Synthesis of novel chiral 6,6′-bis(oxazolyl)-1,1′-biphenyls and their application as ligands in copper(I)-catalyzed asymmetric cyclopropanation

Article abstract of DOI:10.1016/j.tetasy.2006.10.017

The synthesis of a number of novel chiral bis(oxazolyl) ligands with a biphenyl backbone from ellagic acid 1 and their application in the copper(I)-catalyzed asymmetric cyclopropanation was described.

Full text of DOI:10.1016/j.tetasy.2006.10.017

Tetrahedron: Asymmetry 17 (2006) 2804–2812
Synthesis of novel chiral 6,60-bis(oxazolyl)-1,10-biphenyls
and their application as ligands in copper(I)-catalyzed
asymmetric cyclopropanation
Karamali Khanbabaee,* Sinan Basceken and Ulrich Flo¨rke
Faculty of Science, Department of Chemistry, University of Paderborn, Warburger Str. 100, D-33098 Paderborn, Germany
Received 21 September 2006; accepted 5 October 2006
Available online 7 November 2006
Abstract—The synthesis of a number of novel chiral bis(oxazolyl) ligands with a biphenyl backbone from ellagic acid 1 and their appli-
cation in the copper(I)-catalyzed asymmetric cyclopropanation was described.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
oxide in the presence of benzyl chloride gave rac-3 as a
racemic mixture (Scheme 1), which was further character-
ized by X-ray analysis (Fig. 1). Hydrolysis of ester rac-3
under basic conditions led to the formation of the corre-
sponding biaryldicarboxylic acid rac-4 in a quantitative
yield.
Optically active cyclopropane derivatives, especially 2-
substituted cyclopropane carboxylates are present in many
biologically important substances.1 2-Phenyl-1-cyclopro-
pane carboxylates are very useful intermediates in the syn-
thesis of these chiral compounds.2 Up until now, several
strategies have been developed for enantioselective
cyclopropanation.3 In this context, axially chiral binaph-
thyl ligands (BINAP) with a C2-symmetry axis have also
been applied in the copper(I)-catalyzed asymmetric
cyclopropanation.4
At this stage, attempts to separate the racemic mixture rac-
4 into its enantiomerically pure atropisomers (aR)-4 and
(aS)-4 by using cinchonin, was only effective for (aR)-4,
but unfortunately not for (aS)-4 (Scheme 1).
Enantiomerically pure (aR)-4 was treated with achiral
amine 5 to give b-hydroxylamide (aR)-8 (Scheme 2). All
attempts to crystallize (aR)-8 failed. However, we were
able to obtain crystals from rac-8, which was further
characterized by X-ray analysis (Fig. 2).
Herein, we report on the synthesis of a number of optically
active bis(oxazolyl) derivatives with a biphenyl backbone
and their use as chiral ligands in the copper(I)-catalyzed
asymmetric cyclopropanation of styrene 18 with ethyl di-
azoacetate 19.
b-Hydroxamide (aR)-8 was then converted into the novel
chiral ligand (aR)-9 by mesylation and spontaneous cycli-
zation (Scheme 2).
2. Results and discussion
However, rac-4 was treated with the enantiomerically pure
amines 6 and 7 to give two diastereomeric pairs of
b-hydroxylamides 10/11 and 12/13 (Scheme 3), respec-
tively, which could be separated by chromatography on
silica gel. For the synthesis of stereochemically pure
b-hydroxylamides 10–13, prior activation of the carboxylic
groups of dicarboxylic acid 4 was necessary, which could
be achieved upon its treatment with thionyl chloride. The
resulting dichlorides (not shown) from these reactions were
Bis-lactone 2 was prepared by the reaction of ellagic acid 1
with benzyl chloride in the presence of potassium carbon-
ate in water. The opening of the lactone rings of bis-lactone
2 under strongly basic conditions with potassium hydr-
*
Corresponding author. Tel.: +49 5251 602175; fax: +49 5251 603245;
e-mail: kkh@zitmail.uni-paderborn.de
0957-4166/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2006.10.017

K. Khanbabaee et al. / Tetrahedron: Asymmetry 17 (2006) 2804–2812
2805
OH
OBn
OBn
HO
O
BnO
Bn
BnO
Bn
O
BnCl,
K2CO3/H2O
O
O
BnCl, KOH
BnO
BnO
COOBn
COOBn
O
O
O
Δ
O
OH
BnO
Bn
Bn
OBn
OH
OBn
OBn
2
3
rac-
1
KOH/EtOH,
H2O
Δ
OBn
OBn
OBn
BnO
Bn
BnO
Bn
BnO
Bn
1) Cinchonin
2) 1 M H2SO4
BnO
BnO
COOH
COOH
BnO
BnO
COOH
COOH
BnO
BnO
COOH
+
COOH
BnO
Bn
BnO
Bn
BnO
Bn
OBn
OBn
OBn
4
-
rac
4
)-
(a
S
4
R)-
(a
Scheme 1. Synthesis and the separation of rac-4 into its enantiomerically pure atropisomers (aR)-4 and (aS)-4.
Figure 1. Molecular structure of rac-3 by X-ray analysis. The hydrogen atoms are omitted for clarity.
then condensed with one of the commercially available
amines 6 or 7 in situ to furnish the corresponding b-
hydroxylamides 10–13. With these compounds in hand,
we now were able to synthesize four novel chiral ligands
14–17 by mesylation of their hydroxyl groups and sponta-
neous cyclization (Scheme 3).
To investigate the catalytic potential of these chiral ligands,
we decided to run a series of copper(I)-catalyzed stereo-
selective cyclopropanations of styrene 18 with ethyl diazo-
acetate 19 (Scheme 4). As a result, freshly prepared chiral
copper complex II was applied to the stereoselective pro-
cess (Scheme 4). Principally, four stereoisomers [two pairs

2806
K. Khanbabaee et al. / Tetrahedron: Asymmetry 17 (2006) 2804–2812
OBn
OBn
OH
Bn
BnO
Bn
BnO
O
1) SOCl2
MsCl,
Et3N
BnO
N
5
2)
BnO
BnO
CO NH
CO NH
N
OBn
(aR )-4
O
BnO
Bn
Bn
OBn
OH
OBn
OBn
(aR)-9
(aR)-8
Scheme 2. Synthesis of the chiral ligand (aR)-9.
rac-8
Figure 2. Molecular structure of rac-8 by X-ray analysis.
of enantiomers trans-(1S,2S)-20/trans-(1R,2R)-20 and
cis-(1S,2R)-20/cis-(1R,2S)-20] could be formed from this
reaction.
pairs from this reaction, either trans-(1S,2S)-20/cis-
(1S,2R)-20 or trans-(1R,2R)-20/cis-(1R,2S)-20 (Scheme 4).
In this context, we ran the cyclopropanation reaction of
stryrene 18 with ethyl diazoacetate 19 in the presence of
our new chiral ligands 9, 14–17 and analyzed the
products obtained by gas chromatography (GC) on a chi-
ral column. It should be noted, that we used 4–10 mol %
We expected that the application of enantiomeric ligands I
in this reaction would lead favourably to the enantioselec-
tive formation of one of the two possible diastereomeric

K. Khanbabaee et al. / Tetrahedron: Asymmetry 17 (2006) 2804–2812
OBn
2807
OBn
OH
R1
OH
R1
Bn
BnO
Bn
BnO
1) SOCl2
2) 6 or 7
R2
R2
BnO
BnO
BnO
BnO
CO NH
CO NH
CO NH
CO NH
rac-4
+
R2
R1
R2
R1
Bn
BnO
BnO
Bn
OH
OH
OBn
OBn
HO
R1
R2
R1 = Me, R2 = Me;
R1 =
R1 =
i
-Pr, R2 = H; (a
S
,
,
S
,
S
)-10
)-11
R1 =
R1 =
i
-Pr, R2 = H; (a
R
,
,
S
,
S
)-12
)-13
t
-Bu, R2 = H; (a
S
S
,
S
t
-Bu, R2 = H; (a
R
S
,
S
H2N
5
)-6
)-7
MesCl,
Et3N
MesCl,
Et3N
R1 =
R1 =
i
-Pr, R2 = H; (
-Bu, R2 = H; (
S
S
t
OBn
OBn
BnO
Bn
BnO
Bn
O
O
BnO
N
BnO
N
R2
R2
R1
R1
R2
R1
N
OBn
N
OBn
R1
R2
O
O
Bn
OBn
Bn
OBn
OBn
OBn
R1
=
i
-Pr, R2 = H; (a
S
,
S
,
S
)-14
)-15
R1 =
R1 =
i
-Pr, R2 = H; (a
R
,
S
,
S
)-16
R1 =
t
-Bu, R2 = H; (a
S
,
S
,
S
t
-Bu, R2 = H; (a
R
,
S
,
S
)-17
Scheme 3. Synthesis of b-hydroxylamides 10–13 and chiral ligands 14–17.
OBn
OBn
Bn
BnO
Bn
O
BnO
O
R2
R1
R2
R1
chiral biphenyl
copper(I)
complex
BnO
N
BnO
N
Cu(I)OTf
N
OBn
R1
R2
OTf
R1
R2
N
OBn
Cu
O
O
Bn
OBn
Bn
OBn
OBn
OBn
II
I
3
N2HC
2
1
O
OEt
OEt
Complex II
+
O
+
-N2
O
O
cis-(1R,2S)-20
18
trans-(1R,2R)-20
19
Scheme 4. Enantioselective cyclopropanation by using a chiral biphenyl copper(I)-complex.
chiral ligand in each run. To ensure a complete consump-
tion of ethyl diazoacetate 19, stryrene 18 was generally used
in excess (2.5 equiv). Before the product composition was
analyzed by GC, the metal components were separated
by flash chromatography on silica gel. The results of these
reactions are listed in Table 1, which shows that most of
the ligands have an influence on the stereochemical course
of the reaction. As expected, ligands (aS,S,S)-14 preferen-
tially gave rise to the formation of a mixture of the diaste-
reomeric compounds trans-(1R,2R)-20/cis-(1R,2S)-20. The
highest enantioselectivity [83% ee, cis-(1R,2S)-20] was
provided by the chiral ligand (aS,S,S)-15, having bulky
tert-butyl groups in the neighbourhood of the N-atoms
of the catalyst, whereas its diastereomeric counterpart
(aR,S,S)-17 was inactive in the same process at room tem-
perature. This result is similar to the result in the same
asymmetric process [for cyclopropane cis-(1R,2S)-20, 86%
ee] by using a chiral BINAP ligand.4
To study the ability of axially chiral ligands without a car-
bon centred chirality, we next synthesized ligand (aR)-9
with two methyl groups in the neighbourhood of the
N-atoms of the ligands (R1 = R2 = Me). The application
of ligand (aR)-9 in the same catalytic process preferentially
led to the stereoselective formation of cis-(1S,2R)-20 with
61% ee. It should also be noted that not only ligand

2808
K. Khanbabaee et al. / Tetrahedron: Asymmetry 17 (2006) 2804–2812
5.5 (s, 4H, OCH2Ph), 7.16–7.53 (m, 30Ar–H). 13C NMR
(CDCl3): d (ppm) = 32.18 (t, RCH2Ph), 76.06, 76.17 (t,
OCH2Ph), 111.67 (s, biaryl-C-1, biaryl-C-10), 116.28 (s,
biaryl-C-6, biaryl-C-60), 125.85, 127.69, 128.21, 128.39,
128.48, 128.51, 128.53, 128.65, 128.76, 128.85 (d, Ar–C),
135.96 (s, biaryl-C-5, biaryl-C-50), 136.35, 136.69, 140.33,
141.16 (s, biaryl-C-3, biaryl-C-30), 144.27 (s, biaryl-C-2,
biaryl-C-20), 152.64 (s, biaryl-C-4, biaryl-C-40), 157.21 (s,
C–lactone). IR (KBr): m (cmꢀ1): 3064, 3032, 2934, 2874,
1748, 1253, 1096. C56H42O8 (842.928): calcd C, 79.79; H,
5.02; found C, 79.39; H, 4.90.
Table 1. Enantioselective cyclopropanation of styrene 18 with ethyl
diazoacetate 19 catalyzed by transition-metal complexes IIa
Ligand
Yield
(%)b
Ratioc of trans/cis
% eec
(config)d
trans
cis
(aR)-9
43
38/62
45/55
49/51
54/46
—
56 (1S,2S) 61 (1S,2R)
10 (1R,2R) 56 (1R,2S)
60 (1R,2R) 83 (1R,2S)
54 (1R,2R) 28 (1R,2S)
(aS,S,S)-14 44
(aS,S,S)-15 44
(aR,S,S)-16 42
(aR,S,S)-17 41
—
—
a Styrene (4.7 mmol), ethyl diazoacetate (1.9 mmol), [Cu(I)OTf(C6H6)0.5
]
(1 mol %), ligand (4–10 mol %) in chloroform (4 ml) at room temperature
for 24 h.
b Isolated yield of a mixture of trans- and cis-20.
c Determined by GC with FS-Hydrodex-b-3P.
d By comparison of their specific rotation with the one reported for trans-
and cis-20.
3.1.2. Dibenzyl 2,20,3,30,4,40-hexabenzyloxy-5,50-dibenzyl-
1,10-diphenyl-6,60-dicarboxylate rac-3. A stirred suspen-
sion of 3,30,4,40-tetra-O-benzyl-5,50-di-C-benzylellagic acid
2 (5.0 g, 5.93 mmol), potassium carbonate (10 g, 0.075 mol)
and benzyl chloride (50 ml, 0.43 mol) were refluxed for 1 h.
At ca. 100 °C, the reaction mixture suddenly became red.
The crude product was extracted with dichloromethane
and dried over Na2SO4. After filtration of Na2SO4, the sol-
vent was first evaporated. After the evaporation of excess
BnCl under a high vacuum, the crude product was
obtained, which was recrystallized from ethanol to give
the racemic mixture rac-3 as colourless crystals (5.1 g,
70%), mp 108 °C. 1H NMR (CDCl3): d (ppm) = 4.18–
4.25 (dd, 4H, J = 15.4 Hz, J = 20.7 Hz, CH2Ph, ester),
4.73–4.99 (m, 14H, OCH2Ph, RCH2Ph), 5.01 (d, 2H,
J = 10.4 Hz, OCH2Ph), 6.92–7.35 (m, 50 Ar–H). 13C
NMR (CDCl3): d (ppm) = 32.98 (t, RCH2Ph), 67.12 (t,
CH2Ph), 73.82, 75.45, 75.49 (t, OCH2Ph), 125.72 (d, Ar–
C), 126.8 (s, biaryl-C-1, biaryl-C-10), 127.05, 127.23,
127.97, 128.0, 128.04, 128.07, 128.26, 128.28, 128.32,
128.36, 128.4, 128.71, 128.76 (d, Ar–C), 129.16 (s, biaryl-
C-6, biaryl-C-60), 129.68 (s, biaryl-C-5, biaryl-C-50),
135.18, 137.2, 137.25, 138.16, 141.02 (s, Ar–C), 147.6 (s,
biaryl-C-3, biaryl-C-30), 150.32 (s, biaryl-C-2, biaryl-C-
20), 151.51 (s, biaryl-C-4, biaryl-C-40), 167.37 (s, COOBn).
IR (KBr): m (cmꢀ1): 3065, 3035, 2945, 2884, 1752, 1255,
1195. MS (FAB/NBA): m/z (%) = 1240.4 (3) [M]+.
C84H70O10 (1239.5): calcd C, 81.4; H, 5.69; found C,
81.35; H, 5.29.
(aR)-9, but also ligands (aS,S,S)-14 and (aS,S,S)-15 in the
same catalytic process led preferentially to the stereoselec-
tive formation of cis- rather than trans-cyclopropanes,
although with different absolute stereochemistry (Table 1).
The specific rotations for the two stereoisomers 20 are
reported as follows: cis-(1S,2R)-20 (99% ee): [a]D = +18.6
(c 1.01 g/100 ml, CHCl3), trans-(1S,2S)-20 (99% ee):
[a]D = +296 (c 0.88 g/100 ml, CHCl3).5
3. Experimental
3.1. General remarks
All solvents were dried and purified by standard literature
methods prior to use. Melting points were determined on a
¨
BUCHI SMP-20 apparatus. IR spectra (film or KBr) were
measured with a FT-IR-Spectrometer NICOLET 510 P
instrument. Mass spectra were recorded with a FINNI-
GAN MAT 8200 apparatus. Elemental analysis was car-
ried out by the Perkin–Elmer Elemental Analyser 2400,
Reactions were monitored with Merck TLC aluminium
sheets (Kieselgel 60 F254) and preparative chromatography
was carried out with silica gel 60 (70–230 mesh ASTM). 1H
NMR (500 MHz) and 13C NMR (125 MHz) spectra were
acquired on a Bruker AMX 500 spectrometer in CDCl3,
with TMS as the internal standard.
3.1.3. rac-2,20,3,30,4,40-Hexabenzyloxy-5,50-dibenzyl-6,60-
diphenic acid 4. Dibenzylester 3 (5.0 g, 4.0 mmol) was dis-
solved in ethanol under heating and treated with a satu-
rated solution of potassium hydroxide in water (60 ml).
The reaction mixture was then stirred at reflux for 15 h.
Approximately 2/3 of the solvent was removed and the res-
idue was treated with 3 N HCl at 0 °C. The reaction mix-
ture was then diluted with water and the product was
extracted with dichloromethane, dried over Na2SO4. After
filtration of Na2SO4, the solvent was evaporated to give
racemic mixture rac-4 as a yellow solid (4.2 g, 98%), mp
109 °C. 1H NMR (CDCl3): d (ppm) = 4.06 (s, 4H,
RCH2Ph), 4.62–5.04 (m, 12H, OCH2Ph), 6.85–7.34 (m,
40H, Ar–H). 13C NMR (CDCl3): d (ppm) = 33.66 (t,
RCH2Ph), 74.54, 75.04, 75.27 (t, OCH2Ph), 124.5 (s, bia-
ryl-C-1, biaryl-C-10), 125.62, 126.62, 126.98 (d, Ar–C),
127.14 (s, biaryl-C-6, biaryl-C-60), 127.58, 127.81, 127.98,
128.09, 128.14, 128.22, 128.26, 128.34, 128.54, 128.67 (d,
Ar–C), 137.14, 137.34, 137.87, 141.66 (s, Ar–C), 145.95
(s, biaryl-C-3, biaryl-C-30), 149.26 (s, biaryl-C-2, biaryl-C-
3.1.1. 3,30,4,40-Tetra-O-benzyl-5,50-di-C-benzylellagic acid
2. A stirred suspension of ellagic acid (10 g, 0.033 mol),
potassium carbonate (150 g, 1.125 mol) and benzyl chlo-
ride (250 ml, 2.14 mol) in water (430 ml) were refluxed for
15 h. The reaction mixture was then cooled down to room
temperature. The organic layer were separated and the
products extracted from the organic layer with dichloro-
methane and dried over Na2SO4. After filtration of
Na2SO4, the solvent was evaporated and the crude product
was obtained as a dark red oil. Overnight, colourless nee-
dles were formed from this oil, which were separated and
washed with acetone and dried in vacuum to give colour-
1
less needles (8.36 g, 30%), mp 239 °C. H NMR (CDCl3):
d (ppm) = 4.78 (s, 4H, RCH2Ph), 5.02 (s, 4H, OCH2Ph),

K. Khanbabaee et al. / Tetrahedron: Asymmetry 17 (2006) 2804–2812
2809
20), 151.4 (s, biaryl-C-4, biaryl-C-40), 174.08 (s, COOH). IR
(KBr): m (cmꢀ1): 3607, 3431, 3059, 3025, 2938, 2873, 1686,
1561, 1364, 1093. MS (FAB/NBA): m/z (%) = 1059.8 (27)
[M+]. C70H58O10 (1059.204): calcd C, 79.38; H, 5.52; found
C, 75.43; H, 5.23.
aryl-C-6, biaryl-C-60), 128.06 (s, biaryl-C-5, biaryl-C-50),
128.15, 128.22, 128.27, 128.34, 128.39, 128.46, 128.48 (d,
Ar–C), 135.11, 136.91, 136.98, 137.88 (s, Ar–C), 140.69
(s, biaryl-C-3, biaryl-C-30), 146.82 (s, biaryl-C-2, biaryl-C-
20), 152.34 (s, biaryl-C-4, biaryl-C-40), 169.32 (s, CONR).
IR (KBr): m (cmꢀ1): 3405, 3215, 3053, 2960, 1645, 1253.
MS (FAB/NBA): m/z (%) = 1202.2 (50) [M+]. C78H76-
N2O10 (1201.445): calcd C, 77.98; H, 6.38; N, 2.33; found
C, 77.99; H, 6.24; N, 2.19.
3.1.4. (aR)-2,20,3,30,4,40-Hexabenzyloxy-5,50-dibenzyl-6,60-
diphenic acid 4. A suspension of rac-4 (5.0 g, 4.0 mmol)
and cinchonin (2.95 g, 10.0 mmol) in dried ethanol
(150 ml) were stirred at reflux, until a clear solution were
formed. After cooling down the reaction mixture to room
temperature, colourless needles were obtained after 12 h,
which were then separated by filtration. After recrystalliza-
tion of the separated crystals from dry ethanol, the (aR)-
diastereomeric salt was obtained in a stereochemically pure
form. The hydrolysis of the obtained (aR)-diastereomeric
salt with 1 M H2SO4 at 0 °C followed by the extraction
of the resulting product with dichloromethane gave (aR)-
2,20,3,30,4,40-hexabenzyloxy-5,50-dibenzyl-6,60-diphenic acid
4 as a yellow solid (2 g, 47%). [a]D = ꢀ66 (c 1 g/100 ml,
CHCl3).
3.2.2. (aS)-2,20,3,30,4,40-Hexabenzyloxy-5,50-dibenzyl-6,60-
bis[N-(1S)-(1-iso-propyl-2-hydroxyethyl)carboxamido]-1,10-
biphenyl 10. This compound was prepared by general
procedure I from rac-4 (0.5 g, 0.47 mmol), thionyl chloride
(300 ll, 4.14 mmol) and chiral amine (S)-6 to give, after
chromatographic purification, (aS)-2,20,3,30,4,40-hexabenz-
yloxy-5,50-dibenzyl-6,60-bis[N-(1S)-(1-iso-propyl-2-hydroxy-
ethyl)carboxamido]-1,10-biphenyl 10 as a yellow solid
(0.26 g, 45%), mp 51 °C. [a]D = +1.5 (c 1 g/100 ml, CHCl3).
1H NMR (CDCl3):
d (ppm) = 0.58–0.69 (dd, 12H,
J = 16 Hz, J = 6.7 Hz, CH3), 1.49–1.66 (m, 2H, i-Pr),
2.46 (2H, ROH), 3.28–3.39 (m, 4H, H-2), 3.45–3.57 (m,
2H, H-1), 4.21 (d, 2H, J = 15.8 Hz, RCH2Ph), 4.35 (d,
2H, J = 15.8 Hz, RCH2Ph), 4.96–5.28 (m, 12H, OCH2Ph),
3.2. General procedure I for the preparation of
b-hydroxylamides
7.11–7.55 (m, 40H, Ar–H). 13C NMR (CDCl3):
d
A solution of biaryldicarboxylic acid and Et3N in dried
dichloromethane was treated with thionyl chloride at
0 °C and the solution was then stirred for 3 h at room
temperature. Both, the solvent and the excess of thionyl
chloride evaporated at room temperature, first dichloro-
methane on a rotary evaporator and then the excess of
thionyl chloride on an oil pump at a high vacuum. The
resulting dichloride was resolved in dried dichloromethane
and treated dropwise with a solution of the chiral amine in
dried dichloromethane at 0 °C.
(ppm) = 19.17, 19.43 (q, CH3), 29.21 (d, C–i-Pr), 33.8 (t,
RCH2Ph), 58.68 (d, C-1), 63.98 (t, C-2), 75.55, 75.58 (t,
OCH2Ph), 124.57 (s, biaryl-C-1, biaryl-C-10), 126.42,
128.17, 128.28, 128.47, 128.67, 128.8, 128.87, 128.96 (d,
Ar–C), 137.34, 137.47, 138.1, 141.23 (s, Ar–C), 147.28 (s,
biaryl-C-3, biaryl-C-30), 149.63 (s, biaryl-C-2, biaryl-C-
20), 152.69 (s, biaryl-C-4, biaryl-C-40), 169.98 (CONR).
C80H80N2O10 (1229.499): calcd C, 78.15; H, 6.56; N, 2.28;
found C, 77.62; H, 6.25; N, 2.14.
After adding Et3N in dried dichloromethane, the reaction
mixture was stirred for 5 h at room temperature. The reac-
tion mixture was diluted with water and the product was
extracted three times with dichloromethane. The organic
phases were then washed once with a saturated solution
of NaCl in water, dried over Na2SO4. After filtration, the
solvent was evaporated and the resulting product was puri-
fied by chromatography on silica gel.
3.2.3. (aS)-2,20,3,30,4,40-Hexabenzyloxy-5,50-dibenzyl-6,60-
bis[N-(1S)-(1-tert-butyl-2-hydroxyethyl)carboxamido]-1,10-
biphenyl 11. This compound was prepared by general
procedure I from rac-4 (1.0 g, 0.94 mmol), thionyl chloride
(300 ll, 4.14 mmol) and chiral amine (S)-7 to give, after
chromatographic purification, (aS)-2,20,3,30,4,40-hexabenz-
yloxy-5,50-dibenzyl-6,60-bis[N-(1S)-(1-tert-butyl-2-hydroxy-
ethyl)carboxamido]-1,10-biphenyl 11 as a yellow solid
(0.53 g, 45%), mp 58 °C. [a]D = +16.5 (c 1 g/100 ml,
CHCl3). 1H NMR (CDCl3): d (ppm) = 0.59 (s, 18H,
CH3), 3.34–3.37 (dd, 2H, J = 6.3 Hz, J = 11.8 Hz, H-2),
3.44–3.47 (dd, 2H, J = 3.1 Hz, J = 11.8 Hz, H-2), 3.63–
3.67 (m, 2H, H-1), 4.12 (d, 2H, J = 16.1 Hz, RCH2Ph),
4.31 (d, 2H, J = 16.1 Hz, RCH2Ph), 4.93–4.99 (m, 6H,
OCH2Ph), 5.12–5.17 (m, 6H, OCH2Ph), 7.1–7.36 (m,
40H, Ar–H). 13C NMR (CDCl3): d (ppm) = 26.7 (q,
CH3), 33.61 (t, RCH2Ph), 60.92 (d, C-1), 62.95 (t, C-2),
74.83, 75.17, 75.32 (t, OCH2Ph), 124.11 (s, biaryl-C-1, bi-
aryl-C-10), 125.96, 127.78, 127.94, 127.98, 128.02, 128.18,
128.32, 128.34, 128.48 (d, Ar–C), 136.04, 136.92, 137.07,
137.66 (s, Ar–C), 140.81 (s, biaryl-C-3, biaryl-C-30), 146.8
(s, biaryl-C-2, biaryl-C-20), 152.27 (s, biaryl-C-4, biaryl-C-
40), 169.66 (s, CONR). IR (KBr): m (cmꢀ1): 3405, 3215,
3053, 2960, 1645, 1253. MS (FAB/NBA): m/z
(%) = 1258.5 (5) [M+]. C82H84N2O10 (1257.552): calcd C,
78.32; H, 6.73; N, 2.23; found C, 77.99; H, 6.73; N, 2.16.
3.2.1. (aR)-2,20,3,30,4,40-Hexabenzyloxy-5,50-dibenzyl-6,60-
bis[N-(1,1-dimethyl-2-hydroxyethyl)carboxamido]-1,10-bi-
phenyl 8. This compound was prepared by general proce-
dure I from (aR)-4 (0.5 g, 0.47 mmol) and thionyl chloride
(300 ll, 4.14 mmol) and amine 5 to give after chromato-
graphic purification, (aR)-2,20,3,30,4,40-hexabenzyloxy-
5,50-dibenzyl-6,60-bis[N-(1,1-dimethyl-2-hydroxyethyl)carb-
oxamido]-1,10-biphenyl 8 as a yellow solid (0.51 g, 90%),
1
mp 53 °C. [a]D = ꢀ19.6 (c 1 g/100 ml, CHCl3). H NMR
(CDCl3): d (ppm) = 0.82 (s, 9H, CH3), 0.86 (s, 3H, CH3),
3.17–3.24 (dd, 4H, J = 11.6 Hz, J = 18.2 Hz, H-2), 4.1 (d,
2H, J = 15.7 Hz, RCH2Ph), 4.32 (d, 2H, J = 15.7 Hz,
RCH2Ph), 4.95–5.19 (m, 12H, OCH2Ph), 7.01–7.38 (m,
40H, Ar–H). 13C NMR (CDCl3): d (ppm) = 23.31, 23.51
(q, CH3), 32.69 (t, RCH2Ph), 56.64 (s, C-1), 70.38 (C-2),
75.05, 75.48, 75.66 (t, OCH2Ph), 124.08 (s, biaryl-C-1, bi-
aryl-C-10), 125.89, 127.52, 127.71 (d, Ar–C), 127.79 (s, bi-

2810
K. Khanbabaee et al. / Tetrahedron: Asymmetry 17 (2006) 2804–2812
3.2.4. (aR)-2,20,3,30,4,40-Hexabenzyloxy-5,50-dibenzyl-6,60-
bis[N-(1S)-(1-iso-propyl-2-hydroxyethyl)carboxamido]-1,10-
biphenyl 12. This compound was prepared by general
procedure I from rac-4 (0.5 g, 0.47 mmol), thionyl chloride
(300 ll, 4.14 mmol) and chiral amine (S)-6 to give after
chromatographic purification (aR)-2,20,3,30,4,40-hexabenz-
yloxy-5,50-dibenzyl-6,60-bis[N-(1S)-(1-iso-propyl-2-hydroxy-
ethyl)carboxamido]-1,10-biphenyl 12 as a yellow oil (0.26 g,
45%). [a]D = ꢀ45.8 (c 1 g/100 ml, CHCl3). 1H NMR
and the resulting products were purified by chromato-
graphy on silica gel.
3.3.1. (aR)-2,20,3,30,4,40-Hexabenzyloxy-5,50-dibenzyl-6,60-
bis(4,4-dimethyloxazolin-2-yl)-1,10-biphenyl 9. This com-
pound was prepared by general procedure II from (aR)-8
(0.1 g, 0.083 mmol) and mesyl chloride (200 ml, 2.1 mmol)
and Et3N (3 ml). After chromatographic purification, the
(aR)-2,20,3,30,4,40-hexabenzyloxy-5,50-dibenzyl-6,60-bis(4,4-
dimethyloxazolin-2-yl)-1,10-biphenyl 9 was obtained as a
yellow oil (0.087 g, 90%). [a]D = +21.7 (c 1 g/100 ml,
(CDCl3):
d (ppm) = 0.58–0.69 (dd, 12H, J = 16 Hz,
J = 6.7 Hz, CH3), 1.49–1.66 (m, 2H, i-Pr), 2.46 (2H,
ROH), 3.28–3.39 (m, 4H, H-2), 3.45–3.57 (m, 2H, H-1),
4.21 (d, 2H, J = 15.8 Hz, RCH2Ph), 4.35 (d, 2H,
J = 15.8 Hz, RCH2Ph), 4.96–5.28 (m, 12H, OCH2Ph),
1
CHCl3). H NMR (CDCl3): d (ppm) = 0.92 (s, 6H, CH3),
1.04 (s, 6H, CH3), 3.58 (d, 2H, J = 7.7 Hz, H-5), 3.79 (d,
2H, J = 7.7 Hz, H-5), 4.28 (2H, J = 15.4 Hz, RCH2Ph),
4.41 (d, 2H, J = 15.4 Hz, RCH2Ph), 4.94–5.01 (m, 6H,
OCH2Ph), 5.05 (d, 2H, J = 10.7 Hz, OCH2Ph), 5.22 (d,
2H, J = 11.2 Hz, OCH2Ph), 5.42 (d, 2H, J = 11.2 Hz,
OCH2Ph), 7.06–7.42 (m, 40H, Ar–H). 13C NMR (CDCl3):
d (ppm) = 27.71, 27.88 (q, CH3), 33.09 (t, RCH2Ph), 67.52
(s, C-4), 73.72, 75.52, 75.69 (t, OCH2Ph), 78.07 (t, C-5),
125.58 (s, biaryl-C-1, biaryl-C-10), 125.43, 127.08, 127.38,
127.95, 127.96, 128.1, 128.31, 128.36, 128.55, 128.6,
128.67, 128.89, 129.42 (d, Ar–C), 137.45, 137.52, 138.86,
141.29 (s, Ar–C), 147.15 (s, biaryl-C-3, biaryl-C-30),
150.73 (s, biaryl-C-2, biaryl-C-20), 151.42 (s, biaryl-C-4,
biaryl-C-40), 160.11 (s, CNO). IR (KBr): m (cmꢀ1): 3058,
2975, 1650, 1356, 1098. C78H72N2O8 (1165.415): calcd C,
80.39; H, 6.23; N, 2.40; found C, 79.66; H, 6.12; N, 2.26.
7.11–7.55 (m, 40H, Ar–H). 13C NMR (CDCl3):
d
(ppm) = 19.17, 19.43 (q, CH3), 29.21 (d, C–i-Pr), 33.8 (t,
RCH2Ph), 58.68 (d, C-1), 63.98 (t, C-2), 75.55, 75.58 (t,
OCH2Ph), 124.57 (s, biaryl-C-1, biaryl-C-10), 126.42,
128.17, 128.28, 128.47, 128.67, 128.8, 128.87, 128.96 (d,
Ar–C), 137.34, 137.47, 138.1, 141.23 (s, Ar–C), 147.28 (s,
biaryl-C-3, biaryl-C-30), 149.63 (s, biaryl-C-2, biaryl-C-
20), 152.69 (s, biaryl-C-4, biaryl-C-40), 169.98 (CONR).
C80H80N2O10 (1229.499): calcd C, 78.15; H, 6.56; N, 2.28;
found C, 77.62; H, 6.25; N, 2.14.
3.2.5. (aR)-2,20,3,30,4,40-Hexabenzyloxy-5,50-dibenzyl-6,60-
bis[N-(1S)-(1-tert-butyl-2-hydroxyethyl)carboxamido]-1,10-
biphenyl 13. This compound was prepared by general
procedure I from rac-4 (1.0 g, 0.94 mmol), thionyl chloride
(300 ll, 4.14 mmol) and chiral amine (S)-7 to give after
chromatographic purification (aR)-2,20,3,30,4,40-hexabenz-
yloxy-5,50-dibenzyl-6,60-bis[N-(1S)-(1-tert-butyl-2-hydro-
xy-ethyl)carboxamido]-1,10-biphenyl 13 as a yellow solid
(0.53 g, 45%), mp 57 °C. [a]D = ꢀ57 (c 1 g/100 ml, CHCl3).
1H NMR (CDCl3): d (ppm) = 0.59 (s, 18H, CH3), 3.34–
3.37 (dd, 2H, J = 6.3 Hz, J = 11.8 Hz, H-2), 3.44–3.47
(dd, 2H, J = 3.1 Hz, J = 11.8 Hz, H-2), 3.63–3.67 (m, 2H,
H-1), 4.12 (d, 2H, J = 16.1 Hz, RCH2Ph), 4.31 (d, 2H,
J = 16.1 Hz, RCH2Ph), 4.93–4.99 (m, 6H, OCH2Ph),
5.12–5.17 (m, 6H, OCH2Ph), 7.1–7.36 (m, 40H, Ar–H).
13C NMR (CDCl3): d (ppm) = 26.7 (q, CH3), 33.61 (t,
RCH2Ph), 60.92 (d, C-1), 62.95 (t, C-2), 74.83, 75.17,
75.32 (t, OCH2Ph), 124.11 (s, biaryl-C-1, biaryl-C-10),
125.96, 127.78, 127.94, 127.98, 128.02, 128.18, 128.32,
128.34, 128.48 (d, Ar–C), 136.04, 136.92, 137.07, 137.66
(s, Ar–C), 140.81 (s, biaryl-C-3, biaryl-C-30), 146.8 (s, bi-
aryl-C-2, biaryl-C-20), 152.27 (s, biaryl-C-4, biaryl-C-40),
169.66 (s, CONR). IR (KBr): m (cmꢀ1): 3405, 3215, 3053,
2960, 1645, 1253. MS (FAB/NBA): m/z (%) = 1258.5 (5)
[M+]. C82H84N2O10 (1257.552): calcd C, 78.32; H, 6.73;
N, 2.23; found C, 77.72; H, 6.63; N, 2.09.
3.3.2. (aS)-2,20,3,30,4,40-Hexabenzyloxy-5,50-dibenzyl-6,60-
bis[(4S)-(iso-propyloxazolin-2-yl)]-1,10-biphenyl 14. This
compound was prepared by general procedure II from
(aS,S,S)-10 (0.15 g, 0.122 mmol), mesyl chloride (250 ll,
2.6 mmol) and Et3N (4 ml). After chromatographic purifi-
cation, the (aS)-2,20,3,30,4,40-hexabenzyloxy-5,50-dibenzyl-
6,60-bis[(4S)-(iso-propyloxazolin-2-yl)]-1,10-biphenyl
14
was obtained as a yellow oil (0.138 g, 95%). [a]D = ꢀ51 (c
1 g/100 ml, CHCl3). 1H NMR (CDCl3): d (ppm) = 0.77
(d, 6H, J = 6.6 Hz, CH3), 0.86 (d, 6H, J = 6.6 Hz, CH3),
1.44–1.46 (m, 2H, i-Pr), 3.55–3.62 (m, 4H, H-5), 4.04–
4.09 (m, 2H, H-4), 4.21 (d, 2H, J = 15.3 Hz, RCH2Ph),
4.31 (d, 2H, J = 15.3 Hz, RCH2Ph), 4.9 (d, 2H,
J = 10.6 Hz, OCH2Ph), 4.96–4.99 (dd, 4H, J = 4 Hz,
J = 10.6 Hz, OCH2Ph), 5.1 (d, 2H, J = 10.8 Hz, OCH2Ph),
5.2 (d, 2H, J = 11.3 Hz, OCH2Ph), 5.35 (d, 2H,
J = 11.3 Hz, OCH2Ph), 7.08–7.43 (m, 40H, Ar–H). 13C
NMR (CDCl3): d (ppm) = 18.93, 19.53 (q, CH3), 32.75
(d, C–i-Pr), 33.54 (t, RCH2Ph), 69.89 (t, C-5), 73.65 (d,
C-4), 73.78, 75.39, 75.52 (t, OCH2Ph), 125.67 (s, biaryl-
C-1, biaryl-C-10), 125.39, 127.12, 127.41, 127.84, 127.91,
127.96, 128.06, 128.18, 128.31, 128.35, 128.52, 128.57,
128.7 (d, Ar–C), 129.4, 129.68, 137.49, 137.55, 138.83,
141.69 (s, Ar–C), 147.14 (s, biaryl-C-3, biaryl-C-30),
150.62 (s, biaryl-C-2, biaryl-C-20), 151.09 (s, biaryl-C-4,
biaryl-C-40), 161.62 (s, CNO). MS (FAB/NBA): m/z
(%) = 1194.3 (55) [M+]. C80H76N2O8 (1193.468): calcd C,
80.51; H, 6.42; N, 2.35; found C, 79.33; H, 5.47; N, 2.27.
3.3. General procedure II for the preparation of chiral
ligands
A solution of carboxamido-1,10-biphenyl and Et3N in dried
dichloromethane was treated with mesyl chloride at 0 °C.
The reaction mixture was then warmed up to room temper-
ature and stirred for 3 h. After dilution of the reaction mix-
ture with water, the products were extracted three times
with dichloromethane. The organic phases were then dried
over Na2SO4. After filtration, the solvent was evaporated
3.3.3. (aS)-2,20,3,30,4,40-Hexabenzyloxy-5,50-dibenzyl-6,60-
bis[(4S)-(tert-butyloxazolin-2-yl)]-1,10-biphenyl 15. This
compound was prepared by general procedure II from
(aS,S,S)-11 (0.21 g, 0.17 mmol), mesyl chloride (250 ll,

K. Khanbabaee et al. / Tetrahedron: Asymmetry 17 (2006) 2804–2812
2811
2.6 mmol) and Et3N (4 ml). After chromatographic purifi-
cation, the (aS)-2,20,3,30,4,40-hexabenzyloxy-5,50-dibenzyl-
6,60-bis[(4S)-(tert-butyloxazolin-2-yl)]-1,10-biphenyl 15 was
obtained as a colourless solid (0.2 g, 98%), mp 105 °C.
J = 8.3 Hz, J = 10.1 Hz, H-4), 4.22 (d, 2H, J = 15.2 Hz,
RCH2Ph), 4.28 (d, 2H, J = 15.2 Hz, RCH2Ph), 4.84 (d,
2H, J = 10.6 Hz, OCH2Ph), 4.91 (d, 2H, J = 10.7 Hz,
OCH2Ph), 4.99 (d, 2H, J = 10.6 Hz, OCH2Ph), 5.05 (d,
2H, J = 10.7 Hz, OCH2Ph), 5.15 (d, 2H, J = 11 Hz,
OCH2Ph), 5.41 (d, 2H, J = 11 Hz, OCH2Ph), 6.99–7.38
(m, 40H, Ar–H). 13C NMR (CDCl3): d (ppm) = 26.26 (q,
CH3), 33.05 (s, C–t-Bu), 33.57 (t, RCH2Ph), 67.82 (t, C-
5), 73.75, 75.33, 75.65 (t, OCH2Ph), 76.71 (d, C-4),
125.43, 127.09, 127.52, 127.86, 127.91, 127.93, 128.2,
128.31, 128.65, 128.81 (d, Ar–C), 129.81, 137.45, 137.63,
138.64 (s, Ar–C), 141.23 (s, biaryl-C-3, biaryl-C-30),
150.78 (s, biaryl-C-2, biaryl-C-20), 151.53 (s, biaryl-C-4,
biaryl-C-40), 161.66 (s, CNO). MS (FAB/NBA):
m/z (%) = 1222.5 (40) [M+]. C82H80N2O8 (1221.521): calcd
C, 80.63; H, 6.60; N, 2.29; found C, 79.86; H, 6.49; N,
2.21.
1
[a]D = ꢀ58 (c 1 g/100 ml, CHCl3). H NMR (CDCl3): d
(ppm) = 0.7 (s, 18H, CH3), 3.62 (t, 2H, J = 9.8 Hz, H-5),
3.69 (t, 2H, J = 9.6 Hz, H-5), 3.96–3.99 (dd, 2H,
J = 8.1 Hz, J = 9.9 Hz, H-4), 4.15 (d, 2H, J = 15.4 Hz,
RCH2Ph), 4.27 (d, 2H, J = 15.4 Hz, RCH2Ph), 4.82 (d,
2H, J = 10.6 Hz, OCH2Ph), 4.92–4.95 (dd, 4H,
J = 2.4 Hz, J = 10.7 Hz, OCH2Ph), 5.07 (d, 2H, J =
10.7 Hz, OCH2Ph), 5.19 (d, 2H, J = 11.3 Hz, OCH2Ph),
5.35 (d, 2H, J = 11.3 Hz, OCH2Ph), 6.99–7.38 (m, 40H,
Ar–H). 13C NMR (CDCl3): d (ppm) = 25.99 (q, CH3),
33.04 (s, C–t-Bu), 33.54 (t, RCH2Ph), 67.56 (t, C-5),
73.72, 75.37, 75.59 (t, OCH2Ph), 76.93 (d, C-4), 125.74 (s,
biaryl-C-1, biaryl-C-10), 125.37, 127.05, 127.33, 127.85,
127.93, 128.26, 128.29, 128.32, 128.58 (d, Ar–C), 129.47,
129.54, 137.46, 137.56, 138.86, 141.59 (s, Ar–C), 147.15
(s, biaryl-C-3, biaryl-C-30), 150.72 (s, biaryl-C-2, biaryl-
C-20), 151.15 (s, biaryl-C-4, biaryl-C-40), 161.66 (s, CNO).
MS (FAB/NBA): m/z (%) = 1222.6 (70) [M+]. C82H80-
N2O8 (1221.521): calcd C, 80.63; H, 6.60; N, 2.29; found
C, 80.13; H, 6.06; N, 1.79.
3.4. General procedure III for the preparation of the cis- and
the trans-cyclopropanes
Cyclopropanes were prepared from styrene 18 (4.7 mmol),
ethyl diazoacetate 19 (1.9 mmol), [Cu(I)OTf(C6H6)0.5
]
(1 mol %) and chiral biarylligand (4–10 mol %) in chloro-
form (4 ml) at room temperature. The reaction mixture
was then stirred for 24 h at room temperature to give a
mixture of trans- and cis-20 as a yellow oil after purifica-
tion of the crude products on silica gel. The composition
of the obtained products was analyzed by GC on a chiral
column with FS-Hydrodex-b-3P.
3.3.4. (aR)-2,20,3,30,4,40-Hexabenzyloxy-5,50-dibenzyl-6,60-
bis[(4S)-(iso-propyloxazolin-2-yl)]-1,10-biphenyl 16. This
compound was prepared by general procedure II from
(aR,S,S)-12 (0.15 g, 0.122 mmol), mesyl chloride (250 ll,
2.6 mmol) and Et3N (4 ml). After chromatographic purifi-
cation,
6,60-bis[(4S)-(iso-propyloxazolin-2-yl)]-1,10-biphenyl
was obtained as a yellow oil (0.138 g, 95%). [a]D = +35.5
(c 1 g/100 ml, CHCl3). 1H NMR (CDCl3):
(aR)-2,20,3,30,4,40-hexabenzyloxy-5,50-dibenzyl-
16
3.4.1. Compound trans-20. 1H NMR (CDCl3):
d
d
(ppm) = 1.34 (t, J = 7.1 Hz, 3H, CH3), 1.3–1.38 (m, 1H,
H-3), 1.61–1.7 (m, 1H, H-3), 1.92–2.0 (m, 1H, H-1),
2.53–2.63 (m, 1H, H-2), 4.15–4.28 (dd, 2H, J = 7.1 Hz,
J = 14.3 Hz, CH2), 7.13–7.38 (m, 5H, Ar–H). 13C NMR
(CDCl3): d (ppm) = 14.71 (q, CH3), 17.51 (t, C-3), 24.63,
26.61 (d, C-1, C-2), 61.14 (t, CH2), 126.59, 126.9, 128.9
(d, Ar–C), 140.56 (s, Ar–C), 173.85 (s, COOEt).
(ppm) = 0.78–0.81 (t, 12H, CH3), 1.38–1.53 (m, 2H, i-Pr),
3.68–3.93 (m, 4H, H-5), 4.12–4.21 (m, 2H, H-4), 4.24–
4.44 (dd, 4H, J = 15 Hz, J = 23.7 Hz, RCH2Ph), 4.87–
5.15 (m, 8H, OCH2Ph), 5.22 (d, 2H, J = 10.9 Hz,
OCH2Ph), 5.45 (d, 2H, J = 10.9 Hz, OCH2Ph), 7.04–7.48
(m, 40H, Ar–H). 13C NMR (CDCl3): d (ppm) = 19.57,
20.2 (q, CH3), 33.6 (d, C–i-Pr), 34.07 (t, RCH2Ph), 70.43
(t, C-5), 73.87 (d, C-4), 74.25, 75.92, 76.05 (t, OCH2Ph),
126.21 (s, biaryl-C-1, biaryl-C-10), 125.98, 127.62, 128.06,
128.4, 128.68, 128.81, 129.09, 129.24 (d, Ar–C), 129.68,
130.19, 137.91, 138.01, 139.04, 141.82 (s, Ar–C), 147.87
(s, biaryl-C-3, biaryl-C-30), 151.21 (s, biaryl-C-2, biaryl-C-
20), 151.94 (s, biaryl-C-4, biaryl-C-40), 162.13 (s, CNO).
MS (FAB/NBA): m/z (%) = 1194.3 (55) [M+].
C80H76N2O8 (1193.468): calcd C, 80.51; H, 6.42; N, 2.35;
found C, 79.33; H, 5.47; N, 2.27.
3.4.2. Compound cis-20. 1H NMR (CDCl3):
d
(ppm) = 1.01 (t, J = 7.1 Hz, 3H, CH3), 1.31–1.42 (m, 1H,
H-3), 1.71–1.81 (m, 1H, H-3), 2.06–2.18 (m, 1H, H-1),
2.56–2.69 (m, 1H, H-2), 3.86–3.97 (dd, 2H, J = 7.1 Hz,
J = 14.3 Hz, CH2), 7.18–7.34 (m, 5H, Ar–H). 13C NMR
(CDCl3): d (ppm) = 11.5 (t, CH3), 14.42 (q, C-3), 22.22,
25.86 (d, C-1, C-2), 60.56 (t, CH2), 127.04, 128.28, 129.72
(d, Ar–C), 136.99 (s, Ar–C), 171.37 (s, COOEt).
3.3.5. (aR)-2,20,3,30,4,40-Hexabenzyloxy-5,50-dibenzyl-6,60-
bis[(4S)-(tert-butyloxazolin-2-yl)]-1,10-biphenyl 17. This
compound was prepared by general procedure II from
(aR,S,S)-13 (0.21 g, 0.17 mmol), mesyl chloride (250 ll,
2.6 mmol) and Et3N (4 ml). After chromatographic purifi-
Acknowledgements
We gratefully acknowledge financial support from the
Deutsche Forschungsgemeinschaft (DFG, Project No.
KH 21/3-1) and from the University of Paderborn. Spec-
troscopic measurements by PD Dr. Hans Egold (NMR),
and Dr. Heinz Weber (MS) are gratefully acknowledged.
Thanks are also due to Professor Karsten Krohn for his
valuable advice and fruitful discussions.
cation,
(aR)-2,20,3,30,4,40-hexabenzyloxy-5,50-dibenzyl-
17
6,60-bis[(4S)-(tert-butyloxazolin-2-yl)]-1,10-biphenyl
was obtained as a yellow solid (0.2 g, 98%), mp 49 °C.
1
[a]D = +36.5 (c 1 g/100 ml, CHCl3). H NMR (CDCl3): d
(ppm) = 0.7 (s, 18H, CH3), 3.7 (t, 2H, J = 10.3 Hz, H-5),
3.88 (t, 2H, J = 9.1 Hz, H-5), 4.02–4.06 (dd, 2H,

2812
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