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P(NMe
2
)
3
,CCl
4
CH
2
Cl
2,
- 40 °C
or PPh
3,
CCl
4
THF, reflux
Cl
Br
O
O
Ph
3
P-CH
2
Br, Br
tBuOK, THF, reflux
O
1
O
1
2
2
Cl
Br
1
2
Ph
3
P=CHR
R
O
Toluene
O
2
2
+
3
3
R
R = COOMe, 140 °C, 56-90%
R = CN, 110 °C, 83-98%
3
R = COOMe
4
R = CN
Scheme 1 Preparation of different classes of exo-glycals from lactones.
dichloro-exo-glycals 1, the carbon chlorine bond and the double bond,
despite it is an enol ether, were found particularly non-reactive. Never-
theless, the C-C bond can be reduced by hydrogenation, the C-Cl bond
being cleaved at the same time.
6
The later can be cleaved by radical
reaction providing methylene analogues
10
but C-C coupling reactions
involving the C-Cl bonds failed.
Reasoning that a carbon-bromine bond could be more reactive, a
method to prepare dibromo-exo-glycals was sought for. It was found that
the reaction of monobromomethylene phosphorane unexpectedly gave
excellent yields of the dibromo-exo-glycal 2 by a complex mechanism in-
volving a Wittig olefination with in situ generated dibromophosphorane.
11
In the meantime, the Wittig olefination of carbohydrate lactones with
stabilized phosphoranes at high temperature was discovered. This was an
ecient reaction which worked with g- and d- lactones in excellent yields.
This way another new class of exo-glycals 3 was available, as Z/E mixtures
but with an interesting ''push-pull'' substitution of the double bond
(Scheme 1).
12
Over the years, our group tried to develop some chemistry around these
C-glycosylidene compounds in order to enlarge the scope of this
approach and to explore the possibilities to access new compounds of
interest in the carbohydrate and other related fields. Here are described
some recent developments from our group on the chemical manipulation
of some exo-glycals, obtained by new methodologies, giving access to
new, more complex, exo-glycals, to C-glycosyl compounds, spirohetero-
cycles and to sugar b- and g-amino acids.
2 Functionalization of exo-glycal double bond
As mentioned above, C-glycosylidene compounds are not common and
their chemistry remains to be explored, in particular the reactivity of the
anomeric double bond. Given the paucity of methods to directly form
exo-glycals, it seemed interesting to construct new and more complex
exo-glycals by modification of the different classes of the readily available
ones.
2.1 Reactions from halogenated exo-glycals
Interesting results have been obtained by Lopez's group with mono-
bromo-exo-glycals
in different palladium catalyzed cross-coupling
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