New camphor-derived selenonium ylides: Enantioselective synthesis of chiral epoxides

Article abstract of DOI:10.1071/CH05157

Optically pure selenonium salts 3 as the precursors of two new chiral selenonium ylides 4 can be synthesized stereoselectively from natural d-camphor in good yields. It is found that the reaction of the selenonium salt 3b, an aldehyde, and potassium tert-butoxide can take place smoothly in ?one-pot' via the formation of selenonium ylide 4b, to give chiral trans-diaryl epoxides 5 in good yields with good diastereoselectivities and enantioselectivities. CSIRO 2005.

Full text of DOI:10.1071/CH05157

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CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2005, 58, 749–752
New Camphor-Derived SelenoniumYlides: Enantioselective
Synthesis of Chiral Epoxides
Xin-Liang Li,A Yi Wang,A and Zhi-Zhen HuangA,B
A School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China.
B Corresponding author. Email: huangzhizhen0226@163.com
Optically pure selenonium salts 3 as the precursors of two new chiral selenonium ylides 4 can be synthesized
stereoselectively from natural d-camphor in good yields. It is found that the reaction of the selenonium salt 3b, an
aldehyde, and potassium tert-butoxide can take place smoothly in ‘one-pot’ via the formation of selenonium ylide
4b, to give chiral trans-diaryl epoxides 5 in good yields with good diastereoselectivities and enantioselectivities.
Manuscript received: 1 July 2005.
Final version: 19 August 2005.
Introduction
Chiral epoxides are one of the most important syn-
thetic intermediates[1] because of their significant biological
activities[2] and versatile chemical transformations.[3] Nowa-
days, the development of new and efficient syntheses of chiral
epoxides is still a challenging subject. Among various syn-
theses of chiral epoxides, the chiral ylide route is one of the
most important methods. It centres around the tandem for-
mation of carbon–carbon bonds and epoxidation.[4] Recently,
camphor-derived sulfonium ylides have received much atten-
tion in the enantioselective synthesis of three-membered
ring compounds.[5] Asymmetric syntheses of cyclopropanes
and aziridines through the formation of camphor-derived
semistabilized sulfonium ylides achieve excellent enantio-
selectivities (up to 99% e.e.),[5a,5b] however, the enantio-
selectivities for the asymmetric synthesis of epoxides derived
from these ylides are only moderate (19–74% e.e.).[5c] The
steric hindrance of a selenonium ylide is larger than that of
the corresponding sulfonium ylide, and there are few reports
concerned with chiral selenonium ylides.[6] As such, we syn-
thesized camphor-derived selenonium ylides and employed
them in an attempt to improve the enantioselectivity of the
asymmetric synthesis of chiral epoxides.
1. LDA
SeR
O
O
2. RSeBr
1
DIBALH
CH2Cl2
SeR
OH
2
1a, 2a: R ꢀ Me
1b, 2b: R ꢀ Ph
Scheme 1. Stereoselective synthesis of optically pure exo-
α-organoseleno isoborneol 2.
could be achieved using a low temperature, a short reaction
time, and one equivalent of lithium diisopropylamide. The
exo-isomer of α-organoseleno camphor 1a could be isolated
conveniently by chromatography. The optically pure exo-
organoseleno camphor 1a and 1b were reduced by diisobutyl-
aluminium hydride to give exo-α-methylseleno isoborneol
2a and exo-α-phenylseleno isoborneol 2b in 71 and 75%
yield, respectively. No endo-hydroxy (borneol) isomer was
observed, which might be due to the steric interaction of
diisobutylaluminium hydride, as a nucleophile, with the
methyl group in the 7-position.[8]
As expected, exo-α-methylseleno isoborneol 2a was
allowed to react with benzyl bromide at 0◦C to afford the
selenonium salt 3a in 85% yield (Scheme 2). We then
employed salt 3a in the reaction with various aldehydes
in the presence of potassium tert-butoxide. It was found
that the one-pot reaction, in which the selenonium ylide
4a was formed in situ, could take place smoothly at 0◦C
to give trans-(2R,3R)-diaryl epoxides 5 in good yield with
good diastereoselectivity and moderate enantioselectivity
Results and Discussion
We found that α-methylseleno camphor 1a could be syn-
thesized in good yield by the reaction of lithiated cam-
phor with methylselenenyl bromide, which was prepared
in situ (Scheme 1). α-Phenylseleno camphor 1b was pre-
pared according to the literature method.[7] Initially, it was
found that almost equal amounts of exo- and endo-isomers
of α-organoseleno camphor 1a was produced at −40◦C.
After further investigation we found that the formation of
the exo-isomer could be favoured by controlling the reaction
conditions. A ratio of up to 10 : 1 of exo- to endo-isomer
© CSIRO 2005
10.1071/CH05157
0004-9425/05/100749

750
X.-L. Li, Y. Wang, and Z.-Z. Huang
that all the bases shown in Table 2 gave the trans-diphenyl
epoxide 5a with moderate to good yields and good diastereo-
selectivities. Interestingly, sodium hydroxide resulted in
Me
Seꢁ
Brꢂ
Ph
SeMe
OH
PhCH2Br
Et2O
OH
trans-diphenyl epoxide 5a with
a dominant (S,S)-
configuration, which is contrary to that produced using
other bases. In terms of the balance of yield, diastereose-
lectivity, and enantioselectivity, the best base is potassium
tert-butoxide, and the most suitable mole ratio of selenon-
ium salt 3b to base is 1 : 4, to produce trans-diaryl epoxides
in good yield with good diastereoselectivities and enantio-
selectivities.
Under optimal conditions, a variety of aromatic aldehydes
were used in the reaction with selenonium salt 3b in the pres-
ence of potassium tert-butoxide at −40◦C. The experimental
results revealed that the yields, the diastereoselectivities,
and the enantioselectivities of trans-(2R,3R)-diaryl epoxides
5a–5f are good under these conditions (Table 3).
2a
3a
Me
Ph
Se
ꢁ
OH
4a
O
H
Ph
H
THF
3a ꢁ ArCHO ꢁ ButOK
ꢂ40ꢃC
Ar
5
Scheme 2. Enantioselective synthesis of trans-(2R,3R)-diaryl epox-
ides via camphor-derived selenonium ylides 4a.
Ph
Conclusions
PhCH2Br
BF4ꢂ
Ph
Seꢁ
OH
SePh
OH
Optically pure selenonium salts 3 as the precursors of two
new chiral selenonium ylides 4 can be synthesized stere-
oselectively from natural d-camphor in good yields. The
reaction of the selenonium salts 3, an aldehyde, and potas-
sium tert-butoxide can take place smoothly in a one-pot
synthesis and form the selenonium ylides 4 in situ, to give
chiral trans-diaryl epoxides 5 in good yields with good
diastereoselectivities and low to good enantioselectivities.
The enantioselectivity of the chiral epoxides achieved by
the protocol reported here, which utilizes camphor-derived
selenonium ylide 4b in situ, is better than the literature
method that employs the corresponding camphor-derived
sulfonium ylides.[5c]
AgBF4/Et2O
2b
3b
Ph
Se
Ph
ꢁ
OH
4b
O
H
Ph
H
THF
ꢂ40ꢃC
3b ꢁ ArCHO ꢁ ButOK
Ar
5
Scheme 3. Enantioselective synthesis of trans-(2R,3R)-diaryl epox-
ides via camphor-derived selenonium ylides 4b.
Experimental
All reactions were carried out under a nitrogen atmosphere. 1H NMR
spectra were determined in CDCl3 on a Bruker ARX-300 (300 MHz)
with tetramethylsilane as internal standard. Mass spectra (electron
impact, EI) were obtained on a VG-ZAB-HS mass spectrometer. IR
spectra were taken with a 5DX-FT-2 spectrometer. Melting points were
uncorrected. Elemental analyses were determined on a Perkin–Elmer
240C analyzer.
(Table 1). Using a different base, such as sodium hydroxide,
sodium hydride, or sodium methoxide, did not improve the
enantioselectivity. The absolute configuration of 5a–5f was
assigned by a comparison of the sign of their optical rotations
with that of the known compounds.[9]
In order to increase the enantioselectivity of the epoxi-
dation reaction from the selenonium ylide, we prepared the
precursor 3b of the sterically more encumbered phenylse-
lenoniumylide 4b. Selenonium salt 3b could only be formed
with the assistance of silver tetrafluoroborate and the reac-
tion proceeded in good yield. Salt 3b was then employed
in the reaction with benzaldehyde in the presence of potas-
sium tert-butoxide in tetrahydrofuran (THF) at 0◦C. As
expected, the one-pot reaction via the selenonium ylide 4b
took place smoothly to give trans-(2R,3R)-diphenyl epoxide
5a in good yield (81%) with good diastereoselectivity (83%)
and improved enantioselectivity (74%, entry 1 in Table 2;
Scheme 3). Among various solvents, THF is the best for opti-
mal yield, diastereoselectivity, and enantioselectivity of the
reaction.
exo-α-Methylseleno Camphor 1a
To a solution of diisopropylamine (3.68 mL, 24 mmol) in THF (80 mL)
was added a solution of BunLi (2.0 M, 12 mL) in hexane at −78◦C.
After stirring for 5 min, a solution of d-(+)-camphor (3.65 g, 24 mmol)
inTHF (8 mL) was added dropwise.The reaction mixture was stirred for
1 h at −78◦C and then the solution of organoselenenyl bromide (4.2 g,
24 mmol) in THF (10 mL), prepared in situ,[10] was added rapidly. The
reaction mixture was stirred for another 1 h at this temperature and then
poured into an aqueous solution of HCl (0.5 N, 200 mL).After extraction
with ether (100 mL × 2), the organic layer was washed with water, a sat-
urated solution of sodium hydrogen carbonate, and a saturated solution
of sodium chloride, and dried with anhydrous magnesium sulfate. After
the solvent was removed by evaporation, the residue was separated by
column chromatography (silica gel, petroleum ether–ether as eluent) to
yield 1a (75%) as a colorless oil. [α]2D0 +52.9◦ (c 1.0, EtOH). (Found:
C 53.9, H 7.4%. C11H18OSe requires C 53.9, H 7.4%.) δH 2.98 (1H,
s, CH), 2.35 (3H, s, CH3), 2.19 (1H d, J 4.1, CH), 2.06–1.07 (4H, m,
2CH2), 0.98 (3H, s, CH3), 0.94 (3H, s, CH3), 0.92 (3H, s, CH3). m/z (EI)
246 (72.16%, M+), 203 (11.27), 200 (2.45), 135 (85.50), 123 (100.00),
For further optimization, different bases were also used
for the one-pot reaction of selenonium ylide 3b with
benzaldehyde in THF at 0◦C. Experimental results showed

New Camphor-Derived Selenonium Ylides: Enantioselective Synthesis of Chiral Epoxides
751
Table 1. Enantioselective synthesis of trans-diaryl epoxides via camphor-derived selenonium ylides 4a
Entry
Ar
Product
YieldA [%]
DiastereoselectivityB [%]
e.e.C [%]
Config.
1
2
3
4
5
6
C6H5
4-FC6H4
4-ClC6H4
2-ClC6H4
4-CH3C6H4
4-CH3OC6H4
5a
5b
5c
5d
5e
5f
78
83
77
70
80
71
86
80
81
84
85
78
58
60
55
37
50
48
R,R
R,R
R,R
R,R
R,R
R,R
A Isolated yields.
B Determined by 1H NMR spectroscopy or gas chromatography.
C Determined by chiral HPLC on a Chiralcel OD-H column.
Table 2. Reaction of selenonium salt 3b with benzaldehyde in different bases
EntryA
Base
3b/Base
YieldB [%]
DiastereoselectivityC [%]
e.e.D [%]
Config.
1
2
3
4
5
6
7
8
ButOK
—
NaH
—
NaOCH3
—
NaOH
—
1 : 2
1 : 4
1 : 2
1 : 4
1 : 2
1 : 4
1 : 2
1 : 4
81
83
65
71
63
80
73
70
83
86
76
78
86
85
90
85
74
78
70
66
57
60
72
67
R,R
R,R
R,R
R,R
R,R
R,R
S,S
S,S
A THF was used as solvent and reaction time is 12 h.
B Isolated yields.
C Determined by 1H NMR spectroscopy or gas chromatography.
D Determined by chiral HPLC on a Chiralcel OD-H column.
Table 3. Enantioselective synthesis of trans-diaryl epoxides via camphor-derived selenonium ylide 4b
Entry
Ar
Product
Temp. [◦C]
YieldA [%]
DiastereoselectivityB [%]
e.e.C [%]
Config.
1
2
3
4
5
6
7
8
C6H5
C6H5
4-FC6H4
4-FC6H4
4-ClC6H4
4-ClC6H4
2-ClC6H4
4-CH3C6H4
4-CH3C6H4
4-CH3OC6H4
4-CH3OC6H4
5a
0
−40
0
83
85
85
82
86
81
72
87
86
73
70
86
87
86
82
85
91
85
81
86
90
93
78
80
73
82
75
80
70
77
81
70
72
R,R
R,R
R,R
R,R
R,R
R,R
R,R
R,R
R,R
R,R
R,R
5b
5c
−40
0
−40
−40
0
5d
5e
9
10
11
−40
5f
0
−40
A Isolated yields.
B Determined by 1H NMR spectroscopy or gas chromatography.
C Determined by chiral HPLC on a Chiralcel OD-H column.
83 (84.50), 55 (75.51). νmax (KBr)/cm−1 2959, 2873, 1742, 1449, 1391,
1373, 1045, 1031.
temperature, and the reaction mixture was stirred for 0.5 h. The mixture
was then poured into a saturated solution (180 mL) of ammonium chlo-
ride and extracted with petroleum ether (150 mL × 3). The organic layer
was washed with a 2 M aqueous solution (100 mL × 3) of HCl and a sat-
urated solution (100 mL × 3) of sodium hydrogen carbonate, and was
then dried with anhydrous magnesium sulfate. After the solvent was
evaporated, the residue was separated by column chromatography (sil-
ica gel, petroleum ether–ether as eluent) to yield 2a (71%) as a colorless
oil. [α]2D0 −17.2◦ (c 1.0, EtOH). (Found: C 53.5, H 8.2%. C11H20OSe
requires C 53.4, H 8.2%.) δH 3.58 (1H d, J 7.5, CH), 3.16 (1H, d, J 7.5,
CH), 2.67 (1H, br, OH), 2.01 (3H, s, CH3), 1.99 (1H, d, J 3.3, 1 of
CH), 1.79–1.76 (1H, m, 1 of CH2), 1.54–1.45 (1H, m, 1 of CH2), 1.16–
1.06 (2H, m, CH2), 1.03 (3H, s, CH3), 0.98 (3H, s, CH3), 0.80 (3H, s,
CH3). m/z (EI) 248 (68%, M+), 233 (6.60), 177 (1.68), 135 (54.94), 109
(89.81), 95 (96.84), 43 (100.00). νmax (KBr)/cm−1 3425, 2951, 1474,
1456, 1390, 1370, 1289, 1092.
exo-α-Phenylseleno Camphor 1b
This was synthesized by a similar method for 1a to yield 1b (81%) as
colorless oil. [α]2D0 +75.9◦ (c 1.0, EtOH). δH 7.66–7.64 (2H, m, PhH),
7.28–7.24 (3H, m, PhH), 3.42 (1H, s, CH), 2.35 (1H d, J 3.9, CH), 2.08–
1.31 (4H, m, 2CH2), 0.95 (3H, s, CH3), 0.89 (3H, s, CH3), 0.81 (3H, s,
CH3). νmax (KBr)/cm−1 3056, 2959, 1746, 1757, 1477, 1026, 739, 692.
exo-α-Methylseleno Isoborneol 2a
To the solution of exo-α-methylseleno camphor 1 (3.43 g, 14.0 mmol)
in dichloromethane (60 mL) was added a 25% solution of diisobuty-
laluminium hydride (12 mL, 21 mmol) in toluene, dropwise, at room

752
X.-L. Li, Y. Wang, and Z.-Z. Huang
exo-α-Phenylseleno Isoborneol 2b
trans-(2R,3R)-2-p-Tolyl-3-phenyloxirane 5e
This was synthesized by a similar method for 2a to yield 2b (75%) as
a colorless oil. [α]2D0 −13.3◦ (c 1.0, EtOH). (Found: C 62.0, H 7.2%.
C16H22OSe requires C 62.1, H 7.2%.) δH 7.54–7.51 (2H, m, PhH),
7.30–7.26 (3H, m, PhH), 3.74 (1H, d, J 7.5, CH), 3.58 (1H, d, J 7.5,
CH), 2.78 (1H, br, OH), 2.14 (1H, d, J 4.2, CH), 1.90–1.70 (1H, m, 1 of
CH2), 1.57–1.49 (1H, m, 1 of CH2), 1.27–1.15 (2H, m, CH2), 1.13 (3H,
s, CH3), 1.03 (3H, s, CH3), 0.85 (3H, s, CH3). m/z (EI) 310 (56.48%,
M+), 312 (9.75), 246 (3.25), 229 (6.82), 157 (45.21), 135 (52.41), 109
(77.00), 43 (100.00). νmax (KBr)/cm−1 3445, 3070, 3056, 2952, 2879,
1578, 1477, 1060, 732.
White solid, mp 60–61◦C (lit.[9a] 62◦C). δH 7.40–7.19 (9H, m, ArH),
3.87 (1H, d, J 1.8, OCH), 3.85 (1H, d, J 1.8, OCH), 2.38 (3H, s, CH3).
νmax (KBr)/cm−1 3052, 2916, 1406, 1111, 816, 738, 509.
trans-(2R,3R)-2-(4-Methoxyphenyl)-3-phenyloxirane 5f
White solid, mp 76–78◦C (lit.[9d] 76–78◦C). δH 7.40–6.91 (9H, m,ArH),
3.87 (1H, d, J 1.9, OCH), 3.84 (1H, d, J 1.9, OCH), 3.83 (d, J 1.9, 1H).
νmax (KBr)/cm−1 3043, 2967, 1614, 1517, 1254, 1032, 826.
Acknowledgments
Selenonium Salt 3a
We gratefully acknowledge the National Natural Science
Foundation of China for its financial support of the project
20332050.
A solution of exo-α-methylseleno isoborneol 2a (2.10 g, 8.5 mmol) and
benzyl bromide (1.54 g, 9.0 mmol) in ether (10 mL) was stirred at 0◦C
for 24 h. The white solid formed was filtered off, washed with ether, and
recrystallized from ethanol–ethyl acetate to yield 3a (85%) as a white
solid, mp 138◦C. [α]2D0 +153.8◦ (c 1.0, EtOH). (Found: C 51.8, H 6.6%.
C18H27OSeBr requires C 51.7, H 6.5%.) δH 7.62–7.37 (5H, m, PhH),
6.51 (1H, br, OH), 5.56 (1H, d, J 11.1, 1 of CH2Ph), 5.44 (1H, d, J 11.1,
1 of CH2Ph), 4.16–4.13 (2H, m, 2CH), 2.55 (3H, s, CH3), 1.84–1.78
(2H, m, CH + 1 of CH2), 1.70–1.60 (1H, m, 1 of CH2), 1.48–1.40 (1H,
m, 1 of CH2), 1.14 (3H, s, CH3), 1.11–1.07 (1H, m, 1 of CH2), 0.98 (3H,
s, CH3), 0.81 (3H, s, CH3). m/z (EIS, positive mode) 339.0 [M − Br].
νmax (KBr)/cm−1 3200, 3039, 2990, 2944, 1456, 1070, 760, 700.
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56.8, H 6.1%. C23H29BF4OSe requires C 56.7, H 6.0%.) δH 7.67–7.54
(5H, m, PhH), 7.20–7.01 (6H, m, PhH + OH), 5.25 (1H, d, J 11.4, CH),
4.99 (1H, d, J 6.1, 1 of PhCH2), 4.94 (1H, d, J 11.5, CH), 4.80 (1H,
d, J 6.2, 1 of PhCH2), 1.76–1.71 (1H, m, 1 of CH2), 1.62–1.61 (1H,
d, J 4.5, CH), 1.45–1.36 (2H, m, CH2), 1.31 (3H, s, CH3), 1.25–1.14
(1H, m, 1 of CH2), 0.98 (3H, s, CH3), 0.76 (3H, s, CH3). m/z (EIS,
positive mode) 401.0 [M − BF4]. νmax (KBr)/cm−1 3464, 3134, 2953,
1477, 1455, 1062, 750, 696.
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Asymmetric Synthesis of trans-(2R,3R)-Diaryl Epoxides 5;
General Procedure
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To the solution of selenonium salt 3b (1.0 mmol) and an aldehyde
(0.49 g, 1.0 mmol) inTHF (10 mL) was added ButOK (0.44 g, 4.0 mmol)
at −40◦C. The reaction mixture was stirred for 12–18 h at this temper-
ature. The mixture was then filtered through a short silica gel column.
Afterevaporationofthesolvent, theresiduewasseparatedbypreparative
thin-layer chromatography to give the trans-diaryl epoxides 5.
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trans-(2R,3R)-2,3-Diphenyloxirane 5a
White solid, mp 68–69◦C (lit.[9a] 69◦C). δH 7.43–7.28 (10H, m, ArH),
3.90 (2H, s, 2OCH). νmax (KBr)/cm−1 3060, 2987, 1452, 1071, 851,
748, 695, 611.
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trans-(2R,3R)-2-(4-Fluorophenyl)-3-phenyloxirane 5b
White solid, mp 76–78◦C (lit.[9e] 76–77◦C). δH 7.41–7.09 (9H, m,ArH),
3.87 (1H, d, J 1.8, OCH), 3.85 (1H, d, J 1.8, OCH). νmax (KBr)/cm−1
3044, 2990, 1512, 1230, 1087, 831, 778, 697.
[9] (a) M. Imuta, H. Ziffer, J. Org. Chem. 1979, 44, 2505.
doi:10.1021/JO01328A038
(b) R. Hayakawa, M. Shimizu, Synlett 1999, 1328.
(c) L. Wang, Z. Z. Huang, J. Chem. Res. 2003, 305.
(d) S. Futamura, S. Kusunose, J. Chem. Soc., PerkinTrans. 1 1984,
15. doi:10.1039/P19840000015
trans-(2R,3R)-2-(4-Chlorophenyl)-3-phenyloxirane 5c
White solid, mp 99–100◦C (lit.[9a] 100◦C). δH 7.39–7.15 (9H, m, ArH),
3.86 (1H, d, J 1.9, OCH), 3.84 (1H, d, J 1.9, OCH). νmax (KBr)/cm−1
3052, 1492, 1460, 1091, 819, 751, 700.
(e) V. K. Aggarwal, E. Alonso, I. Bae, G. Hynd, K. M. Lydon,
M. J. Palmer, M. Patel, M. Porcelloni, J. Richardson, R. A. Sten-
son, J. R. Studley, J.-L.Vasse, C. L. Winn, J.Am. Chem. Soc. 2003,
125, 10926. doi:10.1021/JA034606+
trans-(2R,3R)-2-(2-Chlorophenyl)-3-phenyloxirane 5d
Colorless oil.[9b] δH 7.42–7.28 (9H, m, ArH), 4.25 (1H, d, J 1.9, OCH),
3.79 (1H, d, J 1.9, OCH). νmax (KBr)/cm−1 3065, 2986, 1478, 1275,
1128, 750, 700, 612.
[10] J. N. Denis, J. Vicens, A. Krief, Tetrahedron Lett. 1979, 20, 2697.
doi:10.1016/S0040-4039(01)86390-5