An improved method for the isolation of l-arabinose and d-galactose from gum arabic

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An Improved Method for the Isolation of
l-Arabinose and d-Galactose from Gum
Arabic
a
a
b
Y. S. Li , X. H. Li & C. X. Tan
a
College of Pharmaceutical Sciences, Zhejiang University of
Technology , Hangzhou , 310014 , P. R. China
b
College of Chemical Engineering and Materials Science, Zhejiang
University of Technology , Hangzhou , 310014 , P. R. China
To cite this article: Y. S. Li , X. H. Li & C. X. Tan (2013) An Improved Method for the Isolation of l-
Arabinose and d-Galactose from Gum Arabic, Organic Preparations and Procedures International: The
New Journal for Organic Synthesis, 45:4, 332-335, DOI: 10.1080/00304948.2013.798574
To link to this article: http://dx.doi.org/10.1080/00304948.2013.798574
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Organic Preparations and Procedures International, 45:332–335, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 0030-4948 print / 1945-5453 online
DOI: 10.1080/00304948.2013.798574
OPPI BRIEF
An Improved Method for the Isolation of
L-Arabinose and D-Galactose from Gum Arabic
1
1
2
Y. S. Li, X. H. Li, and C. X. Tan
1College of Pharmaceutical Sciences, Zhejiang University of Technology,
Hangzhou, 310014, P. R. China
2
College of Chemical Engineering and Materials Science, Zhejiang University
of Technology, Hangzhou, 310014, P. R. China
In recent years, l-arabinose (1) and d-galactose (3) have received much attention as im-
portant pharmaceutical intermediates.1 l-arabinose is a useful starting material for the
,2
3
,4
chemical synthesis of other l-pentoses and derivatives thereof. It has application in food
5
processing as a non-caloric sweetener and is widely used for the treatment of diabetes
6
and obesity because its special function of inhibiting intestinal sucrase activity, thereby
delaying sucrose digestion. d-galactose is also used as a sweetener in the food industry
7
8
and as an intermediate in many chemical syntheses.
There are several commonly used processes for the preparation of 1 and 3. One process
involved hydrolysis arabinose-rich vegetable fibers such as beet pulp and corn hulls with
mineral acid to obtain crude mixtures of 1 and 3, which must then be separated by ion
exchange resins9 or column chromatography.
1
3
–11
12,13
Another described the preparation of
14
by enzymatic hydrolysis and selective fermentation of arabinoxylan with yeast and of
by inoculation of a solution containing lactose with specific yeasts or bacteria. These
15
methods give low yields and are inefficient; furthermore, there are problems associated
with multi-step procedures and increasing production cost due to the waste of disposal.
Because of limitations such as large amounts of solvent needed, the use of ion exchange or
chromatography inherent in these procedures to industrial-scale up, there is a compelling
need to devise a new, practical and efficient preparation of 1 and 3 on a large scale, starting
from natural and easily accessible sources. We now report the preparation of 1 and 3 from
gum arabic by a procedure consisting of hydrolysis, filtration, evaporation, acetalization,
extraction, deacetalization and crystallization. These operations were easily carried out on
a large-scale; the solvents used could be recycled and inexpensive reagents were utilized in
the process. Thus, our procedure is less expensive low than the column method and it can
Received June 14, 2012.
Address correspondence to C. X. Tan, College of Chemical Engineering and Materials Science,
Zhejiang University of Technology, Hangzhou, 310014, P. R. China. E-mail: tanchengxia@zjut.edu.cn
332

Isolation of l-Arabinose and d-Galactose from Gum Arabic
333
be employed to replace the old methods. We report a process involving the above steps for
the preparation of 1 and 3 starting from gum Arabic using inexpensive reagents.
Gum arabic, a polysaccharide which consists mainly of 1 and 3 (42% and 31%
respectively), as well as other sugars (27%)16 is first hydrolyzed in aqueous mineral acid
to give 1 and 3 since they do not usually occur as free monosaccharides in nature. The
hydrolysis mixture is then converted to the corresponding acetone acetals (2 and 4). Selective
extraction with organic solvents of different polarity followed by evaporation and finally
deacetalization and crystallization afforded pure 1 and 3 .
OH
O
O
O
O
CH COCH
3
3
OH
OH
O
O
OH
2
1
OH
OH
OH
O
O
O
O
CH COCH
3
3
OH
OH
O
O
OH
3
4
Scheme 1
After several attempts, the best conditions for the hydrolysis of gum arabic were
◦
determined to be 1–2% aqueous sulfuric acid or hydrochloric acid at 95 C. The use of high
concentration of the acid led to difficulty in neutralizing the excess acids in the work-up.
On the other hand, if the concentration of the mineral acidis too low, the hydrolysis was
incomplete and required longer time. The syrup from the hydrolysis was neutralized by
aqueous calcium hydroxide, because calcium sulfate is insoluble and can be easily filtered
off. Traditionally, the syrup of the hydrolysis of gum arabic is processed by using ion
9
–11
12,13
exchange
or column chromatography.
A very important step of the new process is
the conversion of the hydrolysis syrup into acetone acetals using anhydrous acetone in the
concentrated sulfuric acid. Selective extraction of 4 is possible because its free hydroxyl
group makes it more polar than 2 and thus non-polar solvents preferentially extract 2 from
the mixtures.
In conclusion, a novel procedure to obtain 1 and 3 in moderate yields (27% and
18% respectively; based on the weight of gum arabic) has been developed. The present
method is simple and economical, adaptable to large-scale production from easily available
raw materials. It is noteworthy to mention that the products can be obtained by direct
recrystallization without tedious chromatographic separations and the organic solvents can
be recycled.

334
Tan et al.
Experimental Section
All the starting materials were purchased from commercial suppliers and used without
further purification. Melting points were determined on a Yanagimoto micromelting point
apparatus. IR spectra were obtained as KBr pellets on an AVATAR 370 FI-IR Thermon
1
Nicolet spectrometer. H NMR spectra were recorded on an AVANCE III (500 MHz) with
TMS as an internal standard. The mass spectra were measured on a Finnigan Trace DSQ
spectrometer.
Hydrolysis of Gum Arabic
A mixture of gum arabic (120.0 g), 98% sulfuric acid (6.0 g) and H2O (600 mL) was stirred
◦
at 100 C for 4 h. The progress of the reaction was monitored by TLC (96:4 acetone:H2O).
Upon cooling to room temperature, the reaction mixture was neutralized (pH 7) with
calcium hydroxide solid (about 4.5 g) and filtered to remove CaSO4. The filtrate was then
evaporated to give a syrupy hydrolysis product of gum arabic (99.5 g, 83%).
Preparation of the Acetals of L-Arabinose (2) and D-Galactose (4)
Methanol (120 mL), acetone (1.0 L) and 98% sulfuric acid (60.0 g) were added to a round
bottom flask containing the above syrupy hydrolysis product. The reaction mixture was
◦
then stirred vigorously at 15 C for 18 h. After completion of the reaction (monitored by
TLC, 1:2 ethyl acetate:petroleum ether), the mixture was neutralized (pH 7) with calcium
hydroxide solid (about 45 g) and filtered to remove CaSO4. The filtrate was evaporated
under reduced pressure and the residue was dissolved in methanol/water (1:3, v/v), extracted
with petroleum ether (400 mL × 4). The solvent was removed from the combined extracts
by distillation to give the acetal of l-arabinose (2) (57.6 g, 48%) as a white solid, mp.
◦
17
◦
1
3
3
3
5
9–40 C, lit. mp. 40–41 C. H NMR (500 MHz, CDCl3): δ 1.35 (s, 3H, CH3), 1.36 (s,
H, CH3), 1.50 (s, 3H, CH3), 1.55 (s, 3H, CH3), 3.68 (d, J5a, 5b = 13.0 Hz, 1H, Ha-5),
.85 (d, J5b, 5a = 13.0 Hz, 1H, Hb-5), 4.24 (d, J4, 3 = 8.0 Hz, 1H, H-4), 4.32 (dd, J2, 1 =
.0 Hz, J2,3 = 2.0 Hz, 1H, H-2), 4.58 (dd, J3,4 = 8.0 Hz, J3,2 = 2 Hz, 1H, H-3), 5.52
(
d, J1,2 = 5.0 Hz, 1H, H-1). Subsequently the aqueous phase from above was extracted
with chloroform (400 mL × 4) and the extracts combined. Removal of the chloroform
yielded the acetal of d-galactose (4) (48.5 g, 40%) as a pale yellow oil in agreement with
the literature data.1 H NMR (500 MHz, CDCl3): δ 1.33 (s, 6H, CH3), 1.45 (s, 3H, CH3),
7 1
1
.52 (s, 3H, CH3), 3.71–3.74 (m, 1H, Ha-6), 3.81–3.88 (m, 2H, Hb-6, H-5), 4.27 (dd, J4,3 =
8.0 Hz, J4,5 = 2.0 Hz, 1H, H-4), 4.33 (dd, J2,1 = 5.0 Hz, J2,3 = 2.5, J2 Hz, 1H, H-2), 4.60
(dd, J3,4 = 8.0 Hz, J3,2 = 2.5 Hz, 1H, H-3), 5.56 (d, J1,2 = 5.0 Hz, 1H, H-1).
Preparation of L-Arabinose (1) and D-Galactose (3)
The acetal of l-arabinose 2 (57.6 g), H2O (100 mL), and 98% sulfuric acid (1.0 g) were
◦
heated under vigorous stirring at 80–85 C for 1 h. The mixture was neutralized (pH 7) with
solid calcium hydroxide (about 0.7g), then solid calcium bicarbonate (about 0.1g) after
◦
the reaction was complete. After the mixture was cooled to 60 C, it was decolorized with
activated carbon and filtered. The filtrate was evaporated and then the solid residue was

Isolation of l-Arabinose and d-Galactose from Gum Arabic
335
recrystallized from ethanol to give 32.6 g of l-arabinose (1) (27%) as a white solid mp.
◦
18
◦
1
58–160 C, lit. mp. 157–158 C.
The acetal of d-galactose 4 (48.5 g) was hydrolyzed using the same procedure to afford
◦
18
◦
2
1.3 g of d-galactose (3) (18%) as a white solid, mp. 164–166 C, lit. mp. 164–165 C.
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