4-Heterocyclylpiperidines as selective high-affinity ligands at the human dopamine D4 receptor

Article abstract of DOI:10.1021/jm970111h

5-(4-Chlorophenyl)-3-(1-(4-chlorobenzyl)piperidin-4-yl)pyrazole (3) was identified from screening of the Merck sample collection as a human dopamine D4 (hD4) receptor ligand with moderate affinity (61 nM) and 4-fold selectivity over human D2 (hD2) recepto

Full text of DOI:10.1021/jm970111h

2374
J . Med. Chem. 1997, 40, 2374-2385
4-Heter ocyclylp ip er id in es a s Selective High -Affin ity Liga n d s a t th e Hu m a n
Dop a m in e D4 Recep tor
Michael Rowley,* Ian Collins, Howard B. Broughton, William B. Davey, Raymond Baker, Frances Emms,
Rosemarie Marwood, Shil Patel, Smita Patel, C. Ian Ragan, Stephen B. Freedman, Richard Ball,† and
Paul D. Leeson
Merck Sharp and Dohme, Neuroscience Research Centre, Terlings Park, Eastwick Road, Harlow, Essex CM20 2QR, U.K., and
Merck Research Laboratories, Rahway, New J ersey
Received February 24, 1997X
5-(4-Chlorophenyl)-3-(1-(4-chlorobenzyl)piperidin-4-yl)pyrazole (3) was identified from screening
of the Merck sample collection as a human dopamine D4 (hD4) receptor ligand with moderate
affinity (61 nM) and 4-fold selectivity over human D2 (hD2) receptors. Four separate parts of
the molecule have been examined systematically to explore structure-activity relationships
with respect to hD4 affinity and selectivity over other dopamine receptors. It was found that
the 4-chlorophenyl group attached to the pyrazole is optimal, as is the 4-substituted piperidine.
The lipophilic group on the basic nitrogen is more amenable to change, with the optimal group
found to be a phenethyl. The aromatic heterocyle can be altered to a number of different groups,
with isoxazoles and pyrimidines showing improved affinities. This heterocycle can also be
advantageously alkylated, improving the selectivity of the compounds over D2 receptors. It is
hypothesized that the conformation around the bond joining the aromatic heterocycle to the
piperidine is important for D4 affinity, based on crystal structures of isoxazoles (29 and 30)
and on a conformationally constrained compound (28). Putting all the favorable changes
together led to the discovery that 5-(4-chlorophenyl)-4-methyl-3-(1-(2-phenylethyl)piperidin-
4-yl)isoxazole (36) is a nanomolar antagonist at human dopamine D4 receptors with >500-
fold selectivity over hD2 and >200-fold selectivity over hD3. Compound 36 is an antagonist of
hD4 receptors with good oral bioavailability of 38%, a half life of 2 h, and brain levels 10-fold
higher than plasma levels.
Ta ble 1. Reference Compounds and Lead Pyrazole
Schizophrenia is a mental illness for which there is
still a great need for novel drug therapy. Classical
neuroleptics, which are presumed to act by antagonism
of dopamine D2-like receptors,1 are useful for the
treatment of the positive symptoms of schizophrenia but
cause major movement disorders2 which are thought to
be due to blockade of D2 receptors in the striatum, along
with increases in serum prolactin levels.3 The atypical
neuroleptic clozapine4 (1; Table 1) can be used to treat
both positive and negative symptoms and does not
induce the extrapyramidal side effects. However, in
1-2% of patients the drug induces agranulocytosis, and
its use is therefore limited and must be closely moni-
tored.5 In recent years the application of molecular
biology techniques has seen the cloning of a number of
different subtypes of dopamine receptors6-10 which on
the basis of their pharmacology can be divided into two
classes: D1-like (D1, D5) and D2-like (D2, D3, D4).
a
Affinities at cloned human dopamine receptors stably ex-
Clozapine, among its many actions, has higher affinity
pressed in cell lines.
for the D4 subtype than for D2.9 Recent reports11
suggest that D4 receptor density is elevated in post-
mortem schizophrenic brain, although there is some
disagreement about this.12 Studies of mRNA distribu-
tion indicate that D4 receptors are preferentially located
in the mesolimbic system,13 with low density in the
striatum. For these reasons it was decided to identify
selective D4 receptor antagonists to investigate their
potential as novel antipsychotics, possibly without the
undesired side effects of classical neuroleptics. Until
very lately there have been no reports in the literature
on selective dopamine D4 antagonists, but we14 and
others15 have recently communicated on a number of
series of such ligands. In this paper we fully describe
the synthesis and structure-affinity relationship stud-
ies leading to the discovery of high-affinity selective
ligands for the human dopamine D4 (hD4) receptor.
Our strategy began with the screening of the Merck
sample collection. A representative number of known,
unselective dopamine antagonists were selected, and
using the topological similarity probe method16 as
implemented in the in-house TOPOSIM17 package, the
collection was screened for chemicals that had affinity
* Corresponding author: e-mail, Michael_Rowley@Merck.com.
† Merck Research Laboratories, NJ .
X Abstract published in Advance ACS Abstracts, J une 15, 1997.
S0022-2623(97)00111-8 CCC: $14.00 © 1997 American Chemical Society

Heterocyclylpiperidines as High-Affinity Ligands
J ournal of Medicinal Chemistry, 1997, Vol. 40, No. 15 2375
Sch em e 1a
Sch em e 2a
a
Reagents: (i) BOC2O, CH2Cl2; (ii) carbonyldiimidazole, THF;
(iii) 2 equiv of 4-chloroacetophenone/LDA, -78 °C-room temper-
ature; (iv) HCl, EtOAc; (v) PhCH2Br, Et-i-Pr2N, DMF; (vi)
H2NNH2‚H2O, MeOH.
Table 3 were made in this way, except for the ben-
zylpyrazole 14 where, when this route was tried, only
the product (47; Scheme 3) of enolization toward the
phenyl ring was isolated. It was reasoned that this was
the product of the thermodynamic enolate and that if
the reaction were repeated under nonequilibrating
conditions, then the desired product might be obtained.
Thus a 1:1 mixture of the silyl enol ethers derived from
acetophenone 43 (Scheme 3) was treated with methyl-
lithium to generate the enolates in the absence of a
proton source. Addition of the imidazolide now gave a
mixture of the wrong regioisomer (47) along with the
desired product (46) which could be separated chro-
matographically and taken through to pyrazole 14.
The 3-substituted piperidine 20 (Table 4) and the
tetrahydropyridine 23 were made by the same route as
the 4-piperidine and the 2-substituted piperidine 21
made in a similar way but using a benzoyl rather than
BOC protection for the piperidine nitrogen. The syn-
thesis (Scheme 4) of the pyrrolidine analogue 22 began
with the pyrrolidinone ester 48. Hydrolysis to the acid
49 and then activation and coupling with 4-chloroaceto-
phenone enolate gave the diketone 50 which was con-
densed with hydrazine, and the lactam 51 was reduced
with lithium aluminum hydride to give the desired
product (22). Treatment19 (Scheme 5) of N-pheneth-
ylpiperazine with N-methyl-N-phenylthiosemicarbazide
gave the thiosemicarbazide 53, which was condensed
with phenacyl bromide to give the piperazine analogue
24.
a
Reagents: (i) 4-chlorobenzaldehyde, NaOH, EtOH, reflux; (ii)
H2NNH2‚H2O, KOH, ethylene glycol, 130-200 °C; (iii) 4-ClPhCH2Cl,
Et-i-Pr2N, DMF; (iv) formaldehyde, NaBH4, CF3CO2H, THF.
for dopamine receptors and possibly selectivity for D4
over other subtypes. One of these runs, based on the
structure of the classical neuroleptic haloperidol (2),
gave rise to a number of dopamine subtype selective
ligands, from which 5-(4-chlorophenyl)-3-(1-(4-chloroben-
zyl)piperidin-4-yl)pyrazole (3) was chosen as a suitable
lead. This compound has moderate affinity for cloned
human dopamine D4 receptors (Ki 61 nM) and 4-fold
selectivity over D2. Efforts were initiated within me-
dicinal chemistry to improve on the affinity and selec-
tivity within this series of compounds.
Ch em istr y a n d Biology
The pyrazole 3 was synthesized (Scheme 1) starting
from 3-quinuclidinone (38). Condensation with 4-chlo-
robenzaldehyde gave the unsaturated ketone 39, which
on treatment with hydrazine and base18 ring opened to
give pyrazole 4. Alkylation with 4-chlorobenzyl chloride
gave the lead compound 3. Other compounds in Table
2 were made by alkylation of pyrazole 4 with alkyl
halides, with the exception of the N-methylpiperidine
5, which was made by a reductive amination of pyrazole
4 with formaldehyde and sodium borohydride.
A different route to the pyrazolylpiperidines, which
also allowed access to alternative aromatic heterocycles,
is shown in Scheme 2. Isonipecotic acid (40) was
N-protected and then the acid function activated as its
imidazolide. Displacement with 2 equiv of the enolate
prepared from 4-chloroacetophenone and lithium diiso-
propylamide gave a mixture of the desired â-diketone
and excess acetophenone, which could be most conve-
niently separated by treatment of the crude product
with HCl in EtOAc and isolation of the amine hydro-
chloride salt 42. It is necessary to use 2 equiv of the
enolate, as the â-diketone product is more acidic than
the acetophenone, thus quenching the enolate reagent.
N-Alkylation at the diketone stage followed simply by
treatment with hydrazine in methanol at room temper-
ature gave the desired pyrazole. All the compounds in
The 4-methylated piperidine 27 (Table 5) and its
diketone intermediate 26 were made in the same way
as 13 (Scheme 2) but starting with 4-methylpiperidine-
4-carboxylic acid. The synthesis of the spirofused
compound 28 began with iodobutyrophenone (54; Scheme
6). The ketone function was temporarily protected as
its dimethyl acetal using trimethyl orthoformate and
Montmorillonite K-10 clay. The iodide was then dis-
placed with the anion derived from N-BOC ethyl
isonipecotate, the ketone protection being removed in
the workup of this reaction to yield the keto ester 56. It
was necessary to use potassium hexamethyldisilazide
at low temperature to perform the Dieckman cyclization
of this compound, because if sodium ethoxide in reflux-
ing ethanol was used, only the product (58) of cyclization
followed by retro-Claisen reaction was observed. With

2376 J ournal of Medicinal Chemistry, 1997, Vol. 40, No. 15
Rowley et al.
Sch em e 3a
a
Reagents: (i) LDA/TMSCl; (ii) MeLi, ether; (iii) 41/carbonyldiimidazole, -78 °C-room temperature; (iv) H2NNH2‚H2O, MeOH; (v)
HCl, EtOAc; (vi) PhCH2Br, Et-i-Pr2N, DMF.
Sch em e 4a
gave intermediate 60 which was elaborated to the
ethylpyrazole 33. The â-diketones also served as useful
intermediates for the synthesis of alternative hetero-
cycles (Scheme 8). Thus, condensation of 61 with
hydroxylamine gave intermediate hydroxyisoxazolines,
which were not purified but dehydrated with methane-
sulfonyl chloride to give a mixture of the isoxazoles 29
and 30 which were separated by chromatography. The
regiochemistry of the isoxazoles was proved by X-ray
crystallography on both compounds. Reaction of the
diketone 25 with guanidine hydrochloride and sodium
isopropoxide in refluxing 2-propanol gave the aminopy-
rimidine 31. The aminopyrimidine 37 and the isox-
azoles 35 and 36 were made using these procedures
from the corresponding N-phenethyl diketone.
Compounds were tested for their ability to displace
[3H]spiperone from human cloned receptors, D2 stably
expressed in CHO cells,21 D3 and D4 in HEK-293 cells.22
With the exception of compounds 4, 5, and 12 which
have weak dopamine receptor affinities and were only
tested once, binding data are the geometric mean of
several independent determinations: errors of the mean
were within 2-fold of the mean.
a
Reagents: (i) KOH, EtOH/H2O; (ii) carbonyldiimidazole; (iii)
2 equiv of 4-chloroacetophenone/LDA, -78 °C-room temperature;
(iv) H2NNH2‚H2O, MeOH; (v) LiAlH4, 40 °C.
Sch em e 5a
Discu ssion
It is necessary to have a lipophilic group on the basic
nitrogen of the piperidine, as neither the secondary
amine 4 nor the N-methyl derivative 5 binds to dopam-
ine receptors (Table 2). In the lead compound the
lipophilic group is 4-chlorobenzyl, but it is not a require-
ment that it be aromatic, as the cyclohexylmethyl
analogue 6 shows improved affinities. In the benzyl
series, moving the chlorine atom from the para to the
meta position (7) has little effect, whereas the o-chloro
compound (8) has improved hD4 affinity and reduced
hD2 affinity, leading to overall better selectivity. The
biggest improvement in hD4 affinity however comes
from removal of the chlorine atom altogether, leading
to the unsubstituted benzyl analogue 9 with better than
10 nM affinity at hD4 and greater than 10-fold selectiv-
ity over hD2. Homologation of this to the phenethyl
derivative 10 gives a further doubling of binding affinity
at hD4 receptors and a marked reduction in binding to
hD2, leading to a compound with almost 100-fold
selectivity for hD4 over hD2. There is no measurable
binding to the hD3 subtype in this compound. The two-
carbon chain between the basic nitrogen and the lipo-
a
Reagents: (i) PhNMeCSNHNH2, acetonitrile, reflux; (ii) phen-
acyl bromide, EtOH, room temperature then HCl reflux.
potassium hexamethyldisilazide a 2:1 mixture of the
desired diketone (57) and retro-Claisen product (58) was
obtained. Condensation of diketone 57 with hydrazine,
N-deprotection, and alkylation finished the preparation
of the pyrazole 28.
The 4-methylpyrazoles 32 and 34 (Table 6) were made
in the same way as the 4-unsubstituted pyrazoles but
starting with 4-chloropropiophenone rather than the
acetophenone. It was also possible to alkylate the
intermediate â-diketone 59 (Scheme 7) by evaporation
of a mixture with tetrabutylammonium fluoride20 fol-
lowed by treatment with ethyl iodide in chloroform. This

Heterocyclylpiperidines as High-Affinity Ligands
J ournal of Medicinal Chemistry, 1997, Vol. 40, No. 15 2377
Sch em e 6a
a
Reagents: (i) trimethyl orthoformate, Montmorillonite K-10; (ii) N-BOC ethyl isonipocotate/KHMDS, THF, 0 °C; (iii) KHMDS, THF,
0 °C; (iv) H2NNH2‚H2O, MeOH; (v) HCl, EtOAc; (vi) PhCH2CH2Br, Et-i-Pr2N, DMF, 60 °C.
Sch em e 7a
a
Reagents: (i) (a) TBAF, THF, (b) EtI, CHCl3; (ii) HCl, ether; (iii) PhCH2Br, Et-i-Pr2N, DMF; (iv) H2NNH2‚H2O, MeOH.
Sch em e 8a
Ta ble 2. Substitution on the Basic Nitrogen
a
Reagents: (i) H2NOH2‚H2Cl, Et3N, MeOH; (ii) MsCl, Et3N,
CH2Cl2, 0 °C; (iii) guanidine hydrochloride, NaO-i-Pr, i-PrOH,
reflux.
philic benzene ring is optimal, with the phenylpropyl
analogue 11 losing 20-fold in binding to hD4, with no
change at hD2, giving a compound with a poorer profile
than the lead.
The lipophilic group at the other end of the molecule
is also important for binding (Table 3). When the
4-chlorophenyl ring attached to the pyrazole is replaced
with a simple methyl group (12), binding to hD4 is
reduced by a factor of 50 compared to the analogous
compound (9). Again, the chlorine atom on this benzene
ring is not required for binding, with removal giving a
compound (13) with the same affinity for hD4 as that
of compound 9. However, there is a doubling of hD2
a
Affinities at cloned human dopamine receptors stably ex-
pressed in cell lines.
affinity on removal of this chlorine atom, so selectivity
is correspondingly reduced. The attachment of the
benzene ring directly to the pyrazole is important, with
the homologous benzylpyrazole 14 losing 15-fold in hD4
binding, despite the fact that the distance (in number
of atoms) between the two benzene rings is the same
as for compound 10. Since it had been established that
the best substituent on the basic nitrogen is phenethyl,
further changes to the group attached to the pyrazole
were performed with this arrangement. Reduction of
the benzene ring to a cyclohexyl group (15) has little
effect on hD4 binding but gives a marked increase in
affinity at hD2, reducing selectivity. A similar effect is
seen when the benzene ring is replaced with a thiophene

2378 J ournal of Medicinal Chemistry, 1997, Vol. 40, No. 15
Ta ble 3. Aromatic Substituent on the Pyrazole
Rowley et al.
Ta ble 4. Changes to the Piperidine
a
Affinities at cloned human dopamine receptors stably ex-
pressed in cell lines.
Ta ble 5. Conformational Restriction of the Piperidine
a
Affinities at cloned human dopamine receptors stably ex-
pressed in cell lines.
(16). The three pyridyl isomers (17-19) have similar
profiles, with a small reduction in binding to hD4
receptors when compared to the benzene ring, along
with a smaller reduction in hD2 affinity. None of these
changes show benefit over the (4-chlorophenyl)pyrazole
present in the lead.
Similarly there is little gain to be had when the
position and nature of the basic nitrogen in the piperi-
dine ring are varied (Table 4). The 3- and 2-substituted
piperidines 20 and 21 show marked reduction in hD4
affinity. The pyrrolidine 22, in which the position of
the basic nitrogen is intermediate between that of the
4-substituted (9) and 3-substituted (20) piperidines, has
intermediate hD4 affinity as well. While the tetrahy-
dropyridine 23 retains affinity, selectivity over hD2 is
reduced, so there is no advantage with this change. The
final alteration is the piperazine 24. Again, affinity for
the hD4 receptor is reduced over the corresponding
piperidine 10, as is binding at hD2. In this case, the
reduction in affinity may be due to a reduction in the
pKa of the required basic nitrogen. For the piperidine
10 the pKa is >7.5 (an exact measurement is not possible
due to solubility problems), and for the piperazine 24
this is reduced to a pKa of 7.2.
a
Affinities at cloned human dopamine receptors stably ex-
pressed in cell lines.
F igu r e 1. Solution conformation of the piperidine ring of 26
by NMR.
The importance of the pyrazole ring was investigated
in two ways: by steric constraints to alter its conforma-
tion relative to the piperidine and by replacement with
alternative aromatic heterocycles. The results of the
first of these approaches is shown in Table 5. It was
found that the â-diketone intermediate to the pyrazole
also binds to dopamine receptors (25), but addition of a
further methyl group in the 4-position of the piperidine
severely reduces this binding. This is probably due to
a conformational change of the diketone moiety relative
to the piperidine. In the unmethylated compound 25,
the diketone is equatorial to the ring, but NMR experi-
ments (Figure 1) show that in the methylated analogue
26 it is the methyl group that becomes equatorial, with
the diketone shifted to the axial conformation. For the
methylated pyrazole 27 the effect on binding is less
marked, but in this case molecular mechanics calcula-
tions suggest that there is little energy difference
between the two ring flip conformations, and the pyra-
zole-equatorial conformation required for binding is
energetically accessible. In the final compound (28) in

Heterocyclylpiperidines as High-Affinity Ligands
J ournal of Medicinal Chemistry, 1997, Vol. 40, No. 15 2379
Ta ble 6. Alternative Heterocycles
F igu r e 2. Conformations of compounds. 3, 29, and 30 are
X-ray structures of oxalate salts. The oxalic acid and hydrogens
are omitted for clarity. 28 is an energy-minimized structure
(Chem3D v3.5.1) of the conformation with the pyrazole equa-
torial.
structure of isoxazole 29 is similar to that of pyrazole
3, with the aromatic heterocycle coplanar with the C3-
C4 bond of the piperidine and the heteroatoms of the
isoxazole or pyrazole on the same face of the piperidine
ring as the basic nitrogen lone pair. This is the same
arrangement as for the equatorial conformer of the
inactive spirofused compound 28 above. In contrast to
this, the more active isomer (30) has the heterocycle
turned approximately 180°, with the heteroatoms now
toward the face opposite to the nitrogen lone pair. It
may be that for optimal binding, the hD4 receptor
prefers the nitrogen lone pair (or N-H bond of the
protonated species) on the opposite face of the piperidine
to the aromatic heteroatoms. The energetic price for
achieving this in pyrazole 3 and isoxazole 29 is reflected
in their relatively low affinities, and the inaccessibility
of this conformer to the spirofused compound 28 pre-
cludes binding. Another successful replacement for the
pyrazole is the aminopyrimidine 31 with nanomolar
affinity at hD4, but selectivity over hD2 suffers in this
case. Methylation (32) at the 4-position of the pyrazole
also shows a beneficial effect on binding at hD4 recep-
tors, but this time there is not the increase in hD2
binding, so the compound is highly selective. This again
is probably a conformational effect rather that an
increase in binding due to a lipophilic interaction, as
the 4-ethylpyrazole 33, in which the lipophilicity at the
4-position of the pyrazole is further increased, has
similar hD4 affinity to the methyl analogue 32.
a
Affinities at cloned human dopamine receptors stably ex-
pressed in cell lines.
Table 5, the pyrazole and the piperidine rings are locked
essentially orthogonal to one another by the presence
of an ethylene chain. This compound lacks activity
despite the fact that alkylation at the 4-position of the
piperidine 27 is allowed, as is methylation at the
4-position of the pyrazole 32 (Table 6). Again molecular
mechanics calculations suggest little energetic difference
between the conformers with the pyrazole axial or
equatorial. However, for the equatorial conformer
(Figure 2), the pyrazole nitrogens are locked on the same
side of the piperidine as the basic nitrogen lone pair,
an arrangement that might not be advantageous for
binding (vide infra).
Changing the pyrazole to alternative heterocycles
proved a more successful strategy. While one of the two
regioisomeric isoxazoles (29) had a profile very similar
to the lead, the other (30) gave almost 20-fold improve-
ment in binding to the hD4 receptor, with a correspond-
ing increase in selectivity. This may be due to the
positioning of the heteroatoms within the ring, but
isoxazoles 35 and 36, which are regioisomers with
similar binding affinities, suggest this is not the case.
It is interesting to note that the proof of structure of
these isomers by X-ray crystallography also showed
differences in conformation around the bond joining the
isoxazole to the piperidine (Figure 2). The solid state
Having established that the best basic nitrogen sub-
stituent for selectivity is a phenethyl group, that me-
thylation at the 4-position of the heterocycle is beneficial
to binding, and that isoxazole and aminopyrimidine
offer improvements in affinity, combinations of these
changes were made. The 4-methylpyrazole 34 has
nanomolar affinity, with 200-fold selectivity over hD2,
and the aminopyrimidine 37 has, if anything, better
selectivity but somewhat reduced affinity. The best
combination is seen in the isoxazoles 35 and 36, both
with low nanomolar affinity for the desired hD4 recep-

2380 J ournal of Medicinal Chemistry, 1997, Vol. 40, No. 15
Rowley et al.
3.19 (2 H, m, NCH), 6.95 (1 H, s, CdCH), 7.33 (2 H, d, J )
8.6, ArH o to Cl), 7.98 (2 H, d, J ) 8.6, ArH m to Cl); MS (CI+;
NH3) m/ z ) 248 (M+ + H).
tor. Both have greater than 500-fold selectivity over the
hD2 receptor, and one isomer (36) also has better than
200-fold selectivity over hD3.
3-(4-P iper idin yl)-5-(4-ch lor oph en yl)pyr azole (4). A mix-
ture of 39 (35.0 g, 141 mmol), KOH (15.0 g, 267 mmol), and
hydrazine hydrate (11.0 mL, 354 mmol) in ethylene glycol (100
mL) was heated at 110 °C for 15 min to produce a clear dark
brown solution. Hydrazine hydrate and H2O were distilled
from the solution over 1 h at 110-195 °C. The brown solution
was then heated at reflux for 3 h. The mixture was cooled to
room temperature and diluted with H2O (300 mL). The
resulting white precipitate was collected and washed with H2O
(500 mL) and EtOAc (200 mL). The white powder was dried
This more selective isomer (36) has been further
profiled pharmacologically and in pharmacokinetic stud-
ies. In D4 HEK cells isoxazole 36 (1 µM) alone had no
effect but antagonized the dopamine (1 µM)-mediated
inhibition of forskolin (10 µM)-induced elevation of
cAMP levels.22 Thus isoxazole 36 is an antagonist at
the D4 receptor. It also has affinities weaker than
micromolar at 5HT1A and 5HT2 receptors. In rat it had
38% oral bioavailability after a 3 mg/kg dose, reaching
a peak plasma concentration of 60 ng/mL with brain
concentrations 10-fold higher than this at 630 ng/g. The
half life in this species is 2 h.
1
to give 4 (31.5 g, 85%): mp 174-176 °C; H NMR (250 MHz,
DMSO-d6) δ 1.52 (2 H, qd, J ) 12, 4, CHCHAHB), 1.85 (2 H, d,
J ) 10, CHCHAHB), 2.56 (2 H, td, J ) 12, 2, NCHAHB), 2.71 (1
H, tt, J ) 8.1, 4.5, CH2CH), 2.99 (2 H, d, J ) 12, NCHAHB),
6.48 (1 H, s, NdCCH), 7.44 (2 H, d, J ) 8.6, ArH o to Cl), 7.79
(2 H, d, J ) 8.6, ArH m to Cl); MS (CI+; NH3) m/ z ) 262 (M+
+ H). Anal. (C14H16ClN3‚0.5H2O) C, H, N.
Con clu sion s
A lead dopamine D4 ligand (3) has a number of
important pharmacophoric features which have been
explored to optimize dopamine D4 receptor affinity and
selectivity. The lipophilic benzene rings at either end
of the molecule are important for binding, and homolo-
gation of the benzyl group of the lead to a phenethyl
group improves selectivity. The position of the basic
nitrogen within a 6- or 5-membered ring has been
explored and the 4-substituted piperidine found to be
best for binding. The pyrazole can be replaced with a
variety of other aromatic heterocycles or a â-diketone
to retain or improve binding, but there are subtle
conformational effects which can alter affinity at the
hD4 receptor. By introduction of the best groups from
the exploration of these structure-affinity relationship
studies, the isoxazole 36 has been identified as an
optimal ligand. Compound 36 not only is a high-affinity
selective dopamine D4 antagonist but also has a good
pharmacokinetic profile with high oral bioavailability
and duration and very good brain penetration, making
it an attractive tool to study the relevance and impor-
tance of the dopamine D4 receptor in the treatment of
central nervous system disorders, particularly schizo-
phrenia.
3-(1-(4-Ch lor oben zyl)-4-piper idin yl)-5-(4-ch lor oph en yl)-
p yr a zole (3). A solution of 4 (1.50 g, 5.7 mmol), Et3N (1.60
mL, 11.6 mmol), and 4-chlorobenzyl chloride (0.92 g, 5.7 mmol)
in 3:1 CH2Cl2:DMF (20 mL) was stirred at room temperature
for 4.5 h. CH2Cl2 was removed by evaporation, H2O (25 mL)
was added, and the mixture was extracted with EtOAc (2 ×
25 mL). The combined organic extracts were washed with
brine, dried, and evaporated to give an off-white solid. Re-
crystallization yielded the title compound as fine, white needles
1
(0.575 g, 26%): mp 179-181 °C (from EtOH/H2O); H NMR
(360 MHz, CDCl3) δ 1.71 (2 H, qd, J ) 12, 3, CHCHAHB), 1.84
(2 H, d, J ) 11, CHCHAHB), 1.92 (2 H, t, J ) 11, NCHAHB),
2.41-2.55 (1 H, m, CH2CH), 2.84 (2 H, d, J ) 11, NCHAHB),
3.44 (2 H, s, NCH2Ar), 6.21 (1 H, s, NdCCH), 7.30-7.21 (6 H,
m, ArH), 7.61 (2 H, d, J ) 8.1, ArH), 11.6-12.0 (1 H, br s,
NH); MS (CI+; NH3) m/ z ) 386 (M+ + H). Anal. (C21H21Cl2N3)
C, H, N.
The following compounds were made in the same way as 3
using the appropriate alkyl halide, with yields quoted for the
final step.
3-(1-(Cycloh e xylm e t h yl)-4-p ip e r id in yl)-5-(4-ch lor o-
p h en yl)p yr a zole (6): white granules (5%), mp 186-187 °C
1
(from EtOH/H2O); H NMR (360 MHz, CDCl3) δ 0.88 (2 H, q,
J ) 11.6, CH2), 1.10-1.29 (3 H, m, methylenes), 1.47-1.53 (1
H, m, NCH2CH), 1.69-1.99 (11 H, m, methylenes), 2.14 (2 H,
d, J ) 7.0, NCH2CH), 2.62 (1 H, tt, J ) 11.5, 3.9, NCH2-
CH2CH), 2.93 (2 H, d, J ) 11, NCHAHB), 6.33 (1 H, s, NdCCH),
7.33 (2 H, d, J ) 8.4, ArH o to Cl), 7.65 (2 H, d, J ) 8.4, ArH
m to Cl); MS (CI+; NH3) m/ z ) 358 (M+ + H). Anal.
(C21H28N3Cl) C, H, N.
Exp er im en ta l Section
Melting points were taken on a Reichert Thermovar ap-
paratus and are uncorrected. Proton NMR spectra were
measured on a Bruker AM 360 or AC 250 spectrometer, and
chemical shifts are reported in parts per million (δ) downfield
from tetramethylsilane as internal standard; coupling con-
stants are in hertz. Mass spectra were recorded on a VG 70/
250 spectrometer. Merck Kieselgel (230-400) mesh was used
for column chromatography. For reactions, dry solvents were
used as bought from Aldrich. Organic solutions were dried
with anhydrous magnesium sulfate. Elemental analyses were
done by Butterworth Laboratories Ltd., Teddington, Middle-
sex, U.K.
3-(1-(3-Ch lor oben zyl)-4-piper idin yl)-5-(4-ch lor oph en yl)-
p yr a zole (7): white granules (66%), mp 134-136 °C (from
1
EtOH/H2O); H NMR (360 MHz, CDCl3) δ 1.74 (2 H, qd, J )
12, 4, CHCHAHB), 1.87 (2 H, d, J ) 12, CHCHAHB), 1.97 (2 H,
t, J ) 12, NCHAHB), 2.56 (1 H, tt, J ) 12, 4, CH2CH), 2.87 (2
H, d, J ) 12, NCHAHB), 3.46 (2 H, s, NCH2Ar), 6.31 (1 H, s,
NdCCH), 7.17-7.33 (6 H, m, ArH), 7.62 (2 H, d, J ) 8.4, ArH
m to Cl), 10.07 (1 H, br s, NH); MS (CI+; NH3) m/ z ) 386 (M+
+ H). Anal. (C21H21N3Cl2) C, H, N.
3-(1-(2-Ch lor oben zyl)-4-piper idin yl)-5-(4-ch lor oph en yl)-
p yr a zole (8): fine, pale yellow crystals (24%), mp 137-139
°C (from EtOH/H2O); 1H NMR (360 MHz, DMSO-d6) δ 1.74 (2
H, dq, J ) 3.5, 12, CHCHAHB), 1.92 (2 H, br d, J ) 12,
CHCHAHB), 2.19 (2 H, dt, J ) 3.5, 11, NCHAHB), 2.67 (1 H, m,
CH2CH), 2.91 (2 H, br d, J ) 12, NCHAHB), 3.58 (2 H, s,
ArCH2), 6.53 (1 H, s, NdCCH), 7.26-7.36 (2 H, m, ArH), 7.42-
7.44 (3 H, m, ArH), 7.52 (1 H, dd, J ) 2, 7, ArH), 7.76-7.80 (2
H, d, J ) 8, ArH), 12.69 (1 H, br s, NH); MS (CI+; NH3) m/ z
) 386 (M+ + H). Anal. (C21H21N3Cl2) C, H, N.
2-(4-Ch lor oben zylid en e)qu in u clid in -3-on e (39). A solu-
tion of 3-quinuclidinone (38; 54.5 g, 0.34 mol), 4-chlorobenz-
aldehyde (84.0 g, 0.59 mol), and sodium hydroxide (3.50 g, 88
mmol) in EtOH (400 mL) was heated at reflux for 2.5 h. The
mixture was cooled, and addition of H2O (100 mL) caused the
product to precipitate as an orange solid. The precipitate was
collected and washed with 1:1 EtOH:H2O (400 mL). The
mother liquors stood for 2 days, after which time further
precipitated material was collected. The orange solid was
redissolved in CH2Cl2 (800 mL) and washed with H2O (200
mL). The organic phase was dried and evaporated to give 39
3-(1-Be n zyl-4-p ip e r id in yl)-5-(4-ch lor op h e n yl)p yr a -
zole (9): white needles (33%), mp 164-167 °C (from EtOH/
1
H2O); H NMR (360 MHz, DMSO-d6) δ 1.74 (2 H, qd, J ) 12,
1
as a bright yellow powder (86.2 g, 76%): mp 108-110 °C; H
4, CHCHAHB), 1.86 (2 H, d, J ) 10, CHCHAHB), 1.97 (2 H, t,
J ) 11, NCHAHB), 2.55 (1 H, tt, J ) 11.6, 4, CH2CH), 2.55 (2
H, d, J ) 12, NCHAHB), 3.50 (2 H, br s, NCH2Ph), 6.29 (1 H,
NMR (360 MHz, CDCl3) δ 1.99-2.05 (4 H, m, NCH2CH2), 2.63
(1 H, quintet, J ) 2.9, CHCO), 2.92-3.01 (2 H, m, NCH), 3.11-

Heterocyclylpiperidines as High-Affinity Ligands
J ournal of Medicinal Chemistry, 1997, Vol. 40, No. 15 2381
s, NdCCH), 7.18-7.38 (7 H, m, ArH), 7.61 (2 H, d, J ) 8.3,
ArH); MS (CI+; NH3) m/ z ) 352 (M+ + H). Anal. (C21H22N3-
Cl) C, H, N.
EtOAc, washed with H2O and brine, dried, and evaporated to
give 25 (1.97 g, 56%) as white needles: mp 84-86 °C (from
EtOH/H2O); 1H NMR (360 MHz, DMSO-d6) δ 1.8-2.0 (4 H,
m, CH2CH), 2.0-2.2 (2 H, m, NCHAHB), 2.3-2.4 (1 H, m,
CHCH2), 3.0 (2 H, d, J ) 12, NCHAHB), 3.59 (2 H, s, ArCH2),
6.15 (1 H, s, COCHCO), 7.2-7.4 (5 H, m, Ph), 7.42 (2 H, d, J
) 8.6, ArH o to Cl), 7.81 (2 H, d, J ) 8.6, ArH m to Cl), 15.9
(1 H, br s, OH); MS (CI+; NH3) m/ z ) 356 (M+ + H). Anal.
(C21H22ClNO2) C, H, N.
3-(1-Be n zyl-4-p ip e r id in yl)-5-(4-ch lor op h e n yl)p yr a -
zole (9). A mixture of 25 (0.5 g, 1.4 mmol) and hydrazine
hydrate (0.1 mL, 2 mmol) in EtOH (5 mL) was stirred at room
temperature overnight. H2O was added, the mixture was
extracted with EtOAc (×2), and the combined organic layers
were washed with H2O and brine, dried, evaporated, and
recrystallized from EtOH/H2O to give 9 (0.39 g, 79%).
The following were made in the same way, using the
appropriate ketone and alkylating reagent.
3-(1-(2-P h en yleth yl)-4-p ip er id in yl)-5-(4-ch lor op h en yl)-
p yr a zole (10): beige granules (26%), mp 169-171 °C (from
EtOH/H2O); 1H NMR (360 MHz, DMSO-d6) δ 1.83 (2 H, qd, J
) 12, 4, CHCHAHB), 2.00 (2 H, d, J ) 11, CHCHAHB), 2.14 (2
H, t, J ) 12, NCHAHB), 2.61-2.65 (2 H, m, NCH2CH2Ar), 2.70
(1 H, tt, J ) 11.8, 3.9, CH2CH), 2.81-2.86 (2 H, m, NCH2CH2-
Ar), 3.08 (2 H, d, J ) 12, NCHAHB), 6.34 (1 H, s, NdCCH),
7.35-7.16 (7 H, m, ArH), 7.69 (2 H, d, J ) 8.4, ArH m to Cl);
MS (CI+; NH3) m/ z ) 366 (M+ + H). Anal. (C22H24N3Cl) C,
H, N.
3-(1-(3-P h e n yl-1-p r op yl)-4-p ip e r id in yl)-5-(4-ch lor o-
p h en yl)p yr a zole (11): fine, white crystals (57%), mp 165-
1
167 °C (from EtOH); H NMR (360 MHz, DMSO-d6) δ 1.61-
1.78 (4 H, m, CHCHAHB, CH2CH2CH2), 1.91 (2 H, br d, J )
12, CHCHAHB), 1.99 (2 H, t, J ) 12, NCHAHB), 2.31 (2 H, t, J
) 7, CH2CH2CH2), 2.61 (2 H, t, J ) 7, CH2CH2CH2), 2.72 (1
H, tt, J ) 12, 4, CH2CH), 2.92 (2 H, br d, J ) 12, NCHAHB),
6.51 (1 H, s, NdCCH), 7.14-7.22 (5 H, m, ArH), 7.44 (2 H, d,
J ) 7, ArH o to Cl), 7.79 (2 H, d, J ) 7, ArH m to Cl), 12.67 (1
H, s, NH); MS (CI+; NH3) m/ z ) 380 (M+ + H). Anal.
(C23H26N3Cl) C, H, N.
3-(1-Ben zyl-4-p ip er id in yl)-5-m eth ylp yr a zole (12): ox-
1
alate salt (19% from 41), mp 120-122 °C (from i-PrOH); H
NMR (360 MHz, DMSO-d6) δ 1.8-1.9 (2 H, m, CHAHBCH),
2.02 (2 H, d, J ) 11, CHAHBCH), 2.15 (3 H, s, Me), 2.75-2.85
(1 H, m, CHCH2), 2.95 (2 H, t, J ) 12, NCHAHB), 3.3 (2 H, d,
J
) 12, NCHAHB), 4.21 (2 H, s, NCH2Ph), 5.81 (1 H, s,
NdCCH), 7.4-7.5 (5 H, m, Ph); MS (CI+; NH3) m/ z ) 256
3-(1-Me t h yl-4-p ip e r id in yl)-5-(4-ch lor op h e n yl)p yr a -
zole (5). Trifluoroacetic acid (20 mL, 260 mmol) was added
dropwise over 1 h to a stirred suspension of 4 (1.00 g, 3.8
mmol), sodium borohydride (0.73 g, 19.1 mmol), and paraform-
aldehyde (1.15 g, 38.2 mmol) in dry THF (50 mL) under
nitrogen. After stirring for 20 h the mixture was poured into
ice-cold 10% aqueous NaOH (100 mL) and extracted with CH2-
Cl2 (3 × 50 mL). The combined organic extracts were washed
with brine, dried, and evaporated. Recrystallization yielded
the title compound as white cubes (0.501 g, 48%): mp 209-
211 °C (from EtOH/H2O); 1H NMR (360 MHz, DMSO-d6) δ 1.77
(2 H, qd, J ) 12, 4, CHCHAHB), 1.99 (2 H, d, J ) 12,
CHCHAHB), 2.07 (2 H, t, J ) 12, NCHAHB), 2.30 (3 H, s, NCH3),
2.65 (1 H, tt, J ) 12, 4, CH2CH), 2.91 (2 H, apparent d, J )
12, NCHAHB), 6.32 (1 H, s, NdCCH), 7.32 (2 H, d, J ) 8.5,
ArH), 7.72 (2 H, d, J ) 8.5, ArH), 12.5 (1 H, br s, NH); MS
(CI+; NH3) m/ z ) 276 (M+ + H). Anal. (C15H18N3Cl) C, H, N.
Gen er a l Rou te for th e Syn th esis of P yr a zoles via
â-Dik et on es. 1-Ben zyl-4-(3-h yd r oxy-1-oxo-3-(4-ch lor o-
p h en yl)p r op -2-en -1-yl)p ip er id in e (25). n-BuLi (28 mL, 1.6
M in hexanes, 44.8 mmol) was added to i-Pr2NH (6.2 mL, 44.8
mmol) in THF (150 mL) at 0 °C. The solution was stirred at
0 °C for 15 min and then cooled to -78 °C, and a solution of
4-chloroacetophenone (6.8 g, 44 mmol) in THF (10 mL) was
added over 5 min. In a separate vessel, carbonyldiimidazole
(3.89 g, 24 mmol) was added to 1-(tert-butyloxycarbonyl)-4-
piperidinecarboxylic acid (41; 5 g, 21.8 mmol) in THF (100 mL).
After 45 min, the latter solution was cannulated into the
former, stirred for 45 min at -78 °C, and then warmed to room
temperature. EtOAc was added and the mixture washed with
1 M citric acid, saturated NaHCO3 solution, and brine, dried,
and evaporated to give a mixture of 59 and 4-chloroacetophe-
none: 1H NMR (360 MHz, CDCl3) δ 1.47 (9 H, s , t-Bu), 1.6-
1.7 (2 H, m, CHCHAHB), 1.8-1.9 (2 H, m, CHCHAHB), 2.4-
2.5 (1 H, m, CHCH2), 2.7-2.8 (2 H, m, NCHAHB), 4.1-4.2 (2
H, m, N CHAHB), 6.14 (1 H, s, COCHCO), 7.42 (2 H, d, J ) 7,
ArH o to Cl), 7.81 (2 H, d, J ) 7, ArH m to Cl), 16.0 (1 H, br
s, OH); MS (CI+; NH3) m/ z ) 366 (M+ + H).
(M+ + H). Anal. (C16H21N3‚1.5C2H2O4) C, H, N.
3-(1-Ben zyl-4-p ip er id in yl)-5-p h en ylp yr a zole (13): white
1
plates (39% from 41), mp 130-131 °C (from EtOH); H NMR
(360 MHz, DMSO-d6) δ 1.7-1.8 (2 H, m, CHAHBCH), 1.9 (2 H,
d, J ) 12, CHAHBCH), 2.0-2.1 (2 H, m, NCHAHB), 2.6-2.7 (1
H, m, CHCH2), 2.9 (2 H, d, J ) 12, NCHAHB), 3.48 (2 H, s,
NCH2Ph), 6.49 (1 H, s, NdCCH), 7.2-7.4 (8 H, m, ArH), 7.75
(2 H, d, J ) 7, ArH); MS (CI+; NH3) m/ z ) 318 (M+ + H).
Anal. (C21H23N3‚0.6H2O) C, H, N.
3-(1-(2-P h en yleth yl)-4-p ip er id in yl)-5-cycloh exylp yr a -
zole (15): white needles (8% from 41), mp 102-104 °C (from
EtOH); 1H NMR (360 MHz, DMSO-d6) δ 1.2-1.4 (5 H, m, CH2),
1.5-1.6 (2 H, m, CH2), 1.6-1.8 (2 H, m, CH2), 2.02 (2 H, t, J
) 11, NCHAHB), 2.4-2.5 (4 H, m, CH2), 2.7-2.8 (1 H, m,
CHCH2), 2.96 (2 H, d, J ) 11, NCHAHB), 5.79 (1 H, s, NdCCH),
7.2-7.4 (5 H, m, ArH), 12.03 (1 H, s, NH); MS (CI+; NH3) m/ z
) 323 (M+ + H). Anal. (C22H31N3‚0.2H2O) C, H, N.
3-(1-(2-P h en yleth yl)-4-p ip er id in yl)-5-(2-th ien yl)p yr a -
zole (16): white crystals (4% from 41), mp 183-185 °C (from
EtOH/H2O); 1H NMR (360 MHz, DMSO-d6) δ 1.6-1.7 (2 H,
m, CHCHAHB), 1.9 (2 H, d, J ) 12, CHCHAHB), 2.06 (2 H, t, J
) 12, NCHAHB), 2.53 (2 H, t, J ) 8, PhCH2CH2), 2.6-2.7 (1
H, m, CHCH2), 2.75 (2 H, t, J ) 8, PhCH2), 3.00 (2 H, d, J )
12, NCHAHB), 6.36 (1 H, s, NdCCH), 7.05-7.38 (8 H, m, ArH),
12.6 (1 H, br s, NH); MS (CI+; NH3) m/ z ) 338 (M+ + H).
Anal. (C20H23N3S) C, H, N.
3-(1-(2-P h en yleth yl)-4-p ip er id in yl)-5-(2-p yr id yl)p yr a -
zole (17): orange cubes (3% from 41), mp 155-157 °C (from
EtOH); 1H NMR (360 MHz, DMSO-d6) δ 1.6-1.7 (2 H, m,
CHCHAHB), 1.92 (2 H, d, J ) 11, CHCHAHB), 2.08 (2 H, t, J )
11, NCHAHB), 2.53 (2 H, t, J ) 7.1, PhCH2), 2.56-2.64 (1 H,
m, CHCH2), 2.75 (2 H, t, J ) 7.1, PhCH2CH2), 3.00 (2 H, d, J
) 11, NCHAHB), 6.59 (1 H, s, NdCCH), 7.1-7.3 (6 H, m, ArH),
7.76-7.82 (1 H, m, pyridine-H), 7.84-7.93 (1 H, m, pyridine-
H), 8.52-8.60 (1 H, m, pyridine H-6), 12.75 (1 H, s, NH); MS
(CI+; NH3) m/ z ) 333 (M+ + H). Anal. (C21H24N4) C, H, N.
3-(1-(2-P h en yleth yl)-4-p ip er id in yl)-5-(3-p yr id yl)p yr a -
zole (18): off-white plates (13% from 41), mp 151-152 °C (from
EtOH); 1H NMR (360 MHz, DMSO-d6) δ 1.6-1.7 (2 H, m,
CHCHAHB), 1.92 (2 H, d, J ) 11, CHCHAHB), 2.09 (2 H, t, J )
11, NCHAHB), 2.55 (2 H, t, J ) 7.1, PhCH2), 2.56-2.64 (1 H,
m, CHCH2), 2.76 (2 H, t, J ) 7.1, PhCH2CH2), 3.02 (2 H, d, J
) 11, NCHAHB), 6.61 (1 H, s, NdCCH), 7.1-7.3 (6 H, m, ArH),
7.38-7.42 (1 H, m, pyridine-H), 8.11 (1 H, d, J ) 7.9, pyridine
H-6), 8.47 (1 H, d, J ) 2, pyridine H-2), 12.76 (1 H, s, NH);
MS (CI+; NH3) m/ z ) 333 (M+ + H). Anal. (C21H24N4‚H2O)
C, H, N.
A saturated solution of HCl in EtOAc (50 mL) was added
to the crude oil, and the mixture stirred at room temperature
for 30 min and then stood at 0 °C for 1 h. The resulting solid
was collected, washed with EtOAc, and dried to give 4-(1-(3-
hydroxy-1-oxo-3-(4-chlorophenyl)-2-propenyl))piperidine hy-
drochloride (42) as an off-white solid (5.1 g, 78%): 1H NMR
(250 MHz, DMSO-d6) δ 1.7-2.1 (4 H, m, CHCH2), 2.7-2.8 (1
H, m, CHCH2), 3.3-3.5 (4 H, m, NCH2), 6.6 (1 H, s, COCHCO),
7.62 (2 H, d, J ) 8, ArH o to Cl), 7.98 (2 H, d, J ) 8, ArH m
to Cl); MS (CI+; NH3) m/ z ) 266 (M+ + H).
A 3 g (10 mmol) portion of this solid was suspended in DMF
(30 mL) and CH2Cl2 (30 mL); then Et3N (3.1 mL, 22 mmol)
and benzyl bromide (1.86 g, 10.9 mmol) were added. After
stirring for 24 h the mixture was evaporated, diluted with
3-(1-(2-P h en yleth yl)-4-p ip er id in yl)-5-(4-p yr id yl)p yr a -
zole (19): brown plates (9% from 41), mp 142-144 °C (from
EtOH); 1H NMR (360 MHz, DMSO-d6) δ 1.6-1.7 (2 H, m,
CHCHAHB), 1.92 (2 H, d, J ) 11, CHCHAHB), 2.09 (2 H, t, J )

2382 J ournal of Medicinal Chemistry, 1997, Vol. 40, No. 15
Rowley et al.
11, NCHAHB), 2.54 (2 H, t, J ) 7, PhCH2), 2.6-2.7 (1 H, m,
CHCH2), 2.76 (2 H, t, J ) 7, PhCH2CH2), 3.02 (2 H, d, J ) 11,
NCHAHB), 6.67 (1 H, s, NdCCH), 7.1-7.3 (5 H, m, ArH), 7.72
(1 H, d, J ) 5.4, pyridine H-3), 8.58 (1 H, d, J ) 5.4, pyridine
H-2), 12.92 (1 H, s, NH); MS (CI+; NH3) m/ z ) 333 (M+ + H).
Anal. (C21H24N4) C, H, N.
3-(1-Be n zyl-3-p ip e r id in yl)-5-(4-ch lor op h e n yl)p yr a -
zole (20): white cubes (7% from piperidine-3-carboxylic acid),
mp 215-218 °C (from EtOH/EtOAc); 1H NMR (360 MHz,
CDCl3/CD3OD) δ 2.01-2.26 (4 H, m, piperidinyl CH), 3.28-
3.37 (1 H, m, piperidinyl CH), 4.47 (1 H, br d, J ) 11,
piperidinyl CH), 3.57-3.64 (2 H, m, piperidinyl CH), 3.84-
3.90 (1 H, m, piperidinyl CH), 4.43-4.49 (2 H, m, NCH2Ph),
7.05 (1 H, s, NdCCH), 7.44-7.52 (5 H, m, ArH), 7.69-7.40 (4
H, m, ArH); MS (CI+; NH3) m/ z ) 352 (M+ + H). Anal.
(C21H22N3Cl‚2HBr‚0.25H2O) C, H, N.
3-(1-Be n zyl-2-p ip e r id in yl)-5-(4-ch lor op h e n yl)p yr a -
zole (21): w hite needles (6% from ethylpiperidine-2-carboxyl-
ate), mp 174-176 °C (from EtOH); 1H NMR (360 MHz, CDCl3)
δ 1.30-1.44 (1 H, m, piperidinyl CH), 1.48-1.68 (2 H, m,
piperidinyl CH), 1.70-1.85 (2 H, m, piperidinyl CH), 1.86-
1.98 (1 H, m, piperidinyl CH), 2.05 (1 H, apparent t, J ) 10,
piperidinyl CH), 3.03 (2 H, apparent t, J ) 11, NCH2Ph), 3.45
(1 H, br d, J ) 9, piperidinyl CH), 3.82 (1 H, d, J ) 13, NCHAr),
6.54 (1 H, s, NdCCH), 7.16-7.32 (5 H, m, ArH), 7.33-7.37 (2
H, m, ArH), 7.69-7.40 (2 H, d, J ) 9, ArH); MS (CI+; NH3)
m/ z ) 352 (M+ + H). Anal. (C21H22N3Cl) C, H, N.
NH3) m/ z ) 346 (M+ + H). (In this experiment, the yield of
47 was not determined. However, when 41, carbonyldiimi-
dazole, and phenylacetone were reacted under the conditions
used in the general route for the synthesis of pyrazole via
â-diketones, 47 was obtained in 36% yield.)
The oil 46 (1.15 g, 3.3 mmol) was stirred in MeOH (50 mL)
with hydrazine hydrate (0.67 g, 13.3 mmol) at room temper-
ature for 45 min and evaporated to give 3-(1-(tert-butyloxy-
carbonyl)-4-piperidinyl)-5-benzylpyrazole (1.1 g) as an oil: 1H
NMR (250 MHz, DMSO-d6) δ 1.4 (9 H, s, t-Bu), 1.5-1.6 (2 H,
m, CHCHAHB), 1.8 (2 H, d, J ) 12, CHCHAHB), 2.7-2.9 (3 H,
m, NCHAHB, CHCH2), 3.8 (2 H, s, CH2Ph), 3.9 (2 H, d, J ) 12,
NCHAHB), 5.8 (1 H, s, NdCCH), 7.1-7.4 (5 H, m, Ph).
A saturated solution of HCl in EtOAc (20 mL) was added,
the mixture was warmed to reflux for 2 min and cooled, and
the resulting solid was collected and washed with ether to give
3-(4-piperidinyl)-5-benzylpyrazole hydrochloride (680 mg) as
a solid: 1H NMR (250 MHz, DMSO-d6) δ 1.7-1.8 (2 H, m,
CHCHAHB), 2.0 (2 H, d, J ) 12, CHCHAHB), 2.8-3.0 (3 H, m,
NCHAHB, CHCH2), 3.25 (2 H, d, J ) 12, NCHAHB), 3.9 (2 H, s,
CH2Ph), 5.9 (1 H, s, NdCCH), 7.1-7.4 (5 H, m, Ph).
A 300 mg (1.1 mmol) portion of this solid was dissolved in
DMF (10 mL), and Et-i-Pr2N (280 mg, 2.2 mmol) and benyl
bromide (204 mg, 1.2 mmol) were added. After 16 h H2O was
added, the mixture was extracted with EtOAc (×3), and the
combined organic layers washed with H2O and brine, dried,
evaporated, and purified by flash chromatography eluting with
CH2Cl2:MeOH:880 NH3 (94:5:1, v/v) to give the title compound
as an oil (150 mg, 30% over three steps): oxalate salt, mp 98-
101 °C (from EtOH); 1H NMR (360 MHz, DMSO-d6) δ 1.8-1.9
(2 H, m, CHCHAHB), 2.0 (2 H, d, J ) 12, CHCHAHB), 2.7-2.8
(1 H, m, CHCH2), 2.9 (2 H, t, J ) 12, NCHAHB), 3.2 (2 H, d, J
) 12, NCHAHB), 3.87 (2 H, s, CH2Ph), 4.16 (2 H, s, CH2Ph),
5.81 (1 H, s, NdCCH), 7.1-7.5 (10 H, m, Ph); MS (CI+; NH3)
m/ z ) 332 (M+ + H). Anal. (C22H25N3‚C2H2O4‚0.2H2O) C, H,
N.
3-(1-(2-P h en ylet h yl)-1,2,3,6-t et r a h yd r op yr id -4-yl)-4-
m eth yl-5-p h en ylp yr a zole (23): white crystals (6% from
BOC-protected 1,2,3,4-tetrahydropyridine-4-carboxylic acid),
1
mp 165-167 °C (from EtOH); H NMR (360 MHz, DMSO-d6)
δ 2.18 (3 H, s, Me), 1.6-1.7 (2 H, m, NCH2CHAHB), 2.48-2.52
(2 H, m, CH2), 2.6-2.7 (4 H, m, CH2), 2.80 (2 H, t, J ) 6.8,
CH2), 3.17 (2 H, d, NCH2CH), 5.98 (1 H, t, J ) 2.8, NCH2CH),
7.2-7.6 (10 H, m, ArH), 12.7 (1 H, s, NH); MS (CI+; NH3) m/ z
) 344 (M+ + H). Anal. (C23H25N3) C, H, N.
3-(1-Ben zyl-4-piper idin yl)-4-m eth yl-5-(4-ch lor oph en yl)-
p yr a zole (32): colorless plates (20% from 41), mp 188-190
3-(1-Ben zyl-3-p yr r olid in yl)-5-(4-ch lor op h en yl)p yr a -
zole (22). A solution of methyl 1-benzyl-5-oxo-3-pyrrolidin-
ecarboxylate (48; 10.0 g, 43 mmol) and KOH (2.88 g, 52 mmol)
in 1:1 EtOH:H2O (100 mL) was stirred at room temperature
for 18 h. EtOH was removed by evaporation, and the aqueous
concentrate was washed with Et2O (20 mL) prior to acidifica-
tion with concentrated hydrochloric acid (pH 2) and extraction
with EtOAc (2 × 50 mL). The combined extracts were dried
and concentrated to give 1-benzyl-5-oxo-3-pyrrolidinecarboxylic
acid (49) as a white powder (9.00 g, 96%): 1H NMR (360 MHz,
CDCl3) δ 2.68 (1 H, dd, J ) 17, 10, COCHAHB), 2.78 (1 H, dd,
J ) 17, 7, COCHAHB), 3.14-3.23 (1 H, m, CHCO2H), 3.44-
3.54 (2 H, m, NCH2CH), 4.40 (1 H, d, J ) 15, NCHAHBPh),
4.49 (1 H, d, J ) 15, NCHAHBPh), 7.22-7.35 (5 H, m, Ph); MS
(CI+; NH3) m/ z ) 220 (M+ + H).
A mixture of 49 (3.00 g, 13.7 mmol) and N,N-carbonyldi-
imidazole (2.33 g, 14.4 mmol) in dry THF (20 mL) was stirred
at room temperature under nitrogen for 50 min. Meanwhile,
a solution of LDA in dry THF (30 mL) was prepared by the
addition of n-butyllithium (1.6 M, 17.1 mL, 27.4 mmol) to
diisopropylamine (3.85 mL, 27.4 mmol) at room temperature
under nitrogen. The LDA solution was cooled to -78 °C, and
4-chloroacetophenone (3.55 mL, 27.4 mmol) was added drop-
wise. After the yellow enolate solution stirred for 15 min, the
imidazolide suspension was introduced via cannula. The thick
white suspension was stirred at -78 °C for 2 h, then warmed
to room temperature, and quenched with saturated aqueous
NH4Cl (40 mL). The mixture was diluted with hydrochloric
acid (2 M, 100 mL) and brine (100 mL) and then extracted
with EtOAc (2 × 100 mL). EtOAc was removed by evaporation
to yield a crude mixture (7.7 g) containing 1-benzyl-4-(3-(4-
chlorophenyl)-1-hydroxy-3-oxo-propenyl)pyrrolidin-2-one (50),
4-chloroacetophenone, and unreacted carboxylic acid in a 3:5:1
ratio. The crude material was redissolved in MeOH (20 mL),
then hydrazine hydrate (5 mL) was added, and the solution
was stirred at room temperature for 17 h. MeOH was removed
by evaporation, and the residue was dissolved in aqueous
NaOH (0.3 M, 70 mL) before extraction with EtOAc (2 × 70
mL). The extracts were washed with brine, dried, and
concentrated to give a yellow oil (6.77 g) containing a 1:2
1
°C (from EtOH); H NMR (360 MHz, DMSO-d6) δ 1.61 (2 H,
d, J ) 12, CHCHAHB), 1.90 (2 H, q, J ) 12, CHCHAHB), 2.04-
2.18 (5 H, m, NCHAHB, CH3), 2.60-2.74 (1 H, m, CH2CH), 2.99
(2 H, d, J ) 11, NCHAHB), 3.53 (2 H, s, CH2Ar), 7.12-7.34 (7
H, m, ArH), 7.54 (2 H, d, J ) 8, ArH m to Cl), 11.98 (1 H, br
s, NH); MS (CI+; NH3) m/ z ) 366 (M+ + H). Anal. (C22H24
ClN3) C, H, N.
-
3-(4-(1-(2-P h en yleth yl)p ip er id in yl))-4-m eth yl-5-(4-ch lo-
r op h en yl)p yr a zole (34): white needles (18% from 41), mp
181-183 °C (from EtOH/H2O); 1H NMR (360 MHz, DMSO-
d6) δ 1.92-2.02 (4 H, m, CHCH2), 2.14 (3 H, s, CH3), 2.15-
2.18 (2 H, m, NCHAHB), 2.65-2.77 (3 H, m, CH2CH, CH2Ph),
2.83-2.87 (2 H, m, NCH2CH2Ph), 3.16 (2 H, br d, J ) 12,
NCHAHB), 7.18-7.22 (3 H, m, ArH), 7.25-7.31 (2 H, m, ArH),
7.33-7.40 (2 H, m, ArH), 7.50 (2 H, d, J ) 8, ArH); MS (CI+;
NH3) m/ z ) 380 (M+ + H). Anal. (C23H26ClN3‚0.4H2O) C, H,
N.
3-(1-Ben zyl-4-p ip er id in yl)-5-ben zylp yr a zole (14). Meth-
yllithium (17.4 mL, 1.4 M in ether, 24.3 mmol) was added to
a mixture of 3-phenyl-2-[(trimethylsilyl)oxy]propene and 1-phen-
yl-2-[(trimethylsilyl)oxy]propene23 (5 g, 24.3 mmol, 1:1 mixture)
in ether (150 mL) at room temperature and stirred for 1 h.
Meanwhile a mixture of isonipecotic acid (2.8 g, 12.2 mmol)
and carbonyldiimidazole (1.98 g, 12.2 mmol) was stirred in
THF (80 mL) at room temperature for 15 min and cooled to
-78 °C, and then the enolate solution was added via cannula.
After 30 min at -78 °C, the mixture was warmed to room
temperature for 30 min and then poured into 1 M citric acid
(250 mL). The mixture was extracted with EtOAc (3 × 100
mL), and the combined organic layers were washed with
NaHCO3, H2O, and brine, dried, evaporated, and purified by
flash chromatography eluting with hexanes:EtOAc (5:1, v/v)
to give a mixture of 47 and phenylacetone as the less polar
product and then 46 (1.153 g, 27%) as an oil: 1H NMR (360
MHz, CDCl3) δ 1.50 (9 H, s, t-Bu), 1.56 (2 H, d, J ) 12,
CHCHAHB), 1.6-1.8 (2 H, m, CHCHAHB), 1.87 (2 H, s, CH2-
Ph), 2.2-2.3 (1 H, m, CHCH2), 2.43 (2 H, t, J ) 12, NCHAHB),
4.04 (2 H, d, J ) 12, NCHAHB), 7.2-7.5 (5 H, m, Ph); MS (CI+;

Heterocyclylpiperidines as High-Affinity Ligands
J ournal of Medicinal Chemistry, 1997, Vol. 40, No. 15 2383
mixture of 3-(1-benzyl-5-oxo-3-pyrrolidinyl)-5-(4-chlorophenyl)-
pyrazole (51) and 4-chloroacetophenone hydrazone. Purifica-
tion of a small sample (2.0 g) by flash column chromatography
on silica gel, eluting with EtOAc and then 10% EtOH-EtOAc
gave 51 (0.49 g) as a colorless glass: 1H NMR (250 MHz,
CDCl3) δ 2.72 (1 H, br dd, J ) 17, 6, COCHAHB), 2.95 (2 H, br
dd, J ) 17, 9, COCHAHB), 3.36-3.47 (1 H, m, NCH2CHAr),
3.62-3.76 (2 H, m, NCH2CHAr), 4.52 (2 H, br s, NCH2Ph),
6.30 (1 H, s, NdCCH), 7.20-7.40 (7 H, m, ArH), 7.50-7.58 (2
H, m, ArH).
225 °C (from EtOH/DMF); 1H NMR (360 MHz, DMSO +
CF3CO2H) (a 2.5:1 mixture of invertomers at the protonated
nitrogen was observed) signals for major isomer δ 1.24 (3 H,
s, CH3), 1.86-2.04 (2 H, m, piperidinyl CH), 2.43 (2 H, d, J )
14, piperidinyl CH), 2.86-2.99 (2 H, m, piperidinyl CH), 3.24-
3.36 (2 H, m, piperidinyl CH), 4.28 (2 H, d, J ) 5, NCH2Ph),
6.79 (1 H, s, NdCCH), 7.44-7.52 (7 H, m, ArH), 7.82 (2 H, d,
J ) 9, ArH), 9.5 (1 H, br s, NH); signals visible for minor
isomer δ 2.14-2.23 (2 H, m, piperidinyl CH), 4.45 (2 H, d, J )
5, NCH2Ph), 6.65 (1 H, s, NdCCH), 7.79 (2 H, d, J ) 9, ArH);
MS (CI+; NH3) m/ z ) 366 (M+ + H). Anal. (C22H26N3ClBr2)
C, H, N.
3-P h en yl-1′-(2-p h en ylet h yl)-2,4,5,6-t et r a h yd r ocyclo-
p en ta p yr a zole-6-sp ir o-4′-p ip er id in e (28). Montmorillonite
K-10 clay (40 g) was stirred with trimethyl orthoformate (60
mL) at room temperature for 20 min and then added to a
solution of 4-iodo-1-phenylbutan-1-one (54) (8.0 g, 29 mmol)
in toluene (70 mL). After the suspension stirred for 16 h at
room temperature, the mixture was filtered, washing with
toluene (100 mL). The filtrate was concentrated to give (4-
iodo-1,1-dimethoxybutyl)benzene (55) (6.49 g, 70%) as a green
oil: 1H NMR (250 MHz, C6D6) δ 1.26-1.38 (2 H, m, CH2CH2I),
1.90-1.97 (2 H, m, CH2CH2CH2I), 2.49 (2 H, t, J ) 7, CH2I),
2.96 (6 H, OCH3), 7.00-7.20 (3 H, m, ArH), 7.48-7.53 (2 H,
m, ArH).
A solution of potassium hexamethyldisilylamide in toluene
(0.5 M, 40 mL) was added dropwise to a solution of N-(tert-
butyloxycarbonyl)isonipecotic acid ethyl ester (5.1 g, 20 mmol)
in dry THF (200 mL) at 0 °C under argon. After 0.5 h, a
solution of the iodide 55 (6.49 g, 20 mmol) in dry THF (10 mL)
was added. The cooling bath was removed, and the beige
suspension was stirred for a futher 4 h. The mixture was
diluted with aqueous citric acid (0.5 M, 200 mL) and stirred
for 30 min before extraction with EtOAc (2 × 200 mL). The
organic extracts were dried and concentrated to give a brown
oil. Flash column chromatography on silica gel, eluting with
EtOAc:hexane (9:1, v/v), gave ethyl 4-(4-oxo-4-phenylbutyl)-
1-(tert-butyloxycarbonyl)piperidine-4-carboxylate (56; 4.34 g,
54%) as a yellow oil that crystallized on standing: 1H NMR
(360 MHz, CDCl3) δ 1.26 (3 H, t, J ) 7, OCH2CH3), 1.34-1.42
(2 H, m, CH2), 1.46 (9 H, s, C(CH3)3), 1.56-1.71 (4 H, m, CH2),
2.12 (2 H, br d, J ) 14, piperidinyl CH), 2.89 (2 H, br d, J )
8, piperidinyl CH), 2.94 (2 H, t, J ) 7, COCH2), 3.86 (2 H, br
d, J ) 14, piperidinyl CH), 4.18 (2 H, t, J ) 7, OCH2), 7.45 (2
H, dd, J ) 7, 7, ArH), 7.56 (1 H, tt, J ) 7, 1, ArH), 7.92 (2 H,
dd, J ) 7, 1, ArH); MS (CI+; NH3) m/ z ) 404 (M+ + H).
A solution of potassium hexamethyldisilylamide in toluene
(0.5 M, 20 mL) was added dropwise to a solution of the ester
56 (2.0 g, 4.96 mmol) in dry THF (100 mL) at 0 °C under argon.
After stirring for 1 h at 0 °C, the reaction was quenched with
saturated aqueous NH4Cl (100 mL) and extracted with EtOAc
(100 mL). The extract was dried, concentrated, and purified
by flash column chromatography, eluting with EtOAc:hexane
(9:1, v/v) to give a yellow oil (0.88 g) containing tert-butyl
2-benzoyl-1-oxo-8-azaspiro[4.5]decane-8-carboxate (57) and
tert-butyl 1-oxo-8-azaspiro[4.5]decane-8-carboxylate (58) as a
2:1 mixture by NMR. Analytical samples of the two products
could be separated by preparative thin layer chromatography.
57: 1H NMR (250 MHz, CDCl3) δ 1.42-1.58 (11 H, m, C(CH3)3,
CH2), 1.78-1.96 (4 H, m, piperidinyl CH), 2.84 (2 H, t, J ) 7,
CH2), 3.12 (2 H, ddd, J ) 14, 11, 4, piperidinyl CH), 3.88-
4.03 (2 H, m, piperidinyl CH), 7.40-7.54 (3 H, m, ArH), 7.76-
7.82 (2 H, m, ArH); MS (CI+; NH3) m/ z ) 358 (M+ + H). 58:
1H NMR (250 MHz, CDCl3) δ 1.20-1.50 (11 H, m, C(CH3)3,
CH2), 1.66 (2 H, ddd, J ) 14, 11, 6, piperidinyl CH), 1.87-
1.96 (4 H, m, CH2), 2.26-2.36 (2 H, m, CH2), 3.06 (2 H, ddd,
J ) 14, 11, 4, piperidinyl CH), 4.87 (2 H, ddd, J ) 14, 4, 4,
piperidinyl CH).
A solution of lithium aluminum hydride in THF (1 M, 7 mL)
was added dropwise at room temperature to a stirred solution
of 51 (1.40 g, 3.98 mmol) in THF:Et2O (1:1, 20 mL) under
nitrogen. An initial white precipitate was observed during
addition which redissolved to give a yellow solution. The solu-
tion was stirred at 40 °C for 3.5 h. The reaction was quenched
at room temperature by cautious addition of aqueous Rochelle
salt (1 M, 20 mL). The emulsion was diluted with Et2O (10
mL) and H2O (10 mL) and was stirred vigorously until two
distinct phases formed. The two phases were separated, and
the aqueous phase was extracted with Et2O (2 × 30 mL). The
combined extracts were dried and concentrated. Flash column
chromatography, eluting with EtOAc:EtOH:880 NH3 (95:4.75:
0.25, v/v), gave the title compound 22 as a white foam (0.94 g,
70%) which was characterized as the hydrochloride salt: white
cubes, mp 155-159 °C (from EtOH/EtOAc); 1H NMR (360
MHz, DMSO) δ 2.16-2.38 (1 H, br m, pyrrolidinyl CH), 2.45-
2.64 (1 H, br m, pyrrolidinyl CH), 3.30-3.60 (3 H, br m,
pyrrolidinyl CH), 3.64-3.90 (2 H, br m, pyrrolidinyl CH), 4.42
(2 H, s, NCH2Ph), 6.76 (1 H, s, NdCCH), 7.36-7.41 (5 H, m,
ArH), 7.50-7.53 (2 H, m, ArH), 7.63 (2 H, d, J ) 9, ArH); MS
(CI+; NH3) m/ z ) 338 (M+ + H). Anal. (C20H22N3Cl3) C, H, N.
3-(1-(2-P h en yleth yl)-4-p ip er a zin yl)-5-p h en ylp yr a zole
(24). A solution of 1-(2-phenylethyl)piperazine (52) (1.8 g, 9.5
mmol) and 4-methyl-4-phenyl-3-thiosemicarbazide (1.72 g, 9.5
mmol) in acetonitrile (50 mL) was refluxed for 5 h. The
solution was cooled at -40 °C for 2 h. The resulting white
precipitate was collected and dried in vacuo to give 4-phen-
ethylpiperazine-1-carbothioic acid hydrazide (53) (1.1 g,
35%): 1H NMR (360 MHz, CDCl3) δ 2.56-2.62 (4 H, m,
piperazinyl CH), 2.63-2.72 (2 H, m, NCH2CH2Ph), 2.76-2.88
(2 H, m, NCH2CH2Ph), 3.78-3.88 (4 H, m, piperazinyl CH),
6.98 (1 H, br s, NH), 7.16-7.34 (5 H, m, Ph); MS (CI+; NH3)
m/ z ) 265 (M+ + H).
A supension of 53 (0.50 g, 1.5 mmol) and 2-bromoacetophe-
none (0.30 g, 1.5 mmol) in EtOH (15 mL) was stirred at room
temperature for 24 h, becoming bright yellow. Saturated
ethanolic HCl (5 mL) was added, and the brown solution was
refluxed for 2.5 h. The solution was cooled, poured into H2O
(100 mL), basified with aqueous NaOH (1 M), and extracted
with EtOAc (3 × 50 mL). The combined extracts were dried
and concentrated. Flash column chromatography eluting with
dichloromethane:methanol (95:5, v/v) gave partly purified
material which was recrystallized to yield the title compound
24 (0.159 g, 32%) as brown granules: mp 133-134 °C (from
EtOH/H2O); 1H NMR (360 MHz, DMSO) δ 2.54-2.58 (6 H, m,
NCH2CH2Ph, piperazinyl CH), 2.77 (2 H, br t, J ) 7, NCH2-
CH2Ph), 3.14 (4 H, br t, J ) 3, piperazinyl CH), 6.17 (1 H, br
s, NdCCH), 7.18 (1 H, t, J ) 7, ArH), 7.20-7.30 (5 H, m, ArH),
7.41 (2 H, t, J ) 7, ArH), 7.68 (2 H, d, J ) 7, ArH), 12.63 (1 H,
br s, NH); MS (CI+; NH3) m/ z ) 333 (M+ + H). Anal.
(C21H24N4‚1.2H2O) C, H, N.
1-Ben zyl-4-(3-h yd r oxy-1-oxo-3-(4-ch lor op h en yl)p r op -
2-en -1-yl)-4-m eth ylp ip er id in e (26). This was made in the
same way as 25 starting with 4-methyl-4-piperidinecarboxylic
acid to give white granules: mp 70-71 °C (from EtOH/H2O);
1H NMR (360 MHz, CDCl3) δ 1.22 (3 H, s, CH3), 1.61 (2 H,
ddd, J ) 13, 9, 4, piperidinyl CH), 2.10-2.16 (2 H, m,
piperidinyl CH), 2.33 (2 H, br dd, J ) 9, 9, piperidinyl CH),
2.54-2.59 (2 H, m, piperidinyl CH), 3.48 (2 H, s, NCH2Ph),
6.25 (1 H, s, CdCHCdO), 7.21-7.31 (5 H, m, ArH), 7.43 (2 H,
d, J ) 9, ArH), 7.81 (2 H, d, J ) 9, ArH), 15.59 (1 H, br s,
OH); MS (CI+; NH3) m/ z ) 370 (M+ + H). Anal. (C22H24NO2-
Cl) C, H, N.
The mixture of diketone 57 and ketone 58 (0.85 g) was
dissolved in MeOH (20 mL), and hydrazine hydrate (5 mL,
0.1 mol) was added. After stirring for 45 min the deep orange
solution had become very pale yellow. MeOH was removed
by evaporation, and the residue was partitioned between H2O
(40 mL) and EtOAc (2 × 40 mL). The organic extracts were
dried and concentrated. The crude pyrazole (0.91 g) was
dissolved in EtOAc and treated with a saturated solution of
3-(1-Ben zyl-4-m eth yl-4-piper idin yl)-5-(4-ch lor oph en yl)-
p yr a zole (27): dihydrobromide salt, white granules, mp 222-

2384 J ournal of Medicinal Chemistry, 1997, Vol. 40, No. 15
Rowley et al.
hydrogen chloride in EtOAc (50 mL). After gentle heating to
initiate reaction, the mixture was chilled (4 °C) for 16 h. The
resulting white precipitate was collected and dried in vacuo
to give the crude deprotected piperidine as the hydrochloride
salt (0.62 g). A portion of the crude piperidine salt (0.35 g),
phenethyl bromide (0.25 mL, 1.8 mmol), and diisopropylethyl-
amine (0.70 mL, 4.0 mmol) were dissolved in dimethylforma-
mide (15 mL) and stirred at 60 °C for 24 h. The mixture was
diluted with H2O (40 mL) and extracted with 9:1 dichlo-
romethane:methanol (2 × 20 mL). The organic extracts were
concentrated, and the residue was redissolved in Et2O (20 mL),
washed with H2O (20 mL), dried, and concentrated. Prepara-
tive thin layer chromatography, eluting with 90:8:2 dichlo-
romethane:methanol:880 NH3 (90:8:2, v/v), gave the title
compound 28 (0.08 g, 8% from 56) as a colorless oil: bis oxalate
salt, white powder, mp 182-185 °C (from EtOH); 1H NMR (360
MHz, DMSO) δ 1.86-2.10 (4 H, m, piperidinyl CH), 2.32-
2.48 (2 H, m, CH2), 2.84 (2 H, t, J ) 7, CH2), 3.03-3.07 (2 H,
m, PhCH2), 3.33-3.37 (2 H, m, PhCH2CH2N), 3.40-3.62 (4 H,
m, piperidinyl CH), 7.27-7.38 (6 H, m, ArH), 7.45 (2 H, dd, J
) 8, 8, ArH), 7.65 (2 H, d, J ) 7, ArH); MS (CI+; NH3) m/ z )
358 (M+ + H). Anal. (C24H27N3‚2C2H2O4‚0.5H2O) C, H, N.
3-(1-(4-Ch lor oben zyl)-4-piper idin yl)-5-(4-ch lor oph en yl)-
isoxa zole (29) a n d 5-(1-(4-Ch lor oben zyl)-4-p ip er id in yl)-
3-(4-ch lor op h en yl)isoxa zole (30). Alkylation of 42 (3 g, 10
mmol) with 4-chlorobenzyl chloride (1.77 g, 11 mmol) as above
7.59 (2 H, m, ArH m to Cl); MS (CI+; NH3) m/ z ) 381 (M+
+
H). Anal. (C23H25ClN2O) C, H, N.
3-(4-(1-(2-P h en yleth yl)pip er id in yl))-4-m eth yl-5-(4-ch lo-
r op h en yl)isoxa zole (36): white plates, mp 138-140 °C (from
EtOH/H2O); 1H NMR (360 MHz, CDCl3) δ 2.00-2.10 (4 H, m,
CHCH2), 2.19 (3 H, s, CH3), 2.12-2.32 (2 H, m, NCHACHB),
2.65-2.75 (3 H, m, CH2CH, CH2Ph), 2.85-2.89 (2 H, m, NCH2-
CH2Ph), 3.15 (2 H, br d, J ) 12, NCHACHB), 7.18-7.32 (5 H,
m, Ph), 7.44-7.47 (2 H, m, ArH o to Cl), 7.57-7.64 (2 H, m,
ArH m to Cl); MS (CI+; NH3) m/ z ) 381 (M+ + H). Anal.
(C23H25ClN2O) C, H, N. The isoxazole regiochemistry was
proved by X-ray on this compound.
4-(1-Ben zylpiper idin -4-yl)-6-(4-ch lor oph en yl)pyr im idin -
2-yla m in e (31). Sodium metal (0.15 g, 6.7 mmol) was
dissolved in refluxing anhydrous i-PrOH (10 mL) under
nitrogen. The solution was cooled to 50 °C, and guanidine
hydrochloride (0.42 g, 4.4 mmol) was added. The suspension
was refluxed for 15 min followed by addition of a slurry of
ketone 25 (0.87 g, 2.2 mmol) in i-PrOH (10 mL). The
suspension was refluxed for 17 h and then concentrated. The
residues were diluted with H2O (50 mL) and extracted with
EtOAc (3 × 25 mL). The extracts were dried and concentrated.
Flash column chromatography, eluting with EtOAc:hexane (2:
1, v/v), followed by preparative thin layer chromatography,
eluting with EtOAc:hexane (1:1, v/v), gave the title compound
31 (0.088 g, 10%) as an oil: bishydrochloride, white granules,
1
gave 61 (2.3 g, 59%) as cubes: mp 91-92 °C; H NMR (360
1
mp 251-253 °C (from EtOH); in the H NMR 31 appears as a
MHz, CDCl3) δ 1.8-2.0 (4 H, m, NCH2CH2), 2.0-2.1 (2 H, m,
NCHAHB), 2.3-2.4 (1 H, m, CHCH2), 2.96 (2 H, d, J ) 12,
NCHAHB), 3.5 (2 H, s, ArCH2), 6.14 (1 H, s, CHCO), 7.29 (4 H,
s, ArH), 7.41 (2 H, d, J ) 8.6, ArH o to Cl), 7.81 (2 H, d, J )
8.6, ArH m to Cl), 15.9 (1 H, br s, OH); MS (CI+; NH3) m/ z )
390 (M+ + H). Anal. (C21H21ClNO2) C, H, N.
6:1 ratio of two protonated forms; the purity of the sample was
checked by HPLC on three systems and was always >99.5%;
1H NMR (360 MHz, DMSO-d6) δ 2.10-2.20 (4 H, m, CHCH2),
2.86-3.24 (3 H, m, NCHAHB, CH2CH), 3.42-3.45 (2 H, m,
NCHAHB), 4.32 and 4.49 (2 H, 2 × d, J ) 5, NCH2Ph), 7.24 (1
H, s, CdCH), 7.46-7.48 (3 H, m, 3 of Ph), 7.62-7.72 (4 H, m,
Ph, ArH o to Cl), 8.15 and 8.23 (2 H, 2 × d, J ) 8, ArH o to
Cl), 11.03 and 11.16 (1 H, 2 × br s, NH); MS (CI+; NH3) m/ z
) 379 (M+ + H). Anal. (C21H25Cl2N4‚2HCl‚0.4H2O) C, H, N.
Hydroxylamine hydrochloride (139 mg, 2 mmol) was added
to 61 (399 mg, 1.02 mmol) and Et-i-Pr2N (0.35 mL, 2 mmol)
in EtOH (5 mL) and DMF (2 mL). After stirring for 16 h
hydroxylamine hydrochloride (139 mg, 2 mmol) and Et-i-Pr2N
(0.35 mL, 2 mmol) were added, and the mixture was stirred
at 50 °C for 4 h. H2O (50 mL) was added, the mixture was
extracted with EtOAc (3 × 25 mL), and the combined organic
layers were washed with H2O and brine, dried, evaporated,
and purified by flash chromatography, eluting with CH2Cl2:
EtOH:Et3N (96:4:1, v/v) to give a mixture of 5-(1-(4-chloroben-
zyl)-4-piperidinyl)-3-(4-chlorophenyl)-3-hydroxy-3,4-dihydroisox-
azole and 3-(1-(4-chlorobenzyl)-4-piperidinyl)-5-(4-chlorophenyl)-
3-hydroxy-3,4-dihydroisoxazole (287 mg, 71%); 209 mg (520
µmol) of this oil was dissolved in dichloromethane (5 mL) and
cooled to 0 °C, and Et3N (161 mg, 1.6 mmol) and methane-
sulfonyl chloride (81 mg, 710 µmol) were added. After 1 h
EtOAc (25 mL) was added, and the mixture was washed with
H2O and brine, dried, evaporated, and purified by preparative
thin layer chromatography, eluting with CH2Cl2:EtOH:Et3N
(97:3:1, v/v) to give 30 (107 mg, 27%): oxalate salt, white solid,
2-Am in o-4-(1-(2-p h en yleth yl)-4-p ip er id in yl)-6-(4-ch lo-
r op h en yl)-5-m eth ylp yr im id in e (37): white granules, mp
179-180 °C (from DMF/H2O); 1H NMR (360 MHz, DMSO-d6)
δ 1.65 (2 H, d, J ) 12, CHCHAHB), 1.83 (2 H, q, J ) 12,
CHCHAHB), 2.02-2.08 (5 H, m, CH3, NCHAHB), 2.54 (2 H, t, J
) 8, CH2Ph), 2.73-2.81 (3 H, m, NCH2CH2Ph, CH2CH), 3.04
(2 H, d, J ) 11, ANCHAHB), 6.28 (2 H, s, NH2), 7.16-7.30 (5
H, m, Ph), 7.46-7.52 (4 H, m, ArH); MS (CI+; NH3) m/ z )
407 (M+ + H, 100). Anal. (C24H27ClN4) C, H, N.
3-(1-Ben zyl-4-p ip er id in yl)-4-eth yl-5-(4-ch lor op h en yl)-
p yr a zole (33). A solution of tetrabutylammonium fluoride
in THF (1 M, 2 mL) was added to a solution of diketone 59
(0.705 g, 1.93 mmol) in THF (20 mL). The clear yellow solution
was evaporated to give a yellow oil which was redissolved in
chloroform (20 mL), and ethyl iodide (0.25 mL, 2.9 mmol) was
added. The solution was refluxed under nitrogen for 4 h, then
cooled, and poured into H2O (50 mL). The two phases were
separated, and the aqueous layer was extracted with CH2Cl2
(40 mL). The organic extracts were dried, concentrated, and
purified by flash column chromatography, eluting with hexane:
EtOAc (9:1, v/v) to give 4-(2-(4-chlorobenzoyl)butyryl)-1-(tert-
butylcarbonyloxy)piperidine (60; 0.49 g, 64%) as a yellow oil
(contaminated with some unreacted starting material): 1H
NMR (360 MHz, CDCl3) δ 0.93 (3 H, t, J ) 7, CH2CH3), 1.43
(9 H, s, t-Bu), 1.52-1.70 (4 H, m, piperidinyl CH), 2.01 (2 H,
dq, J ) 7, 7, CH2CH3), 2.58-2.72 (3 H, m, piperidinyl CH),
4.00-4.16 (2 H, m, piperidinyl CH), 4.39 (1 H, t, J ) 7, CHCH2-
CH3), 7.47 (2 H, d, J ) 9, ArH), 7.90 (2 H, d, J ) 9, ArH); MS
(CI+; NH3) m/ z ) 294 (M+ + H).
1
mp 227-230 °C (from EtOH); H NMR (360 MHz, DMSO-d6)
δ 1.7-1.8 (2 H, m, CHCHAHB), 2.08 (2 H, d, J ) 11,
CHCHAHB), 2.3-2.4 (2 H, m, NCHAHB), 3.0-3.1 (3 H, m,
NCHAHB, CH2CH), 3.82 (2 H, s, ArCH2), 6.92 (1 H, s, CHCO),
7.4-7.5 (4 H, m, ArH), 7.59 (2 H, d, J ) 8, ArH o to Cl), 7.88
(2 H, d, J ) 8, ArH m to Cl); MS (CI+; NH3) m/ z ) 387 (M+
+
H). Anal. (C21H20Cl2N2O‚0.8C2H2O4) C, H, N. 29 (51 mg,
13%): oxalate salt, white plates, mp 240-242 °C (from EtOH);
1H NMR (360 MHz, DMSO-d6) δ 1.8-1.9 (2 H, m, CHCHAHB),
2.05 (2 H, d, J ) 11, CHCHAHB), 2.6-2.7 (2 H, m, NCHAHB),
2.9-3.0 (1 H, m, CH2CH), 3.1-3.2 (2 H, m, NCHAHB), 3.97 (2
H, s, ArCH2), 7.07 (1 H, s, CHCO), 7.4-7.5 (4 H, m ArH), 7.61
(2 H, d, J ) 8, ArH o to Cl), 7.85 (2 H, d, J ) 8, ArH m to Cl);
MS (CI+; NH3) m/ z ) 387 (M+ + H). Anal. (C21H20
-
A solution of the diketone (0.47 g, 1.19 mmol) in Et2O (20
mL) was cooled to 0 °C and saturated with hydrogen chloride
gas. The solution was gently warmed until a white precipitate
formed. The suspension was chilled overnight, and the
crystalline material was collected to give 1-(4-chlorophenyl)-
2-ethyl-3-piperidin-4-ylpropane-1,3-dione hydrochloride (0.384
g, 87%): 1H NMR (360 MHz, DMSO) δ 0.85 (3 H, t, J ) 7,
CH2CH3), 1.52-2.98 (6 H, m, piperidinyl CH, CH2CH3), 2.75-
2.98 (3 H, m, piperidinyl CH), 3.18-3.28 (2 H, m, piperidinyl
CH), 5.02 (1 H, t, J ) 6, CHCH2CH3), 7.65 (2 H, d, J ) 8,
Cl2N2O‚C2H2O4) C, H, N. The isoxazole regiochemistry was
proved by X-ray.
3-(4-Ch lor op h en yl)-4-m eth yl-5-(4-(1-(2-p h en yleth yl)p i-
p er id in yl))isoxa zole (35): white needles, mp 130-131 °C
(from EtOH/H2O); 1H NMR (360 MHz, CDCl3) δ 1.40-1.75 (2
H, m, CHCHAHB), 1.90-2.40 (4 H, m, CHCHAHB, NCHAHB),
2.09 (3 H, s, CH3), 2.70-2.80 (2 H, m, CH2Ph), 2.84-2.98 (3
H, m, NCH2CH2Ph, CH2CH), 3.18 (2 H, br d, J ) 12, NCHAHB),
7.20-7.33 (5 H, m, Ph), 7.43-7.46 (2 H, m, ArH o to Cl), 7.57-

Heterocyclylpiperidines as High-Affinity Ligands
J ournal of Medicinal Chemistry, 1997, Vol. 40, No. 15 2385
M.; Niznik, H. B. Cloning of the gene for a human dopamine D5
receptor with higher affinity for dopamine than D1. Nature 1991,
350, 614-619.
ArH), 8.04 (2 H, d, J ) 8, ArH), 8.60 (1 H, br s, NH), 8.90 (1
H, br s, NH); MS (CI+; NH3) m/ z ) 394 (M+ + H).
A solution of the piperidine hydrochloride salt (0.34 g, 1.03
mmol), diisopropylethylamine (0.36 mL, 2.06 mmol), and
benzyl bromide (0.13 mL, 1.08 mmol) in dry DMF (10 mL) was
stirred at 50 °C under nitrogen for 18 h and then at room
temperature for 24 h. The mixture was poured into H2O (50
mL) and extracted with EtOAc (2 × 20 mL). The combined
extracts were dried, concentrated, and purified by flash column
chromatography on silica gel, eluting with EtOAc:hexane (1:
1, v/v) to give 1-(1-benzylpiperidin-4-yl)-3-(4-chlorophenyl)-2-
ethylpropane-1,3-dione (0.146 g, 37%) as a yellow oil: 1H NMR
(250 MHz, CDCl3) δ 0.94 (3 H, t, J ) 6, CH2CH3), 1.52-2.18
(8 H, m, piperidinyl CH, CH2CH3), 2.40-2.55 (1 H, m,
piperidinyl CH), 2.80-2.96 (2 H, m, piperidinyl CH), 3.48 (2
H, s, NCH2Ph), 4.38 (1 H, t, J ) 6, CHCH2CH3), 7.20-7.40 (5
H, m, Ph), 7.46 (2 H, d, J ) 8, ArH), 7.90 (2 H, d, J ) 8, ArH);
MS (CI+; NH3) m/ z ) 384 (M+ + H).
(11) (a) Seeman, P.; Guan, H.-C.; Van Tol, H. H. M. Dopamine D4
receptors elevated in schizophrenia. Nature 1993, 365, 441-445.
(b) Murray, A. M.; Hyde, T. M.; Knable, M. B.; Herman, M. M.;
Bigelow, L. B.; Carter, J . M.; Weinberger, D. R.; Kleinman, J .
E. Distribution of putative D4 dopamine receptors in postmortem
striatum from patients with schizophrenia. J . Neurosci. 1995,
15, 2186-2191. (c) Sumiyoshi, T.; Stockmeier, C. A.; Overholser,
J . C.; Thompson, P. A.; Meltzer, H. Y. Dopamine D4 receptors
and effects of guanine nucleotides on [3H]raclopride binding in
postmortem caudate nucleus of subjects with schizophrenia or
major depression. Brain Res. 1995, 681, 109-116.
(12) Reynolds, G. P.; Mason, S. L. Are Striatal Dopamine D4
Receptors Increased in Schizophrenia? J . Neurochem. 1994, 63,
1576-1577.
(13) Matsumoto, M.; Hidaka, K.; Tada, S.; Tasaki, Y.; Yamaguchi,
T. Full-Length cDNA Cloning and Distribution of the Human
Dopamine D4 Receptor. Mol. Brain Res. 1995, 29, 157-162.
(14) Rowley, M.; Broughton, H. B.; Collins, I.; Baker, R.; Emms, F.;
Marwood, R.; Patel, S.; Patel, S.; Ragan, C. I.; Freedman, S.;
Leeson, P. D. 5-(4-Chlorophenyl)-4-Methyl-3-(1-(2-Phenylethyl)-
piperidin-4-yl)isoxazole (L-741,742): A Potent, Selective An-
tagonist at Human Cloned Dopamine D4 Receptors. J . Med.
Chem. 1996, 39, 1943-1945.
A solution of the diketone (0.14 g, 0.36 mmol) and hydrazine
hydrate (1.0 mL) in methanol (5 mL) was stirred overnight at
room temperature. The mixture was diluted with H2O (15 mL)
and extracted with EtOAc (3 × 10 mL). The combined extracts
were dried, evaporated, and purified by preparative thin layer
chromatography eluting with EtOAc to give a yellow oil that
was triturated with aqueous ethanol and then recrystallized
to yield the title compound 33 (0.036 g, 26%) as colorless
cubes: mp 170-172 °C (from EtOH); 1H NMR (360 MHz,
CDCl3) δ 1.09 (3 H, t, J ) 7.5, CH2CH3), 1.67-1.89 (4 H, m,
piperidinyl CH), 2.07-2.18 (2 H, m, piperidinyl CH), 2.56 (2
H, q, J ) 7.5, CH2CH3), 2.69 (1 H, tt, J ) 8, 8, piperidinyl
CH), 3.03 (2 H, br d, J ) 12, piperidinyl CH), 3.57 (2 H, s,
NCH2Ph), 7.26-7.39 (7 H, m, ArH), 7.50 (2 H, d, J ) 8, ArH);
MS (CI+; NH3) m/ z ) 380 (M+ + H). Anal. (C23H26N3Cl) C,
H, N.
(15) (a) Kulagowski, J . J .; Broughton, H. B.; Curtis, N. R.; Mawer, I.
M.; Ridgil, M. P.; Baker, R.; Emms, F.; Freedman, S.; Marwood,
R.; Patel, S.; Patel, S.; Ragan, C. I.; Leeson, P. D. 3-[[4-(4-Chloro-
phenyl)piperazin-1-yl]methyl]-1H-pyrrolo[2,3-b]pyridine: An An-
tagonist with High Affinity and Selectivity for the Human
Dopamine D4 Receptor. J . Med. Chem. 1996, 39, 1941-1942.
(b) Boyfield, I.; Brown, T. H.; Coldwell, M. C.; Cooper, D. G.;
Hadley, M. S.; Hagan, J . J .; Healy, M. A.; J ohns, A.; King, R. J .;
Middlemiss, D. N.; Nash, D. J .; Riley, G. J .; Scott, E. E.; Smith,
S. A.; Stemp, G. Design and Synthesis of 2-Naphthoate Esters
as Selective Dopamine D4 Antagonists. J . Med. Chem. 1996, 39,
1946-1948. (c) Boyfield, I.; Coldwell, M. C.; Hadley, M. S.; Healy,
M. A.; J ohns, A.; Nash, D. J .; Riley, G. J .; Scott, E. E.; Smith, S.
A.; Stemp, G.; Wilson, K. N-(Substituted-Phenyl)Piperazines:
Antagonists with High Affinity Binding and Functional Selectiv-
ity for Dopamine D4 Receptors. Bioorg. Med. Chem. Lett. 1996,
6, 1227-1232. (d) TenBrink, R. E.; Bergh, C. L.; Duncan, J . N.;
Harris, D. W.; Huff, R. M.; Lahti, R. A.; Lawson, C. F.; Lutzke,
B. S.; Martin, I. J .; Rees, S. A.; Schlachter, S. K.; Sih, J . C.;
Smith, M. W. (S)-(-)-4-[4-[2-(Isochroman-1-yl)ethyl]piperazin-1-
yl]benzenesulfonamide, a Selective Dopamine D4 Antagonist. J .
Med. Chem. 1996, 39, 2435-2437.
Ack n ow led gm en t. We would like to thank Steve
Thomas for NMR experiments and Mike Graham for
pharmacokinetics.
Refer en ces
(16) Carhart, R.; Smith, D. H.; Venkataraghavan, R. Atom pairs as
molecular features in structure-activity studies: definition and
applications. J . Chem. Inf. Comput. Sci. 1985, 25, 64-73.
(17) Sheridan, R.; Kearsley, S. K. Molecular Systems Department,
MRL, Rahway, NJ .
(18) Yanina, A. D.; Trubitsyna, T. K.; Mikhlina, E. E.; Yakhontov,
L. N. Synthesis and pharmalogical properties of 2- and 2,3-
substituted quinuclidines. Khim. Farm. Zh. 1987, 21, 808-
811.
(19) Scovil, J . P. A facile synthesis of thiosemicarbazides and thi-
osemicarbazones by the transamination of 4-methyl-4-phenyl-
3-thiosemicarbazide. Phosphorus, Sulfur Silicon 1991, 60, 15-
19.
(20) Clark, J . H.; Miller, J . M. Hydrogen bonding in organic synthesis.
Part 6. C-Alkylation of â-dicarbonyl compounds using tetraalky-
lammonium fluorides. J . Chem. Soc., Perkin Trans. I 1977,
1743-1745.
(21) Freedman, S. B.; Patel, S.; Marwood, R.; Emms, F.; Seabrook,
G. R.; Knowles, M. R.; McAllister, G. Expression and pharma-
cological characterisation of the human D3 dopamine receptor.
J . Pharmacol. Exp. Ther. 1994, 268, 417-426.
(22) McAllister, G.; Knowles, M. R.; Ward-Booth, S. M.; Sinclair, H.
A.; Patel, S.; Marwood, R.; Emms, F.; Patel, S.; Smith, G. R.;
Seabrook, G. R.; Freedman, S. B. Functional coupling of human
D2, D3, and D4 dopamine receptors in HEK293 cells. J . Recep.
Sig. Trans. Res. 1995, 15, 267-281.
(23) Corey, E. J .; Gross, A. W. Highly selective, kinetically controlled
enolate formation using lithium dialkylamides in the presence
of trimethylchlorosilane. Tetrahedron Lett. 1984, 495-498.
(1) Snyder, S. H. Dopamine receptors, neuroleptics, and schizophre-
nia. Am. J . Psychiatry 1981, 138, 461-464.
(2) Baldessarini, R. J .; Tarsey, D. Dopamine and the pathophysi-
ology of dyskinesias induced by antipsychotic drugs. Annu. Rev.
Neurosci. 1980, 3, 23.
(3) Ben-J ohnathon, N. Dopamine: A prolactin inhibiting hormone.
Endocr. Rev. 1985, 6, 564.
(4) Fitton, A.; Heel, R. C. Clozapine. A review of its pharmacological
properties, and therapeutic use in schizophrenia. Drugs 1990,
40, 722-747.
(5) Schwartz, J . T.; Brotman, A. W. A Clinical Guide to Antipsy-
chotic Drugs. Drugs 1992, 44, 981-992.
(6) Dearry, J . R.; Gingrich, J . A.; Falardeau, R. T.; Fremeau, R. T.;
Bates, M. D.; Caron, M. Molecular cloning and expression of the
gene for a human D1 dopamine receptor. Nature 1990, 347, 72-
76.
(7) Grandy, D. K.; Marchionni, M. A.; Makam, H.; Stofko, R. E.;
Alfano, M.; Frothingham, L.; Fischer, J . B.; Burke-Howie, K. J .;
Bunzow, J . R.; Server, A. C.; Civelli, O. Cloning of the cDNA
and gene for a human D2 dopamine receptor. Proc. Natl. Acad.
Sci. U.S.A. 1989, 86, 9762-9766.
(8) Sokoloff, P.; Giros, B.; Martres, M.-P.; Bouthenet, M.-L.; Schwartz,
J .-C. Molecular cloning and characterization of a novel dopamine
receptor (D3) as a target of neuroleptics. Nature 1990, 347, 146-
151.
(9) Van Tol, H. H. M.; Bunzow, J . R.; Guan, H.-C.; Sunahara, R.
K.; Seeman, P.; Niznik, H. B.; Civelli, O. Cloning of the gene for
a
human dopamine D4 receptor with high affinity for the
antipsychotic clozapine. Nature 1991, 350, 610-614.
(10) Sunahara, R. K.; Guan, H.-C.; O’Dowd, B. F.; Seeman, P.;
Laurier, L. G.; Ng, G.; George, S. R.; Torchia, J .; Van Tol, H. H.
J M970111H