Synthetic study on α(2→8)-linked oligosialic acid employing 1,5-lactamization as a key step

Article abstract of DOI:10.1016/j.tetlet.2009.05.057

An attempt to synthesize α(2→8)-linked oligosialic acid utilizing a 1,5-lactamized sialyl acceptor is described. 1,5-Lactamization was experimentally proven to proceed only for α-sialoside, which was integrated into the synthetic cycle of oligosialic acid

Full text of DOI:10.1016/j.tetlet.2009.05.057

Tetrahedron Letters 50 (2009) 4478–4481
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Tetrahedron Letters
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Synthetic study on
a
(2?8)-linked oligosialic acid employing
1,5-lactamization as a key step
b
Hidenori Tanaka a, Hiromune Ando b,c, , Hideharu Ishida , Makoto Kiso b,c, Hideharu Ishihara a,
*
Mamoru Koketsu d,
*
a Department of Chemistry, Faculty of Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan
b Department of Applied Bioorganic Chemistry, Faculty of Applied Biological Sciences, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan
c Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University, Yoshida Ushinomiya-cho, Sakyo-ku, Kyoto 606-8501, Japan
d Department of Material Science and Technology, Faculty of Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
An attempt to synthesize
described. 1,5-Lactamization was experimentally proven to proceed only for
a
(2?8)-linked oligosialic acid utilizing a 1,5-lactamized sialyl acceptor is
-sialoside, which was inte-
grated into the synthetic cycle of oligosialic acid as a chemical sorting step to collect the desired -sialo-
Received 13 April 2009
Revised 13 May 2009
Accepted 18 May 2009
Available online 23 May 2009
a
a
side and as a transformation step to produce a reactive sialyl acceptor for the next sialylation. Lactamized
oligosialyl acceptors served as favorable coupling partners for sialylation, providing high stereoselectiv-
ities and high yields.
Keywords:
Sialic acid
Ó 2009 Elsevier Ltd. All rights reserved.
Oligosialic acid
Glycosylation
Lactam
Sialic acids are expressed mostly on the glycan chains of glyco-
proteins and glycolipids, which play important roles in biological
events such as cell adhesion, cell differentiation, viral infection,
and neural network development.1 In most cases, they occupy
the distal end of glycan chains, being commonly linked to a galact-
The critical issue in the formation of the a(2?8)linkage
between sialic acids is the extremely low reactivity of the C-8
hydroxyl group, which is probably due to the hydrogen bonding
with either the C-5 acetamido group or the C-1 methoxy carbonyl
group.6 Recently, we have demonstrated that the reactivity of the
C-8 hydroxyl group could be dramatically enhanced by the forma-
tion of the 1,5-lactam of sialic acid, and this finding was success-
fully applied to the synthesis of the glycan moiety of a Hp-s6
ose or N-acetylgalactosamine residue through an
a(2?3) or
a(2?6)linkage. In some cases, a sialic acid residue is further con-
nected with another sialic acid at the C-4, C-8, or C-9 position,
forming dimeric, oligomeric, or polymeric sialic acid. It is known
that the expression of the a(2?8)-linked poly- and oligosialic acids
ganglioside that possesses a 8-O-SO3H-Neu5Ac
quence.4f Based on these results, we developed a synthetic cycle
to produce an (2?8)-linked oligomer of sialic acid as a single
a(2?8)Neu5Ac se-
in mammals is developmentally regulated, closely correlating to
a
the stages of neural network formation,2 and it was also demon-
anomer (Scheme 1). Simply, the synthesis comprises two reac-
strated that a(2?8)-linked di- or trisialic acid-containing ganglio-
tions—sialylation and 1,5-lactamization. Since the 1,5-lactamiza-
sides showed neurite extension activity.3
tion theoretically proceeds in the
lactamization was expected to function as a chemical sorting step
to collect the desired -glycoside exclusively as the lactamized
sialyl acceptor for the next sialylation. Repetition of this cycle
was expected to afford b-anomer-free (2?8)oligosialic acids.
a-configuration, the
Based on these impressive biological properties, the chemical
synthesis of
a(2?8)-linked oligosialic acid has been the subject
a
of intensive investigation in carbohydrate chemistry. So far,
a
(2?8)-linked disialic acid has been successfully prepared using
a
several approaches,4 while tri- and tetrasialic acids have been syn-
thesized only by Tanaka et al. whose approach involved utilizing a
highly reactive 5-N,4-O-oxazolidino sialic acid donor and accep-
To carry out this cycle, we designed key sialyl units 6 and 8,
which bear a phenylthio7 or trifluoroacetimidate group8 at the
anomeric position as a leaving group. A trifluoroacetyl group was
used both to enhance the reactivity of the sialyl donor and to act
as a tentative protecting group for the C-5 amino group.4e,9 Benzyl
groups were incorporated as persistent protecting groups during
the chain extension, and the C-8 hydroxyl group was capped with
a chloroacetyl group. The synthesis of sialyl donors 6 and 8 began
tor.5 Herein, we report an attempt to synthesize
a(2?8)-linked
oligosialic acid based on a reactive 1,5-lactam sialyl acceptor.
* Corresponding authors. Tel./fax: +81 58 293 3452.
E-mail address: hando@gifu-u.ac.jp (H. Ando).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.05.057

H. Tanaka et al. / Tetrahedron Letters 50 (2009) 4478–4481
4479
Y
L
O
PO
O
CO2Z
HN
X
OP
OP
α-Isomer
(Desired)
Glycosylation
H
O
H
O
Y
N
N
ZO2C
O
H
N
O
O
PO
OR
OR
n
PO
O
PO
O
O
HN
O
PO
O
OP
OP
Chemical Sorting
by
OP
OP
X
OP
HO
PO
OP
PO
n+1
1,5-Lactamization
α,β Mixture
O
OH
x
OP
PO
OH
O
x
OP
O
PO
O
H2N
PO
O
Activated Ester
Activated Ester
H2N
PO
O
P: Protecting group
O
L: Leaving group
β-Isomer
(Undesired)
X,Y and Z: Tentative protecting group
2,3-en byproduct
Scheme 1.
with the complete deacetylation of N-Ac thioglycoside 1 with
MsOH in MeOH, followed by a diazo transfer and O-acetylation
to provide 5-azide derivative 210 (80%, 3 steps) (Scheme 2). Com-
pound 2 was then subjected to de-O-acetylation, 8,9-O-isopropyli-
denation, and 4,7-di-O-benzylation to give compound 4 (75%, 4
steps). Subsequent removal of the 8,9-O-isopropylidene acetal
and selective 9-O-benzylation via a stannylene acetal intermediate
afforded compound 5 (91%, 3 steps). Then, azido derivative 5 was
converted into N-TFAc sialyl donor 6 through a reaction sequence
including: (1) reduction of the azide group to an amino group with
1,3-propanedithiol and NEt3 in MeOH11; (2) N-trifluoroacetylation;
and (3) O-chloroacetylation (91%, 3 steps). Furthermore, the treat-
ment of 6 with NBS in wet acetone afforded hemiketal 7 (85%),
which, upon reaction with N-(phenyl)trifluoroacetimidoyl chloride
in the presence of K2CO3 in acetone,8 was converted into imidate
donor 8 in 91% yield as a mixture of - and b-isomers ( :b = 2:3).
a
a
To assess the feasibility of the synthetic cycle, the sialyl unit 6
was subjected to glycosidation and subsequent 1,5-lactamization
(Scheme 3). Then, sialyl donor 6 was reacted with 2-(trimethyl-
silyl)ethanol in the presence of NIS and TfOH12 with the assistance
of the nitrile solvent effect13 at À40 °C to provide sialoside 9 as a
anomeric mixture (a:b = 8:1) in 98% yield. Next, the resulting
anomeric mixture was directly saponified and subsequently 1,5-
lactamized by the treatment with HBTU and DIEA to afford accep-
tor 10 as a single isomer in 75% yield (84% as a conversion yield of
the a-isomer). In this reaction, the b-isomer was transformed into
several unidentified highly polar compounds, which may include
polymerized products,14 thereby greatly facilitating the chromato-
graphic separation of the lactamized
a mixture containing the corresponding 2,3-ene derivative, lacta-
mized -sialoside 10 was easily collected by silica gel column
chromatography. These results proved that 1,5-lactamization func-
tions as a chemical sorting step for -sialoside.
Thus, with the lactamized acceptor 10 in hand, we then incor-
porated it into the cycle. First, we investigated the sialylation reac-
tion of 1,5-lactamized acceptor 10 with donors 6 and 8 (Table 1).
All reactions were carried out in EtCN as a stereocontrolling med-
ium. The initial coupling of acceptor 10 with thioglycoside donor 6
using NIS–TfOH as the activator afforded the desired disaccharide
a-sialoside. In addition, from
a
OR2
SPh
COOMe
OAc
SPh
COOMe
R3O
AcO
a
b
a
O
OR1
O
N3
R
AcO
R1O
91%
OAc
3 R1 = H, R2, R3 = Ispd
4 R1 = Bn, R2, R3 = Ispd
5 R1, R3 = Bn, R2 = H
1 R = NHAc
2 R = N3
c, 82%
80%
d, 91%
OCAc
R
BnO
TFAcHN
e
11 in 27% yield as an anomeric mixture (
a:b = 16:1) (entry 1).
COOMe
O
91%
BnO
The anomeric configuration of the major product in the mixture
OBn
6 R = SPh (β)
7 R = OH (α,β mixture)
8 R = OC(=NPh)CF3 (α:β = 2: 3)
f, 85%
g, 91%
H
O
OCAc
N
COOMe
OSE
BnO
TFAcHN
BnO
a
b
Scheme 2. Reagents and conditions: (a) (i) MsOH/MeOH, reflux; (ii) TfN3, DMAP/
MeOH, rt; (iii) Ac2O, pyr, 0 °C?rt, 80% (3 steps); (b) (i) NaOMe/MeOH, rt; (ii) DMP,
CSA/MeCN, rt, 91% (2 steps); (c) (i) BnBr, TBAI, NaH/DMF, rt; (ii) NaOMe/MeOH, rt,
82% (2 steps); (d) (i) 80% aq AcOH, 80 °C; (ii) DBTO/PhMe, 120 °C; (iii) BnBr, TBAI/
PhMe, 120 °C, 91% (3 steps); (e) (i) 1,3-propanedithiol, Et3N/MeOH, reflux; (ii)
TFAcOMe, NEt3, THF–MeOH (1:1), rt; (iii) CAcOH, DCC, DMAP/CH2Cl2, rt, 91% (3
steps); (f) NBS, wet acetone, rt, 85%; g) CF3C(@NPh)Cl, K2CO3/acetone, rt, 91%.
MsOH = methanesulfonic acid, TfN3 = trifluoromethanesulfonyl azide, DMAP = 4-
dimethylaminopyridine, DMP = 2,2-dimethoxypropane, CSA = ( )-10-camphorsul-
fonic acid, TBAI = tetra-n-butylammonium iodide, DBTO = di-n-butyltin oxide,
TFAc = trifluoroacetyl, CAc = chloroacetyl, DCC = N,N-dicyclohexylcarbodiimide,
Ispd = isopropylidene.
OSE
BnO
HO
6
O
OBn
9
O
98%
(α:β = 8:1)
75%
OBn
(84% from α-isomer)
10
BnO
Scheme 3. Reagents and conditions: (a) SEOH, NIS, TfOH, MS 3 Å/MeCN, À40 °C,
98% ( :b = 8:1); (b) (i) 1.0 M aq NaOH–THF–MeOH (1:2:2), rt; (ii) HBTU, DIEA/
MeCN, rt; (iii) 1.0 M aq NaOH–THF–MeOH (1:2:2), rt, 75% (3 steps). SE = 2-
(trimethylsilyl)ethyl, NIS = N-iodosuccinimide, TfOH = trifluoromethane sulfonic
acid, HBTU = O-(benzotriazol-1-yl)-N,N,N’,N’- tetramethyluronium hexafluorophos-
phate, DIEA = N,N-diisopropylethylamine.
a

4480
H. Tanaka et al. / Tetrahedron Letters 50 (2009) 4478–4481
Table 1
H
O
H
O
6 or 8
(3.0 eq.)
N
N
COOMe
BnO
O
OCAc
OSE
BnO
TFAcHN
OSE
BnO
HO
O
O
O
OBn
OBn
BnO
MS4Å
EtCN
OBn
BnO
BnO
10
(1.0 eq.)
11
Entry
Donor
Promoter
Temp (°C)
Time (h)
Yield (%)
a
:ba
1
2
6 (b)
6 (b)
NIS (4.5 equiv), TfOH (0.9 equiv)
PhSeBr (9.0 equiv), AgOTf (9.0 equiv)
DTBMP (9.0 equiv)
À80 to 0
9
4
27
0
16:1
—
À80 to À70
3
6 (b)
Ph2SO (9.0 equiv), Tf2O (3.3 equiv)
DTBMP (6.0 equiv)
TMSOTf (0.15 equiv)
À80 to rt
À80
12
72
0
—
4
8 (a,b)
86
16:1
a
Determined by 1H NMR analysis of the
a/b mixture.
3
was determined to be
a
from its JC1,H3ax coupling constant
not well suited for the sialylation of lactamized sialyl acceptor 10.
In entry 1, the corresponding N-hydroxy derivative of 10 was iso-
lated as a byproduct, which was produced probably as a result of
N-iodination within the lactam by NIS followed by hydrolysis dur-
ing the aqueous work-up. As shown in entry 4, the coupling of 10
with the imidate donor 8 catalyzed by TMSOTf provided a high
(6.9 Hz)15 while that of the minor product was less than 1.0 Hz,
indicating it to be b.16 When other activators17,18 for thioglycoside
6 were used, none of the desired disaccharide 11 was produced
(entries 2 and 3). It was found that these oxidative conditions were
yield of 11 (86%) with excellent a-selectivity (a:b = 16:1).
Next, the disaccharide 11 as an anomeric mixture was saponi-
fied and 1,5-lactamized to afford the acceptor 12 in pure form
(Scheme 4). In the next cycle, to our delight, the coupling of 8
11
a
83%
H
and 12 provided trisialic acid 13 as
a
single isomer
O
N
H
O
OSE
N
BnO
O
O
BnO
HO
O
OBn
R3
O
OBn
N
R3
O
BnO
12
OSE
BnO
R1O
N
R3
N
O
O
R1O
O
OR1
OR1
O
b
66% (α only)
R1O
R2O
O
H
O
OR1
OR1
O
OR1
N
H
N
O
n
OSE
BnO
O
COOMe
R1O
OCAc
O
BnO
TFAcHN
BnO
n = 0 n = 1 R1
R2
H
R3
H
Ac
H
O
OBn
O
O
12
16
18
20
14
17
19
21
Bn
Bn Ac
Ac Ac
BnO
OBn
13
a
b
c
BnO
OBn
BnO
a
82%
Ac Ac Cbz
H
O
N
d
H
O
OSE
N
BnO
O
H
N
O
O
BnO
O
O
OBn
R2O
O
OR2
R3O2C
OR2
BnO
HO
R3O2C
O
OR2
CO2R3
O
R2O
R1HN
R2O
OBn
BnO
O
OR2
OBn
BnO
O
O
R1HN
OSE
R1HN
14
R2O
OR2
R2O
BnO
b
44% (α only)
n
H
O
N
H
O
OSE
R1
R2
R3
N
n = 0 n = 1
BnO
O
H
O
O
22
24
26
23
25
27
Cbz Ac Me
N
COOMe
OCAc
e
f
BnO
O
OBn
BnO
Ac
Ac
Ac Me
Na
O
BnO
O
O
OBn BnO
O
H
TFAcHN
BnO
OBn
BnO
OBn
Scheme 5. Reagents and conditions: (a) (i) Ac2O, DMAP, pyr, rt, 89% (16), 94% (17),
(ii) H2, Pd(OH)2–C/EtOAc–EtOH (1:1), 40 °C; (iii) Ac2O, DMAP, pyr, rt; (iv) hydrazine
acetate/THF, rt, 84% (18, 3 steps), 85% (19, 3 steps); (b) CbzOSu, DMAP, pyr, rt, 61%
(20), 56% (21); (c) (i) NEt3–MeCN–H2O (1:9:5), 40 °C; (ii) MeI, K2CO3/DMF, rt; (iii)
Ac2O, DMAP, pyr, rt, 37% (22, 3 steps), 22% (23, 3 steps); (d) H2, Pd(OH)2–C/Ac2O–
EtOAc (1:1), rt, 71% (24), 61% (25); (e) 1.0 M aq NaOH, 40 °C, 97% (26), 41% (27).
CbzOSu = N-(benzyloxycarbonyloxy)succinimide.
BnO
15
Scheme 4. Reagents and conditions: (a) (i) 1.0 M aq NaOH–THF–MeOH (1:2:2), rt;
(ii) HBTU, DIEA/MeCN, rt; (iii) 1.0 M aq NaOH–THF–MeOH (1:2:2), rt, 83% (12, 3
steps), 82% (14, 3 steps); (b) 8, TMSOTf/EtCN, À80 °C, 66% (13,
a only), 44% (15, a
only).

H. Tanaka et al. / Tetrahedron Letters 50 (2009) 4478–4481
4481
(3JC1,H3ax = 6.9 Hz)15 in 66% yield. Once again, 13 could be converted
References and notes
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the same reaction conditions to furnish the tetrasaccharide 15
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again as a pure a
-isomer (3JC1,H3ax = 5.8 Hz)15 in 44% yield.
Finally, we attempted to convert the fully lactamized di- and
trisaccharide 12 and 14 into their natural N-acetyl form (Scheme
5). According to our reported procedure,4f compounds 12 and 14
are predisposed for lactam opening. Thus, the following reaction
sequences supplied the suitably protected 20 and 21 in 46% and
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Acknowledgments
This work was financially supported by MEXT of Japan (WPI
program and Grant-in-Aid for Scientific Research to M. K., No.
1701007). We thank Ms. Kiyoko Ito for technical assistance.
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Supplementary data
Supplementary data (1H and 13C NMR spectra of compounds 11,
12, 13, 14, 15, 26, and 27) associated with this article can be found,
in the online version, at doi:10.1016/j.tetlet.2009.05.057.