Synthesis of the trans-fusarinine scaffold

Article abstract of DOI:10.1016/j.tetlet.2010.02.058

The trans-fusarinine backbone is a common feature encountered in many fungal siderophores. This monomer is notably the structural base of N^α-methyl coprogen B and dimerumic acid. Both siderophores are known to be secreted by Scedosporium apiosp

Full text of DOI:10.1016/j.tetlet.2010.02.058

Tetrahedron Letters 51 (2010) 2119–2122
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Tetrahedron Letters
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Synthesis of the trans-fusarinine scaffold
a
b
a
Samuel Bertrand a, Olivier Duval a, , Jean-Jacques Hélesbeux , Gérald Larcher , Pascal Richomme
*
a Laboratoire des Substances d’Origine Naturelle et Analogues Structuraux, UPRES-EA 921, IFR 149 QUASAV, UFR des Sciences Pharmaceutiques et Ingénierie de la Santé,
Université d’Angers, 16 Bd Daviers, 49045 Angers Cedex, France
b Groupe d’Etude des Interactions Hôte–Parasite, UPRES-EA 3142, IFR 132, UFR des Sciences Pharmaceutiques et Ingénierie de la Santé, Université d’Angers, 16 Bd Daviers,
49045 Angers Cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history:
The trans-fusarinine backbone is a common feature encountered in many fungal siderophores. This
monomer is notably the structural base of N -methyl coprogen B and dimerumic acid. Both siderophores
are known to be secreted by Scedosporium apiospermum, an emerging pathogenic fungus studied for its
high involvement in invasive infections of immunocompromised patients. The strategy developed here
for the synthesis of the trans-fusarinine scaffold relies on the preparation of both N-hydroxyornithine
a
Received 27 December 2009
Revised 7 February 2010
Accepted 10 February 2010
Available online 13 February 2010
and 3-anhydroxymevalonic acid subunits starting from
coupling of these two building blocks led to the expected protected backbone.
L-ornithine and 3-butyn-1-ol, respectively. The
Keywords:
Siderophore
Fusarinine
Ó 2010 Elsevier Ltd. All rights reserved.
Multi-step synthesis
Iron is an essential nutriment for all eukaryotes and almost all
prokaryotes as it is required in a large variety of fundamental met-
abolic and informational cellular pathways.1 Despite being one of
the most abundant elements in the Earth’s crust, the bioavailability
of iron either in many environments (soil or sea) or in the human
hosts is limited by the very low solubility of ferric ion. To circum-
vent this problem, many microorganisms synthesize, secrete and
utilize very specific high-affinity iron uptake systems called sidero-
phores.2 These species, known to be amongst the strongest binders
to Fe3+ ion, are able to withdraw iron from human protein–iron
complexes, solubilize and transport it back to the cells. Because
of this high binding affinity, siderophores have attracted much
interest as they can be potentially used for a wide variety of ther-
apeutic applications such as iron detoxification agent, removal of
excess iron resulting from the supportive therapy for b-thalassae-
mia, antioxidant, antimicrobial and anticancer drugs.3
nized as the main building block of various siderophores, mainly
secreted by fungi, such as coprogens, ferrichromes and obviously
the different members of the fusarinine family (Fig. 1).7,8
As part of a previous study from our group on Scedosporium
apiospermum, an emerging opportunistic fungus, we have shown
a
that two siderophores are secreted under iron stress: N -methyl
coprogen B (2) and dimerumic acid.9 In order to study their phys-
icochemical properties and their interactions with the human host,
we decided to prepare quantities of these iron-binders. Thus, we
wanted first to focus on the development of a synthesis of their
common structural monomer, the trans-fusarinine scaffold.
trans-Fusarinine 1 is a small complex moleculebearing four differ-
ent functions: a carboxylic acid, an amine, a primary alcohol and a
hydroxamate. Moreover, prior to embarking on this synthesis, two
critical points have to be highlighted: the configuration of both the
double bond and the asymmetric carbon 2 of the fusarinine chain.
The key step of our retrosynthetic approach, depicted in Fig-
ure 2, consisted in coupling two subunits: (3E)-anhydroxymeva-
lonic acid (5) and N-hydroxyornithine (6), respectively, obtained
Siderophores can be divided into three main classes depending
on the chemical nature of the moieties donating the oxygen ligands
for Fe3+ coordination, which are either phenolates/catecholates,
hydroxamates, or (a-hydroxy)-carboxylates.
from 3-butyn-1-ol (7) and L-ornithine (8). Regarding our strat-
From a chemical point of view, a wide range of natural myco-
bactins and related analogues with an hydroxamate function has
already been prepared.4 Nevertheless, to the best of our knowl-
edge, examples of syntheses of hydroxamate-based siderophores
also bearing a (3E)-anhydroxymevalonic acyl chain are quite rare.5
This specific chain, coupled with N5-hydroxyornithine appendage,
gives trans-fusarinine 1.6 This particular siderophore can be recog-
egy, appropriate protecting groups have to be chosen to allow
a specific cleavage of each one. Carboxylic acid and N -amine
a
functions of N-hydroxyornithine (6) were converted to t-butyl
ester and t-butylcarbamate groups whereas the primary alcohol
function of (3E)-anhydroxymevalonic acid (5) was protected with
t-butyldiphenylsilyl chloride. In this case other protecting groups
such as tetrahydropyranyl ether and tosylate were investigated.
None proved to be successful with our approach.
The primary alcohol 9 was protected according to Nicolaou et al.
* Corresponding author. Tel.: +33 (0) 2 41 22 66 01; fax: +33 (0) 2 41 22 66 34.
E-mail address: olivier.duval@univ-angers.fr (O. Duval).
procedure (Scheme 1).10 Then 4-(t-butyldiphenylsilyloxy)-but-1-
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.02.058

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S. Bertrand et al. / Tetrahedron Letters 51 (2010) 2119–2122
O
O
O
O
O
O
O
HO
N
OH
OH
N
HN
N
O
N
OH
NH2
OH
1
HO
NH
OH
2
NH
OH
O
O
O
H
N
OH
O
O
N
O
HO
H
O
OH
O
O
O
N
HO
O
NH
N
H
N
O
N
O
HN
H
O
O
OH
N
OH
O
NH
H
N
O
O
OH
N
H
O
O
O
O
N
O
N
OH
HN
O
OH
4
3
a
Figure 1. Structures of siderophores based on the trans-fusarinine scaffold (1): N -methyl coprogen B (2), N,N0,N00-triacetylfusarinine C (3) and asperchrome B1 (4).
OH
N
NH2
OH
O
be explained by the steric interactions of the introduced thio-
phenyl appendage with both the silyl-protected hydroxyethyl
chain and the ester function. To confirm this mechanistic hypoth-
esis, we showed that the use of a bulkier ester group such as fluore-
nylmethyl induced the formation of the Z -isomer as the only
product. Unfortunately, we were unable to achieve properly the
next reaction step with this protected ester. Therefore, we kept
working with the methyl ester 11. Separation of the two isomers
of 12 was easily achieved by column chromatography. The thio-
phenyl group of 12 was converted to a methyl group using freshly
prepared methyl copper bromide. The basic hydrolysis of ester 13
was achieved quantitatively using lithium hydroxide affording the
expected (3E)-anhydroxymevalonic acid 14.12,13 The double bond
configuration of compounds 12, 13 and 14 was systematically
assessed by NOESY NMR spectrometry (Fig. 3).
HO
O
1
OH
HN
NH2
OH
HO
OH
+
O
O
5
6
8
NH2
HO
H2N
OH
O
7
Figure 2. Retrosynthetic approach for trans-fusarinine 1.
The synthesis of the N-hydroxy-
mimetic hypothesis, was carried out by oxidizing the terminal
amine of
-ornithine (Scheme 2).14,15 In order to cleave separately
the protecting groups of the two amine functions, commercial
N2-Boc-N5-carbobenzyloxy-
-ornithine was chosen as the starting
L-ornithine (18), based on bio-
yne (10) was converted to ester 13, bearing a E-double bond, in
three steps with a 47% overall yield using Hollowood et al. strat-
egy.11 Following a basic deprotonation with LDA, 10 underwent a
carbanionic attack of methyl chloroformate to yield the unsatu-
rated ester 11. A [1–4] Michael-type addition of thiophenolate on
the alkyne bond afforded the unsaturated ester 12. Both Z and E
configurations were obtained for the resulting double bond. For-
mation of the Z-isomer as the major product (Z/E ratio of 4.8) might
L
L
material (15).16 The use of this protected naturally occurring amino
acid was essential as it gave us the opportunity to introduce the
chiral centre with the expected configuration found in the final
trans-fusarinine structure. All along our multi-step synthesis, our
attention has been mainly focused on the retention of this stereo-
chemical parameter. Then amino ester 16 was synthesized using
dimethylformamide diterbutylacetal under microwave radiation.17
During this reaction the methyl ester is unexpectedly formed with
a 55% yield. Then the pallado-catalyzed cleavage of the benzylcar-
bamate group (Z) under hydrogen atmosphere was almost quanti-
tative. The oxidation of the free terminal amine function with
benzoyl peroxide eventually led to appropriate N2-benzylcarbonyl-
tBDPS
tBDPS
O
OH
a
O
b
O
O
9
10
11
12
c
oxy-L
-ornithine (18).18 This oxidation procedure, previously
reported by Berman and Johnson,19 allowed us to obtain the oxi-
dized product only under anhydrous conditions. Usual biphasic
conditions gave rise to the acetylated amine as a side product.20
Using this strategy, 18 was obtained with a 21% overall yield over
three steps.
O
O
d
O
S
tBDPS
O
O
tBDPS
O
O
13
e
To synthesize the desired trans-fusarinine 19, a coupling reac-
tion had to be achieved with 18 and 14. Thionyl chloride in the
presence of an highly substituted base such as diisopropylethyl-
amine was the first procedure used to activate the carboxylic acid
derivative 14 (Scheme 3).21 Albeit with a low yield (4%), this proce-
dure allowed us to isolate and characterize 19 for the first time.
Coupling agents such as DCC and EDCI were investigated
unsuccessfully.
tBDPS
HO
O
14
Scheme 1. Synthesis of (3E)-anhydroxymevalonic acid appendage 14. Reagents and
conditions: (a) tBDPS-Cl, imidazole, p-dimethylaminopyridine, CH2Cl2, 0 °C, 30 min,
99%; (b) (1) LDA, THF, (2) methyl chloroformate, ꢀ78 °C to rt, overnight, 83%; (c)
thiophenol, sodium methanolate (cat.), MeOH, rt, overnight, 58%; (d) MeCuBr, THF,
ꢀ78 °C to rt, 99%; (e) LiOH 2 M, t-BuOH, 100%.

S. Bertrand et al. / Tetrahedron Letters 51 (2010) 2119–2122
2121
C(CH3)3
CH3
OCH3
aromatic
protons
CH2 (allylic)
CH
CH2O
(allylic)
(vinylic)
O
O
Si
O
C(CH3)3
CH3 (allylic)
CH2 (allylic)
13
OCH3
CH2O
CH (vinylic)
aromatic protons
8
6
4
2 F2 [ppm]
Figure 3. NOESY NMR spectrum of the unsaturated ester 13.
t-Bu
Boc NH OH
O
mechanism known to be mediated by secreted iron chelators. Di-
mers built from a siderophore and a fluorescent marker should also
be accessible using the synthetic approach described herein.24
Such probe-adducts would also be of a great interest to further
understand the specific mechanisms developed by fungi such as
Boc NH O
O
a
c
HN
HN
Z
Z
15
16
b
t-Bu
Aspergillus fumigatus,25 Histoplasma capsulatum26 and
apiospermum.9
S.
Boc NH O
O
t-Bu
Boc NH O
O
HN
O
O
H2N
Acknowledgements
18
17
During his Ph.D., Samuel Bertrand was recipient of a grant from
‘Conseil Général du Maine et Loire, France’. We thank S. Fournier
and M. Lagree from the ‘Plateforme d’Ingénierie et Analyses Molécul-
aires’ (PIAM) of the University of Angers for the MS measurements.
Scheme 2. Synthesis of N-hydroxyornithine subunit 18. Reagents and conditions:
(a) DMF–DBA, toluene, MW, 160 °C, 10 min, 45%; (b) H2, Pd/C 10%, 99%; (c) benzoyl
peroxide, DMF, rt, 1.5 h, 47%.
References and notes
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Boc
O
NH
NH O
HO
O
t-Bu O
+
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O
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14
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O
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7.4 (m, 6H), 5.73 (s, 1H), 3.80 (t, J = 6.3 Hz, 2H), 2.42 (t, J = 6.3 Hz, 2H), 2.13 (s,
3H), 1.06 (s, 9H); 13C NMR (270 MHz, CDCl3) 171.2, 159.9, 135.6, 133.6, 129.7,
127.7, 116.8, 61.8, 44.0, 26.8, 19.3, 19.2; MS (ESIꢀ) 366.93[MꢀH]ꢀ.
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19
Scheme 3. Synthesis of protected trans-fusarinine 19. Reagents and conditions:
SOCl2, iPr2EtN, CH2Cl2, 4%.
We reported here an original strategy for the total synthesis of
trans-fusarinine. So far, we described an advanced protected pre-
cursor of this scaffold. As the final coupling stage has to be im-
proved in terms of yield, we are currently investigating other
coupling reaction alternatives.22 Moreover, this strategy should
also be adapted for the synthesis of the cis-fusarinine scaffold. Thus
addition of dimethyl lithium cuprate to the unsaturated ester 11
should afford the (Z) isomer of 13 as a key subunit of cis-
fusarinine.23
The yield of the final coupling reaction has to be improved to
turn this route into a general synthetic pathway to many major
fungal siderophores so far not prepared using the usual peptide-
chemistry strategies. These derivatives and their synthetic ana-
logues will be the essential tools to study the iron acquisition
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18. Data for amino acid 18: 1H NMR (270, CDCl3) 7.81 (d, 2H, 6.93 Hz), 7.44 (m, 3H),
6.72 (m, 1H), 5.20 (d, 1H, 7.92 Hz), 4.21 (m, 1H), 3.55 (m, 2H), 1.9–1.6 (m, 4H),
1.47 (s, 9H), 1.45 (s, 9H); 13C NMR (270, CDCl3) 171.6, 167.5, 157.9, 134.6,
131.3, 128.5, 127.0, 82.2, 79.9, 53.3, 39.3, 30.8, 28.3, 28.0, 25.2; [a]D
(MeOH) = ꢀ18.4.
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21. Data for protected fusarinine 19: 1H NMR (270 MHz, CDCl3): 7.71 (m, 3H), 7.64
(m, 4H), 7.40 (m, 8H), 5.74 (s, 1H), 5.36 (s, 1H), 4.16 (m, 1H), 3.80 (t, J = 6.4 Hz,
2H), 2.89 (m, 2H), 2.39 (t, J = 6.3 Hz, 2H), 2.12 (s, 3H), 1.9–1.6 (m, 4H), 1.26 (s,
18H), 1.05 (s, 9H); MS (ESIꢀ) 793.41 [M+Cl]ꢀ.
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