Synthetic study of α(2,8) oligosialoside using N-Troc sialyl N-phenyltrifluoroimidate

Article abstract of DOI:10.3987/COM-05-S(T)30

An effective approach for the synthesis of oligo-α(2,8) sialosides using N-Troc sialyl donors is described. Glycosylation of N-Troc sialoside with N-Troc sialyl N-phenyltrifluoroimidate and phosphites smoothly proceeded to provide α(2,8) disialosides in g

Full text of DOI:10.3987/COM-05-S(T)30

HETEROCYCLES, Vol. 67, No. 1, 2006
107
HETEROCYCLES, Vol. 67, No. 1, 2006, pp. 107 - 112. © The Japan Institute of Heterocyclic Chemistry
Received, 8th July, 2005, Accepted, 29th August, 2005, Published online, 30th August, 2005. COM-05-S(T)30
SYNTHETIC STUDY OF α(2,8) OLIGOSIALOSIDE USING N-TROC
SIALYL N-PHENYLTRIFLUOROIMIDATE
Hiroshi Tanaka, Yuji Nishiura, Masaatsu Adachi, and Takashi Takahashi*
Department of Applied Chemistry, Graduate School of Science and Engineering,
Tokyo Institute of Technology 2-12-1 Ookayama, Meguro, Tokyo 152-8552,
Japan; E-mail: ttak@apc.titech.ac.jp
Abstract – An effective approach for the synthesis of oligo-α(2,8) sialosides using
N-Troc sialyl donors is described. Glycosylation of N-Troc sialoside with N-Troc
sialyl N-phenyltrifluoroimidate and phosphites smoothly proceeded to provide
α(2,8) disialosides in good yield and selectivity.
Sialic acids such as Neu5Ac, Neu5Gc and KDN, are often located at the non-reducing end of
glycoconjugates on the cell surface through α-glycosidic bonds, and play a central role in cell surface
recognition phenomena.1 Recently, poly- and oligo-sialosides composed of the α(2,8) disialyl unit (1) are
found in many glycoproteins as well as glycolipids, and would be important fragments to bind proteins as
well as monomeric sialic acid.2 However, low availability of the pure sialo-containing glycoconjugates
from the natural sources makes it difficult to elucidate their biological activity, and requires an effective
methodology for the synthesis of α(2,8) sialosides.
RO
OH
HO
RO
X
HO
CO2H
O
RO
AcHN
O
CO2Me
OH
HO
AcHN
O
RO
2
HO
CO2H
OR
RO
RO
CO2Me
OR
HO
AcHN
O
O
AcHN
HO
RO
1
3
Scheme 1 Strategy for the synthesis of oligo-α(2,8) sialosides
The synthesis of α(2,8) disialyl unit (1) is one of the most problematic processes in chemical
oligosaccharide synthesis.3 The reactivity of the C8 hydroxyl group on sialoside (3) towards glycosylation
is dramatically reduced by the C1 carboxyl and/or the C5 acetamide group. In addition, the C1 carbonyl
group reduces the reactivity of sialyl donor (2) towards glycosidation. Furthermore, lack of the

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HETEROCYCLES, Vol. 67, No. 1, 2006
stereo-directing group adjacent to the anomeric position makes it difficult to stereoselectivity form the
thermodynamically unstable α-glycosidic linkages, and promotes β-elimination during glycosidation.
Recently, the sialy donors possessing N,N-diacetyl,4 azido,5 N-TFA,6 N-Troc,7,8 N-trichloroacetyl8 and
N-Fmoc8 groups at the C5 position have been reported to exhibit the improved reactivity towards
sialylation. We have already reported the one-pot synthesis of sialo-containing oligosaccharides using the
N-Troc β-thiosialoside.8 The N-Troc protected sialyl donors were effective for the synthesis of
sialo-containing amino acids and peptides because modification of the N-Troc protecting group to the
NHAc group can be achieved without racemization of the amino acids and peptides. Herein we report an
efficient synthesis of α(2,8) sialosides by glycosidation of N-Troc sialyl donors.
We first investigated glycosylation of sialoside at the C8 poition with N-Troc sialyl donors varying the
leaving groups (Table 1). We selected N-phenyl trifluoroimidate and phosphites as leaving groups, which
enable activation of the N-acetyl sialy donors at low reaction temperature.9,10 In addition, N-phenyl
trifluoroimidate would be an effective leaving group for glycosylation of low reactive acceptors.11
Treatment of acceptor (4) with three equivalents of thioglycoside12 (5a) in the presence of NIS and TfOH
in MeCN at -35 ºC provided disaccharides (6) in 36% yield with good α-selectivity (Entry 1). Activation
of trifluoroimidate (5b) with a catalytic amount of TMSOTf at -35 ºC resulted in the improved yield of
disaccharide (6). Glycosidation of trifluoroimidate (5b) at -78 ºC provided disaccharide (6) in 67% yield
with good α-selectivity (α:β = 81:19). Glycosyl phosphites (5c) and (5d) resulted in the reduced yields of
6 in comparison with imidate (5b). Use of 1.5 equivalents of glycosyl donor (5b)13 resulted in good yield
of 6. Structure determination of α-sialoside (6α) was achieved by 1H NMR spectral analysis based on the
empirical roles.14
Table 1 Glycosylation of sialoside at the C8 position with N-Troc sialyl donors varying the leaving
groups.
AcO
AcO
OAc
O
OAc
O
X
5a : X = !-SPh
CO2Me
O
AcO
TrocHN
5b : X = "- OC(=NPh)CF3
5c : X = !-OP(OBn)2
5d : X = !-OP(OEt)2
AcO
TrocHN
CO2Me
BnO
OH
CO2Me
O
AcO
BnO
AcO
CO2Me
O
AcO
TrocHN
AcO
TrocHN
AcO
O
O
6
Activator, Solvent
MS-3A, Temp.
6
AcO
4
6
a
Temp (oC)
-35
Entry
eq.
3.0
3.0
3.0
1.5
3.0
3.0
Donor
5a
Activator
Solvent
Yield (%)
" : !
1
2
3
4
5
6
NIS(6.0 eq.)/TfOH(0.1 eq.)
TMSOTf (0.3 eq.)
TMSOTf (0.3 eq.)
TMSOTf (0.15 eq.)
TMSOTf (0.3 eq.)
TMSOTf (0.3 eq.)
MeCN
36
71
67
61
57
47
73 : 27
67 : 33
81 : 19
83 : 17
85 : 15
84 : 16
5b
MeCN
-35
5b
MeCN/CH2Cl2 = 2/3
MeCN/CH2Cl2 = 2/3
MeCN/CH2Cl2 = 2/3
MeCN/CH2Cl2 = 2/3
-78
5b
-78
5c
-78
5d
-78
a
The ":! ratio was determined by HPLC analysis based on refractive index detection

HETEROCYCLES, Vol. 67, No. 1, 2006
109
Next, we investigated the synthesis of oligo-α(2,8)-sialosides. The glycosyl trifluoroimidate (11)
protected with a chloroacetyl group at the C8 hydroxyl group was designed for the synthesis of
oligo-α(2,8) sialosides. The chloroacetyl protecting group can be selectively removed after glycosidation.
The synthesis of the sialyl donor (11) is shown in Scheme 2. Treatment of tetraacetyl N-acetyl
β-thiosialoside (7) with methanesulfonic acid provided amine (8). N-Acylation of amine (8) with
TrocOSu, followed by acetalization of the C8 and C9 hydroxyl groups afforded diol (9). The remaining
hydroxyl group was protected with the acetyl group, followed by regioselectively reductive opening of
the benzylidene acetal provided monool (10) in 54% overall yield from 7. Protection of the resulting
hydroxyl group with chloroacetyl group provided fully protected thioglycoside (11) in 98% yield.
Hydrolysis of the thioglycoside, followed by coupling with imidoyl chloride (13) afforded the sialyl
trifluoroimidate (14) in good yield.
Ph
d. e
O
b. c
AcO
OAc
O
O
HO
OH
SPh
CO2Me
a
SPh
SPh
AcO
AcHN
HO
HO
MsOH•H2N
O
O
CO2Me
CO2Me
TrocHN
AcO
HO
HO
7
8
9
Ph
N
ClAcO
ClAcO
BnO
RO
BnO
Cl
CF3
BnO
AcO
TrocHN
OH
O
CO2Me
SPh
g
13
Ph
h
AcO
N
AcO
TrocHN
O
CO2Me
TrocHN
O
CO2Me
O
CF3
AcO
AcO
AcO
10 : R = H
12
14
f
11 : R = ClAc
ClAc = ClCH2CO-
Scheme 2 Reagents and conditions: (a) MsOH, MeOH, 60 ºC. (b) TrocOSu, NEt3, MeCN, H2O, rt. (c)
PhCH(OMe)2, CSA, MeCN, rt. (d) Ac2O, py, CH2Cl2, DMAP, (e) BH3•NMe3, AlCl3, THF, MS4A, 0 ºC,
54% from 7. (f) chloroacetyl choride, py, CH2Cl2, rt, 98%. (g) NBS, acetone, H2O, 0 ºC, 90%. (h) Cs2CO3,
acetone, 0 ºC, 80%, α:β = 88:12.
Coupling of the sialyl donor (14) with acceptor (4) was examined (Scheme 3). Treatment of acceptor (4)
with three equivalents of the glycosyl imidate (14) in the presence of a catalytic amount of TMSOTf in
CH3CN at -78 ºC provided disaccharides (6) in 58% yield with good α-selectivity. Removal of the
chloroacetyl group with thiourea provided the disaccharide acceptor (16) in 80% yield. Next, the
synthesis of trisialoside (17) was examined. Next we examined the synthesis of tri-α(2,8) sialoside (17).
Treatment of the acceptor (16) with three equivalents of glycosyl imidate (14) under the same
glycosidation conditions unfortunately provided trisaccharide (17) in only 6% as an anomeric mixture,
whose structure was confirmed by MS and 1H NMR spectra. The ratio of the anomeric isomers could not
be estimated by the information. These results indicate that reactivity of disaccharides (16) towards
glycosylation was dramatically reduced in comparison with monosialoside (4).

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HETEROCYCLES, Vol. 67, No. 1, 2006
Finally, deprotection of α(2,8) disialoside was examined. The N-Troc and acetyl groups and methyl esters
of 16 were spontaneously hydrolyzed under the standard basic conditions to provide amino acid (18).
Treatment of 18 with acetic anhydride, followed by exposure to NaOMe in MeOH afforded
α(2,8)-dissialoside (19) possessing the two benzyl groups in 77% yield based on 15. Hydrogenolysis of
the benzyl ethers in the presence of Pd(OH)2 provided the fully deprotected α(2,8)-dissialoside (20)15 in
96% yield. The coupling constants 3JC1-H3ax (5.8 and 5.1 Hz) of 19 indicate that the two glycosidic linkages
are α-configuration.16
ClAcO
BnO
CO2Me
OR
BnO
AcO
AcO
TrocHN
CO2Me
O
O
14
OH
BnO
AcO
TrocHN
AcO
c
O
AcO
O
TrocHN
a
CO2Me
O
BnO
AcO
AcO
CO2Me
O
BnO
O
CO2Me
O
6
O
TrocHN
AcO
O
( 1.0 eq. )
AcO
TrocHN
6
BnO
CO2Me
O
AcO
4
AcO
O
15 : R = ClAc
16 : R = H
TrocHN
6
b
AcO
d
17
OH
HO
CO2H
OH
BnO
HO
AcHN
CO2H
O
f
O
HO
RHN
O
O
HO
HO
HO
CO2H
BnO
CO2H
O
HO
O
HO
AcHN
O
O
6
RHN
HO
6
HO
20
18 : R = H
e
19 : R = Ac
Scheme 3 Reagents and conditions: (a) 14 (3.0 eq.) TMSOTf (0.3 eq.), CH2Cl2/MeCN (2/3), MS3A, -78
ºC, 58%, α:β = 83:17. (b) thiourea, 2,6-lutidine, DMF, 70 ºC. (c) 14 (3.0 eq.) TMSOTf (0.3 eq.),
CH2Cl2/MeCN (2/3), MS3A, -78 ºC, 6% as anomeric mixture. (d) LiOH•H2O, EtOH, H2O, 80 ºC. (e)
Ac2O, MeOH, then NaOMe, MeOH, 77% from 15. (g) Pd(OH)2, H2 (1 atm), H2O, MeOH, 96%.
In conclusion, we reported an effective approach for the synthesis of oligo-α(2,8) sialosides using N-Troc
sialyl donors. Glycosylation of the N-Troc sialoside (4) with the N-Troc sialyl N-phenyltrifluoroimidates
provided α(2,8) sialosides in good yield and selectivity. The coupling methodology would be effective for
the synthesis of α(2,8) sialo-containing glycoconjugates such as glycosyl amino acids and peptides.
Synthesis of various oligosaccharide containing the α(2,8) disialyl unit is in progress.
ACKNOWLEDGEMENTS
This work was supported by a Grand-in Aid for Scientific Research on Priority Area (S) from the
Ministry of Education, Culture, Sports, Science, and Technology. (Grant-in-aid No.14103013).

HETEROCYCLES, Vol. 67, No. 1, 2006
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2H, J = 7.7 Hz), 5.42 (dd, 1H, J6,7 = 1.5 Hz, J7,8 = 6.8 Hz), 5.27 (d, 1H, NH, J = 9.7 Hz), 5.26 (m, 1H),
5.21 (ddd, 1H, J3ax.,4 = 10.1 Hz, J3eq.,4 = 5.3 Hz, J4,5 = 9.7 Hz), 4.92 (d, 1H, J = 12.1 Hz), 4.75 (dd, 1H,
J5,6 = 10.6 Hz, J6,7= 1.5 Hz), 4.51 (d, 1H, CH2CCl3, J = 12.1 Hz), 4.37 (dd, 1H, J8,9=2.4 Hz, Jgem=12.6
Hz), 4.13 (dd, 1H, H-9’, J8,9’ = 5.8 Hz, Jgem = 12.6 Hz), 3.80 (s, 3H, OMe), 3.78 (m, 1H), 2.78 (dd, 1H,
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13
1.98 (4s, 12H, Ac); C NMR (100 MHz, CDCl3) δ 170.7, 170.4, 170.3, 170.0, 167.3, 154.2, 142.6,
128.8, 124.8, 120.6, 119.4, 97.9, 95.5, 74.6, 73.5, 70.0, 67.9, 67.7, 61.8, 53.0, 51.5, 36.7, 20.9, 20.8,
20.7; IR (KBr) 3324, 3027, 2958, 1746, 1539, 1333, 737 (cm-1).
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27
15. Spectra of 20: [α]D +6.4º (c 1.00, H2O); H NMR (400 MHz, D2O) δ 4.16 (m, 1H, H-8), 4.10 (dd,
1
1H, H-9a, J8,9a = 3.9 Hz, J9a,9b = 11.6 Hz ), 3.49 – 3.88 (m, 13H ), 3.39 (dt, 1H, OCH2, J = 7.25 Hz, J
= 8.70 Hz), 2.73 (dd, 1H, H-3’eq., J3’ax.,3’eq. = 12.1 Hz, J3’eq.,4’ = 4.35), 2.61 (dd, 1H, H-3eq., J3ax.,3eq.
12.1 Hz, J3eq.,4 = 4.35 Hz), 2.04, 2,00 (2s, 6H, Ac), 1.71 (dd, 1H, H-3’ax., J3’ax.,3’eq. = 12.1 Hz, J3’ax.,4’
=
=
12.1 ), 1.50 – 1.58 (m, 3H, H- 3ax, OCH2CH2 ), 1.25 (br s, 10H, CH2), 0.83 (t, 3H, J = 6.8 Hz) ; 13C
NMR (100 MHz, D2O, acetone-d6) δ 175.0 x 2, 173.5, 173.4, 101.3, 100.5, 78.7, 74.2, 72.8, 71.8,

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HETEROCYCLES, Vol. 67, No. 1, 2006
69.8, 68.5, 68.3, 68.0, 65.2, 62.8, 61.8, 52.6, 51.9, 49.0, 40.6, 31.2, 28.6, 28.5, 25.3, 22.4, 22.1 x 2,
13.5; IR (KBr) 3530, 1650, 1418, 1378, 1071, 778 (cm-1). MS (ESI-TOF) calcd for C30H53N2O17Na
[M+Na]+ 713.3, found 713.5.
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