Applications of enantiopure 4,5-diphenyl substituted box and pybox ligands in asymmetric catalysis

Article abstract of DOI:10.1016/j.tetasy.2004.08.030

Enantioselectivities of 95-98% have been obtained in the palladium-catalyzed alkylation of rac-3-acetoxy-1,3-diphenyl-1-propene with dimethyl malonate using 4,5-diphenyl substituted bis(oxazoline) as the chiral ligands, whatever the configuration at C-5.

Full text of DOI:10.1016/j.tetasy.2004.08.030

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 15 (2004) 3195–3200
Applications of enantiopure 4,5-diphenyl substituted box and pybox
ligands in asymmetric catalysis
Jerome Bayardon,a Denis Sinou,a,* Matilde Gualab and Giovanni Desimonib,*
a
`
´
´
´
Laboratoire de Synthese Asymetrique, associe au CNRS, CPE Lyon, Universite Claude Bernard Lyon 1,
43, boulevard du 11 novembre 1918, 69622 Villeurbanne cedex, France
bDepartment of Organic Chemistry, University of Pavia, Viale Taramelli, 10-1-27100 Pavia, Italy
Received 28 June 2004; accepted 30 August 2004
Available online 1 October 2004
Abstract—Enantioselectivities of 95–98% have been obtained in the palladium-catalyzed alkylation of rac-3-acetoxy-1,3-diphenyl-1-
propene with dimethyl malonate using 4,5-diphenyl substituted bis(oxazoline) as the chiral ligands, whatever the configuration at
C-5. The copper-catalyzed allylic oxidation of various cycloalkenes gave the corresponding allylic esters with enantioselectivities
up to 84% using the 4,5-diphenyl substituted bis(oxazoline). Lower enantioselectivities were obtained using the corresponding
4,5-diphenyl substituted pyridine-bis(oxazoline), the higher enantioselectivity being observed with the cis-stereoisomer.
ꢀ 2004 Elsevier Ltd. All rights reserved.
1. Introduction
ents on the bridging methylene unit of R1, or the intro-
duction of additional stereogenic centers on the
oxazoline rings (R2 or R3 5 H) (Fig. 1). In these studies
it was noted that there was a reversal of facial selectivity
in alkylation reactions by 4-alkyl- versus 4-aryl-substi-
tuted box ligands.9–12 However if the influence of the
substituent at the 4-position of chiral bis(oxazoline) or
pyridine-bis(oxazoline) ligands on both the activity
and the enantioselectivity of the reaction, has been stud-
ied in detail, there are few reports on the influence of a
supplementary substituent at the stereogenic C-5 center.
Asymmetric catalysis with chiral complexes has received
considerable attention in recent years. Metal complexes
of C2-symmetric enantiopure ligands are among the
most efficient chiral inducers used. In particular,
bis(oxazoline) ligands (box) of general structure A with
two oxazoline rings separated by a spacer (generally a
single carbon atom) have found broad use in the asym-
metric catalysis of a large variety of transformations
during the past decade, such as cyclopropanations,
ene-reactions, Diels–Alder and hetero-Diels–Alder reac-
tions, allylic substitutions, aziridination reactions, and
Mukayama aldol reactions.1–5 The main reasons for
the success of these ligands are their easy preparation
and modification, their versatility, and the high enantio-
selectivities generally obtained. More recently, bis(oxaz-
oline) ligands B with a pyridine ring as the spacer
(pybox) have been synthesized; it was noted that these
ligands generally acted as tridentate ligands.6 Other chi-
ral ligands with sp2 nitrogens as the coordinating atoms
such as 2,20-bipyridines or 1,10-phenantrolines, have
also been studied.7,8
Recently, Pericas et al.13 prepared bis(oxazoline) ligands
`
bearing Ar and OR groups of different steric and elec-
tronic nature at C-4 and C-5, respectively, and studied
their behavior in palladium-catalyzed allylic substitu-
tion. 4-Ph-box and 4,5-di-Ph-box have also been used
as ligands and their steric influences compared in
Diels–Alder and hetero-Diels–Alder reactions,14–21 in
Michael additions,22,23 and Mukayama–Michael and
aldol reactions.24,25 Conversely, there are only two
R
R
R3
O
R3
R2
R3
R2
O
R3
R2
O
O
So far, many variations have been made on the ligands
A and B, including the modification of the R substitu-
N
N
N
R2
N
N
R1
R1
R1
R1
*
B
Corresponding authors. Tel.: +33 (0)4 72 44 81 83; fax: +33 (0)4 78
89 89 14 (D.S.); tel.: +39 0382 507313; fax: +39 0382 507323
(G.D.); e-mail addresses: sinou@univ-lyon1.fr; desimoni@unipv.it
A
Figure 1.
0957-4166/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2004.08.030

3196
J. Bayardon et al. / Tetrahedron: Asymmetry 15 (2004) 3195–3200
Me Me
Me Me
O
O
O
O
C6H5
C6H5
trans-(4R,5R)-4,5-diPhbox 1
C6H5
C6H5
C6H5
cis-(4R,5S)-4,5-diPhbox 2
C6H5
N
N
N
N
C6H5
C6H5
O
O
O
O
N
N
C6H5
C6H5
C6H5
C6H5
C6H5
trans-(4R,5R)-4,5-diPhPybox 3
N
N
N
N
C6H5
C6H5
C6H5
cis-(4R,5S)-4,5-diPhpybox 4
Figure 2.
examples of the use of 4-Ph-Py-box together with 4,5-di-
Ph-Py-box in Diels–22 and Mukayama–Michael reaction.24
and 0.1mol% of KOAc. When the reaction was per-
formed at rt, the transformation was sluggish using
trans-4,5-diPhbox 1 or cis-4,5-diPhbox 2 as the chiral
ligand; ligand 1 gave the alkylated product 7a with
30% chemical yield and 94% ee (Table 1, entry 1), while
40% chemical yield and 96% ee were obtained using lig-
and 2 (Table 1, entry 3), the same enantiomer being ob-
tained using the two ligands. In order to obtain higher
conversions, the reaction was performed at 35ꢁC. The
alkylated product 7a was obtained in 70% and 90%
chemical yield using ligands 1 and 2, respectively, with
enantioselectivities of 97% and 98% (Table 1, entries 2
and 4). So not only the chemical yields were increased
with increasing the reaction temperature, but the
observed enantioselectivities were also a little higher. It
is noteworthy that the bis(oxazoline) ligand bearing a
unique phenyl substituent at C-4 gave an enantioselec-
tivity up to 95% in the same alkylation reaction.13 Hence
it seems that there is a beneficial contribution on enan-
tioselectivity, although low, of the phenyl substituent at
C-5 for the two stereoisomers, the configuration of the
newly created center being the same whatever the config-
As part of a research project aimed at controlling the
activity and enantioselectivity of catalytic reactions
involving bis(oxazoline) ligands 1–4 by variation of the
steric properties of the substituents, and in particular
the substituent at the position 5, and with our previous
results in the field of Diels–Alder reactions, we became
interested in the use of such ligands both in allylic alkyl-
ations and oxidations. Herein we report the use of these
ligands in these two fields (Fig. 2).
2. Results and discussion
We first examined chiral ligands 1 and 2 in the palla-
dium-catalyzed allylic alkylation of rac-1,3-diphenyl-2-
propenyl acetate 5 with dimethyl malonate 6a (Table
1). This reaction was carried in CH2Cl2 in the presence
of a (p-allyl)-palladium-ligand complex generated in situ
from 2mol% [Pd(g3-C3H5)Cl]2 and 5mol% of the
appropriate chiral ligand. The nucleophile was gener-
ated from dimethyl malonate (3equiv) in the presence
of N,O-bis(trimethylsilyl)acetamide (BSA) (3equiv)
uration at C-5 of the bis(oxazoline). This is in contrast
13
`
with the results of Pericas et al. who find that the
trans-bis(oxazoline) bearing
a
C6H5– and
a
Table 1. Asymmetric allylic alkylation of rac-3-acetoxy-1,3-diphenyl-1-propene catalyzed by Pd/bis(oxazoline) catalysta
OAc
Ph
CR(CO2Me)2
Ph
[Pd(η3-C3H5)Cl]2/ligand
Ph
Ph
RCH(CO2Me)2/KOAc/BSA/CH2Cl2
( )-5
7 a-b
a: R = H; b: R = CH3
Entry
Ligand
Nucleophile
T (ꢁC)
Time (day)
Yield (%)b
Ee (%)c (config)d
1
2
3
4
5
6
1
1
2
2
1
2
CH2(CO2Me)2
CH2(CO2Me)2
CH2(CO2Me)2
CH2(CO2Me)2
CH3CH(CO2Me)2
CH3CH(CO2Me)2
25
35
25
35
35
35
5
5
5
3
3
3
30
70
40
90
80
65
94 (R)
97 (R)
96 (R)
98 (R)
95 (R)
95 (R)
a [Substrate]:[nucleophile]:[BSA]:[KOAc]:[Pd]:[ligand] = 25:75:75:2.5:1:2; THF.
b Isolated pure product.
c Determined by HPLC analysis (column Chiralpak AD 0.46 · 25cm, eluent i-PrOH/n-hexane 4:6).
d Determined by comparison of the HPLC retention time with literature data.

J. Bayardon et al. / Tetrahedron: Asymmetry 15 (2004) 3195–3200
3197
Table 2. Enantioselective copper-catalyzed allylic oxidation of cyclo-
alkenes with bis(oxazolines)a
(C6H5)2CHO– substituents at C-4 and C-5, respectively,
gave ees up to 96% in this reaction, when the cis ana-
logue gave no reaction at all.
C6H5CO2
CuOTf.0.5 C6H6/L
Under the above mentioned conditions, dimethyl meth-
ylmalonate 6b afforded alkylated product 7b in 80%
chemical yield, and 95% ee (Table 1, entry 5), and 65%
chemical yield and 95% ee (Table 1, entry 6), using
bis(oxazoline) ligands 1 and 2, respectively.
n
n
C6H5CO3But/acetone
8 a-c
9
a-c
a: n = 1; b: n = 2; c: n = 3
Entry
Alkene
Ligand
Yield (%)b
Ee (%)c (R)d
The observed stereochemistry in this palladium-cata-
lyzed alkylation using 4,5-disubstituted box ligands is
in agreement with the previous results using quite simi-
lar systems.1,11,26,27 As previously postulated, the nucleo-
philic attack occurred on the longer, strained Pd–carbon
bond of the syn/syn p-allyl ligand complex (Fig. 3). As
can be seen, the substituents at C-5, and the configura-
tion of this stereogenic center, are probably too far from
the p-allyl ligand in order to have any important influ-
ence on the enantioselectivity of the process.
1
n = 1
n = 1
n = 1
n = 1
n = 2
n = 2
n = 2
n = 2
n = 3
n = 3
n = 3
n = 3
1
2
3
4
1
2
3
4
1
2
3
4
80
70
26
3
84
80
3
2
3
4
7
15
5
70
78
66
60
50
50
28
42
64
61
28
36
62
80
26
48
6
7
8
9
10
11
12
a [Alkene]:[tert-butyl perbenzoate]:[CuOTfÆ0.5C6H6]:[ligand] = 200:20:
1:2.4; seven days.
+
b Isolated pure product.
O
O
c Determined by HPLC analysis (column Chiralpak AD 0.46 · 25cm,
eluent i-PrOH/n-hexane 1:150).
d Determined by comparison of the HPLC retention time with litera-
ture data.
C6H5
C6H5
C6H5
N
N
Pd
C6H5
4-Phbox under the same conditions. It seems that there
is in this case a beneficial contribution on enantioselec-
tivity of the phenyl substituent at C-5 for the two
stereoisomers.
syn/syn
Figure 3.
The use of cyclohexene 8b as the olefin gave chemical
yields in the same range, when the obtained enantiose-
lectivities using 1 and 2 as the ligand were 64% and
61% ee, respectively (Table 2, entries 5 and 6). These re-
sults (conversion as well as enantioselectivity) are quite
close to those observed using the monosubstituted 4-
Phbox ligand (67%28 and 59%29 ee).
We also used 4,5-diPhpyboxes 3 and 4 as ligands in this
alkylation reaction. However the conversion was low
(less than 19%), whatever the conditions used.
We then turned our attention to the enantioselective
copper-catalyzed allylic oxidation of cyclic olefins. The
catalyst was prepared in situ from CuOTfÆ0.5C6H6 and
the appropriate ligand 1–4 in acetone, with the allylic
oxidation of the alkene performed in the presence of
tert-butyl perbenzoate for seven days. The results are
summarized in Table 2.
Moderate chemical yields (50%) were obtained in the
oxidation of cycloheptene 8c; however ees up to 80%
were obtained using cis-4,5-diPhbox 2, while ees of up
to 62% were only obtained using trans-4,5-diPhbox 1.
It seems that in this case there is some influence of the
stereogenic center at C-5 on the level of
enantioselectivity.
It should be noted that all ligands 1–4 required several
days to allow completion of this oxidation reaction as
previously described for analogous box and pybox lig-
ands, when the 2,20-bipyridines and 1,10-phenantrolines
gave the corresponding allylic esters within 30min at
room temperature.7,8
When using box ligands 1 and 2 associated with copper,
the highest enantioselectivities were obtained in the oxi-
dation of cyclopentene and cyclohexene, in agreement
with the published results concerning this class of
ligands.24,25,30,31
The oxidation of cyclopentene 8a using 4,5-diPhbox lig-
ands 1 and 2 showed good chemical yield and enantiose-
lectivity; the corresponding allylic benzoate 9a being
obtained with ee up to 80% whatever the configuration
at C-5 (Table 2, entries 1 and 2). This observed enantio-
selectivity is higher than those previously published
(71%28 and 69%29 ee) using the monophenyl substituted
When 4,5-diPhpyboxes 3 or 4 were used as the ligand,
low to moderate conversions were observed: 26% and
37% for cyclopentene 8a, 66% and 60% for cyclohexene
8b, and 28% and 42% for cycloheptene 8c, using 3 and 4,
respectively. The obtained enantioselectivities (3% and
15% ee, 28% and 36% ee, 26% and 48% ee, in the

3198
J. Bayardon et al. / Tetrahedron: Asymmetry 15 (2004) 3195–3200
presence of 3 and 4, respectively) are generally lower
than those published in the literature using 4-i-Prpybox
as the chiral ligand.32,33 However we noticed that the
trans-4,5-diPhpybox ligand 3 gave lower enantioselectiv-
ities than the cis isomer. In this case, the stereogenic
center at C-5 plays an important role on the
enantioselectivity of the allylic oxidation. Although the
obtained enantioselectivities are low, we observed the
following sequence of enantioselectivity cyclopen-
tene < cyclohexene < cycloheptene, which is in agree-
ment with the other copper–pybox complexes.6,32,33
These results could be rationalized according to Figures
4 and 5. In the case of the 4,5-diphenylbis(oxazoline) lig-
ands 1 and 2, we can propose the intermediates 10a and
10b (Fig. 4), where the benzoate and the cyclohexenyl
ligands are in the less hindered quadrant of the copper
complex. These intermediates have been put forward
by Zavitsas and co-workers,34,35 and more recently by
Andrus and Zhou.36 The rearrangement of these inter-
mediates gave the (R)-enantiomer, whatever the chirality
at C-5.
For cis-4,5-diphenylpybox 4, we proposed transition
state model 11 (Fig. 5), postulated by Singh and co-
workers.33 The preferred transition state is that where
the benzoate ligand is on the upper less hindered quad-
rant, and the transfer of the benzoate ligand to the
cyclohexyl ligand affords the (R)-allylic benzoate. For
trans-4,5-diphenylpybox 3, the two transition states 12
and 13 are possible (Fig. 5); however, as postulated by
Singh, transition state 12, could be a little more stabi-
lized by some attractive interaction between the two
phenyl rings of the benzoate moiety and at C-5. So in
this case, the (R)-enantiomer would also be formed,
although with lower enantioselectivity.
+1
Me Me
O
O
Ph
Ph
N
N
Ph
Ph
(R)-allylic ester
H
H
Cu
O
O
Ph
O
10a
Me Me
+1
Ph
O
Ph
Ph
N
N
(R)-allylic ester
H
H
Cu
O
Ph
O
3. Conclusion
Ph
4,5-Diphenyl substituted bis(oxazolines) ligands gave
enantioselectivities in the range 95–98% ee in the palla-
dium-catalyzed alkylation of rac-3-acetoxy-1,3-diphen-
yl-1-propene with dimethyl malonate derivatives; there
10b
Figure 4.
+1
O
N
Ph
Ph
O
Ph
Ph
N
O
(R)-allylic ester
O
Cu
N
H
11
+1
+1
Ph
Ph
O
O
N
O
N
O
O
O
N
N
Ph
Ph
Ph
Ph
O
Cu
Cu
N
N
Ph
Ph
H
O
H
13
12
(R)-allylic ester
(S)-allylic ester
Figure 5.

J. Bayardon et al. / Tetrahedron: Asymmetry 15 (2004) 3195–3200
3199
is no influence of the stereogenic center at C-5 on the
enantioselectivity of the reaction.
then stirred at rt for seven days. The solvent was then
removed under reduced pressure, and the residue puri-
fied by column chromatography on silica gel (eluent:
petroleum ether/ethyl acetate 10:1) to give the corre-
sponding allylic benzoate. The ee was determined by
HPLC using a Chiralpak AD (25cm · 0.46cm) and elut-
ing with hexane/i-PrOH (150:1).
The same ligands gave enantioselectivities up to 84% in
the copper-catalyzed allylic oxidation of various cyclo-
alkenes, with again practically no influence of the stereo-
genic center at C-5 on the selectivity. The corresponding
4,5-diphenyl substituted pyridine-bis(oxazolines) gave
lower ees in this reaction, the higher enantioselectivity
being observed with the cis-stereoisomer. The configura-
tion of the newly created center can be rationalized
using the appropriate transition states.
Acknowledgements
J.B. thanks the MENRT for a fellowship.
4. Experimental
4.1. General
References
1. Pfaltz, A. Acc. Chem. Res. 1993, 26, 339–345.
2. Ghosh, A. K.; Mathivanan, P.; Cappiello, J. Tetrahedron:
Asymmetry 1998, 9, 1–45.
3. Jorgensen, K. A.; Johannsen, M.; Yao, S.; Audrain, H.;
Thorhauge, J. Acc. Chem. Res. 1999, 32, 605–613.
4. Evans, D. A.; Rovis, T.; Johnson, J. S. Pure Appl. Chem.
1999, 71, 1407–1415.
5. Johnson, J. S.; Evans, D. A. Acc. Chem. Res. 2000, 33,
325–335.
6. Desimoni, G.; Faita, G.; Quadrelli, P. Chem. Rev. 2003,
103, 3119–3154.
Solvents were purified by standard methods and dried if
necessary. All commercially available reagents were used
as received. Ligands 1,37 2,37 3,24 and 438 have already
been prepared. Reactions involving organometallic
catalysis were carried out in a Schlenk tube under an
inert atmosphere. All reactions were monitored by
TLC (TLC plates GF254 Merck); detection was effected
by UV absorbance. Column chromatography was per-
formed on silica gel 60 (230–240mesh, Merck). NMR
spectra were recorded with a Bruker AMX 300
spectrometer. Enantiomeric excesses were determined
by HPLC with a Chiralpak AD column (25cm ·
4.6cm) using different ratios of hexane/i-propanol as
the eluent.
7. Chelucci, G.; Thummel, R. P. Chem. Rev. 2002, 102, 3129–
3170.
8. Malkov, A. V.; Pernazza, D.; Bell, M.; Bella, M.; Massa,
A.; Teply, F.; Meghani, P.; Kocovsky, P. J. Org. Chem.
2003, 68, 4727–4742, and references cited therein.
9. Johannsen, M.; Jørgensen, K. A. J. Org. Chem. 1995, 60,
5757–5762.
10. Evans, D. A.; Johnson, J. S.; Burgey, C. S.; Campos, K. R.
Tetrahedron Lett. 1999, 40, 2879–2882.
11. Knoch, R.; Wilk, A.; Wannowius, K. J.; Reinen, D.; Elias,
H. Inorg. Chem. 1990, 29, 3799–3805.
12. Thorhauge, J.; Roberson, M.; Hazell, R. G.; Jørgensen,
K. A. Chem. Eur. J. 2002, 8, 1888–1898.
4.2. Procedure for the catalytic allylic alkylation
Ligand (50lmol, 5mol%) and [(g3-C3H5)PdCl]2
(7.3mg, 20 lmol, 2%) were dissolved in CH2Cl2 (8mL)
under nitrogen in a Schlenk tube. The reaction mixture
was stirred for 1h, and rac-1,3-(E)-diphenyl-2-propenyl
acetate 5 (252mg, 1mmol) in CH2Cl2 (3mL) then trans-
ferred into another reaction vessel containing N,O-
bis(trimethylsilyl)acetamide (610mg, 3mmol), KOAc
(9.8mg, 0.1mmol, 10mol%), and the nucleophile
(3mmol) in CH2Cl2 (6mL). The reaction mixture was
stirred at the desired temperature for the appropriate
time. Diethyl ether (20mL) was added, and the solution
washed with a saturated aqueous solution of NH4Cl
(2 · 10mL). The solvent was evaporated, and the resi-
due purified by chromatography on silica gel (eluent:
petroleum ether/ethyl acetate 5:1) to afford alkylated
product 6. The enantioselectivity was determined by
HPLC with a Chiralpak AD (25cm · 0.46cm) and elut-
ing with hexane/i-PrOH (8:2).
`
13. Pericas, M. A.; Puigjaner, C.; Riera, A.; Vidal-Ferran, A.;
Gomez, M.; Jimenez, F.; Muller, G.; Rocamora, M.
´
Chem. Eur. J. 2002, 8, 4164–4178.
14. Desimoni, G.; Faita, G.; Invernizzi, A. G.; Righetti, P.
Tetrahedron 1997, 53, 7671–7688.
15. Desimoni, G.; Faita, G.; Righetti, P.; Sardone, N.
Tetrahedron 1996, 52, 12019–12030.
16. Carbone, P.; Desimoni, G.; Faita, G.; Filippone, S.;
Righetti, P. Tetrahedron 1998, 54, 6099–6110.
17. Desimoni, G.; Faita, G.; Guala, M.; Pratelli, C. Tetrahe-
dron 2002, 58, 2929–2935.
18. Yao, S.; Johannsen, M.; Hazell, R. G.; Jørgensen, K. A.
Angew. Chem., Int. Ed. 1998, 37, 3121–3124.
19. Yao, S.; Johannsen, M.; Audrain, H.; Hazell, R. G.;
Jørgensen, K. A. J. Am. Chem. Soc. 1998, 120, 8599–
8605.
20. Yao, S.; Roberson, M.; Reichel, F.; Hazell, R. G.;
Jørgensen, K. A. J. Org. Chem. 1999, 64, 6677–
6687.
4.3. Procedure for the catalytic allylic oxidation
21. Abblati, G.; Clerici, F.; Gelmi, M. L.; Gambini, A.; Pilati,
T. J. Org. Chem. 2001, 66, 6299–6304.
22. Ji, J.; Barnes, D. M.; Zhang, J.; King, S. A.; Wittenberger,
S. J.; Morton, H. E. J. Am. Chem. Soc. 1999, 121, 10215–
10216.
23. van Lingen, H. L.; Zhuang, W.; Hansen, T.; Rytjes, F. P.
J. T.; Jørgensen, K. A. Org. Biomol. Chem. 2003, 1, 1953–
1958.
To a stirred solution of CuOTFÆ0.5C6H6 (12.6mg,
50lmol, 2.5mol%) in anhydrous acetone (4mL) was
added the bis(oxazoline) (120lmol, 6mol%). After stir-
ring for 1h at rt, the alkene (10mmol, 10equiv) in ace-
tone (2mL) was added to this solution, followed by
the dropwise addition of tert-butyl perbenzoate
(182mg, 1mmol, 1equiv). The resulting solution was

3200
J. Bayardon et al. / Tetrahedron: Asymmetry 15 (2004) 3195–3200
24. Desimoni, G.; Faita, G.; Filippone, S.; Mella, M.;
Zampori, M. G.; Zema, M. Tetrahedron 2001, 57,
10203–102212.
25. van Lingen, H. L.; van de Mortel, J. K. W.; Hekking, K.
F. W.; van Delft, F. L.; Sonke, T.; Rutjes, F. P. J. T. Eur.
J. Org. Chem. 2003, 317–324.
31. Andrus, M. B.; Chen, X. Tetrahedron 1997, 53, 16229–
16240.
32. DattaGupta, A.; Singh, V. K. Tetrahedron Lett. 1996, 37,
2633–2636.
33. Sekar, G.; DattaGupta, A.; Singh, V. K. J. Org. Chem.
1998, 63, 2961–2967.
26. von Matt, P.; Lloyd-Jones, G. C.; Minidis, A. B. E.;
Pfaltz, A.; Macko, L.; Neuburger, M.; Zehnder, M.;
34. Walling, C.; Zavitsas, A. A. J. Am. Chem. Soc. 1963, 85,
2084–2090.
Ruegger, H.; Pregosin, P. S. Helv. Chim. Acta 1995, 78,
¨
265–284.
35. Beckwith, A. L.; Zavitsas, A. A. J. Am. Chem. Soc. 1986,
108, 8230–8234.
27. Pfaltz, A. Acta Chem. Scand. B 1996, 50, 189–194.
28. Gokhale, A. S.; Minidis, A. B. E.; Pfaltz, A. Tetrahedron
Lett. 1995, 36, 1831–1834.
29. Clark, J. S.; Tolhurst, K. F.; Taylor, M.; Swallow, S.
Chem. Soc., Perkin Trans. 1 1998, 1167–1169.
30. Andrus, M. B.; Argade, A. B.; Chen, X.; Pamment, M. G.
Tetrahedron Lett. 1995, 36, 2945–2948.
36. Andrus, M. B.; Zhou, Z. J. Am. Chem. Soc. 2002, 124,
8806–8807.
37. Desimoni, G.; Faita, G.; Mella, M.; Ricci, M.
Righetti, P. P. Tetrahedron 1997, 53, 13495–
13508.
38. Desimoni, G.; Faita, G.; Guala, M.; Pratelli, C. Tetrahe-
dron: Asymmetry 2002, 13, 1651–1654.