LETTER
129
H-Beta Zeolite as an Efficient Catalyst for Per-O-acetylation of Mono - and
Disaccharides
Pallooru Muni Bhaskar and Duraikkannu Loganathan*
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India
Fax 091-44-2350509; E-mail: loganath@pallava.iitm.ernet.in
Received 22 October 1998
and environment-friendly transformation to glycosides
and disaccharides. Thus the preparation of sugars per-O-
acetates was explored using six different zeolites in the
present study. Here, we report H-beta zeolite7a as a highly
efficient catalyst for the per-O-acetylation of various
mono-and disaccharides and alkyl glycosides (Scheme 1).
Abstract: Among the six different zeolites examined, H-beta is
demonstrated to be a highly efficient and selective catalyst for the
per-O-acetylation of various mono-and disaccharides and methyl
glycosides.
Key words: zeolites, acetylation, sugars, methyl glycosides
The application of inorganic solid acids as heterogeneous
catalysts for organic synthesis is an area of intense re-
search. Zeolites and molecular sieves have been shown to
function as effective catalysts for liquid-phase organic
transformations1. The advantages of these heterogeneous
catalysts over the homogeneous systems include stability,
ease of handling, lack of corrosion and other environmen-
tal hazards and ease of recovery and regeneration. More
importantly, these catalysts have already led to the indus-
trial use of enviro-economic processes and are predicted
to play a central role in enabling environmentally sustain-
able development in the 21st century2. Zeolites have been
employed in carbohydrate chemistry to catalyse the for-
mation of O-isopropylidene acetals3a, preparation of 5-hy-
droxymethylfurfural from fructose and precursors3b,
Scheme 1
In a typical experiment, H-beta zeolite (1g) was added to
a suspension of methyl α-D-glucopyranoside (2 mmol) in
acetic anhydride (5 ml) kept stirring at room temperature.
Completion of reaction after 3 h of stirring was indicated
by the disappearance of the substrate on TLC. Filtration of
the catalyst followed by washing the same with dichlo-
romethane and concentration of the filtrate combined with
washings under reduced pressure afforded a colourless
solid (99 %).
isomerization and hydrolysis of di
–
and
Several mono- and disaccharides and methyl glycosides
underwent acetylation in excellent yields (Table 1). As
noted earlier under clay catalysis, the conversion of cello-
biose alone was partial due presumably to its poor solubil-
ity. Interestingly the per-O-acetate formation in the case
of the trisaccharide raffinose was not detectable on TLC
even after 48 h of stirring. Comparison of this result with
the 98% yield of raffinose per-O-acetate obtained using
montmorillonite K-10 clearly reveals the shape selectivity
of H-beta zeolite towards the mono and disaccharides.
Generally, the reaction times were shorter with H-beta
zeolite as compared to those observed with the clay.
Analysis of the 300 MHz 1H-NMR spectra of various sug-
ar per-O-acetates permitted the determination of their
composition4b, 8a-g. The pyranose forms represented 66-
100% of the fully acetylated sugars. Comparison of the
composition of per-O-acetates shown in Table 1 with
those obtained using pyridine and acetic anhydride for D-
glucose and D-mannose (94% α & 6% β and 79% α and
21% β pyranose forms respectively representing the ini-
tial composition of free sugars9) point out the partial ano-
merisation and ring modification occurring under H-beta
zeolite catalysed conditions, as also observed earlier using
the clay6. Likewise, the formation of furanose per-O-ace-
tates mostly from monosaccharides and the hexa-O-ace-
polysaccharides3c,3d and synthesis of alkyl glucoside
surfactants3e.
Per-O-acetylation is commonly employed for protecting
hydroxyl groups in sugars. Fully acetylated sugars are in-
expensive and versatile intermediates for the synthesis of
many naturally occurring glycosides, oligosaccharides
and glycoconjugates. In addition, the isolation and struc-
tural elucidation of such natural products is facilitated by
conversion to their per-O-acetates. Acetylation of sugars
is often carried out using acetic anhydride as the reagent
and the catalysts4a-c employed include pyridine, sodium
acetate, zinc chloride, ferric chloride, sulphuric acid,
ion exchange resin and iodine. The acute toxicity and dis-
agreeable odor of pyridine are well known5. Large scale
handling of pyridine and other homogeneous catalysts is
not only troublesome but their recovery is difficult as
well.
We recently demonstrated the per-O-acetylation of sugars
catalysed by the clay, montmorillonite K-106. The inher-
ent shape selectivity, potential to fine tune the Bronsted /
Lewis acidity and thermal stability of zeolites make them
advantageous over clays. Successful zeolite catalysed
preparation of fully acetylated sugars, it was reasoned,
would potentially allow their subsequent stereoselective
Synlett 1999, No. 1, 129–131 ISSN 0936-5214 © Thieme Stuttgart · New York
Bhaskar, Pallooru Muni
