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OH
Bn
3
N
N
N
N
N
N
N
N
N
N
N
Bn
N
N
N
3
OH
N
N
NN
NN
Bn
HO
3
TBTA
THPTA
N
N
N
N
N
N
N
N
N
N
CO
2
H
OSO
3
N
N
N
N
N
N
NN
NN
BTTES
BTTAA
FIGURE 8.3
Various chelating ligands for biocompatible CuAAC.
Click-activated fluorogenic probes have been developed to address the problem
of background labeling (Scheme 8.4) [59]. In this approach, the click product gen-
erates a strong detectable signal whereas the starting materials remain traceless. The
fluorogenic probe design was based on 4-amino-1,8-naphthalimide, which absorbs
light in the visible region and emits at long wavelengths (
max
540-550 nm) [59]. It
was postulated that the fluorescent signal of such a compound could be modulated by
the formation of a triazole ring because electron-donating groups at the C-4 position
were known to strongly affect fluorescent properties. Indeed, a 1,8-naphthalimide
derivative, which has an azido moiety at the C-4 position, has a very low fluorescence
O
O
N
O
OR
N
OR
Cu(I)
O
+
N
OH
O
OH
O
OH
N
OH
N
HO
HO
N
3
6-alkynyl-Fuc
non-fluorescent
fluorescent
SCHEME 8.4
Fluorescence of 4-azido-1,8-naphthalimide is quenched by the azido moiety.
Upon formation of a triazole, the derivative becomes fluorescent.
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