A first synthesis of sulfonic acid analogues of N-acetylneuraminic acid

Article abstract of DOI:10.1016/j.tetlet.2007.12.043

Sulfonic acid analogues of N-acetylneuraminic are synthesized from 1-thio-l-fucoside derivatives with the introduction of an azido group at C-4 of the fucose moiety and carbanionic addition onto fully protected lactones. The analogues in the form of methy

Full text of DOI:10.1016/j.tetlet.2007.12.043

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 1196–1198
A first synthesis of sulfonic acid analogues of N-acetylneuraminic acid
a
a,
a
a,b
a,c
*
´
´
´
´
´
´
´
´
Zoltan B. Szabo , Aniko Borbas , Istvan Bajza , Andras Liptak , Sandor Antus
a Research Group for Carbohydrates of the Hungarian Academy of Sciences, University of Debrecen, PO Box 94, H-4010, Hungary
b Department of Biochemistry, University of Debrecen, PO Box 55, H-4010, Hungary
c Department of Organic Chemistry, University of Debrecen, PO Box 20, H-4010, Hungary
Received 5 November 2007; revised 27 November 2007; accepted 7 December 2007
Available online 14 December 2007
Abstract
Sulfonic acid analogues of N-acetylneuraminic are synthesized from 1-thio-L-fucoside derivatives with the introduction of an azido
group at C-4 of the fucose moiety and carbanionic addition onto fully protected lactones. The analogues in the form of methyl glycosides
are subjected to a neuraminidase inhibition assay.
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: N-Acetylneuraminic acid analogues; Sulfonic acids; Thioglycosides; Methyl glycosides
N-Acetylneuraminic acid (Neu5Ac 1, Fig. 1), the most
abundant sialic acid, is bound to the exo-terminal b-galac-
tosyl residues of cell-surface glycoproteins or glycolipids,
and plays an important role in the carbohydrate–protein
recognition events leading to cell adhesion, extravasation
of leukocytes and bacterial or viral infections. Mimetics
that are able to inhibit sialic acid binding proteins (such
as sialoadhesins, selectins, and influenza hemagglutinins),
may show potent anti-inflammatory, antiviral or antibacte-
rial effects.1 While phosphonic acid analogues2 and sulfate
ester homologs3 of N-acetylneuraminic acid, containing an
anionic group as a replacement for the carboxylate moiety
are known, a sulfonic acid mimic has not been prepared to
date. Previously, we reported the synthesis of sulfonato-
methyl analogues4 of aldos-2-ulosonic acids, however, the
exact conformation and the N-acetyl group were missing
from these derivatives. The synthesis of sulfonic acid deriv-
atives 2a,b and 3, that contain the 5-NHAc function and
accurately mimic the ring system of 1 is described in this
Letter.
The preparation of 2a and 2b started from an L-fucose
derivative, since L-fucosides have the same chair conformer
as Neu5Ac derivatives. Thus, 45 was mesylated at C-4 and
treated with NaN3 to give 5 (Scheme 1). The thiophenyl
aglycon of 5 was firstly hydrolyzed in the presence of N-
bromosuccinimide and the lactol intermediate was then
oxidized with PCC to give lactone 6 in a good overall yield.
Next, according to our previously published methodology,4
lactone 6 was reacted with CH3SO3Et in the presence of
nBuLi, and the formation of 7, a 1-ethoxysulfonyl-hept-2-
-
CH2SO3 Na+
-
COO-
OH
CH2SO3 Na+
O
OMe
OH
OH
HO
AcHN
H3C
AcHN
H3C
OMe
O
O
OH
AcHN
OH
1
OH
2a,2b
OH
3
Fig. 1. Structures of Neu5Ac (1) and the proposed analogues (2 and 3).
*
Corresponding author. Tel.: +36 52 512 900; fax: +36 52 453836.
´
E-mail address: borbasa@puma.unideb.hu (A. Borbas).
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.12.043

Z. B. Szabo´ et al. / Tetrahedron Letters 49 (2008) 1196–1198
1197
OH
CH2SO3Et
O
OBn
a,b
c,d
H3C
HO
SPh
OBn
H3C
N3
SPh
OBn
e
H3C
O
H3C
N3
O
OBn
5
O
OBn
6
O
OBn
7
N3
OBn
OBn
4
Scheme 1. Reagents and conditions: (a) MsCl (3.5 equiv), Et3N (1 equiv), CH2Cl2, 0 °C, 1 h, 66%; (b) NaN3 (3.5 equiv), DMF, 115 °C, 1 h, 77%; (c) N-
˚
bromosuccinimide (1.3 equiv in 3 portions), acetone (aq), 30 min, 89%; (d) PCC (3.5 equiv), 3 A MS, abs CH2Cl2, 3 h, 75%; (e) CH3SO3Et (1.1 equiv),
nBuLi (1.1 equiv), iPr2NH (1.1 equiv), THF, À78 °C, 1 h, 48%.
ulose sugar occurred in an acceptable (48%) yield. Com-
pound 7 was formed as a single a anomer.
of the acetamido group, but also gave rise to cleavage of
the sulfonate ethyl ester, due to the strong nucleophilic
character of the AcSÀ ion. After chromatography on silica
11 and 12 were isolated as sodium salts, according to
MALDI-TOF MS analysis. Finally, the 3-O- and 4-O-
benzyl groups were removed from 11 and 12 by catalytic
hydrogenation to give 2a and 2b in good yields.
For the preparation of 3 the synthesis of a functional-
ized 2-deoxy-lactone intermediate was necessary. Starting
from 13,9 a methyl xanthate ester was introduced to give
14 (Scheme 3). According to a reported procedure,10 14
was reacted with Bu3SnH in the presence of AIBN result-
ing in a radical reductive elimination yielding 3,4-O-iso-
propylidene-L-fucal, 15. This volatile, 1,2-unsaturated
compound was treated directly with PCC11 to give lactone
16 in an acceptable yield for the two reaction steps. Deiso-
propylidenation of 16 and selective benzylation of 17 gave
rise to lactone 18. Finally, the axial 4-OH group of 18 was
exchanged to an equatorial azido group to give the desired
4-azido-3-O-benzyl-2,4,6-trideoxy-L-arabino-hexopyrano-
1,5-lactone, 19.
The carbanionic addition onto lactone 19 to produce 20
was accomplished, unfortunately, in poor yield. NMR
spectra revealed that the chromatographically uniform 20
turned out to be a mixture of a and b anomers in a ratio
of 7: 3. The loss of stereoselectivity during the addition
reaction is probably due to the absence of the 2-O-substitu-
ent in 19. Subsequent compounds in the reaction sequence
also existed as anomeric mixtures. Azide 20 was reacted
with ethanethiol in the presence of BF3ÁEt2O at a tempera-
ture as low as À20 °C to give 21 (a/b ꢀ 2:1) in a 60% yield.
Methanol was glycosylated with 21 to give 22 (a/b ꢀ 3:2) in
a yield of 81%. Although the anomers of 22 could be sep-
arated, their mixture was reacted further due to the small
amount of substance available. The azide reduction and
salt formation step to produce 23 (a/b ꢀ 3:2) proceeded
in a moderate yield of 38%, while hydrogenolysis of 23
resulted in the formation of sulfonic acid derivative 3
(a/b ꢀ 3:2), in a 68% yield (Scheme 4).
To achieve the synthesis of both 2a and 2b the anomeric
centre of compounds such as 7 should be fixed in the form
of glycosides.6 Thus, 7 was further derivatized to yield thio-
glycoside 8, which occurred also as the a anomer, and was
examined by X-ray crystallography.7 The methanolysis of
8, under glycosylation conditions, gave a separable mixture
of both the a-OMe (9) and the b-OMe (10) glycosides in a
nearly 2: 1 ratio, where the thermodynamically stable a-
anomer was the main product. The anomeric configura-
tions of 9 and 10, were confirmed from the chemical shifts
in the 13C NMR spectra, using previously established
results.6 The C-2 atom of 9 (a-OMe derivative) resonated
at 98.83 ppm, while that of 10 (b-OMe glycoside) appeared
at 100.25 ppm (Scheme 2).
The next step was the transformation of both the sulfo-
nate ethyl ester and the azide moiety of 9 and 10 into a
sulfonate salt and an acetamido group, respectively. A
one-step procedure for the selective reduction and acetyl-
ation of sugar azides has been elaborated by Pavia and
co-workers.8 However, the reaction of 9 and 10 with
KSAc/AcSH in DMF resulted not only in the formation
SEt
a
H3C
N3
CH2SO3Et
O
7
OBn
BnO
8
b
CH2SO3Et
OMe
H3C
N3
H3C
CH2SO3Et
OMe
OBn
O
O
+
N3
OBn
BnO
BnO
10
9
c
c
-
CH2SO3 Na+
OMe
CH2SO3 Na+
OR
-
H3C
AcHN
O
H3C
AcHN
OMe
OR
O
The inhibitory activity of compounds 2a,b and 3 were
measured on Clostridium perfringens neuraminidase using
fetuin as the substrate with periodate-thiobarbiturate
detection.12 Of the three compounds, only 3 showed mod-
erate inhibition with an IC50 value of 4.6 mM. (For the
Neu5Ac2en reference compound, an IC50 value of
0.56 mM was observed.) The inhibitory activity of 3 is
not strong enough for practical applications, nevertheless,
the skeleton of 3 can more or less mimic Neu5Ac and
RO
RO
11 R = Bn
2a R = H
12 R = Bn
2b R = H
d
d
Scheme 2. Reagents and conditions: (a) EtSH (1.8 equiv), BF3.Et2O
(2.5 equiv), abs CH2Cl2, 4 h, 75%; (b) MeOH (15 equiv), NIS (1.5 equiv),
˚
TfOH (0.5 equiv), 3 A MS, abs CH2Cl2, À45 °C to À20 °C, 1 h, 55% for 9,
34% for 10; (c) KSAc (1.1 equiv), 45 min, then KSAc (2.9 equiv), AcSH
(15 equiv), DMF, 24 h; (d) H2, Pd/C, MeOH, 3 days, 76% for 2a from 9,
52% for 2b from 10.

1198
Z. B. Szabo´ et al. / Tetrahedron Letters 49 (2008) 1196–1198
O
H3C
H3C
c
SPh
OH
SPh
OCSSCH3
H3C
a
b
O
O
O
O
O
O
O
O
15
13
14
O
O
e
H3C
O
f, g
O
H3C
HO
O
d
H3C
HO
H3C
N3
BnO
O
O
O
O
OH
O
OBn
19
16
18
17
Scheme 3. Reagents and conditions: (a) NaH (1.3 equiv), imidazole (0.2 equiv), CS2 (7 equiv), 15 min, then MeI (7 equiv), 1 h, quant; (b) Bu3SnH
(4.8 equiv), AIBN, toluene, reflux, 5 min; (c) PCC (5 equiv), abs CH2Cl2, reflux, 24 h, 44% for 16 from 14; (d) HCl (aq), MeOH, 45 °C, 18 h, 91%; (e)
˚
Bu2SnO (1.3 equiv), 3 A MS, CH3CN, reflux, 30 h, then CsF (2 equiv), BnBr (2.5 equiv), Bu4NI, reflux, 72 h, 51%; (f) MsCl (3.5 equiv), Et3N (1 equiv),
CH2Cl2, 0 °C, 1 h, 85%; (g) NaN3 (3.5 equiv), DMF, 115 °C, 1 h, 69%.
all new compounds are available. Supplementary data
associated with this article can be found, in the online ver-
sion, at doi:10.1016/j.tetlet.2007.12.043.
SEt
OH
H3C
N3
H3C
N3
a
O
OBn
21
c
O
OBn
20
b
CH2SO3Et
CH2SO3Et
19
References and notes
OCH3
CH2SO3- Na+
OCH3
H3C
H3C
N3
O
d
O
OBn
CH2SO3Et
AcHN
1. (a) Simanek, E. E.; McGarvey, G. J.; Jablonowski, J. A.; Wong, C.-H.
Chem. Rev. 1998, 98, 833–862; (b) Kiefel, M. J.; von Itzstein, M.
Chem. Rev. 2002, 102, 471–490.
OR
23
R = Bn
R = H
22
e
3
2. (a) Chan, T. H.; Xin, Z. C. Chem. Commun. 1996, 905–906; (b) Gao,
J.; Martichonok, V.; Whitesides, G. M. J. Org. Chem. 1996, 61, 9538–
9540; (c) Hanessian, S.; Rogel, O. J. Org. Chem. 2000, 65, 2667–2674.
3. Sasaki, K.; Nishida, Y.; Uzawa, H.; Kobayashi, K. Bioorg. Med.
Chem. Lett. 2003, 13, 2821–2823.
Scheme 4. Reagents and conditions: (a) CH3SO3Et (1.1 equiv), nBuLi
(1.1 equiv), iPr2NH (1.1 equiv), THF, À78 °C, 1 h, 35%, 20a:20b ꢀ 7:3; (b)
EtSH (1.8 equiv), BF3ÁEt2O (2.5 equiv), abs CH2Cl2, 4 h, 60%,
21a:21b ꢀ 2:1; (c) MeOH (15 equiv), NIS (1.5 equiv), TfOH (0.5 equiv),
˚
´ ´
4. (a) Borbas, A.; Szabovik, G.; Antal, Zs.; Herczegh, P.; Agocs, A.;
3 A MS, abs CH2Cl2, À45 °C, 81%, 22a:22b ꢀ 3:2; (d) KSAc (1.1 equiv),
´
´
45 min, then KSAc (2.9 equiv), AcSH (15 equiv), DMF, 24 h; 38%,
Liptak, A. Tetrahedron Lett. 1999, 40, 3639–3642; (b) Borbas, A.;
23a:23b ꢀ 3:2; (e) H2, Pd/C, MeOH, 3 days, 68%, 3a:3b ꢀ 3:2.
Szabovik, G.; Antal, Zs.; Feher, K.; Csavas, M.; Szilagyi, L.;
´
´ ´
´
´
Herczegh, P.; Liptak, A. Tetrahedron: Asymmetry 2000, 11, 549–566.
´
´
´
5. Szabo, Z. B.; Borbas, A.; Bajza, I.; Liptak, A. Tetrahedron:
may become a substituent of the sialyl moiety in a neo-
glycoconjugate. Comparison of 3 with 2a,b reveals that
the 3-deoxy function is essential for these mimetics.
Asymmetry 2005, 16, 83–95.
6. Majer, G.; Borbas, A.; Illyes, T. Z.; Szilagyi, L.; Benyei, A. Cs.;
´
´
´
´
´
´
Liptak, A. Carbohydr. Res. 2007, 342, 1393–1404.
7. Details of the crystal structure analysis are provided as Supplemen-
tary data. Crystallographic data (excluding structure factors) for the
structure 8 reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as Supplementary Publica-
tion Number CCDC 661647. Copies of the data can be obtained, free
of charge, on application to CCDC, 12 Union Road, Cambridge CB2
1EZ, UK [fax: +44(0)-1223-336033 or e-mail: deposit@ccdc.cam.
ac.uk].
Acknowledgements
This work was supported by the Hungarian Academy of
Sciences and the Hungarian Research Fund (OTKA K
62802 A. B. and AT 48798 A. L.). We thank Dr Attila Csa-
´
ba Benyei (X-ray measurements for compound 8), Dr Lili
8. Rakotomanomana, N.; Lacombe, J.-M.; Pavia, A. A. Carbohydr. Res.
1990, 197, 318–323.
9. Jaramillo, C.; Corrales, G.; Fernandez-Mayoralas, A. Tetrahedron
Lett. 1998, 39, 7783–7786.
´
´
Kandra and Dr Gyo¨ngyi Gyemant (enzymatic and mass
spectrometric measurements) for their enthusiastic help.
Supplementary data
´
10. Fernandes-Mayoralas, A.; Marra, A.; Trumtel, M.; Veyrieres, A.;
Sinay¨, P. Carbohydr. Res. 1989, 188, 81–95.
Experimental procedures for all synthetic steps and
characterization (including 1H and 13C NMR data) for
11. Rollin, P.; Sinay, P. Carbohydr. Res. 1981, 98, 139–142.
12. Warren, L. J. Biol. Chem. 1959, 234, 1971–1975.
¨