Microwave-assisted four-component reaction for the synthesis of a monothiohydantoin inhibitor of a fatty acid amide hydrolase

Article abstract of DOI:10.1016/j.tetlet.2008.08.100

Monothiohydantoin 3a is made in seven steps from 1,4-diiodobenzene and is shown to be an inhibitor of fatty acid amide hydrolase (IC₅₀ = 23.4 ± 1.1 μM). The key step in the sequence involves a microwave-assisted four-component reaction that makes the 5,5′-disubstituted hydantoin nucleus by the sequential formation of two C-C and two C-N bonds.

Full text of DOI:10.1016/j.tetlet.2008.08.100

Tetrahedron Letters 49 (2008) 6495–6497
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Microwave-assisted four-component reaction for the synthesis
of a monothiohydantoin inhibitor of a fatty acid amide hydrolase
Estelle Gallienne a, Giulio G. Muccioli b,c, Didier M. Lambert b,c, Michael Shipman a,
*
a Department of Chemistry, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
b Unité de Chimie pharmaceutique et de Radiopharmacie, Ecole de Pharmacie, Faculté de Médecine, UCL, Avenue E. Mounier 73, UCL-CMFA 7340, B-1200 Brussels, Belgium
c Drug Design and Discovery Center, FUNDP, rue de Bruxelles 61,B-5000 Namur, Belgium
a r t i c l e i n f o
a b s t r a c t
Article history:
Monothiohydantoin 3a is made in seven steps from 1,4-diiodobenzene and is shown to be an inhibitor of
Received 21 July 2008
Revised 18 August 2008
Accepted 27 August 2008
Available online 31 August 2008
fatty acid amide hydrolase (IC50 = 23.4 1.1 lM). The key step in the sequence involves a microwave-
assisted four-component reaction that makes the 5,50-disubstituted hydantoin nucleus by the sequential
formation of two C–C and two C–N bonds.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
FAAH
Thiohydantoins
Bucherer–Bergs reaction
Multi-component reactions
Microwaves
The hydantoin nucleus 1 has many pharmacological effects1
and is found in several clinically important medicines (e.g., niluta-
mide,2 phenytoin3). Hydantoins also serve as useful intermediates
for the preparation of non-natural amino acids via chemical1 or
enzymatic hydrolysis.4 Of existing methods to hydantoins,1,5 the
Bucherer–Bergs reaction provides perhaps the best method for
their preparation.6,7 It is a four-component reaction (4-CR) that in-
volves the condensation of a ketone (or aldehyde) with cyanide,
ammonia and carbon dioxide with the latter two reagents conve-
niently generated from ammonium carbonate (Scheme 1).
Recently, to enhance the scope of the Bucherer–Bergs reaction
in drug discovery, Montagne et al. reported a modification of this
4-CR that uses nitriles and organometallic reagents as the starting
materials.8 A key advantage of this method is that it creates two
points of chemical diversity by combining the R and R1 groups to-
gether in the same vessel as hydantoin assembly (Scheme 1). This
reaction tolerates considerable variation in the structure of the
organometallic reagent and the nitrile, and appears to have broad
scope.8 To test its suitability for drug discovery, we sought to apply
this new methodology to the synthesis of functionalised molecules
that might serve as enzyme inhibitors. In this Letter, we report an
efficient seven-step synthesis of a novel inhibitor of fatty acid
amide hydrolase (FAAH) using this chemistry.
Bucherer—Bergs reaction (4-CR, one point of diversity):
O
O
HN
KCN, NH , CO
3
2
NH
1
O
R
R
1
R
R
1
Modified Bucherer—Bergs reaction (4-CR, two points of diversity):
KCN
O
NM
HN
1
(NH ) CO
4 2
—M
R
3
R
N
NH
O
1
R
R
1
R
R
1
where M = Li or MgX
Scheme 1. Synthesis of 5,50-disubstituted hydantoins.
Fatty acid amide hydrolase (FAAH) is a membrane-bound en-
zyme, responsible for the hydrolysis of bioactive lipids, including
endogenous ligands of the cannabinoid receptors such as ananda-
mide.9 Effective inhibition of FAAH is widely recognised as a prom-
ising approach for the treatment of sleep disorders, anxiety,
epilepsy, cancer and neurodegenerative disorders. Recently, a ser-
ies of monothiohydantoins including 2 (n = 5–13) were identified
by Muccioli et al. as reversible and competitive FAAH inhibitors,
devoid of affinity for cannabinoid receptors (Fig. 1).10,11 Molecular
modelling and docking studies by Michaux et al. using the active
site of rat FAAH suggested that the introduction of a polar group
at the 4-position of one of the aromatic rings might enhance FAAH
* Corresponding author. Tel.: +44 247 652 3186; fax: +44 247 652 4112.
E-mail address: m.shipman@warwick.ac.uk (M. Shipman).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.08.100

6496
E. Gallienne et al. / Tetrahedron Letters 49 (2008) 6495–6497
Next, selective monoalkylation of hydantoin 5 at N-3 was
S
(CH2)nCH3
O
S
(CH2)nCH3
O
N
achieved using bromooctane/K2CO3 to give 6 in 83% yield. The site
of N-alkylation was deduced by the disappearance of the more
downfield NH signal at 11.15 ppm in the 1H NMR spectrum, and
is fully consistent with the literature precedent.1 Further, three-
carbon homologation was effected by palladium-catalysed Heck
coupling with methyl acrylate under microwave conditions15 to
give ester 7 in 84% yield exclusively as the E-isomer. Hydrogena-
tion to remove the alkene double bond followed by thionylation
of the more nucleophilic C@O with Lawesson’s reagent gave
monothiohydantoin 8 in 53% yield over the two steps. The che-
moselectivity of this transformation was readily deduced from
the downfield shift in the 13C NMR (DMSO-d6) of the C-2 carbon
(155.4?181.2 ppm). Finally, conversion of the methyl ester to
the corresponding amide was achieved by hydrolysis to the car-
boxylic acid and amidation using PyBOP/HOBt/aqueous ammonia.
This strategy provided the amide-functionalised monothiohydan-
toin 3a in seven steps and 17% overall yield from 1,4-diiodobenz-
ene (Scheme 2).
N
HN
HN
H2N
3
2
O
where n = 5-13
Figure 1. Monothiohydantoin FAAH inhibitors.
binding through hydrogen bonding to residues of the catalytic triad
(Ser241, Ser217 and Lys142).12 Specifically, monothiohydantoin 3
(n = 5) was calculated to bind with a higher affinity than 2 (n = 5)
within the active site of rat FAAH by 7.0 kcal/mol.
To test the validity of this binding model, we decided to con-
struct monothiohydantoin 3 and measure its inhibition of FAAH.
Based upon early data for 2 (n = 5–13),10 we targetted 3a bearing
an octyl side chain at N-3 to maximise inhibition. Our synthesis
of 3a began with the preparation of 5,50-disubstituted hydantoin
5 using the new 4-CR (Scheme 2). Thus, treatment of 1,4-diiodo-
benzene (4) with n-BuLi at ꢀ78 °C in THF gave the presumed
monolithiated species, to which was added benzonitrile. Upon
warming to 0 °C, EtOH, (NH4)2CO3, KCN and water were succes-
sively added, then the mixture was irradiated in a CEM DiscoverÒ
microwave (50 W) for 1 h.13 After work-up and purification,
hydantoin 5 was isolated in 51% yield (see Supplementary data).
Microwave irradiation is not essential and similar results can be
achieved in a resealable glass pressure tube as reported in the ori-
ginal study.8,14 However, the microwave procedure allows the pro-
cess to be conducted with better control of vessel pressure and
temperature, as well as enhanced reaction containment and safety.
For these reasons, we recommend the use of these new micro-
wave-based conditions.
Inhibition of rat recombinant FAAH by monothiohydantoin 3a
was conducted in accordance to the method of Omeir et al.16 using
labelled anandamide, [3H]-AEA. Whilst good inhibition was
observed (IC50 = 23.4 1.1
potent than the unsubstituted derivative 2 (n = 7) (IC50 = 6.1
0.3 M). Thus, in contradiction to in silico predictions, the introduc-
lM), this compound is four times less
l
tion of the amide group at C-4 at one of the aromatic rings has no
beneficial effect on rat FAAH inhibition.17
To conclude, an efficient seven-step synthesis of amide-func-
tionalised monothiohydantoin 3a has been achieved in 17% overall
yield. The route exploits a microwave-assisted 4-CR to establish
the 5,50-hydantoin nucleus with formation of two C–C and two
C–N bonds via a ‘one-pot’ process. Work to use this methodology
to prepare further FAAH inhibitors is ongoing in our laboratories.
Acknowledgements
This work was supported by the Engineering and Physical Sci-
ences Research Council (EP/E501184/1). We are indebted to the
EPSRC National Mass Spectrometry Service Centre for performing
some of the mass measurements, and to EPSRC (EP/C007999/1)
for the provision of additional mass spectrometers. Recombinant
rat FAAH was kindly produced by Geoffray Labar. G.G.M. is sup-
ported by the National Fund for Scientific Research (FNRS,
Belgium).
O
1. n-BuLi, THF,
–78 ºC
NH
3. KCN, (–NH4)2CO3, 70 ºC
2. PhCN 78→ 0 ºC
EtOH/H2O (1:1), μW
I
I
HN
O
51%
4
5
I
K2CO3, DMF
Me(CH2)7Br
83%
Supplementary data
O
(CH2)7CH3
O
(CH2)7CH3
O
N
MeO2CCH=CH2
N
A supplementary data section is provided, which includes
experimental procedures and characterisation data for 3a, 5–8
and the FAAH assay. Supplementary data associated with this
article can be found, in the online version, at doi:10.1016/j.tetlet.
2008.08.100.
Pd(OAc)2, Bu3N
O
HN
HN
DMF,100 ºC, μW
84%
I
7
6
CO2Me
References and notes
1. H2, Pd/C, MeOH, rt
2. Lawesson's reagent
PhMe, 110 ºC
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S
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N
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Scheme 2. Synthesis of monothiohydantoin 3a from 1,4-diiodobenzene.

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6497
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