Synthesis and biological activity of the calcium modulator (R) and (S)-3-methyl 5-pentyl 2,6-dimethyl-4-(3-nitrophenyl)-1,4-dihydropyridine-3,5-dicarboxylate

Article abstract of DOI:10.1016/j.bmcl.2009.12.104

An efficient total synthesis of (R) and (S)-3-methyl 5-pentyl 2,6-dimethyl-4-(3-nitrophenyl)-1,4-dihydropyridine-3,5-dicarboxylate in high optical purities is reported. The useful step is the resolution of racemic 2, 6-dimethyl-5-methoxycarbonyl-4-(3-nitrophenyl)-1,4-dihydropyridine-3-carboxylic acid by using commercially available Cinchona alkaloids cinchonidine and quinidine as the resolving agents. Under the optimum conditions, the optical purities for R- and S-enantiomers are extremely high (ee >99.5%). The further dihydropyridine receptor binding activity assay shows that the S-enantiomer is more potent than R-enantiomer both in rat cardiac (approximately 19 times) and cerebral cortex membrane (12 times).

Full text of DOI:10.1016/j.bmcl.2009.12.104

Bioorganic & Medicinal Chemistry Letters 20 (2010) 805–808
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and biological activity of the calcium modulator (R) and
(S)-3-methyl 5-pentyl 2,6-dimethyl-4-(3-nitrophenyl)-1,4-dihydropyridine-
3,5-dicarboxylate
Bang-le Zhang a, Wei He b, Xin Shi b, Meng-lei Huan a, Qiu-ju Huang a, Si-yuan Zhou a,
*
a Department of Pharmaceutics, School of Pharmacy, Fourth Military Medical University, Xi’an, Shaanxi 710032, China
b Department of Chemistry, School of Pharmacy, Fourth Military Medical University, Xi’an, Shaanxi 710032, China
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient total synthesis of (R) and (S)-3-methyl 5-pentyl 2,6-dimethyl-4-(3-nitrophenyl)-1,4-dihydro-
pyridine-3,5-dicarboxylate in high optical purities is reported. The useful step is the resolution of racemic
2, 6-dimethyl-5-methoxycarbonyl-4-(3-nitrophenyl)-1,4-dihydropyridine-3-carboxylic acid by using
commercially available Cinchona alkaloids cinchonidine and quinidine as the resolving agents. Under
the optimum conditions, the optical purities for R- and S-enantiomers are extremely high (ee >99.5%).
The further dihydropyridine receptor binding activity assay shows that the S-enantiomer is more potent
than R-enantiomer both in rat cardiac (approximately 19 times) and cerebral cortex membrane
(12 times).
Received 26 November 2009
Revised 23 December 2009
Accepted 28 December 2009
Available online 4 January 2010
Keywords:
1,4-Dihydropyridines
Calcium modulator
Resolution
Ó 2009 Elsevier Ltd. All rights reserved.
Receptor binding activity
1,4-Dihydropyridines (1,4-DHPDs), a class of calcium modula-
tors, are widely used in clinic because of their biological actions
as drugs in vasodilatation, hepatoprotection, neuromodulatory,
cognition, memory enhancing, neuroprotection, anti-atherosclero-
sis, anti-diabetes, antioxidant, anti-mutagenic, and anti-tumor.1
The different activity of 1,4-dihydropyridines derivatives often de-
rives from the functional groups substituted on the 1,4-DHPDs
moiety. When substituents on the left side differ from those on
the right side of 1,4-DHPDs, the molecule is chiral, with the carbon
at position 4 as the stereogenic centre and exists as two enantio-
mers. Some years ago, a great deal of effort was devoted to inves-
tigating the role of the chirality in dihydropyridine ring. The
enantiomers of an unsymmetrical 1,4-DHPDs usually differ in their
biological activities and even have an exactly opposite activity pro-
file.2 For example, the S-enantiomer of manidipine is about 30–80
times more potent than the R-isomer in its antihypertensive ac-
tion.3,4 In some case, the activity of each enantiomer was often to-
tally different, one of them being a calcium antagonist of the
channel and the other an agonist.5 Chiral 4-aryl-1,4-DHPDs have
been extensively investigated as calcium modulator for the last
two decades and some chiral 1,4-DHPDs have been synthesized
by several research groups through an asymmetric induction6 or
through the resolution of racemic mixture.3,7
3-Methyl 5-pentyl 2,6-dimethyl-4-(3-nitrophenyl)-1,4-dihy-
dropyridine-3,5-dicarboxylate (MN9202, 1, Fig. 1) is a promising
1,4-DHPDs analogue screened by our group, which have potent
pharmacological activities8 in the treatment of myocardial hyper-
trophy, membrane lipid peroxidation, platelet aggregation, anti-
proliferation, anticonvulsant, anti-hypercholesterolemia, and
anti-atherosclerosis. All of these earlier studies were, however,
only carried out with racemic MN9202 on calcium channels. The
title compounds have never been prepared in enantiomerically
pure form, and the role of stereoselectivity in MN9202 were not
addressed.
In light of the chiral and chemical peculiarities of dihydropyri-
dines, the investigation whether the chirality at position 4 of
MN9202 influence the pattern of calcium channel modulators be-
came very important for improving the therapeutical application.
For this purpose, the single enantiomers were synthesized and
the dihydropyridine receptor binding activities were also detected
in rat cardiac and cerebral cortex membrane.
NO2
R1
R2O2C
Me
CO2R3
Me
MeO2C
Me
CO2CH2(CH2)3Me
Me
N
N
H
H
* Corresponding authors. Tel.: +86 2984774556; fax: +86 2984774552.
E-mail addresses: blezhang@fmmu.edu.cn (B. Zhang), zhousy@fmmu.edu.cn (S.
Zhou).
1,4-Dihydropyridines (1,4-DHPDs)
MN9202 (1)
Figure 1. Chemical structures of 1,4-dihydropyridine derivatives.
0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2009.12.104

806
B. Zhang et al. / Bioorg. Med. Chem. Lett. 20 (2010) 805–808
NO2
enantiomeric acid 3 was obtained successfully in high ee and yield
(entry 5). Solvent of DMF/H2O (8:5) was also efficient when per-
formed the racemic acid with resolving agent quinidine (entry 8).
The free enantiomeric acid 3 was obtained by treating the salt with
2 M HCl after work-up.11 The cinchonidine and quinidine can be
recovered12 from the mother liquor of resolution due to their alka-
loid feature.13 When the recovered bases were reused in the next
resolution, no significant loss in activity was observed (at least 5,
data were not shown). The absolute configurations at the C-4 car-
bon have been established that (+)-3a and (ꢁ)-3b have S- and
R-configurations at C-4, respectively, which were based on the
optical value of 3a and 3b with the compounds reported in litera-
ture,7b,c whose absolute configurations have been determined by
X-ray crystallography.7f
NO2
i
CH3COCH2CO2CH3
NH3-H2O
MeO2C
Me
CO2Me
Me
76%
CHO
N
H
2
NO2
ii
62%
MeO2C
Me
CO2H
Me
N
H
3
Scheme 1. Reagents and conditions: (i) EtOH, rt, 1 h; refluxing, 7 h; (ii) 10 M NaOH,
After obtained the key intermediates, enantiomeric dihydro-
pyridinecarboxylic acids 3a and 3b, the esterification was carried
out easily (Scheme 2). The resulting enantiomeric dihydropyridin-
ecarboxylic acid 3a or 3b was converted to corresponding acid
chloride by treatment with SOCl2 in CH2Cl2–DMF (4:1) at ꢁ20 °C.
The acid chloride was not isolated and directly treated with n-
pentanol to give the final enantiomeric MN9202 1a or 1b in the
yield 78% and 76%, respectively.14 The absolute configuration of
(ꢁ)-1a was proven to be R which determined by the precursor of
(S)-3a because of no inversion in the esterification reaction. The
racemic MN9202 was synthesized by the method mentioned above
when using the racemic dihydropyridinecarboxylic acid 3 instead
of enantiomeric acid, the structure of MN9202 was identical to that
reported by us using another synthetic method. In order to further
investigate the therapeutical application, the optical purities of
MN9202 enantiomers were also determined by chiral high-pres-
sure-liquid-chromatography columns AD-H. Under the optimum
conditions,15 the racemic MN9202 as a reference standard can be
extremely separated and the enantiomeric excess of 1a or 1b were
both determined.
In order to evaluate pharmacological profiles of the MN9202
enantiomers, we performed competition binding experiments with
[3H]-nitrendipine in rat cardiac and cerebral cortex membrane
homogenate by the general method.7f The concentrations of 50%
inhibition of nitrendipine binding (IC50) are shown in Table 2.
The (S)-MN9202 has higher affinity than the (R)-MN9202 both in
the cardiac and cerebral cortex membrane, approximately 19 times
more potent in cardiac and 12 times in cerebral cortex membrane.
The binding results also indicate that both (R)- and (S)-MN9202 ex-
hibit differences in tissue selectivity and have slightly high affinity
in the cardiac membrane.
MeOH, refluxing, 9 h; 2 M HCl, pH = 3.
The synthesis of racemic 2,6-dimethyl-5-methoxy carbonyl-4-
(3-nitrophenyl)-1,4-dihydropyridine-3-carboxylic acid 3 was illus-
trated in Scheme 1. Using commercially available 3-nitrobenzalde-
hyde, methyl 3-oxobutanoate, and NH3ꢀH2O as starting materials,
the 3,5-dimethyl 2,6-dimethyl-4-(3-nitrophenyl)-1,4-dihydropyri-
dine-3,5-dicarboxylate 2 was prepared in 76% yield via the Han-
tzsch three component condensation in the usual manner
according to similar method described in previous Letter.9 When
treated 2 with 10 M NaOH in MeOH–H2O, the desired product 3
was obtained in 62% yield after recrystallization with MeOH–H2O.
With the acid 3 in hand, the resolution protocol using chiral or-
ganic bases was tried. Shibanuma et al.7b have reported the resolu-
tion method using cinchonine and cinchonidine to obtain the
single enantiomers of 3, but the resolution process need the pro-
tected and deprotected approach for NH in 1,4-DHPDs ring. The
optically active 1,4-dihydropyridine derivatives have been synthe-
sized from the enantiomers of the 1-ethoxymethylated carboxylic
acid followed by removal of the prior NH-protecting group. We at-
tempted to use unprotected racemic acid 3 for direct resolution
and the resolution was investigated in detail. The results were
shown in Table 1. When using cinchonidine and quinidine as chiral
resolving agents, the unprotected racemic acid can be resolved by
the formation of 3-cinchonidine salt and 3-quinidine salt. In the
resolution process, the solvent was very important and has been
chosen carefully because of its effect on the efficiency of resolution
and the ease of crystallization.10 When performed the racemic acid
with resolving agent cinchonidine in EtOH and acetone, the resolu-
tion was totally inefficient and the ee value was very poor (Table 1,
entries 1 and 2). DMF allowed an improvement for ee value to 99%,
but the crystallization yield was very low (26%, entry 3). Addition
of water to the DMF has shown the favorable effect in crystalliza-
tion. But while the water was increased, the ee value was de-
creased (entries 4–7). When DMF/H2O (8:5, v/v) was used, the
In summary, we have identified an efficient process to the syn-
thesis of (R) and (S)-methyl pentyl 1,4-dihydro-2,6-dimethyl-4-(3-
nitrophenyl)-3,5-pyridine dicarboxylate (MN9202) in high optical
purities. The cinchonidine and quinidine-derived resolution proto-
col is completely enantioselective for the dihydropyridinecarboxy-
Table 1
The resolution of racemic 1,4-dihydropyridine acid 3
Entry
Resolving agent
Solvent
Yielda (%)
eeb (%)
Absolute configurationc
1
2
3
4
5
6
7
8
9
Cinchonidine
Cinchonidine
Cinchonidine
Cinchonidine
Cinchonidine
Cinchonidine
Cinchonidine
Quinidine
EtOH
Acetone
DMF
DMF–H2O (8:2)
DMF–H2O (8:5)
DMF–H2O (8:8)
DMF–H2O (8:11)
DMF–H2O (8:5)
DMF–H2O (8:8)
52
49
26
32
41
44
46
43
46
63.4
72.1
99.3
99.5
>99.5
97.8
95.0
>99.5
96.6
S
S
S
S
S
S
S
R
R
Quinidine
a
b
c
Based on the total racemic 3, recrystallized twice.
Determined after esterification by chiral HPLC.
Established by comparing the optical value of 3a or 3b with the compounds reported in the literature.

B. Zhang et al. / Bioorg. Med. Chem. Lett. 20 (2010) 805–808
807
NO2
Supplementary data
iii
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2009.12.104.
ii
MeO2C
Me
CO2H
Me
cinchonidine
3-cinchonidine salt
NO2
N
H
References and notes
i
(+)-(S)-3a
MeO2C
Me
CO2H
Me
1. Safak, C.; Simsek, R. Mini. Rev. Med. Chem. 2006, 6, 747.
NO2
i
2. Inotsume, N.; Nakano, M. J. Biochem. Biophys. Methods 2002, 54, 255.
3. Kajino, M.; Wada, Y.; Nagai, Y.; Nagaoka, A.; Meguro, K. Chem. Pharm. Bull. 1989,
37, 2225.
4. Cataldi, M.; Taglialatela, M.; Palagiano, F.; Secondo, A.; deCaprariis, P.;
Amoroso, S.; diRenzo, G.; Annunziato, L. Eur. J. Pharmacol. 1999, 376, 169.
5. Franckowiak, G.; Bechem, M.; Schramm, M.; Thomas, G. Eur. J. Pharmacol. 1985,
114, 223.
6. (a) Ashworth, I.; Hopes, P.; Levin, D.; Patel, I.; Salloo, R. Tetrahedron Lett. 2002,
43, 4931; (b) Iqbal, N.; Vo, D.; McEwen, C. A.; Wolowyk, M. W.; Knaus, E. E.
Chirality 1994, 6, 515; (c) Martin, N.; Martinez-Grau, A.; Seoane, C.; Marco, J. L.;
Albert, A.; Cano, F. H. Tetrahedron: Asymmetry 1995, 6, 877.
N
H
3-quinidine salt
ii
quinidine
MeO2C
Me
CO2H
Me
3
iii
N
H
(-)-(R)-3b
NO2
7. (a) Boatto, G.; Nieddu, M.; Faedda, M. V.; deCaprariis, P. Chirality 2003, 15, 494;
(b) Shibanuma, T.; Iwanami, M.; Okuda, K.; Takenaka, T.; Murakami, M. Chem.
Pharm. Bull. 1980, 28, 2809; (c) Ashimori, A.; Uchida, T.; Ohtaki, Y.; Tanaka, M.;
Ohe, K.; Fukaya, C.; Watanabe, M.; Kagitani, M.; Yokoyama, K. Chem. Pharm.
Bull. 1991, 39, 108; (d) Peri, R.; Padmanabhan, S.; Rutledge, A.; Singh, S.; Triggle,
D. J. J. Med. Chem. 2000, 43, 2906; (e) Baranda, A. B.; Etxebarria, N.; Jiménez, R.
M.; Alonso, R. M. J. Chromatogr. Sci. 2005, 43, 505; (f) Tamazawa, T.; Arima, H.;
Kojima, T.; Isomura, Y.; Okada, M.; Fujita, S.; Furuya, T.; Takenaka, T.; Inagaki,
O.; Terai, M. J. Med. Chem. 1986, 29, 2504.
IV
(+)-(S)-3a
MeO2C
Me
CO2CH2(CH2)3Me
Me
N
H
(-)-(R)-1a
NO2
8. (a) Yang, Z.; Zhou, S.; Yang, T.; Mei, Q.; Liu, L. J. Fourth. Mil. Med. Univ. 2004, 25,
1794; (b) Liu, L.; Mei, Q.; Zhang, F.; Chen, L.; Zhao, D. Chin. Pharmacol. Bull.
2003, 19, 796; (c) Wang, J.; Guo, W.; Mei, Q.; Wang, S.; Zhao, D. J. Fourth. Mil.
Med. Univ. 2003, 24, 1688; (d) Wang, L.; Mei, Q.; Zhao, D.; Wu, L.; Tian, Q.; Li, J.
Chin. Pharmacol. Bull. 2000, 16, 406; (e) Liu, L.; Mei, Q.; Zhao, D. J. Fourth. Mil.
Med. Univ. 2001, 22, 1456; (f) Wang, L.; Mei, Q.; Zhao, D.; Guo, Z. Acta
Pharmacol. Sin. 2000, 21, 623; (g) Yao, X.; Gan, H.; Jia, M.; Fang, Z.; Mei, Q. J.
Fourth. Mil. Med. Univ. 2000, 21, 311; (h) Wang, L.; Mei, Q.; Zhao, D.; Tian, Q.;
Zhang, Z. Chin. Pharm. J. 1998, 33, 282; (i) Wang, L.; Pei, Q.; Mei, Q.; Zhao, D.; Li,
J.; Tian, Q. Chin. Pharmacol. Bull. 1997, 13, 512.
IV
(-)-(R)-3b
MeO2C
Me
CO2CH2(CH2)3Me
Me
N
H
(+)-(S)-1b
Scheme 2. Reagents and conditions: (i) solvent, refluxing, 15 min; rt, overnight; (ii)
0.5 M HCl, 0 °C; (iii) recovering of cinchonidine or quinidine: 40%NaOH, rt; (iv)
SOCl2, CH2Cl2–DMF (4:1, v/v), ꢁ20 °C.
9. Bao, C.; Chen, Z.; Yuan, F. Chin. J. Pharm. 1992, 23, 8.
10. (a) Borghese, A.; Libert, V.; Zhang, T.; Charles, A. A. Org. Process Res. Dev. 2004, 8,
532; (b) Wang, Y.; Chen, A. M. Org. Process Res. Dev. 2008, 12, 282.
11. Preparation of the optical isomers of 2,6-dimethyl-5-methoxycarbonyl-4-(3-
nitrophenyl)-1,4-dihydropyridine-3-carboxylic acid:
A mixture of 3 (4 g,
12 mmol) and cinchonidine (3.55 g, 12 mmol) in DMF (8 mL) was refluxed
for 15 min and then added heated water (5 mL). The above mixture was cooled
and allowed to stand at ambient temperature for 18 h. The resulting
precipitates were collected by filtration and air-dried to give the impure 3-
cinchonidine salt. After recrystallized twice from DMF–H2O (8:5), the pure salt
was obtained (3.1 g) in 41% yield (82% based on the isomer). Mp 193–195 °C;
Table 2
Inhibition of [3H]-nitrendipine binding to rat cardiac and cerebral cortex membrane
homogenate
½ ꢂ ꢁ58.4(c 0.5, acetone). A suspension of the pure salt (2.0 g, 3 mmol) in
a 2D5
a
Entry
Drug
IC50 (nM)
EtOAc (100 mL) was treated with 0.5 M HCl (40 mL) under stirring and cooling
in an ice-bath and then the aqueous layer was removed and extracted with
EtOAc. The combined organic layer were washed with brine, dried over Na2SO4
and evaporated in vacuo to give (+)-3a (0.88 g). (+)-(S)-3a: 84% yield. Mp 193–
Cardiac membrane
Cerebral cortex membrane
1
2
3
(ꢁ)-(R)-1a
(+)-(S)-1b
Racemic 1
8.14 1.10c
0.42 0.06b,c
0.76 0.09
17.62 1.86
1.48 0.12b
3.62 0.32
195 °C; ½a 2D5
ꢂ
+24.2 (c 0.5, acetone) [lit.7b mp 194–195 °C; ½a 2D2
ꢂ
+19.1 (c 0.556,
acetone)]; 1H NMR (400 MHz, DMSO-d6/CDCl3(1:1)) d 8.62 (s, br, 1H), 8.07 (s,
1H), 7.96 (d, J = 7 Hz, 1H), 7.64 (d, J = 8 Hz, 1H), 7.44–7.43 (m, 1H), 5.06 (s, 1H),
3.60 (s, 3H), 2.34 (s, 3H), 2.33 (s, 3H); ESI-MS m/z: 333 (M+H+); HREI-MS calcd
for C16H16N2O6 332.1008; Found 332.1021; IR (KBr): 3354, 1675, 1662, 1261,
1161, 766, 578 cmꢁ1. The 3-quinidine salt and optical isomer 3b was obtained
as a similar procedure mentioned above when using quinidine as resolution
a
Concentrations of 50% inhibition of [3H]-nitrendipine binding, each value rep-
resents the mean SD (n = 3).
b
P <0.01, compared with (ꢁ)-(R)-1a.
c
P <0.01, compared with cerebral cortex membrane.
agent. 3-quinidine salt: 43% yield. Mp 199–201 °C, ½a D25
ꢂ
+119.8(c 0.5, acetone).
(ꢁ)-(R)-3b: 93%yield. Mp 190–192 °C; ½a D25
ꢂ
ꢁ24.0 (c 0.5, acetone) [lit.7b mp
196–197 °C; ½a 2D2
ꢂ
ꢁ19.6 (c 0.542, acetone)]; ESI-MS m/z: 333 (M+H+); HREI-MS
lic acids 3, and cinchonidine and quinidine can be reused in the
next steps, which is amenable to scale-up for the production of
MN9202. This approach can be also used to synthesis of others chi-
ral dihydropyridines and open up a new field to investigate the chi-
rality influence in MN9202. The dihydropyridine receptor binding
assay demonstrated that the stereochemistry at C-4 is crucial and
S-enantiomer was defined as active isomer which is more potent
than R-enantiomer both in rat cardiac and cerebral cortex
membrane.
calcd for C16H16N2O6 332.1008; found 332.1016; the 1H NMR and IR spectra of
(ꢁ)-(R)-3b were identical to those of another enantiomer (+)-(S)-3a.
12. Recovering of cinchonidine and quinidine: To the corresponding aqueous layer
(obtained after treated with HCl) was added 40% NaOH. The resulted aqueous
layer (pH 10) was extracted with EtOAc. The combined organic layer was
washed with brine, dried over Na2SO4 and evaporated in vacuo to give
cinchonidine (93% yield) and quinidine (95% yield).
13. He, W.; Zhang, B.-L.; Jiang, R.; Liu, P.; Sun, X.-L.; Zhang, S.-Y. Tetrahedron Lett.
2006, 47, 5231.
14. Synthesis of (R) and (S)-3-methyl-5-pentyl-2,6-dimethyl-4-(3-nitrophenyl)-1,4-
dihydropyridine-3,5-dicarboxylate: A suspension of (+)-3a (0.52 g, 1.5 mmol) in
CH2Cl2–DMF (4:1, 5 mL) was cooled at ꢁ20 °C in an ice-bath under N2, and
SOCl2 (0.2 g, 1.6 mmol) was added dropwise under this temperature. After
adding, the mixture was stirred for another 30 min. Then a solution of n-
pentanol in CH2Cl2 (2 mL) was added dropwise to the reaction mixture under
the same condition over a period of 10 min. The reaction mixture was stirred
for 9 h (TLC) and diluted with CH2Cl2 (50 mL) and wished with water (50 mL).
The organic layer was separated and the aqueous layer was extracted with
CH2Cl2 (30 mL ꢃ 3). The combined organic layer were washed with brine, dried
Acknowledgment
The financial support of this research by National Natural Sci-
ence Foundation of China (Nos. 20702063 and 30700887) is grate-
fully acknowledged.

808
B. Zhang et al. / Bioorg. Med. Chem. Lett. 20 (2010) 805–808
over Na2SO4 and evaporated in vacuo. The residue was purified by column
3364, 2957, 2928, 2360, 2341, 1649, cmꢁ1. (+)-(S)-1b: 83%yield. Mp 112–
chromatography on silica gel (elute: petroleum ether/ethyl acetate = 3:1, v/v).
The fractions containing the desired compound were combined and
evaporated in vacuo to give (R)-MN9202 (0.46 g). (S)-MN9202 was obtained
from (R)-3b by using a similar manner. (ꢁ)-(R)-1a: 76%yield. Mp 112–113 °C;
113 °C; ½a 2D5
ꢂ
+9.8 (c 1.0, acetone); ESI-MS m/z: 403 (M+H+); HREI-MS calcd for
C21H26N2O6 402.1791; found 402.1785; the 1H NMR and IR spectra of (+)-(S)-
1b were identical to those of another enantiomer (ꢁ)-(R)-1a.
15. Enantiomeric purity analysis: The optical purity was checked by chiral HPLC
under the following conditions: Chiral AD-H (4.6 mm ꢃ 250 mm); column
temperature, 30 °C; eluent, n-hexane/i-propanol (95:5, v/v); flow rate, 0.7 mL/
min; detection, UV 254 nm (Dionex instrument). Retention times (Shown in
Figure 2, see Supplementary data): t(ꢁ)-(R)-1a = 29 min; t(+)-(S)-1b = 33 min.
½
a 2D5
ꢂ
ꢁ9.7 (c 0.5, acetone); 1H NMR (400 MHz, CDCl3) d 8.11 (s, 1H), 8.01 (d,
J = 8 Hz, 1H), 7.64 (d, J = 8 Hz, 1H), 7.37 (dd, J = 8 Hz, J = 8 Hz, 1H), 5.70 (s, 1H),
5.10 (s, 1H), 4.10–3.96 (m, 2H), 3.65 (s, 3H), 2.38 (s, 3H), 2.37 (s, 3H), 1.62–1.53
(m, 2H), 1.32–1.19 (m, 4H), 0.86 (t, J = 7 Hz, 3H); ESI-MS m/z: 403 (M+H+);
HREI-MS calcd for C21H26N2O6 402.1791; found 402.1802; IR (KBr): 3440,