Experimental and theoretical studies of the absolute configuration of (2S,1′R) and (2R,1′R)-2-acetoxymethyl-3-phenyl-N-(1′- phenylethyl)-propionamide

Article abstract of DOI:10.1016/j.molstruc.2010.07.049

Two diastereoisomers (2S,1′R) and (2R,1′R)-2-acetoxymethyl-3- phenyl-N-(1′-phenylethyl)-propionamide(2), have been studied to investigate their discrimination on the absolute configuration using nuclear magnetic resonance (NMR), infrared (IR) spectroscopy

Full text of DOI:10.1016/j.molstruc.2010.07.049

Journal of Molecular Structure 981 (2010) 159–162
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
0
Experimental and theoretical studies of the absolute configuration of (2S,1 R)
0
0
and (2R,1 R)-2-acetoxymethyl-3-phenyl-N-(1 -phenylethyl)-propionamide
Sihong Wang a,*, Yuhe Kan b
a
Analysis and Inspection Center, Yanbian University, Yanji 133002, PR China
Department of Chemistry, Huaiyin Normal University, Huai’an 223300, PR China
b
a r t i c l e i n f o
a b s t r a c t
0
0
0
Article history:
Two diastereoisomers (2S,1 R) and (2R,1 R)-2-acetoxymethyl-3-phenyl-N-(1 -phenylethyl)-propionamide(2),
have been studied toinvestigatetheir discrimination on the absolute configuration using nuclear magnetic res-
onance (NMR), infrared (IR) spectroscopy were performed. The experimental results were supported by per-
forming density functional theory (DFT) calculations for the NMR, IR spectra using the Becke 3-parameter,
Received 28 March 2007
Received in revised form 22 July 2010
Accepted 27 July 2010
Available online 3 August 2010
ꢀꢀ
Lee, Yang and Parr (B3LYP) functional and the 6-311+G basis set.
Ó 2010 Elsevier B.V. All rights reserved.
Keywords:
Nuclear magnetic resonance spectroscopy
Absolute configuration
Diastereoisomer amide
Quantum chemistry calculation
Infrared spectroscopy
1
. Introduction
and 75.04 MHz for carbon NMR, respectively. Sample was dis-
solved in chloroform-d (0.5 ml) with 0.1% tetramethylsilane
(TMS) as an internal reference and transferred into a 5 mm NMR
tube. Standard pulse programs provided by Bruker WinNMR 3.5
software packages were used to acquire; melting point was deter-
mined in open capillary tube and is uncorrected. Elemental analy-
sis was performed on a Perkin–Elmer 204Q; the mass spectrometer
using a LCQ Advantage (Thermo Finnigan) was operated in electron
impact mode with ionization energy of 70 eV. The ion source tem-
perature was maintained at 280 °C; the Fourier transform infrared
spectrum was measured (in KBr disc) on an IRPrestige-21 (Shima-
2
-Branched carboxylic acids and their derivatives are frequently
found in enzyme inhibitors as optically active compounds with the
chiral center at the -position [1–4]. Optically D-(+)-1-phenylethyl-
amine, is condensed with a carboxylic acid, giving the diastereoiso-
mer which can be transformed into optically 2-branched amide
a
(Scheme 1).
Both diastereoisomers (2S,1 R)-2-acetoxymethyl-3-phenyl-N-
0
0
0
0
(
1 -phenylethyl)-propionamide (2) [2(2S,1 R)] and (2R,1 R)-2-acet-
0
oxymethyl-3-phenyl-N-(1 -phenylethyl)-propionamide (2) [2(2R,-
0
ꢁ1
ꢁ1
1
R)] (Scheme 2) were investigated by using NMR and IR spectra.
dzu) in the 4000–400 cm region with 4 cm resolution.
0
0
In addition, GIAO calculations of 2(2S,1 R) and 2(2R,1 R) have been
performed by using the B3LYP method. These calculations were
valuable for providing insight into NMR and IR spectra. In this article,
basic experimental and theoretical information about the structure
of a 2(2S,1 R) and 2(2R,1 R) was presented. To the best of our knowl-
edge, no evidence of studies for 2(2S,1 R) and 2(2R,1 R) has been re-
2.2. Synthesis
0
0
The racemic 2-(acetoxymethyl)-2-phenylpropionic acid (1) was
0
0
prepared starting from commercially available diethylmalonate
according to literature methods [5–7]. To an ice-chilled stirred solu-
tionof1 (0.4639 g, 2.09 mmol)indichloromethane(5 ml)wasadded
ported to date in the open chemical literature.
2
. Experimental and calculations
isobutylchloroformate (300
line (240 l, 2.09 mmol) in dichloromethane. The resulting mixture
was stirred for 30 min at 0 °C, D-(+)-1-phenylethylamine (240 l,
.09 mmol) was added to the mixture, stirred for 1 h at room tem-
ll, 2.09 mmol) and 4-methylmorpho-
l
2.1. Physical methods
l
2
All NMR spectra were recorded on a Bruker AVANCE 300 super-
conductor spectrometer operating at 300.13 MHz for proton NMR
perature. After removal of the solvent, the residue was extracted
with ethyl acetate (50 ml ꢂ 3) and the organic layer was washed
with 10% citric acid (50 ml ꢂ 3), 5% sodium bicarbonate
(
50 ml ꢂ 3), and dried over anhydrous magnesium sulphate. Evapo-
*
Corresponding author. Tel./fax: +86 433 2732207.
E-mail address: wangquan20080808@yahoo.com.cn (S. Wang).
ration of the organic solvent afforded the product as a white solid.
0
022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2010.07.049

1
60
S. Wang, Y. Kan / Journal of Molecular Structure 981 (2010) 159–162
O
O
Me
Me
O
eters was included using the integral equation formalism model
IEF–PCM [12]. All calculations were performed with the Gaussian
03 package [13,14].
R1
R1
R1
R1
O
N
H
NH
2
R1
R2
R2
OH
R2
O
3. Results and discussions
N
H
NH2
R2
R2
The synthesis procedure is shown in Scheme 2. Two diastereo-
0
0
mers 2(2S,1 R) and 2(2R,1 R) were characterized by elemental anal-
Scheme 1. 2-Branched carboxylic acids and their derivatives.
ysis, mass spectrometry, IR and NMR spectroscopy.
0
In the column chromatography, 2(2S,1 R) isomer eluted faster than
0
2
(2R,1 R). This tendency has been already noticed in the previous work
for simple chain carboxylic acids [15,16]. No crystal suitable for X-ray
0
analysis was obtained for both diastereoisomers 2(2S,1 R) and
0
2
(2R,1 R). Therefore, to further try to investigate their structure, both
D-(+)-1-phenylethylamine
0
0
H
diastereoisomers 2(2S,1 R) and 2(2R,1 R) were studied by using NMR.
O
ΟΗ
O
N
isobutylchloroformate
First of all, proton NMR spectra of the pure diastereoisomers were per-
0
O
O
formed to possibly point out any small difference between 2(2S,1 R)
O
O
2
1
0
and 2(2R,1 R). In fact, it is very well known in literature that chiral re-
agents can enhance shielding/deshielding effects and help resolve dia-
stereoisomers and/or enantiomers [15–18]. In parallel, two-
dimensional NMR experiments such as NOESY were run on diastereo-
isomers and confirm one-dimensional NMR assignments. The proton
0
0
Scheme 2. The synthetic route of 2(2S,1 R) and 2(2R,1 R).
The crude product was separated by column chromatography over
silica gel eluting with ethyl acetate/n-hexane = 3:1–1:1.
NMR chemical shifts of the CH(H25) and CH (H33–H35) of the fast-
3
0
eluting 2(2S,1 R) (Fig. 1) were always more upfield than those of the
slow-eluting 2(2R,1 R), which suggest that the CH(H25) and
0
0
2
(2S,1 R): 195 mg, R
f
value, 0.27; [
a
]
D
= ꢁ63.0° (c, 0.44, CHCl
3
);
+
+
m.p.: 112–114 °C; MS: [M+H] = 326.4, [M+Na] = 348.6; elemental
CH (H33–H35) of the fast-eluting compounds were magnetically
3
analysis: Calcd. (found) C: 73.82 (73.81), H: 7.12 (7.13), N: 4.30
shielded by the benzene ring (C6–C11), thus confirming the fast-elut-
0
0
0
(
4.29); 2(2R,1 R): 105 mg, R
f
value, 0.22; [
a
]
D
= ꢁ33.2° (c, 0.44,
ing compound is 2(2S,1 R) and the slow-eluting is 2(2R,1 R). In the case
+
+
0
CHCl
3
); m.p.: 115–117 °C; MS: [M+H] = 326.4, [M+Na] = 348.6;
of 2(2R,1 R), preferential anisotropic shielding of the CH (H35–H36) by
3
0
elemental analysis: Calcd. (found) C: 73.82(73.80), H: 7.12(7.13),
N: 4.30(4.30).
the phenyl (H43–H47) is resulted. In the case of 2(2S,1 R), preferential
anisotropic shielding of the phenyl (C19–C24) is observed. Observed
magnitudes of chemical shift differences are consistent with others
earlier observation, such as diastereomeric pairs of 2-hydroxy-
2.3. Computational methods
0
methyl-2-methyl-3-phenyl-N-(1 -phenylethyl)-propionamide, 2-hy-
0
Geometries of the model systems were optimized using the hy-
droxymethyl- 3-phenyl-N-(1 -phenylethyl)-propionamide, 2-phenyl-
0
brid density functional method B3LYP [8] with the 6-311G(d,p) ba-
sis set. Frequencies were calculated for the model systems in order
to identify them as minima. The calculated vibrational frequencies
were scaled by the recommended factor of 0.9688 [9] and calcu-
N-(1 -phenylethyl)-propionamide [15,16,19–25].
A useful conformational mode that accounts for the observed
0
signs of the chemical shift differences between the 2(2S,1 R) and
0
2(2R,1 R) is shown in Scheme 3. The proton chemical shift of H26
ꢁ1
lated spectra were convoluted by applying a 10 cm Lorentzian
function with GaussSum program [10]. NMR shielding tensors
were computed with the GIAO method [11]. The calculated values
was clearly identified through correlations in the 2D-NOE with
the H25, it was also correlations in the COSY with the H27 chem-
0
0
ical shift for 2(2S,1 R) and 2(2R,1 R). Conformation [A] will become
much more stable than [B], resulting from rotation around the C1–
C2 bond, owing to the steric interaction of the bulky C6–C11 moi-
ety with the phenyl (C19–C24). This consideration suggests that
1
13
for the H and C isotropic chemical shifts were referenced to the
corresponding values for TMS, which was calculated at the same
level of theory. The effect of solvent on the theoretical NMR param-
0
0
ꢀꢀ
Fig. 1. Atom numberings and optimized structure of 2(2S,1 R) and 2(2R,1 R) from B3LYP/6-311+G calculations.

S. Wang, Y. Kan / Journal of Molecular Structure 981 (2010) 159–162
161
O
Me
O
Me
H
R2
R1
H
R1
N
H
R
2
1
=-CH
R =-CH O(C=O)CH
3
2
2
Ph
N
H
R2
H
H
A
B
2
(2S,1`R)
2(2R,1`R)
0
0
Scheme 3. Conformational mode between the 2(2S,1 R) and 2(2R,1 R).
Table 1
Table 2
Observed and calculated selected frequencies (cmꢁ1).
The experimental and theoretical 1H NMR chemical shifts for 2(2S,1 R) and 2(2R,1 R).
0
0
0
0
0
0
Mode no.
2(2S,1 R)
2(2R,1 R)
Mode no.
2(2S,1 R)
2(2R,1 R)
m
(Exp.)
m
(Calc.)
m
(Exp.)
m
(Calc.)
Exp.
IEF–PCM (calc.)
Exp.
IEF–PCM (calc.)
1
2
4
6
7
8
9
299
385
441
538
695
727
754
899
298
379
461
550
694
725
755
896
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
2.67
4.25
5.06
7.21
7.30
7.29
7.28
7.28
1.21
1.21
1.21
1.93
1.93
1.93
2.79
2.92
2.97
4.42
5.08
7.46
7.74
7.67
7.64
7.60
0.97
1.67
1.18
1.74
2.29
2.23
3.60
3.08
3.73
3.68
7.66
7.56
7.54
7.67
7.32
2.79
6.94
5.07
7.20
7.20
7.10
7.19
7.11
1.42
2.04
1.42
1.42
2.04
2.04
2.71
4.25
2.86
6.30
5.34
7.70
7.70
7.63
7.68
7.53
1.20
1.90
1.49
1.48
1.49
1.94
2.47
4.07
4.35
2.99
8.18
7.62
7.57
7.53
7.45
441.70
538.14
694.37
727.16
754.17
898.83
460.99
549.71
694.37
725.23
754.17
894.97
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
0
1
2
4
5
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
1006.84
1020.34
1031.92
1035.77
1080.14
1101.35
1190.08
1211.30
1226.73
1354.03
1379.10
1427.32
1442.75
1473.61
1489.00
1660.71
1737.86
2941.44
3007.02
3020.52
3072.60
3080.32
3089.96
3518.16
1007
1020
1031
1035
1079
1101
1189
1211
1227
1354
1379
1426
1442
1474
1489
1661
1739
2941
3008
3020
3073
3081
3091
3519
1010.70
1018.41
1033.84
1082.06
1099.42
1195.86
1203.58
1222.87
1354.03
1375.24
1425.39
1435.04
1446.61
1477.47
1498.69
1658.78
1749.44
2939.51
3003.17
3018.60
3072.60
3084.18
3089.96
3479.58
1011
1019
1034
1082
1100
1196
1203
1223
1354
1376
1426
1435
1446
1478
1499
1660
1749
2940
3004
3019
3073
3084
3091
3480
4.25
7.30
7.27
7.25
7.29
7.19
2.91
7.22
7.12
6.97
6.96
6.95
Table 3
The experimental and theoretical 13C NMR chemical shifts for 2(2S,1 R) and 2(2R,1 R).
0
0
0
0
Mode no.
2(2S,1 R)
2(2R,1 R)
Exp.
IEF–PCM calc.
Exp.
IEF–PCM calc.
2
2
2
2
5
6
7
8
48.38
170.72
48.99
142.47
126.06
128.89
127.17
128.49
128.63
21.44
51
176
56
151
131
136
135
135
136
23
48.34
170.96
49.06
142.93
126.05
127.32
127.32
128.57
128.94
20.29
50
177
54
150
131
135
135
135
138
19
the NMR and IR method is possible for the absolute configuration
determination of 2-branched carboxylic acid.
0
0
To confirm the configuration of 2(2S,1 R) and 2(2R,1 R), the
GIAO based carbon and proton NMR chemical shifts calculations
were carried out on the two configurations of 2. The geometries
of the two configurations of 2 were fully optimized by the
29
3
3
3
3
34
35
3
3
3
3
0
1
2
3
ꢀ
ꢀ
B3LYP/6-311++G method [26]. The vibrational frequency analysis
performed at the same level gave no imaginary frequencies, ensur-
ing that they are true energy minimum structures [27–31]. In a
study of the IR spectra of diastereomeric, it was different in
170.96
20.84
35.21
180
23
38
170.71
21.16
35.35
178
22
38
6
7
8
9
65.27
64
65.19
71
4
1
1
1
3
1
1
1
3
41.70, 538.14, 694.37, 727.16, 754.17, 898.83, 1006.84, 1020.34,
031.92, 1035.77, 1080.14, 1101.35, 1190.08, 1211.30, 1226.73,
354.03, 1379.10, 1427.32, 1442.75, 1473.61, 1489.0, 1660.71,
737.86, 2941.44, 3007.02, 3020.52, 3072.60, 3080.32, 3089.96,
138.37
128.89
128.49
126.55
127.17
128.63
48.38
147
138
135
132
134
136
51
138.53
128.94
127.32
126.67
127.32
128.61
48.34
148
138
134
132
134
136
50
40
4
4
4
4
45
46
1
2
3
4
ꢁ
1
518.16 cm ; 460.99, 549.71, 694.37, 725.23, 754.17, 894.97,
010.70, 1018.41, 1033.84, 1082.06, 1099.42, 1195.86, 1203.58,
222.87, 1354.03, 1375.24, 1425.39, 1435.04, 1446.61, 1477.47,
498.69, 1658.78, 1749.44, 2939.51, 3003.17, 3018.60, 3072.60,
170.72
48.99
176
56
170.96
49.06
177
54
4
7
ꢁ1
084.18, 3089.96, 3479.58 cm . The observed and calculated fre-
quencies are summarized in Table 1. The calculated frequencies
0
ꢁ1
0
C@O for the 2(2S,1 R) were 1660.71 and 1737.86 cm , while the
frequencies C@O for the 2(2R,1 R) were 1658.78 and
ꢁ
1
ꢁ1
observed frequencies were 1661 and 1739 cm ; the calculated
1749.44 cm , while the observed frequencies were 1660 and

1
62
S. Wang, Y. Kan / Journal of Molecular Structure 981 (2010) 159–162
ꢁ
1
0
1
2
749 cm . These calculated frequencies for the 2(2S,1 R) and
[3] S.F. Wang, J.Y. Jin, Z.H. Zeng, G.R. Tian, Chin. Chem. Lett. 21 (2010) 159–162.
[4] S.F. Wang, G.R. Tian, W.Z. Zhang, J.Y. Jin, Bioorg. Med. Chem. Lett. 19 (2009)
0
(2R,1 R) conformers were in fair agreement with those of the ob-
served bands, respectively.
5
009–5011.
5] H.S. Lee, D.H. Kim, Bioorg. Med. Chem. 11 (2003) 4685–4691.
[6] H.S. Lee, J.D. Park, D.H. Kim, Bull. Korean Chem. Soc. 24 (2003) 467–472.
[
Meanwhile, the same procedure was applied on TMS to calcu-
late the relative chemical shifts of these two isomers. All the calcu-
lations were performed using the Gaussian 03 program. The plots
were made using experimental proton NMR and carbon NMR data
versus theoretical values, and then analyzed with linear regression.
The correlation coefficient (R ) for the carbon NMR chemical shifts
with the isomer 2(2S,1 R) is 0.9979, for 2(2R,1 R) is 0.9993. The R
for the proton NMR chemical shifts with the isomer 2(2S,1 R) is
.9908, for 2(2R,1 R) is 0.9940 [32].
[
7] D.H. Kim, J.I. Park, S.J. Chung, J.D. Park, N.K. Park, J.H. Han, Bioorg. Med. Chem.
0 (2002) 2553–2560.
8] A.D. Becke, Phys. Rev. A 38 (1988) 3098–3100.
1
[
[9] C.T. Lee, W.T. Yang, R.G. Parr, Theochem-J. Mol. Struct. 40 (1988) 305–313.
10] J.P. Merrick, D. Moran, L. Radom, J. Phys. Chem. A 111 (2007) 11683–11700.
11] N.M. O’Boyle, A.L. Tenderholt, K.M. Langner, J. Comput. Chem. 29 (2008) 839–
[
[
2
845.
0
0
2
[12] K. Wolinski, J.F. Hinton, P. Pulay, J. Am. Chem. Soc. 112 (1990) 8251–8260.
[13] G.W.T.M.J. Frisch, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J.A.
Montgomery, Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J.
Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson,
H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T.
Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian,
J.B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J.
Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A.
Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels,
M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman,
J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A.
Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-
Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W.
Chen, M.W. Wong, C. Gonzalez, J.A. Pople, Gaussian 03, Revision C.02,
Gaussian, Inc., Wallingford, CT, 2004.
0
0
0
The computational results are summarized in Tables 2 and 3. It
can be seen that the chemical shifts deduced by the B3LYP/6-
ꢀ
ꢀ
0
0
3
11++G method for isomer 2(2S,1 R) and 2(2R,1 R) are closer to
the experimental carbon and proton NMR data. Comparison of
the experimental and theoretical data made it clear that 2 has a
greater probability of adopting the structure of 2(2S,1 R) and
(2R,1 R).
0
0
2
4
. Conclusions
[
[
[
14] Y.H. Kan, Q. Li, Chem. J. Chin. Univ.-Chin. 30 (2009) 174–177.
15] J.M. Seco, E. Quinoa, R. Riguera, Chem. Rev. 104 (2004) 17–117.
16] J.M. Seco, E. Quinoa, R. Riguera, Tetrahedron: Asymmetry 12 (2001) 2915–
In conclusion, the NMR and IR strategy for determination of the
absolute configuration of a stereogenic center in the a-position of a
carboxylic acid is reliable. The quantum chemistry calculation re-
sults of NMR and IR spectra suggest a relative stereo structure of
2925.
[
17] F. Freire, J.M. Seco, E. Quinoa, R. Riguera, Org. Lett. 12 (2010) 208–211.
[18] A.E. Aliev, D. Courtier-Murias, S. Bhandal, S. Zhou, Chem. Commun. 46 (2010)
695–697.
[19] N.J.S. Harmat, S. Mangani, E. Perrotta, D. Giannotti, R. Nannicini, M. Altamura,
Tetrahedron Lett. 41 (2000) 1261–1264.
0
0
2
(2S,1 R) and 2(2R,1 R) with a great probability.
[
20] G. Helmchen, H. Völter, W. Schühle, Tetrahedron Lett. 18 (1977) 1417–1420.
[21] T.R. Hoye, A.S.S. Hamad, D.O. Koltun, M.A. Tennakoon, Tetrahedron Lett. 41
2000) 2289–2293.
22] S. Demunari, G. Marazzi, A. Forgione, A. Longo, P. Lombardi, Tetrahedron Lett.
1 (1980) 2273–2276.
[23] S. Demunari, G. Marazzi, F. Faustini, V. Villa, L. Carluccio, J. Fluorine Chem. 34
1986) 157–166.
24] M. Hirota, H. Ohigashi, Y. Oki, K. Koshimizu, Agric. Biol. Chem. 44 (1980) 1351–
356.
[25] G.M. Nicholas, T.F. Molinski, Tetrahedron 56 (2000) 2921–2927.
Acknowledgement
(
[
The Project Sponsored by the Scientific Research Foundation
financially supported the work for the Doctoral Program of Yanbian
University, China. Dr Sihong Wang is grateful to Prof. Lixin Wu (Ji-
lin University), Dr. Shrongshi Lin (Peking University), Prof. Qifan
Yin (Huaiyin Normal University) for their assistance in the proofing
of this manuscript, careful review, measuring IR and MS spectra.
2
(
[
1
[
26] A.E. Aliev, S. Bhandal, D. Courtier-Murias, J. Phys. Chem. A 113 (2009) 10858–
0865.
1
[
27] A.E. Klimovitskii, E.E. Zvereva, G.A. Chmutova, A.B. Dobrynin, V.Y. Fedorenko,
Y.G. Shtyrlin, I.A. Litvinov, T.V. Bulygina, E.N. Klimovitskii, J. Mol. Struct. 828
Appendix A. Supplementary material
(
2007) 147–153.
28] K. Omata, K. Kotani, K. Kabuto, T. Fujiwara, Y. Takeuchi, Chem. Commun. 46
2010) 3610–3612.
[
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molstruc.2010.07.049.
(
[29] K. Akasaka, K. Gyimesi-Forras, M. Lammerhofer, T. Fujita, M. Watanabe, N.
Harada, W. Lindner, Chirality 17 (2005) 544–555.
[
30] V.A. Ershova, V.M. Pogrebnyak, A.V. Golovin, A.V. Virovets, P.P. Semyannikov,
Tetrahedron: Asymmetry 15 (2004) 109–118.
References
[
31] O.M. Nesterova, B.S. Kikot, Khim. Geterotsikl. Soedin. (1979) 181–184.
[
[
1] S.H. Wang, S.F. Wang, W. Xuan, Z.H. Zeng, J.Y. Jin, J. Ma, G.R. Tian, Bioorg. Med.
Chem. 16 (2008) 3596–3601.
2] S.F. Wang, J.Y. Jin, G.R. Tian. <http://www.rcsb.org/pdb/explore/
explore.do?structureId=3FOR>.
[32] X.F. Li, D.H. Zhang, U. Lee, X.G. Li, J.G. Cheng, W.L. Zhu, J.H. Jung, H.D. Choi, B.W.
Son, J. Nat. Prod. 70 (2007) 307–309.