The identification of pyrimidine-diazabicyclo[3.3.0]octane derivatives as 5-HT2C receptor agonists

Article abstract of DOI:10.1016/j.bmcl.2006.03.007

The 5-HT_2C receptor has been implicated in the regulation of appetite. As such, small molecule agonists to this receptor may serve as novel therapies to combat obesity. We describe here the identification, synthesis, and SAR of a 5-HT_2C

Full text of DOI:10.1016/j.bmcl.2006.03.007

Bioorganic & Medicinal Chemistry Letters 16 (2006) 2891–2894
The identification of pyrimidine-diazabicyclo[3.3.0]octane
derivatives as 5-HT2C receptor agonists
Bayard R. Huck,* Luis Llamas, Michael J. Robarge, Thomas C. Dent, Jianping Song,
William F. Hodnick, Chris Crumrine, Alain Stricker-Krongrad, John Harrington,
Kurt R. Brunden and Youssef L. Bennani
Athersys, Inc., 3201 Carnegie Ave., Cleveland, OH 44115, USA
Received 14 December 2005; revised 1 March 2006; accepted 2 March 2006
Available online 20 March 2006
Abstract—The 5-HT2C receptor has been implicated in the regulation of appetite. As such, small molecule agonists to this receptor
may serve as novel therapies to combat obesity. We describe here the identification, synthesis, and SAR of a 5-HT2C agonist from
a unique pyrimidine-diazabicyclo[3.3.0]octane series. This compound displayed good potency at the 5-HT2C receptor, modest
selectivity relative to other 5-HT2 receptors, and was efficacious in an acute feeding study in rats.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Obesity has become a worldwide health issue, exacerbat-
ed by its implication in the onset of cardiovascular
disease, diabetes, and cancer. The increasing prevalence
of obesity has sparked a rush in the pharmaceutical
industry to discover treatments that reduce body weight.
Specifically, much research has been focused on the
development of drugs that reduce food consumption
through action at key CNS regulators of appetite. In
addition to the approved drugs meridia and xenical,
the most advanced new drug candidate is rimonobant,
a cannabinoid receptor antagonist, which has shown
efficacy in phase III clinical trials. Clearly, there is a need
to discover other potential drug candidates that could
mitigate food consumption and contain the widespread
prevalence of obesity.
date is the achievement of sufficient selectivity over the
other 5-HT2 receptors. 5-HT2A has been implicated in
the onset of psychiatric disorders, while 5-HT2B has
been linked with cardiovascular risks.
A high-throughput screen of our compound library
(>80,000 compounds) revealed lead compound 1, which
has EC50 values of 0.1, 0.6, and 1.7 lM in 5-HT2C, 5-
HT2A, and 5-HT2B receptor functional assays,3 respec-
tively. This compound is similar to the known 5-HT2C
partial agonist mCPP 2 (Fig. 1). Unfortunately, mCPP
has a low selectivity over other serotonin receptors,
especially 5-HT2A and 5-HT2B. Our initial research
efforts were directed at improving compound potency
with the added hope of an improved level of selectivity
over other serotonin receptors. We attempted to achieve
these goals through the manipulation of the homopiper-
azine ring via the introduction of a piperazine isostere.
2,7-Diazabicyclo[3.3.0]octane (DABO) was selected be-
cause of several attractive features: two potential sites
for attachment to the pyrimidine core (2-N or 7-N),
Serotonin (5-HT) is a neurotransmitter that regulates
many important physiological processes. There are at
least 14 different G-protein coupled 5-HT receptor sub-
types. There are three 5-HT2 receptors, A–C and
5-HT2C receptors are primarily located in the brain.
5-HT2C receptor agonism has been linked to a decrease
in food consumption, and several classes of 5-HT2c
receptor agonists have been reported.1,2 A concern in
the development of a 5-HT2C agonist as a drug candi-
Keywords: 5-HT2C; Obesity.
*
Corresponding author. Tel.: +1 440 781 4300; e-mail:
brhuck@hotmail.com
Figure 1. Structures of HTS lead 1, mCPP 2, and DABO.
0960-894X/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2006.03.007

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B. R. Huck et al. / Bioorg. Med. Chem. Lett. 16 (2006) 2891–2894
the ability to introduce chirality into the compounds,
and the ease of modulation to introduce additional func-
tionality (Fig. 1).4
A racemic synthesis of DABO was initiated with the
protection of aminoacetaldehyde dimethyl acetal 3 as
the ethyl carbamate.5 The protected acetal 4 was then
alkylated with allyl bromide to provide intermediate 5,
which was subsequently converted to the desired alde-
hyde 6. A 1,3 dipolar cycloaddition of 6 and N-benzyl-
glycine provided the orthogonally protected DABO
derivative 7 (the 2-N is protected as a benzyl amine,
and the 7-N is protected as an ethyl carbamate). Of note,
enantiomerically pure DABO was subsequently synthe-
sized via the use of a chiral amine-protecting group on
the glycine derivative. Additionally, the introduction
of functionality to the DABO unit was accomplished
through the use of N-benzyl-L-alanine, rather than N-
benzylglycine (Scheme 1).
Scheme 2. Synthesis of pyrimidine-DABO derivatives. Reagents: (a)
NH4CO2H, Pd/C, EtOH; (b) pyrimidine chloride, DIEA, and CH2Cl2;
(c) 12 M HCl.
The ability of DABO to serve as a useful homopiper-
azine surrogate was explored via the synthesis of deriv-
atives in which the pyrimidine core was linked to
DABO through either the 2-N or the 7-N position.
The synthesis of derivatives that bound DABO through
the 2-N position was achieved via hydrogenating the 2-
N-benzyl amine on 7, coupling the resulting secondary
amine with an appropriate 2-chloro-pyrimidine deriva-
tive, followed by deprotecting the ethyl carbamate to
give final compounds 11–13. Alternatively, the synthesis
of derivatives with the 7-N position of DABO bound to
the pyrimidine core was achieved via ethyl carbamate
deprotection, coupling with the same 2-chloro-pyrimi-
dine derivative, and deprotection of the benzyl amine
to give compounds 8–10. Three compounds in each ser-
ies were synthesized which included either a trifluoro-
methyl group found in the lead compound or a
dihalogenated bi-aryl moiety that is commonplace in
known 5-HT2C agonists1,2 (Scheme 2).
revealed that this set of compounds is completely inac-
tive (EC50 values greater than 20 lM). However, 2-N
substitution (compounds 11–13) provided one deriva-
tive, compound 11 (EC50 = 180 nM), that displayed
moderate 5-HT2C agonism (Table 1).
The initial success of racemic DABO as a piperazine iso-
stere, in the context of 5-HT2C agonism, prompted the
analysis of the pure enantiomers of compound 11. Enan-
tiomeric compounds 14 and 15 were synthesized through
the use of a chiral (R)-methylbenzylamine-protecting
group, which allowed for silica gel chromatographic
separation of the resulting diastereomers after the 1,3
dipolar cycloaddition. Deprotection of the chiral ben-
zyl-protecting group provided enantiomerically pure
DABO. There was a marked improvement in activity
between the enantiomers of compound 11. In particular,
compound 14 (the S,S enantiomer) has an EC50 of
23 nM, which marks a fourfold increase in potency over
our HTS lead 1 (Table 2). Unfortunately, compound 14
is only slightly selective for 5-HT2C relative to 5-HT2A
(5·) and 5-HT2B (3·). This relatively poor selectivity
could result in clinical side effects related to 5-HT2A
and/or 5-HT2B interaction. Accordingly, an attempt
was made to explore the SAR through the modification
of DABO via the installation of a methyl group during
the synthesis of DABO using either N-benzyl-L- or D-al-
anine instead of N-benzylglycine. This modification of
Analysis of the compounds in which DABO is bound to
pyrimidine through the 7-N position (compounds 8–10)
Table 1. Activities of pyrimidine-DABO derivatives
Compounda
R
5-HT2C EC50 (nM)
8
9
CF3
>20,000
>20,000
>20,000
180
2,5-Di-Cl-phenyl
2,5-Di-F-phenyl
CF3
10
11
12
13
2,5-Di-Cl-phenyl
2,5-Di-F-phenyl
>1000
>1000
Scheme 1. Synthesis of DABO derivative 7. Reagents: (a) ethyl
chloroformate, NaOH, toluene, H2O; (b) allyl bromide, triethylben-
zylammonium chloride, toluene; (c) formic acid; (d) N-benzylglycine,
toluene.
a All compounds were purified by preparative HPLC and were evalu-
ated for proper identity and purity by analytical HPLC–MS and by
1H NMR.

B. R. Huck et al. / Bioorg. Med. Chem. Lett. 16 (2006) 2891–2894
Table 2. Activities of enantiomeric compounds
2893
usually 2–3, away from the aromatic ring. We hypothe-
sized that the second nitrogen, several atoms from the
aromatic ring, was absolutely necessary for potency,
but that the tertiary nitrogen bound to the aromatic ring
was not as important for agonist activity. To address the
issue of whether two nitrogens are necessary for poten-
cy, we designed a mono-amine scaffold based on the
bicyclic ring structure of DABO.
Compounda
R
Chirality 5-HT2C
5-HT2A
EC50 (nM) EC50 (nM) EC50 (nM)
5-HT2B
11
14
15
16
17
CF3 Racemic 180
220
116
320
62
CF3 (S,S)
23
519
57
CF3 (R,R)
CF3 (S,S,R)
CF3 (S,S,S)
25
40
48
151
22
a All compounds were purified by preparative HPLC and were evalu-
ated for proper identity and purity by analytical HPLC–MS and by
1H NMR.
Bicyclic scaffold 21 was synthesized in one step from
commercially available cyclopentenone via [3 + 2] cyclo-
addition with an azomethine ylide. Scaffold 21 was con-
verted to its corresponding triflate and then subjected to
a Suzuki reaction with 3-(trifluoromethyl)phenylboronic
acid. The resulting product was exposed to hydrogena-
tion conditions, which removed the benzyl group and re-
duced the alkene to give the desired final product 22
(Scheme 4). The reduction of the alkene yielded a single
diastereomer, presumably from attack from the less hin-
dered alkene face (Table 3).
DABO gave rise to compounds 16 (from L-Ala) and 17
(from D-Ala) (Fig. 2). Compound 17 (EC50 = 22 nM)
was equipotent to compound 14, but the level of selec-
tivity was still unimpressive (Table 2).
Our attention subsequently turned toward an investiga-
tion of the importance of the pyrimidine ring. A small
selection of compounds was synthesized in which the
pyrimidine ring was replaced with a phenyl ring. These
compounds (18–20) were synthesized via a Buchwald
aryl amination of the 2-N on DABO with an aryl
bromide, followed by acidic deprotection of the
tert-butyl carbamate (Scheme 3).
The racemic bicyclic single-nitrogen compound 22 was
analyzed for 5-HT2C receptor agonism and compared
with the corresponding dual nitrogen pyrimidine-DABO
compound 11 (Table 4). Although compound 11
(EC50 = 180 nM) was more than 2-fold more potent
than compound 22 (EC50 = 420 nM), it seems that the
nitrogen bound to the aromatic ring is not absolutely
necessary for 5-HT2C agonism. Thus, a properly de-
signed, chiral single-nitrogen scaffold may hold promise
as a potent 5-HT2C agonist.
An analysis of the results revealed that there is an
approximate fourfold advantage in having a pyrimidine
ring over a phenyl ring (compound 14 EC50 = 23 nM vs.
compound 18 EC50 = 103 nM). There also appears to be
a clear preference for a trifluoromethyl group in the
meta position over a halogen (Cl, compound 19,
EC50 = 503 nM; F, compound 20, EC50 = 319 nM). Of
some interest, the incorporation of a phenyl ring pro-
vides compounds that display comparable 5-HT2B activ-
ity to those containing pyrimidine.
The prevailing paradigm for 5-HT2C agonists is a need
for two nitrogens.1,2 One nitrogen is bound to the aro-
matic ring, while the second nitrogen is several atoms,
Scheme 4. Synthesis of bicyclic mono-amine derivative 22. Reagents:
(a) N-(methoxymethyl)-N-(trimethylsilylmethyl)benzyl amine, TFA,
CH2Cl2; (b) trifluoromethanesulfonic anhydride, pyridine, CH2Cl2; (c)
3-(trifluoromethyl)phenylboronic acid, Pd(PPh3)4, Na2CO3, NMP; (d)
NH4CO2H, 10% Pd/C, MeOH.
Figure 2. Structures of compound 16 and compound 17.
Table 3. Activities of phenyl-DABO derivatives
Compounda
R
5-HT2C
EC50 (nM)
5-HT2A
EC50 (nM)
5-HT2B
EC50 (nM)
14
18
19
20
CF3
CF3
Cl
23
103
503
319
116
168
62
65
416
1120
59
108
F
a All compounds were purified by preparative HPLC and were evalu-
ated for proper identity and purity by analytical HPLC–MS and by
1H NMR.
Scheme 3. Synthesis of phenyl-DABO derivatives 18–20. Reagents: (a)
Pd(dba)2, DPPF, NaO-t-Bu, toluene; (b) 4 N HCl/dioxane.

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B. R. Huck et al. / Bioorg. Med. Chem. Lett. 16 (2006) 2891–2894
Table 4. Activity of mono-amine derivative
25.0
20.0
15.0
10.0
5.0
Vehicle
Fenfluramine (3 mg/kg)
14 (1 mg/kg)
14 (3 mg/kg)
14 (10 mg/kg)
14 (20 mg/kg)
Compounda
R
5-HT2C
EC50 (nM)
5-HT2A
EC50 (nM)
5-HT2B
EC50 (nM)
*
11
22
CF3
CF3
180
420
220
1080
320
187
*
*
a All compounds were purified by preparative HPLC and were evalu-
ated for proper identity and purity by analytical HPLC–MS and by
1H NMR.
*
*
0.0
Table 5. Pharmacokinetic properties of 14 in rats
4
24
Time After Dosing (hr)
p<0.05 compared to vehicle control (ANOVA; Dunnett's)
Dose
(mg/kg)
Cmax B/P ratio
(mg/kg) AUCcortex
Cl t1/2
((mL/min)/kg) (h)
F
*
/
AUCplasma
(%)
Chart 1. Effect of 14 on food intake in rats.6
1.2 (iv) 2.930
10.6 (po) 0.052
14.5
6.71
1.1
6
its relative lack of selectivity suggests that it would
have an unacceptable safety profile due to possible
5-HT2A and/or 5-HT2B interaction. Moreover, this
compound has sub-optimal pharmacokinetic behavior.
Second-generation compounds are being developed
that focus on improving selectivity over 5-HT2A and
5-HT2B, and improving the overall pharmacokinetic
profile.
Compound 14 was subjected to additional testing to
establish its pharmacokinetic profile (Table 5). The com-
pound displayed a modest half-life of 1.1 h, but parti-
tioned into the brain nicely with a brain-to-plasma
ratio of >14. This should result in a greater compound
exposure for the brain-localized 5-HT2C receptors rela-
tive to the peripheral 5-HT2B receptors, thereby decreas-
ing the potential for side effects related to interaction
with the latter receptor. Unfortunately, the low oral bio-
availability (6%) of compound 14 was less than
desirable.
References and notes
1. Bishop, M. J.; Nilsson, B. M. Expert Opin. Ther. Patents
2003, 13, 1691.
Compound 14 was investigated in a 24 h acute feeding
study in rats. Fenfluramine, a compound known to reg-
ulate food consumption, was used as a positive control.
A dose-dependent decrease in food consumption was
observed, and compound 14 demonstrated a statistical
significance in reduction of food consumption after an
intraperitoneal dose of 20 mg/kg at both the 4 and
24 h time points (Chart 1).
2. Adams, D.; Benardeau, A.; Bickerdike, M. J.; Bentley, J.
M.; Bissantz, C.; Bourson, A.; Cliffe, I. A.; Hebeisen, P.;
Kennett, G. A.; Knight, A. R.; Malcolm, C. S.; Mizrahi, J.;
Plancher, J.-M.; Richter, H.; Rover, S.; Taylor, S.; Vickers,
S. P. Chimia 2004, 58, 613.
3. Compounds were evaluated for the ability to induce 5-HT2
receptor-mediated increases in intracellular calcium using a
fluorescence-based assay and recombinant cell lines
expressing 5-HT2C(HEK293), 5-HT2A (CHO), and 5-
HT2B (CHO) receptors.
4. Ma, Z.; Chu, D. T. W.; Cooper, C. S.; Li, Q.; Fung, A. K.
L.; Wang, S.; Shen, L. L.; Flamm, R. K.; Nilius, A. M.;
Alder, J. D.; Meulbroek, J. A.; Or, Y. S. J. Med. Chem.
1999, 42, 4202.
5. Schenke, T.; Peterson, U. U.S. Patent 5,071,999, 1991.
6. Male Sprague–Dawley rats were injected with either vehi-
cle, fenfluramine or ATH-88895 (n = 10/group) approxi-
mately 30 min before the animals entered their 12 h dark
cycle. Food consumption was measured 4 and 24 h later.
In summary, a potent 5-HT2C agonist was designed
and synthesized based on a lead compound from
HTS. Crucial to the success of this endeavor was
the use of diazabicyclo[3.3.0]octane, which served as
a suitable piperazine isostere when connected to the
pyrimidine core through the 2-N. Compound 14 is rel-
atively potent against 5-HT2C and was proven to be
efficacious at 20 mg/kg in an acute feeding study.
While compound 14 serves as an interesting prototype,