Organocatalytic highly enantioselective Michael addition of 2-hydroxy-1,4-naphthoquinones to nitroalkenes

Article abstract of DOI:10.1021/ol800945e

(Chemical Equation Presented) The first organocatalytic enantioselective Michael addition of 2-hydroxy-1,4-naphthoquinones to nitroalkenes for the direct synthesis of chiral nitroalkylated naphthoquinone derivatives was investigated. Good yields and excel

Full text of DOI:10.1021/ol800945e

ORGANIC
LETTERS
2
008
Vol. 10, No. 13
817-2820
Organocatalytic Highly Enantioselective
Michael Addition of 2-Hydroxy-1,4-
naphthoquinones to Nitroalkenes
2
Wen-Ming Zhou, Han Liu, and Da-Ming Du*
Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of
Bioorganic Chemistry and Molecular Engineering of Ministry of Education,
College of Chemistry and Molecular Engineering, Peking UniVersity,
Beijing 100871, People’s Republic of China
dudm@pku.edu.cn
Received April 24, 2008
ABSTRACT
The first organocatalytic enantioselective Michael addition of 2-hydroxy-1,4-naphthoquinones to nitroalkenes for the direct synthesis of chiral
nitroalkylated naphthoquinone derivatives was investigated. Good yields and excellent enantioselectivities (up to >99% ee) could be achieved.
This organocatalytic asymmetric Michael addition provides an efficient route torward the synthesis of optically active functionalized
naphthoquinones.
In recent years, organocatalysis has been the subject of
intensive development, and many organocatalysts have been
applied in a variety of asymmetric reactions. Organocatalysis
has been recognized as having special features such as being
environmentally benign and having atom economy and
convenient operation. Among the organocatalysts used
currently, proline derivatives, thioureas, and Cinchona
1
2
alkaloids are recognized as the privileged catalysts. Further
(1) For reviews on organocatalysis, see for examples: (a) Berkessel, A.;
Gr o¨ ger, H. Asymmetric Organocatalysis; Wiley-VCH: Weinheim, Germany,
development of the new asymmetric reaction with use of
easily available organocatalysts and the discovery of a more
practical catalytic asymmetric process toward the synthesis
of novel chiral bioactive compounds are still demanding
research areas.
2
005. (b) Dalko, P. I. EnantioselectiVe Organocatalysis; Wiley-VCH:
Weinheim, Germany, 2007. (c) Special issue on organocatalysis: Acc. Chem.
Res. 2004, 37(8). (d) Dalko, P. I.; Moisan, L. Angew. Chem., Int. Ed. 2001,
4
0, 3726. (e) Jarvo, E. R.; Miller, S. J. Tetrahedron 2002, 58, 2481. (f)
Dalko, P. I.; Moisan, L. Angew. Chem., Int. Ed. 2004, 43, 5138. (g)
Takemoto, Y. Org. Biomol. Chem. 2005, 3, 4299. (h) Seayad, J.; List, B.
Org. Biomol. Chem. 2005, 3, 719. (i) List, B. Chem. Commun. 2006, 819.
Quinones and naphthoquinones are important structural
units in many natural products, which can undergo many
important biological transformations. Naphthoquinones are
also used in industry on a large scale as dye reagents or
components. Many quinone-containing compounds have
biological activity owing to the presence of the quinone
(
j) Connon, S. J. Angew. Chem., Int. Ed. 2006, 45, 3909. (k) Connon, S. J.
Chem. Eur. J. 2006, 12, 5418. (l) Brogan, A. P.; Dickerson, T. J.; Janda,
K. D. Angew. Chem., Int. Ed. 2006, 45, 8100. (m) Marcelli, T.; van
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(
n) Akiyama, T.; Itoh, J.; Fuchibe, K. AdV. Synth. Catal. 2006, 348, 999.
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Chem., Int. Ed. 2007, 46, 494. (q) de Figueiredo, R. M.; Christmann, M.
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Acc. Chem. Res. 2007, 40, 1327. (t) Pellissier, H. Tetrahedron 2007, 63,
3
pharmacophore. Quinone-containing antitumor drugs, such
9
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10.1021/ol800945e CCC: $40.75  2008 American Chemical Society
Published on Web 06/06/2008

7
as mitoxantrone, ametantrone, and doxorubicin, demonstrate
potent antitumor activity and have been used in clinics as
one of the most effective classes of anticancer agents with
broad application in the treatment of several leukemias and
withdrawing nature of the nitro group. The conjugate
addition of naphthoquinone to nitroalkenes is particularly
interesting because it produces nitroalkylated compounds
which are precursors of a variety of other functionalized
bioactive compounds. However, to the best of our knowl-
edge, no studies on the catalytic asymmetric synthesis of
nitroalkylated naphthoquinones has been reported.
As one part of our continuing research program on the
catalytic asymmetric addition of nitroalkane and heteroaro-
4
lymphomas. In view of their important biological features
in medicinal chemistry, a large number of quinone derivatives
and related compounds have been prepared in order to search
for novel bioactive agents with improved pharmacological
8
5
properties.
9
matics to nitroalkenes, we would like to report the first
The asymmetric Michael addition with nitroalkenes as
Michael acceptor is an important C-C bond-forming reac-
tion, which provides access to synthetically useful enan-
tioenriched nitroalkanes and has attracted significant interest
organocatalytic enantioselective Michael addition of 2-hy-
droxynaphthoquinones to nitroalkenes for the direct synthesis
of chiral nitroalkylated naphthoquinone derivatives herein.
Good yields and excellent enantioselectivities (up to >99%
ee) could be achieved for most substrates.
Initially, we studied the model Michael addition reaction
of 2-hydroxy-1,4-naphthoquinone 6a to ꢀ-nitrostyrene 7a by
the catalysis of 10 mol % of a series of readily available
organocatalyst 1-5 (Figure 1) at room temperature. The
6
in recent years. Nitroalkanes can be transformed into a
variety of functionalities due to the strong electron-
(
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Figure 1. Thiourea and cinchona alkaloid organocatalysts.
H.-W.; Guh, J.-H.; Chan, Y.-L.; His, C.-P.; Yang, M.-S.; Li, T.-K.; Lee,
C.-H. Bioorg. Med. Chem. 2008, 16, 1006. (d) Weyler, S.; Baqi, Y.;
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6) For selected reviews see: (a) Berner, O. M.; Tedeschi, L.; Enders,
results are presented in Table 1. During the screening of
catalysts, we first performed the reaction by the catalysis of
thiourea catalyst 1 in CH Cl . Gratifyingly, we found that
2 2
the reaction was completed within 12 h and the correspond-
ing addition product was obtained in 82% yield with excellent
enantioselectivity (>99% ee). Thiourea catalyst 2 cannot give
high enantioselectivity. Such a phenomenon can be ascribed
to the lack of tertiary amine groups, which work as the
activator of the pronucleophile. The cinchona alkaloid
D. Eur. J. Org. Chem. 2002, 1877. (b) Tsogoeva, S. B. Eur. J. Org. Chem.
007, 1701. (c) Christoffers, J.; Koripelly, G.; Rosiak, A.; R o¨ ssle, M.
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2
2
For recent examples see: (f) Okino, T.; Hoashi, Y.; Takemoto, Y. J. Am.
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006, 1451. (n) Luo, S.; Zhang, L.; Mi, X.; Qiao, Y.; Cheng, J.-P. J. Org.
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Org. Lett., Vol. 10, No. 13, 2008

Table 1. Screening of the Reaction Condition of Asymmetric
Table 2. Catalytic Asymmetric Michael Addition of
2-Hydroxy-1,4-naphthoquinones to Nitroalkenes
a
Michael Addition of 2-Hydroxy-1,4-naphthoquinone to
a
ꢀ-Nitrostyrene
R1
R2
product yield (%)
b
ee (%)
c
entry
b
c
entry catalyst solvent t (°C) time (h) yield (%) ee (%)
1
2
3
6
4
5
7
8
9
H
H
H
H
H
H
H
H
H
H
H
Me
Me
MeO
C6H5
8a
8b
8c
8d
8e
8f
8g
8h
8i
8j
8k
8l
8m
8n
82
75
75
71
75
81
75
85
77
77
63
77
73
61
>99
>99
>99
>99
>99
>99
89
4-MeC6H4
4-MeOC6H4
4-FC6H4
1
2
3
4
5
6
7
8
9
0
1
a
1
2
3
4
5
1
1
1
1
1
1
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
THF
CH3CN
CH3OH
toluene
CH2Cl2
CH2Cl2
25
25
25
25
25
25
25
25
25
0
12
24
12
12
12
12
12
12
12
12
12
82
65
72
75
72
71
65
72
59
73
78
>99
6
25
-64
56
87
86
69
97
>99
95
4-ClC6H4
4-BrC6H4
2-MeOC6H4
2-furanyl
2-thienyl
2-phenylethyl
cyclohexyl
C6H5
89
>99
d
10
11
12
13
14
93
95
e
1
1
94
94
95
d
25
4-MeC6H4
C6H5
Reaction conditions: 6a (0.2 mmol), 7a (0.2 mmol), and 10 mol % of
organocatalyst in 2 mL of solvent. Isolated yields after chromatography.
Determined by HPLC analysis on Chiralcel OJ-H column. With 5 mol
catalyst.
b
a
c
d
Reaction conditions: 6 (0.2 mmol), 7 (0.2 mmol), and catalyst 1 (10
b
%
mol %) in CH2Cl2 (2 mL) for 12 h at 25 °C. Isolated yields after
chromatography. Determined by HPLC analysis on Chiralcel OJ-H column.
Determined through its O-methyl derivative. Determined by HPLC
c
d
e
analysis on Chiralcel AD column through its O-methyl derivative.
catalysts 3-5 gave lower enantioselectivity than thiourea 1.
Cinchona alkaloid catalysts 3-5 gave modest enantioselec-
tivities (35-64% ee), though the reaction can also be
completed within 12 h (Table 1, entries 3-5). This observa-
tion indicates that the cooperation of thiourea and tertiary
amine functionalities is significant to the enantiocontrol in
this asymmetric addition. From this comparative study, the
bifunctional thiourea catalyst 1 is the best choice.
For further optimization of the reaction condition, we
screened the effect of solvents, temperature, and catalyst
loading. The investigation on the solvent showed that good
to excellent enantioselectivities (86-97% ee) of nitroalky-
tivities (up to >99% ee). For para-substituted aromatic
nitroalkenes, the ee values are higher than 99% (Table 2,
entries 2-6). The aromatic nitroalkene with ortho substitution
gives a little lower enantioselectivity (Table 2, entry 7). Such
a phenomenon can be attributed to the unfavored interaction
between the ortho-substituted group and nucleophile. The
electronic nature of the substitutions has little effect on the
enantioselectivity. Other aromatic nitroalkenes derived from
furan and thiophene can also be applied in this reaction: 89%
and >99% ee can be achieved respectively (Table 2, entries
3
lated naphthoquinones were also obtained in CH CN, THF,
8
and 9). The aliphatic nitroalkenes can also work as good
and toluene. As proton solvent, methanol gave a lower yield
and enantioselectivity, which can be attributed to the
competive hydrogen bond interaction between methanol and
the organocatalyst or the substrate. The screening identified
dichloromethane as the optimal solvent for the reaction
substrates in the Michael addition of 2-hydroxy-1,4-naph-
thoquinone, but the ee value determination of most products
is very difficult since the enantiomers cannot be separated
on many columns. For aliphatic nitroalkenes, such as 1-nitro-
4-phenyl-1-butene and 1-cyclohexyl-2-nitroethylene, higher
(
>99% ee). The yield decreased slightly when the reaction
than 93% ee values can be obtained, which are determined
through their O-methyl derivatives (Table 2, entries 10 and
temperature was reduced to 0 °C, while the enantioselectivity
was not affected significantly (Table 1, entry 10). When the
catalyst loading was lowered to 5 mol %, though the yield
decreased slightly, high enantioselectivity (95% ee) can still
be achieved (Table 1, entry 11), which demonstrates the high
catalytic activity of thiourea catalyst 1.
With the optimized conditions in hand, the scope of the
organocatalyzed asymmetric Michael addition was explored.
A wide range of nitroalkenes were tested under the optimized
1
1).
Adding electron-rich substituents on the 7-position of the
2-hydroxy-1,4-naphthoquinone does not affect the efficiency
of the reaction (Table 2, entries 12-14), 7-methyl- or
7-methoxy-substituted 2-hydroxy-1,4-naphthoquinone can
also afford high enantioselectivities (94% and 95% ee,
respectively). Other quinone derivatives, such as 2-bromo-
1,4-naphthoquinone, 2-morphorlinyl-1,4-naphthoquinone,
2-methoxy-1,4-naphthoquinone, and p-benzoquinone, were
also tested in the reaction. In these cases, no Michael addition
products can be obtained, which indicates the importance
2 2
reaction conditions (10 mol % 1 in CH Cl at room
temperature for 12 h), and the results are summarized in
Table 2. A series of nitroalkylated naphthoquinones 8a-n
were obtained in good yields with excellent enantioselec-
Org. Lett., Vol. 10, No. 13, 2008
2819

of the 2-hydroxy group in the activation mode of the
1
0
nucleophile.
The absolute configuration of product 8f was determined
to be S on the basis of the X-ray crystallographic analysis
1
1
(
CCDC deposition number 682469). In this crystal struc-
ture, there are hydrogen bonds between the 2-hydroxy group
of one molecule and the carbonyl group of another molecule
(
Figure 2).
Figure 3. Proposed transition state for the Michael addition.
enhances the enantiocontrol in the transition state. The
nucleophile approaches the ꢀ-carbon of the nitroalkene from
the Re face under the direction of the tertiary amine group
and forms the S product.
In summary, we developed the first highly enantioselective
asymmetric Michael addition of 2-hydroxy-1,4-naphtho-
quinones to nitroalkenes by using thiourea 1 as the organo-
catalyst. In this efficient catalytic asymmetric reaction, only
1
3
0 mol % of catalyst is sufficient to afford chiral 2-hydroxy-
-functionalized naphthoquinone derivatives in good yields
with excellent enantioselectivities (up to >99% ee). This
reaction provides a valuable route for the synthesis of chiral
naphthoquinone derivatives. Considering the potent trans-
formations and biological activity of the optically pure
nitroalkylated 1,4-naphthoquinone derivatives, further in-
vestigation on the application of the products is underway
in our group.
Figure 2. X-ray crystal structure of 1,4-naphthoquinone 8f.
A mechanism as illustrated in Figure 3 is proposed to
explain the observed results. The nitro group forms hydrogen
bonds with the thiourea segment of organocatalyst 1. The
hydroxy group of the 2-hydroxy-1,4-naphthoquinone is
deprotonated by the tertiary amine segment of the catalyst
and the newly formed anion is associated with the catalyst
through charge interactions. The double interaction between
the hydrogen and the two vicinal oxygen atoms of naththo-
quinone fixes the conformation of the nucleophile and
Acknowledgment. Project partially supported by the
National Natural Science Foundation of China (Grant Nos.
2
0572003, 20772006, and 20521202) and the Program for
New Century Excellent Talents in University (NCET-07-
0
011).
Supporting Information Available: Experimental pro-
1
13
cedures and characterizations, copies of H NMR and
C
(10) (a) Wang, Y.; Li, H.; Wang, Y.-Q.; Liu, Y.; Foxman, B. M.; Deng,
L. J. Am. Chem. Soc. 2007, 129, 6364. (b) Singh, R. P.; Bartelson, K.;
Wang, Y.; Su, H.; Lu, X.; Deng, L. J. Am. Chem. Soc. 2008, 130, 2422.
NMR of new compounds, HPLC profiles, and the crystal
data of 8f (CIF file). This material is available free of charge
via the Internet at http://pubs.acs.org.
(
11) CCDC 682469 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
OL800945E
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Org. Lett., Vol. 10, No. 13, 2008