Towards the synthesis of a Yersinia pestis cell wall polysaccharide: Enantioselective synthesis of an l-glycero-d-manno-heptose building block

Article abstract of DOI:10.1039/c000784f

A short and enantioselective de novo synthesis of an l-glycero-d-manno- heptose building block for the total synthesis of a Yersinia pestis cell wall polysaccharide is described.

Full text of DOI:10.1039/c000784f

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Towards the synthesis of a Yersinia pestis cell wall polysaccharide:
enantioselective synthesis of an L-glycero-D-manno-heptose building
blockw
ab
Takafumi Ohara,z Alexander Adibekian,yab Davide Esposito,ab Pierre Stallforthab and
Peter H. Seeberger*ab
Received 12th January 2010, Accepted 17th April 2010
First published as an Advance Article on the web 7th May 2010
DOI: 10.1039/c000784f
A short and enantioselective de novo synthesis of an L-glycero-D-
manno-heptose building block for the total synthesis of a
Yersinia pestis cell wall polysaccharide is described.
aldol product to obtain ketone 7 in multi-gram quantities
(Scheme 2).6 Reduction of 7 with L-selectride resulted in
1,3-migration of the TBS-group to yield regioisomer 8 in
79% yield. The free hydroxyl was subsequently protected as
para-bromobenzyl (PBB) ether, a protecting group orthogonal
to benzyl ethers.7 The temporary silyl group was then
exchanged for a benzyl group to afford bisacetal 11.
The plague is re-emerging as cases have been reported in more
than 20 countries in Africa and Asia.1 Yersinia pestis is the
causative agent of the plague, an infection of rodents accidentally
transmitted to humans by the bite of infected fleas. The disease
is manifested in bubonic and pneumonic forms and constitutes
a potential bioterrorism threat. The need for vaccines against
Yersinia pestis is becoming urgent.
Treatment with CSA and methanol resulted in the selective
deprotection of the ketal. The resulting primary hydroxyl was
protected with TBDPS-Cl and the secondary hydroxyl of 12
was masked as TBS-ether to afford fully protected dimethyl-
acetal 13. The dimethylacetal was smoothly hydrolyzed using
a catalytic amount of para-toluenesulfonic acid in acetone to
yield aldehyde 3. A second aldol reaction was performed using
silyl enolether 4 and MgBr2ꢀOEt2 as chelating activator.4c,d
Under these conditions the aldol product 14 was formed in
moderate yield but excellent 2,3-anti-3,4-syn-selectivity, as
determined by analysis of coupling constants at a later stage
of the synthesis.11 Treatment with trifluoroacetic acid resulted
in cleavage of the TBS-group and concomitant lactonization
to alcohol 15 as the major product.
As a first step toward a synthetic carbohydrate vaccine we
are pursuing the synthesis of the core pentasaccharide of the
cell wall lipopolysaccharide (LPS) extracted from different
bacterial strains of Yersinia pestis (Fig. 1).2 The trisaccharide
part is entirely composed of a-linked L-glycero-D-manno-
heptoses and is synthetically most challenging. Several syntheses
of L-glycero-D-manno-heptose3 based on traditional carbo-
hydrate chemistry required protection and deprotection steps
to access the desired protecting group pattern. A de novo
approach presents an attractive alternative to access the
building block.4 Here, we report an enantioselective synthesis
of differentially protected L-glycero-D-manno-heptoses. The
retrosynthetic strategy relies on the enantioselective construction
of the seven carbon backbone using two aldol reactions
(Scheme 1). The heptose hemiacetal presented in a linear form
can be accessed by a Mukaiyama-type aldol reaction between
C5-aldehyde 3 and silyl enolether 4.5 The aldehyde can further
be dissected through a second retroaldol step into C3-ketone 5
and C2-aldehyde 6. We established a proline-catalyzed aldol
reaction as a highly enantioselective and readily scalable
transformation.6
Lactone 15 was efficiently reduced using lithium tri-tert-
butoxyaluminium hydride,8 however, partial migration of the
silyl group from C2 to C3 was observed. Both regioisomers 17
and 18 were separated by flash chromatography and were used
for the synthesis of two different building blocks as depicted in
Scheme 3. Diol 17 was diacetylated to diester 19 and the
The synthesis commenced with the anti-selective L-proline
catalyzed aldol reaction followed by TBS-protection of the
a Department of Biomolecular Systems, Max Planck Institute of
Colloids and Interfaces, Am Muhlenberg 1, 14776 Potsdam,
¨
Germany. E-mail: peter.seeberger@mpikg.mpg.de
¨
b Freie Universitat Berlin Institut fur Chemie und Biochemie,
Arnimallee 22, 14195 Berlin, Germany
¨
w Electronic supplementary information (ESI) available: General
information and procedures (S1–S12) and copies of NMR spectra
(S13–S49). See DOI: 10.1039/c000784f
z Current address: Shionogi Research Laboratories, Shionogi & Co.,
Ltd., 12-4, 5-chome, Fukushima-ku, Osaka, 553-0002, Japan.
y Current address: Department of Chemical Physiology, The Scripps
Research Institute, 10550 North Torrey Pines Road, La Jolla,
CA 92037, USA.
Fig. 1 Structure of the Yersinia pestis core pentasaccharide.
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Scheme 1 Retrosynthesis of L-glycero-D-manno-heptose.
Scheme 3 Synthesis of building blocks 1 and 2.
The resulting crude product was treated with hydrogen
fluoride/pyridine and was diacetylated to produce 22. Finally,
the levulinoyl protective group of the C3 position was
selectively removed using hydrazine acetate to afford acceptor 2.
The diagnostic coupling constants10 helped to confirm the
manno-configuration on a-glycoside 22 and 2.11 To demonstrate
the synthetic utility of the heptose building blocks 1 and 2 in glyco-
sylations we synthesized disaccharide 23. Using a catalytic amount
of TMSOTf as promotor, glycosylating agent 1 was coupled with
acceptor 2. The resulting crude disaccharide was treated with
hydrogen fluoride/pyridine to remove both silyl groups to afford
the desired disaccharide 23 as depicted in Scheme 4.12
In conclusion, we describe a short, enantioselective route to
L-glycero-D-manno-heptose building blocks. Compared to
known synthetic routes to these sugars, our synthesis furnishes
building blocks that are orthogonally protected, and suitable
for assembly of a variety of different L-glycero-D-manno-
heptose-containing oligosaccharides. In addition, this route
will allow for the construction of other naturally occurring
heptose epimers, by altering the stereochemical course of the
aldol reactions and of the ketone reduction. These
building blocks are currently employed in the total synthesis
of Yersinia pestis core pentasaccharide.
Scheme 2 Synthesis of lactones 15 and 16.
anomeric acetate was selectively removed using hydrazine
acetate. The resulting hemiacetal was efficiently converted into
N-phenyl-trifluoroacetimidate glycoside 1 in 85% overall yield
over three steps. Levulinoylation of both hydroxyls on diol 18
yielded diketone 21 and the anomeric levulinoate ester was
replaced by N-(benzyl)benzyloxycarbonyl-5-aminopentan-1-ol
under the agency of boron trifluoride etherate. The protected
amine linker was previously described to be robust towards
various glycosylation and deprotection conditions as well as
readily removable by hydrogenation at the end of the synthesis.4c,9
Scheme 4 Synthesis of disaccharide 23.
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12 Compound 23 displays broadened signals in the 1H and 13C NMR
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1
HPLC and H NMR spectroscopy (see Supporting Information).
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This journal is The Royal Society of Chemistry 2010
4108 | Chem. Commun., 2010, 46, 4106–4108