Efficient acylation and sulfation of carbohydrates using sulfamic acid, a mild, eco-friendly catalyst under organic solvent-free conditions

Article abstract of DOI:10.1039/c1gc15122c

A fast and efficient acylation of carbohydrate derivatives and free sugars using a stoichiometric quantity of acylating agents in the presence of sulfamic acid, an environmentally benign catalyst, under organic solvent-free conditions is reported. Excellent yields in the selective acylation and sulfation of carbohydrate derivatives have also been achieved using sulfamic acid as the catalyst. The reaction is fast and the yields were excellent.

Full text of DOI:10.1039/c1gc15122c

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PAPER
Efficient acylation and sulfation of carbohydrates using sulfamic acid,
a mild, eco-friendly catalyst under organic solvent-free conditions
Abhishek Santra, Goutam Guchhait and Anup Kumar Misra*
Received 31st January 2011, Accepted 17th March 2011
DOI: 10.1039/c1gc15122c
A fast and efficient acylation of carbohydrate derivatives and free sugars using a stoichiometric
quantity of acylating agents in the presence of sulfamic acid, an environmentally benign catalyst,
under organic solvent-free conditions is reported. Excellent yields in the selective acylation and
sulfation of carbohydrate derivatives have also been achieved using sulfamic acid as the catalyst.
The reaction is fast and the yields were excellent.
acetylation of unprotected reducing sugars resulted in an iso-
Introduction
merized product mixtures of pyranose and furanose derivatives.
Acylation is one of the frequently used functional group trans-
Therefore, there is a strong need to find a mild, less toxic and
formations for the protection of alcohols, phenols, amines and
efficient catalyst for the acylation of carbohydrates minimizing
thiols.1 In carbohydrate chemistry, acetylated sugar derivatives
isomerization and loss of sensitive functional groups. During the
are used as inexpensive intermediates for the synthesis of com-
synthesis of complex oligosaccharides of biological importance,
plex oligosaccharides and many bioactive natural products.2 The
we needed to prepare a number of acylated carbohydrate
acetylation and benzoylation of hydroxyl groups in carbohydrate
derivatives containing acidic functionalities and experienced
derivatives are conventionally achieved on treatment with large
inconveniences in preparing a large quantity of acetylated
excess of acetic anhydride and benzoyl chloride respectively in
derivatives using heterogeneous catalysts. We sought to find
the presence of pyridine in spite of its well documented toxicity.3
a mild and efficient non-toxic catalyst for the acylation of
The poor nucleophilic property of the hydroxyl groups in
carbohydrate derivatives without affecting the functional groups
carbohydrates necessitates the activation of the acylating agents
present in it. In searching for an acidic catalyst, sulfamic acid
by addition of pyridine derivatives e.g. 4-dimethylaminopyridine
(SA) attracted our attention because of its unique features, e.g.
(DMAP), 4-pyrrolidino pyridine as co-catalyst in some cases to
speed up the reactions.4 A variety of Lewis acid catalysts,5 which
H3N+SO3- zwitterionic form, moderately acidic (pKa = 1.0), non-
volatile, non-corrosive, insoluble in common organic solvent,
include InCl3, In(OTf)3, ZnCl2, Fe2(SO4)3, FeCl3, iodine, scan-
stability, inexpensive and ready availability. Earlier, a number of
dium triflate, copper triflate, trifluoromethanesulfonate triflate,
reports appeared in the literature using sulfamic acid in several
indium triflate, bismuth triflate etc. and some basic catalysts6
e.g. imidazole, sodium acetate etc. have been introduced for
the acid catalyzed acetylations. A number of heterogeneous
catalysts7 and solid supported acids8 have also been applied to
important organic transformations.11 The catalytic features and
zwitterionic form of SA have encouraged us to investigate the
applications of SA as an acidic catalyst in the O-acylation of
carbohydrates. Selective acylation of carbohydrate derivatives is
catalyze acetylation using acetic anhydride. Besides the use of
often required for the preparation of mono- and disaccharide
different acidic activators, the acetylation reactions have been
carried out in ionic liquids,9 under microwave conditions10 etc.
In spite of their usefulness, many of these promoters have
intermediates in the multistep oligosaccharide synthesis. We
envisaged that SA could be an effective catalyst for the selective
acetylation of carbohydrate derivatives via the formation of
shortcomings to be used in the acylation of carbohydrate
orthoester derivatives under organic solvent-free conditions
derivatives, which include environmental toxicity, the use of
(Scheme 1 and 2).
expensive reagents, moisture intolerance, requirement of catalyst
In another aspect, the role of sulfated carbohydrate derivatives
preparation, incompatibility with the carbohydrate derivatives
in the glycobiology research are well documented.12 In general,
containing acid-sensitive functional groups, use of excess acetic
sulfated derivatives are prepared by the treatment of sulfur
anhydride and other acylating agents etc. In some instances,
trioxide–pyridine complex (SO3·Py) using pyridine as the reac-
tion medium.13 In order develop a greener reaction condition
for the preparation of sulfated carbohydrate derivatives, we
Bose Institute, Molecular Medicine Division, P-1/12, C. I. T. Scheme
sought to develop an acid catalyzed reaction using SA as
VII M, Kolkata, 700054, India. E-mail: akmisra69@gmail.com;
catalyst and room temperature ionic liquid (RTIL)14 as solvent
Fax: +91-33-2355 3886; Tel: +91-33-2569 3240
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The findings of the acylation of a variety of carbohydrate
derivatives and unprotected reducing sugars are presented in
Table 1. Products of all known compounds gave 1H NMR
and 13C NMR spectra that matched data reported in the cited
references.
In order to establish the usefulness of the present protocol in
comparison to the established reaction conditions, a compar-
ative study has also been carried out with the earlier reported
reaction conditions, which is presented in Table 2. Most of the
reported catalysts cause the formation of by products, required
prior preparation of catalyst, can not be used with carbohydrate
derivatives with acid sensitive functionalities etc. It was observed
that the present protocol is clean, non-hazardous, does not form
any by products, has good compatibility with acid sensitive
functionalities and is high yielding. Use of protic acids (e.g.
PhCOOH, TFA) as catalyst resulted in the formation of only
degraded products of the substrates.
Scheme 1 Acylation of free sugars and carbohydrate derivatives
catalyzed by sulfamic acid.
Scheme 2 Sulfamic acid catalyzed selective acetylation of carbohydrate
diol derivative via orthoester formation.
Since preparations of selectively acylated carbohydrate deriva-
tives are required for their use in the multistep oligosaccharide
synthesis, we have extended our attention to apply SA in
the preparation of selective acylated carbohydrate derivatives.
Formation of orthoester derivative and its acidic hydrolysis
is one of the most explored methodologies for the selective
acylation of axial hydroxy group between two vicinal diol of
carbohydrate derivatives. Conventionally, this two step pro-
cedure involves formation of an orthoester derivative of the
diol using triethylorthoacetate in the presence of a strong
acid e.g. TsOH, camphorsulfonic acid etc. in DMF, and acidic
hydrolysis of the orthoester to furnish aceylated derivative
of the axial hydroxy group. In order to develop a one-
step, organic solvent free, greener alternative, phenyl 4-O-
benzyl-1-thio-a-L-rhamnopyranoside (49) and triethylorthoac-
etate (2.0 equiv.) was allowed to react in the presence of
SA (0.3 equiv.) at room temperature. The reaction proceeded
smoothly towards the initial formation of orthoester derivative
in 45 min, and furnish phenyl 2-O-acetyl-4-O-benzyl-1-thio-
a-L-rhamnopyranoside (50) in excellent yield on treatment
with water in one-pot. Following similar reaction conditions,
a series of selectively acetylated carbohydrate derivatives were
prepared in excellent yield (Table 3). It is worth mentioning
that during the reaction, no trace of the formation of other
regio isomer or complete acetylation product was observed.
All reaction products were characterized by their spectral
analysis.
After successful acylation of carbohydrate derivatives, we
turned our attention towards the preparation of sulfated car-
bohydrate derivatives. Only a few methods are available in the
literature for the preparation of sulfated carbohydrate deriva-
tives. Conventionally, sulfation of carbohydrate derivatives are
carried out by the treatment of sulfur trioxide–pyridine complex
(SO3·Py) using pyridine as the reaction medium. Although in
some instances pyridine has been replaced by triethyl amine,
application of an acidic catalyst in the sulfation of carbohydrate
derivative was not reported earlier. In order to evaluate the
catalytic potential of SA for the sulfation of carbohydrate
derivatives, methyl 2,3,4-tri-O-benzyl-a-D-glucopyranoside (57)
was treated with sulfur trioxide–pyridine complex (SO3·Py)
(1.5 to 2.0 equiv.) in the presence of SA (0.3 equiv.) in an
ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate
avoiding toxic pyridine (Scheme 3). We report herein sulfamic
acid (SA) catalyzed efficient acylation as well as sulfation of
carbohydrate derivatives using environmentally benign reaction
conditions.
Scheme 3 Sulfamic acid catalyzed sulfation of carbohydrate derivative
using sulfur trioxide–pyridine complex in ionic liquid.
Results and discussion
In a set of initial experiments, methyl b-D-glucopyranoside (1)
was allowed to react with a varied quantity of acetic anhydride
(1.0 to 2.0 equiv. per hydroxy group) in the presence of SA (0.1 to
1.0 equiv.) without using any organic solvent at room tempera-
ture to 100 ◦C. It was observed that per-O-acetylated methyl a-D-
glucopyranoside (2) was achieved in excellent yield using acetic
anhydride (1.2 equiv. per hydroxy group) and SA (0.4 equiv.)
◦
at 60 C in 15 min. It has also been observed that keeping the
reaction mixture at temperature higher than 60 ◦C resulted in the
slow decomposition of the product formed. Presumably, because
of the acidic nature of the catalyst and formation of acetic acid
during the course of reaction, the product gets decomposed at
high temperature. In order to generalize the reaction conditions,
a number of carbohydrate derivatives containing acid-sensitive
functional groups (e.g. benzylidene, isopropylidene, TBDPS etc.)
were acetylated under similar reaction condition. The reaction
condition has been successfully applied for the preparation of
per-O-acetylated glycopyranose derivative of free sugars. No
trace of formation of per-O-acetylated glycofuranose derivatives
was detected. A slight excess of acetic anhydride was required
in the case of hydrated sugars. Per-O-acylation of reducing
sugars furnished a mixture of a- and b-acetylated derivatives, the
ratio of which was determined by NMR spectroscopy. Similar
to the acetylation, benzoylation of carbohydrate derivatives
have been carried out using benzoyl chloride (1.2 equiv. per
◦
hydroxy group) and SA (0.4 equiv.) at 60 C in excellent yield.
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Table 1 Acylation of carbohydrate derivatives and free sugars using sulfamic acid as catalyst at 60 ◦C
Sl. No.
1
Substrates
Products
Time (min)
15
Yield (%)
96
a/b
Ref.
15
—
2
3
15
5
95
92
—
16
17
—
—
4
5
15
10
92
95
18
19
—
6
7
10
15
90
95
—
—
18
20
8
9
15
10
92
90
—
—
6a
21
10
11
20
90
86
—
—
22
—
180
12
150
92
—
—
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Table 1 (Contd.)
Sl. No.
13
Substrates
Products
Time (min)
180
Yield (%)
90
a/b
Ref.
—
—
14
30
90
—
23
15
16
25
25
95
92
6/1
5/1
5i
5i
17
18
25
25
95
94
4/1
5/1
5i
5i
19
20
25
30
96
92
5/1
1/2
5i
24
21
22
30
40
90
90
1/1
—
10
7a
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Table 2 Comparison of acetylation using different reported catalysts
Substrate
Ac2O (equiv./OH)
Catalyst
Time (min)
Temp. (◦C)
Yield (%)
Ref.
5.0
4.0
2.2
2.2
1.2
2.0
2.0
1.2
Pyridine
Iodine
Cu(OTf)2
HClO4-SiO2
BF3·OEt2
PhCOOH
TFA
180
30
6 h
30
10
10
rt
rt
rt
rt
rt
rt
rt
98
3
5g
5d
7a
25
—
—
~10 with degradation
80
~10 with degradation
50
~10 with degradation
degradation
96
10
10
SA
60 ◦
C
This work
Table 3 Sulfamic acid catalyzed one-pot selective acetylation of carbohydrate derivatives via formation of orthoester derivative and its hydrolysis at
room temperaturea
Sl. No.
1
Substrates
Products
Yield (%)
82
Ref.
26
2
3
85
86
27
28
4
88
29
5
6
82
80
30
27
a In all cases, orthoester formed in 45 min and the orthoester was stirred with water for 10 min. Mp: 4-methoxyphenyl; SE: 2-trimethylsilylethyl.
([BMIM]·PF6) as solvent at 60–80 ◦C. Since SO3·Py is a solid
Conclusions
reagent, the minimum quantity of a room temperature ionic
In summary, sulfamic acid (SA) has been used as a mild, non-
liquid [BMIM]·PF6 has been used to make a homogeneous
hazardous, inexpensive bench-top catalyst for the complete and
selective acylation of carbohydrate derivatives. The reaction
conditions have been successfully applied for the acylation of
reducing sugars. For the first time, sulfation of carbohydrate
derivatives has been carried out using SA, an acidic catalyst
in ionic liquid. The reaction conditions are fast, operationally
simple, reasonably clean, high yielding and do not require excess
reagents. With these features, this method may be considered
reaction mixture, avoiding organic solvents. It was observed
that use of sulfur trioxide–pyridine complex (2.0 equiv.) in
[BMIM]·PF6 (1.0 mL g-1 of substrate) at 80 ◦C furnished
excellent yield of the sulfated product. Following similar reaction
conditions a number of carbohydrate derivatives were sulfated
in excellent yield (Table 4). To the best of our knowledge, this
is the first example of acid catalyzed sulfation of carbohydrate
derivatives.
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Table 4 Sulfamic acid catalyzed sulfation of carbohydrate derivatives using sulfur trioxide–pyridine complex at 80 ◦C
Sl. No.
1
Substrates
Products
Time (h)
2.0
Yield (%)
80
2
3
2.0
2.5
82
73
as a better alternative for the acylation and sulfation of
carbohydrates.
Typical experimental procedure for the sulfation of carbohydrate
derivatives
To a suspension of sugar derivative (1.0 mmol) and sul-
fur trioxide–pyridine complex (2.0 mmol) in [BMIM]·PF6
(1.0 mL g-1 of substrate) was added sulfamic acid (0.3 mmol)
and the reaction mixture was allowed to stir at 80 ◦C for the
appropriate time mentioned in Table 4. The reaction mixture
was diluted in methanol and treated with Dowex 50 W X8 (H+)
resin. The reaction mixture was filtered, concentrated and passed
through a short pad of silica to give pure sulfated product.
Experimental
Typical experimental procedure for the acetylation of
carbohydrate derivatives
To a suspension of free sugar or sugar derivative (1.0 mmol)
in acetic anhydride (1.2 equiv. per hydroxy group) was added
sulfamic acid (0.4 mmol) and the reaction mixture was allowed
◦
to stir at 60 C for the appropriate time mentioned in Table 1.
Acknowledgements
The solvents were removed under reduced pressure and the crude
product was passed through a short pad of silica to give pure
acetylated product.
A. S. and G. G. thank CSIR, New Delhi, for providing
Junior and Senior Research Fellowships. This work was partly
supported by the Bose Institute, Kolkata.
Typical experimental procedure for the benzoylation of
carbohydrate derivatives
Notes and references
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