Rapid, one pot preparation of D-mannose and D-mannitol from starch: The effect of microwave irradiation and MoVI catalyst

Article abstract of DOI:10.1016/j.tetasy.2011.06.006

The effect of microwave irradiation upon starch hydrolysis and simultaneous epimerization of the D-glucose to D-mannose obtained was investigated. An acidic aqueous solution of starch was treated with a catalytic amount of hexavalent molybdenum salt in microwave field and the composition of the reaction mixture was analyzed. Rapid starch hydrolysis and subsequent epimerization provided an equilibrium reaction mixture of D-glucose and D-mannose (2:1) without the formation of any undesirable by-products. The reduction of D-mannose with sodium borohydride yielded D-mannitol in very good yield. Microwave irradiation proved to be an efficient tool for the transformation of starch to mannose over an Mo^VI catalyst. This method has the advantages of environmental friendliness, easy operation, good yields and substantial reduction of reaction time.

Full text of DOI:10.1016/j.tetasy.2011.06.006

Tetrahedron: Asymmetry 22 (2011) 1184–1188
Contents lists available at ScienceDirect
Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Rapid, one pot preparation of D-mannose and D-mannitol from starch:
the effect of microwave irradiation and MoVI catalyst
⇑
Zuzana Hricovíniová
Institute of Chemistry, Slovak Academy of Sciences, SK-845 38 Bratislava, Slovakia
a r t i c l e i n f o
a b s t r a c t
Article history:
The effect of microwave irradiation upon starch hydrolysis and simultaneous epimerization of the
Received 16 May 2011
Accepted 6 June 2011
Available online 19 July 2011
D
-glucose to D-mannose obtained was investigated. An acidic aqueous solution of starch was treated with
a catalytic amount of hexavalent molybdenum salt in microwave field and the composition of the
reaction mixture was analyzed. Rapid starch hydrolysis and subsequent epimerization provided an
equilibrium reaction mixture of
by-products. The reduction of
D
-glucose and
D
-mannose (2:1) without the formation of any undesirable
D-mannose with sodium borohydride yielded
D
-mannitol in very good
yield. Microwave irradiation proved to be an efficient tool for the transformation of starch to mannose
over an MoVI catalyst. This method has the advantages of environmental friendliness, easy operation,
good yields and substantial reduction of reaction time.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
discussed.8–10 It is known that the addition of salts to solvents
can increase their conductivity and the solution of substance
The use of renewable resources and the development of envi-
ronmentally friendly processes for the preparation of various
chemical compounds is very important today.1 Carbohydrates are
the most abundant natural organic compounds, mostly present in
polymeric forms, and are amongst the most intensively studied
precursors with a wide range of properties and applications.2
Starch is the second most abundant biopolymer in Nature after
cellulose. It has higher viability than cellulose because its depoly-
merization process is easier. Starch, a polysaccharide consisting
may be heated up much more rapidly than is done by other con-
vective–conductive heating sources such as an oil bath. The pres-
ence of ions can frequently enhance the effects of dielectric loss
and microwave coupling and could produce superheating.11–13
The interaction of metal ions with carbohydrates has been exten-
sively studied for a long time.14,15 Carbohydrate chemistry is cur-
rently dealing with the development of efficient methods for the
conversion of carbohydrates into more valuable and enantiomeri-
cally pure products. One of the significant achievements in this
field is the epimerization reaction of aldoses catalyzed by molyb-
date ions16–18 known as the Bílik reaction.19,20 Molybdate ions
were investigated as catalyst for the isomerization of various
reducing sugars and it has been shown that in a microwave field,
the transformation proceeds very efficiently. This approach was
thus found to be an effective way for the isomerization of a
carbohydrate carbon skeleton with high stereospecificity. This
procedure led to the efficient preparation of aldoses,21 2-C-
(hydroxymethyl)-branched aldoses, ketoses,22 (1?6)-linked disac-
charides23 but it is also effective in the case of aldoses bearing
nitrogen in the branch.24
Recent research has shown that starch strongly interacts with
microwave irradiation and has different properties compared to
conventionally heated reactions. Several papers have reported
the hydrolysis of starch in microwave field.13,25–28 Since one of
the important features of current chemistry methods is the effec-
tive preparation of compounds from naturally-occurring sources,
we have undertaken to find even more efficient method to convert
starch not only to glucose but directly to other important com-
pounds. We examined the properties of molybdate salts to act as
of D-glucose linked with a-glycosidic linkages, could serve as a
sustainable primary resource of carbohydrate molecules for the
chemical industry.3 However, only few purposes have been found
so far for raw, native starch. This natural material is often modified
by various chemical or enzymatic methods to obtain useful
products used in the food and pharmaceutical industries.4–6
Considerable effort has been expended to achieve efficient poly-
saccharide depolymerization and utilization of the monosaccha-
rides obtained as a source for the synthesis of many carbon
feedstocks. Nowadays, the synthesis of organic compounds using
microwaves attracts research due to short reaction times, high
yields and improved selectivity.7 Microwave reactors produce effi-
cient internal heating by the direct coupling of microwave energy
with the molecules (solvents, reagents or catalysts) present in the
reaction mixtures. The different applications of microwave irradia-
tion for fast and reproducible syntheses have been thoroughly
⇑
Tel.: +421 2 59410282; fax: +421 2 59410222.
E-mail addresses: chemhric@savba.sk, Zuzana.Hricoviniova@savba.sk
0957-4166/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2011.06.006

Z. Hricovíniová / Tetrahedron: Asymmetry 22 (2011) 1184–1188
1185
convenient ions to increase the efficiency of heating under condi-
Amylose
tions of microwave irradiation together with their catalytic proper-
ties to improve the process of starch hydrolysis and simultaneous
OH
4
OH
O
OH
O
epimerization of obtained
mannose.
D-glucose to commercially interesting D-
1
4
HO
O
O
OH
HO
HO
O
1
HO
OH
OH
O
OH
Glc
2. Results and discussion
OH
m
MoVI H3O+
MW 3 min
To develop an efficient, rapid and environmentally friendly pro-
cedure for the production of commercially interesting -mannose
HO
HO
OH
MoVI H3O+
O
D
1
from native starch, we explored the use of microwave irradiation
in combination with a hexavalent molybdenum catalyst. Molyb-
date ions act as a powerful catalyst for the isomerization of reduc-
ing sugars and have a number of unique features.29–31 To access the
potential of MoVI ions for the starch transformation a series of pre-
liminary experiments were carried out by irradiating mixtures of
starch in aqueous solutions of hydrochloric acid containing molyb-
date salts in a range of concentrations in a microwave field. A sur-
vey of reaction conditions was undertaken for this transformation.
Based on a broad variable set in terms of acid concentration,
amount of catalyst, reaction time, and microwave power, the influ-
ence of the different variables could be evaluated to identify corre-
lations between the reaction conditions and starch conversion as
well as product distribution.
OH
4
OH
O
6
O
O
OH
O
1
HO
4
O
OH
HO
O
1
HO
HO
OH
O
OH
Man
Amylopectin
n
Starch
Scheme 1. The reaction scheme of starch hydrolysis and simultaneous epimeriza-
tion of the obtained -glucose to -mannose in a microwave field.
D
D
In the first experiment, five aqueous solutions of hydrochloric
acid (0.1; 0.25; 0.5; 0.75 and 1.0 M) were tested for the hydrolysis
of starch samples in a microwave field. The depolymerization of
The effects of starch sources on the yields of D-mannose were
compared using three different starches in subsequent experi-
ments: potato starch, corn starch and rice starch. A solution of each
starch in 0.25 M hydrochloric acid with a catalytic amount of so-
dium molybdate was exposed to microwave irradiation under pre-
viously optimized conditions. The ratio of the sugars present in the
1,4-
smoothly in 0.25 M hydrochloric acid in a microwave field. The
cleavage of -glycosidic linkages was achieved via the addition of
a- and 1,6-a-glycosidic linkages of starch to D-glucose worked
a
water catalyzed by acid. It was observed that starch hydrolysis is
predominantly determined by acid concentration independent of
the starch source used. As expected, faster conversion of starch
equilibrium reaction mixture with respect to the production of D-
mannose was determined by 1H NMR spectroscopy. Fractioniza-
tion of the reaction mixture by column chromatography afforded
to D-glucose can be reached in the reaction performed using higher
D-glucose (62–65%) and D-mannose (30–33%). The efficiency of
concentrations of acid. Our investigations focused on the develop-
ment of a catalytic system that would improve starch hydrolysis
with subsequent epimerization of D-glucose to D-mannose. Hence
the method was also demonstrated in the semi-preparative scale.
The transformation occurs in 3 min reaching comparable final con-
centrations as listed in Table 1. Regarding the influence of the MoVI
catalyst in acidic aqueous solution in combination with microwave
power, minor differences of higher overall conversion to glucose/
mannose among three different starches tested seem insignificant.
Starch hydrolysis is predominantly determined by acid concentra-
in the following experiments, the effects of molybdic acid salts
on hydrolysis were examined. Comprehensive investigations on
the combined hydrolysis and isomerization reactions in one step
were carried out using catalytic amounts of sodium molybdate
and dilute hydrochloric acid. It was found that a 0.5% solution of
sodium molybdate was a sufficiently high concentration of this
catalyst and led to very good conversion to products. It should be
noted that increasing the amount of catalyst did not have a signif-
icant effect on the product composition. The effect of microwave
irradiation on conversion, selectivity and product distribution
was examined using a multimode microwave reactor consisting
of a continuous focused microwave power delivery system with
operator-selectable power. It was observed that the microwave
power significantly influences the course of the reaction. The best
results were obtained by performing the reaction at 200 W of
microwave power for 3–5 min.
tion but the simultaneous mutual interconversion of
-mannose is governed by a highly stereospecific isomerization
reaction that is catalyzed only by molybdate ions.
D-glucose and
D
The microwave heated samples of starch were compared with
samples prepared by conduction heating (90 °C, oil bath). Starch
hydrolysis using the same reaction conditions (0.5% Na2MoO4Á
2H2O in 0.25 M HCl) proceeded very slowly. Under conventional
conditions the equilibrium reaction mixtures were obtained after
35–40 h of heating. The analysis of the 1H NMR spectra indicated
that the starch was completely hydrolyzed and converted to the
equilibrium mixture (3:1) of glucose and mannose. The formation
of
in about 35–40 h. The final equilibrium mixture was comparable to
that found in previous studies of -Glc/
-Man epimerization.16,17
D-mannose started in about 6 h and the maximum was reached
In the optimized reaction conditions, a 5% solution of starch in
0.25 M hydrochloric acid with a catalytic amount of sodium
molybdate was exposed to microwave irradiation in a sealed vessel
for different time periods. Homogenous blue solutions were ob-
tained, indicating the completion of the complex formation. Se-
lected reaction conditions led to complete depolymerization of
D
D
Different reaction kinetics and thermodynamics under conven-
tional conditions, compared to microwave conditions, were seen
in the large differences in both time-course and the composition
of reaction mixture during reaction. After 10 h of heating, the ratio
of Glc/Man was 5:1 while the equilibrium mixture of Glc/Man (3:1)
was reached after 35 h of heating. The composition of the reaction
starch and high conversion by means of D-glucose and D-mannose
content (2:1). It was observed that the transformation occurs with
full selectivity and an equilibrium reaction mixture was reached
within 3 min. The reaction scheme of the combined starch hydro-
mixture in terms of starch,
D-glucose and D-mannose content as
a function of time is depicted in Figure 1.
lysis and simultaneous epimerization reaction of obtained
cose to -mannose is illustrated in Scheme 1.
D-glu-
Different reaction kinetics and thermodynamics were observed
using microwave heating. Starch hydrolysis and molybdic
D

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Z. Hricovíniová / Tetrahedron: Asymmetry 22 (2011) 1184–1188
Table 1
Comparison of microwave and conduction-heated samples of the combined starch hydrolysis and simultaneous epimerization reaction of obtained D-glucose to D-mannose
Starch source
MW field
Starch/Glc/Man (%)
Conventional (%)
Starch/Glc/Man (%)
Heating
Starch/Glc/Man (%)
Time (min)
Time (h)
Time (h)
Potato starch
Corn starch
Rice starch
3
3
3
0/65/33
0/62/30
0/63/31
10
10
10
52/35/7
56/32/6
55/31/6
40
40
40
0/73/24
0/74/24
0/73/22
Both partial and full conversions are shown for conventional heating to demonstrate vast differences between microwave and conventional approaches in reaction kinetics
and amounts of mannose formations.
yields of glucose/mannose in all three starch samples tested. There
were only minor differences in the composition of the reaction
1.2
mixtures of various starches. It should be noted that only a cata-
lytic amount of sodium molybdate is very effective and that a high-
1.0
er amount of catalyst does not significantly change the conversion
to the product (1–2%).
0.8
The rearrangement of the obtained
accomplished after acid hydrolysis of starch in a microwave field
using MoVI ions as a catalytically active species. In this respect,
glucose isomerization is a crucial step in the efficient production
of -mannose as a valuable chemical product. The mechanism of
this transformation is based on the fact that in the presence of
molybdate ions, the hydroxyl groups of the free glucose unit coor-
dinate with the dimolybdate anion and the creation of catalytically
D-glucose to D-mannose was
0.6
0.4
0.2
0.0
D-
D
active complexes leads to isomerization of D-glucose to D-mannose
through high rearrangements of the carbohydrate carbon
0
10
20
30
40
50
skeleton.20,21
Time [h]
It is important to note that different conversions to products
could be obtained in a microwave field as a function of time. The
comparison of the results clearly shows that microwave irradiation
markedly accelerated the isomerization process (by two orders of
magnitude over conventional heating) and that the microwave
field also caused differences in the equilibration of the reaction
mixtures. Microwave irradiation as a non-conventional energy
source thus has a strong influence upon the kinetics and thermody-
namics of this reaction. The data obtained suggest that the syner-
gistic coordination ability of the catalyst used under the given
reaction conditions enables simultaneous efficient starch depoly-
Figure 1. Conversion of potato starch (d) to
function of time under conventional, oil-bath heating. The formation of
starts at approximately 6 h after starch hydrolysis to D-glucose.
D
-glucose (N) and
D
-mannose (ꢀ) as a
D
-mannose
acid-catalyzed isomerization reached thermodynamic equilibrium
after 3 min under identical reaction conditions (concentrations,
temperature) as used for conventional heating (Fig. 2). As seen in
Table 1, the studied conversion of starch to the mixture of Glc/
Man was dramatically faster. Furthermore, the ratio of Glc/Man
was considerably higher (2:1) than that obtained under the classi-
cal conditions (5:1 after 10 h; 3:1 after 35 h). Table 1 summarizes
all of the data obtained for starch samples heated by using conduc-
tion or microwave energy sources. Microwave-induced hydrolysis
in the presence of sodium molybdate as a catalyst afforded similar
merization and the formation of a new compound
a single step. Thus, unlike conventional heating,
D
-mannose in
-mannose is
D
formed virtually simultaneously with D-glucose using microwave
radiation. This approach opens up the way for the preparation of
many interesting sugar derivatives. It should also be noted that
the presence of molybdate ions improves the efficiency of energy
absorption from the microwave source and thus the reaction pro-
ceeds faster compared to that without the presence of these ions.
The MoVI ions thus perform a double role in the present reaction:
the first role is the improvement of the dielectric properties of
the solution, leading to high energy transfer to solution from a
microwave source; secondly, MoVI acts as an efficient catalyst that
enables the stereospecific conversion of glucose to mannose. These
twofold roles of molybdate ions make MoVI a unique catalyst for
the isomerization reactions of carbohydrate molecules.
1.2
1.0
0.8
0.6
0.4
0.2
0.0
The main purpose of this study was the development of a
practical method for starch hydrolysis with subsequent efficient
production of
are widely used in the food and pharmaceutical industry.32–34
The reduction of -mannose with sodium borohydride leads to
-mannitol in a good yield (78%), hence the production of pure
-mannose is needed in significant amounts. At present, -manni-
-fructose from
less-expensive glucose–fructose syrups which leads to a mixture
of -mannitol and
-sorbitol (1:1).35–37 Mixtures of mannitol and
sorbitol are also produced enzymatically from fructose with differ-
ent microorganisms.38,39 The main problem is that
-mannitol is
D-mannose and D-mannitol. Both of these products
D
D
D
D
tol is mainly produced by the reduction of
D
0
50
100
150
200
250
300
350
Time [s]
D
D
Figure 2. Conversion of potato starch (d) to
function of time in a microwave field.
D
-glucose (N) and -mannose (ꢀ) as a
D
D

Z. Hricovíniová / Tetrahedron: Asymmetry 22 (2011) 1184–1188
1187
difficult to separate from its stereoisomer
D
-sorbitol. Compared to
4.2. Hydrolysis of starch with sodium molybdate under
microwave irradiation
enzymatic production, chemical catalysis employing inexpensive
inorganic materials for the isomerization of reducing sugars could
offer attractive advantages. Utilizing an efficient catalytic system
and water as the solvent has many advantages as this approach
is environmentally friendly and inexpensive. Moreover, carbohy-
drates as water-soluble polar molecules, with high dielectric
losses, are suitable chemical systems for microwave irradiation.
They could be modified by this approach without the need for pro-
tection and deprotection steps. Furthermore, the high demands in
activation energy of the isomerization process can be completed in
a very short reaction time, which makes aqueous catalysis more
efficient. The application of this methodology provides an attrac-
tive aspect to the field of microwave-assisted metal-catalyzed
reactions in aqueous media.
Starch (500 mg) was dissolved in 0.25 M HCl (10 ml) and Na2-
MoO4Á2H2O (50 mg) was added to obtain a stock solution. The
sealed tubes (1 ml) were exposed to microwave irradiation
200 W for different lengths of time (10 s–5 min). Samples were
À
treated with Amberlite IRA-400 in the HCO3 form to remove the
catalyst. The reaction mixtures were analyzed by NMR spectros-
copy measurements and the ratio of starch/glucose/mannose was
determined by integration of selected resonances in the 1H NMR
spectra.
4.3. Hydrolysis of starch with sodium molybdate with
conventional heating
Starch (200 mg) was dissolved in 0.25 M HCl (4 ml) and Na2-
MoO4Á2H2O (20 mg) was added. The tube was sealed and heated
in an oil-bath at 90–95 °C for 40 h. Samples (0.3 ml) were taken
at selected intervals, and treated with Amberlite IRA-400 in the
3. Conclusion
We have developed a direct and convenient microwave assisted
one-pot protocol for the preparation of D-mannose from native
starch by a simple chemical process. Microwave irradiation in
combination with a catalytic amount of molybdate ions proved
to be an efficient method for the hydrolysis of starch and isomeri-
À
HCO3 form to remove the catalyst. The reaction mixture was ana-
lyzed by NMR spectroscopy measurements and the ratio of starch/
glucose/mannose was determined by integration of selected reso-
nances in the 1H NMR spectra.
zation of the obtained D-glucose to D-mannose. The nature of heat-
ing had a great impact on this transformation. The rate of starch
hydrolysis considerably increased under microwave irradiation.
In addition, the conversion of glucose to mannose in the subse-
quent isomerization step was much higher compared to conven-
tional heating. The short reaction times, good conversions in
combination with the easy performance and work-up make this
method attractive and also applicable on a semi-preparative scale.
In conclusion, we would like to point out that the application of
MoVI salts and microwave power for depolymerization of starch
leads to its complete hydrolysis and high conversion by means of
4.4. Typical procedure for starch hydrolysis under microwave
conditions on a semi-preparative scale
Potato starch (500 mg) was dissolved in 0.25 M HCl (10 ml) and
Na2MoO4Á2H2O (50 mg) was added. The sealed tube was exposed
to microwave irradiation for 3 min. The reaction mixture was also
treated batch-wise with an excess of the cation/anion ion-ex-
change resin, filtered off, washed with water and the combined fil-
trates were evaporated. The syrupy residue was fractionated by
column chromatography on Dowex 50W X8 (200–400 mesh) in
Ba2+ form with water as eluent. Fractionization of the syrupy resi-
D
-glucose and D-mannose content.
due afforded
33%).
D-glucose (0.325 g, 65%) and D-mannose (0.165 g,
4. Experimental
4.1. General methods
4.5. Reduction of
D-mannose to
D-mannitol
All starches used were purchased from commercial suppliers.
Microwave reactions were performed in a multimode microwave
reactor CEM Discover consisting of a continuous focused micro-
wave power delivery system with operator-selectable power from
0 to 300 W; microwave frequency source of 2.45 GHz. The reac-
tions were performed in sealed glass tubes and were stirred mag-
netically. Conversions and the purities of the products were
determined by NMR spectroscopy. High-resolution NMR spectra
were recorded in a 5 mm cryoprobe on Varian 600 VNMRS spec-
trometer. The experiments were carried out at 25 °C in D2O. The
proton and carbon chemical shifts were referenced to external
TSP. One-dimensional 1H and 13C NMR spectra as well as two-
dimensional COSY and HSQC were used to determine 1H and 13C
chemical shifts.
To a solution of
D
-mannose (1.0 g) in water (15 ml) was added
an aqueous solution of sodium borohydride (0.2 g in 5 ml H2O).
The reaction mixture was kept at room temperature for 2 h. When
the reduction was complete the reaction mixture was acidified
with a drop of acetic acid, deionized with cation/anion ion-
exchange resin and evaporated to dryness. Crystallization from
methanol afforded
D
-mannitol (780 mg; 78%). Mp 165–166 °C;
[a]D = +23.7?+24.5 (c 10, Na2B4O7), 24 h, which was in accordance
with the literature.40
Acknowledgement
This research was supported by VEGA Grant 2/0087/11 and SP
Grant 2003SP200280203.
Optical rotations were determined at 20 °C with an automatic
polarimeter Perkin–Elmer Model 141 using a 10 cm, 1-ml cell.
Melting points were measured on a Kofler hotstage microscope.
Separations of the free sugars were accomplished by column chro-
matography on Dowex 50W X8 resin (Sigma–Aldrich) in the Ba2+
form (200–400 mesh). Paper chromatography was performed by
the descending method on the Whatman No. 1 paper using ethyl
acetate–pyridine–water (8:2:1) as the mobile phase. The chro-
matograms were made visible by means of alkaline silver nitrate.
All chemicals were reagent grade and used without further
purification.
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