2,5-Disubstituted pyridines: The discovery of a novel series of 5-HT2A ligands

Article abstract of DOI:10.1016/j.bmcl.2007.01.098

This report describes the effect of replacing the central basic amine present in many known 5-HT_2A ligands with an aromatic residue. We targeted the isomeric phenethylpyridines 2 and 3 and these compounds proved to be excellent leads, possessin

Full text of DOI:10.1016/j.bmcl.2007.01.098

Bioorganic & Medicinal Chemistry Letters 17 (2007) 2643–2648
2,5-Disubstituted pyridines: The discovery of a novel
series of 5-HT2A ligands
Kevin J. Wilson,* Monique B. van Niel, Laura Cooper, Dawn Bloomfield,
Desmond O’Connor, L. Rebecca Fish and Angus M. MacLeod
The Neuroscience Research Centre, Merck Sharp and Dohme, Terlings Park, Harlow, Essex CM20 2QR, UK
Received 13 December 2006; revised 29 January 2007; accepted 30 January 2007
Available online 2 February 2007
Abstract—This report describes the effect of replacing the central basic amine present in many known 5-HT2A ligands with an aro-
matic residue. We targeted the isomeric phenethylpyridines 2 and 3 and these compounds proved to be excellent leads, possessing
good 5-HT2A receptor binding affinity and selectivity over the 5-HT2C subtype. Optimization of one isomer led to the identification
of 25, a compound with sub-nanomolar 5-HT2A affinity and selectivity over 5-HT2C of greater than 4600-fold.
ꢀ 2007 Elsevier Ltd. All rights reserved.
A recent meta-analysis of epidemiological studies on
insomnia indicated that between 30% and 48% of the
global population find difficulty in initiating or main-
taining sleep.1 Moreover, a significant proportion of
people (9–15%) exhibit insomnia symptoms that are
accompanied by daytime consequences. These effects
range from sleepiness, irritability, and memory impair-
ment to clinically significant anxiety and depression.2
There is also a higher incidence of serious accidents
among insomniacs, along with increased job absenteeism
and health care costs.3 All currently approved
prescription remedies for the condition are sedative hyp-
notic agents that act by allosterically modulating the
GABA-A ion channel. Benzodiazepines are one such class
of compounds that have been in use since the 1960s.
Although they significantly increase sleep bout duration
they are also associated with numerous side effects.4 These
include withdrawal reactions, daytime drowsiness, and
cognitive impairment—effects not unlike the symptoms
of insomnia. Newer, non-benzodiazepine hypnotic
agents, including Ambienꢁ and Lunestaꢁ, have addressed
some of these issues. However, they are listed by the
DEA under Schedule IV as they retain potential for
abuse.5 Hence an unmet medical need persists for novel,
non-sedating therapies that lack abuse potential.
Selective antagonism of the 5-HT2A receptor has
emerged as a promising new mechanism for the treat-
ment of insomnia. In fact, the non-selective 5-HT2
antagonist Ritanserin was first shown to enhance slow-
wave sleep (SWS), a component of deep sleep, in
humans 20 years ago.6 Much more recently, the bench-
mark selective 5-HT2A antagonist M100907 (Fig. 1) has
been shown to increase SWS and total sleep time, and
reduce the time spent awake after sleep onset (WASO)
in elderly patients.7 In-house preclinical studies with rats
fitted with telemetry devices showed that our proprietary
selective antagonist 1 (Fig. 1) significantly enhanced
SWS and reduced the number of awakenings without
effecting REM bout duration.8 No effect was observed
F
F
N
N
N
MeO
NC
OMeOH
NH
O
EMD 281014
F
M100907
F
N
1
F
F
S
O2
Keywords: 5-HT2A ligand; Pyridine; Styrene.
*
Corresponding author. Tel.: +1 617 992 2066; e-mail:
kevin_wilson2@merck.com
Figure 1. Selected 5-HT2A antagonists.
0960-894X/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.01.098

2644
K. J. Wilson et al. / Bioorg. Med. Chem. Lett. 17 (2007) 2643–2648
on sleep onset latency, strongly suggesting that sleep
architecture can be beneficially modified without induc-
ing somnolence. In addition, selective 5-HT2A antago-
nists should lack the adverse effects associated with
GABA-receptor pharmacology, such as memory loss
and residual sedation.
ion channel (Table 1). We were gratified to see the high
affinity and selectivity for the target receptor of these
new pyridine ligands. The 2-sulfonylpyridine isomers
(2 and 7) showed a small increase in binding affinity
for the hERG channel. Blockade of this ion channel
in vivo can lead to prolongation of the QTc interval
and ultimately cardiac arrhythmia. Hence the improved
off-target profile exhibited by 3 and 10 prompted us to
focus on the 5-sulfonylpyridine isomer for further struc-
tural modifications.
The focus of our ongoing program had centered on the
4-sulfonylpiperidine series, exemplified by 1, which we
have previously disclosed.8,9 Recent work in our labora-
tory identified a series of high affinity non-basic piperi-
dines.10 These ligands are structurally related to
‘‘linear’’ antagonists such as M100907, EMD 281014
(Fig. 1), and our own 4-sulfonylpiperidines but lack
the central ammonium ion previously considered to be
an essential recognition feature. We hoped to further
extend existing pharmacophore models11 by replacing
the central cyclic amine with an aromatic unit. Hence
we targeted the isomeric sulfonylpyridines 2 and 3 as
potential new leads.12
The decision was made to concentrate on the styryl side-
chain since 10 had improved selectivity over 3 and was
significantly more stable when incubated with rat (turn-
over: 29% and 83%, respectively) and human (0% and
18%, respectively) liver microsomes. We were able to
efficiently explore the SAR of the distal aromatic units
by constructing a small series of 2-chloro-5-sulfonylpyri-
dines 9 (Scheme 2, steps a and b). The 4-fluoro and
2,4-difluorostyrene15 side-chains were accessible via the
Suzuki chemistry described above (Scheme 2, step d).
All attempts to react 9 directly with substituted styrenes
according to various Heck protocols proved unsuccess-
ful. Alternatively, a vinyl group was installed via a
microwave assisted Stille coupling with tribu-
tyl(vinyl)stannane, yielding 11. A ‘‘reverse’’ Heck cou-
pling was then employed to vary the side-chain
aromatic, again using microwave acceleration (Scheme
3).
Both isomers were accessed from 2-chloro-5-iodopyri-
dine 4 following analogous synthetic routes differing
only in the first step (Schemes 1 and 2). Thus, for 2, sul-
fide 5 was generated by nucleophilic displacement of the
2-chloro substituent. The complementary sulfide 8
(R1 = 4-F) was constructed via a copper-catalyzed Ull-
man reaction of the 5-iodo group.13 Oxidation to the
halo sulfonylpyridines 6 and 9 (R1 = 4-F) was carried
out using either MCPBA or Oxoneꢁ. Subsequent incor-
poration of the styryl side-chain via a microwave14
assisted Suzuki protocol provided 7 and 10 (R1 = 4-F,
R2 = H). Finally, hydrogenation of the olefins gave the
target phenethylpyridines 2 and 3.
Binding affinities of a range of compounds are shown in
Table 1. Moving the fluorine atom from the 4-position
of the phenylsulfone to the 3- and 2-positions or remov-
ing it altogether resulted in a 2- to 3-fold increase in
5-HT2A receptor binding relative to 10 (compounds
12, 13, and 14). The very low binding affinities for the
5-HT2C subtype and hERG ion channel were retained.
The 2-fluorostyrene side-chain imparted further
The binding affinities of compounds 2, 3, 7, and 10 were
measured for the human 5-HT2A receptor, related
GPCRs (h5-HT2C and hD2), and the hERG potassium
I
F
I
a
N
N
Cl
S
4
5
b
F
F
I
c
F
N
N
S
S
O2
O2
7
6
d
F
F
N
S
O2
2
Scheme 1. Reagents and conditions: (a) ArSH, K2CO3, MeCN, reflux; (b) oxone, MeOH, rt; (c) [(E)-2-(4-fluorophenyl)vinyl]boronic acid,
Pd(PPh3)4, THF, 2 N Na2CO3, 150 ꢂC, 10 min, microwave reactor; (d) H2 (50 psi), EtOAc, AcOH, Pd/C.

K. J. Wilson et al. / Bioorg. Med. Chem. Lett. 17 (2007) 2643–2648
2645
N
Cl
R1
N
Cl
a
I
S
4
8
b or c
F
R1
R1
N
Cl
d
N
R2
S
S
O2
O2
10, 12-14, 18, 25
9
R1 = 4-F, R2 = H
e
F
F
N
S
O2
3
Scheme 2. Reagents and conditions: (a) ArSH, K2CO3, CuI, ethylene glycol, IPA, reflux; (b) MCPBA, DCM, rt; (c) oxone, MeOH, rt; (d) [(E)-2-(4-
fluorophenyl)vinyl]boronic acid or [(E)-2-(2,4-difluorophenyl)vinyl]boronic acid, Pd(PPh3)4, THF, 2 N Na2CO3, 150 ꢂC, 10 min, microwave reactor;
(e) H2 (1 atm), AcOH, PtO2.
improvements in 5-HT2A affinity (compound 15), giving
rise to a compound with selectivity in excess of 1500-fold
and lacking hERG binding. The 3-fluorostyrene analog
16 and heterocycles such as pyridine 17 gave a signifi-
cant reduction in potency.
dines in the hope that the more polar sulfoxide unit
would improve the physical properties of the compounds.
The synthetic route is analogous to that of the sulfones
except that only one equivalent of MCPBA is used
for the oxidation step to provide the complementary
racemic chloropyridine intermediates 26 (Scheme 4). No
effect on 5-HT2A binding resulted from the change in
oxidation state (Table 2). Hence affinities of the 5-sulfinyl-
pyridines 27–32 were almost identical to the correspond-
ing analogs from the parent series. However, a dramatic
increase in 5-HT2C affinity was consistently observed,
resulting in diminished selectivity. For example 30, the
sulfoxide analog of 25, showed only a 38-fold separation
over 5-HT2C equating to a decrease of at least two orders
of magnitude. We hoped that resolution of 30 would
furnish a more selective compound but, as shown with
31 and 32, the high affinity for both receptor subtypes
resided with the same enantiomer. An additional draw-
back of the sulfoxides, with the exception of 28, was a
substantial increase in hERG binding.
Contrary to the lack of 5-HT2C binding observed for
4- and 2-fluorostyrenes 13 and 15, an order of magnitude
increase in potency was observed for 18 bearing both
fluorine atoms. Further losses in selectivity were found
with 2-chloro-4-fluorostyrene 19 (36-fold), 2-methyl-4-
fluorostyrene 20 (27-fold), and 2-cyano-4-fluorostyrene
21 (2-fold). In contrast, the 2-hydroxy-4-fluoro analog
22 was found to be especially selective for the 5-HT2A
receptor (3000-fold) by virtue of its sub-nanomolar
binding to this subtype. Removal of the 4-fluoro substitu-
ent to give 2-hydroxystyrene 23 resulted in an order of
magnitude drop in 5-HT2A affinity and selectivity. Cap-
ping the hydroxy with a methyl group was poorly tolerat-
ed, illustrated by the much reduced affinity of 24. All
oxygenated styrene analogs had prohibitive hERG
binding.
Having identified 25 as a potent and selective 5-HT2A li-
gand we now wished to measure the pharmacodynamic
response of this analog in vivo. To investigate the effect
of the sulfur oxidation state we also assayed 30 despite
the shortcomings of this series. The compounds were
dosed in rats at 5 mg/kg po. with co-administration of
a radiolabeled selective 5-HT2A ligand.16 Receptor occu-
pancy17 and drug concentrations in both plasma and
brain were determined at 1 h (Table 3). Using this pro-
tocol we were able to determine brain permeability
and gain some indication of pharmacokinetic parame-
ters and exposure-occupancy relationships.
Interestingly, the loss in selectivity observed for 2,4-di-
substituted styrenes 18–21 was not seen for 2,4-difluoro-
styrene 25 lacking a substituent on the phenylsulfone. In
contrast, a significant enhancement of 5-HT2A binding
was achieved while maintaining the lack of affinity for
the 5-HT2C subtype. Consequently 25 was found to pos-
sess the best in vitro data with an excellent off-target
profile (hERG binding: 27% inh at 10 lM) and excep-
tionally high selectivity over 5-HT2C (>4600-fold). These
data are indicative of a lack of additivity between the
substituents on the phenylsulfone and styrene units.
The sulfone proved to be inferior to the corresponding
sulfoxide, with 30 attaining 3-fold higher plasma levels,
6-fold higher brain levels, and 2-fold higher occupancy.
During the course of our studies with the sulfones we be-
came interested in profiling the related 5-sulfinylpyri-

2646
K. J. Wilson et al. / Bioorg. Med. Chem. Lett. 17 (2007) 2643–2648
Table 1. Binding affinities of the sulfonylpyridines
R
N
Ar
S
O2
Compound
R
Ar
h5-HT2A Ki (nM)a
h5-HT2C Ki (nM)a
2C/2A ratio
hD2 Ki (nM)a
hERG Ki (nM)a
M100907b
0.31
0.39
19
13
180
42
460
1300
310
1100
5600
6900
8200
3300
1
2
3
7
>4000
2700
>4000
>210
120
>140
>1000
>1000
>1000
23
28
F
F
F
10
12
13
4-F
3-F
2-F
17
>4000
>4000
>4000
>230
>420
>740
>1000
>1000
>1000
9500
>9000
>9000
9.6
5.4
F
14
15
H
7.4
2.6
>4000
>4000
>540
>1000
>2000
>9000
>9000
2-F
>1500
F
16
17
2-F
2-F
150
790
>4000
>4000
>27
>5
>2000
>2000
>9000
4200
F
F
N
18
19
20
2-F
2-F
2-F
2.6
1.8
8.2
410
64
160
36
>1000
>2000
>2000
>9000
4800
F
F
F
C
l
220
27
5000
Me
F
F
21
22
2-F
2-F
9.9
19
2.1
>2000
>2000
1500
2100
C
N
H
0.19
580
3000
O
23
2-F
3.6
810
220
>2000
1400
OH

K. J. Wilson et al. / Bioorg. Med. Chem. Lett. 17 (2007) 2643–2648
2647
Table 1 (continued)
Compound
R
Ar
h5-HT2A Ki (nM)a
h5-HT2C Ki (nM)a
2C/2A ratio
22
hD2 Ki (nM)a
hERG Ki (nM)a
F
F
24
2-F
H
65
1400
>2000
1400
OMe
25
0.86
>4000
>4600
>2000
>9000
F
a h5-HT2A, h5-HT2C, hD2, and hERG binding affinities were determined as described in Ref. 9 (n P 2).
b Data from Ref. 9.
R1
R1
N
Cl
N
a
S
S
O2
O2
11
9
b
R2
R1
N
S
O2
15-17, 19-24
Scheme 3. Reagents and conditions: (a) tributyl(vinyl)stannane, Pd(PPh3)4, THF, 150 ꢂC, 10 min, microwave reactor; (b) ArI, Pd(PPh3)2Cl2, tri-o-
tolylphosphine, MeCN, Et3N, 170 ꢂC, 20 min, microwave reactor.
R1
N
Cl
N
Cl
a
I
S
4
8
b
F
R1
N
Cl
R1
N
c
R2
S
S
O
O
27-32
26
Scheme 4. Reagents and conditions: (a) benzenethiol or 2-fluorobenzenethiol, K2CO3, CuI, ethylene glycol, IPA, reflux; (b) MCPBA, DCM, rt; (c)
[(E)-2-(4-fluorophenyl)vinyl]boronic acid or [(E)-2-(2,4-difluorophenyl)vinyl]boronic acid, Pd(PPh3)4, THF, 2 N Na2CO3, 150 ꢂC, 10 min, microwave
reactor.
Both compounds achieved exposure in the brain 250-
fold above their Ki for the rat 5-HT2A receptor
(0.72 nM for 25; 4.1 nM for 30). Receptor occupancy
is often poorly correlated with concentration of drug
in the brain and is highly dependent on the degree of
non-specific protein binding. It is feasible that the en-
hanced occupancy of 30 over 25 is a reflection of their
relative cLogP values (4.2 for 25; 3.4 for 30). Due to this
decrease in lipophilicity, the non-specific binding of 30
will likely be reduced relative to the sulfone resulting
in increased binding to the target receptor. By extrapo-
lation, a logical strategy to improve the pharmacody-
namic response of the sulfonylpyridines is to reduce
lipophilicity by adding polar groups that are compatible
with partitioning across the blood–brain barrier. Work
of this nature, including the application of related bia-
rylsulfones to the modification of sleep architecture, is
the subject of a forthcoming publication.12
In conclusion, we have discovered a novel series of
isomeric sulfonylpyridines that show high affinity for
the 5-HT2A receptor. Initial optimization of one isomer

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K. J. Wilson et al. / Bioorg. Med. Chem. Lett. 17 (2007) 2643–2648
Table 2. Binding affinities of the sulfinylpyridines
F
R1
N
R2
S
O
Compound
R1
R2
h5-HT2A Ki (nM)a
h5-HT2C Ki (nM)a
2C/2A ratio
hD2 Ki (nM)a
hERG Ki (nM)a
27
28
F
H
F
H
F
F
F
4.6
3.5
7.7
0.79
30
160
43
35
12
26
38
80
36
>2000
>2000
>2000
>2000
>2000
>2000
2900
>9000
3900
F
29
30
H
H
H
H
200
30
2800
4200
31 (R or S)
32 (S or R)
2400
25
0.70
6500
a h5-HT2A, h5-HT2C, hD2 and hERG binding affinities were determined as described in Ref. 9 (n P 2).
3. Mendelson, W. B.; Roth, T.; Cassella, J.; Roehrs, T.;
Walsh, J. K.; Woods, J. H.; Buysse, D. J.; Meyer, R. E.
Sleep Med. Rev. 2004, 8, 7, and references cited within.
4. Holbrook, A. M.; Crowther, R.; Lotter, A.; Cheng, C.;
King, D. Can. Med. Assoc. J. 2000, 162(2), 225.
5. Hajak, G.; Muller, W. E.; Wittchen, H. U.; Pittrow, D.;
Kirch, W. Addiction 2003, 98, 1371.
Table 3. Occupancy data
Compound
Occupancy (%)
[Plasma] (nM)
[Brain] (nM)
25
30
45
90
80
230
180
1070
6. Idzikowski, C.; Mills, F. J.; Glennard, R. Brain Res. 1986,
378, 164.
7. Markham, R. J. Aventis 2003 Press and Analyst Confer-
ence, London, UK, February 2004.
demonstrated that selectivity over the 5-HT2C subtype
was highly dependent on the nature of the substituents
carried by the styrene side-chain. Hence, selectivity ran-
ged from as low as 2-fold to greater than 4600-fold. The
corresponding sulfoxide analogs showed consistently
reduced selectivity but improved physicochemical prop-
erties, reflected in their superior in vivo occupancy.
These new compounds illustrate that the central cyclic
aliphatic amine present in many 5-HT2A ligands can
be replaced by an aromatic residue. The sulfonyl and
sulfinyl groups are sufficiently electron-withdrawing to
effectively render the pyridines non-basic, with calculat-
ed pKa values for representative compounds 7, 25, and
30 of À3.0, 0.1, and 0.8, respectively. This provides fur-
ther evidence that a basic amine is not a prerequisite for
efficient 5-HT2A receptor binding. Thus the compounds
presented belong to a new family of ligands that contin-
ue to expand the established pharmacophore models.
8. Fish, L. R.; Gilligan, M. T.; Humphries, A. C.; Ivarsson,
M.; Ladduwahetty, T.; Merchant, K. J.; O’Connor, D.;
Patel, S.; Philipps, E.; Vargas, H. M.; Hutson, P. H.;
MacLeod, A. M. Bioorg. Med. Chem. Lett. 2005, 15, 3665.
9. Fletcher, S.; Burkamp, F.; Blurton, P.; Cheng, S. K. F.;
Clarkson, R.; O’Connor, D.; Spinks, D.; Tudge, M.; van
Niel, M. B.; Patel, S.; Chapman, K.; Marwood, R.;
Shepheard, S.; Bentley, G.; Cook, G. P.; Bristow, L. J.;
Castro, J. L.; Hutson, P. M.; MacLeod, A. M. J. Med.
Chem. 2002, 45, 492.
10. Ladduwahetty, T.; Boase, A. L.; Mitchinson, A.; Quin, C.;
Patel, S.; Chapman, K.; MacLeod, A. M. Bioorg. Med.
Chem. Lett. 2006, 16, 3201.
11. Westkaemper, R. B.; Glennon, R. A. Curr. Top. Med.
Chem. 2002, 575.
12. The evolution and optimization of the related phenylsulf-
ones will be discussed in detail in a separate publication.
13. Kwong, F. K.; Buchwald, S. L. Org. Lett. 2002, 4, 3517.
14. All microwave reactions were performed in a Smith
Synthesizerꢁ microwave reactor from Personal Chemistry.
15. [(E)-2-(2,4-difluorophenyl)vinyl]boronic was prepared
from 1-ethynyl-2,4-difluorobenzene and catechol borane
according to the method described in Brown, H. C.;
Gupta, S. K. J. Am. Chem. Soc. 1975, 97, 5249: dH
(360 MHz, DMSO-d6) 6.16 (1H, d, J = 18.6 Hz), 7.11 (1H,
m), 7.25 (1H, m), 7.34 (1H, d, J = 18.6 Hz), 7.68 (1H, m),
7.87 (2H, s).
Acknowledgments
We thank Mark Chambers for the preparation of com-
pound 29, Steve Thomas for separating the sulfoxide
enantiomers 31 and 32, and David Witter and Jonathan
Young for their helpful suggestions in the preparation of
this manuscript.
16. [3H] radioligand (6 lCi) was administered by tail vein
injection. Full assay details are described in Ref. 8.
17. Occupancy at the 5-HT2A receptor was calculated using
the following equation: Total binding (TB) = Vehicle
counts; Compound binding (CB) = Compound counts;
Occupancy (%) = 100—[100 · (CB/TB)].
References and notes
1. Ohayon, M. M. Sleep Med. Rev. 2002, 6, 97.
2. Taylor, D. J.; Lichstein, K. L.; Durrence, H.; Reidel, B.
W.; Bush, A. J. Sleep 2005, 28(11), 1457.