Highly enantioselective borane reduction of prochiral ketones catalyzed by C3-symmetric tripodal β-hydroxy amides

Article abstract of DOI:10.1055/s-2006-941591

The asymmetric borane reduction of prochiral ketones with a series of easily constructed chiral C₃-symmetric tripodal tris(β- hydroxyamide) ligands was investigated. The borane complex of chiral ligand 1,1′,″,-(1,3,5-benzenetricarbonyl)-tris[(2S)-α, α-diphenyl-2-pyrrolidinemethanol] (1h) was found to be an efficient catalyst in asymmetric borane reduction of prochiral ketones and excellent enantioselectivities were obtained with both electron-deficient and electron-rich prochiral ketones (up to 97% ee). Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2006-941591

LETTER
1559
Highly Enantioselective Borane Reduction of Prochiral Ketones Catalyzed by
C3-Symmetric Tripodal b-Hydroxy Amides
C
-Symmetric
T
a
ripodal
b
-H
o
ydroxyAmide Fang, Jiaxi Xu, Da-Ming Du*
3
Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Bioorganic Chemistry and Molecular Engineering,
College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. of China
Fax +86(10)62751708; E-mail: dudm@pku.edu.cn
Received 30 January 2006
droxyamide ligands have also been demonstrated to be
Abstract: The asymmetric borane reduction of prochiral ketones
excellent catalysts for asymmetric alkynylation of alde-
with a series of easily constructed chiral C3-symmetric tripodal
tris(b-hydroxyamide) ligands was investigated. The borane com-
plex of chiral ligand 1,1¢,1¢¢-(1,3,5-benzenetricarbonyl)-tris[(2S)-
hydes.9 Herein, the synthesis of enantiomerically pure C3-
symmetric tripodal b-hydroxyamide ligands and their
a,a-diphenyl-2-pyrrolidinemethanol] (1h) was found to be an effi- application as chiral ligands in the catalytic enantioselec-
cient catalyst in asymmetric borane reduction of prochiral ketones
tive borane reduction of prochiral ketones were investi-
and excellent enantioselectivities were obtained with both electron-
gated. An easily handled, high efficiency method to obtain
deficient and electron-rich prochiral ketones (up to 97% ee).
highly optically active secondary alcohols was provided.
Key words: asymmetric reduction, borane, ketone, b-hydroxy-
A series of C3-symmetric tris(b-hydroxyamide) ligands
amide, C3 symmetry
were easily prepared from commercially available or eas-
ily synthesized optically pure amino alcohols in excellent
yields. A mixture of 1,3,5-benzenetricarboxylic acid,
Enantiomerically pure secondary alcohols are important
SOCl2 and several drops of DMF was refluxed to afford
intermediates or chiral building blocks for the synthesis of
the 1,3,5-benzenetricarboxylic chloride. After removal of
various functional organic compounds such as halides,
excess SOCl2 and without further purification, the trichlo-
esters, ethers, amines, and many biologically active com-
ride reacted with chiral amino alcohols10 in the presence
pounds.1 Thus, the enantioselective reduction of prochiral
of excess triethylamine in CH2Cl2 at reflux to afford
ketones to enantiomerically pure secondary alcohols has
tris(b-hydroxyamide)s 1a–h (Figure 1) in 70–96%
received considerable attention in recent decades and
yields.11 These ligands have great advantages because
there are many methods available for reducing a wide
they are easy to synthesize and purify.
variety of ketones with high enantioselectivities.2,3 Enan-
tioselective reduction of prochiral ketones by oxaza-
borolidines derived from chiral amino alcohols has been
recognized as one of the most efficient methods for ob-
taining enantiomerically pure secondary alcohols.4 It is
important to develop versatile chiral ligands which are
robust and easy to prepare so that efficient catalysts can be
screened to get excellent enantioselectivity.
The development of C3-symmetric ligands for asymmetric
catalysis and chiral recognition has also attracted attention
in recent years.5 However, to the best of our knowledge,
the use of C3-symmetric hydroxyamide ligands in borane
reduction of ketones has not been reported so far.6 Some
sulfonamido alcohols7 and phosphinamido alcohols8 have
been reported to be efficient catalysts for the enantioselec-
tive reduction of prochiral ketones. Thus, it should be of
interest to further investigate the catalytic ability of b-hy-
droxyamide derivatives. We expect that the cooperation
of amido group and hydroxy group in these ligands may
result in special catalytic activity. In our previous study,
we have reported pyridinylmethylpyrrolidinemethanol
derivatives as catalysts for the borane-mediated reduction
of prochiral ketones.3h Meanwhile, C3-symmetric b-hy-
SYNLETT 2006, No. 10, pp 1559–1563
Advanced online publication: 12.06.2006
2
1
.0
6
.2
0
0
6
Figure 1 C3-symmetric tris(b-hydroxyamide) ligands
DOI: 10.1055/s-2006-941591; Art ID: W02206ST
© Georg Thieme Verlag Stuttgart · New York

1560
T. Fang et al.
LETTER
In order to evaluate the efficiency of these ligands in the of acetophenone was further investigated at various reac-
catalytic asymmetric reaction, we carried out the enantio- tion conditions. When the reaction was carried out at room
selective reduction of prochiral ketones with borane. We temperature in THF, the reduction of acetophenone using
first examined the reduction of acetophenone using in situ 10 mol% catalyst prepared in situ from ligand 1h and
generated borane complex in the presence of 10 mol% BH3·SMe2 afford the corresponding (R)-1-phenylethanol
chiral ligands 1 in THF at 50 °C for one hour. The results in 92% ee. The enantiomeric excess obtained at room
are summarized in Table 1.
temperature was almost same as that at 50 °C. When the
reaction was carried out at refluxing temperature, no
distinct improvement in enantioselectivity, and the corre-
sponding (R)-1-phenylethanol was obtained in 96% ee
(Table 1, entry 13). The results suggest that this catalyst
can tolerate a range of temperature with no significant ef-
fect on the reaction. On the other hand, solvent also has an
effect on the enantioselectivity of the product, THF is bet-
ter than toluene and dichloromethane. In dichloromethane
and toluene, the enantioselectivities decreased slightly
(Table 1, entries 11 and 12). A decrease in the catalyst
loading from 10 mol% to 5 mol% at 50 °C resulted in no
decrease in the enantioselectivity (Table 1, entries 8 and
10). Further decrease in the catalyst loading from 5 mol%
to 1 mol% or 0.5 mol% at 50 °C resulted in a lower enan-
tioselectivity while no loss of yield (Table 1, entries 14
and 15). Catalyst loading of 5 mol% provided the opti-
mum level of enantioselectivity. At this loading the rate of
As shown in Table 1, ligand 1b gave only low enantio-
selectivity less than 5% ee (entry 2). Ligands 1a,d–g gave
only 30% to 40% ee despite different structure (Table 1,
entries 1, 4–7). A promising result was obtained with
ligand 1c, the corresponding chiral 1-phenylethanol was
obtained in 48% ee (Table 1, entry 3). With ligand 1h, the
best result was 95% ee and 91% yield (Table 1, entry 8).
As follows from these results ligand 1h containing a
diphenylprolinol moiety gave the best enantioselectivity.
In each case the (R)-1-phenylethanol was formed prefer-
entially except for the ligand 1c. The starting amino alco-
hols of ligands 1c and 1d are enantiomeric, they afford
secondary alcohols of opposite configuration.
The enantioselectivity of borane reduction may be affect-
ed by reaction conditions such as solvent, temperature and
catalyst loading.12 To examine these effects, the reduction
Table 1 Enantioselective Borane Reduction of Acetophenonea
ligand
O
OH
BH3·SMe2
∗
Ph
Me
Ph
Me
solvent
Entry
1
Ligand
Ligand loading (mol%) Solvent
Temp (°C)
Yield (%)b
80
ee (%)c
32
4
Configd
1a
1b
1c
1d
1e
1f
10
10
10
10
10
10
10
10
10
5
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
CH2Cl2
Toluene
THF
THF
THF
THF
50
R
R
S
2
50
80
3
50
86
48
38
42
38
29
95
92
94
86
90
96
83
74
11
4
50
84
R
R
R
R
R
R
R
R
R
R
R
R
R
5
50
82
6
50
85
7
1g
1h
1h
1h
1h
1h
1h
1h
1h
2
50
76
8
50
91
9
r.t.
50
95
10
11
12
13
14
15
16
96
5
Reflux
50
92
5
96
5
Reflux
50
95
1
93
0.5
10
50
94
50
92
a Reaction was carried out on a 0.5-mmol scale in 2 mL of THF for 1 h, molar ratio of acetophenone–BH3 = 1:1.2.
b Isolated yield by column chromatography.
c Determined by HPLC analysis using Daicel Chiralcel OB column.
d The absolute configuration was assigned by comparison of the sign of the specific rotation to the literature data.
Synlett 2006, No. 10, 1559–1563 © Thieme Stuttgart · New York

LETTER
C3-Symmetric Tripodal b-Hydroxy Amides
1561
the catalyzed reaction is sufficiently faster than the non- Table 2 Enantioselective Borane Reduction of Representative
Ketonesa
catalytic reduction with BH3·SMe2.12 As all factors con-
cerned, the best reaction conditions were chosen (Table 1,
entry 10).
5 mol% 1h
BH3·SMe2
OH
O
R
Me
Me
R
THF, 50 °C
In order to further understand the structural effect of new
C3-symmetric ligands on their catalytic activity, the corre-
sponding C3-symmetric tris(amino alcohol) ligand 2
(Figure 2) was synthesized by the borane reduction for 22
hours of the corresponding C3-symmetric tris(b-hy-
droxyamide) ligand 1h.13 In contrast, in the presence of 10
mol% tris(amino alcohol) ligand 2 at the same reaction
conditions as 1h, much lower enantioselectivity was
obtained (Table 1, entries 8 and 16). This result indicates
that the amido group is important for the catalyst enantio-
selectivity. The ligand 1h is less reactive and only less
than 20% of the amide group was reduced in situ by
borane to ligand 2 at the reaction conditions of ketone
reduction. The reduction of ketone is sufficiently faster
than the reduction of tris(b-hydroxyamide) ligand 1h with
BH3·SMe2. In a control experiment, the mixture of 4
mol% ligand 1h and 1 mol% ligand 2 catalyzed the borane
reduction of acetophenone with 93% ee and >99%
conversion, this means in situ reduction of ligand 1h to 2
partially has no effect on its catalytic activity. When
ligand 1h and BH3·SMe2 mixture was stirred at 50 °C for
30 min, a new peak at d = –13.72 ppm in 11B NMR spectra
(H3BO3 as external standard) was appeared, and this peak
Entry
R
Yield (%)b
ee (%)c,d
94
1
2
Ph
96
94
91
93
90
94
91
94
99e
94
4-FC6H4
97
3
4-ClC6H4
4-BrC6H4
4-CH3OC6H4
4-NO2C6H4
2-MeOC6H4
2-Naphthyl
t-Bu
97
4
96
5
91
6
97
7
87
8
95
9
89
10
3,5-NO2C6H4
74
a Reaction was carried out with 0.5-mmol scale in 2 mL of THF, molar
ratio of ketone–BH3 = 1:1.2.
b Isolated yield by column chromatography.
c Determined by HPLC analysis using Daicel Chiralcel OB column.
d The absolute configuration was assigned by comparison the sign of
the specific rotation with the literature data.15,16
was kept until the reaction was complete (before the e Determined by GC analysis.
addition of water to quench the reaction). This result also
provides a further evidence of the relative stability of
ligand 1h in catalytic system.
rivatization with phenyl isocyanate (Table 2, entry 9). All
ketones afford secondary alcohols with R configuration.
Although the presence of amido group prevents the for-
mation of N–B covalent bond, other borane complexes
may be formed instead of oxazaborolidine. Alternative
catalytic pathway may be available to provide highly
enantioselective control. We assumed that the combina-
tion of two oxygen in two hydroxy groups to borane may
form a rigid macrocyclic complex, the third hydroxy
group may combine another borane to facilitate the enan-
tioselective reduction of ketone associated with the
former borane center. Alternatively, the combination of
three oxygen in three hydroxy groups to borane may form
a concave boron complex, another borane associated with
nitrogen attack prochiral ketone to accomplish the enanti-
oselective reduction. In 11B NMR spectra, the borane spe-
cies in catalyst may has only one type and give a single
peak at d = –13.72 ppm. The detailed mechanism needs
further investigation.
Figure 2 C3-symmetric tris(amino alcohol) ligand
Under the optimized reaction conditions, ligand 1h was
applied to the asymmetric borane reduction of a variety of
aromatic and aliphatic ketones.14 As the results, summa-
rized in Table 2, show, high yields and enantioselectivi-
ties were obtained for prochiral ketones containing
electron-donating or electron-withdrawing groups except
3,5-dinitro acetophenone (Table 2, entries 1–9). This cat-
alyst system is unlike B–H/alkyl-substituted oxazaboroli-
dine system, in the later system the enantioselectivities of
secondary alcohols are influenced by the electronic ef-
fects of substituents in the ketones.15 A slightly decreased
ee was obtained with a substituent at the ortho position
probably due to the steric effect (Table 2, entry 7). Ali-
phatic ketone 3,3-dimethylbutan-2-one also gave high
enantioselectivity determined by HPLC analysis after de-
In summary, a series of chiral C3-symmetric tris(b-hy-
droxyamide) ligands were prepared conveniently and one
of them was used as the efficient catalyst in the enantio-
selective borane reduction of prochiral ketones, affording
chiral secondary alcohols in excellent enantioselectivities.
This paper demonstrates that C3-symmetric hydroxy-
amide ligand can be successfully used in the asymmetric
catalytic borane reduction. The development of other
asymmetric reactions using these C3-symmetric ligands is
ongoing in our group.
Synlett 2006, No. 10, 1559–1563 © Thieme Stuttgart · New York

1562
T. Fang et al.
LETTER
(11) N,N¢,N¢¢-Tris[(1S)-1-hydroxymethyl-2-methylpropyl]-
Acknowledgment
1,3,5-benzenetricarboxamide (1a): 75% yield; colorless
solid, mp 301–303 °C; [a]D20 –47.1 (c 0.73, CH2Cl2–
MeOH = 4:1). IR (KBr): n = 3484, 3241, 2960, 1637, 1556,
1463, 1302, 1051, 729, 692 cm–1. 1H NMR (300 MHz,
DMSO-d6): d = 8.38 (s, 3 H), 8.28 (d, J = 8.7 Hz, 3 H), 4.63
(t, J = 4.8 Hz, 3 H), 3.85 (s, 3 H), 3.55 (s, 6 H), 1.98–1.91 (m,
3 H), 0.95–0.90 (m, 18 H). 13C NMR (75 MHz, DMSO-d6):
d = 166.0, 135.3, 128.7, 61.3, 57.0, 28.6, 19.8, 18.9. HRMS–
FAB: m/z calcd for C24H40N3O6: 466.2911; found: 466.2909
[M + H+].
N,N¢,N¢¢-Tris[(1S)-1-hydroxymethyl-3-methylbutyl]-
1,3,5-benzenetricarboxamide (1b): 91% yield; colorless
solid, mp 283–285 °C; [a]D20 –59.1 (c 0.96, CH2Cl2–
MeOH = 4:1). IR (KBr): n = 3279, 2956, 2868, 1637, 1545,
1469, 1298, 1070, 1031, 706 cm–1. 1H NMR (400 MHz,
DMSO-d6): d = 8.40 (s, 3 H), 8.31 (d, J = 8.6 Hz, 3 H), 4.74
(t, J = 5.6 Hz, 3 H), 4.14–4.08 (m, 3 H), 3.50–3.36 (m, 6 H),
1.68–1.61 (m, 3 H),1.53–1.46 (m, 3 H), 1.42–1.36 (m, 3 H),
0.92–0.89 (m, 18 H). 13C NMR (50 MHz, DMSO-d6): d =
165.5, 135.1, 128.6, 63.9, 49.7, 24.4, 23.5, 21.9. HRMS–
FAB: m/z calcd for C27H46N3O6: 508.3381; found: 508.3383
[M + H+].
N,N¢,N¢¢-Tris[(1S)-1-benzyl-2-hydroxy-2-methylpropyl]-
1,3,5-benzenetricarboxamide (1e): 71% yield; colorless
solid, mp 214–216 °C; [a]D20 –190.8 (c 1.12, CH2Cl2–
MeOH = 5:1). IR (KBr): n = 3307, 2973, 1642, 1533, 1278,
1131, 1083, 915, 747, 697 cm–1. 1H NMR (300 MHz,
DMSO-d6): d = 8.22 (d, J = 9.6 Hz, 3 H), 8.16 (s, 3 H), 7.26–
7.08 (m, 15 H), 4.67 (s, 3 H), 4.20 (t, J = 9.9 Hz, 3 H), 3.15
(d, J = 12.6 Hz, 3 H), 2.77 (d, J = 12.0 Hz, 3 H), 1.24 (s, 9
H), 1.19 (s, 9 H). 13C NMR (75 MHz, DMSO-d6): d = 165.5,
140.3, 134.8, 128.9, 128.5, 127.9, 125.6, 71.9, 59.4, 34.2,
28.0, 25.7. HRMS–FAB: m/z calcd for C42H52N3O6:
694.3850; found: 694.3800 [M + H+].
N,N¢,N¢¢-Tris[(1S)-1-benzyl-2-hydroxy-2,2-diphenyl-
ethyl]-1,3,5-benzenetricarboxamide (1f): 70% yield;
colorless solid, mp 152–154 °C; [a]D20 –207.1 (c 0.94,
EtOAc). IR (KBr): n = 3410, 3060, 3027, 1654, 1509, 1496,
1449, 1061, 906, 748, 699 cm–1. 1H NMR (400 MHz,
CDCl3): d = 7.62 (d, J = 7.6 Hz, 6 H), 7.49 (d, J = 7.3 Hz, 6
H), 7.39–7.03 (m, 36 H), 6.33 (d, J = 7.9 Hz, 3 H), 5.18 (d,
J = 6.7 Hz, 3 H), 4.33 (br s, 3 H), 2.93 (d, J = 6.2 Hz, 6 H).
13C NMR (75 MHz, CDCl3): d = 166.1, 145.2, 144.6, 138.5,
135.0, 129.2, 128.6, 128.4, 128.2, 127.6, 127.2, 126.9,
126.5, 125.6, 125.5, 80.9, 59.3, 35.5. HRMS–FAB: m/z
calcd for C72H63N3O6Na: 1088.4609; found: 1088.4616 [M
+ Na+].
This project was supported by National Natural Science Foundation
of China (Grant No: 20572003, 20372001 and 20521202) and
Peking University.
References and Notes
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Tetrahedron: Asymmetry 2005, 16, 3955. (c) Krzemiński,
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Asymmetry 2004, 15, 1495. (e) Sarvary, I.; Norrby, P.;
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Tetrahedron: Asymmetry 2002, 13, 1241. (k)Basavaiah, D.;
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(5) For reviews, see: (a) Moberg, C. Angew. Chem. Int. Ed.
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34, 664.
N,N¢,N¢¢-Tris[(1S)-2-methyl-1-diphenylhydroxymethyl-
propyl]-1,3,5-benzenetricarboxamide (1g): 87% yield;
colorless solid, mp 177–179 °C; [a]D20 –148.3 (c 1.59,
EtOAc). IR (KBr): n = 3421, 2961, 2873, 1650, 1510, 1448,
1309, 1063, 746, 701 cm–1. 1H NMR (400 MHz, CDCl3):
d = 7.82 (s, 3 H), 7.52 (dd, J = 5.6, 7.2 Hz, 12 H), 7.32 (t,
J = 7.6 Hz, 6 H), 7.24–7.14 (m, 9 H), 7.04 (t, J = 7.2 Hz, 3
H), 6.80 (d, J = 9.6 Hz, 3 H), 5.09 (dd, J = 2.0, 9.8 Hz, 3 H),
3.49 (s, 3 H), 1.99–1.93 (m, 3 H), 0.86 (d, J = 6.7 Hz, 9 H),
0.94 (d, J = 6.7 Hz, 9 H). 13C NMR (75 MHz, CDCl3): d =
166.2, 146.0, 145.2, 135.5, 128.4, 128.3, 127.7, 126.9,
125.33, 125.27, 82.2, 59.0, 29.1, 22.9, 18.1. HRMS–FAB:
m/z calcd for C60H63N3O6Na: 944.4609; found: 944.4626
[M + Na+].
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1,1¢,1¢¢-(1,3,5-Benzenetricarbonyl)-tris[(2S)-a,a-
diphenyl-2-pyrrolidinemethanol] (1h): 80% yield;
colorless solid, mp 170–172 °C; [a]D20 –14.8 (c 0.95,
EtOAc). IR (KBr): n = 3298, 3058, 2965, 2886, 1613, 1447,
1401, 1197, 1033, 759, 700 cm–1. 1H NMR (300 MHz,
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659.
Synlett 2006, No. 10, 1559–1563 © Thieme Stuttgart · New York

LETTER
CDCl3): d = 7.59 (d, J = 6.9 Hz, 6 H), 7.47–7.25 (m, 24 H),
C3-Symmetric Tripodal b-Hydroxy Amides
1563
128.0, 127.2, 126.3, 126.2, 125.6, 125.5, 77.8, 70.4, 60.2,
55.4, 29.8, 24.1. Anal. Calcd for C60H63N3O3: C, 82.44; H,
7.26; N, 4.81. Found: C, 80.81; H, 7.13; N, 4.56; MS (ESI):
m/z = 874.7 [M + H+]. HRMS–FAB: m/z calcd for
C60H64N3O3: 874.49419; found: 874.49175 [M + H+].
(14) Typical Procedure for the Reduction of Prochiral
Ketones.
6.20 (s, 3 H), 5.35 (t, J = 7.8 Hz, 3 H), 3.15–3.00 (m, 6 H),
2.19–1.98 (m, 6 H), 1.61–1.56 (m, 6 H). 13C NMR (75 MHz,
CDCl3): d = 170.3, 145.1, 143.2, 136.5, 127.9, 127.7, 127.6,
127.5, 127.4, 82.0, 67.6, 51.8, 29.6, 24.5. HRMS–FAB: m/z
calcd for C60H57N3O6Na: 938.4139; found: 938.4155 [M +
Na+].
(12) (a) Xu, J.; Wei, T.; Zhang, Q. J. Org. Chem. 2003, 68,
10146; and references cited therein. (b) Gilmore, N. J.;
Jones, S.; Muldowney, M. P. Org. Lett. 2004, 6, 2805.
(13) Typical Procedure for 1,1¢,1¢¢-(1,3,5-Benzene-
trimethylene)tris[(2S)-a,a-diphenyl-2-pyrrolidine-
methanol] (2).
BH3·SMe2 (0.3 mL, 2 M solution in THF) was added by
syringe to a solution of chiral ligand (0.025 mmol) in dry
THF (1.5 mL) under nitrogen at r.t. The mixture was stirred
at 50 °C for 30 min. A solution of acetophenone (0.5 mmol)
in dry THF (0.5 mL) was added dropwise over a period of
1 h at 50 °C. After 1 h, the reaction mixture was cooled and
quenched by dropwise addition of H2O (5 mL). The product
was isolated by extraction with CH2Cl2 (3 × 5 mL). The
organic phase was dried over anhyd Na2SO4. After concen-
tration by rotary evaporation, the product was purified by
column chromatography on silica gel (PE–EtOAc = 5:1) to
afford the corresponding (R)-1-phenylethanol with 96%
yield and 94% ee. The enantiomeric excess was determined
by HPLC on chiral column. (Daicel Chiralcel OB column,
hexane–2-PrOH = 9:1, 1.0 mL/min, 254 nm, tR,major = 8.54
and tR,minor = 6.53 min).
A 50-mL round-bottom flask was charged with tris(b-
hydroxyamide) 1h (0.61 g, 0.67 mmol) and THF (10 mL),
BH3·SMe2 (2 M solution in THF, 7.0 mL) was added via
syringe and the mixture was stirred at reflux for 22 h. After
slow addition of 2 mL of H2O to destroy excess BH3, the
organic solvent was removed by rotary evaporation. Then,
20% KOH solution (15 mL) was added and refluxed for 6 h.
After cooling to r.t., the mixture was extracted with CH2Cl2
(3 × 10 mL), dried over anhyd Na2SO4. Purification by
column chromatography on silica gel (PE–EtOAc = 5:1) to
afford a colorless solid (0.17 g, 29% yield); mp 114–116 °C;
[a]D20 +110.9 (c 1.2, CH2Cl2). IR (KBr): n = 2969, 2803,
1491, 1449, 1371, 1107, 909, 732, 705 cm–1. 1H NMR (400
MHz, CDCl3): d = 7.77 (d, J = 7.6 Hz, 6 H), 7.61 (d, J = 7.5
Hz, 6 H), 7.32–7.17 (m, 12 H), 7.16 (t, J = 7.3 Hz, 3 H), 7.09
(t, J = 7.3 Hz, 3 H), 6.54 (s, 3 H), 4.98 (s, 3 H), 3.97 (dd,
J = 4.6, 9.3 Hz, 3 H), 3.11 (d, J = 12.6 Hz, 3 H), 2.92 (d,
J = 12.6 Hz, 3 H), 2.81–2.77 (m, 3 H), 2.30–2.27 (m, 3 H),
1.99–1.93 (m, 3 H), 1.78–1.68 (m, 3 H), 1.65–1.61 (m, 6 H).
13C NMR (75 MHz, CDCl3): d = 148.1, 146.6, 139.2, 128.1,
(15) (a) Xu, J.; Wei, T.; Zhang, Q. J. Org. Chem. 2004, 69, 6860.
(b) Liu, H.; Xu, J. J. Mol. Catal. A: Chem. 2006, 244, 68; and
references cited therein.
(16) (a) Mathre, D. J.; Thompson, A. S.; Douglas, A. W.;
Hoogsteen, K.; Carroll, J. D.; Corley, E. G.; Grabowski, E.
J. J. J. Org. Chem. 1993, 58, 2880. (b) Kaufman, T. S.
Tetrahedron Lett. 1996, 37, 5329. (c) Hirao, A.; Itsuno, S.;
Nakahama, S.; Yamazaki, N. J. Chem. Soc., Chem.
Commun. 1981, 315.
Synlett 2006, No. 10, 1559–1563 © Thieme Stuttgart · New York