BF3-promoted cyclization reaction of imines and salicylaldehyde with silyl enol ethers: unexpected formation of dioxaspiro compounds

Article abstract of DOI:10.1016/j.tet.2008.07.019

Unexpected spiro cyclic products were formed from the reaction of imines and salicylaldehyde with silyl enol ethers in the presence of BF₃·OEt₂. Different kinds of dioxaspiro products were afforded depending on the nature of starting

Full text of DOI:10.1016/j.tet.2008.07.019

Tetrahedron 64 (2008) 9315–9319
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BF3-promoted cyclization reaction of imines and salicylaldehyde with silyl
enol ethers: unexpected formation of dioxaspiro compounds
*
Yong Xin, Huiling Jiang, Jingwei Zhao, Shizheng Zhu
Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Road, Shanghai 200032, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Unexpected spiro cyclic products were formed from the reaction of imines and salicylaldehyde with silyl
enol ethers in the presence of BF3$OEt2. Different kinds of dioxaspiro products were afforded depending
on the nature of starting materials. Furthermore, salicylaldehyde could also react directly with several
silyl enol ethers, giving three products with different spiro cyclic structure under the same reaction
conditions.
Received 5 March 2008
Received in revised form 1 July 2008
Accepted 4 July 2008
Available online 11 July 2008
Ó 2008 Elsevier Ltd. All rights reserved.
EtOH
1. Introduction
C6F5 NH2
+
CHO
OH
C6F5
N
C
H
HO
60-70 °C
84%
b-Amino carbonyls are key structural units in biologically rele-
vant compounds such as -lactams and -amino acids, and were
b
b
1
efficiently synthesized by addition reaction of silyl enolates to im-
ines promoted by Lewis acid.1 In these reactions, equimolar amount
of Lewis acid is usually required as a promoter due to stronger
basicity of nitrogen in the imines. Since the less reactivity of imines
comparing to corresponding ketone moiety, nucleophilic addition
to imines have been less extensively studied.2 Recently, several
Mannich-type reactions involving addition of enolate reagents to
N-aryl imines,3 N-acyl imines,4 and N-phosphinoyl imines5 have
been reported in the literatures. Nevertheless, the use of imines
derived from salicylaldehyde and aniline as electrophiles is limited
mainly due to the presence of an unprotected active hydroxyl
group. To the best of our knowledge, only few precedents con-
cerning the reaction of silyl enolates with salicyl imines bearing
a protected hydroxyl group were reported, affording usual addi-
tion–elimination products.6 More recently, we have described an
unusual formation of spiro or fused cyclic compounds from the
reaction of fluorine-containing push–pull alkene with silyl enol
ethers in the presence of BF3$OEt2.7 To expand this reaction, herein,
we wish to report a novel BF3$OEt2-promoted cyclization reaction
from imines and salicylaldehyde with silyl enol ethers, different
kinds of dioxaspiro compounds could be generated in this way.
Scheme 1. Preparation of imine 1.
Generally, hydroxyl is considered as an active functional group,
which could easily react with catalysts such as TiCl4 and BF3$OEt2,
and need to be protected before conducting the reaction. However,
we were pleased to find that imine 1 could react directly with five-
membered silyl enol ether 2a in benzene under reflux and afford an
unexpected spiro cyclic product 3a in 42% yield (Table 1, entry 1).
From its 1H and 19F NMR spectra, it was clear that a penta-
fluorophenylamino moiety was still contained in the molecular
structure. The spiro structure was further confirmed by X-ray single
crystal diffraction analysis (Fig. 1). There exists a strong intra-
molecular hydrogen bonding between N1–H2 and O2 (dN1–H2/O2
¼
2.211 Å,
:
¼137.0ꢀ) together with
a
weak intra-
N1–H2–O2
molecular hydrogen bonding (dN1–H2/F1¼2.303 Å,
:
N1–H2–F1
¼
106.9ꢀ) between N1–H2 and F1. Furthermore, the torsion angle
Table 1
Reaction results of imine 1 with silyl enol ether 2a
TMSO
O
O
.
BF3 OEt2
+
C6F5
N
C
H
HO
benzene
reflux
n
2. Results and discussion
NHC6F5
H
n
3
1
2
Imine 1 was easily prepared from the direct condensation of
salicylaldehyde and pentafluoroaniline with good yield (Scheme 1).
Entry
n
2
Time (d)
Product 3
Yielda (%)
1
2
1
3
2a
2c
3
2
3a
3c
42
58
* Corresponding author.
E-mail address: zhusz@mail.sioc.ac.cn (S. Zhu).
a
Isolated yield.
0040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.07.019

9316
Y. Xin et al. / Tetrahedron 64 (2008) 9315–9319
Figure 1. Molecular structure of 3a (dN1–H2/O2¼2.211 Å, :N1–H2–O2¼137.0ꢀ; dN1–H2/F1
2.303 Å, :N1–H2–F1¼106.9ꢀ).
¼
Figure 2. Molecular structure of 4 (C8–C9¼1.330 Å, C13–C14¼1.526 Å).
of :N1–C7–C14–H14 is only 42ꢀ, and the pentafluorophenylamino
group and H14 adopts a syn-configuration. It should be noted that
product 3a is very stable even being heated to 200 ꢀC for 24 h
without any elimination of pentafluoroamido group and ring-
opening reaction happened. We envisioned that the stability of this
configuration could be attributed from the intramolecular hydro-
gen bonding effect. Subsequently, treatment of seven-membered
silyl enol ether 2c with 1 also successfully afforded a similar spiro
cyclic product 3c in 58% yield (Table 1, entry 2), whereas no reaction
was observed between 1 and open-chained trimethyl(pent-2-en-3-
yloxy)silane 2d under the same reaction conditions (Scheme 2).
was isolated from the reaction of salicylaldehyde and five-mem-
bered 2a. In its 1H NMR spectrum, eight phenyl H signals were
distributed in low field and another eight aliphatic H signals dis-
tributed in high field (Table 2, entry 2). In addition, compound 5
was found to possess an absolute symmetrical configuration from
the 13C NMR spectrum, and three C signals (41.52, 31.47, and
28.22 ppm) were assigned to six aliphatic carbon atoms. However,
the normal addition–elimination product 6 was given in good yield
from seven-membered 2c (Table 2, entry 3). Furthermore,
a hydrocarbonic analogue of imine 1 was studied and found that
the difluoroboryl derivative 8 was formed in almost quantitative
yield instead of spiro cyclic compound 3 (Table 2, entry 4).
The plausible mechanism for the formation of 3 was proposed
according to our former work on fluorinated push–pull alkene with
silyl enolates (Scheme 3).7 Imine 1 underwent a similar BF3$Et2O-
catalyzed nucleophilic addition reaction with silyl enol ethers to
OTMS
.
BF3 OEt2
C6F5
N
C
H
HO
N.R.
benzene
reflux
2d
1
Scheme 2. Reaction of 1 with open-chained trimethyl(pent-2-en-3-yloxy)silane.
Table 2
Reaction of salicylaldehyde with silyl enol ethers 2a–ca
Unexpectedly, when six-membered silyl enol ether 2b was
employed to react with 1 under the same reaction conditions,
a spiro cyclic product 4 was isolated without pentafluoroaniline
moiety attached (Scheme 3). The molecular structure was finally
confirmed by X-ray single crystal diffraction analysis with a satu-
rated carbon–carbon bond (C13–C14¼1.526 Å) (Fig. 2).
Entry
Reactant
Silyl enol ether
Product
Yieldb (%)
OTMS
O
O
CHO
1
68
OH
OH
OH
2b
4
OTMS
.
BF3 OEt2
O
O
Benzene
reflux, 48%
OTMS
C6F5
N
C
H
HO
+
O
O
CHO
CHO
2
3
34
1
2b
Scheme 3. Preparation of spiro compound 4.
4
2a
5
OTMS
To investigate the plausible reaction pathway, the reaction of
salicylaldehyde with 2b was conducted and found that the same
spiro cyclic product 4 was obtained in 68% yield under the same
reaction conditions (Table 2, entry 1), which implied that the
reaction might undergo a salicylaldehyde intermediate. Thus, we
hypothesized that imine 1 decomposed gradually during the
reaction to give salicylaldehyde, which reacted with silyl enol ether
2b subsequently to afford the final product 4.
O
O
N
67
96
2c
6
OTMS
C6H5
C
H
C6H5
N
C
H
4
F2B O
8
HO
7
2a
Consequently, other two silyl enol ethers 2a and 2c were
employed to investigate the reaction of salicylaldehyde (Table 2,
entries 2 and 3). Compound 5 containing two saturated C–C bonds
a
Reaction condition: BF3$OEt2, benzene, reflux.
Isolated yields.
b

Y. Xin et al. / Tetrahedron 64 (2008) 9315–9319
9317
Si
O
F
F-
BF2
N
BF2
N
OH
O
C6F5
.
BF3 OEt2
C6F5
N
C6F5
- FSiMe3
HO
A
HO
HO
1
F2B
OH
O
OH
O
O
OH
H
N
HO
BF2
C6F5
O
N
C6F5
BF2
NH
C6F5
NH
C6F5
NH
C6F5
C6F5 N
C
B
H
O
BF2
O
O
O
O
O
O
- C6F5NH2
- BF2OH
NH
C6F5
NH
C6F5
HN
NH
C6F5
H
HN
C6F5
C6F5
D
3a
E
Figure 3. Plausible mechanism for the formation of 3.
give intermediate A, which is in equilibrium with an intermediate
B. The formed intermediate B was then attacked by another 1 to
form neutral intermediate C, which could undergo further cycli-
zation reaction to form a ketal intermediate D, which could con-
tinually eliminate a molecule of pentafluoroaniline and BF2OH
followed by second cyclization reaction to give the final spiro
product 3a. However, the detailed mechanism for the formation of
compounds 4 and 5 is not clear at this time. Studies aimed at
elucidating the mechanism of this transformation are under
investigation in our laboratory and will be reported in due course
(Fig. 3).
4.2. Preparation of imine 1
Pentafluoroaniline (1.83 g, 10 mmol) was added into a 50 mL
flask containing salicylaldehyde (1.22 g, 10 mmol) and the mixture
was heated to reflux overnight. The reaction mixture was purified
by recrystallization (hexane/AcOEt) to give the product 2-((per-
fluorophenylimino)methyl)phenol 1 as a white solid.
4.3. General experimental procedures for reaction of imine 1
with silyl enol ethers
BF3$Et2O (1.2 mmol) was added dropwise to the mixture of 1
(0.287 g, 1 mmol) and silyl enol ethers (1.2 mmol) in CH2Cl2 (5 mL)
at 0 ꢀC. Then the mixture was warmed to room temperature slowly.
After appropriate time TLC analysis showed that the reaction was
over. Water (10 mL) was added to quench the reaction, extracted
with CH2Cl2 (3ꢁ10 mL), and then the organic layer was dried over
Na2SO4. The residue was purified by column chromatography on
silica gel (AcOEt/hexane¼1:200) to give the corresponding
products 3.
3. Conclusion
We have reported an unexpected formation of dioxaspiro
compounds from imines and salicylaldehyde with silyl enol ethers
in the presence of BF3$OEt2. Fluorine-containing spiro products
were given from the reaction of fluorine-containing imines with
five- and seven-membered silyl enol ethers, however, penta-
fluoroaniline-emitted product was resulted when six-membered
silyl enol ether was employed. The normal addition–elimination
product was afforded from salicylaldehyde and seven-membered
silyl enol ether, whereas two unique spiro cyclic products were
obtained, respectively, from five- and six-membered silyl enol
ethers.
4.3.1. Cyclobuta[1,2-b]-2H-chromene-[1,5-b]-N-(perfluorophenyl)-
3,4-dihydro-2H-chromen-4-amine 3a
White solid, mp: 211–214 ꢀC. Yield: 42%. IR (KBr): 3381, 1523,
1485, 999 cmꢂ1. 1H NMR (300 MHz, CDCl3):
d
7.31 (1H, d, 3JHH¼6 Hz,
3
Ph), 7.26–7.18 (3H, m, Ph), 7.02 (2H, t, JHH¼6 Hz, Ph), 6.93 (1H, d,
3
4. Experimental
4.1. General
3JHH¼8 Hz, Ph), 6.79 (1H, d, JHH¼8 Hz, Ph), 6.55 (1H, s, CH), 4.83
3
3
(1H, d, JHH¼11 Hz, NH), 4.72 (1H, d, JHH¼11 Hz, CH), 2.91–2.84
(1H, m, CH2), 2.75–2.65 (2H, m, CH2), 2.03–1.98 (1H, m, CH2), 1.57–
1.48 (1H, m, CH2). 13C NMR (75 MHz, CDCl3):
d 24.2 (C21), 25.7 (C20),
Melting points are measured on a Temp-Melt. apparatus and are
uncorrected. 1H, 13C, and 19F NMR spectra were recorded on Bruker
AM-300 instruments with Me4Si and CFCl3 as the internal and
external standards, respectively. FTIR spectra were obtained with
a Nicolet AV-360 spectrophotometer. Low resolution mass spectra
(LRMS) or high resolution mass spectra (HRMS) were obtained
on a Finnigan GC–MS 4021 or a Finnigan MAT-8430 instrument
using the electron impact ionization technique (70 eV1), re-
spectively. Elemental analyses were performed by this institute.
Single crystal X-ray structure analysis was performed on a Bruker
P4 instrument.
45.1 (C11), 50.9 (C12), 101.4 (C9), 116.7 (C19), 117.1 (C23), 119.2 (C1),
120.9 (C24), 121.9 (C3), 122.2 (C28), 122.5 (C18), 126.7 (C13), 126.8
(C25), 128.5 (C2), 128.9 (C27), 130.1 (C6), 130.6 (C16), 135.0 (C26), 135.8
(C8), 150.1 (C4), 150.4 (C14). 19F NMR (282 MHz, CDCl3):
d
ꢂ153.68 to
ꢂ154.21 (2F, m), ꢂ164.73 to ꢂ164.82 (2F, m), ꢂ170.32 to ꢂ170.47
(1F, m). EIMS m/z (relative intensity): 287 (C6F4NHCHC6H4OHþ,
6.86), 196 (C6F5NHCþ, 1.65), 170 (C6H4C5H7Oþ, 100). Anal. Calcd for
C
25H16F5NO2: C, 65.65; H, 3.53; N, 3.06. Found: C, 65.61; H, 3.55; N,
2.93.
X-ray data of 3a (CCDC no. 653590): C25H16F5NO2; FW¼457.39;
temperature 293 K; monoclinic, P2(1)/c; wavelength 0.71 Å;

9318
Y. Xin et al. / Tetrahedron 64 (2008) 9315–9319
a¼11.826(3) Å,
b¼25.095(7) Å,
c¼6.8451(19) Å,
a
¼90ꢀ,
(C9), 41.52 (C2), 28.22 (C4), 27.75 (C3). EIMS m/z (relative intensity):
278 (Mþ, 48.45), 250 (MþꢂC2H4, 0.73), 171 (MþꢂC7H7Oþ, 100), 107
(C7H7Oþ, 21.71), 77 (C6H5þ, 17.80). HRMS for C19H18O2 (%): calcd
278.1307, found 278.1305.
b
¼103.303(5)ꢀ,
g
¼90ꢀ; V¼1977.0(9) Å3; Z¼4, Dc¼1.537 mg/m3;
absorption coefficient 0.129 mmꢂ1
;
F(000)¼936; size 0.298ꢁ
0.112ꢁ0.087 mm; 1.62<
q<26.49; reflections collected 11,035;
empirical absorption correction; transmission 1.00maxꢂ0.823min
;
goodness of fit on F2 0.838; final R indices R1¼0.1472, wR2¼0.0816.
4.4.3. Cyclohepta-[1,2-b]-2H-chromene-[1,7-b]-2H-chromen-
4-amine 6
4.3.2. Cyclohepta-[1,2-b]-2H-chromene-[1,7-b]-N-(perfluoro-
phenyl)-3,4-dihydro-2H-chromen-4-amine 3c
White solid, mp: 207–208 ꢀC. Yield: 67%. IR (KBr): 2920, 1486,
1226, 926, 754 cmꢂ1
.
1H NMR (300 MHz, CDCl3):
d
7.13 (4H, t,
3
3
White solid, mp: 185–187 ꢀC. Yield: 58%. IR (KBr): 3389, 2931,
3JHH¼7 Hz, Ph), 6.96 (2H, t, JHH¼7 Hz, Ph), 6.79 (2H, d, JHH¼7 Hz,
1518, 1487, 932 cmꢂ1. 1H NMR (300 MHz, CDCl3):
d 7.20–7.09 (3H,
Ph), 6.67 (2H, s, CH]), 2.57 (2H, dd, 3JHH¼5, 12 Hz, CH2), 2.40 (2H, t,
3
m, Ph), 7.03–6.96 (2H, m, Ph), 6.90–6.72 (2H, m, Ph), 6.71–6.69 (1H,
m, Ph), 6.56 (1H, s, CH), 5.64 (1H, d, 3JHH¼11 Hz, NH), 4.84 (1H, dd,
3JHH¼6, 11 Hz, CH), 2.76–2.71 (1H, m, CH), 2.59–2.57 (1H, m, CH2),
2.50–2.46 (1H, m, CH2), 2.16–1.96 (4H, m, CH2), 1.56–1.47 (2H, m,
3JHH¼12 Hz, CH2), 2.05 (2H, t, JHH¼5 Hz, CH2), 1.53 (2H, t,
3JHH¼9 Hz, CH2). 13C NMR (75 MHz, CDCl3):
d 32.3, 33.6,100.8,116.4,
120.9, 121.8, 123.6, 126.2, 128.8, 135.4, 149.8. EIMS m/z (relative
intensity): 302 (Mþ, 100), 285 (MþꢂOH, 73.04), 273 (MþꢂC2Hþ5 ,
43.74), 195 (MþꢂC7H7O, 11.97), 107 (C7H7O, 8.90), 77 (C6Hþ5 , 7.98).
Anal. Calcd for C20H18O2: C, 83.42; H, 6.00. Found C, 83.44; H, 6.00.
CH2). 13C NMR (75 M, CDCl3):
d 26.4, 30.5, 32.2, 32.5, 48.7, 54.0,
101.7, 116.3, 116.4, 117.5, 121.6, 122.1, 121.8–122.5, 122.5, 124.2,
126.4–126.5, 126.8, 128.6, 129.1, 129.5–130.3, 149.9, 151.2. 19F NMR
3
4.5. Typical procedures for reaction of imine 79 with silyl enol
ether 2a
(282 MHz, CDCl3):
d
ꢂ157.76 (2F, d, JHH¼23 Hz), ꢂ164.41 (2F, d,
3JHH¼23 Hz), ꢂ171.47 (1F, t, JHH¼23 Hz). EIMS m/z (relative in-
tensity): 287 (C6F4NHCHC6H4OHþ, 15.09), 198 (C6H4C8H9Oþ, 100).
Anal. Calcd for C27H20F5NO2: C, 66.80; H, 4.15; N, 2.89. Found: C,
66.81; H, 4.04; N, 2.72.
3
BF3$Et2O (1.2 mmol) was added dropwise to the mixture of 7
(0.197 g, 1 mmol) and 2a (1.2 mmol) in CH2Cl2 (5 mL) at 0 ꢀC. Then
the mixture was slowly warmed to room temperature. After one day
TLC analysis showed that the reaction was complete. Water (10 mL)
was added to quench the reaction, extracted with CH2Cl2 (3ꢁ10 mL),
and then the organic layer was dried over Na2SO4. The residue was
purified by column chromatography on silica gel (hexane/
AcOEt¼200:1) to give the corresponding product 8 in 96% yield.
4.4. General experimental procedures for reaction of
salicylaldehyde with silyl enol ethers
BF3$Et2O (1.2 mmol) was added dropwise to the mixture of
salicylaldehyde (0.122 g 1 mmol) and silyl enol ethers (1.2 mmol) in
CH2Cl2 (5 mL) at 0 ꢀC. Then the temperature was slowly warmed to
room temperature. After appropriate time TLC analysis showed that
the reaction was over. Water (10 mL) was added to quench the
reaction, extracted with CH2Cl2 (3ꢁ10 mL), and then the organic
layer was dried over Na2SO4. The residue was purified by column
chromatography on silica gel (AcOEt/hexane¼1:200) to give the
corresponding products.
4.5.1. (E)-N-(2-(Difluoroboryloxy)benzylidene)benzenamine 810
Red solid, mp: 230–232 ꢀC. Yield: 92%. 1H NMR (CDCl3):
d 9.16
3
3
(1H, s, CH), 7.80 (1H, d, JHH¼7 Hz, Ph), 7.73 (1H, t, JHH¼7 Hz, Ph),
7.63 (2H, d, 3JHH¼7 Hz, Ph), 7.58–7.49 (3H, m, Ph), 7.14–7.07 (2H, m,
Ph). 19F NMR (CDCl3):
d
ꢂ148.08 (s). MS (m/z, %): 245 (Mþ, 7.92), 198
(MþþHꢂBF3, 2.22), 105 (C6H5N]CHþ2 , 3.17). IR (KBr): 3048, 1624,
1553, 1385, 1317, 760 cmꢂ1
.
4.4.1. 5a,6,7,8-Tetrahydro-5H-chromeno[3,2-d]xanthene 4
White solid, mp: 129–131 ꢀC. Yield: 65%. IR (KBr): 2934, 1486,
Acknowledgements
1456, 1226, 946 cmꢂ1. 1H NMR (300 MHz, CDCl3):
d 7.13–7.06 (4H,
m, Ph), 6.93–6.88 (2H, m, Ph), 6.88–6.74 (2H, m, Ph), 6.43 (1H, s,
Financial support for this project was provided by The National
Natural Science Foundation of China (NNSFC, No. 20472106) and
National Basic Research Program (No. 20532040).
CH]), 3.59 (1H, J¼6 Hz, CH), 2.60–2.38 (4H, m, CH2), 1.66–1.50 (m,
4H, CH2). 13C NMR (75 MHz, CDCl3):
d 25.0 (C21), 27.3 (C22), 29.0
(C20), 31.5 (C12), 38.9 (C11), 97.7 (C9), 116.0 (C19), 116.9 (C17), 120.4
(C13), 120.6 (C2), 121.0 (C17), 121.5 (C6), 126.0 (C18), 127.3 (C1), 128.3
(C5), 129.5 (C16), 133.4 (C8), 150.9 (C3), 151.3 (C14). EIMS m/z (relative
intensity): 290 (Mþ, 31.47), 184 (C6H4C8H9Oþ, 100), 77 (C6Hþ5 ,
10.03). Anal. Calcd for C20H18O2: C, 82.73; H, 6.25. Found: C, 82.75;
H, 6.26.
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S. J. Am. Chem. Soc. 2005, 127, 15528–15535; (k) Cozzi, P. G.; Rivalta, E. Angew.
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X-ray data of 4 (CCDC no. 653589): C20H18O2; FW¼290.34;
temperature
293 K;
triclinic,
P-1;
wavelength
0.71 Å;
a¼11.1837(14) Å, b¼11.3076(14) Å, c¼14.3656(17) Å,
a
¼71.829(2)ꢀ,
b
¼89.714(2)ꢀ,
g
¼62.197(2)ꢀ; V¼1505.3(3) Å3; Z¼4, Dc¼1.281 mg/
m3; absorption coefficient 0.081 mmꢂ1; F(000)¼616; size 0.506ꢁ
0.375ꢁ0.314 mm; 1.51<
q<27.00; reflections collected 8899;
absorption correction empirical; transmission 1.00maxꢂ0.604min
;
goodness of fit on F2 0.810; final R indices R1¼0.0489, wR2¼0.1037.
4.4.2. Cyclobuta[1,2-b]-2,3,4,5-tetrahydro-2H-chromene-[1,5-b]-
3,4-dihydro-2H-chromene 5
White solid, mp: 151–152 ꢀC. Yield: 34%. IR (KBr): 2953, 1484,
1457, 1043, 755 cmꢂ1. 1H NMR (300 MHz, CDCl3):
d 7.23–7.11 (4H,
m, Ph), 6.95 (2H, t, 3JHH¼7 Hz, Ph), 6.80 (2H, d, 3JHH¼7 Hz, Ph), 3.04–
2.97 (2H, m, CH2), 2.64–2.48 (4H, m, CH2), 2.02–1.97 (2H, m, CH2),
4. (a) Kobayashi, S.; Matsubara, R.; Kitagawa, H. Org. Lett. 2002, 4, 143–145; (b)
Kobayashi, S.; Matsubara, R.; Nakamura, Y.; Kitagawa, H.; Sugiura, M. J. Am.
Chem. Soc. 2003, 125, 2507–2515; (c) Nakamura, Y.; Matsubara, R.; Kiyohara, H.;
1.37–1.25 (2H, m, CH2). 13C NMR (75 MHz, CDCl3):
d
153.15 (C10),
128.55 (C6), 127.42 (C8), 123.77 (C1), 121.66 (C5), 117.42 (C7), 109.12

Y. Xin et al. / Tetrahedron 64 (2008) 9315–9319
9319
Kobayashi, S. Org. Lett. 2003, 5, 2481–2484; (d) Matsunaga, S.; Yoshida, T.; Naoya,
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7. Jiang, H. L.; Zhu, S. Z. Synlett 2006, 1343–1346.
9. Overman, L. E.; Watson, D. A. J. Org. Chem. 2006, 71, 2587–2599.
10. Tripathi, T. Inorg. Nucl. Chem. Lett. 1978, 14, 97–99.