Chiral diphosphites derived from D-glucose in the copper-catalyzed conjugate addition of diethylzinc to cyclohexenone

Article abstract of DOI:10.1016/S0957-4166(01)00514-6

A series of diphosphite ligands 1-3 derived from readily available D-(+)-glucose and bisphenol or binaphthol derivatives have been applied as ligands in the Cu-catalyzed 1,4-addition of diethylzinc to cyclohexenone. Excellent reaction rates (TOF>1200 h

Full text of DOI:10.1016/S0957-4166(01)00514-6

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 12 (2001) 2895–2900
Chiral diphosphites derived from
D-glucose in the
copper-catalyzed conjugate addition of diethylzinc to
cyclohexenone
Montserrat Di e´ guez,* Aurora Ruiz and Carmen Claver
Departament de Qu ´ı mica F ´ı sica i Inorg a` nica, Universitat Rovira i Virgili, Pl. Imperial T a` rraco 1, 43005 Tarragona, Spain
Received 25 October 2001; accepted 12 November 2001
Abstract—A series of diphosphite ligands 1–3 derived from readily available
D-(+)-glucose and bisphenol or binaphthol derivatives
have been applied as ligands in the Cu-catalyzed 1,4-addition of diethylzinc to cyclohexenone. Excellent reaction rates (TOF>1200
−
1
h ) and good enantioselectivities (e.e. of up to 84%) were achieved. The modular nature of these ligands allows easy systematic
variation in the configuration of the stereocenters (C(3), C(5)) at the ligand backbone and in the biaryl substituents, so the
optimum configuration for maximum enantioselectivity in the asymmetric 1,4-addition can be determined. The results obtained
show that the enantioselectivity induced by the ligand depends strongly on the absolute configuration of the C(3) stereogenic
centre, while the sense of the enantiodiscrimination is predominantly controlled by the configuration of the biaryl groups of the
phosphite moieties. © 2001 Published by Elsevier Science Ltd.
1
. Introduction
alized compounds with several stereogenic centers. This
allows a systematic regio- and stereoselective introduc-
tion of different functionalities in the synthesis of a
series of chiral ligands that can be screened for high
activities and enantioselectivities. At the same time,
they can provide useful information about the origin of
The enantioselective conjugate addition of organo-
metallic reagents to a,b-unsaturated substrates is still of
considerable synthetic interest. In particular, adding
organocuprates to enones is an attractive way of form-
ing a CꢀC bond and simultaneously introducing a new
stereogenic center. Diastereoselective additions to chi-
1
8
the stereoselectivity of the reaction. Moreover, carbo-
2
hydrate-based ligands have demonstrated their poten-
6
d,8,9
ral Michael acceptors and the use of stoichiometric
chiral organometallic reagents are firmly established,
but few highly enantioselective catalytic systems have
been reported. Excellent enantioselectivities have been
obtained in the copper-catalyzed Michael addition of
Grignard and diorganozinc reagents to enones and
other a,b-unsaturated carbonyl compounds using phos-
tial utility in other types of catalytic reactions.
Herein, we report the use of a series of 1,2-protected
furanoside diphosphite ligands 1–3 (Fig. 1), derived
from inexpensive
D
-(+)-glucose, in the enantioselective
copper-catalyzed 1,4-addition of diethylzinc to 2-cyclo-
hexenone. The advantage of these types of ligands is
that many structural variations can be made, so the
optimum configuration for maximum stereoselectivity
can be determined. Thus, these ligands allow the sys-
tematic variation of the configurations at the C(3) and
C(5) stereogenic centers of the ligand backbone and in
the biaryl substituents in order to fully study their
influence on activity and stereoselectivity.
3
4
phoroamidites, amido–phosphine, phosphite–oxazoli-
5
6
7
nes, diphosphite and Schiff-based ligands. However,
further investigations are required in order to under-
stand how to obtain efficient enantiocontrol in this
reaction. In this context, the rational design of new
readily available ligands continues to be an active field
of research.
Carbohydrates are particularly advantageous for this
purpose, as they are readily available, highly function-
2. Results and discussion
2
.1. Ligand design
Ligands 1–3 (Fig. 1) consist of a chiral 1,2-O-protected
furanoside backbone, which determines their underly-
*
Corresponding author. Fax: +34-977559563; e-mail: dieguez@
quimica.urv.es
0
957-4166/01/$ - see front matter © 2001 Published by Elsevier Science Ltd.
PII: S0957-4166(01)00514-6

2
896
M. Di e´ guez et al. / Tetrahedron: Asymmetry 12 (2001) 2895–2900
O
O
O
O
O
O
P
P
O
O
P
O O
P
O
O
O
O
O
O
O
O
O
O
O
O
O
O
P
O
P
O
O
(1R,2R,3S,4R,5R)-1
(1R,2R,3R,4R,5R)-2
(1R,2R,3R,4R,5S)-3
R'
R
R''
O
=
O
O
O
O
O
R'
R
R''
e (Sax) R'' = H
(Rax) R'' = H
g (Sax) R'' = SiMe3
h (Rax) R'' = SiMe3
a
b
c
d
R = R' = H
R = R' = t-Bu
R = tert-Bu, R' = OMe
R = SiMe3, R' = H
f
Figure 1.
1
13
31
ing structure, and two hydroxyl groups at positions
C(3) and C(5). To this basic framework several phos-
phoric acid biaryl esters are attached. The influence of
the different groups attached to the ortho- and para-
positions of the bisphenol moieties on the enantioselec-
tivity was investigated using ligands 1a–d, which have
the same configurations on the carbon atoms C(3) and
C(5). The influence of the configuration of the binaph-
thol moiety on the enantioselectivity was studied using
ligands 1e–h.
yield. The H, C and P NMR spectra agree with
those expected for these C1 ligands (see Section 4).
Rapid ring inversions (atropisomerization) of the seven-
membered dioxaphosphepine rings occurred on the
NMR timescale, since diastereoisomers were not
31
10
detected by low temperature P NMR.
2.3. Conjugate addition of 2-cyclohexenone
Ligands 1–3 were tested in the copper-catalyzed conju-
gate addition of diethylzinc to 2-cyclohexenone 4. The
catalytic system was generated in situ by adding 1
We studied the effects of the C(3) stereogenic centre by
comparing ligands 2a–f with diastereomeric ligands 1a–
f, which have the same substituents in the biphenyl
moieties but have opposite configuration at C(3).
equiv. of the ligand to a solution of Cu(OTf) followed
2
by addition of diethylzinc. In general, good to excellent
reaction rates were observed. Moreover, no 1,2-addi-
tion product was observed.
The influence of the configuration of the C(5) stereo-
genic center was studied using ligands 3b–c, whose
configuration at C(5) is opposite to that of ligands 2.
The effects of different reaction parameters were inves-
tigated for the catalytic precursors containing ligands
1
a. The results are summarized in Table 1.
2.2. Ligand synthesis
The new ligands 1a and 2a were easily synthesized in
HO
O
O
O
Toluene
Py
one step from the corresponding diols, which are easily
+
PCl
1a, 2a
prepared on a large scale from 8 -(+)-glucose using a
D
O
HO
O
standard procedure (Scheme 1). The reaction of the
corresponding diol with 2 equiv. of the desired in situ
formed phosphorochloridite in the presence of base
afforded diphosphite ligands 1a and 2a in good overall
Scheme 1. Synthesis of diphosphite ligands 1a and 2a.

M. Di e´ guez et al. / Tetrahedron: Asymmetry 12 (2001) 2895–2900
2897
a
Table 1. Asymmetric 1,4-addition of diethylzinc to 2-cyclohexenone using diphosphite 1a
O
O
Cu(OTf)2 / L*
ZnEt2
*
Et
4
(R)-5/(S)-5
c
d
e
Entry
Ligand
Solvent
T (°C)
TOFb
876
900
816
\1200
648
Conv. (%)
5
(%)
e.e. (%)
1
2
3
4
5
6
1a
1a
1a
1a
1a
1a
1a
Toluene
CH Cl
0
0
0
73
75
68
100
54
28
71
75
65
100
54
58 (R)
60 (R)
55 (R)
52 (R)
56 (R)
43 (R)
60 (R)
2
2
THF
CH Cl
25
2
2
2
2
2
CH Cl
−20
−40
0
2
CH Cl
456
888
27
74
2
f
7
CH Cl
74
2
a
Reaction conditions: Cu(OTf) (0.025 mmol), ligand (0.025 mmol), ZnEt2 (3.5 mmol), 2 (2.5 mmol), solvent (6 mL).
2
b
c
d
e
f
−1
−1
TOF in mol 4×mol Cu ×h determined after 5 min reaction time by GC.
Conversion determined by GC using undecane as internal standard after 5 min.
%
Determined by GC using undecane as internal standard.
Enantiomeric excess measured by GC using Lipodex A column.
0
.05 mmol of ligand used.
The results showed that the efficiency of the process
depended on the nature of the solvent (entries 1–3).
Thus, the catalyst performance (activity and selectiv-
ity) was best when dichloromethane was used (entry
The results with ligands 1e and 1f, which contain
stereogenic binaphthyl moieties, show that the differ-
ent configuration in the binaphthyl moieties has a
strong effect on both activity and enantioselectivity
(entries 5 and 6). Ligand 1e, which has an (S)-
binaphthyl moiety gave (S)-5 with 65% conversion
and 35% e.e. after 5 min, whereas diastereomer 1f,
which has an (R)-binaphthyl moiety, gave (R)-5 with
99% conversion and 55% e.e. Ligand 1g, which con-
tains bulky trimethylsilyl groups at the ortho positions
of the (S)-binaphthyl moieties, resulted in a higher
reaction rate and a higher enantioselectivity (84% e.e.,
entry 7) than ligand 1e. Surprisingly, ligand 1h, with
bulky trimethylsilyl substituents at the ortho position
of the (R)-binaphthyl moieites, showed lower asym-
metric induction than the less hindered ligand 1f
(entry 8 versus 6). This unexpectedly low enantiose-
lectivity can be explained by taking into account that
the bulky-ligand probably reduces its steric congestion
in the species responsible for the catalytic activity by
adopting an unfavorable conformation that induces
lower enantioselectivities.
2).
The effect of the reaction temperature on activity and
selectivity was also studied. As expected, conversions
were better at high temperature but the best enan-
tioselectivity was found at 0°C. The enantiomeric
excess dropped when the temperature was either
raised or lowered (entries 2, 4–6). These temperature-
dependent results are in line with those reported for
6
d,11
other Cu-diphosphite systems.
Varying the ligand-to-copper ratio showed that excess
ligand did not affect the product outcome 5 (entries 2
and 7).
For comparative purposes, the rest of the ligands
were tested under the conditions that gave the opti-
mum compromise between enantioselectivities and
reaction rates i.e. a ligand-to-copper ratio of 1, a
temperature of 0°C and dichloromethane as solvent.
The results are shown in Table 2.
Ligands 2a–f, whose configuration at the C(3) stereo-
center is opposite to those of ligands 1 (Fig. 1), pro-
duced lower enantioselectivities and slightly lower
activities (entries 9–14). Ligands 3, which resulted
from changing the configuration of carbon C(5) from
(R) to (S), led to slightly higher activities but similar
enantioselectivities compared to the catalytic systems
Cu/ligands 2 (entries 10 and 11 versus 15 and 16).
Ligands 1b and 1d, which have sterically demanding
groups at the ortho positions of the biphenyl moieties
(
tert-butyl and trimethylsilyl groups, respectively)
resulted in higher reaction rates (entries 2 and 4), but
lower enantioselectivities than the less sterically
encumbered unsubstituted ligand 1a (entries 2 and 4
versus 1). The presence of methoxy groups instead of
tert-butyl groups in the para positions of the biphenyl
moieties (ligand 1c) had a negative effect on the
activity (entry 3 versus 2).
Comparison of entries 2, 10 and 15 clearly shows that
the enantiomeric excesses depends strongly on the
absolute configuration of C(3) at the ligand back-
bone.

2
898
M. Di e´ guez et al. / Tetrahedron: Asymmetry 12 (2001) 2895–2900
a
Table 2. Asymmetric 1,4-addition of diethylzinc to 2-cyclohexenone 4 using diphosphite ligands 1–3
O
O
Cu(OTf)2 / L*
ZnEt2
*
Et
4
(R)-5/(S)-5
c
d
e
Entry
Ligand
TOFb
900
\1200
900
1080
780
1188
1176
1044
720
Conv. (%)
5
(%)
e.e. (%)
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1f
1g
1h
2a
2b
2c
2d
2e
2f
75
100
75
90
65
99
98
87
60
68
40
100
65
72
71
55
75
99
75
90
64
99
98
87
60
68
39
99
64
70
71
54
60 (R)
25 (R)
26 (R)
20 (R)
35 (S)
55 (R)
84 (S)
8 (R)
3 (R)
3 (R)
4 (R)
8 (R)
10 (S)
4 (R)
5 (S)
10
11
12
13
14
15
16
816
480
\1200
780
864
852
660
3b
3c
5 (S)
a
b
c
Reaction conditions: Cu(OTf) (0.025 mmol), ligand (0.025 mmol), ZnEt2 (3.5 mmol), 2 (2.5 mmol), solvent (6 mL), T=0°C.
2
−1
−1
TOF in mol 4×mol Cu ×h determined after 5 min reaction time by GC.
Conversion determined by GC using undecane as internal standard after 5 min.
%
d
e
Determined by GC using undecane as internal standard.
Enantiomeric excess measured by GC using Lipodex A column.
3
. Conclusions
substituents on the (S)-binaphthyl moieties. Further
research into the application of these ligands in other
reactions is therefore underway.
A series of diphosphite ligands 1–3, derived from inex-
pensive -(+)-glucose, were screened in the highly active
Cu-catalyzed asymmetric 1,4-addition of diethylzinc to
-cyclohexenone (TOF >1200 h and e.e. of up to
4%). The modular nature of these ligands allow a
D
−
1
2
8
4
. Experimental
facile systematic variation in the configuration of the
C(3) and C(5) stereogenic centers at the ligand bridge
and in the biaryl substituents, so their influence on
activity and stereoselectivity can be determined. Sys-
tematic variation of the stereocenters at C(3) and C(5)
in the ligand backbone revealed that the enantioselec-
tivity depends strongly on the absolute configuration at
C(3). The best enantioselectivities were therefore
achieved with ligands 1 with (S)-configuration at C(3)
and (R)-configuration at C(5).
4.1. General comments
All syntheses were performed using standard Schlenk
techniques under argon atmosphere. Solvents were
purified by standard procedures. Diphosphites 1b–h,
2b–f and 3b–c were prepared by previously described
8
,12
methods.
6-Desoxy-1,2-O-isopropylidene-a-
D-allo-
-gluco-
furanose and 6-desoxy-1,2-O-isopropylidene-a-
D
furanose were synthesized as described in the
8
literature. (1,1%-biphenyl-2,2%-diyl)phosphorochloridite
1
3
Variation in chirality at the axial chiral binaphthyl
substituents in ligands 1 indicates that the sense of the
enantiodiscrimination is predominantly controlled by
the configuration of the biaryl groups at the phosphite
moieties. The presence of bulky substituents on the
ortho positions of the biphenyl moieties has a negative
effect on the enantioselectivity, but a positive effect on
activity. However, enantioselectivity is strongly depen-
dent on the configuration of the binaphthyl moieties
when bulky trimethylsilyl groups are introduced at the
ortho positions. Thus, the highest enantiomeric excess
was found for ligand 1g, which has ortho trimethylsilyl
was prepared in analogy with literature procedure. All
other reagents were used as commercially available.
Elemental analyses were performed on a Carlo Erba
1
13
1
31
1
EA-1108 instrument. H, C{ H} and P{ H} NMR
spectra were recorded on a Varian Gemini 400 MHz
1
spectrometer. Chemical shifts are relative to SiMe ( H
4
13
31
and C) as internal standard or H PO ( P) as external
3
4
standard. All assignments in NMR spectra were deter-
mined by COSY and HETCOR spectra. Gas chromato-
graphic analyses were run on a Hewlett–Packard HP
5890A instrument equipped with a Hewlett–Packard
HP 3396 series II integrator.

M. Di e´ guez et al. / Tetrahedron: Asymmetry 12 (2001) 2895–2900
2899
4
.1.1. 3,5-Bis[(1,1%-biphenyl-2,2%-diyl)phosphite]-6-deoxy-
,2-O-isopropylidene-a- -glucofuranose 1a. In situ
in hexanes, 3.5 mL, 3.5 mmol) was added. A solution of
2-cyclohexenone (0.24 mL, 2.5 mmol) and undecane as
GC internal standard (0.25 mL) in dichloromethane (3
mL) was then added. The reaction was monitored by
GC. The reaction was quenched with HCl (2 M) and
filtered twice through silica flash. The conversion and
enantiomeric excesses were obtained by GC using a
1
D
formed phosphorochloridite (2.2 mmol) was dissolved
in toluene (5 mL) to which pyridine (0.36 mL, 4.6
mmol) was added. 6-Desoxy-1,2-O-isopropylidene-a-D-
glucofuranose (0.21 g, 1 mmol) was azeotropically dried
with toluene (3×1 mL) and dissolved in toluene (10 mL)
to which pyridine (0.18 mL, 2.3 mmol) was added. The
diol solution was transferred slowly over 30 min to the
solution of phosphorochloridite at room temperature.
The reaction mixture was stirred overnight under reflux
and the pyridine salts were removed by filtration. Evap-
oration of the solvent gave a white foam which was
purified by flash chromatography (eluent: toluene) to
produce a white powder (0.46 g, 74%). Anal. calcd for
C H O P : C, 62.66; H, 4.78. Found: C, 62.74; H,
14
Lipodex-A column.
Acknowledgements
We thank the Spanish Ministerio de Educaci o´ n, Cultura
y Deportes and the Generalitat de Catalunya (CIRIT)
for their financial support (PB97-0407-CO5-01).
3
3
30
9 2
3
1
6
4
.82%. P NMR: l 146.8 (d, 1P, JP-P=16.7 Hz), 151.8
6 1
P-P
(
d, 1P, J =16.7 Hz). H NMR: l 1.28 (s, 3H, CH ),
3
3
1
.45 (s, 3H, CH ), 1.55 (d, 3H, H-6, J6-5=6.4 Hz), 4.12
References
3
3
3
(
dd, 1H, H-4, J =8.8 Hz, J4-3=2.4 Hz), 4.63 (d, 1H,
4-5
3
H-2, J2-1=3.6 Hz), 4.77 (m, 1H, H-5), 4.94 (dd, 1H,
1. Perlmutter, P. Conjugate Addition Reactions in Organic
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3
H-3, J3-4=2.4 Hz, J3-P=8.8 Hz), 5.87 (d, 1H, H-1,
3
13
J1-2=3.6 Hz), 7.0–7.6 (m, 16H, CHꢁ). C NMR: l
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2
1.2 (d, C-6, JC-P=2.3 Hz), 26.5 (CH ), 26.9 (CH ),
3 3
6
8.9 (d, C-5, JC-P=19.7 Hz), 76.7 (d, C-3, JC-P=12.9
Hz), 83.0 (t, C-4, JC-P=6.8 Hz), 84.5 (d, C-2, JC-P=2.3
Hz), 105.2 (C-1), 112.4 (CMe ), 122.2 (CHꢁ), 122.3
2
(CHꢁ), 122.4 (CHꢁ), 125.3 (CHꢁ), 125.4 (CHꢁ), 125.5
(CHꢁ), 128.4 (CHꢁ), 129.3 (CHꢁ), 129.4 (CHꢁ), 129.5
(CHꢁ), 130.0 (CHꢁ), 130.2 (CHꢁ), 130.3 (CHꢁ), 131.0
(C), 131.1 (C), 131.2 (C), 131.3 (C), 149.3 (C), 149.4
(C), 149.5 (C).
4
1
.1.2. 3,5-Bis[(1,1%-biphenyl-2,2%-diyl)phosphite]-6-deoxy-
,2-O-isopropylidene-a- -allofuranose 2a. Treatment of
D
in situ formed phosphorochloridite (2.2 mmol) and
-desoxy-1,2-O-isopropylidene-a- -allofuranose (0.21
6
D
g, 1 mmol) as described for compound 1a afforded
diphosphite 2a, which was purified by flash chromatog-
raphy (eluent: toluene) to produce a white powder (0.41
g, 65%). Anal. calcd for C H O P : C, 62.66; H, 4.78.
4. Hu, X.; Chen, H.; Zhang, X. Angew. Chem., Int. Ed.
1999, 38, 3518.
5. Kn o¨ bel, A. K. H.; Escher, I. J.; Pfaltz, A. Synlett 1997,
1429.
3
3
30
1
9 2
3
Found: C, 62.85; H, 4.89%. P NMR: l 137.8 (s, 1P),
1
1
3
47.9 (s, 1P). H NMR: l 1.35 (d, 3H, H-6, J6-5=7.2
Hz), 1.38 (s, 3H, CH ), 1.62 (s, 3H, CH ), 4.20 (m, 1H,
6. (a) Alexakis, A.; Vastra, J.; Burton, J.; Benhaim, C.;
Mangeney, P. Tetrahedron Lett. 1998, 39, 7869; (b) Yan,
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try 2000, 11, 4377; (e) Alexakis, A.; Burton, J.; Vastra, J.;
Benhaim, C.; Fournioux, X.; van den Heuvel, A.; Lev eˆ -
que, J. M.; Maz e´ , F.; Rosset, S. Eur. J. Org. Chem. 2000,
4011.
7. (a) Chataigner, I.; Gennari, C.; Piarulli, U.; Ceccarelli, S.
Angew. Chem., Int. Ed. 2000, 39, 916; (b) Chataigner, I.;
Gennari, C.; Ongeri, S.; Piarulli, U.; Ceccarelli, S. Chem.
Eur. J. 2001, 7, 2628.
3
3
3
3
H-4), 4.44 (dd, 1H, H-2, J =3.2 Hz, J2-3=4.0 Hz),
2
-1
4
.63 (m, 1H, H-3), 4.78 (m, 1H, H-5), 5.73 (d, 1H, H-1,
3
13
J1-2=3.2 Hz), 7.0–7.5 (m, 16H, CHꢁ). C NMR: l
1
7.8 (d, C-6, JC-P=3.2 Hz), 26.7 (CH ), 27.0 (CH ),
3
3
7
0.5 (d, C-5, JC-P=13.6 Hz), 71.9 (C-3), 79.4 (C-2), 81.2
(
(
(
(
(
t, C-4, JC-P=3.0 Hz), 103.7 (C-1), 113.4 (CMe ), 122.1
2
CHꢁ), 122.2 (CHꢁ), 122.3 (CHꢁ), 124.9 (CHꢁ), 125.0
CHꢁ), 125.1 (CHꢁ), 125.3 (CHꢁ), 125.4 (CHꢁ), 129.0
CHꢁ), 129.1 (CHꢁ), 129.2 (CHꢁ), 129.3 (CHꢁ), 129.8
CHꢁ), 129.9 (CHꢁ), 130.9 (C), 131.0 (C), 131.1 (C),
1
49.8 (C), 149.9 (C), 150.0 (C), 151.1 (C).
4
.2. General procedure for the catalytic conjugate
addition of diethylzinc to 2-cyclohexenone
8. Di e´ guez, M.; P a` mies, O.; Ruiz, A.; Castill o´ n, S.; Claver,
C. Chem. Eur. J. 2001, 7, 3086.
In a typical experiment, a solution of Cu(OTf) (9 mg,
9. See for instance: (a) Brunner, H.; Zettlmeier, W. In
Handbook of Enantioselective Catalysis; VCH: Weinheim,
1993; (b) Selke, R. J. Organomet. Chem. 1989, 370, 241;
(c) RajanBabu, T. V.; Casalnuovo, A. L. Pure Appl.
2
0
.025 mmol) and diphosphite ligand (0.025 mmol) in
dichloromethane (3 mL) was stirred for 30 min at room
temperature. After cooling to 0°C, diethylzinc (1 M sol.

2
900
M. Di e´ guez et al. / Tetrahedron: Asymmetry 12 (2001) 2895–2900
Chem. 1994, 94, 149; (d) RajanBabu, T. V.; Ayers, T. A.
A. D.; Clarke, F. H. Helv. Chim. Acta 1993, 76, 900.
11. P a` mies, O.; Net, G.; Ruiz, A.; Claver, C. Tetrahedron:
Asymmetry 1999, 10, 2007.
12. Di e´ guez, M.; Ruiz, A.; Claver, C., submitted.
13. Buisman, G. J. H.; Kamer, P. C. J.; van Leeuwen, P. W.
N. M. Tetrahedron: Asymmetry 1993, 4, 1625.
14. Bennet, S. M. W.; Brown, S. M.; Conole, G.; Dennis, M.
R.; Frase, P. K.; Radojevic, S.; McPartin, M.; Topping,
C. M.; Woodward, S. J. Chem. Soc., Perkin Trans. 1
1999, 3127.
Tetrahedron Lett. 1994, 35, 4295; (e) Yonehara, K.;
Hashizuma, T.; Mori, K.; Ohe, K.; Uemura, S. Chem.
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Angew. Chem., Int. Ed. 1999, 38, 179; (g) P a` mies, O.;
Di e´ guez, M.; Net, G.; Ruiz, A.; Claver, C. Chem. Com-
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132.
1
0. Pastor, D. D.; Shum, S. P.; Rodebaugh, R. K.; Debellis,