Axially dissymmetric binaphthyldiimine chiral Salen-type ligands for catalytic asymmetric addition of diethylzinc to aldehyde

Article abstract of DOI:10.1016/S0957-4166(02)00571-2

The axially dissymmetric chiral Schiff base ligand 10, prepared by the reaction of (R)-(+)-1,1′-binaphthyl-2,2′-diamine with (R)-(+)-2,2′-dihydroxy-[1,1′]-binaphthalenyl-3-carbaldehyde, is a fairly effective chiral ligand for the catalytic asymmetric addition reaction of diethylzinc to aldehydes.

Full text of DOI:10.1016/S0957-4166(02)00571-2

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 13 (2002) 2161–2166
Axially dissymmetric binaphthyldiimine chiral Salen-type ligands
for catalytic asymmetric addition of diethylzinc to aldehyde
Min Shi* and Chun-Jiang Wang
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
354 Fenglin Lu, Shanghai 200032, China
Received 26 August 2002; accepted 11 September 2002
Abstract—The axially dissymmetric chiral Schiff base ligand 10, prepared by the reaction of (R)-(+)-1,1%-binaphthyl-2,2%-diamine
with (R)-(+)-2,2%-dihydroxy-[1,1%]-binaphthalenyl-3-carbaldehyde, is a fairly effective chiral ligand for the catalytic asymmetric
addition reaction of diethylzinc to aldehydes. © 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction
being obtained in several preparatively important reac-
tions.1 Previously, we have reported that axially dissym-
metric chiral Salen-type ligands A–D (Fig. 1), derived
from the reaction of (R)-(+)-1,1%-binaphthyl-2,2%-
Axially dissymmetric 1,1%-binaphthyl and 1,1%-biphenyl
ligands bearing identical groups in the 2- and 2%-posi-
tions have proved remarkably useful in enantioselective
catalysis, with enantiomeric purities close to 100%
diamine
with
2,6-dichlorobenzaldehyde,
2,3-
dichlorobenzaldehyde, 3,4-dichlorobenzaldehyde or
Figure 1. The structures of Schiff bases A–D.
* Corresponding author. Fax: 86-21-64166128; e-mail: mshi@pub.sioc.ac.cn
0957-4166/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(02)00571-2

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M. Shi, C.-J. Wang / Tetrahedron: Asymmetry 13 (2002) 2161–2166
salicylaldehyde, are excellent chiral ligands in the
Cu(MeCN)ClO4-catalyzed asymmetric aziridination of
cinnamates.2 In order to extend the scope of the appli-
cations of these ligands in asymmetric reactions, we
further examined the other types of catalytic asymmet-
ric reactions using these chiral ligands and their deriva-
tives. We found that derivatives of these ligands having
phenolic hydroxyl groups or hydroxyl groups are also
very effective in the asymmetric addition reactions of
diethylzinc to aldehydes, although some very common
Salen type ligands, such as Schiff bases D–F having two
phenolic hydroxy groups, showed no catalytic proper-
ties for this reaction (Fig. 2). In some cases, very
high yields and good enantioselectivities were
obtained.3 Herein, we report on the application of
axially dissymmetric chiral ligands L1–L4 as promotors
for the asymmetric addition reactions of diethylzinc to
aldehydes.
2. Results and discussion
It is well known that b-amino alcohols are excellent
chiral ligands for the asymmetric addition reactions of
diethylzinc to aldehydes. Thus, we first investigated the
synthesis of chiral b-amino alcohols 4 and 5 using
(R)-(+)-1,1%-binaphthyl-2,2%-diamine 1 as a chiral scaf-
fold. As Scheme 1 shows, the two chiral b-amino
alcohols 4 and 5 were successfully prepared from (R)-(+)-
N,N%-dimethyl-1,1%-binaphthyl-2,2%-diamine 3 (derived
from (R)-(+)-1,1%-binaphthyl-2,2%-diamine 1 by reaction
with ethyl chloroformate and reduction of the resulting
carbamate with LiAlH4 in THF) by reaction with ethyl-
ene oxide in the presence of HOAc (Scheme 1). We
found that the monosubstituted ligand 4 could be pre-
pared in 86% yield by reaction of 3 with ethylene oxide
at 0°C, whilst diol 5 was formed in slightly lower yield
(80%) by completing the reaction at 10°C.
Figure 2. The Schiff bases having two phenolic hydroxy groups.
Scheme 1.

M. Shi, C.-J. Wang / Tetrahedron: Asymmetry 13 (2002) 2161–2166
2163
10 (L3) and 11 (L4). The results are summarized in
Table 1. The ee of the products was determined by
chiral HPLC analysis using a chiral stationary-phase
column (CHIRALCEL OD and OJ) and the absolute
configuration of the major enantiomer was assigned
according to the sign of the specific rotations. As shown
in Table 1, using L1 as a ligand for the addition of
diethylzinc to benzylaldehyde in toluene at room tem-
perature, the sec-alcohol was obtained in 51% ee (58%
yield) with R configuration. In contrast, using ligand
L2, the sec-alcohol was obtained in 45% ee (40% yield)
with S configuration under the same conditions (Table
1, entries 1 and 2). The axially dissymmetric chiral
Schiff base ligand L3 gave the best results under the
The axially dissymmetric chiral ligands Schiff bases 10
and 11 were prepared by the reaction of (R)-(+)-1,1%-
binaphthyl-2,2%-diamine 1 and (S)-(−)-1,1%-binaphthyl-
2,2%-diamine 1 with (R)-(+)-2,2%-dihydroxy-[1,1%]-bina-
phthalenyl-3-carbaldehyde 9 (derived from (R)-(+)-
binol 6 as shown in Scheme 2,4 in anhydrous ethanol
under reflux, Scheme 2).5 After standard work-up and
purification by silica gel column chromatography or
recrystallization, 10 and 11 were obtained as yellow
solids in about 95% yields, respectively.
We first examined the asymmetric addition reaction of
diethylzinc to benzaldehydes in the presence of a cata-
lytic amount (5 mol%) of chiral ligands 4 (L1), 5 (L2),
Scheme 2.
Table 1. Catalytic asymmetric addition reaction of diethylzinc to benzaldehyde in the presence of chiral ligands L1–L4
Entry
Chiral ligand
Time (h)
Yielda
eeb (%)
Config.c
1
2
3
4
L1
L2
L3
L4
48
48
48
48
58
40
85
40
51
45
75
7
R
S
R
R
a Isolated yield.
b Determined by chiral HPLC.
c Determined by the sign of the specific rotation.

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M. Shi, C.-J. Wang / Tetrahedron: Asymmetry 13 (2002) 2161–2166
also fairly effective in the asymmetric addition reaction
of diethylzinc with aldehydes. At the present time we
cannot clearly explain why 10 (L3) is so effective for
this reaction, while its diastereomer 11 (L4) is a less
effective ligand under the same conditions. For chiral
ligands L1 and L2, we believe that the flexible struc-
tures and the distance of the active site from the
stereogenic center cause the low enantioselectivities.
Efforts are now underway to elucidate the scope and
limitations of (R)-(+)-1,1%-binaphthyl-2,2%-diamine-
derived Schiff base ligands in asymmetric catalysis.
same conditions in 75% ee (85% yield) with R configu-
ration (Table 1, entry 3). Obviously, L4 is not suitable
for this asymmetric addition reaction (Table 1, entry 4).
The results suggest that L3 is an effective chiral ligand
in this asymmetric reaction.
Under the optimized reaction conditions, we employed
L3 as the chiral ligand in this asymmetric reaction for
various aldehydes. The results are summarized in Table
2.
As can be seen from Table 2, the corresponding sec-
alcohols can be formed in excellent yields (82–98%) and
good ee (60–83%) with R configuration for aromatic
aldehydes (Table 2, entries 1–4, 6–8). On the other
hand, in the reactions of aliphatic aldehydes, although
high yields of the sec-alcohols were achieved, poor
enantioselectivities (23–30% ee) were realized (Table 2,
entries 5 and 9). It is known that in the catalytic
asymmetric addition reaction of diethylzinc, the enan-
tioselectivity is usually much lower with aliphatic alde-
hydes than the equivalent reaction with aromatic
aldehydes.
3. Experimental
Mps were obtained with a Yanagimoto micro melting
point apparatus and are uncorrected. Optical rotations
were determined in a solution of CHCl3 or acetone at
20°C by using a Perkin–Elmer-241 MC digital polar-
imeter; [h]D values are given in unit of 10−1 deg cm2 g−1.
1H NMR spectra were determined for solutions in
CDCl3 with tetramethylsilane (TMS) as internal stan-
dard on a Bruker AMX-300 spectrometer; J-values are
in Hz. IR spectra were determined by a Perkin–Elmer
983 spectrometer. Mass spectra were recorded with an
HP-5989 instrument. High mass spectra were recorded
on a Finnigan MA+ instrument. All solid compounds
reported in this paper gave satisfactory CHN micro-
analyses with an Italian Carlo-Erba 1106 analyzer.
To the best of our knowledge, it is only known that
some Salen-type catalysts of transition metals can cata-
lyze the asymmetric addition reaction6 and the chiral
Schiff bases themselves have not been investigated in
detail for this purpose. Also, during our own investiga-
tions, we found that the readily available Schiff bases
D–F have no catalytic activity in this reaction. There-
fore, we have disclosed for the first time that axially
dissymmetric chiral Schiff base ligands having specific
structure can be effective chiral ligands in this reaction.
(R)-(+)-1,1%-Binaphthyl-2,2%-diamine
was
prepared
according to the literature.7
3.1. Preparation of (R)-2,2%-bis(ethoxycarbonylamino)-
1,1%-binaphthyl, 2
This is a known compound and was prepared according
to the literature method.7 To a stirred, ice-cooling
solution of (R)-binaphthyldiamine (568 mg, 2.0 mmol)
in benzene (12 mL) and pyridine (1.5 mL) was added
dropwise a solution of ethyl chloroformate (0.47 mL,
5.0 mmol) in benzene (3.0 mL). After the addition was
completed, the mixture was stirred for 3 h at room
temperature. The reaction was quenched by adding
10% NaOH (20 mL). The resulting organic layer and
benzene extracts from the aqueous layer were com-
bined, washed with water and dried over MgSO4. After
removal of the solvent under reduced pressure, the
residue was purified by column chromatography on
silica gel. Elution with hexane/ethyl acetate (8:1) gave
In conclusion, we have found that axially dissymmetric
chiral ligands Schiff base 10 derived from the reaction
of (R)-(+)-1,1%-binaphthyl-2,2%-diamine with (R)-(+)-
2,2%-dihydroxy-[1,1%]-binaphthalenyl-3-carbaldehyde 9 is
Table 2. Catalytic asymmetric addition reaction of
diethylzinc to aldehydes in the presence of chiral ligand
L3
Entry
R
Yielda (%)
eeb (%)
Config.c
1
compound 2 as a colorless oil (813 mg, 95%). H NMR
(CDCl3, TMS, 300 MHz) l 1.70 (t, J=7.1 Hz, 3H),
4.30 (q, J=7.1 Hz, 2H), 6.27 (s, 2H, 2 NH), 6.96 (d,
J=8.2 Hz, 2H, ArH), 7.14 (m, 2H, ArH), 7.22 (m, 2H,
ArH), 7.93 (d, J=7.9 Hz, 2H, ArH), 8.06 (d, J=9.1
Hz, 2H, ArH), 8.56 (d, J=9.1 Hz, 2H, ArH).
1
2
3
4
5
6
7
8
9
C6H5
85
82
95
88
95
91
97
98
86
75
60
72
60
30
71
83
79
23
R
R
R
R
R
R
R
R
R
p-MeC6H4
p-EtC6H4
1-Naphthyl
PhCHꢀCH2
p-F-C6H4
p-ClC6H4
p-BrC6H4
iso-Butyl
3.2. Preparation of (R)-2,2%-bis(methylamino)-1,1%-
binaphthyl, 3
a Isolated yields.
b Determined by chiral HPLC.
c Determined by the sign of the specific rotation.
This is a known compound and was prepared according
to the literature method.7 To a stirred solution of LAH
(130 mg, 3.4 mmol), in anhydrous THF (15 mL) was

M. Shi, C.-J. Wang / Tetrahedron: Asymmetry 13 (2002) 2161–2166
2165
added dropwise a solution of (R)-2,2%-bis(ethoxycar-
3.5. Preparation of (R)-2,2%-bis(methoxymethoxy)-1,1%-
binaphthyl, 7
bonylamino)-1,1%-binaphthyl 2 (256 mg, 0.6 mmol) in
anhydrous THF (5 mL). The mixture was heated under
reflux for 1 h, the reaction was quenched by water and
10% NaOH. After separation, the aqueous layer was
extracted with ethyl acetate (2×20 mL). The organic
layers were combined and dried over MgSO4. After
removal of the solvent, the residue was purified by
column chromatography on silica gel. Elution with
hexane/ethyl acetate (10:1) gave compound 3 (178 mg,
This is a known compound and was prepared according
to the literature.4 Under a nitrogen atmosphere, (R)-
1,1%-binaphthol (5.72 g, 20 mmol) was added to a
suspension of NaH (2.4 g, 100 mmol) in anhydrous
THF (40 mL) at 0°C with stirring. The resulting solu-
tion was stirred at 0°C for 10 min, and then
methoxymethyl chloride (3.65 mL, 48 mmol) was
slowly added. The mixture was allowed to warm to
room temperature and stirred for 4 h. Then the reaction
was quenched by water. After separation, the aqueous
layer was extracted with ethyl acetate (2×20 mL). The
combined organic layers were washed with brine and
dried over MgSO4. After removal of the solvent, com-
pound 7 was obtained (7.33 g, 98%), which was pure
1
95%) as a colorless oil. H NMR (CDCl3, TMS, 300
MHz) l 2.84 (s, 6H, 2Me), 6.96 (d, J=8.2 Hz, 2H,
ArH), 7.10 (m, 4H, ArH), 7.36 (d, J=9.7 Hz, 2H,
ArH), 7.83 (d, J=7.6 Hz, 2H, ArH), 7.97 (d, J=8.2
Hz, 2H, ArH).
1
3.3. Preparation of (R)-2-[methyl-(2%-methylamino-
[1,1%]-binaphthalenyl-2-yl)-amino]ethanol, 4
enough for the next step. H NMR (CDCl3, TMS, 300
MHz) l 3.14 (s, 6H, 2 Me), 4.97 (d, J=6.6 Hz, 2H,
CH2), 5.08 (d, J=6.6 Hz, 2H, CH2), 7.13–7.37 (m, 6H,
ArH), 7.58 (d, J=9.0 Hz, 2H, ArH), 7.86 (d, J=8.1
Hz, 2H, ArH), 7.94 (d, J=9.0 Hz, 2H, ArH).
To a stirred, ice-cooling solution of (R)-2,2%-bis(methyl-
amino)-1,1%-binaphthyl 3 (508 mg, 1.63 mmol) in
CH2Cl2 (40 mL) and acetic acid (10 mL) was added
ethylene oxide (10 mL, 0.5 mol). The mixture was
stirred for 24 h at 0°C, then the reaction was quenched
by slowly adding 20% NaOH until pH >7. After sepa-
ration, the aqueous layer was extracted with ethyl
acetate (2×20 mL). The organic layers were combined
and dried over MgSO4. After removal of the solvent,
the residue was purified by column chromatography on
silica gel. Elution with hexane/ethyl acetate (3:1) gave
compound 4 (467 mg, 86%) as colorless solid. Mp
141–142°C; [h]D=−152 (c 0.4, CHCl3); IR (CHCl3) w
3383, 3050, 2980, 1681, 1615, 1594, 1506, 1490, 1440,
1421, 1337, 817, 756 cm−1; 1H NMR (CDCl3, TMS, 300
MHz) l 2.32 (s, 3H, Me), 2.82 (s, 3H, Me), 3.10–3.46
(m, 4H, CH2CH2), 6.95–7.54 (m, 8H, ArH), 7.81–7.99
(m, 4H, ArH); MS (EI) m/e 356 (M+, 7.10), 325 (M+−
31, 100), 308 (M+−48, 84.66), 280 (M+−76, 47.80). Anal.
calcd for C24H24N2O requires: C, 80.89; H, 6.74; N,
7.86. Found: C, 80.93; H, 6.77; N, 7.80%.
3.6. Preparation of (R)-3-formyl-2,2%-bis(methoxy-
methoxy)-1,1%-binaphthyl, 8
This is a known compound and was prepared according
to the literature.4 Under a nitrogen atmosphere, n-BuLi
(2 M in hexene, 2.83 mL, 5.65 mmol) was added to a
solution of (R)-2,2%-bis(methoxymethoxy)-1,1%-binaph-
thyl 7 (1.87 g, 5 mmol) in anhydrous THF (30 mL) at
room temperature. The mixture was stirred for 2 h,
which produced a grey suspension. After the mixture
was cooled to 0°C, DMF (0.46 mL, 6 mmol) was
added. The reaction was allowed to warm to room
temperature and stirred for 4 h. The reaction was then
quenched by saturated NH4Cl (20 mL). After separa-
tion, the aqueous layer was extracted with ethyl acetate
(2×20 mL). The organic layers were combined and
dried over MgSO4. After removal of the solvent, the
residue was purified by column chromatography on
silica gel. Elution with hexane/ethyl acetate (10:1) gave
1
compound 8 (1.4 g, 70%). H NMR (CDCl3, TMS, 300
3.4. (R)-2-({2%-[(2-Hydroxyethyl)-methylamino]-[1,1%]-
binaphthalenyl-2-yl}-methylamino)ethanol, 5
MHz) l 3.07 (s, 3H, Me), 3.20 (s, 3H, Me), 4.64 (d,
J=6.1 Hz, 1H, CH), 4.75 (d, J=6.1 Hz, 1H, CH), 5.04
(d, J=7.3 Hz, 1H, CH), 5.16 (d, J=7.3 Hz, 1H, CH),
7.16–7.62 (m, 7H, ArH), 7.82–8.16 (m, 3H, ArH), 8.58
(s, 1H, ArH), 10.59 (s, 1H, CHꢀO).
The preparation of compound 5 was completed in the
same manner as that of compound 4 except that the
mixture was stirred for 24 h at 10°C. Compound 5 was
obtained (520 mg, 80%) as a colorless solid. Mp 152–
153°C; [h]D=−19.1 (c 4.4, CHCl3); IR (CHCl3) w 3381,
3050, 2982, 1685, 1618, 1597, 1506, 1490, 1445, 1420,
1337, 815, 755 cm−1; 1H NMR (CDCl3, TMS, 300
3.7. Preparation of (R)-2,2%-dihydroxy-[1,1%]-binaphthyl-
3-carbaldehyde, 9
This is a known compound and was prepared according
to the literature method.4 To a stirred solution of
(R)-3-formyl-2,2%-bis(methoxymethoxy)-1,1%-binaphthyl
8 (1.44 g, 3.6 mmol) in methanol (20 mL) was added 10
drops of concentrated HCl at 60°C. Then the mixture
was stirred for 0.5 h at the same temperature. After
removal of the solvent, the residue was purified by
column chromatography on silica gel. Elution with
hexene/ethyl acetate (6:1) gave the compound 9 (1.18 g,
MHz)
l 2.52 (s, 6H, Me), 2.83–3.28 (m, 8H,
2CH2CH2), 7.05 (d, J=7.6 Hz, 2H, ArH), 7.17 (m, 2H,
ArH), 7.20 (m, 2H, ArH), 7.57 (d, J=9 Hz, 2H, ArH),
7.86 (d, J=8.1 Hz, 2H, ArH), 7.96 (d, J=8.7 Hz, 2H,
ArH); 13C NMR (CDCl3, TMS, 75 MHz) l 41.95,
58.56, 59.39, 120.76, 124.62, 126.30, 126.32, 126.71,
128.05, 128.29, 129.39, 130.68, 134.74; MS (EI) m/e 400
(M+, 1.28), 369 (M+−31, 38.24), 352 (M+−48, 23.26),
294 (M+−106, 100); HRMS calcd for C26H28N2O2 (M+)
requires 400.2151, found 400.2153.
1
98%) as a yellow solid. H NMR (CDCl3, TMS, 300
MHz) l 4.94 (s, 1H, OH), 7.02–7.46 (m, 7H, ArH),

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M. Shi, C.-J. Wang / Tetrahedron: Asymmetry 13 (2002) 2161–2166
7.81–8.10 (m, 3H, ArH), 8.21 (s, 1H, ArH), 10.21 (s,
1H, OH), 10.65 (s, 1H, CHꢀO).
Acknowledgements
We thank the State Key Project of Basic Research
(Project 973) (No. G2000048007) and the National
Natural Science Foundation of China for financial sup-
port (20025206).
3.8. Preparation of Salen-type ligands, 10 and 11
3.8.1. Preparation of Salen-type ligand, 10. A mixture of
(R)-binaphthyldiamine (57 mg, 0.2 mmol) and (R)-2,2%-
dihydroxy -[1,1%]-binaphthyl-3-carbaldehyde 9 (126 mg,
0.4 mmol) in absolute ethanol (10 mL) was refluxed for
about 10 h to yield a yellow precipitate. The crude
product was filtered at room temperature and washed
with ice-cold ethanol to afford pure Salen-type ligand
10 (166 mg, 95%). Mp 197–198°C; [h]D=−377 (c 0.105,
acetone); IR (CHCl3) w 3521, 3056, 1622, 1603, 1588,
1466, 1433, 1352, 814, 752 cm−1; 1H NMR (CDCl3,
TMS, 300 MHz) l 4.92 (s, 2H, 2OH), 6.92–8.02 (m,
32H, ArH), 8.67 (s, 2H, ArH), 12.22 (s, 2H, 2OH); MS
(ESI) m/e 899.5 (M++Na+). Anal. calcd for
C62H40N2O4·CH2Cl2 requires: C, 78.66; H, 4.57; N,
3.19. Found: C, 78.79, H, 4.63; N, 2.87%.
References
1. (a) Noyori, R.; Takaya, H. Acc. Chem. Res. 1990, 23, 345;
(b) Noyori, R.; Suga, S.; Kawai, K.; Okada, S.; Kitamura,
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Synthesis 1990, 541; (d) Brunner, H. Synthesis 1988, 645;
(e) Narasaka, K. Synthesis 1991, 1; (f) Noyori, R. Chem.
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3. (a) Noyori, R.; Kitamura, M. Angew. Chem., Int. Ed. Engl.
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Asymmetry 2000, 11, 4923.
4. (a) Matsunaga, S.; Das, J.; Roels, J.; Vogl, E. M.;
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3.8.2. The preparation of Salen-type ligand, 11. This
compound was prepared in the same manner as that of
compound 10 except that (S)-binaphthyldiamine was
used. Mp 186–187°C; [h]D=+646 (c 0.12, acetone); IR
(CHCl3) w 3515, 3381, 3056, 1698, 1622, 1603, 1588,
1466, 1434, 1353, 814, 748 cm−1; 1H NMR (CDCl3,
TMS, 300 MHz) l 5.15 (s, 2H, 2OH), 6.94–8.04 (m,
32H, ArH), 8.60 (s, 2H, ArH), 12.30 (s, 2H, 2OH); MS
(ESI) m/e 899.5 (M++Na+). Anal. calcd for C62H40-
N2O4·CH2Cl2 requires: C, 78.66; H, 4.57; N, 3.19.
Found: C, 78.74; H, 4.79; N, 2.92%.
3.9. Typical asymmetric addition procedure: typical
procedure for the asymmetric addition of Et2Zn to
benzaldehyde
To a solution of ligand 10 (22 mg, 0.025 mmol) in
toluene (1.5 mL) was added dropwise a solution of
Et2Zn (1.1 mL, 1 M in hexene) at room temperature.
After the mixture stirred for 1 h, benzaldehyde (53 mg,
0.5 mmol) was added at room temperature, and the
reaction was stirred for 48 h at room temperature.
Optically active 1-phenylpropan-1-ol (58 mg, 85%) was
obtained after acid workup and purification by silica
gel TLC which was subjected to the chiral HPLC for
the analysis of enantioselectivity.
6. (a) Review for using chiral metal (Salen) complexes in
asymmetric catalysis: Canali, L.; Sherrington, D. C. Chem.
Soc. Rev. 1999, 28, 85; (b) Cozzi, P. G.; Papa, A.; Umani-
Ronchi, A. Tetrahedron Lett. 1996, 37, 4613; (c) Rippert,
A. J.; Keller, F. Helv. Chim. Acta 1999, 82, 125; (d)
Kozlowski, M. C.; Dimauro, E. F. Org. Lett. 2001, 3, 3053.
7. Miyano, S.; Nawa, M.; Mori, A.; Hashimoto, H. Bull.
Chem. Soc. Jpn. 1984, 57, 2171.