Total synthesis of antigen Bacillus anthracis tetrasaccharide - Creation of an anthrax vaccine candidate

Article abstract of DOI:10.1002/anie.200502615

Fighting Anthrax by recognizing its coat: The spores of Bacillus anthracis bear unique oligosaccharides on their surface for interaction with a host. These specific oligosaccharides may prove useful in the development of vaccines against this major biowar

Full text of DOI:10.1002/anie.200502615

Angewandte
Chemie
Vaccines
DOI: 10.1002/anie.200502615
Total Synthesis of Antigen Bacillus Anthracis
Tetrasaccharide—Creation of an Anthrax Vaccine
Candidate**
Daniel B. Werz and Peter H. Seeberger*
A few letters containing a seemingly inconspicuous white
powder killed four innocent people, instilled fear in most
Americans for several weeks, and brought the US Postal
Service to the brink of collapse shortly after the events of
September 11, 2001. The white powder was identified as
dormant spores of the Gram-positive soil bacterium Bacillus
anthracis,[1,2] which are highly resistant to extreme temper-
atures, radiation, harsh chemicals, desiccation, and physical
damage. These properties allow them to persist in the soil for
many years.[3] These spores cause anthrax, a serious infection
of herbivores and cattle, but infects humans only rarely,
except when specially prepared and dispensed as biowarfare
agents. If the spores are inhaled, the host is usually killed
within days. Three polypeptides that comprise the anthrax
toxin play a major role in all stages of infection, from
germination to the induction of vascular collapse leading to
host death.[4]
Bacillus anthracis, like most bacteria, bears unique
oligosaccharides on the surface of the spore for interaction
with the host. Specific oligosaccharide antigens can be used to
design an antibacterial vaccine for the induction of an
immune response.[5] Carbohydrates are evolutionarily more
stable than proteins and have been exploited in a series of
commonly employed vaccines.[6] Synthetic oligosaccharide
vaccines have shown very encouraging results against
cancer,[7] malaria,[8] and Haemophilus influenzae type b[9] to
name just a few.
The structure of tetrasaccharide 1, which is found on the
surface of the exosporium glycoprotein BC1A of Bacillus
anthracis was elucidated in 2004 (Figure 1).[10] A unique
characteristic of this antigen is the nonreducing terminal
sugar, the so-called anthrose, which is not even found in
closely related species.[10] Tetrasaccharide1 is therefore a very
attractive target for vaccine development and the elucidation
[*] Dr. D. B. Werz, Prof. Dr. P. H. Seeberger
Laboratory for Organic Chemistry
Swiss Federal Institute of Technology (ETH) Zürich
ETH Hönggerberg, HCI F 315
Wolfgang-Pauli-Strasse 10, 8093 Zürich (Switzerland)
Fax: (+41)44-633-1235
E-mail: seeberger@org.chem.ethz.ch
[**] This research was supported by the ETH Zürich, by a Feodor Lynen
Research Fellowship of the Alexander von Humboldt Foundation,
and by an Emmy Noether Fellowship of the Deutsche Forschungs-
gemeinschaft (to D.B.W.). We thank Prof. Dr. B. Jaun and B.
Brandenberg for NMR measurements.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Figure 1. Structure of the terminal tetrasaccharide 1 of the major sur-
face glycoprotein of Bacillus anthracis and analogue 2 ready for conju-
gation.
Scheme 1. Synthesis of anthrose building block 8. Reagents and condi-
tions: a) Ac2O, pyridine, 12 h, quant.; b) MPOH, BF3·OEt2, acetone,
08C!258C, 12 h, 71%; c) NaOMe, MeOH, 6 h, quant.; d) 2,2-dime-
thoxypropane, BF3·OEt2, acetone, 08C!258C, 12 h, 96%; e) LevOH,
DMAP, DIPC, CH2Cl2, 08C, 3 h, 92%; f) HCl (pH 3), MeOH, 508C,
18 h, 85%; g) nBu2SnO, toluene, Dean–Stark apparatus, reflux, 2 h;
h) BnBr, TBAI, toluene, reflux, 3 h, 95% (two steps); i) Tf2O, pyridine,
08C, 90 min; j) NaN3, DMF, 258C, 10 h, 80% (two steps); k) CAN,
H2O/CH3CN, 258C, 1 h; l) Cl3CCN, NaH, CH2Cl2, 258C, 45 min, 78%
(two steps). MPOH=para-methoxyphenol, LevOH=levulinic acid,
DMAP=4-dimethylaminopyridine, DIPC=diisopropyl carbodiimide,
Bn=benzyl, TBAI=tetrabutylammonium iodide, CAN=cerium am-
monium nitrate.
of a highly specific immune response against Bacillus
anthracis.
Herein, we describe the first total synthesis of tetrasac-
charide 2 through a convergent [2+2] approach that facilitates
access to analogues and shorter sequences. The terminal
pentenyl group can serve as a point of attachment during
conjugation to a carrier protein in vaccine development. A
straightforward synthesis of the unique monosaccharide
anthrose is part of this total synthesis.
Synthesis of the terminal anthrose[10] started from com-
mercially available d-fucose (3) (Scheme 1).
Acetylation of 3, followed by immediate pro-
tection of the anomeric center with para-
methoxyphenol (MPOH) and subsequent cleav-
age of the acetates furnished 4. A levulinoyl
group proved to be the best choice to protect
the C2 hydroxy group during installation of the
b(1!3) glycosidic linkage in anticipation of its
selective removal prior to methylation of O2.
Thus, reaction of 4 with 2,2-dimethoxypropane
and introduction of the levulinic ester at C2
furnished 5. Removal of the isopropylidene and
tin-mediated selective benzylation of the hy-
droxy group at C3 afforded 6. The configuration
of C4 was inverted by reaction of the hydroxy
group with triflic anhydride to install a triflate,
which was displaced by sodium azide in an SN2-
type fashion to give 7.[11] Removal of the
anomeric p-methoxyphenyl group with wet
cerium ammonium nitrate was followed by the
formation of the anthrose trichloroacetimidate
8 by treatment with trichloroacetonitrile and a
catalytic amount of sodium hydride. A com-
pletely different, more lengthy approach to the
synthesis of an anthrose monosaccharide was
reported recently.[12]
Scheme 2. Synthesis of rhamnose building block 13. Reagents and conditions:
a) Ac2O, pyridine, 12 h, quant.; b) MPOH, BF3·OEt2, acetone, 08C!258C, 12 h, 80%;
.
c) NaOMe, MeOH, 12 h, 96%; d) 2,2-dimethoxypropane, BF3 OEt2, acetone, 08C!
258C, 12 h, 84%; e) NaH, BnBr, DMF, 08C!258C, 4 h, quant.; f) HCl (pH 3), MeOH,
508C, 89%; g) 1,1,1-triethoxyethane, p-TsOH (cat.), DMF, 508C, 50 min; h) AcOH/
H2O (4/1, v/v), 108C, 10 min, 98% (two steps); i) FmocCl, pyridine, 258C, 2 h, 88%;
j) CAN, H2O/CH3CN, 258C, 1 h, 76%; k) Cl3CCN, NaH, CH2Cl2, 258C, 1 h, 94%.
TsOH=para-toluenesulfonic acid, DMF=N,N-dimethylformamide, Fmoc=fluorenyl-
methoxycarbonyl.
Rhamnose building block 13, which is equipped with a
robust participating group at C2 to ensure a selectivity and a
readily removable temporary protecting group (Fmoc) at 3-
OH was synthesized next (Scheme 2). First, the anomeric
center was protected with a para-methoxyphenol group under
the conditions described above to give 10.[13] Formation of the
cis-fused acetal and subsequent benzylation afforded 11. T he
transformation of the acetal into the corresponding
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orthoester and ring opening resulted in the kinetically
preferred axial acetate in 12. The remaining hydroxyl function
was protected with an Fmoc group, and the para-meth-
oxyphenyl glycoside was cleaved. Subsequent reaction with
trichloroacetonitrile in the presence of traces of sodium
hydride afforded building block 13.
The assembly of the tetrasaccharide through a [2+2]
approach commenced with the reaction of known building
block 14[14] with 4-penten-1-ol (Scheme 3). The pentenyl
moiety serves at a later stage as a handle for conjugation to a
carrier protein in the preparation of the vaccine candidate.
Cleavage of the acetate at C2, further glycosylation with 13,
and subsequent removal of Fmoc yielded disaccharide 16.
During the cleavage of the Fmoc group, minor acetate
migration ( ꢀ 10%) from the 2-OH to the 3-OH group was
observed, but the undesired product was easily removed by
column chromatography.
The second disaccharide (Scheme 3B) was assembled by
glycosylation of rhamnose 12, an intermedi-
ate in the synthesis of building block 13, with
the anthrose unit 8. The levulinoyl group,
which ensured b selectivity, was replaced by
the final methoxy substituent at C2. The
methylation in the presence of acetate
proved to be challenging. Even powerful
methylating agents such as methyl triflate
and diazomethane failed to facilitate the
transformation. Satisfying yields were only
possible with MeI/Ag2O in the presence of
catalytic amounts of dimethyl sulfide. The
commonly used maneuver to convert the
methoxyphenyl glycoside into the corre-
sponding trichloroacetimidate furnished dis-
accharide unit 18.
To complete the total synthesis, the two
disaccharide units 16 and 18 were coupled to
afford tetrasaccharide 19 (Scheme 4).
Sodium in liquid ammonia removed all
permanent protecting groups and trans-
formed the azide moiety into an amine, thus
achieving global deprotection. The formation
of the amide with 3-hydroxy-3-methylbuta-
noic acid under peptide-coupling condi-
tions[15] led to tetrasaccharide 2, whose struc-
ture was confirmed by comprehensive spec-
troscopic analysis and comparison with the
reported analytical data for 1.
In conclusion, we have reported a con-
vergent total synthesis of a Bacillus anthracis
tetrasaccharide antigen ready for conjugation
Scheme 3. Syntheses of disaccharide building blocks 16 (A) and 18 (B). Reagents and
conditions: a) 4-pentenol, TMSOTf, CH2Cl2, ꢁ208C, 45 min, 79%; b) NaOMe, MeOH,
4 h, 96%; c) 13, TMSOTf, CH2Cl2, 08C, 1 h, 91%; d) piperidine, DMF, 258C, 30 min,
89%; e) 8, TMSOTf, CH2Cl2, 08C, 1 h, 90%; f) hydrazinium acetate, CH2Cl2, MeOH,
258C, 12 h, quant.; g) MeI, Ag2O, THF, Me2S (cat.), 258C, 8 h, 73%; h) CAN, H2O/
CH3CN, 258C, 1 h; i) Cl3CCN, NaH, CH2Cl2, 258C, 1 h 95% (two steps). TMSOTf=tri-
methylsilyl trifluromethanesulfonate.
Scheme 4. Completion of the total synthesis. Reagents and conditions: a) TMSOTf, CH2Cl2, 08C, 70 min, 73%; b) Na/NH3(l), THF, ꢁ788C, 60%;
c) 3-hydroxy-3-methylbutanoic acid, HATU, DIPEA, DMF, 258C, 2 h, 75%. HATU=N-[(dimethylamino)-1H-1,2,3-triazolo[4,5-b]pyridin-1-ylmethyl-
ene]-N-methylmethanaminium hexafluorophosphate, DIPEA=diisopropylethylamine.
Angew. Chem. Int. Ed. 2005, 44, 6315 –6318
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Communications
to carrier proteins. Immunological studies as well as the
preparation of derivatives are currently under investigation.
Received: July 26, 2005
Published online: September 19, 2005
Keywords: anthrose · carbohydrates · oligosaccharides · total
.
synthesis · vaccines
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