Synthesis of alkyl glycosides using trialkyl borates

Article abstract of DOI:10.3987/COM-00-8899

Some trialkyl borates worked as highly reactive glycosyl acceptors of glycosyl acetates. Several allyl glycosides were obtained in good yields by the reaction of glycosyl acetates with triallyl borate using ytterbium(III) trifluoromethanesulfonate as the

Full text of DOI:10.3987/COM-00-8899

HETEROCYCLES, Vol. 53, No. 6, 2000, pp. 1263 - 1267, Received, 14th March, 2000
SYNTHESIS OF ALKYL GLYCOSIDES USING TRIALKYL
BORATES
Takashi Yamanoi,* Yoshihiro Iwai, and Toshiyuki Inazu*
The Noguchi Institute, 1-8-1, Kaga, Itabashi-ku, Tokyo 173-0003, Japan
Abstract-Some trialkyl borates worked as highly reactive glycosyl acceptors of
glycosyl acetates. Several allyl glycosides were obtained in good yields by the
reaction of glycosyl acetates with triallyl borate using ytterbium(III)
trifluoromethanesulfonate as the activator.
In order to synthesize glycosides and oligosaccharides related to natural products and their analogues for the
1
investigation of their biological functions, glycosidation is one of the most important synthetic methods.
Most of the known glycosidation methods are based on the activation of a leaving group at the anomeric
2
center of a glycosyl donor. In some cases, the alcohol derivatives such as ROSnBu and ROSiMe are
3
3
2
used to increase the reactivity of the glycosyl acceptors. 1-O-Acyl sugars are useful glycosyl donors
because they are stable and easy to prepare, but some of them are difficult to activate. It was reported that
2,3,4,6-tetra-O-benzyl-D-glucopyranosyl acetate (1) is not easily activated by the lanthanide
3
trifluoromethanesulfonates.
We have already reported that the glycosidation reactivity of 1 with alcohols was dramatically increased by
the addition of only a 3 mol % amount of boron trifluoride etherate (BF •OEt ) in the presence of
3
2
4
ytterbium(III) trifluoromethanesulfonate (Yb(OTf) ). This reactivity enhancement was caused by the
3
formation of a BF -ROH complex, which made us consider that compounds having a B-OR bond were
3
likely to be highly reactive acceptors. In this communication, we describe glycosidations of glycosyl
acetates using the trialkyl borates (B(OR) ) as new glycosyl acceptors.
3
5
We investigated the reaction of 1 with triallyl borate (B(OCH CH=CH ) ) as one of the trialkyl borates.
2 3
2
The reaction using Yb(OTf) in dichloromethane (CH Cl ) gave the expected corresponding allyl glucoside
2 2
3
in 86% yield, and this result showed that B(OCH CH=CH ) worked as the highly reactive glycosyl
2 3
2
6
acceptor of 1. Furthermore, the reaction between 1 and B(OCH CH=CH ) was examined in detail. The
2 3
2
reaction using tin(II) trifluoromethanesulfonate (Sn(OTf) ) as the trifluoromethanesulfonate salt in CH Cl
2 2
2
also predominantly gave the α-allyl glucoside in excellent yield. Triphenylmethyl perchlorate (TrtClO ) and
4
cyclopentadienylzirconium trifluoromethanesulfonate tetrahydrofuran complex (Cp Zr(OTf) •THF) were
2
2
also effective for this reaction. On the other hand, the reactions using zinc trifluoromethanesulfonate
(Zn(OTf) ) and halide salts such as BF •OEt and tin (IV) chloride (SnCl ) gave the glucoside in low
2
3
2
4
yields. CH Cl , acetonitrile (MeCN), tetrahydrofuran (THF), and benzene (PhH) were used as the solvent.
2 2
The reactions using these solvents in the presence of Yb (OTf) afforded the allyl glycosides in good yields.
3

The reaction using MeCN predominantly gave the β-anomer. This β-stereoselectivity could be explained by
7
the generation of the α-D-glucosylacetonitrilium ion as Fraser-Reid et al. reported. We used the reaction
conditions involving Yb(OTf) in CH Cl because Yb(OTf) is very stable and can be recycled. The
2 2
3
3
reaction using trimethyl borate (B(OMe) ) and triphenyl borate (B(OPh) ) with Yb(OTf) in CH Cl also
2 2
3
3
3
afforded the corresponding methyl and phenyl glucosides in 98 and 68% yields, respectively. These results
are summarized in Table 1.
BnO
BnO
BnO
BnO
Activator
Solvent
O
O
+
B(OR)
3
BnO
BnO
BnO
BnO
OR
OAc
R=
, Me, Ph
1
Table 1. The reaction of 1 with B(OR) using several activators and solvents
3
a)
α/β
Entry
Activator
Yb(OTf)
Solvent
R
Yield(%)
1
2
3
4
CH Cl
86
44
6/5
2/3
3
2
2
2
2
2
BF •OEt
CH Cl
2
3
2
none
TiCl
CH Cl
2
4
SnCl
CH Cl
21
70
N.d.
4/1
4
2
5
6
TrtClO
CH Cl
2
4
2
2
Cp Zr(OTf) •THF
CH Cl
70
1/1
2
2
Zn(OTf)
Mg(OTf)
2
trace
7
8
9
CH Cl
2
2
2
CH Cl
3/2
7/3
1/3
1/1
1/1
3/2
3/1
2
2
2
2
3
3
3
3
3
62
85
Sn(OTf)
Yb(OTf)
Yb(OTf)
Yb(OTf)
Yb(OTf)
Yb(OTf)
CH Cl
2
2
10
11
12
13
14
73
73
74
98
63
MeCN
THF
PhH
Me
Ph
CH Cl
2
2
CH Cl
2
2
a) Molar ratio; 1: Activator: B(OR) =1:1:1. N.d.=Not determined.
3
The effect of the molar ratio among 1, Yb(OTf) and B(OCH CH=CH ) was investigated. Various
2 3
3
2
amounts of Yb(OTf) were used in the reaction with equimolar amounts of 1 and B(OCH CH=CH ) in
2 3
3
2
CH Cl . The reaction using 0.5 molar equivalents of Yb(OTf) gave the allyl glucoside in as good yield as
2 2
3
the reaction using 1 molar equivalent of Yb(OTf) . Various amounts of B(OCH CH=CH ) were added to
2 3
3
2
the reaction using equimolar amounts of 1 and Yb(OTf) . The reaction using less than 0.6 molar
3
equivalents of B(OCH CH=CH ) reduced the yields of the allyl glucoside. This result showed that two of
2 3
2

Table 2. Effect of the molar ratio among 1, Yb(OTf) and B(OCH CH=CH )
3
2
2 3
Entry
mol% of Yb(OTf)
mol% of B(OAll)
Yield(%)
3
3
1
2
100
50
100
100
86
90
3
4
30
10
100
100
60
65
16
82
71
5
6
100
100
40
7
8
100
50
30
60
55
85
the three allyloxy groups of B(OCH CH=CH ) could have the potential to react. Based on these results,
2 3
2
we examined the reaction using 0.5 molar equivalents of Yb(OTf) and 0.6 molar equivalents of
3
B(OCH CH=CH ) toward 1, and found that this reaction condition also afforded the allyl glucoside in
2 3
2
good yield. These results are shown in Table 2.
Table 3. Synthesis of allyl glycosides from glycosyl acetates
Entry
1
α/β
Glycosyl Acetate
Yield(%)
a)
1
85
6/5
OBn
OBn
O
a)
75
3/2
α
2
3
4
BnO
2
OAc
OBn
OBn
BnO
O
BnO
a)
76
BnO
3
OAc
OAc
BnO
O
BnO
b)
BnO
60
2/3
4
5
NHAc
AcO
O
BnO
b)
5
6
83
5/1
BnO
OAc
OBn
OAc
O
BnO
BnO
BnO
b)
72
α
6
OAc
a) Molar ratio ; glycosyl acetate:Yb(OTf) :B(OCH CH=CH ) =1:0.5:0.6
3
2
2 3
b) Molar ratio ; glycosyl acetate:Yb(OTf) :B(OCH CH=CH ) =1:1:1
3
2
2 3

We applied this reaction to the synthesis of allyl glycosides from several 1-O-acetyl glycopyranoses. As the
glycosyl acetates, 2,3,4,6-tetra-O-benzyl-D-galactopyranosyl acetate (2), 2,3,4,6-tetra-O-benzyl-D-
mannopyranosyl acetate (3), 6-O-acetyl-2,3,4-tri-O-benzyl-D-glucopyranosyl acetate (5), 2-O-acetyl-3,4,6-
tri-O-benzyl-D-mannopyranosyl acetate (6), and 2-acetamido-3,4,6-tri-O-benzyl-2-deoxy-D-glucopyranosyl
acetate (4) were used. The reactions of these glycosyl acetates with B(OCH CH=CH ) using Yb(OTf) in
2 3
2
3
CH Cl at room temperature gave the corresponding allyl glycopyranosides in good yields, respectively.
2 2
These results are shown in Table 3.
Allyl glycosides are widely used in synthetic carbohydrate chemistry, and several methods to synthesize
them based on the reaction of glycosyl acetates with allyl alcohol in the presence of some activators have
8
already been reported. While these methods usually use a large excess of the allyl alcohol and activators,
and in some cases, the benzyl group at the C-3 position and the acetyl group at the C-2 position were
9,10
removed,
our method reported here did not require using a large excess of B(OCH CH=CH ) and
2 3
2
Yb(OTf) and did not give any unprotected compounds at all.
3
As mentioned above, we found that several trialkyl borates worked as highly reactive glycosyl acceptors of
the glycosyl acetates and developed a convenient synthetic method of allyl glycosides using
B(OCH CH=CH ) which is relatively stable in air and commercially available.
2 3
2
ACKNOWLEDGEMENT
We are deeply indebted to Professor Teruaki Mukaiyama, Science University of Tokyo, for his helpful
discussions.
REFERENCES AND NOTES
1.
2.
3.
4.
5.
S. Hanessian and A. G. Pernet, Adv. Carbohydr. Chem. Biochem., 1976, 33, 111.
K. Toshima and K. Tatsuta, Chem. Rev., 1993, 93, 1503; other references cited therein.
J. Inanaga, Y. Yokoyama, and T. Hanamoto, Tetrahedron Lett., 1993, 34, 2791.
T. Yamanoi, Y. Iwai, and T. Inazu, J. Carbohydr. Chem., 1998, 17, 819.
B(OCH CH=CH ) was prepared by the reaction of allyl alcohol with B(OH) in the presence of
2 3
2
3
CaH as mentioned in the following references: T. E. Cole, R. Quintanilla, and S. Rodewald,
2
Synth. React. Inorg. Met. -Org. Chem., 1990, 20, 55. B(OCH CH=CH ) is also commercially
2 3
2
available from Gelest, Inc. (Azuma Co., Ltd.) and Sigma-Aldrich Japan Co., Ltd.
The following paper also suggested an enhancement in the reactivity of the alcohol by the formation
of a boron-alkoxide: K. Toshima, H. Nagai, Y. Ushiki, and S. Matsumura, Synlett, 1998, 1007.
A. J. Ratcliffe and B. Fraser-Reid, J. Chem. Soc., Perkin Trans. I, 1990, 747.
The references for the syntheses of the allyl glycoside were cited in the following paper: H. B.
Mereyala and S. R. Gurrala, Carbohydr. Res., 1998, 307, 351.
6.
7.
8.
9.
T. Takano, F. Nakatsubo, and K. Murakami, Carbohydr. Res., 1990, 203, 341.
10. M.-Z. Liu, H.-N. Fan, Z.-W. Guo, and Y.-Z. Hui, Carbohydr. Res., 1996, 290, 233.
-3
11. A typical experimental procedure is as follows: A 0.2 M (1 M=1 mol•dm ) CH Cl solution of
2 2
B(OCH CH=CH ) (1 mL, 0.2 mmol) was added to a solution of compound (1) (0.2 mmol) and
2 3
2

Yb(OTf) (0.2 mmol) in CH Cl (4 mL) at rt. The resulting mixture was stirred overnight. The
2 2
3
reaction was then quenched by addition of sat. NaHCO solution (5 mL). The reaction mixture was
3
extracted with CHCl , and the organic layer was washed with water and sat. NaCl solution. After
3
the organic layer was dried over Na SO , the solvent was evaporated under reduced pressure. The
2
4
crude product was purified by a preparative silica gel TLC (ethyl acetate/hexane) to give the
corresponding alkyl glycosides.