The squaramide-catalyzed asymmetric Michael/cyclization tandem reaction for the synthesis of chiral trifluoromethylated hydroxyimino tetrahydrobenzofuranones

Article abstract of DOI:10.1039/c6ob00119j

An enantioselective synthesis of trifluoromethylated hydroxyimino tetrahydrobenzofuranones has been developed. 1,3-Dicarbonyl carbocyclic compounds react with β-CF₃-β-disubstituted nitroalkenes in the presence of a chiral squaramide catalyst, p

Full text of DOI:10.1039/c6ob00119j

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DOI: 10.1039/C6OB00119J
Journal Name
PAPER
Squaramide-Catalyzed Asymmetric Michael/Cyclization Tandem
Reaction for Synthesis of Chiral Trifluoromethylated Hydroxy-
imino Tetrahydrobenzofuranones
Received 00th January 20xx,
Accepted 00th January 20xx
Wei Liu, Xiaoyan Lai, Gaofeng Zha, Yan Xu, Panpan Sun, Tao Xia and Yongcun Shen*
DOI: 10.1039/x0xx00000x
An enantioselective synthesis of trifluoromethylated hydroxyimino tetrahydrobenzofuranones has been developed. 1, 3-
dicarbonyl carbocyclic compounds react with β-CF3-β-disubstituted nitroalkenes in the presence of chiral squaramide cata-
lyst, providing an efficient access to diverse hydroxyimino tetrahydrobenzofuranones featuring a trifluoromethyl group at
C3-position of all-carbon quaternary stereocenter with good yields (up to 81%) and enantioselectivities (up to 89%ee).
www.rsc.org/
troalkenes and the difficulty in the stereocontrol are underly-
ing challenges. Thus, application of trifluoromethylated ni-
troalkenes to 1, 3-dicarbonylogous Michael/cyclization tandem
reaction is a big challenge since they have never been tested in
this type of reaction. On the other hand, trifluoromethyl-
substituted heterocycles have been found widespread use in
the drug industry due to the ability of fluorinated groups to
alter essential physical properties of active pharmaceutical
Introduction
Benzofuran derivatives are valuable as potential biologically
active substances, most of which exhibit extensively medicinal
properties and biological activities.1 In particular, tetrahydro-
benzofurans, an important class of benzofuran compounds,
can be utilized in many lifesaving drugs and widely used as
building blocks in the preparation of a variety of natural prod-
ucts, such as evodone, pongamol and paniculide.2 Explorations
of facile methods for preparation of these functionalized het-
erocycles have received considerable attention in synthetic
organic chemistry.
Asymmetric Michael addition of 1, 3-dicarbonyl compounds
to nitroalkenes has been demonstrated as an effective proto-
col for the preparation of chiral γ-nitro carbonyl compounds3
which could be easily transformed to nitrile oxide, amines,
ketones, carboxylic acids4 and hydrobenzofuranone1i,5. How-
ever, to our surprise, the application of this methodology for
the construction of all-carbon quaternary stereocenter6 has
remained elusive to date. To the best of our knowledge, only
few examples with good enantioselectivities and moderate
yields have been disclosed. Recently Wang and co-workers7
reported the first example of enantioselective Mi-
chael/cyclization tandem addition of 4-hydroxycoumarin to β-
monosubstituted nitroolefins catalyzed by chiral bifunctional
thiourea catalysts. We wonder whether β-CF3-β-disubstituted
nitroolefins would be suitable substrates to 1, 3-
dicarbonylogous Michael/cyclization acceptors, which would
lead to the construction of all-carbon quaternary stereocenter
featuring a trifluoromethyl group. The intrinsic steric hin-
drance, the poor reactivity of the β-CF3-β-disubstituted ni-
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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Journal Name
ingredients.8 Among them, benzofuran derivatives bearing a dimedone 1a with (E)-(3, 3, 3-trifluoro-1-nitroprop-1-en-2-yl)
View Article Online
12
DOI: 10.1039/C6OB00119J
trifluoromethyl group at the C3-position have been poorly benzene 2a in the presence of 10mol% organocatalyst
(I-VII,
investigated.9 Hence, such enantioselective synthesis is highly Fig. 1) in DCM at room temperature for three days (Table 1).
valuable. To our delight, the bifunctional catalyst II gave the desired
In recent years, bifunctional chiral squaramides have been product 3aa with high enantioselectivity in moderate yield
proven to be effective catalysts in various asymmetric Michael (Table 1, entry 2). In contrast, squaramide catalysts III IV and
additions.10 Inspired by these achievements, we envision that thiourea catalysts
VII were proven to be of lower activity for
the bifunctional chiral squaramide catalyst, which can be easily this transformation (Table 1, entries 3-7). Under the same
prepared from cheap and commercially available chiral dia- conditions, 1, 2-cyclohexanediamine derivative with different
mine11, could both activate the β-CF3-nitroalkenes and cyclo- configuration showed relatively higher catalytic activity in
hexanediones via a synergetic process. Indeed, the enantiose- comparison with catalysts III VII, affording product in 70%
,
V-
I
-
lective Michael/cyclization tandem addition of cyclic 1, 3- yield with adverse configuration in moderate enantioselectivi-
dicarbonyl compounds to β-CF3-nitroalkenes catalyzed by chi- ty (Table 1, entry 1). The above results suggested that 1, 2-
ral squaramide catalyst proceeded successfully, which gave the cyclohexanediamine and the squaramide moiety were essen-
product hydroxyimino tetrahydrobenzofuranones featuring a tial for the reaction. The configuration of 1, 2-
trifluoromethyl group in moderate yields and satisfactory en- cyclohexanediamine has positive influence on the configura-
antioselectivities.
tion of product 3aa.
Solvent screening showed that the reaction was fully sup-
pressed in CH3CN, MTBE, THF and MeOH (Table 1, entries 13-
16). When the reaction was performed in toluene, only a trace
amount of the product was detected (Table 1, entry 12). How-
ever, products were obtained with comparable enantioselec-
tivities and yields in dichloromethane, chloroform and dichlo-
roethane (Table 1, entries 2, 10 and 11), probably due to high
solubility of the substrates in halohydrocarbon solvents. Mixed
solvent DCM/Et3N (20/1, V/V) and DCM/AcOH (20/1, V/V)
failed to improve the reaction (Table 1, entries 8 and 9).
Results and discussion
To obtain the optimized reaction conditions, we initiated our
study by examining Michael/cyclization tandem reaction of
Table 1 Optimization of the reaction conditionsa
Moreover, the reaction temperature has considerable ef-
fects on the reaction. When temperature was lowered from r.t.
to -10
slightly decreased (Table 1, entries 2, 17 and 18). However,
when temperature was further lower to -20 , enantioselec-
tivity had no change, but the yield obviously decreased (Table
1, entries 18 and 19). Thus, -10 was chosen for the reaction.
The amount of the catalyst has a considerable effect on the
reaction as well (Table 1, Entries 2, 20 and 21). When the
amount of Cat. II increased from 5 mol% to 20 mol%, enanti-
oselectivity hardly changed, but yield dramatically increased
(Table 1, entries 2, 16 and 17). Overall, 10mol% of catalyst was
used.
℃, enantioselectivity obviously increased and the yield
Entry
1
2
3
4
5
6
7
8
Cat.
I
II
III
IV
V
VI
VII
II
II
II
II
II
II
II
II
II
II
II
II
II
Solvent
DCM
DCM
DCM
DCM
DCM
DCM
DCM
Yieldb(%)
70
eec(%)
-61
75
45
-
33
-37
38
30
-
75
73
-
-
-
℃
76
55
℃
n.d.d
60
64
59
73
n.d.
63
DCM/Et3N (20/1)
DCM/AcOH (20/1)
CHCl3
9
10
11
12
13
14
15
16
17e
18f
19g
20h
21i
Based on these screenings, a combination of 10mol% Cat. II
dichloromethane solvent and -10 was chosen as the opti-
mized conditions to test the scope of the reaction.
,
ClCH2CH2Cl
toluene
CH3CN
MTBE
THF
MeOH
DCM
72
℃
trace
n.d.
n.d.
n.d.
n.d.
75
74
70
68
77
With the optimized conditions in hand, a wide range of tri-
fluoromethylated nitroalkenes and different 1, 3-
cyclohexanediones were investigated (Table 2). When dim-
edone 1a was used as Michael donator, all the trifluoromethyl-
ated nitroalkenes gave hydroxyimino tetrahydrobenzofu-
ranones in moderate to good yields with good enantioselectiv-
ities (Table 2, entries 1-9). The electronic feature and the posi-
tion of the substitutents on the aromatic ring have general
effects on the yields while striking effects on the enantioselec-
tivities. The strongly electron-rich β-CF3-4-methoxynitro sty-
rene 2f gave the highest enantioselectivity and moderate yield
(Table 2, entry 6). When 1, 3-cyclohexanedione 1b was used as
Michael donator, all the trifluoromethylated nitroalkenes gave
-
-
79
84
84
75
74
DCM
DCM
DCM
DCM
II
aUnless otherwise noted, reactions were carried out with 1a (0.1mmol), 2a
(0.1mmol) and 10mol% catalyst in solvent (1.0 mL) at room temperature
for 3 days. bIsolated yield after column chromatography purification.
cDetermined by chiral HPLC analysis. dNot detected. eThe reaction was
f
performed at 0
℃
days. gThe reaction was performed at -20
used. i20mol% catalyst was used.
for 5 days. The reaction was performed at -10
℃
℃
for 5 days. h5mol% catalyst was
for 5
2 | J. Name., 2012, 00, 1-3
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Table 2 Asymmetric tandem reaction of cyclohexanediones with trifluoro-
methylated nitroolefinsa
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DOI: 10.1039/C6OB00119J
Entry
R1
R2
Product Yieldb(%) eec(%)
Fig. 2 X-ray crystal structure of 3bg.
1
2
3
4
5
6
7
8
9
Me(1a
Me(1a
Me(1a
Me(1a
Me(1a
Me(1a
Me(1a
Me(1a
Me(1a
Me(1a
)
)
)
)
)
)
)
)
)
)
H(2a
)
3aa
3ab
3ac
3ad
3ae
3af
3ag
3ah
3ai
74
78
70
69
75
65
81
75
66
n.d.d
77
73
73
65
69
66
70
78
68
n.d.
84
81
77
85
74
88
67
56
72
-
71
86
78
85
87
89
88
58
65
-
3-Me(2b
3-F(2c
3,5-diMe(2d
4-Me(2e
4-OMe(2f
4-F(2g
4-Br(2h
4-CF3(2i
2-OMe(2j
H(2a
3-Me(2b
3-F(2c
3,5-diMe(2d
4-Me(2e
4-OMe(2f
4-F(2g
4-Br(2h
4-CF3(2i
2-OMe(2j
)
)
)
)
)
)
)
)
10
11
12
13
14
15
16
17
18
19
20
)
3aj
H(1b
H(1b
H(1b
H(1b
H(1b
H(1b
H(1b
H(1b
H(1b
H(1b
)
)
)
)
)
)
)
)
)
)
)
3ba
3bb
3bc
3bd
3be
3bf
3bg
3bh
3bi
)
)
)
)
)
)
)
)
)
3bj
aThe reactions were carried out with
1
(0.2mmol), 2 (0.2mmol) and 10mol%
catalyst II in DCM (1.0 mL) at -10
℃.
bIsolated yield after column chroma-
tography purification. cDetermined by chiral HPLC analysis. dNot detected.
hydroxyimino tetrahydrobenzofuranones
3 with moderate to
good yields and enantioselectivities (Table 2, entries 11-19).
The electronic feature and the substituent effects were similar
to the case with dimedone 1a. The highest enantioselectivity
was also gained from β-CF3-4-methoxy nitrostyrene 2f (Table 2,
entry 16). However, ortho-methoxy substituted β-CF3-
nitrostyrene 2j gave no product (Table 2, entries 10 and 20). In
addition, dimethyl group on C-5 position of 1, 3- cyclohex-
anedione has a little effect on the yields and the enantioselec-
tivities (Table 2, entries 1-10 vs 11-20).
The absolute configuration of the product 3bg was unam-
biguously assigned as R-configuration by single crystal X-ray
analysis (Fig. 2). According to the reported literatures7,13 and
our results, a possible model for the asymmetric induction was
then developed (Scheme 1). Initially, 5,5-dimethylcyclohexane-
1,3-dione underwent asymmetric Michael addition with β-CF3-
Conclusions
In conclusion, we have developed the asymmetric Mi-
chael/cyclization tandem reaction of cyclohexanediones with
trifluoromethylated nitroolefins catalyzed by chiral bifunction-
al squaramide catalysts. This protocol provides a mild and effi-
cient method for preparation of hydroxyimino tetrahydroben-
zofuranone type adducts with trifluoromethylated all-carbon
quaternary stereocenter in moderate yields (65-81%) and good
enantioselectivities (56-89%ee). Moreover, this is the first ap-
plication of β, β-disubstituted nitroalkenes in
a Mi-
chael/cyclization tandem reaction. Further extension of this
methodology in organic synthesis is currently underway in our
laboratory.
nitrostyrene to give chiral Michael adduct 4, which was pro-
moted by bifunctional chiral squaramide catalyst II via hydro-
gen bonds. One of them was formed between enol and amine,
and the other was between nitro and squaramide. During the
reaction, the Re-face attack of enol at the β-position of the
Acknowledgements
We are grateful for the grants from the National Natural Sci-
ence Foundation of China (No. 20972123).
nitroalkene was favored. Then Michael adduct 4 would tend to
form the hydroxyimino tetrahydrobenzofuranone 3aa accord-
ing to procedure reported7.
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