E. J. Roh et al. / Bioorg. Med. Chem. 16 (2008) 9349–9358
9355
gers central sensitization. However, in active pain states, an enhan-
cer may obviate the use of a vanilloid agonist.
2H), 1.32 (t, J = 7.2 Hz, 3H), 0.86–0.97 (m, 6H) MS: 332.2 (M-ethyl,
base). Anal. (C20H24FNO4) C, H, N: calcd 66.47, 6.69, 3.88; found
66.39, 6.69, 3.91.
In conclusion, the finding that a well-described compound class,
that is, the DHPs, can modulate TRPV1 channels in a positive fash-
ion represents a useful lead for future pain therapeutics. This study
has focused on the SAR of the novel DHP enhancers, which differs
considerably from their known action at other target sites, such as
L-type calcium channels and adenosine receptors. In future studies
it will be desirable to further separate the activity of the TRPV1
enhancers from activity at the purine receptors. The present study
revealed that 6-phenyl-, 3-O/S-ester-, and small alkyl-substitutions
at the 2 and 4 positions of the DHP pharmacophore are required for
the maximal-enhancing effect. The development of analogues that
are more potent and selective in their action at the TRPV1 channel
and the validation of this approach in pain models, including
in vivo studies, will be required.
4.1.2.3. 3,5-Diethyl-2-methyl-4-ethyl-6-(4-fluorophenyl)-1,4-(+)-
dihydropyridine-3,5-dicarboxylate (11). C20H24FNO4 1H NMR
(CDCl3) 7.26–7.31 (m, 2H), 7.05–7.11 (m, 2H), 5.57 (s, 1H), 4.18–
4.24 (m, 2H), 4.02 (t, J = 5.6 Hz, 1H), 3.90–3.97 (m, 2H), 2.32 (s,
3H), 1.48–1.54 (m, 2H), 1.31 (t, J = 7.2 Hz, 3H), 0.96 (t, J = 7.2 Hz,
3H), 0.88 (t, J = 7.5 Hz, 3H) MS: 362.2 (M+H), 332.2 (M-ethyl, base)
Anal. (C20H24FNO4) C, H, N: calcd 66.47, 6.69, 3.88; found 66.41,
6.93, 3.76.
4.1.2.4. 3-Ethylthio-5-ethyl-2-methyl-4-ethyl-6-(2-fluorophenyl)-
1,4-(+)-dihydropyridine-3,5-dicarboxylate
(13). C20H24FNO3S
1H NMR (CDCl3) 7.31–7.34 (m, 1H), 7.01–7.19 (m, 3H), 5.62 (s,
1H), 4.16 (t, J = 5.4 Hz, 1H), 3.87–3.94 (m, 2H), 2.87 (q, J = 7.5 Hz,
2H), 2.25 (s, 3H), 1.47–1.57 (m, 2H), 1.22 (t, J = 7.4 Hz, 3H), 0.82–
0.92 (m, 6H) MS: 348.2 (M-ethyl, base). Anal. (C20H24FNO3S) C, H,
N: calcd 63.64, 6.41, 3.71; found 63.47, 6.39, 3.64.
4. Experimental procedures
4.1. Chemical synthesis
4.1.1. Materials and instrumentation
4.1.2.5. 3-Ethylthio-5-ethyl-2-methyl-4-ethyl-6-(3-fluorophenyl)-
Ethyl-3-aminocrotonate, aldehydes, ethylacetoacetate, 2,2,6-
trimethyl-4H-1,3-dioxin-4-one, all acid chlorides, 2,2-dimethyl-
1,3-dioxane-4,6-dione, ethanethiol, propanethiol, and other gen-
eral reagents were purchased from Aldrich (Milwaukee, WI), and
synthetic intermediates were of analytical grade. Compounds 7,
8, 12, 18, and 21–23 were prepared as reported by Li, et al.34 All
other materials were obtained from commercial sources. Proton
nuclear magnetic resonance spectroscopy was performed on a Var-
ian GEMINI-300 spectrometer, and all spectra were obtained in
CDCl3. Chemical shifts (d) relative to TMS are given. Chemical ion-
ization (CI) mass spectrometry was performed with a Finnigan
4600 mass spectrometer, and electron-impact (EI) mass spectrom-
etry with a VG7070F mass spectrometer at 6 kV. High-resolution
FAB (fast atom bombardment) mass spectra were taken with a
JEOL SX102 spectrometer using nitrobenzoic acid as matrix. clogp
values were calculated using ChemDraw Ultra, version 11.0.
1,4-(+)-dihydropyridine-3,5-dicarboxylate
(14). C20H24FNO3S
1H NMR (CDCl3) 7.34–7.38 (m, 1H), 7.01–7.12 (m, 3H), 5.72 (s,
1H), 4.18 (t, J = 5.7 Hz, 1H), 3.93–4.01 (m, 2H), 2.94 (q, J = 7.2 Hz,
2H), 2.33 (s, 3H), 1.54–1.60 (m, 2H), 1.26–1.32 (m, 3H), 0.89–0.99
(m, 6H). HRMS calcd for C20H25FNO3S 378.1539; found 378.1525.
4.1.2.6. 3-Thioethyl-5-ethyl-2-methyl-4-ethyl-6-(4-fluorophenyl)-
1,4-(+)-dihydropyridine-3,5-dicarboxylate
(15). C20H24FNO3S
1H NMR (CDCl3) 7.27–7.32 (m, 2H), 7.06–7.12 (m, 2H), 5.69 (s, 1H),
4.18 (t, J = 6.0 Hz, 1H), 3.93–4.00 (m, 2H), 2.94 (q, J = 7.4 Hz, 2H),
2.33 (s, 3H), 1.53–1.60 (m, 2H), 1.29 (t, J = 7.2 Hz, 3H), 0.89–1.02 (m,
6H) MS: 348.2 (M-Et) base, 316.2 (M-EtS). Anal. (C20H24FNO3S) C, H,
N: calcd 63.64, 6.41, 3.71; found 63.49, 6.43, 3.58.
4.1.2.7. 3-Ethylthio-5-ethyl-2-methyl-4-ethyl-6-(4-cyanophenyl)-
1,4-(+)-dihydropyridine-3,5-dicarboxylate
(16). C21H24N2O3S
1H NMR (CDCl3) 7.69 (d, J = 8.1 Hz, 2H), 7.43 (d, J = 8.1 Hz, 2H),
5.71 (s, 1H), 4.19 (t, J = 5.9 Hz, 1H), 3.94–3.99 (m, 2H), 2.95 (q,
J = 7.5 Hz, 2H), 2.32 (s, 3H), 1.56–1.61 (m, 2H), 1.29 (t, J = 7.4 Hz,
3H), 0.89–1.02 (m, 6H) MS: 355.2 (M-Et), 323.2 (M-EtS). HRMS calcd
for C21H24N2O3NaS 407.1403; found 407.1385.
4.1.2. General procedure for preparation of substituted 1,4-DHP
derivatives
The synthesis of DHP derivatives was carried out by knownmeth-
ods.29,34 The methods shown in Schemes 1 and 2 were used depend-
ing on the substitution on C-6 position of DHP. Equimolar amounts
(0.5–1.0 mmol) of the appropriate b-enaminoester (33, 38, or 58),
aldehyde (36 or 40), and b-ketoester (34, 35, or 37) were dissolved
in 2–5 mL of absolute ethanol. The mixture was sealed in a Pyrex
tube and heated, with stirring, to 80–90 °C for 18–36 h. After the
mixture was cooled to room temperature, the solvent was evapo-
rated, and the residue was purified using preparative TLC (silica
60, Aldrich; petroleum ether/ethylacetate (5:1–10:1)). The products
were shownto be homogeneous by analytical TLC and were stored at
ꢀ20 °C.
4.1.2.8. 3-Ethylthio-5-ethyl-2-methyl-4-ethyl-6-cyclohexyl-1,4-
(+)-dihydropyridine-3,5-dicarboxylate (17). C20H31NO3S 1H
NMR (CDCl3) 5.81 (s, 1H), 4.08–4.27 (m, 3H), 3.72–3.86 (m, 1H),
2.90 (q, J = 7.2 Hz, 2H), 2.30 (s, 3H), 1.78–1.81 (m, 4H), 1.15–1.43
(m, 14H), 0.77 (t, J = 7.7 Hz, 3H) 336.3 (M-ethyl, base). HRMS calcd
for C20H31NO3SNa 388.1922; found 388.1914.
4.1.2.9. 3-Ethylthio-5-ethyl-2-methyl-4-propyl-6-(3-chlorophenyl)-
1,4-(+)-dihydropyridine-3,5-dicarboxylate (18). C21H26ClNO3S
1H NMR (CDCl3) 7.17–7.41 (m, 4H), 5.74 (s, 1H), 4.19 (t,
J = 5.9 Hz, 1H), 3.98 (q, J = 7.2 Hz, 2H), 2.92 (q, J = 7.5 Hz, 2H),
2.32 (s, 3H), 1.26–1.55 (m, 7H), 0.89–1.17 (m, 6H) MS: 364.2 (M-
propyl), 346.2 (M-EtS). Anal. (C21H26ClNO3S) C, H, N: calcd 61.83,
6.42, 3.43; found 61.56, 6.47, 3.33.
4.1.2.1. 3,5-Diethyl-2-methyl-4-ethyl-6-(3-chlorophenyl)-1,4-(+)-
dihydropyridine-3,5-dicarboxylate (9). C20H24ClNO4 1H NMR
(CDCl3) 7.17–7.41 (m, 4H), 5.57 (s, 1H), 4.15–4.24 (m, 2H), 4.03 (t,
J = 5.7 Hz, 1H), 3.94 (q, J = 7.1 Hz, 2H), 2.32 (s, 3H), 1.47–1.54 (m,
2H), 1.31 (t, J = 7.2 Hz, 3H), 0.86–0.97 (m, 6H) 348.2 (M-ethyl, base).
Anal. (C20H24ClNO4) C, H, N: calcd 63.57, 6.40, 3.71; found 63.29,
6.59, 3.53.
4.1.2.10. 3-Ethylthio-5-ethyl-2-methyl-4-cyclopropyl-6-(4-fluo-
rophenyl)-1,4-(+)-dihydropyridine-3,5-dicarboxylate (19). C21H24
FNO3S 1H NMR (CDCl3) 7.28–7.33 (m, 2H), 7.07–7.13 (m, 2H),
5.74 (s, 1H), 3.94–4.03 (m, 3H), 2.95 (q, J = 7.2 Hz, 2H), 2.35 (s,
3H), 1.29 (t, J = 7.2 Hz, 3H), 1.01 (t, J = 7.2 Hz, 3H), 0.25–0.45 (m,
5H) 348.2 (M-cyclopropyl), 328.2 (M-EtS). Anal. (C21H24FNO3S) C,
H, N: calcd 64.76, 6.21, 3.60; found 64.85, 6.22, 3.54.
4.1.2.2. 3,5-Diethyl-2-methyl-4-ethyl-6-(3-fluorophenyl)-1,4-(+)-
dihydropyridine-3,5-dicarboxylate (10). C20H24FNO4 1H NMR
(CDCl3) 6.95–7.41 (m, 4H), 5.58 (s, 1H), 4.18–4.24 (m, 2H), 4.03 (t,
J = 5.7 Hz, 1H), 3.93 (q, J = 7.1 Hz, 2H), 2.32 (s, 3H), 1.47–1.54 (m,