A mild, efficient and α-selective glycosidation by using potassium dodecatungstocobaltate trihydrate as catalyst

Article abstract of DOI:10.1016/j.bmcl.2004.03.110

Treatment of tri-O-acetyl-D-glucal3,4,6-Tri-O-acetyl-1,5-anhyro-2-deoxy-D- arabinahex-1-enitol.¹ 1 with several alcohols in the presence of catalytic amount of POM (K₅CoW₁₂O₄₀· 3H₂O) as a heterogeneous, reusable, efficient and environmentally benign catalyst under neutral conditions and ambient temperature gave the corresponding 2,3-unsaturated glycopyranosides in excellent yields, with good anomeric selectivity. This catalyst proved to be not efficient for phenols.

Full text of DOI:10.1016/j.bmcl.2004.03.110

Bioorganic & Medicinal Chemistry Letters 14 (2004) 3611–3614
A mild, efficient and a-selective glycosidation by using
potassium dodecatungstocobaltate trihydrate as catalyst
Ezzat Rafiee, Shahram Tangestaninejad,* Mohammad H. Habibi
and Valiollah Mirkhani
Department of Chemistry, University of Isfahan, Isfahan 81746-73441, Iran
Received 8 February 2004; revised 13 March 2004; accepted 15 March 2004
Available online
Abstract—Treatment of tri-O-acetyl-D 1 with several alcohols in the presence of catalytic amount of POM
-glucalꢀ
(K5CoW12O40Æ3H2O) as a heterogeneous, reusable, efficient and environmentally benign catalyst under neutral conditions and
ambient temperature gave the corresponding 2,3-unsaturated glycopyranosides in excellent yields, with good anomeric selectivity.
This catalyst proved to be not efficient for phenols.
ꢀ 2004 Elsevier Ltd. All rights reserved.
1. Introduction
announced an oxidative alternative to this rearrange-
ment by using 3-O-pentenoyl glycols and iodonium
dicollidinium perchlorate under mild conditions.11
Recently, the ability of several Lewis acid catalysts;
LiClO4, LiBF4, BF3ÆEt2O, SnCl4, InCl3, TaCl5 and
LnCl3 was also examined for the Ferrier rearrangement
The stereoselective O-glycosidation of glycals is one
of the most important and straight-forward methods for
the preparation of glycosides, which are utilized for the
biologically active substances and the substrates for
synthesis of medicines such as antibiotics.1 Among gly-
cosides the 2,3-unsaturated glycosides2 has a unique
place and importance in carbohydrate chemistry since
this unsaturation can be further functionalized. Signifi-
cantly, as well as participation in simple addition reac-
tion across the glucal double bond, the presence of a
good leaving group at C-3 facilitate SN20 reactions
allowing for the introduction of a wide variety of nu-
cleophiles at C-1 of the sugar nucleus with concomitant
migration of the double bond.3 Lewis acid-catalyzed
allylic rearrangement of glycols is well known as the
Ferrier reaction4 and widely employed to obtain the 2,3-
unsaturated glycosides. Different reagents such as, bor-
on trifluride-ether,4a;b;5 tin(IV) chloride,6 clay catalyst
of 3,4,6-tri-O-acetyl
D-glucal with selected alcohols; the
best result was achieved using InCl3.12 Therefore, the
introduction of new effective and environmentally
friendly glycosidation method have attracted consider-
able attention in current synthetic organic chemistry
related to both biomolecules and functional materi-
als.13;4f
The successful applications of K5CoW12O40Æ3H2O as
electron transfer14 catalyst for a wide variety of organic
transformations15 prompted us to explore the potential
of this polyoxometale as catalyst for glycosidation
reaction.
9
montmorillonite K-10,7 DDQ,8 BiCl3 and N-iodosuc-
cinimide10 have been introduced for the Ferrier rear-
rangement under mild conditions. Fraser-Reid et al.
2. Results and discussion
Since, K5CoW12O40Æ3H2O, behaved just like an electron
transfer catalyst in the later cases,14;15 we pursued the
utilization of such POM’s properties in glycosidation
reaction. We first examined the glycosidation of 1 with
benzyl alcohol in several solvents (Table 1, entries 1–4).
Keywords: Glycosidation; Ferrier rearrangement; Glucal; Polyoxometa-
late.
* Corresponding author. Tel.: +98-311-7932705; fax: +98-311-66897-
32; e-mail: e.rafiee@sci.ui.ac.ir
3,4,6-Tri-O-acetyl-1,5-anhyro-2-deoxy-
ꢀ
D
-arabinahex-1-enitol.
Tri-O-acetyl-D-glucal (1.0 mmol) was treated with
0960-894X/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.03.110

3612
E. Rafiee et al. / Bioorg. Med. Chem. Lett. 14 (2004) 3611–3614
Table 1. Glycosidation of 1 with benzyl alcohol by K5CoW12O40Æ3H2O
ambient temperature to give the benzyl 2,3-unsaturated
glycopyranoside 10. Acetonitrile was shown to be
superior to the other solvents examined (Scheme 1).
Next, we optimized the quantity of catalyst used in this
reaction (Table 1, entries 4–6). The reaction was
extended to other alcohols (3–7) under such conditions
(Scheme 2).
Entry
Solvent
POM (mol %)
T (min) Yield (%)a
1
2
3
4
5
6
CH2Cl2
PhH
Et2O
10
10
10
10
20
5
10
30
60
10
10
30
60
10
20
95
96
80
CH3CN
CH3CN
CH3CN
a Isolated yield after purification by column chromatography.
As shown in Table 2, entries 1–6, these glycosidation
reactions performed to afford the corresponding 2,3-
unsaturated glycosides in excellent yields with the a-
anomer as the major products. The orientation of the
glycoside bond is a or b depends on some combination
benzyl alcohol (1.5 mmol) and K5CoW12O40Æ3H2O
(0.1 mmol) in CH3CN, CH2Cl2, benzene or Et2O at
OAc
OAc
AcO
O
K5CoW12O40 . 3H2O
R.T., Solvent
OH
O
AcO
AcO
+
O
1
2
10
C--Me
O
Ac=
major (α) 84%
minor (β) 16%
Scheme 1.
OAc
OAc
O
K5CoW12O40 . 3H2O (10%mol)
R.T., CH3CN
AcO
O
AcO
AcO
R-OH
+
OR
10-17
2- 9
1
major (α) (84-89) %
minor (β) (11-16) %
Scheme 2.
Table 2. Glycosidation of 1 with several alcohols and phenols by K5CoW12O40Æ3H2O
Entry
R–OH (R ¼ )
Product
Time
Yield (%)a
a:bb
OH
1
2
3
4
10
10 (min)
95
93
95
6:1
OH
2
3
11
12
48 (h)
1 (h)
9:1
7:1
OH
MeO
OH
4
5
13
12 (h)
90
9:1
NO2
5
6
CH3–
C2H5–
6
7
14
15
10 (min)
15 (min)
95
95
6:1
6:1
7
8
8
9
––
––
24 (h)
24 (h)
0
0
––
––
Cl
OH
OH
H3C
a Isolated yield after purification by column chromatography and characterized by 1H NMR.2a;8;12;18
b a:b Ratios were determined by 1H NMR (200 MHz) spectroscopy.

E. Rafiee et al. / Bioorg. Med. Chem. Lett. 14 (2004) 3611–3614
3613
methodology offers an efficient glycosidation of glucal
by a cheap, recyclable with very easy work-up, relatively
nontoxic and easily prepared catalyst, which performed
the corresponding 2,3-unsaturated glycosides under
neutral and catalytic conditions.
AcO
AcO
AcO
AcO
O
O
I
O
OAc
Ac
o
+
-
o
POM
POM
AcO
AcO
AcO
AcO
4. Experimental
O
O+
o
OR'
+
OR
4.1. Preparation of the catalyst
ROH
R'OH
II
The synthesis of potassium dodecatungstocobaltate tri-
hydrate (K5CoW12O40Æ3H2O) starts with the preparation
of sodium tungstodicobalt(II)ate from cobaltous acetate
(2.5 g, 0.01 mol) and sodium tungstate (19.8 g, 0.06 mol)
in acetic acid and water at pH 6.5–7.5. The sodium salt is
then converted to the potassium salt by treatment with
potassium chloride (13 g).
Scheme 3.
of control elements, one presumed control element is
commonly known as the ‘kinetic anomeric effect’. There
is a well-known thermodynamic effect (‘the anomeric
effect’) in tetrahydropyrans, which favours axial linkage
of electronegative functional groups to C-1. It is thus
assumed that this thermodynamic ‘anomeric effect’ ex-
erts its influence in the transition state for bond forming
to the electrophilic C-1, hence axial (or a) bond for-
mation. In fact, the axial preference for the incoming
nucleophile may be better viewed as the general stereo-
electronic effect that favours axial attack in any six-
membered ring.16
Finally the cobalt(II) complex is oxidized to the
cobalt(III) complex by potassium persulfate (10 g)
in 40 mL of 2 M H2SO4. The crystals of
K5CoW12O40Æ20H2O were dried at 200 ꢁC, after recrys-
tallization with methanol, potassium dodecatungstoco-
baltate trihydrate (K5CoW12O40Æ3H2O) was obtained.
4.2. Typical procedure for glycosidation of tri-O-acetyl-
glucal with K5CoW12O40ꢀ3H2O
D-
The experimental results show that secondary alcohol
(Table 2, entry 2) and benzylic alcohol with electron
withdrawing substitution (Table 2, entry 4) a long
reaction time was required to get a high yields. This
catalyst is not efficient for phenols with different sub-
stitutions (Table 2, entries 7 and 8).
To a solution of tri-O-acetyl-D-glucal 1 (1.0 mmol) and
the alcohol (1.5 mmol) in acetonitrile (5 mL) was added
K5CoW12O40Æ3H2O (0.1 mmol). The mixture was stirred
for the appropriate time according to Table 1 at ambient
temperature. The progress of the reaction was moni-
tored by TLC. The reaction mixture was filtered and the
filtrate was quenched with saturated NaHCO3 (10%,
25 mL) and then extracted with dichloromethane
(3 · 25 mL). The organic layer was dried over anhydrous
sodium sulfate, filtered and concentrated. The residue
was purified by plate silica gel chromatography to afford
the pure products. Products were characterized from
their NMR spectral data.2a;8;12;18
Mechanistically, since potassium dodecatungstocobal-
tate, K5CoW12O40Æ3H2O, is apparently a perfect outer
sphere one-electron oxidant due to the presence of a
sheet of chemically inert oxygen atoms, which protect
the central ion from undesired inner-sphere substitution
reactions,14 this glycosidation reaction probably pro-
ceeds via a one-electron transfer with the initial forma-
tion of the radical cation I8a;17 and the allylic oxonium
intermediate II6a as shown in Scheme 3. The possibility
of a concerted electron transfer mechanism was strongly
supported by a large decrease of the reaction rate upon
addition of a small amount of acrilonitrile or 2,6-di-tert-
butylphenol as a radical scavenger.
Acknowledgements
We thank the University of Isfahan Research Council
for support of this work.
In addition, we have found that K5CoW12O40Æ3H2O can
be reused several times without loss of activity by fil-
tering the catalyst, washing with acetone, drying and
immediately reusing. The yield of 10 promoted by
recovered catalyst for four times reminded 90%.
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3. Conclusion
To the best of our knowledge, this is the first report on
POM mediated Ferrier rearrangements. The present

3614
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