Stereoselective preparation of 1,2,4-oxadiazole derivatives substituted by pentafluorophenyl by 1,3-dipolar cycloaddition reaction

Article abstract of DOI:10.1016/j.tet.2006.06.119

1,3-Dipolar cycloaddition reactions of chiral imines obtained from optically active amino acids with nitrile oxides afforded 1,2,4-oxadiazole derivatives in moderate to good yields with good stereoselectivity. Investigation on the effect of bases suggeste

Full text of DOI:10.1016/j.tet.2006.06.119

Tetrahedron 62 (2006) 11008–11011
Stereoselective preparation of 1,2,4-oxadiazole
derivatives substituted by pentafluorophenyl
by 1,3-dipolar cycloaddition reaction
Huiling Jiang,a Jingwei Zhao,a Xiaobing Hanb and Shizheng Zhua,
*
aKey Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
354 Fenglin Road, Shanghai 200032, China
bShanghai Leadchem Co. Ltd, China
Received 26 June 2006; revised 29 June 2006; accepted 29 June 2006
Available online 18 September 2006
Abstract—1,3-Dipolar cycloaddition reactions of chiral imines obtained from optically active amino acids with nitrile oxides afforded
1,2,4-oxadiazole derivatives in moderate to good yields with good stereoselectivity. Investigation on the effect of bases suggested that triethyl-
amine was prone to afford better stereoselectivity, while NaHCO3 was prone to increase the reaction rates and yields.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
the potential versatility of this reaction for the construction
of chiral compounds,5 the demand for asymmetric versions
of this reaction has increased over the last 20 years. Thus,
several publications on asymmetric 1,3-DC reactions of
nitrile oxides with alkenes have appeared.6 However, to the
best of our knowledge, only a few applications of imino-
1,3-DC reaction were reported;7 furthermore, no literature
has reported the reaction of optically active imine in the
reaction. As a continuation of our research interests in chem-
ical transformation of fluorine-containing imine,8 we report
herein the 1,3-DC reaction of chiral imine 1 prepared from
perfluorobenzaldehyde and (S)-ethyl 2-amino-3-methyl-
butanoate with nitrile oxides in the presence of triethylamine
or NaHCO3. It was favorable that the reaction can afford
the corresponding five-membered heterocycles with good
stereoselectivity.
1,3-Diploar cycloaddition (1,3-DC) reaction for the synthe-
sis of five-membered heterocycles is a classic reaction in
organic chemistry, due to a high degree of site-, regio-,
and stereoselectivity.1 Therefore, these reactions are widely
used for the preparation of molecules with significance for
both academia and industry. In recent years, the develop-
ment of new stereoselective version has been a major
challenge. The stereochemistry of the 1,3-DC reaction can
be controlled either by the appropriate substrates or choosing
a metal complex as a catalyst.2 Compared with the utiliza-
tion of a metal catalyst, it is straightforward to employ the
chiral substrate in the reaction system. Generally, the most
commonly used chiral dipoles or philodipoles were derived
from optically active amino acids or their derivatives.3
CN
COOR
R1
Bn
N
O
2. Results and discussion
Ph
N
Ph
N
O
OMe
H
BnCOO
Compound 1 can be conveniently prepared by the dehydra-
tion reaction of perfluorobenzaldehyde with (S)-ethyl 2-
amino-3-methylbutanoate (Scheme 1).9 Compared with
unfluorinated imine, pentafluorophenyl reinforced the stabil-
ity of 1 to react with nitrile oxides.
Nitrile oxides are known to be remarkably active dipoles in
1,3-DC reactions, and have been extensively investigated for
their synthetic application and for elucidation of the reaction
mechanism of 1,3-DC reaction.4 Generally, the 1,3-DC
reaction of nitrile oxides with philodipoles can afford two
regioisomers, each as a pair of enantiomers in which the
relative configuration between the 4- and 5-substituents is
determined by the geometry of the phliodipoles. Due to
O
O
O
OEt
C6F5CHO
EtOH
OH
OEt
N
NH2
NH2
Keywords: 1,3-Dipolar cycloaddition; Nitrile oxides; Pentafluorophenyl-
containing imine; Stereoselectivity.
C6F5
1
L-valine
* Corresponding author. Tel.: +86 21 54925184; fax: +86 21 64166128;
e-mail: zhusz@mail.sioc.ac.cn
Scheme 1.
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.06.119

H. Jiang et al. / Tetrahedron 62 (2006) 11008–11011
Table 1. Results of 1,3-DC reaction of 1 in the presence of TEA
11009
Nitrile oxides are almost always generated in situ in order to
COOEt
avoid dimerization. Generally, triethylamine (TEA) was
utilized prevalently to prepare nitrile oxides in situ from benzo-
hydroximinoyl chlorides.10 On the other hand, NaHCO3
could be employed in some 1,3-DC reactions as an alterna-
tive to TEA.11 In this work, we compared base effects in the
1,3-DC reaction of optically active imine with nitrile oxides.
First, we investigated the 1,3-DC reaction of 1 and p-bromo-
phenyl nitrile oxide in the presence of TEA and the expected
oxadiazole derivative 3a was isolated in a moderate yield
and with good diastereoselectivity and enantioselectivity
(Scheme 2). Furthermore, the X-ray single crystal diffrac-
tion analysis was carried out to get more stereochemistry
information (Fig. 1).
R
COOEt
R
OH
Et3N
N
+
N
N
Cl
CH2Cl2,
0°C-r.t.
N
O
C6F5
deb
C6F5
Entry
1
2
3
2 (R¼)
Time
(d)
Yielda 3
(%)
eec
(%)
(%)
1
2
3
4
5
6
7
2a (p-C6H4Br)
2b (o-C6H4Cl)
2c (p-C6H4F)
2d (o-C6H4F)
2e (p-C6H4Me)
2
3
3
2
2
2
3
3a (56)
3b (44)
3c (45)
3d (54)
3e (42)
3f (57)
Nod
98
95
99
96
91
90
—
90
78
78
68
86
77
—
2f (CH]CHC6H5)
2g (m-C6H4NO2)
a
b
c
d
Isolated yield.
Br
Br
COOEt
N
1H NMR.
Chiral HPLC.
No product was isolated.
COOEt
Et3N
+
N
OH
CH2Cl2,
0°C-r.t.
N
N
O
C6F5
3a (d.e.: 98%; e.e.: 90%)
C6F5
Cl
1
2a
56%
Table 2. Results of 1,3-DC reaction of 1 in the presence of NaHCO3
Scheme 2.
COOEt
R
COOEt
R
OH
N
NaHCO3
Based on the same reaction conditions, a series of structur-
ally diversified nitrile oxides were used in the reaction and
all results are summarized in Table 1. Just like 2a, other
substituted-phenyl nitrile oxides reacted with optically
active imine in the presence of TEA (Table 1, entries 2–5).
The aliphatic nitrile oxide 2f also afforded the oxadiazole
derivative in considerable yield. It showed that all the yields
were not satisfactory and it was attributed to the slow and
uncontrollable decomposition of the imine, because penta-
fluorobenzaldehyde and valinoethylate were detected in
the reaction mixture. It was worthy to note that no product
was isolated when a strong electron-withdrawing group
such as NO2 was present (Table 1, entry 7).
+
N
N
Cl
benzene,
0°C-r.t.
N
O
C6F5
C6F5
Entry
1
2
3
2 (R¼)
Time
(d)
Yielda 3
(%)
deb
(%)
eec
(%)
1
2
3
4
5
6
2a (p-C6H4Br)
2b (o-C6H4Cl)
2c (p-C6H4F)
2d (o-C6H4F)
2e (p-C6H4Me)
1
1
1
1
1
1
3a (90)
3b (78)
3c (84)
3d (74)
3e (54)
3g (69)
97
92
97
95
99
89
57
54
55
55
59
d
2g (m-C6H4NO2)
a
b
c
d
Isolated yield.
1H NMR.
Chiral HPLC.
Due to the limitation of the apparatus the ee value cannot be detected.
Compared with TEA, NaHCO3 shortened the reaction time
obviously from 3 days to 1 day as well as improved the yield
(Table 2, entries 1–5). In addition, it was worthy to note that
nitrile oxide containing the strong electron-withdrawing
group such as nitryl could also afford the 1,2,4-oxadiazole
product in the presence of NaHCO3 (Table 2, entry 6). How-
ever, lower enantioselectivity was observed in all cases,
which could be attributed to the greater racemization of
imine in the presence of NaHCO3. On the other hand, the
value of de in both tables indicated that the ratio of the
diastereoisomers was not influenced by the experimental
conditions.
3. Conclusion
We have demonstrated a new version of enantioselective
1,3-DC reaction of pentafluorophenyl-substituted imines
and nitrile oxides. When TEA was utilized as the base, the
1,2,4-oxadiazole derivatives were isolated in moderate
yields with good enantioselectivity. On the other hand, if
NaHCO3 was employed, higher yields were obtained
even with nitrile oxides substituted by strong electron-
withdrawing group; furthermore, the mild reaction condi-
tions, ease of manipulation, straightforward procedure, and
considerable yield of useful products make this transforma-
tion potentially useful in organic synthesis.
Figure 1. Molecular structure of 3a.

11010
H. Jiang et al. / Tetrahedron 62 (2006) 11008–11011
4. Experimental
66.52, 61.52, 29.71 (CH), 19.46 (CH3), 19.30 (CH3), 14.22
(CH3). MS [ESI] (m/z, %): 521.2 (M++H). IR (cmꢀ1):
2967, 1734, 1522, 1506, 1153, 1003. HRMS calcd for
C21H19BrN2O3F5: 521.0499; found: 521.0494.
4.1. General
Melting points were measured on a Temp-Melt. apparatus
1
and are uncorrected. H, 13C, and 19F NMR spectra were
X-ray data of compound 3a: C21H19BrF5N2O3: FW¼521.28;
temperature 293(2) K; monoclinic, P2(1)/c; wavelength
recorded on Bruker AM-300 or AM-400 instruments with
Me4Si and CFCl3 as the internal standards, respectively.
FTIR spectra were obtained with a Nicolet AV-360 spectro-
photometer. Low resolution mass spectra (LRMS) or high
resolution mass spectra (HRMS) were obtained on a Finni-
gan GC–MS 4021 or a Finnigan MAT-8430 instrument,
respectively, using the electron impact ionization technique
(70 eV). Single crystal X-ray structure analysis was
performed on a Bruker P4 instrument.
˚
˚
˚
˚
0.71 A; a¼11.934(3) A, b¼12.730(3) A, c¼15.089(14) A,
a¼90.00ꢁ, b¼108.035(4)ꢁ, g¼90.00ꢁ; V¼2179.7(9) A;
˚
Z¼4, Dc¼1.589 mg/m3; absorption coefficient 1.954 mmꢀ1
;
F (000)¼1048; 1.79<q<27.00; reflections collected 12,234;
absorption correction empirical; transmission 1.000max
0.5946min; final R indices R1¼0.0511, wR2¼0.0867. The
ꢀ
CCDC number is 612390.
4.3.2. (S)-Ethyl-2-((R)-3-(2-chlorophenyl)-5-(perfluoro-
phenyl)-1,2,4-oxadiazol-4(5H)-yl)-3-methylbutanoate
4.2. Preparation of compound 1
1
3b. Mp: 73–75 ꢁC. H NMR (CDCl3): d 7.59 (1H, s, Ph),
7.53–7.40 (3H, m, Ph), 7.29 (1H, s, CH), 4.30–4.14 (2H,
(S)-Ethyl 2-amino-3-methylbutanoate (50 mmol) and penta-
fluorobenzaldehyde (50 mmol) were refluxed in EtOH
(10 ml) for 20 h. Then TLC analysis showed that the
reaction was over and the product was purified by column
chromatography on silica gel to give compound 1 (92%).
3
m, CH2), 3.52 (1H, d, JHH¼11 Hz, CH), 1.99–1.92 (1H,
3
m, CH), 1.34 (3H, t, JHH¼7 Hz, CH3), 0.89 (3H, d,
3
3JHH¼7 Hz, CH3), 0.84 (3H, d, JHH¼7 Hz, CH3). 19F
NMR (CDCl3): ꢀ142.6 to ꢀ142.79 (2F, m), ꢀ151.45 (1F,
2
t, JFF¼20 Hz), ꢀ160.35 to ꢀ160.54 (2F, m). 13C NMR
(CDCl3): d 169.34 (C]O), 156.22 (C]N), 135.16 (Ph),
131.26 (Ph), 130.45 (Ph), 128.48 (Ph), 126.45 (Ph), 126.34
(Ph), 85.79, 66.44, 61.54, 29.69 (CH), 19.44 (CH3), 19.28
(CH3), 14.19 (CH3). MS [ESI] (m/z, %): 477.2 (M++H). IR
(cmꢀ1): 2970, 1735, 1653, 1523, 1508, 1004. HRMS calcd
for C21H18ClN2O3F5: (M++H) 477.1008; found: 477.0999.
4.2.1. (S,E)-Ethyl 3-methyl-2-(perfluorobenzylidene-
amino)butanoate 1. Yellow oil. H NMR (CDCl3): d 8.27
1
3
(1H, s, CH]), 4.16 (2H, dd, JHH¼14, 7 Hz, CH2), 3.64
3
(1H, d, JHH¼7 Hz, CH), 2.37–2.34 (1H, m, CH), 1.22
3
3
(3H, t, JHH¼7 Hz, CH3), 0.90 (6H, t, JHH¼6 Hz, CH3).
19F NMR (CDCl3): ꢀ142.00 to ꢀ142.11 (2F, m), ꢀ150.61
(1F, t, 2JFF¼20 Hz), ꢀ161.70 to ꢀ161.90 (2F, m).
4.3.3. (S)-Ethyl-2-((R)-3-(4-fluorophenyl)-5-(perfluoro-
phenyl)-1,2,4-oxadiazol-4(5H)-yl)-3-methylbutanoate
4.3. Experimental procedure
1
3c. Mp: 109–111 ꢁC. H NMR (CDCl3): d 7.60 (2H, t,
3JHH¼6 Hz, Ph), 7.298 (1H, s, CH), 7.19 (2H, d, 3JHH¼6 Hz,
Method a: Et3N (0.5 mmol) in 1 mL of CH2Cl2 was added
slowly to the mixture of 1 (0.3 mmol) and nitrile oxide 2a
(0.3 mmol) in 2 mL CH2Cl2 at 0 ꢁC. Then the mixture was
warmed to room temperature. TLC analysis was used to
monitor the reaction. After 2 days, the reaction was com-
pleted. After general work-up, the residue was purified by
column chromatography on silica gel to give the product
3a in a yield of 56%. Method b: the mixture of 1
(0.3 mmol) and nitrile oxide 2a (0.3 mmol) in 2 mL of benz-
ene was added slowly into the mixture of NaHCO3
(0.6 mmol) in 1 mL benzene at 0 ꢁC. Then the mixture
was warmed to room temperature. TLC analysis was used
to monitor the reaction. After 1 day, the reaction was over
completely. After general work-up, the residue was purified
by column chromatography on silica gel to give the product
3a in a yield of 90%.
3
Ph), 4.29–4.14 (2H, m, CH2), 3.51 (1H, d, JHH¼11 Hz,
3
CH), 1.99–1.92 (1H, m, CH), 1.26 (3H, t, JHH¼7 Hz,
3
3
CH3), 0.89 (3H, d, JHH¼7 Hz, CH3), 0.84 (3H, d, JHH
¼
7 Hz, CH3). 19F NMR (CDCl3): ꢀ108.09 (1F), ꢀ142.76 to
2
ꢀ143.92 (2F, m), ꢀ151.73 (1F, t, JFF¼20 Hz), ꢀ160.51
to ꢀ160.70 (2F, m). 13C NMR (CDCl3): d 169.43 (C]O),
156.45 (C]N), 130.56 (Ph), 130.47 (Ph), 116.56 (Ph),
116.33 (Ph), 85.57, 66.40, 61.49, 29.72 (CH), 19.46 (CH3),
19.30 (CH3), 14.21 (CH3). MS [ESI] (m/z, %): 461.2
(M++H). IR (cmꢀ1): 2973, 1735, 1605, 1523, 1508, 1003.
HRMS calcd for C21H18N2O3F6: (M++H) 461.1298; found:
461.1294.
4.3.4. (S)-Ethyl-2-((R)-3-(2-fluorophenyl)-5-(perfluoro-
phenyl)-1,2,4-oxadiazol-4(5H)-yl)-3-methylbutanoate
3d. Oil. H NMR (CDCl3): d 7.56–7.54 (2H, m, Ph), 7.35
1
4.3.1. (S)-Ethyl-2-((R)-3-(4-bromophenyl)-5-(perfluoro-
phenyl)-1,2,4-oxadiazol-4(5H)-yl)-3-methylbutanoate
3a. Mp: 56–57 ꢁC. 1H NMR (CDCl3): d 7.56 (2H, d,
3JHH¼8 Hz, Ph), 7.40 (2H, d, 3JHH¼8 Hz, Ph), 7.19 (1H, s,
(1H, s, CH), 7.29–7.21 (2H, m, Ph), 4.24–4.18 (2H, m,
CH2), 3.38 (1H, d, JHH¼11 Hz, CH), 1.96–1.84 (1H, m,
3
3
3
CH), 1.27 (3H, t, JHH¼7 Hz, CH3), 0.85 (3H, d, JHH
¼
3
7 Hz, CH3), 0.75 (3H, d, JHH¼7 Hz, CH3). 19F NMR
3
CH), 4.21–4.05 (2H, m, CH2), 3.42 (1H, d, JHH¼11 Hz,
(CDCl3): ꢀ111.16 (1F), ꢀ142.59 to ꢀ142.66 (2F, m),
3
2
CH), 1.89–1.85 (1H, m, CH), 1.19 (3H, t, JHH¼7 Hz,
ꢀ151.76 (1F, t, JFF¼20 Hz), ꢀ160.69 to ꢀ160.91 (2F,
CH3), 0.809 (3H, d, 3JHH¼6 Hz, CH3), 0.76 (3H, d, 3JHH
¼
m). 13C NMR (CDCl3): d 169.59 (C]O), 158.99 (C]N),
133.36 (Ph), 133.02 (Ph), 131.25 (Ph), 124.90 (Ph), 116.79
(Ph), 116.58 (Ph), 85.59, 66.22, 61.45, 29.59 (CH), 19.44
(CH3), 19.01 (CH3), 14.09 (CH3). MS [ESI] (m/z, %):
461.2 (M++H). IR (cmꢀ1): 2972, 1734, 1522, 1508, 1003.
6 Hz, CH3). 19F NMR (CDCl3): ꢀ142.72 to ꢀ142.82 (2F,
2
m), ꢀ151.5 (1F, t, JFF¼20 Hz), ꢀ160.42 to ꢀ160.61 (2F,
m). 13C NMR (CDCl3): d 169.36 (C]O), 156.53 (C]N),
132.49 (Ph), 129.85 (Ph), 125.64 (Ph), 123.49 (Ph), 85.69,

H. Jiang et al. / Tetrahedron 62 (2006) 11008–11011
11011
HRMS calcd for C21H18N2O3F6: (M++H) 461.1302; found:
461.1294.
References and notes
1. (a) Torssell, K. B. G. Nitrile Oxides, Nitrones and Nitronates in
Organic Synthesis; VCH: Weinheim, 1988; (b) Frederickson,
M. Tetrahedron 1997, 53, 403–425; (c) Annunziata, R.;
Benaglia, M.; Cinquini, M.; Raimondi, L. Tetrahedron 1993,
4.3.5. (R)-Ethyl-3-methyl-2-((R)-5-(perfluorophenyl)-3-
p-tolyl-1,2,4-oxadiazol-4(5H)-yl)butanoate 3e. Mp: 72–
1
3
73 ꢁC. H NMR (CDCl3): d 7.42 (2H, d, JHH¼7 Hz, Ph),
3
7.28 (2H, d, JHH¼7 Hz, Ph), 7.27 (1H, s, CH), 4.26–4.14
€
49, 8629–8636; (d) Caramella, P.; Grunanger, P. 1,3-Dipolar
3
(2H, m, CH2), 3.56 (1H, d, JHH¼10 Hz, CH), 2.42 (3H, s,
Cycloaddition Chemistry; Padwa, A., Ed.; Wiley: New York,
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3
CH3), 1.96–1.92 (1H, m, CH), 1.28 (3H, t, JHH¼7 Hz,
3
3
CH3), 0.87 (3H, d, JHH¼7 Hz, CH3), 0.84 (3H, d, JHH
¼
7 Hz, CH3). 19F NMR (CDCl3): ꢀ142.63 to ꢀ142.73 (2F,
2
m), ꢀ151.99 (1F, t, JFF¼20 Hz), ꢀ160.68 to ꢀ160.86
2. (a) DeShong, P.; Dicken, C. M.; Leginus, J. M.; Whittle, R. R.
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Uemura, T.; Wada, E. Tetrahedron Lett. 1992, 33, 7889–
7892; (c) Martin, S. F.; Colapret, J. A.; Dappen, M. S.;
(2F, m). 13C NMR (CDCl3): 169.62 (C]O), 157.30
(C]N), 141.48 (Ph), 129.94 (Ph), 128.37 (Ph), 121.48
(Ph), 85.42, 66.31, 61.39, 29.74 (CH), 21.54 (CH3), 19.50
(CH3), 19.31 (CH3), 14.23 (CH3). MS [ESI] (m/z, %):
457.2 (M++H). IR (cmꢀ1): 2966, 1733, 1522, 1507, 1154.
HRMS calcd for C22H21N2O3F5: 456.1472; found:
456.1465.
´
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1
3
59–61 ꢁC. H NMR (CDCl3): d 7.50 (2H, d, JHH¼6 Hz,
Ph), 7.42–7.30 (3H, m, Ph), 7.25 (1H, s, CH), 7.24 (1H, d,
3JHH¼11 Hz, CH]), 6.56 (1H, d, JHH¼11 Hz, CH]),
3
3
4.24–4.19 (2H, m, CH2), 3.69 (1H, d, JHH¼10 Hz, CH),
3
2.09–1.98 (1H, m, CH), 1.27 (3H, t, JHH¼7 Hz, CH3),
3
3
0.96 (3H, d, JHH¼7 Hz, CH3), 0.86 (3H, d, JHH¼7 Hz,
CH3). 19F NMR (CDCl3): ꢀ142.54 to ꢀ142.67 (2F, m),
2
ꢀ151.81 (1F, t, JFF¼20 Hz), ꢀ160.72 to ꢀ160.91 (2F,
m). 13C NMR (CDCl3): d 169.65 (C]O), 155.08 (C]N),
138.75 (Ph), 138.57, 135.13 (Ph), 129.68 (Ph), 129.57
(Ph), 128.88 (Ph), 127.37, 110.04, 86.07, 66.41, 61.48,
29.86 (CH), 19.55 (CH3), 19.29 (CH3), 14.15 (CH3). MS
[ESI] (m/z, %): 469.2 (M++H). IR (cmꢀ1): 2970, 1735,
1653, 1522, 1508, 1003. HRMS calcd for C23H21N2O3F5:
(M++H) 469.1559; found: 469.1545.
4.3.7. (S)-Ethyl-3-methyl-2-((R)-3-(3-nitrophenyl)-5-
(perfluorophenyl)-1,2,4-oxadiazol-4(5H)-yl)butanoate
1
3g. Mp: 87–89 ꢁC. H NMR (CDCl3): d 8.46 (1H, s, Ph),
8.41 (1H, d, 3JHH¼8 Hz, Ph), 7.98 (1H, d, 3JHH¼7 Hz, Ph),
3
7.75 (1H, d, JHH¼7 Hz, Ph), 7.34 (1H, s, CH), 4.32–4.18
3
(2H, m, CH2), 3.50 (1H, d, JHH¼11 Hz, CH), 2.02–1.98
3
(1H, m, CH), 1.27 (3H, t, JHH¼7 Hz, CH3), 0.96–0.86
(6H, m, CH3). 19F NMR (CDCl3): ꢀ142.75 to ꢀ142.85
2
(2F, m), ꢀ151.03 (1F, t, JFF¼20 Hz), ꢀ160.08 to
ꢀ160.26 (2F, m). 13C NMR (CDCl3): d 169.03 (C]O),
155.59 (C]N), 148.64 (Ph), 133.91 (Ph), 130.44 (Ph),
126.51 (Ph), 125.96 (Ph), 123.25 (Ph), 86.08, 66.67, 61.72,
29.67 (CH), 19.40 (CH3), 19.26 (CH3), 14.18 (CH3). MS
[ESI] (m/z, %): 488 (M++H). IR (cmꢀ1): 2970, 1736,
1730, 1523, 1508, 1350, 1003. HRMS calcd for
C20H18N3O5F5: 487.1167; found: 487.1174.
10. (a) Molteni, G.; Buttero, P. D. Tetrahedron: Asymmetry 2005,
16, 1983–1987; (b) Gerald, E.; Claude, T.; Claude, D.; Yves,
C. Tetrahedron: Asymmetry 2005, 16, 2459–2474; (c) Xu,
W. M.; Tang, E.; Huang, X. Tetrahedron 2005, 61, 501–506.
11. (a) Cinquini, E.; Freccero, M.; Gandolfi, R.; Amade’, S. M.;
Rastelli, A. Tetrahedron 1997, 53, 9279–9292; (b) Denmark,
S. E.; Kallemeyn, J. M. J. Org. Chem. 2005, 70, 2839–2842;
(c) Zorn, C.; Anichini, B.; Goti, A.; Brandi, A.; Kozhushkov,
S. I.; Meijere, A.; Citti, L. J. Org. Chem. 1999, 64, 7846–7855.
Acknowledgements
Financial support for this project was provided by The
National Natural Science Foundation of China (NNSFC)
(Nos. 20472106 and 20532040), and Pudong Science and
Technology Bureau (No. PKZ2004-03).