Chain-branched 1,3-dibenzylthioureas as vanilloid receptor 1 antagonists

Article abstract of DOI:10.1016/j.bmcl.2004.01.066

A series of chain-branched 1,3-dibenzylthiourea derivatives were synthesized, and tested their antagonist activity against vanilloid receptor 1. Chain-branching led to a significant change in the mode of action and the potency. (R)-Methyl or ethyl-branched 1,3-dibenzylthiourea derivatives showed the most potent antagonist activity up to the IC₅₀ value of 0.05 μM which is 10-fold more potent than capsazepine.

Full text of DOI:10.1016/j.bmcl.2004.01.066

Bioorganic & Medicinal Chemistry Letters 14 (2004) 1751–1755
Chain-branched 1,3-dibenzylthioureas as vanilloid
receptor 1 antagonists
Chong Hyun Ryu,a Mi Jung Jang,a Jeong Wha Jung,a Ju-Hyun Park,a Hye Young Choi,a
Young-ger Suh,b Uhtaek Oh,b Hyeung-geun Park,b Jeewoo Lee,b Hyun-Joo Koh,c
a,
Joo-Hyun Mo,c Yung Hyup Joo,c Young-Ho Parkc andHee-Doo Kim *
aCollege of Pharmacy, Sookmyung Women’s University, Seoul 140-742, Republic of Korea
bCollege of Pharmacy, Seoul National University, Seoul 151-742, Republic of Korea
cAmorePacific R& D Center, Youngin-Si, Kyounggi-do 449-900, Republic of Korea
Received15 December 2003; accepted15 January 2004
Abstract—A series of chain-branched 1,3-dibenzylthiourea derivatives were synthesized, and tested their antagonist activity against
vanilloidreceptor 1. Chain-branching ledto a significant change in the mode of action andthe potency. (
R)-Methyl or ethyl-
branched 1,3-dibenzylthiourea derivatives showed the most potent antagonist activity up to the IC50 value of 0.05 mM which is
10-foldmore potent than capsazepine.
# 2004 Elsevier Ltd. All rights reserved.
Among the several therapeutic approaches for the
treatment of pain, main are the modulation of opioid
receptors andcyclooxygenase. However, these approa-
change in the mode of action on VR1, thereby convert-
ing the agonist binding mode into the antagonist one.
But still, there is a needto develop the more potent VR1
antagonists. As far as we know, thiourea is the most
powerful functionality foundin VR1 modulators such
as SDZ249482 (2), capsazepine, MK056 (3), and
SC0030 (4).9 In line with the previous result, this work
was initiatedwhen the thiourea 5, the methyl-branched
form of the strong agonist 1, was discovered as a VR1
antagonist from our preliminary study. Thus we set
about to explore the SAR of chain-branched1,3-diben-
zylthioureas as VR1 antagonist (Fig. 1).
1
ches have undesirable side effects associated with their
uses,2 andthis has prompteda search for mechan-
istically different analgesic agents.1 In this context,
vanilloidreceptor 1 (VR1) is at present one of the
attractive targets for the discovery of novel analgesics.3
VR1 is a ligand-gated nonselective cation channel acti-
vatedby capsaicin. By acting on VR1, capsaicin excites
andthen edsensitizes a subset of primary neurons
involvedin nociception, neurogenic inflammation, and
a variety of local regulatory functions.4
The synthetic routes to various alkyl-branched1,3-
dibenzylthiourea analogues are outlined in Schemes 1–
4. The optically active methyl-branched1,3-idben-
zylthioureas (6–11, 17–19 and 21) were chosen as the
first targets in order to clarify the stereo effect of
the stereogenic center andthe substituent effect on the
phenyl ring. Scheme 1 shows one-step conversion of
the commercially available chiral amines into the
corresponding thioureas (6–11) using 4-tert-butylbenzyl-
isothiocyanate in basic conditions.
Despite the concentratedworks on VR1 agonists includ-
ing capsaicin, their excitatory side effects such as pungency
andhypothermia responses couldnot be separatedfrom
the antinociceptive properties.5 Since the discovery of
capsazepine as a first competitive VR1 antagonist,6 the
possibility of VR1 antagonist as an ideal analgesic has
been suggestedcarefully, andfollowedby the continued
efforts to discover the novel antagonists.7À9
From our previous work,8 we foundthat chain-branch-
ing of the acyclic thiocarbamates ledto a significant
Scheme 2 depicts the synthesis of the methyl-branched
1,3-dibenzylthioureas having acetamide or methane-
sulfonamide functionality. The commercially available
chiral compound 12 was protectedwith di- tert-butyldi-
* Corresponding author. Tel. +82-2-710-9567; fax: +82-2-703-0736;
e-mail: hdkim@sookmyung.ac.kr
0960-894X/$ - see front matter # 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.01.066

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C. H. Ryu et al. / Bioorg. Med. Chem. Lett. 14 (2004) 1751–1755
carbonate to give the N-Boct 13. Compound 13 was
then reduced to the amine 14 by catalytic hydrogena-
tion. The acetylation or mesylation of the amine 14 with
acetic anhydride or methanesulfonic anhydride, followed
by deprotection of t-Boc group with trifluoroacetic acid
gave the amine 16. Compound 16 was then condensed
with 4-tert-butylbenzylisothiocyanate to furnish the
desired compound 17, 18, and 19, respectively. Depro-
tection of compound 14 with trifluoroacetic acidgave
the diamine 20, which is condensed with 4-tert-butyl-
benzylisothiocyanate to affordcompound 21.
As shown in Scheme 3, a series of the racemic alkyl-
branched methanesulfonamide derivatives 26–30 were
also synthesizedin order to gauge the effect of the R on
the antagonist activity against VR1. Friedel–Crafts
acylation of the methanesulfonamide 22, followedby
reductive amination via oxime 24 afforded the amine 25.
Figure 1. Structures of capsaicin, capsazepine, and1,3-dibenzylthiourea analogues.
Scheme 1. Reagents andconditions: (a) 4- tert-butylbenzylisothiocyanate, triethylamine, CH2Cl2, 84–99%.
Scheme 2. Reagents and conditions: (a) di-tert-butyldicarbonate, aqueous NaHCO3-CH2Cl2, 94%, (b) H2, Pd-C, MeOH, 95%, (c) acetic anhydride,
or methanesulfonic anhydride, pyridine, CH2Cl2, (d ) CFCO2H, (e) 4-tert-butylbenzylisothiocyanate, triethylamine, CH2Cl2, 86% (2 steps).
3

C. H. Ryu et al. / Bioorg. Med. Chem. Lett. 14 (2004) 1751–1755
1753
Scheme 3. Reagents andconditions: (a) (RCO) 2O, AlCl3, ClCH2CH2Cl, 35–50%; (b) NH2OH, HCl, AcONa, 73–94%, (c) H2, Pd/C, MeOH,
75–95%, (d) 4-tert-butylbenzylisothiocyanate, THF, 59–87%.
Scheme 4. Reagents and conditions: (a) formic acid, acetic anhydride, 96%, (b) HNO3, H2SO4, À20 ꢀC, 58%. (c) H2, Pd/C, MeOH, 88%, (d) 1N-
KOH, MeOH, reflux, 77%, (e) 4-tert-butylbenzylisothiocyanate, THF, 71%, (f) (CH3SO2)2O, pyridine, CH2Cl2, À40 ꢀC, 91%.
Compound 25 was then condensed with 4-tert-butyl-
benzylisothiocyanate to furnish the desired compounds
26–30, respectively.
binding from the agonist mode to the antagonist mode.
Until now, this was the only way for the 1,3-diben-
zylthiourea compounds to exert the antagonism. Here,
however, we foundthe interesting cases that, if being
chain-branched, the 1,3-dibenzylthioureas could retain
the antagonist activity even without methanesulfona-
mide functionality. Most of alkyl-branched compounds
in this study possessed the antagonist activity ranged
from the IC50 value of 25 mM to 0.05 mM. Even without
polar substituents on the phenyl ring, compounds 6, 7, 8
and 9 showedthe antagonist activity. In particular,
compound 8 having methyl group on 4-position of
phenyl ring showedthe comparable antagonist activity
as capsazepine did. In view of a general trend that polar
substitution on phenyl ring is the primary requisite for
receptor binding, this is an unusual phenomenon.
Chain-branching appears to be the most important fac-
tor for the antagonist activity. Compound 19 having
methanesulfonamide group was one of the most potent
analogues (IC50=0.05 mM) foundfrom our SAR stud-
ies. This compoundis approximately 10-foldmore
potent than capsazepine. Consistent with the previous
result,9 methanesulfonamide group was found to be the
best functionality for 4-position on the phenyl ring.
The ethyl-branched( R)-isomer 37 was prepared
through the route described in Scheme 4. The synthesis
startedwith the commercially available ( R)-(+)-1-phe-
nylpropylamine (31, ChiProsTM, 98% ee). Compound
31 was formylatedwith HCO H and acetic anhydride,
2
followedby nitration to affordcompound 33. Hydro-
genation of compound 33 with H2 on Pd/C, followed by
deprotection with KOH gave the diamine 35. Chemo-
selective formation of the thiourea 36 was successively
achievedby treatment of the diamine 35 with 4-tert-
butylbenzylisothiocyanate at À40 ꢀC. Mesylation of 36
with methanesulfonic anhydride furnished the thiourea
37.
The biological activities of the 1,3-dibenzylthioureas
were evaluatedas both agonists andantagonists in the
45Ca2+-influx assay using the neonatal rat cultured
spinal sensory neurons by the methoddescribedin the
literature.10 The results are summarizedin Table 1.
Stereochemistry of the chiral center is critical. R-enan-
tiomers are uniformly more potent than the S-enantio-
mers. The eudismic ratios are ranged from 5 to 50. The
substitution on the phenyl ring of 1,3-dibenzylthiourea
compounds had a dramatic effect on the potency and
selectivity. As we have reportedpreviously, 9 4-substitution
by the methanesulfonamide group changed receptor
Having optimizedthe 4-position of phenyl ring, atten-
tion was turnedto explore the appropriate alkyl group
to maximize the chain-branching effect. At first, we
preparedcompounds 26–30 into the racemic form for
convenience. The examination of the effect of R
revealedthat the smaller groups showedthe better

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C. H. Ryu et al. / Bioorg. Med. Chem. Lett. 14 (2004) 1751–1755
Table 1. 45Ca2+-Influx activity of the alkyl-branched 1,3-dibenzylthiourea derivatives
No.
X
R
Absolute
configuration
45Ca2+ influx activity (mM)a
Agonist (EC50 Antagonist (IC50
)
)
6
7
8
9
10
11
21
17
18
19
26
27
28
H
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Et
S
R
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
25.5
4.96
0.40
3.49
0.58
H
CH3
But
NO2
NO2
NH2
R
R
S
R
R
S
R
14.7
3.30
3.60
1.47
0.05
0.15
0.36
3.24
1.49
CH3CONH
CH3SO2NH
CH3SO2NH
CH3SO2NH
CH3SO2NH
CH3SO2NH
CH3SO2NH
CH3SO2NH
CH3SO2NH
CH3SO2NH
Racemate
Racemate
Racemate
Racemate
Racemate
R
i-Pr
Pr
Phenyl
Et
29
30
16.7
37
3 (MK056)
Capsazepine
0.05
0.11
0.60
H
—
a EC50 (the concentration of derivatives necessary to produce 50% of the maximal response) and IC50 values (the concentration of derivatives
necessary to reduce to 0.5 mM capsaicin by 50%) were estimatedwith at least 3 replicates at each concentration. Each compoundwas testedin two
independent experiments. Antagonist data were fitted with a sigmoid function.
antagonist activity than the bulkier ones. The racemic
methyl-branchedanalogue 26 hadsimilar activity com-
paredto the ethyl analogue 27, whereas the i-propyl,
propyl, andphenyl analogues 28–30 showedweak VR1
antagonist activities. Further investigation of chiral
ethyl analogue revealedthat ( R)-ethyl analogue 37 was
as potent as (R)-methyl analogue 19 with the IC50 value
of 0.05 mM, suggesting that ethyl group appearedto be
the maximum size permittedfor R group. Compounds
19 and 37 were also 2-foldmore potent as chain-
branched1,3-dibenzylureas comparedto non-branched
analogue MK056 (3), indicating that chain-branching is
not detrimental, but beneficial to the potency in the
present cases.
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In summary, a series of chain-branched1,3-idben-
zylthiourea derivatives were synthesized, and tested
their antagonist activity against VR1. The most notable
fact is that the agonist couldbe changedinto the
antagonist simply by introducing an alkyl chain at
the benzylic position of 1,3-dibenzylthioureas. (R)-
Methyl or ethyl-branched dibenzylthiourea derivatives
(19, 37) showedthe most potent antagonist activity up
to the IC50 value of 0.05 mM which is 10-foldmore
potent than capsazepine. The chain-branching method
described here appears to be a promising strategy for
the development of a novel antagonist.
Acknowledgements
This research was supportedby a grant of the Korea
Health R&D Project, Ministry of Health & Welfare,
Republic of Korea: 02-PJ2-PG4-PT-01-0014.

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