Analogues of capsaicin with agonist activity as novel analgesic agents: Structure-activity studies. 4. Potent, orally active analgesics

Article abstract of DOI:10.1021/jm960512h

Structural features of three regions of the capsaicin molecule necessary for agonist properties were delineated by a previously reported modular approach. These in vitro agonist effects were shown to correlate with analgesic potency in rodent models. Comb

Full text of DOI:10.1021/jm960512h

4942
J . Med. Chem. 1996, 39, 4942-4951
An a logu es of Ca p sa icin w ith Agon ist Activity a s Novel An a lgesic Agen ts:
Str u ctu r e-Activity Stu d ies. 4. P oten t, Or a lly Active An a lgesics
Roger Wrigglesworth,* Christopher S. J . Walpole,* Stuart Bevan, Elizabeth A. Campbell, Andy Dray,
Glyn A. Hughes, Iain J ames, Kay J . Masdin, and J anet Winter
Sandoz Institute for Medical Research, Gower Place, London WC1E 6BN, England
Received J uly 16, 1996X
Structural features of three regions of the capsaicin molecule necessary for agonist properties
were delineated by a previously reported modular approach. These in vitro agonist effects
were shown to correlate with analgesic potency in rodent models. Combination of optimal
structural features from each of these regions of the capsaicin molecule have led to highly
potent agonists (e.g., 1b). Evaluation in vivo established that 1b had analgesic properties but
poor oral activity, short duration of action, and excitatory side effects which precluded further
development of this compound. Preliminary metabolism studies had shown that the phenol
moiety of 1b was rapidly glucuronidated in vivo, providing a possible explanation for the poor
pharmacokinetic profile. Subsequent specific modification of the phenol group led to compounds
2a -j, which retained in vitro potency. The in vivo profiles of two representatives of this series,
2a ,h , were much improved over the “parent” phenol series, and they are candidates for
development as analgesic agents.
Ch a r t 1
In tr od u ction
Earlier papers in this series have described the
potential use of capsaicin-like agonists as analgesic
agents.1-3 These papers described in detail the modular
variations of the capsaicin structure which was conve-
niently broken down into three separate parts: the
aromatic “A” region, the amide bond “B” region, and the
hydrophobic side-chain “C” region. Each region was
varied in turn while holding the others constant. The
compounds were evaluated in an in vitro screen which
provides a measure of capsaicin-like agonism (45Ca2+
influx into neonatal rat dorsal root ganglia (DRG)
neurons4) and which we have established is predictive
of analgesia in animal models.1
This work has formed the basis of a molecular
approach toward more potent capsaicin agonists which
are anticipated to be useful as novel analgesic agents.
In parallel with our studies, workers at Procter and
Gamble Co. have explored structural modifications of
the capsaicin molecule. Based on antinociceptive stud-
ies, a series of aliphatic vanillylamides were described5
culminating in the identification of Olvanil (NE 19950,
oleyl vanillylamide) as a putative analgesic. The struc-
ture-activity relationship (SAR) conclusions from both
groups are broadly in concert. More recently Park et
al.6 have described capsaicin-like agonists in the patent
literature.
Specifically, from our earlier studies, it was estab-
lished that the natural substitution pattern was optimal
in the A region, that a thiourea moiety conferred high
potency when incorporated in the B region, and that a
hydrophobic unit of limited size, e.g., a substituted
aralkyl or aralkenyl substituent, was necessary in the
C region. The present paper describes the com bin a -
tion of these individual potency-enhancing features in
the same molecule with the aim of making compounds
with increased in vitro potency which might therefore
be expected to have enhanced analgesic properties in
vivo. Realization of this goal and the subsequent refine-
ments of the target structures based on in vivo data are
described below and have led to compounds with
therapeutic potential.
Ch em istr y
The synthetic approaches to the target molecules are
unexceptional and not discussed here in detail. The
skeletal assembly, involving the coupling of an amine
component with an isothiocyanate component, is shown
in outline in Scheme 1, and the resulting target struc-
tures are listed in Table 1. The protecting group
strategies (described by X in Scheme 1 and Table 1)
varied from compound to compound and are described
in detail in the Experimental Section. For the “parent”
phenol series 1a -i, the phenolic group was either left
unprotected or carried through the isothiocyanate cou-
pling step as the ethoxyethyl acetal which was subse-
quently removed using dilute acid. For the 2-aminoet-
hyl phenolic ether series 2a -j, the primary amino group
was protected as the phthalimide in the coupling step
and then removed using hydrazine under standard
conditions.
The (Z)-ethenyl compound 1d was made by photo-
chemical isomerization of the E compound 1c. The
X Abstract published in Advance ACS Abstracts, November 1, 1996.
S0022-2623(96)00512-2 CCC: $12.00 © 1996 American Chemical Society

Capsaicin Analogues as Potent Analgesics
J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 25 4943
Ta ble 1. Routes of Synthesis (Scheme 2) and 45Ca2+ Influx
Activity of Thioureas
Sch em e 1
(methylamino)ethyl side chain of 4 was protected as the
FMoc derivative for the coupling step. The protecting
group was subsequently removed using piperidine under
standard conditions. The (dimethylamino)ethyl phenol
ether 5 was prepared from the aminoethyl compound
2a by treatment with paraformaldehyde and subsequent
reduction with sodium cyanoborohydride. The acylated
compounds 8-10 were made from 2a using acetyl
chloride, ethyl chloroformate, and di-tert-butyl dicar-
bonate, respectively.
Biologica l Resu lts a n d Discu ssion
Combination of the optimal features of each of the
three regions of the capsaicin molecule has led to the
aralkyl and aralkenyl thioureas 1a -i. As anticipated,
evaluation of these compounds in the 45Ca2+ influx assay
has established that 1a -i are capsaicin-like agonists
with potencies, in some cases, greater than that of
capsaicin itself (see Table 1). Indeed, with the exception
of the complex natural product resiniferatoxin and its
analogues,7-9 these compounds are the most potent
capsaicin-like agonists reported.
The C region substituents (R2), exemplified in 1a -i,
fall within the molecular size limit (Mr e 55) for the
“hydrophobic substituent of limited size” compatible
with high potency which had been proposed earlier.3 The
significant difference in activity between the isomers
1c,d is interesting and may reflect a further manifesta-
tion of the shape and size limitation in this region.
Further consideration of the SAR establishes the im-
portance of substitution of the aromatic ring (i.e., the
unsubstituted analogue 1g is less potent), but further
attempts to rationalize the nature or pattern of substi-
tution seem pointless from these data given the similar
activities of these analogues.
The p-chlorophenethyl thiourea 1b was selected for
in vivo evaluation to assess its potential as an analgesic
agent. The potent (E)-ethenyl compound 1d was pre-
cluded from consideration because of its limited chemi-
cal stability.
From the data shown in Table 2 the analgesic proper-
ties of 1b support the principle established earlier,1-3
namely, that potent capsaicin-like agonism correlates
with analgesic effects in rodent models. This molecule
was however precluded from further development as a
therapeutic agent because of several unwanted proper-
ties. Although 1b was effective in several rodent
analgesia models given subcutaneously (ED50 ) 11.8 (
3.6 µmol/kg, 4.0 ( 1.2 mg/kg), oral activity was poor
(ED50 ) 370 ( 54 µmol/kg, 113 ( 16 mg/kg) and the

4944 J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 25
Wrigglesworth et al.
Ta ble 2. Comparative in Vivo Data for Thioureas 1b and 2a ,h and Reference Compounds
tail-flick latency
ED50 (µmol/kg)
writhing ED50 (µmol/kg)
duration of
action (h)a
bronchoconstriction
compound
sc
po
sc
po
threshold doseb (µmol/kg iv)
capsaicin
1b
oleyl vanillylamide
2a
10.3 ( 2.9
11.8 ( 3.6
962 ( 272
370 ( 54
5.72 ( 0.63
10.13 ( 1.16
2.10 ( 0.59
0.018 ( 0.003
0.025 ( 0.005
>0.100
0.137 ( 0.037
0.062 ( 0.012
>0.100
<2
11.3 ( 1.6 >400
4.8 ( 1.2 22.87 ( 3.18
0.40 ( 0.09 1.45 ( 0.55
6.3 ( 1.0
21.61 ( 2.98
5
>6
3.32 ( 0.36 11.45 ( 2.81
0.49 ( 0.04 2.55 ( 0.50
3.06 ( 0.57
2h
oleyl 4-O-(aminoethyl)vanillylamide
morphine
184 ( 45
4.27 ( 0.16
a
b
At ED50 dose. Threshold dose required to induce an increase in Pao
.
duration of the response was short (total loss of activity
at 2 h after an ED50 dose sc). The spectrum of excitatory
side effects of 1b was similar to that of capsaicin.10 In
particular, bronchoconstrictive effects of 1b (threshold
dose 0.025 ( 0.005 µmol/kg, 8.9 ( 1.8 µg/kg, iv, guinea
pig), which are a characteristic property of capsaicin,11
were observed at similar doses to the latter. The
excitatory properties of 1b were also manifest in its
pungency and irritant properties which were akin to
those of capsaicin itself. Olvanil (oleyl vanillylamide),
while clearly less pungent than capsaicin,5b suffers from
some of the same defects as 1b, particularly, poor oral
activity.
the amino group all lead to less active compounds than
the unsubstituted compound 2a . The lack of activity
of the quaternary ammonium compound 6 is particu-
larly noteworthy, possibly implying the existence of an
access barrier, e.g., membrane penetration, to such a
compound. Further investigation of this hypothesis is
in progress.
On the basis of the in vitro data compounds 2a ,h were
evaluated in vivo in analgesia models in comparison
with morphine as a representative established analgesic
agent and oleyl 4-O-(aminoethyl)vanillylamide (NE
21610), another capsaicin agonist. These data are
shown in Table 2.
The medicinal chemistry goal to develop a therapeutic
entity therefore was focused to address these issues,
namely, the improvement of the bioavailability and the
reduction of the excitatory properties of 1b. Preliminary
metabolism studies on phenol 1b established that the
molecule was rapidly metabolized to the O-glucuronide,
which was the only detectable component in plasma at
all time points after oral dosing in rats and dogs.12 This
marked glucuronidation and thereby probable inactiva-
tion of the parent molecule 1b would explain the low
oral bioavailability and the relatively short duration of
action of this compound.
The modification of the phenol group described above
was undertaken to improve the pharmacokinetic prop-
erties of these molecules. It can be seen from the data
in Table 2 that this improvement has been achieved.
While the potency of the phenol 1b is comparable with
that of 2a by the subcutaneous route, the oral potency
of the latter is much improved. In contrast the same
improvement is not observed with oleyl 4-O-(aminoet-
hyl)vanillylamide, where, from the tail-flick latency data
(see Table 2), it appears that this molecule has only poor
oral activity. We have no explanation for this difference
which clearly lies elsewhere than in vitro potency as
oleyl 4-O-(aminoethyl)vanillylamide is comparable in
the Ca2+ influx assay to 2h . The potency of 2h is
significantly increased by both routes of administration
over that of 2a , perhaps reflecting the greater in vitro
activity of the former; moreover 2h is more potent than
the reference molecule morphine. Another improve-
ment is illustrated by the increased duration of action
of both compounds, 2a ,h , over the “parent” phenol 1b.
Fortuitously this structural change, which has im-
proved the analgesic profile of analogues such as 2a ,h ,
also reduced the excitatory properties of these com-
pounds in comparison to the phenolic compounds. In
contrast to 1b, a clear separation of bronchoconstrictive
effects from analgesia was achieved with 2a and (to a
lesser extent) with the more potent compound 2h .
These compounds are also virtually nonpungent in
comparison to capsaicin and 1b. A possible explanation
for the reduced excitatory properties of O-substituted
compounds such as 2h over the phenol series, e.g., 1b,
appears to lie in their rate of excitation of the sensory
neuron.14
It had been established from our earlier studies1 that,
in general, blocking of the phenolic OH group with a
variety of substituents removed or drastically reduced
agonist activity in capsaicin analogues; however, sub-
stitution with the 2-aminoethyl group led to the series
of compounds 2a -j in which agonist potency in vitro
was substantially maintained. Comparison of struc-
tural pairs (e.g., 1b and 2a , 1e and 2c, 1f and 2d , and
1g and 2b) showed relatively lower potency in the
O-substituted analogues over the parent phenols with
one clear exception (1i and 2h ). Contemporaneously
the Procter and Gamble group claimed that this same
substituent attached to the phenolic moiety of their
aliphatic vanillylamides improved the water solubility,
reduced the irritant effects, and retained the antinoci-
ceptive properties of these compounds.13 A representa-
tive of this chemical class is the compound oleyl 4-O-
(aminoethyl)vanillylamide (NE 21610).
With the identification of the 2-aminoethyl substitu-
ent as an acceptable replacement for the metabolically
labile parent phenolic hydrogen, it was considered
important to explore the SAR of this substituent. The
results of this structural investigation are shown in
Table 1 from the series of analogues of 2a . It is clear
from the in vitro data presented in Table 1 that all
modifications of the 2-aminoethyl substituent are del-
eterious. Thus lengthening of the methylene bridge (in
3) and alkylation (in 4-6) and acylation (in 7-10) of
On the basis of the combination of its antinociceptive
properties and its reduced excitatory effects compared
with other capsaicin agonists, 2h has been selected as
a clinical development candidate with a novel mode of
analgesic action. Mechanistic studies are ongoing to
elaborate a molecular explanation for the improved
properties of 2h and congeners which involves the rate

Capsaicin Analogues as Potent Analgesics
J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 25 4945
between EtOAc (75 mL) and water (50 mL). The organic phase
was washed with water (50 mL) and brine (50 mL) and then
dried over Na2SO4. After filtration and removal of solvent in
vacuo, the residue was dissolved in THF (20 mL) and cooled
on ice; 1 M HClaq (5 mL) was added, and the reaction mixture
was stirred for 3 h. The THF was removed in vacuo; the
residue was taken up in EtOAc (50 mL), washed with 1 M
HClaq (25 mL), water (2 × 25 mL), and brine (25 mL), and
then dried over Na2SO4. The mixture was filtered and the
solvent removed in vacuo. The residue was purified by
recrystallization from MeOH to give 350 mg of an off-white
crystalline solid (12% yield): mp 147-150 °C; TLC (silica,
cyclohexane/EtOAc, 1:1) Rf 0.24; 1H-NMR (DMSO-d6, 60 MHz)
δ 2.85 (2H, m, ArCH2CH2N), 3.65 (2H, m, ArCH2CH2N), 3.75
(3H, s, ArOCH3), 4.55 (2H, d, J ) 6.0 Hz, ArCH2N), 6.7-7.0
(3H, m, ArH), 7.1-7.35 (4H, m, ArH), 7.50 (1H, br m, CH2NH),
of membrane penetration, and this will be reported in
due course.
Exp er im en ta l Section
Gen er a l In for m a tion . Melting points were determined
using a Reichert hot-stage microscope and are uncorrected.
Routine NMR spectra were recorded using Hitachi-Perkin
Elmer R12B and Varian Gemini 200 machines. High-field
spectra were recorded using Varian VX400 400 MHz (Univer-
sity College London Chemistry Department) and Bruker
AM360 360 MHz (Sandoz, Basle) instruments. All spectra
were recorded using tetramethylsilane (TMS) as an internal
standard, and chemical shifts are reported in parts per million
(δ) downfield from TMS. Coupling constants are reported in
hertz. A Perkin-Elmer 781 machine was used to record IR
spectra. Elemental analyses were performed by the Analytical
Department of University College London and were within
0.4% of theory unless otherwise indicated. Mass spectra were
recorded by the Mass Spectrometry Department of University
College London, using a VG 7070F/H spectrometer, and FAB
spectra were recorded in Sandoz, Basle, using a VG 70-SE
spectrometer. Accurate mass determinations were made by
M. Cocksedge and Dr. D. Carter, London School of Pharmacy,
using a VG ZAB SE mass spectrometer and FAB ionization.
TLC was performed using Merck Kieselgel 60 F254 silica
plates or Merck aluminum oxide 60 F254 plates, and compo-
nents were visualized using UV light and iodine vapor. HPLC
was performed using a Waters 600 system (µ-Bondapak C-18
column (RP18), using CH3CN/0.1% aqueous TFA gradients of
compositions stated in the text. Compounds were purified by
flash column chromatography15 using Merck Kieselgel 60
(230-400 mesh) unless otherwise indicated. Solvents were
HLPC grade and used without further purification. Solvents
were dried according to the standard procedures.16 Test
compounds were homogenous by TLC or HPLC unless other-
wise stated. Chemical yields were not optimized.
7.80 (1H, br m, ArCH2NH); MS m/e 334 (M+). Anal. (C17H19
-
N2O2SF) C,H,N.
Gen er a l P r oced u r e for th e Syn th esis of Isoth iocya n -
a tes. Unless commercially available, or otherwise indicated,
all isothiocyanates mentioned herein were synthesized by the
method illustrated by the synthesis of 4-tert-butylbenzyl
isothiocyanate described below.
4-ter t-Bu tylben zyl Isoth iocya n a te. 4-tert-Butylbenzy-
lamine (2.0 g, 0.0123 mol) was dissolved in EtOAc (20 mL)
with triethylamine (2.53 g, 0.025 mol) and added dropwise to
a solution of thiophosgene (1.41 g, 0.0123 mol) in EtOAc (50
mL), previously cooled on ice, with stirring. The reaction
mixture was allowed to come to room temperature and stirred
for 2 h, until complete by TLC. The reaction mixture was
washed with water (2 × 50 mL) and brine (50 mL) before
drying over Na2SO4. The solution was filtered, and the solvent
was removed in vacuo to leave a brown solid, which was
purified by flash column chromatography (silica, cyclohexane/
EtOAc, 4:1) to give 1.71 g of a yellow-brown solid (68% yield):
TLC (silica, cyclohexane/EtOAc, 1:1) Rf 0.61.
N-(4-Hydr oxy-3-m eth oxyben zyl)-N′-(2-(4-ch lor oph en yl)-
eth yl)th iou r ea (1b): synthesized from vanillylamine hydro-
chloride and 2-(4-chlorophenyl)ethyl isothiocyanate by method
A; purification by recrystallization from aqueous MeOH gave
a white crystalline solid, 48% yield; mp 143-144 °C; TLC
(silica, CH2Cl2/MeOH, 5:1) Rf 0.69; 1H-NMR (DMSO-d6, 400
MHz) δ 2.82 (2H, t, J ) 7.16 Hz, ArCH2CH2), 3.65 (2H, br m,
ArCH2CH2N), 3.76 (3H, s, ArOCH3), 4.52 (2H, br m, ArCH2N),
6.73 (2H, m, ArH), 6.9 (1H, s, ArH), 7.31 (4H, m, ArH), 7.40
(1H, br m, thiourea NH), 7.75 (1H, br m, thiourea NH), 8.90
(1H, s, ArOH); MS m/e 350 (M+). Anal. (C17H19N2O2SCl)
C,H,N.
Gen er a l P r oced u r e for th e Syn th esis of Va n illyl Th io-
u r ea s. The thioureas described herein were prepared either
by coupling vanillylamine (or a 4-alkyloxy derivative) with the
relevant isothiocyanate (method A) or by coupling vanillyl
isothiocyanate (or a 4-alkyloxy derivative) with the relevant
amine (method B). Both are exemplified below.
CAUTION. Some of the vanillyl thioureas described below
are pungent and are irritant to broken skin and mucous
membranes (akin to the effects of capsaicin itself). Appropriate
handling precautions should be taken.
Meth od A: N-(4-Hyd r oxy-3-m eth oxyben zyl)-N′-(4-ch lo-
r oben zyl)th iou r ea (1a ). Vanillylamine hydrochloride (2.5
g, 0.013 mol) was dissolved in DMF (20 mL) with 5 M NaOH
(5.2 mL). The mixture was stirred and cooled on ice. 4-Chlo-
robenzyl isothiocyanate (2.66 g, 0.04 mol) was dissolved in
DMF (10 mL) and added dropwise. The mixture was stirred
for 18 h and then poured into water (250 mL). The aqueous
mixture was extracted with diethyl ether (4 × 100 mL); the
combined organic phase was washed with 1 M HClaq (50 mL),
saturated NaHCO3aq (50 mL), and brine (50 mL) and then
dried over MgSO4 with stirring. The solution was filtered and
the solvent removed in vacuo to give an oil. The residue was
purified by flash column chromatography (silica, CH2Cl2,
changing to CH2Cl2/MeOH, 50:1) and recrystallized from
MeOH to give 1.15 g of a white crystalline solid (38% yield):
4-Ch lor oben zoyl Nitr ile. This compound was prepared
from 4-chlorobenzoyl chloride, according to the method of
Burger and Hornbaker.17
2-(4-Ch lor op h en yl)-2-h yd r oxyeth yla m in e. Lithium alu-
minum hydride (50 g) was suspended in dry diethyl ether (500
mL) and stirred on a salt/ice bath. A solution of 4-chloroben-
zoyl nitrile (115.0 g, 0.695 mol) in diethyl ether (500 mL) was
added dropwise over 30 min. The mixture was stirred under
N2 at room temperature for 18 h and then refluxed for 6 h.
The mixture was cooled to room temperature and then on ice,
and wet diethyl ether (500 mL) was added to deactivate the
LiAlH4 followed by the slow addition of 5 N NaOHaq
. The
mixture was filtered, the solution was dried over Na2SO4 and
then filtered, and the solvent was removed in vacuo. The
residue was purified by flash column chromatography (silica,
CH2Cl2/MeOH, 10:1) to give 39.0 g of a white crystalline solid
(33% yield): TLC (CH2Cl2/MeOH/AcOH, 120:90:5) Rf 0.45.
2-(4-Ch lor op h en yl)-2-ch lor oet h yla m in e H yd r och lo-
r id e. 2-(4-Chlorophenyl)-2-hydroxyethylamine (38.2 g, 0.223
mol) was dissolved in chloroform (500 mL). Thionyl chloride
(55 mL, 0.765 mol) in chloroform (100 mL) was added slowly
with stirring, and a dense beige precipitate formed. The
mixture was stirred and refluxed for 3 h and then cooled, and
the solvent was removed in vacuo. The residue was sonicated
in MeOH, filtered, washed with diethyl ether, and then dried
to give 25.9 g of an off-white crystalline solid (51% yield): TLC
(silica, CH2Cl2/MeOH, 5:1) Rf 0.6; 1H-NMR (CD3OD, 60 MHz)
1
mp 135-138 °C; TLC (silica, CH2Cl2/MeOH, 5:1) Rf 0.59; H-
NMR (DMSO-d6, 400 MHz) δ 3.74 (3H, s, ArOCH3), 4.56 (2H,
br m, ArCH2NH), 4.70 (2H, br m, ArCH2NH), 6.72 (2H, m,
benzyl ArH5,6), 6.91 (1H, s, vanillyl ArH2), 7.35 (4H, m, ArH),
7.92 (2H, br m, thiourea NHs), 8.92 (1H, s, ArOH); MS m/e
336 (M+). Anal. (C16H17N2O2SCl) C, H, N.
Meth od B: N-(4-Hyd r oxy-3-m eth oxyben zyl)-N′-(2-(4-
flu or op h en yl)eth yl)th iou r ea (1e). 2-(4-Fluorophenyl)ethy-
lamine hydrochloride (1.56 g, 0.0089 mol) was dissolved in dry
DMF (10 mL) with triethylamine (0.987 g, 9.75 mL) and
stirred. 4-(1-Ethoxyethoxy)-3-methoxybenzyl isothiocyanate
(2.37 g, 0.0089 mol) was dissolved in dry DMF (2 mL), added
slowly to the reaction mixture, and stirred overnight. The
DMF was removed in vacuo and the residue partitioned

4946 J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 25
Wrigglesworth et al.
δ 3.55 (2H, d, J ) 7.2 Hz, CHClCH2NH2), 5.4 (1H, t, J ) 7.2
Hz, ArCHClCH2), 7.6 (4H, s, ArH); FABMS m/e 190 (MH+).
136-138 °C; TLC (silica, cyclohexane/EtOAc, 1:1) Rf 0.28; 1H-
NMR (DMSO-d6, 60 MHz) δ 2.95 (2H, m, NHCH2CH2Ar), 3.65
(2H, m, NHCH2CH2Ar), 3.75 (3H, s, ArOCH3), 4.55 (2H, d, J
) 6.0 Hz, ArCH2NH), 6.75-7.0 (3H, m, ArH), 7.1-7.9 (5H, m,
3 × ArH + 2 × NH), 8.85 (1H, s, ArH); MS m/e 384 (M+). Anal.
(C17H18N2O2SCl) C,H,N.
2-P h en yleth yl isoth iocya n a te: synthesized from 2-phe-
nylethylamine, to give an orange oil, which was used without
further purification after workup, crude yield 97%.
2-(4-Ch lor op h en yl)-2-ch lor oeth yl Isoth iocya n a te. 2-(4-
Chlorophenyl)-2-chloroethylamine hydrochloride (25.9 g, 0.114
mol) was suspended in water (300 mL), with a few crystals of
phenolphthalein. Thiophosgene (9.2 mL, 0.120 mol) was added
in CH2Cl2 (200 mL) with stirring followed by the gradual
addition of 2 M NaOHaq until the aqueous layer retained a
permanent purple coloration. After stirring for 30 min, the
layers were partitioned using a separating funnel. The organic
phase was washed with saturated NaClaq, dried over Na2SO4,
and then filtered and the solvent removed in vacuo. The
residue was purified by flash column chromatography (silica,
cyclohexane/EtOAc, 50:1) to give 22.3 g of a yellow crystalline
solid (84% yield): TLC (silica, cyclohexane/EtOAc, 50:1) Rf
0.25; 1H-NMR (CDCl3, 60 MHz) δ 3.95 (2H, d, J ) 7.0 Hz,
CHCH2NCS), 5.0 (1H, t, J ) 7.0 Hz, ArCHClCH2), 7.4 (4H,
m, ArH); IR ν 2090 cm-1 (NCS stretch).
(E)-2-(4-Ch lor op h en yl)eth en yl Isoth iocya n a te. 2-(4-
Chlorophenyl)-2-chloroethyl isothiocyanate (9.35 g, 0.04 mol)
was dissolved in toluene (100 mL) and stirred at 100 °C;
triethylamine (4.5 g, 0.045 mol) was added, and the reaction
mixture was stirred for 18 h, at which point a further 4.5 g of
triethylamine was added. The reaction mixture was stirred
at 100 °C for another 18 h. The reaction mixture was cooled
to room temperature and filtered, and the solvent was removed
in vacuo. The residue was partially purified by flash column
chromatography (silica, cyclohexane/EtOAc, 50:1); the mixture
of cis and trans isomers was then recrystallized from n-hexane
to give 1.55 g of the pure trans isomer, a white crystalline solid
(20% yield): TLC (silica, cyclohexane/EtOAc, 50:1) Rf 0.45; 1H-
NMR (acetone-d6, 200 MHz) δ 6.84 (1H, d, J ) 14.13 Hz,
ArCHdCHNCS), 7.05 (1H, d, J ) 13.96 Hz, ArCHdCHNCS),
7.33 (4H, m, ArH).
N -(E )-(2-(4-Ch lor op h e n yl)e t h e n yl)-N ′-(4-h yd r oxy-3-
m eth oxyben zyl)th iou r ea (1c): synthesized from vanilly-
lamine hydrochloride and (E)-2-(4-chlorophenyl)ethenyl isothio-
cyanate by method A; purification by recrystallization from
EtOAc/cyclohexane gave a white crystalline solid, yield 87%;
mp 180-183 °C; TLC (silica, CH2Cl2/MeOH, 25:1) Rf 0.32; 1H-
NMR (acetone-d6, 400 MHz) δ 3.8 (3H, s, ArOCH3), 4.7 (2H,
d, J ) 5 Hz, ArCH2NH), 6.1 (1H, d, J ) 14.6 Hz, ArCHd
CHNH), 6.7-7.4 (7H, m, ArH), 7.6 (2H, br m, ArCH2NH +
ArOH), 8.1 (1H, m, ArCHdCHNH), 9.3 (1H, br d, ArCHd
CHNH); MS m/e 348 (M+). Anal. (C17H17N2O2SCl) C,H,N.
N -(Z)-(2-(4-Ch lor op h e n yl)e t h e n yl)-N ′-(4-h yd r oxy-3-
m eth oxyben zyl)th iou r ea (1d ). A solution of 1c (300 mg,
0.000 86 mol) in anhydrous THF (120 mL) was prepared, and
dry Ar was bubbled through the solution for 40 min. The
solution was irradiated for 47 min with a mercury lamp, until
no starting material remained. The solvent was removed in
vacuo, and the residue was purified by flash column chroma-
tography (silica, cyclohexane/EtOAc, 1:1) and recrystallization
from EtOAc/cyclohexane to give 170 mg of a white crystalline
solid (57% yield): mp 143 °C; TLC (silica, cyclohexane/EtOAc,
1:1) Rf 0.28; 1H-NMR (acetone-d6, 400 MHz) δ 3.82 (3H, s,
ArOCH3), 4.70 (2H, br s, ArCH2N), 5.61 (1H, d, J ) 9.6 Hz,
ArCHdCHNH), 6.75-6.85 (2H, m, ArH), 7.01 (1H, d, J ) 1.6
Hz, ArH), 7.33 (4H, m, ArH), 7.55 (1H, m, ArCHdCHNH), 7.58
(1H, s, ArOH), 7.85 (1H, br m, thiourea NH), 8.85 (1H, br m,
CHdCHNH); MS m/e 348 (M+). Anal. (C17H17N2O2SCl)
C,H,N.
N-(4-Hyd r oxy-3-m eth oxyben zyl)-N′-(2-p h en yleth yl)th -
iou r ea (1g): synthesized from 2-phenylethyl isothiocyanate
and vanillylamine hydrochloride by method A; purification by
recrystallization from EtOAc/petroleum ether gave a white
crystalline solid, yield 32%; mp 105-107 °C; 1H-NMR (DMSO-
d6, 60 MHz) δ 2.80 (2H, t, J ) 7 Hz, ArCH2CH2), 3.70 (2H, m,
ArCH2CH2N), 3.75 (3H, s, ArOCH3), 4.55 (2H, d, J ) 6 Hz,
ArCH2N), 6.75-6.95 (3H, m, ArH), 7.2-7.4 (5H, m, ArH), 7.40
(1H, br t, thiourea NH), 7.75 (1H, br t, thiourea NH), 8.80 (1H,
s, ArOH); MS m/e 316 (M+). Anal. (C17H20N2O2S) C,H,N.
3-(4-Ch lor oph en yl)pr opyl Meth yloxim e. Methoxylamine
hydrochloride (4.8 g, 0.0575 mol) and sodium acetate (4.80 g,
0.0575 mol) were suspended in MeOH (40 mL) and added to a
solution of 3-(4-chlorophenyl)propanal (3.0 g, 0.0178 mol) in
MeOH (30 mL). The reaction mixture was refluxed for 90 min
and cooled and the solvent removed in vacuo. The residue was
partitioned between CH2Cl2 and water; the organic phase was
washed with brine, dried over MgSO4, and filtered and the
solvent removed in vacuo. The residue was purified by flash
column chromatography (silica, cyclohexane/EtOAc, 30:1) to
give 1.50 g of a colorless oil (43% yield): TLC (silica, cyclo-
hexane/EtOAc, 1:1) Rf 0.60; 1H-NMR (CDCl3, 60 MHz) δ 2.80
(4H, m, ArCH2CH2CH), 3.80 (3H, s, NOCH3), 6.70 (1H, m,
CH2CHdNOCH3), 7.20 (4H, m, ArH); MS m/e 197 (M+).
3-(4-Ch lor op h en yl)p r op yla m in e. 3-(4-Chlorophenyl)pro-
pyl methyloxime (1.71 g, 0.0087 mol) was dissolved in dry THF
(10 mL) and stirred under Ar on ice; 1 M borane-THF complex
in THF (4.5 mL) was added slowly to the oxime solution, and
the reaction mixture was refluxed for 24 h. Another 4.5 mL
of 1 M borane-THF solution was added and the reaction
mixture refluxed for a further 4 h and then cooled on ice.
MeOH (60 mL) and 5 M NaOHaq (14 mL) were slowly added,
and the reaction mixture was refluxed for 2 h. The solvent
was removed in vacuo, and the residue was partitioned
between water and EtOAc. The organic phase was dried over
MgSO4 and filtered, and the solvent was removed in vacuo to
give 1.45 g (98%) of a pale brown oil, which was used without
purification/characterization: TLC (silica, CH2Cl2/MeOH/
AcOH, 120:90:5) Rf 0.45 (ninhydrin positive).
Va n illyl Isoth iocya n a te. Vanillylamine hydrochloride
(1.89 g, 0.010 mol) was dissolved in water (20 mL) with CH2-
Cl2 (20 mL) and calcium carbonate (3.0 g, 0.030 mol) and
stirred. Thiophosgene (0.9 mL, 0.012 mol) was added slowly
in CH2Cl2 (10 mL) over 90 min. The reaction mixture was
stirred at room temperature for 24 h and then filtered, and
the organic phase was purified by flash column chromatogra-
phy (silica, cyclohexane/EtOAc, 4:1) to give 1.1 g of a dark
brown oil (56% yield): TLC (silica, cyclohexane/EtOAc, 4:1)
Rf 0.43.
N-(3-(4-Ch lor op h en yl)p r op yl)-N′-(4-h yd r oxy-3-m et h -
oxyben zyl)th iou r ea (1h ): synthesized from vanillyl isothio-
cyanate and 3-(4-chlorophenyl)propylamine by method B in
dry THF; purification by flash column chromatography (silica,
CH2Cl2/MeOH, 50:1) and recrystallization from EtOAc/n-
hexane gave a white crystalline solid, yield 44%; mp 109-111
°C; TLC (silica, CH2Cl2/MeOH, 25:1) Rf 0.45; 1H-NMR (CDCl3,
400 MHz) δ 2.60 (2H, t, J ) 7.58 Hz, ArCH2CH2), 3.43 (2H, br
m, CH2CH2NH), 3.88 (3H, s, ArOCH3), 4.47 (2H, br m,
ArCH2N), 5.63 (1H, s, ArOH), 5.73 (1H, br m, thiourea NH),
6.00 (1H, br m, thiourea NH), 6.80 (3H, m, vanillyl ArH), 7.15
(4H, m, ArH): MS m/e 364 (M+). Anal. (C18H21N2O2SCl)
C,H,N.
4-(1-Eth oxyeth oxy)-3-m eth oxyben zyl isoth iocya n a te:
from 4-(1-ethoxyethoxy)-3-methoxybenzylamine,12 to give a
brown oil, used without further purification after workup,
crude yield 100%; TLC (silica, cyclohexane/EtOAc, 1:1) Rf 0.46.
2-(2,4-Dich lor op h en yl)eth yl isoth iocya n a te: synthe-
sized from 2-(2,4-dichlorophenyl)ethylamine, to give a brown
oil, which was used without further purification after workup,
crude yield 98%; TLC (silica, cyclohexane/EtOAc, 1:1) Rf 0.7.
N-(2-(2,4-Dich lor op h en yl)eth yl)-N′-(4-h yd r oxy-3-m eth -
oxyben zyl)th iou r ea (1f): synthesized from vanillylamine
hydrochloride and 2-(2,4-dichlorophenyl)ethyl isothiocyanate
by method A; purification by recrystallization from EtOAc/
petroleum ether gave a white crystalline solid, yield 33%; mp
N-(4-ter t-Bu tylben zyl)-N′-(4-h ydr oxy-3-m eth oxyben zyl)-
th iou r ea (1i): synthesized from vanillylamine hydrochloride
and 4-tert-butylbenzyl isothiocyanate by method A; purification
by flash column chromatography (silica, cyclohexane/EtOAc,
3:1) gave a pure white foamed solid (50% yield), which was

Capsaicin Analogues as Potent Analgesics
J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 25 4947
recrystallized from EtOAc/n-hexane to give a white crystalline
solid, yield 21%; mp 110-111 °C; TLC (silica, cyclohexane/
EtOAc, 1:1) Rf 0.30; 1H-NMR (CDCl3, 200 MHz) δ 1.30 (9H, s,
tert-butyl), 3.84 (3H, s, ArOCH3), 4.51 (2H, d, J ) 5.12 Hz,
ArCH2NH), 4.58 (2H, d, J ) 4.76 Hz, ArCH2NH), 5.61 (1H, s,
ArOH), 6.02 (2H, br m, thiourea NHs), 6.73-6.86 (3H, m,
vanillyl ArH), 7.26 (4H, m, ArH); MS m/e 358 (M+). Anal.
(C20H26N2O2S) C,H,N.
Boc-va n illyla m in e. Vanillylamine hydrochloride (18.0 g,
0.095 mol) and triethylamine (10.6 g, 0.11 mol) were dissolved
in water (250 mL). Di-tert-butyl dicarbonate (20.5 g, 0.095
mol) in dioxane (200 mL) was added with stirring over a period
of 15 min and the resulting mixture stirred overnight at room
temperature. The dioxane was removed in vacuo and the
aqueous residue extracted with CHCl3 (3 × 150 mL). The
combined extracts were dried over MgSO4 and filtered and the
solvent removed in vacuo to leave a brown oil which was
purified by flash column chromatography (silica, cyclohexane/
EtOAc, 5:2) to give 21.5 g of a colorless oil (89% yield) which
crystallized on standing: TLC (silica, cyclohexane/EtOAc, 1:1)
Rf 0.5.
Boc-4-(2-br om oeth oxy)-3-m eth oxyben zyla m in e. Boc-
vanillylamine (21.0 g, 0.083 mol), 1,2-dibromoethane (250 mL),
40% aqueous KOH (66 mL), and 40% aqueous tetrabutylam-
monium hydroxide (6.6 mL) were combined and heated at 50
°C for 3 h with rapid stirring. The mixture was cooled, diluted
with CH2Cl2 (200 mL), and washed with water (3 × 200 mL),
and the combined aqueous washings were extracted once with
CH2Cl2 (600 mL). The combined organic layers were washed
with brine, dried over MgSO4, and filtered, and the solvent
was removed in vacuo to leave 23.0 g of a white solid (77%
yield) which was used without further purification: TLC
(silica, cyclohexane/EtOAc, 1:1) Rf 0.6.
and dried in vacuo to give 1.35 g of a glassy solid (90% yield).
The hydrochloride salt was prepared by dissolving 2a in boiling
1 M HClaq, cooling to room temperature, and filtering. The
filtrate was recrystallized from EtOH/water and dried in vacuo
over P2O5 to give 1.40 g of a colorless crystalline solid (78%
overall yield): mp 83-84 °C; TLC (silica, CHCl3/Et3N/MeOH,
93:2:5) Rf 0.3, (silica, nBuOH/AcOH/H2O, 4:1:1) Rf 0.3; 1H-NMR
(DMSO-d6, 400 MHz) δ 2.80 (2H, t, J ) 6.8 Hz, ArCH2CH2N),
3.00 (2H, t, J ) 5.2 Hz, OCH2CH2N), 3.62 (2H, m, ArCH2CH2N),
3.80 (3H, s, ArOCH3), 4.00 (2H, t, J ) 5.2 Hz, OCH2CH2N),
4.57 (2H, br s, ArCH2N), 6.78-6.95 (3H, m, ArH), 7.30 (4H,
m, ArH), 7.67 (1H, br s, NH), 7.98 (1H, br s, NH); FABMS m/e
394 (MH+). Anal. (C19H24N3O2SCl‚HCl‚2.5H2O) C,H,N,O,S,-
Cl.
N-(4-(2-P h t h a lim id oet h oxy)-3-m et h oxyb en zyl)-N′-(2-
p h en yleth yl)th iou r ea : synthesized from 4-(2-phthalimido-
ethoxy)-3-methoxybenzylamine trifluoroacetate and 2-phenyl-
ethyl isothiocyanate by method A in EtOAc with the addition
of triethylamine, which gave the product in 54% yield; TLC
(silica, cyclohexane/EtOAc, 1:1) Rf 0.2.
N-(4-(2-Am in oeth oxy)-3-m eth oxyben zyl)-N′-(2-p h en yl-
eth yl)th iou r ea (2b): synthesized from N-(4-(2-phthalimido-
ethoxy)-3-methoxybenzyl)-N′-(2-phenylethyl)thiourea, which
gave a pale yellow glassy solid, 93% yield; TLC (silica, MeOH)
Rf 0.1; HPLC reverse phase C-18 (CH3CN/0.1% TFAaq gradient
1
10-100%) tR ) 8.9 min (>98% purity); H-NMR (CDCl3, 400
MHz) δ 2.86 (2H, t, J ) 7.0 Hz, ArCH2CH2N), 3.08 (2H, t, J )
5.0 Hz, OCH2CH2N), 3.16 (2H, br m, NH2), 3.71 (2H, t, J )
7.0 Hz, ArCH2CH2N), 3.77 (3H, s, ArOCH3), 4.00 (2H, t, J )
5.0 Hz, OCH2CH2N), 4.44 (2H, d, J ) 4.2 Hz, ArCH2N), 6.09
(1H, br m, NH), 6.43 (1H, br m, NH), 6.68-6.79 (3H, m, ArH),
7.11-7.31 (5H, m, ArH); HRMS (C21H29N3O2ClS) calcd 422.1669,
found 422.1662.
B o c -4-(2-p h t h a li m i d o e t h o x y )-3-m e t h o x y b e n z y l-
a m in e. Boc-4-(2-bromoethoxy)-3-methoxybenzylamine (23.0
g, 0.064 mol) and potassium phthalimide (11.8 g, 0.064 mol)
were suspended in dry DMF (500 mL). The resulting suspen-
sion was heated at 50 °C for 2 h with rapid stirring (after 30
min the mixture became homogeneous). The mixture was then
cooled and the DMF removed under high vacuum. The
resulting solid residue was purified by flash column chroma-
tography (silica, cyclohexane/EtOAc, 1:1) to give 26.5 g of a
white solid (97% yield): TLC (silica, cyclohexane/EtOAc, 1:1)
Rf 0.45.
(4-(2-P h t h a lim id oe t h oxy)-3-m e t h oxyb e n zyl)a m m o-
n iu m Tr iflu or oa cet a t e. Boc-4-(2-phthalimidoethoxy)-3-
methoxybenzylamine (26.5 g, 0.062 mol) was dissolved in
CH2Cl2 (200 mL), and trifluoroacetic acid (15 mL) was added
dropwise with stirring. On completion of the addition, the
mixture was stirred for a further 2 h at room temperature until
the reaction was complete by TLC (silica, cyclohexane/EtOAc,
1:1). The solvent was removed in vacuo, and the resulting
colorless oil solidified on standing to give 24.5 g of a white
crystalline solid (90% yield): 1H-NMR (DMSO-d6, 200 MHz)
δ 3.63 (3H, s, ArOCH3), 3.95 (4H, m, OCH2CH2N + ArCH2N),
4.23 (2H, t, J ) 5.9 Hz, OCH2CH2N), 6.95-7.10 (3H, m, ArH),
7.89 (4H, m, ArH).
N-(4-(2-P h t h a lim id oet h oxy)-3-m et h oxyb en zyl)-N′-(2-
(4-flu or op h en yl)eth yl)th iou r ea : synthesized from 4-(2-
phthalimidoethoxy)-3-methoxybenzylamine trifluoroacetate and
2-(4-fluorophenyl)ethyl isothiocyanate by method A in EtOAc
with the addition of triethylamine, which gave the product in
47% yield; TLC (silica, cyclohexane/EtOAc, 1:2) Rf 0.3.
N-(4-(2-Am in oeth oxy)-3-m eth oxyben zyl)-N′-(2-(4-flu o-
r op h en yl)eth yl)th iou r ea (2c): synthesized from N-(4-(2-
phthalimidoethoxy)-3-methoxybenzyl)-N′-(2-phenylethyl)thio-
urea, to give a pale yellow glassy solid, 42% yield; TLC (MeOH)
Rf 0.2; HPLC reverse phase C-18 (CH3CN/0.1% TFAaq gradient
1
10-100%) tR ) 8.7 min (>98% purity); H-NMR (CDCl3, 400
MHz) δ 2.53 (2H, br m, NH2), 2.99 (2H, t, J ) 7.2 Hz, ArCH2-
CH2N), 3.11 (2H, t, J ) 5.2 Hz, OCH2CH2N), 3.75 (2H, br m,
ArCH2CH2N), 3.84 (3H, s, ArOCH3), 4.03 (2H, t, J ) 5.2 Hz,
OCH2CH2N), 4.45 (2H, br m, ArCH2N), 5.82 (1H, br m, NH),
6.18 (1H, br m, NH), 6.82, 7.17, 7.35 (7H, m, ArH); HRMS
(C19H25N3O2FS) calcd 378.1652, found 378.1650.
N-(4-(2-P h t h a lim id oet h oxy)-3-m et h oxyb en zyl)-N′-(2-
(2,4-d ich lor op h en yl)eth yl)th iou r ea: synthesized from 4-(2-
phthalimidoethoxy)-3-methoxybenzylamine trifluoroacetate and
2-(2,4-dichlorophenylethyl isothiocyanate by method A in
EtOAc with the addition of triethylamine, which gave the
product in 44% yield; TLC (silica, cyclohexane/ethyl acetate,
1:1) Rf 0.35.
N -(4-(2-Am in oe t h oxy)-3-m e t h oxyb e n zyl)-N ′-(2-(2,4-
dich lor oph en yl)eth yl)th iou r ea (2d): synthesized from N-(4-
(2-phthalimidoethoxy)-3-methoxybenzyl)-N′-(2-(2,4-dichlorophe-
nyl)ethyl)thiourea, to give a pale yellow glassy solid, 35% yield;
TLC (silica, MeOH) Rf 0.15; HPLC reverse phase C-18 (CH3-
CN/0.1% TFAaq gradient 10-100%) tR ) 9.3 min (>98% purity);
1H-NMR (CDCl3, 400 MHz) δ 1.54 (2H, br m, NH2), 2.84 (2H,
t, J ) 7.2 Hz, ArCH2CH2N), 3.10 (2H, t, J ) 5.4 Hz,
OCH2CH2N), 3.72 (2H, br m, ArCH2CH2N), 3.83 (3H, s,
ArOCH3), 4.02 (2H, t, J ) 5.4 Hz, OCH2CH2N), 4.42 (2H, br
m, NH), 5.72 (1H, br m, NH), 6.20 (1H, br m, NH), 6.32-7.13
(6H, m, ArH); HRMS (C19H24N3O2Cl2S) calcd 428.0966, found
428.0962.
Gen er a l P r oced u r e for th e Syn th esis of Am in oeth oxy
An a logu es. All deprotections of phthalimidoethoxy com-
pounds to give the desired aminoethoxy compounds were
carried out by the method exemplified by the synthesis of
compound 2a .
N-(4-(2-Am in oeth oxy)-3-m eth oxyben zyl)-N′-(2-(4-ch lo-
r op h en yl)eth yl)th iou r ea (2a ). A suspension of 7 (2.0 g,
0.0038 mol) in ethanol (10 mL) was heated to 60 °C, until the
solution became homogeneous. Hydrazine monohydrate (0.95
mL, 980 mg, 0.0195 mol) was added and the mixture heated
for 90 min. After 15 min, a white precipitate was formed, and
a small amount of ethanol was added to keep the mixture
mobile. The reaction mixture was cooled and transferred to a
separating funnel, and methyl tert-butyl ether (50 mL) and
0.5 M NaOHaq (50 mL) were added. The organic layer was
extracted, washed with brine, dried over Na2SO4, and filtered
and the solvent removed in vacuo. The residue was purified
by flash column chromatography (silica, CH2Cl2/MeOH, 10:1,
changing to MeOH). The product fractions were evaporated
4-(2-P h t h a lim id oet h oxy)-3-m et h oxyb en zyl isot h io-
cya n a te: prepared by the reaction of 4-(2-phthalimidoethoxy)-
3-methoxybenzylamine trifluoroacetate (5.0 g, 0.011 mol) with
thiophosgene, as described for 2-(4-chlorophenyl)-2-chloroethyl
isothiocyanate, and purified by flash column chromatography

4948 J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 25
Wrigglesworth et al.
(silica, cyclohexane/ethyl acetate, 50:1) to give 2.8 g of a yellow
oil (67% yield) which was used immediately.
benzyl)thiourea, to give a yellow glassy solid (58% yield); TLC
(silica, MeOH) Rf 0.1; HPLC reverse phase C-18 (CH3CN/0.1%
1
TFAaq gradient 10-100%) tR ) 10.9 min (>98% purity); H-
N-(4-(2-P h t h a lim id oet h oxy)-3-m et h oxyb en zyl)-N′-(4-
iod oben zyl)th iou r ea : synthesized from 4-(2-phthalimido-
ethoxy)-3-methoxybenzyl isothiocyanate and 4-iodobenzy-
lamine by method B, in anhydrous THF; purification by flash
column chromatography (silica, cyclohexane/ethyl acetate, 2:1)
gave 1.6 g of the product (78% yield); TLC (silica, cyclohexane/
ethyl acetate, 1:1) Rf 0.25; 1H-NMR (DMSO-d6, 200 MHz) δ
3.54 (3H, s, ArOCH3), 3.93 (2H, t, J ) 6 Hz, OCH2CH2N), 4.17
(2H, t, J ) 6 Hz, OCH2CH2N), 4.50-4.67 (4H, br d, 2 × CH2-
Ar), 6.70-6.96 (3H, m, ArH), 7.07 (2H, d, J ) 9 Hz, I-ArH),
7.67 (2H, d, J ) 9 Hz, I-ArH), 7.80-7.98 (5H, br s, ArH).
N-(4-(2-Am in oeth oxy)-3-m eth oxyben zyl)-N′-(4-iodoben -
zyl)th iou r ea (2e): synthesized from N-(4-(2-phthalimidoet-
hoxy)-3-methoxybenzyl)-N′-(4-iodobenzyl)thiourea, to give a
yellow glassy solid (13% yield); TLC (silica, MeOH) Rf 0.2;
HPLC reverse phase C-18 (CH3CN/0.1% TFAaq gradient 10-
100%) tR ) 9.8 min (>98% purity); 1H-NMR (CD3OD, 200 MHz)
δ 3.15 (2H, t, J ) 6 Hz, OCH2CH2NH2), 3.81 (3H, s, ArOCH3),
4.10 (2H, t, J ) 6 Hz, OCH2CH2N), 4.77 and 4.80 (4H, 2 × br
s, 2 × CH2Ar), 6.79-7.02 (3H, m, ArH), 7.05 (2H, d, J ) 9 Hz,
I-ArH), 7.63 (2H, d, J ) 9 Hz, I-ArH); HRMS (C18H23N3O2IS)
calcd 472.0556, found 472.0550.
4-(Tr im eth ylsilyl)tolu en e. 4-Chlorotoluene (4.0 g, 0.032
mol), trimethylsilyl chloride (84 mL), and Mg dust (840 mg,
0.045 mol) were suspended in anhydrous THF, and the
resultant mixture was refluxed for 18 h, after which time all
the magnesium had been consumed. The solution was cooled
to room temperature and poured into water (150 mL), and the
product was extracted into diethyl ether. The organic layer
was dried over MgSO4 and filtered, and the solvent was
removed in vacuo. The product was purified by vacuum
distillation (0.1 mmHg, bp 28 °C) to give 2.9 g of a colorless
oil (55% yield).
NMR (CDCl3, 200 MHz) δ 0.26 (9H, s, Si(CH3)3), 3.08 (2H, t,
J ) 6 Hz, OCH2CH2N), 3.80 (3H, s, ArOCH3), 4.00 (2H, t, J )
6 Hz, OCH2CH2N), 4.56 and 4.66 (2 × 2H, 2 × br d, J ) 7 Hz,
2 × ArCH2NH), 4.80 (∼2H, br s, NH2), 6.43 (2H, br s, 2 × NH),
6.72-6.83 (3H, m, ArH), 7.25 (2H, d, J ) 8 Hz, ArH), 7.50
(2H, d, J ) 8 Hz, ArH); HRMS (C21H32N3O2SSi) calcd 418.1985,
found 418.1980.
N-(4-(2-P h t h a lim id oet h oxy)-3-m et h oxyb en zyl)-N′-(4-
ter t-bu tylp h en yl)th iou r ea : synthesized from (4-(2-phthal-
imidoethoxy)-3-methoxybenzyl)ammonium trifluoroacetate and
4-tert-butylphenyl isothiocyanate by method A; purification by
flash column chromatography (silica, CHCl3) gave 1.11 g of a
white powdered solid (41% yield); TLC (silica, cyclohexane/
EtOAc, 1:1) Rf 0.38; 1H-NMR (CDCl3, 200 MHz) δ 1.26 (9H, s,
tert-butyl), 3.70 (3H, s, ArOCH3), 4.08 (2H, t, J ) 5.6 Hz,
OCH2CH2N), 4.25 (2H, t, J ) 5.6 Hz, OCH2CH2N), 4.75 (2H,
d, J ) 5.3 Hz, ArCH2NH), 6.17 (1H, br t, NH), 6.80 (3H, m,
ArH), 7.24 (4H, m, ArH), 7.76 (4H, m, ArH).
N-(4-(2-Am in oeth oxy)-3-m eth oxyben zyl)-N′-(4-ter t-bu -
tylp h en yl)th iou r ea (2g): synthesized from N-(4-(2-phthal-
imidoethoxy)-3-methoxybenzyl)-N′-(4-tert-butylphenyl)thio-
urea; purification by flash column chromatography (silica,
CH2Cl2/MeOH, 10:1) gave a colorless glassy solid (48% yield);
TLC (silica, CH2Cl2/MeOH, 10:1) Rf 0.15; HPLC reverse phase
C-18 (CH3CN/0.1% TFAaq gradient 10-100%) tR ) 9.9 min
(>98% purity); 1H-NMR (CDCl3, 200 MHz) δ 1.30 (9H, s, tert-
butyl), 2.32 (2H, br m, NH2), 3.12 (2H, m, OCH2CH2N), 3.85
(3H, s, ArOCH3), 4.03 (2H, t, J ) 5.5 Hz, OCH2CH2), 4.82 (2H,
d, J ) 5.5 Hz, ArCH2NH), 6.31 (1H, br t, NH), 6.86 (3H, m,
ArH), 7.30 (4H, m, ArH), 7.85 (1H, br m, NH); HRMS
(C21H30N3O2S) calcd 388.2059, found 388.2055.
N-(4-(2-P h t h a lim id oet h oxy)-3-m et h oxyb en zyl)-N′-(4-
ter t-bu tylben zyl)th iou r ea : synthesized from 4-(2-phthal-
imidoethoxy)-3-methoxybenzyl isothiocyanate and 4-tert-
butylbenzylamine by method B, 97% yield; used directly
without further purification; TLC (silica, cyclohexane/EtOAc,
1:1) Rf 0.3; 1H-NMR (CDCl3, 200 MHz) δ 1.26 (9H, s, tert-butyl),
3.62 (3H, s, ArOCH3), 4.03 (2H, t, J ) 5.6 Hz, OCH2CH2N),
4.19 (2H, t, J ) 5.6 Hz, OCH2CH2), 4.50 and 4.57 (2 × 2H, 2
× br d, J ≈ 5 Hz, 2 × ArCH2NH), 6.19 (2H, br s, 2NH), 6.64-
6.85 (3H, m, ArH), 7.12-7.34 (4H, m, ArH), 7.65-7.87 (4H,
m, ArH).
4-(Tr im eth ylsilyl)ben zyl Br om id e. 4-(Trimethylsilyl)-
toluene (2.0 g, 0.012 mol), N-bromosuccinimide (2.2 g, 0.012
mol), and a catalytic amount of dibenzoyl peroxide were
combined in 60 mL of dry carbon tetrachloride and stirred
under nitrogen. The mixture was refluxed for 6 h, until
conversion was complete by 1H-NMR, and then poured into
water. The organic phase was dried over MgSO4 and filtered,
and the solvent was removed in vacuo. Purification by flash
column chromatography (silica, cyclohexane) gave 1.6 g of a
pale yellow oil (55% yield): TLC (silica, cyclohexane/EtOAc,
1:1) Rf 0.8; 1H-NMR (CDCl3, 200 MHz) δ 0.28 (9H, s, ArSi-
(CH3)3), 4.50 (2H, s, ArCH2Br), 7.35-7.55 (4H, m, ArH).
(4-(Tr im eth ylsilyl)ben zyl)ph th alim ide: synthesized from
4-(trimethylsilyl)benzyl bromide, as described for Boc-4-(2-
phthalimidoethoxy)-3-methoxybenzylamine; purification by
flash column chromatography (silica, cyclohexane/EtOAc, 10:
1) gave a white crystalline solid (77% yield); TLC (silica,
cyclohexane/EtOAc, 10:1) Rf 0.22; 1H-NMR (CDCl3, 200 MHz)
δ 0.24 (9H, s, ArSi(CH3)3), 4.85 (2H, s, ArCH2N), 7.47 (4H, s,
ArH), 7.70 (2H, m, ArH), 7.85 (2H, m, ArH).
4-(Tr im eth ylsilyl)ben zyla m in e: synthesized from (4-
(trimethylsilyl)benzyl)phthalimide, as described for 2a ; puri-
fication by flash column chromatography (silica, CH2Cl2/
MeOH, 10:1) gave a white solid (77% yield); TLC (silica,
CH2Cl2/MeOH, 10:1) Rf 0.2; 1H-NMR (CDCl3, 200 MHz) δ 0.24
(9H, s, ArSi(CH3)3), 3.80 (2H, br s, ArCH2NH2), 3.90 (2H, s,
ArCH2NH2), 7.37 (2H, m, ArH), 7.52 (2H, m, ArH).
N-(4-(2-P h t h a lim id oet h oxy)-3-m et h oxyb en zyl)-N′-(4-
(tr im eth ylsilyl)ben zyl)th iou r ea : synthesized from 4-(2-
phthalimidoethoxy)-3-methoxybenzyl isothiocyanate and 4-(t-
rimethylsilyl)benzylamine by method B, 97% yield; used
without purification; TLC (silica, cyclohexane/ethyl acetate,
1:1) Rf 0.35; 1H-NMR (CDCl3, 200 MHz) δ 0.26 (9H, s,
Si(CH3)3), 3.67 (3H, s, ArOCH3), 4.06 (2H, t, J ) 6 Hz,
OCH2CH2N), 4.24 (2H, t, J ) 6 Hz, OCH2CH2N), 4.54 and 4.66
(2 × 2H, d, J ) 7 Hz, 2 × ArCH2NH), 6.26 (2H, br m, 2 ×
NH), 6.69-6.88 (3H, m, ArH), 7.27 (2H, d, J ) 8 Hz, ArH),
7.50 (2H, d, J ) 8 Hz, ArH), 7.70-7.90 (4H, m, ArH).
N-(4-(2-Am in oeth oxy)-3-m eth oxyben zyl)-N′-(4-(tr im e-
t h ylsilyl)b en zyl)t h iou r ea (2f): synthesized from N-(4-(2-
phthalimidoethoxy)-3-methoxybenzyl)-N′-(4-(trimethylsilyl)-
N-(4-(2-Am in oeth oxy)-3-m eth oxyben zyl)-N′-(4-ter t-bu -
tylben zyl)th iou r ea (2h ). 2h was synthesized from N-(4-(2-
phthalimidoethoxy)-3-methoxybenzyl)-N′-(4-tert-butylbenzyl)-
thiourea, to give a colorless glassy solid (66% yield). The
hydrochloride salt was formed by the addition of aqueous HCl
to a solution of 2h in MeOH; the salt was filtered, recrystal-
lized from EtOH/water, and dried in vacuo at 75 °C for 48 h,
to give a colorless crystalline solid (46% overall yield): TLC
1
(silica, MeOH) Rf 0.2; mp 125-130 °C; H-NMR (CDCl3, 400
MHz) δ 1.30 (9H, s, tert-butyl), 1.54 (2H, br s, NH2), 3.08 (2H,
t, J ) 5 Hz, OCH2CH2N), 3.80 (2H, s, ArOCH3), 4.00 (2H, t, J
) 5 Hz, OCH2CH2N), 4.54 (2H, br s, ArCH2N), 4.59 (2H, br s,
ArCH2N), 6.25 (2H, br s, 2 × NH), 6.76 (3H, m, ArH), 7.26
(4H, m, ArH); MS m/e 401 (M+). Anal. (C22H31N3O2S‚-
HCl‚H2O) C,H,N,O,S,Cl.
3,5-Di-ter t-bu tylben zoyl Ch lor id e. 3,5-Di-tert-butylben-
zoic acid (500 mg, 0.002 mol) was suspended in thionyl chloride
and refluxed for 3 h. The thionyl chloride was removed in
vacuo, and the crude product was used without purification.
3,5-Di-ter t-bu tylben zyl Alcoh ol. 3,5-Di-tert-butylbenzoyl
chloride (200 mg, 0.8 mmol) was dissolved in anhydrous THF
(5 mL), added dropwise to a stirred suspension of LiAlH4 (100
mg, 0.0024 mol) in anhydrous THF (5 mL) under nitrogen,
and stirred at room temperature for 3 h. ‘Wet’ THF was then
cautiously added to decompose the excess LiAlH4; the mixture
was filtered, the supernatant was dried over MgSO4 and
filtered, and the solvent was removed in vacuo, to leave a
colorless oil, which was used without further purification: 1H-
NMR (CDCl3, 200 MHz) δ 1.31 (18H, s, 2 × tBu), 4.65 (2H, br
s, ArCH2OH), 7.22 (2H, m, ArH), 7.38 (1H, m, ArH).

Capsaicin Analogues as Potent Analgesics
J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 25 4949
3,5-Di-ter t-bu tylben za ld eh yd e. 3,5-Di-tert-butylbenzyl
alcohol (200 mg, 0.9 mmol) and MnO2 (400 mg, 0.0044 mol)
were suspended in chloroform and refluxed for 5 h. After
cooling and filtration, the solvent was removed to leave a
colorless crystalline solid which was used without further
purification: TLC (silica, cyclohexane/EtOAc, 1:1) Rf 0.65; 1H-
NMR (CDCl3, 200 MHz) δ 1.38 (18H, s, 2 × tBu), 7.72 (3H, s,
ArH), 10.00 (1H, s, ArCHO).
B o c -4-(3-p h t h a lim id o p r o p o x y )-3-m e t h o x y b e n zy l-
a m in e: prepared as described for Boc-4-(2-phthalimidoet-
hoxy)-3-methoxybenzylamine from Boc-4-(3-bromopropoxy)-3-
methoxybenzylamine (2.80 g, 0.0075 mol); purified by flash
column chromatography (silica, cyclohexane/EtOAc, 2:1) to give
2.80 g of a white solid (85% yield); TLC (silica, cyclohexane/
EtOAc, 1:1) Rf 0.44.
(4-(3-P h t h a lim id op r op oxy)-3-m et h oxyb en zyl)a m m o-
n iu m tr iflu or oa ceta te: prepared as described for (4-(3-
phthalimidoethoxy)-3-methoxybenzyl)ammonium trifluoroac-
etate, from Boc-4-(3-phthalimidopropoxy)-3-methoxybenzyl-
amine; used without purification or characterization.
N-(4-(3-P h th a lim id op r op oxy)-3-m eth oxyben zyl)-N′-(2-
(4-ch lor op h en yl)eth yl)th iou r ea : synthesized from (4-(3-
phthalimidopropoxy)-3-methoxybenzyl)ammonium trifluoro-
acetate and 2-(4-chlorophenyl)ethyl isothiocyanate by method
A; purification by flash column chromatography (silica, cyclo-
hexane/EtOAc, 1:1) gave a white powdered solid (85% yield),
TLC (silica, cyclohexane/EtOAc, 1:1) Rf 0.29.
N-(4-(3-Am in opr opoxy)-3-m eth oxyben zyl)-N′-(2-(4-ch lo-
r op h en yl)eth yl)th iou r ea (3): synthesized from N-(4-(3-
phthalimidopropoxy)-3-methoxybenzyl)-N′-(2-(4-chlorophenyl)-
ethyl)thiourea; purification by flash column chromatography
(silica, CH2Cl2/MeOH, 10:1, changing to MeOH) gave a color-
less glassy solid (77% yield); TLC (silica, CH2Cl2/MeOH/AcOH,
90:9:1) Rf 0.17; HPLC reverse phase C-18 (CH3CN/0.1% TFAaq
gradient 10-100%) tR ) 8.9 min (>98% purity); 1H-NMR
(DMSO-d6, 400 MHz) δ 1.76 (2H, m, OCH2CH2CH2N), 2.67
(2H, t, J ) 6.63 Hz, OCH2CH2CH2N), 2.82 (2H, t, J ) 7.30
Hz, ArCH2CH2N), 3.62 (2H, br m, ArCH2CH2N), 3.72 (3H, s,
ArOCH3), 3.98 (2H, t, J ) 6.35 Hz, OCH2CH2CH2N), 4.54 (2H,
br m, ArCH2N), 6.78 (1H, m, ArH), 6.90 (2H, m, ArH), 7.30
(4H, m, ArH), 7.63 (1H, br m, NH), 7.98 (1H, br m, NH); HRMS
(C20H27N3O2SCl) calcd 408.1513, found 408.1520.
N-Boc-3-m et h oxy-4-(2-(m et h yla m m on io)et h oxy)b en -
zyla m in e Hyd r obr om id e. Boc-4-(2-bromoethoxy)-3-meth-
oxybenzylamine (2.0 g, 0.0056 mol) was dissolved in a 33%
solution of methylamine in EtOH (250 mL) and refluxed under
a positive pressure of argon for 6 h. Solvent and excess
methylamine were removed in vacuo. Purification by flash
column chromatography (CHCl3/MeOH, 3:2) gave 1.65 g of a
colorless glassy solid (76% yield): TLC (silica, CH2Cl2/MeOH/
AcOH, 120:90:5) Rf 0.65; 1H-NMR (CD3OD, 60 MHz) δ 1.45
(9H, s, tBu), 2.80 (3H, s, NCH3), 3.35 (2H, m, CH2CH2NCH3),
3.90 (3H, s, ArOCH3), 4.1-4.4 (4H, m, ArCH2N, OCH2CH2),
6.45 (3H, m, ArH).
3,5-Di-ter t-bu tylben zyl m eth yla ld oxim e: synthesized
from 3,5-di-tert-butylbenzaldehyde as described for 3-(4-chlo-
rophenyl)propyl methyloxime to give an off-white solid (87%
1
yield); H-NMR (CDCl3, 200 MHz) δ 1.31 (18H, s, 2 × tBu),
3.95 (3H, s, NOCH3), 7.41 (3H, m, ArH), 8.06 (1H, s, ArCHdN).
3,5-Di-ter t-bu tylben zyla m in e: synthesized from 3,5-di-
tert-butylbenzyl methyloxime as described for 3-(4-chlorophe-
nyl)propylamine; purification by flash column chromatography
(silica, cyclohexane/EtOAc, 1:10) gave 850 mg of a colorless
oil (70% yield); 1H-NMR (CDCl3, 200 MHz) δ 1.31 (18H, s, 2 ×
tBu), 1.75 (2H, br s, NH2), 3.85 (2H, s, ArCH2N), 7.20 (3H, m,
ArH).
N-(4-(2-P h th a lim id oeth oxy)-3-m eth oxyben zyl)-N′-(3,5-
d i-ter t-bu tylben zyl)th iou r ea : synthesized from 4-(2-ph-
thalimidoethoxy)-3-methoxybenzyl isothiocyanate and 3,5-di-
tert-butylbenzylamine by method B, in EtOAc (75% yield); TLC
(silica, cyclohexane/EtOAc, 1:1) Rf 0.16; 1H-NMR (CDCl3, 200
MHz) δ 1.28 (18H, s, 2 × tBu), 3.65 (3H, s, ArOCH3), 4.15 (4H,
m, OCH2CH2N), 4.55 (4H, br s, 2 × ArCH2N), 6.10 (2H, 2 ×
br s, 2 × NH), 6.75 (3H, m, ArH), 7.10 (2H, m, ArH), 7.35 (1H,
m, ArH), 7.75 (4H, m, ArH).
N-(4-(2-Am in oeth oxy)-3-m eth oxyben zyl)-N′-(3,5-d i-ter t-
bu tylben zyl)th iou r ea (2i): synthesized from N-(4-(2-ph-
thalimidoethoxy)-3-methoxybenzyl)-N′-(3,5-di-tert-butylbenzyl)-
thiourea; purification by flash column chromatography (silica,
CH2Cl2/MeOH, 10:1, changing to MeOH) gave a colorless
glassy solid (33% yield); TLC (silica, MeOH) Rf 0.15; HPLC
reverse phase C-18 (CH3CN/0.1% TFAaq gradient 10-100%)
tR ) 9.8 min (>98% purity); 1H-NMR (CDCl3, 200 MHz) δ 1.28
(18H, s, 2 × tBu), 2.62 (2H, br, NH2), 3.04 (2H, t, J ) 5 Hz,
OCH2CH2N), 3.72 (3H, s, ArOCH3), 3.95 (2H, t, J ) 5 Hz,
OCH2CH2N), 4.55 (4H, br s, 2 × ArCH2N), 6.35 (2H, 2 × br s,
2 × NH), 6.75 (3H, m, ArH), 7.10 (2H, m, ArH), 7.30 (1H, m,
ArH): HRMS (C26H40N3O2S) calcd 458.2841, found 458.2847.
2-(4-ter t-Bu tylp h en yl)eth yl isoth iocya n a te: synthesized
from 2-(4-tert-butylphenyl)ethylamine; purification by flash
column chromatography (silica, cyclohexane) gave a yellow oil
(80% yield).
Boc-4-(2-(N-((9-flu or en ylm et h yloxy)ca r b on yl)-N-m e-
th yla m in o)eth oxy)-3-m eth oxyben zyla m in e. Boc-3-meth-
oxy-4-(2-(methylammonio)ethoxy)benzylamine hydrobromide
(1.65 g, 0.0042 mol) and Fmoc-O-Su (1.57 g, 0.0046 mol) were
dissolved in 75 mL of dioxane (plus a few drops of water) with
triethylamine (650 µL, 470 mg, 0.0047 mol), and the solution
was stirred at room temperature for 6 h. The solvents were
removed in vacuo, and the residue was partitioned between
EtOAc and water. The organic phase was washed with brine
and dried over anhydrous Na2SO4. The mixture was filtered
and the solvent removed in vacuo. The product was purified
by flash column chromatography (silica, CHCl3) to give 2.1 g
of a colorless oil (93% yield): TLC (silica, cyclohexane/EtOAc,
1:1) Rf 0.37; 1H-NMR (CDCl3, 60 MHz) δ 1.45 (9H, s, tBu),
3.05 (3H, s, NCH3), 3.70 (2H, m, CH2CH2NCH3), 3.80 (3H, s,
ArOCH3), 4.1-4.6 (6H, m, fluorenylCH2CH, ArCH2N, OCH2-
CH2), 4.85 (1H, m, CH), 6.80 (3H, m, ArH), 7.2-7.9 (8H, m,
fluoreneArH).
(4-(2-(N-((9-F lu or en ylm et h yloxy)ca r b on yl)-N-m et h y-
la m in o)eth oxy)-3-m eth oxyben zyl)a m m on iu m tr iflu or o-
a ceta te: prepared as described for (4-(2-phthalimidoethoxy)-
3-methoxybenzyl)ammonium trifluoroacetate, from Boc-4-(2-
(N-((9-fluorenylmethyloxy)carbonyl)-N-methylamino)ethoxy)-
3-methoxybenzylamine (2.0 g, 0.0038 mol), to give 2.05 g of a
colorless glassy solid (100% yield); 1H-NMR (CD3OD, 60 MHz)
δ 2.95 (3H, s, NCH3), 3.20-4.60 (12H, m, 4 × CH2, ArOCH3,
CH), 6.70-7.9 (11H, m, ArH).
N-(4-(2-P h t h a lim id oet h oxy)-3-m et h oxyb en zyl)-N′-(2-
(4-ter t-bu tylp h en yl)eth yl)th iou r ea : synthesized from 4-(2-
phthalimidoethoxy)-3-methoxybenzyl isothiocyanate and 2-(4-
tert-butylphenyl)ethylamine by method B; purification by flash
column chromatography (silica, cyclohexane/EtOAc, 1:1) gave
a white solid (38% yield).
N-(4-(2-Am in oeth oxy)-3-m eth oxyben zyl)-N′-(2-(4-ter t-
bu t ylp h en yl)et h yl)t h iou r ea (2j): synthesized from N-(4-
(2-phthalimidoethoxy)-3-methoxybenzyl)-N′-(2-(4-tert-butylphe-
nyl)ethyl)thiourea; purification by flash column chromatography
(silica, CH2Cl2/MeOH, 10:1, changing to MeOH) gave a color-
less glassy solid (56% yield); TLC (silica, MeOH) Rf 0.1; HPLC
reverse phase C-18 (CH3CN/0.1% TFAaq gradient 10-100%)
tR ) 9.3 min (>98% purity); 1H-NMR (CDCl3, 200 MHz) δ 1.30
(9H, s, tert-butyl), 2.84 (2H, t, J ) 7 Hz, ArCH2CH2N), 3.07
(2H, br m, OCH2CH2N), 3.70 (2H, br m, ArCH2CH2N), 3.82
(3H, s, ArOCH3), 4.00 (2H, t, J ) 5 Hz, OCH2CH2N), 4.48 (2H,
br m, ArCH2NH), 5.90 (1H, br m, NH), 6.15 (1H, br m, NH),
6.70-6.85 (3H, m, ArH), 7.12-7.34 (4H, m, ArH); HRMS
(C23H34N3O2S) calcd 416.2372, found 416.2378.
Boc-4-(3-br om op r op oxy)-3-m eth oxyben zyla m in e: pre-
pared as described for Boc-4-(2-bromoethoxy)-3-methoxyben-
zylamine from Boc-vanillylamine (2.0 g, 0.008 mol) and 1,3-
dibromopropane (50 mL) to give a colorless oil, yield 2.91 g
1
(97%); TLC (silica, cyclohexane/EtOAc, 1:1) Rf 0.63; H-NMR
(CDCl3, 60 MHz) δ 1.46 (9H, s, tert-butyl), 2.35 (2H, m,
OCH2CH2CH2Br), 3.65 (2H, m, OCH2CH2CH2Br), 3.85 (3H, s,
ArOCH3), 4.05-4.30 (4H, m, ArCH2N, OCH2CH2CH2Br), 4.82
(1H, br m, NH), 6.85 (3H, s, ArH).
N-(4-(2-(N-((9-F lu or en ylm eth yloxy)ca r bon yl)-N-m eth -
ylam in o)eth oxy)-3-m eth oxyben zyl)-N′-(2-(4-ch lor oph en yl)-
eth yl)th iou r ea : synthesized from (4-(2-(N-((9-fluorenylme-

4950 J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 25
Wrigglesworth et al.
t h yloxy)ca r bon yl)-N -m et h yla m in o)et h oxy)-3-m et h oxy-
benzyl)ammonium trifluoroacetate and 2-(4-chlorophenyl)ethyl
isothiocyanate by method A; purification by flash column
chromatography (silica, cyclohexane/EtOAc, 1:1) gave a color-
less glassy solid (96% yield); TLC (silica, cyclohexane/EtOAc,
1:1) Rf 0.23.
NH), 8.38 (1H, br m, NH); MS m/e 436 (M+). Anal. (C20H30
N3O2SCl‚CF3CO2H‚H2O) C,H,N,O,F.
-
N-(4-(2-P h t h a lim id oet h oxy)-3-m et h oxyb en zyl)-N′-(2-
(4-ch lor op h en yl)eth yl)th iou r ea (7): synthesized from 4-(2-
phthalimidoethoxy)-3-methoxybenzylamine trifluoroacetate and
2-(4-chlorophenyl)ethyl isothiocyanate by method A, in EtOAc
with triethylamine; purification by flash column chromatog-
raphy (silica, cyclohexane/EtOAc, 1:1) gave a pale yellow
crystalline solid in 69% yield; mp 59-61 °C; TLC (silica,
N-(3-Meth oxy-4-(2-(m eth yla m in o)eth oxy)ben zyl-N′-(2-
(4-ch lor op h en yl)eth yl)th iou r ea (4). N-(4-(2-(N-((9-Fluo-
renylmethyloxy)carbonyl)-N-methylamino)ethoxy)-3-methox-
ybenzyl)-N′-(2-(4-chlorophenyl)ethyl)thiourea (1.90 g, 0.003
mol) was dissolved in CH2Cl2 (50 mL) with piperidine (50 mL)
and stirred for 30 min at room temperature. The solution was
diluted with CH2Cl2 and washed with water and brine, and
the solvent was removed in vacuo. Purification by flash
column chromatography (silica, CH2Cl2/MeOH, 5:1) gave 300
mg of a colorless glassy solid (24% yield): TLC (silica, CH2-
Cl2/MeOH, 5:1) Rf 0.35; HPLC reverse phase C-18 (CH3CN/
0.1% TFAaq gradient 10-100%) tR ) 9.1 min (>98% purity);
1H-NMR (DMSO-d6, 400 MHz) δ 2.62 (3H, s, NCH3), 2.80 (2H,
t, J ) 7.33 Hz, ArCH2CH2NH), 3.26 (2H, br m, OCH2CH2NH),
3.60 (2H, br m, ArCH2CH2NH), 3.76 (3H, s, ArOCH3), 4.21 (2H,
t, J ) 5.1 Hz, OCH2CH2NH), 4.60 (2H, br m, ArCH2NH), 6.80
(1H, m, ArH), 7.00 (2H, m, ArH), 7.30 (4H, m, ArH), 7.80 (1H,
t, J ) 5.2 Hz, NH), 8.07 (1H, t, J ) 5.6 Hz, NH); HRMS
(C20H27N3O2SCl) calcd 408.1513, found 408.1508.
N-(4-(2-(N,N-Dim eth ylam in o)eth oxy)-3-m eth oxyben zyl)-
N′-(2-(4-ch lor op h en yl)eth yl)th iou r ea (5). The free base of
compound 2a (250 mg, 0.000 63 mol) was dissolved in 15 mL
of MeOH with paraformaldehyde (160 mg, 0.0053 mol) and
refluxed for 3 h. After cooling to room temperature, sodium
cyanoborohydride (130 mg, 0.002 mol) was added, and the
reaction mixture was stirred at room temperature for 30 min.
Water (10 mL) was added, and the solvent was removed in
vacuo. The product was extracted into CH2Cl2, the solvent
removed in vacuo, and the product purified by flash column
chromatography (silica, CH2Cl2/MeOH, 10:1) to give 80 mg of
a colorless glassy solid (30% yield): TLC (silica, CH2Cl2/MeOH,
10:1) Rf 0.17; HPLC reverse phase C-18 (CH3CN/0.1% TFAaq
gradient 10-100%) tR ) 9.1 min (>98% purity); 1H-NMR
(DMSO-d6, 400 MHz) δ 2.23 (6H, s, N(CH3)2), 2.62 (2H, t, J )
5.97 Hz, OCH2CH2N), 2.82 (2H, t, J ) 7.18 Hz, ArCH2CH2N),
3.63 (2H, br m, ArCH2CH2N), 3.75 (3H, s, ArOCH3), 4.02 (2H,
t, J ) 5.93 Hz, OCH2CH2N), 4.57 (2H, br m, ArCH2N), 6.78
(1H, m, ArH), 6.92 (2H, m, ArH), 7.30 (4H, m, ArH), 7.44 (1H,
br m, NH), 7.80 (1H, br m, NH); HRMS (C21H29N3O2SCl) calcd
422.1669, found 422.1662.
Boc-4-(2-(t r im et h yla m m on io)et h oxy)-3-m et h oxyb en -
zyla m in e Br om id e. Boc-4-(2-bromoethoxy)-3-methoxyben-
zylamine (1.0 g, 0.0027 mol) was dissolved in a saturated
solution of trimethylamine in MeOH, and the solution was
refluxed under a positive pressure of N2 for 18 h, until the
reaction was complete by TLC. Solvent was removed in vacuo
to give 1.15 g of a colorless oil (100% yield); this was used
without purification: TLC (silica, CH2Cl2/MeOH/AcOH, 120:
90:5) Rf 0.30.
(4-(2-(Tr im eth yla m m on io)eth oxy)-3-m eth oxyben zyl)-
a m m on iu m d itr iflu or oa ceta te: prepared as described for
(4-(3-phthalimidoethoxy)-3-methoxybenzyl)ammonium trifluo-
roacetate, from Boc-4-(2-(trimethylammonio)ethoxy)-3-meth-
oxybenzylamine bromide; used without purification or char-
acterization.
N-(4-(2-(Tr im eth ylam m on io)eth oxy)-3-m eth oxyben zyl)-
N′-(2-(4-ch lor op h en yl)et h yl)t h iou r ea t r iflu or oa cet a t e
(6): synthesized from (4-(2-(trimethylammonio)ethoxy)-3-
methoxybenzyl)ammonium ditrifluoroacetate and 2-(4-chlo-
rophenyl)ethyl isothiocyanate by method A; purification by
preparative HPLC (reverse phase C-18, silica, gradient 90:10
acetonitrile/0.1% TFAaq, changing to 100% acetonitrile) gave
280 mg of a colorless glassy solid (19% yield); TLC (silica, CH2-
Cl2/MeOH/AcOH, 120:90:5) Rf 0.2; 1H-NMR (DMSO-d6, 400
MHz) δ 2.80 (2H, t, J ) 7.30 Hz, ArCH2CH2N), 3.20 (9H, s,
N(CH3)2), 3.60 (2H, br m, ArCH2CH2N), 3.75 (3H, s, ArOCH3),
3.78 (2H, t, J ) 4.81 Hz, ArOCH2CH2N), 4.39 (2H, br m,
ArOCH2CH2N(CH3)2), 4.59 (2H, br m, ArCH2N), 6.82 (1H, m,
ArH), 7.02 (2H, m, ArH), 7.30 (4H, m, ArH), 8.13 (1H, br m,
1
cyclohexane/EtOAc, 1:2) Rf 0.4; H-NMR (CDCl3, 400 MHz) δ
2.84 (2H, t, J ) 7.0 Hz, ArCH2CH2N), 3.62 (5H, m, ArCH2CH2N,
ArOCH3), 4.11 (2H, t, J ) 5.7 Hz, OCH2CH2N), 4.27 (2H, t, J
) 5.7 Hz, OCH2CH2N), 4.39 (2H, d, J ) 5.5 Hz, ArCH2N), 5.62
(1H, br s, NH), 6.03 (1H, br s, NH), 6.70-6.85 (3H, m, ArH),
7.15 (4H, m, ArH), 7.80 (4H, m, ArH): MS m/e 523 (M+). Anal.
(C27H26N3O4SCl) C,H,N.
N-(4-(2-Aceta m id oeth oxy)-3-m eth oxyben zyl)-N′-(2-(4-
ch lor op h en yl)eth yl)th iou r ea (8). Compound 2a (400 mg,
0.001 mol) was dissolved in anhydrous CH2Cl2 (30 mL) with
triethylamine (0.160 mL, 116 mg, 0.0012 mol) and acetyl
chloride (80 mg, 0.001 mol) and stirred at room temperature
under an atmosphere of nitrogen for 2.5 h, until the reaction
was complete by TLC. The solution was diluted to 150 mL
with CH2Cl2 and washed with water and brine. The solvent
was removed in vacuo. The residue was purified by flash
column chromatography (silica, CH2Cl2/MeOH, 20:1) and
recrystallized from EtOAc/n-hexane to give 250 mg of a white
crystalline solid (56% yield): mp 121-122 °C; TLC (silica, CH2-
Cl2/MeOH, 10:1) Rf 0.30; 1H-NMR (DMSO-d6, 400 MHz) δ 1.82
(3H, s, CH3), 2.80 (2H, t, J ) 7.2 Hz, ArCH2CH2NH), 3.33 (2H,
br m, OCH2CH2NH), 3.62 (2H, br m, ArCH2CH2NH), 3.72 (3H,
s, ArOCH3), 3.93 (2H, t, J ) 5.9 Hz, OCH2CH2NH), 4.55 (2H,
br m, ArCH2NH), 6.75 (1H, m, ArH), 6.92 (2H, m, ArH), 7.30
(4H, m, ArH), 7.45 (1H, br m, NH), 7.80 (1H, br m, NH), 8.10
(1H, br m, NH); FABMS m/e 436 (MH+). Anal. (C21H26N3O3-
SCl) C,H,N.
N-(4-(2-((E t h oxyca r b on yl)a m in o)et h oxy)-3-m et h oxy-
b en zyl)-N′-(2-(4-ch lor op h en yl)et h yl)t h iou r ea (9): pre-
pared as described for compound 8 from the free base of
compound 2a (300 mg, 0.000 76 mol) and ethyl chloroformate
(0.075 mL, 86 mg, 0.0008 mol); purification by flash column
chromatography (silica, cyclohexane/EtOAc, 1:1) and recrys-
tallization from EtOAc/n-hexane gave 140 mg of a white
crystalline solid (40% yield); mp 73-74 °C; TLC (silica, CH2-
1
Cl2/MeOH, 20:1) Rf 0.46; H-NMR (CD3OD, 400 MHz) δ 1.23
(3H, t, J ) 7.1 Hz, CH2CH3), 2.86 (2H, t, J ) 7.08 Hz, ArCH2-
CH2NH), 3.46 (2H, t, J ) 5.55 Hz, OCH2CH2N), 3.72 (2H, br
m, ArCH2CH2NH), 3.83 (3H, s, ArOCH3), 4.02 (2H, t, J ) 5.55
Hz, OCH2CH2), 4.08 (2H, q, J ) 6.9 Hz, OC(dO)CH2CH3)), 4.58
(2H, br m, ArCH2NH), 6.80-7.00 (3H, m, ArH), 7.20 (4H, m,
ArH); FABMS m/e 466 (MH+). Anal. (C22H28N3O4SCl) C,H,N.
N-(4-(2-(Boc-a m in o)eth oxy)-3-m eth oxyben zyl)-N′-(2-(4-
ch lor op h en yl)eth yl)th iou r ea (10): prepared as described
for compound 8 from the free base of compound 2a (300 mg,
0.000 76 mol) and di-tert-butyl dicarbonate (340 mg, 0.0015
mol); purification by flash column chromatography (silica,
cyclohexane/EtOAc, 1:1) and recrystallization from EtOAc/n-
hexane gave 105 mg of a white crystalline solid (28% yield);
1
mp 117 °C; TLC (silica, cyclohexane/EtOAc, 1:1) Rf 0.21; H-
NMR (CD3OD, 400 MHz) δ 1.42 (9H, s, tert-butyl), 2.86 (2H,
t, J ) 7.14 Hz, ArCH2CH2N), 3.41 (2H, t, J ) 5.55 Hz,
OCH2CH2N), 3.70 (2H, br m, ArCH2CH2N), 3.83 (3H, s,
ArOCH3), 4.00 (2H, t, J ) 5.57 Hz, OCH2CH2), 4.58 (2H, br
m, ArCH2NH), 6.79-6.96 (3H, m, ArH), 7.20 (4H, m, ArH);
FABMS m/e 494 (MH+). Anal. (C24H32N3O4SCl) C,H,N.
Biology. The in vitro assay (45Ca2+ influx into neonatal rat
DRG neurons) and the mouse tail-flick in vivo antinociceptive
assay have been described in part 1 of this series.1
Mou se Wr ith in g An tin ocicep tive Assa y. Female mice
(CD-1, Charles River, weight 20g) were maintained in a
controlled lighting environment (12 h on/12 h off) and fasted
overnight prior to testing. Animals received an intraperitoneal
injection of 0.3 mL of an acetic acid solution (200 mM), and 5
min later the number of abdominal constrictions was counted
in the subsequent 5 min period. Animals received drug or
vehicle (10 animals/group) subcutaneously or orally 55 min

Capsaicin Analogues as Potent Analgesics
J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 25 4951
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number of writhes (drug))/(mean number of writhes (vehicle))
× 100. A logistic function (ORIGIN software) was fitted to
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error, as the dose required to produce a 50% reduction in the
number of writhes.
Br on ch ocon str iction . Capsaicin is known to cause an
acute bronchoconstrictor response when administered as an
intravenous injection in anesthetized guinea pigs.10 Dunkin-
Hartley guinea pigs (400-450 g, n ) 4 animals/treatment
group) were anesthetized with urethane (2 g/kg of body weight)
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at 37 °C. The carotid artery was cannulated for blood pressure
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tracheal cannula, as an index of changes in tracheobronchial
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J M960512H