Catalytic enantioselective Mukaiyama-Michael reaction of 2-(trimethylsilyloxy)furan with α′-phenylsulfonyl enones

Article abstract of DOI:10.1055/s-2008-1032074

The Mukaiyama-Michael reaction of 2-(trimethylsilyloxy)furan with α′-phenylsulfonyl enones using bis(oxazoline)-copper(II) complexes as chiral catalyst provides the γ-butenolides in high enantio- and diastereoselectivity. This approach is very useful for

Full text of DOI:10.1055/s-2008-1032074

LETTER
555
Catalytic Enantioselective Mukaiyama–Michael Reaction of 2-(Trimethyl-
silyloxy)furan with a¢-Phenylsulfonyl Enones
Enantioselective
M
y
ukaiyama–M
e
ichael
R
ea
y
ction eon Yang, Sunggak Kim*
Center for Molecular Design & Synthesis and Department of Chemistry, School of Molecular Science, Korea Advanced Institute of Science
and Technology, Daejeon 305701, Korea
Fax +82(42)8698370; E-mail: skim@kaist.ac.kr
Received 5 November 2007
purpose with various electrophiles such as carbonyl
Abstract: The Mukaiyama–Michael reaction of 2-(trimethylsi-
compounds9 and acetals10 in the presence of a Lewis acid
to afford the corresponding 4-substituted butenolides. The
Michael reaction of 2-(trialkylsilyloxy)furans provides
lyloxy)furan with a¢-phenylsulfonyl enones using bis(oxazoline)–
copper(II) complexes as chiral catalyst provides the g-butenolides
in high enantio- and diastereoselectivity. This approach is very use-
ful for the enantioselective synthesis of g-butenolide derivatives un- the butenolides bearing a C4-substituent functionalized at
der chiral Lewis acid catalysis.
its terminal carbon which allows the further extension of
the substituent. Accordingly, this approach is expected to
become an efficient tool for the enantioselective synthesis
of various butenolide derivatives under chiral Lewis acid
catalysis. In fact, several reports on the Michael addition
reactions of 2-silyloxyfurans catalyzed by chiral Lewis
acids have appeared. Katsuki first examined the Mukaiya-
ma–Michael reaction of 2-(trimethylsilyloxy)furan with
oxazolidinone enoates using chiral scandium(III)–BINOL
derivatives or a copper(II)–bis(oxazoline) complex as
chiral catalysts.11 The former catalyst gave very high anti/
syn selectivity (>50:1), but only modest enantioselectivity
(68% ee), whereas the latter exhibited high enantioselec-
tivity (95% ee), but somewhat low anti/syn selectivity of
(8.5:1). Desimoni12 and Suga13 also reported the Mukaiya-
ma–Michael reaction of 2-(trimethylsilyloxy)furans using
(E)-3-crotonyl-1,3-oxazolidin-2-one under chiral Lewis
acid catalysis. In addition, it is noteworthy that an asym-
metric Michael reaction of 2-silyloxyfurans with a,b-un-
saturated aldehydes using an organic catalyst afforded
chiral g-butenolides with high syn-selectivity and enan-
tioselectivity.14
Key words: asymmetric catalysis, Michael additions, Lewis acids,
ligands, lactones
The Michael reaction is widely recognized as one of the
most important organic reactions for the formation of a
new carbon–carbon bond.1 Among the various Michael
reactions, the Mukaiyama reaction involving the Lewis
acid catalyzed reaction of a silyl enol ether with an a,b-
unsaturated carbonyl derivative has attracted a great deal
of attention due to its synthetic usefulness.2 The asymmet-
ric version of the Mukaiyama–Michael reactions has been
studied by several groups and includes the use of
(BINOL)Ti–oxo complex,3 (–)-trans-a,a¢-(dimethyl-1,3-
dioxolane-4,5-diyl)bis(diphenylmethanol) (TADDOL)-
derived titanium chloride,4a and bis(oxazoline)–Cu(II)
complexes.4b,5
The enantioselective synthesis of g-butenolide synthons
has become synthetically important6 because chiral g-
butenolides and their derivatives are common structural
subunits in natural products and biologically active com-
pounds,7 and chiral building blocks in organic synthesis.8
2-(Trialkylsilyloxy)furans have been widely used for this
In connection with our continued effort to develop enanti-
oselective conjugate addition reactions using a¢-phospho-
O
O
O
O
O
O
N
N
N
N
N
N
Ph
Ph t-Bu
t-Bu
1a (R)-PhBox
1b (S)-t-BuBox
1c (S)-InBox
O
O
O
O
Ph
Ph
Ph
Ph
N
N
N
N
Ph
1d (4R,5S)-diPhBox
Ph
Ph
1e (4R,5R)-diPhBox
Ph
Figure 1
ric enone 2, we initially investigated the Mukaiyama–
Michael reaction of 2-(trimethylsilyloxy)furan 3. Based
on our previous results of the Friedel–Crafts reactions,15
SYNLETT 2008, No. 4, pp 0555–0560
Advanced online publication: 23.01.2008
DOI: 10.1055/s-2008-1032074; Art ID: U10607ST
© Georg Thieme Verlag Stuttgart · New York
x
x.
x
x.
2
0
0
8

556
H. Yang, S. Kim
LETTER
O
O
O
H
MX/L*
Xc
+
R
OTMS
Xc
O
O
R
2 X = (MeO)2P(O)
5 X = PhO2S
4 X = (MeO)2P(O)
6 X = PhO2S
3
Scheme 1
Table 1 Screening of the Chiral Catalysts with 2aa
O
O
Cu(OTf)2/L*
O
O
H
+
(OMe)2P
CH2Cl2
0 °C
Me
OTMS
P(OMe)2
O
O
O
Me
4a
2a
3
Entry
L*
1a
1b
1c
Time (h)
Yield (%)
ee (%)b
anti/syn (%)c
1
2
3
0.2
7
92
85
88
82
–1
84
97:3
99:1
94:6
0.2
a All reactions were carried out on a 0.1-mmol scale.
b Enantiomeric excess was determined by chiral HPLC.
c The anti/syn ratio was determined by 1H NMR.
Table 2 Screening of the Chiral Catalysts with 5aa
O
MX/L*
O
H
O
(10 mol%)
+
SO2Ph
CHCl3
0 °C
Me
OTMS
SO2Ph
O
O
Me
5a
3
6a
Entry
MX
L*
1a
1a
1a
1a
1a
1b
1c
1d
1e
Time (h)
Yield (%)
ee (%)b
anti/syn (%)c
1
2
3
4
5
6
7
8
9
Cu(OTf)2
Zn(OTf)2
Mg(OTf)2
Sc(OTf)3
Yb(OTf)3
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
2
99
99
31
35
39
18
95
95
18
98
88
8
98:2
99:1
94:6
96:4
93:7
n.d.d
97:3
>99:1
83:17
18
72
72
72
2
10
–2
–3
64
99
18
2
2
2
a All reactions were carried out on a 0.1-mmol scale.
b Enantiomeric excess was determined by chiral HPLC.
c The anti/syn ratio was determined by 1H NMR.
d The ratio was not determined.
the reaction was carried out with several chiral bis(oxazo- yield and enantioselectivity (Scheme 1). However, (S)-t-
line) (Box) ligands (Figure 1) using copper(II) triflate (20 BuBox (1b) slowed down the reaction significantly and
mol%) in dichloromethane. As shown in Table 1, the re- almost no enantiomeric excess was obtained. Further im-
action was highly stereospecific and gave almost exclu- provements using a¢-phosphoric enone were not success-
sively the anti product. Good results were obtained with ful. We next turned our attention to a¢-phenylsulfonyl
(R)-PhBox (1a) and (S)-InBox (1c) in terms of chemical
Synlett 2008, No. 4, 555–560 © Thieme Stuttgart · New York

LETTER
Enantioselective Mukaiyama–Michael Reaction
557
Table 3 Effect of Solvent in Mukaiyama–Michael Reaction of 2-(Trimethylsilyloxy)furan 3 with a¢-Phenylsulfonyl Enone 5aa
O
Cu(OTf)2/1d
O
(5 mol%)
O
H
+
SO2Ph
solvent
0 °C
Me
OTMS
SO2Ph
O
O
Me
5a
3
6a
Entry
Solvent
CH2Cl2
CHCl3
THF
Time (h)
Yield (%)
ee (%)b
94
anti/syn (%)c
91:9
1
2
3
4
2
80
90
8
2
99
>99:1
72
72
80
90:10
toluene
14
62
94:6
a All reactions were carried out on a 0.1-mmol scale.
b Enantiomeric excess was determined by chiral HPLC.
c The anti/syn ratio was determined by 1H NMR.
enone. Previously, Kanemasa utilized the a¢-phenylsulfo- Generally, better ee anti/syn selectivities were realized
nyl enone 5 in asymmetric Diels–Alder reactions.16
with the use of more bis(oxazoline)–copper(II) catalyst.
When the reaction was carried out with 10 mol% of 1d–
Cu(OTf)2 complex, the best ee (>97% ee) and anti/syn se-
lectivity (99:1) were obtained. In the case of sterically
bulky 5f (entries 23 and 24), the reaction was incomplete
using 10 mol% of 1d–Cu(OTf)2 complex and required 20
mol% of 1d–Cu(OTf)2 complex for completion along
with a slightly lower anti/syn selectivity and enantioselec-
tivity.
To study the enantiomeric efficiency of the a¢-phenylsul-
fonyl enone, the reaction was carried out with enone 5a
and 2-(trimethylsilyloxy)furan 3 in the presence of several
chiral Box ligand–metal complexes (10 mol%) in chloro-
form at 0 °C and the experimental results are summarized
in Table 2. From experimental results obtained in this
study, several features are noteworthy. First, the a¢-phen-
ylsulfonyl enone exhibited higher enantioselectivity than
the a¢-phosphoric enone (98% ee vs 82% ee; entry 1). Sec-
ond, (4R,5S)-diPhBox (1d)17 and copper(II) complex gave
a slightly better result in terms of enantioselectivity and
anti selectivity than 1a (entry 8). Third, 1a and Zn(OTf)2
complex was also effective but slowed down the reaction
significantly (entry 2). Finally, other metal–ligand com-
plexes derived from Mg(OTf)2, Sc(OTf)3, and Yb(OTf)3
were ineffective, yielding very low ee along with slightly
lower anti/syn selectivities (entries 3–5).
O
O
O
O
1) H2, Pd/C
2) NaBH4
H
H
SO2Ph
SO2Ph
O
OH
6g (95% ee)
7 (71%)
[α]D +13.8 (c 1.1 in CH2Cl2)
O
O
1) Ac2O, DMAP,
pyridine
O
O
H2, Pd/C
H
H
2) Na(Hg),
Na2HPO4
We also studied the solvent effect using 5 mol% of 1d–
Cu(OTf)2 complex (Table 3). Chloroform gave the best
result in terms of chemical yield, enantio- and diastereo-
selectivity. However, tetrahydrofuran and toluene slowed
the reaction down drastically and reduced the ee signifi-
cantly (entries 3 and 4). To test the catalytic ability of 1d–
Cu(OTf)2 complex, when we reduced the amount of the
catalyst from 10 mol% to 3 mol%, the reaction was
slowed down and required five hours for completion with-
out a significant loss of the ee (98% ee) and the same anti/
syn selectivity. Thus, the remaining reactions were carried
out with several a¢-phenylsulfonyl enones 5 and 3 using 5
mol% and 10 mol% 1d–Cu(OTf)2 complex in chloroform
at 0 °C.
8 (81%)
9 (87%)
(S)-(–)-γ-butyrolactone
[α]D –47.1 (c 0.9 in MeOH)
lit. [α]D –48
O
O
1) OsO4, NaIO4
O
H
O
H
2) LiAlH4
SO2Ph
O
Me
Me
6a (99% ee)
10 (75%)
[α]D = +58.7 (c 1.0 in CH2Cl2)
Me
Me
TBDPSCl
HO
OH
RO
OR
DMAP,
pyridine
OH
OH
11 (61%)
12: R = TBDPS (85%)
Table 4 summarizes the experimental results and illus-
trates the scope and the efficiency of the present method.
The use of 5 mol% of 1a–Cu(OTf)2 complex slowed
down the reaction significantly and also reduced the anti/
syn ratio to some extent. Although the similar phenomena
were observed with 1d, 1a was more noticeable than 1d.
[α]D +2.6 (c 2.0 in CHCl3)
Scheme 2 Conversion of 6g and 6a into 9 and 12, respectively
To establish the absolute configuration and to demon-
strate the utility of the present approach, butenolide ad-
Synlett 2008, No. 4, 555–560 © Thieme Stuttgart · New York

558
H. Yang, S. Kim
LETTER
Table 4 Mukaiyama–Michael Reaction of 3 with a¢-Phenylsulfonyl Enones 5 Using Cu(II)–Bis(oxazoline) Complexesa,18
O
Cu(OTf)2/L*
O
O
(x mol%)
H
+
SO2Ph
CHCl3
0 °C
R
OTMS
SO2Ph
O
O
R
3
5
6
R = Me (6a), Ph (6e),
Et (6b), i-Pr (6f),
Pr (6c), H (6g),
R = Me (5a), Ph (5e),
Et (5b), i-Pr (5f),
Pr (5c), H (5g),
CH2CH2Ph (6d)
CH2CH2Ph (5d)
Entry
L*
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1d
1d
1d
1d
1d
1d
1d
1d
1d
1d
1d
1d
1d
R
x (mol%)
Time (h)
Yield (%)
85 (6g)
99 (6a)
82 (6a)
80 (6b)
17 (6b)
99 (6c)
16 (6c)
92 (6d)
45 (6d)
95 (6e)
72 (6e)
91 (6g)
95 (6a)
90 (6a)
79 (6b)
71 (6b)
92 (6c)
72 (6c)
88 (6d)
85 (6d)
>99 (6e)
>99 (6e)
75 (6f)
28 (6f)
ee (%)b
91
anti/syn (%)c
–
1
H (5g)
10
10
5
2
2
Me (5a)
Me (5a)
Et (5b)
Et (5b)
Pr (5c)
Pr (5c)
2
98
98:2
3
72
8
82
91:9
4
10
5
99
>99:1
92:8
5
96
3
95
6
10
5
99
83:17
69:31
95:5
7
96
3
65
8
CH2CH2Ph(5d)
CH2CH2Ph(5d)
Ph (5e)
10
5
>99
69
9
96
3
91:9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
10
5
97
>99:1
64:35
–
Ph (5e)
96
2
96
H (5g)
5
95
Me (5a)
10
5
2
99
>99:1
>99:1
98:2
Me (5a)
2
99
Et (5b)
10
5
72
96
18
96
24
96
2
99
Et (5b)
99
>99:1
>99:1
>99:1
>99:1
95:5
Pr (5c)
10
5
98
Pr (5c)
99
CH2CH2Ph(5d)
CH2CH2Ph(5d)
Ph (5e)
10
5
97
97
10
5
99
>99:1
75:25
97:3
Ph (5e)
12
96
96
99
i-Pr (5f)
20
10
97
i-Pr (5f)
97
95:5
a All reactions were carried out on a 0.1-mmol scale.
b Enantiomeric excess was determined by chiral HPLC.
c The anti/syn ratio was determined by 1H NMR.
duct 6g was converted to the natural product, (S)-(–)-g- rotation value of 9 and the previously known compound,
butyrolactone 9 by routine operations (Scheme 2). Hydro- the absolute configuration was assigned as S.19 Further-
genation and reduction of the keto group afforded 7 in more, 6a was similarly converted into 10 by routine oper-
71% yield. Acetylation of 7 followed by desulfonation ations. Treatment of 10 with osmium tetroxide and
provided 8 in 81% yield, which was further reduced to the sodium periodate in aqueous THF followed by reduction
previously known product 9. Compared with the optical with lithium aluminum hydride afforded 11, which was
Synlett 2008, No. 4, 555–560 © Thieme Stuttgart · New York

LETTER
Enantioselective Mukaiyama–Michael Reaction
559
further converted into 12 (Scheme 2). According to the
optical rotation value of 12, it was an enantiomer of the
previously known product {[a]D –2.5 (c = 2.2 in
CHCl3)}.11b Thus, the configuration of (+)-anti-6a was
proven to be S,S.
References and Notes
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8921.
(5) Evans, D. A.; Rovis, T.; Kozlowski, M. C.; Tedrow, J. S.
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M.; García, F.; Batalla, C. Tetrahedron Lett. 2004, 45, 8549.
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Tetrahedron Lett. 2005, 46, 5889.
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(b) Szlosek, M.; Franck, X.; Figadère, B.; Cavé, A. J. Org.
Chem. 1998, 63, 5169. (c) Evans, D. A.; Kozlowski, M. C.;
Murry, J. A.; Burgey, C. S.; Campos, K. R.; Connell, B. T.;
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Lett. 1991, 32, 3795. (b) Morimoto, Y.; Nishida, K.;
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Figure
Box]Cu(OTf)2(H2O)2
2
X-ray
crystallography
of
[(4R,5S)-diPh-
The structure of [(4R,5S)-diPhBox]Cu(OTf)2(H2O)2 was
determined by X-ray crystallography (Figure 1). This
complex shows the octahedral geometry, the counter ions
weakly coordinate to the apical site and two H2O’s are lo-
cated in equatorial site. Based on X-ray crystallographic
information and the observed enantioselectivity in this
study, a tentative model would have the s-trans enone
template in a distorted square planar geometry. The s-
trans conformation would be attributed to a p–p stabiliza-
tion of the transition state as shown in Figure 2.20
Ph
O
Ph
N
Cu
O
O
Ph
O
N
O
S
Ph
Si-face
Figure 3 The expected model of the substrate–catalyst complex
(11) (a) Kitajima, H.; Katsuki, T. Synlett 1997, 568.
(b) Kitajima, H.; Ito, K.; Katsuki, T. Tetrahedron 1997, 53,
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Asymmetry 1998, 9, 1165.
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M. G.; Zema, M. Tetrahedron 2001, 57, 10203.
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Commun. 2004, 1414.
(14) Brown, S. P.; Goodwin, N. C.; MacMillan, D. W. C. J. Am.
Chem. Soc. 2003, 125, 1192.
In summary, we have shown that the a¢-phenylsulfonyl
enone templates are highly efficient for the catalytic enan-
tioselective Mukaiyama–Michael reactions. Using 1d–
Cu(OTf)2 complex, the Mukaiyama–Michael reaction of
2-(trimethylsilyloxy)furan with a¢-phenylsulfonyl enones
affords g-butenolides in high enantioselectivities up to
99% ee and high anti/syn selectivities (up to 99:1).
(15) Yang, H.; Hong, Y.-T.; Kim, S. Org. Lett. 2007, 9, 2281.
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23, 145. (b) Wada, E.; Yasuoka, H.; Kanemasa, S. Chem.
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Acknowledgment
We thank KOSEF (R01-2007-000-20315-0), CMDS, and KEPCO
for financial support.
Synlett 2008, No. 4, 555–560 © Thieme Stuttgart · New York

560
H. Yang, S. Kim
LETTER
(18) Typical Procedure for the Mukaiyama–Michael
Reaction of 2-(Trimethylsilyloxy)furan (3) with a¢-
Phenylsulfonyl Enone 5a: To the flask of freshly prepared
[(4R,5S)-diPhBox]Cu(OTf)2 (5 mol%) in CHCl3 (1 mL) was
added a solution of a¢-phenylsulfonyl enone 5a (0.11 mmol,
25 mg) in CHCl3 (1 mL). After being stirred at r.t. for 0.5 h,
the reaction mixture was cooled to 0 °C, and the solution of
3 (0.22 mmol, 35 mg) in CHCl3 (1 mL) was added to the
reaction mixture. The reaction was maintained at the desired
temperature until a complete consumption of the a¢-
2360, 1731, 1612, 1566, 1490, 1432, 1353, 1223, 1116,
1060, 1005, 946, 870, 783, 720, 669 cm–1. HRMS (ESI):
m/z [M + H]+ calcd for C15H17O5S: 309.0798; found:
309.0791. Product ratio was determined by HPLC analysis
(Chiralcel OD-H, 30%
i-PrOH–hexane, 0.6 mL/min, l = 254 nm); S,S-isomer
(major): tR = 42.4 min and R,R-isomer (minor): tR = 53.7
min; [a]D25 +58.7° (c = 1.0 in CH2Cl2).
(19) (a) Utaka, M.; Watabu, H.; Takeda, A. J. Org. Chem. 1987,
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phenylsulfonyl enone 5a as monitored by TLC. After
completion of the reaction, the reaction was quenched with
sat. aq NaHCO3 solution, extracted with CH2Cl2, washed
with H2O, dried over MgSO4, filtered, and concentrated
under reduced pressure. The residue was purified by silica
gel column chromatography to give 6a as an oil in 99% yield
(34 mg). Spectroscopic data for 6a: 1H NMR (400 MHz,
CDCl3): d = 1.01 (d, J = 6.9 Hz, 3 H), 2.41 (m, 1 H), 2.60 (dd,
J = 18.8, 7.2 Hz, 1 H), 2.84 (dd, J = 18.8, 5.3 Hz, 1 H), 4.14
(dd, J = 21.4, 13.3 Hz, 2 H), 4.90 (dd, J = 6.1, 3.2 Hz, 1 H),
6.08 (dd, J = 5.8, 1.9 Hz, 1 H), 7.43 (d, J = 5.9 Hz, 1 H), 7.56
(t, J = 8.0 Hz, 2 H), 7.67 (t, J = 7.7 Hz, 1 H), 7.84 (d, J = 7.4
Hz, 2 H). 13C NMR (100 MHz, CDCl3): d = 16.02, 31.88,
46.12, 66.86, 85.73, 122.26, 128.05, 129.34, 134.32, 138.60,
154.66, 172.39, 196.34. IR (KBr): 3700, 3139, 3022, 2953,
Synlett 2008, No. 4, 555–560 © Thieme Stuttgart · New York

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