Exploring the interactions of unsaturated glucuronides with influenza virus sialidase

Article abstract of DOI:10.1021/jm301145k

A series of C3 O-functionalized 2-acetamido-2-deoxy-⁴-β-d- glucuronides were synthesized to explore noncharge interactions in subsite 2 of the influenza virus sialidase active site. In complex with A/N8 sialidase, the parent compound (C3 OH) inverts its solution conformation to bind with all substituents well positioned in the active site. The parent compound inhibits influenza virus sialidase at a sub-μM level; the introduction of small alkyl substituents or an acetyl group at C3 is also tolerated.

Full text of DOI:10.1021/jm301145k

Brief Article
pubs.acs.org/jmc
Exploring the Interactions of Unsaturated Glucuronides with
Influenza Virus Sialidase†
Beenu Bhatt,‡ Raphael Bohm,‡ Philip S. Kerry,§ Jeffrey C. Dyason,‡ Rupert J. M. Russell,§
̈
Robin J. Thomson,‡ and Mark von Itzstein*,‡
‡Institute for Glycomics, Gold Coast Campus, Griffith University, Queensland 4222, Australia
§Interdisciplinary Centre for Human and Avian Influenza Research, University of St. Andrews, North Haugh, St. Andrews, Fife KY16
9ST, U.K.
S
* Supporting Information
ABSTRACT: A series of C3 O-functionalized 2-acetamido-2-deoxy-Δ4-β-D-glucuronides were synthesized to explore noncharge
interactions in subsite 2 of the influenza virus sialidase active site. In complex with A/N8 sialidase, the parent compound (C3
OH) inverts its solution conformation to bind with all substituents well positioned in the active site. The parent compound
inhibits influenza virus sialidase at a sub-μM level; the introduction of small alkyl substituents or an acetyl group at C3 is also
tolerated.
INTRODUCTION
■
The continual emergence of new strains of influenza virus
presents an ongoing threat to humankind.1 In addition to the
burden of seasonal epidemic human influenza, there are serious
concerns about the zoonotic potential of nonhuman, in
particular swine and avian, influenza viruses.2 A number of
influenza-specific antiviral therapeutics are currently available,
or are in development, which target different stages of virus
replication.1 The drugs recommended by the World Health
Organization as a first option for treatment during the 2009
influenza A pandemic (A/H1N1/pdm09), and in human
infections with highly pathogenic avian influenza (eg “bird
flu” H5N1), are inhibitors of the viral enzyme sialidase
(neuraminidase, NA).3 This enzyme plays an important role
Figure 1. Influenza virus sialidase inhibitors. Drugs 1−3 exploit H-
bonding and electrostatic interactions in subsite 2 of the active site,
in the release of new virus particles as they bud from the surface
whereas inhibitors 4−6 exploit hydrophobic interactions.
of an infected host cell; its inhibition leaves the virus particles
clumped at the cell surface, thereby reducing virus spread.4
showing that the amino acids of subsite 2 can also engage in
hydrophobic interactions with an appropriately positioned
The first sialidase inhibitor drugs were the potent (Ki sub-
nM) inhibitors zanamivir 1 and oseltamivir carboxylate 2 (OC,
nonpolar group (as in 4 and 5).6,8 This finding was further
supported with the synthesis of OC analogue 6, in which the
basic primary amine of OC was replaced by a vinyl group. 6
showed significant inhibition of influenza virus sialidase (Ki 45
nM),9 despite the smaller 2-propyl O-alkyl side chain compared
to 2. The introduction of a hydrophobic, rather than a charged,
substituent to bind subsite 2 could be advantageous in terms of
drug pharmacokinetics: the zwitterionic character of inhibitors
1−3 containing a guanidino or amino group limits the drugs’
oral bioavailability.4,5 Importantly, presumably by virtue of the
hydrophobic side-chain binding subsite 2, 4 retains potent
inhibition against influenza A viruses harboring the E119 V
mutation that confers resistance to OC 2.10
the active form of oseltamivir) (Figure 1).4 A third inhibitor,
peramivir 3, was given temporary Emergency Use Author-
ization during the 2009 influenza pandemic.5 During that
pandemic, the sialidase inhibitors provided a means of
therapeutic intervention while an appropriate vaccine was
being developed and manufactured. Development of next-
generation influenza virus sialidase inhibitors is ongoing, based
in particular upon new insights into the sialidase structure.6,7
Both during and since the development of inhibitors 1−3,
emphasis has been on structures that incorporate functional
groups to engage in predominantly hydrogen bonding and
electrostatic interactions with conserved acidic amino acids in a
pocket of the active site (subsite 2), which is dominated by
negatively charged residues (Glu119, Glu277, Asp151).4
Subsite 2 accommodates the guanidino and amino groups of
inhibitors 1−3. The development of potent inhibitor A-315675
4 (e.g., Ki 0.2 nM against A/N2 NA),8 however, gave a new
dimension to probing this area of the sialidase active site by
We are interested in the development of readily accessible
uronic acid-based sialidase inhibitors of type 7 (Figure 2),11,12
Received: August 6, 2012
© XXXX American Chemical Society
A
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Journal of Medicinal Chemistry
Brief Article
which would not have been possible using the previously
described12 3-O-pivaloylated analogue of 16. High-yielding
preparation of the GlcNAc 3-pentyl β-glycoside 12 was
achieved through TMSOTf-mediated14 activation of GlcNAc
penta-O-acetate 11 to give the oxazolinium ion intermediate
and subsequent one-pot reaction with 3-pentanol. This
glycosidation method was found to be significantly higher
yielding than glycosidation on the preformed GlcNAc 1,2-
oxazoline.12,15 Conversion of trihydroxy derivative 13, obtained
by base-catalyzed de-O-acetylation of 12, to the corresponding
6-carboxy derivative took advantage of the now well-established
selective oxidation of primary over secondary hydroxyl groups
mediated by the 2,2,6,6-tetramethylpiperidine 1-oxide radical
(TEMPO).16 Accordingly, 13 was reacted with NaOCl in the
presence of catalytic TEMPO and KBr in aqueous sodium
bicarbonate solution; oxidation was most efficient when the
reaction temperature was maintained between 0−4 °C. The
resulting carboxylic acid derivative was not isolated but was
instead converted17 directly to the corresponding acetylated
methyl ester 14. The dried crude product was treated17 with
methyl iodide in DMF for esterification, followed by addition of
acetic anhydride and catalytic DMAP, affording acetate-
protected glucuronide methyl ester 14 in 61% yield over
three steps in a one-pot reaction. Also isolated as a byproduct
was the 3,6-lactone 15 (19% yield), characterized by the smaller
3J coupling constants associated with the 1C4 chair con-
Figure 2. Sialidase inhibitors based on a dihydropyran scaffold:
Neu5Ac2en 8 and uronic acid based 7, a “cross-mimic” of 8 and
oseltamivir carboxylate (OC) 2.
2-acetamido-2-deoxy-Δ4-β-D-glucuronides which mimic the
naturally occurring sialidase inhibitor 5-acetamido-2,6-anhy-
dro-3,5-dideoxy-D-glycero-D-galacto-non-2-enonic acid (Neu5A-
c2en 8) but in which the glycerol side-chain of 8 is replaced by
an ether (the glycoside of the glucuronide). An advantage of
the uronic acid scaffold is its derivation from inexpensive 2-
acetamido-2-deoxy-glucose (GlcNAc) compared to N-acetyl-
neuraminic acid and quinic acid used for the syntheses of 1 and
2, respectively.13 A number of examples of 7 (R = alkyl), but
not the OC mimic 3-pentyl analogue 9, have been evaluated as
inhibitors of influenza virus sialidase, where they were found to
inhibit in the micromolar range.11 The objectives of the work
described here were two-fold: to evaluate the binding mode of
the Δ4-β-D-glucuronide 9, which contains the 3-pentyl side-
chain of 2 on a dihydropyran scaffold, in complex with
influenza virus sialidase, and to explore the hydrophobic
paradigm of subsite 2 binding, through C3 O-substituted
derivatives of 9.
3
formation of 15 (including a change in J1,2 from 8.1 Hz in 14
to a broad singlet18 in 15). Conversion of glucuronide 14 to the
4,5-unsaturated derivative, Δ4-β-D-glucuronide 16, was achieved
through DBU-mediated β-elimination.11,12 To obtain the 3-
hydroxy glucuronide 17 for further functionalization at C3,
chemoselective cleavage of the C3 O-acetate in the presence of
the methyl ester was undertaken using sodium methoxide in
methanol. The overall yield of key intermediate 17 in 8 steps
from GlcNAc 10 was 40%. Further saponification of 17 in
aqueous methanolic NaOH (pH 13) gave the “parent” inhibitor
on the Δ4-β-D-glucuronide scaffold, 9.12
RESULTS AND DISCUSSION
■
Synthesis of the Δ4-β-D-Glucuronide Scaffold. The
route to the target 2-acetamido-Δ4-β-D-glucuronides began with
an efficient synthesis of key 3-hydroxy intermediate, methyl (3-
pentyl 2-acetamido-2-deoxy-Δ4-β-D-glucopyranosid)uronate
17, from GlcNAc (10) as outlined in Scheme 1. This approach
was used as it allowed selective deprotection at the C3 position,
Structural Evaluation of the Δ4-β-D-Glucuronide
Scaffold in Complex with Influenza Virus Sialidase. To
assess the binding mode of the 3-pentyl 2-acetamido-Δ4-β-D-
glucuronide scaffold with influenza virus sialidase, 9 was soaked
into crystals of influenza A virus N8 sialidase [A/duck/
Ukraine/1/63 (H3N8)], and the structure of the resultant
complex was determined by X-ray crystallography (Figure 3).
The N8 sialidase is representative of influenza A virus sialidases
as it is equally sensitive to inhibitors 1 and 2 when compared
with other influenza A virus sialidases.19 This study provides the
first structural information for a glucuronide-based inhibitor in
complex with an influenza virus sialidase. The apo-N8 sialidase
has an open conformation of the flexible “150-loop”.7 In the
N8:9 complex, this loop has closed over the bound inhibitor, as
seen in N8 complexes of 8 and 2, under similar soaking
conditions.7 Despite 9 having a solution half-chair conforma-
tion in which the substituents on the dihydropyran ring are in
a
Scheme 1. Synthetic Route to 9 and 17
1
pseudoaxial positions, as indicated by H NMR spectrosco-
py,12,15 the compound is clearly bound in the sialidase active
site in the alternative half chair conformation which positions
all substituents equatorially (Figure 3D). Superimposition of
corresponding ring carbons (1, 3, and 5 in glucuronide
numbering) of 9 and Neu5Ac2en 8 shows their half-chair
conformations to be virtually identical (Figure 3C). By virtue of
this conformation, the C1 carboxylate, C4 hydroxyl, and C5
acetamido groups of 9 are oriented, and interacting with key
a
Reagents and conditions: (a) Ac2O, py, DMAP, DCM, rt, 24 h
(96%); (b) (i) TMSOTf, DCE, 50 °C, 12 h, (ii) 4 Å mol. sieves, 3-
pentanol, rt, 48 h (95%); (c) NaOMe, MeOH, 0 °C−rt, 2 h (99%);
(d) (i) TEMPO, KBr, aq NaOCl, aq NaHCO3, 0−4 °C, 12 h, (ii) MeI,
DMF, rt, 24 h, (iii) Ac2O, DMAP, rt, 12 h (61% over 3 steps); (e)
DBU, DCM, rt, 24 h (isolated 52%; 74% based on recovered starting
material 14); (f) NaOMe, MeOH, rt, 12 h (97%); (g) NaOH,
MeOH/H2O (1:1), pH 13, 0 °C−rt, 16 h (95%).
B
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Journal of Medicinal Chemistry
Brief Article
Figure 4. 3-Pentyl 3-O-ethyl-2-acetamido-2-deoxy-Δ4-β-D-glucuronide
18 docked into the active site of influenza A virus sialidase (PDB
2HU4), superimposed with pyrrolidine-based inhibitor 5 (magenta;
from PDB 1XOE). The surface is colored by electrostatic potential
(blue, positive; red, negative).
2). Fully or partially trans-esterified product was observed in
reactions with ethyl iodide and allyl bromide, respectively. Base-
Figure 3. X-ray crystal structure of 3-pentyl 2-acetamido-Δ4-β-D-
glucuronide 9 in complex with influenza A virus N8 sialidase. (A) N8:9
complex superimposed with N8:Neu5Ac2en 8 complex (8 magenta,
PDB 2HTR). (B) N8:9 complex superimposed with N8:OC 2
complex (2 cyan, PDB 2HT8). (C) Superimposition of ligands 9 and
8 (8 magenta, PDB 2HTR) (the glycerol side-chain of 8 has been
removed in this image) showing the very similar dihydropyran ring
conformations of 9 and 8. (D) Half-chair conformation of 9 showing
equatorial conformation of substituents.
a
Scheme 2. Synthetic Route to 18 and 23−25
highly conserved residues in the active site, as seen7 for
Neu5Ac2en (Figure 3A). Specifically, the C1 carboxylate of 9
interacts with the three strictly conserved arginine residues
(Arg118, Arg292, and Arg371) in the active site; the C4
hydroxyl group extends toward the pocket (subsite 2) bordered
by Glu119 and Asp151 and the C5 acetamido group is directed
toward Trp178. Furthermore, in the N8:9 complex, the
conserved residue Glu276 is oriented to form a salt bridge
with Arg224, providing hydrophobic character to the active site
pocket that accommodates the 3-pentyl side-chain of 9 and of
OC 2 (Figure 3B). Comparing 9 and 2 (Figure 3B): while the
conformation of the central ring and orientation of the pendant
substituents are very similar (not shown), the position of 9 in
the active site is slightly shifted from that of 2, and the arms of
the 3-pentyl side chain occupy slightly different regions of the
binding pocket.
a
Reagents and conditions: (a) RX, NaH, TBAI, DMF, 0 °C−rt, 24−48
h (EtI, 19 56%; MeI, 20 66%; AllylBr, 21, 34%; 22, 42%); (b) NaOH,
MeOH/H2O (1:1), pH 13, 0 °C−rt, 16 h; (c) Ac2O, py, rt, 16 h
(38%).
catalyzed saponification of the 3-O-alkylated compounds
yielded the corresponding free acids 18, 23, and 24. The final
target 3-O-acetylated derivative was obtained through reaction
of fully deprotected glucuronic acid 9 with acetic anhydride in
pyridine at room temperature, affording 3-O-acetyl derivative
25 in 38% yield (unoptimized).
Biological Evaluation. The 3-O-substituted 2-acetamido-
Δ4-β-D-glucuronides synthesized were evaluated for their ability
to inhibit the enzyme activity of influenza A virus sialidase,
using an in vitro fluorometric assay.21,22 The inhibitory
activities of 18 and 23−25, with comparison to 9, the general21
sialidase inhibitor Neu5Ac2en 8, and previously synthesized23
4-O-ethyl-Neu5Ac2en 26, are presented in Table 1.
The free C3 hydroxyl group of 9 is positioned appropriately
for substitution to explore hydrophobic interactions in subsite 2
of the active site. To evaluate the potential for a substituent at
C3 on 9 to reach the area of subsite 2 accessed by the
pyrrolidine-based inhibitors 4 and 5, the model compound 3-O-
ethyl substituted Δ4-β-D-glucuronide 18 was docked into the
active site of influenza A sialidase (PDB 2HU4) and
superimposed with pyrrolidine-based inhibitor 520 (from PDB
1XOE), the only example of this series available in the PDB.
The overlay of the structures of 5 and 18 (Figure 4) shows
similar extension of the methyl ester of 5 and the 3-O-ethyl
group of 18 into subsite 2. To examine if Δ4-β-D-glucuronides
with small hydrophobic substituents at C3 could enhance
binding affinity, synthesis and biological evaluation of a small
series of compounds with a methyl, ethyl, or allyl ether, or an
acetate ester at the C3 position of 9 was undertaken.
Synthesis of 3-O-Substituted Δ4-β-D-Glucuronides. For
synthesis of the target 3-O-alkylated derivatives, the 3-hydroxy
methyl ester derivative 17 was reacted with NaH in DMF in the
presence of the alkyl halide (EtI, MeI, allyl bromide) (Scheme
The structure of 3-pentyl Δ4-β-D-glucuronide 9 in complex
with N8 sialidase (Figure 3) shows 9 positioned in the active
site with the central ring and the carboxylate, hydroxyl, and
C
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Journal of Medicinal Chemistry
Brief Article
pyrrolidine and furan based inhibitors 5 and 3020 (inhibition of
A/N2 NA:20 5 IC50 0.041 μM; 30 0.41 μM). The particular
hydrophobic substituents introduced onto 9, however,
apparently fail to engage in interactions in subsite 2 that
improve overall binding affinity.
Table 1. Inhibitory Activity (IC50) of 9, 18, and 23−26
against Influenza A Virus (N2) Sialidase
The 3-atom extension from the pyrrolidine ring into subsite
2 seen, for example, in 4, 5, and 30, equates to only a 2-atom
extension from the C3 hydroxyl group of 9. As such, the fine-
tuning of the nature and “directionality” of this substituent
(e.g., 10-fold greater activity on introduction of the cis- versus
trans-propenyl substituent on scaffold 56), which was required
during optimization on the pyrrolidine template,6 is not as
readily accessible on the current Δ4-β-D-glucuronide template.
An exception to this is variation of the acetyl group of 25 that
enables further examination of the hydrophobic paradigm.
In conclusion, the 3-pentyl Δ4-β-D-glucuronide inhibitor
scaffold can be efficiently synthesized from an inexpensive and
readily accessible starting material. The first structural study of
this scaffold in complex with influenza virus sialidase shows that
the parent compound 9 binds well into the sialidase active site:
this binding results in good inhibitory activity for the
“unfunctionalized” template. The single available hydroxyl
group can be efficiently functionalized to explore possibilities
for binding interactions in subsite 2 such as those previously
identified on the pyrrolidine and cyclohexene scaffolds.
ab
,
ab
,
R or R2
O−H
O−CH2CH3
O−CH3
compd
IC50 (μM)
compd
C50 (μM)
9
0.32
8
10.5
479
18
23
24
25
28.0 (88)
1.45 (5)
212 (690)
5.39 (17)
26
O−CH2CHCH3
O−C(O)CH3
a
Results are given as means for three experimentally determined
values; IC50 95% confidence intervals are within 15% of the IC50
b
value. Number in square parentheses is the fold-difference of IC50
from the “parent” inhibitor (R or R2 = OH). In this assay, benchmark
inhibitors were: 4-amino-4-deoxy-Neu5Ac2en 27, IC50 0.68 μM; 4-
deoxy-4-guanidino-Neu5Ac2en 1; IC50 0.012 μM.
EXPERIMENTAL SECTION
■
General. A full description of materials and methods is provided in
the Supporting Information (SI). Final compounds were purified by
reversed phase (RP) HPLC. Purities of synthetic intermediates after
chromatographic purification were judged to be >90% by analysis of
acetamido substituents in similar orientations within the active
site to those seen for Neu5Ac2en 8 and OC 2. In terms of
inhibitory activity against A/N2 sialidase, 9 exhibits ∼30-fold
greater inhibitory potency than Neu5Ac2en 8, making the 3-
pentyl derivative the most potent inhibitor on the glucuronic
acid scaffold, with O-substitution at C3, to date.11 The
inhibitory potency of the 3-pentyl Δ4-β-D-glucuronide (dihy-
dropyran-based) scaffold appears to lie midway between that of
the natural dihydropyran scaffold with the glycerol side chain
(8) and the cyclohexene-based scaffold with the 3-pentyl ether
side chain, hydroxy-OC analogue 2824 (inhibition of A/N2
NA:24 28 IC50 0.015 μM, compared to Neu5Ac2en 8 IC50 1.7
μM). It is interesting that on the 3-pentyl Δ4-β-D-glucuronide
scaffold, the C3-hydroxy derivative (9) appears to have a similar
inhibitory potency to the corresponding C3-amino analogue
2915 reported by Smith et al. (inhibition of A/N2 NA:15 29
IC50 0.770 μM; compared to zanamivir 1 IC50 0.005 μM),
although comparison across assays should be made with
caution. This is in contrast to the Neu5Ac2en scaffold where
change in C4 substituent from a hydroxyl (8) to an amino (27)
group gives significant improvement in inhibitory activity21
(Table 1: see data for 8 and 27 in footnote).
With regard to the effect of functionality introduced to
explore hydrophobic interaction in subsite 2, the 3-O-alkylated
(18, 23, 24) or acetylated (25) glucuronide derivatives showed
weaker inhibition than the parent C3-hydroxy glucuronide 9,
with activity decreasing as the size of the alkyl chain increased
(Table 1). The weaker activity of the ethyl ether derivative 18
compared to 9 is mirrored in that of 4-O-ethyl-Neu5Ac2en 26
compared to Neu5Ac2en 8. Modeling had indicated adequate
space in subsite 2 for, in particular, the 3-O-ethyl and acetyl
substituents on the Δ4-β-D-glucuronide scaffold, placing them
potentially in a similar region to the methyl ester of the
1
the H and 13C NMR spectra (SI). Purity of tested compounds was
≥95% (HPLC analysis). 1,25 8,26 26,23 and 2725 used in the sialidase
assay were synthesized according to literature procedures and purified
by RP-HPLC (≥95% purity). Methods for the N8:9 complex
formation and X-ray structure determination,27 computational
chemistry,27 and sialidase activity assay,22 were as previously described.
General Procedure for De-esterification of 2-Acetamido-2-
deoxy-Δ4-β-D-glucuronides. To a solution of ester in MeOH:H2O
(1:1) at 0 °C, was added aq NaOH to achieve pH 13. The reaction
mixture was stirred at rt for 16 h and then neutralized with Amberlite
IR-120 (H+) resin. The crude product was purified by chromatography
on silica (EtOAc/MeOH/H2O 8:1.5:0.5) followed by RP-HPLC.
General Procedure for Synthesis of 3-O-Alkylated 2-
Acetamido-2-deoxy-Δ4-β-D-glucuronides 18, 23, and 24. To a
solution of 17 in anhyd DMF (0.12 M) under Ar was added alkyl
halide (7 mol equiv) and nBu4NI (0.2 mol equiv). The reaction was
cooled to 0 °C, NaH (1.3 mol equiv) was added, and the reaction was
stirred at 0 °C for 30 min and at rt for 24−48 h. The reaction was
quenched at 0 °C by addition of a drop of AcOH, followed by addition
of water and extractive workup with EtOAc. The crude product was
purified by chromatography on silica to give the 3-O-alkylated
derivative (19−22; see Scheme 2). 19−22 were de-esterified and
the products (18, 23, and 24) purified according to the general
procedure (yields and spectral characterization are given in the SI).
3-Pentyl 2-Acetamido-3-O-acetyl-2,4-dideoxy-α-L-threo-hex-
4-enopyranosiduronic Acid (25). To a solution of 912 (0.03 g, 0.10
mmol) in anhyd pyridine (1 mL) under N2 was added acetic anhydride
(0.5 mL). The reaction was stirred at rt for 16 h, concentrated, and the
crude product was purified by chromatography on silica to give 25
(0.013 g, 38%) (spectral characterization is given in the SI).
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures and spectral data for synthesized
compounds. Protocols for: molecular modeling; N8−9
D
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Journal of Medicinal Chemistry
Brief Article
(9) Hanessian, S.; Wang, J.; Montgomery, D.; Stoll, V.; Stewart, K.
D.; Kati, W.; Maring, C.; Kempf, D.; Hutchins, C.; Laver, W. G.
Design, synthesis, and neuraminidase inhibitory activity of GS-4071
analogues that utilize a novel hydrophobic paradigm. Bioorg. Med.
Chem. Lett. 2002, 12, 3425−3429.
structure determination (including crystallographic statistics);
sialidase activity assay. This material is available free of charge
via the Internet at http://pubs.acs.org.
Accession Codes
PDB ID: 4D8S.
́
(10) Abed, Y.; Nehme, B.; Baz, M.; Boivin, G. Activity of the
neuraminidase inhibitor A-315675 against oseltamivir-resistant influ-
enza neuraminidases of N1 and N2 subtypes. Antiviral Res. 2008, 77,
163−166.
(11) Mann, M. C.; Islam, T.; Dyason, J. C.; Florio, P.; Trower, C. J.;
Thomson, R. J.; von Itzstein, M. Unsaturated N-acetyl-D-glucosami-
nuronic acid glycosides as inhibitors of influenza virus sialidase.
Glycoconjugate J. 2006, 23, 127−133.
(12) Mann, M. C.; Thomson, R. J.; Dyason, J. C.; McAtamney, S.;
von Itzstein, M. Modelling, synthesis and biological evaluation of novel
glucuronide-based probes of Vibrio cholerae sialidase. Bioorg. Med.
Chem. 2006, 14, 1518−1537.
(13) Magano, J. Synthetic approaches to the neuraminidase inhibitors
zanamivir (Relenza) and oseltamivir phosphate (Tamiflu) for the
treatment of influenza. Chem. Rev. 2009, 109, 4398−438.
(14) Yu, B.; Ouyang, Q.; Li, C.; Hui, Y. The TMSOTf-promoted
“one pot” β-glycosidations of peracetylated chitobiose. J. Carbohydr.
Chem. 1996, 15, 297−302.
AUTHOR INFORMATION
Corresponding Author
*Phone: +61-(0)7-5552-7025. Fax: +61-(0)7-5552-9040. E-
mail: m.vonitzstein@griffith.edu.au.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
M.v.I. thanks the Honda Foundation (Australia). R.J.M.R.
thanks the Medical Research Council and the Scottish Funding
Council for financial support. B.B. and R.B. thank Griffith
University for the award of scholarships.
DEDICATION
■
†This paper is dedicated to the memory of Professor Stephen J.
Angyal.
(15) Smith, P. W.; Robinson, J. E.; Evans, D. N.; Sollis, S. L.; Howes,
P. D.; Trivedi, N.; Bethell, R. C. Sialidase inhibitors related to
zanamivir: synthesis and biological evaluation of 4H-pyran 6-ether and
ketone. Bioorg. Med. Chem. Lett. 1999, 9, 601−604.
ABBREVIATIONS USED
(16) See for example: (a) Davis, N. J.; Flitsch, S. L. Selective
oxidation of monosaccharide derivatives to uronic acids. Tetrahedron
Lett. 1993, 34, 1181−1184. (b) Lin, F.; Peng, W.; Xu, W.; Han, X.; Yu,
B. A facile preparation of uronates via selective oxidation with
TEMPO/KBr/Ca(OCl)2 under aqueous conditions. Carbohydr. Res.
2004, 339, 1219−1223.
■
NA, neuraminidase/sialidase; A/N8 or A/N2, influenza A virus
N8 or N2 sialidase; OC, oseltamivir carboxylate; GlcNAc, 2-
acetamido-2-deoxy-glucose; Neu5Ac2en, 5-acetamido-2,6-anhy-
dro-3,5-dideoxy-D-glycero-D-galacto-non-2-enoic acid
́
(17) Gyorgydeak, Z.; Thiem, J. Synthesis of methyl (D-glycopyr-
̈
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