Highly enantioselective organocatalyzed vinylogous Michael-type reaction for the construction of trifluoromethylated all-carbon quaternary stereocenters

Article abstract of DOI:10.1021/ol500157b

The first example of a highly enantioselective vinylogous Michael-type reaction of β,β-disubstituted nitroalkenes is disclosed. A series of biologically important chiral oxindoles, featuring a trifluoromethylated all-carbon quaternary chiral center, were obtained in good yields with excellent enantioselectivities (up to >99% ee).

Full text of DOI:10.1021/ol500157b

Letter
pubs.acs.org/OrgLett
Highly Enantioselective Organocatalyzed Vinylogous Michael-Type
Reaction for the Construction of Trifluoromethylated All-Carbon
Quaternary Stereocenters
Qiao Chen, Guoqiang Wang, Xianxing Jiang, Zhaoqing Xu,* Li Lin, and Rui Wang*
Key Laboratory of Preclinical Study for New Drugs of Gansu Province, School of Basic Medical Sciences, Lanzhou University,
Lanzhou, 730000, China
S
* Supporting Information
ABSTRACT: The first example of a highly enantioselective
vinylogous Michael-type reaction of β,β-disubstituted nitro-
alkenes is disclosed. A series of biologically important chiral
oxindoles, featuring a trifluoromethylated all-carbon quater-
nary chiral center, were obtained in good yields with excellent
enantioselectivities (up to >99% ee).
ptically active organofluorine compounds are fundamen-
tally used in the realm of pharmaceutical and agricultural
olefins would be proper vinylogous Michael acceptors and lead
to the construction of all-carbon quaternary stereocenters
featuring a trifluoromethyl group. The intrinsic steric hindrance
and poor reactivity of the β,β-disubstituted α,β-unsaturated
compounds, and the difficulty in the stereocontrol are
underlying challenges. Moreover, it is clear that methods
established for nonfluorinated substrates are often not
appropriate for CF3-containing molecules. Thus, application
of trifluoromethylated nitroalkenes to a vinylogous Michael-
type reaction is also a big challenge since they have never been
tested in this type of reaction.
On the basis of our continuing interest in the stereo-
controlled γ functionalization of α,β-unsaturated carbonyl
compounds12 and asymmetric modification of oxindoles,13 we
disclose herein the first asymmetric vinylogous Michael-type
addition of olefinic oxindoles to β,β-disubstituted nitroalkenes
for the construction of trifluoromethylated all-carbon quater-
nary stereocenters. The reaction proceeded smoothly at room
temperature and gave the desired products with uniformly
excellent enantioselectivities and high yields. It is known that 3-
alkylidene oxindoles can react in the α- or γ-position with
electrophilic reagents and increase the complexity of the
reaction’s pathway. What is more, the geometry of the double
bond in the substrate is lost during the dienolation process,
which may lead to an inseparable mixture of the products with
Z- and E-isomers. Notably, under our optimized conditions, 3-
alkylidene oxindoles exhibited complete γ-selectivity and the
alkene geometries in the products were excellently controlled
(in most cases, Z/E > 20/1).
O
chemistry.1 Particularly, those bearing a trifluoromethyl group
are fascinating building blocks for new drug candidates.2 Thus,
tremendous efforts have been devoted toward the development
of effective and reliable methods for the easy construction of
trifluoromethylated compounds.3 Although a variety of
processes have been reported to generate CF3-substituted
tertiary or heteroquaternary stereogenic centers, the enantio-
selective construction of trifluoromethylated all-carbon quater-
nary stereocenters remains not only a demand for biochemists
and medicinal chemists but also a great challenge for synthetic
organic chemists.4 In 2012, the groups of Shibata developed a
highly enantioselective cyanation of β,β-CF3 enones to provide
addition adducts having trifluoromethylated all-carbon quater-
nary stereocenters; they next succeeded in the enantioselective
conjugate addition of nitromethane to β-aryl-β-CF3 enones.5
Gade and co-workers reported a copper-catalyzed electrophilic
trifluoromethylation of β-ketoesters.6 Very recently, Jia et al.
demonstrated that the construction of trifluoromethylated all-
carbon quaternary stereocenters could be achieved via a Ni-
catalzyed Fridel−Craft reaction.7 Despite these notable
advances, the development of efficient methodologies is still
highly desirable.
The vinylogous Michael-type reaction has been demon-
strated as an effective protocol for the enantioselective carbon−
carbon bond formation at the γ-position and was intensively
studied in recent years.8−12 However, much to our surprise, the
application of this methodology for the synthesis of all-carbon
quaternary stereocenters has remained elusive until now. To
the best of our knowledge, only one single example with good
ee and moderate yield was disclosed by the group of
Melchiorre.10 Recently, Cashiraghi and co-workers reported
the first example of an organocatalyzed vinylogous Michael-
type reaction of 3-alkylidene oxindoles to β-monosubstituted
nitroolefins, with outstanding levels of yields and enantiose-
lectivity.11 We wondered whether β-CF3-β-disubstituted nitro-
We initiated our study by examining the reaction of 3-
alkylidene oxindole 2a with trifluoromethylated nitroalkene 3a
in the presence of 10 mol % organocatalyst 1 in toluene, and at
room temperature (Table 1). To our delight, the quinine-
derived bifunctional catalyst 1a gave the desired product 4aa
Received: January 17, 2014
Published: February 25, 2014
© 2014 American Chemical Society
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Organic Letters
Letter
a
of enantioselectivity. The quinine-derived thiourea catalyst 1c
was found to be the most promising catalyst and was selected
for further investigation.
Table 1. Optimization of the Reaction Conditions
In order to obviate unproductive reaction pathways and to
further improve the yield, we therefore examined a series of
solvents (entries 8−12). However, other solvents did not show
any positive effect on the reaction reactivity. A trace amount or
even no desired product was obtained when methanol or
acetonitrile was used as solvent (entries 10 and 11). Using an
excess of oxindole 2a or 15 mol % catalyst gave a sharp increase
of the reaction yield14 (entries 13 and 14). Finally, the best
result was obtained when the reaction was performed with 1.5
equiv of 2a, using 15 mol % of 1c in 0.5 M toluene, and stirred
at room temperature for 36 h (98% conversion, >20:1 dr, 99%
ee; entry 15).
Under the optimized reaction conditions outlined above, a
wide range of trifluoromethylated nitroalkenes (3a−3n) were
investigated (Table 2). Both electron-donating and -with-
a
Table 2. Scope of Trifluoromethylated Nitroalkene 3
b
c
d
entry
R1 (3)
Ph (3a)
product yield (%) dr (Z/E)
ee (%)
b
b
c
1
4aa
4ab
4ac
4ad
4ae
4af
92
87
91
84
90
88
82
90
83
92
91
82
93
60
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
20:1
99 (99)
99 (99)
99 (99)
99 (99)
99 (98)
99 (99)
99 (99)
99 (99)
99 (99)
99 (99)
98 (98)
99 (98)
99 (99)
99 (99)
entry
cat.
solvent
conv (%)
dr (Z:E)
ee (%)
2
4-MePh (3b)
4-ClPh (3c)
1
1a
1b
1c
1d
1e
1f
toluene
toluene
toluene
toluene
toluene
toluene
toluene
CH2Cl2
THF
56
0
>20:1
−
>20:1
>20:1
−
95
−
99
98
−
3
2
4
4-FPh (3d)
3
78
64
0
5
4-CF3Ph (3e)
3-ClPh (3f)
4
6
5
7
3-MeOPh (3g)
3-MePh (3h)
2-FPh (3i)
4ag
4ah
4ai
6
0
−
−
8
7
1g
1c
1c
1c
1c
1c
1c
1c
1c
37
72
52
trace
trace
70
89
84
98
>20:1
>20:1
>20:1
−
98
98
97
−
9
8
10
11
12
13
14
3,5-Me2Ph (3j)
2-thienyl (3k)
2-naphthyl (3l)
2-phenylethyl (3m)
1-octyl (3n)
4aj
>20:1
>20:1
>20:1
>20:1
15:1
9
4ak
4al
10
11
12
MeOH
MeCN
CHCl3
toluene
toluene
toluene
−
−
4am
4an
>20:1
>20:1
>20:1
>20:1
98
99
99
99
d
13
a
e
All reactions were carried out by using 0.15 mmol of 3, 1.5 equiv of
14
b
2a, 15 mol % of 1c, and 0.75 mL of toluene. Isolated yield.
f
15
c
d
Determined by 19F NMR analysis. Determined by chiral HPLC
a
Unless otherwise noted, the reactions were carried out by using 0.1
analysis. Values in parentheses refer to reactions catalyzed by the
corresponding opposite enantiomer of 1c (derived from quinidine).
mmol of 3a, 1.2 equiv of 2a, 10 mol % of the catalyst, and 1 mL of
b
solvent; stirred for 24 h. Conversions and diastereomeric ratios were
determined by 19F NMR analysis of the crude reaction mixtures.
c
d
e
Determined by chiral HPLC analysis. 1.5 equiv of 2a was used. 15
f
mol % of 1c was used. 0.15 mmol of 3a, 1.5 equiv of 2a, 15 mol % of
drawing substituents at different positions on the aryl ring of
the nitroalkenes (entries 2−9) afforded the products in high
yields and excellent stereoselectivities. Notably, 3,5-disubsti-
tuted product 4aj (entry 10) and heteroaromatic-substituted
product 4ak (entry 11) were also isolated with unexceptionable
yields and stereoselectivities. Moreover, the sterically demand-
ing naphthyl-substituted nitroalkene 3l and the 2-phenylethyl
substituted nitroalkene 3m were well tolerated substrates for
this transformation. It is very interesting that substrate 3n
bearing an aliphatic side chain still gave the desired product
with the same remarkable enantioselectivity, albeit with a
decrease in diastereoselectivity and yield. It is worth noting that
the corresponding opposite enantiomers of the products were
also obtained with uniformly excellent enantioselectivities,
which are displayed in the parentheses in the last column of
Table 2.
1c, 0.75 mL of toluene, and 36 h.
with high enantioselectivity and moderate yield (entry 1). In
contrast, thiourea catalyst 1b was proven to be inactive for this
transformation (entry 2). Under the same conditions, quinine
derivative 1c and cinchonidine derivative 1d showed relatively
higher catalytic activities in comparison with 1a, affording the
product 4aa in excellent diastereoselectivities and promising
yields (entries 3 and 4). The reaction did not proceed in the
presence of quinine 1e or primary amine-thiourea catalyst 1f
(entries 5 and 6), while quinine-derived squaramide 1g gave the
product 4aa with almost the same ee values as that obtained
with 1c, albeit with poor yield (entry 7). The above results
suggested that both the cinchona group and the thiourea
moiety are essential for reaction progress and the high degree
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Organic Letters
Letter
With positive results obtained for the alkene 3 as the Michael
acceptors, our attention was turned to the reactivity of different
oxindole donors (Table 3). Variation of the substituents on the
determined by X-ray crystal structure analysis of 516 (see
Supporting Information).
In summary, an unprecedented organocatalytic enantiose-
lective vinylogous Michael-type reaction between 3-alkylidene
oxindoles and trifluoromethylated nitroalkenes under mild
reaction conditions has been developed. The reaction
proceeded with a very high level of site selectivity and excellent
enantioselectivities (up to 93% yield, >20:1 dr, and >99% ee),
providing a mild and efficient method for the construction of
trifluoromethylated all-carbon quaternary stereocenters. More-
over, this is the first application of β,β-disubstituted α,β-
unsaturated compounds in a vinylogous Michael-type reaction.
Since the oxindole enjoys a privileged role in the realm of
medicinal chemistry, and the CF3 modification often enhances
the efficacy of the drug, the compounds prepared herein will be
potential candidates for new drug discovery. Further
applications of this methodology as well as the biological
evaluation of the products are underway.
a
Table 3. Scope of 3-Alkylidene Oxindole 2
yield
b
dr
(Z:E)
c
d
entry
R2, R3 (2)
PG
product
(%)
ee (%)
1
2
3
4
5
6
7
8
5-F, H (2b)
5-Br, H (2c)
Boc
Boc
4ba
4ca
4da
4ea
4fa
86
81
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
N. D.
>20:1
99 (>99)
98 (99)
99 (99)
99 (99)
99 (99)
99 (99)
N. D.
5-Me, H (2d) Boc
81
6-Cl, H (2e)
H, Ph (2f)
H, ethyl (2g)
H, − (2h)
H, H (2i)
Boc
Boc
Boc
Boc
Moc
87
91
4ga
4ha
4ia
72
<10
81
99 (99)
ASSOCIATED CONTENT
* Supporting Information
■
a
All reactions were carried out by using 0.15 mmol of 3a, 1.5 equiv of
S
b
2, 15 mol % of 1c, and 0.75 mL of toluene. Isolated yield.
c
d
Experimental procedures and characterization of the products.
CCDC 986456 and the CIF data are available in the Supporting
Information. This material is available free of charge via the
Internet at http://pubs.acs.org.
Determined by 19F NMR analysis. Determined by chiral HPLC
analysis. Values in parentheses refer to reactions catalyzed by the
corresponding opposite enantiomer of 1c (derived from quinidine).
AUTHOR INFORMATION
Corresponding Authors
benzene ring of the oxindole had little influence on the
stereoselectivities or yields of the products (entries 1−4). Even
the benzylidene oxindole 2f was well tolerated, and a high level
of yield and enantioselectivity were observed (entry 5). 3-
Alkylidene oxindole 2g also proved to be efficient for this
progress, providing the addition product 4ga in admirable
diastereo- and enantioselectivities with a moderate yield (entry
6), whereas oxindole 2h was relatively unreactive and only trace
amounts of 4ha were detected (entry 7). Additionally, as
expected, oxindole 2i, protected by a methoxy carbonyl (Moc)
group, was a competent donor in the reaction (entry 8). In
sharp contrast, protection-free oxindole, as well as a methyl
protected derivative, did not demonstrate any sign of reactivity,
thus highlighting the importance of a proper N-substituent for
this organocatalytic progress15 (for details, see Figure S2 in the
Supporting Information). Significantly, the Boc group of the
product could be easily removed via a single step under acidic
conditions, and the corresponding N-unprotected oxindole ring
5 was formed with unexceptionable yield and stereoselectivities
(Scheme 1). This makes up for the sluggishness of unprotected
oxindole, and the NH of 5 could be further modified. The
relative and absolute configurations of the products were
■
*E-mail: zqxu@lzu.edu.cn.
*E-mail: wangrui@lzu.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for the grants from the NSFC (Nos. 91213303,
21102141, 21272107, and 21202072), the National S&T Major
Project of China, and the Fundamental Research Funds for the
Central Universities (Nos. 860976 and 861188).
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