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equiv. of acetic anhydride in 2 molar equiv. of [bmIm][dca]
ionic liquid at room temperature (25 °C). When the same
reaction was conducted at 50 °C the reaction was complete
within 5 min. The acetylation of b-methyl glucopyranoside
organic solvents illustrate the usefulness of dicyanamide based
ionic liquids compared to more traditional solvents. This is
especially evident when considering the solvent-related ‘green’
benefits of the re-usable ionic liquid. To confirm the latter
possibility, recovered [emIm][dca] was re-used in an acetyla-
tion reaction; a similar reaction time was required for complete
acetylation (data not shown).
proceeded just as quickly and efficiently as with -glucose and
D
without any observed effect on the glycosidic linkage.
N-Acetylneuraminic acid [3] (Fig. 2), a polyfunctional
19
ulosonic acid with five hydroxy groups of differing reactivity,
The observation that the reactions in Table 1 proceed just as
rapidly, in the absence of catalyst, as the catalysed reactions in
Table 2 indicates that the ionic liquid has a more crucial role
than simply as an inert solvent. In order to investigate whether
the dicyanamide compound may have a role as a reactant, or as
a base catalyst, a reaction (final entry in Table 2) was carried out
using only 0.5 molar equiv. of ionic liquid and no catalyst. This
reaction proceeded to full acetylation despite the dicyanamide
compound only representing 0.1 molar equiv. with respect to
hydroxy groups. This suggests that the ionic liquid is acting as
a regenerating catalyst. The mechanism of this catalysis is
currently under investigation, but it is most likely related to the
basicity of the dicyanamide anion. The absence of any reaction
in the case of the [bmIm][tfsa] ionic liquid (Table 1) further
supports this proposition.
the disaccharide sucrose [4], and the trisaccharide raffinose [5],
represent complex saccharides that have a variety of different
hydroxy groups. All of these hydroxy groups were acetylated
within 24 hours at room temperature. It is expected that an
increase in reaction temperature will speed up the reaction rate
for the more complex saccharides by increasing their solubility
in the ionic liquid.
In conclusion, we have developed a rapid, clean and mild
method for O-acetylation of alcohols and saccharides using a
dicyanamide based ionic liquid. The ionic liquid is not only an
excellent solvent for a range of hydroxylated compounds but
also an effective base catalyst for O-acetylation.
Notes and references
1
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2
Fig. 2 [3] Neu5Ac, [4] sucrose and [5] raffinose.
2
3
R. A. Sheldon, Chem. Commun., 2001, 2399.
D. R. MacFarlane, J. Golding, S. Forsyth, M. Forsyth and G. B. Deacon,
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G. Hoefle and W. Steglich, Synthesis, 1972, 619.
A. I. Vogel, VogelAs Textbook Of Practical Organic Chemistry, 5th edn.,
Wiley, New York, 1989, 644.
J. Gelas, Adv. Carbohydr. Chem. Biochem., 1981, 39, 71.
Table 2 details a variety of acetylation reactions of a- -
D
4
5
6
glucose in the presence of different added catalysts and
solvents. The traditional acetylation catalysts, pyridine and
sodium acetate, continue to achieve the anomeric selectivity
6
that they are known to produce, although the presence of some
7
8
alternate anomer suggests that the dicyanamide ionic liquid is
playing a competitive catalytic role in the reaction.
F. Dasgupta, P. P. Singh and H. C. Srivastava, Carbohydr. Res., 1980,
8
0, 346.
Reactions with triethylamine (Et
3
N) as catalyst were carried
9 J. A. Hyatt and G. W. Tindall, Heterocycles, 1993, 35, 227.
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2 R. Mueller and A. Oftring, in Anionic surfactants as catalysts for
complete acylation of polyols, BASF, Germany, 1994, 4.
out at room temperature in either [bmIm][dca] or the common
volatile organic solvents; acetone, acetonitrile or dimethylfor-
mamide (DMF). The long reaction times and lower yields in the
1
1
3 P. M. Bhaskar and D. Loganathan, Tetrahedron Lett., 1998, 39, 2215.
Table 2 Acetylation reactions of a-
D
-glucose using added catalyst
14 P. M. Bhaskar and D. Loganathan, Synlett, 1999, 129.
1
5 M. Curini, F. Epifano, M. C. Marcotullio, O. Rosati and M. Rossi, Synth.
Commun., 2000, 30, 1319.
Ac
2
O
Temp. Time
(h)
Yield %
(a/b)
Solvent (eq)a
1 [emIm][dca]
(eq) Catalyst (eq) (°C)
16 A. Sharma and S. Chattopadhyay, Biotechnol. Lett., 1993, 15, 1145; C.
C. Akoh, J. Am. Oil Chem. Soc., 1994, 71, 319; N. Junot, J. C. Meslin
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1
2
3
4
5
6
7
8
9
0
5
0.5 NaOAc 50
10 Pyridine 50
0.5
0.2
4
0.2
4
0.1
24
48
48
95 (11/89)
98 (95/5)
10 [bmIm][dca] 10
1 [bmIm][dca]
10 [bmIm][dca]
2 [bmIm][dca]
2 [bmIm][dca]
10 DMF
10 Acetone
10 Acetonitrile
0.5[emIm][dca]
5
5
5
5
5
5
5
5
0.5 Pyridine
0
rtb
0
rt
rt
83 (75/25)
95 (60/40)
80 (58/42)
97 (17/83)
88 (28/72)
75 (11/89)
69 (8/91)
5 Et
5 Et
5 Et
5 Et
5 Et
5 Et
3
3
3
3
3
3
N
N
N
N
N
N
rt
rt
18 S. K. Spear, G. A. Broker, M. A. Klingshirn, L. Moens, M. A. Godshall,
T. P. Johnson and R. D. Rogers, Abstr. Pap.-221st Am. Chem. Soc.,
2001, IEC-053.
1
No catalyst 50
0.2
92 (46/54)
a
Denotes molar equivalents. b Room temperature is approx. 25 °C.
19 M. von Itzstein and R. J. Thomson, Top. Curr. Chem., 1997, 186,
1
19.
CHEM. COMMUN., 2002, 714–715
715