Synthesis of thioglycosides in room temperature ionic liquid

Article abstract of DOI:10.1080/07328303.2011.605195

An eco-friendly reaction for the preparation of thioglycosides has been developed using an ionic liquid as the solvent. Thioglycosides were obtained in excellent yields on treatment of per-O-acetylated sugar derivatives with thiols in the presence of boro

Full text of DOI:10.1080/07328303.2011.605195

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Synthesis of Thioglycosides in Room
Temperature Ionic Liquid
a
a
a
Abhishek Santra , Abhijit Sau & Anup Kumar Misra
a
Bose Institute, Division of Molecular Medicine, P-1/12, C.I.T.
Scheme VII-M, Kolkata, 700054, India
Published online: 21 Sep 2011.
To cite this article: Abhishek Santra , Abhijit Sau & Anup Kumar Misra (2011) Synthesis of
Thioglycosides in Room Temperature Ionic Liquid, Journal of Carbohydrate Chemistry, 30:2, 85-93,
DOI: 10.1080/07328303.2011.605195
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Journal of Carbohydrate Chemistry, 30:85–93, 2011
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 0732-8303 print / 1532-2327 online
DOI: 10.1080/07328303.2011.605195
Synthesis of Thioglycosides
in Room Temperature
Ionic Liquid
Abhishek Santra, Abhijit Sau, and Anup Kumar Misra
Bose Institute, Division of Molecular Medicine, P-1/12, C.I.T. Scheme VII-M,
Kolkata-700054, India
An eco-friendly reaction for the preparation of thioglycosides has been developed us-
ing an ionic liquid as the solvent. Thioglycosides were obtained in excellent yields on
treatment of per-O-acetylated sugar derivatives with thiols in the presence of boron tri-
◦
fluoride diethyl etherate in [Bmim][BF
4
] as solvent at 20 C. Supplemental materials are
available for this article. Go to the publisher’s online edition of Journal of Carbohydrate
Chemistry to view the free supplemental file.
Keywords Thioglycoside; Ionic liquid; Eco-friendly; Boron trifluoride diethyl etherate;
[
4
Bmim][BF ]
INTRODUCTION
Thioglycosides are widely used glycosyl donors in the synthesis of oligosaccha-
rides.[
1–4]
They have been used as intermediates for the preparation of glycosyl
fluorides,[ sulfoxides, and sulfone derivatives, which are also considered as
5]
useful glycosyl donors in glycoside synthesis.[
6–9]
Because of their close similar-
ity with their O-glycoside counterparts and stability against enzymatic cleav-
age, a number of reports appeared using thioglycosides in several biochemical
[
10–14]
and structural studies.
Because of the great stability of thioglycosides
under a wide range of reaction conditions for protecting group manipulations,
they can serve as effective glycosyl donors as well as glycosyl acceptors. This
orthogonal property of thioglycosides has been successfully exploited in the
[
15,16]
synthesis of complex oligosaccharides.
A number of reports have appeared
in the literature for the preparation of thioglycosides, essentially following a
general concept of reaction of acetylated sugar derivatives with thiols or thiol
Received March 12, 2011; accepted July 11, 2011.
Address correspondence to Anup Kumar Misra, Bose Institute, Division of Molecular
Medicine, P-1/12, C.I.T. Scheme VII-M, Kolkata-700054, India. E-mail: akmisra69@
gmail.com
8
5

A. Santra et al.
8
6
derivatives in the presence of a Lewis acid.[17–24] However, these methods suffer
from a number of drawbacks such as use of excess chlorinated organic solvents,
malodorous thiols, control of temperature, and anomerization of products. A
few odorless reaction protocols have also appeared in the literature for the
preparation of thioglycosides including either treatment of glycosyl thiouro-
[
25,26]
nium salts
with alkyl halides or reductive cleavage of disulfides followed
[27]
by treatment with glycosyl halides.
Although these methods are compara-
tively odorless, they require either pregeneration of S-glycosyl isothiouronium
salts from relatively unstable glycosyl halides or reductive cleavage of disul-
fides before reacting with glycosyl halides. Despite their usefulness, the above-
mentioned methods have a major shortcoming because of the use of a large
quantity of chlorinated organic solvents. In an endeavor to develop a clean, or-
ganic, solvent-free synthesis of thioglycosides, we sought to evaluate the use of
eco-friendly room temperature ionic liquids (RTIL) in the preparation of thio-
glycosides. In the recent past, ionic liquids have gained considerable attention
from synthetic organic chemists because of their potential to act as solvents
in several organic transformations under green reaction conditions.[
28,29]
Ionic
liquids have been used in various synthetic methodologies for the transfor-
[
30]
[31]
mation of carbohydrates such as acetylation,
benzylidenation,
orthoes-
[
32]
[33]
Fischer glycosylation,[34]
terification,
and activation of thioglycosides
Ferrier rearrangement of glycols,
and glycosyl trichloroacetimidate deriva-
in glycosylations. There is no report for the preparation of thioglyco-
[
35,36]
tives[
37,38]
side derivatives using ionic liquids. We report herein an efficient preparation
of thioglycoside derivatives using 1-butyl-3-methylimidazolium tetrafluorobo-
rate ([Bmim][BF4]), a frequently available rt ionic liquid, avoiding chlorinated
organic solvents (Sch. 1).
O
.
O
AcO
ArSH, BF OEt
AcO
3
2
SAr
.
[
BMIM] BF , r. t.
OAc
4
Scheme 1: Preparation of thioglycoside by treatment of sugar acetate with thiol in the
presence of BF ·OEt2 in ionic liquid.
3
RESULTS AND DISCUSSION
Initially, β-D-glucose penta-acetate (1) was allowed to react with a varied
quantity of thiophenol (1.0 to 2.0 equiv.) in the presence of borontrifluoride
diethyletherate (BF3·OEt2; 1.0 to 2.0 equiv.) in 1-butyl-3-methylimidazolium
tetrafluoroborate ([Bmim][BF4]) (0.5 mL to 2.0 mL per mmol of substrate) at
◦
0
C to rt. After a series of experiments, it was observed that the use of 1.2 equiv.
thiophenol in the presence of 1.2 equiv. of BF3·OEt2 in 1.0 mL [Bmim][BF4] per
mmol of substrate could furnish 91% yield of phenyl 2,3,4,6-tetra-O-acetyl-1-
◦
thio-β-D-glucopyranoside (9) in 40 min at 20 C. Following a similar reaction

Synthesis of Thioglycosides
8
7
condition a series of per-O-acetylated thioglycoside derivatives were prepared
from different acetylated sugar derivatives in excellent yields (80% to 92%),
which are presented in Table 1. The products were obtained by extraction with
diethyl ether and aqueous workup. Exclusive 1,2-trans thioglycoside deriva-
tives were obtained in most of the cases, except minor quantities (∼5% to 10%)
of 1,2-cis products in some cases. Presumably the 1,2-cis products were formed
due to the anomerization of the kinetically controlled 1,2-trans products un-
der acidic reaction conditions. A scale-up reaction using 10 mmol of compound
1
and p-thiocresol under similar reaction conditions furnished 84% yield of
compound 10. All known products gave acceptable spectral data matched with
the cited references. A number of imidazolium ionic liquids (e.g., [Bmim][PF6],
[Bmim][OTf]) have also been examined as reaction solvents. The best results
were obtained using [Bmim][BF4] in terms of solubility of the substrates and
yield of the products obtained (Table 2). Presumably, the structure of ionic
−
liquid [Bmim][BF4] influences the reaction and BF4 could be a better coun-
teranion to facilitate the thioglycosylation using BF3·OEt2.
In order to study the recycling property of the ionic liquid, after completion
of the reaction it was extracted with diethyl ether. The remaining ionic liquid
was successively washed with toluene and water. The solvents were removed
◦
under reduced pressure to give the ionic liquid, which was dried at 70 C under
reduced pressure. The recovered ionic liquid was reused for thioglycosylation
at least four times without significant loss of efficiency, providing a similar
yield of thioglycoside derivatives (Table 1, entry 1).
In summary, a novel, eco-friendly, reasonably fast, high-yielding method-
ology has been developed for the preparation of thioglycosides using
[
Bmim][BF4] ionic liquid. [Bmim][BF4] can be recycled and reused several
times without any loss of efficiency.
EXPERIMENTAL
General Methods
All reactions were monitored by thin layer chromatography over silica
gel–coated TLC plates. The spots on TLC were visualized by warming ceric
sulphate (2% Ce(SO4)2 in 2N H2SO4) sprayed plates in hot plate. Silica gel
1
13
2
30–400 mesh was used for column chromatography. H and C NMR were
recorded on Brucker Avance DRX 500 MHz using TMS as internal reference.
Chemical shift value is expressed in δ ppm.
Typical Experimental Condition
To a mixture of penta-O-acetyl-β-D-glucopyranose (1; 390 mg; 1.0 mmol)
and thiophenol (125 µL, 1.2 mmol) in [Bmim][BF4] (1.0 mL) was added

A. Santra et al.
8
8
Table 1: BF
3
·OEt
2 4
-catalyzed synthesis of thiglycosides in ionic liquid [Bmim][BF ] at
◦
2
0 C
Sl.
No.
Time
(min)
Yield
(%)
Substrate
Thiol
Product
Ref.
1
OAc
Thiophenol
OAc
O
40
91
[39]
AcO
AcO
O
AcO
AcO
a
9
9
8
0
0
7
OAc
SPh
OAc
OAc
b
c
1
9
2
p-Thiocresol
OAc
O
40
92
[23]
AcO
84d
88e
AcO
STol
OAc
0
1
3
4
5
2-Thionapthol
OAc
O
60
40
40
[40]
—
AcO
AcO
SNap
OAc
1
1
4-Methoxy
thiophenol
OAc
O
90
92
AcO
AcO
SMp
OAc
1
2
AcO OAc
Thiophenol
AcO OAc
[41]
O
O
AcO
SPh
AcO
OAc
OAc
3
OAc
1
2
6
7
8
p-Thiocresol
2-Thionapthol
Thiophenol
AcO OAc
40
60
40
93f
82g
87
[23]
[40]
[42]
O
AcO
STol
OAc
4
1
AcO OAc
O
AcO
SNap
OAc
1
5
OAc
OAc
O
AcO
AcO
AcO
AcO
O
AcO
AcO
SPh
OAc
1
6
3
9
p-Thiocresol
Thiophenol
p-Thiocresol
Thiophenol
OAc
O
40
30
30
45
85
90
88
86
[23]
[43]
[23]
[44]
AcO
AcO
AcO
STol
SPh
1
7
1
1
1
0
1
2
OAc
Me
AcO
O
Me
AcO
O
OAc
OAc
OAc
OAc
4
1
8
STol
Me
AcO
O
OAc
OAc
1
9
OAc
OAc
AcO
AcO
O
AcO
AcO
O
OAc
NPhth
SPh
NPhth
5
2
0
(
Continued on next page)

Synthesis of Thioglycosides
8
9
Table 1: BF
3
·OEt
2 4
-catalyzed synthesis of thiglycosides in ionic liquid [Bmim][BF ] at
◦
2
0 C (Continued)
Sl.
No.
Time Yield
(min) (%) Ref.
Substrate
Thiol
Product
AcO OAc
AcO
AcO
1
3
Thiophenol
OAc
45
90
[45]
O
OAc
OAc
O
O
AcO
O
O
AcO AcO
OAc
O
OAc
AcO AcO
SPh
OAc
6
2
1
1
1
4
5
4-Methoxy
thiophenol
AcO OAc
60
60
82
80
—
O
OAc
O
AcO
O
AcO AcO
SMp
OAc
2
OAc
O
2
OAc
O
Thiophenol
Thiophenol
[45]
AcO
AcO
OAc
AcO
AcO
AcO
O
O
OAc
O
AcO
OAc
OAc
AcO
O
OAc
AcO
SPh
SPh
7
OAc
2
3
OAc
O
1
6
OAc
O
60
85
[45]
AcO
AcO
OAc
O
AcO
AcO
OAc
O
O
AcO AcO
O
OAc
AcO
AcO
OAc
8
2
4
a
Yield in first recycled IL.
Yield in second recycled IL.
In third recycled IL.
b
c
d
e
Reaction carried out with 4.0 g (10.25 mmol) of compound 1.
Together with ∼8% α-product.
f
Together with ∼5% α-product.
Together with ∼10% α-product.
g
Ph, Phenyl; Tol, 4-methylphenyl; Nap, 2-Napthyl; Mp, 4-methoxyphenyl.
BF3·OEt2 (150 µL, 1.2 mmol) and the reaction mixture was allowed to stir
◦
at 20 C for 40 min. After completion, the reaction mixture was extracted with
diethyl ether. The organic layer was successively washed with satd. NaHCO3
and water, dried (Na2SO4), and concentrated under reduced pressure to give
crude product, which was purified by column chromatography using hexane-
EtOAc (5:1) as eluent to furnish pure phenyl 2,3,4,6-tetra-O-acetyl-1-thio-β-
D-glucopyranoside (9; 400 mg; 91%). To the remaining ionic liquid was added
Table 2: Comparison of the yields of the thioglycosylation of D-glucose
penta-O-acetate (1) using thiophenol and BF
3
·OEt
2
in different ionic liquids (IL)
Sl. No.
1
2
3
4
Substrate
Product
Ionic liquid Yield (%)
OAc
OAc
O
[Bmim][BF
[BMIM][PF
]
]
91
4
AcO
AcO
O
AcO
AcO
6
72
63
30
OAc
SPh
OAc
OAc
[BMIM] [OTf]
[BMIM] [Cl]
1
9

A. Santra et al.
9
0
water and it was washed with toluene. The water layer was concentrated under
◦
reduced pressure to give ionic liquid, which was dried at 70 C under reduced
pressure before its reuse.
Spectral Data of New Compounds
4
-Methoxyphenyl 2,3,4,6-tetra-O-acetyl-1-thio-β-D-glucopyranoside (12)
◦
25
1
White solid; m.p. 164–165 C; [α]D −22 (c 1.2, CHCl3); H NMR (CDCl3,
00 MHz): δ 7.44 (d, J = 9.2 Hz, 2 H, Ar-H), 6.84 (d, J = 9.2 Hz, 2 H, Ar-H), 5.19
5
(
t, J = 9.2 Hz each, 1 H, H-3), 4.99 (t, J = 9.8 Hz each, 1 H, H-2), 4.89 (t, J =
9
.2 Hz each, 1 H, H-4), 4.54 (d, J = 9.8 Hz, 1 H, H-1), 4.22–4.15 (m, 2 H, H-6),
.81 (s, 3 H, OCH3), 3.69–3.65 (m, 1 H, H-5), 2.10, 2.07, 2.01, 1.98 (4 s, 12 H, 4
3
1
3
COCH3); C NMR (CDCl3, 125 MHz): δ 170.6, 170.2, 169.4, 169.2 (4 COCH3),
1
60.4-114.4 (Ar-C), 85.6 (C-1), 75.7, 74.0, 69.8, 68.2, 62.0 (C-6), 55.3 (OCH3),
+
20.8, 20.7 (2 C), 20.6 (4 COCH3); ESI-MS: m/z = 493.1 [M+Na] ; Anal. Calcd.
for C21H26O10S (470.12): C, 53.61; H, 5.57%; found: C, 53.40; H, 5.83%.
4
-Methoxyphenyl (2,3,4,6-tetra-O-acetyl-β-D-galactopyranosyl)-(1→4)-2,3,6-
tri-O-acetyl-1-thio-β-D-glucopyranoside (22)
White solid; m.p. 167–168 C; [α]D25 −21 (c 1.2, CHCl3); H NMR (CDCl3,
◦
1
500 MHz): δ 7.42 (d, J = 9.2 Hz, 2 H, Ar-H), 6.84 (d, J = 9.2 Hz, 2 H, Ar-H),
5
7
.33 (br s, 1 H, H-4B), 5.18 (t, J = 9.2 Hz each, 1 H, H-3 ), 5.08 (dd, J = 7.9,
A
.9 Hz, 1 H, H-2 ), 4.94 (dd, J = 10.3, 3.4 Hz, 1 H, H-3B), 4.80 (t, J = 9.7 Hz
A
each, 1 H, H-2B), 4.55–4.52 (dd, J = 11.9, 1.7 Hz, 1 H, H-6 ), 4.50 (d, J =
aA
1
6
9
1
1
0.0 Hz, 1 H, H-1B), 4.44 (d, J = 7.9 Hz, 1 H, H-1 ), 4.13–4.0 (m, 3 H, H-
A
and H-6 ), 3.86–3.82 (m, 1 H, H-5 ), 3.80 (s, 3 H, OCH3), 3.70 (t, J =
abB
bA
A
.8 Hz each, 1 H, H-4 ), 3.59–3.56 (m, 1 H, H-5B), 2.14, 2.10, 2.03, 2.02, 2.00,
A
.95 (6 s, 21 H, 7 COCH3); 13C NMR (CDCl3, 125 MHz): δ 170.3, 170.2, 170.1,
ꢁ
ꢁ
70.0, 169.7, 169.5, 169.0 (7 COCH3), 160.3–114.3 (Ar-C), 100.0 (C-1 ), 85.4
(
(
[
C-1), 76.6, 76.0, 73.9, 70.9, 70.6, 70.2, 69.0, 66.6, 61.9 (C-6), 60.7 (C-6 ), 55.3
OCH3), 20.9, 20.8, 20.7, 20.6 (3 C), 20.5 (7 COCH3); ESI-MS: m/z = 781.2
+
M+Na] ; Anal. Calcd. for C33H42O18S (758.21): C, 52.24; H, 5.58%; found: C,
5
2.00; H, 5.85%.
ACKNOWLEDGEMENT
A. S. and A. S. thank CSIR, New Delhi, for providing Junior Research Fellow-
ships. This work was supported by Bose Institute, Kolkata.

Synthesis of Thioglycosides
9
1
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