Synthesis of molecular tweezers bearing pentafluorophenyl and several pendant aryl groups. Effective application of the modified Suzuki-Miyaura coupling reaction

Article abstract of DOI:10.1016/j.jfluchem.2005.12.007

A molecular tweezer bearing two pentafluorophenyl groups 2b and its o-methylated aryl homologue 2c were newly synthesized by the slight modification of the known procedure. The low yield in the introduction of pentafluorophenyl group to the precursor dibr

Full text of DOI:10.1016/j.jfluchem.2005.12.007

Journal of Fluorine Chemistry 127 (2006) 604–609
www.elsevier.com/locate/fluor
Synthesis of molecular tweezers bearing pentafluorophenyl
and several pendant aryl groups
Effective application of the modified Suzuki–Miyaura coupling reaction
*
Toshinobu Korenaga , Takahiro Kosaki, Yusuke Kawauchi, Tadashi Ema, Takashi Sakai
*
Division of Chemistry and Biochemistry, Graduate School of Natural Science and Technology, Okayama University, Okayama 700-8530, Japan
Received 12 December 2005; accepted 16 December 2005
Available online 24 January 2006
Abstract
A molecular tweezer bearing two pentafluorophenyl groups 2b and its o-methylated aryl homologue 2c were newly synthesized by the slight
modification of the known procedure. The low yield in the introduction of pentafluorophenyl group to the precursor dibromide 6 was improved by
application of the modified Suzuki–Miyaura coupling reaction using C6F5B(OH)2 with Pd2(dba)3/P(t-Bu)3/CsF/Ag2O catalytic system, which was
recently developed by us.
# 2006 Elsevier B.V. All rights reserved.
Keywords: Molecular tweezer; Pentafluorophenyl group; Stacking interaction; Suzuki–Miyaura coupling
1. Introduction
the guest compounds decrease, and (2) the p–p interaction is
observable even in a polar organic solvent such as THF [3].
The difference in the nature of molecular tweezers between 1
and 2a was demonstrated by the calculations of electrostatic
potential surfaces (Fig. 2) [4,5]. The surface of acridine ring of 1
has a negative electrostatic potential (black), which indicates that
electron-deficient guests can be intercalated into the concave slit.
On the other hand, the electrostatic potential surfaces on the
pentafluorophenyl rings have positive values (pale grey), sugge-
sting that electron-rich compounds such as benzene derivatives
having electron-donating substituents are the possible guests.
The unique nature of pentafluoropheyl group sometimes
causes synthetic problems, for instance, in the Suzuki–
Miyaura coupling reaction, which is one of the most useful
methods for the synthesis of biaryl compounds [6]. Typical
example of the reaction is the cross-coupling of arylboronic
acids with aryl halides in the presence of Pd(PPh3)4 catalyst
and a base such as K2CO3. However, a similar reaction of
pentafluorophenylboronic acid (C6F5B(OH)2) with aryl
halides did not proceed (Eqs. (1) and (2)) [7] because of the
strong electron-withdrawing nature of the pentafluorophenyl
group [8]. Recently, Pd-catalyzed and Ag2O-promoted
coupling reactions of C6F5BF3K with aryl iodides have been
reported by Frohn et al. (Eq. (3)) [9]. Although the reaction
yielded the coupling products in more than 90% yield, the
method was not applicable to aryl bromides. Quite recently,
Intra- and intermolecular p–p and CH–p aromatic–
aromatic interactions are important phenomena for the host–
guest chemistry [1]. Among a variety of molecular systems
developed for investigation of the interactions, the molecular
tweezer 1 bearing two acridine groups, which was synthesized
by Zimmerman et al., is quite interesting (Fig. 1) [2]. In the
tweezer 1, the cofacially arranged two acridines have the ability
to capture aromatic guest compounds by sandwiching. The
following interesting features were clarified: (a) the rigid spacer
moiety of the tweezer 1, which keeps the suitable distance
between two acridines, plays an important role, and (b) the
association constants between 1 and guest compounds increase
as the electron density of the acridine groups increase [2]. We
have recently synthesized the molecular tweezer 2a having two
pentafluorophenyl groups, which accepts electron-rich aro-
matic guests in the concave (Fig. 1) [3] in contrast to the
opposite feature of 1. Thus, we demonstrated that: (1) the
association constants (Ka) between the tweezer 2a and aromatic
guests increase as the negative quadrupole moment values of
* Corresponding authors. Tel.: +81 86 251 8092; fax: +81 86 251 8092.
E-mail address: tsakai@cc.okayama-u.ac.jp (T. Sakai).
0022-1139/$ – see front matter # 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2005.12.007

T. Korenaga et al. / Journal of Fluorine Chemistry 127 (2006) 604–609
605
Fig. 1. Comparison of molecular tweezers.
we have developed an efficient coupling reaction of
C6F5B(OH)2 with aryl bromides in the presence of
Pd2(dba)3/P(t-Bu)3/CsF/Ag2O (Eq. (4)) [10], in which the
two points are the keys: (a) the use of CsF as a base, which
does not disturb the generation of active species from Pd
catalyst together with Ag2O, and (b) the use of P(t-Bu)3 ligand
for Pd catalyst, which enhanced the catalytic activity in the
reaction with aryl bromides. The newly developed conditions
should be more useful to facilitate the introduction of the
pentafluorophenyl group into aromatic compounds.
On the basis of above investigations, we report here
synthesis of a new tweezer 2c (Ar = 2,4-dimethylphenyl),
which is expected to prevent its dimerization by the bulkier
o-substituent in the aryl spacer (Ar) as compared to the easily
dimerizable tweezer 2a (Ar = 3,5-di-t-butylphenyl) as reported
previously [3]. The X-ray analysis showed that the pyridine
ring of the one molecule of 2a is locked into the slit between the
two C6F5 groups of the other molecule, in a single crystal [3].
The dimerization might negatively affect the host–guest inter-
action. Here, we further attempted to synthesize 2d (Ar = 2,4,
Thiemann et al. [7c]
(1)
(2)
Hocek and coworkers [7b]
Frohn et al. [9]
(3)
Korenaga et al. [10]
(4)

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T. Korenaga et al. / Journal of Fluorine Chemistry 127 (2006) 604–609
Fig. 2. Electrostatic potential surfaces of molecular tweezers.
6-trimethylphenyl) with a more hindered aryl spacer. The
problem which should be solved here is the low yield in the
final stage for introduction of the bispentafluorophenyl groups
to the precursor dibromide 6. We demonstrated that the yield
can be successfully improved by application of the modified
Suzuki–Miyaura coupling with C6F5B(OH)2 under the condi-
tions developed by us as mentioned above [10]. The reaction
was carried out by using compound 2b (Ar = Ph) which was
prepared as a typical compound for establishing these synthetic
pathways.
48 h, giving 6b (47% yield) [2e]. The dibromide 6c was
synthesized in a similar pathway but with a slight modification.
Thus, in the aldol reaction step, the reaction of aldehyde 3c with
4 under similar conditions (KOH in MeOH/H2O) gave no
product due to the steric hindrance of o-methyl group of 3c.
Therefore, ketone 4 was converted to the enolate anion by the
treatment with LDA in THF at ꢁ78 8C for 30 min and then
allowed to react with aldehyde 3c, giving 5c successfully in
83% yield. Cyclization of 5c to 6c was performed in a similar
way [2e]. The modified condition for the aldol reaction was also
effective for the reaction of 4 with 2,4,6-trimethylaldehyde 3d
to give 5d in 51% yield. However, the subsequent reaction of 5d
with 4 did not proceed; hence, synthesis of tweezer 2d could not
be accomplished. The dibromides 6b and 6c thus obtained were
converted to the molecular tweezers 2b (44%) and 2c (22%),
respectively, by treatment with t-BuLi in THF at ꢁ78 8C for
30 min and then with 40 eq. of C6F6 at 20 8C for 80 h [3].
The above-mentioned final step for introduction of the
pentafluorophenyl groups was successful, but resulted in a low
2. Results and discussion
New dibromide of 6c was synthesized in a similar pathway
to that for known compounds 6a and 6b (Scheme 1). Synthetic
pathway for 6b is briefly described as a typical example.
Aldol reaction of 4 [11] with 3b (KOH in MeOH/H2O, rt, 12 h)
gave 5b (91% yield), which was allowed to react with 4
(BF3ꢀEt2O, 100 8C, 4 h) and then with NH3/MeOH at 20 8C for
Scheme 1. Synthesis of molecular tweezers bearing pentafluorophenyl groups.

T. Korenaga et al. / Journal of Fluorine Chemistry 127 (2006) 604–609
607
Table 1
Suzuki–Miyaura coupling reaction for synthesis of 2b
Substrate
C6F5BRR0
Pd catalyst
Base
Solvent
Temperature (8C)
2b (%)
6b
6b
7
C6F5B(OH)2
C6F5BF3K
C6F5BF3K
C6F5B(OH)2
10% Pd(PPh3)4
K2CO3
K2CO3
K2CO3
CsF
DME
70
100
100
100
0
0
10% Pd(OAc)2/2 eq. PPh3
10% Pd(OAc)2/2 eq. PPh3
5% Pd2(dba)3/1.2 eq. P(t-Bu)3
Toluene
Toluene
DMF
20
62
6b
yield, and required a large excess of C6F6 with a long reaction
time. Therefore, we then tried to improve the yield by using
the Suzuki–Miyaura coupling reaction. The coupling reaction
of 6b with C6F5B(OH)2 under the usual conditions (Pd(PPh3)4,
K2CO3, DME, 70 8C for 12 h) did not proceed as was expected
(Table 1). Next, we tried the coupling reaction under the
Frohn’s condition [9]. However, our attempts for the coupling
reaction of dibromide 6b with C6F5BF3K gave no product
(Table 1). Hence, dibromide 6b was converted to the
corresponding diiodide 7 [(1) n-BuLi, THF, ꢁ78 8C, 30
min; (2) I2, 4 h; 80% for two steps] [2b], and then allowed to
react with C6F5BF3K in the presence of 10% Pd(OAc)2/PPh3/
Ag2O/K2CO3 in toluene at 100 8C for 14 h, giving the tweezer
2b in only 20% yield (Table 1). Finally, we performed the
coupling reaction of dibromide 6b with C6F5B(OH)2 in the
presence of 5% Pd2(dba)3/P(t-Bu)3/CsF/Ag2O (DMF, 100 8C,
12 h) [10]. The yield of the tweezer 2b was dramatically
improved up to 62% yield (Table 1). The method was found to
be effective for the preparation of molecular tweezers bearing
pentafluorophenyl groups.
4. Experimental
4.1. General methods
All reactions were carried out under a nitrogen atmosphere
with dry and freshly distilled solvents under anhydrous
conditions, unless otherwise noted. Tetrahydrofuran (THF)
was distilled from sodium, and dichloromethane (CH2Cl2) was
distilled from calcium hydride. Reactions were monitored by
thin-layer chromatography with precoated silica gel plates
(Merck 60 F254, plate length 40 mm). As a usual workup
procedure, the reaction mixture was extracted with ethyl acetate
(EtOAc). The organic layer was dried over MgSO4, filtrated
with suction, and then concentrated in vacuo. Preparative
column chromatography was carried out by using silica gel (Fuji
Silysia BW-127 ZH, 100–270 mesh). 1H and 13C NMR spectra
were measured at 200 MHz (or 300 MHz or 500 MHz) and
50 MHz (or 75 MHz), respectively, and chemical shifts are
given relative totetramethylsilane(TMS).19F NMRspectrawere
measured at 282 MHz, and chemical shifts are given relative to
CCl3F using C6F6 as secondary reference (ꢁ162.9 ppm).
X-ray analysis of the new tweezers 2b and 2c and estimation
of Ka values between these tweezwes and aromatic guests are
in progress.
4.2. 7-Benzylidene-2-bromo-a-tetralone (5b)
A mixture of 7-bromo-1-tetralone (4, 6.00 g, 26.7 mmol),
benzaldehyde (3b, 10.8 mL, 107 mmol), and potassium
hydroxide (1.28 g, 22.8 mmol) in methanol (75 mL) and water
(15 mL) was stirred at room temperature for 12 h. The mixture
was filtered, and then filtrate was washed with water. After
being dried in vacuo, 7-benzylidene-2-bromo-a-tetralone (5b)
was obtained in 91% yield (7.57 g) as a light yellow solid: mp
3. Conclusion
We succeeded in preparing new molecular tweezers 2b
and 2c. We demonstrated that the modified Suzuki–Miyaura
coupling reaction (C6F5B(OH)2, 5% Pd2(dba)3/P(t-Bu)3
/CsF/Ag2O, DMF) is effectively applicable to dibromide 6b
to prepare the new tweezer 2b. The established synthetic
pathway would be usable for design of new tweezers bearing
the pentafluorophenyl groups and an appropriate pendant
moiety.
1
135–137 8C; Rf = 0.40 (SiO2, hexane/EtOAc, 7:1); H NMR
(200 MHz, CDCl3) d 2.89 (t, J = 5.9 Hz, 2H), 3.13 (t, J = 5.9 Hz,
2H), 7.15 (d, J = 8.0 Hz, 1H), 7.39–7.45 (m, 5H), 7.60 (dd,
J = 2.0, 8.0 Hz, 1H), 7.88 (s, 1H), 8.25 (d, J = 2.0 Hz, 1H); 13
C

608
T. Korenaga et al. / Journal of Fluorine Chemistry 127 (2006) 604–609
NMR (50 MHz, CDCl3) d 26.9, 28.4, 121.0, 128.4, 128.7, 129.9,
130.0, 130.9, 134.5, 134.9, 135.5, 135.9, 137.5, 141.8, 186.5; IR
of 6c: mp 269–272 8C; Rf = 0.45 (SiO2, hexane/EtOAc, 7:1);
1H NMR (300 MHz, CDCl3) d 2.00 (s, 3H), 2.42 (s, 3H), 2.43–
2.80 (m, 8H), 6.92 (d, J = 7.8 Hz, 1H), 7.08 (d, J = 8.1 Hz, 2H),
7.12 (d, J = 7.8 Hz, 1H), 7.16 (s, 1H), 7.44 (dd, J = 2.1, 8.1 Hz,
2H), 8.68 (d, J = 2.1 Hz, 2H); 13C NMR (75 MHz, CDCl3) d
19.6, 21.3, 25.3, 27.6, 121.0, 126.9, 128.0, 129.1, 129.7, 130.9,
131.4, 133.8, 135.0, 136.5, 136.9, 137.6, 147.4, 148.9; IR (KBr)
3064, 3012, 2947, 1547 cmꢁ1; Anal. Calcd for C29H23Br2O: C,
63.87; H, 4.25; N, 2.57. Found: C, 63.55; H, 4.31; N, 2.53;
EIMS 70 eV, m/z (rel. int.) 543 [M]+ (54), 545 [M + 2]+ (100),
547 [M + 4]+ (50).
(KBr) 3061, 2951, 2839, 1666, 1601, 1582 cmꢁ1
.
4.3. 7-(2,4-Dimethyl)benzylidene-2-bromo-a-tetralone (5c)
To a solution of diisopropylamine (77.5 mL, 0.55 mmol) in
THF (27 mL) was added n-butyllithium (1.57 M in n-hexane,
0.32 mL, 0.50 mmol) at ꢁ20 8C, and then the solution was
stirred at ꢁ78 8C for 0.5 h. 7-Bromo-1-tetralone (4, 113 mg,
0.50 mmol) in THF (1.0 mL) was added to the resulting
solution. The solution was stirred at ꢁ78 8C for 0.5 h, and a
solution of 2,4-dimethylbenzaldehyde (3c, 66.0 mL,
0.50 mmol) in THF (0.50 mL) was added to the mixture.
After being stirred at room temperature for 29 h, the mixture
was quenched with 10% HCl aq. (50 mL). The resulting
mixture was treated in a usual manner. The resulting solid was
purified by column chromatography (toluene/hexane, 2:1) to
give 5c (142 mg, 83% yield) as an orange solid: mp 125–
129 8C; Rf = 0.63 (SiO2, toluene/EtOAc, 30:1); 1H NMR
(200 MHz, CDCl3) d 2.32 (s, 3H), 2.36 (s, 3H), 2.86 (t,
J = 5.9 Hz, 2H), 2.99 (t, J = 5.9 Hz, 2H), 7.01–7.10 (m, 3H),
7.14 (d, J = 8.0 Hz, 1H), 7.59 (dd, J = 1.8, 8.0 Hz, 1H), 7.92 (s,
1H), 8.26 (d, J = 1.8 Hz, 1H); 13C NMR (50 MHz, CDCl3) d
20.0, 21.3, 27.1, 28.7, 120.9, 126.1, 128.8, 130.0, 130.9, 131.1,
131.7, 134.5, 135.0, 135.9, 136.6, 137.9, 138.8, 142.0, 186.7;
IR (KBr) 3078, 2989, 2897, 1659, 1582 cmꢁ1; Anal. Calcd for
C19H17BrO: C, 66.87; H, 5.02. Found: C, 66.50; H, 4.91.
4.6. 2,12-Bis(pentafluorophenyl)-7-phenyl-5,6,8,9-
tetrahydro-dibenzo[c,h]acridine (2b) by nucleophilic
substitution
To a solution of 6b (207 mg, 0.40 mmol) in dry THF
(10 mL) was added t-BuLi (1.5 M in n-hexane, 0.64 mL,
0.96 mmol) at ꢁ78 8C. After this mixture had been stirred for
0.5 h, hexafluorobenzene (1.8 mL, 16 mmol) was added. The
resulting mixture was stirred at ꢁ78 8C for 84 h. The solvent
was removed under reduced pressure, and CHCl3 was added.
The solution was washed with water, and dried over Na2SO4.
The solution was concentrated under reduced pressure. The
residue was recrystallized from CHCl3 and MeOH to give 10
(121 mg, 44% yield) as a white powder: mp > 300 8C;
Rf = 0.40 (SiO2, hexane/toluene, 3:1); 1H NMR (500 MHz,
CDCl3) d 2.70–2.91 (m, 8H), 7.23 (m, 2H), 7.30–7.37 (m, 4H),
7.44–7.54 (m, 3H), 8.58 (s, 2H); 19F NMR (282 MHz, CDCl3) d
ꢁ172.4 (m, 4F), ꢁ155.4 (t, J = 20.9 Hz, 2F), ꢁ142.7 (m, 4F);
IR (KBr) 2934, 2845, 1558, 1495 cmꢁ1; HR-MS (EI) required
for C39H19NF10: 691.1358; found (molecular ion, m/z):
691.1379.
4.4. 2,12-Dibromo-7-(2,4-dimethyl)phenyl-5,6,8,9-
tetrahydro-dibenzo[c,h]acridine (6b)
7-Bromo-1-tetralone (4, 2.8 g, 12.6 mmol) and 5b (4.14 g,
13.2 mmol) with boron trifluoride–ether complex (2.53 mL,
20.2 mmol) were heated at 100 8C for 4 h. The reaction mixture
was permitted to cool to room temperature and was dissolved in
CH2Cl2 (20 mL) and methanol (5.0 mL). To the solution was
added an ammonia–methanol solution (7.0 M, 234 mL,
1.64 mol) at 0 8C. After being warmed to room temperature,
the mixture was stirred for 48 h. The solution was concentrated
under reduced pressure. The residue was filtered, washed with
methanol, and dried in vacuo. The resulting solid was
recrystallized from CH2Cl2 to give 6b (3.06 g, 47% yield):
mp 245–248 8C; Rf = 0.35 (SiO2, hexane/acetone, 2:1); 1H
NMR (300 MHz, CDCl3) d 2.62–2.80 (m, 8H), 7.08 (d,
J = 8.1 Hz, 2H), 7.19 (dd, J = 1.8, 8.1 Hz, 2H), 7.42–7.53 (m,
5H), 8.65 (d, J = 1.8 Hz, 2H); 13C NMR (75 MHz, CDCl3) d
25.7, 27.6, 121.0, 127.7, 128.1, 128.4, 128.6, 129.1, 129.5,
131.5, 136.4, 136.8, 137.3, 147.7, 148.9; IR (KBr) 3048, 2937,
2830, 1547 cmꢁ1; EIMS 70 eV, m/z (rel. int.) 515 [M]+ (56),
517 [M + 2]+ (100), 519 [M + 4]+ (48).
4.7. 2,12-Bis(pentafluorophenyl)-7-(2,4-dimethyl)phenyl-
5,6,8,9-tetrahydro-dibenzo[c,h]acridine (2c)
According to the same procedure as described for 2b, 16 mg
(22%) of 2c was obtained from 55 mg of 6c as a white powder:
mp > 300 8C; Rf = 0.40 (SiO2, hexane/toluene, 3:1); 1H NMR
(500 MHz, CDCl3) d 2.03 (s, 3H), 2.42 (s, 3H), 2.52–2.89 (m,
8H), 6.96 (d, J = 7.5 Hz, 1H), 7.14 (d, J = 8.0 Hz, 1H), 7.18 (s,
1H), 7.34 (m, 4H), 8.60 (s, 2H); 19F NMR (282 MHz, CDCl3) d
ꢁ163.6 (m, 4F), ꢁ157.1 (t, J = 21.2 Hz, 2F), ꢁ144.5 (m, 4F);
IR (KBr) 2935, 2843, 2361, 1558, 1520, 1497, 1435, 1366,
1061, 991, 891, 826, 772, 610, 563, 513 cmꢁ1; Anal. Calcd for
C41H23F10N: C, 68.04; H, 3.32; N, 1.97. Found: C, 68.43; H,
3.22; N, 1.95.
4.8. 2,12-Diiodo-7-(2,4-dimethyl)phenyl-5,6,8,9-
tetrahydro-dibenzo[c,h]acridine (7)
4.5. 2,12-Dibromo-7-(2,4-dimethyl)phenyl-5,6,8,9-
tetrahydro-dibenzo[c,h]acridine (6c)
To a solution of 6b (1.39 g, 2.68 mmol) in dry THF
(130 mL) was added n-butyllithium (1.58 M in n-hexane,
3.73 mL, 5.89 mmol) at ꢁ78 8C, and the mixture was stirred for
0.5 h. A solution of iodine (3.40 g, 13.4 mmol) in dry THF
(40 mL) was added to the reaction mixture, and the mixture was
According to the same procedure as described for 6b,
58 mg (35%) of 6c was obtained from 68 mg of 4 and 55 mg

T. Korenaga et al. / Journal of Fluorine Chemistry 127 (2006) 604–609
609
stirred for 4 h. An aqueous solution of sodium thiosulfate
(26 mL) was added, and the mixture was stirred at room
temperature for 0.5 h. The mixture was treated in a usual
manner. The residue was recrystallized from EtOAc twice to
give 7 (982 mg, 60% yield) as a light yellow solid: mp 262–
266 8C; Rf = 0.45 (SiO2, hexane/CH2Cl2, 2:1); 1H NMR
(300 MHz, CDCl3) d 2.63–2.75 (m, 8H), 6.95 (d, J = 8.1 Hz,
2H), 7.18 (dd, J = 1.8, 8.1 Hz, 2H), 7.43–7.52 (m, 3H), 7.64
(dd, J = 1.8, 8.1 Hz, 2H), 8.81 (d, J = 1.8 Hz, 2H); 13C NMR
(75 MHz, CDCl3) d 25.7, 27.7, 92.4, 127.7, 128.4, 128.6, 129.3,
129.4, 134.0, 137.0, 137.1, 137.3, 137.5, 147.7, 148.9; IR (KBr)
Laboratory of Okayama University for 1H, 13C, and 19F NMR
measurements. We are grateful to Dr. Teruhiko Nitoda in
Okayama University for his kind assistance in the measurement
of mass spectra.
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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. Komar-
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Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W.
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4.10. 2,12-Bis(pentafluorophenyl)-7-phenyl-5,6,8,9-
tetrahydro-dibenzo[c,h]acridine (2b) by Suzuki–Miyaura
coupling reaction using C6F5B(OH)2
A 20 mL of a dry Schlenk tube was charged with 6b (52 mg,
0.10 mmol) and DMF (2 mL) under an argon atmosphere.
Argon was bubbled through the solution for 10 min. To the
solution were added pentafluorophenylboronic acid (47 mg,
0.22 mmol), tris(dibenzylideneacetone)dipalladium (4.8 mg,
5.0 mmol), CsF (61 mg, 0.40 mmol), Ag2O (56 mg, 0.24 mmol),
and P(t-Bu)3 (4 mL, 16 mmol) with a gentle flow of argon. The
reaction mixture was stirred at 100 8C for 12 h. The resulting
mixture was filtered through a pad of Celite and washed with
CHCl3. The filtrate was concentrated under reduced pressure.
The residuewas purified by recrystallization from CHCl3/MeOH
(2:1) to give 2b (43 mg, 62% yield) as a white powder.
(b) F. Bellina, A. Carpita, R. Rossi, Synthesis (2004) 2419;
(c) S. Kotha, K. Lahiri, D. Kashinath, Tetrahedron 58 (2002) 9633;
(d) J. Hassan, M. Sevignon, C. Gozzi, E. Schulz, M. Lemaire, Chem. Rev.
102 (2002) 1359.
´ ˇ´
[7] (a) M. Havelkova, D. Dvorak, M. Hocek, Synthesis (2000) 1704;
ˇ
(b) M. Havelkova, M. Hocek, M. Cesnek, D. Dvorak, Synlett (1999)
´
ˇ´
1145;
(c) T. Thiemann, K. Umeno, D. Ohira, E. Inohae, T. Sawada, S. Mataka,
New J. Chem. 23 (1999) 1067.
Acknowledgements
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7340.
[9] H.-J. Frohn, N.Y. Adonin, V.V. Bardin, V.F. Starichenko, Tetrahedron Lett.
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This work was partially supported by Okayama Foundation
for Science and Technology, and a Grant-in-Aid for Young
Scientists (B) from the Ministry of Education, Culture, Sports,
Science, and Technology, Japan. We thank the SC-NMR
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