Diphenyl sulfoxides as selective antagonists of the muscarinic M2 receptor

Article abstract of DOI:10.1016/S0960-894X(00)00438-8

Structure-activity studies on [4-(phenylsulfonyl)phenyl]methylpiperazine led to the discovery of 4-cyclohexyl-α-[4-[[4-methoxyphenyl](S)-sulfinyl]phenyl]-1-piperazineacetoni trile, 1, an M₂ selective muscarinic antagonist. Affinity at the cloned human M₂ receptor was 2.7 nM; the M₁/M₂ selectivity is 40-fold. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0960-894X(00)00438-8

Bioorganic & Medicinal Chemistry Letters 10 (2000) 2255±2257
Diphenyl Sulfoxides as Selective Antagonists of the
Muscarinic M2 Receptor
Joseph A. Kozlowski,* Derek B. Lowe, Henry S. Guzik, Guowei Zhou,
Vilma B. Ruperto, Ruth A. Du€y, Robert McQuade, Gordon Crosby, Jr., Lisa A. Taylor,
William Billard, Herbert Binch, III and Jean E. Lachowicz
Schering-Plough Research Institute, 2015 Galloping Hill Road, Kenilworth, NJ 07033-0539, USA
Received 30 May 2000; accepted 24 July 2000
AbstractÐStructure±activity studies on [4-(phenylsulfonyl)phenyl]methylpiperazine led to the discovery of 4-cyclohexyl-a-[4-[[4-
methoxyphenyl](S)-sul®nyl]phenyl]-1-piperazineacetonitrile, 1, an M2 selective muscarinic antagonist. Anity at the cloned human
M2 receptor was 2.7 nM; the M1/M2 selectivity is 40-fold. # 2000 Elsevier Science Ltd. All rights reserved.
Introduction
the M2 receptor. This paper describes the discovery of 1,
a potent M2 selective muscarinic antagonist.11
Considering that acetylcholine plays a key role in gov-
erning learning and memory processes,1 and Alzheimer's
disease has been associated with a reduction in choli-
nergic activity,2 it is understandable that e€orts to treat
the disease have focused on enhancement of acetylcho-
line levels. Current approaches involve administration
of acetylcholinesterase inhibitors,3 or acetylcholine
mimics such as post-synaptic M1 agonists.4 However,
clinical trials with either have produced moderate or
limited improvements in function.4,5 An alternate
approach is to develop drugs that enhance the release of
acetylcholine.2,6 In the CNS, data suggests that pre-
synaptic M2 receptors regulate acetylcholine release, and
an M2 antagonist will produce increased acetylcholine
levels.7,8 The M1/M2 selectivity is crucial since the post-
synaptic receptor is of the M1 subtype, and nonselective
muscarinic antagonists such as scopolamine are known
to produce cognitive de®cits. Himbacine9 and BIBN-9910
are reported to be M2 antagonists with 10- to 20-fold
selectivity versus other muscarinic receptor subtypes,
but in general the literature is devoid of compounds
with high M1/M2 selectivity (Fig. 1). Based on this hypoth-
esis we initiated a drug discovery program with the goal
of identifying an M2 selective muscarinic antagonist.
Screening identi®ed the lead compound, [4-(phenyl-
sulfonyl)phenyl]methylpiperazine, with Ki <50 nM at
Figure 1.
Chemistry
The diphenylsulfones described in this paper were pre-
pared as shown in Scheme 1. Reaction of the sodium
salt of a benzenethiol and p-¯uorobenzaldehyde, was
followed by methyl Grignard addition. Oxidation of
sulfur with m-chloroperoxybenzoic acid (MCPBA) gave
the diphenylsulfone. The alcohol to chloride conversion
was accomplished with thionyl chloride. Displacement
of chloride with a substituted piperazine produced target
compounds. Various benzenethiols are available com-
mercially. N-Substituted piperazines were purchased or
prepared from N-BOC piperazine via reductive amination
with aldehydes, or alkylation with alkyl halides, fol-
lowed by removal of the protecting group.
*Corresponding author. Tel.: +1-908-740-3488; fax: +1-908-740-7152;
e-mail: joseph.kozlowski@spcorp.com
0960-894X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(00)00438-8

2256
J. A. Kozlowski et al. / Bioorg. Med. Chem. Lett. 10 (2000) 2255±2257
Table 1.
Scheme 1. (a) NaH, p-¯uorobenzaldehyde; (b) methylmagnesium
bromide; (c) MCPBA; (d) SOCl2; (e) NaI, Et3N, 4-(R)-piperazine.
Ki (nM)
Compound
R
M1
M2
M1/M2
2
3
4
5
6
7
H
CH3
CH2CH2OH
CO2Et
Cyclohexyl
Ph
57
96
77
200
1.3
4770
37
34
26
135
0.2
4600
1.6
3
3
1.5
6
1
With the piperazine group ®xed as cyclohexyl, Table 2
summarizes data for substitution on the phenylsulphonyl.
M1/M2 selectivity was improved with 4-methoxy, 4-
methyl, or 3-chloro. However, bulky groups such as
tert-butyl or methylsulfonyl reduced anity for the M2
receptor.
Scheme 2. (a) SOCl2; (b) trans-2-(R)-1-(S)-2-Ph-cyclohexanol, pyri-
dine, THF; (c) p-MgBr-benzaldehyde diethylacetal; (d) p-TsOH; (e)
cyclohexyl piperazine, Ti(i-OPr)4, Et2AlCN.
Compounds containing either the (R)- or (S)-sulfoxide
were prepared from the reaction of a Grignard reagent
and an optically active sul®nate ester12,13 (Scheme 2).
Typically one enantiomeric series is readily accessible,
as one of the diastereomeric sul®nate esters tends to be
crystalline.14 However, since both the (R)- and (S)-sulf-
oxides were targets, we sought a method where both
diastereomeric sul®nate esters were readily isolable. We
found that the reaction of 4-methoxysul®nyl chloride
with trans-2-(R)-1-(S)-2-phenylcyclohexanol,14 gave a
mixture of diastereomers (3:1 (R) to (S) sulfoxide) which
were separable via ¯ash chromatography. Menthol13
was less e€ective as the chiral auxiliary because the dia-
stereomeric sul®nate esters were not readily separable.
Addition of the Grignard reagent derived from p-bromo-
benzaldehydedimethylacetal proceeded with inversion of
con®guration at sulfur and regeneration of the chiral
alcohol. Following acetal hydrolysis, the Strecker amine
was prepared by reaction of the aldehyde with cyclohexyl
piperazine, titanium(IV) isopropoxide, and diethyl
aluminum cyanide.
Table 2.
Ki (nM)
Compound
R
M1
M2
M1/M2
8
9
10
11
12
OCH3
CH3
Cl
t-Butyl
SO2CH3
6
11
8
85
0.6
0.8
0.6
33
10
14
13
2.6
3
106
36
As shown in Table 3, modi®cation of the benzylic
group, in combination with a sulfoxide bridging the
phenyl rings led to compounds with anity and selec-
tivity comparable to the sulfone series. In addition,
small groups were preferred at the benzylic position.
Separation of diastereomers at the benzylic center, with
the (R)-sulfoxide, was accomplished on a Chiracel OD
column (20% IPA/hexane). Diastereomers of the corre-
sponding (S)-sulfoxide were separated by fractional
crystallization (ethyl acetate/hexane).
The in¯uence of chirality at sulfur and the benzylic site
on anity and selectivity is summarized in Table 4.
Con®guration A was preferred at the benzylic site. The
Table 3.
Results
The binding data15 in Table 1 show that variation of the
N-(4)-R-group on the piperazine had a direct in¯uence
on anity at the M2 receptor. A lipophilic group, such
as cyclohexyl, improved both the M2 anity and selec-
tivity. However, M2 anity was decreased when the
piperazine N-(4)-nitrogen was rendered nonbasic with
an electron withdrawing substituent. This observation is
consistent with reports of agonist binding to muscarinic
receptors, where it is proposed that a basic nitrogen
interacts with Asp 103 on the third transmembrane
domain.16,17
Ki (nM)
Compound
R
M1
M2
M1/M2
13
14
15
16
17
CH3
CN
Cyclohexyl
CONH2
CF3
6
117
52
242
23
1
8
17
40
5
6
14
3
6
5

J. A. Kozlowski et al. / Bioorg. Med. Chem. Lett. 10 (2000) 2255±2257
2257
Table 4.
series of potent and M2 selective ligands. Compound 1
displays an M1/M2 selectivity of 40-fold with an M2
Ki=2.7 nM. The compound has been characterized as
an antagonist, was shown to increase acetylcholine
levels in vivo and was e€ective in animal models of
cognition.18 These initial results are promising for the
development of M2 selective muscarinic antagonists for
the treatment of Alzheimer's disease.
CN
Ki (nM)
Compound
SO
Isomer
M1
M2
M1/M2
1
(S)
(S)
(S)
(R)
(R)
Mix
A
B
A
B
112
83
180
140
253
2.7
3.8
20
9
17
40
22
9
15
15
18
19
20
21
References and Notes
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combination of the (S)-sulfoxide with benzylic con®g-
uration A gave the highest anity and selectivity for
M2. However, we found that the benzylic cyanide
undergoes facile racemization and were unable to isolate
pure enantiomers. Therefore, the data in Table 4 re¯ect
partial racemization of the benzylic center. Knowing
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used to de®ne nonspeci®c binding. Separation of bound from
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Figure 2.
Conclusion
Structure±activity relationship studies on [4-(phenyl-
sulfonyl)phenyl]methylpiperazine led to discovery of a