Microwave-accelerated methylation of starch

Article abstract of DOI:10.1016/j.carres.2007.09.006

A novel microwave-accelerated method for methylating soluble starch is described. Soluble starch could be fully methylated in 72% yield within 4.66 min using iodomethane and 30% potassium hydroxide under microwave irradiation. The completely methylated starch thus obtained was hydrolyzed with 60% HCO₂H for 1.5 min under 80% MW power, followed by 0.05 M H₂SO₄ for 2.0 min under 100% MW power. The partially methylated monosaccharides were separated by preparative paper chromatography and identified by their melting points and optical rotations.

Full text of DOI:10.1016/j.carres.2007.09.006

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Carbohydrate Research 343 (2008) 151–154
Note
Microwave-accelerated methylation of starch
Vandana Singh* and Ashutosh Tiwari
Department of Chemistry, University of Allahabad, Allahabad 211 002, India
Received 29 July 2007; received in revised form 18 September 2007; accepted 20 September 2007
Available online 24 October 2007
Abstract—A novel microwave-accelerated method for methylating soluble starch is described. Soluble starch could be fully methyl-
ated in 72% yield within 4.66 min using iodomethane and 30% potassium hydroxide under microwave irradiation. The completely
methylated starch thus obtained was hydrolyzed with 60% HCO2H for 1.5 min under 80% MW power, followed by 0.05 M H2SO4
for 2.0 min under 100% MW power. The partially methylated monosaccharides were separated by preparative paper chromato-
graphy and identified by their melting points and optical rotations.
Ó 2007 Published by Elsevier Ltd.
Keywords: Starch soluble; Irradiation microwave; Iodomethane; Methylation
Conventionally starch can be methylated1 by the dimethyl
sulfate (DMS)-alkali procedure of Haworth to provide
solubility in N,N-dimethylformamide and then by Kuhn
methylation with silver oxide and iodomethane. Amy-
lose and amylopectin can also be methylated directly
in liquid ammonia with sodium and iodomethane, but
native starch granules require some form of pretreat-
ment. The Freudenberg and Boppel method1 involves
dissolution of the starch in a mixture of liquid ammonia,
sodium, and iodomethane in which the ammonia must
be removed after completing the methylation. Methyl-
ation2 of starch is also reported in DMSO using the
lithium methylsulfinyl carbanion and iodomethane (a
modification of the Hakomori method), which requires
stirring for long periods and sonication. Microwave
(MW) irradiation3 as an efficient source of thermal
energy is becoming a standard technique in various
fields of chemistry including carbohydrate chemistry.
Microwave-assisted selective and unselective protection
and deprotection of hydroxyl functionalities of carbo-
hydrates and many other aspects of carbohydrate chemis-
try have been reviewed by Corsaro et al.4 Hydrolyses of
starch5 and plant seed gums,6 synthesis of alkyl glyco-
sides,7 graft copolymerization8 of acrylic acid with
starch, graft copolymerization of acrylonitrile,9 and
acrylamide10 with guar gum have been recently
reported. Our group has reported a modification of
Haworth’s method where complete methylation of guar
gum could be done under microwave irradiation using
dimethyl sulfate in aqueous alkali.11 The methylation
could be completed in a very short reaction time under
milder conditions, but with the disadvantage of SO2
evolution during the reaction. In the present study com-
plete methylation of starch was undertaken (Scheme 1)
to avoid SO2 evolution, and iodomethane instead of
dimethyl sulfate was used as methylating agent. Com-
plete methylation of the soluble starch could be achieved
under MW irradiation without using any catalyst or an
inert atmosphere. Starch was fully methylated12 with
iodomethane and 30% aqueous potassium hydroxide
under MW irradiation. An IR spectrum of the methyl-
ated product obtained after 3 min exposure showed a
small peak in the –OH region, indicating that methyl-
ation is incomplete at this stage. The O–H stretching
peak completely disappeared after 4.66 min exposure,
indicating that complete methylation requires 4.66 min
exposure (Fig. 1). The fully methylated product had
*
Corresponding author. Tel.: +91 532 2461518; fax: +91 532
2540858; e-mail: singhvandanasingh@rediffmail.com
Present address: Biomolecular Electronics and Conducting Polymer
Research Group, National Physical Laboratory, Dr. K. S. Krishnan
Road, New Delhi 110 012, India.
no absorption in the IR (Fig. 1) at 3600–3400 cmÀ1
,
0008-6215/$ - see front matter Ó 2007 Published by Elsevier Ltd.
doi:10.1016/j.carres.2007.09.006

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V. Singh, A. Tiwari / Carbohydrate Research 343 (2008) 151–154
Figure 2. 1H NMR of fully methylated soluble starch.
while the starch originally had a broad peak at
Figure 1. FTIR spectra of (A) soluble starch; (B) hydrolyzate of
completely methylated soluble starch; (C) partially methylated starch
recovered after 2nd addition; (D) partially methylated starch obtained
after 3rd addition; (E) fully methylated starch after 4th addition.
1
3440 cmÀ1 due to the –OH stretching. The H NMR
spectrum in CDCl3 (Fig. 2) showed strong, sharp,
three-proton singlets between peaks at d 3.31 and 3.48
OH
O
O
HO
OH
HO
O
O
HO
MeI, 30%KOH
OH
HO
O
O
O
100% MW for 4.66 min
HO
HO
O
HO
O
HO
Soluble starch
OMe
O
O
MeO
OMe
MeO
60% HCO2H; 80% MW power for 1.5 min
0.1 N H2SO4; 100% MW power for 2 min
O
O
MeO
OMe
MeO
O
O
O
MeO
MeO
O
O
MeO
MeO
Fully methylated soluble starch
OMe
O
OMe
OH
O
O
+
+
HO
MeO
HO
MeO
MeO
MeO
MeO
MeO
OH
MeO
OH
OH
2,3,4,6-tetra-O-methyl-D-glucose
2,3,6-tri-O-methyl-D-glucose
2,3,-di-O-methyl-D-glucose
Scheme 1. Methylation and hydrolysis of soluble starch under MW irradiation.

V. Singh, A. Tiwari / Carbohydrate Research 343 (2008) 151–154
153
for the –OCH3 groups besides the other usual peaks
1.2. Methylation under microwave irradiationà
(Scheme 1).
The fully methylated starch was hydrolyzed in 3.5 min
using MW irradiation, using 60% HCO2H, followed by
0.1 N H2SO4.
To a solution of starch (2.0 g in 20 mL of 15% aq KOH),
5 mL of CH3I and 5 mL of 30% KOH were added in
four equal installments. After each addition of the
reagents, the reaction mixture was given a 1-min exposure
to 100% MW power. The progress of the reaction was
monitored by IR spectroscopy. For complete methyl-
ation the reaction mixture required an additional
exposure of 0.66 min. Thus complete methylation
required a total exposure time of 4.66 min. On addition
of each installment of the reagents, the pH of the reac-
tion mixture was 13.6, which decreased to pH 7.1 after
exposure. Finally the reaction mixture was cooled to
room temperature and was thoroughly extracted with
CHCl3. The extract was dried over anhyd Na2SO4,
and the solvent was evaporated under reduced pressure.
The methylated product was obtained as a light-yellow
powder: yield 1.82 g, (72%); methoxyl19 (42.4%). An
additional methylation reaction did not increase the
methoxyl content of the product.
Though starch, being a macromolecule cannot show
rotation or migration when exposed to an electric field
of microwaves, its hydroxyl groups show localized
rotation13 on an essentially immobile starch molecule.
However, such localized rotations13 cannot correspond
instantaneously to the rapidly changing direction of
the field, which creates friction that manifests itself as
heat resulting in O–H bond breaking. The dielectric
heating results in rapid energy transfer from these
groups to neighboring molecules (CH3I and NaOH),
as it is not possible to store the energy in a specific part
of the molecule. Further MW are also reported to have
special effects14 on lowering of Gibbs free-energy of
activation of the reactions. This occurs either through
the storage of microwave energy in the vibrational
energy of a molecule (enthalpy effect) or by the align-
ment of molecules (entropy effect). In view of the above
two effects, methylation under MW conditions is possible
under mild conditions in a very short time.
1.3. Hydrolysis of methylated starch using MW
irradiation
Thus, under MW irradiation starch can be fully
methylated in good yield and thereafter hydrolyzed very
efficiently into a mixture of the methylated monosaccha-
rides within a very short reaction time. The hydrolyzates
of the methylated starch, methylated conventionally and
by the MW method, were co-chromatographed in solvent
A. Three methylated monosaccharides, namely 2,3,4,6-
tetra-O-methyl-D-glucose, 2,3,6-tri-O-methyl-D-glucose,
and 2,3-di-O-methyl-D-glucose were detected. The prod-
ucts were fractionated by paper chromatography on
Whatman 3 MM paper (solvent A). The methylated
sugars were identified15 by their mp, optical rotation,
RTMG, and by preparing their crystalline derivatives.
The methylated starch (250 mg) was dissolved in 25 mL
of 60% HCO2H, and the solution was exposed to 80%
MW power for 1.5 min. The solution was then concen-
trated, and the last traces of HCO2H were removed
under vacuum. The residue was then dissolved in
15 mL of 0.1 N H2SO4 and exposed to full MW power
for 2.0 min. The hydrolyzate was cooled, neutralized
with barium carbonate, filtered, and concentrated under
reduced pressure to a light-yellow syrup. The partially
methylated monosaccharides were identified by paper
chromatography (pc) in solvent A.
GLC of the partially methylated alditol acetates20
(obtained by the reduction of the hydrolyzate with
NaBH4, followed by acetylation) also confirmed the
presence of 2,3,4,6-tetra-O-methyl-D-glucose, 2,3,6-tri-
O-methyl-D-glucose, and 2,3-di-O-methyl-D-glucose in
the hydrolyzate.
1. Experimental
1.1. General
Complete methylation and hydrolysis of the fully
methylated starch could be successfully achieved under
microwave irradiation in very short time. Besides being
rapid, the method does not necessitate an inert
atmosphere for the methylation.
A Kenstar (Model No. MOW 9811, 1200 W) domestic
microwave (MW) oven was used for all the experiments.
Solutions were concentrated at diminished pressure at
60–62 °C. Paper chromatography was carried out at
room temperature with solvent system A, 4:1:5 butanol–
ethanol–water;16 B, 169:47:15 benzene–ethanol–water;17
and C, 10:1 butanone–water,18 with detection using
aniline hydrogen phthalate. IR spectra were recorded
on Bomem ABB FTLA2000 spectrophotometer in
Acknowledgments
The authors are thankful to PAR lab, Telierganj Allaha-
bad for providing their GLC facility and to the Indian
1
CHCl3. H spectra were recorded on Jeol 400 MHz FT
NMR in CDCl3. Soluble starch (Qualligen) was used
for the study. All the reactions were carried out in open
glass vessels.
à This operation should be conducted in a well-ventilated hood. CH3I
(bp 41–43 °C) is volatile and toxic.

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NMR facility.
12. Harris, P. J.; Henry, R. J.; Blakeney, A. B.; Stone, B. A.
Carbohydr. Res. 1984, 127, 59–73.
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