Seleno-imine: A new class of versatile, modular N,Se ligands for asymmetric palladium-catalyzed allylic alkylation

Article abstract of DOI:10.1055/s-2005-871546

The palladium-catalyzed asymmetric allylic alkylation of 1,3-diphenyl-2-propenyl acetate with dimethyl malonate in the presence of chiral seleno-imine ligands derived from an inexpensive and easily available chiral pool was investigated. Excellent yield a

Full text of DOI:10.1055/s-2005-871546

LETTER
1675
Seleno-Imine: A New Class of Versatile, Modular N,Se Ligands for
Asymmetric Palladium-Catalyzed Allylic Alkylation
Seleno-Imine:
A
N
n
ew Class of
t
Versati
o
le, Modular
n
N
,Se Ligand
i
s
o L. Braga,* Márcio W. Paixão, Graciane Marin
Departamento de Química, Universidade Federal de Santa Maria, 97105-900, Santa Maria, RS, Brazil
Fax +55(55)32208998; E-mail: albraga@quimica.ufsm.br
Received 28 April 2005
H
O
Abstract: The palladium-catalyzed asymmetric allylic alkylation
of 1,3-diphenyl-2-propenyl acetate with dimethyl malonate in the
presence of chiral seleno-imine ligands derived from an inexpen-
sive and easily available chiral pool was investigated. Excellent
yield and enantioselectivity (up to 97% ee) was achieved when
ligand 4a was used.
O
N
N
SePh
Fe
SeBn
Hiroi
PPh2
Hou
Key words: seleno-imines, allylic alkylations, palladium-catalyzed
O
O
Asymmetric catalysis of organic reactions to provide
enantiomerically enriched products is of central impor-
tance to modern synthetic chemistry.1 Within this field
palladium-catalyzed allylic nucleophilic substitution is
one of the most important tools for the enantioselective
formation of carbon–carbon and carbon–heteroatom
bonds. The reaction mechanism is well understood and
the process has already found numerous applications in
the synthesis of natural products.2
N
SePh
N
H
SePh
Hou
Helmchen
Figure 1
The modular structure of seleno-imines in general allows
the preparation of a ligand library with enormous diversity
(Scheme 1).
Development of new chiral ligands is an important chal-
lenge of current research in this area. Various C2- and C1-
symmetric bidentate chiral ligands have been applied to
this process to provide high enantioselectivities.2c,3
The chiral imino-selenides 4a–i were easily prepared
from the corresponding, commercially available a-amino
alcohols or readily prepared8 precursors 1a–d, which were
further quantitatively converted into the Boc-protected
derivatives by reaction with di-tert-butyl dicarbonate in
acetonitrile. The chiral aziridines 2a–d was obtained in
good yields by treatment of N-Boc aminoalcohols with p-
toluenesulfonyl chloride and potassium hydroxide in boil-
ing THF. Finally, the selenium moieties were efficiently
Recently, some reports exploring imines containing b-
phosphines4 or sulfides5 as effective chiral ligands have
been successfully applied to the palladium-catalyzed
asymmetric allylic substitution. Furthermore, the efficient
use of chiral organoselenium ligands has been recently re-
ported for this reaction.6 For example, Hou and co-work-
ers have evaluated the planar chiral [2,2]-paracyclophane
ligands for the palladium-catalyzed allylic substitution,
obtaining good to excellent enantioselectivities.6a Other
groups also showed that high enantioselectivity was ob-
tained with this type of N,Se-bidentate ligand (Figure 1).
R1
R1
R1
SeR2
(iii); (iv)
(i); (ii)
OH
N
NH2
3a–e
NH2
Boc
1a–d
2a–d
As part of our broader program to explore the preparation
and use of chiral organochalcogen compounds in asym-
metric catalysis,7 we now give a preliminary account of
our efforts towards the synthesis of a focused library of
chiral seleno-imine ligands as a new class of sterically and
electronically adjustable chiral ligands and their applica-
tion for the palladium-catalyzed asymmetric allylic alkyl-
ation.
4a = R1 = i-Pr, R2 = Bn, R3 = Ph
4b = R1 = Bn, R2 = Bn, R3 = Ph
4c = R1 = i-Bu, R2 = Bn, R3 = Ph
4d = R1 = s-Bu, R2 = Bn, R3 = Ph
4e = R1 = i-Pr, R2 = Ph, R3 = Ph
(v)
R1
R3
SeR2
4f = R1 = i-Pr, R2 = Bn, R3 = o-OMePh
4g = R1 = i-Pr, R2 = Bn, R3 = p-OMePh
4h = R1 = i-Pr, R2 = Bn, R3 = o-ClPh
N
4i =
R
1 = i-Pr, R2 = Bn, R3 = p-ClPh
Scheme 1 Synthesis of a seleno-imine ligand library. Reagents and
conditions: (i) Boc2O, CH3CN, r.t., 3 h; (ii) KOH, TsCl, THF, reflux,
4 h; (iii) R2SeSeR2/NaBH4, THF, r.t., 24 h; (iv) TFA, CH2Cl2, r.t., 1
h; (v) R3CHO, EtOH, MgSO4, r.t., 12 h.
SYNLETT 2005, No. 11, pp 1675–1678
Advanced online publication: 14.06.2005
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DOI: 10.1055/s-2005-871546; Art ID: S04505ST
© Georg Thieme Verlag Stuttgart · New York

1676
A. L. Braga et al.
LETTER
introduced by regioselective nucleophilic ring-opening by and using dichloromethane as solvent, the alkylated prod-
9
attack of R2SeSeR2/NaBH4 at the less hindered carbon of uct was obtained with an excellent level of enantiomeric
the aziridines 2a–d,10 furnishing the aliphatic chiral amino excess in quantitative yield (Table 1, entry 2).
selenide. Deprotection of the chiral selenide 3a–e using
Other solvents were also tested, however it was found that
TFA proceeded smoothly at room temperature to give the
dichloromethane gave the highest selectivity.14
free amino-selenide. Subsequently, imines 4a–i were syn-
Under the same conditions, we have examined the steric
thesized by condensation with the requisite aromatic alde-
and the electronic effect based on the different structures
of ligand 4 derived from different chiral resources
(Table 2).
hyde in the presence of magnesium sulfate. After
filtration, and removal of solvent and excess aldehyde in
high vacuum at elevated temperature, compounds 4a–i
were obtained as practically pure materials and single ste-
reoisomers. The imines were unstable towards moisture as
well as SiO2, thus, no further attempts at purification were
undertaken (Scheme 1).11
Table 2 shows that the more bulky selenenyl substituents
(R1) in 4a and 4e provided a higher degree of asymmetric
induction in proportion to the steric bulk (Table 2, entries
1 vs 5).
With the target ligands in hand, we focused our attention
on investigating their potential in asymmetric catalysis.
As a starting point, we chose to explore the enantioselec-
tive palladium-catalyzed allylic alkylation.
In the next stage we examined how the amino acid side
chain (R1) of the ligands influences the stereoselectivity of
the alkylation reaction. We found that i-Pr (Table 2, entry
1) is superior to Bn, i-Bu, or s-Bu (Table 2, entries 2, 3,
and 4).
In order to optimize the reaction conditions, the amount of
catalyst, base, and solvents were varied for ligand 4a
(Table 1). Varying the ligand-to-metal ratio had a small
influence (Table 1, entries 1– 4); using 5 mol% of 4a,
which corresponds to a 1:1 ratio of bidentate ligand/palla-
dium, gave the best results (Table 1, entry 2). In terms of
base, some differences in the asymmetric induction be-
tween the use of dimethyl malonate/BSA and dimethyl so-
diomalonate (Table 1, entries 2 vs 5) were observed.12
Recently, the majority of substitution reactions have been
conducted according to Trost’s procedure13 using BSA in
the presence of metal acetate salts. Under these conditions
With both the substituent at the selenium atom and the
amino acid side chain determined, we finally examined
the effect of the imine group on the stereoselectivity
(Table 2, entries 6–9). Varying the electronic and steric
properties resulted in a remarkable change in the enantio-
selectivity. Reactions with o- and p-methoxy on the
phenyl group were employed; in this case we observed de-
creased ees of the corresponding products (Table 2,
entries 6 and 7). An electron-withdrawing group present
on the phenyl group also lowers the enantioselectivity
when compared with an unsubstituted phenyl group.
Table 1 Optimization of the Allylic Alkylation Using Ligand 4a
[Pd(n3-C3H5)Cl]2
OAc
Ph
MeO2C
CO2Me
Ph
Chiral ligand 4a
MeO2C CO2Me
Solvent
Se
Ph
N
Ph
Ph
4a
Ph
R/S
Entrya
Loading of 4a (mol%)
Solvent
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
Toluene
CH3CN
THF
Yield (%)c
ee (%)d
90
1
2
3
4
5b
6
7
8
10
5
100
100
92
97
2.5
1
85
72
82
5
55
35
5
74
82
5
92
73
5
75
78
a N,O-bis(trimethylsilyl)acetamide (BSA, 3 equiv), dimethyl malonate (3 equiv), KOAc (0.06 mmol), and acetate (1 equiv).
b NaH (1.5 equiv), dimethyl malonate (2 equiv), and acetate (1 equiv).
c Isolated yield.
d Determined by HPLC with a chiralcel OD column and the absolute configuration of the product was assigned through comparison of the sign
of specific rotation with literature data.
Synlett 2005, No. 11, 1675–1678 © Thieme Stuttgart · New York

LETTER
Seleno-Imine: A New Class of Versatile, Modular N,Se Ligands
1677
Table 2 Asymmetric Palladium-Catalyzed Allylic Alkylation with Dimethyl Malonate
2.5 mol%
[Pd(n3-C3H5)Cl]2
MeO2C
CO2Me
Ph
R1
R3
OAc
Ph
5 mol%
Chiral ligand 4a–i
SeR2
N
Ph
Ph
MeO2C
CO2Me
R/S
CH2Cl2
4 a–i
Entrya
Ligand
4a
R1
R2
R3
Yield (%)b
ee (%)c
97
1
2
3
4
5
6
7
8
9
i-Pr
Bn
Bn
Bn
Bn
Bn
Ph
Bn
Bn
Bn
Bn
Ph
93
89
86
94
92
95
94
100
99
4b
4c
Ph
78
i-Bu
s-Bu
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
Ph
75
4d
4e
Ph
80
Ph
75
4f
o-OMePh
p-OMePh
o-ClPh
p-ClPh
87
4g
85
4h
4i
85
84
a BSA (3 equiv), dimethyl malonate (3 equiv), KOAc (0.06 mmol), and acetate (1 equiv).
b Isolated yield.
c Determined by HPLC with a chiralcel OD column and the absolute configuration of the product was assigned through comparison of the sign
of specific rotation with literature data.
(3) Masdeu-Bultó, A. M.; Diéguez, M.; Martin, E.; Gómez, M.
Coord. Chem. Rev. 2003, 242, 159.
(4) For chiral P-imine ligands, see: (a) Saitoh, A.; Misawa, M.;
Morimoto, T. Synlett 1994, 483. (b) Saitoh, A.; Achiwa, K.;
Tanaka, K.; Morimoto, T. J. Org. Chem. 2000, 65, 4228.
(c) Hu, X.; Dai, H.; Bai, C.; Chen, H.; Zheng, Z.
In the present study, the absolute configuration of the
major enantiomer was determined to be R in all cases by
the optical rotation and the comparison of peaks in chiral
HPLC analysis.
In summary, we have developed a new class of modular
ligands that are derived from amino acids for the palladi-
um-catalyzed asymmetric allylic alkylation, the products
were obtained in high yields and enantiomeric excess.
This work paves the way for the synthesis and evaluation
of larger libraries of ligands based upon the same general
structure. Further studies are in progress in our laborato-
ries concerning new metal-catalyzed asymmetric reac-
tions and will be reported in due course.
Tetrahedron: Asymmetry 2004, 15, 1065. (d) Lee, J. H.;
Son, S. U.; Chung, Y. K. Tetrahedron: Asymmetry 2003, 14,
2109. (e) Fukuda, T.; Takehara, A.; Iwao, M. Tetrahedron:
Asymmetry 2001, 12, 2793. (f) Mino, T.; Ogawa, T.;
Yamashita, M. J. Organomet. Chem. 2003, 665, 122.
(5) For chiral S-imine ligands, see: (a) Anderson, J. C.; James,
D. S.; Mathias, J. P. Tetrahedron: Asymmetry 1998, 9, 753.
(b) Adams, H.; Anderson, J. C.; Cubbon, R.; James, D. S.;
Mathias, J. P. J. Org. Chem. 1999, 64, 8256.
(6) (a) Hou, X.-L.; Wu, X.-W.; Dai, L.-X.; Cao, B.-X.; Sun, J.
Chem. Commun. 2000, 1195. (b) Sprinz, J.; Kiefer, M.;
Helmchen, G. Tetrahedron Lett. 1994, 35, 1523. (c) Hiroi,
K.; Suzuki, Y.; Abe, I. Tetrahedron: Asymmetry 1999, 10,
1173. (d) You, S.-L.; Hou, X.-L.; Dai, L.-X. Tetrahedron:
Asymmetry 2000, 11, 1495.
(7) (a) Braga, A. L.; Silva, S. J. N.; Lüdtke, D. S.; Drekener, R.
L.; Silveira, C. C.; Rocha, J. B. T.; Wessjohann, L. A.
Tetrahedron Lett. 2002, 43, 7329. (b) Braga, A. L.;
Rodrigues, O. E. D.; Paixão, M. W.; Appelt, H. R.; Silveira,
C. C.; Bottega, D. P. Synthesis 2002, 2338. (c) Braga, A. L.;
Appelt, H. R.; Schneider, P. H.; Silveira, C. C.; Wessjohann,
L. A. Tetrahedron: Asymmetry 1999, 10, 1733. (d) Braga,
A. L.; Vargas, F.; Andrade, L. H.; Silveira, C. C.
Acknowledgment
We are grateful to the CAPES, CNPq, and FAPERGS for financial
support. M. W. P. thanks CAPES for a PhD fellowship and G. M.
thanks CNPq for a Master fellowship.
References
(1) Comprehensive Asymmetric Catalysis; Jacobsen, E. N.;
Pfaltz, A.; Yamamoto, H., Eds.; Springer: Berlin, 1999.
(2) (a) Trost, B. M.; Crawley, M. L. Chem. Rev. 2003, 103,
2921. (b) Trost, B. M. J. Org. Chem. 2004, 69, 5813.
(c) Johannsen, M.; Jorgensen, K. A. Chem. Rev. 1998, 98,
1689. (d) Trost, B. M.; Van Vranken, D. L. Chem. Rev.
1996, 96, 395.
Tetrahedron Lett. 2002, 43, 2335. (e) Braga, A. L.; Paixão,
M. W.; Lüdtke, D. S.; Silveira, C. C.; Rodrigues, O. E. D.
Org. Lett. 2003, 5, 2635. (f) Braga, A. L.; Paixão, M. W.;
Milani, P.; Silveira, C. C.; Rodrigues, O. E. D.; Alves, E. F.
Synlett 2005, No. 11, 1675–1678 © Thieme Stuttgart · New York

1678
A. L. Braga et al.
LETTER
Chem. Commun. 2004, 2488. (g) Braga, A. L.; Paixão, M.
W.; Milani, P.; Silveira, C. C.; Rodrigues, O. E. D.; Alves,
E. F. Synlett 2004, 1297. (h) Schneider, P. H.; Schrekker, H.
S.; Silveira, C. C.; Wessjohann, L. A.; Braga, A. L. Eur. J.
Org. Chem. 2004, 2715.
128.87, 126.46, 77.00, 32.74, 32.42, 19.71, 18.57. HRMS-
ESI: m/z calcd for C18H21NSe + H+: 332.0911; found:
332.0915.
4f: Yield: 78%; [a]D20 = +23 (c 0.6, CH2Cl2). 1H NMR
(CDCl3, 400 MHz): d = 8.60 (s, 1 H), 7.99–7.97 (m, 1 H),
7.35–7.16 (m, 6 H), 6.97–6.88 (m, 2 H), 3.88 (s, 3 H), 3.76–
3.69 (m, 2 H), 3.01–2.99 (m, 1 H), 2.87–2.77 (m, 2 H), 1.95–
1.90 (m, 1 H), 0.89 (d, 6 H, J = 6,72). 13C NMR (CDCl3, 100
MHz): d = 158.80, 156.24, 139.60, 131.60, 128.90, 128.27,
127.52, 126.42, 124.68, 120.70, 111.56, 77.87, 55.50, 33.26,
28.32, 27.37, 19.71, 18.81. HRMS-ESI: m/z calcd for
C20H25NOSe + H+: 376.1179; found: 376.1189.
(8) Chiral aminoalcohols were easily obtained by reduction of
the corresponding amino acids: McKennon, M. J.; Meyers,
A. I. J. Org. Chem. 1993, 58, 3568.
(9) Osborn, H. M. I.; Sweeney, J. B. Synlett 1993, 145.
(10) (a) Tanner, D. Angew. Chem., Int. Ed. Engl. 1994, 33, 599.
(b) McCoull, W.; Davis, F. A. Synthesis 2000, 1347.
(11) General Procedure for the Synthesis of Seleno-Imine
Ligands 4a–i: An equimolar mixture of 3 (1 mmol) and the
requisite aromatic aldehyde (1 mmol) with MgSO4 (0.54 g/
mmol) was stirred in EtOH (2.5 mL/mmol) for 24 h.
Filtration and concentration in vacuo gave ligands 4a–i as
oils, which were unstable to chromatography, the
4g: Yield: 90%; [a]D20 +71 (c 0.58, CH2Cl2). 1H NMR
(CDCl3, 400 MHz): d = 8.09 (s, 1 H), 7.71–7.69 (m, 2 H),
7.29–7.16 (m, 5 H), 7.00–6.91 (m, 2 H), 3.81 (s, 3 H), 3.75–
3.67 (m, 2 H), 2.95–2.76 (m, 3 H), 1.93–1.89 (m, 1 H), 0.94–
0.81 (m, 6 H). 13C NMR (CDCl3, 100 MHz): d = 161.51,
159.61, 139.62, 131.91, 129.95, 129.80, 129.17, 128.31,
126.47, 114.30, 113.93, 77.91, 55.50, 33.37, 28.80, 28.48,
19.72, 18.72. HRMS-ESI: m/z calcd for C20H25NOSe + H+:
376.1179; found: 376.1193.
compounds were obtained as practically pure materials.
4a: Yield: 90%; [a]D20 +54 (c 0.6, CH2Cl2). 1H NMR
(CDCl3, 400 MHz): d = 8.14 (s, 1 H), 7.76–7.71 (m, 2 H),
7.37–7.04 (m, 8 H), 3.69 (dd, 2 H, J = 12, J = 19), 2.97–2.76
(m, 3 H), 1.92–1.90 (m, 1 H), 0.88 (d, 6 H, J = 5.8). 13
C
4h: Yield: 88%; [a]D20 +52 (c 0.75, CH2Cl2). 1H NMR
(CDCl3, 400 MHz): d = 8.57 (s, 1 H), 8.03–8.01 (m, 1 H),
7.37–7.19 (m, 8 H), 3.85 (s, 2 H), 3.01–2.97 (m, 1 H), 2.89–
NMR (CDCl3, 100 MHz): d = 160.36, 139.55, 136.18,
130.44, 129.66, 128.93, 128.88, 128.50, 128.25, 77.93,
33.17, 28.98, 28.32, 22.63, 19.70. HRMS-ESI: m/z calcd for
C19H23NSe + H+: 346.1073; found: 346.1063.
2.74 (m, 2 H), 1.94–1.92 (m, 1 H), 0.94–0.88 (m, 6 H). 13
NMR (CDCl3, 100 MHz): d = 157.05, 139.41, 134.99,
133.25, 131.22, 129.61, 128.87, 128.60, 128.50, 126.85,
126.50, 77.32, 33.15, 28.43, 27.40, 19.62, 18.53. HRMS-
ESI: m/z calcd for C19H22NClSe + H+: 380.070; found:
380.0856.
C
4b: Yield: 65%; [a]D20 –29 (c 0.276, CH2Cl2). 1H NMR
(CDCl3, 400 MHz): d = 7.88 (s, 1 H), 7.67–7.64 (m, 2 H),
7.39–6.93 (m, 13 H), 3.75 (dd, 2 H, J = 12, J = 21), 3.46–
3.44 (m, 1 H), 3.04–2.80 (m, 4 H). 13C NMR (CDCl3, 100
MHz): d = 160.81, 139.41, 138.55, 135.84, 130.50, 129.60,
129.22, 128.80, 128.59, 128.42, 128.31, 126.46, 126.03,
76.68, 42.95, 30.99, 27.50. HRMS-ESI: m/z calcd for
C23H23NSe + H+: 394.1074; found: 394.1084.
4i: Yield: 80%; [a]D20 +61 (c 0.9, CH2Cl2). 1H NMR (CDCl3,
400 MHz): d = 8.10 (s, 1 H), 7.70–7.67 (m, 3 H), 7.38–7.21
(m, 6 H), 3.70 (dd, 2 H, J = 12, J = 16), 2.95–2.73 (m, 3 H),
1.94–1.86 (m, 1 H), 0.89–0.86 (m, 6 H). 13C NMR (CDCl3,
100 MHz): d = 158.98, 139.47, 136.40, 134.63, 129.42,
128.90, 128.78, 128.71, 128.54, 128.35, 126.56, 77.85,
33.36, 28.60, 27.81, 19.66, 19.61. HRMS-ESI: m/z calcd for
C19H22NClSe + H+: 380.0684; found: 380.0673.
4c: Yield: 72%; [a]D20 +21 (c 1.13, CH2Cl2). 1H NMR
(CDCl3, 400 MHz): d = 8.16 (s, 1 H), 7.76–7.74 (m, 2 H),
7.42–7.40 (m, 3 H), 7.28–7.17 (m, 5 H), 3.77–3.73 (m, 2 H),
3.31–3.29 (m, 1 H), 2.77–2.61 (m, 2 H), 1.69–1.63 (m, 1 H),
1.48–1.39 (m, 2 H), 0.89–0.81 (m, 6 H). 13C NMR (CDCl3,
100 MHz): d = 160.40, 139.57, 136.05, 130.60, 129.72,
128.57, 128.40, 128.25, 126.57, 69.91, 45.35, 31.20, 27.70,
24.67, 23.55, 21.38. HRMS-ESI: m/z calcd for C20H25NSe +
H+: 360.1230; found: 360.1224.
(12) (a) Brown, J. M.; Hulmes, D. I.; Guiry, P. J. Tetrahedron
1994, 50, 4493. (b) Yamaguchi, M.; Shima, T.; Yamagishi,
T.; Hida, M. Tetrahedron: Asymmetry 1991, 2, 663.
(13) Trost, B. M.; Murphy, D. J. Organometallics 1985, 4, 1143.
(14) General Procedure for the Allylic Alkylation of 1,3-
Diphenyl-2-propenyl Acetate with Dimethyl Malonate:
A solution of [PdCl(h3-C3H5)]2 (50 mmol) and chiral ligand
(5 mmol%) in dichloromethane (1.5 mL) was stirred for 30
min at r.t. Subsequently, a solution of rac-1,3-diphenyl-2-
propenyl acetate (1.0 mmol), dimethyl malonate (3.0 mmol),
BSA (3.0 mmol), and KOAc (3 mg, cat. quantity) in
dichloromethane (0.8 mL) were added. The reaction mixture
was stirred for 48 h. After this time, sat. NH4Cl (aq) was
added to quench the reaction, followed by extraction with
dichloromethane (3 × 15 mL). The combined organic layers
were dried over MgSO4. The solvent was removed in vacuo.
The yield was determined by isolated product and the ee by
HPLC (Chiralcel OD, 0.5% 2-propanol–hexane, flow 0.5
mL/min, l = 254nm).
4d: Yield: 70%; [a]D20 +13 (c 1.08, CH2Cl2). 1H NMR
(CDCl3, 400 MHz): d = 8.16 (s, 1 H), 7.77–7.75 (m, 2 H),
7.41–7.39 (m, 3 H), 7.26–7.17 (m, 5 H), 3.74–3.66 (m, 2 H),
3.07–3.03 (m, 1 H), 2.88–2.75 (m, 2 H), 1.70–1.68 (m, 1 H),
1.50–1.47 (m, 1 H), 1.11–1.09 (m, 1 H), 0.90–0.83 (m, 6 H).
13C NMR (CDCl3, 100 MHz): d = 160.31, 138.51, 136.14,
130.46, 128.53, 128.33, 128.25, 126.87, 126.50, 76.69,
40.04, 28.38, 27.50, 25.41, 15.63, 11.51. HRMS-ESI: m/z
calcd for C20H25NSe + H+: 360.1230; found: 360.1224.
4e: Yield: 76%; [a]D 20 +52 (c 0.7, CH2Cl2). 1H NMR
(CDCl3, 400 MHz): d = 8.16 (s, 1 H), 7.72–7.71 (m, 2 H),
7.47–7.15 (m, 8 H), 3.34–2.96 (m, 3 H), 2.04–1.99 (m, 1 H),
0.95–0.91 (m, 6 H). 13C NMR (CDCl3, 100 MHz): d =
160.68, 136.07, 133.55, 132.30, 130.82, 130.43, 129.34,
Synlett 2005, No. 11, 1675–1678 © Thieme Stuttgart · New York