Reductive radical reaction of gem-difluorinated organoselenium compounds with an indium(III) chloride-sodium borohydride system

Article abstract of DOI:10.1055/s-2006-926420

As for gem-difluorinated phenylseleno compounds, indium hydride, in situ generated by transmetallation between InCl₃ and NaBH₄, was found to be a convenient radical reagent as an alternative to tributyltin hydride. Besides its excell

Full text of DOI:10.1055/s-2006-926420

PAPER
1475
Reductive Radical Reaction of gem-Difluorinated Organoselenium
Compounds with an Indium(III) Chloride–Sodium Borohydride System
R
eductive Radica
l
i
Reactio
n
n
of ge
m
-D
i
fluorinated O
-
r
ganose
Y
lenium
C
ompound
i
s
ng Qin,a Yan-Yan Yang,b Xiao-Long Qiu,b Feng-Ling Qing*a,b
a
College of Chemistry and Chemical Engineering, Donghua University, 1882 West Yanan Lu, Shanghai 200051, P. R. of China
Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
354 Fenglin Lu, Shanghai 200032, P. R. of China
b
Fax +86(21)64166128; E-mail: flq@mail.sioc.ac.cn
Received 9 November 2005; revised 6 December 2005
According to our reported approach,5 we firstly synthe-
sized a series of a,a-difluoro-b-hydroxy selenides 1,
which were supposed to be converted into cyclization pre-
cursors 2 by allylation with allyl bromide. Considering
Abstract: As for gem-difluorinated phenylseleno compounds, indi-
um hydride, in situ generated by transmetallation between InCl3 and
NaBH4, was found to be a convenient radical reagent as an alterna-
tive to tributyltin hydride. Besides its excellent performance in in-
tramolecular cyclization and radical deselenylation, the NaBH4/ that the strong basicity of NaH would result in the dehy-
InCl3 system also succeeded in intermolecular radical addition be-
tween organoselenium compounds and styrene.
drofluorination of a,a-difluoro-b-hydroxy selenides, the
inorganic base KOH10 was used for the allylation of alco-
Key words: radical deselenylation, radical addition, organoseleni-
um compounds, bromodifluoromethyl phenyl selenide
hol 1 providing the desired organoseleno compounds 2 in
85–92% yield (Table 1). With cyclization precursors 2 in
hand, we carried out the reductive radical cyclization us-
ing the Baba group protocol.4 Surprisingly, we found that
As the most useful and most common radical reagent, although an InCl3 catalyst loading of 10 mol% was report-
tributyltin hydride (Bu3SnH) was extensively used in rad- ed to be effective for the cyclization of alkyl bromide and
ical intramolecular cyclization,1 radical dehalogenation2 aryl iodide,4 one equivalent of InCl3 is needed to complete
and intermolecular radical addition.3 However, the toxic- the cyclization of organoseleno compounds 2. The result-
ity of tributyltin compounds renders themselves defective, ant tetrahydrofuran derivatives 3 were obtained as mix-
which limits their large-scale utilization in industry ex- tures of trans and cis isomers in moderate yields (Table 1,
tremely. Exhilaratingly, the convenient and nontoxic sys- entries 1, 2). It should be mentioned that although reac-
tem NaBH4/InCl3, in which indium hydride (Cl2InH) was tions of organoseleno compounds 2c,d with the InCl3/
generated in situ, was recently reported by Baba and co- NaBH4 system were found to proceed well (19F NMR
workers to be effective as an alternative to tributyltin hy- spectroscopy), there are no corresponding products isolat-
dride.4 It is noteworthy that this system works well for ed because of the high volatility and low boiling point of
radical dehalogenation, radical intramolecular cyclization the products (Table 1, entries 3, 4).
and intermolecular radical addition of alkyl halides under
very mild conditions without using any initiators. On the
other hand, our group recently developed a novel fluori-
nating reagent, PhSeCF2TMS, which could be conve-
4-methyltetrahydrofuran in moderate to high yield and in
Engman’s group reported that a series of Bu3SnH-mediat-
ed reductive radical cyclizations of O-allylated b-hy-
droxyalkyl phenyl selenides furnished the corresponding
niently applied to synthesize a,a-difluoro-b-hydroxy
selenides.5 As vinylic pseudohalide groups,6 the phenylse-
lenium compounds were reported to undergo reductive
radical cyclization,7radical deselenylation8 and intermo-
lecular radical addition9 in a similar manner to the
Bu3SnH-mediated reaction of alkyl halides, which reflect-
ed the great potential of PhSeCF2TMS as a fluorinated
building block. Based on our ongoing project on the utili-
zation of PhSeCF2TMS and as an extension to the utiliza-
tion scope of the NaBH4/InCl3 system, we wish to report
herein the NaBH4/InCl3-mediated radical intramolecular
cyclization, radical deselenylation and radical intermolec-
ular addition of gem-difluoromethyl-containing phenylse-
leno compounds.
low cis/trans selectivity (cis/trans = 2:3 to 1:10).7a Al-
though lowering the reaction temperature can enhance the
cis/trans selectivity to a certain extent, the yield decreased
at the same time.7a Slightly surprisingly, InCl3/NaBH4-
mediated reductive radical cyclization of O-allylated b-
hydroxy-a,a-difluoroalkyl phenyl selenides 2 gave 4-meth-
yltetrahydrofurans 3 with very high cis/trans selectivity,
rendering the predominant trans isomer with a ratio of
16:1 to 20:1 over the cis isomer. In our opinion, higher di-
astereoselectivity may result from the utilization of the
NaBH4/InCl3 system and/or the steric hindrance of the
fluoride atoms, although the exact reason is not clear yet
and remains to be investigated.
The assignment of trans/cis isomers of tetrahydrofuran
derivatives 3 was based on NOESY experiments. As a
representative example, the NOESY experiment observed
for the trans isomer of compound 3a is shown in Figure 1.
No NOESY correlation between H1 and H4 of the furan
ring was observed.
SYNTHESIS 2006, No. 9, pp 1475–1479
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Advanced online publication: 04.04.2006
DOI: 10.1055/s-2006-926420; Art ID: F19505SS
© Georg Thieme Verlag Stuttgart · New York

1476
Y.-Y. Qin et al.
PAPER
Table 1 Reductive Radical Cyclization Products and Organoselenium Intermediates
PhSe
R
OH
F2C
Br
, KOH
CF2
InCl3, NaBH4
MeCN
R
C
H
CF2SePh
R
O
TBAB, CH2Cl2
O
1
2
3
a,a-Difluoro-b-hydroxy selenides 1
O-Allylation intermediates 2
Reductive radical cyclization products 3
(% yield,b trans/cis ratioc)
(% yield)a
OH
PhSe
CF2
F2C
NC
C
H
CF2SePh
O
O
1a
NC
NC
3a (56%; 20/1)
2a (92%)
PhSe
CF2
Cl
F2C
OH
Cl
C
H
CF2SePh
O
O
Cl
1b
Cl
Cl
Cl
3b (65%; 16/1)
2b (86%)
d
OH
PhSe
CF2
O
C
H
CF2SePh
1c
2c (90%)
d
OH
PhSe
C3H7
CF2
C3H7
C
H
1d
CF2SePh
O
2d (85%)
a Isolated yields.
b Isolated yields of trans isomers.
c Determined by 19F NMR spectroscopy.
d Product could not be isolated due to high volatility and low boiling point.
no NOE
H4
NOE
NaBH4/InCl3, the desired difluoromethyl alcohol 4 was
formed in 83% yield (Scheme 1), which was a bit lower
than the yield obtained by using the Bu3SnH/AIBN sys-
tem. Although Baba’s group reported that the alkyl halide
underwent radical dehalogenation with a catalytic amount
of InCl3 (0.1 equiv),4 we found that even one equivalent of
InCl3 was not enough for completion of the deselenylation
reaction of compound 1c. Using an amount of about ten
equivalents of NaBH4/InCl3 led to completion of the reac-
tion within six hours.
H1
CF2
H
O
H
NOE
CN
Figure 1 NOE correlation from NOESY spectra of trans-3a
Although the reduction of the phenylseleno group to a hy-
drogen atom could be readily achieved by traditional
treatment with Bu3SnH/AIBN,5,8 there is, to the best of our
knowledge, no correlative report on the reduction of the
PhSe group to a hydrogen atom with the NaBH4/InCl3 sys-
tem. In view of the distinct advantage of the NaBH4/InCl3
system over the Bu3SnH/AIBN system,4 we also investi-
gated the NaBH4/InCl3-mediated radical deselenylation of
a,a-difluoro-b-hydroxy phenyl selenide. We were pleased
to discover that, upon treatment of compound 1c with
It has been discovered that phenylseleno esters were ef-
fective precursors of acyl radicals for the intermolecular
alkene addition,9 which is a potentially useful method for
carbon–carbon bond formation. Thus, a,a-difluoro phenyl
selenide was also envisioned as the precusor of the gem-
difluoromethyl radical for intermolecular alkene addition.
Considering that alkyl bromides could also undergo a rad-
ical reaction in the NaBH4/InCl3 system4 and the fact that
the bromine atom bound to the fluorocarbon moiety is
more reactive than the one bound to the hydrocarbon moi-
ety,11 PhSeCF2CH2CH2Ph (5), prepared via SmI2-mediat-
ed reaction of bromodifluoromethyl phenyl selenide with
OH
OH
InCl3, NaBH4
MeCN
C
H
CF2SePh
C
H
CF2H
1c
4 (83%)
Scheme 1
Synthesis 2006, No. 9, 1475–1479 © Thieme Stuttgart · New York

PAPER
Reductive Radical Reaction of gem-Difluorinated Organoselenium Compounds
1477
SmI2, i-PrOH
THF
F
F
F
F
+
PhSeCF2CH2CH2Ph
PhSeCF2Br
Ph
Bu3SnH, AIBN
benzene
Ph
+
Se
Ph
Ph
Ph
Ph
5 (58%)
5
6 (72%)
Scheme 2
F
F
Bu3SnH, AIBN
benzene
+
PhSeCF2Br
Ph
Ph
Ph
styrene (Scheme 2), was used as the substrate for studying
this kind of reaction.
6 (32%)
Scheme 3
With compound 5 in hand, we studied the NaBH4/InCl3-
mediated intermolecular radical addition of compound 5
to styrene (Table 2). Besides by-product 1,1-difluoro-3-
phenylpropane (7), the addition product 6 was afforded in
46% yield upon treatment of phenyl selenide 5 with sty-
rene using the NaBH4 (1.5 equiv)/InCl3(1.5 equiv) sys-
tem. As it can be seen from Table 2, the amount of InCl3
had a great influence on the reaction products. An excess
of InCl3 and NaBH4 greatly reduced the yield of addition
product 6 and increased yield of by-product 7.
In summary, we investigated the NaBH4/InCl3-mediated
radical intramolecular cyclization of O-allylated b-hy-
droxy-a,a-difluoroalkyl phenyl selenides 2 and found that
the reaction proceeded well to afford 3,3-difluoro-4-meth-
yl-tetrahydrofuran derivatives in high cis/trans selectivity.
In addition, NaBH4/InCl3-mediated radical deselenylation
and radical intermolecular addition of gem-difluorometh-
yl-containing phenylseleno compounds were also ad-
dressed. These studies demonstrate that the NaBH4/InCl3
system could not entirely substitute the Bu3SnH/AIBN
system in radical reactions despite its non-toxicity. Both
systems can be efficiently utilized in different radical
reactions, respectively, and they could supplement each
other in some of them.
Bu3SnH/AIBN-mediated intermolecular radical addition
of PhSeCF2CH2CH2Ph with styrene was also carried out
in order to compare this system with the InCl3/NaBH4
one. The desired addition product 6 was provided in 72%
yield (Scheme 3), which indicated that the NaBH4/InCl3
system was less effective in the intermolecular radical ad-
dition of phenylselenium compounds than the Bu3SnH/
AIBN system. In addition, we were pleasantly surprised to THF and benzene were distilled from Na metal. CH2Cl2 was dis-
tilled from CaH2. All melting points are uncorrected. 1H NMR and
find that addition product 6 could be also afforded in 32%
13C NMR spectra were recorded on a Bruker AM300 spectrometer.
yield by Bu3SnH/AIBN-mediated reaction of PhSeCF2Br
19F NMR was recorded on a Bruker AM300 spectrometer (FCCl3 as
with styrene in one step, which further demonstrated the
outside standard and low field is positive). Chemical shifts (d) are
great potential of PhSeCF2Br as gem-difluoromethylene
reported in ppm, and coupling constants (J) in Hz. IR spectra were
diradical precursor. Although bromodifluorophenylsulfa-
nylmethane (PhSCF2Br) was also reported recently as
synthetic equivalent of the gem-difluoromethylene dirad-
ical synthon,12 three-step procedures, involved in the rad-
ical addition of the bromide atom (SmI2, Bu3SnH/AIBN
or Et3B/Bu3SnH/O2), the oxidation of the phenylthio
group (m-CPBA) and the final radical addition of sulfone
(SmI2), were needed to profit from its function as biradical
synthon. Therefore, PhSeCF2Br presented a distinct ad-
vantage over PhSCF2Br when acting as gem-difluoro-
methylene diradical synthon.
recorded on a Shimadzu IR-440 spectrometer and MS spectra on a
Finnigan-MAT-8430 instrument using EI at 70 eV. Petroleum ether
(PE) used for flash chromatography had a boiling range of 30–60
°C.
1,1-Difluoro-2-(allyloxy)-2-(4¢-cyanophenyl)ethyl Phenyl
Selenide (2a)
To a solution of the alcohol 1a (163 mg, 0.48 mmol), allyl bromide
(0.21 mL, 2.4 mmol) and tetrabutylammonium bromide (50 mg,
0.16 mmol) in CH2Cl2 (0.69 mL) was added aq KOH (5 M, 0.96
mL). The resultant mixture was vigorously stirred at r.t. After about
4 h, the biphasic system was treated with CH2Cl2 (10 mL) and sat.
aq NH4Cl (20 mL). The two layers were separated and the aqueous
Table 2 Indium-Catalyzed Intermolecular Radical Addition of Phenylseleno Group
F
F
F
F
InCl3, NaBH4
MeCN
CF2H
Ph
+
+
Ph
Ph
Ph
Ph
Se
Ph
5
6
7
Entry
InCl3 (equiv)
NaBH4 (equiv)
Ratio of 6/7a
1/24
1
2
3
4
5
10
5
12
6
1/10
2.5
1.5
1
3
1/2.7
1.5
1
1/0.9
1/0.5
a Determined by 19F NMR.
Synthesis 2006, No. 9, 1475–1479 © Thieme Stuttgart · New York

1478
Y.-Y. Qin et al.
PAPER
phase was extracted with CH2Cl2 (2 × 20 mL). The combined organ-
ic phases were dried (Na2SO4). After filtration and removal of the
solvent in vacuo, the resultant residue was purified by flash chroma-
tography (PE) to give pure compound 2a (167 mg, 92%) as a color-
less oil.
1,1-Difluoro-3-phenylpropyl Phenyl Selenide (5)
Under a N2 atmosphere, samarium powder (40 mesh, 190 mg) was
added to a solution of CH2I2 (60 mL, 0.75 mmol) in THF (10 mL).
The mixture was stirred for 1 h at r.t. Then, a solution of bromodi-
fluoromethyl phenyl selenide (143 mg, 0.5 mmol), styrene (0.12 mL,
1 mmol) and i-PrOH (77 mL, 1 mmol) in THF (2.5 mL) was added
at 0 °C. The resultant mixture was stirred for 15 min at 0 °C and
then allowed to warm up to r.t. within an additional 2 h. The reaction
was quenched with sat. aq NaHCO3 solution (10 mL). The organic
layer was separated and the aqueous layer was extracted with
EtOAc (3 × 10 mL). The combined organic layers were dried
(Na2SO4). After filtration and removal of the solvent in vacuo, the
resultant residue was purified by flash chromatography with PE to
give compound 5 (90 mg, 58%) as a yellow oil.
IR (neat): 2871, 2231, 1579, 1478, 1440, 1098 cm–1.
1H NMR (300 MHz, CDCl3): d = 3.96 (dd, J = 12.6, 6.3 Hz, 1 H),
4.14 (dd, J = 12.9, 5.4 Hz, 1 H), 4.76 (dd, J = 12.9, 7.2 Hz, 1 H),
5.25–5.32 (m, 2 H), 5.90 (ddd, J = 23.4, 11.4, 6.6 Hz, 1 H), 7.26–
7.42 (m, 3 H), 7.53–7.56 (m, 2 H), 7.62–7.68 (m, 4 H).
13C NMR (75 MHz, CDCl3): d = 71.5, 82.7 (dd, J = 27.2, 24.0 Hz),
113.1, 118.4, 118.8, 123.8, 125.0 (t, J = 298 Hz), 129.1, 129.2,
129.4, 132.0, 133.0, 137.2, 139.4.
19F NMR (282 MHz, CDCl3): d = –75.7 (dd, J = 212, 5.1 Hz, 1 F),
–80.7 (dd, J = 210, 10.2 Hz, 1 F).
MS (EI, 70 eV): m/z (%) = 378 (M+, 2), 322 (3), 172 (41), 157 (10),
IR (neat): 3064, 3030, 2930, 1580, 1478, 1440, 1164, 1033 cm–1.
1H NMR (300 MHz, CDCl3): d = 2.33–2.49 (m, 2 H), 2.87–2.92 (m,
2 H), 7.14–7.44 (m, 8 H), 7.72–7.75 (m, 2 H).
13C NMR (75 MHz, CDCl3): d = 29.7 (t, J = 4.0 Hz), 42.0 (t,
J = 21.4 Hz), 126.4, 128.3, 128.5 (t, J = 301 Hz), 128.6, 129.3,
129.4, 130.2, 137.0, 139.8.
130 (17), 77 (12), 41 (100).
HRMS (EI): m/z calcd for C18H15F2NOSe: 379.0286; found:
379.0301.
19F NMR (282 MHz, CDCl3): d = –70.2 (t, J = 14.1 Hz).
MS (EI): m/z (%) = 312 (M+, 19), 158 (18), 91 (100), 77 (11).
3,3-Difluoro-4-methyl-2-(4¢-cyanophenyl)-tetrahydrofuran (3a)
A round-bottomed flask charged with InCl3 (62 mg, 0.28 mmol)
was heated at 150 °C in vacuo for 1 h. After N2 was filled in, NaBH4
(11 mg, 0.28 mmol) and MeCN (0.6 mL) were added and the result-
ant mixture was stirred at –78 °C for 5 min. After the heterogeneous
solution was warmed up to r.t., 2a (76 mg, 0.2 mmol) was added and
the resulting mixture was stirred for 6 h. Then, deionized water was
added and the reaction mixture was extracted with Et2O (3 × 20
mL). The combined organic phases were dried (Na2SO4). After fil-
tration and removal of the solvent in vacuo, the residue was purified
by flash chromatography with PE to give pure compound 3a (25
mg, 56%) as a pale-yellow oil.
Anal. Calcd for C15H14F2Se: C, 57.89; H, 4.53. Found: C, 58.26; H,
4.72.
3,3-Difluoro-1,5-diphenylpentane (6)14
Method A: InCl3 (33 mg, 0.15 mmol) was heated at 150 °C in vacuo
for 1 h. Then, N2 was filled into the vacuo system and a solution of
NaBH4 (6 mg, 0.15 mmol) in MeCN (0.3 mL) was added. The mix-
ture was stirred at –78 °C for 5 min. After the heterogeneous mix-
ture was warmed up to r.t., 5 (31 mg, 0.1 mmol) and styrene (0.12
mL, 1.0 mmol) were added and the resulting mixture was stirred for
6 h. The reaction was quenched with deionized water. The reaction
mixture was extracted with Et2O (3 × 20 mL) and the combined or-
ganic phases were dried (Na2SO4). After filtration and removal of
the solvent in vacuo, the resultant residue was purified by flash
chromatography with PE to give compound 6 (12 mg, 46%) as a
pale-yellow oil.
IR (neat): 3065, 2984, 2231, 1613, 1507, 1459, 1195, 1068 cm–1.
1H NMR (300 MHz, CDCl3): d = 1.16 (d, J = 6.9 Hz, 3 H), 2.60 (tt,
J = 25.5, 8.1 Hz, 1 H), 3.69 (t, J = 9.0 Hz, 1 H), 4.41 (t, J = 8.3 Hz,
1 H), 4.89 (t, J = 12.3 Hz, 1 H), 7.50–7.70 (m, 4 H).
13C NMR (75 MHz, CDCl3): d = 10.0 (dd, J = 7.4, 5.1 Hz), 40.2 (dd,
J = 24.8, 21.2 Hz), 72.7 (dd, J = 5.4, 2.6 Hz), 82.3 (dd, J = 32.9,
25.4 Hz), 112.4, 118.6, 127.4, 127.8 (t, J = 255 Hz), 132.1, 140.0.
19F NMR (282 MHz, CDCl3): d = –105.6 (ddd, J = 230, 18.0, 11.8
Hz, 1 F), –113.5 (dt, J = 228, 10.7 Hz, 1 F).
MS (EI): m/z (%) = 223 (M+, 30), 132 (34), 102 (11), 92 (29), 77
Method B: Under N2, to a solution of compound 5 (15 mg, 0.05
mmol) in anhyd benzene (0.8 mL) was added AIBN (3 mg, 0.015
mmol). Then, the mixture was heated to 90 °C and tributyltin hy-
dride (17 ml, 0.06 mmol) was added dropwise. The reaction mixture
was heated to reflux until TLC showed the complete consumption
of the starting materi al. After cooling the solution to r.t., the solvent
was removed in vacuo. The residue was purified by flash chroma-
tography with PE to give compound 6 (9 mg, 72%) as a pale-yellow
oil.
(100).
HRMS (EI): m/z calcd for C12H11F2NO: 223.0809; found:
223.0811.
Method C: Under N2, to a solution of bromodifluoromethyl phenyl
selenide (29 mg, 0.1 mmol) in anhyd benzene (1.5 mL) was added
AIBN (10 mg, 0.06 mmol). Then, the mixture was heated to 90 °C
and tributyltin hydride (69 ml, 0.25 mmol) was added dropwise. The
reaction mixture was heated to reflux until TLC showed the com-
plete consumption of the starting material. After cooling the solu-
tion to r.t., the solvent was removed in vacuo. The residue was
purified by flash chromatography with PE to give compound 6
(7 mg, 32%) as a pale-yellow oil.
1H NMR (300 MHz, CDCl3): d = 2.09–2.26 (m, 4 H), 2.82 (t,
J = 8.4 Hz, 4 H), 7.18–7.33 (m, 10 H).
13C NMR (75 MHz, CDCl3): d = 28.6 (t, J = 5.25 Hz), 38.5 (t,
J = 25.1 Hz), 124.2 (t, J = 247 Hz), 126.3, 128.3, 128.6, 140.7.
2,2-Difluoro-1-phenylethanol (4)13
A round-bottomed flask charged with InCl3 (220 mg, 1.0 mmol)
was heated at 150 °C in vacuo for 1 h. After N2 was filled in, NaBH4
(45 mg, 1.2 mmol) and MeCN (3 mL) were added and the mixture
was stirred at –78 °C for 5 min. After the heterogeneous solution
was warmed up to r.t., 1c (31 mg, 0.1 mmol) was added and the re-
sulting mixture was stirred for 6 h. Then, deionized water was added
and the reaction mixture was extracted with Et2O (3 × 20 mL). The
combined organic phases were dried (Na2SO4). After filtration and
removal of the solvent in vacuo, the resultant residue was purified
by flash chromatography (PE–EtOAc, 9:1) to give compound 4 (13
mg, 83%) as a pale-yellow oil.
1H NMR (400 MHz, CDCl3): d = 2.34 (br s, 1 H), 4.80–4.86 (m, 1
H), 5.77 (td, J = 56.0, 4.8 Hz, 1 H), 7.35–7.44 (m, 5 H).
19F NMR (282 MHz, CDCl3): d = –127.4 (ddd, J = 286, 56, 9.6 Hz,
1 F), –128.0 (ddd, J = 286, 56, 9.6 Hz, 1 F).
19F NMR (282 MHz, CDCl3): d = –99.8 (q, J = 15.8 Hz).
Synthesis 2006, No. 9, 1475–1479 © Thieme Stuttgart · New York

PAPER
Reductive Radical Reaction of gem-Difluorinated Organoselenium Compounds
1479
(5) Qin, Y.-Y.; Qiu, X.-L.; Yang, Y.-Y.; Meng, W.-D.; Qing, F.-
L. J. Org. Chem. 2005, 70, 9040.
Acknowledgment
We thank the National Natural Science Foundation of China, Mini-
stry of Education of China and Shanghai Municipal Scientific Com-
mittee for funding this work.
(6) (a) Huang, X.; Ma, Y. Synthesis 1997, 417. (b) Comasseto,
J. V.; Lo, W. L.; Petragnani, N. Tetrahedron 1997, 53, 7445.
(c) Denis, J. N.; Krief, A. Tetrahedron Lett. 1982, 23, 3411.
(d) Procter, D. J. J. Chem. Soc., Perkin Trans. 1 2001, 335.
(7) (a) Engman, L.; Gupta, V. J. Org. Chem. 1997, 62, 157.
(b) Rawal, V. H.; Singh, S. P.; Dufour, C.; Michoud, C. J.
Org. Chem. 1993, 58, 7718.
(8) (a) Miyashita, M.; Suzuki, T.; Yoshikoshi, A. J. Am. Chem.
Soc. 1989, 111, 3728. (b) Kang, Y. H.; Kim, K. Tetrahedron
1999, 55, 4271. (c) Ternon, M.; Outurquin, F.; Paulmier, C.
Tetrahedron 2001, 57, 10259. (d) Billimoria, A. D.; Cava,
M. P. J. Org. Chem. 1994, 59, 6777. (e) Kamimura, A.;
Omata, Y.; Mitsudera, H.; Kakehi, A. J. Chem. Soc., Perkin
Trans. 1 2000, 4499.
References
(1) (a) Walton, R.; Fraser-Reid, B. J. Am. Chem. Soc. 1991, 113,
5791. (b) Baguley, P. A.; Binmore, G.; Milne, A.; Walton, J.
C. Chem. Commun. 1996, 18, 2199. (c) Crimmins, M. T.;
O’Mahony, R. J. Org. Chem. 1989, 54, 1157. (d) Marco-
Contelles, J.; Gallego, P.; Rodriguez-Fernandez, M.; Khiar,
N.; Destabel, C. J. Org. Chem. 1997, 62, 7397.
(2) (a) Roush, W. R.; Lin, X.-F. J. Am. Chem. Soc. 1995, 117,
2236. (b) Bonini, C.; Federici, C.; Rossi, L.; Righi, G. J.
Org. Chem. 1995, 60, 4803. (c) Curran, D. P.; Jasperse, C.
P.; Totleben, M. J. J. Org. Chem. 1991, 56, 7169.
(d) Nakamura, E.; Machii, D.; Inubushi, T. J. Am. Chem.
Soc. 1989, 111, 6849. (e) Neeson, S. J.; Stevenson, P. J.
Tetrahedron 1989, 45, 6239.
(3) (a) Harrowven, D. C.; Dainty, R. F. Tetrahedron 1997, 53,
15771. (b) Chai, C. L. L.; King, A. R. J. Chem. Soc., Perkin
Trans. 1 1999, 1173. (c) Giese, B.; Damm, W.; Dickhaut, J.;
Wetterich, F.; Sun, S.; Curran, D. P. Tetrahedron Lett. 1991,
32, 6097. (d) Balczewski, P.; Mikolajczyk, M. Synthesis
1995, 392.
(9) Boger, D. L.; Mathvink, R. J. J. Org. Chem. 1989, 54, 1777.
(10) Arnone, A.; Bernardi, R.; Bravo, P.; Cardillo, R.;
Ghiringhelli, D.; Cavicchio, G. J. Chem. Soc., Perkin Trans.
1 1991, 1887.
(11) Hu, C.-M.; Qiu, Y.-L. J. Chem. Soc., Perkin Trans. 1 1993,
331.
(12) Reutrakul, V.; Thongpaisanwong, T.; Tuchinda, P.;
Kuhakarn, C.; Pohmakotr, M. J. Org. Chem. 2004, 69, 6913.
(13) (a) Prober, M. J. Am. Chem. Soc. 1953, 75, 968.
(b) Capriel, P.; Binsch, G. Tetrahedron 1979, 35, 2661.
(14) Reutrakul, V.; Thongpaisanwong, T.; Tuchinda, P.;
Kuhakarrn, C.; Pohmakotr, M. J. Org. Chem. 2004, 69,
6913.
(4) Inoue, K.; Sawada, A.; Shibata, I.; Baba, A. J. Am. Chem.
Soc. 2002, 124, 906.
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