Chemistry Reference
In-Depth Information
Scheme 1
First use of ferric chloride in a glycosylation.
We will focus particularly on such transformations induced by ferric
salts. This review is thus not intended to present exhaustive work done in
carbohydrate chemistry with these salts. It will rather focus on a few
transformations promoted by ferric salts used as a Lewis acid at our la-
boratories at Orsay and Gif sur Yvette. Dedicating this very specific topic
to Andr
´
Lubineau seems most appropriate.
Iron salts have recently attracted considerable attention as inexpensive
and environmentally friendly agents in a wide range of selective pro-
cesses in organic synthesis.
1
Over the past decade, FeCl
3
6H
2
O has
shown to be an effective catalyst in the formation of carbon-carbon
2
or
carbon-nitrogen
3
bonds, in intramolecular Friedel-Craft reactions
4
and in
the reduction of ketones
5
or allylic alcohols.
6
In carbohydrate chemistry, the use of iron salts to promote a
selective transformation has not always been a first choice, in spite of
their being readily accessible and abundant. One of the first uses of
iron(
III
) chloride of which we are aware of is the preparation of oxazoline
2, by treatment of 2-acetamido-1,3,4,6-tetra-O-acetyl-2-deoxy-b-
D
-
glucopyranose 1 with ferric chloride in dichloromethane, described by
Matta and Bahl (Scheme 1).
7
When an alcohol in excess was added to the
mixture, the b-glycoside 3 was formed instead of the oxazoline,
8,9
and this
has been used on several occasions using interesting modified versions
of glycosylations.
10-12
Another pioneering work using anhydrous iron(
III
) chloride in carbo-
hydrate chemistry was the selective cleavage of benzyl ethers, protecting
groups widely used for carbohydrates, leaving other ethers, acyl groups
and unsaturations unaffected (Scheme 2).
13
This procedure has been
conveniently included in a number of multi-step synthetic plans,
14
as
shown in the removal of the four benzyl groups on a late tetrasaccharide
intermediate 6 to deliver tetra-ol 7.
15
2 Brief historical background including Andr
´
Lubineau's contribution
Emil Fischer established that the sulfuric acid-catalyzed reaction of
D
-glucose with methanol provides the corresponding methyl
D
-glucopyr-
anoside 9, a reaction now known as the Fischer glycoside synthesis
(Scheme 3).
16
This remarkable transformation does not need any pro-
tection on the hydroxyl groups of glucose, an approach that remains
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