Novel tetrahydropyrido[3,2-c]pyrroles as 5-HT7 antagonists

Article abstract of DOI:10.1016/j.bmcl.2010.11.078

The synthesis and SAR for a novel series of tetrahydropyrido[3,2-c]pyrroles is described. Optimization of the pendant aryl ring lead to high binding affinity at the 5-HT₇ receptor when small lipophilic groups were placed in the para position. Modification of the N-benzyl group and secondary amine were not well tolerated. A representative set of compounds was shown to be functional antagonists of the 5-HT₇ receptor.

Full text of DOI:10.1016/j.bmcl.2010.11.078

Bioorganic & Medicinal Chemistry Letters 21 (2011) 42–44
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Novel tetrahydropyrido[3,2-c]pyrroles as 5-HT7 antagonists
⇑
Dale A. Rudolph , Curt A. Dvorak, Lisa Dvorak, Diane Nepomuceno, Pascal Bonaventure,
Timothy W. Lovenberg, Nicholas I. Carruthers
Johnson & Johnson Pharmaceutical Research and Development, L.L.C., 3210 Merryfield Row, San Diego, CA 92121, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and SAR for a novel series of tetrahydropyrido[3,2-c]pyrroles is described. Optimization of
the pendant aryl ring lead to high binding affinity at the 5-HT7 receptor when small lipophilic groups
were placed in the para position. Modification of the N-benzyl group and secondary amine were not well
tolerated. A representative set of compounds was shown to be functional antagonists of the 5-HT7
receptor.
Received 4 October 2010
Accepted 17 November 2010
Available online 24 November 2010
Keywords:
5-HT7
Ó 2010 Elsevier Ltd. All rights reserved.
5-HT7 receptor
Serotonin
The 5-HT7 receptor is the most recently described member of
the 5-HT receptor family.1 The 5-HT7 receptor has been detected
both in the periphery and the CNS. In the periphery the receptor
is found in the smooth muscle cells of blood vessels2 and the gas-
trointestinal tract.3 In human and rodent brain, the highest densi-
ties of 5-HT7 receptors have been observed in the hypothalamus
(including the suprachiasmatic nucleus), thalamus, hippocampus,
cortex and dorsal raphe.4–6 Important roles for the central 5-HT7
receptor have been identified in circadian rhythmicity,7 thermo-
regulation,8 sleep and endocrine regulation.9 Indications of an
involvement of the 5-HT7 receptor in mood disorders come from
the anatomical localization of the 5-HT7 receptor in the limbic sys-
tem and from several behavioral studies showing that SB-26997010
1 in Figure 1, a selective 5-HT7 antagonist, like classical serotonin
reuptake inhibitors (SSRIs), decreases immobility in both tail sus-
pension and forced swim test.11,12 In agreement with these phar-
macological data, 5-HT7 knockout mice showed reduced
immobility in both the forced swim test and the tail suspension
test.11
The Boc group is then removed with 1.0 M HCl in diethyl ether
with a 10:1 mixture of dichloromethane and methanol as solvent
to afford the desired secondary amines 6 in moderate to excellent
yield as the HCl salts after filtration of the reaction mixture.
The SAR associated with substitution of the pendant aryl ring is
shown in Table 1.17 Since our HTS hit contained a 4-chlorophenyl
N
N
N
N
HO
S
O
O
Cl
N
H
1, 5-HT7 Ki = 1 nM
2, 5-HT7 Ki = 14 nM
Figure 1. SB-269970 and HTS hit.
One strategy for our program entailed replacement of the cen-
tral pyrazole core of HTS hit 213 with a pyrrole. This was attractive
to us since the corresponding tetrahydropyrido[3,2-c]pyrroles
could be assembled14 in essentially one step with three points of
diversity as depicted in Scheme 1.15 Treatment of commercially
available 4-oxo-piperidine-1-carboxylic acid tert-butyl ester 3 with
a benzylamine in the presence of silica gel with toluene as the sol-
vent generates the iminium/enamine intermediate that is then
trapped by easily synthesized nitrostyrenes16 4 to afford the Boc-
protected tetrahydropyrido[3,2-c]pyrroles 5 in moderate yield.
R2
O
N
N
O2N
a
+
R1
R2
N
R3
Boc
3
4
5, R3=Boc
b
6, R3=H
⇑
Corresponding author.
E-mail address: drudolph@its.jnj.com (D.A. Rudolph).
Scheme 1. Reagents and conditions: (a) ArCH2NH2, SiO2, toluene, rt then nitrosty-
rene, 20–55% yield; (b) 1.0 M HCl in Et2O, 10:1 CH2Cl2/MeOH, rt, 30–85% yield.
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.11.078

D. A. Rudolph et al. / Bioorg. Med. Chem. Lett. 21 (2011) 42–44
43
Table 1
Table 3
Pendant aryl ring analogs and their affinity at the 5-HT7 receptor
Substitution of the basic piperidine nitrogen
N
N
Cl
R1
N
H
N
R3
No.
R1=
Binding affinity Kia (nM)
No.
R3
=
Binding Affinity Ki, nMa
7
8
9
H
4-Cl
220 ( 15)
107 ( 18)
10,000
4467 ( 610)
167 ( 19)
2067 ( 202)
450 ( 28)
35 ( 3)
10,000
10,000
2727 ( 1322)
45 ( 7)
290 ( 35)
10,000
25
26
27
Me
Et
36b
250b
4-OCF3
3,4-diCl
4-CF3
2,4-diCl
4-NO2
4-CH3
4-tBu
3-Cl
3-F
4-F
4-OCH3
4-OH
iPr
370 ( 38)
10
11
12
13
14
15
16
17
18
19
20
a
Values are means of three experiments in triplicate unless indicated, SEM is in
parentheses.
b
Values are single experiments.
Next we turned our attention to exploring substitution on the
N-benzyl group of the pyrrole nucleus. The previously explored
pendant aryl ring was held constant while chloro substitution
was walked around the aromatic ring of the benzyl group.
In all cases substitution on the phenyl ring of the benzyl group
resulted in reduction of 5-HT7 activity. Substitution at the ortho po-
sition (21, Ki = 2287 nM) caused a 10-fold drop in potency, meta
substitution (22, Ki = 6600 nM) brought about a 30-fold reduction
while the outcome of para substitution (21, Ki = 753 nM) was only
a threefold reduction in 5-HT7 activity compared to compound 7.
Table 2 also illustrates that the negative effects of the meta chloro
substitution on the benzyl group can be ameliorated by using a
preferred substituent on the pendant aryl ring, such as 4-chloro
(24, Ki = 533 nM) which improves potency over compound 22 by
12-fold. But even with that improvement, it does not approach
the affinity of the corresponding unsubstituted benzyl compound
8 (Ki = 107 nM).
Substitution of the basic nitrogen was also investigated. Table 3
illustrates that a methyl substituted tertiary amine (25, Ki = 36 nM)
showed a modest threefold improvement in potency over the sec-
ondary amine analog 8. This improvement in activity was com-
pletely eliminated with only a small increase in steric size to an
ethyl group (26, Ki = 250 nM). The isopropyl analog (27,
Ki = 370 nM) confirms that the basic piperidine nitrogen prefers
small or no substituents.
a
Values are means of three experiments in triplicate unless indicated, SEM is in
parentheses.
Table 2
N-Benzyl analogs and their affinity at the 5-HT7 receptor
R2
N
R1
N
H
No.
R1=
R2=
Binding affinity Kia (nM)
21
22
23
24
H
H
H
4-Cl
2-Cl
3-Cl
4-Cl
3-Cl
2287 ( 1357)
6600 ( 1513)
753 ( 52)
533 ( 75)
a
Values are means of three experiments in triplicate unless indicated, SEM is in
parentheses.
ring, we commenced our investigation of the aromatic group with
a classic Topliss approach.18 The para chloro substituent (8,
Ki = 107 nM) was slightly more potent than the unsubstituted phe-
nyl analog (7, Ki = 220 nM). These results dictated the synthesis of
the 3,4-dichlorophenyl compound (10, Ki = 4467 nM) that was
much less active than either the 4-chlorophenyl or phenyl analogs.
This outcome combined with the results of the meta chloro (16,
Ki = 10,000 nM) and meta fluoro (17, Ki = 2727 nM) analogs clearly
demonstrate a deleterious effect of steric demand on the meta po-
sition of the pendant aryl ring. Increasing the lipophilicity of the
para substituent by replacing the chloro group with a trifluoro-
methyl (11, Ki = 167 nM) did not improve potency at the 5-HT7
receptor. Exploration of ortho substitution, in the form of the 2,4-
dichlorophenyl compound (12, Ki = 2067 nM), indicated that
increasing steric size in the ortho position was not tolerated. Based
on these SAR results, we focused our remaining attention on a di-
verse set of para substituents.
Both the para-substituted methyl compound (14, Ki = 35 nM)
and the 4-fluorophenyl analog (18, Ki = 45 nM) show a 5- to 6-fold
improvement in 5-HT7 potency versus the unsubstituted phenyl
ring. This improvement in affinity is relinquished if the steric bulk
of the para substituent is increased substantially, as in the 4-tert-
butyl group (15, Ki = 10,000 nM), or if the substituent is
more hydrophilic, such as the para methoxy (19, Ki = 290 nM) or
4-hydroxy analogs (20, Ki = 10,000 nM).
In an assay to determine functional activity19 a representative
set of compounds, 7 (pKb = 6.7), 8 (pKb = 7.2), 14 (pKb = 7.6) and
25 (pKb = 7.8), were determined to be high affinity antagonists of
the 5-HT7 receptor. Compound 14 was chosen for further in vitro
and in vivo profiling. Selectivity screening for off target activity
at the a1 adrenergic receptor revealed troubling potency at a1 of
5 nM. In spite of this liability, we were interested in the in vivo
properties of 14. When 14 was dosed at 10 mg/kg ip in the rat after
1 h the brain concentration was 27 lM compared to 3 lM in the
plasma. This indicated the compound was able to readily penetrate
the blood–brain barrier. This result encouraged us to test com-
pound 14 in a pharmacodynamic model of 5-HT7 activity. The
functional 5-HT7 antagonist activity of compound 14, given ip
was assessed using the 5-CT-induced hypothermia model in con-
scious rats.20 It has been shown that mice lacking the 5-HT7 recep-
tor do not experience either 5-HT- or 5-CT-induced hypothermia,1
and experimental data suggest this hypothermia is a centrally
mediated effect.1,20 Compound 14 was shown to partially, but sig-
nificantly inhibit 5-CT induced hypothermia at 10 mg/kg ip.
In conclusion, by substituting a pyrrole for a pyrazole in HTS hit
2 we were able to rapidly develop SAR around the tetrahydropyr-
ido[3,2-c]pyrrole core. Optimization of the pendant aryl ring lead
to high binding affinity at the 5-HT7 receptor when small lipophilic

44
D. A. Rudolph et al. / Bioorg. Med. Chem. Lett. 21 (2011) 42–44
15. Representative procedures for Scheme 1: To a solution of 4-oxo-piperidine-1-
carboxylic acid tert-butyl ester 3 (1 equiv) in toluene (0.65 M) was added a
benzyl amine (1 equiv) followed by SiO2 (1:1 wt to 3). This mixture was aged
overnight and treated with a nitrostyrene 4 (1 equiv) in a minimal amount of
EtOH. The reaction was stirred at rt for 24 h and then filtered through Celite
and concentrated in vacuo. The residue was purified by SiO2 chromatography
(EtOAc/Hex) to afford compounds 5. Compounds 5 were treated with 1.0 M HCl
in Et2O (10 equiv) in a mixture of Et2O and CH2Cl2 (ratio dictated by solubility)
for 12 h. In most cases the reaction mixture was filtered and the solid was
washed with cold Et2O to afford the desired compounds 6 as their HCl salts.
16. Gairaud, C. B.; Lappin, G. R. J. Org. Chem. 1953, 18, 1.
17. Receptor binding was performed using the rat recombinant 5-HT7 receptors.
The affinity of the described compounds for the rat 5-HT7 receptor subtypes
was evaluated by competitive radioligand binding assays using [3H]5-CT. The
assay was performed on membranes prepared from HEK-293 stably
transfected with r5-HT7. Following centrifugation, membranes were
resuspended and incubated for 60 min at room temperature with 1 nM
[3H]5-CT in the presence of increasing concentration of test compounds.
groups were placed in the para position. Substitution at the ortho
and meta positions along with larger or polar groups in the para po-
sition was not tolerated. Modification of the N-benzyl group and
secondary amine were not well tolerated. Further profiling of ana-
log 14 indicated off target liability at the a1 adrenergic receptor,
but promising drug-like properties resulted in excellent brain pen-
etration after ip dosing and activity in an in vivo model of func-
tional 5-HT7 antagonist activity. In summary, we have identified
a promising series of potent 5-HT7 antagonists that merit further
medicinal chemistry effort in order to minimize off target activity
while maintaining the good drug-like properties of the template,
details of which will be reported in due course.
References and notes
Nonspecific binding was defined in the presence of 10 lM 5-HT. Incubation
was stopped by rapid filtration using the cell harvester (Packard). Radioactivity
was counted in a TopCount-NXT (Packard). Sigmoidal inhibition curves were
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values (concentration producing 50% inhibition of specific radioligand binding)
were calculated. Ki values were derived according to Cheng and Prussoff
(Cheng and Prusoff, 1973). pKi = Àlog Ki. Experiments were conducted in
triplicate. Stock drug solutions (10 mM) were prepared in DMSO (the final
assay concentration of DMSO not exceeding 0.4%). Drug dilutions were
prepared in assay buffer.
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