Stereocontrolled dopamine receptor binding and subtype selectivity of clebopride analogues synthesized from aspartic acid

Article abstract of DOI:10.1016/S0960-894X(03)00678-4

Employing the achiral 4-aminopiperidine derivative clebopride as a lead compound, chiral analogues were developed displaying dopamine receptor binding profiles that proved to be strongly dependent on the stereochemistry. Compared to the D1 receptor, the t

Full text of DOI:10.1016/S0960-894X(03)00678-4

Bioorganic & Medicinal Chemistry Letters 13 (2003) 3293–3296
Stereocontrolled Dopamine Receptor Binding and Subtype
Selectivity of Clebopride Analogues Synthesized from Aspartic Acid
Jurgen Einsiedel, Klaus Weber, Christoph Thomas, Thomas Lehmann, Harald Hubner
and Peter Gmeiner*
Department of Medicinal Chemistry, Emil Fischer Center, Friedrich Alexander University, Schuhstraße 19, D-91052 Erlangen, Germany
Received 21 March 2003; revised 14 May 2003; accepted 10 June 2003
Abstract—Employing the achiral 4-aminopiperidine derivative clebopride as a lead compound, chiral analogues were developed displaying
dopamine receptor binding profiles that proved to be strongly dependent on the stereochemistry. Compared to the D1 receptor, the test
compounds showed high selectivity for the D2-like subtypes including D2long, D2short, D3 and D4. The highest D4 and D3 affinities were
observed for the cis-3-amino-4-methylpyrrolidines 3e and the enantiomer ent3e resulting in Ki values of 0.23 and 1.8 nM, respectively. The
benzamides of type 3 and 5 were synthesized in enantiopure form starting from (S)-aspartic acid and its unnatural optical antipode.
# 2003 Elsevier Ltd. All rights reserved.
In connection with our program on peptide promoted
receptor modulation, we reported on a practical synthesis
of the enantiopure aminolactams 1 and 2, which proved to
be valuable scaffolds for the design of biologically active
peptide mimetics. Compounds of this type, which were
accessible from (S)-aspartic acid, proved to adopt b-turn
like conformations and exhibited pharmacological effects
comparable to the genuine dopamine receptor modulating
peptide PLG.1 As a consequence of our very recent find-
ings indicating that a modification of the relative topicity
of the pharmacophoric benzamide and the basic amine
function of nemonapride derivatives strongly influences
the binding preferences towards the subtypes of the D2
receptor family,2 we were intrigued by the question,
whether the building blocks 1 and 2 could also give
access to novel chiral dopamine receptor ligands to serve
as new atypical neuroleptics. Thus, we chose the 4-ami-
nopiperidine clebopride (4) as a further pharmacological
lead.3 Clebopride displays a dual D2 and D4 affinity.4
On the other hand, benzamide derivatives such as sul-
piride5 and amisulpride,6 being therapeutically used as
antipsychotic drugs, additionally exhibit a significant
D3 affinity. In order to design pharmacological tools
that might help to figure out the in vivo consequences of
the fine tuning of subtype profiles, clebopride analogues
of type 3 and the regioisomeric 3-aminopiperidines of
type 5 should be investigated. Systematic SAR studies
involving the modification of the heterocyclic core structure
and the benzamide unit should help to find a potentially
antipsychotic drug candidate with a well-balanced dopa-
mine receptor binding profile. Modifications of the piper-
idine should be realized by the introduction of a methyl
group into the position 3 or by formally rearranging an
endocyclic sp3-carbon into an exocyclic position leading
to a methylpyrrolidine substructure. SAR studies in the
benzamide moiety should involve suitably substituted
methoxybenzene and methoxynaphthalene derivatives.
The synthesis of the clebopride regioisomer of type 5
should proceed through the key intermediate 2. To study
whether the receptor binding and selectivity proceeds in a
stereocontrolled way, the stereoisomers of the target
*Corresponding author. Tel.: +49-9131-8529-383; fax: +49-9131-
8522-585; e-mail: gmeiner@pharmazie.uni-erlangen.de
0960-894X/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0960-894X(03)00678-4

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J. Einsiedel et al. / Bioorg. Med. Chem. Lett. 13 (2003) 3293–3296
compounds should be prepared in both enantiomeric
forms starting from (S)-aspartic acid and its unnatural
antipode as the respective precursors.
Starting from the chiral building blocks 1 and 2 being
readily available by chemo- and regioselective transfor-
mations of natural aspartic acid, efficient N- and C-
alkylations gave the chiral intermediates 7 (via 6) and
10, according to our previously reported protocol
(Scheme 1).1 The preparation of the aminopiperidine
building blocks 9 and 12 was planned by hydrogenolytic
debenzylation of the tertiary amine and reduction of the
lactam functionality. Applying Perlman’s catalyst and
an atmospheric pressure of hydrogen, we were able to
perform a chemoselective deprotection of the exocyclic
nitrogen to give the primary amine derivative 8 in 97%
yield. Subsequently, LiAlH4 promoted reduction was
performed to furnish the diamine 9 as an acylation pre-
cursor. Starting from the regioisomer 10, hydro-
genolytic deprotection gave access to the primary amine
11. Subsequent hydride mediated reduction of the lac-
tame functionality afforded the regioisomeric piperidine
derivative 12. The enantiomers ent9 and ent12 were
prepared analogously starting from (R)-aspartic acid.
Scheme 2. (a) NaN3, DMSO, 60 ꢁC, 18: 5 h, 21 and 23: 14 h (18: 78%,
21: 78%, 23: 81%); (b) LiAlH4, Et2O, 0 ꢁC to rt, 19a: 0.5 h, 19b,c: 3 h
(19a: 92%; 19b,c: crude); (c) (1) H2, Pd(OH)2/C, EtOH, rt, H2-con-
sumption monitored; (2) NEt3, CHCl3, rt (20: 38%, 22: 18%).
logical leads for the development of D3 active drugs, we
decided to synthesize the corresponding 4-ethynyl and 4-
phenylethynyl surrogates as well as iodo-substituted naph-
thoic acid derivatives 30 and 31 as promising building
blocks.
Scheme 1. (a) H2, Pd(OH)2/C, MeOH, rt, 15 h (8: 97%, 11: 96%);
(b) LiAlH4, THF, reflux, 15 min, then, rt, 12 h (9: 88%, 12: 61%).
Starting from the readily available 1-methoxy-2-naphthoic
acid methyl ester 24,13 the 4-iodo derivative 25 was acces-
sible in 83% yield by the reaction with ICl (Scheme 3).
Sonogashira-reaction14 allowed the introduction of
TMS-acetylene and phenylacetylene to afford the
respective ethynyl derivatives 27 and 28 in high yield.
Desilylation of 27 by NBu4F furnished the correspond-
ing ethynyl derivative 29. Finally, we performed a
smooth saponification of the carboxylic esters 25, 28
and 29 using KHCO3/MeOH to give the carboxylic acid
derivatives 26, 30 and 31.
The 3-amino-4-methyl-pyrrolidines 19b,c were synthe-
sized by exploiting a short and effective reaction sequence,
that has been developed in our laboratories giving access
to the enantiopure mesyloxypyrrolidinium methanesulfo-
nates 16a,b starting from (R)-aspartic acid (Scheme 2). In
contrast to our previously reported protocol,2 we con-
veniently prepared 16a,b as a mixture of diastereomers,
which was subjected to a reductive debenzylation, neu-
tralized with NEt3 and subsequently separated by flash
chromatography to give the pure mesyloxypyrrolidines 20
and 22 in acceptable overall yields. SN2 reaction using
NaN3/DMSO proceeded under complete inversion giving
access to the diastereomers 21 and 23, respectively.7 Sub-
sequent LiAlH4 reduction furnished the corresponding
primary amine derivatives 19b,c. Starting from the
mesyloxypyrrolidinium salt 15, which was hydrogenated
to give the monobenzyl derivative 17, the diamines 13, 14
and 19a (via 18) were prepared as described
recently.2,8ꢀ10 The corresponding enantiomers ent19a,b,c
were synthesized starting from (S)-aspartic acid.
Scheme 3. (a) ICl, HOAc, rt, 1 h, 80 ꢁC, 2 h (83%); (b) KHCO3,
MeOH, reflux, 3 h (26: 89%, 30: 98%, 31: 99%); (c) 25, TMS-acetyl-
ene or phenylacetylene, PdCl2(PPh3)2 2 mol%, CuI 4 mol%, THF, rt,
18 h (27: 83%, 28: 84%); (d) Bu4NF, THF, rt, 30 min (94%).
Considering the 4-cyanonaphthamide nafadotride11 and
bromonaphthamide analogues12 as promising pharmaco-

J. Einsiedel et al. / Bioorg. Med. Chem. Lett. 13 (2003) 3293–3296
3295
The chiral diamines 9,12,13,14,19a,b,c and their enan-
tiomers were acylated using the aminobenzoic acid 32
and its N-methyl derivative 33 as well as the naphthoic
acids 26,30,31 and the 4-bromo analogue 3415 applying
DCC/HOBt as the coupling reagent to give the corre-
sponding amides 3a–l, 5a,b and the optical antipodes
ent3a-l, ent5a,b (Scheme 4, Table 1).16ꢀ18
D1 affinity was determined by employing bovine striatal
membrane preparations and the D1 selective antagonist
[3H]SCH 23390.22
Generally the test compounds showed weak D1 binding.
Compared to clebopride (4) and N-methylclebopride,
the formal migration of the basic nitrogen within the
chiral regioisomers 5a,b and ent5a,b led to a significant
decrease of the dopamine receptor recognition. On the
other hand, the trans-configured 3-methyl-4-amino-
piperidine derivatives ent3a and ent3b showed sub-
stantial D2, D3 and D4 affinities resulting in Ki values
of 20 and 13 nM, respectively, for D3 as well as 7.4 and
1.6 nM, respectively, for D4. Thus, the introduction of
the methyl substituent resulted in a significant increase
of D3 affinity for the (3R,4R)-isomers ent3a,b and in a
decrease of D3 binding for the (3S,4S)-enantiomers
3a,b. Investigating the methylpyrrolidine derivative
ent3c with the same spatial orientation of the sub-
stituents as for ent3a,b, substantial D3 affinity could be
observed as well (Ki=27). For the enantiomer 3c, strong
and selective D4 binding was observed (Ki=0.77 nM).
For the cis-isomer ent3e, the highest D3 affinity was
measured (Ki=1.8 nM), leading to the observation, that
both the absolute configuration at the attachment posi-
Scheme 4.
The test compounds 3a–l, clebopride (4) and 5a,b
and the optical antipodes ent3a-l and ent5a,b as well as
N-methyl-clebopride18 were evaluated in vitro for their
abilities to displace [3H]spiperone from the cloned
19
human dopamine receptors D2long, D2short
,
D320 and
D4.421 being stably expressed in CHO cells (Table 1).22
a
Table 1. Chemical reaction and receptor binding data [Ki values (nM) based on the means of 2–4 experiments performed in triplicate at eight
concentrations]
Product
–X
ArCO2H
R
R0
Yield (%)
94
D1
D2long
D2short
D3
D4
35
7.4
4.7
1.6
3a
32
32
33
33
H
H
Me
Me
—
—
—
—
19,000
7100
8200
4400
470
8.6
54
8
340
7.4
39
2200
20
510
13
ent3a
3b
ent3b
94
6.4
3c
33
33
34
34
Me
Me
—
—
—
Br
Br
63 (two steps)
66 (two steps)
12,000
5300
8400
4200
21
15
1000
490
19
11
700
260
220
27
740
140
0.77
4.3
17
59
ent3c
3d
ent3d
—
3e
33
33
34
34
Me
Me
—
—
—
Br
Br
64 (two steps)
69 (two steps)
4100
4700
4700
4000
3.7
2.3
200
290
3
2
135
170
100
1.8
390
130
0.23
0.84
2.7
ent3e
3f
ent3f
—
27
3g
ent3g
3h
ent3 h
3i
ent3i
3j
ent3j
34
34
26
26
31
31
30
30
—
—
—
—
—
—
—
—
Br
Br
I
80
70
85
77
7200
7400
3000
6100
3300
6100
2800
3300
180
150
60
190
190
500
6400
7700
130
150
23
70
82
250
4000
5500
250
84
34
33
86
79
1000
1000
5
21
1.2
3.3
2.5
7
I
Ethynyl
Ethynyl
Phenylethynyl
Phenylethynyl
500
1500
3k
ent3k
34
34
—
—
Br
Br
69
89
2100
2900
640
1300
420
870
410
1000
130
78
3l
ent3l
34
34
—
—
Br
Br
1700
2100
2000
2500
1900
2800
580
1000
500
610
Clebopride (4)
N-Methyl-clebopride
32
33
H
Me
—
—
—
95
6000
6800
18
24
12
15
170
160
3.2
8.3
5a
32
32
33
33
H
H
Me
Me
—
—
—
—
85
86
17,000
13,000
10,000
5600
1600
7900
170
1100
4200
130
2900
8700
560
200
530
25
ent5a
5b
ent5b
250
210
1500
64
aD1 binding was determined with the radioligand [3H]SCH23390 (KD=0.35 nM) at 0.3 nM; D2, D3 and D4 data were derived from experiments
employing [3H]spiperone at a concentration of 0.5 nM and were calculated with KD values of 0.1 nM, 0.2–0.45 nM and 0.1–0.45 nM, respectively.

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J. Einsiedel et al. / Bioorg. Med. Chem. Lett. 13 (2003) 3293–3296
tion of the amine and the spatial disposition of the
methyl substituent strongly controls D3 binding. On the
other hand, comparable D2 and D4 affinities were
observed for 3e and ent3e. Interestingly, 3e turned out
to be one of the most potent D4 ligands reported
(Ki=0.23 nM). The 4-bromonaphthamide analogues 3d,
ent3d and ent3f exhibited significantly diminished
dopamine receptor affinities. On the other hand, 3f dis-
played a strong and quite selective D4 binding (Ki=2.7
nM; D2long/D4=74, D2short/D4=50, D3/D4=144).
Within our assay, the binding profiles of the recently
described aminopyrrolidines 3g and ent3g12 renouncing
the C-methyl substitution were comparable to those of 3f
and ent3f. Interestingly, the 4-iodo and 4-ethynyl deriva-
tives 3h,i/ent3h,i displayed only poor stereodifferentiation.
The phenylethynyl derivatives 3j, ent3j only exhibited
weak receptor recognition, obviously due to unfavorable
interactions in the binding pocket of the receptor. This
effect could also be observed, when we investigated the
homologous pyrrolidine derivatives 3k,l and ent3k,l.
4. The dopamine receptor binding profile was evaluated in
our laboratories (Table 1).
5. O’Connor, S. E.; Brown, R. A. Gen. Pharmacol. 1982, 13,
185.
6. Schoemaker, H.; Claustre, Y.; Fage, D.; Rouquier, L.;
Chergui, K.; Curet, O.; Oblin, A.; Gonon, F.; Carter, C.;
Benavides, J.; Scatton, B. J. Pharmacol. Exp. Ther. 1997, 280,
83.
7. Asahina, Y.; Fukuda, Y.; Fukuda, H. Eur. Patent 443,489,
1991; Chem. Abstr. 1991, 115, 256018.
8. Thomas, C.; Orecher, F.; Gmeiner, P. Synthesis 1998, 1491.
9. For a previous preparation, see: Rosen, T.; Chu, D. T. W.;
Lico, I. M.; Fernandes, P. B.; Shen, L.; Borodkin, S.; Pernet,
A. G. J. Med. Chem. 1988, 31, 1586.
10. Einsiedel, J.; Hubner, H.; Gmeiner, P. Bioorg. Med. Chem.
Lett. 2000, 10, 2041.
11. Sokoloff, P.; Schwartz, J. C. TIPS 1995, 16, 270.
12. Huang, Y.; Luedtke, R. R.; Freeman, R. A.; Wu, L.;
Mach, R. H. J. Med. Chem. 2001, 44, 1815.
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Soc. Jpn. 1993, 66, 613.
14. Thorand, S.; Krause, N. J. Org. Chem. 1998, 63, 8551.
15. Rognan, D.; Mann, A.; Wermuth, C. G.; Martres, M.-P.;
Giros, B.; Sokoloff, P.; Schwartz, J.-C.; Lecomte, J.-M.; Gar-
rido, F. Eur. Patent 539,281, 1992; Chem. Abstr. 1993, 119,
139085.
In conclusion, SAR studies on novel chiral benzamide
derivatives revealed insights into the three-dimensional
structural requirements of subtype specific binding.
Within future studies, a series of pharmacological tools
with graduated receptor binding profiles might help to
establish relationships between the balance of subtype
recognition and atypical antipsychotic properties.
16. 3b: a2D0=+17.3ꢁ (c 1.0, CHCl3); ent3b: a2D0=ꢀ17.5ꢁ (c 1.0,
CHCl3); mp: 184 ꢁC; 1H NMR (CDCl3, 360 MHz): d=0.92 (d,
J=6.4 Hz, 3H, CHꢀCH3), 1.50 (dddd, J=12.0, 12.0, 11.4, 3.6
Hz, 1H, H-5a), 1.60–1.72 (m, 1H, H-5b), 1.78–1.88 (m, 1H, H-
3), 2.01–2.18 (m, 2H, 2-Ha, 6-Ha); 2.84–2.93 (m, 2H, 2-Hb, 6-
Hb); 2.95 (d, J=5.3 Hz, 3H, NHCH3); 3.50 (s, 2H, NCH2Ph);
3.72 (dddd, J=11.4, 10.8, 8.7, 3.9 Hz, 1H, H-4), 3.93 (s, 3H,
OCH3), 4.65–4.75 (m, 1H, NHCH3), 6.10 (s, 1H, Ar), 7.16–7.37
(m, 5H, Ph), 7.51 (d, J=8.7 Hz, 1H, NHCO), 8.10 (s, 1H, Ar).
IR (KBr) 3390, 2935, 2800, 1640, 1600, 1520, 1280, 1245, 755,
700 cmꢀ1; Analysis calcd for C22H28ClN3O2: C, 65.74H, 7.02N,
10.45. Found: C, 65.74H, 7.08 N, 10.30. EIMS (m/z): 401 (M+).
17. For the preparation of 3c,e in racemic form, see: UK
Patent 2,037,740, 1980; Chem. Abstr. 1981, 94, 156736.
18. N-methylclebopride was synthesized by DCC/HOBt-
coupling using 4-amino-1-benzylpiperidine; see also: De Pau-
lis, T.; Hall, Y.; Kumar, S.; Ramsby, S. O.; Ogren, S. O.;
Hogberg, T. Eur. J. Med. Chem. 1990, 25, 507.
19. Hayes, G.; Biden, T. J.; Selbie, L. A.; Shine, J. Mol.
Endocrinol. 1992, 6, 920.
Acknowledgements
The BMBF and the Fonds der Chemischen Industrie is
acknowledged for financial support. We thank Dr.
H. H. M. Van Tol (Clarke Institute of Psychiatry, Tor-
onto), Dr. J.-C. Schwartz and Dr. P. Sokoloff
(INSERM, Paris) and Dr. J. Shine (The Garvan Insti-
tute of Medical Research, Sydney) for providing D4.4,
D3 and D2 receptor expressing cell lines.
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