Asymmetric michael addition of malonates to enones catalyzed by a primary Β-amino acid and its lithium salt

Article abstract of DOI:10.1021/jo201429w

Highly enantioselective Michael addition of malonates to enones was achieved using a mixed catalyst consisting of a primary Β-amino acid, O-TBDPS (S)-Β-homoserine, and its lithium salt. Various cyclic and acyclic enones were converted into 1,5-ketoesters in high yields (up to 92%) with high enantioselectivity (up to 97% ee) under mild reaction conditions. Details of synthesis of the catalyst, optimization of the reaction conditions for the Michael addition reaction, and a plausible reaction mechanism are described.

Full text of DOI:10.1021/jo201429w

NOTE
pubs.acs.org/joc
Asymmetric Michael Addition of Malonates to Enones Catalyzed by a
Primary β-Amino Acid and Its Lithium Salt
Masanori Yoshida,* Mao Narita, and Shoji Hara
Division of Chemical Process Engineering, Graduate School of Engineering, Hokkaido University, Kita 13-jo Nishi 8, Kita-ku, Sapporo,
Hokkaido 060-8628, Japan
S Supporting Information
b
ABSTRACT: Highly enantioselective Michael addition of
malonates to enones was achieved using a mixed catalyst
consisting of a primary β-amino acid, O-TBDPS (S)-β-homo-
serine, and its lithium salt. Various cyclic and acyclic enones
were converted into 1,5-ketoesters in high yields (up to 92%)
with high enantioselectivity (up to 97% ee) under mild reaction
conditions. Details of synthesis of the catalyst, optimization of
the reaction conditions for the Michael addition reaction, and a
plausible reaction mechanism are described.
ichael addition reaction of a nucleophile to an α,β-un-
saturated carbonyl compound is a strong candidate for the
various β-amino acid lithium salts to carry out a catalyst screen.
While esters 1c,d and a dipeptide 1e gave the Michael adduct 4a
in low to moderate yields (16À42%) with moderate to good
enantioselectivity (43À77% ee), a siloxymethyl-substituted β-amino
acid salt, O-TBDPS (S)-β-homoserine lithium salt (1f), provided
4a in a good yield (65%) with high enantioselectivity (82% ee)
(Table 1, entry 8). Since the use of siloxymethyl-substituted
α- and γ-amino acid salts, 1h5 and 1i, as catalysts resulted in
lower enantioselectivity than that from using the β-amino acid
salt catalyst 1f, a β-amino acid moiety was found to be necessary
to obtain the Michael adduct with high enantioselectivity
(Table 1, entries 9 and 10).6 Hence, we chose 1f as a catalyst
for further investigations.
After carrying out a solvent screen for Michael addition reaction
of 2a to 3a with catalyst 1f (see Supporting Information), we
found that a mixed solvent consisting of DMSO and dichlor-
oethane afforded the Michael adduct 4a with the best enantios-
electivity among common organic solvents (Table 2, entry 1).
We then attempted to improve the conversion of the Michael
addition reaction to increase the yield of 4a. Results obtained by a
screening of additives showed that the addition of a catalytic
amount of benzoic or acetic acid greatly increased the yield of 4a
without any loss of enantioselectivity (Table 2, entries 2 and 3).
The addition of a weaker or stronger acid or water also increased
the yield of 4a; however, the enantioselectivity was decreased
(Table 2, entries 4À6). Since amino acid salt catalyst 1f is a base,
we assumed that the addition of an acid generated an amino acid
1g in situ by an acidÀbase equilibrium reaction. This assumption
was supported by the fact that Michael addition reaction in
the presence of catalyst 1g and lithium benzoate gave a result
similar to that in the presence of catalyst 1f and benzoic acid
M
purpose of introducing a substituent into the β-position of the
carbonyl group since a large number of examples using various
Michael donors and acceptors have been reported. In the case of
creating a carbonÀcarbon bond by the Michael addition reac-
tion, malonates are recognized as useful Michael donors, and a
catalytic asymmetric version of the reaction has been achieved
using various catalysts including both organocatalysts and orga-
nometallic catalysts.1 As pioneering works in organocatalytic
asymmetric Michael addition of carbon nucleophiles to α,β-un-
saturated carbonyl compounds,2 Yamaguchi’s group reported
that an alkali metal salt of proline efficiently promoted the
Michael addition of malonates to enones.1mÀo In the course
of using a primary amino acid as an asymmetric catalyst,3,4 we
recently found that a lithium salt of primary α-amino acid was
also an effective catalyst for the asymmetric Michael addition of
malonates to enones; however, there was room for improvement
in the yields and enantioselectivity of the reaction.5 To investi-
gate the reaction using a primary amino acid catalyst in more
detail, we then planned to employ a β-amino acid as a catalyst
because it is known that both yield and enantioselectivity of
amino acid catalysis are greatly affected by the position of the
amino and carboxyl groups in the catalyst.6 In this report, we
present details of the Michael addition reaction of malonates to
enones catalyzed by a primary β-amino acid and its lithium salt
(Figure 1).
Initially, lithium salts of 4- and 1-benzyl L-aspartates 1a,b were
employed for the asymmetric Michael addition of dimethyl malonate
(2a) to 2-cyclohexen-1-one (3a) to evaluate the difference between
an α- and a β-amino acids as catalysts (Table 1, entries 1 and 2).
Although the reaction using a β-amino acid salt catalyst 1b was slow,
enantioselectivity was much higher than that in the reaction using
an α-amino acid salt catalyst 1a. Therefore, we decided to employ
Received: July 14, 2011
Published: September 06, 2011
r
2011 American Chemical Society
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|

The Journal of Organic Chemistry
NOTE
Table 2. Additive Screen for Michael Addition Reaction of 2a
to 3a in the Presence of Catalyst 1f and/or 1ga
entry
catalyst
additive
yieldb (%)
eec (%)
1
2
1f
1f
1f
1f
1f
1f
none
43
69
70
55
77
54
73
76
59
64
65
26
61
71
76
74
90
92
92
87
78
87
93
74
43
44
33
13
92
93
93
90
PhCO2H
CH3CO2H
p-NO2C6H4OH
p-CH3C6H4SO3H
H2Od
3
4
5
6
Figure 1. Catalysts used in the Michael addition of malonates to
enones.
7
1g
PhCO2Li
PhCO2Na
PhCO2K
PhCO2Rb
PhCO2Cs
none
8
1g
9
1g
Table 1. Catalyst Screen for Michael Addition of 2a to 3aa
10
11
12
13
14
15
16
1g
1g
1g
1f/1g (3:1)
1f/1g (1:1)
1f/1g (1:3)
1f/1g (1:6)
none
none
none
entry
catalyst
yieldb (%)
eec (%)
none
a Unless otherwise mentioned, the reactions were carried out with 1f
and/or 1g (total 0.05 mmol), 2a (1.0 mmol), 3a (0.5 mmol), and an
additive (0.05 mmol) in DMSO/(CH2Cl)2 (1:2, 1 mL) at 25 °C for
24 h. b Isolated yield of 4a based on 3a. c Determined by chiral HPLC
analysis. d 0.5 mmol of H2O was added.
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
81
22
16
42
25
65
76
70
63 (S)
72 (R)
74 (R)
43 (R)
77 (R)
82 (R)
69 (S)
51 (R)
We then employed other cyclic enones as substrates. Reaction of
2-cyclohepten-1-one (3b) was also successfully completed with-
in 2 days to give Michael adduct 4f in 89% yield with 97% ee,
while the reaction with 2-cyclopenten-1-one (3c) was very slow
and provided Michael adduct 4g in 61% yield after 7 days (Table 3,
entries 7 and 8).1a,g,l Acyclic enones were then subjected to the
reaction conditions. Benzalacetone (3d), 4-phenyl-3-buten-2-one,
was found to be a good substrate to provide Michael adduct 4h in a
good yield (92%) with high enantioselectivity (86% ee) (Table 3,
entry 9). Other 4-aryl-3-buten-2-ones, (E)-4-(furan-2-yl)- (3e)
and (E)-4-(thiophen-2-yl)-3-buten-2-one (3f), also gave similar
results, and the corresponding Michael adducts 4i (88%, 90% ee)
and 4j (86%, 85% ee) were successfully obtained (Table 3, entries
10 and 11). Although a 4-alkyl-3-buten-2-one, (E)-3-octen-2-one
(3g), required longer reaction time, Michael adduct 4k was
obtained in a good yield with high enantioselectivity (Table 3,
entry 12). Unfortunately, chalcone (3h), an aryl vinyl ketone, was
found to be an unsuitable substrate for the present reaction
conditions since a large amount of 3h was recovered after carrying
out the reaction for 7 days (Table 3, entry 13).
A plausible reaction mechanism of Michael addition reaction
of 2a to 3a catalyzed by a mixed catalyst 1f/1g is depicted in
Scheme 1. Since the use of amino acid 1g as a sole catalyst gave
the Michael adduct 4a in a low yield with very low enantioselec-
tivity (Table 2, entry 12), amino acid lithium salt 1f would
favorably react with 3a to form carbinolamine I. Probably, amino
acid 1g converted the carbinolamine I into carbinolamine II by
an acidÀbase equilibrium reaction to promote the formation of
imine III since dehydration of carbinolamine generally proceeds
smoothly in a weak acidic condition. Then malonate 2a was
added to the imine III from the less hindered side to generate
1h
1i
a Reactions were carried out with 1 (0.15 mmol), 2a (0.6 mmol), and 3a
(0.5 mmol) in DMSO/CH2Cl2 (1:1, 1 mL) at 25 °C for 24 h. b Isolated
yield of 4a based on 3a. c Determined by chiral HPLC analysis. Absolute
configurations of 4a determined by comparison of the specific rotation
with that of the literatures are shown in parentheses (see Supporting
Information).
(Table 2, entries 2 and 7). Since Michael addition reaction using
amino acid catalyst 1g with other alkali metal (Na, K, Rb, and Cs)
salts of benzoic acid or without any additives led to lower yields
(except for the reaction with PhCO2Na) and lower enantios-
electivity, both amino acid 1g and its lithium salt 1f would be
necessary to achieve high yield and enantioselectivity (Table 2,
entries 8À12). Indeed, a mixed catalyst consisting of 1f and 1g
afforded the Michael adduct 4a in good yields with high
enantioselectivity (Table 2, entries 13À16). The best molar ratio
of 1f to 1g was determined to be 1:3 (Table 3, entry 15).
Finally, we examined substrate scope with various malonates
and enones (Table 3). Michael addition reaction of 2a to 3a was
completed within 96 h to give 4a in 91% yield with 93% ee
(Table 3, entry 1). By using moderately bulky malonates, diethyl
malonate (2b) and diisopropyl malonate (2c), Michael adducts
4b,c were synthesized with slightly better enantioselectivity (95% ee)
than that of 2a, although di-tert-butyl malonate (2d) was too
bulky to react with 3a (Table 3, entries 2À4). Although the
reaction of 2b with 3a required 5 days to consume 3a completely,
the reaction time could be reduced to 2 days by increasing the
amount of catalyst to 20 mol % from 10 mol % (Table 2, entry 6).
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NOTE
Table 3. Substrate Scopea
a Unless otherwise mentioned, the reactions were carried out with a mixed catalyst 1f/1g (1:3, 0.05 mmol), 2 (1.0 mmol), and 3 (0.5 mmol) in
DMSO/(CH2Cl)2 (1:2, 0.5 mL) at 25 °C. b Isolated yield of 4 based on 3. c Determined by chiral HPLC analysis. Absolute configurations of
4 determined by comparison of the specific rotation with that of the literatures are shown in parentheses (see Supporting Information).
d Conversion: 83%. e The amount of catalyst was increased to 0.1 mmol. f Conversion: 40%.
imine IV. Finally, hydration of the imine IV provided the Michael
adduct 4a and catalyst 1f/1g.
Butyldiphenylsilyl L-serine was synthesized according to the literature7
and converted into its lithium salt 1h.
Synthesis of O-tert-Butyldiphenylsilyl (S)-β-Homoserine
(1g). To a mixture of NaHCO3 (4.20 g, 50 mmol), H2O (100 mL),
and THF (50 mL) was added 4-benzyl L-aspartate (4.46 g, 20 mmol).
After cessation of gas evolution, benzyl chloroformate (3.75 g, 22 mmol)
was added dropwise. After stirring for 1 h at room temperature, the
resulting reaction mixture was washed with Et2O (50 mL Â 2) and
acidified to pH 2 with aq 1 N HCl. The obtained cloudy solution was
extracted with ethyl acetate (50 mL Â 3). The combined organic phase
was washed with brine, dried over MgSO4, filtered, and concentrated
under reduced pressure. The obtained white solid was washed with
hexane and dried in air to give 4-benzyl N-benzyloxycarbonyl L-aspartate
[Z-Asp(OBn)-OH]8,9 in 98% yield (7.00 g, 19.6 mmol). In a round-
bottomed flask, trimethyl borate (6.7 mL, 60 mmol) was added to a
solution of Z-Asp(OBn)-OH (7.14 g, 20 mmol) in dry THF (80 mL)
In summary, we found that a mixed catalyst consisting of a
primary β-amino acid, O-TBDPS β-homoserine, and its lithium
salt was an effective catalyst for Michael addition reaction of
malonates to enones to give various 1,5-ketoesters in good yields
with high enantioselectivity.
’ EXPERIMENTAL SECTION
Materials. Enones and malonates were used after purification by
distillation or column chromatography. 4-Benzyl L-aspartate, 1-benzyl
L-aspartate, 1-methyl L-aspartate, 1-t-butyl L-aspartate, and aspartame
were commercially available. Their lithium salts 1aÀe were prepared by
treatment with lithium hydroxide according to the literature.1m O-tert-
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NOTE
Scheme 1. Plausible Reaction Mechanism
obtained white solid was ground well and dried in vacuo. The obtained
powder was used as a mixed catalyst 1f/1g (1:3).
(S)-4-Amino-5-tert-butyldiphenylsiloxy Valeric Acid Litium
Salt (1i). 5-Benzyl L-glutamate was synthesized by a modified procedure of
the literature method.11 In a round-bottomed flask, benzyl alcohol (12 g, 110
mmol) was added to a mixture of L-glutamic acid (14.7 g, 100 mmol), concd
H2SO4 (9.8 g) and H2O (6.5 mL), and the whole reaction mixture was
stirred for 45 min at 70 °C. Then water was removed from the flask under
reduced pressure for 4 h at 70 °C. The resulting syrup was poured into a
500 mL beaker containing NaHCO3 (16.8 g, 200 mmol) and H2O (50 mL)
at 0 °C, and the remaining syrup in the flask was washed off with cold water
(30 mL). After the combined reaction mixture was left standing overnight at
room temperature, a precipitated white solid was collected by filtration and
recrystallized from H2O. The obtained white solid was washed with ethyl
acetate to give 5-benzyl L-glutamate in 33% yield (7.88 g, 33 mmol). Z
Protection (86%), reduction of carboxyl group (60%), TBDPS protection
(74%), and removal of benzyl and Z groups (82%) were performed by the
same procedure for the synthesis of 1g to give (S)-4-amino-5-t-butyldiphe-
nylsiloxy valeric acid (pre-1i): mp 126À127 °C; [α]19D = +18.5 (c = 1.0,
MeOH), δH (CDCl3) 1.02 (9H, s), 1.60À1.87 (2H, m), 2.20À2.40 (2H,
m), 3.17À3.22 (1H, m), 3.68 (2H, d, J = 5.4 Hz), 7.31À7.62 (10H, m); δC
(CDCl3) 19.2, 25.8, 26.8, 34.6, 53.5, 64.7, 127.8, 129.9, 132.55, 132.59,
135.51, 135.54, 179.1; ν (KBr)/cmÀ1 1577 (CdO); HR ESI-MS calcd for
C21H29NO3Si + Na (M + Na) 394.1809, found M+ + Na, 394.1805.
Treatment with pre-1i with lithium hydroxyde1m gave the title compound 1i.
Typical Procedure for the Michael Addition of Malonates
2 to Enones 3. In a 7 mL vial, 2b (160 mg, 1 mmol) was added to a
solution of a mixed catalyst 1f/1g (1:3, 36 mg, 0.1 mmol) and 3a (48 mg,
0.5 mmol) in DMSO/(CH2Cl)2 (1:2, 0.5 mL) at 25 °C. After the
reaction mixture was stirred for 48 h at 25 °C, saturated aq NaCl (1 mL)
was added to the vial and extracted with Et2O (1.5 mL Â 4). The
combined organic phase was dried over MgSO4, filtered, and concen-
trated under reduced pressure. (R)-3-[Bis(ethoxycarbonyl)methyl]cyclo-
hexanone (4b)12a,b was isolated by column chromatography (silica gel,
hexane/Et2O 1:1) in 91% yield (116 mg, 0.455 mmol) as oil. The enantio-
selectivity was determined by HPLC analysis (95% ee). The absolute
configuration was determined by comparison of the specific rotation
with that of the literature: δH (CDCl3) 1.26À1.30 (6H, t  2, J = 7.2 Hz),
1.46À1.56 (1H, m), 1.62À1.74 (1H, m), 1.95À1.98 (1H, m), 2.05À2.11
(1H, m), 2.22À2.31 (2H, m), 2.38À2.59 (3H, m), 3.30 (1H, d, J = 7.9 Hz),
4.18À4.24 (4H, q  2, J = 7.2 Hz).
at 0 °C. After stirring for 10 min at 0 °C, borane dimethylsulfide complex
(12 mL, 120 mmol) was added, and the whole reaction mixture was
stirred for 2 h at room temperature. Then H2O (100 mL) and saturated
aq NaHCO3 (100 mL) were successively added carefully, and the
resulting solution was extracted with ethyl acetate (100 mL Â 3). The
combined organic phase was washed with a diluted aq NaCl (made from
10 mL of H2O and 100 mL of saturated aq NaCl), dried over MgSO4,
filtered, and concentrated under reduced pressure. The obtained crude
product was purified by column chromatography (silica gel; hexane/
ethyl acetate 1:1) to give N-benzyloxycarbonyl (S)-β-homoserine
benzyl ester [Z-Asp(OBn)-ol]10 in 66% yield (4.53 g, 13.2 mmol). To
a solution of Z-Asp(OBn)-ol (4.53 g, 13.2 mmol) in DMF (25 mL) were
successfully added at 0 °C imidazole (0.9 g, 13.2 mmol) and t-butyl-
diphenylchlorosilane (3.63 g, 13.2 mmol). After stirring for 14 h at room
temperature, the resulting solution was poured into saturated aq NH4Cl
(100 mL) and extracted with ethyl acetate (50 mL Â 3). The combined
organic phase was washed with saturated aq NaCl (50 mL), dried over
MgSO4, filtered, and concentrated under reduced pressure. The obtained
crude product was purified by column chromatography (silica gel; hexane/
ethyl acetate 3:1) to give N-benzyloxycarbonyl O-t-butyldiphenylsilyl
(S)-β-homoserine benzyl ester in 89% yield (6.79 g, 11.7 mmol).
N-Benzyloxycarbonyl O-t-butyldiphenylsilyl (S)-β-homoserine benzyl
ester (2.91 g, 5 mmol) was dissolved in MeOH (40 mL), and the solu-
tion was added on Pd/C (10%, 300 mg) under nitrogen atmosphere in a
round-bottomed flask. After the atmosphere in the flask was replaced
with hydrogen, the reaction mixture was stirred for 18 h under hydrogen
atmosphere at room temperature. Pd/C was filtered with Celite, and the
filtrate was concentrated under reduced pressure. The obtained white
solid was washed with H2O and hexane successively and dried in air to
give pure O-t-butyldiphenylsilyl (S)-β-homoserine (1g) in 79% yield
(1.41 g, 3.95 mmol) as a white solid: mp 154À155 °C; [α]19D = À13.7
(c = 1.0, MeOH), δH (CD3OD) 1.09 (9H, s), 2.43À2.53 (2H, m),
3.53À3.58 (1H, m), 3.70À3.81 (2H, m), 7.41À7.70 (10H, m); δC
(CD3OD) 20.0, 27.3, 36.3, 52.2, 65.1, 129.0, 131.2, 131.3, 133.68,
133.71, 136.71, 136.73, 177.3; ν (KBr)/cmÀ1 1594 (CdO); HR ESI-
MS calcd for C20H27NO3Si + Na (M + Na) 380.1652, found M+ + Na,
380.1653.
4-[Bis(ethoxycarbonyl)methyl]-2-decanone (4k): δH (CDCl3)
0.87 (3H, t, J = 7.1 Hz), 1.25À1.38 (16H, m), 2.14 (3H, s), 2.51 (1H, dd,
J = 6.7, 17.1 Hz), 2.65À2.69 (1H, m), 2.75 (1H, dd, J = 5.3, 17.1 Hz), 3.53
(1H, d, J = 5.5 Hz), 4.180 (2H, q, J = 7.1 Hz), 4.184 (2H, q, J = 7.1 Hz);
δC (CDCl3) 14.15, 14.18, 22.7, 27.0, 29.3, 30.4, 31.8, 32.3, 33.6, 45.4, 54.1,
61.3, 61.4, 168.9, 169.1, 207.7; ν(neat)/cmÀ1 1751, 1730 (CdO); HR ESI-
MS calcd for C17H30O5 + Na (M + Na) 337.1986, found: M+ + Na,
337.1992.
Spectroscopic data of 4a,12a,b 4b,12a,b 4c,1m,12a 4d,12c 4e,1f,12c,12d 4f,12a
4g,12a,b 4h,1k,12a,12d 4i,1g 4j,1g and 4l1k,12e,12f are in agreement with the
published data.
’ ASSOCIATED CONTENT
S
Supporting Information. Solvent screen for Michael
b
addition of 2a to 3a with catalyst 1f, specific rotation and HPLC
data of 4, and NMR spectra of 1g, pre-1i, and 4. This material is
available free of charge via the Internet at http://pubs.acs.org.
Preparation of a Mixed Catalyst 1f/1g (1:3). In a vial, ground
LiOH monohydrate (42 mg, 1 mmol) was added to a solution of 1g (1.43 g,
4 mmol) in MeOH (4 mL) at 0 °C. After stirring overnight at room temp-
erature, the reaction mixture was concentrated under reduced pressure. The
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: myoshida@eng.hokudai.ac.jp.
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NOTE
’ ACKNOWLEDGMENT
Asymmetry 2001, 12, 2463. (c) Prabagaran, N.; Sundararajan, G. Tetra-
hedron: Asymmetry 2002, 13, 1053. (d) Naka, H.; Kanase, N.; Ueno, M.;
Kondo, Y. Chem.—Eur. J. 2008, 14, 5267. (e) Wang, J.; Li, H.; Zu, L.;
Jiang, W.; Xie, H.; Duan, W.; Wang, W. J. Am. Chem. Soc. 2006,
128, 12652. (f) Agostinho, M.; Kobayashi, S. J. Am. Chem. Soc. 2008,
130, 2430.
This work was partly supported by the Global COE Program
(Project No. B01: Catalysis asthe Basis for Innovation in Materials
Science) from the Ministry of Education, Culture, Sports, Science
and Technology, Japan.
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