Asymmetric catalysis of the Friedel-Crafts reaction with fluoral by chiral binaphthol-derived titanium catalysts through asymmetric activation

Full text of DOI:10.1021/jo991691o

J . Org. Chem. 2000, 65, 1597-1599
1597
Sch em e 1
Asym m etr ic Ca ta lysis of th e F r ied el-Cr a fts
Rea ction w ith F lu or a l by Ch ir a l
Bin a p h th ol-Der ived Tita n iu m Ca ta lysts
th r ou gh Asym m etr ic Activa tion
Akihiro Ishii,† Vadim A. Soloshonok, and
Koichi Mikami*
Department of Chemical Technology, Tokyo Institute of
Technology, Ookayama, Meguro-ku, Tokyo 152-8552, J apan
report a practical synthetic route to chiral 1-aryl-2,2,2-
trifluoroethanol derivatives of synthetic importance8
through the F-C reaction with fluoral using chiral
binaphthol-derived titanium (BINOL-Ti) catalysts9 via
asymmetric activation.10 In combination with chiral
activators, the catalytic activity and enantioselectivity of
BINOL-Ti catalysts can be enhanced (Scheme 1).
Received October 27, 1999
In tr od u ction
Asymmetric synthesis of organofluorine compounds is
an important issue in pharmaceutical chemistry1 and
optoelectronic material science.2 In particular, asym-
metric catalysis of carbon-carbon bond forming reactions
is the most attractive method, because the carbon
skeleton of chiral organofluorine molecules can be con-
structed at the time of asymmetric induction.3 The
Friedel-Crafts (F-C) reaction is one of the most funda-
mental carbon-carbon bond forming reactions in organic
synthesis.4 However, its application to catalytic asym-
metric synthesis has been quite limited.5,6,7 Herein, we
Resu lt a n d Discu ssion
In the F-C reaction, the catalytic activity and enan-
tioselectivity of BINOL-Ti catalysts11 were found to be
critically influenced by the substituents of BINOL de-
rivatives (Table 1). (1) (R)-6,6′-Br2-BINOL-Ti catalyst was
the most effective catalyst. This F-C reaction did not
proceed easily as compared with the carbonyl-ene
reaction3d,e or the Mukaiyama-aldol reaction3d with fluo-
ral. Therefore, the role of the electron-withdrawing group
at the 6,6′-position of BINOL was found to be very
important for increasing the Lewis acidity (runs 1-3).
Relatively high enantioselectivity was obtained even
when using 1 mol % of (R)-6,6′-Br2-BINOL-Ti catalyst
(run 4). (2) Polar solvent was more effective for producing
higher para regioselectivity (run 5). When toluene was
used as a solvent, the enantio-enriched adducts of fluoral
to toluene were also obtained, along with the expected
F-C product with anisole. (3) Interestingly, a lower
reaction temperature leads to a decrease in the enantio-
selectivity of para-isomer, presumably because of the
oligomeric nature of the BINOL-Ti catalysts at lower
temperature (run 6). (4) The steric bulkiness of the alkyl
ether portion of the aromatic substrates was essential
for producing higher para regioselectivity (run 7). Inter-
estingly, the bis-adduct with fluoral was not obtained
even when using a large excess of fluoral in the reaction
of diphenyl ether (run 8).
* To whom correspondence should be addressed.
† On leave from Chemical Research Center, Central Glass Co. Ltd.,
Kawagoe, Saitama 350-1151.
(1) Reviews: (a) Ojima, I.; McCarthy, J . R.; Welch, J . T., Eds.
Biomedical Frontiers of Fluorine Chemistry; American Chemical
Society: Washington, D. C., 1996. (b) Resnati, G. Tetrahedron 1993,
49, 9385.
(2) Reviews: (a) Resnati, G.; Soloshonok, V. A., Eds. Tetrahedron
1996, 52, 1. (b) Olah, G. A.; Chambers, R. D.; Prakash, G. K. S.
Synthetic Fluorine Chemistry; Wiley: New York, 1992.
(3) (a) Reviews: Iseki, K. Tetrahedron 1998, 54, 13887. (b) Iseki,
K.; Kuroki, Y.; Kobayashi, Y. Tetrahedron Lett. 1997, 38, 7209. (c) Iseki,
K.; Oishi, S.; Sasai, H.; Shibasaki, M. Tetrahedron Lett. 1996, 37, 9081.
(d) Mikami, K.; Yajima, T.; Takasaki, T.; Matsukawa, S.; Terada, M.;
Uchimaru, T.; Maruta, M. Tetrahedron 1996, 52, 85. (e) Mikami, K.;
Yajima, T.; Terada, M.; Uchimaru, T. Tetrahedron Lett. 1993, 34, 7591.
(f) Soloshonok, V. A.; Hayashi, T. Tetrahedron Lett. 1994, 35, 2713.
(g) Hayashi, T.; Soloshonok, V. A. Tetrahedron: Asymmetry 1994, 5,
1091, and references therein.
(4) Reviews: (a) Smith, M. B. Organic Synthesis; McGraw-Hill: New
York, 1994;
p 1313. (b) Heaney, H. In Comprehensive Organic
Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon Press: Oxford,
1991; Vol. 2, p 733. (c) Roberts, R. M.; Khalaf, A. A. in Friedel-Crafts
Alkylation Chemistry. A Century of Discovery; Dekker: New York,
1984. (d) Olah, G. A. Friedel-Crafts Chemistry; Wiley-Interscience:
New York, 1973.
The sense of asymmetric induction was the same as
observed in BINOL-Ti-catalyzed asymmetric reactions
such as the carbonyl-ene reaction11,13 and the Mu-
(5) Diastereoselective F-C type reaction: (a) Costa, P. R. R.; Cabral,
L. M.; Alencar, K. G.; Schmidt, L. L.; Vasconcellos, M. L. A. A.
Tetrahedron Lett. 1997, 38, 7021. (b) El Kaim, L.; Guyoton, S.; Meyer,
C. Tetrahedron Lett. 1996, 37, 375. (c) Bigi, F.; Sartori, G.; Maggi, R.;
Cantarelli, E.; Galaverna, G. Tetrahedron: Asymmetry 1993, 4, 2411,
and references therein. Double asymmetric synthesis: (d) Terada, M.;
Sayo, N.; Mikami, K. Synlett 1995, 411. (e) Casiraghi, G.; Bigi, F.;
Casnati, G.; Sartori, G.; Soncini, P.; Gasparri Fava, G.; Ferrari Belicchi,
M. J . Org. Chem. 1988, 53, 1779. Pictet-Spengler reaction: (f) Cox,
E. D.; Hameker, L. K.; Li, J .; Yu, P.; Czerwinski, K. M.; Deng, L.;
Bennett. D. W.; Cook, J . M. J . Org. Chem. 1997, 62, 44. (g) Dai, W.-
M.; Zhu, H. J .; Hao, X.-J . Tetrahedron Lett. 1996, 37, 5971. (h) Soe,
T.; Kawate, T.; Fukui, N.; Hino, T.; Nakagawa, M. Heterocycles 1996,
42, 347.
(6) Enantioselective F-C type reaction: (a) Erker, G.; van der
Zeijden, A. A. H. Angew. Chem., Int. Ed. Engl. 1990, 29, 512. (b) Bigi,
F.; Casiraghi, G.; Casnati, G.; Sartori, G.; Gasparri Fava, G.; Ferrari
Belicchi, M. J . Org. Chem. 1985, 50, 5018. Pictet-Spengler reaction:
(c) Kawate, T.; Yamada, H.; Soe, T.; Nakagawa, M. Tetrahedron:
Asymmetry 1996, 7, 1249.
(8) (a) Corey, E. J .; Bakshi, R. K. Tetrahedron Lett. 1990, 31, 611.
(b) Chong, J . M.; Mar, E. K. J . Org. Chem. 1991, 56, 893, and references
therein. (c) Corey, E. J .; Cheng, X.-M.; Cimprich, K. A.; Sarshar, S.
Tetrahedron Lett. 1991, 32, 6835. (d) Ramachandran, P. V.; Teodorovic,
A. V.; Gong, B.; Brown, H. C. Tetrahedron: Asymmetry 1994, 5, 1075.
(e) Fujisawa, T.; Sugimoto, T.; Shimizu, M. Tetrahedron: Asymmetry
1994, 5, 1095.
(9) Reviews: (a) Mikami, K. Pure and Appl. Chem. 1996, 68, 639.
(b) Mikami, K.; Terada, M.; Narisawa, S.; Nakai, T. Synlett 1992, 255.
(10) (a) Mikami, K.; Matsukawa, S. Nature 1997, 385, 613. (b)
Matsukawa, S.; Mikami, K. Tetrahedron: Asymmetry 1997, 8, 815. (c)
Matsukawa, S.; Mikami, K. Enantiomer 1996, 1, 69. (d) Matsukawa,
S.; Mikami, K. Tetrahedron: Asymmetry 1995, 6, 2571.
(11) Mikami, K.; Terada, M.; Nakai, T. J . Am. Chem. Soc. 1990, 112,
3949; 1989, 111, 1940; Mikami, K.; Terada, M.; Narisawa, S.; Nakai,
T. Org. Synth. 1993, 71, 14.
(7) Stereospecific F-C type reaction: (a) Toshimitsu, A.; Hirosawa,
C.; Tamao, K. Synlett 1996, 465. (b) Muehldorf, A. V.; Guzman-Perez,
A.; Kluge, A. F. Tetrahedron Lett. 1994, 35, 8755.
(12) Ohno, A.; Nakai, J .; Nakamura, K.; Goto, T.; Oka, S. Bull.
Chem. Soc. J pn., 1981, 54, 3486 [R]20D -35.9° (c 1.35, EtOH) for 80.2%
ee of R isomer.
10.1021/jo991691o CCC: $19.00 © 2000 American Chemical Society
Published on Web 02/08/2000

1598 J . Org. Chem., Vol. 65, No. 5, 2000
Ta ble 1. Th e F -C Rea ction s w ith F lu or a l Ca ta lyzed by (R)-BINOLs/Cl2Ti(OP r i)2/MS 4Aa
Notes
run
1
chiral ligand
cat. (mol%)
solvent
temp (°C)
yield (%)c
p-2:o-2c
ee (%)d
1
2
3
4
5
6
7
8
a
a
a
a
a
a
b
c
(R)-BINOL
30
5
5
1
30
5
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
toluene
CH2Cl2
CH2Cl2
CH2Cl2
0
0
0
0
0
82
11
94
99
99
94
85
90
4:1
4:1
4:1
4:1
2:1
4:1
8:1
3:1
73 (R)
22 (R)
84 (R)
72 (R)
83 (R)
79 (R)
83 (Re)
54 (Re)
(R)-H8-BINOLb
(R)-6,6′-Br2-BINOL
(R)-6,6′-Br2-BINOL
(R)-6,6′-Br2-BINOL
(R)-6,6′-Br2-BINOL
(R)-6,6′-Br2-BINOL
(R)-6,6′-Br2-BINOL
-30
15
10
0
0
a
BINOL-Ti catalysts were prepared as previously reported (ref 11). An excess of fluoral was used in all runs, because of the self-
polymerization. (R)-Octahydrobinaphthol. c Isolated yield after silica gel column chromatography. The enantiomeric excess of p-2.
Determined by chiral HPLC. p-2a : Daicel, CHIRALPAK OD-H, n-hexane:i-PrOH ) 98:2, 0.8 mL/min, 254 nm, tR ) 43 min (S), 49 min
(R). p-2b: Daicel, CHIRALPAK AS, n-hexane:i-PrOH ) 99:1, 0.8 mL/min, 254 nm, tR ) 40 min (S), 42 min (R). p-2c: Daicel, CHIRALPAK
OD-H, n-hexane:i-PrOH ) 98:2, 0.8 mL/min, 254 nm, tR ) 34 min (R), 44 min (S). The absolute configuration of p-2a was determined by
the comparison of optical rotation with the literature values (ref 12). e The absolute configurations of p-2b and p-2c were assumed to be
R from a similarity.
b
d
Ta ble 2. Th e F -C Rea ction s w ith F lu or a l Ca ta lyzed by
(R)-6,6′-Br2-BINOL obviously decreases the catalytic
activity.15 The highly acidic catalyst was prepared by the
coordination of the acidic 6,6′-Br2-BINOL with the (R)-
6,6′-Br2-BINOL-Ti(OPri)2, in particular.10a,b In combina-
tion with the matched chiral activator, the enantioselec-
tivity can be improved up to 90% ee as well as producing
a high chemical yield (runs 5, 6).
We have thus reported the first example of asymmetric
catalysis of the Friedel-Crafts reaction with fluoral.
Chiral 1-aryl-2,2,2-trifluoroethanol derivatives of syn-
thetic importance are obtained by the catalysis of chiral
binaphthol-derived titanium complexes through asym-
metric activation.
BINOL-Ti Com p lex th r ou gh Asym m etr ic Activa tion a
cat.
(mol%)
temp yield
ee
run
1
additive
(°C) (%)c p-2:o-2c (%)d
1
2
3
4
5
6
a
a
a
a
a
b
10
10
10
10
10
10
-
0
0
0
0
0
0
66
94
97
88
89
90
4:1
3:1
3:1
4:1
4:1
8:1
70 (R)
68 (R)
64 (R)
78 (R)
90 (R)
90 (R)
pentafluorophenol
(R)-BINOL
(R)-5-Cl-BIPOLb
(R)-6,6′-Br2-BINOL
(R)-6,6′-Br2-BINOL
a
(R)-6,6′-Br2-BINOL-Ti(OPri)2 was activated by the additive in
a molar ratio of 1:1 in dichloromethane (2 mL) at room tempera-
ture under an argon atomosphere for 1 h. The F-C reaction was
carried out in situ by the addition of anisole (1 mmol) in
dichloromethane (1 ml) and then passing an excess amount of
b
fluoral. 5,5′-Dichloro-4,4′,6,6′-tetramethyl-2,2′-biphenol. c Isolat-
Exp er im en ta l Section
d
ed yield after silica gel chromatography. Refers to that of p-2.
Gen er a l. Melting point is uncorrected. 1H NMR spectra were
recorded at 300 MHz;13C NMR spectra were recorded at 75 MHz.
Tetramethylsilane (δ ) 0 ppm) was used as an internal standard,
and CDCl3 was used as the solvent. Analytical thin-layer
chromatography (TLC) were performed on a glass plates pre-
coated with silica gel (Merck Kieselgel 60 F254, layer thickness
0.25 mm). Visualization was accomplished by UV light (254 nm)
and phosphomolybdic acid. Column chromatography was per-
formed on Silica Gel 60 (70-230 mesh) purchased from Kanto
Chemical Co., Inc. Molecular sieves (MS) 4A (activated powder)
was purchased from Aldrich Chemical Co., Inc. Dehydrated
dichloromethane and toluene were purchased from Kanto Chemi-
cal Co., Inc. Fluoral was generated by the addition of fluoral
hydrate to concentrated H2SO4 at 100 °C.
kaiyama-aldol reaction14 regardless of the preparative
procedure of the catalysts; (R)-BINOL-Ti catalyst pro-
duces an (R)-alcohol product.
This F-C reaction cannot proceed through a six-
membered transition state (A) involving a chiral Lewis
acid, which has been reported to preferentially produce
an ortho-F-C-product in the reaction of phenol5e,6b or
1-naphthol.6a In our case, however, the para-isomer is
the major product.
Gen er a l P r oced u r e for t h e F r ied el-Cr a ft s R ea ct ion s
w ith F lu or a l Ca ta lyzed by BINOL-Ti Com p lexes th r ou gh
Asym m etr ic Activa tion . To a solution of Ti(OPri)4 (28.4 mg,
0.1 mmol) in dehydrated dichloromethane (1 mL) was added (R)-
6,6′-Br2-BINOL (44.4 mg, 0.1 mmol) at room temperature under
an argon atomosphere. After stirring for 1 h, the additive (0.1
mmol) in dehydrated dichloromethane (1 mL) was added to the
mixture. After stirring for additional 1 h, aromatic substrate (1
mmol) in dehydrated dichloromethane (1 mL) was added, and
then an excess amount of freshly dehydrated and distilled fluoral
was passed to the catalyst solution at 0 °C. The reaction mixture
was stirred for 12 h at the same temperature. Dichloromethane
(5 mL) and H2O (3 mL) were added to the mixture. Insoluble
material was filtered off through a pad of Celite, and the filtrate
was extracted three times with dichloromethane. The combined
organic layer was washed with brine, dried over MgSO4, and
evaporated under reduced pressure. Chromatographic separa-
tion by silica gel (dichloromethane:n-hexane ) 3:2) gave the
product.
The catalyst efficiency and enantioselectivity of BINOL-
Ti complexes can be further increased through asym-
metric activation.10 Thus, the (R)-6,6′-Br2-BINOL-Ti-
(OPri)2, prepared from Ti(OPri)4 and (R)-6,6′-Br2-BINOL,
is activated by the addition of acidic activators such as
(R)-5-Cl-BIPOL and (R)-6,6′-Br2-BINOL (Table 2). In all
runs, chemical yields are obviously improved by the
addition of acidic activators, in sharp contrast to a similar
reaction catalyzed by Yb(OTf)3, in which the addition of
p-2a : colorless oil. 1H NMR (CDCl3) δ 2.73 (d, J ) 4.5 Hz,
1H), 3.81 (s, 3H), 4.93 (dq, J ) 4.5, 6.6 Hz, 1H), 6.91 (d, J ) 9.0
(13) Corey, E. J .; Barnes-Seeman, D.; Lee, T. W.; Goodman, S. N.
Tetrahedron Lett. 1997, 37, 6513.
(14) (a) Mikami, K.; Matsukawa, S.; Sawa, E.; Harada, A.; Koga,
N. Tetrahedron Lett. 1997, 38, 1951. (b) Mikami, K.; Matsukawa, S.;
Nagashima, M.; Funabashi, H.; Morishima, H. Tetrahedron Lett. 1997,
38, 579, and references therein.
(15) The same reaction in the presence of Yb(OTf)3 (10 mol %) gave
the F-C products in 78% yield (para/ortho ) 7). The addition of (R)-
6,6′-Br2-BINOL (1 or 2 equiv) decreased the chemical yields (42% (para/
ortho ) 13) or 45% (para/ortho ) 16), respectively).

Notes
J . Org. Chem., Vol. 65, No. 5, 2000 1599
Hz, 2H), 7.38 (d, J ) 9.0 Hz, 2H). 13C NMR (CDCl3) δ 55.4, 72.5
(q, J ) 32 Hz), 114.1, 124.4 (q, J ) 282 Hz), 126.2, 128.9, 160.5.
[M+]. [R]30 -0.8° (c 0.51, EtOH) (6% ee of R isomer). Chiral
D
HPLC (Daicel, CHIRALPAK OD-H, n-hexane:i-PrOH ) 98:2, 0.8
mL/min, 254 nm) tR ) 26 min (S), 34 min (R). Rf (Merck Kieselgel
60 F254/dichloromethane:n-hexane ) 3:2) 0.48.
IR (neat) 3440, 1170 cm-1. MS (EI) m/z 206 [M+], 137. [R]29
D
-23.0° (c 1.62, EtOH) (61% ee of R isomer). Chiral HPLC (Daicel,
CHIRALPAK OD-H, n-hexane:i-PrOH ) 98:2, 0.8 mL/min, 254
p-2c: colorless oil. 1H NMR (CDCl3) δ 2.53 (d, J ) 4.2 Hz,
1H), 5.01 (dq, J ) 4.2, 6.7 Hz, 1H), 7.02 (d, J ) 9.0 Hz, 2H),
7.04 (d, J ) 7.5 Hz, 2H), 7.15 (t, J ) 7.5 Hz, 1H), 7.37 (t, J ) 7.5
Hz, 2H), 7.44 (d, J ) 9.0 Hz, 2H). 13C NMR (CDCl3) δ 72.5 (q, J
) 32 Hz), 118.5, 119.5, 124.0, 124.3 (q, J ) 282 Hz), 128.5, 129.1,
130.0, 156.5, 158.7. IR (neat) 3440, 1170 cm-1. MS (EI) m/z 268
nm) tR ) 43 min (S), 49 min (R). Rf (Merck Kieselgel 60 F254
dichloromethane:n-hexane ) 3:2) 0.24.
/
o-2a : colorless oil. 1H NMR (CDCl3) δ 3.66 (d, J ) 7.8 Hz,
1H), 3.88 (s, 3H), 5.27 (quin, J ) 7.8 Hz, 1H), 6.95 (d, J ) 7.8
Hz, 1H), 7.02 (t, J ) 7.8 Hz, 1H), 7.37 (t, J ) 7.8 Hz, 1H), 7.39
(d, J ) 7.8 Hz, 1H). 13C NMR (CDCl3) δ 55.8, 69.8 (q, J ) 33
Hz), 111.4, 121.1, 122.2, 124.7 (q, J ) 283 Hz), 129.3, 130.6,
157.6. IR (neat) 3450, 1174 cm-1. MS (EI) m/z 206 [M+], 137.
[M+], 199. [R]29 -21.6° (c 0.90, EtOH) (77% ee of R isomer).
D
Chiral HPLC (Daicel, CHIRALPAK OD-H, n-hexane:i-PrOH )
98:2, 0.8 mL/min, 254 nm) tR ) 34 min (R), 44 min (S). Rf (Merck
Kieselgel 60 F254/dichloromethane: n-hexane ) 3:2) 0.27.
o-2c: colorless oil. 1H NMR (CDCl3) δ 3.16 (d, J ) 6.9 Hz, 1H),
5.48 (quin, J ) 6.9 Hz, 1H), 6.83 (d, J ) 8.3 Hz, 1H), 7.03 (d, J
) 8.3 Hz, 2H), 7.16 (t, J ) 8.3 Hz, 2H), 7.31 (t, J ) 8.3 Hz, 1H),
7.37 (t, J ) 8.4 Hz, 2H), 7.58 (d, J ) 8.3 Hz, 1H). 13C NMR
(CDCl3) δ 68.4 (q, J ) 33 Hz), 117.9, 119.6, 123.6, 124.3, 124.7,
124.7 (q, J ) 283 Hz), 129.4, 130.2, 130.8, 155.9, 156.4. IR (neat)
[R]31 -11.8° (c 2.05, EtOH) (50% ee of R isomer). Chiral HPLC
D
(Daicel, CHIRALPAK OD-H, n-hexane:i-PrOH ) 98:2, 0.8 mL/
min, 254 nm) tR ) 20 min (R), 27 min (S). Rf (Merck Kieselgel
60 F254/dichloromethane:n-hexane ) 3:2) 0.33.
p-2b: colorless needles. mp 65 °C (n-hexane). 1H NMR (CDCl3)
δ 0.98 (t, J ) 7.5 Hz, 3H), 1.49 (sex., J ) 7.5 Hz, 2H), 1.77 (quin,
J ) 7.5 Hz, 2H), 2.43 (d, J ) 4.5 Hz, 1H), 3.97 (t, J ) 7.5 Hz,
2H), 4.96 (dq, J ) 4.5, 6.6 Hz, 1H), 6.92 (d, J ) 9.0 Hz, 2H),
7.38 (d, J ) 9.0 Hz, 2H). 13C NMR (CDCl3) δ 13.9, 19.3, 31.3,
67.9, 72.6 (q, J ) 32 Hz), 114.7, 124.4 (q, J ) 282 Hz), 126.0,
128.8, 160.2. IR (neat) 3400, 1174 cm-1. MS (EI) m/z 248 [M+],
3440, 1176 cm-1. MS (EI) m/z 268 [M+]. [R]29 -15.1° (c 1.59,
D
EtOH) (63% ee of R isomer). Chiral HPLC (Daicel, CHIRALPAK
OD-H, n-hexane:i-PrOH ) 99:1, 0.8 mL/min, 254 nm) tR ) 23
min (R), 28 min (S). Rf (Merck Kieselgel 60 F254/dichloromethane:
n-hexane ) 3:2) 0.36.
179. [R]29 -34.1° (c 1.08, EtOH) (100% ee of R isomer). Chiral
D
HPLC (Daicel, CHIRALPAK AS, n-hexane:i-PrOH ) 99:1, 0.8
mL/min, 254 nm) tR ) 40 min (S), 42 min (R). Rf (Merck Kieselgel
60 F254/dichloromethane:n-hexane ) 3:2) 0.27.
Ack n ow led gm en t. Grants-in-Aid for Scientific Re-
search (Nos. 09238209 and 10208204) are gratefully
acknowledged.
o-2b: colorless oil. 1H NMR (CDCl3) δ 0.99 (t, J ) 7.5 Hz, 3H),
1.50 (sex., J ) 7.5 Hz, 2H), 1.81 (m, 2H), 3.83 (d, J ) 7.5 Hz,
1H), 4.05 (m, 2H), 5.24 (quin, J ) 7.5 Hz, 1H), 6.94 (d, J ) 7.5
Hz, 1H), 6.99 (t, J ) 7.5 Hz, 1H), 7.35 (t, J ) 7.5 Hz, 1H), 7.36
(d, J ) 7.5 Hz, 1H). 13C NMR (CDCl3) δ 13.6, 19.1, 31.1, 68.3,
70.6 (q, J ) 33 Hz), 112.2, 121.1, 122.2, 125.0 (q, J ) 284 Hz),
129.8, 130.8, 157.5. IR (neat) 3440, 1172 cm-1. MS (EI) m/z 248
Su p p or tin g In for m a tion Ava ila ble: 1H NMR spectra of
compounds p- or o-2a -c. This material is available free of
charge via the Internet at http://pubs.acs.org.
J O991691O