Catalytic enantioselective [2+2]-cycloaddition reaction of 2-methoxycarbonyl-2-cyclopenten-1-one by chiral copper catalyst

Article abstract of DOI:10.1016/j.tet.2006.01.050

A new catalytic system for enantioselective [2+2]-cycloaddition reaction of 2-methoxycarbonyl-2-cyclopenten-1-one with thioacetylene derivatives is described. The use of a catalytic amount (20-30 mol%) of copper(II) salt with chiral bis-pyridine ligand wa

Full text of DOI:10.1016/j.tet.2006.01.050

Tetrahedron 62 (2006) 3380–3388
Catalytic enantioselective [2D2]-cycloaddition reaction of
2-methoxycarbonyl-2-cyclopenten-1-one by chiral copper catalyst
*
Yosuke Takenaka, Hisanaka Ito, Mineki Hasegawa and Kazuo Iguchi
*
School of Life Science, Tokyo University of Pharmacy and Life Science, 1432-1 Horinouchi, Hachioji, Tokyo 192-0392, Japan
Received 10 November 2005; accepted 16 January 2006
Available online 13 February 2006
Abstract—A new catalytic system for enantioselective [2C2]-cycloaddition reaction of 2-methoxycarbonyl-2-cyclopenten-1-one with
thioacetylene derivatives is described. The use of a catalytic amount (20–30 mol%) of copper(II) salt with chiral bis-pyridine ligand was
found to be effective in promoting the [2C2]-cycloaddition reaction, furnishing the corresponding bicyclic compound in good yield and good
enantioselectivity.
q 2006 Elsevier Ltd. All rights reserved.
1. Introduction
the addition of a nucleophile to 2-methoxycarbonyl-2-
cyclopenten-1-one (1) and report herein a catalytic
enantioselective [2C2]-cycloaddition of 2-alkoxy-
carbonyl-2-cycloalken-1-one with thioacetylene deriva-
tives for the construction of a chiral bicyclic system.11
[2C2]-Cycloaddition reaction has been recognized as a
powerful tool for the construction of a functionalized
four-membered ring system.1 The reaction usually
proceeds under photo-irradiation or Lewis acid (also
Brønsted acid)-catalyzed conditions.1b Among them, the
Lewis acid-catalyzed condition is an attractive method,
because the reaction proceeds under mild conditions with
high siteselectivity.2–4 Chiral Lewis acid-catalyzed
enantioselective [2C2]-cycloaddition reaction is a useful
tool for the synthesis of a poly-functionalized four-
membered ring system as an optically active form.
Narasaka et al. reported a Ti–TADDOL complex
catalyzed enantioselective [2C2]-cycloaddition reaction
of acryloyl oxazolidinone derivatives and the reaction
provided excellent enantioselectivity (Eq. 1).5,6 On the
other hand, the enantioselective [2C2]-cycloaddition
reaction of 2-methoxycarbonyl-2-cyclopenten-1-one (1),
which can be useful for preparation of a chiral bicyclic
system, has not yet been reported (Eq. 2). The preparation
of a bicyclic system containing a four-membered ring as
optically active form is important for the synthesis of
natural and biologically active compounds. Although
asymmetric 1,4-addition of a nucleophile such as ketene
silyl acetal to 2-methoxycarbonyl-2-cyclopenten-1-one (1)
have been reported by several groups, the yield and
enantio- or diastereoselectivity were moderate.7–10 We
tried to develop a chiral catalytic system (Scheme 1) for
2. Results and discussion
2.1. The [2D2]-cycloaddition reaction of 2-methoxy-
carbonyl-2-cyclopenten-1-one catalyzed by Lewis acid
The Lewis acid catalyst for the [2C2]-cycloaddition
reaction of 2-methoxycarbonyl-2-cyclopenten-1-one (1)
was examined. The substrates 1 and 2 were prepared
according to the literature procedure.12,13 Substrate 1 was
used for [2C2]-cycloaddition reaction without purification
due to instability (O90% from 1H NMR). The result of the
[2C2]-cycloaddition reaction is shown in Table 1. In the
presence of 30 mol% of titanium chloride or zirconium
chloride, the reaction proceeded to give cycloaddition
product 3 along with a significant amount of by-product 4
and other by-products (entries 1 and 2). Another haloge-
nated metal-catalyzed reaction also gave by-product 4. Only
under zinc bromide-catalyzed conditions, the reaction
smoothly proceeded to provide the desired product 3 in
59% yield without a halogenated by-product (entry 5). The
best result was obtained by employing 10 mol% of zinc
bromide as a catalyst to prevent the decomposition of the
product 3 (entry 6).
Keywords: [2C2]-Cycloaddition reaction; Ligand; Copper salt.
*
Corresponding authors. Tel.: C81 426 76 5473; fax: C81 426 76 7282
(H.I.); tel.: C81 426 76 7273; fax: C81 426 76 7282 (K.I.); e-mail
addresses: itohisa@ls.toyaku.ac.jp; onocerin@ls.toyaku.ac.jp
A plausible mechanism for the formation of 4 is shown in
Scheme 2. The 1,4-addition of phenylthioacetylene (2) to
0040–4020/$ - see front matter q 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.01.050

Y. Takenaka et al. / Tetrahedron 62 (2006) 3380–3388
3381
L
Ti
SR'
SR'
n
O
O
O
O
O
O
R
N
O
Ti-TADDOL
(1)
(2)
R
N
O
R
N
O
SR'
SR'
Narasaka etal.
L
n
M
O
O
SR'
chiral cat.
O
O
O
CO Me
2
SR'
OMe
OMe
O
O
N
X
X
Cu
O
N
O
Chiral catalyst
Scheme 1.
the chloride anion to the carbocation in the intermediate 5
proceeded competitively to give by-product 4. Another
plausible route for the formation of 4 is as follows. After the
formation of cycloadduct 3, the decomposition of 3
proceeded based on the coordination of Lewis acid to the
carbonyl group on the cyclopentane ring. Actually,
compound 4 was obtained by the treatment of 3 with
titanium chloride (vide infra).
Table 1. Survey of metal salt catalysts for the [2C2]-cycloaddition reaction
O
O
O
CO Me
SPh
2
CO Me
2
30 mol% MX
n
SPh
CO Me
X
2
+
+
CH Cl , 0 ˚C
2
2
SPh
1
2
3
4 X = Cl or Br
Entry
MXn
Time (h)
3:4a
Yield of 3 (%)b
1
2
3
4
TiCl4
ZrCl4
MgBr2$OEt2
CuCl2
ZnBr2
0.5
0.5
1
48
0.5
2
0.5
0.5
0.5
1.3
0.5
0.5
1:1.1
1:1.9
1:0.9
1:0.8
1:0
1:0
1:0
1:0
1:0
32
26
16
33
59
84
42
65
57
75
51
80
To prevent the side-reaction, metal triflate catalyst was
examined due to the low nucleophilicity of triflate anion and
the results are shown in Table 1 (entries 7–12). Under the
metal triflate-catalyzed conditions, theuncyclized compound
such as 4 was not detected. Most of the examined triflates
worked nicely, and the reaction in the presence of 10 mol% of
copper triflate at 0 8C gave the best result (entry 12).
5
6c
7
ZnBr2
Sc(OTf)3
Sc(OTf)3
Yb(OTf)3
Yb(OTf)3
Cu(OTf)2
Cu(OTf)2
8c
9
10c
11
12c
1:0
1:0
1:0
2.2. Enantioselective [2D2]-cycloaddition reaction of
2-methoxycarbonyl-2-cyclopenten-1-one by reported
chiral catalyst
a Ratio was determined by 1H NMR analysis.
b Isolated yield.
c Catalyst (10 mol%) was employed.
Based on the result of the examination of Lewis acid
catalysts, several reported chiral Lewis acid catalysts were
examined for the asymmetric [2C2]-cycloaddition reaction
of 2-methoxycarbonyl-2-cyclopenten-1-one (1) with phenyl-
thioacethylene (2) prior to the development of a novel
catalyst. The result is shown in Scheme 3.
Cl TiO
3
Cl TiO
O
4
CO Me
2
Cl
SPh
2
CO Me
CO Me
2
2
TiCl , CH Cl
2
4
2
SPh
SPh
5
1
+
H O
3
As mentioned above, the Ti–TADDOL catalyst was
effective for the [2C2]-cycloaddition reaction of acryloyl-
oxazolidinone derivatives with thioacetylenes or ketene
dithioacetals.5 Therefore, this catalytic system was exam-
ined at first. Although moderate yield (53% yield) along
with by-product 4 was obtained under 30 mol% of
Ti–TADDOL complex 6-catalyzed conditions, the enantio-
meric excess of the product was not satisfactory (22% ee).
We also examined the reaction catalyzed by a chiral copper
catalyst.9d,14 Under the presence of 20 mol% of a copper-
BOX complex 7, the reaction proceeded to give cycloadduct
O
O
Cl Ti O
4
CO Me
2
CO Me
2
CO Me
2
Cl
SPh
SPh
SPh
4
3
Scheme 2.
the substrate 1 gave intermediate 5. The intramolecular
reaction of the resulting titanium enolate to the carbocation
on the carbon bearing the thiophenyl group gave the desired
[2C2]-cycloadduct 3. During this pathway, the reaction of

3382
Y. Takenaka et al. / Tetrahedron 62 (2006) 3380–3388
2.3. Enantioselective [2D2]-cycloaddition reaction of
2-methoxycarbonyl-2-cyclopenten-1-one by novel
copper catalyst
Ph Ph
O
O
OH
OH
O
O
O
Ph Ph
CO Me
SPh
2
CO Me
2
Cl
Based on the examination of the reported chiral catalyst as
shown in Scheme 3, the development of a novel catalyst was
examined. We employed copper(II) salt as Lewis acid and a
novel and simple C2-symmetric ligand having a binaphthyl
moiety as a chiral source and two pyridine moieties as a
coordination site to copper salt with appropriate binding
affinity.
6
SPh
CO Me
2
TiCl (Oi-Pr)
2 2
+
CH Cl , -78 ˚C
2
2
SPh
H
1
2
4
3
53% yield, 22% ee
O
O
N
N
O
O
Novel chiral ligand 10 having two pyridine moieties was
prepared from (S)-binaphthol (8) with 2-pyridinemethanol
(9) by Mitsunobu reaction16 (Scheme 4). Other modified
chiral ligands 11–13 were also prepared in the same manner.
The results of the enantioselective [2C2]-cycloaddition
reaction of compound 1 with phenylthioacetylene (2) are
shown in Table 2. In the case of a catalyst prepared from
ligand 10 with copper triflate, the reaction proceeded at 0 8C
and moderate ee was obtained but the yield was
unsatisfactory (entry 1). The use of toluene or THF as a
solvent instead of CH2Cl2 significantly decreased both the
yield and ee (entries 2 and 3). In these cases, the low yield
CO Me
SPh
2
7
SPh
CuCl •AgSbF
2
6
CO Me
2
+
CH Cl , –30 - 0 ˚C
2
2
H
1
2
3
20% yield, 32% ee
Scheme 3.
3 in moderate enantiomeric excess (32% ee), but the yield
was very low (20% yield). The titanium–BINOL catalyst15
was also examined. Although good enantioselectivity was
observed (79% ee), the yield was quite low (8% yield).
DIAD
O
O
N
N
PPh
OH
OH
3
+
OH
THF
N
8
9
10
Br
O
O
O
N
N
O
O
N
N
O
O
N
N
N
N
O
Br
11
12
13
14
Scheme 4.
Table 2. Chiral copper-catalyzed [2C2]-cycloaddition reaction
O
O
CO Me
SPh
2
cat.
SPh
CO Me
2
+
H
12
3
Entry
Ligand
Copper salt
Mol%
Solvent
CH2Cl2
Toluene
THF
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
Temperature (8C)
Time (h)
Yield (%)a
ee (%)b
1
2
3
4
5
6
7
8
10
10
10
10
10
10
10
11
11
12
13
14
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Sc(OTf)2
CuCl2CAgSbF6
CuCl2CAgSbF6
CuCl2CAgSbF6
Cu(OTf)2
CuCl2CAgSbF6
CuCl2CAgSbF6
CuCl2CAgSbF6
CuCl2CAgSbF6
30
30
30
30
30
50
100
30
30
30
30
30
K78 to 0
K78 to 0
K78 to 0
0
K78 to K30
K78 to K30
K30
K30 to 0
K30 to 0
0
0
K30
13
17
17
17
14
14
19
13
19
12
5
13
7
44
20
—
11
41
40
39
16
—
30
29
25
NR
13
56
69
65
6
!5
24
18
50
9
10
11
12
10
a Isolated yield.
b Ratio was determined by chiral HPLC analysis.

Y. Takenaka et al. / Tetrahedron 62 (2006) 3380–3388
3383
O
DCC
DMAP
OH
OH
O
O
N
N
+
CH Cl
2
2
N
CO H
2
O
8
15
16
Br
O
O
O
O
O
N
N
O
O
N
N
O
O
N
N
O
O
O
O
N
N
O
O
O
Br
17
18
19
20
O
Br
Br
O
N
O
O
O
O
N
N
N
O
O
21
22
Scheme 5.
was observed due to the decomposition of substrate 1 during
the long reaction time at 0 8C. To improve the ee and yield,
other metal salts were examined and the best results were
obtained with the use of hexafluoroantimonate as a counter
anion of copper9 (entries 5–7). With the increase of the
Lewis acidity, the reaction proceeded even at K30 8C and
the yield was significantly improved due to avoid the
decomposition of substrate 1. In the presence of 50 mol% of
catalyst, best yield was obtained (69%, entry 6). The use of
other modified ligands 11–14 decreased both the yield and
ee (entries 8–12). These results indicated that the catalytic
activity is not enough in the present catalysts due to the
strong Lewis basicity of the nitrogen atom on the ligand.
Therefore, novel ligands having a picolinate moiety to
decrease the Lewis basicity of the coordination site to the
metal center were designed.
dicyclohexylcarbodiimide in 94% yield (Scheme 5). Other
modified ligands 17–22 were prepared in the same manner.
The reaction of compound 1 with 2 catalyzed by the
picolinate catalyst prepared from ligand 16 with copper
triflate proceeded at 0 8C to give the [2C2]-adduct 3 in 31%
yield with 24% ee (Table 3). This result showed significant
improvement of the yield compared with the result of
the 30 mol% of catalyst prepared from ligand 10 with
copper triflate. The use of hexafluoroantimonate as a
counter anion of copper instead of triflate increased the
catalytic activity and the reaction proceeded at low
temperature and the product 3 was obtained at K30 8C in
62% yield with 53% ee (entry 2). The reaction also
proceeded at K78 8C and 64% ee was obtained (entry 3).
The catalytic activities of the modified chiral ligands 17–22
were also examined. The ligand 17 and 18, which is larger
than pyridine ring, were prepared to increase the steric
repulsion between catalyst and substrate and the reaction
The ligand 16 was prepared from (S)-binaphthol (8) with
picolinic acid (15) by a condensation reaction using
Table 3. Chiral copper catalyst catalyzed [2C2]-cycloaddition reaction
O
O
CO2Me
SPh
30 mol% cat.
SPh
CO2Me
+
CH2Cl2
H
3
1
2
Entry
Ligand
Copper salt
Temperature (8C)
Time (h)
Yield (%)a
ee (%)b,c
1
2
3
4
5
6
7
8
16
16
16
17
17
18
18
19
20
21
22
Cu(OTf)2
0
2.5
0.6
3
2
3
1.5
0.8
12
3
1.5
4
31
62
48
35
38
14
45
7
24
53
64
23
24
12
19
13
0
CuCl2CAgSbF6
CuCl2CAgSbF6
Cu(OTf)2
CuCl2CAgSbF6
Cu(OTf)2
CuCl2CAgSbF6
CuCl2CAgSbF6
CuCl2CAgSbF6
CuCl2CAgSbF6
CuCl2CAgSbF6
K30
K78
K30
K30
K30 to 0
K30
K30
K30
K78
K30
9
10
11
35
13
41
73
13
a Isolated yield.
b Ratio was determined by chiral HPLC analysis.
c The absolute configurations of the major enatiomer were (1R,5S) except for entry 11.

3384
Y. Takenaka et al. / Tetrahedron 62 (2006) 3380–3388
Table 4. Chiral copper-catalyzed [2C2]-cycloaddition reaction of substituted thioacetylene derivatives
30 mol% ligand
CuCl + AgSbF
O
O
2
CO Me
SR
2
2
2
6
SR
CO Me
2
+
CH Cl , –78 ˚C
2
2
1
R
1
R
23
24
1
Entry
23
Ligand
Time (h)
Product
Yield (%)a
ee (%)b
1
2
3
4
5
R1ZnBu, R2ZPh (23a)
R1ZnBu, R2ZPh (23a)
R1ZTMS, R2ZPh (23b)
R1ZTMS, R2ZMe (23c)
R1ZnBu, R2ZMe (23c)
10
16
16
10
16
16
16
16
16
2
24a
24a
—
24c
24c
9
45
32
—
73
15
75
NR
3
51
a Isolated yield.
b Ratio was determined by chiral HPLC analysis.
gave product 3 but the ee was decreased in both cases
(entries 4–7). The ligand 19 having a nicotinate moiety
instead of picolinate was also examined and the product 3
was obtained with low yield and ee (entry 8). To increase
the steric bulkiness of the naphthalene ring, modification of
the BINOL moiety was also examined. The reaction of
the ligand 20 prepared from 3,30-dibromobinaphthol gave
racemate 3. In the case of ligand 21 derived from
6,60-dibromobinaphthol, the reaction proceeded at K78 8C
and the product 3 was obtained in 13% yield with 73% ee
(entry 10).
enantiomer of the [2C2]-adduct 3 could be determined as
(1R,5S) configuration.
The present enantioselective [2C2]-cycloaddition reaction
was applied to the first step of an enantioselective total
synthesis of (C)-tricycloclavulone (26) as shown in
Scheme 7.11 For the synthesis of tricycloclavulone as a
natural form, the ligand ent-16 derived from (R)-binaphthol
was employed. In a 20 mmol scale reaction under the
presence of 20 mol% of the catalyst, the reaction proceeded
and the best enantioselectivity was achieved (73% ee). One
cause of the improvement of the enantioselectivity is that
both substrates were prepared just before use.
The [2C2]-cycloaddition reaction of 2-methoxycarbonyl-2-
cyclopenten-1-one (1) with thioacetylene derivatives 2317
was also examined and the results are shown in Table 4. In
the case of compound 23a, good enantiomeric excess was
found by using both ligand 10 and 16 (entries 1 and 2). As
mentioned above, the use of ligand 10 decreased the yield of
the cycloaddition product (entries 1 and 4). Although the
yield was low, high enantiomeric excess was achieved in
the case of TMS-substituted substrate 23c (entry 4).
O
O
O
N
N
CuCl , AgSbF
2
6
O
ent-16
O
O
CO Me
2
SPh
SPh
20 mol% cat.
CH Cl , -78 ˚C
CO Me
The absolute stereochemistry of the product was determined
by comparison of the [a]D value after conversion
of compound 3 to known compound 2518 as shown in
Scheme 6. Compound 3 (60% ee) prepared by the use of
ligand 16 derived from (S)-binaphthol was treated with
titanium chloride to cleave the four-membered ring to give
chlorinated compound 4 (vide supra). The acid hydrolysis
and decarboxylation of compound 4 with concentrated HCl
in MeOH gave known compound 25. A comparison of the
optical rotation value with that of (S)-25 suggested that the
absolute configuration of the chiral center of compound 25
was R configuration. According to the above-mentioned
conversion, the absolute configuration of the major
2
+
2
2
H
67% yield
73% ee
1
2
(1S,5R )-3
OAc
O
AcO
CO Me
2
H
tricycloclavulone (26)
Scheme 7.
3. Conclusion
O
O
We have shown our efforts for the development of a
catalytic system for [2C2]-cycloaddition reaction of
2-methoxycarbonyl-2-cyclopenten-1-one with thioacetyl-
ene derivatives. Among the survey of Lewis acid catalysts
to accelerate the cycloaddition reaction, zinc bromide and
copper triflate worked efficiently to give a four-membered
ring product. Based on this result, we prepared a simple
chiral ligand from BINOL and picolinic acid having the
coordination ability to copper salt. The presented chiral
O
CO Me
2
CO Me
2
SPh
TiCl
c. HCl
MeOH
4
Cl
CH Cl
CO H
2
2
2
SPh
H
H
H
91%
33%
4
3
25
[ ] = +65.1˚ (c 0.45, CHCl )
(60% ee)
a
D
3
a
lit. [ ] = –116.5˚ (c 6.99, CHCl ) for (S)-25
D
3
Scheme 6.

Y. Takenaka et al. / Tetrahedron 62 (2006) 3380–3388
3385
catalyst worked to give [2C2]-cycloadduct in good yield
and enantiomeric excess. This catalytic system was applied
to the enantioselective total synthesis of marine prostanoid
tricycloclavulone.
d; 2.81 (4H, td, JZ6.3, 1.7 Hz), 4.24 (4H, t, JZ6.3 Hz),
6.26 (2H, d, JZ7.8 Hz), 6.89 (2H, ddd, JZ1.0, 4.9, 7.4 Hz),
6.99–7.21 (6H, m), 7.30 (2H, ddd, JZ1.5, 6.5, 8.0 Hz), 7.38
(2H, d, JZ9.0 Hz), 7.86 (2H, d, JZ8.2 Hz), 7.93 (2H, d,
JZ9.0 Hz), 8.33 (2H, dt, JZ0.9, 4.9 Hz). 13C NMR
(75 MHz, CDCl3) d; 38.2, 68.3, 114.9, 120.1, 120.9,
123.4, 123.7, 125.3, 126.0, 127.6, 129.0, 129.1, 134.0,
135.6, 148.6, 153.9, 158.3. HRESIMS calcd for
C34H29N2O2: 497.2229 (MCH)C, found: 497.2201. Anal.
Calcd for C34H28N2O2: C, 82.23; H, 5.68; N, 5.64. Found:
C, 82.17; H, 5.74; N, 5.45.
4. Experimental
4.1. General procedure for the preparation of ligand
10–13
4.1.1. 2,20-Bis(2-pyridylmethoxy)-(1S)-1,10-binaph-
thalene (10). To a solution of (S)-BINOL (1.0 g,
3.49 mmol) in THF (30 mL) was added triphenylphosphine
(2.0 g, 7.68 mmol), 2-(hydroxymethyl)pyridine (0.70 mL,
7.68 mmol) and a solution of diisopropyl azodicarboxylate
(1.5 mL, 7.68 mmol) in THF (2 mL) at room temperature.
After being stirred for 15 h, the reaction mixture was
concentrated under reduced pressure. The crude material
was purified by silica gel column chromatography (hexane/
AcOEt, 1:1–1:3 and CHCl3/AcOEt, 2:1) to afford com-
pound 10 (1.31 g, 2.80 mmol, 80% yield). White amorphous
solid. [a]2D8 K63.6 (c 0.53, CHCl3). IR (KBr) n cmK1; 1685,
1654, 1636, 1618, 749. 1H NMR (300 MHz, CDCl3) d; 5.22
(4H, s), 6.72 (2H, d, JZ7.9 Hz), 7.04 (2H, dd, JZ5.2,
7.2 Hz), 7.23–7.28 (6H, m), 7.35 (2H, m), 7.46 (2H, d, JZ
9.0 Hz), 7.90 (2H, d, JZ8.2 Hz), 7.98 (2H, d, JZ9.0 Hz),
8.45 (2H, d, JZ4.5 Hz). 13C NMR (75 MHz, CDCl3) d;
71.4, 115.0, 120.1, 120.7, 122.1, 123.8, 125.4, 126.5, 127.9,
129.5, 134.1, 136.4, 148.6, 153.6, 157.7, 162.3. HRESIMS
calcd for C32H25N2O2: 469.1916 (MCH)C, found:
469.1911. Anal. Calcd for C32H24N2O2: C, 82.03; H, 5.16;
N, 5.98. Found: C, 81.85; H, 5.21; N, 5.97.
4.1.5. (R,R)-1,2-Diphenyl-1,2-bis(2-pyridylethoxy)ethane
(14). Colorless crystals. Mp 78–80 8C (from hexane–
AcOEt). [a]2D8 K59.1 (c 0.65, CHCl3). IR (KBr) n cmK1
;
1597, 1589, 760, 698. 1H NMR (300 MHz, CDCl3) d; 4.57
(2H, d, JZ14.0 Hz), 4.70 (2H, s), 4.70 (2H, d, JZ14.0 Hz),
7.08–7.26 (12H, m), 7.48 (2H, d, JZ7.8 Hz), 7.61 (2H, dt,
JZ1.8, 7.7 Hz), 8.47 (2H, d, JZ4.8 Hz). 13C NMR
(75 MHz, CDCl3) d; 72.1, 85.8, 121.2, 122.0, 127.7,
127.8, 127.9, 136.5, 138.1, 148.8, 159.0. HRESIMS calcd
for C26H25N2O2: 397.1916 (MCH)C, found: 397.1915.
Anal. Calcd for C26H24N2O2: C, 78.76; H, 6.10; N, 7.07.
Found: C, 78.66; H, 6.01; N, 6.94.
4.2. General procedure for the preparation of
ligand 16–22
4.2.1. 2,20-Bis(picolinyloxy)-(1S)-1,10-binaphthalene (16).
To a solution of (S)-BINOL (1.0 g, 3.49 mmol), picolinic
acid (946 mg, 7.68 mmol) and 4-dimethylaminopyridine in
CH2Cl2 (10 mL) was added a solution of dicyclohexylcar-
bodiimide (1.66 g, 8.03 mmol) in CH2Cl2 (5 mL) at room
temperature. After being stirred for 15 h, the mixture was
filtered through Celite and the filtrate was concentrated
under reduced pressure. The residue was purified by silica
gel column chromatography (hexane/AcOEt, 1:5) to afford
compound 16 (1.63 g, 3.28 mmol) in 94% yield. White
amorphous solid. [a]D28 K84.9 (c 0.59, CHCl3). IR (KBr)
n cmK1; 1752, 1685, 1654, 1582, 745. 1H NMR (300 MHz,
CDCl3) d; 7.31–7.48 (14H, m), 7.56–7.61 (4H, m), 7.90
(2H, d, JZ8.1 Hz), 8.00 (2H, d, JZ8.9 Hz), 8.63 (2H, d,
JZ4.6 Hz). 13C NMR (75 MHz, CDCl3) d; 121.5, 123.5,
125.4, 125.9, 126.1, 126.9, 127.1, 127.9, 129.8, 131.6,
133.3, 136.9, 146.8, 147.0, 150.0, 162.8. HRESIMS calcd
for C32H21N2O4: 497.1501 (MCH)C, found: 497.1496.
Anal. Calcd for C32H20N2O4: C, 77.41; H, 4.06; N, 5.64.
Found: C, 77.20; H, 4.15; N, 5.58.
4.1.2. 2,20-Bis[6-methyl(2-pyridylmethoxy)]-(1S)-1,10-
binaphthalene (11). Colorless crystals. Mp 130–132 8C
(from hexane–AcOEt). [a]2D8 K56.0 (c 0.53, CHCl3). IR
1
(KBr) n cmK1; 1618, 1593, 806, 775. H NMR (300 MHz,
CDCl3) d; 2.47 (6H, s), 5.18 (4H, s), 6.53 (2H, d, JZ
7.7 Hz), 6.88 (2H, d, JZ7.6 Hz), 7.15 (2H, t, JZ7.7 Hz),
7.24–7.26 (4H, m), 7.30–7.37 (2H, m), 7.44 (2H, d, JZ
4.0 Hz), 7.88 (2H, d, JZ8.1 Hz), 7.96 (2H, d, JZ9.0 Hz).
13C NMR (75 MHz, CDCl3) d; 24.2, 71.4, 114.9, 117.5,
120.0, 121.6, 123.7, 125.4, 126.4, 127.9, 129.3, 129.5,
134.1, 136.7, 153.7, 157.0, 157.2. HRESIMS calcd for
C34H29N2O2: 497.2229 (MCH)C, found: 497.2225.
4.1.3. 2,20-Bis(2-pyridylmethoxy)-(1S)-[3,30-dibromo-
1,10-binaphthalene] (12). Colorless crystals. Mp 138–
140 8C (from ether). [a]2D7 C40.0 (c 0.87, CHCl3). IR
(KBr) n cmK1; 1595, 1572, 760. 1H NMR (300 MHz,
CDCl3) d; 4.71 (2H, d, JZ13.1 Hz), 5.12 (2H, d, JZ
13.1 Hz), 6.89 (2H, d, JZ7.9 Hz), 7.01 (2H, t, JZ8.4 Hz),
7.42 (4H, q, JZ8.1, 15.9 Hz), 7.75 (2H, d, JZ8.1 Hz), 8.12
(2H, s), 8.31 (2H, br d, JZ4.3 Hz). 13C NMR (75 MHz,
CDCl3) d; 75.5, 117.3, 120.6, 122.0, 125.8, 126.1, 126.7,
127.0, 127.2, 131.5, 132.9, 133.0, 136.4, 148.3, 151.8,
157.0. HRESIMS calcd for C32H23N2O2Br2: 625.0126
(MCH)C, found: 625.0132.
4.2.2.
2,20-Bis(2-quinolinecarbonyloxy)-(1S)-1,10-
binaphthalene (17). Colorless crystals. Mp 193–195 8C
(from hexane–AcOEt). [a]2D8 C53.7 (c 0.51, CHCl3). IR
(KBr) n cmK1; 1768, 1744, 1654, 839, 777. 1H NMR
(300 MHz, CDCl3) d; 7.34–7.79 (16H, m), 7.91 (2H, d, JZ
8.0 Hz), 8.02 (2H, d, JZ8.9 Hz), 8.06 (2H, d, JZ8.6 Hz),
8.18 (2H, d, JZ8.3 Hz). 13C NMR (75 MHz, CDCl3) d;
121.5, 122.0, 123.9, 126.3, 126.6, 127.6, 127.8, 128.4,
129.0, 129.6, 130.2, 130.5, 131.0, 131.2, 133.7, 137.6,
147.4, 147.5, 148.1, 163.4. HRESIMS calcd for
C40H25N2O4: 597.1814 (MCH)C, found: 597.1768. Anal.
Calcd for C40H24N2O4: C, 80.52; H, 4.05; N, 4.70. Found:
C, 80.43; H, 4.04; N, 4.71.
4.1.4. 2,20-Bis(2-pyridylethoxy)-(1S)-1,10-binaphthalene
(13). Colorless oil. [a]D22 K43.2 (c 0.75, CHCl3). IR (neat)
n cmK1; 1591, 1508, 809, 748. 1H NMR (300 MHz, CDCl3)

3386
Y. Takenaka et al. / Tetrahedron 62 (2006) 3380–3388
4.2.3. 2,20-Bis(2-isoquinolinecarbonyloxy)-(1S)-1,10-
binaphthalene (18). Colorless crystals. Mp 209–211 8C
(from hexane–AcOEt). [a]2D8 C6.4 (c 0.5, CHCl3). IR (KBr)
n cmK1; 1735, 1701, 1654, 1560, 801. 1H NMR d;
(300 MHz, CDCl3) 712–7.22 (4H, m), 7.39–7.44 (2H, m),
7.49–7.59 (6H, m), 7.63 (2H, d, JZ8.9 Hz), 7.65–7.75 (4H,
m), 7.98 (2H, d, JZ8.9 Hz), 8.04 (2H, d, JZ8.9 Hz), 8.49
(2H, d, JZ5.6 Hz). 13C NMR (75 MHz, CDCl3) d; 121.9,
123.7, 124.0, 125.7, 126.0, 126.1, 126.6, 126.7, 127.1,
128.0, 128.2, 130.1, 130.4, 132.0, 133.5, 136.4, 141.4,
147.4, 148.9, 164.6. HRESIMS calcd for C40H25N2O4:
597.1814 (MCH)C, found: 597.1768. Anal. Calcd for
C40H24N2O4: C, 80.52; H, 4.05; N, 4.70. Found: C, 80.31;
H, 4.28; N, 4.67.
135.5, 137.0, 147.7, 150.1, 163.7. HRESIMS calcd for
C26H21N2O4: 425.1501 (MCH)C, found: 425.1536.
4.3. General procedure for the enantioselective [2D2]-
cycloaddition reaction of 2-methoxycarbonyl-2-
cyclopenten-1-one (1) with thioacetylene derivatives
4.3.1. (1S,5R)-1-Methoxycarbonyl-2-oxo-7-(phenylthio)-
bicyclo[3.2.0]hept-6-ene (3). To a suspension of copper(II)
chloride (1.3 g, 9.7 mmol) and ligand ent-16 (5.76 g,
11.6 mmol) in CH2Cl2 (110 mL) was stirred at ambient
temperature for 30 min and silver hexafluoroantimonate
(7.0 g, 20.4 mmol) was added to the resulting mixture in the
dark. After being stirred for 1 h at ambient temperature, a
solution of 2-methoxycarbonyl-2-cyclohexen-1-one (1)
(6.40 g, 45.7 mmol) and phenylthioacetylene (2) (7.24 g,
54.8 mmol) in CH2Cl2 (130 mL) was added to the mixture
at K78 8C. After being stirred for 1.5 h at the same
temperature, phosphate buffer (pH 6.86, 150 mL) was added
to the mixture and the resulting mixture was extracted with
AcOEt. The organic layer was washed with brine and dried
over MgSO4. Filtration and concentration of the mixture
under reduced pressure gave crude material, which was
purified by silica gel column chromatography (hexane/
AcOEt, 7:1) to afford compound 3 (8.44 g, 30.8 mmol) in
67% yield. 73% ee (Chiralcel OJ-H, hexane/IPAZ95:5).
Pale yellow oil. [a]D22 C499 (c 1.33, CHCl3). IR (neat)
n cmK1; 1746, 1731, 1294, 747. 1H NMR (300 MHz,
CDCl3) d; 1.89 (1H, bdd, JZ9.1, 13.5 Hz), 2.13 (1H, dddd,
JZ6.9, 8.7, 11.9, 13.5 Hz), 2.39 (1H, ddd, JZ1.0, 8.7,
18.3 Hz), 3.02 (1H, ddd, JZ9.1, 11.9, 18.3 Hz), 3.67 (1H, d,
JZ6.9 Hz), 3.75 (3H, s), 5.88 (1H, s), 7.26–7.38 (3H, m),
7.49–7.53 (2H, m). 13C NMR (75 MHz, CDCl3) d; 22.4
(CH2), 34.9 (CH2), 48.3 (CH), 52.3 (CH3), 66.2 (C), 128.8
(CH), 129.4 (CH), 129.7 (C), 133.0 (CH), 133.8 (CH), 140.5
(C), 167.6 (C), 208.4 (C). EI-MS m/z: 274 (MC). Anal.
Calcd for C15H14O3S: C, 65.67; H, 5.14. Found: C, 65.55;
H, 5.11.
4.2.4. 2,20-Bis(nicotinyloxy)-(1S)-1,10-binaphthalene
(19). White amorphous solid. [a]2D8 K74.1 (c 0.56,
CHCl3). IR (KBr) n cmK1; 1741, 1589, 1508, 1420, 730.
1H NMR (300 MHz, CDCl3) d; 7.20 (2H, ddd, JZ0.8, 4.9,
8.0 Hz), 7.32–7.42 (4H, m), 7.49 (2H, ddd, JZ2.3, 5.8,
8.3 Hz), 7.55 (2H, d, JZ8.9 Hz), 7.86 (2H, td, JZ2.0,
8.0 Hz), 7.93 (2H, d, JZ8.2 Hz), 8.01 (2H, d, JZ8.9 Hz),
8.65 (2H, dd, JZ1.7, 4.9 Hz), 8.75 (2H, dd, JZ0.8, 2.2 Hz).
13C NMR (75 MHz, CDCl3) d; 121.4, 123.2, 123.5, 125.1,
125.9, 126.1, 127.1, 128.2, 129.9, 131.6, 133.2, 146.5,
151.0, 153.6, 163.4. HRESIMS calcd for C32H21N2O4:
497.1510 (MCH)C, found: 497.1496. Anal. Calcd for
C32H21N2O4: C, 77.41; H, 4.06; N, 5.64. Found: C, 77.07;
H, 4.21; N, 5.61.
4.2.5. 2,20-Bis(picolinyloxy)-(1S)-[3,30-dibromo-1,10-
binaphthalene] (20). Colorless crystals. Mp 156–158 8C
(from acetone). [a]D24 K2.7 (c 0.75, CHCl3). IR (KBr)
n cmK1; 1764, 1578, 745. 1H NMR (300 MHz, acetone-d6)
d; 7.23 (2H, d, JZ8.3 Hz), 7.41 (2H, t, JZ7.6 Hz), 7.47–
7.61 (4H, m), 7.83–8.09 (6H, m), 8.43 (2H, s), 8.61 (2H, br
d, JZ4.5 Hz). 13C NMR (75 MHz, acetone-d6) d; 115.6,
125.9, 126.3, 126.5, 127.5, 127.8, 127.9, 128.0, 132.4,
133.6, 137.5, 144.9, 146.9, 150.5, 162.1. HRESIMS calcd
for C32H19N2O4Br2: 652.9712 (MCH)C, found: 652.9655.
4.3.2. 1-Methoxycarbonyl-2-oxo-6-butyl-7-(phenylthio)-
bicyclo[3.2.0]hept-6-ene (24a). Pale yellow oil. [a]D26
K148 (32% ee, c 0.71, CHCl3). IR (neat) n cmK1; 1743,
1734, 1730. 1H NMR (300 MHz, CDCl3) d; 0.88 (3H, t, JZ
7.3 Hz), 1.19–1.50 (4H, m), 1.88–2.25 (4H, m), 2.34 (1H,
ddd, JZ1.3, 8.6, 18.3 Hz), 2.84 (1H, ddd, JZ9.3, 11.9,
18.3 Hz), 3.53 (3H, s), 3.64 (1H, d, JZ7.0 Hz), 7.20–7.33
(3H, m), 7.36–7.45 (2H, m). 13C NMR (75 MHz, CDCl3)
d; 13.4, 20.3, 22.6, 27.5, 28.4, 35.0, 49.4, 51.9, 65.3,
127.5, 128.8, 129.5, 132.1, 132.4, 160.0, 168.1, 207.7.
HRESIMS calcd for C19H23O3S: 331.1368 (MCH)C,
found: 331.1384.
4.2.6. 2,20-Bis(picolinyloxy) -(1S)-[6,60-dibromo-1,10-
binaphthalene] (21). Colorless crystals. Mp 78–80 8C
(from ether–hexane). [a]2D4 K80.1 (c 0.71, CHCl3). IR
(KBr) n cmK1; 1756, 1584, 744. 1H NMR (300 MHz,
CDCl3) d; 7.21 (2H, d, JZ9.0 Hz), 7.36 (2H, d, JZ6.1 Hz),
7.41 (2H, dd, JZ1.3, 9.2 Hz), 7.54–7.69 (6H, m), 7.90 (2H,
d, JZ9.0 Hz), 8.06 (2H, s), 8.65 (2H, br d, JZ4.7 Hz). 13C
NMR (75 MHz, CDCl3) d; 122.0, 122.7, 123.2, 125.4,
127.1, 127.6, 129.1, 130.0, 130.1, 131.6, 132.6, 137.0,
146.6, 147.1. HRESIMS calcd for C32H19N2O4Br2:
652.9712 (MCH)C, found: 652.9692. Anal. Calcd for
C32H18N2O4Br2: C, 58.74; H, 2.77; N, 4.28. Found: C,
58.53; H, 2.85; N, 4.05.
4.3.3. 1-Methoxycarbonyl-2-oxo-6-trimethylsilyl-7-
(methylthio)bicyclo[3.2.0]hept-6-ene (24c). Pale yellow
4.2.7. (R,R)-1,2-Diphenyl-1,2-bis(picolinyloxy)ethane
(22). White amorphous solid. [a]2D8 C72.4 (c 0.51,
CHCl3). IR (KBr) n cmK1; 1733, 1582, 697. 1H NMR
(300 MHz, CDCl3) d; 6.58 (2H, s), 7.20–7.35 (10H, m), 7.41
(2H, ddd, JZ0.9, 4.6, 7.6 Hz), 7.78 (2H, dt, JZ1.7, 7.7 Hz),
8.11 (2H, d, JZ7.8 Hz), 8.71 (2H, d, JZ4.6 Hz). 13C NMR
(75 MHz, CDCl3) d; 78.5, 125.2, 126.8, 127.9, 128.3, 128.7,
oil. [a]2D6 K383 (73% ee, c 0.35, CHCl3). IR (neat) n cmK1
;
1
1747, 1732. H NMR (300 MHz, CDCl3) d; 0.16 (9H, s),
1.89 (1H, dd, JZ9.0, 13.5 Hz HH), 2.06–2.13 (1H, m),
2.33 (3H, s), 2.29–2.41 (1H, m), 2.96 (1H, ddd, JZ9.0,
12.2, 18.2 Hz), 3.54 (1H, d, JZ6.8 Hz), 3.74 (3H, s).
13C NMR (75 MHz, CDCl3) d; K1.7, 13.9, 22.9, 35.1,
49.9, 52.2, 68.5, 149.0, 151.4, 168.4, 210.0. HRESIMS

Y. Takenaka et al. / Tetrahedron 62 (2006) 3380–3388
3387
calcd for C13H20O3NaSiS: 307.0800 (MCNa)C, found:
307.0808.
3. (a) Takasu, K.; Ueno, M.; Inanaga, K.; Ihara, M. J. Org. Chem.
2004, 69, 517–521. (b) Takasu, K.; Nagao, S.; Ueno, M.;
Ihara, M. Tetrahedron 2004, 60, 2071–2078.
4. Lewis acid-catalyzed [2C2]-cycloaddition of g,g-dialkoxy-
allylic zirconium species to a,b-unsaturated carbonyl
compounds, see: Ito, H.; Sato, A.; Taguchi, T. Tetrahedron
Lett. 1999, 40, 3217–3220.
4.3.4.
2-(Methoxycarbonyl)-3-(2-chloro-2-phenyl-
thioethenyl)cyclopentan-1-one (4). To a solution of 3
(60% ee) (135 mg, 0.429 mmol) in CH2Cl2 (2 mL) was
added titanium tetrachloride (53 mL, 0.428 mmol) at 0 8C.
After being stirred for 30 min at same temperature, neutral
phosphate buffer solution (pH 6.86, 1 mL) was added to the
reaction mixture. The resulting mixture was extracted with
AcOEt and the organic layer was dried over MgSO4 and
consentrated under vacuum. The residue was purified by
silica gel column chromatography (hexane/AcOEt, 5:1) to
afford compound 4 (140 mg, 0.45 mmol) in 91% yield. [a]D28
C267 (c 0.51, CHCl3). IR (neat) n cmK1; 1760, 1730,
5. For review, see: (a) Hayashi, Y.; Narasaka, K. In Compre-
hensive Asymmetric Catalysis; Jacobsen, E. N., Pfalts, A.,
Yamamoto, H., Eds.; Springer: Berlin, 1999; pp 1255–1269.
See: also; (b) Hayashi, Y.; Narasaka, K. Chem. Lett. 1989,
793–796. (c) Ichikawa, Y.; Narita, A.; Shiozawa, A.; Hayashi,
Y.; Narasaka, K. J. Chem. Soc., Chem. Commun. 1989,
1919–1921. (d) Narasaka, K.; Hayashi, Y.; Shimadzu, H.;
Niihara, S. J. Am. Chem. Soc. 1992, 114, 8869–8885.
6. (a) Narasaka, K.; Kusama, H.; Hayashi, Y. Bull. Chem.
Soc. Jpn. 1991, 64, 1471–1478. (b) Hayashi, Y.; Otaka, K.;
Saito, N.; Narasaka, K. Bull. Chem. Soc. Jpn. 1991, 64,
2122–2127. (c) Hayashi, Y.; Narasaka, K. Chem. Lett. 1990,
1295–1298. (d) Narasaka, K.; Hayashi, K.; Hayashi, Y.
Tetrahedron 1994, 50, 4529–4542.
1
1584. H NMR (300 MHz, CDCl3) d; 1.68–1.79 (1H, m),
2.25–2.23 (3H, m), 3.08 (1H, d, JZ11.3 Hz), 3.73 (3H, s),
3.88–3.98 (1H, m), 6.15 (1H, d, JZ9.5 Hz), 7.30–7.39 (5H,
m). 13C NMR (75 MHz, CDCl3) d; 27.5, 37.8, 42.8,
52.6, 60.6, 127.8, 129.2, 129.5, 130.5, 132.1, 139.1, 168.4,
209.3. HRESIMS calcd for C15H15O3NaSCl: 333.0328
(MCNa)C, found: 333.0302.
7. For examples of diastereoselective addition of nucleophile
to chiral 2-cyclopentenecarboxylate, see: (a) Berl, V.;
Helmchen, G.; Preston, S. Tetrahedron Lett. 1994, 35,
233–236. (b) Urban, E.; Knu¨hl, G.; Helmchen, G. Tetrahedron
Lett. 1995, 36, 7229–7232. (c) Urban, E.; Knu¨hl, G.;
Helmchen, G. Tetrahedron 1995, 51, 13031–13038. (d)
Urban, E.; Knu¨hl, G.; Helmchen, G. Tetrahedron 1996, 52,
971–986. (e) Spreitzer, H.; Pichler, A.; Holzer, W.; Toth, I.;
Zuchart, B. Helv. Chim. Acta 1997, 80, 139–145. (f)
Spreitzer, H.; Pichler, A.; Holzer, W.; Schlager, C. Helv.
Chim. Acta 1998, 81, 40–45. (g) Chapdelaine, D.; Belzile, J.;
Deslongchamps, P. J. Org. Chem. 2002, 67, 5669–5672.
8. For examples of diastereoselective addition of chiral nucleo-
phile to 2-cyclopentenecarboxylate, see: (a) Enders, D.;
Kaiser, A. Liebigs Ann. 1997, 485–493. (b) Enders, D.;
Kaiser, A. Heterocycles 1997, 46, 631–635.
4.3.5. 3-(Hydroxycarbonylmethyl)cyclopentan-1-one
(25). The mixture of compound 4 (60 mg, 0.193 mmol)
and concentrated HCl (0.1 mL) in MeOH (1 mL) was
heated at 60 8C. After being stirred for 12 h at same
temperature, the mixture was concentrated under reduced
pressure. The residue was purified by silica gel column
chromatography (hexane/isopropanol, 1:5) to afford com-
pound 2518 (9 mg, 0.063 mmol) in 33% yield. [a]2D8 C65.1
(c 0.45, CHCl3). IR (neat) n cmK1; 1737. 1H NMR
(300 MHz, CDCl3) d; 1.20–1.25 (1H, m), 1.53–1.70 (1H,
m), 1.86–1.96 (1H, m), 2.14–2.39 (3H, m), 2.48–2.68 (3H,
m). 13C NMR (75 MHz, CDCl3) d; 29.1, 33.1, 38.3, 39.3,
44.5, 177.7, 218.3.
9. For examples of enantioselective addition of nucleophile
to 2-cyclopentenecarboxylate using chiral catalyst, see: (a)
Bernardi, A.; Karamfilova, K.; Boschin, G.; Scolastico, C.
Tetrahedron Lett. 1995, 36, 1363–1364. (b) Bernardi, A.;
Colombo, G.; Scolastico, C. Tetrahedron Lett. 1996, 37,
8921–8924. (c) Bernardi, A.; Karamfilova, K.; Sanguinetti, S.;
Scolastico, C. Tetrahedron 1997, 53, 13009–13026. (d)
Evans, D. A.; Rovis, T.; Kozlowski, M. C.; Downey,
C. W.; Tedrow, J. S. J. Am. Chem. Soc. 2000, 122,
9134–9142.
Acknowledgements
We thank the Uehara Memorial Foundation for financial
support.
References and notes
10. For examples of asymmetric cyclopropanation of 2-cyclo-
pentenecarboxylate, see: (a) Fioravanti, S.; Morreale, A.;
Pellacani, L.; Tardella, P. A. J. Org. Chem. 2002, 67,
4972–4974. (b) Fioravanti, S.; Morreale, A.; Pellacani, L.;
Tardella, P. A. Tetrahedron Lett. 2003, 44, 3031–3034.
11. One example of enantioselective [2C2]-cycloaddition of
2-cyclopentenecarboxylate was already reported as a key
step of a total synthesis of tricycloclavulone by our group, see:
(a) Ito, H.; Hasegawa, M.; Takenaka, Y.; Kobayashi, T.;
Iguchi, K. J. Am. Chem. Soc. 2004, 126, 4520–4521. See also,
(b) Ito, H.; Kobayashi, T.; Hasegawa, M.; Iguchi, K.
Tetrahedron Lett. 2003, 44, 1259–1261.
1. For recent reviews on cyclobutane derivatives, see: (a)
Crimmins, M. T. In Trost, B. M., Fleming, I., Eds.;
Comprehensive Organic Synthesis; Pergamon: Oxford, 1991;
Vol. 5; Chapter 2.3. (b) Lee-Ruff, E.; Mladenova, G. Chem.
Rev. 2003, 103, 1449–1483. (c) Namyslo, J. C.; Kaufmann,
D. E. Chem. Rev. 2003, 103, 1485–1537.
2. For examples, see: (a) Meyers, A. I.; Tschantz, M. A.;
Brengel, G. P. J. Org. Chem. 1995, 60, 4359–4362. (b)
Groaning, M. D.; Brengel, G. P.; Meyers, A. I. Tetrahedron
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