Organocatalytic enantioselective Michael addition of a kojic acid derivative to nitro olefins

Article abstract of DOI:10.1039/c2ob07192d

By employing a chiral bifunctional thiourea-tertiary amine as catalyst, enantioselective Michael addition of a kojic acid derivative to nitro olefins was realised. The reactions afforded the products with good yields (up to 99%) in good enantioselectivities (up to 97% ee). In addition, the absolute configuration of one product was determined. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2ob07192d

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Organocatalytic enantioselective Michael addition of a kojic acid derivative to
nitro olefins†
Jiyu Wang,a,b Qing Zhang,a Hui Zhang,a,b Yujun Feng,a Weicheng Yuana and Xiaomei Zhang*a
Received 29th December 2011, Accepted 15th February 2012
DOI: 10.1039/c2ob07192d
reported.13 Owing to the good nucleophilicity of kojic acid, we
By employing a chiral bifunctional thiourea–tertiary amine
envisioned that chiral bifunctional organocatalysts 1 would cata-
lyze enantioselective Michael addition of a kojic acid derivative
(for example, 2-((tert-butyldimethylsilyloxy)methyl)-5-hydroxy-
4H-pyran-4-one 214) to nitro olefin 3 as outlined in Scheme 1.
This transformation would be important because a novel sort of
enantioenriched kojic acid derivatives could be obtained. In
addition, the corresponding product 4 could undergo oxidative
fragmentation to generate β-nitro acid 5. Following reduction of
the nitro group would accomplish the synthesis of enantio-
enriched β2-amino acid 6. β2-Amino acids are important com-
ponents in β-peptides.15 In contrast to β3-homo-amino acids, β2-
amino acids cannot be obtained simply by homologation of the
(natural) α-amino acids, but have to be synthesized by enantio-
selective reactions. Therefore, synthesis of β2-amino acids has
attracted increasing attention16,17 and development of a facile
strategy for preparation of enantioenriched β2-amino acids is of
great importance.
Herein we present the enantioselective Michael addition of a
kojic acid derivative to a wide variety of nitro olefins promoted
by bifunctional chiral thiourea–tertiary amine organocatalysts.
The reactions proceeded smoothly to provide the desired pro-
ducts in good yields (up to 99%) and good enantioselectivities
(up to 97% ee). Subsequent oxidative fragmentation of one of
the product afforded the corresponding β-nitro acid. Hence this
transformation enables easy access to enantioenriched β2-amino
acids.
as catalyst, enantioselective Michael addition of a kojic acid
derivative to nitro olefins was realised. The reactions
afforded the products with good yields (up to 99%) in good
enantioselectivities (up to 97% ee). In addition, the absolute
configuration of one product was determined.
Introduction
The nitro olefins are important building blocks and intermediates
in organic synthesis because the nitro group can be easily trans-
formed into other useful groups such as amines and carbonyls.1
Asymmetric Michael addition of nitro olefins represents one of
the most versatile entries to enantioenriched nitroalkanes. As a
result, many efforts have been devoted to the development of
this process.2 Impressive progress has been made with both
chiral metal3 and organocatalysts.4 Notably, in recent years,
many chiral organocatalysts have been successfully employed to
promote asymmetric Michael addition of various nucleophiles
such as 1,3-dicarbonyl compounds,5 aldehydes or ketones,6 oxi-
ndoles,7 arenes,8 hetero nucleophiles9 and other nucleophiles10
to nitro olefins to provide a wide variety of enantioenriched
nitroalkanes. While the organocatalytic asymmetric Michael
addition of nitro olefins has been explored intensively, we noted
that the asymmetric Michael addition of kojic acid to nitro
olefins has not been developed yet.
Kojic acid is a natural fungal metabolite. Owing to the diverse
biological activities of many kojic acid derivatives, such as che-
lating activity and inhibition of tyrosinase, a large number of
kojic acid derivatives have been prepared for biological evalu-
ation.11 Kojic acid derivatives were also employed in construc-
tion of various biomedicinally important natural products and
analogues.12 However, only a few examples of organocatalyzed
asymmetric reactions of kojic acid and its derivatives have been
Results and discussion
First,
2-((tert-butyldimethylsilyloxy)methyl)-5-hydroxy-4H-
pyran-4-one 214 was selected as the standard substrate in the
reaction due to its good solubility. A range of bifunctional
thiourea–tertiary amine organocatalysts (Fig. 1) were screened in
the Michael addition of 2 to nitro olefin 3a in toluene at 20 °C
for 2 days. The results are summarized in Table 1.
As can be seen in Table 1, all of the catalysts 1a–1j (Fig. 1)
catalyzed the reaction to provide the adduct 4a in moderate
yields. Cinchona alkaloids derived thiourea–tertiary amine cata-
lysts 1a–1d afforded low enantioselectivities (Table 1, entries
1–4). (1R,2R)-1,2-Diphenyl-1,2-diamine derived catalyst 1e also
resulted in very low enantioselectivity (Table 1, entry 5). Better
aChengdu Institute of Organic Chemistry, Chinese Academy of Sciences,
Chengdu, China. E-mail: xmzhang@cioc.ac.cn; Fax: (+86)28-
85257883; Tel: (+86)28-85257883
bGraduate School of Chinese Academy of Sciences, Beijing, China
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c2ob07192d
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Scheme 1 Proposed chiral bifunctional thiourea–tertiary amine 1 catalyzed Michael addition of kojic acid derivative 2 to nitro olefin 3 and sub-
sequent conversion to enantioenriched β2-amino acid 6.
Fig. 1 Chiral bifunctional thiourea–tertiary amine organocatalysts evaluated in this study.
results were observed with trans-cyclohexane-1,2-diamine
derived thiourea–tertiary amine catalysts 1f–1j (Table 1, entries
6–10), in which 1j provided the highest ee value (Table 1, entry
10).
Afterwards the temperature was also investigated. Lowering the
temperature to 0 °C caused no change in ee value (Table 1, entry
24). To our delight, when the reaction was conducted at −10 °C
for 4 days, the product was obtained in excellent yield with good
enantioselectivity (Table 1, entry 25). Further lowering the temp-
erature to −20 °C resulted in decrease in ee value (Table 1, entry
26).
Having established the optimal conditions, the enantioselec-
tive Michael addition was expanded to a wide variety of nitro
olefins. The results are summarized in Table 2. Generally, the
reaction of aromatic nitro olefins bearing electron-withdrawing
substituents in the para position of the phenyl groups with kojic
acid derivative 2 proceeded smoothly to provide the correspond-
ing products in good enantioselectivities (Table 2, entries 2–5).
Meanwhile, electron-donating substituents in the para position
of the phenyl groups caused slightly decrease in ee values
(Table 2, entries 6 and 7). Aromatic nitro olefins bearing substi-
tuents in the ortho position or the meta position of the phenyl
groups also afforded the corresponding products in inferior enan-
tioselectivities (Table 2, entries 8–12). However, 1-naphthyl
nitro olefin 3m exhibited good enantioselectivity (Table 2, entry
13). By employing 20 mol% of catalyst 1j, the highest ee value
of 97% ee was obtained with heteroaryl nitro olefin 3n, although
Therefore, 1j was determined to be the optimal catalyst and
was employed in further investigations. Subsequently, various
solvents were evaluated. Several aromatic solvents other than
toluene delivered very poor yields and ee values (Table 1, entries
11–14). Particularly, no product was obtained in mesitylene,
perhaps due to the poor solubility of compound 2 in mesitylene
(Table 1, entry 14). Dichloromethane and chloroform also gave
only moderate yields and poor ee values (Table 1, entries 15 and
16). Ether led to even lower yield as well as poor enantioselec-
tivity (Table 1, entry 17). Gratifyingly, we found that methanol
provided the product in much higher yields with good ee value
(Table 1, entry 18), which suggested that alcoholic solvents
would benefit the reaction. Consequently several other alcoholic
solvents were also tested. However they did not result in better
results (Table 1, entries 19–21). Hence methanol was determined
as the most favourable solvent to the reaction. Addition of 4 Å
molecular sieve led to erosion both in yield and ee value
(Table 1, entry 22). Enhancing the catalyst loading to 15 mol%
did not improve the enantioselectivity (Table 1, entry 23).
This journal is © The Royal Society of Chemistry 2012
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Table 1 Enantioselective Michael addition of kojic acid derivative 2 to
nitro olefin 3aa
Table 2 Enantioselective Michael addition of kojic acid derivative 2 to
nitro olefins 3a–3oa
Yieldb (%)
eec (%)
Entry
3 (R)
Yieldb (%)
eec (%)
Conf.d
Entry
Cat*
Solvent
T (°C)
1
2
3
4
5
6
7
8
3a (Ph)
95
85
86
83
84
77
80
99
92
87
80
89
86
58
68
90
92
93
94
89
87
88
81
83
83
86
87
91
97
89
R (+)
(+)
(+)
(+)
(+)
(+)
(+)
(+)
(+)
(+)
(+)
(+)
(+)
(+)
(+)
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1j
1j
1j
1j
1j
1j
1j
1j
1j
1j
1j
1j
1j
1j
1j
1j
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Xylene
Mesitylene
C6H5Cl
C6H5CF3
CH2Cl2
CHCl3
Et2O
MeOH
EtOH
i-PrOH
HOCH2CH2OH
MeOH
MeOH
MeOH
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
0
69
65
71
68
54
84
64
84
80
82
57
NR
59
59
59
69
20
95
75
95
27
74
87
82
95
81
−31
−35
30
40
29
48
52
−70
77
79
56
—
45
48
46
54
42
83
81
81
45
71
80
82
90
85
3b (4-FC6H4)
3c (4-ClC6H4)
3d (4-BrC6H4)
3e (4-O2NC6H4)
3f (4-MeC6H4)
3g (4-MeOC6H4)
3h (2-ClC6H4)
3i (2,4-Cl2C6H3)
3j (2-BrC6H4)
3k (2-O2NC6H4)
3l (3-BrC6H4)
3m (1-Naphthyl)
3n (2-Thienyl)
3o (c-C6H11)
9
9
10
11
12
13
14e
15f
10
11
12
13
14
15
16
17
18
19
20
21
22d
23e
24f
25g
26h
a Unless specified otherwise, reactions were carried out with
2
(0.15 mmol), 3 (0.1 mmol), and the catalyst 1j (10 mol%) in methanol
(3 mL) at −10 °C for 4 days. b Isolated yield based on 3. c The ee values
were determined by using chiral HPLC. d The absolute configuration of
4a was determined by comparison of the optical rotation value with the
literature datum after being converted into a known compound.17b,18a,b
f
e 20 mol% of 1j was employed. The ee value was determined by using
HPLC after 4o was deprotected.
MeOH
MeOH
−10
−20
a Unless specified otherwise, reactions were carried out with
2
(0.15 mmol), 3a (0.1 mmol) and the catalyst in the solvent (3 mL) at
20 °C for 2 days. b Isolated yield based on 3a. c The ee values were
determined by using chiral HPLC. d With 4 Å MS as additive. e 15 mol
% of 1j was used. f The reaction was carried at 0 °C for 3 days. g The
reaction was carried at −10 °C for 4 days. h The reaction was carried at
−20 °C for 6 days.
Conclusions
In conclusion, we have developed an efficient enantioselective
bifunctional chiral thiourea–tertiary amine catalyzed Michael
addition of a kojic acid derivative to nitro olefins. The reactions
proceeded smoothly to provide the corresponding products with
good yields (up to 99%) in good enantioselectivities (up to
97%). By oxidative fragmentation and following esterification,
product 4a was readily converted into known β-nitro ester 7. The
absolute configuration of 4a was determined as R by comparison
of the optical rotation value of 7 with the literature datum. This
transformation also provided a facile way to synthesize enan-
tioenriched β2-amino acids. Further investigations of the scope
and synthetic utility of this chemistry are under way.
in poor yield (Table 2, entry 14). Furthermore, cyclohexyl nitro
olefin 3o was also subjected to the reaction and gave the product
with good enantioselectivity in moderate yield (Table 2, entry
15).
To illustrate the synthetic utility of this methodology and
further confirm the absolute stereochemistry of this reaction, the
product 4a was subjected to oxidative fragmentation18 to give
β-nitro acid 5a. Subsequent esterification of 5a afforded ester 7
which is a known compound.17b,18a,b By comparison of the
optical rotation value of 7 with the literature datum, the configur-
ation was determined as R. Notably, enantioenriched β2-amino
acid 6a can be easily prepared from β-nitro acid 5a via hydrogen-
ation according to the literature.17b Thus a valuable strategy for
the synthesis of enantioenriched β2-amino acids has been well
established (Scheme 2).
Acknowledgements
We are grateful for the financial support from National Sciences
Foundation of China (21172217) and National Basic Research
Program of China (973 Program) (2010CB833300).
2952 | Org. Biomol. Chem., 2012, 10, 2950–2954
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Scheme 2 Oxidative fragmentation of the product 4a and determination of the absolute configuration.
F. A. Sahr, M. Kuhn, M. Civera, C. Gennari and U. Piarulli, Eur. J. Org.
Chem., 2011, 5599.
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This journal is © The Royal Society of Chemistry 2012
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