Use of boron enolates in water. The first boron enolate-mediated diastereoselective aldol reactions using catalytic boron sources

Article abstract of DOI:10.1016/S0040-4020(02)00976-6

Highly diastereoselective aldol reactions in water using a catalytic amount of diarylborinic acid have been developed. The reactions proceeded smoothly in the presence of a small amount of an anionic surfactant and a Br?nsted acid. Water was the most suit

Full text of DOI:10.1016/S0040-4020(02)00976-6

Tetrahedron 58 (2002) 8263–8268
Use of boron enolates in water. The first boron enolate-mediated
diastereoselective aldol reactions using catalytic boron sources
*
Yuichiro Mori, Juta Kobayashi, Kei Manabe and Shu¯ Kobayashi
Graduate School of Pharmaceutical Sciences, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Received 9 May 2002; accepted 15 May 2002
Abstract—Highly diastereoselective aldol reactions in water using a catalytic amount of diarylborinic acid have been developed. The
reactions proceeded smoothly in the presence of a small amount of an anionic surfactant and a Brønsted acid. Water was the most suitable
solvent, and organic solvents such as ether and dichloromethane were ineffective in this system. Use of bis(4-trifluoromethylphenyl)borinic
acid gave high catalytic activity. It is most plausible to conclude that the active species of the reactions are boron enolates, and this is the first
example of catalytic use of a boron source in boron enolate-mediated diastereoselective aldol reactions. q 2002 Elsevier Science Ltd. All
rights reserved.
1. Introduction
acid–surfactant-combined catalyst (LASC),9–19 and the
representative LASC is scandium tris(dodecyl sulfate)
(Sc(DS)3). In the presence of a catalytic amount of a
LASC, useful synthetic reactions such as aldol, allylation,
Mannich, and Michael reactions proceed smoothly in water.
The third one is a system using a Brønsted acid–surfactant-
combined catalyst (BASC).20–25 In the presence of a
catalytic amount of a BASC, Mannich-type reactions,
esterification, etherification, and thioetherification reactions
are successfully conducted in water. In these surfactant-
aided systems, organic substrates form emulsion droplets
that function as reaction media in water.
Boron enolate-mediated aldol reactions provide powerful
tools for diastereoselective aldol methodologies.1 Since
they were first developed in 1973 by Mukaiyama,2 many
3
groups (Koster’s group, Masamune’s group,4 Evans’
¨
group,5 etc.) have improved the reaction systems. Boron
enolate-mediated aldol reactions proceed via six-membered
transition states, and the small size of boron atom results in
high diastereoselectivity because the steric effect plays a
significant role. Boron enolates are usually generated from a
boron source (e.g. dialkylboron triflates), a tertiary amine
(e.g. triethylamine), and a substrate (carbonyl compound).
To the best of our knowledge, stoichiometric amounts of
boron sources are necessary in all examples previously
reported. Therefore, development of boron enolate-
mediated reactions using a catalytic amount of a boron
source is a challenging subject in synthetic organic
chemistry.
In the course of our investigation on new reaction systems in
water, we have developed the first example of a catalytic use
of a boron source in boron enolate-mediated diastereo-
selective aldol reactions.26 In our system, boron enolates are
generated in situ from silyl enol ethers and diarylborinic
acid. A remarkable feature of the reaction system compared
with conventional boron enolate chemistry is that the
reactions proceed smoothly in water, not in organic
solvents. It seems curious because boron enolates are
known to be easily decomposed by water. Here, we describe
detailed studies in the boron-catalyzed reactions in water.
Recently, organic reactions in water have received much
attention because water is a cheap, safe, and environ-
mentally benign solvent. In our group, several types of
catalyst systems that work efficiently in water have already
been developed so far. The first one is a system composed
of scandium trifluoromethanesulfonate (Sc(OTf)3) and a
surfactant.6–8 By applying this system, various C–C bond
forming reactions such as Mukaiyama aldol reactions and
allylation reactions proceed in water without using any
organic solvents. The second one is a system using a Lewis
2. Results and discussion
In our search for new reaction systems in water, we tested
various compounds that might act as better catalysts in
water. We chose Mukaiyama aldol reaction as a model
reaction. After screening various compounds, we found that
diphenylborinic acid (Ph2BOH, 1),27 which was prepared
from its ethanolamine ester,28 was an effective catalyst in
the presence of sodium dodecyl sulfate (SDS), an anionic
Keywords: boron enolate; water; aldol reaction; surfactant; aldehyde;
electron-withdrawing group.
*
Corresponding author. Tel.: þ81-3-5841-4790; fax: þ81-3-5684-0634;
e-mail: skobayas@mol.f.u-tokyo.ac.jp
0040–4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(02)00976-6

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Y. Mori et al. / Tetrahedron 58 (2002) 8263–8268
Table 1. Mukaiyama aldol reaction using 1 in water
Table 3. Effect of solvents
Run
Solvent
H2O
Et2O
CH2Cl2
–
SDS (equiv.)
Yield (%)
syn/anti
Run
1 (equiv.)
PhCO2H (equiv.)
Yield (%)
syn/anti
1
2
3
4
5
6
0.1
0.1
0.1
0.1
–
90
92/8
–
–
90/10
–
73/27
Trace
Trace
24
Trace
13
1
2
3
0.1
0.1
–
–
0.1
0.1
–
0.01
–
0.01
0.01
0.01
Trace
90
2
27
4
–
92/8
53/47
58/42
91/9
94/6
H2O/THF (1/9)
H2O/EtOH (1/9)
4
5a
6b
–
93
a
Without SDS.
08C.
diastereoselectivity was maintained (run 4). Under water/
THF and water/ethanol conditions, which were often used
for aqueous reactions, the reaction was significantly
suppressed (runs 5, 6).
b
surfactant. We chose the reaction of benzaldehyde with the
silyl enol ether derived from propiophenone (2), and several
reaction conditions were examined (Table 1).
The effect of Brønsted acids was also investigated (Table 4).
The use of acetic acid gave almost the same result as that of
benzoic acid (run 2). When stronger acids such as
hydrochloric acid and p-toluenesulfonic acid were used,
the yields became lower because the hydrolysis of the silyl
enol ether was also accelerated (runs 3, 4). In these cases,
diastereoselectivities were also lowered probably because
these strong acids themselves function as catalysts of the
While the reaction proceeded very slowly in the presence of
1 and SDS, a dramatic improvement of the yield was
observed when a small amount of benzoic acid was added to
this system (runs 1, 2). To our surprise, the diastereoselec-
tivity (syn/anti) of the aldol product was much better than
that of the previously reported LASC system.17 In the
absence of 1, not only the diastereoselectivity but also the
yield were significantly lower (runs 3, 4). The reaction
proceeded very slowly with high diastereoselectivity in the
absence of SDS (run 5). The best yield and selectivity were
obtained when 1 (0.1 equiv.), SDS (0.1 equiv.), and benzoic
acid (0.01 equiv.) were used at 08C (run 6).
Table 4. Effect of Brønsted acids
The effect of other surfactants was investigated (Table 2).
Sodium dodecylbenzenesulfonate, which is also an anionic
surfactant, accelerated the reaction and gave high syn
selectivity (run 2). Triton X-100 (nonionic surfactant) and
cetyltrimethylammonium bromide (cationic surfactant)
were much less effective (runs 3, 4).
Run
Brønsted acid
Time (h)
Yield (%)
syn/anti
1
2
3
4
PhCO2H
CH3CO2H
HCl
24
18
2
90
93
76
69
92/8
92/8
89/11
89/11
TsOH
2
In order to know whether the reaction proceeded in water
specifically, the solvent effect was studied (Table 3). Ether
and dichloromethane, which had been used as representative
solvents for boron enolates-mediated aldol reactions, were
not suitable for this reaction system (runs 2, 3). Under neat
conditions, the reaction was very slow, though high
Table 5. Effect of catalysts
Table 2. Effect of surfactants
Run
Ar
Yield (%)
syn/anti
1
2
3
4
5
6
7
8
Ph (1)
90
87
91
57
77
78
32
33
9
92/8
4-Trifluoromethylphenyl (3)29
3-Trifluoromethylphenyl
3,5-Bis(trifluoromethyl)phenyl30
4-Fluorophenyl31
4-Bromophenyl32
4-Tolyl33
89/11
85/15
64/36
90/10
87/13
72/28
74/26
55/45
70/30
88/12
Run
Surfactant
Yield (%)
syn/anti
1
2
3
4
SDS
NaO3SC6H4C12H25
TritonX-100
C16H33Nþ(CH3)3Br2
90
81
Trace
8
92/8
93/7
–
4-Methoxyphenyl34
Mesityl
9
10
11
1-Naphthyl35
2-Naphthyl
38
70
87/13

Y. Mori et al. / Tetrahedron 58 (2002) 8263–8268
8265
an electron-donating group such as methyl or methoxy
group was at para position, the yield and diastereoselec-
tivity were lowered. On the other hand, an electron-
withdrawing group such as bromo group, fluoro group, or
trifluoromethyl group was at para position, high diastereo-
selectivity was maintained. When trifluoromethyl group
was at para position, the reaction proceeded significantly
faster than that using 1. Comparison of the reaction rate is
shown in Fig. 1. Under standard conditions (Ar2BOH
0.1 equiv.; PhCO2H 0.01 equiv.; SDS 0.1 equiv.), bis(4-tri-
fluoromethylphenyl)borinic acid (3) accelerated the aldol
reaction of cyclohexanecarboxaldehyde with 2 more
significantly than 1 did (Fig. 1).
Various aldehydes and silyl enol ethers were applied to this
reaction system (Table 6). Not only aromatic aldehydes but
also a,b-unsaturated, and aliphatic aldehydes could be used.
Silyl enol ethers derived from aliphatic ketones and
thioesters could also be applied. The highest diastereo-
selectivity (syn/anti¼97/3) was obtained when 2-naph-
thaldehyde and the silyl enol ether derived from
3-pentanone were employed (run 16). A remarkable
point was that low selectivity or reverse selectivity was
obtained when trans-enolates were used (runs 4, 5, 7, 10, 11,
and 15). The diastereoselectivity was dependent on the
geometry of the enolates when both regioisomers of the
silyl enol ethers from tert-butyl thiopropionate were used
(runs 8–11).
Figure 1. Catalytic activity of 1 and 3.
aldol reaction between the aldehyde and the silyl enol ether,
which gave the same product with low selectivity.
Catalytic activity of other diarylborinic acids30 was
examined (Table 5). These diarylborinic acids were
prepared from the corresponding aryl bromides and
trialkylborates (trimethylborate or tributylborate). When
The high diastereoselectivity observed in these reactions
Table 6. Mukaiyama aldol reactions using 1 and 3 in water
Run
Catalyst
R1
R2
R3
R4
Ph
Ph
Et
Et
Et
iPr
Z/E
Yield (%)
syn/anti
1
1
3
1
1
3
1
1
1
3
1
3
1
1
1
1
1
1
3
1
3
3
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Me
Me
Me
H
H
Me
H
Me
Me
H
H
H
H
Me
Me
H
(CH2)4
H
.99/,1
.99/,1
.99/,1
19/81
19/81
96/4
0/100
2/98
2/98
97/3
90
87
60
72
60
72
58
62
76
84
43
92
51
80
79
74
76
80
61
65
78
92/8
89/11
96/4
53/47
29/71
95/5
56/44
96/4
95/5
39/61
18/82
90/10
95/5
2
3a
4a
5a
6a
7
8a,b
9a–c
10a,b,d
11a–c
12
StBu
StBu
StBu
StBu
Ph
H
Me
Me
H
H
H
(CH2)4
H
H
H
H
H
97/3
4-ClC6H4
4-ClC6H4
Me
Me
Me
H
Me
Me
Me
Me
Me
Me
.99/,1
.99/,1
.99/,1
0/100
.99/,1
.99/,1
.99/,1
.99/,1
.99/,1
.99/,1
13a
14
Et
Ph
1-Naphthyl
1-Naphthyl
2-Naphthyl
PhCHvCH
PhCHvCH
PhCH2CH2
PhCH2CH2
Cyclohexyl
92/8
15
47/53
97/3
91/9
90/10
92/8
93/7
16a
17e
18b
19
Et
Ph
Ph
Ph
Ph
Ph
20
21
H
H
96/4
a
Silyl enol ether (3.0 equiv.)
At 08C.
2 days.
3 days.
PhCO2H (0.1 equiv.).
b
c
d
e

8266
Y. Mori et al. / Tetrahedron 58 (2002) 8263–8268
Figure 2. Changes of the amount of 2 during the reactions using 1 as a
catalyst in water (A: benzaldehyde, V: cyclohexanecarboxaldehyde, W:
no aldehyde, C: concentration of 2 in water (mol/L)). Conditions: 2
(1.5 equiv.), 1 (0.1 equiv.), PhCO2H (0.1 equiv.), SDS (0.1 equiv.), H2O
(aldehyde: 167 mM), 308C. The amount of 2 was determined by HPLC
(acetophenone was used as an internal standard).
Scheme 1. Assumed catalytic cycle.
was remarkable because lower diastereoselectivity had been
obtained in the Lewis acid-catalyzed Mukaiyama aldol
reactions in water so far. For the metal-catalyzed
Mukaiyama aldol reactions of aldehydes with silyl enol
ethers, only two reaction mechanisms are known. One is the
Lewis acid mechanism, in which an aldehyde activated by a
Lewis acid reacts with a silyl enol ether, and the other is the
metal enolate mechanism in which a reactive metal enolate
is formed from a silyl enol ether by silicon–metal exchange
and reacts with an aldehyde.36–39 In the present system,
we proposed the latter reaction mechanism because it could
explain the results shown above more clearly. In boron
enolate chemistry, the aldol reaction proceeds via chair-like
six-membered transition state and the diastereoselectivity is
dependent on the geometry of the starting boron enolate.
Furthermore, the trend that syn selectivity is higher than anti
selectivity is observed.40 In the present reaction system, the
same trend was observed. As shown in Table 6, high syn
selectivity was observed when a cis enolate was used.
Additionally, lower selectivity was observed when trans
enolate was used. Thus, we proposed the mechanism which
involved a boron enolate as a reaction intermediate (the
boron enolate mechanism, shown in Scheme 1). The
mechanism is based on the hypothesis that 1 can react
with a silyl enol ether to form the corresponding boron
enolate under the conditions.41,42 When a cis enolate was
used, an aldehyde and the boron enolate would react via a
chair-like six-membered transition state to give the syn aldol
product. The initial aldol product is presumed to be easily
cleaved by hydrolysis, and 1 can be regenerated. The role
of benzoic acid may be acceleration of the Si–B
exchange step, which is thought to be the rate-determining
step. Since the leaving group in the step is hydroxyl group,
which is a weak leaving group, of 1, it seems necessary to
protonate it to increase leaving ability by adding a proton
source.
because nonselective Brønsted acid-mediated aldol pathway
became significant.
To support the proposed mechanism, kinetic studies were
undertaken. The rate of disappearance of silyl enol ether 2
was examined (Fig. 2 (left)). Benzaldehyde (more reactive)
and cyclohexanecarboxaldehyde (less reactive, 34% yield
under the conditions of Fig. 2) were chosen as substrates for
investigation of the relation between the reactivity of these
aldehydes and disappearance rate of 2. It was found that the
rate was independent of the reactivity of the aldehydes. The
reactivity of the aldehydes has no significant effect on
the disappearance rate of 2. Furthermore, the amount of the
silyl enol ether also decreased at the same rate in the case
that no aldehyde was added to the reaction system. In
addition, the rate was found to be dependent only on the
amount of the remaining 2, following first-order kinetics
(k¼3.0£1024 s21, Fig. 2 (right)). These facts support the
boron enolate mechanism rather than the Lewis acid
mechanism, because the reactivity and the absence of the
aldehyde are presumed to affect the rate of the disappear-
ance of 2 in the Lewis acid mechanism. In our mechanism,
the aldol addition step should be very fast because of the
instability of the boron enolate in water. The results of the
kinetic studies suggest the Si–B exchange step, not the aldol
addition step, is the rate-determining step because the rate
depends only on the amount of the remaining 2. The formed
boron enolate would react with an aldehyde rapidly or be
hydrolyzed to form the corresponding ketone. The cleavage
of the B–O bond of the initial aldol product should be
fast because of the acidity of the reaction media (pH ca. 4).
Thus, the boron enolate mechanism is much more likely
than the Lewis acid mechanism. The same mechanism
can be proposed for the case of using 3 because the same
trend of diastereoselectivity was observed. When trans-
enolates were used, higher diastereoselectivities were
obtained by using 3 (Table 6, runs 4, 5, 10, and 11).
When a more Lewis acidic catalyst was used,30 diastereo-
selectivity was lowered probably because of side pathways
such as Lewis acid-mediated aldol pathway and Brønsted
acid-mediated aldol pathway (Table 5, run 4). As far as
we know, this is the first example of the formation of a
boron enolate using a catalytic amount of a boron source.
In addition, it is noted that the reactions proceed
The substituents on boron atom would affect the Si–B
exchange step. When an electron-withdrawing group such
as trifluoromethyl group was introduced, electron density on
boron atom was decreased and thus, the boron atom seems
to be more easily attacked by oxygen atom of a silyl enol
ether. An electron-donating group would have an opposite
effect, and diastereoselectivity would be lower probably

Y. Mori et al. / Tetrahedron 58 (2002) 8263–8268
8267
smoothly in water as a solvent to attain high yields and
selectivities.
4.1.2. Bis(3-trifluoromethylphenyl)borinic acid. Mp 37–
1
388C; IR (KBr) 3349, 1610, 1346, 1164, 934 cm21; H
NMR (CDCl3) d 7.60 (dd, J¼7.7, 7.7 Hz, 2H), 7.80 (d, J¼
7.9 Hz, 2H), 7.95 (d, J¼7.3 Hz, 2H), 8.06 (s, 2H); 13C NMR
(CDCl3) d 124.2 (q, J¼272.0 Hz), 128.0 (q, J¼3.7 Hz),
128.5, 130.5 (q, J¼32.1 Hz), 130.9 (q, J¼3.7 Hz), 138.0;
MS (m/z) 318 [Mþ], 173. Anal. calcd for C14H9BF6O: C,
52.87; H, 2.85. Found: C, 53.04; H, 3.14.
3. Conclusion
We have developed a new reaction system in water. Highly
diastereoselective Mukaiyama aldol reactions proceed
smoothly in the presence of a catalytic amount of
diarylborinic acid such as 1 or 3, benzoic acid, and SDS.
A remarkable point is that the reaction intermediate is a
boron enolate which is formed in the reaction system. It
should be noted that the reactions proceed smoothly in water
at ambient temperature, while traditional boron enolate-
mediated aldol reactions need lower temperatures and
strictly anhydrous conditions. Water is used as the sole
solvent, and this system is thought to lead to environ-
mentally friendly systems. The new system described here
will be a guideline for development of the methods of
treating water-unstable compounds in water.
4.1.3. Bis(4-fluorophenyl)borinic acid. Mp 31–328C; IR
(KBr) 3447, 1597, 1230, 1159, 836 cm21; 1H NMR (CDCl3)
d 7.14 (dd, J¼8.9, 8.9 Hz, 4H), 7.78 (dd, J¼6.3, 8.5 Hz, 4H);
13C NMR (CDCl3) d 115.1, 115.3, 136.9, 137.0; HRMS.
Calcd for C12H9BF2O: m/z 218.0715. Found: 218.0717.
4.1.4. Bis(4-bromophenyl)borinic acid. Mp 80–818C; IR
(KBr) 3569, 1584, 1389, 1072, 1009 cm21 1H NMR
;
(CDCl3) d 7.59 (d, J¼8.3 Hz, 4H), 7.63 (d, J¼8.3 Hz,
4H); 13C NMR (CDCl3) d 126.4, 131.3, 136.1; HRMS.
Calcd for C12H9BBr2O: m/z 337.9113. Found: 337.9117.
4.1.5. Bis(4-tolyl)borinic acid. Mp 40–418C; IR (KBr)
1
3399, 1605, 1328, 1184, 824 cm21; H NMR (CDCl3) d
2.41 (s, 6H), 7.26 (d, J¼8.1 Hz, 4H), 7.71 (d, J¼7.9 Hz,
4H); 13C NMR (CDCl3) d 21.7, 128.7, 134.8, 141.2; HRMS.
Calcd for C14H15BO: m/z 210.1216. Found: 210.1219.
4. Experimental
4.1. A typical experimental procedure for diastereo-
selective aldol reactions in water
4.1.6. Bis(4-methoxyphenyl)borinic acid. Mp 92–938C;
1
IR (KBr) 3602, 2962, 2041, 1918, 1279 cm21; H NMR
To a stirred white suspension of diarylborinic acid
(0.025 mmol), benzoic acid (0.0025 mmol), and SDS
(0.025 mmol) in water (1.5 mL), an aldehyde (0.25 mmol)
and a silyl enol ether (0.375 mmol) were added successively
at 308C. After 24 h, saturated aq. NaHCO3 and brine were
added, and the mixture was extracted with ethyl acetate,
dried over Na2SO4, and concentrated. The aldol product
was purified by preparative TLC (SiO2, ethyl acetate/
hexane¼1/3). All the aldol products are literature-known
compounds.
(CDCl3) d 3.87 (s, 6H), 6.98 (d, J¼8.8 Hz, 4H), 7.77 (d, J¼
8.6 Hz, 4H); 13C NMR (CDCl3) d 55.1, 113.5, 136.6, 161.9;
MS (m/z) 242 [Mþ]. Anal. calcd for C14H15BO3: C, 69.46;
H, 6.25. Found: C, 69.22; H, 6.16.
4.1.7. Di-1-naphthylborinic acid. Mp 107–1088C; IR
(KBr) 3399, 3046, 1571, 1287, 782 cm21 1H NMR
;
(CDCl3) d 7.43–7.54 (m, 6H), 7.73 (dd, J¼1.3, 6.8 Hz,
2H), 7.90–7.98 (m, 4H), 8.39 (d, J¼7.9 Hz, 2H); 13C NMR
(CDCl3) d 125.1, 125.7, 126.4, 128.4, 128.7, 131.0, 133.4,
134.9, 136.0; MS (m/z) 282 [Mþ]. Anal. calcd for
C20H15BO: C, 85.14; H, 5.36. Found: C, 85.29; H, 5.50.
4.1.1. Bis(4-trifluoromethylphenyl)borinic acid (3). To a
stirred solution of 4-bromobenzotrifluoride (5.0 mmol) in
ether (20 mL) were added sec-butyllithium (1.0 M cyclo-
hexane and n-hexane solution, 5 mmol) and tributylborate
(1.7 mmol) successively at 2788C. The reaction mixture
was stirred overnight at room temperature and quenched
with water. The ether layer was washed with 1N HCl, water,
and brine, and dried over Na2SO4, and concentrated.
The crude product was dissolved in 75% ethanol, and
2-ethanolamine (2.3 mmol) was added. The mixture was
stirred for 1 h at 508C and poured into ether. The ether layer
was washed with water and brine, and dried over Na2SO4,
and concentrated. The mixture was washed with chloro-
form on a glass filter and dried in vacuo. The product
(ethanolamine ester) was hydrolyzed by 1N HCl aq./acetone/
methanol at room temperature. The reaction mixture was
extracted with ether and washed with water and brine, and
dried over Na2SO4, and concentrated. The obtained crystal
was dried in vacuo (56%). Mp 79–808C; IR (KBr) 3399,
4.1.8. Di-2-naphthylborinic acid. Mp 129–1308C; IR
(KBr) 3581, 3054, 1626, 1348, 744 cm21 1H NMR
;
(CDCl3) d 7.51–7.60 (m, 4H), 7.91–7.95 (m, 8H), 8.37 (s,
2H); 13C NMR (CDCl3) d 126.1, 127.2, 127.3, 127.8, 128.9,
130.2, 132.9, 134.8, 136.7; MS (m/z) 282 [Mþ]. Anal. calcd
for C20H15BO: C, 85.14; H, 5.36. Found: C, 84.85; H, 5.51.
Acknowledgements
This work was partially supported by CREST and SORST,
Japan Science and Technology Corporation (JST), and a
Grant-in-Aid for Scientific Research from Japan Society
of the Promotion of Sciences. Y. M. thanks the JSPS
fellowship for Japanese Junior Scientists.
1
1323, 1127, 838, 762 cm21; H NMR (CDCl3) d 7.72 (d,
J¼7.9 Hz, 4H), 7.88 (d, J¼7.7 Hz, 4H); 13C NMR (CDCl3)
d 124.0 (q, J¼272.1 Hz), 124.8, 133.1 (q, J¼32.2 Hz),
134.8; MS (m/z) 318 [Mþ], 173. Anal. calcd for
C14H9BF6O: C, 52.87; H, 2.85. Found: C, 53.17; H, 3.09.
References
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Y. Mori et al. / Tetrahedron 58 (2002) 8263–8268
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