Thiazoles and thiopyridines: Novel series of high affinity h5HT 7 ligands

Article abstract of DOI:10.1016/j.bmcl.2003.11.050

A series of thiazole based 5HT₇ ligands has been identified from screening. Optimisation of the pendent aryl group and modification of the core gave a related series of high affinity, selective thiopyridine based 5HT₇ ligands, the most active of which behaves as a partial agonist.

Full text of DOI:10.1016/j.bmcl.2003.11.050

Bioorganic & Medicinal Chemistry Letters 14 (2004) 677–680
Thiazoles and thiopyridines: novel series of high affinity
h5HT7 ligands
Christopher G. Thomson,a,* Margaret S. Beer,b Neil R. Curtis,a
Helen J. Diggle,a Emma Handfordb and Janusz J. Kulagowskia
aDepartment of Medicinal Chemistry, Merck Sharp & Dohme Research Laboratories,
The Neuroscience Research Centre, Terlings Park, Harlow, Essex CM20 2QR, UK
bDepartment of Biochemistry, Merck Sharp & Dohme Research Laboratories,
The Neuroscience Research Centre, Terlings Park, Harlow, Essex CM20 2QR, UK
Received 24 July 2003; revised 15 October 2003; accepted 17 November 2003
Abstract—A series ofthiazole based 5HT ligands has been identified from screening. Optimisation of the pendent aryl group and
7
modification ofthe core gave a related series ofhigh affinity, selective thiopyridine based 5HT ligands, the most active ofwhich
7
behaves as a partial agonist.
# 2003 Elsevier Ltd. All rights reserved.
Based on localisation and pharmacological studies mainly
with non-selective ligands,1 the human 5HT7 receptor has
been implicated in a variety ofCNS ufnctions. These
include depression,2,3 schizophrenia4 and control ofcir-
cadian rhythms.5 New, high affinity and selective ligands
for this receptor would greatly facilitate our under-
standing ofthe role of5HT receptors in the brain and
7
potentially lead to new drug entities in multiple ther-
apeutic areas.
Compound 3a was identified from in-house screening as
having reasonable binding affinity at the cloned h5HT7
receptor and some selectivity over other receptors in
preliminary counter-screening (Table 1). All compounds
ofinterest were routinely screened against a small
selection ofother common G-protein coupled receptors.
These included 5HT1A, as currently used h5HT7 ligands
such as 5-carboxamidotryptamine (5-CT) and 8-OH-
DPAT show poor selectivity over this subtype,6 and
5HT2A and D2 receptors, as activity at these could poten-
tially confound any subsequent behavioural studies.
Current ligands in use include 8-OH-DPAT (1), an
agonist at 5HT7 which has varying selectivity over other
5HT receptors.6 More recently selective antagonists
have been developed, such as SB-2699707 (2). We
therefore set out to discover a novel series of high affi-
nity and selective h5HT7 ligands to further probe the
role ofthis receptor.
Two areas ofthe lead molecule were initially selected for
modification — the pyridyl ring and the central thiazole
core. Chemistry to synthesise these analogues was con-
veniently carried out from commercially available
chlorides or thiols as shown in Scheme 1.
Where neither ofthese starting materials was available,
for example, in the synthesis of the tert-butylthiazole 4,
condensation ofthe relevant a-bromoketone with
ammonium dithiocarbamate8 (Scheme 2) yielded the
Keywords: Thiazoles; Thiopyridines; h5HT7 ligands; Partial agonist.
* Corresponding author. Tel.: +44-1279-440563; fax: +44-1279-
440390; e-mail: christopher_thomson@merck.com
0960-894X/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2003.11.050

678
C. G. Thomson et al. / Bioorg. Med. Chem. Lett. 14 (2004) 677–680
Table 1.
thiol which could then be alkylated as before. For
examples 3d and 3f, condensation ofammonium
dithiocarbamate with bromophenylacetaldehyde9 and 2-
bromopropiophenone gave the desired thiols.
In the case ofpyridyl analogue 3j, synthesis was carried
out by rearrangement ofthe N-oxide of2-phenylpyr-
idine, followed by subsequent displacement of the
resulting 4-pyridyl chloride by the sidechain thiol
(Scheme 3). Isomer 5a was formed from 6-phenyl-2-
pyridone, which was converted to the thiopyridone, and
subsequent alkylation occurred exclusively on sulfur
(Scheme 4).
No.
Ar
h5HT7
Ki/nMa
h5HT1A
Ki/nMb
h5HT2A
Ki/nMc
hD2
Ki/nMd
3a
3b
3c
27
6800
12
90e
>200e
130e
Synthesis of further pyridine analogues started from
2,6-dibromopyridine (Scheme 5). Displacement ofone
bromine could be carried out with the anions ofbenzene
methanethiol, benzyl alcohol or 2-dimethyl aminoetha-
nethiol. Displacement with benzene methanethiol
directly introducing the sulphur gave compound 6. This
intermediate could be lithiated and quenched with
ketones to give hydroxyl substituted compounds, but
attempts to couple Grignard reagents using nickel cata-
lysis10 failed, possibly due to the presence of sulfur. The
hydroxyalkyl intermediates were debenzylated to give
the thiopyridones, which were alkylated on sulfur with
the chloroethyl amine as before.
210e
640e
>1000e
3d
3e
3f
3200
84
830
1000e
>1000
170e
22
In the case ofthe benzyloxy compound 7, substitution
ofthe remaining bromine with Grignard reagents under
nickel catalysis went smoothly, except for tert-butyl-
magnesium chloride where no substitution was
observed. The protected pyridones could be debenzyl-
ated and converted to the thiopyridones with Law-
esson’s reagent. Again, alkylation on sulfur with the
chloroethyl amine yielded the final compounds 5.
3g
2200
1200e
>1000
Values represent the geometric mean of3–6 determinations unless
specified.
a Displacement of[ 3H]-5HT from the cloned receptor expressed in
CHO cells.12
b Displacement of[ 3H]-5HT from the cloned receptor expressed in
HeLa cells.13
Compound 5e was prepared from 5b using copper
mediated addition11 of tert-butylmagnesium chloride.
c Displacement of[ H]-Ketanserin from the cloned receptor expressed
3
in CHO cells.14
d Displacement of[ 3H]-Spiperone from the cloned receptor expressed
in CHO cells.15
e Geometric mean of2 determinations.
Scheme 3. Reagents: (i) H2O2, AcOH; (ii) POCl3, CHCl3.
.
Scheme 1. Reagents: (i) ClCH2CH2NMe2 HCl, NaH, DMF; (ii)
.
HSCH2CH2NMe2 HCl, NaH, DMF.
Scheme 4. Reagents: (i) Lawesson’s reagent, DME; (ii) ClCH2-
.
CH2NMe2 HCl, NaH, DMF.
Scheme 2. (i) EtOH.

C. G. Thomson et al. / Bioorg. Med. Chem. Lett. 14 (2004) 677–680
679
Table 2.
No.
Ar
h5HT7
Ki/nMa
h5HT1A
Ki/nMb
h5HT2A
Ki/nMc
hD2
Ki/nMd
3c
12
1200e
>4000
100
210e
640e
>1000e
3h
3i
3j
1200e
Scheme 5. Reagents: (i) PhCH2SH, NaH, DMF; (ii) BuLi, cyclo-
hexanone, THF; (iii) Pd/C, EtOH, H2; (iv) PhCH2OH, NaH, DMF;
(v) cyclo-hexylmagnesium bromide, NiCl2(dppe), Et2O; (vi) Lawesson’s
.
reagent, DME; (vii) HSCH2CH2NMe2 HCl, NaH, DMF; (viii) tert-
butylmagnesium chloride, CuCN, THF.
5a
0.6
16
320e
450
Variation ofthe pyridyl group of 3a showed some
interesting structure–activity relationships (Table 1).
To obtain good binding affinity at h5HT7, substitution
is necessary, but has to be at C4, with phenyl (3c)
preferred over pyridyl. Benzofusion (3e) or 4,5-dis-
ubstitution (3f) are not tolerated. Both aromatic and
tert-butyl groups are tolerated, but the tert-butyl
group shows greater selectivity compared with aro-
matic substitutions.
a,b,c,d,e See Table 1 for details.
Table 3.
No.
R
h5HT7
Ki/nMa
h5HT1A
Ki/nMb
h5HT2A
Ki/nMc
hD2
Ki/nMd
With the phenyl group in place ofthe pyridyl, variation
ofthe core heterocycle was explored ( Table 2). Two
alternative phenyl substituted heterocycles were intro-
duced (3h and 3i) but were found to be detrimental.
However, when the thiazole was replaced by 2,6-dis-
ubstituted pyridine (5a), the most potent compound at
h5HT7 from these series was found, with improved
selectivity over h5HT2A and hD2. The position of
nitrogen is important, as 2,4-substituted pyridine (3j)
does not show this high affinity binding.
5a
0.6
16
320e
450
5b
5c
Br
4.1
0.7
37e
8.7
270e
230e
870e
1300e
5d
5e
7.2
1.3
41e
17
480e
100e
>1000e
Table 3 shows the effect ofvarying substituents at the
6-position of 5a. A variety oflipophilic groups were
tolerated giving high h5HT7 binding. Larger groups
seem to show an advantage in h5HT7 binding over
smaller or more polar substitution (5d). In the thiazole
series, selectivity over the counter-screen receptors was
increased by replacing phenyl with tert-butyl. However,
in the pyridine series, aromatic and tert-butyl groups are
very similar in profile. Pyridine 5a still remains the
highest affinity and most selective compound from
the two series. This compound was therefore tested in a
functional assay16 and found to behave as a partial ago-
nist, giving 80% ofthe response ofthe full agonist 5-CT
in h5HT7 receptors (Fig. 1).
330
a,b,c,d,e See Table 1 for details.
In conclusion, two novel, related series ofh5HT
7
ligands have been identified by elaboration ofa screen-
ing lead, resulting in compound 5a which shows high
affinity and partial agonism for h5HT7, and selectivity
over counter-screen receptors. Affinity for h5HT7, and
selectivity over hD2, h5HT1A and h5HT2A receptors can
be manipulated in both series by modification ofthe
pendent alkyl or aromatic groups.

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C. G. Thomson et al. / Bioorg. Med. Chem. Lett. 14 (2004) 677–680
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12. Binding assays were performed in 96-well microtitre plates
in a total volume of500 mL, containing 1 nM [3H] 5-HT in
assay buffer (50 mm Tris–HCl, pH 7.6 rt, containing
0.05% ascorbic acid and 10mM pargyline), with or with-
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7
30 min at 24 ꢀC, followed by termination by rapid fil-
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Figure 1. Stimulation of b-lactamase activity by 5-CT and compound
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Acknowledgements
The authors would like to thank F. Pelaez, M. Mojena,
M. Maroto and A. Cabello at Merck, Sharp & Dohme,
CIBE (Spain), and J. Stanton and M. Heald for counter
screening results, and especially S. Patel and M. Guscott
for scientific guidance.
References and notes
16. Functional responses were assessed with a b-lactamase
reporter gene assay using Aurora 3, HEK293/CRE b-lac-
tamase reporter cells stably transfected with the h5HT7
receptor. The reporter system is based on cAMP response
element (CRE) regulated transcription of b-lactamase
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membrane permeable green fluorescent substrate, CCF4.
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Cytofluor fluorescence spectrophotometer (excitation
wavelength 409 nm, emission wavelengths of460 nm of r
blue spectrum and 530 nm for green spectrum readings).
Briefly, cells were plated out in Eagle’s modified essential
media (EMEM)+10% dialysed foetal bovine serum
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microtitre plates. 24 h later, cells were washed in phos-
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in a CO2 incubator for 4 h. The resulting b-lactamase was
detected with the addition of20 mL CCF4 substrate. After
2 h, plates were read using a Cytofluor fluorescence spec-
trophotometer at the wavelengths specified above.
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