N-Substituted pyrrolidines and tetrahydrofurans as novel AMPAR positive modulators

Article abstract of DOI:10.1016/j.bmcl.2010.09.062

A series of novel AMPA receptor positive modulators displaying CNS penetration have been discovered with sub-micromolar activity and good selectivity over the cardiac channel receptor, hERG. We describe here the synthesis of these compounds which are biaryl pyrrolidine and tetrahydrofuran sulfonamides and disclose their activities against the human GluA2 flip isoform homotetrameric receptor.

Full text of DOI:10.1016/j.bmcl.2010.09.062

Bioorganic & Medicinal Chemistry Letters 20 (2010) 7116–7119
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
N-Substituted pyrrolidines and tetrahydrofurans as novel AMPAR
positive modulators
⇑
Kevin M. Thewlis , Laura Aldegheri, Mark H. Harries, Claudette Mookherjee, Beatrice Oliosi, Simon E. Ward
Neurosciences Centre of Excellence for Drug Discovery, GlaxoSmithKline, New Frontiers Science Park, Third Avenue, Harlow, Essex CM19 5AW, United Kingdom
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of novel AMPA receptor positive modulators displaying CNS penetration have been discovered
with sub-micromolar activity and good selectivity over the cardiac channel receptor, hERG. We describe
here the synthesis of these compounds which are biaryl pyrrolidine and tetrahydrofuran sulfonamides
and disclose their activities against the human GluA2 flip isoform homotetrameric receptor.
Ó 2010 Published by Elsevier Ltd.
Received 2 August 2010
Revised 7 September 2010
Accepted 10 September 2010
Available online 17 September 2010
Keywords:
AMPA
Receptor
Modulators
Synthesis
AMPA receptors
(
a
-amino-3-hydroxyl-5-methyl-4-isoxazole-
have been clinically investigated, such as 6 (LY450108) and 7
(LY451395). In broad terms the molecules have generally displayed
higher affinities for the AMPAR over time, with the Lilly pheneth-
ylsulfonamides being considerably more potent than the original
benzamide class.11
From evaluation of these chemical chemotypes, we proposed a
medicinal chemistry strategy with the purpose of creating our own
series of CNS-penetrant AMPAR positive modulators, with the fo-
cus of combining high potency with desirable physicochemical
and pharmacokinetic properties.
One area of exploration was to build on existing literature for
the phenethylsulfonamide series of molecules, and seek to modu-
late the physicochemical properties such as solubility and lipophil-
icity by introduction of heteroatoms into the phenethyl linker. This
led to the synthesis and evaluation of a set of 3,4-disubstituted
pyrrolidines and 3,4-disubstituted tetrahydrofurans with trans
and cis relative stereochemistry, respectively. These targets were
chosen for their synthetic tractability and, although structurally
similar, should be considered as two distinct series. It was antici-
pated that the introduction of a heteroatom into the linker would
reduce the lipophilicity and hence improve the physicochemical
profile of the series. Furthermore, we sought to discover com-
pounds with reduced polar surface area allowing for a greater
probability of achieving CNS penetration.12
Molecules with a pyrrolidine linker were prepared by the
route described in Scheme 1. The trans-N-substituted pyrrolidine
skeletons (10a–d) were constructed via a paraformaldehyde med-
iated [3+2] cycloaddition reaction on commercially available
trans-4-bromonitrostyrene (8) with the equivalent N-substituted
glycines (9a–d). The nitro group was then reduced using indium
propionic acid (AMPA)) belong to the ligand-gated, ionotropic
glutamatergic receptor family. They are widely distributed in the
mammalian central nervous system and mediate the vast majority
of fast excitatory neurotransmission in the CNS.1,2 The glutamater-
gic neurotransmitter system has been a recent focus for the
advancement of innovative treatments for psychiatric and
neurodegenerative conditions as its dysfunction underlies the
pathophysiology of diseases such as schizophrenia, Alzheimer’s
disease, Parkinson’s disease and mood disorders.3,4 Preclinical
studies have identified modulators of the AMPAR which slow the
rate of receptor deactivation and/or desensitization leading to
enhanced synaptic activity and efficacy in cognition models.5–7
There are a number of molecules exemplified by a variety of
chemotypes that have been described as AMPAR positive modula-
tors.8,9 This can be highlighted by describing three major, well-
established, classes that have been investigated clinically which
exist alongside a number of less extensively exemplified structures
(Fig. 1). The first group was derived from the nootropic agent 1
(Aniracetam) which was shown to improve different phases of
learning and memory impairment in rats and mice.10 This mole-
cule was subsequently developed into a range of benzamide
derivatives by Cortex which included 2 (CX-516) and 3 (CX-691).
The second major class, known as the benzothiadiazines, can be
exemplified by the diuretic agent 4 (cyclothiazide) and was devel-
oped into 5 (S-18986) by Servier. Thirdly, there are the phenethyl-
sulfonamides discovered by Lilly, which include molecules that
⇑
Corresponding author.
E-mail address: Kevin.M.Thewlis@gsk.com (K.M. Thewlis).
0960-894X/$ - see front matter Ó 2010 Published by Elsevier Ltd.
doi:10.1016/j.bmcl.2010.09.062

K. M. Thewlis et al. / Bioorg. Med. Chem. Lett. 20 (2010) 7116–7119
7117
O
O
O
O
O
O
O
H2N
S
S
N
N
N
N
NH
N
N
O
N
H
O
N
H
Cl
O
H
1
2
3
4
H
N
H
N
O
O
S
O
S
S
NH
O
O
O
O
F
H
N
H
N
O
O
S
N
H
5
6
7
F
Figure 1. Clinically evaluated AMPAR positive modulators.
Table 1
R
Biological activity of trans-3,4-disubstituted pyrrolidine analoguesa,b
O
N+
N
R
N
R
OH
a
O
N
H
+
O
N+
Br
9a-d
8
O
Br
O
O
10a-d
HN
S
Ar
R
R
O
M) Asym max hERGc IC50
(lM)
N
N
c
b
Compds Ar
R
EC50
(
l
O
13a
13b
13c
13d
14a
14b
14c
14d
15
(3-NHSO2Me)Ph Me
(3-NHSO2Me)Ph Et
(3-NHSO2Me)Ph iPr
(3-NHSO2Me)Ph Ph
0.3
1.6
6.3
5.0
5.0
7.9
124
111
89
38
109
90
30
48
75
79
31.6
2.5
6.3
1.3
2.0
0.4
ND
0.8
>63
NH2
NH
S
Br
Br
11a-d
12a-d
O
R
(4-CN)Ph
(4-CN)Ph
(4-CN)Ph
Me
Et
N
iPr 20.0
13a-d
14a-d
15-21
d
(4-CN)Ph
Ph
6.2
5.0
4.0
6.3
O
(3-SO2Me)Ph
(3-NHCOMe)Ph
(3-COMe)Ph
(3-CF3)Ph
Me
Me
Me
22a, 22d, 23, 24
NH
S
16
17
18
12.6
7.9
4.0
Ar
73
26
O
Me 79.4
19
20
21
22a
22d
23
(2-F)Ph
3-Pyridyl
Me
Me
Me
Me
Ph
2.5
4.0
2.5
5.0
4.0
20
ND
31.6
6.3
Scheme 1. General procedure for the synthesis of pyrrolidine analogues. Reagents
and conditions:13 (a) paraformaldehyde, toluene, reflux, Dean–Stark; (b) indium
100
101
117
74
29
106
i
3-(2-F)pyridyl
3-(6-F)pyridyl
3-(6-F)pyridyl
2-Thiophene
3-Thiophene
metal, concd HCl, THF, rt; (c) PrSO2Cl, DBU, CH2Cl2; (d) ArB(OH)2, Pd(OAc)2, PPh3,
1.0
Cs2CO3, dioxane, water, reflux.
0.08
>63
4.0
Me 20.0
Me 3.2
24
powder in concentrated hydrochloric acid to give the correspond-
ing amines (11a–d) which were functionalized to form the sul-
a
Drawn with relative stereochemistry.
FLIPR generated EC50 against hGluA2. Asym max is the fitted maximum
b
fonamides
(12a–d).
However,
this
seemingly
simple
response, relative to 100% defined as the maximal response of cyclothiazide
standard.
transformation was difficult to optimise, the best results were ob-
tained when using two equivalents of isopropylsulfonyl chloride
at 0 °C in the presence of DBU (1,8-diazabicyclo[5.4.0]undec-7-
ene) as base. These key intermediates were reacted under stan-
dard Suzuki coupling conditions affording a range of analogues
(13–24).
As detailed in Table 1, initial lead 13a remained the most potent
compound in this series. However, activity at the human GluA2 flip
isoform homotetrameric receptor (hGluA2) can be achieved with a
range of substituted phenyl and heterocyclic groups (Ar) some of
which were aimed at lowering the polar surface area of the mole-
cule in a bid to improve CNS penetration. Sulfonamide 13a is over
10-fold more active at hGluA2 than the amide equivalent (16).
Furthermore, the sulfone (15) and ketone (17) equivalents are
equipotent with 16. Switching to other electron withdrawing
substituents such as trifluoromethyl (18) sees a dramatic loss in
activity. Pyridines offered no advantage in terms of hGluA2 activity
versus phenyl as compounds 19 and 21 are equipotent, but 21 has
the higher maximal response relative to the maximal response of
the cyclothiazide (4) [defined as 100%] standard used in the assay.
hERG affinity from 3H-dofetilide displacement assay.
c
Moving the fluorine from the 2-position to the 6-position of the
pyridyl ring, as in 22a, saw a reduction in hGluA2 potency as well
as an increase in hERG (KCNH2 voltage-sensitive potassium chan-
nel) affinity. But removing the fluorine altogether as in compound
20 reduced hERG affinity whilst maintaining hGluA2 activity.
Other heterocycles such as thiophenes 23 and 24 showed a con-
trast between 2- and 3-substitution with compound 24 being the
more active.
Activity is also possible with a range of substituents on the
nitrogen of the pyrrolidine ring (R). However, the trend is that
activity decreases with increasing alkyl steric bulk on the pyrroli-
dine nitrogen, that is, Me > Et > iPr. This trend is demonstrated by
compounds 13a–c and 14a–c. Interestingly, when R was phenyl
(13d and 14d) it was found that the activity was greater than when
R was isopropyl (13c and 14c), and the activities were equipotent
in the case of 22a (R = Me) and 22d.

7118
K. M. Thewlis et al. / Bioorg. Med. Chem. Lett. 20 (2010) 7116–7119
This series of compounds whilst active also produced many
product is obtained presumably because the hydride is delivered
from the least hindered face during the reduction of the intermedi-
ate imine complex. Subsequent iodination using N-iodo-succini-
mide predominantly affords the 4-iodo regioisomer to give
intermediate 29. Sulfonylation is carried out using the same condi-
tions as described above for intermediates 12a–d to make com-
pound 30. Subsequent conversion of the iodo into the pinacolato
boronate ester (31) under Suzuki style conditions with bis(pinaco-
lato)diboron and [1,1-bis(diphenylphosphino)ferrocene] palladium
(II) chloride dichloromethane complex as catalyst, before standard
Suzuki conditions using polymer supported tetrakis(triphenyl-
phosphine)palladium(0) are employed to form a range of targets
(32–38).
examples with significant affinity in the hERG cardiac channel
binding assay. For example, a comparison of the N–R substitution
showed that when R is methyl the level of hERG affinity is lower
than with other, larger, groups. Interestingly, when R is the more
lipophilic phenyl substituent hERG affinity is at a significantly
higher level than hGluA2 activity; in particular compound 22d
(daylight c log P of 4.20) has approximately 50-fold more affinity
for hERG. Therefore when R is a small, less lipophilic, group such
as methyl improved potency at hGluA2 was possible with greater
selectivity over hERG. Furthermore, compounds 13a and 14a have
favourable aqueous solubility achieving levels of >9 mg/mL and
>2.7 mg/mL, respectively.
It was also seen with many examples that the N-methylated
pyrrolidines had high intrinsic clearance in both human and rat li-
ver microsomes. A metabolite identification study was performed
confirming that the major metabolite of compound 13a in rat liver
microsomes was the anticipated N-demethylated analogue. A trace
of a +16 Da metabolite was also detected, but not characterised
further, which would indicate oxidation has occurred somewhere
in the molecule. We therefore needed to identify a series of mole-
cules free of hERG affinity as well as exhibiting improved metabolic
stability. It was proposed to synthesise a series of tetrahydrofurans
where the NR of the pyrrolidine was replaced with oxygen.
Molecules with a tetrahydrofuran (THF) linker were prepared
by the route described in Scheme 2. Commercially available 3,6-
dioxabicyclo[3.1.0]hexane (25) underwent epoxide ring opening
by way of a Grignard reaction with phenylmagnesium bromide
to form the trans-THF product (26). This material was oxidised to
the ketone (27) followed by a reductive amination reaction with
benzylamine with subsequent debenzylation under hydrogenation
conditions to give the cis-THF amine intermediate (28). The cis-
It is evident from Table 2 that the THF analogues are more ac-
tive at hGluA2 than their N-methyl pyrrolidine equivalents. This
could be due to steric effects, consistent with the trend observed
in the pyrrolidine series. The THF series is generally less lipophilic,
for example, the daylight c log P of 32 is 3.03, whereas the
N-methyl pyrrolidine analogue 14a has a daylight c log P value of
3.30. The THF series also has low polar surface area and would also
be free from the metabolic issues associated with an N-methyl pyr-
rolidine as highlighted above. Therefore, it could be predicted that
these compounds would be more soluble and more CNS penetrant.
These molecules were also free from significant hERG affinity, with
only 32 and 34 showing <100-fold selectivity. This could be attrib-
uted to the non-basic nature of the THF.
Table 3 gives a summary of key physicochemical properties and
metabolic data collected for some of these compounds. In the pyr-
rolidine series, when the amino substituent is larger than methyl,
low intrinsic clearances can be achieved, but these molecules have
higher lipophilicities which potentially contributes to poorer selec-
tivity versus hERG. Compound 22a represents a pyrrolidine of low-
er lipophilicity with a daylight c log P of 2.60 (although hERG
affinity is still high). PSA is lower also due to the use of a fluori-
nated pyridine as the Ar group. Unfortunately this compound
had high rat intrinsic clearance (5.6 mL/min/g) and so was not
evaluated in vivo. Similarly, THF compounds 35 and 36 also dis-
played lower PSA and lipophilicity (daylight c log P 2.33 and 2.54,
respectively) but had high intrinsic clearance in rat liver micro-
somes, whereas in human liver microsomes both had a clearance
of <1 ml/min/g. However, compound 14a had a rat intrinsic clear-
ance of <0.5 mL/min/g despite being a methylated pyrrolidine and
a PSA of 73 which gave us confidence to test this compound in a rat
O
O
O
O
a
b
OH
O
25
26
27
O
O
e
c, d
NH2
NH2
29
28
I
Table 2
O
Biological activity of cis-3,4-disubstituted tetrahydrofuran analoguesa,b
O
O
f
O
O
g
S
N
H
S
N
H
O
O
O
31
HN
30
S
Ar
O
B
I
O
O
hERGc IC50
(lM)
O
Compds
Ar
EC50
(lM)
Asym max
O
h
32
33
34
35
36
37
38
(4-CN)Ph
0.5
0.1
0.3
0.3
0.3
0.3
0.4
112
124
125
131
117
117
120
12.6
>63
12.6
>63
>63
>63
>63
(3-COMe)Ph
3-(6-F)pyridyl
3-(5-F)pyridyl
2-(5-F)pyridyl
2-Thiophene
3-Thiophene
S
N
H
O
32-38
Ar
Scheme 2. General procedure for the synthesis of THF analogues. Reagents and
conditions:14 (a) PhMgBr, CuI, THF 0 °C; (b) trichloroisocyanuric acid, TEMPO,
CH2Cl2, 0 °C; (c) PhCH2NH2 NaBH(OAc)3, acetic acid, CH2Cl2; (d) Pd(OH)2/C, H2,
MeOH; (e) CF3SO3H, NIS, MeCN; (f) iPrSO2Cl, DBU, CH2Cl2; (g) [1,1-bis(diphenyl-
phosphino)ferrocene]dichloropalladium(II) CH2Cl2 complex, bis(pinacolato)dibo-
ron, KOAc, DMSO; (h) ArBr, polymer supported Pd(PPh3)4, Na2CO3, dioxane, water.
a
Drawn with relative stereochemistry.
FLIPR generated EC50 against hGluA2. Asym max is the fitted maximum
b
response, relative to 100% defined as the maximal response of cyclothiazide
standard.
c
hERG affinity from 3H-dofetilide displacement assay.

K. M. Thewlis et al. / Bioorg. Med. Chem. Lett. 20 (2010) 7116–7119
7119
Table 3
We have demonstrated that a series of pyrrolidines and tetra-
hydrofurans display high activity against the AMPA receptor, with
some molecules showing sub-micromolar potencies. We have also
successfully developed this series to afford molecules with a good
selectivity profile, in particular against hERG.
Moreover, we identified examples which show a good in vitro
pharmacokinetic profile with low intrinsic clearance in both rat
and human liver microsomes and for the example tested, good
CNS penetration. Further studies are ongoing.
Physicochemical properties and intrinsic clearance of selected molecules
Compds
PSA (Å2)
c log Pa
hCLi (mL/min/g)
rCLi (mL/min/g)
13b
13c
14a
14b
22a
35
96
96
73
73
62
62
68
3.33
3.64
3.30
3.96
2.60
2.33
2.54
0.7
0.7
0.8
1.2
0.8
0.6
0.8
0.5
0.6
<0.5
0.9
5.6
3.6
36
5.1
a
c log P Daylight Chemical Information Systems Inc., Aliso Viejo, CA,
References and notes
http://www.daylight.com.
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PK model. The results showed that 14a did indeed show good lev-
els of brain exposure with CNS penetration (brain to blood ratio of
1.8), it also exhibited a half-life of over 2 h.
It is difficult to make direct comparisons between the pyrrol-
idines and THFs as they have trans and cis regioisomerism, respec-
tively. For example, the selectivity that the THFs have over hERG
may be due in part to the cis configuration rather than the THF it-
self, although a reduction in the lipophilicity and basicity may also
add to a reduction in hERG affinity. Furthermore, the cis and trans
isomers may have differing solubilities and lipophilicities. These
isomers discussed above were made due to the tractability of their
synthesis but it may be useful to make a cis-pyrrolidine and a
trans-THF to be able to make a full SAR comparison.