Catalytic and stereoselective glycosylation with disarmed glycosyl fluoride by using a combination of stannous(II) chloride (SnCl2) and silver tetrakis(pentafluorophenyl)borate [AgB(C6F5)4] as a catalyst

Article abstract of DOI:10.1246/cl.2001.388

A catalytic and stereoselective glycosylation of various glycosyl acceptors with disarmed glycosyl fluorides is successfully carried out by using a combination of stannous(II) chloride (SnCl₂) and silver tetrakis(pentafluorophenyl)borate [AgB(C₆F₅)₄] as a catalyst in the coexistence of MS5A in toluene and the corresponding 1,2-trans di-or trisaccharides were obtained in high yields.

Full text of DOI:10.1246/cl.2001.388

388
Chemistry Letters 2001
Catalytic and Stereoselective Glycosylation with Disarmed Glycosyl Fluoride
by Using a Combination of Stannous(II) Chloride (SnCl2) and
Silver Tetrakis(pentafluorophenyl)borate [AgB(C6F5)4] as a Catalyst
Teruaki Mukaiyama, Hisashi Maeshima, and Hideki Jona
Department of Applied Chemistry, Faculty of Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162-8601
(Received January 26, 2001; CL-010076)
A catalytic and stereoselective glycosylation of various
glycosyl acceptors with disarmed glycosyl fluorides is success-
fully carried out by using a combination of stannous(II) chlo-
ride (SnCl2) and silver tetrakis(pentafluorophenyl)borate
[AgB(C6F5)4] as a catalyst in the coexistence of MS5A in
toluene and the corresponding 1,2-trans di- or trisaccharides
were obtained in high yields.
To develop a new and useful method for stereoselective
glycosylation is one of the most fundamental research targets in
carbohydrate chemistry.1 In the synthesis of complex oligosac-
charides chains, glycosyl fluorides are one of the most frequent-
ly used donors as they are quite stable compared with the corre-
sponding other 1-halogenated sugars. Although many activat-
ing reagents for glycosyl fluorides had been studied2 after our
publication in 1981,3 only five examples were reported for acti-
vation of ‘disarmed’ glycosyl fluorides by using stoichiometric
amounts of activators such as SnCl2–AgClO4,4
5
7
Cp2ZrCl2–AgClO4, Cp2HfCl2–AgOTf,6 BF3·OEt2, or rare
earth metal salts.8 As for catalytic glycosylation, there is an
example which uses trimethylsilylated acceptors catalyzed by
30 mol% of La(ClO4)3·nH2O.9 Then, an efficient method for
the reaction of various free hydroxy groups containing glycosyl
acceptors with disarmed glycosyl fluorides was tried. Recently,
it was reported from our laboratory that several catalysts con-
taining soft and weakly-coordinating counter anions such as
B(C6F5)4– could effectively be employed in glycosylation10 and
aldol reactions.11 It was then considered that the facile genera-
tion of active Sn(II) cationic species would take place by the
combined use of SnCl2 and AgB(C6F5)412 to work as an effec-
tive catalyst for the activation of ‘disarmed’ glycosyl fluorides.
In this communication, we would like to report an improved
method for activation of ‘disarmed’ glycosyl fluorides by the
combined use of catalytic amounts of the respective SnCl2 and
AgB(C6F5)4, which afford the corresponding β-D-di- or tri-sac-
charides in excellent yields on treatment with several glycosyl
acceptors.
In the first place, 1.2 equivalents of SnCl2 and AgClO4
were used together with MS4A in Et2O by taking up the reac-
tion of 2,3,4,6-tetra-O-benzoyl-β-D-glucopyranosyl fluoride (1),
disarmed glycosyl fluoride, with methyl 2,3,4-tri-O-benzoyl-α-
D-glucopyranoside (2) according to our previous procedure3
(Table 1, Entry 4). As a result, it was observed that the desired
disaccharide was obtained in only 13% yield and a large
amount of starting material was recovered. Then, various addi-
tives and silver salts were examined to improve the yields of the
desired disaccharides (Table 1). Among additives, MS5A
proved to be most effective while almost no reaction took place
in the absence of additives (Entries from 1 to 5).13 It was noted
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
389
that AgB(C6F5)4,12 a useful reagent neither hydroscopic nor
explosive, generated an active catalyst more effectively than
any other silver salts on treatment with SnCl2: that is, a catalyst
generated by using the silver borate smoothly promoted the gly-
cosylation and the desired disaccharide was obtained in higher
yield (Entries from 5 to 8). The optimized reaction conditions
were demonstrated in Entry 9 (AgB(C6F5)4, MS5A = 3 g/mmol,
quant.). No reaction took place when SnCl2 or silver salt was
used alone (Entries 11 and 12).
gave the corresponding β-D-trisaccharide in good yield.
The typical experimental procedure is as follows: to a stirred
suspension of MS5A (150 mg), SnCl2 (1.9 mg, 10 µmol) and
AgB(C6F5)4 (7.9 mg, 10 µmol) in toluene (1.0 mL) was added 1
(35.9 mg, 60.0 µmol) and 2 (25.3 mg, 50.0 µmol) in toluene (1.0
mL) at 0 °C. After the reaction mixture was stirred for 2 h, it was
quenched by adding saturated aqueous NaHCO3. The mixture
was filtered through Celite and extracted with CH2Cl2 (× 3). The
combined organic layer was washed with H2O and brine, and
dried over Na2SO4. After being filtered and evaporated, the
resulting residue was purified by preparative TLC (silica gel) to
give the desired product 3 (54.2 mg, 99.9%).15
Thus, a catalytic and stereoselective glycosylation using ‘dis-
armed’ glycosyl donor, the fluoride, was efficiently performed by
using 20 mol% each of SnCl2 and AgB(C6F5)4 in the coexistence
of MS5A in toluene. Further study on this glycosylation using
various donors and acceptors including amino sugars, and the
mechanistic study on the catalytic cycle is now in progress.
Next, in order to achieve the catalytic process of this glyco-
sylation, both the amount of catalyst and the effect of solvents
were examined to find the best conditions (Table 2). As a
result, the combined use of 20 mol% each of SnCl2 and
AgB(C6F5)4 effectively activated disarmed glycosyl fluoride
and the corresponding disaccharide was obtained in 91% (Entry
3). When 10 mol% of SnCl2 and 20 mol% of AgB(C6F5)4 were
treated, ca. 20 mol% of AgCl precipitated, which suggested the
formation of Sn[B(C6F5)4]2 (Entry 6). On the other hand, only a
trace amount of disaccharide was obtained when glycosyl donor
1 and acceptor 2 were allowed to react in the above solution
while the desired disaccharide was smoothly afforded in 92%
yield by further addition of 10 mol% of SnCl2 to the solution
(Entry 7). These results consequently indicate that
ClSnB(C6F5)4 might be the important active species generated
from equimolar amounts of SnCl2 and AgB(C6F5)4. Therefore,
Sn[B(C6F5)4]2 which was possibly generated from
ClSnB(C6F5)4 together with SnCl2 by disproportionation would
not behave as an effective catalyst for this glycosylation.
Concerning the solvent, toluene gave the best result (Entry 8)
while no reaction took place in polar solvents such as THF and
MeCN (Entries 13 and 14), which is probably due to deactiva-
tion of the Lewis Acid by their strong coordination to stannous
cation
The present research is partially supported by Grant-in-
Aids for Scientific Research from Ministry of Education,
Culture, Sports, Science and Technology.
References and Notes
1
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Reviews on glycosyl fluoride; M. Shimizu, H. Togo, and M.
Yokoyama, Synthesis, 1998, 799; K. Toshima, Carbohydr. Res.,
327, 15 (2000).
2
3
4
T. Mukaiyama, Y. Murai, and S. Shoda, Chem. Lett., 1981, 431.
K. C. Nicolaou, T. J. Caulfield, H. Kataoka, and N. A. Stylianides,
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T. Matsumoto, H. Maeta, K. Suzuki, and G. Tsuchihashi,
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Finally, glycosylation using various donors and acceptors
were examined in order to extend the scope of the present
method (Table 3). As a result, glycosylation reaction smoothly
proceeded to afford the corresponding β-D-disaccharides in
excellent yields even in the cases of using acceptors having sec-
ondary alcohol, 6 and 7. In addition, glycosylation using disac-
charide14 having disarmed glycosyl fluoride as a donor also
8
9
S. Hosono, W. S. Kim, H. Sasai, and M. Shibasaki, J. Org. Chem.,
60, 4, (1995).
W. S. Kim, S. Hosono, H. Sasai, and M. Shibasaki, Tetrahedron
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10 K. Takeuchi and T. Mukaiyama, Chem. Lett., 1998, 555, and relat-
ed references are also sited.
11 T. Mukaiyama, M. Yanagisawa, D. Iida, and I. Hachiya, Chem.
Lett., 2000, 606; M. Yanagisawa, T. Shimamura, D. Iida, J.
Matsuo, and T. Mukaiyama, Chem. Pharm. Bull., 48, 1838 (2000).
12 Silver tetrakis(pentafluorophenyl)borate [AgB(C6F5)4] was pre-
pared by adding an ethereal solution of LiB(C6F5)4 (purchased
from Tokyo Chemical Industry Co., Ltd.) to an aqueous solution
of AgNO3. After azeotropic removal of water and diethyl ether
with toluene, AgB(C6F5)4 was obtained as a brown solid contain-
ing 1–3 mol of toluene. Y. Hayashi, J. J. Rohde, and E. J. Corey,
J. Am. Chem. Soc., 118, 5502 (1996).
13 There are two reports concerning the use of MS5A as the most
effective additive in the glycosylation with glycosyl fluoride; K.
Toshima, K. Kasumi, and S. Matsumura, Synlett, 1998, 643; H.
Jona, K. Takeuchi, and T. Mukaiyama, Chem. Lett., 2000, 1278.
14 T. Mukaiyama, K. Takeuchi, H. Jona, H. Maeshima, and T.
Saitoh, Helv. Chim. Acta, 83, 1901 (2000).
15 The disaccharide was also obtained in nearly quantitative yield
when the glycosylation was carried out even in a scale 5 times as
large as the above procedure.