Discovery of N-{[1-(propylsulfonyl)-4-pyridin-2-ylpiperidin-4-yl]methyl}benzamides as novel, selective and potent GlyT1 inhibitors

Article abstract of DOI:10.1016/j.bmcl.2008.12.115

Employing an iterative analogue library approach, novel potent and selective glycine transporter 1 (GlyT1) inhibitors containing a 4-pyridin-2-ylpiperidine sulfonamide have been discovered. These inhibitors are devoid of time-dependent CYP inhibition activity and exhibit improved aqueous solubility versus the corresponding 4-phenylpiperidine analogues.

Full text of DOI:10.1016/j.bmcl.2008.12.115

Bioorganic & Medicinal Chemistry Letters 19 (2009) 1488–1491
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Discovery of N-{[1-(propylsulfonyl)-4-pyridin-2-ylpiperidin-4-yl]methyl}-
benzamides as novel, selective and potent GlyT1 inhibitors
a
b
b
b
Zhijian Zhao a, , William H. Leister , Julie A. O’Brien , Wei Lemaire , David L. Williams Jr. ,
Marlene A. Jacobson b, Cyrille Sur b, Gene G. Kinney b, Doug J. Pettibone b, Philip R. Tiller c,
Sheri Smith c, George D. Hartman a, Craig W. Lindsley a, Scott E. Wolkenberg a
*
a Department of Medicinal Chemistry, Technology Enabled Synthesis Group, Merck & Co., Inc., WP-14-1, PO Box 4 Sumneytown Pike, West Point, PA 19486, USA
b Department of Neuroscience, Merck & Co., Inc., PO Box 4, West Point, PA 19486, USA
c Department of Drug Metabolism, Merck & Co., Inc., PO Box 4, West Point, PA 19486, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Employing an iterative analogue library approach, novel potent and selective glycine transporter 1
(GlyT1) inhibitors containing a 4-pyridin-2-ylpiperidine sulfonamide have been discovered. These inhib-
itors are devoid of time-dependent CYP inhibition activity and exhibit improved aqueous solubility ver-
sus the corresponding 4-phenylpiperidine analogues.
Received 22 October 2008
Revised 22 December 2008
Accepted 30 December 2008
Available online 9 January 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
GlyT1
Schizophrenia
NMDA receptor
Inhibitor
N-{[1-(Propylsulfonyl)-4-pyridin-2-
ylpiperidin-4-yl]methyl}benzamides
A growing body of evidence suggests that hypofunction of N-
methyl- -aspartate (NMDA) glutamatergic receptors may play an
phrenic patients with glycine itself or sarcosine, a weak GlyT1
inhibitor, improved symptoms.8 Because of the drawbacks with
these compounds (weak GlyT1 inhibition, poor pharmacokinetics
and brain penetration), numerous efforts have been reported to
identify improved GlyT1 inhibitors.9,10
Recently, we reported the optimization of HTS lead 1 using a
focused library approach which identified 2-amino-6-chloro-N-
((4-phenyl-1-(propylsulfonyl)piperidin-4-yl)methyl)benzamide
(ACPPB, 2) as a novel potent (IC50 = 2.6 nM) and selective (vs gly-
cine transporter 2 (GlyT2) and taurine transporter (TauT)) GlyT1
inhibitor based on a 4,4-disubstituted piperidine core structure
(Fig. 1). Further in vivo evaluation demonstrated that 2 not only
increases glycine levels in rat prefrontal cortex (PFC), but also
D
important role in the pathophysiological development of schizo-
phrenia.1 To test this NMDA hypofunction hypothesis of schizo-
phrenia, compounds that activate NMDA receptors are needed.
However, NMDA receptor agonists that target the glutamate bind-
ing site tend to be neurotoxic and lead to cell death.2 An alternative
approach is to modulate NMDA receptor activation indirectly, for
example by increasing the levels of glycine, an endogenous, oblig-
atory co-agonist with glutamate for NMDA receptor activation.3 It
is known that NMDA receptor activity can be enhanced by elevat-
ing the concentration of glycine in the local microenvironment of
NMDA receptor-containing synapses.4 Glycine levels in the brain
are regulated by glycine transporter 1 (GlyT1) which appears to
be largely expressed at glutamatergic synapses,5 and inhibition of
GlyT1 leads to increased local glycine concentrations and activa-
tion of synaptic NMDA receptors.6 Therefore, increasing the activ-
ity of NMDA receptors by inhibition of GlyT1 has emerged as an
attractive approach to test the NMDA hypofunction hypothesis
and potentially provide a treatment of schizophrenia.7 This ap-
proach is supported by clinical trials in which treatment of schizo-
O
OMe
O
NH2
N
H
N
H
Cl
N
S
N
S
O
O
O
O
1 (IC50 = 130 nM)
2 (ACPPB, IC50 = 2.6 nM)
* Corresponding author.
E-mail address: Zhijian_Zhao@merck.com (Z. Zhao).
Figure 1. 4,4-Disubstituted piperidine GlyT1 inhibitors.
0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2008.12.115

Z. Zhao et al. / Bioorg. Med. Chem. Lett. 19 (2009) 1488–1491
1489
In this letter, we report the results of our search for new selec-
tive and potent GlyT1 inhibitors with improved physical properties
and diminished time-dependent CYP inhibition. Early SAR studies
in the 4,4-disubstituted piperidine series suggested that a benzam-
ide and an n-propyl sulfonyl group on the 1-nitrogen of the piper-
idine core are required for maximizing GlyT1 potency. Therefore,
effort was focused on replacing the phenyl group on the C4 posi-
tion of the piperidine core. Our first library simply replaced the
C4-phenyl with a 2-pyridyl. As illustrated in Scheme 1, deprotona-
tion of Boc-protected 4-cyanopiperidine 3 and subsequent reaction
with 2-fluoropyridine generated 4.11 Deprotection of the Boc
group, sulfonylation, and Raney Ni-catalyzed hydrogenation affor-
ded the key intermediate amine 6. Amide formation could be
achieved by either direct acylation of amine 6 with an acid chloride
CN
CN
CN
a
b,c
N
N
N
Boc
N
N
S
Boc
O
O
3
4
5
O
N
NH2
N
N
H
Ar
d
e
N
S
N
S
O
O
O
O
7
6
Table 2
Scheme 1. Reagents and conditions: (a) KHMDS, THF, 25 °C, then 2-fluoropyridine;
(b) 4 M HCl/dioxane; (c) n-PrSO2Cl, i-Pr2NEt, CH2Cl2, 0–25 °C; (d) H2, Raney Ni; 2M
NH3 in MeOH, 25 °C; (e) i—ArCOCl, i-Pr2NEt, CH2Cl2, 0–25 °C, or ii—ArCO2H, PS-DCC,
HOBt, i-Pr2NEt, CH2Cl2, 25 °C, high throughput LC–MS purification.12
Human GlyT1 inhibition of substituted 2-pyridyl substitution at piperidine 4-position
R1
O
R2
N
N
H
Ar
significantly enhances prepulse inhibition (PPI) in DBA/2J mice
without impairing basal startle amplitude indicating an antipsy-
chotic effect without sedation.10 However, time-dependent CYP
inhibition and poor water solubility prevented further develop-
ment of 2.
N
S
O
O
8--11
R1
a
Compound
Ar
R2
H
GlyT1 IC50 (nM)
8a
CH3
CH3
129
Table 1
Human GlyT1 inhibition of 2-pyridyl substitution at piperidine 4-position
O
8b
8c
9a
9b
H
H
38.3
Cl
Cl
N
N
H
Ar
N
S
CH3
H
3.5
54.7
3.0
Cl
O
O
7
a
a
CH3
CH3
Compound Ar
GlyT1 IC50 Compound Ar
(nM)
GlyT1 IC50
(nM)
7a
107
7h
7i
103
517
H
Cl
Cl
Cl
Cl
7b
49.4
9c
H
H
H
H
H
H
H
CH3
2.5
48.3
6.9
F
Cl
Cl
Cl
7c
7d
7e
19.5
7j,CPyPB
4.4
131
6.6
10a
10b
10c
11a
11b
11c
OCH3
OCH3
Cl
Cl
Cl
833
7k
7l
Cl
Cl
Cl
Cl
OCH3
7.4
31.3
19.4
11.9
F3C
Cl
Cl
F
O
>5000
482
NH2
N
N
N
7f
7m
6.9
5.7
F3CO
O
O
Cl
Cl
Cl
F
7g
7n,CFPyPB
412
Cl
All compounds >30,000 nM versus GlyT2 and TauT.
Compounds 8a–8c, 9a–9c, and 10a–10c >30,000 nM versus GlyT2 and TauT.
a
a
IC50 are average of at least 3 measurements.
IC50 are average of at least 3 measurements.

1490
Z. Zhao et al. / Bioorg. Med. Chem. Lett. 19 (2009) 1488–1491
O
Cl
N
O
Cl
O
Cl
N
N
H
N
H
N
N
H
Cl
Cl
Cl
N
S
N
S
N
S
O
O
O
O
O
O
12
(
IC50 = 69.3 nM)
13 (IC50 = 79.3 nM)
7j (IC50 = 4.4 nM)
Figure 2. IC50 of 2-, 3- and 4-pyridyl in the same benzamide.
or coupling of 6 with a carboxylic acid, polymer-bound DCC (PS-
DCC), HOBt, and i-Pr2NEt. Accordingly, >200 benzamide analogues
were synthesized in iterative focused libraries employing solution
phase parallel synthesis. Selected compounds and data are summa-
rized in Table 1.
the same route shown in Scheme 1.). Replacement of 2-pyridyl in
7j with 3- or 4-pyridyl leads to a significant potency loss as indi-
cated in Figure 2.
In an effort to evaluate the effects of a chiral methyl group a-po-
sition to the benzamide nitrogen, libraries (R)-15 and (S)-15 were
prepared (Scheme 2). The chiral methyl group was introduced to 5
by methyl Grignard addition followed by NaBH4 reduction of the
intermediate imine.14 The resulting racemic amines were sepa-
rated by chiral HPLC to provide the enantiomerically pure (R)-14
and (S)-14. Amide formation of pure (R)-14 or (S)-14 with 80 car-
boxylic acids or acid chlorides afforded the libraries (R)-15 or (S)-
15, respectively. Selected compounds and data are presented in Ta-
Toourdelight, the2-pyridylsubstitutedcompounds, exemplified
by 7a–7n, not only retain GlyT1 potency, but also maintain high
selectivity versus GlyT2 and (TauT) even without the
a-methyl
group present in 2. Table 1 reveals the GlyT1 inhibition SAR in the
2-pyridyl series. The GlyT1 potency is enhanced by an ortho-substi-
tution on the phenyl ring of the benzamide moiety. This is clearly
demonstrated by the 10Â increase in potency from compound 7a
to 7g, where an o-chlorine is incorporated. Significantly, in this 2-
pyridyl series, benzamides lacking the aniline present in 2 were
identified which exhibit potency equivalent to 2 (compare 7j and
7nto7m). Furtherevaluationof7j(2,4-dichloro-N-((1-(propylsulfo-
nyl)-4-(pyridin-2-yl)piperidin-4-yl)benzamide, CPyPB) and 7n(3,6-
dichloro-2-fluoro-N-((1-(propylsulfonyl)-4-(pyridine-2-yL)piperi-
din-4-yl)methyl)benzamide, CFPyPB) showed that they are supe-
rior to 1: (1) Neither 7j nor 7n are P-gp substrates, a preferred
for any compound to be developed as a CNS drug; (2) Neither 7j
nor 7n are time-dependent CYP inhibitors which is an issue for
2; and (3) both 7j and 7n have improved water solubility compared
ble 3. Two features stand out in this chiral
a-methyl benzamide
series. Firstly, this series displays highly enantioselective inhibition
of GlyT1; all S isomers are much more potent then their R ana-
logues. Secondly, the S-methyl analogues are equipotent to the
analogues lacking an a-methyl group. This is in contrast to the 4-
phenylpiperidine series where
a-methyl substitution boosted po-
tency up to 10-fold.10
To explore how more dramatic structural changes affect GlyT1
activity, the planar 2-pyridyl group in the compound 7 series
was replaced with the nonplanar 2-(1-methylpiperidinyl) moiety
which resulted in a bispiperidine library 18 (48 analogues were
prepared). The chemistry employed to incorporate the 2-(N-meth-
ylpiperidyl) group into library 18 is shown in Scheme 3. Although
the potency decreases slightly (Table 4) from the 2-pyridyl to the
to 2 (2 and its HCl salt are insoluble in water (<50
lg/mL); the HCl
salt of 7n and 7j achieve water solubility of 570 g/mL and 850
l
lg/
mL, respectively). Moreover, 7n exhibits effects in rodent microdi-
alysis and pre-pulse inhibition studies equivalent to 2.13
Following the same route illustrated in Scheme 1, different
substituted 2-pyridyl libraries were prepared to investigate substi-
tuent effects at the 2-pyridyl ring. Selected compounds and GlyT1
inhibition data are presented in Table 2. In general, small substitu-
ents at the 4-position (4-Me, 8a–8c) or 6-position (6-Me, 9a–9c; 6-
MeO, 10a–10c) on the 2-pyridyl ring are well tolerated. In fact, in
the cases of both 4-methyl and 6-methyl group substitution, GlyT1
potencies are slightly increased by the addition of a methyl group
to the pyridyl ring (compare 8c and 9c with 7j). However, a large,
polar group such as morpholinyl, on the 6-position of the 2-pyridyl
ring is not tolerated and dramatically decreases GlyT1 inhibition as
illustrated by the comparison of 7j and 11c.
Table 3
Human GlyT1 activity of substituted 2-pyridyl substitution at piperidine 4-position
CH3
O
CH3
O
N
N
H
Ar
N
N
H
Ar
N
S
N
S
O
O
O
-15
O
R
S
-15
Compound Ar
GlyT1
IC50
Compound Ar
GlyT1
IC50
a
a
It should be mentioned that the nitrogen position at the pyri-
dine ring is very important for GlyT1 potency based on our obser-
vations from libraries of 3- and 4-pyridyl substitution at the
piperidine C4-position (These libraries were prepared following
(nM)
(nM)
(R)-15a
2592
(S)-15a
17.7
F3CO
Cl
F3CO
(R)-15b
(R)-15c
661
576
(S)-15b
(S)-15c
3.0
4.0
CH3
NH2
CH3 O
Cl
Cl
Cl
CN
N
N
N
N
H
Ar
a,b
c
Cl
Cl
F
F
F
N
S
N
S
N
S
O
O
O
O
O
O
5
F
(R)-14 and (S)-14
(R)-15 and (S)-15
Compounds (S)-15a–c >30,000 nM versus GlyT2 and TauT.
The R and S configurations were determined by X-ray analysis of the Morsher amide
of amine (S)-14.
Scheme 2. Reagents and conditions: (a) i—MeMgBr, toluene, 25 °C; ii—NaBH4,
MeOH; (b) ChiralPak AD; (c) i—ArCOCl, i-Pr2NEt, CH2Cl2, 0– 25 °C, or ii—ArCO2H, PS-
DCC, HOBt, i-Pr2NEt, CH2Cl2, 25 °C, high throughput LC–MS purification.12
a
IC50 are average of at least 3 measurements.

Z. Zhao et al. / Bioorg. Med. Chem. Lett. 19 (2009) 1488–1491
1491
O
CN
CN
N+
b,c
d
N
N
NH2
N
N
H
Ar
a
-
CF3SO3
N
S
N
S
N
S
N
S
O
O
O
O
O
O
O
O
17
18
5
16
Scheme 3. Reagents and conditions: (a) CF3SO3Me, 0–25 °C; (b) MPBH4, MeOH 25 °C; (c) H2, Raney Ni; 2 M NH3 in MeOH, 25 °C; (d) i—ArCOCl, i-Pr2NEt, CH2Cl2, 0–25 °C, or
ii—ArCO2H, PS-DCC, HOBt, i-Pr2NEt, CH2Cl2, 25 °C, high throughput LC–MS purification.12
came undesirable time-dependent CYP inhibition. Further results
within this series will be disclosed in due course.
Table 4
Human GlyT1 activity of N-methylpiperidine substitution at piperidine 4-position
O
Acknowledgments
N
N
H
Ar
We thank Carl Homnick for performing chiral separations, Joan
MurphyandCharles W. Ross, III, forHRMS measurements, andNancy
Tsou for X-ray crystal analysis of the Mosher’s amide of (S)-14.
N
S
O
O
18
References and notes
Compound Ar
GlyT1
IC50
(nM)
Compound Ar
GlyT1
IC50
(nM)
a
a
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3314
18d
58.9
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18e
9.5
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Cl
Cl
F
F
18c
21.2
18f
43.1
7. Sur, C.; Kinney, G. G. Expert Opin. Investig. Drugs 2004, 13, 515. and references
therein.
F3CO
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a
IC50 are average of at least 3 measurements.
10. (a) Lindsley, C. W.; Zhao, Z.; Leister, W. H.; O’Brien, J.; Lemaire, W.; Williams, D.
L.; Chen, T.-B.; Chang, R. S. L.; Burno, M.; Jacobson, M. A.; Sur, C.; Kinney, G. G.;
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2-(N-methylpiperidinyl) substitution at the central piperidine C4
position, the addition of a more basic amine is anticipated to im-
prove physical properties within this series.
In summary, by modification of the piperidine C4-substituent of
lead compounds 1 and 2, we have discovered two series of potent
and selective GlyT1 inhibitors through a library approach. New
compounds 7j and 7n from 2-pyridyl library not only improved
physical properties (e.g. greater water solubility) but also over-
13. Manuscript in progress.
14. Brussee, J.; Dofferhoff, F.; Kruse, C. G.; Van der Gen, A. Tetrahedron 1990, 46,
1653.