Solvent-free and stereoselective synthesis of C-glycosides by michael-type addition of enamino esters to 2-nitro- d -glucal

Article abstract of DOI:10.1055/s-0030-1258498

Michael-type addition of enamino esters to 3,4,6-tri-O-benzyl-2-nitro-d- glucal under solvent-free conditions formed C-glycosides in excellent yields with high stereoselectivity. Reduction of the nitro group afforded the corresponding bicyclic 2-amino C-g

Full text of DOI:10.1055/s-0030-1258498

2174
LETTER
Solvent-Free and Stereoselective Synthesis of C-Glycosides by Michael-Type
Addition of Enamino Esters to 2-Nitro-D-glucal
S
olvent-Free St
i
Z
Synthesis of
C
-
h
G
lycosides ang,a,b Chu-Yi Yu,*a Zhi-Tang Huang,*a Yue-Mei Jiaa
a
Beijing National Laboratory for Molecular Science (BNLMS), CAS Key Laboratory of Molecular Recognition and Function,
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. of China
Fax +86(10)82616433; E-mail: yucy@iccas.ac.cn
b
Graduate University of Chinese Academy of Sciences, Beijing 100049, P. R. of China
Received 13 April 2010
diverse reactive properties, and they have been widely
used in the synthesis of alicyclic, aromatic and heterocy-
clic compounds.5 Hence, it would be significant not only
in carbohydrate chemistry but also in synthetic organic
chemistry to develop a convenient, efficient and practical
method for the C-glycosidation of enamino esters.
Abstract: Michael-type addition of enamino esters to 3,4,6-tri-O-
benzyl-2-nitro-D-glucal under solvent-free conditions formed
C-glycosides in excellent yields with high stereoselectivity. Reduc-
tion of the nitro group afforded the corresponding bicyclic 2-amino
C-glycosides.
Key words: solvent-free, C-glycosides, Michael-type addition
Solvent-free reactions, due to the advantages of less pol-
lution, lower expense, and easier procedures, have attract-
ed considerable attention from synthetic organic
chemists.6 However, the application of solvent-free reac-
tions in carbohydrate chemistry has been limited.7 Herein,
we report a solvent-free C-glycosidation method by using
Michael-type addition of enamino esters to 3,4,6-tri-O-
benzyl-2-nitro-D-glucal (1).8
C-Glycosides, either synthetic or naturally occurring, are
an important class of bioactive compounds.1 In contrast to
their O-analogues, C-glycosides are less vulnerable to en-
zymatic and chemical hydrolyses, which has led to signif-
icant interest in their viability as drug candidates and
enzyme inhibitors. The synthesis of C-glycosides has
hence attracted much attention from synthetic organic
chemists and methods for the formation of C-glycosides
are well documented in the literature.2 The synthetic strat-
egy for C-glycosides usually involves the construction of
the anomeric C–C bond using the nucleophilic, electro-
philic, or radical character of the anomeric center, and
concerted reactions have also been employed for C-gly-
cosidation.3 Electrophilic reactions were probably most
widely applied to C-glycosidation and commonly used C-
nucleophiles include organometallics, cyanides, C-sily-
late compounds, alkenes, and activated aromatic and b-di-
carbonyl compounds. Only a few applications of enamino
esters as nucleophiles for C-glycosidation have been re-
ported in the literature.4 Due to their easy preparation,
considerable stability, and bisnucleophility, enamino es-
ters are a remarkable class of synthetic intermediates with
The 2-nitro-D-glucal 1 was prepared according to a report-
ed method.9 Enamino ester 2d was chosen as a model sub-
strate in the test reactions with 1 (Scheme 1; R1 = Ph,
R2 = Bn). Considering that the Michael addition reactions
were usually performed with the aid of a base,10 the reac-
tions between 1 and 2d were initially carried out in the
presence of various bases, such as: NaH, t-BuOK,
CH3ONa, DBU, Et3N or DMAP, and in various solvents
(CH2Cl2, MeCN, THF, 1,4-dioxane, toluene or DMF).
Unfortunately, either no reaction occurred or complex
mixtures of products were generated under such reaction
conditions. Subsequently, in order to optimize the reac-
tion conditions, the influence of concentration on the out-
come of the glycosidation was examined, and the reaction
between 1 and 2d was attempted in concentrations rang-
ing from 0.05 M to solvent-free conditions. To our de-
NHR2
OBn
O
OBn
COOEt
O
solvent-free
BnO
BnO
BnO
BnO
COOEt
NHR2
+
R1
R1
NO2
H
NO2
1
2
3
OBn
O
OBn
O
COOEt
Zn, NH4Cl
BnO
BnO
BnO
BnO
+
COOEt
Ph
EtOH, reflux
HN
HN
6
7
Ph
Scheme 1 Solvent-free and stereoselective C-glycosidation by Michael-type addition of enamino esters to 2-nitro-D-glucal
SYNLETT 2010, No. 14, pp 2174–2178
x
x
.x
x
.2
0
1
0
Advanced online publication: 16.07.2010
DOI: 10.1055/s-0030-1258498; Art ID: W05910ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
Solvent-Free Stereoselective Synthesis of C-Glycosides
2175
light, the reaction proceeded smoothly to give C- the other side. The X-ray single crystal analyses of 3d
glycoside 3d under solvent-free conditions. The appropri- (Figure 1) and 3i (Figure 2) unambiguously confirmed
ate temperature for the reaction, in terms both of yield and these assignments.13
reaction time, was found to be 110 °C.11 The reaction be-
Subsequently, we focused our attention on the C-glycosi-
tween a variety of enamino esters12 2a–k and 1 were then
dation of heterocyclic enamines 4 (Table 2). The solvent-
attempted (Table 1). All reactions were complete within
5–8 hours and resulted in the formation of the desired C-
glycosides 3a–k in good yield (85–94%) with excellent b-
stereoselectivity. The reactions of the enamino esters de-
rived from ethyl acetoacetate (2i–k) with 1 should be per-
formed at temperatures below 95 °C (Table 1, entries 9,
10, and 11) to avoid low yields. Although both b-C- and
N-positions of enamino esters can act as nucleophilic cen-
ters and either could, in principle, be involved in the reac-
tions with 1, no N-glycoside was detected. Therefore, the
reactions between 1 and 2 proceeded smoothly with excel-
lent regio- and stereoselectivity. The presence of a nitro
group at C-2 seemed to allow anchimeric assistance and
consequently orientated the glycosylation in the direction
of the b-anomer.
free reactions of heterocyclic enamino esters 4a and 4b14
were conducted at 85 °C and were complete within 30
minutes, whereas the reactions of nitro-substituted hetero-
cyclic enamines 4c and 4d15 required longer reaction
times and did not go to completion. The conversion ratio
was not improved by addition of a base; on the contrary,
the yields decreased dramatically (20–25%), due to the
decomposition of 2-nitroglucal 1. C-Glycosidation of het-
The configurations of C-glycosides were determined on
the basis of their spectroscopic properties. For example, in
the 1H NMR spectra of 3d, the anomeric hydrogen signal
appeared as a doublet with J1¢,2¢ = 9.7 Hz. Similarly, the H-
2¢ signal appeared as a triplet with J1 = J2 = 9.7 Hz. This
clearly indicated a trans relationship between H-1¢ and H-
2¢, and between H-2¢ and H-3¢. The structure assignment
was also supported by an NOE experiment that indicated
strong interactions between H-1¢ and H-3¢/H-5¢ and be-
Figure 1 X-ray crystal structure of 3d; partial hydrogen atoms bon-
ded to carbon atoms have been omitted for clarity. The intramolecular
hydrogen bond is indicated by a dashed line.
tween H-2¢ and H-4¢. It was inferred that H-1¢ and H-3¢/H-
5¢ were on the same side, whereas H-2¢ and H-4¢ were on
Table 1 Michael-Type Addition of Enamino Esters to 2-Nitro-D-glucal
OBn
5'
OBn
O
NHR2
COOEt
NHR2
4'
solvent-free
O
BnO
BnO
BnO
BnO
COOEt
+
1'
R1
2'
3'
H
R1
NO2
NO2
1
2a–k
3a–k
Entry
Enamino ester
R1
R2
Temp (°C)
110
110
110
110
110
110
110
110
95
Time (h)
Yield (%)a
1
2
2a
2b
2c
2d
2e
2f
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Me
Me
Me
H
8
6
5
5
7
5
8
8
6
5
3
85
87
90
94
88
92
92
91
91
93
94
Me
Ph
3
4
Bn
n-Pr
i-Pr
5
6
7
2g
2h
2i
CH2CH=CH2
CH2CO2Et
H
8
9
10
11
2j
CH2CH=CH2
Bn
95
2k
95
a Combined isolated yields after column chromatography.
Synlett 2010, No. 14, 2174–2178 © Thieme Stuttgart · New York

2176
T. Zhang et al.
LETTER
H-2¢; while the H-2¢ signal appeared as a double doublet
with J2¢,3¢ = 3.7 Hz, indicating axial/equatorial arrange-
1
ment for H-2¢ and H-3¢ and a C4 conformation for 5a-.
This structural assignment was also supported by an NOE
experiment, which demonstrated a strong interaction be-
tween H-1¢ and H-6¢.
The presence of a nitro group at C-2¢ has several advan-
tages; in particular, it would provide access to various 2-
amino-C-glycosides,16 which are important components
of many glycoproteins that are attractive targets for the
design of C-linked carbohydrate mimetics.17 Accordingly,
we then attempted the reduction of the nitro group. A va-
riety of reagents and conditions for reduction of the nitro
groups18 were screened, including Raney Ni, Zn/Ac2O
and Zn/AcOH, but all these conditions led to unidentifi-
able mixtures of products. Fortunately, reduction of com-
pound 3 with four equivalents of zinc dust and four
equivalents of NH4Cl in ethanol proved to be a relatively
clean reaction, resulting in the formation of the unexpect-
ed bicyclic compounds 6 and 7 in good yields. Bicyclic
compounds 6 and 7 were clearly formed through a se-
quence of reactions, i.e., reduction of the nitro group, fol-
lowed by subsequent intramolecular cyclization along
with the elimination of the amino group of the aglycon.
Figure 2 X-ray crystal structure of 3i; partial hydrogen atoms bon-
ded to carbon atoms have been omitted for clarity. The intramolecular
hydrogen bond is indicated by a dashed line.
erocyclic enamino esters could proceed when refluxing in
MeCN, albeit in lower yields. Because 2-nitroglucal 1 did
not react completely (Table 2, entries 2 and 5), a catalytic
amount of Et3N was added to solve this problem (Table 2,
entries 3 and 6). For six-membered heterocyclic enam-
ines, only the b-anomer was obtained, whereas five-mem-
bered enamines afforded a mixture of two anomers. The
results indicated that compounds with six-membered
rings showed better stereoselectivity than those with five-
membered rings.
The ratio of products 6 and 7 ranged between 3:1 to 1.2:1
(Table 3); the configurations of the products were deter-
mined by 1H NMR and NOE experiments. For compound
6, the H-3 signal appeared at d = 3.18 ppm as a double
doublet with J1 = 9.8 Hz and J2 = 8.2 Hz, and the H-7a
signal appeared at d = 3.32 ppm as a triplet with
J1 = J2 = 9.2 Hz. NOE experiments showed interactions
between H-3a and H-2/H-5/H-7 and between H-7a and
The configurations of the C-glycosides were determined
by analysis of their 1H NMR and NOE spectra. For the a-
anomer, the H-1¢ signal appeared as a doublet with
J
1¢,2¢ = 9.8 Hz, indicating a trans arrangement for H-1¢ and
Table 2 Michael-Type Addition of Heterocyclic Enamines to 2-Nitro-D-glucal
OBn
OBn
OBn
OBn
4'
R3
H
5'
2'
O
H
O
BnO
BnO
BnO
BnO
O2N
+
N
1'
O
R3
+
n
NH
R3
OBn
3'
NO2
NO2
n
NH
n
β-5a–d
4a–d
α-5a–d
1
Entry
4
n, R3
Solvent
–
Temp (°C)
85
Additive
Recovered 1 (%) Time (h)
Yield (%)
96a
b/a
1
2
3
4
5
6
7
8
4a
4a
4a
4b
4b
4b
4c
4d
0, CO2Et
0, CO2Et
0, CO2Et
1, CO2Et
1, CO2Et
1, CO2Et
0, NO2
–
–
0.5
5
98:2b
90:10b
90:10b
1:0
MeCN
MeCN
–
reflux
reflux
85
–
10
–
74a
Et3N
5
89a
–
–
0.5
8
94
MeCN
MeCN
–
reflux
reflux
85
–
15
–
70
1:0
Et3N
8
84
1:0
–
–
7
4
86a
93:7c
1:0
1, NO2
–
85
9
6
79
a Combined isolated yield after column chromatography.
b Ratio was determined from the isolated pure diastereomers.
c Ratio was determined by integration of the H-2¢ signal in the 1H NMR spectra (300 MHz).
Synlett 2010, No. 14, 2174–2178 © Thieme Stuttgart · New York

LETTER
Solvent-Free Stereoselective Synthesis of C-Glycosides
2177
Table 3 Reduction of 2-Nitro-C-glycosides
OBn
OBn
5
OBn
COOEt
6
COOEt
O4
BnO
O
Zn, NH4Cl
BnO
BnO
O
3
NHR2
BnO
+
BnO
COOEt
Ph
BnO
EtOH, reflux
7
HN
HN
NO2
2
Ph
7
Ph
6
3
Entry
2-Nitro-C-glycoside
R2
Time (h)
0.5
Yield (%)a
Ratio 6/7b
1.2:1
3:1
1
2
3
4
3a
3d
3e
3h
H
82
87
83
85
Bn
0.5
n-Pr
CH2CO2Et
0.5
1.6:1
2:1
0.5
a Combined isolated yield after column chromatography.
b The ratio of 6 and 7 was determined by the isolation of pure diastereomers.
H-3/H-6, which indicated a trans relationship between H- References and Notes
2/H-3, H-3/H-3a and H-3a/H-7a. The H-3 signal of com-
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appeared at d = 3.24 ppm as a triplet with J1 = J2 = 9.2 Hz.
The trans relationship between H-2/H-3 and H-7a/H-3a
and the cis relationship between H-3/H-3a could be ob-
served from an NOE experiment (Figure 3).
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H6
H6
OBn
H7a
OBn
H7a
H3
COOEt
H3
O
O
BnO
BnO
BnO
BnO
COOEt
Ph
H2
HN
HN
H5
H7
H5
H7
H3a
H3a
H2
Ph
7
6
Figure 3 Key NOE correlations for bicyclic compounds 6 and 7
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In conclusion, an efficient, solvent-free C-glycosidation
method has been developed based on the Michael-type ad-
dition of enamino esters to 3,4,6-tri-O-benzyl-2-nitro-D-
glucal, which results in the formation of the desired C-gly-
cosides in good yields and with excellent regio- and stere-
oselectivity. Reduction of the nitro group gave access to
novel bicyclic 2-amino C-glycosides that are potentially
useful as bioactive compounds, such as inhibitors of gly-
cosidases.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
Acknowledgment
This work is supported by The National Natural Science Foundation
of China (No. 20472086), The National Basic Research Program of
China (No. 2009CB526511), The Ministry of Science and Techno-
logy and the Ministry of Health of the P. R. of China (No.
2009ZX09501-006), and The Chinese Academy of Sciences.
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Synlett 2010, No. 14, 2174–2178 © Thieme Stuttgart · New York

2178
T. Zhang et al.
LETTER
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Synlett 2010, No. 14, 2174–2178 © Thieme Stuttgart · New York