Glycosidation promoted by a reusable solid superacid in supercritical carbon dioxide

Article abstract of DOI:10.1002/anie.200602161

Superduper: A practical and cleaner synthetic protocol for various glycosides was established by means of a recyclable solid superacid promoter in supercritical carbon dioxide (scCO₂). Anomeric mixtures composed of α- and β-isomers were success

Full text of DOI:10.1002/anie.200602161

Communications
Glycosidation
DOI: 10.1002/anie.200602161
Glycosidation Promoted by a Reusable Solid
Superacid in Supercritical Carbon Dioxide**
Xue-Bing Li, Masato Ogawa, Toshiki Monden,
Takahiro Maeda, Eri Yamashita, Mariko Naka,
Masao Matsuda, Hiroshi Hinou, and
Shin-Ichiro Nishimura*
Functional roles of glycoconjugates are of growing impor-
tance owing to carbohydrate-mediated signal transductions,
which are crucial steps for a variety of biological phenomena
such as cellular differentiation, aging, immune systems,
infections, and malignant alterations of cells. Synthetic studies
of carbohydrates are indispensable both for elucidating
essential structures of the functional glycoconjugates and
for producing therapeutic reagents of carbohydrate-related
diseases.[1]
Glycosidation is one of the most important reactions in
chemical synthesis of glycoconjugates.[2] In recent years,
although there has been an increasing interest in environ-
mentally more-sustainable chemical processes,[3] a practical
and cleaner chemical synthetic process of carbohydrates has
been discussed very little except for a few pioneering efforts.[4]
The advent of a highly efficient, safe, and environmentally
benign method for the glycosidation is strongly expected
because this key technology will allow for practical and
industrial scale production of natural and unnatural glyco-
sides. Enzyme-assisted synthesis has been recognized to be a
much more sustainable and efficient strategy than pure
chemical protocols in terms of the construction of various
seeds libraries for the discovery and research of therapeutic
reagents.[5] However, the present potentials are not high
enough for large scale synthesis of drug candidates required
for the following clinical trials.
[*] Dr. X.-B. Li, T. Monden, T. Maeda, E. Yamashita, M. Naka,
Dr. M. Matsuda, Dr. H. Hinou, Prof. Dr. S.-I. Nishimura
Division of Advanced Chemical Biology
Graduate School of Advanced Life Science
Frontier ResearchCenter for Post-Genome Science and Technology
Hokkaido University
N21, W11, Sapporo 001-0021 (Japan)
Fax: (+81)11-706-9042
E-mail: shin@glyco.sci.hokudai.ac.jp
M. Ogawa
Department of Technical Engineering
Faculty of Science, Hokkaido University
N10, W8, Sapporo 060-0010 (Japan)
[**] This work was supported by a grant for the National Project
“Researchand Development on Glycocluster Controlling Biomole-
cules” from the New Energy and Industrial Technology Develop-
ment Organization (NEDO).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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In the present study, our interest is focused on the
feasibility of recyclable solid-superacid-promoted glycosida-
tion in supercritical fluids (SCFs). Solid superacids are
industrially potent catalysts that can act both as Lewis acids
and Brønstead acids. SCFs used as reaction media for
homogeneous catalysts or as compressed carriers for hetero-
geneous catalysts have great potential for manipulation of a
variety of catalytic reactions.[3] As CO2 has a critical point
close to ambient temperature (Tc = 31.38C, Pc = 7.4 MPa) and
is nonflammable, nontoxic, and inexpensive, supercritical
(sc)CO2 has received immense interest as an alternative
reaction media to petroleum-derived common organic sol-
vents used in large-scale chemical synthesis. However, there
has been no example of the glycosidation by means of scCO2
as a reaction media. Although it seemed that this might be
owing to the poor solubility of polar biomolecules, such as
amino acids and sugars toward scCO2, recent reports[6] on the
improved solubility of some sugar acetates in scCO2 greatly
motivated us to challenge “green” replacement of conven-
tional synthetic protocols by using hazardous promoters and
flammable/toxic organic solvents.[7] We hypothesized that the
use of a nonpolar scCO2 may promote glycosidation by
increasing the interaction of relatively polar species such as
sugar derivatives and alcohols on the surface of solid acid
catalysts. Furthermore, this effect will encourage the phase
separation of contaminated H2O, which appears to reduce the
yields of the glycosidation due to hydrolysis of the products/
intermediates through rehydration at the glycoside bonds. We
thought that an original and desired apparatus suited for
monitoring glycosidation process must allow for investigating
versatile and general conditions of solid-superacid-promoted
cleaner glycosidation in scCO2.
tion of various donors and acceptors with different solubility
to scCO2. As indicated in Figure 2, the reaction promoted by
sulfated zirconia (SO4/ZrO2),[8] one of the common and
industrially available solid superacid catalysts, was success-
Figure 2. Photographs showing the process of the dissolution of
glycosyl donor 1 (126 mg) and acceptor 4 (17 mg) during the
glycosidation promoted by sulfated zirconia (126 mg) in scCO2 at
348C: a) a mixture of solid materials before addition of CO2; b) most
of the donor and acceptor substrates were dissolved in scCO2 at a
pressure of 7.5 MPa; c) at a pressure of 8.0 MPa, only sulfated zirconia
remained in the solid state.
fully monitored through the quartz window by using 3D
digital fine scope. In accordance with the progress of the
glycosidation, the reaction mixture exhibited a significant
phase-transition state from emulsion to a clear supercritical
fluidic phase, suggesting the existence of some sweet spots in
the individual reaction condition. The present synthetic
system made possible a precise optimization study for
searching for suitable reaction conditions. We finally con-
cluded that all the glycosidation reactions between donors 1,[9]
2,[10] 3,[11] and acceptors 4–8 by using SO4/ZrO2 proceeded
smoothly in scCO2 under just over the critical point at 348C
and 8.0 MPa for 6 h.
Table 1 indicates the results of glycosidation of 2,3,4,6-
tetra-O-acetyl-a-d-galactopyranosyl trichloroacetimidate 1
with acceptors 4–7. Coupling of 1 with all acceptors in
scCO2 gave only b-glycosides 9,[12] 10,[13] 11, and 12 in 76–87%
yields, suggesting that the reactions proceeded by neighboring
To monitor the solubility of typical glycosyl donors and
acceptors toward scCO2, a novel apparatus with a quartz-
window-equipped reactor was developed and examined in
this study. Figure 1 shows schematic view of the apparatus
used for glycosidation in supercritical fluids. In addition to the
convenient quartz window, it should also be noted that a
tailored stirrer connected with the reactor allowed for
accelerating heterogeneous solid-acid-promoted glycosida-
Table 1: Glycosidations of glycosyl donor 1 withvarious glycosyl accept-
ors promoted by sulfated zirconia in scCO2.[b]
Entry
Acceptor
Products
Yield [%][c]
a/b ratio[d]
1
2
3
4
4
5
6
7
9
10
11
12
85
87
76
81
0/1
0/1
0/1
0/1
[a] SO4/ZrO2 was obtained commercially from Wako Pure Chemical
Industries, Ltd. and activated by heating at 6508C for 2 hbefore use.
[b] Glycosidations were carried out by use of 1.1 equiv of alcohol and
100 wt% of SO4/ZrO2 to the donor 1. [c] Yield of isolated product after
purification by SFC. [d] The stereochemistry (a/b ratio) was determined
by SFC and 1H NMR spectroscopy (500 MHz).
Figure 1. Equipment used for glycosidation reactions in scCO2. Th e
tailored SCF synthesizer is composed of a window-equipped stainless-
steel 10-mL reactor with a circulatory heating apparatus, a high
pressure pump, a pressure regulator, and a motor-driven stirrer.
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Communications
group participation mechanism as observed in the reactions of
common donors having 2-O-acyl protections conducted in
common organic solvents. Glycosidation by using glycosyl
donors 2 and 3 protected by O-benzyl groups were also
examined under the same conditions, in scCO2 at 348C and
8.0 MPa. The results summarized in Table 2suggest that both
Table 2: Glycosidations of glycosyl donor 2 or 3 withvarious glycosyl
acceptors promoted by sulfated zirconia in scCO2.[b]
Figure 3. SFC profile for the separation of the anomeric mixture of the
glycosides produced by the glycosidation of 2,3,4,6-tetra-O-benzyl-a-d-
galactopyranosyl imidate (3) with n-octylalcohol (4). Purification was
performed witha Gilson SF3 SFC system equipped withChiralpak AD
(4.6”250 mm2, Daicel Chemical Industries, Ltd.) under a scCO2–
MeOH gradient program (MeOH, 5% (0 min)!50% (15 min), flow
rate=2.5 mLminꢀ1) at 14mPa and 308C. Monitoring of the com-
pounds was carried out witha UV detector at 254 nm or an
Entry
Donor
Acceptor
Products
Yield [%][c]
a/b ratio[d]
1
2
3
4
5
6
7
8
2
2
2
2
3
3
3
3
4
5
7
8
4
5
7
8
13
14
15
16
13
14
15
16
86
91
80
54
88
82
78
51
79/21
81/19
72/28
68/32
24/76
45/56
32/68
37/63
evaporative light scattering detector (ELSD) in cases for the purifica-
tion of O-acetylated products derived from glycosyl donor 1).
[a] SO4/ZrO2 was obtained commercially from Wako Pure Chemical
Industries, Ltd. and activated by heating at 6508C for 2 hbefore use.
[b] Glycosidations were carried out by use of 1.1 equiv of alcohol and
100 wt% of SO4/ZrO2 to the donor 2 or 3. [c] Yield of isolated product
after purification by SFC. [d] The stereochemistry (a/b ratio) was
determined by SFC and 1H NMR spectroscopy (500 MHz). Bn=benzyl.
lytic by-products.[18] Purification of the stereoisomers of the
glycosides by means of SFC allowed for totally sustainable
and environmentally friendly protocol for the practical and
large-scale synthesis of biologically and industrially important
glycoconjugates. We are currently investigating the versatility
of the present protocols by the combined use of SCF and SFC
in the combinatorial synthesis of glycoside-based drug seeds
libraries as well as the construction of commercially available
SCF synthesizer that can be implemented in existing organic
synthesis and carbohydrate chemistry laboratories (com-
pounds used in this study are listed in Scheme 1).
glycosyl donors 2 and 3 were efficiently activated by SO4/
ZrO2 in scCO2 and afforded the expected glycosides 13,[14]
14,[15] 15,[16] and 16[17] in 51–91% yields as a mixture of
stereoisomers. These results indicate that the combination of
SO4/ZrO2 with SFC did not work for the improvement of
stereoselectivity in the selected glycosidation. The crude
products, which contained solid catalyst, were recovered free
from any solvent and thus they were dissolved in a small
amount of methanol and subjected to simple filtration to
separate the reusable catalyst. To minimize the use of organic
solvents in the whole synthetic protocols, we decided to
employ supercritical fluid chromatography (SFC) with scCO2
as the mobile phase for the final purification of the glycosides
instead of common silica gel chromatography. As indicated in
Figure 3, it was revealed that SFC, by using scCO2 under
methanol-gradient (5–50%) at 308C and 14 MPa, allowed for
separation of the mixture of a/b glycosides. Yields of the
isolated products represent similar characteristics that are
observed in the conventional methods with common organic
solvents.
Experimental Section
A quartz vessel charged with glycosyl donor (1 equiv), accepter
(1.1 equiv), and sulfated zirconia (100 wt% of the donor) was
attached to the reactor (10 mL) as shown in Figure 2. The reactor was
heated to 348C and CO2 was introduced successively from a reservoir
by a high-pressure pump. The start of the reaction is defined as the
time at which the CO2 pressure reaches 8.0 MPa. After stirring the
reaction mixture at 348C and 8.0 MPa for 6 h, the reactor was cooled
to 208C and CO2 was then slowly leaked out. The residual solid in the
vessel was washed out with methanol. Sulfated zirconia was recovered
by filtration. The filtrate was evaporated to give a residue that was
subjected to SFC for the isolation and analysis (yield and a/b ratio) of
the product.
In conclusion, we have reported the first example of the
cleaner chemical synthesis of glycosides in supercritical fluid
(scCO2). Our results show that CO2 can provide significant
advantages in reusable heterogeneous solid acid (SO4/ZrO2)-
promoted glycosidation of common glycosyl donors with
various acceptors over a wide range of temperatures and
pressures. All of our glycosidation reactions show that there is
a clear preference for glycoside formation over their hydro-
Received: May 31, 2006
Published online: July 26, 2006
Keywords: chromatography · glycosidation · green chemistry ·
.
superacidic systems · supercritical fluids
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Scheme 1. Chemical structures of glycosyl donors, acceptors, and
products used in this study. MPM=4-methoxybenzyl
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Angew. Chem. Int. Ed. 2006, 45, 5652 –5655
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