Chain-branched acyclic phenethylthiocarbamates as vanilloid receptor antagonists

Article abstract of DOI:10.1016/S0960-894X(03)00178-1

A series of acyclic phenethylthiocarbamate derivatives have been synthesized, and their antagonist effect against vanilloid receptor tested. Chain branching led to a significant change in antagonist activity of the parent molecule. Ethyl-branched 1e showe

Full text of DOI:10.1016/S0960-894X(03)00178-1

Bioorganic & Medicinal Chemistry Letters 13 (2003) 1549–1552
Chain-Branched Acyclic Phenethylthiocarbamates as
Vanilloid Receptor Antagonists
JungWha Yoon,a HyeYoung Choi,a Hyun Joo Lee,a Chong Hyun Ryu,a
Hyeung-geun Park,b Young-ger Suh,b Uhtaek Oh,b Yeon Su Jeong,c
Jin Kyu Choi,c Young-Ho Parkc and Hee-Doo Kima,*
aCollege of Pharmacy, Sookmyung Women’s University, 53-12, Chungpa-dong,
Yongsan-ku, Seoul 140-742, South Korea
bCollege of Pharmacy, Seoul National University, Seoul 151-742, South Korea
cAmorePacific R & D Center, Youngin-Si, Kyounggi-do 449-900, South Korea
Received 7 December 2002; accepted 13 February 2003
Abstract—A series of acyclic phenethylthiocarbamate derivatives have been synthesized, and their antagonist effect against vanil-
loid receptor tested. Chain branching led to a significant change in antagonist activity of the parent molecule. Ethyl-branched 1e
showed a 6.3 mM of IC50 value in 45Ca2+-influx assay.
# 2003 Elsevier Science Ltd. All rights reserved.
Capsaicin and its vanilloid analogues have been exten-
sively studied for the development of novel analgesics
with entirely different mechanism either from opioids or
NSAIDs. By acting on vanilloid receptor (VR1), cap-
saicin excites and then desensitizes a subset of primary
neurons involved in nociception, neurogenic inflamma-
tion, and a variety of local regulatory functions.1 Due to
this unique biological activity, VR1 is at present one of
the most attractive targets for the treatment of pain.2
structural modification of capsazepine. The noticeable
difference between the agonist and the antagonist is in
their binding mode on VR1. In contrast to the agonist’s
coplanar conformation, the antagonists are known to
have an orthogonal conformation between the vanilloid
aromatic ring (A region) and the amide/thiourea bond
(B region).7a
Our basic strategy for structural modification is to seek
the chain-branched acyclic compounds deviated from
coplanar conformation with minimal structural dis-
turbance from cyclic capsazepine. Due to their flex-
ibility, acyclic compounds are often better than the
corresponding rigid cyclic ones to achieve the pharmaco-
phoric conformation. However, all of the acyclic vanilloid
compounds known to date act as receptor agonists. In this
sense, the development of acyclic vanilloid compounds
with antagonist activity looks like a very difficult task to
be achieved, but if possible, is of importance in this
research field. Here, we examined the chain branching
method as a solution for the above-mentioned two
incompatible facts. Chain branching often interferes
with receptor binding by steric effect, thereby reducing
the potency or altering the pharmacological profiles of
the ligand. It is anticipated that chain branching may
destabilize the agonist binding mode (conformation) of
acyclic vanilloid compounds for steric reason, thereby
Resiniferatoxin,3
olvanil,4
SDZ-249482,5
and
KR-250186 are well-known potent VR1 agonists devel-
oped to date. However, despite the concentrated effort
on agonists, they have been exposed to the side effects
such as pungency and/or hypothermia responses.7 In
this context, the possibility of VR1 antagonist as an
ideal analgesic has been suggested carefully, and fol-
lowed by some efforts to discover the novel antagonists
in the last decade.8 In 1994, capsazepine has been
developed by Sandoz group as a first competitive VR1
antagonist,9 however, the utility of capsazepine is lim-
ited by its moderate in vivo activity and nonspecificity.10
Coupled with the recent cloning of VR1,11 the renewed
interest on VR antagonist prompts us to examine the
*Corresponding author. Tel.: +82-2-710-9567; fax: +82-2-703-0736;
e-mail: hdkim@sookmyung.ac.kr
0960-894X/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0960-894X(03)00178-1

1550
J. Yoon et al. / Bioorg. Med. Chem. Lett. 13 (2003) 1549–1552
leading to more favorable conformation for antagonist
binding mode. As shown in Figure 1, a series of
compounds 1a–s were designed as the first targets for
preparation.
literature.12 The results are summarized in Table 1. As
expected, the antagonist activities of the compounds 1a–
k are strongly influenced by chain length. Optimal car-
bon chain length between A and B region was found to
be propyl. Further examination of the effect of R1
reveals that the smaller groups showed better antagonist
activity than the bulkier one. The methyl (1b) and ethyl
(1e) derivatives had IC50 values ranging from 6.3 to 9.9
mM. When R1 is hydrogen, however, activity is reduced
greatly. This implies the important contribution of R1
branching for antagonist activity. Moderate activities
were also observed for phenethyl derivatives (1i, 1j).
Thus, compound 1e with ethyl group and optimal pro-
pyl chain was chosen as a lead for next SAR study.
A series of the thiocarbamate compounds with a sys-
temic variation of the alkyl groups have been synthe-
sized in order to gauge the effect of the R1 on the
antagonist activity of these compounds against VR1
receptor. The thiocarbamates 1b, and 1d–k were pre-
pared from 3-methoxy-4-hydroxycinamaldehyde (2) in
five steps as shown in Scheme 1. Protection of phenolic
OH with the TBS group, followed by addition of
Grignard reagent afforded the corresponding allylic
alcohol 4 in good yield. Catalytic hydrogenation of 4 on
Pd/C gave the saturated alcohol 5. After forming the
alkoxide with NaH in THF, 5 was treated with phen-
ethylisothiocyanate to give the corresponding thio-
carbamate 6. Deprotection of the TBS group with
TBAF gave the desired compounds 1b and 1d–k,
respectively. Compounds 1a and 1c were also prepared
in similar ways with different starting material such as
4-hydroxy-3-methoxyphenylacetone and 4-(40-hydroxy-
30-methoxyphenyl)butan-2-one.
It is well known that modification of B-region both in
capsaicin and capsazepine has a great influence on their
Table 1. 45Ca2+-influx activity of the phenethylthiocarbamate
derivatives
The biological activities of the thiocarbamate 1a–k were
evaluated as both agonists and antagonists in the
45Ca2+-influx assay using the neonatal rat cultured
spinal sensory neurons by the method described in the
Compd
n
R1
45Ca2+ influx activity
(mM)a
Agonist Antagonist
(EC50)
(IC50)
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1
2
3
2
2
2
2
2
2
Methyl
Methyl
Methyl
H
>100
>100
>100
>100
>100
>100
>100
>100
>100
>30
9.9
>30
>30
6.3
27.9
>30
25
19.2
10.7
Ethyl
Propyl
Phenyl
Benzyl
Phenethyl
22-(4-Trifluoromethylphenyl)ethyl >100
2
Phenylpropyl
—
>100
>100
>30
0.6
Capsazepine
aEC50 (the concentration of derivatives necessary to produce 50% of
the maximal response) and IC50 values (the concentration of deriva-
tives necessary to reduce to 0.5 mM capsaicin by 50%) were estimated
with at least 3 replicates at each concentration. Each compound was
tested in two independent experiments. Antagonist data were fitted
with a sigmoid function.
Figure 1. Structures of capsaicin, capsazepine and target molecules
1a–s.
Scheme 1. Reagents and conditions: (a) NaH, THF; then TBSCl, 99%; (b) R1MgBr, THF, 80–99%; (c) H2, Pd/C, MeOH, 80–100%; (d) NaH,
R2NCS, THF, 75–95%; (e) TBAF, THF, 90–99%.

J. Yoon et al. / Bioorg. Med. Chem. Lett. 13 (2003) 1549–1552
1551
Scheme 2. Reagents and conditions: (a) PCC, CH2Cl2, 98%; (b) RNH2; H2, Pd/C 40ꢁ85%; (c) PhCH2CH2NCS, toluene, reflux, 45–85%;
(d) TBAF, THF, 75–95%; (e) (EtO)2POCH2CO2Et, NaH, THF, 58%; H2, Pd/C, 81%; (f) PhCH2CH2NH2, Me3Al, CH2Cl2, 98%; (g) Lawesson’s
reagent, toluene, 80 ꢀC, 47%; (h) phenethylamine, diethyl dicarbonate, DMAP, CH2Cl2, 21%.
agonist or antagonist activity. At the present time,
thiourea has its most potent functionality for the B-region.
Thus, we studied the SAR of B-region modification. Car-
bamate, thiocarbamate, amide, thioamide, urea, and
thiourea analogues were prepared according to Scheme
2. Alcohol 5e was oxidized with PCC to give the ketone
7 in 98% yield. Reductive amination of 7 followed by
deprotection gave 1o, 1p, 1q and 1r, respectively. Hor-
ner–Worthward–Emmons olefination of 7 with triethyl
phosphonoacetate, followed by catalytic hydrogenation
afforded ester 10 in 47% yield. Direct amide formation
from ester was achieved with trimethylaluminium and
phenethylamine to afford 11 in 98% yield. Treatment of
11 with Lawesson’s reagent in toluene at 80 ꢀC gave the
thioamide 12 in 47% yield. Subsequent deprotection of
TBS group from 11 and 12 with TBAF gave 1n and 1m,
respectively.
Table 2. 45Ca2+-Influx activity of the ethyl branched vanilloid
derivatives
Compd
X
Y
45Ca2+ influx activity (mM)
Agonist (EC50
)
Antagonist (IC50)
1e
1l
1m
1n
1o
1p
1q
O
O
CH2
CH2
S
O
S
O
S
O
S
S
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
6.3
>30
>30
>30
28.5
>30
27.2
20.9
>30
0.6
NH
NH
NCH3
NCH2Ph
NCH2CH2Ph
1r
As shown in Table 2, amide 1n and thioamide 1m were
found to be inactive, which implies that hetero atom is
needed at X position for activity. Carbamate 1l and
urea 1p are less reactive than the corresponding thio-
carbamate 1e and thiourea 1o, indicating that sulfur is
better than oxygen at the Y position. In comparison of
IC50 values of the alkyl thiourea derivatives (1o, 1q–s)
turned out that N-alkylation was not fruitful way to
increase the antagonist activity of the thiourea 1o. At
the present time, thiocarbamate is the best functionality
for this new scaffold.
1s
Capsazepine
S
—
still lower than that of capsazepine at this stage, the
most important thing is that the first acyclic vanilloid
compounds with antagonist activity are prepared by
chain branching method. In addition, the chain branch-
ing method presented here appears to be a promising
strategy for the development of novel antagonists.
In summary, a series of chain-branched acyclic phen-
ethylthiocarbamate derivatives have been synthesized,
and their antagonist effect against vanilloid receptor
tested. Ethyl-branched phenethylthiocarbamate 1e
showed 6.3 mM of IC50 value. Although the potency is
Acknowledgements
This study was supported by a grant from the Korea
Health R&D Project, Ministry of Health & Welfare,
Republic of Korea.

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J. Yoon et al. / Bioorg. Med. Chem. Lett. 13 (2003) 1549–1552
Lee, J. W.; Park, H. G.; Park, O. H.; Lee, Y. S.; Park, Y. H.;
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