this siderophore is very weak.4d The present study de-
scribes the synthesis of two fluorescently labeled pyo-
chelins 1 and 2, which are suitable for further applications
in biological media. The 4-nitro-benzo[1,2,5]oxadiazole
(NBD) fluorophore was chosen according to its size and
photophysical properties in aqueous media.4a-c The NBD
and pyochelin were connected through two types of spacer
arm, a short succinic spacer arm, and a longer spacer which
minimized the steric hindrances with the proteins involved
in ferric pyochelin uptake. The two NBD-spacer arm
building blocks were prepared under the form of the two
pentafluorophenyl esters 3 and 4. These building blocks
were connected to the free amine 5 of the functionalized
pyochelin 6. Pyochelin analog 6 was synthesized starting
from nor-pyochelin 7 (Scheme 1).
used without further purification to synthesize the corre-
sponding Weinreb amide 10 using N,O-dimethylhydrox-
ylamine in the presence of EDCI. The hydroxamic ester 10
was then reduced to the corresponding aldehyde 11 using
LiAlH4.7 This aldehyde is very sensitive and was used
straight away without further purification. Condensation
with L-cysteine, in the presence of potassium acetate in a
hydroethanolic medium, led to the expected nor-pyochelin
7. The latter was converted into the functionalized pyoche-
lin 6 by coupling with the methanesulfonic acid 3-tert-
butoxycarbonyl-aminopropyl ester 12.8 The best results
for this reaction were obtained when the nor-pyochelin 7
and the linker precursor 12 were reacted in the presence
of K2CO3 and crown-ether[18.6] in acetonitrile at 80 °C
(Scheme 2). In these conditions, the expected compound
7 was cleanly isolated with a 62% yield. Other methods
using different bases (Cs2CO3, tertiary amines), solvents
(acetone, dioxane), or temperature conditions led to poor
yields of the desired product. The O-alkylation product on
phenol has never been isolated. This observation could be
due tothe presence of a hydrogen bond between the phenol
and thiazoline nitrogen.5a
Scheme 1. Structure and Retrosynthesis of Fluorescent
Conjuguates 1 and 2. Structure of Pyochelin (colored in red)
Scheme 2. Synthesis of the Functionalized Pyochelin 6
The two NBD-spacer arm building blocks 3 and 4 were
prepared in parallel. The corresponding N-hydroxysucci-
nimide ester for pentafluorophenyl ester 3 has been pre-
viously described in the literature.4b In our hands, this
compound was produced ata moderate yield and presented
technical difficulties when used in our approach (low stabi-
lity, lack of solubility, final product contamination with N-
hydroxysuccinimide). We then developed the synthesis of
the alternative building block 3. The tert-butyl-hemisucci-
nate 13 was converted into the corresponding pentafluoro-
phenyl ester 14, a white crystalline solid, stable and easy to
handle. This succinic diester 14 was reacted with piperazi-
nyl-NBD 15,4a and the tert-butyl protecting group of the
resulting adduct 16 was further cleaved in the presence
Although the thiazolidine nitrogen N300 of pyochelin
is involved in iron chelation,5 this atom can host a three-
carbon long aliphatic extension bearing a terminal amine
function. Other positions were shown to be crucial for either
metal chelation or biological recognition and were not
suitable for substitution.5 The synthesis of nor-pyochelin 7
started with the condensation of D-cysteine with the 2-
hydroxybenzonitrile 8 under buffered conditions in an hy-
dromethanolic medium.6 The resulting thiazoline 9 was
(5) (a) Tseng, C.-F.; Burger, A.; Mislin, G. L. A.; Schalk, I. J.; Yu, S.-
F.; Chan, S. I.; Abdallah, M. A. J. Biol. Inorg. Chem. (JBIC) 2006, 11,
419–432. (b) Schlegel, K.; Lex, J.; Taraz, K.; Budzikiewicz, H. Z.
Naturforsch. 2006, 61c, 263–266.
(6) (a) Bergeron, R. J.; Wiegand, J.; Dionis, J. B.; Egli-Karmakka,
M.; Frei, J.; Huxley-Tencer, A.; Peter, H. H. J. Med. Chem. 1991, 34,
2072–2078. (b) Zamri, A.; Abdallah, M. A. Tetrahedron 1999, 56, 249–
256.
(7) Fehrentz, J. A.; Castro, B. Synthesis 1983, 676–678.
(8) Brouwer, A. J.; Liskamp, R. M. J. Eur. J. Org. Chem. 2005, 487–
495.
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