A novel resin-bound CF3-containing building block: Application to the solid-phase synthesis of CF3-containing acrylates and 4-isoxazolecarboxylates

Article abstract of DOI:10.1016/S0022-1139(01)00461-4

A novel resin-bound CF₃-containing building block (3) was prepared. Stereospecific Suzuki cross-coupling of compound 3 with a variety of aryl boronic acids gave the corresponding resin-bound coupling products (4), which can be cleaved with sodium methoxide to afford methyl (E)-3-aryl-4,4,4-trifluoro-2-butenoate (5). 1,3-Dipolar cycloaddition of compound 3 with aryl nitrile oxides followed by cleavage with TFA led to methyl 3-aryl-5-trifluoromethyl-4-isoxazolecarboxylate regioselectively, which was determined by ¹⁹F NMR analysis.

Full text of DOI:10.1016/S0022-1139(01)00461-4

Journal of Fluorine Chemistry 111 *2001) 241±246
A novel resin-bound CF3-containing building block: application
to the solid-phase synthesis of CF3-containing acrylates
and 4-isoxazolecarboxylates
*
Hong-Jun Wang, Wen Ling, Long Lu
Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Lu, Shanghai 200032, PRChina
Received 24 April 2001; accepted 28 June 2001
Abstract
A novel resin-bound CF3-containing building block *3) was prepared. Stereospeci®c Suzuki cross-coupling of compound 3 with a variety
of aryl boronic acids gave the corresponding resin-bound coupling products *4), which can be cleaved with sodium methoxide to afford
methyl *E)-3-aryl-4,4,4-tri¯uoro-2-butenoate *5). 1,3-Dipolar cycloaddition of compound 3 with aryl nitrile oxides followed by cleavage
with TFA led to methyl 3-aryl-5-tri¯uoromethyl-4-isoxazolecarboxylate regioselectively, which was determined by 19F NMR analysis.
# 2001 Elsevier Science B.V. All rights reserved.
Keywords: Tri¯uoromethyl; Solid-phase synthesis; 1,3-Dipolar cycloaddition
1. Introduction
dithionite-initiated addition on reaction of 1,1,1-tribromo-
2,2,2-tri¯uoroethane with vinyl ethyl ether followed by
Jones' oxidation [19], and is considered as a suitable
CF3-containing building block *Scheme 1).
Solid-phase organic synthesis *SPOS) has attracted much
attention over the past decade [1±4], as a powerful tool in the
rapid synthesis of small molecule libraries for drug discov-
ery [5±10]. It is known that introduction of ¯uorine into an
organic molecule often leads to changes in biological activ-
ities [11,12]. Recently, great efforts have been made in the
search for practical and ef®cient methods for the synthesis of
selectively ¯uorinated organic compounds [13,14]. Due to
our continuing interest in organo¯uorine chemistry [15±18],
we tried to investigate the preparation and application of
polymer-supported ¯uoro-containing building blocks.
Herein we report the preliminary results for the preparation
of a novel resin-bound CF3-containing building block and
its palladium-catalyzed cross-coupling reactions with aryl
boronic acids and 1,3-dipolar cyclo-addition reaction with
aryl nitrile oxides.
For attaching 3,3-dibromo-4,4,4-tri¯uorobutyric acid *1)
onto Wang resin *Scheme 1), initial attempts using 1-*3-
dimethylaminopropyl)-3-ethylcarbodiimide hydrchloride,
1-hydroxybenzotriazole hydrate and diisopropylethylamine
*EDC-HOBt-DIPEA) or Mitsunobu reaction conditions
have been made but gave very poor results. So we activated
Wang resin by using Hanessian's procedure [20], namely
treatment of Wang resin with trichloroacetonitrile in CH2Cl2
in the presence of 1,8-diazabicyclo [5.4.0]undec-7-ene
*DBU) to give Wang trichloroacetimidate resin 2, which
reacted with 3,3-dibromo-4,4,4-tri¯uorobutyric acid *1)
in CH2Cl2 followed by elimination with triethylamine
to afford the corresponding resin-bound CF3-containing
building block 3. The reaction of Wang trichloroacetimi-
date resin 2 with acid 1 was monitored by FT-IR spectro-
scopy which showed the disappearance of bands *3340,
2. Results and discussion
1664 cmÀ1
) associated with trichloroacetimidate and
appearance of a strong band at 1749 cmÀ1. Similarly, 19F
NMR indicated that the con®guration of the double bond
was Z form [19] after the cleavage from the resin. The
structure and loading of the resin-bound CF3-containing
building block 3 was con®rmed by cleavage with MeONa
in MeOH/THF *1:4) [21] to give the corresponding methyl
ester [19] *Scheme 2).
3,3-Dibromo-4,4,4-tri¯uorobutyrid acid *1) was prepa-
red previously as an intermediate through the sodium
* Corresponding author. Tel.: ‡86-21-64163300;
fax: ‡86-21-64166128.
E-mail address: lulong@pub.sioc.ac.cn *L. Lu).
0022-1139/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 1 1 3 9 * 0 1 ) 0 0 4 6 1 - 4

242
H.-J. Wang et al. / Journal of Fluorine Chemistry 111 -2001) 241±246
Scheme 1.
Scheme 2.
Suzuki coupling reaction of the resin-bound CF3-contain-
For compound 5d, a byproduct was also isolated in yield
of 26% based on the initial loading of the Wang resin
*1.7 mmol/g) and proved to be the Michael addition product
*6) of 5d with methanol *Scheme 4).
Solid-phase 1,3-dipolar cycloaddition reaction has been
well studied recently [29±38] as a very powerful method
to synthesize a variety of heterocycle compounds such
as isoxazolines, isoxazoles, pyrrolidines, tetrahydrofurans
and so on. For the preparation of CF3-containing hetero-
cycles, the 1,3-dipolar cycloaddition reaction of CF3-
containing building block 3 with aryl nitrile oxides was
studied in detail. A variety of oximes 7, which are the
precursors of nitrile oxides in the presence of base, were
ing building block 3 with aryl boronic acids was ®rst
investigated. Recently, solid-phase Suzuki coupling reac-
tions have been widely investigated [22±28]. Under normal
Suzuki conditions, the coupling reaction with a variety of
aryl boronic acids proceeded smoothly, affording the corre-
sponding resin-bound coupling products 4 in good yields.
Treatment of resin-bound coupling products with MeONa
in MeOH/THF *1:4) followed by the puri®cation on a silica
gel column led to the corresponding methyl esters 5, which
1
are characterized by MS, H NMR, 19F NMR and HRMS,
etc. *Scheme 3). The detailed results of Suzuki coupling
reactions were summarized in Table 1.
Scheme 3.

H.-J. Wang et al. / Journal of Fluorine Chemistry 111 -2001) 241±246
243
Table 1
Suzuki coupling reactions of resin-bound CF3-containing building block 3
‡
Entry
1
ArB*OH)2
Final coupling products
MS *EI) *M )
Yielda *%)
230
43
2
236
27
3
4
5
260
275
274
35
49
36
6
7
248
260
33
42
8
276
49
a Isolated yields were based on the initial loading of Wang resin *1.7 mmol/g).
prepared according to the literature procedures [39].
The CF3-containing building block 3 reacted with oximes
in the presence of Et3N in DMF, affording a mixture of
1,3-dipolar cycloaddition products *8 and 9) after cleavage
with 10% TFA/DCM solution followed by the treatment
with diazomethane in Et2O *Scheme 5). The ratio of the
two isomers was determined by 19F NMR analysis and
the detailed result summarized in Table 2. The major
isomers, 3-aryl-5-tri¯uoromethyl-4-isoxazolecarboxylate
*8), were isolated by chromatography on a silica gel
column, and characterized by MS *EI), HRMS, 1H and
19F NMR [40].
Scheme 4.

244
H.-J. Wang et al. / Journal of Fluorine Chemistry 111 -2001) 241±246
Scheme 5.
Table 2
1,3-Dipolar cycloaddition reaction of CF3-containing building block 3 with oximes
Entry
R±
dCF of 8 *ppm)
dCF of 9 *ppm)
Ratio of 8/9a
Yieldb *%)
3
3
1
2
3
4
5
6
7
MeO
EtO
Me
21.9
21.9
22.0
22.0
22.0
22.0
21.9
14.0
14.0
14.0
14.1
14.1
14.0
14.0
13:1
13:1
14:1
8:1
48 *8a)
43 *8b)
32 *8c)
27 *8d)
21 *8e)
38 *8f)
34 *8g)
H
Cl
18:1
14:1
13:1
PhO
BnO
a The ratio of 8/9 was determined by 19F NMR analysis of the crude methyl ester mixture.
b Isolated yields based on initial loading of Wang resin *1.7 mmol/g).
3. Conclusions
nated compound libraries by using resin-bound ¯uorinated
building blocks is on the way.
3,3-Dibromo-4,4,4-tri¯uorobutyric acid, which was iso-
lated for the ®rst time, reacted with Wang resin through
trichloroacetonitrile activation to afford a novel resin-bound
CF3-containing building block *Z)-Wang 3-bromo-4,4,4-tri-
¯uoro-2-butenoate. The resin-bound CF3-containing build-
ing block underwent Suzuki coupling reactions with a
variety of aryl boronic acids to give methyl *E)-3-aryl-
4,4,4-tri¯uoro-2-butenoates in good yield. Furthermore, a
series of methyl 3-aryl-5-tri¯uoromethyl-4-isoxazolecar-
boxylates were prepared by 1,3-dipolar cycloaddition reac-
tions of *Z)-Wang 3-bromo-4,4,4-tri¯uoro-2-butenoate with
the precursors of aryl nitrile oxides. Construction of ¯uori-
4. Experimental
4.1. General
Wang resin *1.7 mmol/g) was obtained from Polymer
1
Laboratories Inc. H and 19F NMR spectra were recorded
at 300 and 282 MHz, respectively, in CDCl3 solutions with a
Bruker AM300 instrument. Chemical shifts are expressed in
ppm down®eld from internal tetramethylsilane *for 1H NMR)
or tri¯uoroacetic acid *for 19F NMR). Mass spectra were

H.-J. Wang et al. / Journal of Fluorine Chemistry 111 -2001) 241±246
245
obtained on a HP5989A analytical spectrometer. Infrared
spectra were recorded on a Bio-Rad FTS-185 spectrometer.
Methyl *E)-3-*3-nitrophenyl)-4,4,4-tri¯uoro-2-butenoate
*5d): oil, yield: 49%. 1H NMR d 8.32 *m, 1H), 8.18
*s, 1H), 7.64 *m, 2H), 6.73 *d, J ˆ 1:5 Hz, 1H), 3.65
*s, 3H) ppm; 19F NMR d 9.4 *s, 3F) ppm; MS *EI) m/z
275 *M ). HRMS *EI) C11H8NF3O4, calcd.: 275.0405;
found: 275.0410.
4.1.1. Preparation of resin-bound CF3-containing
building block -3)
‡
Resin 2 was prepared from Wang resin according to
Hanessian's method [20]. For high loaded Wang resin
*1.70 mmol/g), the reaction should be repeated. Theoretical
loading for resin 2 was 1.31 mmol/g. The CF3-containing
acid was prepared according to the literature [19].
To a suspension of resin 2 *3.0 g, 3.93 mmol) in CH2Cl2
*30 ml), acid 1 *3.54 g, 11.79 mmol) was added. The result-
ing mixture was shaken for 30 h at 258C. The resin was
®ltered, washed successively with CH2Cl2, DMSO, THF and
CH2Cl2 *three times for each solvent) and dried in vacuum.
To a suspension of this resin intermediate in CH2Cl2 *30 ml),
Et3N *1.64 ml, 11.79 mmol) was added. The mixture was
shaken for 8 h at 258C. The resin was ®ltered, washed
successively with CH2Cl2, DMSO, THF and CH2Cl2 *three
times for each solvent) and dried in vacuum to give to give
resin-bound CF3-containing building block *3) *3.19 g).
Theoretical loading for resin 3 was 1.23 mmol/g.
Methyl *E)-3-*3-ethoxyphenyl)-4,4,4-tri¯uoro-2-buteno-
1
ate *5e): oil, yield: 36%. H NMR d 7.30 *m, 1H), 6.95
*dd, J ˆ 8:2 Hz, 2.3 Hz, 1H), 6.83 *m, 2H), 6.59 *d,
J ˆ 1:4 Hz, 1H), 4.03 *q, J ˆ 7:0 Hz, 2H), 3.62 *s, 3H),
1.41 *t, J ˆ 7:0 Hz, 3H) ppm; 19F NMR d 9.5 *s, 3F) ppm;
‡
MS *EI) m/z 274 *M ).
Methyl *E)-3-*4-¯uorophenyl)-4,4,4-tri¯uoro-2-buteno-
1
ate *5f): oil, yield: 33%. H NMR d 7.28 *m, 2H), 7.11
*m, 2H), 6.62 *d, J ˆ 1:4 Hz, 1H), 3.63 *s, 3H) ppm; 19F
NMR d 9.3 *s, 3F), À34.4 *m, 1F) ppm; MS *EI) m/z 248
‡
*M ). HRMS *EI) C11H8F4O2, calcd.: 248.0460; found:
248.0463.
Methyl *E)-3-*4-methoxyphenyl)-4,4,4-tri¯uoro-2-bute-
noate *5g): oil, yield: 42%. 1H NMR d 7.24 *d, J ˆ 8:7 Hz,
2H), 6.93 *m, 2H), 6.57 *d, J ˆ 1:0 Hz, 1H), 3.84 *s, 3H),
3.64*s, 3H)ppm; 19F NMRd 9.6*s, 3F)ppm; MS*EI) m/z 260
‡
*M ).
4.1.2. General procedure for the Suzuki coupling
Methyl *E)-3-*4-methylthiophenyl)-4,4,4-tri¯uoro-2-bute-
1
To a degassed suspension of resin-bound CF3-containing
building block *3, 200 mg, 0.246 mmol) in xylene *5.0 ml),
Pd*PPh3)4 *29 mg, 0.025 mmol), 2 M Na2CO3 *0.31 ml,
0.615 mmol), EtOH *0.2 ml) and aryl boronic acid
*0.984 mmol, 4.0 eq.) were added. The resulting mixture
was stirred under argon for 24 h at 808C. After cooling to
room temperature, the resin was ®ltered, washed succes-
sively with DMF, DMF/H2O *1:1), THF, MeOH and CH2Cl2
*three times for each solvent) and dried in vacuum to give
resin 4. Resin 4 was treated with a solution of MeONa
*0.05 mmol) in MeOH/THF *1:4, 4.0 ml) for 5 h, removed
by ®ltration and washed with THF *5.0 ml) and CH2Cl2
*5.0 ml). The combined ®ltrates were concentrated and
puri®ed by ¯ash chromatography to give methyl *E)-3-
aryl-4,4,4-tri¯uoro-2-butenoates.
noate *5h): oil, yield: 49%. H NMR d 7.24 *m, 4H), 6.60
*d,J ˆ 1:3 Hz,1H),3.64*s,3H),2.50*s,3H)ppm;19FNMRd
‡
9.6 *s, 3F) ppm; MS *EI) m/z 276 *M ). HRMS *EI)
C12H11F3O2S, calcd.: 276.0432; found: 276.0627.
Methyl 3-methoxy-3-*3-nitrophenyl)-4,4,4-tri¯uorobuty-
1
rate *6): oil, yield: 28%. H NMR d 8.46 *s, 1H), 8.26 *dd,
J ˆ 8:2 Hz, 2.0 Hz, 1H), 7.90 *d, J ˆ 8:2 Hz, 1H), 3.59 *s,
3H), 3.60 *s, 3H), 3.30 *dd, J ˆ 46:1 Hz, 15.7 Hz, 2H) ppm;
‡
19F NMR d 1.57 *s, 3F) ppm; MS *EI) m/z 307 *M ). HRMS
*EI) C12H12NF3O5, calcd.: 307.0667; found: 307.0660.
4.1.3. General procedure for the 1,3-dipolar
cycloaddition reaction
To a suspension of resin-bound CF3-containing building
block 3 *200 mg, 0.246 mmol) in DMF, chloro oxime 7
*5.0 eq.) and Et3N *0.51 ml, 3.69 mmol) were added. The
mixture was shaken for 12 h. After that, the resin was
®ltered, washed successively with DMF, DMF/H2O *1:1),
THF, MeOH and CH2Cl2 *three times for each solvent),
dried in vacuum. The resin was treated with 10% TFA/DCM
for 3 h, removed by ®ltration and washed with CH2Cl2
*5:0 ml  2). The combined ®ltrates were concentrated.
The residue was then treated with CH2N2/Et2O *>0.1 N,
2.0 ml) for 10 min. The solution was concentrated to
give the crude ester mixture. The ratio was determined by
19F NMR analysis. Chromatography of the crude ester
mixture gave the major products, 3-aryl-5-tri¯uoromethyl-
4-isoxazolecarboxylates *8).
Methyl *E)-3-phenyl-4,4,4-tri¯uoro-2-butenoate *5a): oil,
yield: 43%. 1H NMR d 7.42 *m, 3H), 7.34 *m, 2H), 6.61 *dd,
J ˆ 2:6 Hz, 1.4 Hz, 1H), 3.61 *s, 3H) ppm; 19F NMR d 9.4
‡
*s, 3F) ppm; MS *EI) m/z 230 *M ). HRMS *EI) C11H9F3O2,
calcd.: 230.0555; found: 230.0561.
Methyl *E)-3-*3-thienyl)-4,4,4-tri¯uoro-2-butenoate *5b):
1
oil, yield: 27%. H NMR d 7.49 *d, J ˆ 2:7 Hz, 1H), 7.36
*m, 1H), 7.12 *d, J ˆ 5:0 Hz, 1H), 6.57 *d, J ˆ 1:4 Hz, 1H),
3.70 *s, 3H) ppm; 19F NMR d 9.7 *s, 3F) ppm; MS *EI) m/z
236 *M ). HRMS *EI) C9H7F3O2S, calcd.: 236.0119; found:
236.0110.
‡
Methyl *E)-3-*3-methoxyphenyl)-4,4,4-tri¯uoro-2-bute-
1
noate *5c): oil, yield: 35%. H NMR d 7.32 *t, J ˆ 8:0 Hz,
1H), 6.97 *m, 1H), 6.85 *m, 2H), 6.60 *d, J ˆ 1:5 Hz, 1H),
Methyl 3-*4-methoxyphenyl)-5-tri¯uoromethyl-4-isoxa-
zolecarboxylate *8a): oil, yield: 48%. 1H NMR d 7.64
*dd, J ˆ 7:0 Hz, 2.0 Hz, 2H), 7.00 *dd, J ˆ 6:7 Hz,
2.0 Hz, 2H), 3.89 *s, 3H), 3.87 *s, 3H) ppm; 19F NMR d
3.82*s, 3H), 3.62*s, 3H)ppm; 19F NMRd 9.5*s, 3F)ppm; MS
*EI) m/z 260 *M ). HRMS *EI) C12H11F3O3, calcd.:
260.0660; found: 260.0661.
‡

246
H.-J. Wang et al. / Journal of Fluorine Chemistry 111 -2001) 241±246
‡
[3] P.H.H. Hermkens, H.C.J. Ottenheijm, D. Rees, Tetrahedron 52 *1996)
4527.
14.0 *s, 3F) ppm; MS *EI) m/z 301 *M ). HRMS *EI)
C13H10NF3O4, calcd.: 301.0562; found: 301.0570.
[4] C.J. Andres, D.L. Whitehouse, M.S. Deshpande, Curr. Opin. Chem.
Biol. 2 *1998) 353.
Methyl 3-*4-ethoxyphenyl)-5-tri¯uoromethyl-4-isoxazo-
lecarboxylate *8b): oil, yield: 43%. 1H NMR d 7.63 *dd, J ˆ
6:7 Hz, 2.0 Hz, 2H), 6.99 *dd, J ˆ 7:0 Hz, 2.0 Hz, 2H), 4.1
*q, J ˆ 7:0 Hz, 2H), 3.90 *s, 3H), 1.46 *t, J ˆ 7:0 Hz, 3H)
[5] R.G.J. Franze, Comb. Chem. 2 *2000) 195.
[6] R.E.J. Dolle, Comb. Chem. 2 *2000) 383.
[7] R.E. Dolle, K.H. Nelson Jr., J. Comb. Chem. 1 *1999) 235.
[8] A. Nefzi, J.M. Ostresh, R.A. Houghten, Chem. Rev. 97 *1997) 449.
[9] L.A. Thompson, J.A. Ellman, Chem. Rev. 96 *1996) 555.
[10] J.S. Fruchtel, G. Jung, Angew. Chem. Int. Ed. Engl. 35 *1996) 17.
[11] J.T. Welch, Tetrahedron 43 *1987) 3123.
‡
ppm; 19F NMR d 14.0 *s, 3F) ppm; MS *EI) m/z 315 *M ).
Methyl 3-*4-methylphenyl)-5-tri¯uoromethyl-4-isoxazo-
lecarboxylate *8c) [40]: oil, yield: 32%. 1H NMR d 7.57 *m,
2H), 7.30 *d, J ˆ 8:6 Hz, 2H) 3.89 *s, 3H), 2.43 *s, 3H) ppm;
[12] J.T. Welch, S. Eswarakrishnan, Fluorine in Bioorganic Chemistry,
Wiley, New York, 1991.
‡
19F NMR d 14.0 *s, 3F) ppm; MS *EI): m/z 285 *M ).
[13] J.T. Welch, Selective Fluorination in Organic and Bioorganic
Chemistry, American Chemical Society, Washington, DC, 1991.
[14] M. Hudlicky, Chemistry of Organic Fluorine Compounds, Ellis
Horwood, Chichester, UK, 1976.
Methyl 3-phenyl-5-tri¯uoromethyl-4-isoxazolecarboxy-
1
late *8d) [40]: oil, yield: 27%. H NMR d 7.67 *m, 2H),
7.52 *m, 3H), 3.88 *s, 3H) ppm; 19F NMR d 14.1 *s, 3F) ppm;
MS *EI): m/z 271 *M ).
‡
[15] Q.S. Zhang, L. Lu, Tetrahedron Lett. 41 *2000) 8545.
[16] L. Lu, D.J. Burton, Tetrahedron Lett. 38 *1997) 7673.
[17] L. Xue, L. Lu S. Pederson, Q. Liu, R. Narske, D.J. Burton, J. Org.
Chem. 62 *1997) 1064.
Methyl 3-*4-chlorophenyl)-5-tri¯uoromethyl-4-isoxazo-
1
lecarboxylate *8e) [40]: oil, yield: 21%. H NMR d 7.64
*m, 2H), 7.48 *m, 2H), 3.90 *s, 3H) ppm; 19F NMR d 14.1 *s,
3F) ppm; MS *EI): m/z 307, 305 *M ).
[18] Y. Wang, L. Lu, to be published.
‡
[19] W.Y. Huang, L. Lu, Chin. Chem. Lett. 2 *1991) 769.
[20] S. Hanessian, F. Xie, Tetrahedron Lett. 39 *1998) 733.
[21] R. Frenette, R.W. Friesen, Tetrahedron Lett. 35 *1994) 9177.
[22] H. Juteau, Y. Gareau, M. Labelle, S. Lamontagne, N. Tremblay, M.-
C. Carriere, N. Sawyer, D. Denis, K.M. Metters, Bioorg. Med. Chem.
Lett. 11 *2001) 747.
Methyl 3-*4-phenoxyphenyl)-5-tri¯uoromethyl-4-isoxa-
zolecarboxylate *8f): oil, yield: 38%. 1H NMR d 7.66
*dd, J ˆ 6:8 Hz, 2.0 Hz, 2H), 7.40 *m, 2H), 7.20 *m, 1H),
7.09 *m, 4H), 3.90 *s, 3H) ppm; 19F NMR d 14.0 *s, 3F) ppm;
MS *EI) m/z 363 *M ). HRMS *EI) C18H12NF3O4, calcd.:
363.0718; found: 363.0726.
‡
[23] C.A. Briehn, T. Kirschbaum, P. Bouerle, J. Org. Chem. 65 *2000)
352.
[24] M. Gravel, C.D. Berube, D.G. Hall, J. Comb. Chem. 2 *2000) 228.
[25] W. Li, K. Burgess, Tetrahedron Lett. 40 *1999) 6527.
[26] S.-E. Yoo, Y.-D. Gong, M.-Y. Choi, J.-S. Seo, K.Y. Yi, Tetrahedron
Lett. 41 *2000) 6415.
Methyl 3-*4-benzyloxyphenyl)-5-tri¯uoromethyl-4-iso-
1
xazolecarboxylate *8g): oil, yield: 34%. H NMR d 7.65
*m, 2H), 7.42 *m, 5H), 7.09 *m, 2H), 5.14 *s, 2H), 3.90 *s,
3H) ppm; 19F NMR d 14.0 *s, 3F) ppm; MS *EI) m/z 377
*M ). HRMS *EI) C19H14NF3O4, calcd.: 377.0875; found:
377.0882.
[27] C.M. Huwe, H. Kunzer, Tetrahedron Lett. 40 *1999) 683.
[28] C.G. Blettner, W.A. Kinig, G. Ruhter, W. Stenzel, T. Schotten,
Synlett. *1999) 307.
‡
[29] W.J. Haap, D. Kaiser, T.B. Walk, G. Jung, Tetrahedron 54 *1998)
3705.
[30] J.-F. Cheng, A.M.M. Mjalli, Tetrahedron Lett. 39 *1998) 939.
[31] S. Kobayashi, R. Akiyama, Tetrahedron Lett. 39 *1998) 9211.
[32] B.B. Shankar, D.Y. Yang, S. Girton, A.K. Ganguly, Tetrahedron Lett.
39 *1998) 2447.
Acknowledgements
We thank the National Natural Science Foundation of
China *Grant nos. 29825104 and 29632003) and the Chinese
Academy of Sciences for ®nancial support of this work.
[33] E.J. Kantorowski, M.J. Kurth, J. Org. Chem. 62 *1997) 6797.
[34] D.L. Whitehouse, K.H. Nelson Jr., S.N. Savinov, D.J. Austin,
Tetrahedron Lett. 38 *1997) 7139.
[35] M.R. Gowravaram, M.A. Gallop, Tetrahedron Lett. 38 *1997) 6973.
[36] B.C. Hamper, D.R. Dukesherer, M.S. South, Tetrahedron Lett. 37
*1996) 3671.
References
[37] A.M.M. Mjalli, S. Sarshar, T.J. Baiga, Tetrahedron Lett. 37 *1996)
2943.
[1] S. Booth, P.H.H. Hermkens, H.C.J. Ottenheijm, D. Rees, Tetrahedron
54 *1998) 15385.
[38] D.A. Goff, Tetrahedron Lett. 40 *1999) 8741.
[39] K.C. Liu, B.R. Shelton, R.K. Howe, J. Org. Chem. 45 *1980) 3916.
[40] Y.C. Shen, J.H. Zheng, Y.Z. Huang, Synthesis *1985) 970.
[2] P.H.H. Hermkens, H.C.J. Ottenheijm, D. Rees, Tetrahedron 53 *1997)
5647.