A General Method for the Divergent Synthesis of C-9 Functionalised Sialic Acid Derivatives

Article abstract of DOI:10.1002/ejoc.202001049

Sialic acids, a ubiquitous family of sugars shown to be involved in numerous biologically important processes, exhibit remarkable structural diversity in nature. Access to these derivatives by chemical and enzymatic means is a major bottle neck in understanding the role played by each particular modification. As part of a program to study such roles and determine the substrate specificity of novel sialic acid aldolases, a general and robust synthetic protocol was devised to gain access to all naturally occurring C-9 functionalised N-acetylneuraminic acid derivatives including esters. These derivatives were synthesised in 11 linear steps from a common advanced intermediate, which allowed for divergent modification at the C-9 position. Four substitutions were installed in this study: O-acetyl, O-lactyl, O-SO₃, O-PO₃. This synthetic pathway includes both an effective way to benzylate neuraminic acid derivatives, as well as a working methodology towards unnatural β-linked neuraminic acid glycosides.

Full text of DOI:10.1002/ejoc.202001049

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: A General Method for the Divergent Synthesis of C-9
Functionalised Sialic Acid Derivatives
Authors: Abdullah A Hassan and Stefan Oscarson
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202001049
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0
1/2020

European Journal of Organic Chemistry
10.1002/ejoc.202001049
FULL PAPER
A General Method for the Divergent Synthesis of C-9
Functionalised Sialic Acid Derivatives
[a]
[a]
Abdullah A. Hassan and Stefan Oscarson*
[
a]
A. A. Hassan. Prof S. Oscarson
Centre for Synthesis and Chemical Biology
University College Dublin,
Dublin 4, Ireland
https://oscarsonresearch.wordpress.com/
E-mail: Stefan.oscarson@ucd.ie
Supporting information for this article is given via a link at the end of the document
Abstract: Sialic acids, a ubiquitous family of sugars shown to be
involved in numerous biologically important processes, exhibit
remarkable structural diversity in nature. Access to these derivatives
by chemical and enzymatic means is a major bottle neck in
understanding the role played by each particular modification. As part
of a program to study such roles and determine the substrate
specificity of novel sialic acid aldolases, a general and robust
synthetic protocol was devised to gain access to all naturally occurring
C-9 functionalised N-acetylneuraminic acid derivatives including
esters. These derivatives were synthesised in 11 linear steps from a
common advanced intermediate, which allowed for divergent
modification at the C-9 position. Four substitutions were installed in
N-acetylneuraminate lyases) are a group of enzymes that
catalyse the reversible aldol condensation of appropriately
modified ManNAc precursors and pyruvate to generate the
corresponding neuraminic acid derivatives (Scheme 1)[
Subsequently, we discovered there was a need for an efficient
synthetic protocol to access these important class of sialic acid
derivatives.
8],[9]
3 3
this study: O-acetyl, O-lactyl, O-SO , O-PO . This synthetic pathway
includes both an effective way to benzylate neuraminic acid
derivatives, as well as a working methodology towards unnatural -
linked neuraminic acid glycosides.
Scheme 1: Chemoenzymatic synthesis of C-9 functionalised
Neu5Ac derivatives
Introduction
Although there are a few chemoenzymatic & chemical syntheses
of C-9 analogues of Neu5Ac reported in literature,[10],[11],[12] there
is no general synthetic route for the preparation of C-9 modified
Neu5Ac derivatives that tolerate a diverse range of functionality
Sialic acids are a family of nonulosonic acids that occur as
terminal glycans in
a large variety of glycoproteins and
glycolipids.[ Due to their peripheral location, in conjunction with
their negative charge at physiological pH, sialic acids have
regulatory and protective properties through their interactions with
carbohydrate binding proteins.[2] This implicates them in
numerous biologically important processes such as cellular
recognition, adhesion and signalling phenomena.[3],[4] N-
acetylneuraminic acid (Neu5Ac), the most abundant form of sialic
acid found in mammalian cells, also exhibits remarkable structural
diversity, with more than 50 different derivatives identified in
nature.[5],[6] These structural variations can occur as a variety of
O-substitutions (acetylation, lactylation, sulfation & methylation)
at C-4,C-7,C-8 and C-9, or as N-substitutions at C-5. With a
notable exception to the highly studied acetylated C-9 Neu5Ac
derivatives,[7] the exact biological significance of the remaining
derivatives is poorly understood; mostly due to limited access to
these compounds. As the modification at the C-9 position of
Neu5Ac has been shown to modulate a range of biological
processes,[4] a robust and reliable protocol to functionalise the C-
1]
(
ethers, esters, phosphates & sulfates etc). It should be noted that
regioselective introduction of our desired functionalities at C-9
was troublesome due to the known high reactivity of the C-4
hydroxy group of Neu5Ac leading to di- and poly-functionalised
derivatives.[
11],[13],[14]
Our synthetic route facilitates the use of
benzyl ether protecting groups, which are typically not tolerated in
nonulosonic acids due to the presence of the carboxylic acid
moiety and 3-deoxy methylene group; resulting in the competing
elimination side reaction, lactamisation, ring contraction products
and other indistinguishable side products.[15],[16] Once introduced
the benzyl groups are excellent protecting groups due to their
stability but also their mild removal under neutral- or slightly acidic
conditions allowing almost any substituent in the target structure,
especially ester groups where known C-9 to C-7 acyl migration
can be prevented during deprotection.
In order to utilise these C-9 modified derivatives in the retro-aldol
condensation reaction and obtain synthetic pure model
compounds to further elucidate their biologically significance, we
embarked on their divergent synthesis.
9
position of Neu5Ac is described in this report.
Originally in our research group, the corresponding C-6
functionalised N-acetylmannosamine (ManNAc) biosynthetic
precursors were prepared as part of a program to study the
substrate specificity of novel sialic acid aldolases isolated by
biological collaborators. Sialic acid aldolases (also referred to as
Results and Discussion
Initially, it was envisaged that we could gain access to C-9
derivatised neuraminic acid analogues by regioselectively
1
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European Journal of Organic Chemistry
10.1002/ejoc.202001049
FULL PAPER
protecting the primary C-9 alcohol of a Neu5Ac thiophenol
derivative, followed by global benzylation of the remaining
hydroxy moieties (Scheme 2). With this synthetic strategy, we
planned to orthogonally mask our anomeric position with a stable
thiophenol functionality and hence the known compound 5[16] was
prepared and tritylated to give 6. However, subsequent
benzylation of the triol 6 proved to be non-trivial.
yield. The concomitant formation of the benzyl ester was
beneficial to our synthesis as it facilitated a single global
debenzylation step as opposed to an additional methyl hydrolysis
step, which does not allow C-9 ester functionalities in our final
target structures.
Scheme 2: Failed synthetic sequence
Although there are a handful reported cases of benzylated
neuraminic acid derivatives (albeit in low yield) mainly by Sharma
and others, we found this reaction to be quite difficult (See SI for
detailed benzylations attempts).[17],[18] One of the reasons for this
difficulty can be attributed to the unique structural features of
neuraminic acid namely, the electron withdrawing nature of the
carboxylic acid moiety and the presence of the acidic 3-deoxy
3 2 2
Scheme 3: Reagents & Conditions; (a) Et N, CH Cl , rt, 91%,
(b) NaOMe, MeOH, rt, 9 h (c) TrCl, pyridine, 60 °C, 48 h, 68%
(d) BnBr, Ba(OH) ·8H O, BaO, DMF, 0 °C – rt, 16 h, 75%.
2
2
With our desired benzylated glycal in hand, we turned our
attention to the olefin addition reaction used widely in the
synthesis of 2-deoxy sugars.[25] Initially, we treated 12 with N-
iodosuccinimide (NIS) in acetic acid at 65 °C (Scheme 4),
however the harsh acidic conditions resulted in the cleavage of
the trityl ether protecting group.[ Subsequent replacement of
acetic acid with benzyl alcohol as our nucleophile was beneficial
in two folds; it prevented premature cleavage of the trityl moiety,
whilst also masking our newly installed hydroxy anomer as a
benzyl ether – ultimately reducing the total number of steps.
However, this reaction resulted in a mixture of inseparable
diastereomers as analysed by TLC and NMR. In the complex
[
19],[20]
methylene protons.
This led us to explore several
benzylation methods using a variety of bases, acids, benzylating
reagents and solvents. We found that strong bases, such as NaH,
as expected resulted in a complex mixture of products (elimination,
partial O- and N-benzylation, lactamisation, ring contraction
amongst other indistinguishable products). When we switched to
relatively milder bases, the desired per-benzylated compound 7
was not formed, instead partial benzylation with minor elimination
26]
product was observed when using Ba(OH)
of the reaction was detected with Ag CO
temperatures. We then switched to acidic conditions using benzyl
trichloroacetimidate and triflic acid in CH Cl
,[21] the results were
2
, while no progression
2
3
, even at elevated
NMR, three distinct C-1 signals could be observed (in the ratio of
3
1: 0.7: 0.7), one with a J
C1-H3
of 3.7 Hz (compound 13) and the
2
2
other two being broad singlets, which we tentatively assign as
compounds 14 and 15. Although the anomeric configuration of
our target compounds is insignificant to our studies, having some
sort of diastereoselectivity would ease characterisation and
purification attempts. In 2017, Li and co-workers reported the
diastereoselective preparation of 2-deoxy-2-iodo-glycosides from
wistfully similar to their base mediated counterparts in addition to
the observance of the trityl ether cleavage.
With these results, and inspired by work on Kdo (keto-3-deoxy‐α‐
D‐manno‐oct‐2‐ulosonic acid) glycals by Kosma and our own
group,[
22],[23]
we decided to revise our synthetic plan to incorporate
their corresponding glycals and glycosyl acceptors using I
2
and
the main competing elimination product into our synthetic
sequence. Accordingly, per-acetylated glycal 9 was prepared by
elimination of chloride 8 in a 91% yield (Scheme 3).[24] De-
esterfication using freshly prepared 1 M sodium methoxide
solution furnished 4,7,8,9-tetrahydroxy neuraminic acid glycal 10.
The alkaline solution was subsequently stirred with an acidic resin
overnight to restore the methyl ester functionality.
Complementary to our previous synthetic endeavour (Scheme 2),
the primary alcohol at C-9 was temporarily masked as a trityl ether
diacetoxy(iodo)benzene to furnish the desired -glycosides.[
27]
Extending Li’s protocol by using I , PhI(OAc) and benzyl alcohol
2
2
in anhydrous CH
3
CN, we were able to predominantly obtain the
2
,3-diaxial 3-iodo--benzyl glycoside in an 85% yield. The
1
presence of additional benzylic H NMR peaks at 4.71 & 4.34 ppm,
and the upfield shift of C-3 proton from 6.13 ppm to 4.83 ppm,
J
confirmed formation of 13 (9:1, 13:14; based on 3 C1-H3).[28],[29]
Whilst we could have reductively removed the 3-iodo substituent
at this stage, we decided to retain it until the final step to avoid
future elimination issues when functionalising the C-9 position. In
(
11, 68%), followed by subsequent benzylation of the triol hydroxy
moieties and transesterification to give benzyl ester 12 in 75%
2
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European Journal of Organic Chemistry
10.1002/ejoc.202001049
FULL PAPER
our hands, 3-iodo Neu5Ac compounds were stable under basic
and acidic conditions.
Scheme 4: Glycal addition reaction for formation of 13
With the C-9 hydroxy liberated using miniscule equivalents of
formic acid in CH Cl , we were able to functionalise the advanced
2 2
common intermediate 16 (Scheme 5) with our desired
substituents. The acetyl substituent was installed under
conventional conditions using acetic anhydride in pyridine (17,
83%). Similarly, 16 was sulfated by the addition of 5 equivalents
of SO Et N complex in DMF, furnishing sulfate 18 in 75% yield.
3
3
The lactyl ester analogue was introduced via an acylation reaction
between 16 and silyl protected lactyl chloride 19, which itself was
prepared in 3 steps (89% yield) from commercially available L-
methyl lactate, to afford 20 in a 73% yield. Finally, phosphate 22
was prepared using diphenyl phosphoryl chloride in pyridine in a
96% yield.
Scheme 5: Divergent synthesis of C-9 functionalised Neu5Ac
derivatives
3
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European Journal of Organic Chemistry
10.1002/ejoc.202001049
FULL PAPER
(
101 MHZ or 125 MHz) spectra were recorded on Varian-inova
With our divergently synthesised C-9 modified neuraminic acid
derivatives in hand, we turned to their deprotection (Scheme 6).
spectrometers at 25 °C in chloroform-d1 (CDCl
3
), MeOH-d4
((CD CO),
(CD
3
OD),
water-d2
(D
2
O),
SO).
acetone-d6
H
3 2
)
1
Hydrogenolysis of 22 using Adam’s catalyst (PtO
2
) selectively
dimethylsulfoxide-d6 ((CD
3
)
2
NMR spectra were
, δ = 7.26
O, δ = 4.79 ppm; (CD SO δ =
CO, δ = 2.84 ppm); or internal trimethylsilane, δ
= 0.00 ppm). 13C NMR spectra were standardised against the
residual solvent peak (CDCl , δ = 77.16 ppm; CD OD, δ = 49.00
ppm; (CD SO δ = 39.52 ppm; (CD
CO, δ = 29.84 ppm). All 13
cleaved the phenyl and 3-iodo moieties to afford 23 in a
quantitative yield. Removal of the TBS group in 20 was
standardised against the residual solvent peak (CDCl
ppm; CD OD, δ = 3.31 ppm; D
2.50 ppm; (CD
3
3
2
3 2
)
accomplished by treatment with acetic acid in THF and CH
2
Cl
2
to
3 2
)
give 21 in 93% yield. All four derivatives 17, 18, 21, and 23, were
then globally de-benzylated by palladium (10% mol) catalysed
3
3
hydrogenolysis under an atmosphere of H
2
in a solvent mixture of
3
1
)
2
3
)
2
C
EtOAc and MeOH, which also concomitantly reduced the 3-iodo
function in 17, 18 and 21. The polarity of the solvent mixture was
gradually increased as the reaction progressed (80:20 → 10:90
v/v; EtOAc: MeOH). Following chromatography, we were able to
isolate all four desired C-9 functionalised neuraminic acid
derivatives in yields ranging from 61-85%.
NMR are H decoupled. All NMR data is represented as follows:
chemical shift (δ ppm), multiplicity (s = singlet, d = doublet, t =
triplet, q = quartet, dd = doublet of doublets, ddd = doublet of
doublets of doublets, dt = doublet of triplets, m = multiplet, br =
broad signal, ad = apparent doublet, at = apparent triplet),
coupling constant in Hz, integration. Mass spectrometry was
determined using either Waters Quattro Micro LC-MS/MS in
electronspray ionisation (ESI) mode or by matrix -assisted laser
deposition/ionization (MALDI) modes.
Experimental Procedure and Data:
Methyl 5-acetamido-4,7,8,9-tetra-O-acetyl-2,6-anhydro-3,5-
dideoxy-D-glycero-D-galacto-non-2-enonate (9) A pre-oven
dried round bottom flask containing compound 8 (11.9 g, 23.3
mmol), pre-activated 4 Å molecular sieves and a stirring bar were
dried on a Schlenk line for 15 mins. The reaction vessel was then
Scheme 6: Catalytic hydrogenolysis to furnish desired target
2 2 3
charged with CH Cl (233 mL) and anhydrous Et N (260 mL, 1.87
structures
mol) was added dropwise to the flask in an ice-water bath. The
suspension was stirred at room temperature for 2 h upon which
the reaction mixture was concentrated down under reduced
pressure. The resulting yellow coloured oil was diluted with
Conclusion
In summary, we describe herein a general and robust synthetic
protocol for the divergent preparation of naturally occurring C-9
functionalised sialic acid derivatives from N-acetylneuraminic acid
in 9 short linear steps. The synthesis includes both an effective
way to benzylated neuraminic acid derivatives as well as a
working methodology towards -linked neuraminic acid
glycosides. The target compounds will be used as substrates for
chemoenzymatic studies (i.e. retro-aldol condensation reactions),
as a well as model synthetic compounds for understanding the
biological implications of diversification of N-acetylneuraminic
acid analogues at the terminal primary alcohol.
2 2 3
CH Cl (250 mL) and washed consecutively with NaHCO (2 x
100 mL), H O (2 x 100 mL) and brine (2 x 100 mL). The combined
2
2 4
organic fractions were dried over Na SO and evaporated in
vacuo. The crude glycal was purified by flash column
chromatography (cHEX/EtOAc, 30:70 → 10:90 cHEX/EtOAc v/v)
to give 9 as a white amorphous solid (10.0 g, 21.20 mmol, 91%).
1
R
f
: 0.52, cHEX/EtOAc (10:90); H NMR (300 MHz, CD
3
OD) δ 5.97
(
d, J = 2.8 Hz, 1H, H-3), 5.57 (dd, J = 8.4, 2.6 Hz, 1H, H-7), 5.52
(dd, J = 6.1, 2.6 Hz, 1H, H-8), 5.39 (ddd, J = 6.3, 2.8 Hz, 1H, H-
4), 4.62 (dd, J = 12.4, 2.9 Hz, 1H, H-9a), 4.47 (dd, J = 10.2, 2.6
Hz, 1H, H-6), 4.24 – 4.12 (m, 2H, H-9b, H-5), 3.83 (s, 3H, OMe),
2
3
1
1
6
.13 (s, 3H, OAc), 2.08 (s, 3H, OAc), 2.08 (s, 3H, OAc), 2.07 (s,
H, OAc), 1.92 (s, 3H, NAc) ppm; 13C NMR (126 MHz, CDCl
) δ
3
70.76, 170.56, 170.19, 170.13, 170.08 (C=O x 5), 161.59 (C-1),
45.02 (C-2), 108.0 (C-3), 70.81 (C-6), 68.14 (C-7), 67.6 (C-4),
1.9 (C-9), 52.51 (CH ), 46.34 (C-5), 23.02, 20.81, 20.79, 20.68,
3
Experimental Section
20 27 12
20.66 (5 x Ac) ppm; HRMS (ESI): m/z calcd for C H NNaO :
+
[30]
4
96.1431 [M + Na] ; found 496.1457.
General Methods:
All the starting material chemicals were purchased from
commercial suppliers (Carbosynth, Sigma Aldrich, Flourochem
and Acros) and used without further purification. Unless otherwise
stated, all reactions containing air- and moisture sensitive
reagents were carried out under an inert atmosphere of nitrogen
in oven-dried glassware with magnetic stirring. Anhydrous
Methyl
5-acetamido-2,6-anhydro-3,5-dideoxy-9-O-trityl-D-
glycero-D-galacto-non-2-enonate (11) Na (0.32 g, 13.7 mmol)
was added to a vessel containing MeOH (21 mL) and the resulting
solution was added dropwise to 9 (6.5 g, 13.7 mmol) in MeOH (5
mL). The reaction mixture was stirred at room temperature for 9
h. Upon complete de-acetylation (monitored by TLC analysis
EtOAc, 100 %), the reaction was quenched with Amberlyst® 15
hydrogen form resin and stirred overnight. The resin was
subsequently filtered off and the reaction mixture was
concentrated in vacuo to give compound 10, which was used in
the next step without further purification. 10 (4.18 g, 13.7 mmol)
was dissolved in anhydrous pyridine (45 mL) and trityl chloride
TM
solvents were obtained from PureSolv-EN solvent purification
system or purchased from Sigma-Aldrich in AcrosSeal® bottles.
All reactions were monitored by thin-layer chromatography (TLC)
on Merck DC-Alufolien plates precoated with silica gel 60 F254.
TLC plates were visualised with UV-light (254 nm) and stained
(
4.6 g, 16.44 mmol) was added. The reaction mixture was heated
2 4
with H SO (8%) and/or ninhydrin solution. Silica gel column
to 60 °C and stirred at this temperature overnight. The reaction
mixture was poured into ice-water, extracted with an excess of
CH
Na
chromatography was carried out using Davisil silica gel or with
automated flash chromatography suite (Buchi Reveleris X2 AND
Biotage SP4 HPFC). H NMR (300, 400 or 500 MHz), 13C NMR
2
Cl
2
(50 mL) and the combined organic layers was dried over
4
. Toluene was added to the solution for co-evaporation
1
2
SO
4
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European Journal of Organic Chemistry
10.1002/ejoc.202001049
FULL PAPER
and the solvents were removed under reduced pressure. The
crude residue was purified by flash column chromatography
H-7, CH
2
Ph), 4.26 (dd, J = 8.7, 1.4 Hz, 1H, H-6), 4.04 (dt, J = 8.9,
2
2.6 Hz, 1H, C-5), 3.98 (d, J = 11.3 Hz, 1H, CH Ph), 3.85 (dd, J =
(
Toluene/EtOAc, 40:60 → 10:90 v/v) to give 11 as a white foam
10.7, 2.2 Hz, 1H, H-9a), 3.40 (dd, J = 9.9, 2.5 Hz, 1H, H-4), 3.28
(dd, J = 10.7, 3.1 Hz, 1H, H-9b), 2.06 (s, 3H, NAc); C NMR (151
MHz, CD OD) δ 172.1 (C=O), 166.3 (C=O), 143.9, 138.5, 138.3,
3
137.8, 136.1, 135.1, 128.2, 128.1, 127.92, 127.90, 127.7, 127.5,
127.3, 127.1, 126.8, 126.7, 126.6, 125.8 (35 x C-Ar), 100.6 (C-2),
1
13
(
5.1 g, 9.32 mmol, 68%). R
OD) δ 7.53 – 7.47 (m, 6H, ArH), 7.35 – 7.28 (m, 6H,
ArH), 7.26 – 7.22 (m, 3H, ArH) 5.96 (d, J = 2.5 Hz, 1H, H-3), 4.46
dd, J = 8.8, 2.5 Hz, 1H, H-4), 4.21 (dd, J = 10.8, 1.3 Hz, 1H, H-
), 3.98 (dd, J = 10.9, 8.8 Hz, 1H, H-5), 3.78 (s, 3H, OMe), 3.75
dd, J = 9.5, 1.3 Hz, 1H, H-9a), 3.43 (dd, J = 9.5, 2.4 Hz, 1H, H-
), 3.27 (dd, J = 9.5, 5.4 Hz, 1H, H-8), 2.04 (s, 3H, NAc) ppm; 13
NMR (126 MHz, CD OD) δ 175.2 (NC=O), 164.9, 164.4 (C-1),
f
: 0.15, EtOAc (100%); H NMR (500
MHz, CD
3
(
6
(
7
86.3 (C-6), 77.2 (CH
2
Ph), 74.6 (C-8), 74.4 (C-7), 73.6 (C-5), 71.8
Ph), 66.7 (C-4), 60.5 (C-9), 34.2
(CH Ph), 70.3 (CH Ph), 67.2 (CH
2
2
2
C
(C-3), 21.6 (NAc) ppm; HRMS (ESI): m/z calcd for C65
1150.3367 [M + Na] ; found 1150.3401
H
62INNaO
9
:
+
3
1
3
49.0 (C-2), 145.7, 145.2, 130.1, 129.3, 128.8 (C-Ar), 113.6 (C-
), 87.7 (C-Tr)), 78.2, 70.4, 69.9 (C-6), 67.8 (C-7), 66.9 (C-4), 61.6
Benzyl (benzyl 5-acetamido-4,7,8-tri-O-benzyl-3,5-dideoxy-3-
iodo-β-D-erythro-L-manno-non-2-ulopyranosid)onate (16)
Formic acid (0.2 mL) was added to a solution of compound 13
2.01 g, 1.78 mmol) in CH Cl (9 mL) at 0 °C. The resulting yellow
(
3
C-9), 52.9 (CH ), 52.0 (C-5), 22.8 (NAc); HRMS (ESI): m/z calcd
+
[31]
8
for C31H33NO : 570.2104 [M + Na] , found 570.2098.
(
2
2
Benzyl
dideoxy-9-O-trityl-D-glycero-D-galacto-non-2-enonate
Compound 11 (2.38 g, 4.35 mmol) was solubilised in anhydrous
DMF (21.7 mL) and at O °C Ba(OH) ·8H O (4.12 g, 13.05 mmol),
5-acetamido-2,6-anhydro-4,7,8-tri-O-benzyl-3,5-
coloured reaction mixture was warmed to room temperature and
stirred at this temperature until TLC showed complete cleavage
of the trityl group (~ 10 min). MeOH (0.2 mL) was added to quench
excess acid, and the solution was concentrated in vacuo and
(12)
2
2
BaO (1.33 g, 8.70 mmol) and BnBr (3.1 mL, 26 mmol) were added. subsequently purified by flash column chromatography
The reaction mixture was allowed to warm up to room
temperature and the suspension was stirred overnight. Upon
completion (analysed by TLC), the reaction mixture was diluted
(cHex/EtOAc, 50:50 v/v) to give compound 17 as a pale yellow
1
foam (1.50 g, 1.69 mmol, 95%). R
NMR (500 MHz, CD OD) δ 7.64 – 7.50 (m, 2H, ArH), 7.37 – 7.20
(m, 16H, ArH), 7.20 – 7.07 (m, 5H, ArH), 6.98 – 6.64 (m, 2H, ArH),
f
: 0.41, cHEX/EtOAc (60:40); H
3
with CH
were filtered through a bed of Celite and washed sequentially with
NaHCO (2 x 50 mL), H O (2 x 50 mL) and brine (2 x 50 mL). The
2 2
Cl (100 mL), and the resulting insoluble inorganic salts
5.19 (d, J = 12.1 Hz, 1H, CH
5.08 (d, J = 12.2 Hz, 1H, CH
2
Ph), 5.15 (d, J = 12.1 Hz, 1H, CH
Ph), 5.02 (d, J = 12.2 Hz, 1H, CH
2
Ph),
Ph),
3
2
2
2
combined organic fractions were concentrated and the resulting
crude product was purified by flash column chromatography
4.83 (d, J = 3.2 Hz, 1H, H-3), 4.79 (d, J = 11.8 Hz, 1H, CH
4.73 (d, J = 10.2 Hz, 1H, CH Ph), 4.69 – 4.62 (m, 3H, CH Ph, H-
7,), 4.55 (dd, J = 11.0, 9.3 Hz, 1H, H-6), 4.48 – 4.41 (m, 2H,
CH Ph), 4.41 – 4.31 (m, 1H), 4.02 (d, J = 11.2 Hz, 1H, CH Ph),
2
Ph),
2
2
(
cHEX/EtOAc, 70:30 → 10:90 v/v) to furnish compound 12 as an
off-white amorphous foam (2.91 g, 3.26 mmol, 75%). R : 0.55,
) δ 7.55 – 7.48
m, 6H, ArH), 7.36 – 7.29 (m, 8H, ArH), 7.29 – 7.17 (m, 19H, ArH),
f
2
2
1
cHEX/EtOAc (50:50); H NMR (500 MHz, CDCl
(
3
3.99 (dd, J = 12.1, 2.5 Hz, 1H H-9a), 3.96 – 3.89 (m, 1H, H-4),
3.86 – 3.80 (m, 1H, H-8), 3.72 (dd, J = 12.1, 3.0 Hz, 1H, H-9b),
1
3
7
=
(
.17 – 7.12 (m, 2H, ArH), 6.13 (d, J = 3.6 Hz, 1H, H-3), 5.23 (d, J
12.3 Hz, 1H, CH Ph), 5.16 (d, J = 12.3 Hz, 2H, CH Ph), 4.74
ddd, J = 6.3, 4.3, 4.1 Hz, 1H, H-8), 4.68 (d, J = 11.9 Hz, 1H,
CH Ph), 4.51 (d, J = 11.9 Hz, 1H, CH Ph), 4.44 (dd, J = 11.5, 6.3
Hz, 2H, H-7), 4.35 (d, J = 11.4 Hz, 1H, CH Ph), 4.27 – 4.23 (m,
H, CH Ph, H-4), 4.21 – 4.17 (m, 1H, H-5), 3.79 (dt, J = 6.0, 4.1
Hz, 1Hz, H-6), 3.75 (dd, J = 10.1, 4.3 Hz, 1H, H-9a), 3.31 (dd, J =
3.65 (dt, J = 8.9, 2.8 Hz, 1H, H-5), 1.94 (s, 3H, NAc). C NMR
(126 MHz, CDCl ) δ 170.8 (C=O), 166.2, 138.2, 138.1, 137.6,
136.0, 134.6, 129.2 – 127.1 (m) (C-Ar), 100.4 (C-Tr), 81.8 (C-6),
75.0 (C-8), 72.6 (C-7), 72.1 (CH Ph), 71.3,(CH Ph) 71.2 (C-6),
71.0 (CH Ph), 68.0 (CH Ph), 67.1 (C-9), 60.4 (C-4), 52.3, 36.9 (C-
2
2
3
2
2
2
2
2
2
2
2
2
3), 23.5 (NAc); HRMS (ESI): m/z calcd for C46
908.2271 [M + Na] ; found 908.2263.
H
48INNaO
9
:
+
13
1
0.1, 4.0 Hz, 1H, H-9b), 1.84 (s, 3H, NAc) ppm; C NMR (126
MHz, CDCl ) δ 169.4 (C=O), 133.9, 128.8, 128.55, 128.50, 128.41,
28.19, 128.16, 128 (C-Ar x 10), 127.7, 127.5, 126.9 (C-Ar), 109.6
C-3), 78.1 (C-6), 77.3 (C-8) 74.72 (CH Ph), 74.1 (C-7), 72.51 (C-
), 71.6 (C-4), 70.70 (CH Ph), 67.11 (CH Ph), 62.41 (C-9), 23.5
NAc) ppm; HRMS (ESI): m/z calcd for C58 : 916.3825
3
Benzyl (benzyl 5-acetamido-9-O-acetyl-4,7,8-tri-O-benzyl-3,5-
dideoxy-3-iodo-β-D-erythro-L-manno-non-2-
1
(
5
(
2
ulopyranosid)onate (17) Acetic anhydride (0.25 mL, 2.67 mmol)
was added to solution of 16 (1.57 g, 1.78 mmol) in anhydrous
pyridine (3.56 mL). The solution was stirred at room temperature
overnight (17 h). Upon completion, the reaction mixture was
diluted in EtOAc and washed sequentially with NaHCO3 (2 x 10
mL), water (2 x 10mL) and brine (2 x 10 mL). The combined
organic fractions were dried over MgSO4 and concentrated under
reduced pressure. The crude acetate was purified by flash column
chromatography (70:30, cHex: EtOAc v/v) to give 17 as pale-
2
2
H55NNaO
8
+
[
M + Na] ; found 916.3822.
Benzyl (benzyl 5-acetamido-4,7,8-tri-O-benzyl-3,5-dideoxy-3-
iodo-9-O-trityl-β-D-erythro-L-manno-non-2-
ulopyranosid)onate (13) Compound 12 (0.83 g, 0.92 mmol) was
dissolved in anhydrous CH
alcohol (0.9 mL, 5.10 mmol), (Diacetoxyiodo)benzene (0.36 g,
.10 mmol) and iodine (0.14 g, 0.55 mmol) were added
3
CN (3.7 mL) and at 0 °C benzyl
yellow foam (1.36 g, 1.47 mmol, 83%). R
f
: 0.45, cHEX/EtOAc
1
(60:40); 1H NMR (600 MHz, CD
3
OD, conformational rotamers
sequentially. The reaction vessel was then removed from the ice
bath and stirred at ambient temperature for 10 min. The reaction
mixture was diluted in EtOAc and washed sequentially with
observed. Spectral data for 1 of the rotamers is reported) δ 7.47
– 7.42 (m, 3H, ArH), 7.40 – 7.23 (m, 15H, ArH), 7.24 – 7.14 (m,
6H, ArH), 7.14 – 7.06 (m, 1H, ArH), 5.22 – 5.17 (m, 1H, CH
5.14 (d, J = 12.1 Hz, 1H, CH Ph), 4.89 (d, J = 2.8 Hz, 1H, H-3),
4.86 – 4.83 (m, 2H, C-8), 4.80 (d, J = 12.1 Hz, 1H, CH Ph), 4.66
– 4.59 (m, 2H), 4.58 – 4.52 (m, 1H, H-4), 4.51 – 4.45 (m, 1H, H-
7), 4.35 (d, J = 12.0 Hz, 1H, CH Ph), 4.15 – 4.07 (m, 2H, CH Ph,
2
Ph),
Na
2
SO
3
(2 x 5 mL), water (2 x 5 mL) and brine (2 x 5 mL). The
and
2
combined organic fractions were dried over MgSO
4
2
evaporated under reduced pressure. The crude product was
purified by flash column chromatography (cHEX/EtOAc, 70:30
2
2
v/v) to afford 13 (0.88 g, 0.78 mmol, 85%) as an amorphous foam.
H9a), 4.01 (ddd, J = 8.6, 5.1, 2.3 Hz, 1H, H-6), 3.91 – 3.82 (m, 1H,
H-5), 3.62 (dd, J = 7.8, 1.4 Hz, 1H, H-9b), 2.04 (s, 3H, OAc), 1.74
1
R
f
: 0.68, cHEX/EtOAc (60:40); H NMR (500 MHz, CDCl
3
) δ 7.52
–
2
7.41 (m, 6H, ArH), 7.40 – 7.27 (m, 10H, ArH), 7.29 – 7.12 (m,
0H, ArH), 7.07 (dd, J = 8.4, 6.9 Hz, 2H, ArH), 6.74 – 6.68 (m, 1H,
NH), 4.83 (d, J = 2.5 Hz, 1H, H-3), 4.80 (d, J = 10.3 Hz, 2H,
CH Ph), 4.71 (d, J = 10.3 Hz, 1H, CH Ph), 4.65 (d, J = 11.9 Hz,
Ph), 4.60 (d, J = 8.8 Hz, 1H, CH Ph), 4.52 (d, J = 12.4 Hz,
Ph), 4.45 – 4.42 (m, 2H, H-8, CH Ph), 4.41 – 4.35 (m, 3H,
(s, 3H, NAc). ppm; 13C NMR (151 MHz, CDCl
3
) δ 170.7 (C=O),
165.9 (NC=O), 138.2, 138.0, 137.8, 137.7, 137.5, 136.3, 129.0,
128.9, 128.7, 128.57, 128.52, 128.47, 128.45, 128.41, 128.3 (C x
5, C-Ar), 128.1, 127.9, 127.88, 127.86, 127.79, 127.75, 127.6,
127.59, 127.50, 127.3 ( C-Ar), 100.5 (C-2), 99.9 (C-8), 81.1 (C-6),
2
2
1
1
H, CH
H, CH
2
2
2
2
2 2 2
78.2, 75.3, 74.6, 74.0 (CH Ph), 73.4, 73.2 (CH Ph), 72.9 (CH Ph),
5
This article is protected by copyright. All rights reserved.

European Journal of Organic Chemistry
10.1002/ejoc.202001049
FULL PAPER
7
6
2.8, 72.1, 71.8, 71.4, 70.7, 67.7 (C-5), 67.5 (C-9), 66.9 (CH
6.2, 63.4 (C-4), 62.8, 30.85 (C-3), 23.5 (OAc), 21.0 (NAc) ppm;
50INNaO10: 950.2377 [M
2
Ph),
71.8 (CH
2
Ph), 71.4 (C-8), 70.7, 67.5 (C-9), 66.2 (C-5), 51.7 (C-4),
36.3 (C-3), 21.00 (NAc); HRMS (MALDI-TOF): m/z calcd for
C H66INNaO11Si: 1094.3348 [M + Na] ; found 1094.3352.
55
+
HRMS (MALDI-TOF): m/z calcd for C48
+
H
+
Na] ; found 950.2379
Benzyl (benzyl 5-acetamido-4,7,8-tri-O-benzyl-3,5-dideoxy-3-
iodo-9-O-(diphenyl)phosphoryl-β-D-erythro-L-manno-non-2-
ulopyranosid)onate (22) Compound 16 (0.27 g, 0.31 mmol) was
dissolved in anhydrous pyridine (1.1 mL), and, at 0 °C, diphenyl
phosphoryl chloride (0.17 g, 0.62 mmol) was added portion-wise.
The reaction mixture was then stirred at room temperature (12 h).
The reaction was quenched with MeOH (1 mL) before being
evaporated under reduced pressure (co-evaporation with toluene).
The resulting crude phosphate was purified by silica gel flash
column chromatography (70:30, cHEX: EtOAc v/v) to furnish 23
Benzyl (benzyl 5-acetamido-4,7,8-tri-O-benzyl-3,5-dideoxy-3-
iodo-9-O-sulfo-β-D-erythro-L-manno-non-2-
ulopyranosid)onate (18) Compound 16 (0.5 g, 0.58 mmol) was
solubilised in dry DMF (5.8 mL) and at O °C, SO
3 3
.Et N (0.525 g,
2
.9 mmol) powder was added portion-wise. Et N (0.40 mL, 2.9
3
mmol) was subsequently injected into the pale-yellow solution.
The reaction mixture warmed to room temperature and stirred at
this temperature overnight. Upon completion of the reaction
(
monitored by TLC), the solution was concentrated under reduced
pressure (co-evaporation toluene) and re-dissolved in CH Cl
before being washed sequentially with NaHCO (2 x 10 mL),
water (2 x 10 mL) and brine solution (2 x 10 mL). The combined
organic fractions were dried over Na SO and concentrated in
vacuo before being purified by flash column chromatography
2
2
f
as an off-white amorphous solid (0.33 g, 0.29 mmol, 96%). R :
1
3
0.31, cHEX:EtOAc, (1:1); H NMR (500 MHz, CDCl
(m, 2H, ArH), 7.35 – 7.18 (m, 33H, ArH),, 5.17 – 5.11 (m, 2H,
CH Ph, NH), 5.08 (dd, J = 12.1, 7.8 Hz, 1H, CH Ph), 4.96 (d, J =
3.8 Hz, 1H, H-3), 4.80 – 4.77 (m, 2H, H-7, CH Ph), 4.74 (d, J =
11.7 Hz, 2H, CH Ph), 4.60 – 4.54 (m, 2H, H-6, CH Ph), 4.52 –
4.46 (m, 2H, CH Ph), 4.45 – 4.39 (m, 3H, H-9a, H-8, CH Ph), 4.10
) δ 7.40 – 7.35 (m, 5H, ArH), 7.33 – 7.16 (m, 14H, – 4.02 (m, 2H, H-4), 3.96 (ddd, J = 8.6, 5.1, 2.3 Hz, 1H, H-5), 3.85
3
) δ 7.41 – 7.36
2
4
2
2
2
(
(
(
60:40, cHex: EtOAc v/v) to give 18 as a white amorphous solid
2
2
1
0.42 g, 0.43 mmol, 75%). R
600 MHz, CDCl
f
: 0.34, cHEX/EtOAc (60:40); H NMR
2
2
3
ArH), 7.17 – 7.07 (m, 6H, ArH), 4.82 (d, J = 3.1 Hz, 1H, C-3), 4.79
– 3.76 (m, 1H, H9b), 3.73 (dd, J = 5.0, 1.9 Hz, 1H, H-7), 3.56 (dd,
–
2
2
4.76 (m, 1H, CH
H,CH Ph, H8), 4.59 – 4.52 (m, 3H, CH
H, CH Ph), 4.44 – 4.38 (m, 2H, H-9b) 4.08 – 4.00 (m, 2H, H-9b,
2
Ph), 4.73 (d, J = 11.9 Hz, 1H, CH
2
Ph), 4.65 (m,
J = 7.8, 1.9 Hz, 1H, H-6), 1.98 (s, 3H, NAc) ppm; 13C NMR (126
2
2
Ph, H-7), 4.51 – 4.45 (m,
3
MHz, cdcl ) δ 171.3 (C=O), 166.6 (C=O), 138.6, 138.5, 138.0,
136.5, 135.0, 129.5, 129.4, 129.1, 129.0, 128.88, 128.85, 128.7,
128.48, 128.3, 128.2, 128.1, 127.95, 127.90 (C-Ar), 100.8 (C-2),
2
H-6, H-4), 3.75 (ddd, J = 7.4, 4.6, 2.5 Hz, 1H, H-5), 1.97 (s, 3H,
1
3
NAc) ppm; C NMR (126 MHz, CDCl
1
1
3
) δ, 170.7 (C=O), 166.1 (C-
), 136.3, 134.8, 134.7, 134.65, 134.61, 128.36, 128.34, 128.32,
28.31, 128.2, 127.3, (C-Ar x 20), 100.3 (C-2), 74.9 (C-6), 72.5
Ph), 71.2 (C-5), 71.1 (CH Ph), 70.9 (CH Ph), 67.9
C-9), 67.0 (C-8), 60.5 (CH Ph), 52.2 (C-4), 36.8 (C-3), 23.4 (NAc)
82.2 (C-6), 73.0 (C-7), 72.5 (CH
71.4 (CH Ph), 68.50 (CH Ph), 67.53 (C-5), 60.8 (CH
4), 37.3 (C-3), 23.9 (NAc); HRM S (MALDI-TOF): m/z calcd for
2
Ph), 71.7 (C-8), 71.6 (CH
2
Ph),
2
2
2
Ph), 52.7 (C-
+
(
(
C-7), 72.0 (CH
2
2
2
58
C H57INNaO12P: 1140.2561 [M + Na] ; found 1140.2592
2
ppm; HRMS (MALDI-TOF): m/z calcd for C46H48INNaO12S:
+
9
88.1840 [M + Na] ; found 988.1856.
General Procedure for catalytic de-benzylation of C-9
functionalised Neu5Ac derivatives:
C-9 functionalised Neu5Ac (1 eq) was solubilised in a mixture of
EtOAc and MeOH (80:20 0.2 M). Pd/C (10% wt) was added and
Benzyl (benzyl 5-acetamido-4,7,8-tri-O-benzyl-3,5-dideoxy-3-
iodo-9-O-L-lactyl-β-D-erythro-L-manno-non-2-ulopyranosid)
onate (21) L-Lactyl chloride 19 (75 mg, 0.33 mmol) in DMF (0.8
mL) was added slowly to 16 (0.250 g, 0.28 mmol) solubilised in
anhydrous pyridine (2 mL) at O °C. The reaction mixture was then
allowed to warm to room temperature and stirred at this ambient
temperature upon completion of the reaction (5 h). MeOH (1 mL)
was added to the solution before evaporation under reduced
2
the reaction mixture was stirred under H atmosphere (10 bar).
After stirring the mixture at room temperature overnight, the
polarity of the reaction solvent was increased to 10:90 (EtOAc:
MeOH v/v) and the reaction was continued for a further 12 h. The
reaction was then filtered, concentrated in vacuo and purified by
flash column chromatography (EtOAc:MeOH 85:15 v/v).
pressure. The crude material was diluted in CH
2 2
Cl and washed
sequentially with NaHCO (2 x 10 mL), water (2 x 10 mL) and
3
brine (2 x 10 mL). The combined organic fractions were dried over
MgSO4, concentrated in vacuo and then purified by column
chromatography (50:50, cHEX:EtOAc v/v) to generate 20 as a
pale-yellow foam (0.19 g, 0.18 mmol, 73%). Subsequently, glacial
acetic acid (0.5 mL) was added to 20 dissolved in THF (1.8 mL)
and the solution was stirred at room temperature for 30 min. The
resulting solution was concentrated down and purified by flash
flash column chromatography (60:40. cHex:EtOAc v/v) to furnish
5
-Acetamido-9-O-acetyl-3,5-dideoxy-D-glycero-D-galacto-
non-2-ulopyranosonic acid (1) Compound 1 was prepared from
7 according to the General Procedure to afford the title
compound as an off-white amorphous solid (162 mg, 462 mmol,
1
1
7
8
8%). H NMR (500 MHz, DMSO) δ 4.11 – 4.02 (m, 2H, H-9a, H-
), 3.93 (t, J = 10.2 Hz, 1H, H-7), 3.84 (dd, J = 11.9, 2.7 Hz, 1H,
H-9b), 3.79 – 3.71 (m, 1H, H-5), 3.71 – 3.53 (m, 2H, H-6, H-4),
.72 (dd, J = 12.9, 4.5 Hz, 0.05H, H-3eq), 2.32 (dd, J = 13.1, 5.0
2
1 ( 0.16 g, 0.17 mmol, 93%) as white foam. R
f
: 0.49, cHex:EtOAc
) δ 7.46 – 7.41 (m, 6H, ArH),
.37 – 7.27 (m, 13H, ArH), 7.24 – 7.16 (m, 3H, ArH), 7.14 – 7.06
PH), 5.14 (dd, J = 12.1, 7.8
Ph), 4.86 – 4.84 (m, 2H, C-3, CH Ph), 4.80 (d, J =
Ph), 4.64 – 4.59 (m, 2H, CH Ph, H-7), 4.56 –
Ph, H-8), 4.50 (d, J = 6.1 Hz, 1H), 4.48 – 4.46
Ph), 4.03 – 3.99
m, 2H, H9b), 3.78 (dd, J = 5.0, 1.9 Hz, 1H, H-4), 3.75 (ddd, J =
2
1
(
50:50); H NMR (600 MHz, CDCl
3
Hz, 1H, H-3eq), 2.05 (s, 3H, NAc), 1.88 (dd, J = 13.1, 11.5 Hz, 1H,
H-3ax); C NMR (125 MHz, DMSO) δ 173.2 (C1), 171.5 (C=O),
7
13
(
m, 3H, ArH), 5.23 – 5.16 (m, 4H, CH
2
1
6
69.9 (C=O), 95.3 (C-2), 70.5 (C-6), 69.0 (C-8), 66.79 (C-7),
6.75 (C-4), 65.7 (C-9), 53.0 (C-5), 39.4 (C-3), 23.4 (OAc), 20.7
Hz, 1H, CH
2
2
1
4
2.0 Hz, 1H, CH
.53 (m, 2H, CH
2
2
(
NAc) ppm; HRMS (ESI): m/z calcd for C13
H21NNaO10: 374.1063
2
+
[
M + Na] ; found 374.1097. NMR signals consistent with
(
(
m, 2H,H-6, H-9a), 4.13 (d, J = 11.4 Hz, 1H, CH
2
[32]
previously reported spectral data
7
.4, 5.0, 2.5 Hz, 1H, H-5), 2.04 (s, 3H, NAc), 1.74 (s, 3H, lactyl
13
3 3
CH ) ; C NMR (151 MHz, CDCl ) δ 170.7 (C=O), 170.6 (C=O),
5
-Acetamido-3,5-dideoxy-9-O-L-lactyl-D-glycero-D-galacto-
non-2-ulopyranosonic acid (2) Compound 2 was prepared from
1 according to the General Procedure to afford the title
1
1
1
1
69.7 (C=O), 136.3, 134.8, 129.0, 128.9, 128.7, 128.57, 128.55,
28.52, 128.46, 128.45, 128.41, 128.38, 128.36, 128.34, 128.31,
28.13, 128.10, 128.0, 127.9, 127.8, 127.79, 127.75, 127.6,
27.59, 127.50, 127.3 (C-Ar), 99.9 (C-2), 81.1 (C-6), 75.3
2
1
compound as a white foamy solid (56 mg, 148 mmol, 82%). H
NMR (500 MHz, D O) 3.94 (dt, J = 11.3, 2.7 Hz, 2H, H-9a, H-4),
.87 – 3.76 (m, 1H, H-6), 3.73 (dd, J = 11.7, 2.6 Hz, 1H, H-9b),
2
(
2 2 2 2
CH Ph), 74.0 (CH Ph), 73.4 (CH Ph), 72.8 (CH Ph), 72.1 (C-7),
3
6
This article is protected by copyright. All rights reserved.

European Journal of Organic Chemistry
10.1002/ejoc.202001049
FULL PAPER
3
5
.64 (ddd, J = 8.8, 6.1, 2.4 Hz, 1H, H-8), 3.56 – 3.49 (m, 1H, H-
), 3.45 (t, J = 7.2 Hz, 1H, H-7), 2.68 – 2.54 (m, 1H, minor alpha
[
7]
K. N. Barnard, B. R. Wasik, J. R. Laclair, D. W. Buchholz, W. S.
Weichert, B. K. Alford-Lawrence, H. C. Aguilar, C. R. Parrish, MBio
2019, 10, DOI 10.1128/mBio.02490-19.
H-3eq), 2.20 (dd, J = 13.0, 4.7 Hz, 1H, H-3eq), 1.94 (s, 3H, NAc),
1
3
1.89 – 1.63 (m, 4H, H-3ax, lactyl CH
3
); C NMR (125 MHz,
DMSO-d ) δ 173.8 (C=O), 172.5 (C=O), 170.1 (C=O), 107.3 (C-
6
[8]
[9]
G. W. Hart, Y. Akimoto, Essentials Glycobiol. 2009.
2), 71.0 (C-4), 69.4 (C-5), 68.4 (C-7), 66.63 (C-8), 66.61 (C-6),
M. J. Kim, W. J. Hennen, H. M. Sweers, C. H. Wong, J. Am. Chem.
Soc. 1988, 110, 6481–6486.
58.1 (C-9), 41.9 (C-3), 24.6 (NAc), 21.7 (Ac) ppm; HRMS
+
(
MALDI-TOF): m/z calcd for C14
H
23NNaO11: 404.1169 [M + Na] ;
[
10]
H. Prescher, S. Gütgemann, M. Frank, E. Kuhfeldt, C. Watzl, R.
Brossmer, Bioorganic Med. Chem. 2015, 23, 5915–5921.
S. S. Park, J. Gervay-Hague, Org. Lett. 2014, 16, 5044–5047.
M. Shadrick, C. Yu, S. Geringer, S. Ritter, A. Behm, A. Cox, M.
Lohman, C. De Meo, New J. Chem. 2018, 42, 14138–14141.
K. Furuhata, Trends Glycosci. Glycotechnol. 2004, 16, 143–169.
O. Popik, B. Dhakal, D. Crich, J. Org. Chem. 2017, 82, 6142–6152.
M. Hawsawi, A. Wickramasinghe, D. Crich, J. Org. Chem. 2019, 84,
found 404.1206. NMR signals consistent with previously reported
spectral data [33]
[
[
11]
12]
5
2
-Acetamido-3,5-dideoxy-9-O-sulfo-D-glycero-D-galacto-non-
-ulopyranosonic acid (3) Compound 3 was prepared from 18
according to the General Procedure to afford the title compound
as a white solid (134 mg, 343 mmol, 85%). H NMR (500 MHz,
[13]
[14]
1
D
3
2
O) δ 3.98 – 3.87 (m, 2H, H-6, H-7), 3.82 – 3.74 (m, 1H, H-8),
.69 (dd, J = 11.8, 2.6 Hz, 1H, H-9a), 3.60 (ddd, J = 9.2, 6.3, 2.7
Hz, 1H, H- H-5), 3.47 (dd, J = 11.8, 6.3 Hz, 1H, H-9b), 3.41 (dd, J
9.2, 1.2 Hz, 1H, H-4), 2.17 (dd, J = 13.1, 5.0 Hz, 1H, H-3eq),
.90 (s, 3H, NAc), 1.73 (dd, J = 13.1, 11.4 Hz, 1H, H-3ax) ppm;
[
[
[
15]
16]
17]
14688–14700.
H. G. Bazin, Y. Du, T. Polat, R. J. Linhardt, J. Org. Chem. 1999, 64,
7254–7259.
=
1
1
3
2
C NMR (125 MHz, D O) δ 173.4 (C=O), 172.7 (C=O), 103.3 (C-
M. Sharma, C. R. Petrie, W. Korytnyk, Carbohydr. Res. 1988, 175,
2
), 73.3 (C-6), 71.5 (C-8), 70.73 (C-7), 70.71 (C-4), 68.7 (C-9),
25–34.
52.8 (C-5), 35.2 (C-3), 23.2 (OAc) ppm; HRMS (MALDI-TOF): m/z
+
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Tetrahedron 1993, 49, 1619–1624.
calcd for C11
H
19NNaO12S: 412.0526 [M + Na] ; found 412.0610
[
34]
NMR signals consistent with previously reported spectral data
[
[
[
19]
20]
21]
5
-Acetamido-3,5-dideoxy-9-O-phosphoryl-D-glycero-D-
galacto-non-2-ulopyranosonic acid (4) Compound 4 was
prepared from 23 according to the General Procedure to afford
the title compound as an off white solid (78 mg, 201 mmol, 61%).
[
[
[
22]
23]
24]
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1
H NMR (500 MHz, DMSO) δ 4.24 – 4.16 (m, 2H), 4.06 (t, J = 10.2
Hz, 1H), 3.97 (dd, J = 11.9, 2.7 Hz, 1H), 3.91 – 3.86 (m, 1H), 3.75
(
dd, J = 11.8, 6.3 Hz, 1H), 3.69 (dd, J = 9.3, 1.2 Hz, 1H), 2.85 (dd,
J = 12.9, 4.5 Hz, 5% alpha), 2.45 (dd, J = 13.1, 5.0 Hz, 1H), 2.18
(
(
6
d, J = 0.9 Hz, 3H), 2.01 (dd, J = 13.1, 11.5 Hz, 1H) ppm; 13C NMR
125 MHz DMSO-d ) δ 172.7, 170.9, 103.2, 71.9, 70.1, 69.7, 68.3,
8.1, 58.2, 43.4, 22.9 ppm; HRMS (MALDI-TOF): m/z calcd for
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6
[
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C
11
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[
34]
[27]
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consistent with reported spectral data.
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[
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(
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13/IA/1959) is gratefully acknowledged.
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Keywords: Carbohydrates • sialic acid • aldolase • total
synthesis • neuraminic acid
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7
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Entry for the Table of Contents
Graphical Abstract 1:
A general and operationally simple protocol for the synthesis of C-9 functionalised neuraminic acid derivatives from N-acetylneuraminic
acid is described. During the synthetic endeavor, an efficient way to introduce sialic acid ‘unfriendly’ benzyl groups was established.
Additionally, a working methodology towards unnatural β-configured neuraminic acid glycosides is reported.
Institute and/or researcher Twitter usernames: @Abdi509
Key Topic: Carbohydrate synthesis
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European Journal of Organic Chemistry
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Scheme 1: Chemoenzymatic synthesis of C-9 functionalised Neu5Ac derivatives
Scheme 2: Failed synthetic sequence
Scheme 3: Reagents & Conditions; (a) Et
3 2 2
N, CH Cl , rt, 91%, (b) NaOMe, MeOH, rt, 9 h (c)
TrCl, pyridine, 60 °C, 48 h, 68% (d) BnBr, Ba(OH)
2 2
·8H O, BaO, DMF, 0 °C – rt, 16 h, 75%.
Scheme 4: Glycal addition reaction for formation of 13
1
0
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European Journal of Organic Chemistry
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Scheme 5: Divergent synthesis of C-9 functionalised Neu5Ac derivatives
Scheme 6: Catalytic hydrogenolysis to furnish desired target structures
1
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