M. S. Kim et al. / Bioorg. Med. Chem. Lett. 24 (2014) 382–385
383
The asymmetric synthesis of the C-region utilized the highly
enantioselective phase-transfer catalytic mono-alkylation of malo-
namic ester as a key step, previously reported by Park and Jew et al.
(Schemes 3 and 4).15 The substrate for asymmetric alkylation,
N,N-bis(p-methoxyphenyl) malonamide tert-butyl ester (11), was
prepared from malonic monoester 10. The phase-transfer catalytic
O
O
O
H
H
O
OCH3
OH
20
3
OH
a-alkylation of 11 in the presence of (R,R)-3,4,5-trifluorophenyl-
O
O
NAS bromide using 4-t-benzyl bromide afforded the highly
enantioselective 12R (99%, 92% ee). The LiAlH4 reduction of 12R
produced the 3-aminopropanol 13R, whose di-PMP protecting
group was converted into the corresponding Boc group to give
14R. The alcohol of 14R was pivaloylated and then the N-Boc group
was deprotected to yield the C-region amine 15R. Finally, the cou-
pling of 15R with isothiocyanate 6 followed by MOM-deprotection
provided the final compound 2R. The amine of 15R was converted
to the corresponding isothiocyanate, which was coupled with
amine 9 to provide the final 3R.
1 Resiniferatoxin (RTX)
R1
R2
H
N
H
N
O
O
S
R1=OH,
R2=OCH3
2
3 R1=NHSO2CH3, R2=F
To prepare the corresponding S-isomers, the phase-transfer cat-
alytic a-alkylation of 11 employing (S,S)-ligand provided 12S with
Figure 1. Resinifertoxin (RTX) and simplified RTX (sRTX).
high enantioselectivity (99%, 99% ee). With 12S, the same routes
used in Scheme 3 produced the final 2S and 3S, respectively. The
structures and optical purities of final compounds were confirmed
by spectroscopic data and chiral HPLC.16
OMOM
OMe
OMOM
OMe
OH
a, b
c, d
SCN
H
HO
OMe
O
The binding affinities and potencies as agonists/antagonists of
the synthesized TRPV1 ligands were assessed in vitro by a binding
competition assay with [3H]RTX and by a functional 45Ca2+ uptake
assay using human TRPV1 heterologously expressed in Chinese
hamster ovary (CHO) cells, as previously described.17,18 The results
are summarized in Table 1, together with the potencies of RTX,
I-RTX and racemates 2 and 3.
The receptor activities of compounds 2 and 3 were previously
reported for the rat TRPV1/CHO system10,13 and are reported here
for human TRPV1 compared to the activities of RTX and I-RTX. Com-
pound 2 proved to be a potent agonist for hTRPV1 with Ki = 6.1 nM
and EC50 = 1.34 nM; it was thus within ca. 5- and 1.5-fold of the
potency of RTX in binding affinity and agonism, respectively, with
human TRPV1. Compound 3 was a potent hTRPV1 antagonist with
Ki = 23 nM and Ki(ant) = 122 nM, which was ca. 2- and 20-fold less
potent than I-RTX in binding affinity and antagonism, respectively.
Analysis of the chiral isoforms indicated that the R-isomer was
the more active isomer for both compounds 2 and 3. In the case of
5
6
4
Scheme 1. Synthesis of the A-region of compound 2. Reagents and conditions:
(a) DIEA, MOMCl, CH2Cl2, rt, 2 h, 99%; (b) NaBH4, LiCl, THF, EtOH, 0 °C, 1 h , 99%;
(c) DPPA, DBU, toluene, rt, 1 h, 99%; (d) CS2, PPh3, THF, reflux, 2 h, 60%.
NH2
F
NHSO2CH3
F
NHSO2CH3
F
a, b
c
H3N
I
NC
7
8
9
Scheme 2. Synthesis of the A-region of compound 3. Reagents and conditions:
(a) MsCl, pyridine, rt, 2 h, 98%; (b) Zn(CN)2, Pd(PPh3)4, DMF, 150 °C, 15 h, 85%;
(c) 2 M BH3-SMe2 in THF, reflux, 3 h then 1 M HCl, reflux, 15 h, 92%.
(Scheme 2). The amine of 7 was mesylated and then its iodide was
converted into the corresponding nitrile to provide nitrile 8, which
was reduced to afford the amine 9.
PMP
PMP
N
PMP
N
O
OH
O
N
a
b
c
PMP
O
HO
PMP
PMP
O
O
O
O
O
O
12R
10
11
13
R
OH
h, i
H
N
H
O
O
N
OMe
f, g
d, e
O
O
S
NH3
O
O
HO
NHBoc
2R
j, k
14R
O
H
15R
N
S
H
H
N
O
N
F
S
3
R
Scheme 3. Syntheses of (R)-sRTX isomers Reagents and conditions: (a) 4,40-dimethoxydiphenyl amine, EDC, DMAP, CH2Cl2, rt, 20 h, 98%; (b) (R,R)-3,4,5-trifluorophenyl-NAS
bromide, 4-t-butylbenzyl bromide, 50% KOH, toluene, ꢀ40 °C, 24 h, 99%; (c) LiAlH4, dibutyl ether, reflux, 1 h, 70%; (d) CAN, H2O, CH3CN, 0 °C, 30 min; (e) 8 N NaOH, Boc2O, rt,
7 h, 60% for 2 steps; (f) C(CH3)3COCl, DMAP, CH2Cl2, rt, 4 h, 95%; (g) CF3CO2H, CH2Cl2, rt, 2 h; (h) compound 6, NEt3, CH2Cl2, rt, 20 h, 80% for 2 steps; (i) CF3CO2H, CH2Cl2, 0 °C,
1 h, 60%; (j) 1,10-thiocarbonyldi-2-pyridone, NEt3, DMF, rt, 15 h, 75% for 2 steps; (k) compound 9, NEt3, CH2Cl2, rt, 15 h, 70%.