Highly enantioselective michael addition of cyclic 1,3-Dicarbonyl compounds to β,γ-Unsaturated α-Keto esters

Article abstract of DOI:10.1002/adsc.201000045

A highly enantioselective Michael addition of cyclic 1,3-dicarbonyl compounds to β,γ-unsaturated α-keto esters catalyzed by amino acid-derived thiourea-tertiary-amine catalysts is presented. Using 5 mol% of a novel tyrosine-derived thiourea catalyst, a series of chiral coumarin derivatives were obtained in excellent yields (up to 99%) and with up to 96% ee under very mild conditions within a short reaction time.

Full text of DOI:10.1002/adsc.201000045

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DOI: 10.1002/adsc.201000045
Highly Enantioselective Michael Addition of Cyclic
1,3-Dicarbonyl Compounds to b,g-Unsaturated a-Keto Esters
Xing-Kuan Chen,a,b Chang-Wu Zheng,b Sheng-Li Zhao,b Zhuo Chai,b
Ying-Quan Yang,b Gang Zhao,b,* and Wei-Guo Caoa,*
a
Department of Chemistry, Shanghai University, Shanghai 200444, Peopleꢀs Republic of China
E-mail: wgcao@staff.shu.edu.cn
Key Laboratory of Synthetic Chemistry of Natural Substances, Shanghai Institute of Organic Chemistry, Chinese
b
Academy of Sciences, 345 Lingling Lu, Shanghai 200032, Peopleꢀs Republic of China
Fax : (+86)-21-64166128; e-mail: zhaog@mail.sioc.ac.cn
Received: January 19, 2010; Revised: April 20, 2010; Published online: July 2, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000045.
Abstract: A highly enantioselective Michael addi-
tion of cyclic 1,3-dicarbonyl compounds to b,g-unsa-
turated a-keto esters catalyzed by amino acid-de-
rived thiourea-tertiary-amine catalysts is presented.
Using 5 mol% of a novel tyrosine-derived thiourea
catalyst, a series of chiral coumarin derivatives
were obtained in excellent yields (up to 99%) and
with up to 96% ee under very mild conditions
within a short reaction time.
it was not until very recently that an similar organoca-
talyzed reaction between cyclic diketones and b,g-un-
saturated a-keto esters using Cinchona alkaloid-de-
rived pyrimidines was reported by Calter et al.[5] In
contrast, several organocatalysts have been developed
for the Michael addition of cyclic diketones to a,b-un-
saturated ketones, in which the “imine catalysis” usu-
ally controls the enantioselectivity.[6]
On the other hand, following the pioneering works
of Jacobsen and Takemoto,[7] remarkable advances
have been made in the development and application
of bifunctional chiral thiourea catalysts in catalytic
asymmetric reactions.[8] Recently, the use of inexpen-
sive and readily available amino acids as the chiral
scaffold to develop novel thiourea catalysts has at-
tracted considerable interest and proved to be effec-
tive in several asymmetric reactions.[9] Our group has
Keywords: 1,3-dicarbonyl compounds; Michael ad-
dition; organocatalysts; thioureas; b,g-unsaturated
a-keto esters
Increased focus has recently been placed on the appli- been interested in the development of organocatalytic
cation of organocatalysis in reactions allowing the reactions utilizing b,g-unsaturated a-keto esters as a
rapid construction of functionalized molecules from reaction partner to synthesize useful compounds.[10]
simple and readily available starting materials.[1] In We report herein the development of several amino
this field, the asymmetric Michael addition of 1,3- acid-derived thiourea-tertiary amine catalysts and
cyclic dicarbonyl compounds to a,b-unsaturated car- their applications to the asymmetric Michael addition
bonyl systems, which could provide many chiral com- of cyclic 1,3-dicarbonyl compounds to b,g-unsaturated
pounds with biological and pharmaceutical activities a-keto esters.
such as neo-flavonoids and coumarins, has attracted
Using the reaction between (E)-methyl 2-oxo-4-
much attention.[2] As a class of activated a,b-unsatu- phenylbut-3-enoate (3a) and 4-hydroxycoumarin (4a)
rated carbonyl systems, b,g-unsaturated a-keto esters as the model reaction, a series of thiourea-tertiary
sometimes are preferred electrophiles in this kind of amine catalysts (Figure 1) was evaluated and the re-
reaction due to their high reactivity and the easy con- sults are summarized in Table 1. Similar to Takemo-
vertibility of the resultant products to other useful toꢀs catalyst 1, all the amino acid-derived thiourea-ter-
structures.[3] In 2003, Jørgensen et al. reported the tiary amine catalysts 2a–2j could catalyze the reaction
enantioselective Michael addition of cyclic 1,3-dicar- efficiently to give excellent yields of the product 5a in
bonyl compounds to b,g-unsaturated a-keto esters two hours at room temperature,[11] but the enantiose-
catalyzed by chiral bisoxazoline-copper(II) complexes lectivities were different. While catalysts 2a–2d and
with moderate to high enantioselectivities.[4] However, 2e and 2f with several different amino acid skeletons
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Adv. Synth. Catal. 2010, 352, 1648 – 1652

Highly Enantioselective Michael Addition of Cyclic 1,3-Dicarbonyl Compounds
Figure 1. Organocatalysts tested in this study.
did not produce considerable changes in the enantio- reduced ee values (entries 8 and 9). In addition,
selectivity (Table 1, entries 2–7), catalysts 2g and 2h changing the thiourea moiety to the electron-donating
bearing increased steric hindrance at the tertiary 2,6-diisopropylphenyl group (2j) also gave a much
amine moiety catalyzed the reaction with significantly lower ee value (entry 11). Reducing the loading
amount of the best catalyst 2f to 1.0 mol%, 2.0 mol%
or increasing the loading amount to 10 mol% have
little influence on both the yield and enantioselectivi-
ty (entries 12–14).
Table 1. The effect of catalysts on the reaction yield and se-
lectivity[a]
Table 2. Screening of solvents for the addition of 3a to 4a in
the presence of 2f.[a]
Entry
Catalyst
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
1
95
99
98
99
98
98
99
97
98
98
97
99
98
99
79
78
77
82
78
80
85
45
27
81
38
80
83
86
2a
2b
2c
2d
2e
2f
2g
2h
2i
Entry
Solvent
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
toluene
n-hexane
CHCl3
Et2O
99
62
98
99
99
99
98
98
99
99
86
83
83
93
91
83
70
92
93
94
9
10
11
12[d]
13[e]
14[f]
THF
2j
1,4-dioxane
MTBE
DME
Et2O
Et2O
2f
2f
2f
9[d]
10[e]
[a]
Unless otherwise noted, the reaction was conducted with
0.1 mmol of 3a and 0.11 mmol of 4a in the presence of
5.0 mol% of catalyst in 2.0 mL of solvent at room tem-
perature for 2 h.
[a]
Unless otherwise noted, the reaction was conducted with
0.1 mmol of 3a and 0.11 mmol of 4a in the presence of
5.0 mol% of 2f in 2.0 mL of solvent at room temperature
for 2 h.
[b]
[c]
Isolated yields.
[b]
[c]
Determined by chiral HPLC analysis on a chiral column.
The absolute configuration was determined by compari-
son with the literature (see ref.[4]).
1.0 mol% of 2f was used.
2.0 mol% of 2f were used.
Isolated yields.
Determined by chiral HPLC analysis on a chiral column.
The absolute configuration was determined by compari-
son with the literature (see ref.[4]).
The reaction was conducted at 08C for 3 h.
The reaction was conducted at À258C for 15 h.
[d]
[e]
[f]
[d]
[e]
10 mol% of 2f were used.
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Xing-Kuan Chen et al.
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Table 3. Examination of the generality of the reaction with
different b,g-unsaturated a-keto esters 3 and 4a.[a]
Table 4. Scope of the reaction with different 1,3-dicarbonyl
compounds as donors.[a]
Entry 3 4[14]
Product t
Yield
ee
Entry R1 R2
5
Yield [%][b] ee [%][c]
,
(h) [%][b]
[%][c]
1
Ph, Me (3a)
5a
99
93
92
90
92
96
92
90
90
93
91
90
92
84
–
2
3
4
4-F-C6H4, Me (3b)
4-Cl-C6H4, Me (3c)
4-Br-C6H4, Me (3d)
5b 98
5c 99
5d 96
1
2
3a
3c
5o
5p
1
1
99
99
94
93
5
6
7
4-NO2-C6H4, Me (3e) 5e
4-CH3-C6H4, Me (3f) 5f
4-EtO-C6H4, Me (3g) 5g
99
95
92
8
9
2-Br-C6H4, Me (3h)
2-Furyl, Me (3i)
Ph, Et (3j)
Ph, i-Pr (3k)
Ph, Bn (3l)
PhCH2CH2, Et (3m)
n-Bu, Me (3n)
5h 94
5i
5j
5k 97
5l 93
5m 78
97
91
10
11
12
13
14[d]
3
4
3a
3a
5q
5r
5
5
98
99
90
91
–
–
[a]
Unless otherwise noted, the reaction was conducted with
0.1 mmol of 3 and 0.11 mmol of 4a in the presence of
5.0 mol% of 2f in 2.0 mL of Et2O at room temperature
for 2 h.
Isolated yields.
Determined by HPLC analysis on a chiral column.
A complicated mixture was obtained.
[b]
[c]
[d]
5
3a
5s
8
96
77
[a]
Unless otherwise noted, the reaction was conducted with
0.1 mmol of 3 and 0.11 mmol of 4 in the presence of
5.0 mol% of 2f in 2.0 mL of Et2O at room temperature.
Isolated yields.
As a compromise between catalyst amount and
[b]
[c]
enantioselectivity, 5.0 mol% of the newly developed
catalyst 2f was used for further screening of solvents
and reaction temperature (Table 2). In general, the re-
actions proceeded well in the solvents examined
giving the desired 5a in excellent yields except for n-
Determined by HPLC analysis on a chiral column
hexane, which may be ascribed to the poor solubility the ester moiety (R2) also gave good results (en-
of the reactants in this solvent. The highest ee value tries 10–12). However, less satisfactory results were
of 93% was obtained in diethyl ether with up to 99% obtained with aliphatic substrates 3m and 3n: de-
yield (entry 4). However, no obvious improvement in creased yield and ee value with 3m and only a compli-
enantioselectivity was observed when reducing the re- cated mixture were obtained in the case of 3n (en-
action temperature to 08C or À258C, except that pro- tries 14 and 15).
longed reaction times were required (entries 9 and
To further extend the scope of the present reaction,
10). To conclude, the present reaction is best per- several other cyclic 1,3-dicarbonyl compounds 4b–4e
formed with 5 mol% of 2f in diethyl ether at room were also studied as the Michael donors and the re-
temperature.
sults are listed in Table 4. To our delight, all the sub-
With the optimal reaction conditions achieved strates underwent the reaction smoothly to give the
above, we subsequently examined the reaction of 4a desired products 5o–5s in excellent yields and high ee
with a range of b,g-unsaturated a-keto esters 3 to ex- values. The electronic nature of the substrates seemed
plore the generality of this reaction, and the results to have an influence on the reaction rate: a longer re-
are summarized in Table 3. High yields and enantiose- action time of 5 h was required for substrates 4c and
lectivities were achieved with b,g-unsaturated a-keto 4d with all-carbon skeletons and 8 h were needed in
esters bearing various g-aryl substituents, irrespective the case of substrate 4e bearing an additional elec-
of their electronic nature or steric hindrance (en- tron-withdrawing carbonyl group (entries 3–5). It is
tries 1–9). Substrates 3j–3l with bulkier substituents at worth mentioning that a much longer reaction time
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Adv. Synth. Catal. 2010, 352, 1648 – 1652

Highly Enantioselective Michael Addition of Cyclic 1,3-Dicarbonyl Compounds
Figure 2. X-ray structure of 5v.
(24 h) was required for the reaction of 4c and 4d with lyst, a broad range of adducts was obtained in high
3a in Calterꢀs system.[5]
yields and with up to 96% ee. Further elaboration of
Warfarin and its derivatives are widely used as anti- the products to other useful compounds and the appli-
coagulant drugs and their fluorine-containing counter- cation of the catalysts in other reaction systems are
parts may have interesting biological activities. now under investigation in our laboratory.
Herein the Michael addition of cyclic 1,3-dicarbonyl
compounds to a,b-unsaturated trifluoromethyl ke-
tones was further investigated.[12] A model reaction Experimental Section
between 4a and (E)-1,1,1-trifluoro-4-phenylbut-3-en-
2-one showed that good results could be obtained in Typical Procedure
toluene in the presence of 2f (5mol%) within 5 h.
The generality of this reaction was also extended to
other substrates (Scheme 1), which gave the CF3-con-
To a solution of b,g-unsaturated a-keto ester 3a (0.10 mmol)
and 4-hydroxy-2H-chromen-2-one 4 (0.11 mmol) in 2.0 mL
of diethyl ether (Et2O), 2f (0.005 mmol) was added. The
mixture was stirred at room temperature for 2 h. Then the
crude product was purified directly by column chromatogra-
phy on silica gel (hexane/acetate=5:1) to afford the corre-
sponding product 5a as a white solid; yield: 99%; mp 88–
898C; [a]2D4:7: À22.36 (c 1.0, DCM). 1H NMR (300 MHz,
CDCl3): d=7.81 (dd, J=8.4, 7.8 Hz, 0.92H), 7.48–7.56 (m,
1H), 7.22–7.36 (m, 7H), 4.98 (br s, 0.57H), 4.73 (br s,
0.26H), 4.28 (d, J=7.8 Hz, 0.32H), 4.17 (t, J=7.7 Hz,
1.15H), 3.92 (s, 0.96H), 3.89 (s, 2.04H), 2.79 (dd, J=7.2,
7.8 Hz, 0.31H), 2.44 (m, 1.68H), enantiomeric excess: 93%,
determined by HPLC (Chiralpak ADH column, hexane/
i-PrOH=80:20, flow rate 0.75 mLminÀ1, tmajor =16.4 min,
tminor =28.3 min, l=254 nm).
Scheme 1. Michael addtion between cyclic 1,3-dicarbonyl
compounds and a,b-unsaturated trifluoromethyl ketones.
Acknowledgements
We thank the National Natural Science Foundation of China
(No. 20172064, 203900502, 20532040), QT Program, Shang-
hai Natural Science Council for financial support; we are
grateful to Professor Yue-peng Cai for X-ray analyses.
taining products in high yields (98–99%) and moder-
ate enantioselectivities (61–75% ee). The ee value of
product 5v could be improved to up to 99.9% after a
single recrystallization from hexane/ethyl acetate (5:1,
v/v).[13] The absolute configurations of 5t–5w were de-
termined according to the X-ray structure of 5v
(Figure 2).
In conclusion, we have accomplished a highly enan-
tioselective Michael addition of cyclic 1,3-dicarbonyl
compounds to b,g-unsaturated a-keto esters using
amino acid-derived thiourea-tertiary amine catalysts
under mild conditions. With 5 mol% of the best cata-
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COMMUNICATIONS
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Adv. Synth. Catal. 2010, 352, 1648 – 1652