Efficient synthesis of the siderophore petrobactin via antimony triethoxide mediated coupling

Article abstract of DOI:10.1016/j.tetlet.2012.01.074

Chemical synthesis of petrobactin, a siderophore for Bacillus anthracis, has been achieved via Sb(OEt)₃-mediated ester-amide exchange.

Full text of DOI:10.1016/j.tetlet.2012.01.074

Tetrahedron Letters 53 (2012) 1627–1629
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Efficient synthesis of the siderophore petrobactin via antimony triethoxide
mediated coupling
⇑
⇑
Rajesh K. Pandey , Gregory G. Jarvis, Philip S. Low
Department of Chemistry, Purdue University, 560 Oval Dr., West Lafayette, IN 47907, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Chemical synthesis of petrobactin, a siderophore for Bacillus anthracis, has been achieved via Sb(OEt)3-
mediated ester–amide exchange.
Received 23 December 2011
Revised 17 January 2012
Accepted 18 January 2012
Available online 28 January 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Petrobactin
Siderophore
Antimony triethoxide
Bacillus anthracis
Iron acquisition
Ester–amide exchange
Introduction
the iron–siderophore complexes for their cognate bacterial recep-
tors are similarly high.6–9
Infections caused by Bacillus bacteria are often life threatening
and consequently remain a worldwide health concern. Bacillus
anthracis and Bacillus cereus are two such Bacillus species that re-
main in the public eye, because they can emerge in the human
population via transmission from animals, contaminated food, or
bioterrorism.1,2
Bacillus species, like most other pathogenic bacteria, have devel-
oped a mechanism to obtain the iron required for their prolifera-
tion by releasing an iron chelating agent called a siderophore
into their environment. After iron chelation, the resulting iron–
siderophore complex is retrieved by association with a sidero-
phore-specific receptor expressed on the microbial cell surface,
which then internalizes and delivers the ferric chelate complex
into the cell cytoplasm. Because ferric iron is essential for bacterial
survival,3 the human immune system attempts to suppress bacte-
rial growth by sequestering all extracellular iron immediately upon
detection of bacterial components.4,5 Survival of the pathogen in its
host thus depends on the pathogen’s ability to outcompete the host
for extracellular iron. Not surprisingly, the affinities of most sidero-
phores for iron are in the subpicomolar range, and the affinities of
Because of the high affinities and specificities of siderophores
for their cognate pathogens,10 siderophore–antibiotic conjugates
have been recently exploited as a tool for the selective killing of
the source pathogen.11–13 Bacillus species, including B. anthracis
and B. cereus, recognize the 3,4-catecholate siderophore called pet-
robactin (Fig. 1).14 Petrobactin–iron chelate complexes in turn bind
cell surface receptor-like proteins, FpuA, FatB, and YclQ, and are
transported into the bacteria.15,16 In order to examine whether pet-
robactin might be similarly exploited for the selective therapy of
Bacillus infections, we sought to synthesize the siderophore in
quantifiable amounts. While some chemical and enzymatic syn-
theses have been established in the literature,7,17–20 they have pro-
ven to be inadequate in providing sufficient yield for subsequent
use in large scale combinatorial screening for therapeutic use.
Recently, high-yielding Sb(OEt)3-mediated ester–amide transfor-
mation has been outlined in the literature,21–23 prompting us to
apply the same coupling conditions for the synthesis of petrobac-
tin. Herein, we report Sb(OEt)3-mediated coupling enables the high
yield production of preclinical quantities of petrobactin
(Scheme 1–3).
Results and discussion
⇑
Corresponding authors. Tel.: +1 765 494 4395; fax: +1 765 494 5272 (R.K.P.);
Total synthesis of petrobactin, 1, begins with the synthesis of
intermediate 6, which was prepared in 3 steps, starting from a
commercially available N-dibenzyl-aminoalcohol, 2 (Scheme 1).
tel.: +1 765 494 5273; fax: +1 765 494 5272 (P.S.L.).
E-mail addresses: rkpandey1@yahoo.com (R.K. Pandey), plow@purdue.edu
(P.S. Low).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2012.01.074

1628
R. K. Pandey et al. / Tetrahedron Letters 53 (2012) 1627–1629
O
O
O
HO
H
H
HO
HO
OH
OH
N
N
N
N
N
N
H
H
H
H
O
OH
O
Figure 1. Structure of petrobactin, 1.
NHBoc
Formation of a diamine byproduct was observed in 5–10% abun-
dance during this nucleophilic substitution reaction. Debenzyla-
tion was quantitatively achieved under H2-Pd/C conditions in
ethanol to yield 6. Separately, compound 9 was prepared from
commercially available 3,4-dihydroxybenzoic acid in 2 steps with
an 85% yield (Scheme 2). Esterification of 7 was achieved using
H2SO4 and methanol under reflux conditions, followed by benzyl
protection of the catechol unit without any purification.
Ester–amide exchange was carried out with amine 6 and acid 9
under neat conditions using Sb(OEt)3 to produce 10 with a 75%
yield. The N-Boc group was removed from 10 by treatment with
trifluoroacetic acid (TFA) to give amine 11 as a TFA salt. A separate
component for the coupling reaction, 13, was prepared as an acti-
vated NHS ester by a reported literature procedure.24
H2N
a
4
Bn
Bn
Bn
Bn
N
N
N
OMs
OH
b
2
3
Bn
Bn
NHBoc
NHBoc
N
H
c
H2N
N
H
5
6
Scheme 1. Reagents and conditions: (a) MsCl, TEA, DCM, 0 °C to rt, 3 h, 85%; (b) 4
(1.5 equiv), DIPEA (15 equiv), DMSO, 70 °C, 24 h, 75%; (c) Pd/C, H2, EtOH, reflux 4 h,
95%.
The hydroxyl group was mesyl (Ms) protected to provide com-
pound 3 with an 85% yield. Nucleophilic displacement of O-Ms
with amine, 4, provided 5 with a 75% yield of the isolated product.
O
O
O
HO
HO
OH
a
BnO
BnO
b
OMe
OMe
HO
HO
7
8
9
Scheme 2. Reagents and conditions: (a) MeOH, H2SO4 (cat), reflux 8 h, quantitative; (b) K2CO3 (6 equiv), BnBr (4 equiv), DMF, 120 °C, 36 h, 85%.
O
a
b
BnO
BnO
NHBoc
N
H
N
H
6 + 9
10
O
BnO
BnO
NH
O
2
N
H
N
H
TFA salt
11
O
N
O
O
HO
4 steps
O
O
OH
HO
N
O
c
OH
12
O
O
O
O
OH
13
O
O
O
O
O
O
H
N
H
N
BnO
BnO
OBn
OBn
N
H
N
H
N
H
N
H
O
OH
O
14
d, e
O
O
O
HO
H
N
H
N
HO
HO
OH
OH
N
H
N
H
N
H
N
H
O
O
OH
1
Scheme 3. Reagents and conditions: (a) Sb(OEt)3 (1 equiv), neat, 80 °C, 24 h, 75%; (b) TFA, DCM, 0 °C, 0.5 h, 93%; (c) 13 (1 equiv), TEA, dioxane, DCM, 1.5 h, 82%; (d) TFA, DCM/
H2O (100:1), 0 °C to rt, 2 h; (e) H2, PdCl2, AcOH/H2O (20:1), 2 h, 65%.

R. K. Pandey et al. / Tetrahedron Letters 53 (2012) 1627–1629
1629
4. Park, C. H.; Valore, E. V.; Waring, A. J.; Ganz, T. J. Biol. Chem. 2001, 276, 7806–
7810.
5. Peyssonnaux, C.; Zinkernagel, A. S.; Datta, V.; Lauth, X.; Johnson, R. S.; Nizet, V.
Blood 2006, 107, 3727–3732.
6. Reid, R. T.; Livet, D. H.; Faulkner, D. J.; Butler, A. Nature 1993, 366, 455–458.
7. Lee, J. Y.; Janes, B. K.; Passalacqua, K. D.; Pfleger, B. F.; Bergman, N. H.; Liu, H.;
Hakansson, K.; Somu, R. V.; Aldrich, C. C.; Cendrowski, S.; Hanna, P. C.;
Sherman, D. H. J. Bacteriol. 2007, 189, 1698–1710.
8. Zawadzka, A. M.; Abergel, R. J.; Nichiporuk, R.; Andersen, U. N.; Raymond, K. N.
Biochemistry 2009, 48, 3645–3657.
9. Abergel, R. J.; Zawadzka, A. M.; Raymond, K. N. J. Am. Chem. Soc. 2008, 130,
2124–2125.
10. Diarra, M. S.; Lavoie, M. C.; Jacques, M.; Darwish, I.; Dolence, E. K.; Dolence, J.
A.; Ghosh, A.; Ghosh, M.; Miller, M. J.; Malouin, F. Antimicrob. Agents Chemother.
1996, 40, 2610–2617.
11. Miller, M. J.; Malouin, F. Acc. Chem. Res. 1993, 26, 241–249.
12. Wencewicz, T.; Möllmann, U.; Long, T.; Miller, M. Biometals 2009, 22, 633–648.
13. Roosenberg, J. M., 2nd; Lin, Y. M.; Lu, Y.; Miller, M. J. Curr. Med. Chem. 2000, 7,
159–197.
14. Barbeau, K.; Zhang, G.; Live, D. H.; Butler, A. J. Am. Chem. Soc. 2001, 124, 378–
379.
15. Zawadzka, A. M.; Kim, Y.; Maltseva, N.; Nichiporuk, R.; Fan, Y.; Joachimiak, A.;
Raymond, K. N. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 21854–21859.
16. Carlson, P. E., Jr.; Dixon, S. D.; Janes, B. K.; Carr, K. A.; Nusca, T. D.; Anderson, E.
C.; Keene, S. E.; Sherman, D. H.; Hanna, P. C. Mol. Microbiol. 2010, 75, 900–909.
17. Gardner, R. A.; Kinkade, R.; Wang, C.; Phanstiel, O. t. J. Org. Chem. 2004, 69,
3530–3537.
18. Bugdahn, N.; Peuckert, F.; Albrecht, A. G.; Miethke, M.; Marahiel, M. A.;
Oberthür, M. Angew. Chem. Int. Ed. 2010, 49, 10210–10213.
19. Bergeron, R. J.; Huang, G.; Smith, R. E.; Bharti, N.; McManis, J. S.; Butler, A.
Tetrahedron 2003, 59, 2007–2014.
20. Oves-Costales, D.; Song, L.; Challis, G. L. Chem. Commun. 2009, 1389–1391.
21. Ishihara, K.; Kuroki, Y.; Hanaki, N.; Ohara, S.; Yamamoto, H. ChemInform. 1996,
27.
22. Kuroki, Y.; Ishihara, K.; Hanaki, N.; Ohara, S.; Yamamoto, H. ChemInform. 1998,
29.
Coupling of 11 and 13 was achieved using triethylamine (TEA)
to give protected petrobactin, 14, with an 82% conversion. The t-
butyl protecting group was removed from the citric acid moiety
of compound 14 under TFA conditions. After completion, the reac-
tion mixture was concentrated using high vacuum without heating
to avoid intramolecular imine formation between amide bonds and
tertiary acid groups of citric acid moieties.17,24 14 was debenzylat-
ed using H2-PdCl2 conditions to provide petrobactin, 1, with a 65%
yield (Scheme 3). The physical and spectroscopic data of petrobac-
tin were in full agreement with the literature data.17
Conclusion
We have shown the application of antimony triethoxide-medi-
ated ester–amide exchange for the chemical synthesis of petrobac-
tin from commercially available starting materials in 8 steps with
good overall yield (22.5%).
Acknowledgment
This work was financially supported by Department of Defense
(Contract No. W9 11SR-08-C-0001).
References and notes
1. Helgason, E.; Okstad, O. A.; Caugant, D. A.; Johansen, H. A.; Fouet, A.; Mock, M.;
Hegna, I.; Kolsto, A. B. Appl. Environ. Microbiol. 2000, 66, 2627–2630.
2. Koppisch, A. T.; Dhungana, S.; Hill, K. K.; Boukhalfa, H.; Heine, H. S.; Colip, L. A.;
Romero, R. B.; Shou, Y.; Ticknor, L. O.; Marrone, B. L.; Hersman, L. E.; Iyer, S.;
Ruggiero, C. E. Biometals 2008, 21, 581–589.
23. Sakakura, A.; Umemura, S.; Ishihara, K. Chem. Commun. 2008, 3561–3563.
24. Guo, H.; Naser, S. A.; Ghobrial, G.; Phanstiel J. Med. Chem. 2002, 45, 2056–2063.
3. Schaible, U. E.; Kaufmann, S. H. E. Nat. Rev. Microbiol. 2004, 2, 946–953.