Pd/C
(0.28 g) in MeOH (20 mL) was hydrogenated under 1 atm of
H2 for 3 h. The resulting mixture was filtered over Celite and
evaporated to dryness. The resulting pale yellow oil was
purified by flash chromatography on silica gel using ether/
Et3N, 9.5/0.5, as eluent to afford the title compound (0.6 g,
2‚HCl: 1H NMR (methanol-d4) δ 2.04-2.16 (m, 2H), 2.97
(s, 3H), 3.0-3.3 (m, 4H), 3.47 (s, 3H), 3.56-3.66 (m, 1H), 3.7-
3.8 (m, 1H), 7.25 (d, 2H, J ) 8.4 Hz), 7.34 (d, 2H, J ) 8.4 Hz).
Anal. (C14H18ClNO2‚HCl) C, H, N.
98%) as a colorless oil: [R]25 -54° (c 1, EtOH); 1H NMR
D
(CDCl3) δ 1.76-1.9 (m, H5eq), 2.09 (dt, H6ax, J ) 2.7 and 11.1
Hz), 2.29 (s, 3H), 2.37 (dd, H2′, J ) 3.6 and 11.7 Hz), 2.70 (dq,
H5ax, J ) 3.9 and 12.3 Hz), 2.85 (dt, H4, J ) 3.9 and 11.7 Hz),
2.92-3.06 (m, H3 and H6eq), 3.18 (br d, H2′′, J ) 12.0 Hz), 3.50
(s, 3H), 7.1-7.4 (m, 5H); 13C NMR (CDCl3) δ 26.6, 41.8, 46.2,
46.6, 51.2, 55.9, 58.3, 126.1, 127.6, 128.0, 143.0, 172.7; MS m/z
233 (M+, 13), 232 (6), 174 (17), 70 (26), 44 (100).
(-)-Meth yl 4â-(4-Ch lor op h en yl)-1-m eth ylp ip er id in e-
3â-ca r boxyla te (3). To a solution of 1 (6.4 g, 24 mmol) in
MeOH (200 mL) was added a solution of dibenzoyl-L-tartaric
acid (8.9 g, 24 mmol) in MeOH (100 mL). The resulting
mixture was stirred at room temperature for 5 h and then
filtered, and the white precipitate was washed with MeOH (20
mL). This tartrate salt was treated with a saturated solution
of NaHCO3 (150 mL) and the mixture extracted with CHCl3
(3 × 100 mL). The combined organic phases were washed with
brine (150 mL), dried, and concentrated under reduced pres-
sure to afford the title compound (2.0 g) as a white solid: mp
Preparation of the hydrochloride salt was made by dissolu-
tion of the free base in a methanolic solution of HCl(g),
concentration, and final trituration of the crude salts with
ether: [R]25 -130° (c 1.0, EtOH); mp 168-169 °C; H NMR
1
D
(methanol-d4) δ 2.0-2.1 (m, 1H), 2.5-2.7 (m, 1H), 2.95 (s, 3H),
3.1-3.5 (m, 4H), 3.42 (s, 3H), 3.6-3.7 (m, 2H), 3.7-3.85 (m,
1H), 7.2-7.4 (m, 5H). Anal. (C14H19NO2‚HCl) C, H, N.
98-99 °C; [R]25 -56° (c 1.0, EtOH).
D
(-)-4â-(4-Ch lor op h en yl)-3â-(h yd r oxym et h yl)-1-m et h -
ylp ip er id in e (5). To a solution of 3 (1.0 g, 3.7 mmol) in THF
(30 mL) was added portionwise LiAlH4 (0.3 g, 7.5 mmol). The
resulting mixture was stirred at room temperature for 2 h,
and then a saturated solution of Rochelle salt (30 mL) was
added followed by extraction with EtOAc (100 mL). The
organic phase was washed with brine (100 mL), dried, and
concentrated under reduced pressure to afford the title com-
pound (0.9 g, 98%) as a colorless oil: [R]25D -70° (c 1.0, EtOH);
The hydrochloride salt was prepared by dissolution of the
free base in a methanolic solution of HCl(g), concentration,
and final trituration of the crude salt with ether: [R]25D -130°
(c 1.0, EtOH). Anal. (C14H18ClNO2‚HCl‚H2O) C, H, N.
(+)-Meth yl 4â-(4-Ch lor op h en yl)-1-m eth ylp ip er id in e-
3â-ca r boxyla te (4). To the mixture of enantiomers derived
from the mother liquor of previous separation (4.2 g, 15.7
mmol) in MeOH (150 mL) was added a solution of dibenzoyl-
D-tartaric acid (5.8 g, 15.7 mmol) in MeOH (50 mL). The
resulting mixture was stirred at room temperature for 5 h and
then filtered, and the white precipitate was washed with
1H NMR (CDCl3) δ 1.64-1.84 (m, H3 and H5eq), 2.11 (dt, H6ax
,
J ) 3.3 and 11.7 Hz), 2.29 (s, 3H), 2.45 (dt, H1′, J ) 2.7 and
11.4 Hz), 2.55 (dq, H5ax, J ) 4.2 and 12.6 Hz), 2.84 (dt, H4, J
) 4.5 and 13.5 Hz), 3.0-3.1 (m, H6eq), 3.14 (br d, H2′′, J ) 11.4
Hz), 3.54 (dt, H8, J ) 2.4 and 10.8 Hz), 3.70 (dd, H8, J ) 3.3
and 11.1 Hz), 7.24 (d, 2H, J ) 8.7 Hz), 7.29 (d, 2H, J ) 8.7
Hz); 13C NMR (CDCl3) δ 27.9 (C4), 40.2 (C2), 43.5 (C3), 46.3
MeOH (10 mL). This tartrate salt was treated with
a
saturated solution of NaHCO3 (100 mL) and the mixture
extracted with CHCl3 (3 × 70 mL). The combined organic
phases were washed with brine (150 mL), dried, and concen-
trated under reduced pressure to afford the title compound
(2.2 g) as a white solid: mp 98-99 °C; [R]25 +56° (c 1.0,
(C6), 56.2 (C1), 61.4 (C5), 64.5 (C8), 128.4 (C11, C15), 129.2 (C12
14), 131.9 (C13), 142.1 (C10); MS m/z 239 (M+, 6), 208 (6), 100
(16), 44 (100). Anal. (C13H18ClNO) C, H, N.
,
D
C
EtOH).
Preparation of the hydrochloride salt was made by dissolu-
tion of the free base in a methanolic solution of HCl(g),
concentration, and final trituration of the crude salts with
(+)-4â-(4-Ch lor op h en yl)-3â-(h yd r oxym et h yl)-1-m et h -
ylp ip er id in e (6) was prepared similarly to 5. From 4 there
was obtained 6 (82%) as a colorless oil, [R]25D +67° (c 1, EtOH).
Anal. (C13H18ClNO) C, H, N.
ether: [R]25 +126° (c 1.0, EtOH). Anal. (C14H18ClNO2‚HCl)
D
C, H, N.
(-)-3â-(Acet oxym et h yl)-4â-(4-ch lor op h en yl)-1-m et h -
ylp ip er id in e (7). To a solution of 5 (90 mg, 0.38 mmol) in
pyridine (2 mL) was added acetic anhydride (0.5 mL). The
resulting solution was stirred at room temperature for 15 h,
then concentrated under reduced pressure, diluted with EtOAc
(30 mL), and washed with a saturated solution of NH4Cl (2 ×
20 mL). The organic solution was dried and concentrated
under reduced pressure to afford the title compound (0.10 g,
(-)-Meth yl 4â-(4-Ch lor op h en yl)-1-m eth ylp ip er id in e-
3r-ca r boxyla te (12). To a solution of 4 (0.4 g, 1.49 mmol) in
MeOH (3 mL) was added a 30% methanolic solution of sodium
methoxide (0.01 mL). The resulting solution was stirred at
reflux for 11 h and concentrated under reduced pressure. CH2-
Cl2 and a saturated solution of NH4Cl were added. The organic
layer was washed with brine, dried over sodium sulfate, and
concentrated under reduced pressure to afford 4 and 12 in a
1:5.6 ratio (determined by GC-MS analysis). Purification of
the crude product by silica gel flash chromatography using
ether/Et3N, 9.8/0.2, as eluent afforded the title compound (0.35
95%) as a white solid: mp 76 °C; [R]25 -109° (c 0.75; EtOH);
D
Rf 0.6 (ether/Et3N, 9.5/0.5); 1H NMR (benzene-d6) δ 1.21 (br d,
1H, J ) 11.4 Hz), 1.52 (s, 3H), 1.72 (dq, 1H, J ) 3.0 and 12.3
Hz), 1.6-1.7 (m, 1H), 1.86 (dd, 1H, J ) 2.7 and 11.4 Hz), 2.0-
2.1 (m, 1H), 2.09 (s, 3H), 2.40 (dt, 1H, J ) 3.9 and 11.4 Hz),
2.67 (br d, 1H, J ) 8.1 Hz), 2.91 (d, 1H, J ) 11.4 Hz), 3.90
(dd, 1H, J ) 4.5 and 10.8 Hz), 4.47 (dd, 1H, J ) 9.6 and 10.5
Hz), 6.68 (d, 2H, J ) 8.4 Hz), 7.09 (d, 2H, J ) 8.4 Hz); 13C
NMR (CDCl3) δ 20.8, 25.6, 39.6, 41.9, 46.5, 56.2, 57.8, 62.5,
128.4, 128.5, 132.0, 141.5, 170.9; MS m/z 281 (M+, 6), 238 (6),
208 (15), 142 (7), 44 (100). Anal. (C15H20ClNO2) C, H, N.
(+)-3â-(Acet oxym et h yl)-4â-(4-ch lor op h en yl)-1-m et h -
ylp ip er id in e (8) was prepared similarly to 7. From 6 there
g, 85%) as a colorless oil: [R]25 -50° (c 1.0, EtOH). Anal.
D
(C14H18ClNO2) C, H, N.
(+)-Meth yl 4â-(4-Ch lor op h en yl)-1-m eth ylp ip er id in e-
3r-ca r boxyla te (11). To a solution of 3 (0.5 g, 1.87 mmol) in
MeOH (6 mL) was added a 30% methanolic solution of sodium
methoxide (0.04 mL). The resulting solution was stirred at
reflux for 24 h and concentrated under reduced pressure. CH2-
Cl2 and brine were added, and the organic layer was washed
with brine. Concentration of the combined organic phase
afforded 3 and 11 in a 1:32 ratio (determined by GC-MS
analysis). Purification of the crude product by silica gel flash
chromatography using ether/Et3N, 9.8/0.2, as eluent afforded
the title compound (0.43 g, 86%) as a colorless oil: [R]25D +46°
(c 1.0, EtOH).
was obtained 5 (93%) as a white solid: [R]25 +107° (c 0.35,
D
EtOH); MS m/z 281 (M+, 6). Anal. (C15H20ClNO2) C, H, N.
(-) 4â-(4-ch lor op h en yl)-1-m et h yl-3â-n -p r op ylp ip er i-
d in e (9). Oxalyl chloride (0.19 mL) was dissolved in anhy-

1968 J ournal of Medicinal Chemistry, 1998, Vol. 41, No. 11
Kozikowski et al.
drous CH2Cl2 (15 mL), and the solution was cooled to -78 °C.
Dimethyl sulfoxide (0.32 mL) was added, after 5 min the
alcohol 5 (0.5 g, 2.08 mmol) was added in CH2Cl2 (5 mL), and
stirring was continued for 30 min. The reaction mixture was
quenched by adding Et3N (2.84 mL), and the resulting solution
was warmed to room temperature, diluted with CH2Cl2 (30
mL), washed with NH4Cl (2 × 30 mL), dried, and concentrated
under reduced pressure to provide the intermediate aldehyde
(0.45 g, 91%) as a colorless oil used in the next step without
further purification: 1H NMR (CDCl3) δ 1.9-2.0 (m, 1H), 2.10
(dt, 1H, J ) 2.4 and 11.4 Hz), 2.29 (s, 3H), 2.2-2.4 (m, 2H),
2.64-2.74 (m, 1H), 2.92 (dt, 1H, J ) 3.9 and 12.9 Hz), 3.0-
3.1 (m, 1H), 3.28 (br d, 1H, J ) 11.4 Hz), 7.2 (d, 2H, J ) 8.4
Hz), 7.29 (d, 2H, J ) 8.4 Hz), 8.7 (s, 1H); 13C NMR (CDCl3) δ
27.2, 40.9, 46.5, 51.9, 55.9, 57.0, 128.6, 128.7, 132.3, 140.6,
203.9.
A solution of n-BuLi (2.28 mL, 1 M in hexane, 5.7 mmol)
was dissolved in THF (10 mL) and cooled to 0 °C. Ethyltri-
phenylphosphonium bromide (2.1 g, 5.7 mmol) was added
slowly under nitrogen. The resulting yellow-orange solution
was stirred at 0 °C for 30 min, and then the cooling bath was
removed. The crude aldehyde (0.45 g, 1.9 mmol) was added
in THF (2 mL), and the reaction mixture was stirred for 15 h
at room temperature, diluted with EtOAc (20 mL), and washed
with a saturated solution of NH4Cl (2 × 30 mL). The organic
phase was extracted with 10% HCl (3 × 10 mL). The com-
bined aqueous phases were washed with EtOAc (30 mL),
neutralized with a saturated solution of NaHCO3, and ex-
tracted with CH2Cl2 (2 × 30 mL). The combined organic
phases were dried and concentrated under reduced pressure,
and the residue was purified by flash chromatography on silica
gel using ether/Et3N, 9.5/0.5, as eluent to afford the olefin
intermediate as a mixture of the cis and trans isomers (0.3 g,
63%): MS m/z 248 (M+, 6), 57 (100).
J ) 8.4 Hz); 13C NMR (CDCl3) δ 20.7, 34.4, 41.0, 44.2, 46.4,
56.0, 59.3, 65.2, 128.7, 128.8, 132.2, 142.1, 170.9. Anal.
(C15H20ClNO) C, H, N.
(+)-4â-(4-ch lor op h en yl)-1-m et h yl-3r-n -p r op ylp ip er i-
d in e (15) was prepared similarly to 9. From 13 there was
obtained 15 (70%) as a colorless oil: [R]25D +41° (c 1.0, EtOH);
1H NMR (CDCl3) δ 0.73 (t, 3 H, J ) 7.2 Hz), 0.8-1.0 (m, 1 H),
1.0-1.2 (m, 2 H), 1.2-1.4 (m, 1 H), 1.65 (t, 1 H, J ) 10.8 Hz),
1.7-1.9 (m, 3 H), 1.9-2.15 (m, 2 H), 2.32 (s, 3 H), 2.93 (d, 1 H,
J ) 11.1 Hz), 3.05 (d, 1 H, J ) 10.8 Hz), 7.10 (d, 2 H, J ) 8.4
Hz), 7.25 (d, J ) 8.1 Hz); 13C NMR (CDCl3) δ 14.1, 19.7, 33.9,
35.0, 40.8, 46.5, 48.2, 56.3, 61.6, 128.5, 129.0, 131.6, 143.8.
Preparation of the hydrochloride salt was made by dissolu-
tion of the free base in a methanolic solution of HCl(g),
concentration, and final trituration of the crude salts with
ether: [R]25D +34° (c 0.25, EtOH); mp 216 °C (EtOAc); 1H NMR
(methanol-d4) δ 0.77 (t, 3 H, J ) 6.9 Hz), 1.0-1.4 (m, 4 H),
1.9-2.2 (m, 3 H), 2.56 (q, 1 H, J ) 10.8 Hz), 2.86 (t, 1 H, J )
12.6 Hz), 2.93 (s, 3 H), 3.0-3.2 (m, 1 H), 3.5-3.7 (m, 2 H),
7.23 (d, 2 H, J ) 8.4 Hz), 7.35 (d, 2 H, J ) 8.4 Hz). Anal.
(C15H22ClN‚HCl) C, H, N.
Sin gle-Cr ysta l X-r a y An a lysis of th e (-)-Diben zoyl-
ta r tr a te of 3. A clear rectangular 0.06 × 0.08 × 0.52 mm
crystal, C14H19O2ClN+C18H13O8-, FW ) 626.04, was selected
for data collection. Data were collected on
a computer-
controlled Siemens CCD 1K area detector system with a
Siemens PLATFORM goniometer using a Rigaku rotating
anode source and Gobel mirrors (Cu KR radiation, λ ) 1.541 78
Å, T ) 295 K). Data collection nominally covered a hemisphere
in reciprocal space by combining six sets of exposures with
different 2θ and φ angles: each exposure covered a range of
0.75° in ω. The crystal to detector distance was 5.09 cm, and
coverage of a unique set was 98% complete to 1.0 Å resolution.
The crystal decay was monitored by repeating 50 of the initial
frames at the end of data collection and was found to be 2.7%.
A least-squares refinement16 using 176 centered reflections
within 16.2 < 2θ < 34.4° gave the orthorhombic P212121 cell,
a ) 7.752(3) Å, b ) 14.691(5) Å, c ) 27.502(8) Å, with V )
3132.2(17) Å3, Z ) 4, and dcalc ) 1.328 gm/cm3. A total of 8342
reflections were to 2θmax ) 100°, of which there were 2923
independent reflections. Corrections were applied for Lorentz
and polarization effects. An empirical absorption correction
was applied using equivalent reflections (SADABS17), µ )
1.577 mm-1. Maximum and minimum transmission were 0.44
and 0.88, respectively. The structure was solved by direct
methods with the aid of the program SHELXTl18 and refined
on F2 with full matrix least-squares.18 The 398 parameters
refined include the coordinates and anisotropic thermal pa-
rameters for all non-hydrogen atoms. Hydrogens were in-
cluded using a riding model. The final R values for the 2244
observed reflections with Fo > 4σ(|Fo|) were R ) 0.086 and wR-
(F2) ) 0.208. The goodness of fit parameter was 1.07, and final
difference Fourier excursions were 0.41 and -0.27 e Å-3. The
absolute configuration determination was based on a method
suggested by D. Rogers.19 The absolute structure parameter
which should be near 0.0 for the correct choice of chirality and
1.0 for an incorrect choice was 0.04(6). The compound also
contained a chiral anion, (-)-dibenzoyltartaric acid.
To a solution of the intermediate olefins (0.2 g, 0.80 mmol)
in cyclohexane (20 mL) was added 5% Pt/C (0.2 g). The
mixture was stirred at room temperature for 30 min under 40
psi of H2. The resulting solution was filtered over Celite and
evaporated to dryness. The resulting colorless oil was purified
by flash chromatography on silica gel using ether/Et3N, 9.5/
0.5, as eluent to afford the title compound 9 (0.19 g, 94%) as
1
a colorless oil: [R]25 -84° (c 0.5, EtOH); H NMR (benzene-
D
d6) δ 0.71 (t, 3H, J ) 6.9 Hz), 0.75-1.0 (m, 2H), 1.2-1.4 (m,
2H), 1.52-1.65 (m, 1H), 1.65-1.84 (m, 2H), 1.84-2.0 (m, 2H),
2.14 (s, 3H), 2.47 (dt, 1H, J ) 3.6 and 12.3 Hz), 2.7-2.84 (m,
1H), 6.77 (d, 2H, J ) 8.4 Hz), 7.15 (d, 2H, J ) 8.4 Hz); 13C
NMR (CDCl3) δ 14.0, 21.1, 25.4, 27.6, 40.2, 43.9, 46.8, 56.5,
59.4, 128.1, 128.8, 131.4, 142.9; MS m/z 251 (M+, 8), 208 (8),
112 (24), 44 (100).
Preparation of the hydrochloride salt was made by dis-
solution of the free base in a methanolic solution of HCl(g),
concentration, and final trituration of the crude salt with
ether: mp >230 °C; [R]25 -73° (c 0.25, EtOH); 1H NMR
D
(methanol-d4) δ 0.78 (t, 3H, J ) 6.6 Hz), 0.9-1.1 (m, 2H),
1.28-1.5 (m, 2H), 1.94-2.06 (m, 1H), 2.14-2.38 (m, 2H),
2.92 (s, 3H), 3.04-3.4 (m, 3H), 3.54-3.7 (m, 2H), 7.24 (d, 2H,
J ) 7.8 Hz), 7.35 (d, 2H, J ) 7.8 Hz). Anal. (C15H22ClN‚HCl)
C, H, N.
(+)-4â-(4-Ch lor op h en yl)-3r-(h yd r oxym et h yl)-1-m et h -
ylp ip er id in e (13) was prepared similarly to 5. From 11 there
Ack n ow led gm en t. We are indebted to the National
Institutes of Health, National Institute on Drug Abuse
(DA11546), for their support of these studies.
was obtained 13 (84%) as a colorless oil: [R]25 +38° (c 0.5,
D
EtOH); mp 148-150 °C; 1H NMR (CDCl3) δ 1.4 (br s, OH),
1.7-2.1 (m, 5 H), 2.29 (dd, 1 H, J ) 5.4 and 10.5 Hz), 2.36 (s,
3 H), 2.95 (d, 1 H, J ) 10.8 Hz), 3.15 (d, 1 H, J ) 10.8 Hz),
3.24 (dd, 1 H, J ) 6.6 and 10.8 Hz), 3.41 (dd, 1 H, J ) 3.0 and
10.8 Hz), 7.14 (d, 2 H, J ) 8.4 Hz), 7.27 (d, 2 H, J ) 8.4 Hz).
Anal. (C13H18ClNO) C, H, N.
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was obtained 14 (80%) as a white solid: 1H NMR (CDCl3) δ
1.7-1.9 (m, 3 H), 1.97 (s, 3 H), 1.95-2.1 (m, 1 H), 2.1-2.3 (m,
2 H), 2.35 (s, 3 H), 2.95 (d, 1 H, J ) 11.4 Hz), 3.07 (d, 1 H, J
) 9.6 Hz), 3.63 (dd, 1 H, J ) 7.5 and 11.4 Hz), 3.82 (dd, 1 H,
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