A method for obtaining equilibrium tautomeric mixtures of reducing sugars via glycosylamines using nonaqueous media

Article abstract of DOI:10.1016/S0008-6215(02)00075-7

Equilibrium tautomeric mixtures of several mono- and disaccharides are obtained in anhydrous form, without the use of water, by reacting the commercially available reducing sugars with ammonia gas in dry methanol, followed by the concentration of the resultant solution to dryness. Mutarotation and hydrolysis of the initially formed glycosylamine in the resultant medium account for the transformation. Equilibrium anomeric mixtures enriched in the β-form of commercially available sugars such as α-D-glucose and α-lactose have not only vastly increased solubility, but are also synthetically valuable as these can be readily converted to the methyl/benzyl/trimethylsilyl ether and other derivatives for further transformations.

Full text of DOI:10.1016/S0008-6215(02)00075-7

Carbohydrate Research 337 (2002) 965–968
www.elsevier.com/locate/carres
Note
A method for obtaining equilibrium tautomeric mixtures of
reducing sugars via glycosylamines using nonaqueous media
Sikkander Sulthan Allavudeen, Balagurunathan Kuberan, Duraikkannu Loganathan*
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India
Received 11 September 2001; accepted 11 March 2002
Dedicated to Professor Girish K. Trivedi on his 62nd birthday
Abstract
Equilibrium tautomeric mixtures of several mono- and disaccharides are obtained in anhydrous form, without the use of water,
by reacting the commercially available reducing sugars with ammonia gas in dry methanol, followed by the concentration of the
resultant solution to dryness. Mutarotation and hydrolysis of the initially formed glycosylamine in the resultant medium account
for the transformation. Equilibrium anomeric mixtures enriched in the b-form of commercially available sugars such as
a- -glucose and a-lactose have not only vastly increased solubility, but are also synthetically valuable as these can be readily
D
converted to the methyl/benzyl/trimethylsilyl ether and other derivatives for further transformations. © 2002 Elsevier Science Ltd.
All rights reserved.
Keywords: Carbohydrates; Equilibrium composition; Mutarotation; Nonaqueous media; NMR spectroscopy
Mutarotation of reducing sugars is a unique struc-
tural attribute of carbohydrates. The kinetic, thermody-
namic and mechanistic aspects of mutarotation have
been widely studied.1–3 Catalysis of mutarotation in
aqueous medium is attributed to the bifunctional na-
ture of water molecules. In aqueous organic media,
catalysts such as 2-hydroxypyridine4 have been shown
to accelerate the mutarotation. Knowledge of the equi-
librium composition of the tautomeric mixture of sug-
ars formed by mutarotation in solution is fundamental
to carbohydrate chemistry and glycobiology, as their
physical, chemical, and biological properties depend on
the nature and proportions of various forms.
Commercially manufactured large-volume sugars,
such as glucose and lactose, exist in the a-pyranose
form as their ordinary crystalline forms. Their consider-
ably reduced solubility,5,6 as compared to the b-forms,
apparently poses difficulties in processing, and a mathe-
matical model for lactose dissolution has been devel-
oped.7 Anomerically pure b-forms are obtained by
crystallizing concentrated solutions of their a-forms at
high temperature (\93 °C) and are relatively very ex-
pensive. Hence, large-scale anhydrous preparations of
an equilibrium anomeric mixture of sugars enriched in
b-form would be potentially valuable in this regard. An
earlier method8 for preparing an anomeric mixture of
glucose rich in the b-form consists of spraying a hot
concentrated glucose syrup onto a moving bed of previ-
ously made product at high temperature (85–105 °C) in
a rotary drier. Besides the industrial importance, anhy-
drous equilibrium anomeric mixtures of sugars also
have synthetic utility as they could be readily converted
to the methyl/benzyl/trimethylsilyl ether and other
derivatives for further transformations. We report
herein on the serendipitous development of a mild and
efficient method for obtaining an equilibrium composi-
tion of reducing sugars using nonaqueous media.
As part of our work on the synthesis of novel glyco-
sylasparagine mimics as acceptors for glycosidases,9
glycosylamines were chosen as synthons. The reported
procedure10 for the preparation of b-
D
-glucopyran-
osylamine involves the reaction of a-
D-glucose with
* Corresponding author. Tel.: +91-44-2578264; fax: +91-
44-2350509.
E-mail address: loganath@iitm.ernet.in (D. Loganathan).
ammonia gas in dry methanol and subsequent storage
of the reaction mixture in the freezer for 6–8 weeks to
0008-6215/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0008-6215(02)00075-7

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S.S. Alla6udeen et al. / Carbohydrate Research 337 (2002) 965–968
cose using methanol as the medium. Interestingly, there
was good agreement between the compositions of the
-glucose peracetate obtained from 0.36 and 100 g
D
batches of dextrose, illustrating the efficiency in scaleup
of the present process.
The above transformation appears to be general as
demonstrated using various reducing mono- and disac-
charides. The tautomeric compositions obtained from
1
the specific rotation data and H NMR data are listed
in Tables 1 and 2, respectively. The observed anomeric
compositions of sugars, which are known to undergo
simple mutarotation, such as
and
D-glucose, D-mannose,
D
-xylose agree very well with the reported equi-
Scheme 1.
librium compositions in water.2,15 On the other hand,
the observed composition of furanose forms (Table 2)
for the sugars that are known to undergo complex
afford the product in crystalline form. Our efforts to
shorten the time for obtaining the amine by removal of
the solvent have led instead to the formation of an
mutarotation viz., D-arabinose and D-ribose differ from
the values of 2.5% a and 2% b and 6.5% a and 13.5%
b, respectively.16 Considering that the transformation is
brought about in methanol medium, the observed com-
positions are generally in accordance with earlier re-
ports on the effect of organic solvents on the
equilibrium proportions of sugar tautomers.17,18
The use of anhydrous ethanol as a medium for the
transformation of dextrose afforded an equilibrium
mixture with the composition of 38% and 62% a- and
b-glucopyranose, respectively. However, aprotic sol-
vents such as N,N-dimethylformamide and acetonitrile
failed to bring about the transformation, as glucose
remained mostly undissolved even after 30 min of bub-
bling with ammonia. In a control experiment, crys-
equilibrium anomeric mixture of D-glucose. In a typical
procedure11 (Scheme 1), dry ammonia gas was bubbled
through a stirred suspension of commercial dextrose
(1.08 g, 6 mmol, a:b pyranoses=94:6) in anhydrous
methanol (6 mL) containing 1% (w/v) of ammonium
chloride as a catalyst for 30 min at 0 °C, followed by
concentration of the resultant clear solution to dryness
under reduced pressure to afford a syrup. The identity
of the syrup as an equilibrium mixture of a- and
b-D-glucopyranoses was established based on optical
1
rotation data (Table 1) and H NMR spectral data, as
well as H NMR data of its peracetate,12–14 prepared
1
using acetic anhydride and pyridine at 0 °C. The com-
position of the peracetate turned out to be 41 and 59%
a and b anomers, respectively, while that of the product
obtained under identical acetylation conditions of com-
mercial dextrose, used in the present study, was deter-
mined to be 94% a and 6% b. Thus efforts to simplify
the procedure for the preparation of b-
osylamine from a- -glucose have instead led to the
formation of an equilibrium anomeric mixture of
Table 1
talline b-D-glucopyranosylamine prepared according to
the procedure of Isbell and Frush10 was dissolved in
anhydrous methanol (the concentration chosen being 1
mmol/mL, the same as the one employed in the test
experiment), and evaporated to dryness. The solid sub-
sequently obtained was fully acetylated under similar
conditions employed in the test experiment. The charac-
terization of the only product obtained above as 1-N-
D-glucopyran-
D
D-glu-
Composition of the tautomeric mixtures obtained based on specific rotation data
Sugar no.
Sugar
Observed composition
Pyranose
Reported equilibrium composition in
water2,19
Pyranose
a
b
a
b
1
2
3
4
5
D
D
D
-glucose
-galactose
-mannose
36
30
69
74
36
64
70
31
26
64
36.2
29.6
68.8
61
63.8
70.4
31.2
39
2-acetamido-2-deoxy-
-xylose
D-glucose
D
34.8
65.2

S.S. Alla6udeen et al. / Carbohydrate Research 337 (2002) 965–968
967
Table 2
1H NMR (400 MHz, CDCl3) spectral data and composition of peracetylated tautomeric mixtures
Sugar
no.
Peracetylated product
1H NMR chemical shift (ppm) a
Composition
H-1ap
H-1bp
H-1af
H-1bf ap
bp
af
bf
1
2
3
4
5
6
7
8
9
penta-O-acetyl-
tetra-O-acetyl-2-deoxy-
D
-glucose
-arabino-hexose
6.25 (3.4)
6.18
5.67 (8.3)
5.73 (8.7)
5.47 (2.9)
5.63 (8.3)
5.73 (6.8)
6.28
6.03 (4.9)
5.64 (8.3)
5.67 (8.3)
5.59 (8.3)
5.61
41
49
58
27
38
50
23
45
48
40
25
59
51
42
66
62
25
D
tetra-O-acetyl-
tetra-O-acetyl-
tetra-O-acetyl-
tetra-O-acetyl-
tetra-O-acetyl-
L
L
-rhamnose
-fucose
-xylose
-arabinose
-ribose
6.00 (1.5)
6.26 (2.4)
6.25 (1.5)
5.63 (6.8)
6.07 (3.3)
6.20 (3.9)
6.16 (3.9)
6.18 (3.4)
6.22
6.23 (4.0)
6.12
4
3
D
D
D
6.14
6.43 (3.6)
6.32
6.17
16
9
4.5
54.5 18
octa-O-acetyl-lactose
octa-O-acetyl-maltose
octa-O-acetyl-cellobiose
octa-O-acetyl-melibiose
55
52
60
10
11
6.17
6.00
58
14
3
a Coupling constants are given in parentheses.
1. Experimental
acetyl-2,3,4,6-tetra-O-acetyl-b-D-glucopyranosylamine,
based on H NMR comparison with that of an authen-
tic sample, rules out the hydrolysis of the initially
1
General methods.—The melting point was deter-
mined on a Toshniwal melting point apparatus and is
uncorrected. Optical rotation was measured on a
JASCO-DIP 200 digital polarimeter using a cell of
length 10 mm. NMR spectra were recorded on JEOL
formed b-D-glucopyranosylamine arising from exposure
to atmospheric moisture, if any, during the process.
Based on the above observations and the proposed
mechanism for the hydrolysis of glycopyranosyl-
amines,19 the present transformation could be rational-
ized as follows: Ammonolysis of the reducing sugar in
anhydrous methanol results in the formation of
glycopyranosylamine and water, the byproduct. The
resultant medium, which is essentially 1.8% (v/v) water
in methanol, becomes increasingly enriched in water as
methanol and ammonia are removed under reduced
pressure, facilitating the mutarotation and hydrolysis of
1
GSX-400 (operating at 400 for H and at 100.5 MHz
for 13C) spectrometer. The anomeric ratios of various
peracetylated sugars were obtained based on the inte-
gral intensity of the respective anomeric protons. All
sugars were obtained from Pfanstiehl Laboratories,
Waukegan, IL, USA.
General procedure for obtaining equilibrium tautomeric
mixture of reducing sugars.—Ammonia gas, dried over
a column of KOH, was bubbled through a stirred
suspension of sugar (6 mmol) in anhyd MeOH (6 mL)
containing 1% w/v of ammonium chloride (60 mg) for
30 min at 0 °C. The resultant clear solution was concen-
trated to dryness under reduced pressure at rt to afford
a solid/syrup in quantitative yield. This syrup/solid was
quickly dissolved in a known volume of water (20 mL),
and optical rotations were measured immediately.
Peracetylation of the tautomeric mixtures obtained:
general procedure.—A suspension of sugar (2 mmol) in
MeOH (2 mL) was subjected to the procedure de-
scribed above. The solid/syrup obtained was cooled to
0 °C, followed by the addition of a pre-cooled mixture
of Ac2O (3.5 mL for monosaccharides and 5 mL for
disaccharides) and pyridine (7 and 10 mL, respectively)
under stirring for 1 h. The resultant clear solution was
stirred at rt for 12 h. The reaction mixture was concen-
trated on a rotoevaporator at 70 °C to a minimum
volume. Addition of toluene (5 mL), followed by con-
the amine. Reaction of
D-glucose with liquid ammonia
in the presence of a dehydrating agent such as anhy-
drous calcium sulfate, followed by removal of ammo-
nia, has been reported to afford
a mixture of
b-glucopyranosylamine and diglucosylamine.20 It is sat-
isfying to note that the transformation is clean without
any formation of diglycosylamine.
In view of the fact that the tautomeric forms of many
sugars are yet to be isolated in pure form and fully
characterized,16 the present process, being mild, might
facilitate the rapid preparation of an equilibrium com-
position of various sugars in the anhydrous state that
would be amenable for direct structural investigation
by methods such as solid-state NMR spectroscopy.
Such mixtures would also lend themselves to ready
derivatization, enabling chromatographic separation of
the various tautomeric forms and their subsequent
structural analysis.

968
S.S. Alla6udeen et al. / Carbohydrate Research 337 (2002) 965–968
centration in vacuo, was repeated several times to re-
move the remaining pyridine. The residual toluene was
also co-evaporated using dichloromethane (5 mL) twice
in vacuo to afford a solid or syrup in quantitative yield.
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D-glucose
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Acknowledgements
NMR spectral facilities provided by the RSIC, IIT
Madras are gratefully acknowledged.