Indoline and piperazine containing derivatives as a novel class of mixed D2/D4 receptor antagonists. Part 1: Identification and structure-activity relationships

Article abstract of DOI:10.1016/S0960-894X(02)00655-8

Optimization of the lead compound 2-[-4-(4-chloro-benzyl)-piperazin-1-yl]-1-(2,3-dihydro-indol-1-yl)-ethanone 1 by systematic structure-activity relation (SAR) studies lead to two potent compounds 2-[-4-(4-chloro-benzyl)-piperazin-1-yl]-1-(2-methy-2,3-dihydro-indol-1-yl)- ethanone 2n and 2-[-4-(4-chloro-benzyl)-piperazin-1-yl]-1-(2-methy-2,3-dihydro-indol-1-yl)- ethanone 7b. Their related synthesis was also reported.

Full text of DOI:10.1016/S0960-894X(02)00655-8

Bioorganic & Medicinal Chemistry Letters 12 (2002) 3105–3109
Indoline and Piperazine Containing Derivatives as a Novel Class
of Mixed D2/D4 Receptor Antagonists.
Part 1: Identification and Structure–Activity Relationships
He Zhao,* Andrew Thurkauf, Xiaoshu He, Kevin Hodgetts, Xiaoyan Zhang,
Stanislaw Rachwal, Renata X. Kover, Alan Hutchison, John Peterson, Andrzej Kieltyka,
Robbin Brodbeck, Renee Primus and Jan W. F. Wasley
Neurogen Corporation, 35 Northeast Industrial Road, Branford, CT 06405, USA
Received 5 June 2002; accepted 31 July 2002
Abstract—Optimization of the lead compound 2-[-4-(4-chloro-benzyl)-piperazin-1-yl]-1-(2,3-dihydro-indol-1-yl)-ethanone 1 by sys-
tematic structure–activity relation (SAR) studies lead to two potent compounds 2-[-4-(4-chloro-benzyl)-piperazin-1-yl]-1-(2-methy-
2,3-dihydro-indol-1-yl)-ethanone 2n and 2-[-4-(4-chloro-benzyl)-piperazin-1-yl]-1-(2-methy-2,3-dihydro-indol-1-yl)-ethanone 7b.
Their related synthesis was also reported.
# 2002 Elsevier Science Ltd. All rights reserved.
The oldest and most enduring hypothesis for the etiology
of schizophrenia is that it results from a hyperfunctioning
of central dopaminergic systems.1 In 1963, Carlsson and
Lindquist reported that dopamine turnover as measured
by homovanillic acid levels was increased in laboratory
animals following the administration of neuroleptic
drugs.2 Major support for this hypothesis has also come
from the observation that antipsychotic medications
exert their therapeutic effects via dopamine receptor
blockade.3
identification of a number of D4 specific antagonists5ꢀ15
some of which advanced to clinical trials.16ꢀ21
L-745,870 is the first selective dopamine D4 receptor
antagonist to have been tested in both behavioral
experimental animal studies and in clinical trials.
L-745,870 was ineffective in reversing apomorphine-
induced disruption of pre-pulse inhibition,22 although
this result is controversial.23 In two other classic tests of
neuroleptic activity and EPS liability, L-745,870 was
ineffective in reversing amphetamine-induced hyper-
activity and apomorphine-induced stereotypy.22 In a
small phase II trial in acutely psychotic, neuroleptic
responsive patients L-745,870 was ineffective.24 As these
studies have cast a shadow on the importance of the D4
receptor, we considered the possibility that a combination
of D4 and D2 receptor affinities could be a more reason-
able target for antipsychotic drugs like clozapine,25,26
based on the fact that this agent shows a unique high
D4/moderate D2 binding affinity feature. In addition,
although clozapine binds to a wide array of receptors
which may influence its atypical profile, there are no
effective antipsychotic agents which lack significant D2
affinity. Therefore, we set out a research program to
obtain a compound with a receptor profile having high
D4 (<10 nM) and moderate D2 (<200 nM) receptor
affinities and also with lower binding affinities to a1
(>1000 nM) to avert undesirable cardiovascular
Although most clinically available drugs for schizo-
phrenia are dopamine D2 receptor antagonists, they are
not efficacious in treating all schizophrenic patients and
generally cause motor disorders such as extrapyramidal
side effects (EPS). Clozapine and olanzepine are regarded
as an atypical antipsychotics in that they provide relief
from many of the positive florid symptoms of the disease
without any significant EPS. The observation that cloza-
pine has a high affinity for the D4 receptor subtype raised
the possibility that clozapine’s efficacy might, at least in
part, be attributed to D4 antagonism.4 Therefore, the
development of selective D4 receptor antagonists has
become an active field of research, and this lead to the
*Corresponding author. Tel.:+1-203-488-8201x3077; fax:+1-203-483-
7027; e-mail: hzhao@nrgn.com
0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00655-8

3106
H. Zhao et al. / Bioorg. Med. Chem. Lett. 12 (2002) 3105–3109
Figure 1.
effects.27,28 Ideally the target would possess a similar
D2/D4 binding ratio to that of clozapine (2ꢁ10/1,
D2=138 nM, D4=9 nM).4
found to be fully D4 selective compounds. This finding
encouraged us to explore other heterocyclic groups to
replace oxindole (4b–4h). Each of these isosteric replace-
ments resulted in D4 selective antagonists.
Affinities at D2 and D4 receptors were determined via
standard competitive displacement assays using human
D2 and D4 clones with [3H]YM 09151 as the competitive
ligand. Affinity at the a1 receptor was determined via
standard competitive displacement assays using rat
brain homogenate with [3H]prazosine as the competitive
ligand. Screening of a chemical library uncovered 2-[-4-
(4-chloro-benzyl)-piperazin-1-yl]-1-(2,3-dihydro-indol-1-
yl)-ethanone 1 showing high affinity binding for D4 (1.6
nM), relatively modest binding for D2 (690 nM) and
significant affinity for a1 (88 nM). For the purposes of
our SAR studies, compound 1 was divided into five
subunits (Parts A–E) from left to right as shown in
Figure 1.
As a result of our studies on Parts A and B, we had
promoted 2n as the target reference compound to be
used for Part C studies. Examination of the isomeric
piperidines 5a and 5b indicated that a nitrogen at posi-
tion 4 (Y=N) was required for receptor binding (Table
Table 1. Effect of substitution within the indoline ring system
Compd
R
R0
Ki (nM )
Exploration of the SAR began by varying the aromatic
substitutions on the indoline ring (Fig. 1, Part A). It was
found that both electron-withdrawing and electron-
releasing groups decrease dopamine receptor and a1
binding activities relative to the parent compound 1
(Table 1, compounds 2a–2m). We then turned our
attention to exploration of indoline positions 2 and 3.
Alkyl substitutions provided three promising com-
pounds (2n, 2s and 2w) fitting our criteria.
D2
D4
a1
1
H
4-Cl
6-Cl
4-F
H
H
H
H
H
H
H
H
H
H
H
H
H
690
2145
607
>1000
2552
3431
1692
6357
4623
487
>1000
450
3780
3780
209
4375
509
2524
341
117
129
187
1.6
992
701
145
51
462
37
771
48
6392
3394
>1000
235
235
5.4
96
6
549
622
4.3
88
988
3545
261
3122
—
—
2805
2624
972
>1000
—
>1000
>1000
2655
1800
2202
—
>1000
5006
2834
909
2a
2b
2c
2d
2e
2f
2g
2h
2i
6-F
4-Me
5-Me
6-Me
7-Me
4-OMe
5-NO2
6-NO2
5-SMe
In order to better understand the importance of indoline
as a pharmacophoric subunit, a number of heterocyclic
ring systems were examined (Table 2). It is notable that
the directly homologous examples of the tetra-
hydroquinolines 3a and 3b are less active than the cor-
responding indolines 1 and 2n.
2j
2k
2l
2m
2n
2o
2p
2q
2r
2s
2t
2u
2v
2w
2x
2y
2z
5-SO2NMe2
H
H
H
H
H
H
H
H
H
H
H
H
H
H
2-Me
3-Me
2-Et
2-isoPr
2-isoBu
2,2-diMe
2,2-diEt
2,2-(CH2)4–
2,2-(CH2)5–
cis-2,3-diMe
trans-2,3-diMe
2,3,3-triMe
We next studied the glycine spacer, Part B in Figure 1
(Table 3). Compound 4a, with a reverse glycine, is
essentially inactive. Substitution at a position (4i) or chain
length changes (4j and 4k) resulted in loss of affinity at all
three binding sites. These results appear to demonstrate
the importance of spacer length and oxygen position26 for
activity. Shifting the carbonyl out of the chain gave oxin-
dole 4b and its sulfonamide analogue 4c which were
31
36
1563
7
206
108
726
3325
120
183
883
983
2544
3386
2845
8994
cis-2,3-(CH2)4–
377

H. Zhao et al. / Bioorg. Med. Chem. Lett. 12 (2002) 3105–3109
Table 3. Effect of changes to the glycine spacer
3107
Table 2. Effect of non-indoline ring
Compd
3a
R
Ki (nM )
D4
Compd
R
Ki (nM )
D4
D2
a1
D2
a1
2629
47
>10000
4a
>10,000
3854
—
3b
4126
2220
95
37
—
4b
4c
4d
4e
4f
1642
>10,000
1225
8
7
598
161
3c
>1000
>1000
>1000
>1000
—
3d
35
50
259
73
8445
1694
1052
5136
36
3e
1926
81
8053
3f
5847
726
429
14.2
1207
3
3g
2610
4g
4h
4i
>1000
979
522
13
3h
>10,000
2298
3i
>5000
59
—
>7000
>1000
281
>1000
4819
>1000
4j
4). Furthermore, the C-1 carbon led to potent a1 binding
affinity which might increase cardiovascular effect risks.
Various mono and dimethylated piperazines displayed
generally unfavorable D2 affinities.
4k
5847
726
1000
The next area to explore was the benzylic methylene
spacer of 2n. The results of this study (Table 5) indicate
that either shortening or lengthening of the carbon
chain as well as substitution onto the methylene of 2n
resulted in significant loss of affinity at dopamine D2
and D4 receptors.
GTPg35S binding from cell membranes. Functional
assessment of compounds 2n and 7b at both the D2 and
D4 receptors indicates no agonist properties up to 10
mM, while demonstrating functional Ki values of 960
nM and 140 nM, respectively, at D2 receptor, and 9 nM
and 7 nM, at D4 receptor.
Finally, we attempted to optimize the activities of 2n
and 5a by varying the substituent pattern of the N-benzyl
group (Table 6). The 4-methylbenzyl compound 7b was
identified having similar binding potency as 2n. This
indicated that the steric rather than electronic effects
influence biological properties for this substitution. In
addition, all three pyridylmethyl (2, 3 and 4) analogues
8 (Fig. 1) show no activity.
Scheme 1 represents the general procedure used to pre-
pare most indoline piperazine compounds 12. Indoline
and 2-methylindoline are commercially available. Other
substituted indolines (9a–9m, 9o, and 9w–9z relative to
Table 2) were prepared by reduction of the correspond-
ing indoles with trimethylamine-borane complex fol-
lowing Berger’s method.29 The synthesis of 2-
ethylindoline,30 2-isopropyl and 2-isobutylindolines31
have been previously reported. 2,2-Disubstituted indo-
lines (9s–9v relative to 2s–2v) were prepared by treatment
of 2,2-disubstituted-1,2-dihydro-3H-indol-3-ones32 with
LAH–AlCl3 complex (rt, 1 h). The fragments selected
in Table 2 are commercially available and have been
Compounds were also assessed as to their functional
activity both at the D2 and D4 receptors. D2 functional
activity was assessed via compound reversal of quinpir-
ole inhibited, forskolin stimulated cAMPproduction
from whole cells, while D4 functional activity was
assessed via inhibition of quinpirole stimulated

3108
H. Zhao et al. / Bioorg. Med. Chem. Lett. 12 (2002) 3105–3109
Table 6. Effect of substitution on benzylic ring
Table 4. Effect of piperidine and substituted piperazine
Compd
R
X
Ki (nM )
Compd
R
R0
X
Y
Ki (nM )
D2
D4
a1
D2
D4
a1
2n
7a
7b
7c
7d
7e
7f
7g
7h
7i
4-Cl
4-F
4-Me
4-Et
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
209
887
188
753
1784
1918
5287
2094
3823
620
1530
225
3011
214
119
1414
52
71
834
62
166
39
158
>10,000
306
23
5.4
7
3
2655
671
1257
1184
504
320
7125
872
4165
>1000
1324
>1000
2056
>1000
—
—
118
236
—
127
95
327
142
—
118
103
—
2n
5a
5b
5c
5d
5g
5h
5i
H
H
H
Me
Me CH
N
N
209
92
—
4742
8620 >10,000
6740
5839
5.4
4
7264
1361
2655
162
>10,000
508
—
1300
594
—
N
CH
N
N
N
Me
H
N
N
N
N
N
N
6
cis-1,6-diMe
cis-3,5-diMe
trans-2,5-diMe
(2R)-Me
(5R)-Me
4-isoPr
4-tertBu
4-CF3
4-OMe
4-OCF3
3-OCF3
2-Me
2-OMe
3,5-diF
3,5-diCl
2,5-diMe
2-F
3-F
4-F
2-Cl
3-Cl
3-Me
4-Me
12
12
11
15
5
H
H
335
14
—
H
N
N
H
>1000
6
7
7j
7k
7l
25
12
15
8
40
7
6
29
4
5
2
10
2
4
Table 5. Effect of methylene spacer change
7m
7n
7o
7p
7q
7r
7s
7t
7u
7v
7w
7x
7y
7z
Compd
6a
R
Ki (nM)
D4
4-OMe
4-NO2
3,4-diMe
3-F, 4-Me
2-Cl, 4-F
D2
—
a1
5745
—
3
21
518
6b
5247
1252
134
67
115
937
6c
6d
>1000
>1000
2076
used for preparation of 3 in the same manner as indicated
in Scheme 1.
1-Bromo-2-chloroethane and 1-bromo-3-chloropropane
were employed to react with various amides in the pre-
sence of sodium hydride in DMF for 24 h, yielding 4b–
4g and 4h (Table 3). Using the appropriate chloro acid
chlorides gave 4i and 4k, while changing the reaction
order of Scheme 1 generated compound 4a. Urea 4j was
prepared by treatment of indoline with sodium hydride
and ethyl 4-(4-chlorobenzyl)piperazine-1-carboxylate33
in THF (reflux, 24 h, 83% yield).
Scheme 1.
applied to the preparation of compounds 6 (Table 5)
and 7a–7n (Table 6).
Using the lead compound 1 as reference, a systematic
SAR study has been carried out with the goal of identi-
fying compounds with a D2/D4 affinity ratio similar to
that found in the atypical antipsychotic clozapine. The
compounds 2-[-4-(4-chloro-benzyl)-piperazin-1-yl]-1-(2-
methy-2,3-dihydro-indol-1-yl)-ethanone 2n and 2-[-4-(4-
chloro-benzyl)-piperazin-1-yl]-1-(2-methy-2,3-dihydro-
indol-1-yl)-ethanone 7b, were found to meet this
criteria. The asymmetric synthesis, further SAR and
biological evaluation are decribed in the proceeding
paper.35
Piperidine containing compounds (5a in Table 4 and
7o–7z in Table 6) were prepared as illustrated in Scheme
2. Compound 5b was prepared by the same procedure
as Scheme 1. Compounds 5c–5i in Table 4 were synthe-
sized by controlling the coupling sequences using steric
effects. Substituted benzyl piperazines were usually
formed by the reported procedures34 and further

H. Zhao et al. / Bioorg. Med. Chem. Lett. 12 (2002) 3105–3109
3109
12. Kesten, S. R.; Heffner, T. G.; Johnson, S. J.; Pugsley,
T. A.; Wright, J. L.; Wise, L. D. J. Med. Chem. 1999, 42, 3718.
13. Belliotti, T. R.; Wustrow, D. J.; Brink, W. A.; Zoski,
K. T.; Shih, Y.-H.; Whetzel, S. Z.; Georgic, L. M.; Corbin,
A. E.; Akunne, H. C.; Heffiner, T. G.; Pugsley, T. A.; Wise,
L. D. J. Med. Chem. 1999, 42, 5181.
14. Perrone, R.; Berardi, F.; Colabufo, N. A.; Leopoldo, M.;
Tortorella, V. J. Med. Chem. 2000, 43, 270.
15. Hodgetts, K. J.; Kieltyka, A.; Brodbeck, R.; Tran, J. N.;
Wasley, J. W. F.; Thurkauf, A. Bioorg. Med. Chem. 2001, 9,
3207.
16. Lober, S.; Hubner, H.; Utz, W.; Gmeiner, P. J. Med.
Chem. 2001, 44, 2691.
17. Faraci, W. S.; Zorn, S. H.; Sanner, M. A.; Fliri, A. Curr.
Opin. Chem. Biol. 1998, 2, 535.
18. Thurkauf, A.; Yuan, J.; Chen, X.; Wasley, J. W. F.;
Meade, R.; Woodruff, K. H.; Huston, K.; Ross, P. C. J. Med.
Chem. 1996, 38, 4950.
19. Kulagowski, J. J.; Broughton, H. B.; Curtis, N. R.;
Mawer, I. M.; Ridgill, M. P.; Baker, R.; Emms, F.; Freedman,
S. B.; Marwood, R.; Patel, S.; Ragan, C. I.; Leeson, P. D. J.
Med. Chem. 1996, 39, 1941.
20. Ten Brink, 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. J. Med. Chem. 1996, 39, 2435.
Scheme 2.
21. Sanner, M. A.; Chppie, T. A.; Dunaiskis, A. R.; Fliri,
A. F.; Desai, K. A.; Zorn, S. H.; Jackson, C. G.; Faraci, W. S.;
Collins, J. L.; Duignan, D. B.; Di Prete, C. C.; Lee, J. S.;
Trozzi, A. Bioorg. Med. Chem. Lett. 1998, 8, 725.
22. Bristow, L. J.; Collinson, N.; Cook, J. P.; Curtis, N.;
Freedman, S. B.; Kulagowski, J. J.; Leeson, P. D.; Patel, S.;
Ragan, C. I.; Ridgill, M.; Saywell, K. L.; Trickleback, M. D.
J. Pharmacol. Exp. Ther. 1997, 283, 1256.
References and Notes
1. Egan, M. F.; Weinberger, D. R. Curr. Opin. Neurobiol.
1997, 7, 701.
2. Carlsson, A.; Linquist, M. Acta Pharmacol. Toxicol. 1963,
20, 140.
3. Seeman, P.; Van Tol, H. M. M. Trends Pharmacol. Sci.
1994, 15, 264.
4. Van Tol, H. H. M.; Bunzov, J. R.; Guan, H.-C.; Sunahara,
R. K.; Seeman, P.; Niznik, H. B.; Civelli, O. Nature 1991, 350,
610.
5. Kulagowski, J. J.; Patel, S. Curr. Pharm. Des. 1997, 3, 355.
6. Belliotti, T. R.; Brink, W. A.; Kesten, S. R.; Rubin, J. R.;
Wustrow, D. J.; Zoski, K. T.; Whetzel, S. Z.; Corbin, A. E.;
Pugsley, T. A.; Heffner, T. G.; Wise, L. D. Bioorg. Med.
Chem. Lett. 1998, 8, 1499.
7. Arlt, M.; Bottcher, H.; Riethmuller, A.; Schneider, G.;
Bartoszyk, G. D.; Greiner, H.; Seyfried, C. A. Bioorg. Med.
Chem. Lett. 1998, 8, 2033.
8. Moore, K. W.; Bonner, K.; Jones, E. A.; Emms, F.; Lee-
son, P. D.; Marwood, R.; Patel, S.; Patel, S.; Rowley, M.;
Thomas, S.; Carling, R. W. Bioorg. Med. Chem. Lett. 1999, 9,
1285.
9. Haubmann, C.; Hubner, H.; Gmeiner, P. Bioorg. Med.
Chem. Lett. 1999, 9, 1969.
23. Mansbach, R. S.; Brooks, E. W.; Sanner, M. A.; Zorn,
S. H. Psychopharmacology 1998, 135, 194.
24. Bristow, L. T. M.; Kramer, J.; Kulagowski, S.; Patel, C.;
Ragan, G. Seabrook. Trends Pharmacol. Sci. 1997, 18, 186.
25. Zhang, X.; Hodgetts, K.; Rachwal, S.; Zhao, H.; Wasley,
J. W. F.; Craven, K.; Brodbeck, R.; Kieltyka, A.; Hoffman,
D.; Bacolod, M. D.; Girard, B.; Tran, J.; Thurkauf, T. J. Med.
Chem. 2000, 43, 3923.
26. Zhao, H.; Thurkauf, K.; Braun, J.; Brodbeck, R.; Kiel-
tyka, A. Bioorg. Med. Chem. Lett. 2000, 10, 2119.
27. Pang, C. C. Y. Pharmacology and Therapeutics 2001, 90, 179.
28. Michelotti, G. A.; Price, D. T.; Schwinn, D. A. Pharma-
cology and Therapeutics 2000, 88, 281.
29. Berger, J. G. Synthesis 1974, 508.
30. Meyers, A. I.; Hellring, S. Tetrahedron Lett. 1981, 22,
5119.
31. Allbrecht, H.; Schmitt, C. Synthesis 1994, 983.
32. Azadi-Ardakani, M.; Alkhader, M. A.; Lippiatt, J. H.;
Patel, D. I.; Smalley, R. K.; Higson, S. J. Chem. Soc., Perkin
Trans. I 1986, 1107.
10. Haubmann, C.; Hubner, H.; Gmeiner, P. Bioorg. Med.
Chem. Lett. 1999, 9, 3143.
33. Buckle, D. R.; Outred, J.; Smith, H.; Spicer, B. A. J. Med.
Chem. 1984, 27, 1452.
11. Carling, R. W.; Moore, K. W.; Moyes, C. R.; Jones, E. A.;
Bonner, K.; Emms, F.; Marwood, R.; Patel, S.; Patel, S.;
Pletcher, A. E.; Beer, M.; Sohal, B.; Pike, A.; Leeson, P. D. J.
Med. Chem. 1999, 42, 2706.
34. Craig, J. C.; Young, R. J. Org. Synth. 1973, 5, 88.
35. Zhao, H.; He, X.; Thurkauf, A.; Hoffman, D.; Kieltyka,
A.; Brodbeck, R.; Primus, R.; Wasley, J. Bioorg. Med. Chem.
Lett. 2002, 12, 3111.