Indolinol-catalyzed asymmetric Michael reaction of aldehydes to nitroalkenes in brine

Article abstract of DOI:10.1016/j.tetasy.2016.03.006

(2S,3aS,7aS)-Perhydroindolinol A facilitated the reaction of a wide range of aldehyde and nitroalkene substrates to provide Michael adducts with excellent enantioselectivities (up to 98% ee), excellent yields and high diastereoselectivities (syn/anti up t

Full text of DOI:10.1016/j.tetasy.2016.03.006

Tetrahedron: Asymmetry xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Indolinol-catalyzed asymmetric Michael reaction of aldehydes
to nitroalkenes in brine
Xin Hu a, Yi-Fei Wei a, Nan Wu b, Zhiguo Jiang c, Can Liu d, Ren-Shi Luo a,
⇑
a School of Pharmaceutical Sciences, Gannan Medical University, Yixueyuan Road, Ganzhou, Jiangxi 341000, People’s Republic of China
b Department of Aviation Oil and Material, Air Force Logistics College, Xuzhou, Jiangsu 221000, People’s Republic of China
c Department of Clinical Medicine, Xuzhou Medical College, Xuzhou, Jiangsu 221004, People’s Republic of China
d School of Pharmacy, Guangzhou Medical University, Guangzhou, Guangdong 510006, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
(2S,3aS,7aS)-Perhydroindolinol A facilitated the reaction of a wide range of aldehyde and nitroalkene sub-
strates to provide Michael adducts with excellent enantioselectivities (up to 98% ee), excellent yields and
high diastereoselectivities (syn/anti up to 99:1). Our results indicated that perhydroindolinols were also
highly efficient organocatalysts for asymmetric Michael reactions in brine, while also being environmen-
tally friendly.
Received 25 November 2015
Revised 7 March 2016
Accepted 10 March 2016
Available online xxxx
Ó 2016 Published by Elsevier Ltd.
1. Introduction
Additional efforts have been made to develop efficient
organocatalysts for asymmetric reactions in water.14 However,
Organomolecule-catalyzed asymmetric Michael additions play a
significant role in various organocatalytic C–C bond-forming reac-
tions.1 In particular, the conjugate addition reactions of carbonyl
there are very limited examples of reported organocatalytic
Michael reactions under aqueous conditions. For example, Wang
et al. reported on multifluoro-substituted phenyl silicone ether
organocatalysts for asymmetric Michael addition reactions.5b
Headley and Ni developed two examples of water-soluble
imidazole and aniline-substituted silicone ether organocatalysts
for asymmetric Michael addition reactions of aldehydes to
nitroalkenes.5g
We found that chiral proline derivatives were of significant
importance in many enantioselective organocatalytic transforma-
tions,15 and that slight changes on catalyst structure can sometimes
improve the catalytic activity dramatically. We previously reported
on effective diphenylperhydroindolinol silyl ether organocatalysts
for asymmetric Michael addition reactions of aldehydes to
nitroalkenes, providing Michael adducts in nearly enantiomerically
pure form (99%), good yields and high diastereoselectivities
(syn/trans 99:1).16 In our previous report, however, organic solvents
were used, which is not consistent with ‘green chemistry’. Herein
we report the synthesis of new perhydroindole derivatives and then
applied them to the asymmetric Michael reactions of aldehydes to
nitroalkenes in aqueous solution (Scheme 2).
compounds to nitroalkenes result in the formation of c-nitro car-
bonyl compounds, which are versatile synthetic intermediates.2
Since Barbas et al. demonstrated the first organocatalytic
asymmetric Michael reaction of unmodified aldehydes to
nitroalkenes using chiral secondary amine catalyst,3a more and
more selective and efficient organocatalysts have been developed
for organocatalyzed Michael reactions of carbonyl compounds to
nitroalkenes, such as proline-derived diamines,3 pyrrolidinyltetra-
zoles,4 ethers 1,5 amides 2,6 thioureas7 and chiral quaternary
ammonium salt oganocatalysts.8 Other organocatalysts such as cin-
chona alkaloid derivatives 3,9 cyclohexylene diamine derivatives
410 and some special organocatalysts11 have also been developed
for the asymmetric Michael reactions of nitroalkenes (Scheme 1).
Although great progress has been achieved in asymmetric
Michael additions of nitroalkenes, most of the organocatalysts
were designed to be poorly water soluble, or even water insoluble
because of a large hydrophobic group. Recently, environmentally
friendly and metal-free reactions have been gained intensive
focus.12 Therefore, the design of organocatalysts within the princi-
ple of ‘green chemistry’ is a challenging task. Aqueous reactions
catalyzed by organocatalysts are recognized as ‘green chemistry’
for the advantages of being environmentally safe.13
2. Results and discussions
2.1. Effects of catalysts
At first, we synthesized the perhydroindole derivatives
organocatalysts according to the literature.17 With these catalysts
⇑
Corresponding author. Fax: +86 7978169775.
E-mail address: renshiluo@126.com (R.-S. Luo).
http://dx.doi.org/10.1016/j.tetasy.2016.03.006
0957-4166/Ó 2016 Published by Elsevier Ltd.
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2
X. Hu et al. / Tetrahedron: Asymmetry xxx (2016) xxx–xxx
OMe
H
N
t-Bu
S
Ph
H
N
NHTf
Bn
N
H
N
H
Ph
OTMS
N
H
NH2
N
H
O
NH2
N
4
2
1
3
Scheme 1. Some representative organocatalysts for asymmetric Michael reactions.
although a further increase to 30 mol % increased the reactivity
H
H
H
and decreased the reaction time to 0.5 h (Table 1, entry 5). From
the catalyst loading optimization, we found that lower stereoselec-
tivity was obtained with 30 mol % catalyst loading or lower enan-
tioselectivity was obtained with 10 mol % catalyst loading. Hence
we used 20 mol % catalyst loading in order to further study this
catalytic system.
OH
R1
Si R2
R3
N
H
O
N
H
H
A
B: R1 = Me; R2 = Me; R3 = t-Bu
C: R1 = Ph; R2 = Ph; R3 = t-Bu
Scheme 2. Perhydroindole derivatives for asymmetric Michael reaction.
2.2. Influence of the solvent
in hand, we next investigated the asymmetric Michael addition of
aldehydes to nitroalkenes. The Michael reaction of propanal and
(E)-(2-nitrovinyl)-benzene was chosen as the model reaction. The
reaction was performed with 20 mol % catalyst as the catalytic sys-
tem in NaCl aq at room temperature. Initial screening of the cata-
lysts demonstrated that the cyclohexyl ring plays an important
role both in terms of the stereoselectivity and enantioselectivity
when compared with (S)-prolinol (Table 1). Good enantioselectiv-
ity (93% ee) and yield (98%) were achieved when (2S,3aS,7aS)-
(octahydro-indol-2-yl)-methanol A was employed (Table 1, entry
1). Chiral perhydroindolinol silyl ethers B and C, the hydroxyl
groups of which were protected by a siloxy group, exhibited lower
reactivities and enantioselectivities (Table 1, entries 2–3). In the
presence of B and C, the reaction was complete in 4 h at room tem-
perature to afford the corresponding Michael adduct with the same
enantioselectivity (90%) (Table 1, entries 2, 3). Only 23% ee was
observed when using (S)-prolinol as the catalyst (Table 1, entry
4). In comparison to prolinol, (2S,3aS,7aS)-diphenylperhydroin-
dolinol A, prolinol silyl ethers B and C, which incorporates a rigid
cyclohexyl ring in the backbone, which in turn contributes toward
differentiating between the diastereofacial approaches, showed
better efficiencies in Michael reactions between aldehydes and
nitroalkenes.16 Catalyst loading can also be reduced to 10 mol %
without compromising the enantioselectivity (Table 1, entry 6),
In order to obtain better reaction conditions (both reactivity
and selectivity) of A, we next screened the effects of several com-
mon organic solvents (Table 2, entries 1–10). Throughout this
study, we used 20 mol % catalyst and 10 equiv of aldehyde. As
can be seen in Table 2, a strong solvent effect was observed. CH2-
Cl2, hexane, toluene, THF and MeOH were the solvents of choice
in this reaction, providing good ees and excellent diastereoselectiv-
ities (syn/trans 99:1) (Table 2, entries 1–6). No Michael adduct was
observed when DMF was used as the solvent (Table 2, entry 7).
When H2O was used as the solvent, the enantioselectivity was
increased to 90% (Table 2, entry 8), while in saturated NaCl, 93%
ee was obtained and the reaction was complete in 1 h (Table 2,
entry 9). The reaction can even be performed at 0 °C to give the
desired product with 98% ee (Table 2, entry 10). From the solvent
screening, it can be seen that H2O and saturated NaCl gave good
results, with saturated NaCl providing the best results. We believe
catalyst A with a hydroxy group interacts with brine by hydrogen
bonding and thus strengthens the catalytic effects.
2.3. Michael addition of various aldehydes and nitroalkenes
Under the optimized reaction conditions, the asymmetric
Michael addition of aldehydes and nitroalkenes were screened
(Table 3). As can be seen in Table 3, a variety of aldehydes and
Table 2
Table 1
Influences of solvents on Michael addition of propanal and (E)-(2-nitrovinyl)-benzene
The effects of catalysts in the Michael reaction of propanal and (E)-(2-nitrovinyl)-
benzene
Ph
catalyst A
Ph
OHC
NO2
NO2
CH3CH2CHO
catalyst
+
Ph
OHC
NO2
solvent, rt
NO2
CH3CH2CHO
+
CH3
Ph
NaCl aq., rt
5
7
6
CH3
Yield [%]a
syn/antib
ee [%]c
5
7
6
Entry
Solvent
Time [h]
1
3
4
5
6
7
8
CH2Cl2
Hexane
Toluene
THF
MeOH
DMF
H2O
NaCl aq
NaCl aq
5
5
5
3
5
12
5
1
95
91
93
90
85
<5
93
98
95
99:1
99:1
99:1
99:1
99:1
ND
99:1
99:1
99:1
83
89
88
77
86
ND
90
93
98
Entry
Catalyst
Time [h]
Yield [%]a
syn/antib
ee [%]c
1
2
3
A
B
C
1
4
4
1
0.5
5
98
85
98
98
98
90
99:1
99:1
99:1
85:15
97:3
99:1
93
90
90
23
93
91
4
Prolinol
A
A
5d
6e
9
10d
5
a
b
c
Yield of isolated product.
Determined by HPLC analysis.
The ee value was determined by HPLC analysis on a chiral phase (Chiralcel
a
b
c
Yield of isolated product.
Determined by 1H NMR Spectroscopy (400 MHz).
The ee value was determined by HPLC analysis on a chiral phase (Chiralcel
OD-H).
d
30 mol % catalyst loading.
10 mol % catalyst loading.
OD-H).
e
d
The reaction was performed at 0 °C.
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X. Hu et al. / Tetrahedron: Asymmetry xxx (2016) xxx–xxx
3
Table 3
Asymmetric Michael reaction of aldehydes and nitroalkenes
R1
20 mol % A
NO2
OHC
NO2
R2
CHO
R1
+
NaCl aq. 0 o C
R2
Yield [%]a
8
10
9
b
Entry
1
Product
Time [h]
syn/anti
ee [%]c
5
98
99:1
98
OHC
NO2
CH3
OCH3
NO2
2
3
7
7
95
96
97:3
98:2
96
92
OHC
OHC
CH3
OCH3
NO2
CH3
Cl
4
5
98
98:2
94
OHC
OHC
NO2
CH3
Cl
5
6
9
8
91
93
99:1
95
95
NO2
CH3
Cl
88:12
OHC
OHC
NO2
CH3
7
8
7
91
92
99:1
93
NO2
CH3
16
45:55
91(80)
OHC
OHC
OHC
NO2
NO2
NO2
CH3
O
9
18
18
93
93
97:3
90
CH3
S
10
82:18
84(84)
CH3
(continued on next page)
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X. Hu et al. / Tetrahedron: Asymmetry xxx (2016) xxx–xxx
Table 3 (continued)
b
Entry
Product
Time [h]
Yield [%]a
syn/anti
ee [%]c
11
12
13
7
93
99:1
96
OHC
Et
NO2
12
93
93
—
88
92
OHC
H3C
NO2
NO2
CH3
7
95:5
OHC
nPr
14
15
16
18
91
93
91
99:1
98:2
93:7
88
97
95
OHC
H3C
NO2
CH3
7
OHC
OHC
NO2
NO2
nBu
8
n-Bu
a
Yield of isolated product.
Determined by HPLC.
The ee value was determined by HPLC analysis on a chiral phase (Chiralcel OD-H, AD-H or AS-H).
b
c
nitroalkenes could be used in the conjugate addition reaction, to
afford the desired -nitroaldehyde products in excellent yields
results indicated that perhydroindolinols are also highly efficient
organocatalysts for asymmetric Michael reactions in water. Further
applications of this catalytic system to other asymmetric reactions
are currently in progress.
c
and enantioselectivities (Table 3, entries 1–16). Aryl-substituted
(Table 3, entries 1–7), alkyl-substituted (Table 3, entry 8) and
heteroaryl-substituted nitroalkenes (Table 3, entries 9 and 10)
were excellent Michael acceptors. However in comparison to aryl
substituted nitroalkenes, longer reaction times were needed for
alkyl-substituted (Table 3, entry 8) and heteroaryl-substituted
nitroalkenes (Table 3, entries 9 and 10). For example, 16–18 h were
needed to ensure complete conversion, while worse diastereose-
lectivities (syn/trans 45:55) were obtained when alkyl-substituted
or heteroaryl-substituted nitroalkenes were used (Table 3, entries
8–10). Straight and branched aldehydes were good Michael donors
(Table 3, entries 11–16), although longer reaction times were also
required for branched aldehydes (Table 3, entries 12 and 14).
4. Experimental
4.1. General
All reactions were carried out under air in glassware with mag-
netic stirring. The solvents were directly used with commercial
purchase. Unless otherwise stated, commercial reagents purchased
from Alfa Aesar, Acros and Aldrich chemical companies were used
without further purification. Purification of the reaction products
was carried out by flash chromatography using Qing Dao Sea
chemical reagent silica gel (200–300 mesh). 1H NMR spectra were
recorded on a Bruker XL 400 (400 MHz) spectrometer and the
spectra are referenced internally to the residual proton resonance
in CDCl3 (d = 7.26 ppm), or with tetramethylsilane (TMS,
d = 0.00 ppm) as the internal standard. Chemical shifts re reported
as parts per million (ppm) in the d scale downfield from TMS. 13C
NMR spectra were recorded on Bruker (400 MHz) spectrometer
with complete proton decoupling, and chemical shifts are reported
in ppm from TMS with the solvent as the internal reference (CDCl3,
d = 77.0 ppm). HPLC analyses were conducted on a Shimadzu 10A
3. Conclusions
In conclusion, new perhydroindole derivatives have been syn-
thesized in excellent yields and evaluated as chiral organocatalysts
in asymmetric Michael reactions of aldehydes to nitroalkenes.
(2S,3aS,7aS)-Perhydroindolinol A facilitated the reaction of a wide
range of aldehydes and nitroalkenes substrates, providing Michael
adducts with excellent enantioselectivities (up to 98% ee), excellent
yields and high diastereoselectivities (syn/trans up to 99:1). These
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X. Hu et al. / Tetrahedron: Asymmetry xxx (2016) xxx–xxx
5
instrument using CHIRALCEL OD-H, AD-H or AS-H columns
4.3.1. (2R,3S)-2-Methyl-4-nitro-3-phenylbutanal16
(0.46 cm diameter  25 cm length). Analytical TLC was performed
using EM separations percolated silica gel 0.2 mm layer UV 254 flu-
orescent sheets.
OHC
NO2
4.2. Procedures for the preparation of organocatalysts
4.2.1. Synthesis of catalyst A17
CH3
Prepared from (E)-(2-nitrovinyl)-benzene and propanal accord-
ing to the general procedure. Purified by preparative chromatogra-
phy on silica gel (petroleum ether/ethyl acetate 20:1); Colorless
oil; ee = 98%. The enantiomeric excess was determined by HPLC
using Daicel Chiralcel OD-H column (k = 208 nm, hexane/i-
PrOH = 85:15, 1.0 mL/min), tR = 15.05 min (minor), tR = 17.26 min
(major). 1H NMR (400 MHz, CDCl3): d 0.89 (3H, d, J = 7.3 Hz),
2.66–2.72 (1H, m), 3.70–3.76 (1H, m), 4.57–4.62 (1H, m), 4.69–
4.74 (1H, m), 7.07–7.27 (5H, m), 9.61 (1H, d, J = 1.5 Hz).
Amino alcohol
A was prepared from (2S,3aS,7aS)-methyl
octahydro-1H-indole-2-carboxylate (3.66 g, 20 mmol) as the mate-
rial by using lithium aluminum hydride (1.9 g, 50 mmol) as the
reductant in THF in excellent yield as a yellow oil. A (2.8 g, 90%
yield). 1H NMR (400 MHZ, CDCl3): d 1.25–1.30 (3H, m), 1.44–1.88
(8H, m), 1.97–2.02 (1H, m), 3.04–3.08 (1H, m), 3.26–3.29 (2H,
m), 3.39–3.44 (1H, m), 3.58–3.62 (1H, m). 13C (100 MHz, CDCl3):
d 22.4, 23.6, 28.2, 29.6, 33.1, 38.4, 57.6, 59.2, 65.9.
4.2.2. Synthesis of catalyst B16
A solution of (CH3)2t-BuSiCl (3.45 mL, 20 mmol) in 10 mL of
CH2Cl2 was added dropwise to a solution of (2S,3aS,7aS)-octahy-
4.3.2. (2R,3S)-2-Methyl-4-nitro-3-(2-methoxyphenyl)-butanal16
dro-1H-indol-2-yl)methanol
A
(1.55 g, 10 mmol) and Et3N
(4.16 mL, 30 mmol) in 20 mL of CH2Cl2 at 0 °C. The reaction was
stirred for 24 h at ambient temperature until full conversion of
the starting material. The reaction was quenched with water,
extracted with CH2Cl2 (3 Â 25 mL), dried over anhydrous Na2SO4,
filtered and concentrated in vacuum. The crude product was puri-
fied by flash chromatography and B (2.53 g, 94% yield) was isolated
OCH3
OHC
NO2
CH3
Prepared from (E)-1-methoxy-2-(2-nitrovinyl)-benzene and
propanal according to the general procedure. Purified by prepara-
tive chromatography on silica gel (petroleum ether/ethyl acetate
20:1); Colorless oil; ee = 96%. The enantiomeric excess was deter-
mined by HPLC using Daicel Chiralcel OD-H column (k = 208 nm,
hexane/i-PrOH = 85:15, 1.0 mL/min), tR = 15.89 min (minor),
tR = 16.85 min (major). 1H NMR (400 MHz, CDCl3): d 0.84 (3H, d,
J = 7.3 Hz), 2.90–2.94 (1H, m), 3.74 (3H, s), 3.92–3.98 (1H, m),
4.63–4.68 (1H, m), 4.75–4.80 (1H, m), 6.79–6.84 (2H, m), 6.98–
7.00 (1H, m), 7.16–7.20 (1H, m), 9.62 (1H, d, J = 1.8 Hz).
as yellow oil. [
a
]
D
20 = À9.5 (c 0.10, CHCl3); 1H NMR (400 MHZ,
CDCl3): d 0.03 (6H, s), 0.86 (9H, s),1.21–1.32 (2H, m), 1.41–1.48
(5H, m),1.64–1.68 (2H, m), 1.79–1.82 (1H, m), 2.05–2.10 (1H, m),
3.12–3.17 (1H, m), 3.33–3.37 (1H, m), 3.58–3.62 (1H, m), 3.72–
3.76 (1H, m), 4.34 (1H, s). 13C (100 MHz, CDCl3): d À5.33, À5.27,
18.43, 22.22, 23.30, 26, 32.83, 37.95, 57.98, 59.42, 64.70.; HRMS
(ESI) m/z: calculated for C15H31NOSi 269.2175; found: 269.2179.
4.2.3. Synthesis of catalyst C16
A solution of Ph2t-BuSiCl (4.50 mL, 20 mmol) in 10 mL of CH2Cl2
was added dropwise to a solution of (2S,3aS,7aS)-octahydro-1H-
indol-2-yl) methanol A (1.55 g, 10 mmol) and Et3N (4.16 mL,
30 mmol) in 20 mL of CH2Cl2 at 0 °C. The reaction was stirred for
24 h at ambient temperature until full conversion of the starting
material. The reaction was quenched with water, extracted with
CH2Cl2 (3 Â 25 mL), dried over anhydrous Na2SO4, filtered and con-
centrated in vacuum. The crude product was purified by flash chro-
matography and C (3.45 g, 88% yield) was isolated as a yellow oil.
4.3.3. (2R,3S)-2-Methyl-4-nitro-3-(4-methoxyphenyl)-butanal16
OCH3
OHC
NO2
CH3
[
a]
D
20 = À8.7 (c 0.25, CHCl3); 1H NMR (400 MHZ, CDCl3): d 1.09 (9H,
Prepared from (E)-1-methoxy-4-(2-nitrovinyl)-benzene and
propanal according to the general procedure. Purified by prepara-
tive chromatography on silica gel (petroleum ether/ethyl acetate
20:1); Colorless oil; ee = 92%. The enantiomeric excess was deter-
mined by HPLC using Daicel Chiralcel AS-H column (k = 208 nm,
hexane/i-PrOH = 85:15, 1.0 mL/min), tR = 28.68 min (minor),
tR = 32.91 min (major). 1H NMR (400 MHz, CDCl3): d 0.88 (3H, d,
J = 7.3 Hz), 2.60–2.66 (1H, m), 3.66 (3H, s), 3.68–3.71 (1H, m),
4.51–4.57 (1H, m), 4.64–4.70 (1H, m), 6.75–6.78 (2H, m), 6.98–
7.05 (2H, m), 9.58 (1H, d, J = 1.6 Hz).
s), 1.22–1.71 (9H, m), 1.82–1.89 (1H, m), 2.01–2.03 (1H, m), 2.22
(1H, s), 3.03–3.07 (1H, m), 3.25–3.31 (1H, m), 3.65–3.68 (1H, m),
7.39–7.44 (6H, m), 7.69–7.72 (4H, m). 13C (100 MHz, CDCl3): d
19.29, 21.91, 23.96, 26.89, 28.37, 29.03, 33.86, 39.14, 57.76,
59.24, 67.02, 127.52, 127.63, 127.67, 129.60, 133.60, 133.66,
134.93, 135.69.; HRMS (ESI) m/z: calculated for
C25H35NOSi
393.2488; found: 393.2493.
4.3. Typical experimental procedure16
4.3.4. (2R,3S)-3-(4-Chlorophenyl)-2-methyl-4-nitrobutanal16
Propanal (0.48 mL, 6.7 mmol) was added to a solution of
nitroalkenes (100 mg, 0.67 mmol) and A (20.8 mg, 0.134 mmol)
in NaCl aq (2.0 mL) at 0 °C. After the reaction mixture had been
stirred for 5 h at that temperature, the reaction mixture was
purified by flash chromatography to afford the Michael adduct
Cl
(136 mg, 98%) as
a
clear oil. syn/trans 99:1 (by 1H NMR
spectroscopy of the crude mixture), 98% ee (by HPLC using Daicel
Chiralcel OD-H column, k = 208 nm, hexane/i-PrOH = 85:15,
1.0 mL/min), tR = 15.05 min (minor), tR = 17.26 min (major).16
OHC
NO2
CH3
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6
X. Hu et al. / Tetrahedron: Asymmetry xxx (2016) xxx–xxx
by HPLC using Daicel Chiralcel OD-H column (k = 208 nm, hex-
Prepared from (E)-1-chloro-4-(2-nitrovinyl)-benzene and pro-
ane/i-PrOH = 85:15,
1.0 mL/min),
tR = 26.93 min
(minor),
panal according to the general procedure. Purified by preparative
chromatography on silica gel (petroleum ether/ethyl acetate
20:1); Colorless oil; ee = 93.5%. The enantiomeric excess was deter-
mined by HPLC using Daicel Chiralcel AD-H column (k = 208 nm,
hexane/i-PrOH = 99:1, 1.0 mL/min), tR = 15.41 min (minor),
tR = 16.98 min (major). 1H NMR (400 MHz, CDCl3): d 0.91 (3H, d,
J = 7.3 Hz), 2.66–2.71 (1H, m), 3.69–3.75 (1H, m), 4.54–4.60 (1H,
m), 4.67–4.74 (1H, m), 7.03–7.10 (2H, m), 7.20–7.25 (2H, m),
9.61 (1H, s).
tR = 28.67 min (major). 1H NMR (400 MHz, CDCl3): d 0.87 (3H, d,
J = 7.3 Hz), 2.86–2.91 (1H, m), 4.67–4.85 (3H, m), 7.25–7.50 (4H,
m), 7.71 (1H, d, J = 8.2 Hz), 7.78 (1H, d, J = 8.0 Hz), 8.01–8.03 (1H,
m), 9.65 (1H, d, J = 1.6 Hz).
4.3.8. (2R,3R)-3-Cyclohexyl-2-methyl-4-nitro-butanal16
4.3.5. (2R,3S)-3-(2-Chlorophenyl)-2-methyl-4-nitrobutanal18
OHC
NO2
CH3
Cl
Prepared from (E)-(2-nitrovinyl)-cyclohexane and propanal
according to the general procedure. Purified by preparative chro-
matography on silica gel (petroleum ether/ethyl acetate 20:1);
Colorless oil; ee = 91%. The enantiomeric excess was determined
by HPLC using Daicel Chiralcel AD-H column (k = 210 nm,
hexane/i-PrOH = 85:15, 1.0 mL/min), tR = 6.78 min (major), tR = 7.51
min (minor). 1H NMR (400 MHz, CDCl3): d 0.81–0.93 (3H, m),
0.96–1.00 (2H, m), 1.13 (3H, d, J = 6.8 Hz), 1.34–1.70 (6H, m),
2.48–2.67 (2H, m), 4.29–4.36 (1H, m), 4.50–4.55 (1H, m), 9.61
(1H, s).
OHC
NO2
CH3
Prepared from (E)-1-chloro-2-(2-nitrovinyl)-benzene and pro-
panal according to the general procedure. Purified by preparative
chromatography on silica gel (petroleum ether/ethyl acetate
20:1); Colorless oil; ee = 95%. The enantiomeric excess was deter-
mined by HPLC using Daicel Chiralcel OD-H column (k = 210 nm,
hexane/i-PrOH = 85:15, 1.0 mL/min), tR = 11.68 min (minor),
tR = 16.98 min (major). 1H NMR (400 MHz, CDCl3): d 0.91 (3H, d,
J = 7.3 Hz), 2.66–2.71 (1H, m), 3.69–3.75 (1H, m), 4.54–4.60 (1H,
m), 4.67–4.74 (1H, m), 7.03–7.10 (2H, m), 7.20–7.25 (2H, m),
9.61 (1H, s).
4.3.9. (2R,3R)-3-Furyl-2-methyl-4-nitrobutanal16
4.3.6. (2R,3S)-3-(3-Chlorophenyl)-2-methyl-4-nitrobutanal19
O
OHC
NO2
CH3
Cl
Prepared from (E)-2-(2-nitrovinyl)-furan and propanal accord-
ing to the general procedure. Purified by preparative chromatogra-
phy on silica gel (petroleum ether/ethyl acetate 20:1); Colorless
oil; ee = 90%. The enantiomeric excess was determined by HPLC
using Daicel Chiralcel AD-H column (k = 210 nm, hexane/i-
PrOH = 85:15, 1.0 mL/min), tR = 19.47 min (major), tR = 24.68 min
(minor). 1H NMR (400 MHz, CDCl3): d 0.99 (3H, d, J = 7.3 Hz),
2.69–2.79 (1H, m), 3.91–4.05 (1H, m), 4.60–4.71 (2H, m), 6.06–
6.23 (2H, m), 7.29 (1H, s), 9.63 (1H, s).
OHC
NO2
CH3
Prepared from (E)-1-chloro-3-(2-nitrovinyl)-benzene and pro-
panal according to the general procedure. Purified by preparative
chromatography on silica gel (petroleum ether/ethyl acetate
20:1); Colorless oil; ee = 95%. The enantiomeric excess was deter-
mined by HPLC using Daicel Chiralcel OD-H column (k = 210 nm,
hexane/i-PrOH = 85:15, 1.0 mL/min), tR = 13.52 min (minor),
tR = 17.02 min (major). 1H NMR (400 MHz, CDCl3) d 2.82–2.70 (m,
1H), 1.01 (d, J = 7.3 Hz, 3H), 3.82–3.74 (m, 1H), 4.69–4.61 (m,
1H), 4.84–4.75 (m, 1H), 7.32–6.97 (m, 4H), 9.69 (d, J = 1.5 Hz, 1H).
4.3.10. (2R,3R)-2-Methyl-4-nitro-3-(2-thiophyl)butanal16
S
4.3.7. (2R,3S)-2-Methyl-3-(2-naphthyl)-4-nitrobutanal16
OHC
NO2
CH3
Prepared from (E)-2-(2-nitrovinyl)-thiophene and propanal
according to the general procedure. Purified by preparative
chromatography on silica gel (petroleum ether/ethyl acetate
20:1); Colorless oil; ee = 84%. The enantiomeric excess was
determined by HPLC using Daicel Chiralcel AD-H column
(k = 238 nm, hexane/i-PrOH = 85:15, 1.0 mL/min), tR = 16.69 min
(major), tR = 27.95 min (minor). 1H NMR (400 MHz, CDCl3): d 0.87
(3H, d, J = 7.3 Hz), 2.74–2.85 (1H, m), 4.16–4.27 (1H, m),
4.65–4.80 (2H, m), 6.89–6.96 (2H, m), 7.21–7.27 (1H, m), 9.67
(1H, d, J = 0.9 Hz).
OHC
NO2
CH3
Prepared from (E)-2-(2-nitrovinyl)-naphthalene and propanal
according to the general procedure. Purified by preparative chro-
matography on silica gel (petroleum ether/ethyl acetate 20:1);
Colorless oil; ee = 93%. The enantiomeric excess was determined
Please cite this article in press as: Hu, X.; et al. Tetrahedron: Asymmetry (2016), http://dx.doi.org/10.1016/j.tetasy.2016.03.006

X. Hu et al. / Tetrahedron: Asymmetry xxx (2016) xxx–xxx
7
4.3.11. (2R,3S)-2-Ethyl-4-nitro-3-phenylbutanal16
Prepared from (E)-(2-nitrovinyl)-benzene and 3-methylbutanal
according to the general procedure. Purified by preparative chro-
matography on silica gel (petroleum ether/ethyl acetate 20:1);
Colorless oil; ee = 88%. The enantiomeric excess was determined
by HPLC using Daicel Chiralcel OD-H column (k = 208 nm, hexane/
i-PrOH = 85:15, 1.0 mL/min), tR = 14.65 min (minor), tR = 16.51 min
(major). 1H NMR (400 MHz, CDCl3): d 0.87 (3H, d, J = 7.0 Hz), 1.09
(3H, d, J = 7.2 Hz), 1.66–1.75 (1H, m), 2.76–2.80 (1H, m), 4.54–
4.69 (2H, m), 7.18–7.36 (5H, m), 9.91 (1H, d, J = 2.4 Hz).
OHC
NO2
Et
Prepared from (E)-(2-nitrovinyl)-benzene and butyraldehyde
according to the general procedure. Purified by preparative chro-
matography on silica gel (petroleum ether/ethyl acetate 20:1);
Colorless oil; ee = 96%. The enantiomeric excess was determined
by HPLC using Daicel Chiralcel OD-H column (k = 208 nm, hex-
4.3.15. (2R,3S)-2-Butyl-4-nitro-3-phenylbutanal16
ane/i-PrOH = 85:15,
1.0 mL/min),
tR = 11.26 min
(major),
tR = 14.01 min (minor). 1H NMR (400 MHz, CDCl3): d 0.71 (3H, t,
J = 7.5 Hz), 1.35–1.44 (2H, m), 2.56–2.62 (1H, m), 3.68–3.74 (1H,
m), 4.51–4.69 (2H, m), 7.09–7.25 (5H, m), 9.67 (1H, d, J = 2.5 Hz).
OHC
NO2
nBu
4.3.12. (R)-2,2-Dimethyl-4-nitro-3-phenylbutanal16
Prepared from (E)-(2-nitrovinyl)-benzene and hexanal accord-
ing to the general procedure. Purified by preparative chromatogra-
phy on silica gel (petroleum ether/ethyl acetate 20:1); Colorless oil;
ee = 97%. The enantiomeric excess was determined by HPLC using
Daicel Chiralcel OD-H column (k = 208 nm, hexane/i-PrOH = 85:15,
1.0 mL/min), tR = 15.70 min (minor), tR = 18.68 min (major). 1H
NMR (400 MHz, CDCl3): d 0.87 (3H, t, J = 6.7 Hz), 1.02–1.18 (4H,
m), 1.30–1.41 (2H, m), 2.59–2.65 (1H, m), 3.67–3.73 (1H, m),
4.53–4.66 (2H, m), 7.09–7.27 (5H, m), 9.61 (1H, d, J = 2.7 Hz).
OHC
H3C
NO2
CH3
Prepared from (E)-(2-nitrovinyl)-benzene and isobutyraldehyde
according to the general procedure. Purified by preparative chro-
matography on silica gel (petroleum ether/ethyl acetate 20:1);
Colorless oil; ee = 88%. The enantiomeric excess was determined
by HPLC using Daicel Chiralcel OD-H column (k = 208 nm, hex-
4.3.16. (2R,3S)-2-Pentyl-4-nitro-3-phenylbutanal16
ane/i-PrOH = 85:15,
1.0 mL/min),
tR = 14.26 min
(minor),
tR = 20.65 min (major). 1H NMR (400 MHz, CDCl3): d 0.86 (3H, s),
0.99 (3H, s), 3.67–3.71 (2H, m), 4.59 (1H, dd, J = 4.2, 13.1 Hz),
4.73–4.79 (1H, m), 7.09–7.21 (5H, m), 9.39 (1H, s).
OHC
NO2
n-Bu
4.3.13. (2R,3S)-2-Propyl-4-nitro-3-phenylbutanal16
Prepared from (E)-(2-nitrovinyl)-benzene and heptanal accord-
ing to the general procedure. Purified by preparative chromatogra-
phy on silica gel (petroleum ether/ethyl acetate 20:1); Colorless oil;
ee = 95%. The enantiomeric excess was determined by HPLC using
Daicel Chiralcel OD-H column (k = 208 nm, hexane/i-PrOH = 85:15,
1.0 mL/min), tR = 15.12 min (minor), tR = 17.31 min (major). 1H
NMR (400 MHz, CDCl3): d 0.72 (3H, t, J = 6.8 Hz), 1.01–1.42 (8H,
m), 2.59–2.65 (1H, m), 3.67–3.73 (1H, m), 4.53–4.65 (2H, m), 7.09
(2H, d, J = 7.2 Hz), 7.18–7.27 (3H, m), 9.61 (1H, d, J = 2.6 Hz).
OHC
NO2
nPr
Prepared from (E)-(2-nitrovinyl)-benzene and pentanal accord-
ing to the general procedure. Purified by preparative chromatogra-
phy on silica gel (petroleum ether/ethyl acetate 20:1); Colorless
oil; ee = 92%. The enantiomeric excess was determined by HPLC
using Daicel Chiralcel OD-H column (k = 208 nm, hexane/i-
PrOH = 85:15, 1.0 mL/min), tR = 14.47 min (minor), tR = 16.19 min
(major). 1H NMR (400 MHz, CDCl3): d 0.69 (3H, t, J = 7.1 Hz),
1.06–1.42 (4H, m), 2.59–2.66 (1H, m), 3.67–3.73 (1H, m), 4.53–
4.65 (2H, m), 7.08–7.27 (5H, m), 9.60 (1H, d, J = 2.5 Hz).
Acknowledgements
We are grateful to the start fund of the School of Pharmaceutical
Sciences, Gannan Medical University (QD201303, YB201415), the
National Natural Science Foundation of China (21562004) for par-
tial financial support of this study. We also thank the Medical Sci-
entific Research Foundation of Guangdong Province (B2014191),
The youth research project of colleges and universities in Guangz-
hou (2012C208), The PhD Start-up Fund of Guangzhou medical
university (L110506).
4.3.14. (2R,3S)-2-Isopropyl-4-nitro-3-phenylbutanal16
Supplementary data
OHC
H3C
NO2
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetasy.2016.03.
006.
CH3
Please cite this article in press as: Hu, X.; et al. Tetrahedron: Asymmetry (2016), http://dx.doi.org/10.1016/j.tetasy.2016.03.006

8
X. Hu et al. / Tetrahedron: Asymmetry xxx (2016) xxx–xxx
Rahman, M. B. Catal. Lett. 2014, 144, 222; (q) Wang, Y.; Li, D.; Lin, J.; Wei, K. RSC
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