Design, synthesis, and evaluation of tetrahydroquinoline and pyrrolidine sulfonamide carbamates as γ-secretase inhibitors

Article abstract of DOI:10.1016/j.bmcl.2007.03.055

γ-Secretase is a key enzyme involved in the production of β-amyloid peptides which are believed to play a critical role in the onset and progression of Alzheimer's disease (AD). As such, inhibition of γ-secretase has been an attractive approach to AD ther

Full text of DOI:10.1016/j.bmcl.2007.03.055

Bioorganic & Medicinal Chemistry Letters 17 (2007) 3010–3013
Design, synthesis, and evaluation of tetrahydroquinoline
and pyrrolidine sulfonamide carbamates as c-secretase inhibitors
Tao Guo,a,* Huizhong Gu,a Doug W. Hobbs,a Laura L. Rokosz,a Tara M. Stauffer,a
Biji Jacoba and John W. Claderb
aPharmacopeia Drug Discovery, Inc., P.O. Box 5350, Princeton, NJ 08543-5350, USA
bSchering-Plough Research Institute, 2015 Galloping Hill Road, Kenilworth, NJ 07033, USA
Received 27 February 2007; revised 15 March 2007; accepted 19 March 2007
Available online 23 March 2007
Abstract—c-Secretase is a key enzyme involved in the production of b-amyloid peptides which are believed to play a critical role in
the onset and progression of Alzheimer’s disease (AD). As such, inhibition of c-secretase has been an attractive approach to AD
therapy. In this paper, the design, synthesis, and evaluation of tetrahydroquinoline and pyrrolidine sulfonamide carbamates as c
-secretase inhibitors are described.
Ó 2007 Elsevier Ltd. All rights reserved.
Alzheimer’s disease (AD) has become the most common
cause of dementia in the elderly, affecting approximately
4.5 million people in the US and 15 million people world-
wide.1a,b AD is a devastating disease characterized by a
progressive loss in memory and cognitive function which
ultimately leads to death. The pathological hallmark of
AD includes extraneuronal amyloid plaques containing
primarily aggregated b-amyloid peptides (Ab) and intra-
cellular neurofibrillary tangles (NFTs).1a,b Although
both amyloid plaques and NFTs are thought to contrib-
ute to neuronal cell death and the symptoms observed in
AD, the formation of amyloid plaques is unique to AD,
whereas NFTs have been found in some other uncom-
mon neurodegenerative diseases.1c Therefore, the pro-
duction and deposition of Ab peptides in the brain are
believed to play a key role in the onset and progression
of AD (the ‘amyloid cascade hypothesis’).1d
for novel c-secretase inhibitors, various cyclic amine sul-
fonamide carbamates (2–6) were designed based on a
screening hit 1 (IC50 = 2.5 lM).3d The SAR of tetrahy-
droquinoline sulfonamide carbamates 2 and piperidine
N
O
S
O
Cl
1
γ-secretase IC50 = 2.5 μM
Ab peptides are produced via the sequential cleavage of
b-amyloid precursor protein (b-APP) by two proteases:
b- and c-secretases. Inhibition of either b- or c-secreta-
ses has become an attractive approach to the treatment
of AD. c-Secretase is a novel membrane-bound aspartyl
protease composed of presenilin-1/2, nicastrin, Aph-1,
and pen-2, and to date a number of small molecule c-
secretase inhibitors have been reported.2,3 In our search
n
NR1R2
O
NR1R2
O
N
S
N
S
m
O
O
O
O
O
O
Cl
Cl
Keywords: Alzheimer’s disease; c-Secretase inhibitors.
Corresponding author. Tel.: +1 609 452 3746; fax: +1 609 452
3699; e-mail: tguo@pcop.com
5: n = 1
6: n = 0
2: m = 1
3: m = 2
4: m = 3
*
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.03.055

T. Guo et al. / Bioorg. Med. Chem. Lett. 17 (2007) 3010–3013
3011
sulfonamide carbamates 5 have been recently dis-
closed.3d–f In this paper, the synthesis and SAR of tetra-
hydroquinoline sulfonamide carbamates 3, 4, and
pyrrolidine sulfonamide carbamates 6 are described.
N
S
CHO
a-f
g,h
O
O
N
CO2H
The synthesis of carbamates 3 is illustrated in Scheme 1.
Hydrogenation of quinoline-2-acetic acid methyl ester 74
(prepared from quinoline-N-oxide according to the liter-
ature procedure in Ref. 4) using H2/PtO2 followed by
sulfonylation with 4-chlorobenzenesulfonyl chloride
gave sulfonamide 8. Then, reduction of 8 using DI-
BAL-H provided alcohol 9. Finally, conversion of 9 to
4-nitrophenylcarbonate followed by treatment with
various amines yielded target compounds 3.
10
Cl
11
OH
O
NR1R2
N
N
S
O
S
O
O
O
O
i,j
The preparation of carbamates 4 is shown in Scheme 2.
The synthesis began with hydrogenation of quinaldic
acid 10, followed by LAH reduction, TMS-protection
of the resulting alcohol, sulfonylation, TMS-deprotec-
tion, and Swern oxidation to provide aldehyde 11. Next,
after Wittig coupling of 11 with methyl (triphenylphos-
phoranylidene)acetate, DIBAL-H reduction gave
propanol 12. Finally, conversion of 12 to 4-nitrophenyl-
carbonate followed by reactions with various amines
furnished target compounds 4.
Cl
Cl
12
4
Scheme 2. Preparation of tetrahydroquinoline sulfonamides 4.
Reagents and conditions: (a) H2, PtO2, MeOH, rt, 6 h, 100%; (b)
LiAlH4, THF, reflux, 4 h, 87%; (c) TMSCl, Et3N, CH2Cl2, 0 °C,
15 min, 93%; (d) 4-chlorobenzenesulfonyl chloride, Et3N,
ClCH2CH2Cl, 70 °C, 16 h, 47%; (e) K2CO3, MeOH, 0 °C, 30 min,
96%; (f) Swern oxidation, 61%; (g) methyl (triphenyl-phosphoranylid-
ene)acetate, THF, rt, 16 h, 100%; (h) DIBAL-H, toluene/hexane, rt,
3 h, 95%; (i) 4-nitrophenyl chloroformate, Et3N, CH2Cl2, rt, 16 h; (j)
R1R2NH, Et3N, CH2Cl2, rt, 6 h, 50–95% (steps i–j).
The synthesis of carbamates 6 is illustrated in Scheme 3.
Commercially available (2R,5S)-Boc-5-phenyl-pyrroli-
dine-2-carboxylic acid 13 (SNPE North America LLC)
was treated with HCl to remove Boc, followed by
LAH reduction, TMS-protection of the resulting alco-
hol, and sulfonylation to give pyrrolidine sulfonamide
silylether 14. The TMS protecting group in 14 was then
removed using K2CO3 in MeOH to provide alcohol 15.5
Finally, conversion of alcohol 15 to 4-nitrophenylcar-
OTMS
N
a-d
OH
O
S
O
e
N
O
Boc
Cl
13
14
CO2Me
a,b
N
c
O S O
CO2Me
N
1
2
O
N
R R
OH
N
S
N
O
f,g
7
O
O
O
S
O
Cl
8
Cl
Cl
O
6
15
N
OH
N
O
NR1R2
Scheme 3. Preparation of pyrrolidine sulfonamides 6. Reagents and
conditions: (a) 4N HCl, dioxane, rt, 2 h; (b) LiAlH4, THF, reflux, 4 h;
(c) TMSCl, Et3N, CH2Cl2, 0 °C, 45 min, 100% (steps a–c); (d) 4-
chlorobenzenesulfonyl chloride, Et3N, ClCH2CH2Cl, 70 °C, 16 h, 52%;
(e) K2CO3, MeOH, rt, 30 min, 96%; (f) 4-nitrophenyl chloroformate,
Et3N, CH2Cl2, rt, 16 h; (g) R1R2NH, Et3N, CH2Cl2, rt, 6 h, 50–95%
(steps f–g).
d,e
O S O
O S O
Cl
Cl
9
3
Scheme 1. Preparation of tetrahydroquinoline sulfonamides 3.
Reagents and conditions: (a) H2, PtO2, MeOH, rt, 4 h; (b) 4-
chlorobenzenesulfonyl chloride, Et3N, ClCH2CH2Cl, 70 °C, 16 h,
28% (steps a–b); (c) DIBAL-H, toluene/hexane, rt, 16 h, 60%; (d) 4-
nitrophenyl chloroformate, Et3N, CH2Cl2, rt, 16 h; (e) R1R2NH, Et3N,
CH2Cl2, rt, 6 h, 50–95% (steps d-e).
bonate followed by reactions with various amines gave
target compounds 6.
Table 1 lists the SAR6,7 of tetrahydroquinoline sulfon-
amide carbamates 3 and 4. For the two carbon chain

3012
T. Guo et al. / Bioorg. Med. Chem. Lett. 17 (2007) 3010–3013
linked carbamates (3a–g, m = 2), IC50 values ranging
from 280 nM (3b) to 1900 nM (3c) were achieved. For
the three carbon chain linked carbamates (4a–g,
m = 3), potency enhancements of ꢀ5-fold were observed
for carbamates of piperazine and piperidine with bulky
side chains as compared to the corresponding two car-
bon chain carbamates (4e–g vs. 3e–g). In contrast, little
change in potency was observed for primary amine car-
bamates (4a,b vs. 3a,b) or for carbamates of piperazine
and piperidine with small side chains (4c,d vs 3c,d).
The potency levels observed for the three carbon chain
linked carbamates are similar in magnitude to those
found for the one carbon chain linked carbamates (2,
m = 1).3d As an example, carbamate 4g (IC50 = 60 nM,
m = 3) is almost equipotent to carbamate 2g3d
(IC50 = 73 nM, m = 1). It should be noted that all the
compounds shown in Table 1 are racemic mixtures.
The two enantiomers of 2g had been separated by chiral
preparative HPLC, with one enantiomer displaying an
IC50 of 39 nM, while the other enantiomer showing an
IC50 of >1000 nM (see Ref. 3d). However, the absolute
stereochemistry for the active enantiomer was not
established.
Table 1. SAR of tetrahydroquinoline sulfonamide carbamates
O
m
NR1R2
N
S
O
O
O
Cl
Compound
m
NR1R2
IC50 (nM)a
H
N
N
3a
2
530
H
N
N
4a
3b
4b
3c
4c
3d
4d
3e
4e
3f
3
2
3
2
3
2
3
2
3
2
3
2
3
1
220
280
86
Table 2 summarizes the SAR6,7 of pyrrolidine sulfon-
amide carbamates 6. As can be seen from Table 2,
N
N
H
N
N
Table 2. SAR of pyrrolidine sulfonamides
H
N
N
n
O
NR1R2
1900
780
880
680
1000
180
900
190
360
60
N
N
N
S
N
N
O
O
O
Cl
OH
OH
N
N
Compound
n
NR1R2
IC50 (nM)a
H
N
N
N
N
N
N
N
6a
0
600
N
H
N
6b
6c
0
0
0
0
0
0
1
650
450
540
120
140
180
163e
N
N
N
N
H
N
OH
6d
6e
N
N
N
N
N
H
N
4f
N
H
N
3g
4g
2g3d
N
N
N
N
6f
N
N
N
6g
5g3e
N
N
N
733d
N
a IC50’s are mean values of two or more determinations with the
standard deviations no greater than 50% from the mean.6
a IC50’s are mean values of two or more determinations with the
standard deviations no greater than 50% from the mean.6

T. Guo et al. / Bioorg. Med. Chem. Lett. 17 (2007) 3010–3013
3013
3. (a) Rishton, G. M.; Retz, D. M.; Tempest, P. A.; Novotny,
J.; Kahn, S.; Treanor, J. J. S.; Lile, J. D.; Citron, M. J.
Med. Chem. 2000, 43, 2297; (b) Smith, D.W.; Munoz, B.;
Srinivasan, K.; Bergstrom, C.P.; Chaturvedula, P.V.;
Deshpande, M.S.; Kevy, D.J.; Lau, W.Y.; Parker, M.F.;
Sloan, C.P.; Wallace, O.B.; Wang, H.H. PCT Int. Appl.
WO2000050391; (c) Churcher, I.; Beher, D.; Best, J. D.;
Castro, J. L.; Clarke, E. E.; Gentry, A.; Harrison, T.;
Hitzel, L.; Kay, E.; Kerrad, S.; Lewis, H. D.; Morentin-
Gutierrez, P.; Mortishire-Smith, R.; Oakley, P. J.; Reilly,
M.; Shaw, D. E.; Shearman, M. S.; Teall, M. R.; Williams,
S.; Wrigley, J. D. Bioorg. Med. Chem. Lett. 2007, 17, 280;
(d) Asberom, T.; Bara, T. A.; Clader, J. W.; Greenlee, W.
J.; Guzik, H. S.; Josien, H. B.; Li, W.; Parker, E. M.;
Pissarnitski, D. A.; Song, L.; Zhang, L.; Zhao, Z. Bioorg.
Med. Chem. Lett. 2007, 17, 205; (e) Pissarnitski, D. A.;
Asberom, T.; Bara, T. A.; Buevich, A. V.; Clader, J. W.;
Greenlee, W. J.; Guzik, H. S.; Josien, H. B.; Li, W.;
McEwan, M.; McKittrick, B. A.; Nechuta, T. L.; Parker, E.
M.; Sinning, L.; Smith, E. M.; Song, L.; Vaccaro, H. A.;
Voigt, J. H.; Zhang, L.; Zhang, Q.; Zhao, Z. Bioorg. Med.
Chem. Lett. 2007, 17, 57; (f) Asberom, T.; Zhao, Z.; Bara,
T. A.; Clader, J. W.; Hyde, L. A.; Josien, H. B.; Li, W.;
McPhail, A. T.; Nomeir, A. A.; Parker, E. M.; Rajagopa-
lan, M.; Song, L.; Wong, G. T.; Zhang, L.; Zhang, Q.;
Pissarnitski, D. A. Bioorg. Med. Chem. Lett. 2007, 17, 511.
4. Gilchrist, T. L.; Rahman, A. J. Chem. Soc. Perkin Trans. 1
1998, 7, 1203.
carbamates bearing piperazines and piperidines with
bulky side chains (6e–g) are more active than carbamates
bearing acyclic amines (6a–b) or piperazines and piperi-
dines with small side chains (6c–d). Compound 6e
showed the highest potency in this series with an IC50 va-
lue of 120 nM. It should be noted that compounds 6a–g
were prepared as pure (2R,5S)-enantiomers.5 The antip-
odes, (2S,5R)-enantiomers, were also prepared but all
gave IC50 > 10,000 nM (data not shown).5 As a result,
the strong preference for (2R,5S)-configuration in c-
secretase inhibition is clearly established. It should also
be noted that the pyrrolidine carbamates 6 are in general
ꢀ10-fold less potent than the corresponding piperidine
carbamates 5.3e For example, pyrrolidine analog 6g
(IC50 = 180 nM) is 11-fold less potent than piperidine
analog 5g3e (IC50 = 16 nM). Interestingly, using chiral
HPLC and Mosher’s method, the absolute stereochemis-
try required for c-secretase inhibition by the piperidine
carbamates 5 had been established to have the (2R,6S)-
configuration (see Ref. 3e), consistent with that for
pyrrolidine carbamates 6. Based on these results, the
absolute stereochemistry for the active enantiomers in
the tetrahydroquinoline carbamates could be inferred
as the (2R)-configuration. However, this hypothesis
needs further experimental confirmation.
5. The alcohol intermediate 15 was analyzed by chiral HPLC
using analytical Chiralcel OD column, 10% isopropanol in
hexane, 1 mL/min, 254 nm: tR = 9.9 min, >99% e.e. The
antipode, (2S,5R)-15, was also prepared (from (2S,5R)-
Boc-5-phenyl-pyrrolidine-2-carboxylic acid) and analyzed:
tR = 12.3 min, >99% e.e. Each pure enantiomer was inde-
pendently converted to the corresponding carbamates.
6. To measure c-secretase activity, compounds were diluted
serially in DMSO, followed by a 37.5-fold dilution in start
buffer (Hepes, pH 7.5, and 5 mM EDTA). 10 lL of
membranes (5.0 lg total protein) in 20 mM Hepes, pH
5.0, and 5 mM EDTA, prepared as described in Ref. 7, was
mixed with 30 lL of start buffer containing either com-
pound or 2.7% DMSO (2% final) in a black 96-well Corning
Costar HTRF-compatible plate and incubated for 2 h at
37 °C. The enzymatic reaction was quenched with the
addition of 40 lL of a mixture containing 100 mM Tris–
HCl, pH 7.5, 50 mM NaCl, 0.5% BSA, 2.0% Triton X-100,
17 nM biotin-W02 (Ab40-specific antibody), 85 nM allo-
phycocyanin-streptavidin (Wallac), and ꢀ2 nM Europilat-
ed-G2-10 (Ab40-specific antibody labeled using Europium
chelate from Wallac according to the manufacturers’
procedures). The mixture was incubated for two to 16 h
at room temperature to allow the signal to develop and the
plate was read on a Wallac Victor. Fluorescence was
recorded at 665 nm. The signal at 615 nm was also
monitored to check for fluorescence interference or other
artifacts. All IC50’s represent the mean values of two or
more determinations with the standard deviations no
greater than 50% from the mean.
In summary, novel cyclic amine sulfonamide carbamates
have been discovered as potent c-secretase inhibitors.
Tetrahydroquinoline sulfonamide carbamates with one
and three carbon chain linkers are more potent than
those with a two carbon chain linker. Pyrrolidine sulfon-
amide carbamates are in general less potent than the
corresponding piperidine sulfonamide carbamates. A
clear preference for the (2R,5S)-configuration was firmly
established for the pyrrolidine sulfonamide carbamate
c-secretase inhibitors.
Acknowledgment
The authors thank Dr. William J. Greenlee of Schering-
Plough Research Institute for helpful discussions, sup-
port, and encouragement.
References and notes
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Parker, E. Biochemistry 2001, 40, 5049.