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HO
H
HO
HO
H
HO
HO
H
HO
R-O-NH
2
O
OH
OH
O
H
N
H
OH
OR
18
F
18
F
18
F
1
95
NH
HO
H
HO
OH
N
HN
NH
2
N
O
O
18
F
O
O
NH
HN
96
R' = OH
97
R' = H
R'
O
NH
HN
N
O
O
COOH
SCHEME 7.20
Oxime formation as a click coupling between FDG and a cyclic RGD
peptide.
7.4.3 Glycosylation
Although not yet considered as a click chemistry method, glycosylation of sugar has
been envisioned in radiochemistry. The construction of glucosylated 2-nitroimidazole
has been reported [95]. 2-nitroimidazole derivatives are considered as markers of
hypoxia. Under hypoxia, they are reduced and the resulting species bind to macro-
molecules of cells and then accumulate in hypoxic cells. This cell status is crucial
in cancer treatment, oxygen-deprived tumor cells being less reactive to treatment,
and the noninvasive detection of hypoxia is still a challenge. In a practical way, the
N
-glycosylation of imidazole with cold tetra-
O
-acetyl-2-deoxy-2-fluoro-glucose was
achieved in 36% yield in acetonitrile at 60
◦
C for 3 hours in the presence of SnCl
4
and
Hg(CN)
2
. When using the
18
F analog
98
, a higher yield of
99
(80%) was obtained in
60 minutes. (Scheme 7.21)
The direct glycosylation of the free hydroxyl of serine or threonine by the widely
used acetylated [
18
F]FDG
98
under the Lewis acid catalyst has been reported
O
2
N
N
NO
2
AcO
Ac
AcO
AcO
Ac
AcO
N
N
O
O
OAc
SnCl
4
N
18
F
18
F
Hg(CN)
2
CH
3
CN, 60 °C
9
9
9
SCHEME 7.21
Glycosylation as a click coupling reaction between FDG and a nitro-
imidazole.
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