H8-MonoPhos and its application in catalytic enantioselective hydrogenation of α-dehydroamino acids

Article abstract of DOI:10.1016/S0040-4020(02)01062-1

H₈-MonoPhos, a new stable and readily soluble monodentate phosphoramidite ligand, has been facilely prepared from H₈-BINOL. The ligand achieved up to 99.9% ee and 96.7% ee in hydrogenation of dehydroalanine and dehydrohomophenylalani

Full text of DOI:10.1016/S0040-4020(02)01062-1

Tetrahedron 58 (2002) 8799–8803
H8-MonoPhos and its application in catalytic enantioselective
hydrogenation of a-dehydroamino acids
b
b
Qingle Zeng,a Hui Liu,a Aiqiao Mi,a Yaozhong Jiang,a Xingshu Li, Michael C. K. Choi
,
*
and Albert S. C. Chanb
aUnion Laboratory of Asymmetric Synthesis, Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041,
People’s Republic of China
bOpen Laboratory of Chirotechnology and Department of Applied Biology and Chemical Technology,
The Hong Kong Polytechnic University, Hong Kong, People’s Republic of China
Received 17 May 2002; revised 26 July 2002; accepted 22 August 2002
Abstract—H8-MonoPhos, a new stable and readily soluble monodentate phosphoramidite ligand, has been facilely prepared from
H8-BINOL. The ligand achieved up to 99.9% ee and 96.7% ee in hydrogenation of dehydroalanine and dehydrohomophenylalanine in a S/C
ratio of 500:1, respectively, which are among the best results to date. For dehydrophenylalanine derivatives, it gave good to excellent
enantioselectivity. Some factors controlling the enantioselectivity and conversion were examined and are discussed. The interesting effects of
ligand/rhodium ratio on the enantioselectivity and conversion were observed, which a mechanism was proposed to explain. q 2002 Elsevier
Science Ltd. All rights reserved.
1. Introduction
if the BINOL backbone was partially hydrogenated into
H8-BINOL, the resulting compound would have better
solubility.
In the catalytic enantioselective hydrogenation of function-
alized prochiral olefins, bidentate phosphorus-containing
ligands are superior to monodentate analogues in asym-
metric induction, for example, DIPAMP and PAMP,1 and
all of the best ligands are bidentate in the past nearly 30
years;2 but recently Pringle3 and Feringa4 found that
monodentate ligands could be superior to bidentate
analogues, and Feringa’s MonoPhos 1 (Scheme 1) was
even comparable to the best bidentate ligands in hydrogen-
ation in some solvents. In our preliminary research, we
found the crystalline MonoPhos was not readily soluble in
some solvents, which decreased the rate of hydrogenation as
noted in Feringa’s paper, and will limit its application to
some substrates, which only dissolve in the solvents in
which selectivity is poor for the MonoPhos. We thought that
Furthermore, the H8-BINOL skeleton is more electron-rich
and larger dihedral angle than BINOL one as Takaya’s
report.5 Takaya’s H8-BINAP shows higher asymmetric
induction in the hydrogenation of olefins than BINAP.5 In
our Laboratory, H8-BINAM based aminophosphite ligands
were better ligands than their analogues in hydrogenation;6
H8-BINOL was used as a ligand in the asymmetric
alkylation of aldehydes7 and the hetero-Diels–Alder
reaction8 and gave better enantioselectivity than BINOL.
Prompted by these results and the facile preparation of
H8-BINOL from BINOL,9 we 0 synthesized N,N-dimethyl
5,50,6,60,7,70,8,80-octahydro-1,1 -bi-2-naphthyl phosphor-
amidite 2 (labeled as H8-MonoPhos, Scheme 1) and
investigated its application in the enantioselective hydro-
genation of a-dehydroamino acids in detail.10
2. Results and discussion
2.1. Synthesis of chiral H8-MonoPhos
Scheme 1. (S)-MonoPhos 1 and (R)-H8-MonoPhos 2.
(R)-H8-MonoPhos 2 was readily obtained by refluxing the
benzene solution of (R)-H8-BINOL and hexamethyl-
phosphorous triamide. Unlike MonoPhos, H8-MonoPhos is
a white solid and is soluble in various common organic
solvents, which provide a wide scope of solvents for
Keywords:
catalytic
dehydrohomophenylalanine; monodentate phosphoramidite ligand.
enantioselective
hydrogenation;
*
Corresponding author. Tel.: þ86-28-5229250; fax: þ86-28-5223978;
e-mail: ulas@cioc.ac.cn
0040–4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(02)01062-1

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Q. Zeng et al. / Tetrahedron 58 (2002) 8799–8803
Scheme 3.
Scheme 2.
optimizing the reaction conditions for various substrates.
H8-MonoPhos 2 is stable to purification by flash column
chromatography and storage for 1 year under a nitrogen
atmosphere. The stability is especially beneficial to its
potential use in large-scale processes.
Phos (entries 14 and 15), which implied that H8-MonoPhos
was more efficient (Table 1).
2.3. Enantioselective hydrogenation of ethyl (Z)-2-
acetoamido-4-phenylcrotonate by Rh–H8-MonoPhos
catalyst
2.2. Enantioselective hydrogenation of methyl (Z)-
acetoamidocinnaminate by Rh–H8-MonoPhos catalyst
As a key intermediate of most commercially important
angiotensin converting enzyme (ACE) inhibitors, asym-
metric synthesis of L-homophenylalanine has received
increasing attention,12 but only a few excellent results
were achieved by asymmetric catalytic method.13 So we
tested this catalyst system in the hydrogenation of ethyl
(Z)-2-acetamido-4-phenylcrotonate to prepare homophenyl-
alanine (Scheme 3). Excellent enantioselectivities and
conversions were achieved in various solvents except
MeOH (Table 2, entries 1–5). Under ambient hydrogen
pressure the reaction was completed within 2 h (entry 6),
which was the most efficient for the substrate among all of
the reported ligands. Even in the presence of only 0.2 mol%
Rh-complex of H8-MonoPhos, the substrate was hydrogen-
ated into homophenylalanine with up to 96.7% ee (entry
11). Compared with the known chiral ligands for the
catalytic synthesis of homophenylalanine such as SpirOP
(96.2% ee, 1 mol% catalyst)13b and DPAMPP (95.7% ee,
1 mol% catalyst),13d H8-MonoPhos 2 showed the most
efficient asymmetric catalytic property for the synthesis of
optical pure homophenylalanine. The facile preparation,
stability and high efficiency demonstrate the promising
prospect of H8-MonoPhos 2 in large scale production of
homophenylalanine.
With (R)-H8-MonoPhos in hand, we first investigated
various conditions in the enantioselective hydrogenation
of methyl (Z)-acetoamidocinnaminate (Scheme 2). The
catalyst was prepared in situ by mixing Rh(COD)2BF4 and
(R)-H8-MonoPhos in acetone solution. All of the ee values
in the tested solvents were higher than or comparable to
those of MonoPhos (entries 1–8). Importantly, the substrate
could not be completely hydrogenated in toluene with
MonoPhos,4a but it could with H8-MonoPhos, which shows
the advantage of the soluble H8-MonoPhos (entry 1). The
effects of solvents on the chiral induction were small except
for MeOH (entry 7). The conversion was low and the ee
value decreased a little, probably because the nucleo-
philicity and low steric hindrance of MeOH resulted in the
decomposition of H8-MonoPhos by MeOH in the reaction
conditions, which has been observed for monodentate
phosphites.11 Hydrogen pressure accelerated the reaction
but played a minor role in enantioselectivity (entries 8–11).
Like MonoPhos, the ee value increased a little when the
temperature decreased to 08C (entry 12). Interestingly, when
the substrate to catalyst ratio increased from 100 to 500, the
reaction gave a consistent ee value (entries 8 and 13). As a
comparison, the substrate could not be completely hydro-
genated in 0.5 h in the optimized solvents with (R)-Mono-
2.4. Enantioselective hydrogenation of other
dehydroamino acids catalyzed by Rh–H8-MonoPhos
complex
Table 1. Rhodium-catalyzed enantioselective hydrogenation of methyl (Z)-
acetoamidocinnaminate 3a
The hydrogenation of other dehydroamino acids were also
investigated (Scheme 4, Table 3). Interestingly, the simple
dehydroalanine, for which it is hard to achieve a high ee
value with a number of bidentate phosphine ligands, was
Entry
P (bar)
t (h)
Solvent
%eea
%conv.a
1
2
3
4
5
6
7
8
20
20
20
20
20
20
20
20
1
10
40
20
20
20
20
2
0.5
0.5
0.5
0.5
1
0.5
0.5
6
Toluene
94.9
95.0
94.4
95.7
94.2
95.4
90.8
95.5
94.8
95.6
95.6
97.0
96.4
92.3
93.0
99.1
99.7
99.3
96.9
90.2
99.2
47.4
98.0
99.7
96.8
99.6
99.3
99.9
25.6
56.8
ClCH2CH2Cl
CH2Cl2
AcOEt
THF
Acetone
MeOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
Acetone
CH2Cl2
AcOEt
Table 2. Enantioselective hydrogenation of ethyl (Z)-2-acetomido-4-
phenylcrotonate 3b by H8-MonoPhos–Rh complex
Entry
t (h)
Solvent
%eea
%conv.a
9
10
11
12b
13c
14d
15d
1
1
2
3
4
0.5
0.5
0.5
0.5
0.5
2
i-PrOH
Acetone
CH2Cl2
THF
MeOH
i-PrOH
Acetone
95.9
95.0
92.9
96.6
96.2
95.9
96.7
99.0
99.9
99.9
99.9
42.0
99.6
99.9
0.5
12.5
8
0.5
0.5
5
6b
7c
8
Reactions were performed with 0.1 M solutions of substrates at S/C¼100:1
at rt and 20 bar initial hydrogen pressure unless otherwise noted.
Ee and conversion were determined by GC on a CP-Chirasil-L-Val
column. The configurations of all the predominant products were in S
Reactions were performed with 0.1 M solutions of substrates at S/C¼100:1
a
at rt and 20 bar initial hydrogen pressure unless otherwise noted.
Ee and conversion were determined by GC on a CP-Chirasil-L-Val
column. The configurations of all the predominant products were in S
a
form.
08C.
b
form.
The hydrogen pressure is ambient pressure.
The substrate to catalyst mole ratio is 500.
c
b
The substrate to catalyst mole ratio is 500.
Ligand is (R)-MonoPhos.
d
c

Q. Zeng et al. / Tetrahedron 58 (2002) 8799–8803
8801
effectively hydrogenated with Rh–H8-MonoPhos catalyst
system (entries 16–18).
2.5. Effect of the ligand/Rh ratio on enantioselective
hydrogenation
Scheme 4.
Different from the finding by Reetz14 and Feringa,4a we
Table 3. Enantioselective hydrogenation of other a-dehydroamino acids 3 by H8-MonoPhos–Rh complex
Entry
Substrates
R1
R3
R4
t (h)
Solvents
%eea
%conv.a
1
3c
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
3r
3s
H
H
Ph
3-Cl–Ph
4-Cl–Ph
4-Me–Ph
Ph
Me
Me
Me
Me
Me
Me
Ph
Me
Me
H
H
H
0.5
8
Acetone
Acetone
i-PrOH
Acetone
Acetone
MeOH
Acetone
Acetone
Acetone
Acetone
CH2Cl2
Acetone
Acetone
CH2Cl2
Acetone
Acetone
Acetone
Acetone
99.9
99.9
93.3
74.0
82.6
82.1
84.3
49.3
82.9
92.5
98.0
91.0
94.4
96.0
95.9
80.8
54.2
23.3
99.9
99.9
92.8
99.9
99.9
77.8
98.2
54.4
96.6
98.6
99.9
77.1
95.9
99.9
99.9
62.7
12.6
60.0
2b
3
0.5
0.5
0.5
0.3
2
4
5
6
H
7
Me
Me
Me
Me
Me
Me
Me
Me
H
8c
9
H
4-F–Ph
Ph
2
Me
Me
Me
Me
Me
Me
Me
Me
Me
Ph
0.5
0.5
5
0.5
0.5
5
10
11d
12
13
14d
15
16
17
18e
2-Cl–Ph
4-Cl–Ph
4-Br–Ph
4-MeO–Ph
4-NO2–Ph
4-AcO–3-MeO–Ph
Furyl
2
Me
Me
Me
0.5
0.5
10
(E)–PhCHvCH
Me
Reactions were performed with 0.1 M solutions of substrates at S/C¼100:1 at rt and 20 bar initial hydrogen pressure unless otherwise noted; R2¼H unless other
mentioned.
Ee and conversion were determined by GC on a CP-Chirasil-L-Val column. The configurations of all the predominant products were in S form.
The substrate to catalyst mole ratio is 500.
a
b
c
R2¼Ph
d
7 bar of H2.
60 bar H2 pressure; R2¼Me.
e
hydrogenated in extremely high ee values with H8-Mono-
Phos 2, even with 0.2 mol% catalyst loading (entries 1 and
2). The acid substrates gave lower enantioselectivities
(entries 3–6). Replacement of the acetamido group by
benzamido decreased the enantioselectivity obviously
(entry 7). If the substrate was an E isomer, both the ee
value and conversion were very low (entries 7 and 8). For
most of the substituted dehydrophenylalanine methyl ester
derivatives good to excellent ee values were achieved, but
there is no clear tendency for the effect on the enantio-
selectivity and conversion of the substituted group (entries
9–14). The L-Dopa precursor could be obtained in excellent
optical purity by hydrogenation in the presence of H8-
MonoPhos–Rh complex (entry 15). Hetero-aromatic or
alkenyl or b,b-di-substituted dehydroalanine could not be
found that, with 3 equiv. or more of ligand 2, the
hydrogenation of dehydro-N-acetylphenylalanine methyl
ester became much slower; while with 1 equiv. of the
ligand, the conversion dropped obviously. A 2:1 mole ratio
of ligand/Rh is preferential (Scheme 2, Table 4). We
suppose, the RhL2S2 (S¼solvent or COD) is the precursor to
coordination with the dehydroamino acid substrate. RhL4
and RhL3S should be dissociated into RhL2, and two RhLS3
will turn into a RhL2S2 and RhS4, but RhS4 has much lower
activity than RhL2S2 and can easily be reduced into rhodium
black in catalytic hydrogenation (Scheme 5). So the ratio of
ligand/Rh does not effect enantioselectivity but conversion
(entries 1–4). The mechanism can well explain why the ee
values are consistent but the conversions differ from each
other.
Table 4. The effects of the ligand/Rh ratios on enantioselective
hydrogenation of methyl (Z)-acetoamidocinnaminate 3a
Entry
L/Rh
%eea
%conv.a
b
1
2
3
4
4.4
3.3
2.2
1.1
–
1.2
78.9
98.0
53.8
95.9
95.5
94.8
Reactions were performed with 0.1 M solutions of substrates at S/C¼100:1
at rt and 20 bar initial hydrogen pressure unless otherwise noted.
Ee and conversion were determined by GC on a CP-Chirasil-L-Val
column. The configurations of all the predominant products were in S
a
form.
Only one enantiomer peak detected on GC but too low to be measured
accurately.
b
Scheme 5.

8802
Q. Zeng et al. / Tetrahedron 58 (2002) 8799–8803
3. Conclusion
6.99 (d, H, J¼3.9 Hz), 7.01 (d, H, J¼3.9 Hz), 7.07 (d, H,
J¼6.3 Hz); 13C NMR (75.5 MHz, CDCl3) d 22.5, 22.7,
22.8, 27.6, 27.7, 22.7, 29.0, 29.1, 35.6, 35.9, 118.5, 118.6,
128.2, 128.3, 129.2, 132.9, 137.4, 137.8, 137.9, 148.2,
148.3, 148.6; 31P NMR (121.5 MHz, CDCl3) d 143.6; MS
(EI, 70 eV, m/z): 367 (Mþ, 100%), 324 (76%); IR (KBr):
3010, 1585, 1469, 1248, 1233, 1222, 982, 937 cm21. Anal.
calcd for C22H26NO2P: C, 71.92; H, 7.13; N, 3.81; P, 8.43;
found: C, 71.32; H, 7.10; N, 3.89; P, 8.34.
In summary, H8-MonoPhos, a new stable and easily
prepared monodentate phosphoramidite ligand, holds
much better solubility and gives higher ee values and
reaction rate than MonoPhos in most of solvents in the
hydrogenation of dehydrophenylalanine. With the H8-
MonoPhos–rhodium complex, the highest enantio-
selectivity (96.7% ee at a S/C ratio of 500:1) and reaction
rate were achieved in the hydrogenation of ethyl (Z)-2-
acetoamido-4-phenylcrotonate; 99.9% ee was achieved for
dehydroalanine, which is comparable to the result obtained
by the best bidentate ligands. But the catalyst system was
not suitable for some other substrates. The interesting
effects of ligand/Rh mole ratio in hydrogenation with H8-
MonoPhos were observed. A reasonable mechanism was
proposed to explain this.
4.3. A general procedure for enantioselective
hydrogenation catalyzed by Rh–H8-MonoPhos complex
In a dry box, 40 ml 0.025 M Rh(COD)2BF4 acetone
solution, 80 ml 0.0275 M (R)-H8-MonoPhos acetone
solution, 0.1 mmol substrate and 1 ml solvent were added
into a glass tube equipped with a stirrer in a 50 ml autoclave
under Ar atmosphere. The autoclave was pressurized with
H2 and the hydrogenation was carried out under the chosen
conditions. After the hydrogen was released, the mixture
was filtered through a short silica gel column to remove the
catalyst. The methyl or ethyl ester was directly analyzed via
chiral capillary GC with a 25 m Chrowpak capillary column
(CP-Chirasil-L-Val). The acid was converted to the
corresponding methyl ester with methyl iodide/KHCO3/
DMF before GC analysis.
4. Experimental
4.1. General aspects
All melting points were determined on a digital melting
1
point apparatus and were uncorrected. H NMR and 31P
NMR spectra were recorded on Brucker AC-E 300 and
Brucker AC-400 spectrometers. Infrared spectra were
recorded on a Nicolet MX-1 spectrometer. Optical rotations
were measured on a Perkin–Elmer 241 polarimeter. All
reactions involving air- and moisture-sensitive compounds
were carried out under a dry argon atmosphere using
standard Schlenk line techniques. THF, benzene and
pyridine were distilled from sodium benzophenone ketyl;
solvents used in hydrogenation were degassed by three
freeze–thaw cycles prior to use.
Acknowledgements
We are grateful for the financial support from the National
Science Foundation of China (29872036) and The Hong
Kong Polytechnic University ASD Fund.
4.2. Materials
References
(R)-H8-BINOL was prepared according to the literature
procedure.8b Rh(COD)2BF4 was purchased from Aldrich
Chemical Co. Hexamethylphosphorous triamide and
2-acetoamidoacrylic acid were purchased from Acros
Chemical Co. All 2-acylamidoacrylic acids were syn-
thesized in accordance with the process developed by
Blatt.15 Their corresponding methyl esters were prepared by
the reaction of free acids with MeI in the presence of
KHCO3 in DMF. The preparation of ethyl (Z)-2-acetamido-
4-phenylcrotonate was achieved using the literature pro-
cedure with slight modification.13a
1. Vineyard, B. D.; Knowles, W. S.; Sabacky, M. J.; Bachman,
G. L.; Weinkauff, D. J. J. Am. Chem. Soc. 1977, 99,
5946–5952.
2. (a) In Catalytic Asymmetric Synthesis. 2nd ed. Ojima, I., Ed.;
VCH: Wheinheim, 2000. (b) Noyori, R. Asymmetric Catalysis
in Organic Synthesis. Wiley: New York, 1994. (c) Jacobsen,
E. N.; Pfaltz, A.; Yamamoto, H. Comprehensive Asymmetric
Catalysis. Springer: Berlin, 1999.
3. Claver, C.; Fernandez, E.; Gillon, A.; Heslop, K.; Hyett, D. J.;
Martorell, A.; Orpen, A. G.; Pringle, P. G. Chem. Commun.
2000, 961–962.
4. Berg van den, M.; Minnaard, A. J.; Schudde, E. P.; Van Esch,
J.; De Vries, A. H. M.; De Vries, J. G.; Feringa, B. L. J. Am.
Chem. Soc. 2000, 122, 11539–11540.
4.2.1. N,N-Dimethyl (R)-5,50,6,60,7,70,8,80-octahydro-1,10-
bi-2-naphthyl phosphoramidite (H8-MonoPhos). Under
N2 atmosphere, (R)-(þ)-H8-BINOL (147 mg, 0.5 mmol),
hexamethylphosphorous triamide (109 mg, 0.74 mmol),
4 mg NH4Cl and 4 ml dry benzene were added into a flask
equipped with a stirrer and heated to reflux for 3 h. The
mixture was directly purified by flash column chromato-
graphy on silica gel eluted with petroleum ether/ethyl
acetate (5:1) to gave a white solid compound. Yield 121 mg
5. (a) Zhang, X.; Mashima, K.; Koyano, K.; Sayo, N.;
Kumobayashi, H.; Akutagawa, S.; Takaya, H. Tetrahedron
Lett. 1991, 32, 7283–7286. (b) Zhang, X.; Mashima, K.;
Koyano, K.; Sayo, N.; Kumobayashi, H.; Akutagawa, S.;
Takaya, H. J. Chem. Soc., Perkin Trans. 1 1994, 2309–2322.
(c) Zhang, X.; Uemura, T.; Matsumura, K.; Kumobayashi, H.;
Sayo, N.; Takaya, H. Synlett 1994, 7, 501–503. (d) Uemura,
T.; Zhang, X.; Matsumura, K.; Sayo, N.; Kumobayashi, H.;
Ohta, T.; Nozaki, K.; Takaya, H. J. Org. Chem. 1996, 61,
5510–5516.
1
(66.2%); mp: 56–578C; [a]3D2¼2310 (c 0.610, THF); H
NMR (400 MHz, CDCl3) d 1.52–1.62 (m, 2H), 1.74–1.81
(m, 6H), 2.18–2.36 (m, 2H), 2.49 (d, 6H, J¼8.8 Hz), 2.56–
2.71 (m, 2H), 2.74–2.87 (m, 4H), 6.84 (d, H, J¼6.0 Hz),
6. (a) Zhang, F. Y.; Pai, C. C.; Chan, A. S. C. J. Am. Chem. Soc.

Q. Zeng et al. / Tetrahedron 58 (2002) 8799–8803
8803
1998, 120, 5808–5809. (b) Zhang, F. Y.; Kwok, W. H.; Chan,
A. S. C. Tetrahedron: Asymmetry 2001, 12, 2337–2342.
7. (a) Chan, A. S. C.; Zhang, F. Y.; Yip, C. W. J. Am. Chem. Soc.
1997, 119, 4080–4081. (b) Zhang, F. Y.; Yip, C. W.; Cao, R.;
Chan, A. S. C. Tetrahedron: Asymmetry 1997, 8, 585–590.
(c) Zhang, F. Y.; Chan, A. S. C. Tetrahedron: Asymmetry
1997, 8, 3651–3656.
1998, 63, 7875–7884. (b) Xu, Q.; Wang, G.; Wang, X.; Wu,
T.; Pan, X.; Chan, A. S. C.; Yang, T. Tetrahedron: Asymmetry
2000, 11, 2309–2314. (c) Lin, W. Q.; He, Z.; Jing, Y.; Cui, X.;
Liu, H.; Mi, A. Q. Tetrahedron: Asymmetry 2001, 11,
1583–1588. (d) Lin, W.; He, Z.; Zhang, H.; Zhang, X.; Mi,
A.; Jiang, Y. Synthesis 2001, 7, 1007–1009. (e) Long, A.;
Baldwin, S. W. Tetrahedron Lett. 2001, 42, 5343.
(f) Kolarovic, Al; Berkes, D.; Baran, P.; Povazanec, F.
Tetrahedron Lett. 2001, 42, 2579–2582.
8. (a) Wang, B.; Feng, X.; Cui, X.; Liu, H.; Jiang, Y. Chem.
Commun. 2000, 1605–1606. (b) Wang, B.; Feng, X.; Huang,
Y.; Liu, H.; Cui, X.; Jiang, Y. J. Org. Chem. 2002, 67,
2175–2182.
13. (a) Melillo, D. G.; Larsen, R. D.; Mathre, D. J.; Shukis, W. F.;
Wood, A. W.; Colleluori, J. R. J. Org. Chem. 1987, 52,
5143–5150. (b) Li, X.; Yeung, C.-H.; Chan, A. S. C.; Lee,
D.-S.; Yang, T.-K. Tetrahedron: Asymmetry 1999, 10,
3863–3867. (c) Lou, R.; Mi, A.; Jiang, Y.; Qin, Y.; Li, Z.;
Fu, F.; Chan, A. S. C. Tetrahedron 2000, 56, 5857–5864.
(d) Xie, Y.; Lou, R.; Li, Z.; Mi, A. Q.; Jiang, Y. Z.
Tetrahedron: Asymmetry 2000, 11, 1487–1494.
9. (a) Cram, D. J.; Helgeson, R. C.; Peacock, S. C.; Kaplan, L. J.;
Domeier, L. A.; Moreau, P.; Koga, K.; Mayer, J. M.; Chao, Y.;
Siegel, M. G.; Hoffman, D. H.; Sogah, G. D. Y. J. Org. Chem.
1978, 43, 1930–1946. (b) Guo, H.; Ding, K. L. Tetrahedron
Lett. 2000, 41, 10061–10064.
10. Zeng, Q.; Liu, H.; Cui, X.; Mi, A.; Jiang, Y.; Li, X.; Choi,
M. C. K.; Chan, A. S. C. Tetrahedron: Asymmetry 2002, 13,
115–117.
14. Reetz, M. T.; Mehler, G. Angew Chem., Int. Ed. Engl. 2000,
39, 3889–3890.
11. Chen, W.; Xiao, J. Tetrahedron Lett. 2001, 42, 2897–2900.
12. (a) Jackson, R. F. W.; Moore, R. J.; Dexter, C. S. J. Org. Chem.
15. Blatt, A. H. Organic Syntheses, Wiley: New York, 1964;
Collect. Vol. II. p 1–4.