Selective synthesis of Neu5Ac2en and its oxazoline derivative using BF3·Et2O

Article abstract of DOI:10.1016/j.tetlet.2009.01.117

Application of the Lewis acid BF₃·Et₂O to the selective synthesis of 5-acetamido-2,6-anhydro-3,5-dideoxy-d-glycero-d-galacto-non-2-enonic acid (Neu5Ac2en) and the related oxazoline, methyl 7,8,9-tri-O-acetyl-2,3,4,5-tetradeoxy-2,3-didehydro-2,3-trideoxy-4′,5′-dihydro-2′-methyloxazolo[5,4-d]- d-glycero-d-talo-non-2-enonate is described.

Full text of DOI:10.1016/j.tetlet.2009.01.117

Tetrahedron Letters 50 (2009) 1642–1644
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Selective synthesis of Neu5Ac2en and its oxazoline derivative using BF3ꢀEt2O
*
Goreti Ribeiro Morais, Rudi Santiago Oliveira, Robert A. Falconer
Institute of Cancer Therapeutics, School of Life Sciences, University of Bradford, Bradford BD7 1DP, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Application of the Lewis acid BF3ꢀEt2O to the selective synthesis of 5-acetamido-2,6-anhydro-3,5-dide-
Received 19 November 2008
Revised 9 January 2009
Accepted 23 January 2009
Available online 27 January 2009
oxy-D-glycero-D-galacto-non-2-enonic acid (Neu5Ac2en) and the related oxazoline, methyl 7,8,9-tri-O-
acetyl-2,3,4,5-tetradeoxy-2,3-didehydro-2,3-trideoxy-40,50-dihydro-20-methyloxazolo[5,4-d]-
-talo-non-2-enonate is described.
D-glycero-
D
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Lewis acid
BF3ꢀEt2O
Sialic acid
Glycal
Neu5Ac2en
Oxazoline
N-Acetylneuraminic acid 1a (sialic acid, Neu5Ac, Fig. 1) is com-
monly found on the surface of mammalian cells and is of particular
importance in cellular recognition processes, cell adhesion and dis-
ease states.1 Hence there is great interest in the synthesis of sialo-
sides, either as biological probes or as enzyme inhibitors. Many of
the procedures to obtain such analogues proceed through the
intermediate 2-chlorosialic acid 2 (Fig. 1), but a disadvantage to
its use is competitive elimination of the chlorine atom, which is
aided by the electron-withdrawing nature of the protected carbox-
ylic acid at the 2-position. This commonly leads to the formation of
a 2-deoxy-2,3-dehydro-Neu5Ac derivative (Neu5Ac2en 3a, Fig. 1).
Neu5Ac2en and its N-glycolyl analogue are metabolic products
found in body fluids and secretions2 and as a result are of interest
per se. Neu5Ac2en3 is a potent sialidase inhibitor, as are analogues
modified at C-4.4–6 It has been used as the basis for the design and
synthesis of sialidase inhibitors with anti-influenza activity,7–9 the
best example being Zanamivir (4-guanidino-Neu5Ac2en). In
addition, inhibition of other sialidases has been achieved by
z2,3-unsaturated analogues.10–12 Chemically, per-O-acetylated
2,3-unsaturated sialic acid derivative 3b has been used for the
preparation of analogues of sialic acid via the epoxide,13–16 synthe-
sis of N-acetyl-3-fluoroneuraminic acid by treatment of the glycal
3b with X2Fe–BF3ꢀEt2O,17 molecular fluorine18 or TEDA–CH2Clꢀ2BF4
(Selectfluor)19 and for the synthesis of C-4 substituted sialic acid
analogues via the 4,5-oxazoline derivative 4.
DBU,13,20,21 anhydrous H2PO4 in refluxing MeCN22 and pyridine.16
In addition, one group has reported the synthesis of 3b from per-
O-acetyl methyl ester 1c when treated with PPh3ꢀHBr in MeCN.19
The respective 4-epimer can be readily prepared from methyl ester
1b when treated with Ac2O in H2SO4 at 80 °C,10,23 or from the 2-
bromosialic acid derivative when treated with syn-collidine.20
Also of importance is the corresponding 2,3-unsaturated 4,5-
oxazoline derivative 4, particularly in the synthesis of C-4 modified
sialosides.4,7,24 Compound 4 is prepared easily by the Lewis acid-
promoted internal nucleophilic attack of the acetamido group
(lone pair of the oxygen) on the electrophilic centre at the 4-posi-
tion with inversion of configuration. Two different strategies have
OR2
OR2
OAc
OAc
OR2
R2O
Cl
AcO
COOR1
O
COOMe
O
AcHN
AcHN
R2O
AcO
1a R1=H; R2=H (Neu5Ac)
2
1b R1=Me; R2=H
1c R1=Me; R2=Ac
OR2
R2O
OAc
OR2
O
AcO
COOR1
N
O
O
COOMe
AcHN
R2O
AcO
Typically, the preparation of 3b involves the base-promoted
elimination of 2-chlorosialoside 2, using bases such as
3a R1=H; R2=H (Neu5Ac2en)
3b R1=Me; R2=Ac
4
* Corresponding author. Tel.: +44 1274 235842; fax: +44 1274 233234.
E-mail address: r.a.falconer1@bradford.ac.uk (R.A. Falconer).
Figure 1.
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.01.117

G. R. Morais et al. / Tetrahedron Letters 50 (2009) 1642–1644
1643
Table 1
been reported in the literature: (a) synthesis of the 4,5-oxazoline 4
26
from the Neu5Ac2en per-O-acetate 3b using BF3ꢀEt2O25 or SnCl4
1c Lewis acid
!
3b þ 4
and (b) synthesis of the 4,5-oxazoline 4 from per-O-acetylated sia-
lic acid 1c, using trimethylsilyl trifluoromethanesulfonate
(TMSOTf) as the catalyst.4,6,27
overnight
Entry
Lewis acid
Equivalents
Solvent
Yielda (%)
Ratio 3b:4b
The chemical scope of oxazoline 4 is diverse. Catalytic hydroge-
nation (Pd/C) produces the respective 4-deoxy Neu5Ac2en
derivative4 whereas acidic cleavage by trifluoroacetic acid gives
the 4-epi-Neu5Ac2en derivative.4 This 4-epi-Neu5Ac2en has been
prepared previously by refluxing Neu5Ac with sulfuric acid and
acetic anhydride.10 Nucleophiles have also been introduced to this
molecule, for example, sulfur and nitrogen at C-4; the oxazoline
ring was opened stereoselectively via an SN2 reaction to obtain
1
2
3
4
5
6
7
8
9
BF3ꢀEt2O
BF3ꢀEt2O
BF3ꢀEt2O
BF3ꢀEt2O
BF3ꢀEt2O
BF3ꢀEt2O
BF3ꢀEt2O
BF3ꢀEt2O
SnCl4
6.0
3.0
3.0
3.0
1.2
1.0
1.0
0.5e
1.2
CH2Cl2
CH2Cl2
CH3CNc
93
74
92
51
95
ND
98
56
91
1:99
1:99
9:1
5:1
1:99
2:3
1:5
1:4
1:1
c
CH2Cl2
CH2Cl2
CH2Cl2
d
CH2Cl2
CH2Cl2
CH3CN
the 4-
a
diastereomer.25 2,3-Unsaturated derivatives were also
synthesised by acetolysis of b-methyl glycosides of sialic acid in
a one-pot reaction using Ac2O in concentrated sulfuric acid with
further hydrolysis to the 4-epi-acetate or 4-epi-hydroxy derivative
at pH 5 and pH 2, respectively.28 Recently, a 3,4-unsaturated sialic
acid derivative was prepared via a Lewis acid-catalysed Ferrier
reaction of the activated allylic oxazoline 4.29
ND—not determined.
a
Total isolated yield of 3b + 4.
Ratio determined by 1H NMR.
1.5 h, 25 °C.
b
c
d
e
4.5 h, rt.
7.5% of starting material was recovered.
Our attempts to prepare a C-allyl-sialoside directly from the
per-O-acetate using BF3ꢀEt2O, in a manner similar to that reported
with aldoses,30 resulted in a mixture of products with an olefinic
hydrogen. Prompted by this finding, a series of experiments were
carried out to investigate the effect of BF3ꢀEt2O on sialic acid deriv-
ative 1c in the absence of a nucleophile (Scheme 1). Conditions
were varied, including equivalents of BF3ꢀEt2O, reaction times
and solvents (Table 1). Interestingly, selectivity as to the formation
of either per-O-acetylated Neu5Ac2en 3b or the 2,3-unsaturated
4,5-oxazoline sialic acid 4 could be achieved according to the
solvent used. Performing the reaction in acetonitrile resulted in
2,3-unsaturated sialic acid 3b whereas use of dichloromethane
led almost exclusively to 2,3-unsaturated 4,5-oxazoline sialic acid
4 (entries 2 and 3). It was found that selectivity was both time and
temperature dependent. By monitoring the reaction using 1H NMR
spectroscopy, it was observed that reaction times longer than 1.5 h
increased the proportion of 4 (decreasing the ratio of 3b:4) in
dichloromethane. Compound 4 was selectively formed when the
reactions were carried out in dichloromethane overnight. It was
found that 1.2 equiv of the Lewis acid BF3ꢀEt2O was sufficient to
complete the reaction (entry 5). Fewer equivalents of BF3ꢀEt2O
led to a slight increase in formation of compound 4 (entry 7) and
incomplete conversion (entry 8). In contrast, compound 1c was
completely converted to 3b after 1.5 h when the reaction was car-
ried out in acetonitrile at 25 °C (entry 3). As a comparison, the reac-
tions were also attempted using SnCl4, which produced a ratio of
3b:4 of 1:1 over a similar reaction time. BF3ꢀEt2O clearly delivers
a significantly higher degree of selectivity.
ingly, an excess of Ac2O led to that mixture whereas an excess of
BF3ꢀEt2O led to only 3b and 4, with a predominance of 4 (Scheme
2). The von Itzstein group has reported the synthesis of 4 in one
step when treating 1-methylsialoside methyl ester with acetic
anhydride using TMSOTf as catalyst, but the use of BF3ꢀEt2O led
only to peracetylation and no elimination product was observed.28
The structural assignment of 4 was aided by data reported in
the literature.10 It should be noted that the coupling constant be-
tween H-4 and H-5 changes from 2.5 to approximately 9 Hz on for-
mation of the oxazoline ring. In addition, the double doublet
attributed to H-6 is shielded by the oxazoline ring and is therefore
shifted to higher field (d 3.41).
It is believed that the stability of the oxazoline 4 is limited,
showing a high susceptibility to ring opening and consequent for-
mation of C-4 substituted analogues. Thus, the preparation of oxaz-
oline 4 directly from methyl ester 1b or from the per-O-acetate
sialic acid 1c to obtain Neu5Ac2en derivatives is a significant
advantage, given that transformations such as hydrogenation,
halogenation or oxidations have been shown to have drawbacks
when carried out with Neu5Ac2en itself.
In summary, the Lewis acid BF3ꢀEt2O can be utilised to selec-
tively prepare Neu5Ac2en 3b or the related oxazoline 4, depending
on the conditions used. Acetonitrile affords Neu5Ac2en per-acetate
3b whereas dichloromethane affords oxazoline 4 (Scheme 1). The
methodology to synthesise Neu5Ac2en 3b, in particular, offers sig-
nificant advantages over published methods involving chloro-
sialoside 2.
Synthesis of methyl 5-acetamido-4,7,8,9-tetra-O-acetyl-2,6-anhy-
In further experiments, we observed that reacting sialic acid
methyl ester 1b with BF3ꢀEt2O and acetic anhydride also promoted
formation of oxazoline 4. It should be noted that these conditions
were recently used in a similar manner for per-O-acetylated aldos-
es.31 In this case, a mixture of 1c, 3b and 4 was obtained. Interest-
dro-3,5-dideoxy-
D
-glycero-
D
-talo-non-2-enonate
L, 0.57 mmol) was added to a solution of
-gly-
cero-D-galacto-2-a/b-nonulopyranosonate (100 mg, 0.19 mmol) in
(Neu5Ac2en,
3b):13,32 BF3ꢀEt2O (72
l
methyl 5-acetamido-2,4,7,8,9-penta-O-acetyl-3,5-dideoxy-
D
OAc
OAc
AcO
O
COOMe
BF3.Et2O
AcHN
4 (9%)
+
OAc
OAc
AcO
OAc
COOMe
AcO
CH3CN
3b (83%)
OAc
O
AcHN
AcO
O
AcO
BF3.Et2O
N
O
1c
COOMe
3b (1%)
+
CH2Cl2
AcO
4 (92%)
Scheme 1.

1644
G. R. Morais et al. / Tetrahedron Letters 50 (2009) 1642–1644
OAc
OH
OH
OAc
OH
COOMe
HO
AcO
OAc
O
AcO
O
N
O
COOMe
COOMe
BF3.Et2O
Ac2O
O
+
AcHN
AcHN
AcO
HO
AcO
1b
3b
4
Scheme 2.
9. Smith, P. W.; Starkey, I. D.; Howes, P. D.; Sollis, S. L.; Keeling, S. P.; Cherry, P. C.;
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dry acetonitrile (2 mL) under a nitrogen atmosphere at 25 °C. After
90 min, the reaction mixture was diluted with dichloromethane
(25 mL) and NaHCO3 powder was added. The reaction mixture was
filtered and concentrated to give 3b (quant.).
Synthesis of methyl 7,8,9-tri-O-acetyl-2,3,4,5-tetradeoxy-2,3-dide-
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hydro-2,3-trideoxy-40,50-dihydro-20-methyloxazolo[5,4-d]-
D
-glycero-
D-
talo-non-2-enonate (4):10,33 BF3ꢀEt2O (29
lL, 0.23 mmol) was added
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dideoxy-D-glycero-D-galacto-2-a/b-nonulopyranosonate (100 mg,
0.19 mmol) in dry dichloromethane (2 mL) under a nitrogen atmo-
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Acknowledgements
This work was supported by EPSRC (GRM) and Yorkshire Cancer
Research (RAF). The authors thank Andrew Healey for running low
resolution mass spectra.
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32. Compound 3b:13 1H NMR (CDCl3) d 1.90 (s, 3H, NAc), 2.01, 2.02, 2.05, 2.09 (4s,
12H, 4OAc), 3.77 (s, 3H, COOMe), 4.17 (dd, 1H, H-9a, J8,9a = 6.9 Hz,
J9a,9b = 12.3 Hz), 4.35–4.38 (m, 2H), 4.57 (dd, 1H, H-9b, J8,9b = 3.4 Hz,
J9a,9b 12.3 Hz), 5.33–5.34 (m, 1H), 5.42–5.34 (m, 2H), 5.58 (m, 1H), 5.98 (d,
1H, 3J 2.7 Hz); ESI MS C20H27O12 N (473.15) m/z 474 [M+H]+, 496 [M+Na]+.
33. Compound 4:10 1H NMR (CDCl3) d 1.99, 2.04, 2.04, 2.14 (4s, 12H, 3OAc and CH3),
3.41 (dd, 1H, H-6, J6,7 = 2.5 Hz, J5,6 = 9.9 Hz), 3.79 (s, 3H, COOMe), 3.93 (dd, 1H,
H-5, J4,5 = 8.7 Hz, J5,6 = 9.9 Hz), 4.22 (dd, 1H, H-9a, J8,9a = 6.3 Hz, J9a,9b = 12.5 Hz),
4.58 (dd, 1H, H-9b, J8,9a 2.5 Hz, J9a,9b = 12.5 Hz), 4.81 (dd, 1H, H-4, J3,4 = 3.9 Hz,
J4,5 = 8.7 Hz), 5.53 (ddd, 1H, H-8, J8,9a = 2.5 Hz, J6,7 = 6.0 Hz, J8,9a = 6.3 Hz), 5.62
(dd, 1H, H-7, J6,7 = 2.5 Hz, J6,7 = 6.0 Hz), 6.37 (d, 1H, H-3, J3,4 = 3.9 Hz); ESI MS
C18
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H23NO10 (413.38) m/z 414 [M+H]+.