Synthesis and SAR of tolylamine 5-HT6 antagonists

Article abstract of DOI:10.1016/j.bmcl.2009.03.077

The synthesis and SAR of tolylamines with 5-HT₆ receptor antagonist activity is presented. The amine, core aromatic, peripheral aromatic, and ether linker moieties of HTS hit 1 were modulated and the effect on potency at 5-HT₆ examin

Full text of DOI:10.1016/j.bmcl.2009.03.077

Bioorganic & Medicinal Chemistry Letters 19 (2009) 2409–2412
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and SAR of tolylamine 5-HT6 antagonists
Jamie M. Singer, Michael W. Wilson, Paul D. Johnson, Shelley R. Graham, Leonard W. Cooke, Robin L. Roof,
Peter A. Boxer, Lisa H. Gold, Leonard T. Meltzer, Ann Janssen, Nicole Roush, Jeffrey E. Campbell, Ti-Zhi Su,
*
Susan I. Hurst, Chad L. Stoner, Jacob B. Schwarz
Pfizer Global Research and Development, Eastern Point Road, Groton, CT 06340, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and SAR of tolylamines with 5-HT6 receptor antagonist activity is presented. The amine,
core aromatic, peripheral aromatic, and ether linker moieties of HTS hit 1 were modulated and the effect
on potency at 5-HT6 examined. Tolylpiperidine ether 9h was found to possess desirable pharmacokinetic
(PK) properties, and was also shown to enhance cognition in the rat novel object recognition paradigm.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 8 December 2008
Revised 18 March 2009
Accepted 20 March 2009
Available online 25 March 2009
Keywords:
5-HT6
Phenylpiperidine
Object recognition
The serotonin-6 (5-HT6) receptor is the most recently discov-
ered in the family of serotonin receptors and has generated intense
research activity from a number of groups.1 It has been postulated
that 5-HT6 ligands may afford useful therapies for the treatment of
obesity,2 as well as cognitive enhancement in schizophrenia3 and
Alzheimer’s disease4 owing to preferential expression of the recep-
tor in the central nervous system. Numerous clinical candidates
have been advanced including SB-742457, PRX-07034, and SGS-
518, all for which positive clinical study data have been reported.5
Herein, we wish to disclose the discovery of a series of novel tolyl-
amine 5-HT6 antagonists and their evaluation as potential drug
candidates.
zine with the nitrogen mustard. Protection with Boc anhydride
afforded the desired phenol 4 in addition to some bis-BOC interme-
diate 3. The mixture was of no consequence as hydrolysis with car-
bonate liberated pure phenol 4 for further manipulation. The
aromatic moiety could be introduced either via Mitsunobu displace-
ment with various benzyl alcohols or alkylation in the presence of a
potassium base, and subsequent removal of the carbamate with
acid provided tolylpiperazines 5.
Tolylpiperazines with close chemical similarity had in fact been
reported6 which led us to examine the corresponding piperidine
analogs (Scheme 2). This modification also served to eliminate
the aniline moiety, which has been shown in the case of nefazo-
done to form a reactive metabolite via para-hydroxylation and for-
HN
N
O
BocN
a,b
c
OH
H2N
N
OBoc
1
High throughput screening led to the identification of tolylpiper-
azine ether 1 as a potent 5-HT6 receptor ligand. The toluene methyl
group enhanced the potency for 5-HT6 eightfold compared to the
des-methyl version, and as such this moiety was included in all sub-
sequent analogs. Initially we focused on the effect of modulating the
aromatic periphery of 1 on potency for the 5-HT6 receptor, owing to
the potential for rapid analog generation (Scheme 1). Hence, 2-
methyl-3-amino-2-methyl-phenol 2 was converted to the pipera-
2
3
HN
BocN
d,e
Ar
OH
N
O
N
R
5
4
Scheme 1. Preparation of tolylpiperazine analogs 5. Reagents and conditions: (a)
(ClCH2CH2)2NH, NaI, iPr2NEt, n-hexanol, PhCl; (b) Boc2O, NaOH; (c) K2CO3, MeOH
(27%, 3 steps); (d) ArCH(R)OH, DIAD, PPh3, or ArCH2Br, KHMDS, THF; (e) 4 M HCl in
dioxane.
* Corresponding author. Tel.: +1 860 441 0744; fax: +1 860 686 7254.
E-mail address: jacob.schwarz@pfizer.com (J.B. Schwarz).
0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2009.03.077

2410
J. M. Singer et al. / Bioorg. Med. Chem. Lett. 19 (2009) 2409–2412
BocN
d,e
BocN
a,b
c
a
b
OBn
HO
I
F
OBn
F
N
N
6
7
10
11
HN
HN
BocN
BocN
OH
O
Ar
c,d
F
O
Ar
R
N
N
R
8
9
12
13
Scheme 2. Preparation of tolylpiperidine analogs 9. Reagents and conditions: (a)
Tf2O, Et3N (64%,); (b) 4-(5,5-dimethyl-[1,3,2]dioxaborinan-2-yl)-3,6-dihydro-2H-
pyridine-1-carboxylic acid tert-butyl ester,10 30% Pd(dppf)Cl2, Cs2CO3, CsF, DMF
(37%); (c) H2, Pd/C (61%); (d) ArCH(R)OH, DIAD, PPh3, or ArCH2Br, KHMDS; (e) 4 M
HCl in dioxane.
Scheme 3. Preparation of piperidinylpyridine analogs 13. Reagents and conditions:
(a) 4-(4,4,5,5-Tetramethyl-[1,3,2]dioxaborolan-2-yl)-3,6-dihydro-2H-pyridine-1-
carboxylic acid tert-butyl ester, Pd(PPh3)2Cl2, K2CO3, toluene/EtOH (27%,); (b) H2,
PtO2, MeOH/THF (88%); (c) ArCH(R)OH, NaH, THF; (d) 4 M HCl in dioxane.
mation of the iminoquinone species.7 Hence, starting with 2-
methyl-3-benzyloxy-phenol 6,8 conversion to the triflate and cou-
pling to a dehydropiperidine boronate afforded tetrahydropyridine
7.9 Saturation of the piperidine ring was accomplished by hydroge-
nation, with concomitant unmasking of the phenol to furnish inter-
mediate 8. As before, either Mitsunobu displacement or direct
alkylation followed by removal of the carbamate provided tolylpi-
peridine ethers 9.
(5c) were tolerated. Mono-halogenation was preferred for the
piperidines (9a,b vs 9c–f). Pyridine was less potent in both series
(5g, 9g). The potency of benzylic substitution depended on stereo-
chemistry, with the (S)-configuration being preferred as in 5h and
9h,k. Benzylic hydroxylation on the piperidine motif (9p) was not
tolerated by 5-HT6.
Next, we examined the effect of installing a heteroatom into the
core aromatic ring as a pyridine in an effort to attenuate the clogP,
as an increased likelihood of in vivo toxic events may be expected
from drug candidates with high lipophilicity.12 Suzuki coupling of
2-fluoro-4-iodo-3-methyl-pyridine 1013 with a similar dehydropi-
peridine boronate9 to that used previously furnished fluoropyri-
dine 11 (Scheme 3). Hydrogenation of the olefin, displacement of
the fluoropyridine with a suite of benzyl alcohols, and finally car-
bamate deblocking afforded pyridyl ethers 13.
Compounds 5a–j and 9a–p were tested for their ability to dis-
place [3H]-LSD from human recombinant 5-HT6 receptors (Table
1).11 For the piperazines, aromatic halogens as well as a nitrile
Table 1
Affinity of tolylpiperazines 5 and tolylpiperidines 9 for h5-HT6 receptors
Binding data for the pyridyl ethers 13 is shown in Table 2. As
previously observed, meta-chlorination of the aromatic was pre-
ferred as in 13a,m,p. The most potent compounds from the series
resulted from substitution of CF3 at the benzylic ether carbon
[( )-13q,r].
HN
X
O
Y
R
Finally, we sought to examine the effect of modifying the ether
linker of piperidines 9 on potency. Transposition of the ether oxy-
Compound
X
R
Y
5-HT6 binding Kia (nM)
SB-271046
1
—
N
—
H
H
H
H
H
H
H
H
—
H
0.66b
1.3
10.3
0.60
15.3
11.6
9.3
61.6
126
45.6
70.7
2.5
5a
N
m-F
5b
5c
N
N
m-Cl
m-CN
o-F
3,4-Di-F
3,4-Di-Cl
3-Pyridinec
H
Table 2
Affinity of pyridylpiperidines 13 for h5-HT6 receptors
5d
5e
N
N
5f
N
HN
5g
N
5h
5j
N
N
Me (S)
Me (R)
H
X
O
H
Y
9a
9b
9c
9d
9e
9f
9g
9h
9j
9k
9m
9n
9p
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
C–OHd
m-F
m-Cl
N
R
H
3.3
H
H
H
H
m-CN
m-Me
3,4-Di-F
3,4-Di-Cl
3-Pyridinec
m-F
m-F
m-Cl
m-Cl
m-Cl
174
92.5
42.0
219
>440
6.9
39.8
2.8
39.5
17.9
>440
Compound
X
R
Y
5-HT6 binding Ki (nM)
13a
13b
13c
13d
13e
13f
13g
13h
13j
13k
13m
13n
13p
13q
13r
13s
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
C–OHa
H
H
H
H
H
H
H
H
m-Cl
m-F
m-CF3
o-Cl
p-Cl
o-F
p-F
2,4-Di-F
H
8.1
17.8
19.1
>440
50.0
>440
>440
239
48.5
>440
10.5
208
6.4
H
Me (S)
Me (R)
Me (S)
Me (R)
Et (R/S)
H
m-Cl
Me (S)
Me (R)
Me (S)
Me (R)
Et (R/S)
CF3 (R/S)
CF3 (R/S)
H
H
a
Ki = IC50/{1 + ([free radioligand]/Kd)}, where IC50 is the concentration (nM)
producing half-maximal inhibition of the specific binding of [3H]LSD to human
recombinant 5-HT6, and Kd is the dissociation constant determined by saturation
binding (=3.5). Each Ki represents an average of three replicates run in three sep-
arate experiments (n = 3).
m-Cl
m-Cl
m-Cl
m-Cl
m-F
m-F
2.4
4.3
>440
b
Lit.11b Ki = 0.7 nM.
Y = H.
c
d
a
Prepared by lithiation of TBS-protected 3-bromo-2-methyl-phenol, addition to
Prepared by addition of m-fluorobenzyl alcohol to 10 followed by lithiation and
1-Boc-4-piperidone, desilylation, and alkylation as per Scheme 2.
addition to 1-Boc-4-piperidone, and subsequent deprotection.

J. M. Singer et al. / Bioorg. Med. Chem. Lett. 19 (2009) 2409–2412
2411
Table 4
BocN
Rat pharmacokinetic parameters for tolylpiperidine 9h
c
a,b
Br
OH
OTBS
Parameter
Tolylpiperidine 9h
Bioavailability (%F)
Clearance (mL/min/kg)
Vdssa (L/kg)
90
66
12
5.9
23
14
15
HN
Plasma half-lifeb (t1/2, h)
Brain/plasma (total)
BocN
d,e
Ar
a
OH
HN
O
Volume of distribution at steady state.
From PO dosing.
b
16
17
profile. Although highly lipophilic, owing to the free amine some
free drug was available (Fu ꢀ 0.05). PK data is shown in Table 4.
High clearance in rat accompanied with large volume of distri-
bution resulted in a moderate half life of 5.9 h. Importantly, 9h
was selectively partitioned into rat brain, which would be re-
quired for achieving in vivo activity against neuropsychiatric
indications.
Functional efficacy of 9h was determined by quantifying the
accumulation of cAMP levels by ELISA in cells expressing recombi-
nant human or rat 5-HT6 receptors. While 5-HT markedly in-
creased cAMP levels, 9h did not stimulate levels of cAMP
f,e
O
Ar
O
8
S
O
18
Scheme 4. Preparation of transposed ethers 17 and sulfonyl esters 18. Reagents
and conditions: (a) TBSCl, imidazole (64%,); (b) 4-piperidinylzinc iodide, 30%
Pd(dppf)Cl2, CuI, DMA (53%); (c) TBAF, THF (34%); (d) ArCH2OH, DIAD, PPh3; (e) 4 M
HCl in dioxane; (f) ArSO2Cl, K2CO3, 1:1 acetone:CH3CN.
gen and carbon atoms was performed according to Scheme 4. 2-
Bromo-6-hydroxymethyltoluene 1414 was first protected (TBS),
then subjected to a Negishi coupling reaction with 4-piperidinyl-
zinc iodide15 to give 15. Deprotection to benzyl alcohol 16 and
ether formation under Mitsunobu conditions provided the desired
reverse ethers 17. A second linker modification was envisioned
based on the literature agent diF-BAMPI containing a sulfonyl es-
ter.16 These analogs were prepared in high yield from phenol 8
via straightforward coupling with various sulfonyl chlorides, fol-
lowed by deprotection to give 18.
Binding data for the reverse ethers 17 and sulfonyl esters 18 is
shown in Table 3. Potency of 17a and 17b was similar to that of the
tolylamines 5a,b and 9a,b in Table 1. Interestingly, sulfonyl esters
18 seemed to slightly prefer ortho-substitution although in general
were widely tolerant. Naphthylsulfonates 18m and 18n repre-
sented the most potent analogs from the series.
production in either rat or human cell line at 1, 3 and 10 lM, indi-
cating no agonist activity for the 5-HT6 receptor. Compound 9h did
behave as an antagonist by inhibiting 5-HT stimulated cAMP accu-
mulation in both human and rat 5-HT6 cells (Kb values of 159 and
153 nM for human and rat 5-HT6 receptors, respectively). No
appreciable binding of 9h to the 5-HT1A, 5-HT2A, or 5-HT2C receptor
subtypes was observed (all IC50 > 1.0 l
M).17 In addition, there ap-
peared to be no agonist activity and nominal antagonist activity
in the FLIPR 5HT2A and 5HT2B functional assays.18 Compound 9h
was tested in the novel object recognition paradigm19 using a 24
hour delay procedure in which non-treated rats no longer show
memory for a previously viewed object. Doses of 3 and 10 mg/kg
9h resulted in significantly greater exploration of the novel versus
the familiar object, thereby demonstrating efficacy for reversing
natural forgetting (Fig. 1).
In summary, a number of tolylamine analogs were prepared and
tested to assess their 5-HT6 antagonist activity. The piperazine mo-
tif of HTS hit 1 could be replaced by piperidine as in 9. The core tol-
uene group could be replaced by methylpyridine as in 13. The ether
linker could be reversed or replaced by a sulfonate ester (17 and
18). Finally, tolylpiperidine 9h was tested in rats and found to have
an acceptable PK profile including excellent CNS exposure, and in
addition improved cognition in the novel object recognition
paradigm.
Tolylpiperidine 9h was selected for further biological profiling
owing to its favorable in vitro ADME and receptor selectivity
Table 3
Affinity of tolylpiperidines 17 and 18 for h5-HT6 receptors
HN
A
B
Y
35
Familiar
*
Novel
30
25
20
15
10
5
Compound
A
B
Y
5-HT6 binding Ki (nM)
*
17a
17b
18a
18b
18c
18d
18e
18f
18g
18h
18j
18k
18m
18n
CH2
CH2
O
O
O
O
O
O
O
O
O
O
O
O
O
m-Cl
m-F
H
m-Cl
m-F
o-Cl
p-Cl
o-F
3.5
6.5
SO2
SO2
SO2
SO2
SO2
SO2
SO2
SO2
SO2
SO2
SO2
SO2
19.6
26.1
18.4
5.8
24.7
13.6
10.1
15.1
28.9
11.3
1.5
o-CH3
m-CH3
p-CH3
p-Et
0
Vehicle
0.3
1.0
Dose (mg/kg)
3.0
10.0
1-Naphthyla
2-Naphthyla
O
2.7
Figure 1. Rat novel object recognition assay data for tolylpiperidine 9h at t = 1 h
post IP dose (* = P < 0.05 for novel versus familiar object. N = 9–10 rats per dose
group).
a
Fused.

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J. M. Singer et al. / Bioorg. Med. Chem. Lett. 19 (2009) 2409–2412
8. Moorman, A. E.; Pitzele, B. S.; Jones, P. H.; Gullilkson, G. W.; Albin, D.; Yu, S. S.;
Acknowledgements
Bianchi, R. G.; Sanguinetti, E. L.; Rubin, B.; Grebner, M.; Monroy, M.; Kellar, P.;
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The authors wish to thank Janice Brown, Susan Borosky, Greg
Campbell, and Lynne Denny for their expertise in obtaining the
serotonin receptor selectivity data.
9. For a similar coupling of a dehydropiperidine boronate with an aryl triflate, see:
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