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V. K. Rajput et al. / Tetrahedron Letters 47 (2006) 6987–6991
systems for various carbohydrate reactions we found
that H2SO4–silica17,18 could act as a good protic acid
source under milder and safer conditions than other
silica supported reagents such as HClO4–silica. Extend-
ing its application to synthetic carbohydrate chemistry,
we now report a very simple method for the hydrolysis
of terminal O-isopropylidene groups in the presence of
a series of protecting groups frequently used for oligo-
saccharide synthesis.
In addition to the simplicity of the product isolation,
the catalyst can be recycled several times. Over seven
cycles, the catalyst did not show any significant change
in reactivity. Moreover, reaction with compound 1 on
20 g scale showed no difference in the outcome confirm-
ing the applicability of the reagent system on a large-
scale.
In conclusion, a simple work-up and purification-free
method has been reported for deprotection of terminal
O-isopropylidene groups using reusable H2SO4–silica
which is equally applicable on large-scale. The time
required for the desired transformations is remarkably
shorter compared to the other methods available in
the literature. It is expected that this cheap, environmen-
tally friendly, safe and easy to handle reagent system will
find applications in oligosaccharide synthesis. Com-
pounds derived from these experiments are being used
in our laboratory for the synthesis of furanoside-con-
taining oligosaccharides.
To start, 3-O-acetyl-1,2:5,6-di-O-isopropylidene-a-D-
glucofuranose (1, 1 mmol) was dissolved in commercial
grade methanol (5 mL) and treated with H2SO4–silica19
(100 mg) at room temperature. After 30 min, TLC
showed complete conversion of the starting material to
a slower running component.20 The product obtained
through filtration and evaporation of the solvent was
confirmed as 3-O-acetyl-1,2-O-isopropylidene-a-D-gluco-
furanose (2) by NMR and mass spectrometric analy-
sis. It is worth noting that the reaction was very
sluggish in acetonitrile (ꢀ20% conversion after 24 h)
and no reaction occurred in dichloromethane. A similar
reaction with normal silica did not produce any of the
desired products even after 3 days at room tempera-
ture. When the same transformation was carried out
using aq AcOH, the commonly used method for the
deprotection of terminal isopropylidene groups, it took
12 h for complete conversion of the starting material.
This evidence affirms the potential of the method
(Scheme 1).
Acknowledgements
V.R. and B.R. are thankful to CSIR, New Delhi, for
providing fellowships. Instrumentation facilities from
SAIF and CDRI are gratefully acknowledged.
Supplementary data
Our next target was to ascertain the compatibility of this
reagent system with various protecting groups including
acid labile p-methoxybenzyl (PMB), tert-butyldimethyl-
silyl (TBDMS), o-nitrobenzyl (ONB) groups, etc. For
this purpose, a set of di-O-isopropylidene glucofuranose
derivatives having different protecting groups at 3-OH
were prepared and subjected to the above reaction con-
ditions. In all cases, satisfactory yields of the desired
mono-O-isopropylidenes were obtained without affect-
ing the protecting group at position 3. The results are
summarized in Table 1.
Supplementary data associated with this article can be
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1
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