Catalytic asymmetric borane reduction of prochiral ketones by using (S)-2-(anilinomethyl)pyrrolidine

Article abstract of DOI:10.1246/bcsj.81.274

Catalytic asymmetric borane reduction of prochiral ketones was examined in the presence of (S)-2-(anilinornethyl)pyrrolidine. Chiral secondary alcohols were obtained with moderate to high enantiomeric excesses (up to 96% ee).

Full text of DOI:10.1246/bcsj.81.274

274 Bull. Chem. Soc. Jpn. Vol. 81, No. 2, 274–277 (2008)
Ó 2008 The Chemical Society of Japan
Catalytic Asymmetric Borane Reduction of Prochiral Ketones by
Using (S)-2-(Anilinomethyl)pyrrolidine
Ã
Naoya Hosoda, Yoshihiro Iogawa, Yuichi Shimada, and Masatoshi Asami
Department of Advanced Materials Chemistry, Graduate School of Engineering, Yokohama National University,
79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501
Received August 2, 2007; E-mail: m-asami@ynu.ac.jp
Catalytic asymmetric borane reduction of prochiral ketones was examined in the presence of (S)-2-(anilinomethyl)-
pyrrolidine. Chiral secondary alcohols were obtained with moderate to high enantiomeric excesses (up to 96% ee).
Asymmetric synthesis of enantiomerically enriched secon-
H
H
dary alcohols has been widely studied, because of their signif-
icance as intermediates in the syntheses of natural products
and pharmaceuticals. Enantioselective borane reduction of
prochiral ketones to chiral secondary alcohols using various
chiral amino alcohols has been studied extensively,1 since
Itsuno et al. reported asymmetric reduction of aromatic ke-
tones in the presence of 1.25 molar amount of a chiral amino
alcohol derived from (S)-valine.2 In 1987, Corey et al. reported
a catalytic borane reduction of prochiral ketones with high
enantioselectivity by using oxazaborolidine prepared from
borane and (S)-2-(hydroxydiphenylmethyl)pyrrolidine.3 Other
Ph
N
N
Ph
N
N
B
H
2
H
H
1
Scheme 1. Structures of (S)-2-(anilinomethyl)pyrrolidine (1)
and diazaborolidine 2.
borolidine 2, which acts as a chiral Lewis acid catalyst, is im-
portant to achieve high enantioselectivity in the reduction. In
our previous work,12b we confirmed by 11B NMR spectroscopy
that diazaborolidine was formed from the reaction of (S)-2-
(anilinomethyl)indoline and borane in THF at 0 ꢀC, whereas
diazaborolidine 2 was not observed when (S)-2-(anilinometh-
yl)pyrrolidine (1) was used under the same reaction conditions.
Thus, the reaction conditions for the formation of 2 from 1
and borane were investigated first. Although Quirion et al.
have prepared some diazaborolidines in dimethyl sulfide at
100 ꢀC,8 we examined the preparation of diazaborolidine 2 in
situ in refluxing THF for 45 min after borane (6.0 molar
amount) was added to a THF solution of 1 (1.0 molar amount)
at 0 ꢀC. 11B NMR spectroscopy was performed after removal
of the solvent and excess borane under reduced pressure. The
spectrum of the resulting residue in benzene-d6 showed a
broad peak at +29.5 ppm from BF3-OEt2, which was assigned
to diazaborolidine 2. Thus, it became apparent that 2 could be
formed under the forced reaction conditions using diamine 1.
Next, the effects of reflux time of the reaction of diamine 1
and borane on the enantioselectivity were examined using ace-
tophenone. After refluxing a mixture of 1 (0.1 molar amount)
and borane (1.1 molar amount) in THF for 20 min, aceto-
phenone (1.0 molar amount) was added to the resulting solu-
tion over 2 h at 0 ꢀC, and stirring was continued for 2 h at
room temperature. (R)-1-Phenylethanol was obtained in 95%
yield with 69% ee (Table 1, Entry 1), which indicates that
the formation of 2 as a chiral Lewis acid catalyst is essential
to achieve high enantioselectivity. The selectivity was further
improved to 75% ee by refluxing the mixture of 1 and borane
for 45 min (Entries 2–4), which is higher than that obtained
with a diazaborolidine prepared from borane and (S)-2-(N-
chiral catalysts, such as sulfoximines,4 phosphinamides,5,6
a
mercapto alcohol,7 sulfonamides,8,9 and BINOL derivatives,10
have also been employed for asymmetric borane reduction.
On the other hand, we have been working on asymmetric
syntheses using chiral ꢀ-diamines prepared from (S)-pro-
line11,12 or (S)-indoline-2-carboxylic acid.12,13 In 2000, we re-
ported the asymmetric borane reduction of prochiral ketones
using chiral ꢀ-diamine catalysts.12 Although the reduction of
acetophenone in the presence of a catalytic amount of (S)-2-
(anilinomethyl)indoline gave (R)-1-phenylethanol with good
enantioselectivity (83% ee), selectivity was low (14% ee) when
(S)-2-(anilinomethyl)pyrrolidine (1) was employed. As 1 is
commercially available and easily prepared from a natural
amino acid, (S)-proline, it would be advantageous if high
enantioselectivity could be achieved using 1. We anticipated
that the low selectivity was caused by incomplete formation
of the actual catalyst, diazaborolidine 2, from 1 and borane
in situ (Scheme 1). Consequently, we examined the reaction
conditions for each step, namely, the formation of 2 and the
asymmetric reduction of ketones. As it turns out, we found
that selectivity was dramatically increased by carrying out
the preparation of 2 in refluxing THF, followed by reduction
of the substrates at room temperature, and we have reported
the preliminary results.14,15 In this paper, we report the results
of optimization of the reaction conditions in detail and the
asymmetric reduction of various prochiral ketones.
Results and Discussion
As mentioned in the introduction, the formation of diaza-
Published on the web February 12, 2008; doi:10.1246/bcsj.81.274

N. Hosoda et al.
Bull. Chem. Soc. Jpn. Vol. 81, No. 2 (2008)
275
Table 1. Effect of Reflux Time of Diamine 1 and Borane on
Enantioselectivity
Table 3. Effect of Amounts of Diamine 1 and Borane on
Enantioselectivity
H
BH (1.1 mol. amt.)
3
Entry 1/mol amt. BH3/mol amt. Yield/%a) ee/%b)
Ph
N
N
1
2
3
4
5
6
0.10
0.15
0.20
0.25
0.30
0.10
1.10
1.15
1.20
1.25
1.30
1.25
88
86
97
95
86
91
75
78
81
85
85
59
THF, reflux
H
H
1 (0.1 mol. amt.)
H
Ph, BH
3
N
N
B
H
O
HO
Ph
H
2
a) Isolated yield. b) ee was determined by HPLC analysis.
THF, rt, 2h
Me
Ph
Me
(1.0 mol. amt.)
Table 4. Effect of Solvents on Enantioselectivity
Entry
Reflux time/min
Yield/%a)
ee/%b)
Entry
Solvent
Yield/%a)
ee/%b)
1
2
3
4
5
6
20
30
40
45
50
55
95
86
88
88
80
82
69
71
71
75
75
73
1
2
3
4
5
6c)
THF
Benzene
Toluene
95
87
84
79
78
86
85
73
73
73
53
93
Cyclohexane
Dichloromethane
THF
a) Isolated yield. b) ee was determined by HPLC analysis.
a) Isolated yield. b) ee was determined by HPLC analysis.
c) Acetophenone was added at room temperature.
Table 2. Effect of Reduction Temperature of Acetophenone
on Enantioselectivity
(Entry 4). The use of 0.10 molar amount of 1 and 1.25 molar
amount of borane resulted in a lower ee (Entry 6).
Entry
Temperature/ꢀC
Yield/%a)
ee/%b)
The effect of solvent was also examined. The reduction was
carried out in various solvents in the presence of 0.25 molar
amount of 1 and 1.25 molar amount of borane. The results
are summarized in Table 4. The best result (95% yield, 85%
ee) was obtained when THF was used as the solvent (Entry 1).
The selectivity of the reaction decreased to 73% ee in benzene,
toluene, or cyclohexane (Entries 2–4), and 53% ee in dichloro-
methane (Entry 5). Furthermore, acetophenone was added to
the THF solution at room temperature, instead of 0 ꢀC, over
2 h, because the selectivity of the reduction at room tempera-
ture was greater than that at 0 ꢀC (Table 2, Entries 3 and 4).
As shown in Entry 6, the enantioselectivity improved to
93% ee.
Finally, the reaction was applied to other prochiral ketones
using the optimized reaction conditions in order to examine
the usefulness of 2 as a catalyst. As shown in Table 5, good
to high enantioselectivities were achieved for aromatic ketones
(74–96% ee, Entry 1–9). In particular, the reduction of phen-
acyl chloride showed excellent selectivity (96% ee, Entry 1).
An aliphatic ketone, 3,3-dimethyl-2-butanone, was also reduced
to a chiral secondary alcohol with high ee (95% ee, Entry 10).
Moderate selectivities were obtained in the cases of other
aliphatic ketones (51 and 47% ee, Entries 11 and 12, respec-
tively) and an ꢁ,ꢀ-unsaturated ketone (67% ee, Entry 13).
Scheme 2 shows the stereochemical model that is proposed
for the asymmetric borane reduction with catalyst 2. At room
temperature, the prochiral ketone is reduced via the six-mem-
bered transition state TS-1, in which the larger substituent of
the ketone occupies the equatorial position to yield the major
enantiomer with a high ee. However, the less preferred transi-
tion state TS-2 can form at higher temperatures, which would
cause a decrease in the enantioselectivity. On the other hand, at
lower temperatures, a noncatalytic reaction probably occurs,
1
2
3
4
5
6
40
30
rt
87
85
88
86
86
60
69
72
75
74
69
48
0
À15
À30
a) Isolated yield. b) ee was determined by HPLC analysis.
tosylaminomethyl)piperidine reported by Quirion et al. (72%
ee).8 However, yield and selectivity decreased when the reflux
time was longer than 45 min (Entries 5 and 6). These results
imply that 2 is formed efficiently by refluxing for 45 min.
Next, the effect of the reduction temperature of acetophe-
none was examined. Asymmetric reduction of acetophenone
(1.0 molar amount) was carried out at various temperatures
after a mixture of 1 (0.1 molar amount) and borane (1.1 molar
amount) was refluxed in THF for 45 min and acetophenone
was added to the solution over 2 h at 0 ꢀC. The results are sum-
marized in Table 2. The best selectivity and yield were ob-
tained when the reduction was carried out at room temperature
(Table 2, Entry 3). When the reduction was carried out at
40 or 30 ꢀC, the enantiomeric excess decreased to 69 or 72%
ee, respectively (Entries 1 and 2). Reduction at a temperature
lower than room temperature resulted in the production of
(R)-1-phenylethanol with lower ee (Entries 4–6).
Next, the effect of the amounts of 1 and borane was exam-
ined. Diamine 1 was added in the range of 0.10 to 0.30 molar
amount and borane was added in the range of 1.10 to 1.30 mo-
lar amount based on acetophenone. The results are summariz-
ed in Table 3. The enantioselectivity improved by increasing
the amounts of 1 and borane (Entries 2–5). The best result
(95% yield, 85% ee) was obtained when 0.25 molar amount
of 1 and 1.25 molar amount of borane were employed

276 Bull. Chem. Soc. Jpn. Vol. 81, No. 2 (2008)
Asymmetric Reduction with a Diamine Catalyst
Table 5. Asymmetric Reduction of Prochiral Ketones Using
Diamine 1
BH
3
N
N
H
N
N
H
B
H
H
BH (1.25 mol. amt.)
3
BH
1
3
Ph
2
N
N
O
THF, reflux,
45 min
H
H
L
S
1 (0.25 mol. amt.)
R
R
H
Ph, BH
3
N
N
H
B
N
H
S
R
B
O
N
HO
H
R
2
HO
H
S
H B
2
R
O
H
2
1
2
1
L
THF, rt, 2h
R
R
R
R
L
(1.0 mol. amt.)
Ketone
(major)
R
TS-1 (favored)
Entry
Yield/%a)
ee/%
(Config.)b)
H
N
1
2
Phenacyl chloride
3,4-Dihydro-1(2H)-
naphthalenone
Acetophenone
Propiophenone
2-Methoxyacetophenone
1-Acetonaphthone
1-Indanone
2-Bromoacetophenone
2-(Dimethylamino)-
acetophenone
3,3-Dimethyl-
2-butanone
4-Phenyl-2-butanone
2-Octanone
Benzalacetone
89
96
96(S)
93(R)
L
B
H B
R
H
N
L
2
R
O
HO
S
R
H
3
4
5
6
7
8
9
86
93
81
96
89
90
61
93(R)
88(R)
87(S)
82(R)
81(R)
78(R)
74(S)
S
R
(minor)
TS-2 (disfavored)
Scheme 2. Stereochemical model for asymmetric borane
reduction with diazaborolidine catalyst 2.
to Me4Si and BF3-OEt2, respectively. Specific rotations were
measured on a JASCO P-1010 polarimeter. High-performance
liquid chromatography (HPLC) analyses were carried out with
JASCO instruments (pump, PU-2080 Plus; detector, UV-2075
plus) and TOSOH instruments (pump, CCPS; detector, UV-
8020). Gas chromatography (GC) analysis was carried out on a
SHIMADZU GC-14B. Thin layer chromatography (TLC) analy-
ses were carried out on silica-gel 60 F254-coated plates (E. Merck).
Preparative TLC was performed on silica-gel-coated plates
(Wakogel B-5F, 20 cm  20 cm). A syringe pump (Furue Science
Co., Ltd., Micro Feeder JP-V) was used for the asymmetric borane
reduction of prochiral ketones.
11B NMR Measurement of Diazaborolidine 2. To a THF
(0.5 mL) solution of (S)-2-(anilinomethyl)pyrrolidine (1) (35.6
mg, 0.2 mmol) was added borane–dimethyl sulfide complex
(0.11 mL, 1.2 mmol) at 0 ꢀC and stirring was continued for 45
min under reflux. The solvent and excess borane were removed
under reduced pressure. 11B NMR spectrum of the resulting resi-
due was obtained in C6D6. A broad peak corresponding to diaza-
borolidine 2 was observed at +29.5 ppm from BF3-OEt2.
The Asymmetric Reduction of Acetophenone Catalyzed
by (S)-2-(Anilinomethyl)pyrrolidine (1): Typical Procedure
(Table 4, Entry 6). To a THF (1.25 mL) solution of (S)-2-(ani-
linomethyl)pyrrolidine (1) (88.8 mg, 0.5 mmol) was added bo-
rane–dimethyl sulfide complex (0.24 mL, 2.5 mmol) at 0 ꢀC, and
the mixture was refluxed for 45 min with stirring. After the
mixture was cooled to room temperature, acetophenone (0.67
mol dmÀ3 THF solution, 3.0 mL, 2.0 mmol) was added dropwise
via syringe pump over 2 h, and stirring at room temperature
was continued for 2 h. Then, 1 mol dmÀ3 aqueous HCl was added
at 0 ꢀC, and the reaction mixture was stirred at room temperature
for 30 min. The aqueous layer was separated and extracted
with ether, and the combined organic layer was washed with
1 mol dmÀ3 aqueous HCl, water, and brine successively. The
organic layer was dried over anhydrous sodium sulfate, and the
solvent was removed under reduced pressure. The resulting crude
product was purified by preparative TLC (chloroform) to give
10
91c)
95d)(R)
11
12
13
98
51(R)
47d)(R)
67(R)
93e)
75
a) Isolated yield. b) ee was determined by HPLC analysis,
and the absolute configuration was determined by specific
rotation.5,16–19 c) Determined by GC analysis. d) Determined
by HPLC analysis of p-nitrobenzoate. e) Isolated as p-nitro-
benzoate.
because the regeneration of 2 is slower, which leads to a de-
crease in the rate of catalytic reaction to yield the major enan-
tiomer with a low ee.
Conclusion
The catalytic asymmetric borane reduction of prochiral ke-
tones using (S)-2-(anilinomethyl)pyrrolidine (1) was studied. It
was found that diazaborolidine 2, which acted as an efficient
catalyst of the reduction, was prepared efficiently from 1 and
borane by the examination of the reaction conditions. Using
catalyst 2, chiral secondary alcohols were obtained with mod-
erate to high enantiomeric excesses from prochiral ketones
under the optimized reaction conditions.
Experimental
General. Most manipulations were carried out under an at-
mosphere of argon. Solvents were dried and purified in the usual
manner. (S)-2-(Anilinomethyl)pyrrolidine,17 2-(dimethylamino)-
acetophenone,18 and 2-methoxyacetophenone19 were prepared in
a manner reported previously.
1H and 11B NMR spectra were obtained on a JEOL JNM-EX-
270 spectrometer using CDCl3 and C6D6 as solvents, respectively.
1H and 11B NMR chemical shift values are given in ppm relative

N. Hosoda et al.
Bull. Chem. Soc. Jpn. Vol. 81, No. 2 (2008)
277
(R)-1-phenylethanol (210 mg, 86%). The ee was determined to be
93% by HPLC analysis using a chiral column (Daicel Chiralcel
OD-H (25 Â 0:46 cm i.d.); eluent, 5% 2-propanol in hexane;
flow rate, 0.5 mL minÀ1; tR, 17.5 min for major peak, 21.0 min for
minor peak. Asymmetric reduction of the other ketones was per-
formed in a similar manner. The ee’s were determined by HPLC
using Daicel Chiralcel OD-H (25 Â 0:46 cm i.d.) or Waters Opti-
pak TA (30 Â 0:39 cm i.d.) columns. 2-Chloro-1-phenylethanol:
Soderquist, Tetrahedron Lett. 2003, 44, 4435. j) J. Xu, T. Wei,
Q. Zhang, J. Org. Chem. 2004, 69, 6860.
2 S. Itsuno, K. Ito, A. Hirao, S. Nakahama, J. Chem. Soc.,
Chem. Commun. 1983, 469.
3 E. J. Corey, R. K. Bakshi, S. Shibata, J. Am. Chem. Soc.
1987, 109, 5551.
4
5
C. Bolm, M. Felder, Tetrahedron Lett. 1993, 34, 6041.
M. P. Gamble, A. R. C. Smith, M. Wills, J. Org. Chem.
OD-H; eluent, 3% 2-propanol in hexane; flow rate, 0.5 mL minÀ1
;
1998, 63, 6068.
tR, 33.7 min for major peak, 38.8 min for minor peak. 1,2,3,4-
Tetrahydro-1-naphthol: OD-H; eluent, 2% 2-propanol in hexane;
flow rate, 0.5 mL minÀ1; tR, 28.4 min for minor peak, 30.7 min
for major peak. 1-Phenyl-1-propanol: OD-H; eluent, 5% 2-pro-
panol in hexane; flow rate, 0.5 mL minÀ1; tR, 15.5 min for major
peak, 17.0 min for minor peak. 2-Methoxy-1-phenylethanol:
6
Tang, Tetrahedron: Asymmetry 2003, 14, 95.
K. Li, Z. Zhou, L. Wang, Q. Chen, G. Zhao, Q. Zhou, C.
7 T.-K. Yang, D.-S. Lee, Tetrahedron: Asymmetry 1999,
10, 405.
8
O. Froelich, M. Bonin, J.-C. Quirion, H.-P. Husson, Tetra-
hedron: Asymmetry 1993, 4, 2335.
OD-H; eluent, 5% 2-propanol in hexane; flow rate, 0.5 mL minÀ1
;
9 S.-D. Yang, Y. Shi, Z.-H. Sun, Y.-B. Zhao, Y.-M. Liang,
Tetrahedron: Asymmetry 2006, 17, 1895.
tR, 26.8 min for major peak, 30.6 min for minor peak. 1-(1-Naph-
thyl)ethanol: OD-H; eluent, 5% 2-propanol in hexane; flow rate,
0.5 mL minÀ1; tR, 31.5 min for minor peak, 57.0 min for major
peak. 1-Indanol: OD-H; eluent, 5% 2-propanol in hexane; flow
rate, 0.5 mL minÀ1; tR, 18.9 min for minor peak, 21.0 min for ma-
jor peak. 1-(o-Bromophenyl)ethanol: OD-H; eluent, 2% 2-propa-
nol in hexane; flow rate, 0.5 mL minÀ1; tR, 24.7 min for major
peak, 27.7 min for minor peak. 2-Dimethylamino-1-phenyletha-
nol: OD-H; eluent, 20% 2-propanol in hexane; flow rate, 0.5
mL minÀ1; tR, 11.3 min for minor peak, 13.9 min for major peak.
3,3-Dimethyl-2-butanol (as p-nitrobenzoate): OD-H; eluent, 0.1%
2-propanol in hexane; flow rate, 0.5 mL minÀ1; tR, 28.2 min for
minor peak, 31.1 min for major peak. 4-Phenyl-2-butanol: OD-
H; eluent, 5% 2-propanol in hexane; flow rate, 0.5 mL minÀ1; tR,
21.2 min for major peak, 30.4 min for minor peak. 2-Octanol (as
p-nitrobenzoate): TA; eluent, 0.1% 2-propanol in hexane; flow
rate, 0.5 mL minÀ1; tR, 21.5 min for minor peak, 29.9 min for
major peak. 4-Phenyl-3-buten-2-ol: OD-H; eluent, 10% 2-propa-
nol in hexane; flow rate, 0.5 mL minÀ1; tR, 16.5 min for major
peak, 25.2 min for minor peak.
10 Y.-M. Lin, I.-P. Fu, B.-J. Uang, Tetrahedron: Asymmetry
2001, 12, 3217.
11 a) M. Asami, J. Synth. Org. Chem., Jpn. 1996, 54, 188, and
references cited therein. b) M. Asami, S. Inoue, Bull. Chem. Soc.
Jpn. 1997, 70, 1687. c) M. Asami, S. Sato, K. Honda, S. Inoue,
Heterocycles 2000, 52, 1029. d) A. Seki, M. Asami, Tetrahedron
2002, 58, 4655.
12 a) M. Asami, S. Sato, H. Watanabe, Chem. Lett. 2000, 990.
b) S. Sato, H. Watanabe, M. Asami, Tetrahedron: Asymmetry
2000, 11, 4329.
13 a) M. Asami, H. Watanabe, K. Honda, S. Inoue, Tetra-
hedron: Asymmetry 1998, 9, 4165. b) M. Asami, M. Wada, S.
Furuya, Chem. Lett. 2001, 1110.
14 a) M. Asami, N. Hosoda, Y. Iogawa, S. Sato, 43rd Kanto
Branch Symposium of the Society of Synthetic Organic Chemis-
try, Japan, Yamanashi, May 17 and 18, 2002, Abstr., No. B-08.
b) M. Asami, Y. Shimada, Y. Iogawa, N. Hosoda, 85th Annual
Meeting of the Chemical Society of Japan, Yokohama, March
26–29, 2005, Abstr., No. 3C2-33.
15 During our investigation, an Indian group also reported
that borane reduction of prochiral ketones with (S)-2-(anilino-
methyl)pyrrolidine in refluxing toluene leads to improvement of
stereoselectivity: D. Basavaiah, K. V. Rao, B. S. Reddy, Tetra-
hedron: Asymmetry 2006, 17, 1041.
The authors are grateful to Dr. Hiroko Suezawa and
Mr. Shinji Ishihara, Instrumental Analysis Center, Yokohama
National University, for 11B NMR measurement.
16 a) S. Itsuno, K. Ito, A. Hirao, S. Nakahama J. Org. Chem.
1984, 49, 555. b) K. Naemura, M. Murata, R. Tanaka, M. Yano,
K. Hirose, Y. Tobe, Tetrahedron: Asymmetry 1996, 7, 3285.
c) W.-S. Huang, Q.-S. Hu, L. Pu, J. Org. Chem. 1999, 64, 7940.
d) R. Kuwano, M. Sawamura, J. Shirai, M. Takahashi, Y. Ito, Bull.
Chem. Soc. Jpn. 2000, 73, 485.
17 M. Asami, H. Ohno, S. Kobayashi, T. Mukaiyama, Bull.
Chem. Soc. Jpn. 1978, 51, 1869.
18 S. Miyano, L. D.-L. Lu, S. M. Viti, K. B. Sharpless, J. Org.
Chem. 1985, 50, 4350.
References
1
a) L. Deloux, M. Srebnik, Chem. Rev. 1993, 93, 763.
b) E. J. Corey, C. J. Helal, Angew. Chem., Int. Ed. 1998, 37,
1986. c) S. Itsuno, Org. React. 1998, 52, 395. d) P. Pinho, D.
Guijarro, P. G. Andersson, Tetrahedron 1998, 54, 7897. e) M.
Shimizu, K. Tsukamoto, T. Matsutani, T. Fujisawa, Tetrahedron
1998, 54, 10265. f) T. Yanagi, K. Kikuchi, H. Takeuchi, T.
Ishikawa, T. Nishimura, T. Kamijo, Chem. Lett. 1999, 1203.
g) C. Schunicht, A. Biffis, G. Wulff, Tetrahedron 2000, 56,
1693. h) K. Molvinger, M. Lopez, J. Court, Tetrahedron:
Asymmetry 2000, 11, 2263. i) R. E. Huertas, J. A. Corella, J. A.
19 P. Ferraboschi, P. Grisenti, A. Manzocchi, E. Santaniello,
Tetrahedron 1994, 50, 10539.