5-(4-Chlorophenyl)-4-methyl-3-(1-(2-phenylethyl)piperidin-4-yl)isoxazole: A potent, selective antagonist at human cloned dopamine D4 receptors

Full text of DOI:10.1021/jm960072u

J . Med. Chem. 1996, 39, 1943-1945
1943
Sch em e 1a
5-(4-Ch lor op h en yl)-4-m eth yl-3-(1-(2-
p h en yleth yl)p ip er id in -4-yl)isoxa zole: A
P oten t, Selective An ta gon ist a t Hu m a n
Clon ed Dop a m in e D4 Recep tor s
Michael Rowley,* Howard B. Broughton, Ian Collins,
Raymond Baker, Frances Emms,
Rosemarie Marwood, Shil Patel, Smita Patel,
C. Ian Ragan, Stephen B. Freedman, and
Paul D. Leeson
Merck Sharp and Dohme, Neuroscience Research Centre,
Terlings Park, Eastwick Road,
Harlow, Essex CM20 2QR, U.K.
Received J anuary 22, 1996
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 receptors,1 are useful for the treatment
of the positive symptoms, but suffer from various motor
side effects2 along with increases in serum prolactin
levels.3 The atypical neuroleptic clozapine4 (1) can be
used to treat both positive and negative symptoms of
schizophrenia and does not induce extrapyramidal side
effects. However, in 1-2% of patients the drug induces
agranulocytosis, and its use is therefore limited and
must be closely monitored.5 In recent years the ap-
plication 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). Clozapine, among its many
actions, has higher affinity for the D4 subtype than for
D2.9 Recent reports11 suggests that D4 receptor density
is elevated in postmortem schizophrenic brain. For
these reasons we decided to identify selective D4 recep-
tor antagonists to investigate their potential as novel
antipsychotics.
a
Reagents: (a) 4-Chlorobenzaldehyde, NaOH, EtOH, reflux; (b)
hydrazine hydrate, KOH, ethylene glycol, 130-200 °C; (c)
4-ClPhCH2Cl, EtPr2N, DMF, room temperature; (d) PhCH2CH2Br,
EtjPr2N, DMF 60 °C.
Sch em e 2a
a
Reagents: (a) BOC2O, CH2Cl2, room temperature; (b) car-
bonyldiimidazole, THF, room temperature; (c) hydrazine hydrate,
EtOH, room temperature; (d) CF3CO2H; (e) PhCH2CH2Br, EtjPr2N,
DMF, 60 °C; (f) H2NOH‚HCl, Et3N, MeOH; (g) CH3SO2Cl, Et3N,
CH2Cl2.
Our strategy began with the screening of our sample
collection. Compounds were tested for their ability to
displace [3H]spiperone from human cloned receptors, D2
and D3 stably expressed in CHO cells12 and D4 in HEK-
293 cells.13 A representative number of known dopa-
mine antagonists were selected and, using the topologi-
cal similarity probe method14 as implemented in the in-
house TOPOSIM15 package, the collection was screened
for chemicals that have affinity for dopamine receptors
and may show 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-chlorobenzyl)piperidin-4-yl)pyra-
zole (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. Our
initial approach to improve on the affinity and selectiv-
ity of this lead was to make variations to the substituent
on the basic nitrogen, which can be done by alkylation
of a common precursor and by the synthesis of alterna-
tive aromatic heterocycles to replace the pyrazole.
The pyrazole (3) was synthesized (Scheme 1) starting
from 3-quinuclidinone (4). Condensation with 4-chlo-
robenzaldehyde gave the unsaturated ketone 5, which
on treatment with hydrazine and base16 ring opened to
give pyrazole 6. Alkylation with 4-chlorobenzyl chloride
gave 3. The N-phenylethyl analogue 7 was made by
using 2-phenethyl bromide as the alkylating reagent.
A more general route (Scheme 2) allowing variations to
the pyrazole portion of the molecule started with
S0022-2623(96)00072-6 CCC: $12.00 © 1996 American Chemical Society

1944 J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 10
Ta ble 1. Affinities for Cloned Human Dopamine Receptors
Communications to the Editor
different biological activities were obtained. One of the
isomers, 12, was very similar in its dopamine receptor
binding profile to the lead structure, whereas the other,
13, showed much higher affinity for D4 receptors than
did 3. The reasons for this are not clear, although it is
interesting to note that the solid state conformations,
as determined by X-ray crystallography, show that the
isoxazole ring is turned by approximately 180° in one
isomer relative to the other. The differences in D4
affinity may therefore be due again to differences in the
preferred conformation around the bond between the
two aromatic rings. In the last two compounds in Table
1, the changes made to the lead structure above were
all incorporated. The two isoxazoles (14 and 15) with
methyl groups in the 4-position and phenethyl groups
attached to the piperidine nitrogen both show low
nanomolar affinity for human dopamine D4 receptors
and have greater than micromolar affinity for D2,
leading to selectivities of over 500-fold. There is a small
difference in their binding to D3, with the former
compound, 14, having >200-fold selectivity for D4 over
D3, as well as its high selectivity over D2. In D4 HEK
cells, compound 14 (1 µM) alone had no effect, but
antagonized the dopamine (1 µM)-mediated inhibition
of forskolin (10 µM)-induced elevation of cAMP levels.13
Thus 14 is an antagonist at the D4 receptor.
With these tools in hand it is now possible to move
forward to explore the relevance and importance of the
D4 subtype of dopamine receptor, particularly in the
possible treatment of schizophrenia. Full structure-
activity relationships and experimental detail on this
series of compounds will be published later.
a
All compounds were characterized by proton NMR and mass
b
spectra and gave satisfactory elemental analyses. Binding data
Ack n ow led gm en t. We are grateful to Richard Ball,
Merck Research Laboratories, Rahway, NJ , for deter-
mination of crystal structures and to Steven Thomas
and Richard Herbert for NMR.
are the means of two to four independent determinations.
isonipecotic acid (8). N-Protection with di-tert-butyl
dicarbonate gave acid 9, which was activated with
carbonyldiimidazole and treated with 2 equiv of the
enolate derived from 4-chloropropiophenone to give the
diketone 10. Condensation with hydrazine converted
the diketone to the pyrazole, and then deprotection of
the nitrogen with trifluoroacetic acid, followed by alky-
lation with 2-phenethyl bromide, gave 11. Alterna-
tively, the nitrogen of 10 was deprotected and alkylated,
followed by formation of a mixture of regioisomeric
isoxazoles by treatment with hydroxylamine and dehy-
dration of the intermediate hydroxyisoxazoline with
methanesulfonyl chloride. The isomeric isoxazoles (14,
L-741,742, and 15) were formed in similar proportions,
depending on precise reaction conditions, and separated
chromatographically, and their regiochemistry was
proved by X-ray analysis. Isoxazoles 12 and 13 were
made in an analogous fashion using 4-chloroacetophe-
none and alkylating the piperidine nitrogen with 4-chlo-
robenzyl bromide.
Extension of the alkyl chain in the substituent on the
basic nitrogen from benzyl to phenethyl (7, Table 1) gave
a 10-fold improvement in affinity for the D4 receptor,
along with a small reduction in binding to D2, leading
overall to a compound with 100-fold selectivity. Intro-
duction of a methyl group to the 4-position of the
pyrazole 11 gave another increase in D4 affinity, pos-
sibly due to an alteration in the preferred conformation
around the phenyl-heterocycle bond. On alteration of
the pyrazole heterocycle to isoxazole, two regioisomers
are possible. When this change is made on its own to
the lead structure, two compounds with markedly
Su p p or tin g In for m a tion Ava ila ble: Analytical data for
compounds (3 pages). Ordering information is given on any
current masthead page.
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Communications to the Editor
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