Chemistry Reference
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OH
OH
HO
O
HO
CH
2
R
1
h
ν
HO
O
OH
R
1
CH
2
CHO
HO
R
1
OH
HO
O
OH
O
HO
O
R
2
CH
2
CHO
OH
O
R
OH
2R
1
CH
2
CHO
CH
2
R
1
R= R
1
HO
with 2 equiv of R
1
CH
2
CHO
O
HO
R= R
2
with 1 equiv of R
1
CH
2
CHO and
then 1 equiv of R
2
CH
2
CHO
OH
O
Scheme 17
The formation of b-C-glycosyl (E)-a,b-unsaturated ketones via such an
aldol condensation is well documented either with protected,
92
partially
protected
93
or unprotected
94
b-C-glycosyl ketones. In the galactose series,
the derivatives were found to be effective as antimycobacterial agents.
95
Ferrocene grafted b-C-glycosyl (E)-a,b-unsaturated ketone were prepared
by this way and used as building blocks for the synthesis of ferrocenyl
b-C-glycosyl spiropyrrolidines and pyrrolizidines.
96
b-C-Glycosyl (E)-a,b-unsaturated ketones could also be converted to
glycosyl dienes which gave access to anthraquinone-based C-glycosyl aryl
derivatives through a sequential Diels-Alder reaction and oxidative
aromatisation.
97
They could also be transformed into macrocyclic
triazolyl glycoconjugates.
98
The e
ciency of Knoevenagel condensation has allowed to prepare
various useful building blocks. Thus, Lubineau has developed a highly
effective method to introduce a formyl group at the anomeric position of
pyranosides via enolisation of protected b-C-
D
-glycopyranosylpropan-2-
ones and oxidative cleavage of the more substituted double bond.
99
For
purification and storage, b-C-glycosylformaldehyde could be isolated as
aminal (Scheme 18).
Scherrmann used a radical coupling between glyoxylic oxime ethers
and b-C-glycosyl 2-iodopropane easily obtained from the corresponding
b-C-glycosylpropan-2-one, giving access to C-glycosylthreonine and
allothreonine.
100
Protected C-galactosyl methyl ketones were transformed into b-C-
galactosyl isobutyl derivative (IBCG) through a Wittig reaction with
methylene phosphorane followed by double bond hydrogenation.
101
IBCG is a promising inducer of gene expression in mammalian cells.
Protected C-galactosyl methyl ketones were also converted into vinyl
triflates before undergoing a Suzuki coupling in order to prepare stable
and selective inhibitors of human galectins.
102
Heptoses as reducing sugars could be opposed to penta-2,4-dione
to give the corresponding C-glycosyl derivatives. After acetylation
and reduction of the carbonyl group, the compound was coupled to
2,3-di-O-acetyl adenosine using a phosphoramidite approach leading
to
analogues
of C-glycosylated ADP
D
-glycero-b-
D
-manno-heptose
(Scheme 19).
103
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