Chemistry Reference
In-Depth Information
Exo-glycals as useful tools for anomeric
functionalization of sugars
Nadia Pellegrini-Mo
ยจ
se,*
a,b
Myl
`
ne Richard
a,b
and
Yves Chapleur
a,b
DOI: 10.1039/9781849739986-00099
Exo-glycals or C-glycosylidene compounds having an exocyclic double bond at
the anomeric centre are readily available from sugar lactones through direct olefination
reactions. These unsaturated carbohydrates proved useful intermediates. In this chapter
are described recent studies from our group on functionalization or addition reactions
on the double bond allowing the transformation of exo-glycals into more complex
exo-glycals, spiroheterocycles, C-glycosyl, bis-C-glycosyl compounds and sugar
b
- and
g
-amino acids of interest.
1 Introduction
C-glycosyl compounds have attracted much interest in synthetic
carbohydrate chemistry due to their close analogy with O-glycosides, their
occurrence in Nature and their proven stability toward glycosidases for
example. C-glycosylidenes having an exocyclic double bond at the
anomeric centre are scarce compounds which have been less studied
until recently because only a few complex syntheses were available. The
term exo-glycals has been coined to name these compounds by analogy
with the term glycal which refers to the well-known 1,2-unsaturated
sugars.
1
It is important to note that 5,6-unsaturated carbohydrates are
also exo-glycals more conveniently named 5,6-exo-glycals.
The development of the chemistry of exo-glycals is in relation with the
almost simultaneous discovery of direct methods for the olefination of
readily available sugar lactones by Chapleur
2
and Wilcox
3
in 1984. Since
then, several useful direct or stepwise approaches to exo-glycals have
been proposed.
1,4,5
Over the years, these compounds proved to be useful
compounds and can be further manipulated in different ways to provide
more complex compounds.
The first direct olefination of carbohydrate lactones was discovered
using the tris(dimethylamino)phosphine-carbon tetrachloride couple
which reacted at low temperature to yield dichloroolefins in excellent
yields (Scheme 1).
2,6
The reaction was limited to certain lactones in
particular those having a dioxolane protecting group at O-2-O-3.
6,7
This
limitation was later bypassed by using triphenylphosphine which is less
reactive but reacts at higher temperature allowing reaction with poorly
reactive d-lactones.
8,9
This first class of exo-glycals was not as reactive as methylene-
exo-glycals obtained by reaction of lactones with Tebbe reagent.
3
In
a
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