Heterocyclic sulfoxide and sulfone inhibitors of fatty acid amide hydrolase

Article abstract of DOI:10.1016/j.bmcl.2004.10.025

A novel series of heterocyclic sulfoxides and sulfones was prepared and examined as potential inhibitors of fatty acid amide hydrolase (FAAH), the enzyme responsible for inactivation of neuromodulating fatty acid amides including anandamide and oleamide. A novel series of heterocyclic sulfoxides and sulfones was prepared and examined as potential inhibitors of fatty acid amide hydrolase (FAAH), the enzyme responsible for inactivation of neuromodulating fatty acid amides including anandamide and oleamide.

Full text of DOI:10.1016/j.bmcl.2004.10.025

Bioorganic & Medicinal Chemistry Letters 15 (2005) 103–106
Heterocyclic sulfoxide and sulfone inhibitors of
fatty acid amide hydrolase
Wu Du, Christophe Hardouin, Heng Cheng, Inkyu Hwang and Dale L. Boger*
Department of Chemistry and The Skaggs Institute for Chemical Biology, The Scripps Research Institute,
10550 North Torrey Pines Road, La Jolla, CA 92037, USA
Received 23 July 2004; revised 5 October 2004; accepted 9 October 2004
Available online 28 October 2004
Abstract—A novel series of heterocyclic sulfoxides and sulfones was prepared and examined as potential inhibitors of fatty acid
amide hydrolase (FAAH), the enzyme responsible for inactivation of neuromodulating fatty acid amides including anandamide
and oleamide.
Ó 2004 Elsevier Ltd. All rights reserved.
O
1. Introduction
NH2
O
Oleamide (1)1,2 and anandamide (2)3,4 are prototypical
members of a class of endogenous fatty acid amides that
serve as chemical messengers.5 Anandamide is an endo-
genous fatty acid ethanolamide that binds and activates
the central (CB1) and peripheral (CB2) cannabinoid
receptors, and the vanilloid (VR1) receptor through
which it is thought to exhibit its analgesic and cannabi-
noid effects.4 Oleamide was found of accumulate in the
cerebrospinal fluid under conditions of sleep deprivation
and to induce physiological sleep in animals where it
reduces motility, shortens the sleep induction period,
and lengthens the time spent in slow wave sleep 2 at
the expense of wakening.1,6
Oleamide (1)
OH
N
H
Anandamide (2)
Fatty Acid Amide Hydrolase (FAAH)
O
OH
O
Oleic Acid (3)
OH
Arachidonic Acid (4)
Figure 1.
targetable catalytic mechanism (Ser-Ser-Lys triad), and
the consequences of inhibition (increase endogenous
level of anandamide and/or oleamide) have made FAAH
an attractive therapeutic target8,9 for the development of
a new class of analgesics, nonhypnotic sleep-aids, and a
range of additional clinical disorders.
Fatty acid amide hydrolase (FAAH)7 is an integral
membrane protein that degrades fatty acid primary
amides and ethanolamides including oleamide and
anandamide (Fig. 1).8,9 The distribution of FAAH in
the central nervous system suggests that it degrades neuro-
modulating fatty acid amides at their sites of action
and is responsible for or intimately involved in their regu-
lation.10 FAAH hydrolyzes a wide range of fatty acid
substrates and it appears to work most effectively on
arachidonyl and oleyl substrates.11 This enzyme belongs
to the amidase family of hydrolytic enzymes for which
FAAH is the only member found in mammalians.12 This
unique mammalian distribution,12–14 its attractive and
In spite of this interest, only a selected series of FAAH
inhibitors have been disclosed. These include the discov-
ery that the endogenous sleep-inducing compound 2-
octyl-c-bromoacetoacetate is an effective inhibitor of
FAAH,15 an early series of irreversible FAAH inhibitors
(fatty acid sulfonyl fluorides,16 fluorophosphonates,17
and a-diazoketones18), and an early series of fatty
acid-based reversible FAAH inhibitors bearing non-
selective electrophilic carbonyls.19–21 More recently,
two additional promising classes of FAAH inhibitors
*
Corresponding author. Tel.: +1 8587847522; fax: +1 8587847550;
e-mail: boger@scripps.edu
0960-894X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.10.025

104
W. Du et al. / Bioorg. Med. Chem. Lett. 15 (2005) 103–106
have emerged. The first of these is a series of reversible
a-ketoheterocycle inhibitors that exhibit an extraordi-
nary potency (Ki = 100–200pM) and/or selectivity,22
and the second constitutes a class of aryl carbamates
that acylate (carbamoylate) a FAAH active site catalytic
serine.23 Herein, we report the extension of the studies
on the former class to the examination of a unique series
of heterocyclic sulfoxide and sulfone FAAH inhibitors.
N
O
N
O
Ph(CH2)5Br
X
HS
14, X = S (66%)
15, X = SO (50%)
16, X = SO2 (46%)
13
mCPBA
(1.5 equiv)
Scheme 2.
N
N
N
RBr, Et3N
41−91%
N
RX
HS
2. Chemistry
N
N
O
O
17
The heterocyclic sulfoxides and sulfones examined con-
tain three classes of heterocycles established to be effec-
tive with the a-ketoheterocyclic FAAH inhibitors:22
substituted oxazoles, a fused oxazole, and substituted
oxadiazoles. They were prepared by oxidation of the
corresponding oleyl and phenylalkyl sulfides, which
were prepared by alkylation of corresponding thiols.
18a−d, X = S
mCPBA
19a−d, X = SO
20a−d, X = SO2
(1.5 equiv)
a, R =
b, R = Ph(CH2)5−
c, R = Ph(CH2)6−
d, R = Ph(CH2)7−
Scheme 3.
The synthesis of the substituted 2-mercaptooxazoles is
shown in Scheme 1. The Boc protected 2-aminoketones
5 (Ar = 2-pyridyl, 2-furanyl, 4-oxazolyl groups) were
first prepared from the corresponding heterocycles.24
Treatment of 5 with HCl followed by condensation of
the liberated free amine with thiophosgene in the pres-
ence of CaCO3 as a mild base yielded the isothio-
cyanates 6.25 Without purification, these labile
intermediates were treated with Et3N to give the 2-
mercaptooxazoles 7 through a base-catalyzed cycliza-
tion reaction. Without purification, crude 7 was treated
with cis-1-bromoheptadec-8-ene26 or 6-bromohexyl-
benzene in the presence of Et3N to give sulfides 8
or 9, respectively.
14, which was treated with mCPBA to afford sulfoxide
15 and sulfone 16 (Scheme 2).
The synthesis of the substituted 1,3,4-oxadiazole sulfox-
ides and sulfones is shown in Scheme 3. 2-Mercapto-
1,3,4-oxadiazole 17 was prepared as previously
described through condensation of pyridine-2-carbo-
xhydrazide with CS2 in the presence of KOH in refluxing
ethanol.27 Alkylation of 17 with four alkyl bromides
provided sulfides 18a–d. When sulfide 18a was treated
with 1equiv of mCPBA, the reaction gave a mixture of
the desired sulfoxide 19a in 35% yield and other epoxi-
dation products. The poor chemoselectivity in this reac-
tion compared to sulfide 8 may be attributed to the
greater electron deficient nature of oxadiazole, which
further deactivates sulfide 18a. Treatment of sulfides
18b–d with 1.5equiv of mCPBA yielded the sulfoxides
19b–d (33–56%) and sulfones 20b–d (42–62%) accord-
ingly (81–98% combined).
Oxidation of sulfides 8 with 1equiv mCPBA gave the
sulfoxides 10 in approximately 80% yields with good
chemoselectivity over epoxidation. When
8 were
exposed to a larger excess of mCPBA, epoxidation
occurred in preference of the formation of sulfone.
Oxidation of the sulfides 9 with ca 1.5equiv mCPBA
provided a mixture of the sulfoxides 11 (60–82%) and
sulfones 12 (15–32%) in excellent combined yields (92–
97%), which were easily separated by chromatography.
0.8
0.7
Alkylation of commercially available 2-mercaptobenz-
oxazole (13) with 5-bromopentylbenzene gave sulfide
0.6
1/v
0.5
0.4
0.3
0.2
0.1
0
S
C
N
N
1. HCl
Et3N
O
HS
O
BocHN
2. Cl2C=S
CaCO3
Ar
O
7a−c
Ar
Ar
5a−c
6a−c
8, R =
N
RBr, Et3N
-40000
-20000
0
20000
40000
60000
80000
RX
-0.1
35−54%
O
1/[S], (M–1)
Ar
for 4 steps
9, R =
8a−c, X = S
10a−c, X = SO
mCPBA
(1 equiv)
Figure 2. Lineweaver–Burk plot of competitive FAAH inhibition by
11a and 16. FAAH activity was monitored in the presence of 0lM
(filled diamond), 10lM (filled triangle), and 20lM (filled square) 11a;
and 0lM (filled diamond), 5lM (unfilled square), and 10lM (unfilled
diamond) 16 with an oleamide concentration varied from 13.6 to
108.8lM.
N
N
Ar =
9a−c, X = S
11a−c, X = SO
12a−c, X = SO2
mCPBA
O
O
(1.5 equiv)
a
c
b
Scheme 1.

W. Du et al. / Bioorg. Med. Chem. Lett. 15 (2005) 103–106
105
3. Inhibition studies
and KiÕs established as described (Dixon plot).22a
Lineweaver–Burk analysis established reversible, com-
petitive inhibition (Fig. 2).
Enzyme assays were performed at 20–23°C with purified
recombinant rat FAAH expressed in E. coli28 in a
125mM Tris/1mM EDTA/0.2% glycerol/0.2% Triton
X-100/0.4mM Hepes, pH9.0 buffer. The initial rates of
hydrolysis (610–20% reaction) were monitored using
enzyme concentrations at least 3 times below the
measured Ki by following the hydrolysis of 14C-oleamide
4. Results and discussion
The results of the examination of the sulfoxide and sulf-
one FAAH inhibitors are summarized in Figure 3.29,30
In each case, the sulfoxides and sulfones proved to be
10–100-fold more potent than the corresponding sul-
fides, consistent with an expected enhanced binding
affinity derived from a polarized oxygen binding an en-
zyme oxyanion pocket mimicking the tetrahedral inter-
mediate of the amide hydrolysis reaction. Moreover,
little distinction and no discernable trends were ob-
served in comparing the sulfoxides with the correspond-
ing sulfones and both inhibit FAAH to the same extent.
N
X
O
Ar
Ki
Compd
Aryl
N
X
—
8a
10a
CO22d
0.018 0.005 µM
160 7 µM
S
SO
8
0.5 µM
—
8b
10b
CO22d
S
0.012 0.001 µM
130 10 µM
N
O
O
SO
2.2 0.2 µM
Unlike the behavior of the corresponding a-ketohetero-
cycles, the potency of the corresponding sulfoxides and
sulfones was relatively insensitive to the nature of the at-
tached heterocycle as well as the side chain, although
each system examined represent those known to convey
effective FAAH active site binding.22 Finally, the sul-
fides were roughly 104–105-fold less potent than the cor-
responding a-ketoheterocycles, whereas the sulfoxides
and sulfones were roughly 102–103-fold less potent than
the a-ketoheterocyles. The notable exceptions are 15 and
16, which proved to be only 10-fold less potent than the
corresponding a-ketobenzoxazole, which constitutes one
of the least potent a-ketoheterocycles examined in this
series. Thus, the heterocyclic sulfoxides and sulfones
exhibit a potency intermediate of the corresponding
sulfides and a-ketoheterocycles illustrating that the
polarized oxygen binding in an enzyme oxyanion bind-
ing pocket increases affinity up to 100-fold and that
the reversible covalent binding of the a-ketoheterocycles
to an active site serine increases binding up to 1000-fold
beyond that of the corresponding sulfoxide and sulfone.
—
8c
10c
CO22d
S
SO
0.054 0.004 µM
180 10 µM
8
1 µM
N
X
Ph
O
Ar
Ki
Compd
Aryl
N
X
—
CO22d
0.0047 0.0013 µM
110 10 µM
11 0.4 µM
16 4.5 µM
9a
11a
12a
S
SO
SO2
—
CO22d
S
0.0046 0.0004 µM
80 5 µM
N
O
9b
11b
12b
SO
20 4 µM
SO2
10 2 µM
—
CO22d
0.012 0.001 µM
120 10 µM
30 10 µM
O
9c
11c
12c
S
SO
SO2
40 10 µM
N
O
Ph
Compd
X
Ki
X
—
14
15
16
CO22a
0.22 0.01 µM
150 10 µM
2.6 0.1 µM
3.3 0.7 µM
S
SO
SO2
Acknowledgements
N
O
N
X
We gratefully acknowledge the financial support of the
National Institute of Health (DA156480) and the
Skaggs Institute for Chemical Biology. We wish to espe-
cially thank Professor B. F. Cravatt and his group for a
generous supply of recombinant rat FAAH.
N
Ki
Compd
—
18a
19a
X
CO22f
S
0.003 0.0002 µM
140 10 µM
SO
13 1 µM29
N
N
Ph(CH2)n
n
X
N
O
Supplementary data
Ki
Compd
X
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.bmcl.
2004.10.025. Details of the synthesis and characteriza-
tion of all new compounds is provided.
18b
19b
20b
S
1
1
1
720 50 µM
SO
8
1 µM
SO2
7.5 1 µM
18c
19c
20c
S
SO
SO2
110 10 µM
4.9 1.0 µM
2
2
2
6.8 0.7 µM30
860 50 µM
18d
19d
20d
S
SO
SO2
3
3
3
References and notes
7
0.3 µM
3.3 0.5 µM
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Ki = 590 50lM.
a