Enantioselective darzens reaction using organoselenide-lithium hydroxide complexes

Article abstract of DOI:10.1016/j.tetlet.2010.08.082

Asymmetric Darzens reaction catalyzed by chiral selenides is described. A novel Lewis acid/Br?nsted base catalyst formed by C₂ symmetric chiral selenide-bearing isoborneol skeletons, which were readily prepared from (1S)-10-camphorsulfonic acid, and LiOH promoted the reaction of phenacyl bromide with aldehydes to afford the desired trans oxiranes with up to 62% ee.

Full text of DOI:10.1016/j.tetlet.2010.08.082

Tetrahedron Letters 51 (2010) 5778–5780
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Enantioselective Darzens reaction using organoselenide–lithium
hydroxide complexes
a,
⇑
b
b
b
c
Shin-ichi Watanabe , Risa Hasebe , Jun Ouchi , Hideko Nagasawa , Tadashi Kataoka
a
College of Pharmacy, Kinjo Gakuin University, 1723 Omori 2 chome, Moriyama-ku, Nagoya 463-8521, Japan
Gifu Pharmaceutical University, 1-25-4 Daigaku-nishi, Gifu 501-1196, Japan
Yokohama College of Pharmacy, 601 Matanocho, Totsuka-ku, Yokohama 245-0066, Japan
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Asymmetric Darzens reaction catalyzed by chiral selenides is described. A novel Lewis acid/Brønsted base
catalyst formed by C symmetric chiral selenide-bearing isoborneol skeletons, which were readily pre-
pared from (1S)-10-camphorsulfonic acid, and LiOH promoted the reaction of phenacyl bromide with
aldehydes to afford the desired trans oxiranes with up to 62% ee.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 5 July 2010
Revised 25 August 2010
Accepted 27 August 2010
2
The Darzens reaction is considered to be one of the most com-
mon and powerful tool for the synthesis of oxiranes bearing elec-
tron-withdrawing groups.1 Enantiomerically enriched oxirane
derivatives have been particularly demonstrated as useful chiral
intermediates in organic synthesis.2 The vigorously investigated
methods to approach catalytic asymmetric synthesis of oxiranes
are represented by ylide-mediated epoxidation in the category of
mide and benzaldehyde under basic conditions using lithium
hydroxide (Table 1). The reaction in CH Cl without a chalcogenide
2
2
proceeded hardly at all within 12 h (entries 1 and 2). In the case of
using 0.3 equiv of dimethyl selenide 2a, a 10% yield of 3a was de-
tected after 5 h. However, a prolonged reaction time up to 12 h
gave the trans oxirane 3a in 45% yield with a small amount of
7
trans-1,2,3-tribenzoylcyclopropane (entries 3 and 4). Surprisingly,
2
e
direct oxirane formation with carbonyl-containing compounds.
dimethyl sulfide also catalyzed the reaction to afford the epoxide
in 50% yield with trans selectivity after 12 h (entries 5 and 6). In
general, sulfur ylides bearing an anion-stabilizing group are too
Despite this developing research, relatively few procedures using
chiral phase-transfer agents are available for the enantioselective
3
8
catalytic Darzens reaction. More recently, an asymmetric Darzens
stable to react with an aldehyde. The results indicate that chalcog-
reaction with a chiral Lewis acid giving moderate to high enanti-
enides promoted the reaction and that the oxirane formation pro-
ceeds not via chalcogen-ylides but via a Darzens reaction.
To develop enantioselective oxirane synthesis using a chalco-
oselectivity has been reported.4
In the course of our investigations into the reactivities of sele-
nonium salts, we disclosed the reactivities of selenonium ylides,
which were formed by the reactions of selenonium salts bearing
2
genide, we designed new chiral C symmetric chalcogenides with
a bornane structure because there are many examples for the
ylide-mediated epoxidation using bornane derivatives as a chiral
5
unsaturated carbon–carbon bond groups with nucleophiles. Due
9
to the interesting features of these compounds, we focused on
source. In addition, high enantioselectivities have been reported
the catalytic formation of chiral b-ketoselenonium ylides, which
were generated by the treatment of
a-halocarbonyl compounds
Table 1
with optically pure selenides, and applications reacted to the
asymmetric epoxidation, because selenium ylides react easily with
aldehydes to give oxiranes.5 Interestingly, the reaction of phena-
cyl bromide with aldehydes was found to be promoted by a sele-
nide catalyst under basic conditions to give oxiranes. We report
herein a novel type of oxirane formation catalyzed by selenides
and the development of an asymmetric version of the reaction with
a novel chiral Lewis acid/Brønsted base catalyst derived from the
optically active selenide and lithium hydroxide.
Reactions of PhC(O)CH
2
Br and PhCHO with LiOH in the presence of a chalcogenide
i,6
Me2Ch 2 (0.3 eq.)
O
LiOH
(4.0 eq.)
Ph
Br
O
+
PhCHO
Ph
Ph
O
rt
1a
3a
(4.0 eq.)
(1.0 eq.)
Entry
Chalcogenide 2
Time (H)
Yield of 3a (%)
In order to verify the efficacy of a chalcogenide as catalyst, the
oxirane formation reaction was undertaken with phenacyl bro-
1
2
3
4
5
6
—
—
5
12
5
12
5
Trace
Trace
10
45
Trace
50
2a (Ch@Se)
2a (Ch@Se)
2b (Ch@S)
2b (Ch@S)
⇑
Corresponding author. Tel.: +81 52 798 0180; fax: +81 52 798 0754.
E-mail address: swatana@kinjo-u.ac.jp (S. Watanabe).
12
0
040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.08.082

S. Watanabe et al. / Tetrahedron Letters 51 (2010) 5778–5780
5779
from the reaction with C
2
symmetric sulfide and selenide cata-
Table 2
1
0
Asymmetric Darzens reaction with aldehydes using chiral chalcogenides
lysts. Novel C
2
symmetric selenides can be easily prepared from
11
(
1S)-10-iodocamphor 4. Thus (1S)-10-iodocamphor 4 was trea-
ted with lithium selenide, which was prepared from elemental
selenium and lithium triethylborohydride,12 to afford di-10-cam-
phoryl selenide 5 in 87% yield. Fortunately, the reduction of ketone
chalcogenide
(0.3 eq.)
LiOH
O
(4.0 eq.)
Ph
Br + RCHO
Ph
R
groups with LiAlH
4
gave highly symmetrical di-exo-2-hydroxy-10-
O
O
solvent
rt, 5 h
1
bornyl selenide 6 bearing isoborneol skeletons as a single diaste-
reomer in good yield (Scheme 1).1 The stereochemistry of the hy-
droxy groups was determined to be an exo-configuration by
comparison of the NMR chemical shift of the hydroxymethine pro-
ton with those of analogs of 6.14 Hydroxymethine protons of 10-
chalcogenomethyl-substituted borneol and isoborneol were ob-
served at d 4.46–4.53 and d 3.86–3.94, respectively. The 10-meth-
ylselenomethylisoborneol showed a methine signal at d 3.86,
which is very close to that at d 3.85 of 6.
3
(4.0 eq.)
(1.0 eq.)
3
Eea (%)
Entry
R
Solvent
Chalcogenide Yield of 3 (%)
1
2
3
4
5
6
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
CH
CH
2
Cl
Cl
2
5
6
9
10
6
6
6
6
6
—
6
6
6
6
6
6
6
3a (14)
3a (66)
3a (3)
1 (2S,3R)
48 (2S,3R)
0
5 (2S,3R)
62 (2S,3R)
0
1 (2S,3R)
4 (2S,3R)
2 (2S,3R)
—
2 (2S,3R)
46 (2S,3R)
48 (2S,3R)
56 (2S,3R)
45 (2S,3R)
4 (2S,3R)
7 (2R,3S)
2
2
CH Cl2
2
CH
CHCl
CCl
THF
Et
2
Cl
2
3a (8)
3
3a (71)
3a (42)
3a (89)
3a (74)
3a (85)
3a (40)
3a (61)
3b (86)
3c (95)
3d (65)
3e (28)
3f (49)
3g (20)
4
b
7
Di-exo-2-methoxy-10-bornyl selenide 9 was synthesized in 51%
yield by the reaction of exo-2-methoxy-10-bornyl iodide 8, which
was derived by methylation from exo-2-hydoxy-10-bornyl iodide
8
9
2
O
b
MeCN
MeCN
Toluene
10
11
c
1
1
7
using methyl triflate, with lithium selenide at 80 °C. The com-
1
1
2
3
4-NO
4-ClC
4-BrC H4
2
C
6
H
4
CHCl
CHCl
3
pound 7 reacted with lithium sulfide in DMF at room temperature
to give di-exo-2-hydroxy-10-bornyl sulfide 10 (31%). However, the
reaction with lithium selenide yielded a complex mixture, and
compound 6 was not obtained (Scheme 2).
We first investigated the asymmetric Darzens reaction with chi-
ral chalcogenide catalysts (Table 2). The reaction was carried out
with a variety of catalysts bearing bornane structures for 5 h under
the same conditions as shown in Table 1. The selenide catalysts 5
and 9 behaved similarly to dimethyl selenide to give the oxirane
6 4
H
3
14
15
6
CHCl3
4-MeC
Et
6
H
4
CHCl
CHCl
CHCl
3
3
3
1
1
6
7
b
2 2
Ph(CH )
a
b
c
Determined by HPLC with CHIRALCEL OD and CHIRALPAK IB.
Reaction time is 3 h.
Reaction time is 12 h.
3
a in low yields without enantioselectivity (entries 1 and 3). How-
3
aldehydes in CHCl . The reaction with aromatic aldehydes gave
ever, the reaction was dramatically promoted by using selenide 6,
and the yield of 3a was improved to 66% with 48% ee (entry 2). On
the other hand, when sulfide 10, the sulfur analog of 6, was em-
ployed, the outcome was unsuccessful (entry 4). The best result
for 3a was obtained with CHCl
although the use of CCl , which is one of the chlorinated solvents,
gave no enantioselectivity (entries 5 and 6). In the cases of other
solvents, the reactions gave good chemical yields with very low
enantiomeric excess (entries 7–10). To broaden the scope, we
carried out the asymmetric Darzens reaction with a variety of
3
b, 3c, and 3d in good yields except for 3e with moderate enanti-
oselectivities (entries 12–15). The cis isomers of 3 were not ob-
tained in these reactions. Meanwhile, the reaction with aliphatic
aldehydes produced complicated mixtures, and the trans epoxides
3
up to a 71% yield and 62% ee,
3
f and 3g were isolated with lower enantiomeric excesses (entries
4
1
6 and 17). These reactions proceeded under heterogeneous condi-
tions. The absolute configurations of the major enantiomers of 3
were determined to be (2S,3R) (3g is (2R,3S)) by comparison of
the optical rotation and HPLC data with the reported ones.3
To determine the mechanism for the formation of oxiranes
through the ylide pathway or the Darzens reaction, we prepared
the selenonium salt 11 corresponding to the intermediate for the
ylide formation pathway and carried out its reaction with benzal-
dehyde. The synthesis of selenonium triflate 11 was achieved by
the reaction of selenide 6 and phenacyl bromide with silver triflate
in 85% yield. Next, the reaction of 11 with benzaldehyde using
4 equiv of LiOH toward the selenonium salt hardly proceeded
and gave low enantioselectivity (Scheme 3). The findings suggest
that the Darzens reaction is more feasible for oxirane formation
using the selenide catalyst than the ylide pathway.
j,15
(
a)
(b)
OH
O
O
Se
Se
I
O
HO
4
5
6
Scheme 1. Reagents and conditions: (a) Se, LiEt
THF, 0 °C–rt (88%).
3 4
BH, DMF, 80 °C (87%); (b) LiAlH ,
Only a modest accelerative effect of a chalcogenide on oxirane
formation would be caused by the Lewis acid–Lewis base interac-
tion between the lithium cation and the chalcogen atom as solva-
tion. The use of solvents having a higher coordinating ability for
lithium cations increased the yields of oxiranes (entry 10 in Table
(a)
(b)
OMe
OH
OMe
Se
I
I
MeO
7
8
9
(
c)
O
OH (a)
OH
O
(b)
OH
Ph
Se
Se
Ph
O
S
HO
Ph
HO
HO
3a
TfO
6
10
11
Scheme 2. Reagents and conditions: (a) TfOMe, proton sponge, DCM, reflux (70%);
b) Se, LiEt BH, DMF, 80 °C (51%); (c) Li S, DMF, 80 °C (31%).
Scheme 3. Reagents and conditions: (a) PhC(O)CH
2
Br, AgTfO, acetone, rt (85%); (b)
(
3
2
PhCHO, LiOH, CH Cl , rt, 3 h (5%, 8% ee) or 24 h (8%, 0% ee).
2
2

5
780
S. Watanabe et al. / Tetrahedron Letters 51 (2010) 5778–5780
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Figure 1. Formation of the complex between selenide 6 and LiOH.
2
2 2
). The lower solubility of lithium hydroxide for CH Cl was im-
proved by the addition of chalcogenides as a Lewis base, which
can interact with lithium cations. On the other hand, the yields
and enantioselectivities are greatly enhanced by using bis hydroxy
selenide 6, which would form the chair conformation with the lith-
ium cation, selenium atom, and oxygen atom after the deprotona-
tion from one of the hydroxy groups. In addition, the weak
interaction between selenium atoms and lithium cations makes
the selenium atom electron-deficient, and the oxygen atom in an-
other hydroxy group would then interact with the electron-poor
selenium atom to construct a selenurane-type intermediate pos-
sesses both a Lewis acid part and a Brønsted base part (Fig. 1). The
difference in the reactions using 6 and 10 is attributed to easy for-
mation of selenuranes compared to that of sulfuranes. Unfortu-
nately, the observation of the interaction between Se and Li by
NMR was unsuccessful.
In conclusion, we found that a catalytic amount of chalcoge-
nides accelerated the oxirane preparation reaction of phenacyl bro-
mide and an aldehyde with lithium hydroxide. In particular, the
complexation of di-exo-2-hydroxy-10-bornyl selenide 6 with lith-
ium hydroxide formed a novel Lewis acid/Brønsted base catalyst
to give good yield and moderate stereoselectivities through the
Darzens reaction pathway. The mechanism for the enantioselectiv-
ity in the reaction is currently under investigation.
2007, 48, 813.
6
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4c
8
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Acknowledgment
1
1
1. Sell, T.; Laschat, S.; Dix, I.; Jones, P. G. Eur. J. Org. Chem. 2000, 4119.
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This research was partially supported by a Grant-in-Aid (No.
0590021) from the Ministry of Education, Culture, Sports, Science
and Technology (Japan).
2
13. (1R,1
0
R,2R,2
0
R,4R,4
0
R)-1,1
0
-(Selenobis(methylene)bis(7,7-dimethylbicyclo[2.2.1]-
heptan-2-ol) (6): To
a
mixture of lithium aluminum hydride (209 mg,
5
.51 mmol) in 6 mL of dry THF a solution of di-10-camphoryl selenide (5)
(
1.50 g, 3.93 mmol) in 39 mL of dry THF was added at 0 °C under Ar in small
References and notes
portions. The mixture was stirred at room temperature overnight. The excess
of hydride was cautiously quenched with water and finally with 15% sodium
hydroxide. The resulting mixture was filtered through a Celite pad and
washed thoroughly with ethyl acetate, and the filtrate was washed with
brine. The ethyl acetate extracts were dried over anhydrous magnesium
sulfate and concentrated under reduced pressure. The residue was purified
1
.
For reviews, see: (a) Rosen, T. In Comprehensive Organic Synthesis; Trost, B. M.,
Fleming, I., Eds.; Pergamom: Oxford, 1991; Vol. 2, pp 409–439; (b) Newman, M.
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2
.
(a) Smith, J. G. Synthesis 1984, 629; (b) Berkessel, A.; Gröger, H. In Asymmetric
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by chromatography on silica gel (benzene/diethyl ether = 10:1) to afford
1
1.33 g (88%) of 6 as a white solid: mp 212–224 °C; H NMR (400 MHz) (CDCl
3
)
d: 0.85 (6H, s, 2CH
(2H, m, 2CH of CH
2CH
3
), 1.01–1.08 (2H, m, 2CH), 1.06 (6H, s, 2CH
), 1.53 (2H, td, J = 12.1, 4.2, 2CH of CH
2
3
), 1.16–1.22
), 1.67–1.82 (8H, m,
), 3.85
2
2
), 2.64 (2H, d, J = 10.5, 2CH of CH
2
), 2.86 (2H, d, J = 10.5, 2CH of CH
2
13
2
007, 107, 5841; (f) Wong, O. A.; Shi, Y. Chem. Rev. 2008, 108, 3958.
(2H, dd, J = 7.9, 3.5, 2CH); C NMR (100 MHz) d: 19.9 (q), 20.6 (q), 25.6 (t),
3
.
(a) Colonna, S.; Fornasier, R.; Pfeiffer, U. J. Chem. Soc., Perkin Trans. 1 1978, 8; (b)
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27.1 (t), 31.9 (t), 39.3 (t), 45.3 (d), 47.8 (s), 52.7 (s), 77.6 (d); MS (EI) m/z (rel
+
int. %) 386 (M , 5%), 135 (100); HRMS calcd for C20
H
34
O
2
Se: 386.1724. Found:
2
1
386.1727; ½
aꢀ
¼ ꢁ66:7 (c 1.0 in CH
2 2
Cl ).
D
14. (a) De Lucchi, O.; Lucchini, V.; Marchioro, C.; Vallem, G.; Modena, G. J. Org.
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1
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