Preparation of a protected 3-deoxy-d-manno-oct-2-ulosonate glycal donor for the synthesis of β-KDO-containing oligosaccharides

Article abstract of DOI:10.1021/ol500275j

A practical method for the synthesis of KDO glycal donors was developed. The prepared KDO donors exhibited excellent disastereoselectivity of glycosylation in a CH₂Cl₂-CH₃CN solvent mixture, which was found to be associated with the isopropylidene protection at the C-4 and C-5 hydroxyls. The synthetic use of the KDO donor was demonstrated in the preparation of β-KDO-containing oligosaccharides.

Full text of DOI:10.1021/ol500275j

Letter
pubs.acs.org/OrgLett
Preparation of a Protected 3‑Deoxy‑D-manno-oct-2-ulosonate Glycal
Donor for the Synthesis of β‑KDO-Containing Oligosaccharides
Tapan Kumar Pradhan,† Chun Cheng Lin,‡ and Kwok-Kong Tony Mong*,†
†Applied Chemistry Department, National Chiao Tung University, 1001 Ta Hsueh Road, Taiwan
‡Chemistry Department, National Tsing Hua University, Guang Fu Road, Taiwan
S
* Supporting Information
ABSTRACT: A practical method for the synthesis of KDO glycal donors was developed. The prepared KDO donors exhibited
excellent disastereoselectivity of glycosylation in a CH2Cl2−CH3CN solvent mixture, which was found to be associated with the
isopropylidene protection at the C-4 and C-5 hydroxyls. The synthetic use of the KDO donor was demonstrated in the
preparation of β-KDO-containing oligosaccharides.
lycosides of 3-deoxy-D-manno-oct-2-ulosonoic acid
prepared 1-C-aryl β-KDO glycoside analogues from a 1-C-
arylglycal, yet dearomatization was needed to obtain the natural
KDO glycosides; thus, elaboration to oligosaccharide synthesis
would be tedious.13 Herein, we report a synthetic route for the
preparation of β-selective KDO glycal donors that can be used for
the synthesis of simple β-KDO glycosides and bacterial-related β-
KDO-containing oligosaccharides.
Our synthetic route to the various KDO glycal donors started
with the diacetonide-protected mannofuranose 1, which was
prepared via known procedures that included the addition of
lithiated trimethylsilylacetylide and desilylation to give the alkyne
derivative 2 (Scheme 1).15 Subsequent benzylation of the
hydroxyl groups and bromination transformed 2 into the
1
G(KDO) are prominent in bacteria.
Natural KDO
glycosides are present in α- and β-anomeric configurations. α-
KDO glycosides are found in the core oligosaccharides of
lipopolysaccharides (LPS) in Gram-negative bacteria, whereas β-
KDO glycosides commonly occur in capsular polysaccharides
(CPSs) of Gram-positive and Gram-negative bacteria.2,3 Recent
studies have shown that the reducing-end terminus of CPSs from
various pathogenic bacteria, including Escherichia coli4 and
Neisseria meningitides,5 is composed of a β-KDO-containing
oligosaccharide linker.6 The CPSs of pathogenic bacteria play
defensive roles in the prevention of phagocytosis and comple-
ment-mediated killing of the immune system, indicating their
potential as new therapeutic targets.3a,7−9
The chemical synthesis of KDO-containing oligosaccharides is
nontrivial. As the KDO sugar is not commercially available, a
practical method is needed to provide the starting material
required for the preparation of the KDO-glycosyl substrates that
are used to assemble the oligosaccharides. More challenging is
the absence of a C-3 hydroxyl function in the KDO sugar which
can be used to control the stereochemistry of glycosylation
through neighboring group participation (NGP).10
Scheme 1. Synthesis of KDO Glycal 6
The first synthesis of a KDO glycoside can be traced back to
1978.8a Later studies mainly focused on the synthesis of an α-
KDO glycoside.11 Although an increasing number of β-KDO-
containing oligosaccharides have been identified in pathogenic
bacteria,4−6 glycosylation methods for the synthesis of β-KDO
glycosides remain scarce.12−14 Therefore, a practical synthetic
route to stereoselective KDO glycosyl donors and β-KDO-
containing oligosaccharides is highly desirable.
Sinay et al. described the synthesis of a β-KDO-containing
̈
disaccharide through Wittig condensation of a mannose-derived
aldehyde and a glycosyl phosphonate, but only the E-isomeric
product could be used for β-glycoside formation.12 Ling et al.
Received: January 27, 2014
Published: February 26, 2014
© 2014 American Chemical Society
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Letter
bromoalkyne derivative 3. The bromoalkyne 3 underwent
oxidative cleavage with KMnO4 in methanol−H2O to provide
the methyl α-keto ester 4 in 70% yield.16 Deprotection of the
benzyl ether functions in 4 by hydrogenolysis freed the C-4 and
C-6 hydroxyl groups, followed by instantaneous cyclization of
the C-6 hydroxyl group with the C-2 ketone function to produce
the hemiacetal 5.17 Of note is that a prolonged hydrogenolysis
reaction time led to the migration of the acetonide function.
Finally, subjecting the hemiacetal 5 to the Corey−Winter
procedure18 furnished a KDO glycal 6,14,19 which was employed
as a donor for β-glycoside synthesis. Besides the synthesis of the
glycal 6, other KDO-glycals 7−9 were prepared for the
glycosylation studies (Scheme 2).
Table 1. Glycosylation of 10a−f with KDO Glycal Esters 6 and
8
11a−g
NIS,
TMSOTf
(equiv)
donor,
entry acceptor
time
solvent
a
(h) (A or B)
(%)
dr
b
b
b
c
1
2
3
4
5
6
7
8
9
6, 10a
6, 10a
6, 10a
6, 10b
6, 10c
6, 10d
6, 10d
6, 10e
8, 10e
1.5, 1.5
2.0, 1.5
2.0, 1.5
2.0, 1.5
2.0, 1.5
2.0, 1.5
2.0, 1.5
2.0, 1.5
2.0, 1.5
18
3
A
A
B
B
B
B
A
B
B
11a, 50
11a, 70
11a, 68
11b, 65
11c, 72
11d, 74
11d, 80
11e, 74
11f, 75
5:1
5:1
,
c
3
>20:1
>20:1
14:1
13:1
6:1
6
c
3
c
1
c
1
c
12
18
13:1
d
Scheme 2. Synthesis of KDO Glycal Substrates 7−9
6:1
a
b
A is CH2Cl2 and B is 1:2 (v/v) CH2Cl2/CH3CN mixture. The dr
c
ratios were obtained by HPLC analysis (integral of H-3′). The dr
ratios were determined on the basis of the H NMR spectra of the
1
deiodinated compounds of the diastereomers 11 and 12, i.e., 13a−f
(refer to the corresponding 1H NMR spectra in the Supporting
d
Information). The dr ratio of 11f was determined by isolation of the
diastereomers.
3
octulosonyl saccharide in 11a on the base of the JH‑3ax′/H‑4′
value of 8.4 Hz (indicating the axial position of H-4′).21
In the literature, nitrile solvents have long been regarded as a
means to promote α-glycosylation for sialyl donors,22 but the
stereodirecting effect of the nitrile solvent on KDO donors
remains controversal.14,23 In the present study, we repeated the
glycosylation of 10a in a 1:2 CH2Cl2/CH3CN mixture (entry 3).
This improved the dr to 20:1 in favor of 11a.
With the KDO glycal substrates in hand, we established a β-
selective glycosylation method for the synthesis of β-KDO
glycosides (Scheme 3, Table 1). In the beginning, the
Scheme 3. β-Selective Glycosylation Method for KDO Glycal
Donors 6 and 8
Unlike aldose sugars, the assignment of anomeric config-
uration for KDO glycosides is nontrivial because of the lack of an
anomeric proton. Unger et al. employed the coupling constant
between the axial proton at C-3 and the 13C carbon at the C-1
position (3JC‑1/H‑3ax) for assignment of the anomeric config-
24
5
uration of a KDO glycoside with a C2 conformation.
A
3JC‑1/H‑3ax value of ca. 5.0−6.0 Hz indicates a β-configuration,
while a value of ≤1.0 Hz indicates an α-configuration.
Thus, glycoside 11a was reduced with triphenyl (or tributyl)
tin hydride and azo-bis(isobutyro)nitrile (AIBN)25 to produce
the deoxy derivative 13a (Scheme 4). However, the 13C NMR
spectrum of 13a was indiscernible, probably because it adopted a
3
non-5C2 conformation (as evidenced by a JH‑3ax′/H‑4′ value of
Scheme 4. De-iodination of 11a−f and Removal of the
Isopropylidene Functions of the Deoxy Derivatives 13a,b,e,f
glycosylation of the methyl glucosyl acceptor 10a with KDO
glycal 6 was conducted in CH2Cl2 using N-iodosuccinimide
(NIS, 1.5 equiv) and trimethylsilyl trifluoromethanesulfonate
(TMSOTf, 1.5 equiv) as the promoters (entry 1).20 The reaction
produced the desired diastereomer 11a in 50% yield (isolated)
with a moderate dr of 5:1, but ca. 20% of 6 was recovered even
when the reaction time was prolonged to 18 h. Nevertheless, 2.0
equiv of NIS (with respect to 6) boosted the reaction yield to
70%, though with no improvement in stereoselectivity (entry 2).
A
2C5 conformation was assigned to the iodo-substituted
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Letter
≤6.0 Hz).25 Accordingly, we removed the isopropylidene
functions of 13a to form the glycoside 14a that gave a
3JC‑1/H‑3ax value of ≥5.0 Hz (see Table SII of the Supporting
Information), thus confirming a β-configuration.
Scheme 5. Synthesis of α-Gal-(1→5)-β-KDO-(2→3)-Gal
Trisaccharide 18
Thereafter, the cosolvent system described in entry 3 was used
for the glycosylations of the other acceptors 10b−e. The
reactions of 10b−e with the glycal donor 6 produced the
expected β-linked glycosides 11b−e with high dr (≥13:1) and
good yield (>65%) (entries 4−8). To confirm the nitrile solvent
effect, the glycosylation of 10d was repeated in CH2Cl2, which
yielded 11d at a much lower 6:1 dr (entries 6 and 7).
To probe the influence of the isopropylidene protection on the
stereochemistry, the glycal donor 8 bearing a single 4,5-O-
isopropylidene group was used for the glycosylation of alcohol
10e (entry 9). The diastereoselectivity achieved with 8 decreased
significantly (compared with 6, in entry 9), yet an acceptable dr
of 6:1 for the desired disastereomer 11f was obtained.
Interestingly, the replacement of the 4,5-O-isopropylidene in 6
with a benzyl ether function eroded the selectivity of the
glycosylation.26 Taken together, the results indicate a β-
stereodirecting effect for the 4,5-O-isopropylidene protecting
group in KDO glycal donors.
(Scheme 6a) as found in the CPS of Sinorhizobium meliloti.1a
First, the KDO glycal 9 was coupled with the alcohol 10e to
Scheme 6. Synthesis of β-(2→7)-KDO Trisaccharide
To confirm the β-anomeric configurations of the glycosides
11b−f, the same procedures including the deiodination and
deprotection of the isopropylidene functions as used for 11a
were performed (Scheme 4). Among the deoxy derivatives 13b−
3
f, only 13c gave a JC‑1/H‑3ax value of 5.0 Hz in the coupled 13C
NMR experiment. With the exception of 13d,27 the other deoxy
derivatives underwent deprotection of the isopropylidene
functions to afford the glycosides 14b, 14e, and 14f. Eventually,
these glycosides gave 3JC‑1/H‑3ax values of ≥5.0 Hz (see Table SI of
the Supporting Information), thus confirming a β-configuration.
3
It should be noted that other than the JC‑1/H‑3ax value, we
obtained the difference of the chemical shift (Δδ) between the
axial and equatorial H-3 protons of 13c, 14a, 14b, 14e, and 14f
(and trisaccharide 18) in CDCl3, which varied from 0.12 to 0.81
ppm (see Table SI in S38 of the Supporting Information). These
data clearly reveal the influence of the acceptor structure on the
Δδ of the H-3 geminal protons. As shown in the present study, a
great caution should be taken when employing Δδ to determine
the anomeric configuration of a KDO glycoside.28
After establishing conditions for the β-selective glycosylation,
the KDO glycal donors were employed for the synthesis of
protected β-KDO-containing oligosaccharides, which to the best
of our knowledge, is unprecedented.
The first synthetic target was the protected α-Gal-(1→5)-β-
KDO-(2→3)-Gal trisaccharide 18. This trisaccharide unit is part
of the CPS in the cell wall of Rhizobium fredii.1b The synthetic
route to 18 started from the disaccharide 13c, which was treated
with p-TsOH in MeOH to remove the isopropylidene groups
(Scheme 5a). Selective protection of the C7- and C8-hydroxyls
then furnished the disaccharide intermediate 15 (Scheme 5a).
Subsequent benzylation of the C4-hydroxyl in 15 with dibutyltin
oxide (Bu2SnO) followed by benzyl bromide gave the desired
acceptor 16, which was coupled with the thiogalactosyl donor 17
using the N-formylmorpholine (NFM)-modulated glycosylation
method to produce the target trisaccharide 18 with high α-
selectivity (Scheme 5b).29 The α- and β-configurations of the
nonreducing end Gal and KDO residues in 18 were confirmed by
the 1JH‑1′′/C‑1′′ value of 173 Hz and the 3JC‑1′/H‑3′ax value of 5.2 Hz,
respectively.
produce the 3-iodo β-glycoside 20 with a dr of 6:1 (Scheme 6b).
The subsequent glycosylation of 20 with the KDO glycal 6 gave
the diiodo-substituted β-KDO-(2→7)-KDO disaccharide 21 in
60% isolated yield with a dr of ca.10:1 (estimated from TLC).
Treatment of the disaccharide 21 with a dilute Zn(NO3)2
solution at 50 °C selectively removed the 7,8-O-isopropylidene
group and produced a diol intermediate, which underwent
Encouraged by the synthesis of 18, we synthesized a protected
β-(2→7)-KDO trisaccharide 19 from the glycal donors 6 and 9
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Letter
3, 896−899. (c) Tanaka, H.; Takahashi, D.; Takahashi, T. Angew. Chem.,
Int. Ed. 2006, 45, 770−773. (d) Shimoyama, A.; Saeki, A.; Tanimura, N.;
Tsutsui, H.; Miyake, K.; Suda, Y.; Fujimoto, Y.; Fukase, K. Chem.Eur.
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benzylation of the C-8 hydroxyl to produce the disaccharide
acceptor 22. Iterative glycosylation of 22 with the KDO glycal 6
afforded the triiodo-substituted β-KDO trisaccharide 23 in 65%
isolated yield with a high dr of >10:1 (estimated from TLC). The
trisaccharide 23 was reduced by treatment with Ph3SnH and
AIBN to furnish the target β-KDO trisaccharide 19. The
3JC‑1/H‑3ax values of the internal KDO glycosidic bonds in 19 were
found to be 6.1 and 4.4 Hz, confirming a β-anomeric
configuration.
In summary, we describe a practical synthetic route for the
preparation of KDO glycals, which are efficient glycosyl donors
in the synthesis of β-KDO glycosides and oligosaccharides. The
reported synthetic method provides access to these valuable
oligosaccharide compounds.
(12) Paquet, F.; Sinay, P. J. Am. Chem. Soc. 1984, 106, 8313−8315.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details for preparation of KDO glycals 6−9,
glycosides 11a−f, 13a−f, 14a,b,e,f, 15, and 16, β-KDO-
containing oligosaccharides 18 and 19, and relevant NMR
spectra. This material is available free of charge via the Internet at
http://pubs.acs.org.
(20) We initially used TfOH acid promoters as described in ref 11c
(and ref 22a), but the β-selectivity of glycosylation was lower than that
obtained from the use of TMSOTf.
AUTHOR INFORMATION
Corresponding Author
■
3
(21) The JH‑3ax′/H‑4′ coupling constants for glycosides 11c and 11f
5
*E-mail: tmong@mail.nctu.edu.tw.
Notes
were also obtained (≥ 8.0 Hz), which supported a C2 conformation.
Accordingly, such 2C5 conformation was assigned to other 3-iodo-
substituted KDO-glycosides including 11b,d−e and 20−23. See:
Birnbaum, G. I.; Roy, R.; Brisson, J.-R.; Jennings, H. J. J. Carbohydr.
Chem. 1987, 6, 17−39.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(22) (a) Crich, D.; Li, W. J. Org. Chem. 2007, 28, 7794−7797. (b) Liu,
Y.; Ruan, X.; Li, X.; Li, Y. J. Org. Chem. 2008, 73, 4287−4290.
Thanks are given to the National Science Council of Taiwan
(NSC 102-2113-M-009-009) and the Center for Interdiscipli-
nary Science of NCTU for support.
(23) (a) Mannerstedt, K.; Ekelof, K.; Oscarson, S. Carbohydr. Res.
̈
2007, 342, 631−637. (b) Boons, G. J.; van Delft, F. L.; van der Klein, P.
A. M.; van der Marel, G. A.; van Boom, J. H. Tetrahedron 1992, 48, 885−
904.
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