Design, synthesis, and evaluation of hexahydrobenz[f]isoquinolines as a novel class of dopamine 3 receptor ligands

Article abstract of DOI:10.1016/j.bmcl.2004.09.047

We previously identified hexahydrobenz[f]isoquinoline (4a) as a new class of dopamine 3 receptor (D₃) ligand. Herein, we described the design, synthesis, and preliminary structure-activity relationships of new analogues of 4a as a novel class of D₃ ligands. Among these new analogues, compound 4h is a potent D₃ ligand (K_i = 6.1 nM) and has a selectivity of 133-fold between D₃- and D₂-like receptors, and of 163-fold between D₃- and D₁-like receptors, respectively. Thus, compound 4h represents a promising new lead compound for further design and optimization toward achieving highly potent and selective D₃ ligands.

Full text of DOI:10.1016/j.bmcl.2004.09.047

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Bioorganic & Medicinal Chemistry Letters 14(2004) 5813–5816
Design, synthesis, and evaluation of hexahydrobenz[f ]isoquinolines
as a novel class of dopamine 3 receptor ligands
Xihan Wu,a Jianyong Chen,a Min Ji,a Judith Varady,a
Beth Levantb and Shaomeng Wanga,*
aDepartments of Internal Medicine and Medicinal Chemistry, University of Michigan, CCGC/3316,
1500 East Medical Center Drive, Ann Arbor, MI 48109-0934, USA
bDepartment of Pharmacology, Toxicology, and Therapeutics, University of Kansas Medical Center, Kansas City,
KS 66160-7417, USA
Received 25 June 2004; revised 16 September 2004; accepted 17 September 2004
Available online 6 October 2004
Abstract—We previously identified hexahydrobenz[f]isoquinoline (4a) as a new class of dopamine 3 receptor (D3) ligand. Herein, we
described the design, synthesis, and preliminary structure–activity relationships of new analogues of 4a as a novel class of D3 lig-
ands. Among these new analogues, compound 4h is a potent D3 ligand (Ki = 6.1nM) and has a selectivity of 133-fold between D3-
and D2-like receptors, and of 163-fold between D3- and D1-like receptors, respectively. Thus, compound 4h represents a promising
new lead compound for further design and optimization toward achieving highly potent and selective D3 ligands.
Ó 2004 Published by Elsevier Ltd.
Dopamine receptors belong to the G-protein coupled
receptors (GPCR). The dopamine 3 (D3) subtype recep-
tor has been implicated in several neurological condi-
tions, and potent and selective D3 ligands may have
the therapeutic potential for the treatment of drug
addiction, ParkinsonÕs disease, and schizophrenia.1–3
Accordingly, there has been an enormous research inter-
est in the design and development of novel, potent, and
selective D3 ligands in recent years.3–10
basic core structures.4 Indeed, the majority of those re-
cently reported selective D3 ligands were based upon
the core structure of BP897.6–9 Accordingly, D3 ligands
with novel chemical core structures or scaffolds would
have considerable value to increase the chemical diver-
sity in D3 ligand design and may lead to the develop-
ment of highly potent and selective D3 ligands with
different in vitro and in vivo pharmacological properties.
Potent and highly selective D3 ligands with novel chem-
ical scaffolds may serve as additional pharmacological
tools to investigate the potential role of the D3 receptor
in several neurological conditions and may be developed
as potentially useful therapeutic agents for the treatment
of drug addiction, ParkinsonÕs disease, and schizophre-
nia. To this end, we have employed a novel computa-
tional three-dimensional database screening strategy to
discover novel D3 ligands.10
N
O
N
OCH3
N
H
N
4a
BP897
Compound 4a was identified as a novel D3 ligand using
a computational three-dimensional database searching
strategy.10 This compound has a novel tricyclic hexa-
hydrobenz[f]isoquinoline core structure that is not
found in other known D3 ligands.4 In our in vitro bind-
ing assays, 4a has a Ki value of 84nM for its binding
affinity to the D3 receptor and a moderate selectivity
over the D1- and D2-like receptors, being 10- and 39-
fold, respectively (Table 1). Compound 4a represents a
Although several classes of D3 ligands have been discov-
ered in the last decade, many of the previously reported
D3 ligands were derived from the very limited number of
Keywords: D3 receptor; Novel ligands; Selectivity.
*
Corresponding author. Tel.: +1 734615 0362; fax: +1 734674
9647; e-mail: shaomeng@umich.edu
0960-894X/$ - see front matter Ó 2004Published by Elsevier Ltd.
doi:10.1016/j.bmcl.2004.09.047

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X. Wu et al. / Bioorg. Med. Chem. Lett. 14 (2004) 5813–5816
Table 1. Binding affinities of novel and reference compounds at D1-, D2-, and D3-like receptors in binding assays using rat brain
Ligands
Ki (nM)a
Selectivity
D1-like [3H]SCH 23390b
D2-like [3H]spiperonec
D3 [3H]PD 128907d
D1-like/D3
D2-like/D3
4a
4b
819 96
n.t.e
3290 441
n.t.
84 13
103
10
—
39
—
4c
4d
696 58
n.t.
5
59 474
n.t.
84
9
1
183
63 10
—
18
—
1
4e
1117 120
1566 155
1055 83
993 123
636 103
8
4f
4g
122 11
46 11
25
5
62
28
5
38 13
6.1 1.3
1.1 0.2
1
4h
BP897
811 43
162 48
163
578
133
147
a Data represent the mean SEM of 3–5 independent determinations.
b [3H]SCH 23390 binding assays for D1-like dopamine receptors were performed as previously described in detail20 using membranes prepared from
the caudate-putamen of adult male Sprague–Dawley rats (Harlan, Indianapolis, IN). All compounds were dissolved in 100% EtOH at a con-
centration of 5mM. The assay buffer was 50mM Tris–HCl, 5mM KCl, 2mM MgCl2, and 2mM CaCl2, pH7.4at 23 °C; the concentration of
[3H]SCH 23390 (73Ci/mmol; Amersham) was 0.3nM; and nonspecific binding was determined in the presence of 1lM (+)-butaclamol. Assay tubes
were incubated at 23°C for 90min followed by rapid vacuum filtration. Data were analyzed using SigmaPlot 8.0.2 to determine Ki values using the
KD value for [3H]SCH 23390 of 0.3nM.20
c [3H]spiperone binding assays were performed as previously described in detail20,21 and as described for [3H]SCH 23390 except the concentration of
[3H]spiperone (24Ci/mmol; Amersham) was 0.2nM. Ki values were determined using the KD value for [3H]spiperone of 0.1nM.21
d [3H]PD 128907 binding assays were performed as previously described in detail20,22 using ventral striatal (nucleus accumbens and olfactory
tubercles) membranes prepared in assay buffer (50mM Tris, 1mM EDTA; pH7.4at 23 °C). The concentration of [3H]PD 128907 (116Ci/mmol;
Amersham, Arlington Heights, IL) was 0.3nM; nonspecific binding was defined by 1lM spiperone; and the incubation time was 3h. Ki values were
determined using the KD value for [3H]PD 128907 of 0.3nM.22
e n.t.—not tested due to low affinity in the D3 binding assay.
promising starting point for further optimization to im-
prove its binding affinity to the D3 receptor and its selec-
tivity over the closely related D1- and D2-like receptors.
In this paper, we wish to report the design, synthesis,
and preliminary structure–activity relationship studies
of a series of new analogues of 4a as new ligands for
the D3 receptors and their selectivity over D1- and D2-
like subtype receptors.
diminishing the binding of the ligands to D2-like
receptors.
To further test this idea, compound 4h with a 2-naphthyl
tail was designed and synthesized. Compound 4h has a Ki
value of 6.1nM to the D3 receptor. Furthermore, its
binding affinity to the D2 receptor is decreased by seven
times as compared to 4f, and 18 times as compared to 4g.
Interestingly, its binding affinity at D1-like receptors re-
mained virtually the same as compared to compounds
4f and 4g. As a result, compound 4h has a selectivity of
163- and 133-fold over D1- and D2-like receptors, respec-
tively. Hence, 4h represents a novel, potent D3 ligand
with a good selectivity over D1- and D2-like receptors.
Compound 4a may be divided into three segments, the
tricyclic hexahydrobenz[f]isoquinoline core structure as
the ÔheadÕ, the phenyl ring as the ÔtailÕ, and the linker be-
tween the head and the tail. Compounds 4b and 4c were
designed to test the influence of the length of the linker
to the binding affinity and selectivity. Although 4a, 4b,
and 4c have similar affinities to the D3 receptor, the
selectivity of 4c over the D2-like receptor is significantly
decreased as compared to 4a, primarily due to its in-
creased binding affinity at D2-like receptors (Ki value
changed from 3290nM for 4a to 59nM for 4c at D2-like
receptors).
To directly compare 4h with other known D3 ligands, we
have evaluated BP897, a known selective D3 ligand,3 in
our assay conditions and the results are provided in Ta-
ble 1. As can be seen, BP897 has Ki values of 1.1nM at
the D3 receptor, 162nM at the D2-like receptors, and
636nM at the D1-like receptors, respectively. These val-
ues are in good agreement with the reported Ki values of
0.92nM at the D3 receptor, 61nM at the D2 receptor,
and 3lM at the D1 receptor, respectively, using the
CHO cells expressing recombinant human D1, D2, and
D3 receptors.3 Of note, although it is known that assay
conditions can have a significant influence on the bind-
ing affinity of a ligand at the D3 receptor and the selecti-
vity over other dopamine subtype receptors,11 our
results on BP897 show our assays using membranes pre-
pared from rat brains and assays using CHO cells
expressing recombinant human D1, D2, and D3 appear
to produce quite consistent results. Furthermore, our
data indicate that BP897 and compound 4h have a
similar selectivity between the D3 and D2 receptors
The diminished selectivity from 4a to 4c could be due to
the change either in the direct interactions between the
linker and the receptors, or in the interactions between
the phenyl ring and the receptors, or in both. To clarify
this point, several new analogues (4d–g) with different
lengths of the linker but without the phenyl ring ÔtailÕ
were designed and synthesized. While compound 4d is
approximately two times less potent than 4a to the D3
receptor, compound 4e is as potent as 4a, and com-
pounds 4f and 4g are three- and two times more potent
than 4a, respectively. Compound 4f is also the most
selective ligand among 4d–g. These data suggest that
the ÔtailÕ primarily contributes to the selectivity by

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X. Wu et al. / Bioorg. Med. Chem. Lett. 14 (2004) 5813–5816
5815
i
ii
iii
OH
O
N
R
1
2
3
( )
n
4d n=2, R= H
4e n=3, R= H
4a n=2, R=
4b n=3, R=
4c n=4, R=
4f n=4, R= H
4g n=5, R= H
Scheme 1. Synthesis of new analogues 4b–g. Reagents and conditions: (i) 1.2equiv MeMgBr, Et2O, 0°C then reflux for 0.5h; (ii) p-toluenesulfonic
acid, toluene, reflux, 4h, yield 72% in two steps; (iii) CH2O, RNH3Cl, AcOH, H2O, 6h, yield 18–28%.
iii
ClHH2N
i
BocHN
ii
H2N
BocHN
NHCOCF3
NH2
NH2
NHCOCF3
6
7
5
8
iv
O
v
N
N
N
H
NHCOCF3
4h
9
Scheme 2. Synthesis of compound 4h. Reagents and conditions: (i) 5/Boc2O = 7:1, dioxane, room temperature (rt), overnight; (ii) 1.2equiv
trifluoroacetic anhydride, Et3N, CH2Cl2, rt, overnight, yield 88% in two steps; (iii) AcCl, MeOH, rt, 10h, quantitative; (iv) 3, CH2O, AcOH, H2O,
6h, yield 38%; (v) (a) K2CO3, MeOH, rt, overnight; (b) 1.2equiv 2-naphthoyl chloride, Et3N, CH2Cl2, 0° C, rt, 1h, yield 83% in two steps.
(147-fold vs 133-fold) but BP897 is four times more
selective than 4h between the D3 and D1 receptors
(578-fold vs 163-fold) under our assay conditions.
tors, and between D3- and D2-like receptors,
respectively. Hence, compound 4h represents a promis-
ing lead compound for further optimization toward
obtaining novel, highly potent, and selective D3 ligands.
The synthesis of compounds 4a–g is shown in Scheme
1.12–14 Briefly, treatment of the 1-tetralone 1 with meth-
ylmagnesium bromide, followed by dehydration with p-
toluenesulfonic acid in refluxing toluene, generated the
corresponding 1-methyl-3,4-dihydronaphthalene 3. Amino-
methylation of 3 with the appropriate amine hydrochlo-
ride salt and formaldehyde in aqueous acetic acid
afforded the hexahydrobenz[f]isoquinolines 4a–g.
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acid yielded the hexahydrobenz[f]isoquinoline 9 using
the same procedure as described in Scheme 1. The target
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300MHz) d 1.79 (1.70–1.86, m, 4H), 1.95 (t, J = 7.8Hz,
2H), 2.48–2.58 (m, 4H), 2.67–2.75 (m, 4H), 2.99 (s, 2H),