Highly enantioselective Michael addition of malonates to β-CF 3-β-(3-indolyl)nitroalkenes: Construction of trifluoromethylated all-carbon quaternary stereogenic centres

Article abstract of DOI:10.1002/ejoc.201402243

A family of γ-nitrobutyric acid esters bearing an all-carbon quaternary stereogenic centre have been synthesized. Chiral thioureas catalysed the conjugate addition of malonates to nitroalkenes containing a trifluoromethyl and indole motif at the β-position to afford the corresponding γ-nitrobutyric acid esters in good yields (up to 89-% yield) and with good to excellent enantioselectivities (up to 90-% ee). This protocol provides an efficient access to optically enriched γ-amino acids and β-disubstituted γ-butyrolactams. Trifluoromethylated all-carbon quaternary stereogenic nitroalkanes have been synthesized in good yields and with good to excellent enantioselectivities by organocatalysed asymmetric Michael addition of malonates to nitroalkenes containing a trifluoromethyl and indole motif. This protocol provides an efficient access to optically enriched γ-amino acids and β-disubstituted γ-butyrolactams. Copyright

Full text of DOI:10.1002/ejoc.201402243

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201402243
Highly Enantioselective Michael Addition of Malonates to β-CF3-β-(3-Indolyl)-
nitroalkenes: Construction of Trifluoromethylated All-Carbon Quaternary
Stereogenic Centres
Chun-Hui Ma,[a] Tai-Ran Kang,*[a] Long He,*[a] and Quan-Zhong Liu*[a]
Keywords: Enantioselectivity / Michael addition / Organocatalysis / Amino acids
A family of γ-nitrobutyric acid esters bearing an all-carbon
quaternary stereogenic centre have been synthesized. Chiral
thioureas catalysed the conjugate addition of malonates to
nitroalkenes containing a trifluoromethyl and indole motif at
the β-position to afford the corresponding γ-nitrobutyric acid
esters in good yields (up to 89% yield) and with good to ex-
cellent enantioselectivities (up to 90% ee). This protocol pro-
vides an efficient access to optically enriched γ-amino acids
and β-disubstituted γ-butyrolactams.
Introduction
Results and Discussion
Recently, the enantioselective conjugate addition of ni-
troalkenes has been established as a versatile methodology
in organic synthesis.[6] However, applications of this meth-
odology for the synthesis of molecules bearing all-carbon
quaternary stereocentre(s) are very limited.[7] Furthermore,
β,β-disubstituted nitroalkenes bearing CF3 and indolyl as
substrates have not yet been disclosed. Herein we wish to
report the enantioselective conjugate addition of malonates
to β-CF3-β-(3-indolyl)nitroalkene catalysed by chiral thio-
ureas,[8] which provided γ-nitrobutyric acid esters bearing
all-carbon quaternary stereogenic centres in good yields
and with good to excellent enantioselectivities.
The indole nucleus is found in numerous natural prod-
ucts and biologically active molecules, including commer-
cial drugs.[9] For this reason, β-CF3-β-(3-indolyl)nitroethyl-
enes 2 were chosen for enantioselective conjugate addition
reactions with malonates. Compounds 2 were readily pre-
pared by a modified literature procedure.[7a,10] Indole deriv-
atives were converted into 3 upon reaction with trifluoro-
acetic anhydride followed by treatment with TsCl in the
presence of Et3N.[11] The nitro-aldol products 4 were ob-
tained by nitro-Henry reactions of 3 and nitromethane in
the presence of tetramethylguanidine (TMG) as base
(Scheme 1). Note that other bases were not effective for the
reaction. Finally, the desired β-CF3-β-(3-indolyl)nitroalk-
enes 2 were obtained as single isomers in yields of 67–87%
(see the Supporting Information). The olefins were tenta-
tively assigned the (E) configuration.[7a]
Trifluoromethylated organic compounds, as an impor-
tant class of halogen compounds, have received increasing
attention because of their broad spectrum of intriguing
physicochemical properties compared with isosteric de-
halogenated analogues.[1] It is believed that the trifluoro-
methyl motif plays a pivotal role in the enhancement and
modification of their original biological activities.[2] For ex-
ample, nearly 20% of all pharmaceuticals and 40% of agro-
chemicals under development contain fluorine.[3] Therefore
efficient approaches to such valuable CF3-bearing organic
compounds are of considerable synthetic and biological im-
portance. To date, many strategies have been developed to
achieve the enantioselective synthesis of CF3-containing
compounds with tertiary or quaternary stereogenic
centre(s). Direct asymmetric trifluoromethylations involve
the use of nucleophilic, electrophilic or radical trifluoro-
methylation reagents and enantioselective transformations
of prochiral trifluoromethylated substrates.[4] However, the
construction of all-carbon quaternary stereocentre(s) bear-
ing a trifluoromethyl group has been a great challenge.
Consequently, the development of novel and efficient meth-
ods to meet this challenge would be invaluable for asym-
metric synthesis.[5]
[a] Chemical Synthesis and Pollution Control Key Laboratory of
Sichuan Province, College of Chemistry and Chemical
Engineering, China West Normal University,
Nanchong 637009, P. R. China
Initially, we explored the reaction of β-CF3-β-(3-indolyl)-
nitroethylene (2a) and diethyl malonate (5a) in toluene at
room temperature in the presence of 20 mol-% of Na2CO3
as base and 10 mol-% of chiral thiourea 1 as catalyst (Fig-
ure 1); the results are summarized in Table 1. Encourag-
ingly, the reaction proceeded smoothly to yield the corre-
E-mail: quanzhongliu@cwnu.edu.cn
longhe@cwnu.edu.cn
http://210.41.193.83:555/lab
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201402243.
Eur. J. Org. Chem. 2014, 3981–3985
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3981

C.-H. Ma, T.-R. Kang, L. He, Q.-Z. Liu
SHORT COMMUNICATION
Scheme 1. Preparation of β-trifluoromethyl-β-(3-indolyl)nitroalkenes. TMG = tetramethylguanidine.
Figure 1. Chiral thioureas investigated in the work.
sponding γ-nitrobutyric acid ester 6a, which bears an all- Table 1. Optimization of reaction conditions.[a]
carbon quaternary stereogenic centre (Table 1), even though
β,β-disubstituted nitroalkenes are much less reactive than
trans-β-nitrostyrene. Catalyst screening revealed that the
catalyst employed had a significant effect on the reactivity
and enantioselectivity of the Michael reaction (Entries 1–
7). For example, the use of amino thiourea 1a and bis-
(thiourea) 1b resulted in low yields and enantioselectivities
(Entries 1 and 2). Alkaloid-derived thioureas 1d and 1e fur-
nished poor yields and moderate enantioselectivities (17
and 12% yields, 73 and 50% ee, respectively; Entries 4 and
5). Catalyst 1c afforded the Michael product with the high-
est enantioselectivity of 87%, but a yield of only 18% was
observed (Entry 3). In particular, thiourea 1f derived from
a chiral amine developed by our group exhibited both poor
reactivity and low enantioselectivity (Entry 6).[12] The intro-
duction of phenyl substituents onto the naphthyl motifs of
1f, as shown in 1g, led to a much lower catalytic efficacy
and stereocontrol than with 1f (Entry 6 vs. 7). To further
optimize the procedure, this Michael reaction was chosen
to screen different bases and solvents. It was found that the
yield and enantioselectivity were both greatly dependent on
the base (Entries 8–13). Strong bases such as Cs2CO3 and
K2CO3 afforded excellent yields but with almost no
enantioselectivity (Entries 8 and 9), whereas organic bases
were not suitable, only providing traces of products (En-
tries 11 and 12). Of the bases screened, AcONa was the best
choice in terms of yield and enantioselectivity (Entry 13).
The screening of solvents indicated that toluene delivered
Entry
1
Base
Solvent
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
1a
1b
1c
1d
1e
1f
1g
1c
1c
1c
1c
1c
1c
1c
1c
1c
1c
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Cs2CO3
K2CO3
Na3PO4
Et3N
DABCO
CH3CO2Na toluene
CH3CO2Na CH2Cl2
CH3CO2Na CHCl3
CH3CO2Na
CH3CO2Na
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
27
13
18
17
12
18
9
99
98
23
9
8
24
5
24
13
21
4
4
87
73[d]
50[d]
20[d]
2
0
2
63
86
65
94
86
91
67
10
Et2O
THF
[a] Reaction conditions: 2a (0.05 mmol), 5a (0.3 mmol), catalysts 1
(10 mol-%), solvent (0.5 mL), base (20 mol-%), room temp., 24–
72 h. [b] Isolated yield. [c] Determined by chiral HPLC. [d] The
opposite enantiomer was obtained as the major enantiomer.
To further optimize the procedure, we investigated the
the best enantioselectivity, but unfortunately all attempts to influence of the amount of AcONa and the ratio of 2a/
further improve the yield failed (Entries 13–17). 5a on the reaction outcome (Table 2). Pleasingly, when the
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Eur. J. Org. Chem. 2014, 3981–3985

Michael Reaction of Malonates to Nitroalkenes
reaction was carried out in the presence of 80 mol-% the Supporting Information).[13] On the basis of the ob-
AcONa with a ratio of 2a/5a of 1:20, the desired product served stereochemistry, a plausible transition state was pro-
6a was obtained in 54% yield and with 92% ee (Table 2, posed (Figure 2). The thiourea 1c coordinates to the nitro
Entry 6). Both the yield and ee were almost unchanged olefin 2a through two hydrogen bonds to form an eight-
when the amount of AcONa was increased to 1 equiv. with membered transition state in which the tertiary amine motif
respect to 2a (Entry 4). The reaction performed with acts as a base to deprotonate the malonate. The resulting
20 equiv. of malonate with respect to 2a was sufficient to enolate approaches the nitro olefin 2a from the Si face to
achieve the best selectivity (Entry 6 vs. 7). Further studies yield the (R) product as the major enantiomer.
on catalyst loading indicated that 20 mol-% of 1c afforded
Table 3. Scope of the enantioselective Michael addition reaction.[a]
the desired Michael product in 72% yield with a slight ero-
sion of enantioselectivity (90% ee; Entry 8). In addition,
lower enantioselectivity was observed when the reaction
was carried out at a higher temperature, although the yield
was increased (Entry 9 vs. 6).
Table 2. Effects of the amount of base and substrate ratio on the
reaction.[a]
Entry
R
R1
R2
6
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Ts
Ts
Ts
Ts
Ts
Ts
Ts
Ts
Ts
Ts
Ts
Ts
Ts
Ts
Boc
Boc
H
H
H
Et
Me
Bn
iPr
Et
Et
Et
Et
Et
Et
Et
Bn
Bn
Et
Et
Me
6a
6b
6c
6d
6e
6f
6g
6h
6i
6j
6k
6l
6m
6n
6o
6p
72
75
78
49
64
62
61
67
70
74
63
93
77
0
90
82
90
90
85
84
83
84
82
82
84
88
90
–
H
5-Br
5-MeO
5-Me
5-Cl
6-Cl
6-F
7-Br
6-F
5-Me
4-Br
H
Entry Base (amount [%])[b]
2a/5a
Yield [%][c]
ee [%][d]
1
2
3
4
5
6
7
8
9
AcONa (40)
AcONa (60)
AcONa (80)
AcONa (100)
AcONa (80)
AcONa (80)
AcONa (80)
AcONa (80)
AcONa (80)
1:6
1:6
1:6
29
34
45
44
50
54
71
72
72
92
92
92
91
92
1:6
1:10
1:20
1:30
1:20
1:20
92
75
89
82
90
90
90[e]
85[f]
H
[a] Unless stated otherwise, the reaction was carried out with 2
(0.05 mmol), malonate 5 (1 mmol) and 1c (20 mol-%) using NaOAc
(80 mol-%) as base in toluene (0.5 mL) at room temperature for
24–96 h. [b] Isolated yield. [c] Determined by chiral HPLC.
[a] The reaction was carried out with 2a (0.05 mmol), ethyl malon-
ate (5a), and catalyst 1c (10 mol-%) in toluene (0.5 mL) at room
temp. for 24–96 h, unless indicated otherwise. [b] Molar ratio with
respect to nitro olefin 2a. [c] Isolated yield. [d] Determined by chiral
HPLC. [e] 20 mol-% of 1c was used. [f] The reaction was carried
out at 35 °C.
Having optimized the reaction as outlined above, we then
explored the generality of the protocol for different malon-
ates and nitroalkenes under the optimal conditions. As
shown in Table 3, malonates with various ester moieties,
such as methyl, isopropyl and benzyl, were investigated. In
all cases, chiral γ-nitrobutyric acid esters were obtained
with the same levels of yields and enantioselectivities (En-
tries 1–4), although the enantioselectivity was slightly re-
duced with dimethyl malonate. Various β,β-disubstituted ni-
Figure 2. Possible transition state for the Michael addition reac-
tion.
With the optically enriched γ-nitrobutyric acid esters 6
troalkene derivatives with electron-withdrawing as well as in hand, we turned our attention to the further transforma-
-donating substituents at the 5-, 6- and 7-positions of the tion of 6a into a γ-butyrolactam and γ-aminobutyric acid
indole motif proved to be suitable substrates, giving the (GABA). γ-Aminobutyric acid derivatives are important in-
Michael products in good yields and with high enantio- hibitory neurotransmitters of mammalian central nervous
selectivities (Entries 5–13). Substitution at the 4-position of systems. Moreover, they also serve as chiral building blocks
the indole motif afforded no product (Entry 14). Note also or intermediates in the synthesis of medicinally relevant
that N-Boc-protected nitroalkenes afforded better yields compounds (Figure 3).[14] First, 6a was reduced to the ex-
and enantioselectivities than the corresponding N-Ts deriv- pected ester 7a as a single isomer by using NaBH4 as the
atives (Entries 15 and 16 vs. 1 and 2). The absolute configu- reducing agent (Scheme 2). The absolute configuration at
rations of the nitroalkanes were unambiguously assigned to C-2 was not assigned, because the newly formed stereocen-
be (R) by single-crystal X-ray diffraction analysis of 6k (see tre was removed in a later transformation. No erosion of
Eur. J. Org. Chem. 2014, 3981–3985
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3983

C.-H. Ma, T.-R. Kang, L. He, Q.-Z. Liu
SHORT COMMUNICATION
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures, 1H, 19F, and 13C NMR spectra,
HPLC data of 6–8, and crystal data of 6k.
optical purity was observed during the reduction (64%
yield, 90% ee).[15] Saponification of the ester 7a with al-
coholic sodium hydroxide and subsequent acidification gave
the corresponding acid, which was decarboxylated upon
heating in xylene at reflux to afford the corresponding γ-
butyrolactam 8a, with trifluoromethyl and 3-indolyl both
attached to the β-position, in a total yield of 84% over three
steps with no erosion of enantioselectivity (see the Support-
ing Information).[16] Note that the tosyl group was also re-
moved under the base hydrolysis conditions. The GABA
could be easily synthesized from optically enriched 8a ac-
cording to a literature procedure.[17]
Acknowledgments
This work was supported by the National Natural Science Founda-
tion of China (NSFC) (21102116) and the Innovative Research
Team in the College of Sichuan Province (Grant no. 14TD0016).
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Conclusions
We have developed an enantioselective Michael addition
reaction between malonates and β-CF3-β-(3-indolyl)nitro-
alkenes that affords γ-nitrobutyric acid esters bearing an
all-carbon quaternary stereogenic centre in moderate to
high yields and with good to excellent enantioselectivities.
The Michael products can also be conveniently transformed
into the optically enriched γ-butyrolactams and γ-amino-
butyric acids.
Experimental Section
General Procedure for the Asymmetric Michael Addition Reactions:
A thiourea catalyst (20 mol-%), malonate (1 mmol), and AcONa
(80 mol-%) were added to a solution of β-CF3-β-(3-indolyl)nitroal-
kene (0.05 mmol) in toluene (0.5 mL) at room temperature. The
reaction mixture was stirred at room temperature until the reaction
was complete (monitored by TLC) and then purified by flash col-
umn chromatography on silica gel (petroleum ether/ethyl acetate,
50:1 to 10:1) to yield γ-nitrobutyric acid esters 6a–6p.
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Published Online: May 23, 2014
Eur. J. Org. Chem. 2014, 3981–3985
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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