Environmentally benign synthesis of 2,3-unsaturated glycopyranosides in task-specific ionic liquid

Article abstract of DOI:10.1007/s10562-011-0646-7

A green reaction condition has been developed for the synthesis of 2,3-unsaturated glycopyranosides by the Ferrier rearrangement of glycals using alcohols and thiols in 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIM]?OTf) in excellent yield. [BMIM]?OTf has been applied as a task specific ionic liquid organo-catalyst. Operational simplicity, environmentally benign reaction condition, use of task specific ionic liquid, short reaction time, high yields are the notable features of this methodology.

Full text of DOI:10.1007/s10562-011-0646-7

Catal Lett (2011) 141:925–930
DOI 10.1007/s10562-011-0646-7
Environmentally Benign Synthesis of 2,3-Unsaturated
Glycopyranosides in Task-Specific Ionic Liquid
•
Goutam Guchhait Anup Kumar Misra
Received: 9 May 2011 / Accepted: 1 June 2011 / Published online: 22 June 2011
Ó Springer Science+Business Media, LLC 2011
Abstract A green reaction condition has been developed
for the synthesis of 2,3-unsaturated glycopyranosides by the
Ferrier rearrangement of glycals using alcohols and thiols in
improvement of the Ferrier reaction using a variety of
metallic and non-metallic catalysts [7–17]. Besides these, a
number of heterogeneous catalysts have also been applied
for this transformation [18–22]. Ferrier rearrangement of
acetoxy glycals has also been carried out under microwave
irradiation [23–26]. Despite of their strong influence in the
preparation of 2,3-dideoxy glycopyranosides, many of the
above-mentioned catalysts suffer from several shortcom-
ings, such as lack of environmentally friendliness, use of
strong acidic catalyst, use of expensive catalysts, require-
ment for the preparation of catalysts, oxidizing conditions,
longer reaction times, use of organic solvents, unsatisfac-
tory yields, use of an excess of reagent or catalysts etc. As a
consequence, there is a constant need to develop an envi-
ronmentally benign mild reaction condition for the prepa-
ration of 2,3-unsaturated glycopyranosides using less
hazardous catalyst and reaction medium.
1-butyl-3-methylimidazolium
trifluoromethanesulfonate
([BMIM]ꢀOTf) in excellent yield. [BMIM]ꢀOTf has been
applied as a task specific ionic liquid organo-catalyst.
Operational simplicity, environmentally benign reaction
condition, use of task specific ionic liquid, short reaction
time, high yields are the notable features of this
methodology.
Keywords Ferrier rearrangement ꢀ Ionic liquid ꢀ
Environmentally benign ꢀ 2,3-unsaturated glycosides ꢀ
Glycal
1 Introduction
As a part of the current thrust in the development of
environmentally friendly catalytic processes, we set out to
explore the potential of 1-butyl-3-methylimidazolium tri-
fluoromethanesulfonate ([BMIM]ꢀOTf) as a task-specific
ionic liquid in the Ferrier rearrangement of acetoxy glycals
as catalyst and reaction medium. Room temperature ionic
liquids have attracted significant attention of synthetic
chemists for their capability to act as green reaction med-
ium as well as organo-catalyst [27–35]. In the recent past,
several organic transformations have been carried out in
the ionic liquids under green reaction conditions [36–43].
Earlier, Salunkhe et. al. [44] reported a comparative
investigation on the metal nitrate catalyzed Ferrier rear-
rangement of glycal derivatives in organic solvents and
ionic liquids and showed that the use of ionic liquid
improves yield of the products and stereoselectivity. We
report herein, Ferrier rearrangement of acetoxy glycals
with alcohols and thiols for the preparation of 2,3-
2,3-Unsaturated glycosides or pseudoglycals are useful
intermediates for the synthesis of pharmacologically active
molecules [1, 2]. They have been successfully used as
chiral building blocks in the multi-step synthesis of several
antibiotics [3, 4]. Conventionally, 2,3-unsaturated glyco-
sides are prepared by the allylic rearrangement of acyloxy
glycals (popularly known as Ferrier Rearrangement) cata-
lyzed by a Lewis acid [5, 6]. Because of their synthetic
utility several reports have appeared in the literature for the
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-011-0646-7) contains supplementary
material, which is available to authorized users.
G. Guchhait ꢀ A. K. Misra (&)
Molecular Medicine Division, Bose Institute, P-1/12, C. I. T.
Scheme VII M, Kolkata 700054, West Bengal, India
e-mail: akmisra69@gmail.com
123

926
G. Guchhait, A. K. Misra
RXH (1.5 equiv.), TMSOTf (cat.)
[BMIM].OTf , room temperature
30-90 min.
2,3-unsaturated glycosides (a and b were obtained, the ratio
of which were determined by comparing the integration
OAc
O
OAc
O
AcO
AcO
AcO
XR
1
values of the peaks in H NMR, 13C NMR (Table 1). The
R = alkyl, aryl, glycosyl
X = O; S
a-configuration of the major product was confirmed from
1
the positions of the anomeric protons in the H NMR and
13C NMR spectra. In the 1H NMR spectrum, the H-1 of the
a-glycoside appears in an up field position compared to its
b-isomer. The a-stereochemistry of the anomeric center of
the major products was further confirmed from the NOE
study. A strong NOE between H-1 and H-4 was observed in
the D-glucal derived and such interaction was absent in the
D-galactal derived product. [BMIM]ꢀOTf acts as a task
specific ionic liquid which plays dual role as an organo-
catalyst as well as reaction medium. Addition of a catalytic
amount of TMSOTf as activator to the reaction medium
drives the reaction towards the formation of products and
[BMIM]ꢀOTf acts as a source of triflate ion to maintain the
catalyst concentration in the reaction. In a control reaction,
treatment of compound 1 with 2-propanol in [BMIM]ꢀOTf
in the absence of catalytic TMSOTf did not produce any
product and the substrate remained unaffected even after
24 h at room temperature justifying the requirement of a
catalytic TMSOTf for the initiation of the reaction. Other
available room temperature ionic liquids ([BMIM]ꢀCl,
[BMIM]ꢀBF4 and [BMIM]ꢀPF6) were also tested for their
efficacy in the Ferrier reaction and [BMIM]ꢀOTf was found
best in terms of solubility of the substrates, yield of product
and reaction time (Table 2). Acting as an organo-catalyst,
[BMIM]ꢀOTf has strong influence on the reaction providing
the OTf- counter anion to facilitate the formation of the
products. The experimental condition is simple and isola-
tion of the products and recovery of the ionic liquid can be
achieved using EtOAc and H2O which are recommended
solvents [45] to carry out green reactions.
Scheme 1 Ferrier rearrangement of acetoxy glycals in ionic liquid
[BMIM]ꢀOTf at room temperature
unsaturated O- and S-glycosides using [BMIM]ꢀOTf as
organo-catalyst as well as solvent under an environmen-
tally benign reaction condition avoiding organic solvents
and hazardous chemicals (Scheme 1).
2 Results and Discussion
In a set of initial experiments, triacetoxy-D-glucal (1) was
allowed to react with 2-propanol (1.5 equiv. to 5.0 equiv.)
in 1-butyl-3-methylimidazolium trifluoromethanesulfonate
([BMIM]ꢀOTf; 0.5–1.0 mL/mmol of substrate) by addition
of a catalytic amount (*1.0 lL) of trimethylsilyl trif-
luormethanesulfonate (TMSOTf) at room temperature.
After some experimentation, it was observed that use of
2-propanol (1.5 equiv.) in [BMIM]ꢀOTf (0.5 mL/1 mmol
of substrate) in the presence of catalytic TMSOTf fur-
nished 82% yield of compound 3 (a/b: 4/1) at room tem-
perature in 40 min. Under similar reaction condition
various aliphatic alcohols and hydroxy sugar derivatives
reacted with triacetoxy-D-glucal (1) to furnish alkyl 2,3-
unsaturated glucopyranosides and disaccharides in excel-
lent yield (Table 1). A variety of functional groups such as
allyl, propargyl, isopropylidene, benzyl, isopropyl groups
remained unaffected under the reaction conditions and
formation of by-products was not observed. The reaction
condition has been successfully applied for the reaction of
aryl thiols with compound 1 to furnish 2,3-unsaturated
thioglucosides in good to excellent yield. It is worth
mentioning that formation of further rearranged product
was not observed using thiols, which were evidenced ear-
lier in this reaction. Competitive reaction for the formation
of 2-deoxy glycoside derivatives was not observed in all
cases. Similar to triacetoxy-D-glucal (1), a set of aliphatic
alcohols and monosaccharide acceptor were reacted with
triacetoxy-D-galactal (2) under similar reaction condition to
furnish 2,3-unsaturated galactopyranosides in satisfactory
yield. In the case of triacetyl-D-galactal (2) the reactions
took longer time and the yields were slightly lower in
comparison to its D-glucal counterpart. Reaction of triace-
tyl-D-galactal (2) with thiols resulted in the formation
of some 2-deoxy products together with the expected
2,3-unsaturated products, which can be explained from
the structure of the starting substrate (Table 1, entry 20).
In most of the cases, a mixture two anomers of
3 Conclusion
In conclusion, a green reaction condition has been devel-
oped for the Ferrier rearrangement of acetoxy glycal
derivatives using alcohols and thiols in a task-specific room
temperature ionic liquid ([BMIM]ꢀOTf) avoiding the use of
hazardous chemicals. The reaction is reasonably fast, clean,
environmentally benign, high yielding with high anomeric
selectivity. Therefore, this reaction condition provides a
better and practical alternative to the existing procedures
for this reaction.
4 Experimental
Typical experimental procedure for the Ferrier rear-
rangement of 3,4,6-tri-O-acetylated-D-glucal (1): To a
123

Environmentally Benign Synthesis of 2,3-Unsaturated Glycopyranosides
927
Table 1 Ferrier rearrangement
Sl.
No.
Glycal
Alcohol/thiol
Product
Time
(min)
Yielda
(%)
a/bb Ref.
of glycals in [BMIM]ꢀOTf
catalyzed by TMSOTf at room
temperature
OAc
1
2
3
OAc 2-Propanol
40
45
40
82
80
82
4:1
6:1
4:1
[5]
AcO
AcO
O
AcO
AcO
O
OCH(CH3)2
1
3
OAc
1
1
Allyl alcohol
1-Butanol
[46]
[47]
O
OCH2CH=CH2
4
OAc
AcO
O
OC4H9
OC8H17
OCH2Ph
O
5
OAc
O
4
5
1
1
1-Octanol
45
30
77
78
4:1
6:1
[47]
[48]
AcO
AcO
6
OAc
O
Benzyl alcohol
7
OAc
O
6
7
1
1
Propargyl
alcohol
30
40
74
72
10:1 [48]
AcO
8
OAc
OH
O
6:1
[5]
AcO
O
O
O
O
O
O
O
O
O
O
O
9
OAc
O
OH
8
9
1
1
40
40
74
70
9:1
8:1
[22]
AcO
AcO
O
BnO
BnO
BnO
O
OMe
O
BnO
BnO
BnO
OMe
10
OAc
O
OH
O
–
BnO
OMe
OBn
OBn
O
O
BnO
OMe
OBn
OBn
11
OAc
OBn
O
10
1
40
72
10:1
–
HO
AcO
O
BnO
BnO
O
OBn
O
OMe
BnO
BnO
OMe
12
OAc
11
12
1
1
30
30
76
74
1:0
[49]
HS
HS
AcO
O
S
13
14:1 [50]
OAc
O
AcO
S
14
123

928
G. Guchhait, A. K. Misra
Table 1 continued
Sl. No. Glycal
Alcohol/thiol
Product
Time (min) Yielda (%) a/bb Ref.
OAc
O
13
1
30
75
72
12:1 [22]
HS
AcO
S
15
OAc
O
HS
14
1
30
1:0
–
AcO
OCH3
S
OCH3
16
AcO
15
16
2-Propanol
1-Octanol
100
100
72
74
1:0
2:1
[22]
[22]
AcO
OAc
OAc
O
O
AcO
OCH(CH3)2
2
17
2
AcO
AcO
OAc
O
OC8H17
OCH2Ph
18
17
2
Benzyl alcohol
Allyl alcohol
100
76
1:0
[22]
OAc
O
19
AcO
AcO
18
19
2
2
90
74
72
16:1 [25]
OAc
O
OCH2CH=CH2
20
OH
O
100
1:0
9:1
[22]
OAc
O
O
O
O
O
O
O
O
O
O
O
a
21
OAc
O
Isolated yield
20
90
52c
–
b
AcO
2
HS
a/b-ratio was determined
from the integration values of
S
1
the peaks in H NMR spectrum
c
Together with 2-deoxy
thioglycoside (*25%)
22
Table 2 Comparison of the yields of the Ferrier rearrangement of tri-O-acetyl-D-glucal (1) in the presence of 2-propanol using different ionic
liquids (IL)
Sl. No.
Substrate
Product
Ionic liquid
Time (min)
Yield (%)
1
2
3
4
[BMIM]ꢀ[BF4]
[BMIM]ꢀ[PF6]
[BMIM]ꢀ[OTf]
[BMIM]ꢀ[Cl]
150
150
40
65
52
82
46
OAc
O
OAc
O
AcO
AcO
AcO
OCH(CH3)2
1
3
180
mixture of compound 1 (1.0 mmol) and aliphatic alcohol
(1.5 mmol) or sugar alcohol (1.2 mmol) or thiol
(1.2 mmol) in [BMIM]ꢀOTf (0.5 mL) was added TMSOTf
(1.0 lL) and the reaction mixture was allowed to stir at
room temperature for appropriate time mentioned in
Table 1. The reaction mixture was poured into water and
extracted with ethyl acetate. The EtOAc layer was suc-
cessively washed with satd. NaHCO3 and water, dried
(Na2SO4) and concentrated under reduced pressure to give
the crude product, which was passed through a short pad of
SiO2 using EtOAc-hexane (3:1) as eluant to furnish ano-
meric mixture of the product. The aqueous layer was
123

Environmentally Benign Synthesis of 2,3-Unsaturated Glycopyranosides
929
Acknowledgment G.G. thanks CSIR, New Delhi for providing a
Senior Research Fellowship. This project was funded by Bose Insti-
tute, Kolkata.
evaporated to dryness and the crude mass was passed
through a short pad of SiO2 using EtOAc as eluant to
furnish [BMIM]ꢀOTf, which was dried at 70–80 °C under
reduced pressure before its reuse. Spectral data of the
major isomers those are not reported earlier are as follows:
Methyl 6-O-(4,6-di-O-acetyl-2,3-dideoxy-a-D-erythro-
hex-2-enopyranosyl)-2,3,4-tri-O-benzyl-b-D-allopyranoside
(11): Yellow oil; Rf: 0.5 (hexane:EtOAc; 3:1); IR (neat):
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