Novel pyrazolopiperazinone- and pyrrolopiperazinone-based MCH-R1 antagonists

Article abstract of DOI:10.1016/j.bmcl.2006.10.096

The synthesis and biological testing of novel classes of potent melanin-concentrating hormone (MCH-R1) antagonists based on pyrazolopiperazinone and pyrrolopiperazinone scaffolds are described.

Full text of DOI:10.1016/j.bmcl.2006.10.096

Bioorganic & Medicinal Chemistry Letters 17 (2007) 657–661
Novel pyrazolopiperazinone- and pyrrolopiperazinone-based
MCH-R1 antagonists
ꢀ
ꢁ
*
´
´
Kenneth M. Meyers, Jose Mendez-Andino, Anny-Odile Colson, X. Eric Hu,
John A. Wos, Maria C. Mitchell, Karen Hodge, Jeremy Howard, Jennifer L. Paris,
Martin E. Dowty, Cindy M. Obringer and Ofer Reizes§
Procter and Gamble Pharmaceuticals, 8700 Mason-Montgomery Road, Mason, OH 45039, USA
Received 11 September 2006; revised 30 October 2006; accepted 31 October 2006
Available online 6 November 2006
Abstract—The synthesis and biological testing of novel classes of potent melanin-concentrating hormone (MCH-R1) antagonists
based on pyrazolopiperazinone and pyrrolopiperazinone scaffolds are described.
Ó 2006 Elsevier Ltd. All rights reserved.
The World Health Organization considers obesity a
major health concern for many countries, especially
the United States. Estimates indicate that more than
30% of the US adult population is overweight, and that
obesity-related health care costs are in the range of $100
billion per year.1 These trends have promoted increased
research activity in the pharmaceutical industry directed
to find anti-obesity therapies.2
(Ki = 11 7 nM).6 But despite its high in vitro MCH
antagonist activity, the compound did not promote
any weight loss in a mouse Diet Induced Obesity study
(mDIO). We hypothesized that compound 1 was not ac-
tive in vivo likely due to insufficient brain exposure. In
addition, we hypothesized that the hydrophilic nature
of 1 (water solubility >45 mg/mL at pH 7.4) could have
contributed to its inability to effectively cross the blood–
brain barrier. This led us to consider making com-
pounds with increased lipophilicity to address potential
The G-protein-coupled receptor MCH-R1 has received
significant attention in recent years as a potential target
for effective anti-obesity therapy.3 It has been suggested
that CNS-located MCH-R1 is involved in biological
processes related to mammalian feeding behaviors and
energy expenditure.4 Identification of a small molecule
MCH-R1 antagonist is being heavily pursued by many
laboratories trying to find an effective drug-molecule
that may be effective for the treatment of obesity.5
O
N
N
N
1
Y
O
O
N
N
N
N
Recently, we have disclosed phenethyl ketopiperazine
compound 1 as a potent MCH-R1 antagonist in vitro
X
R
2
O
Y
Keywords: Melanin-concentrating hormone; MCH-R1 antagonists;
5HT-2C receptor; Obesity; Pyrazolopiperazinone; Pyrrolopiperazinone.
O
N
N
*
Corresponding author. Tel.: +1 513 622 0304; fax: +1 513 622
1433; e-mail: mendezandino.jl@pg.com
N
R
X
3
ꢀ
Present address: Boehringer Ingelheim Pharmaceuticals, 900 Ridge-
bury Road, Ridgefield, CT 06877, USA.
Y = dimethylamino,diethylamino,piperidyl
R = H or Me
ꢁ
Present address: Genome Research Institute, University of Cincin-
nati, 2180 E. Galbraith Road, Cincinnati, OH 45237, USA.
Present address: Lerner Research Institute, Cleveland Clinic Foun-
dation, 9500 Euclid Avenue-NC10, Cleveland, OH 44195, USA.
§
Figure 1. MCH-R1 antagonist 1, and alternatives based on pyrazolo-
piperazinone 2 and pyrrolopiperazinone 3 scaffolds.
0960-894X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2006.10.096

658
K. M. Meyers et al. / Bioorg. Med. Chem. Lett. 17 (2007) 657–661
blood–brain barrier penetration issues associated with
compound 1.
HOBT gave amide compounds 5. Reduction of the
amide with BH3 Æ THF followed by Boc-deprotection
with trifluoroacetic acid gave diamine compounds 6.
Condensation with 78 under basic conditions gave pyr-
azolopiperazinone compounds 8. Ester saponification
followed by amide formation yielded final compounds
2a–g.
We have identified pyrazolopiperazinone 2 and pyrrolo-
piperazinone 3 scaffolds as alternatives to 1 (Fig. 1).
These scaffolds were derived from 1 by fusing a pyrazole
or pyrrole ring with the ketopiperazine moiety. As a re-
sult, 2 and 3 should have decreased solubility compared
to 1 due to increased lipophilic character and decreased
pKa of the ketopiperazine nitrogen. We predicted that
scaffold 2 would lead to potent MCH-R1 binders be-
cause key interactions observed between the dimethyl-
amine analog of 1 and the MCH-R1 homology model7
were conserved with 2 (Fig. 2).
In vitro MCH-R1 and 5HT2C binding data for com-
pounds 2a–g are summarized in Table 1.9,10 We desired
selectivity over 5HT2C due to the relatively high homol-
ogy of the receptor with MCH-R1 and its suggested role
in the modulation of feeding behavior that could inter-
fere with effects related to inhibition of MCH-R1. Simple
variations at the tertiary alkyl amine position did not sig-
nificantly affect the MCH-R1 activity. These results were
consistent with our findings in the earlier ketopiperazine
(1) series.6 Fluorine substituted aniline compounds 2a–c
were more potent MCH-R1 binders than trifluoromethyl
substituted aniline compounds 2d–e. Interestingly, in
both the fluoro-aniline and trifluoromethyl-aniline ana-
log pairs, the meta-substituted compound displayed
higher in vitro activity than the corresponding para-
substituted analog. The substitution of a methyl group
for hydrogen at the amide position decreased MCH
in vitro activity for reasons unclear to us.
Pyrazolopiperazinone analogs 2a–g were synthesized
according to Scheme 1. Coupling of 4-(2-Boc-aminoeth-
yl)benzoic acid 4 and secondary amines with EDCI and
R284
R284
Q279
F289
Q279
Compounds 2a–b and 2f–g were selective for MCH-R1
over 5HT2C (Table 1). Compound 2b displayed good
MCH-R1 activity, high selectivity over 5HT2C and
decreased water solubility (26.1 mg/mL at pH 7.4)
compared to 1. However, this compound was not effica-
cious in our 4-day mDIO weight loss model likely due to
limited blood–brain barrier penetration. This hypothesis
was supported by Caco-2 cell assay11 data identifying 2b
as an efflux substrate (A À B/B À A = 4.1/21).12
R197
R197
D123
The analogous pyrrolopiperazinones were prepared
according to Scheme 2. Aldehyde 913 was oxidized to
the acid with KMnO4 and converted to methyl ester
10. Alkylation with 1,2-dibromoethane gave pyrrole
compound 11. Unlike the pyrazolopiperazinone synthe-
Figure 2. Key interactions observed between the dimethylamine
analog of 1 and MCH-R1 homology model (left). Analog of 1 (green)
and 2c (red) in MCH-R1 homology model (right).
O
OEt
O
O
b
Y
a
OH
Y
N
O
N
BOC
BOC
H2N
N
H
N
OEt
Y
H
Br
4
5
6
7
O
O
Y
c
d
O
N
O
X
N
N
N
N
R
N
N
EtO
8
2a-g
Y = dimethylamino, diethylamino, piperidyl
R = H or Me
Scheme 1. Reagents and conditions: (a) secondary amine, EDCI, HOBT, NMM, DMF, rt, 81–89%; (b) i—BH3 Æ THF, 65 °C, 63–81%; ii—
trifluoroacetic acid, CH2Cl2, rt, 91–99%; (c) K2CO3, CH3CN, 80 °C, 68–82%; (d) i—LiOH, THF, H2O, rt, 90–98%; ii—corresponding aniline,
PyBop, N,N-diisopropylethylamine, DMF, rt, 58–76%.

K. M. Meyers et al. / Bioorg. Med. Chem. Lett. 17 (2007) 657–661
Table 1. MCH-R1 in vitro activity for compounds 2a–f9,10
659
Compound
R
X
Y
MCH-R1 Binding Ki (nM)10
5HT2C Ki (lM)
2a
2b
2c
2d
2e
2f
H
3-F
4-F
Piperidyl
22
78
>100
>100
—
H
Diethylamino
Dimethylamino
Diethylamino
Diethylamino
Piperidyl
H
4-F
82
H
3-CF3
4-CF3
4-F
497
2482
5294
NA
—
—
H
Me
Me
>100
>100
2g
H
Piperidyl
Br
H
N
H
O
N
O
N
a
b
O
N
H
O
MeO
OMe
OMe
H2N
MeO
OMe
O
O
9
12
10
11
O
Y
O
N
O
N
X
c
O
d
N
N
N
N
HO
R2
3a R = H, X = 4-F, Y = piperidyl
3b R = Me, X = H, Y = piperidyl
13
Scheme 2. Reagents and conditions: (a) i—KMnO4, acetone, H2O, rt, 88%; ii—TMSCl, MeOH, rt, 94%; (b) 1,2-dibromoethane, NaH, DMF, 65 °C,
70%; (c) NaH, CH3CN, 80 °C; then H2O, 37–46%; (d) aniline, PyBop, N,N-diisopropylethylamine, DMF, rt, 61–73%.
O
N
O
N
sis, condensation of 11 and 12 required the stronger base
sodium hydride. Upon aqueous workup, acid 13 was
isolated. Amide formation with PyBoP yielded the final
compounds 3a–b.
O
N
N
N
N
14
15
F
F
Compound 3a had a MCH Ki of 620 nM and 3b was inac-
tive in our in vitro binding assay. Similar to the pyrazol-
opiperazinone series, N-methyl amide 3b had decreased
activity compared to secondary amide 3a. Comparison
across scaffolds revealed that substitution of the pyrazole
moiety by a pyrrole resulted in decreased activity, there-
fore suggesting that the second nitrogen of the pyrazole
moiety interacts favorably with the receptor.
N
O
O
N
N
N
N
F
F
N
16
17
Figure 3. Amide linker replacements in the pyrrolopiperazinone series.
In an attempt to improve the in vitro efficacy of the pyr-
rolopiperazinone series, we examined the role of the
amide linker between the pyrrole and the halogenated
aromatic ring. Based on structure 1 and its high potency,
we hypothesized that the amide linker was not essential
for biological activity. Modifications to the amide linker
were performed in order to test this hypothesis (Fig. 3).
Br
H
O
N
c
O
b
a
N
F
15
14
OEt
OEt
O
19
18
Scheme 3. Reagents and conditions: (a) i—4-fluorobenzoyl chloride,
AlCl3, 1,2-dichloroethane À20 °C, 71%; ii—1,2-dibromoethane, NaH,
DMF, 65 °C, 58%; (b) 12, NaH, CH3CN, 80 °C, 51%; (c) triethylsilane,
trifluoroacetic acid, rt, 82%.
The synthesis of 14 and 15 is outlined in Scheme 3. Fri-
edel–Crafts acylation of ethyl-pyrrole-2-carboxylate 18
with 4-fluorobenzoyl chloride followed by N-alkylation
with 1,2-dibromoethane gave 19. Condensation with
12 yielded ketone linked 14. Ketone reduction with tri-
ethylsilane in trifluoroacetic acid14 gave methylene
linked variant 15.
chloride 20 gave a mixture of alkene isomers that was
reduced with hydrogen in the presence of Pd/C to yield
21. Alkylation of the pyrrole nitrogen with 1,2-di-
bromoethane followed by condensation with 12 in the
presence of sodium hydride gave 16.
The synthesis of 16 is outlined in Scheme 4. Wittig olef-
ination of 9 with 4-fluorobenzyl triphenylphosphonium

660
K. M. Meyers et al. / Bioorg. Med. Chem. Lett. 17 (2007) 657–661
NH
antagonists in vitro. Despite an attractive in vitro pro-
O
PPh3Cl
a
b
file, the lead pyrazolopiperazinone compound 2b was
not active in vivo likely due to limited blood–brain
barrier penetration. Substitution of the pyrazole moiety
with pyrrole led to decreased MCH-R1 activity.
Through modification of the amide linker between the
4-fluorophenyl and the pyrrole rings in the pyrrolopiper-
azinone series, potency similar to that of pyrazolopiper-
azinones was achieved. Unfortunately, these changes led
16
OMe
F
F
20
21
Scheme 4. Reagents and conditions: (a) i—KHMDS, THF, 0 °C; 9, rt;
ii—H2, Pd/C, MeOH, rt, 73% two steps; (b) i—1,2-dibromoethane,
NaH, DMF, 65 °C, 67%; ii—12, NaH, CH3CN, 80 °C, 41–47%.
to significant losses in MCH-R1 selectivity over 5HT2C
.
The synthesis of 17 is outlined in Scheme 5. Bromination
of ethyl-pyrrole-2-carboxylate 18 gave a mixture of
4-bromo, 5-bromo, and 4,5-dibromo products, which
was easily separable employing reverse-phase prepara-
tive HLPC. 4-Bromo product 22 was N-alkylated with
1,2-dibromoethane and condensed with 12 in the pres-
ence of sodium hydride to give compound 23. Suzuki
coupling of 23 and 4-fluorophenyl boronic acid yielded
final compound 17.
References and notes
1. The Surgeon General’s Call to Action to Prevent and
Decrease Overweight and Obesity 2001, Public Health
Service: Office of the Surgeon General, US Department of
Health and Human Sevices: Rockville, MD, 2001;
www.surgeongeneral.gov/topics/obesity.
The MCH-R1 in vitro activity of 3a–b and 14–17 is
shown in Table 2. Through linker modifications in the
pyrrolopiperazinone series, MCH-R1 binding activity
comparable to the pyrazolopiperazinones was achieved.
This suggests that the detrimental effect of switching the
pyrazole to pyrrole may be circumvented by modifica-
tions to the linker between the 4-fluorophenyl and pyr-
rolopiperazinone rings. Reduction of ketone 14 to
methylene 15 did not significantly affect MCH activity.
Chain-length changes resulted in the most potent
MCH binders of the pyrrolopiperazinone series (16
and 17). These results suggest that the overall orienta-
tion plays a more significant role for binding than the
polar character of the linker. In addition, these results
are in agreement with our hypothesis that the amide of
3a is not required for high-affinity binding with MCH-
R1. However, removal of the amide functionality be-
tween the pyrrolopiperazinone and 4-fluorophenyl rings
resulted in complete loss of MCH-R1 selectivity over
5HT2C (compounds 14–17).
2. (a) Powell, D. R. Obesity reviews 2006, 7, 89; (b) Fong, T.
M. Expert Opin. Investig. Drugs 2005, 14, 243; (c) Heal, D.
J.; Rowley, H. L.; Jackson, H. C. Progress in obesity
research 2003, 9, 260.
3. (a) Dyke, H. J.; Ray, N. C. Expert Opin. Ther. Patents
2005, 15, 1303; (b) Carpenter, A. J.; Hertzog, D. L. Expert
Opin. Ther. Patents 2002, 12, 1639.
4. (a) Kowalski, T. J.; McBriar, M. D. Expert Opin.
Investig. Drugs 2004, 13, 1113; (b) Receveur, J. M.;
Bjurling, E.; Ulven, T.; Little, P. B.; Norregaard, P. K.;
Hogberg, T. Bioorg. Med. Chem. Lett. 2004, 14, 5075;
(c) Shearman, L. P.; Camacho, R. E.; Sloan, S. D.;
Zhou, D.; Bednarek, M. A.; Hreniuk, D. L.; Feighner,
S. D.; Tan, C. P.; Howard, A. D.; Van der Ploeg, L. H.;
MacIntyre, D. E.; Hickey, G. J.; Strack, A. M. Eur.
J. Pharmacol. 2003, 475, 37.
5. (a) Kanuma, K.; Omodera, K.; Nishiguchi, M.; Funako-
shi, T.; Chki, S.; Semple, G.; Tran, T.-A.; Kramer, B.; Hsu,
D.; Casper, M.; Thomsen, B.; Sekiguchi, Y. Bioorg. Med.
Chem. Lett. 2005, 15, 3853; (b) Souers, A. J.; Gao, J.;
Brune, M.; Bush, E.; Wodka, D.; Vasudevan, A.; Judd, A.
S.; Mulhern, M.; Brodjian, S.; Dayton, B.; Shapiro, R.;
Hernandez, L. E.; Marsh, K. C.; Sham, H. L.; Collins, C.
A.; Kym, P. R. J. Med. Chem. 2005, 48, 1318; (c) Ulven, T.;
Little, P. B.; Receveur, J.-M.; Frimurer, T. M.; Rist, Ø.;
Nørregaard, P. K.; Ho¨gberg, T. Bioorg. Med. Chem. Lett.
2006, 16, 1070; (d) Ulven, T.; Frimurer, T. M.; Receveur,
J.-M.; Little, P. B.; Rist, Ø.; Nørregaard, P. K.; Ho¨gberg,
T. J. Med. Chem. 2005, 48, 5684; (e) Palani, A.; Shapiro, S.;
McBriar, M. D.; Clader, J. W.; Greenlee, W. J.; Spar, B.;
Kowalski, T. J.; Farley, C.; Cook, J.; van Heek, M.; Weig,
B.; O’Neill, K.; Graziano, M.; Hawes, B. E. J. Med. Chem.
2005, 48, 4746; (f) Huang, C. Q.; Baker, T.; Schwarz, D.;
Fan, J.; Heise, C. E.; Zhang, M.; Goodfellow, V. S.;
Markison, S.; Gogas, K. R.; Chen, T.; Wang, X.-C.; Zhu,
Y.-F. Bioorg. Med. Chem. Lett. 2005, 15, 3701.
In summary, we have identified pyrazolopiperazinone
and pyrrolopiperazinone scaffolds as potent MCH-R1
O
N
H
a
N
O
N
c
b
Br
17
18
N
OMe
Br
22
23
Scheme 5. Reagents and conditions: (a) i—Br2, CCl4, 0 °C, 53%; (b)
i—1,2-dibromoethane, NaH, DMF, 65 °C, 71%; ii—12, NaH,
CH3CN, 80 °C, 48–53%; (c) 4-fluorophenyl boronic acid, Pd(PPh3)4,
Na2CO3, H2O, DMF, 100 °C, 80–83%.
´
6. Mendez-Andino, J. L.; Colson, A.-O.; Meyers, K. M.;
Mitchell, M. C.; Hodge, K. M.; Howard, J. M.; Ackley, D.
C.; Holbert, J. K.; Mittelstadt, S. W.; Dowty, M. E.;
Obringer, C. M.; Suchanek, P; Reizes, O.; Hu, X. E.; Wos,
J. A. Bioorg. Med. Chem., submitted for publication.
7. (a) Mieling, G. E.; Mieling, K. K.; Bush, R. D.; Colson,
A.-O. US 2005170433 A1 2005; (b) Colson, A.-O.;
Perlman, J. H.; Smolyar, A.; Gershengorn, M. C.; Osman,
R. Biophys. J. 1998, 74, 1087.
Table 2. MCH-R1 in vitro activity for compounds 3a–b and14–17
Compound
MCH-R1 Ki (nM)
5HT2C Ki (lM)
3a
3b
14
15
16
17
625
NA
148
228
15
>100
—
5.3
0.7
2.5
0.1
8. Askew, B. C.; McIntyre, C. J.; Hunt, C. A.; Claremon, D.
A.; Gould, R. J.; Lynch, R. J.; Armstrong, D. J. Bioorg.
Med. Chem. Lett. 1995, 5, 475.
23

K. M. Meyers et al. / Bioorg. Med. Chem. Lett. 17 (2007) 657–661
661
9. MCH binding using a Flashplate radioligand binding
assay was performed by Perkin-Elmer Biosignal (Toronto,
Ontario). Full competition curves were generated with
compound concentration varied from 100 lM to 11 fM.
Potency (Ki) and maximal efficacy were determined and
used to define structure–activity relationship.
(Millipore). Bound radioactive mesulergine was detected
using a scintillation counter.
11. Artursson, P.; Palm, K.; Luthman, K. Advanced Drug
Delivery Reviews 2001, 46, 27.
12. (a) Within our MCH program, we observed that com-
pounds predicted to transport passively across the blood–
brain barrier in the Caco-2 assay usually promoted
statistically significant weight loss in our 4-day mDIO
model, and displayed [brain]/[serum] > 2. Compounds
predicted efflux substrates did not show in vivo activity
and displayed [brain]/[serum] < 1; (b) Please refer to:
10. 5HT2C assay was performed using a membrane prepara-
tion from cells that over-express the receptor. Membranes
were purchased from Euroscreen (ES-318-M) and used
according to established protocols. Briefly, membranes
were incubated in a reaction mixture containing varying
concentrations of the compound and [3H] mesulergine
(final concentration 0.33 nM; Amersham), a compound
known to bind to the 5HT2C receptor. The binding buffer
contained 50 mM Tris–HCl, 0.1% Ascorbic Acid, 5 mM
CaCl2, and 10 lg/ml Saponin. Nonspecific binding was
assessed by incubating the membranes with [3H] mesuler-
gine and excess 10 lM mianserin (ICN). After incubating
the mixture at room temperature for 60 min, bound
mesulergine was separated from unbound mesulergine by
filtration through glass fiber B filter (GF/B) plates
´
Meyers, K. M.; Mendez-Andino, J. L.; Colson, A. O.;
Warshakoon, N. C.; Wos, J. A.; Mitchell, M. C.; Hodge,
K. M.; Howard, J. M.; Ackley, D. C; Holbert, J. K.;
Mittelstadt, S. W.; Dowty, M. E.; Obringer, C. M.; Ofer
Reizes, O.; Hu, X. E. Bioorg. Med. Chem. Lett. 2006, 16,
in press.
13. Barker, P.; Gendler, R.; Rapoport, H. J. Org. Chem. 1978,
43, 4849.
14. West, C. T.; Donnelly, S. J.; Kooistra, D. A.; Doyle, M. P.
J. Org. Chem. 1973, 15, 2675.