Efficient synthesis of 4-amido-N5-acetyl-4-deoxyneuraminic acid and its application to the C-4 modification of sialic acids

Article abstract of DOI:10.1021/ol901511x

Image Presented A straightforward synthesis of 4-amido-N ⁵-acetyl-4-deoxyneuraminic acid, a key precursor to various 4-amidoneuraminic acid analogues, has been achieved using a highly regioselective and diastereoselective [3+ 2]-cycloaddition o

Full text of DOI:10.1021/ol901511x

ORGANIC
LETTERS
2009
Vol. 11, No. 16
3678-3681
Efficient Synthesis of
4-Amido-N5-acetyl-4-deoxyneuraminic
Acid and Its Application to the C-4
Modification of Sialic Acids
Zheng-Xi Gao,† Meng Wang,‡ Shaozhong Wang,‡ and Zhu-Jun Yao*,†,‡
State Key Laboratory of Bioorganic & Natural Products Chemistry, Shanghai Institute
of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road,
Shanghai 200032, China, and Nanjing National Laboratory of Microstructures,
School of Chemistry and Chemical Engineering, Nanjing UniVersity, 22 Hankou Road,
Nanjing, Jiangsu 210093, China
yaoz@mail.sioc.ac.cn; yaoz@nju.edu.cn
Received July 2, 2009
ABSTRACT
A straightforward synthesis of 4-amido-N5-acetyl-4-deoxyneuraminic acid, a key precursor to various 4-amidoneuraminic acid analogues, has
been achieved using a highly regioselective and diastereoselective [3 + 2]-cycloaddition of D-mannose-derived nitrone with methyl acrylate.
Advantages of this newly developed synthesis include the use of economically available materials and reagents, the ease of operations and
the excellent control of stereochemistry, as well as the convenience in application to the C-4 modifications of sialic acids.
Sialic acids are a unique family of monosaccharides in which
more than 50 natural sialic acid derivatives have been
identified.1 Among these natural nine-carbon sugars, N-
acetylneuraminic acid (Neu5Ac, Figure 1, 1) is one of the
most well-known members. In biological systems, they
represent one of the most important constituents of glyco-
conjugates.2 More importantly, some sialic acid-derived
analogues have even achieved commercial successes in the
drug market. For example, Relenza (2), a Neu5Ac analogue
with variation at the C-4 position by substituting the 4-OH
group with a guanidinyl group, has been developed as a
useful drug for the treatment of type A influenza.3 For
obtaining more diverse C-4 modified Relenza-like 4-ami-
(3) (a) Kiefel, M. J.; von Itzstein, M. Prog. Med. Chem. 1999, 36, 1.
(b) von Itzstein, M.; Wu, W.-Y.; Kok, G. B.; Pegg, M. S.; Dyason, J. C.;
Jin, B.; Phan, T. V.; Smythe, M. L.; White, H. F.; Oliver, S. W.; Colman,
P. M.; Varghese, J. N.; Ryan, D. M.; Woods, J. M.; Bethell, R. C.; Hotham,
V. J.; Cameron, J. M.; Penn, C. R. Nature 1993, 363, 418. (c) Fleming,
D. M. Expert Opin. Pharmacol. 2003, 4, 799.
† Chinese Academy of Sciences.
‡ Nanjing University.
(1) Angata, T.; Varki, A. Chem. ReV. 2002, 102, 439.
(2) (a) Sears, P.; Wong, C.-H. Cell. Mol. Life Sci. 1998, 54, 223. (b)
Varki, A. Glycobiology 1993, 3, 97.
10.1021/ol901511x CCC: $40.75
Published on Web 07/24/2009
 2009 American Chemical Society

through the concerted mechanisms.10 For instance, 1,3-
dipolar cycloaddition reaction of nitrone and olefin can
introduce a new amino functionality and create as many as
three new contiguous stereogenic centers in a single step.
Retrosynthetically, the R-keto ester of the open chain
precursor 4 for 4-amido-N5-acetyl-4-deoxyneuraminic acid
(3) can be prepared from the corresponding R-hydroxy
carboxylate 5 (Figure 3). Linking C-2 oxygen and C-4
Figure 1.
Neu5Ac (1), Relenza (2), and 4-amido-N5-acetyl-4-
deoxyneuraminic acid (3).
dosialic acid analogues, 4-amido-N-acetyl-4-deoxyneuramin-
ic acid (3) would be an excellent common intermediate
perfectly suiting the principles of diverse organic synthesis.
However, few success has been achieved since the first
synthesis of 3 using nonselective nitrile oxide cycloaddition
with silylenopyruvate and catalytic hydrogenation of 4-oxime
intermediate (1.3:1 diastereoselectivity) as key steps in 1998.4
For today’s global demand for new antiflu drugs, further
studies of this useful 4-amidosialic acid derivative are thus
of extreme importance for medicinal chemistry and related
biomedical investigations with various sialic acid-processing
enzymes.
Neu5Ac has been used as the common starting material
in most commercial syntheses of sialic acid analogues,
including the industrial production of Relenza.5 Considering
the relatively high cost of Neu5Ac,6 it would be of
considerable value to establish alternative routes to those
useful sialic acid analogues using more economic and readily
available starting materials. With such considerations, we
once developed a formal synthesis of Relenza from the
inexpensive sugar material D-glucono-δ-lactone in 2004.7 In
this paper, we want to report a new efficient and straight-
forward synthesis of 4-amido-N5-acetyl-4-deoxyneuraminic
acid (3) from D-glucono-δ-lactone. An azide-free protocol
was adopted in this work for introducing the C-4 amido
functionality by using a highly regioselective and diastereo-
selective 1,3-dipolar cycloaddition reaction of the corre-
sponding nitrone and methyl acrylate.
The 1,3-dipolar cycloaddition reaction is useful in the
syntheses of various natural and unnatural compounds.8,9
Isoxazolidines, the products of cycloaddition of nitrone and
olefin, are frequently employed as building blocks in
synthesizing various bioactive alkaloids, amino sugars, amino
acids, as well as some useful 1,3-amino alcohols after
cleavage of the N-O bond (Figure 2). With proper optimi-
Figure 3.
Retrosynthesis of 4-amido-N5-acetyl-4-deoxyneuraminic
acid (3).
nitrogen functionalities of compound 5 gives the isoxazoli-
dine 6. This isoxazolidine 6 can be easily prepared by the
1,3-dipolar cycloaddition of a proper nitrone derived from
the known aldehyde 711 and commercially available methyl
acrylate. Aldehyde 7 can be conveniently prepared from the
cheap material D-glucono-δ-lactone 8 in large scales.7,11
(4) Mack, H.; Brossmer, R. Tetrahedron 1998, 54, 4539.
(5) (a) Scheigetz, J.; Zamboni, R.; Bernstein, M. A.; Roy, B. Org. Prep.
Proced. Int. 1995, 27, 637. (b) von Itzstein, M.; Jin, B.; Wu, W.-Y.;
Chandler, M. Carbohydr. Res. 1993, 244, 181. (c) Schreiner, E.; Zbiral,
E.; Kleineidam, R. G.; Schauer, R. Liebigs Ann. Chem. 1991, 129. (d)
Chandler, M.; Bamford, M. J.; Conroy, R.; Lamont, B.; Patel, B.; Patel,
V. K.; Steeples, I. P.; Storer, R.; Weir, N. G.; Wright, M.; Williamson, C.
J. Chem. Soc., Perkin Trans. 1 1995, 1173.
(6) For recent reviews on the syntheses of sialic acids: (a) Li, L. S.;
Wu, Y. L. Curr. Org. Chem. 2003, 7, 447. (b) Kiefel, M. J.; von Itzstein,
M. Chem. ReV. 2002, 102, 471. (c) von Itzstein, M.; Thomson, R. J. Top.
Curr. Chem. 1997, 186, 119. (d) DeNinno, M. P. Synthesis 1991, 583.
(7) Liu, K. G.; Yan, S.; Wu, Y. L.; Yao, Z. J. Org. Lett. 2004, 6, 2269.
(8) A review on 1,3-dipolar cycloaddition, see: Gothelf, K. V.; Jørgensen,
K. A. Chem. ReV. 1998, 98, 863.
(9) Several examples: (a) Pandey, G.; Sahoo, A. K.; Gadre, S. R.; Bagul,
T. D.; Phalgune, U. D. J. Org. Chem. 1999, 64, 4990. (b) Werner, K. M.;
de los Santos, J. M.; Weinreb, S. M. J. Org. Chem. 1999, 64, 4865. (c)
Young, D. G.; Gomez-Bengoa, E.; Hoveyda, A. H. J. Org. Chem. 1999,
64, 692. (d) Snider, B. B.; Lin, H. J. Am. Chem. Soc. 1999, 121, 7778. (e)
Angle, S. R.; Qian, X. L.; Pletnev, A. A.; Chinn, J. J. Org. Chem. 2007,
72, 2015. (f) Kambe, M.; Arai, E.; Suzuki, M.; Tokuyama, H.; Fukuyama,
T. Org. Lett. 2001, 3, 2575.
Figure 2. Substituted 1,3-amino alcohols via 1,3-dipolar cycload-
dition of nitrones.
(10) (a) Padwa, A.; Pearson, W. H. Synthetic Applications of 1,3-Dipolar
Cycloaddition Chemistry Toward Heterocycles and Natural Products;
Wiley: New York, 2002. (b) Frederickson, M. Tetrahedron 1997, 53, 403.
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zations of conditions and substrates, satisfactory regioselec-
tivity, stereoselectivity, and efficiency can be achieved
Org. Lett., Vol. 11, No. 16, 2009
3679

Sugar-aldehyde 7 was prepared from D-glucono-δ-lactone
8 using the procedures of our previous work11 (Scheme 1).
single-crystallographic analysis (see the Supporting Informa-
tion).
Isoxazolidine 11 was then converted to the corresponding
1,3-amino alcohol by reductive cleavage of the N-O bond.
At first, sugar auxiliary was removed from isoxazolidine 11
with hydroxylamine hydrochloride and NaOAc in a mixed
solvent of MeOH-H2O (3:1) at 70 °C (Scheme 2). Optically
Scheme 1
.
1,3-Dipolar Cycloaddition of Nitrone 10 and Methyl
Acrylate
Scheme 2. Completion of the Synthesis of 3
active isoxazolidine 6 can be isolated in 59% yield after
purification using routine silica gel column chromatography.
However, the following reductive cleavage of the N-O bond
of isoxazolidine 6 was troublesome in our initial attempts.
Reactions using the commonly used conditions16 such as
Zn-acetic acid, H2-Raney Ni, or H2-Pd/C all failed to give
the desired amino alcohol. Finally, in the presence of Boc2O,
hydrogenation of isoxazolidine 6 with catalytic amount of
20% Pd(OH)2 proceeded well (40 kg/cm3) and completed
in 24 h, affording the N4-Boc R-hydroxyl carboxylate 5 in
quantitative yield. Dess-Martin oxidation of the newly born
hydroxyl group of 5 provided the R-keto carboxylic acid
methyl ester 4. Without further purification, crude material
4 was directly treated with 4 N HCl in THF and then
hydrolyzed with 20% Et3N-H2O to give the target 4-amido-
N5-acetyl-4-deoxyneuraminic acid (3, 70% yield for two
steps). All physical data of 3 are in good agreement with
those reported.4
As mentioned above, this newly established synthesis of
4-amido-N5-acetyl-4-deoxyneuraminic acid (3) opens a new
entrance to various C-4 modifications of Neu5Ac with
biological interests. With considerations of future applications
to the cell-surface engineering by ligation reactions,17 a C-4
azido-functionalized Neu5Ac derivative 16 was therefore
designed and synthesized using isoxazolidine 6 as the starting
intermediate (Scheme 3). N-Acylation of 6 with 4-azido
Treatment of aldehyde 7 with the sugar-derived hydroxy-
lamine 912 in the presence of MgSO4 afforded the nitrone
10 as an air-stable solid. To our delight, 1,3-dipolar cycload-
dition of nitrone 10 (bearing Vasella’s sugar auxiliary13) and
methyl acrylate provided the excellent diastereoselectiv-
ity.14,15 Simply heating the mixture of the nitrone 10 and
methyl acrylate in toluene afforded a major adduct isoxazo-
lidine 11 in 90% isolated yield. However, our further attempts
to expand the reaction scope of nitrone 10 with other
unsaturated substrates failed to achieve satisfactory regiose-
lectivity and diastereoselectivity, including the reaction with
methyl propiolate. These results indicate that the influencing
factors of the above 1,3-dipolar cycloaddition are consider-
ably complicated and sensitive. Cycloaddition between
nitrone 10 and methyl acrylate is one successful case in
which all the structural elements are perfectly matched to
give the desired adduct 11 with correct stereochemistries.
Characterizations of isoxazolidine 11 were accomplished by
1D and 2D NMR studies and finally confirmed by an X-ray
(11) (a) Liu, K.-G.; Yan, S.; Wu, Y.-L.; Yao, Z.-J. J. Org. Chem. 2002,
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3680
Org. Lett., Vol. 11, No. 16, 2009

In summary, a new straightforward efficient synthesis of
4-amido-N5-acetyl-4-deoxyneuraminic acid starting from the
cheap material D-glucono-δ-lactone has been reported. The
highly regioselective and diastereoselective 1,3-dipolar cy-
cloaddition of D-mannose-derived nitrone with methyl acry-
late was developed and successfully applied in the synthesis.
This newly established azide-free route is advantageous in
using economically available materials, excellent control of
stereochemistry, as well as the convenience in application
to the C-4 modifications of sialic acids. Utilizing the
isoxazolidine intermediate, a new C-4 azido-functionalized
Neu5Ac analogue was synthesized with the established
methodologies. Further application of our protocol to ad-
ditional C-4 amido neuraminic acid derivatives of biological
interests is currently under investigation in this laboratory.
Scheme 3
.
Synthesis of the C-4-Modified Sialic Acid Derivative
16
Acknowledgment. This project is financially supported
by MOST (2010CB833200), NSFC (20672128), and CAS
(KJCX2-YW-H08) and Shanghai Municipal Committee of
Science and Technology (07DZ22001).
Supporting Information Available: Experimental details
and characterizations of new compounds, copies of 1H NMR
and 13C NMR spectra of new compounds and X-ray single-
crystal data of compound 11 (CIF). This material is available
free of charge via the Internet at http://pubs.acs.org.
benzoic acid chloride followed by catalytic hydrogenation
(40 kg/cm3) afforded the amide 13 (60% in two steps).
Treatment of aniline 13 with triflyl azide18 in the presence
of anhydrous CuSO4 and Et3N in DCM and MeOH provided
the corresponding azido compound 14 in 20% yield.
Dess-Martin oxidation of the alcohol 14 afforded the
corresponding R-keto ester 15. Final treatment of 15 with 4
N HCl in THF provided the C-4 azido functionalized
Neu5Ac derivative 16 (40% yield for two steps).
OL901511X
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