Stereoselective synthesis of α(2,9) di- to tetrasialic acids, using a 5,4-N,O-carbonyl protected thiosialoside

Article abstract of DOI:10.1021/jo900176e

(Chemical Equation Presented) An efficient stereoselective synthesis of α(2,9) tetra- to disialic acids 1-3, using the 5,4-N,O-carbonyl protected thiosialoside 4, is described. The cyclic protecting group was effective for α-sialylation without the need f

Full text of DOI:10.1021/jo900176e

would be highly desirable, in that it would facilitate our
understanding of their biological roles.
Stereoselective Synthesis of r(2,9) Di- to
Tetrasialic Acids, Using a 5,4-N,O-Carbonyl
Protected Thiosialoside
The synthesis of R-linked sialic acid derivatives represents
one of the most difficult and challenging processes in the
chemical synthesis of oligosaccharides.6 The carboxyl group at
the C1 position reduces the reactivity of the anomeric position
toward glycosidation. The lack of a participating auxiliary
adjacent to the anomeric center makes it difficult to form
thermodynamically and kinetically glycosylation-disfavored
R-sialosides and promotes ꢀ-elimination with the production
of a glycal. Acetonitrile and propionitrile are frequently used
as effective solvents in direct R-sialylations because these
solvents block the ꢀ face of the intermediate oxonium cation,
thus promoting R-selective sialylation.7 The nitrile-promoted
R-sialylation is more efficient when secondary alcohols are used
in the glycosylation compared to primary alcohols. Several
reports have appeared regarding the synthesis of R(2,9) disialic
acid based on nitrile-promoted R-sialylation.8 The 8,9 diols were
found to be effective acceptors for R(2,9) sialylation reactions.8b
The conversion of the acetamide group at the C5 position of
the sialyl donor to N,N-diacetyl, azido, N-TFA, N-Troc, N-Fmoc,
N-trichloroacetyl, and N-phthalimide groups has also recently
been reported to be effective for improving the reactivity of
the sialyl donor toward glycosidation.9,10 These donors undergo
R-sialylation in nitrile solvents. Wong and co-workers reported
on the synthesis of protected R(2,9) tetrasialic acids using
5-azido sialyl phosphates.10b Lin and co-workers successfully
prepared a protected R(2,9) pentasialic acid based on an iterative
glycosidation strategy using an N-TFA protected sialyl donor.10f
However, although dimerization proceeded in excellent yield
and selectivity, the formation of tetra- and trimers as donors
resulted in a significant reduction in R-selectivity. We recently
reported on a new and effective method for R-sialylation using
the 5,4-N,O-carbonyl protected sialyl donor.11 The donors
undergo R-sialylation and a nitrile solvent is not required in
Hiroshi Tanaka,* Yuji Nishiura, 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
thiroshi@apc.titech.ac.jp; ttak@apc.titech.ac.jp
ReceiVed January 28, 2009
An efficient stereoselective synthesis of R(2,9) tetra- to
disialic acids 1-3, using the 5,4-N,O-carbonyl protected
thiosialoside 4, is described. The cyclic protecting group was
effective for R-sialylation without the need for acetonitrile
as the solvent. The donor 4 enabled the formation of a
tetramer in excellent yield and selectivity. Deprotection of
the cyclic protecting groups of the protected di- to tetrasialica
acids proceeded smoothly to give the fully deprotected R(2,9)
tetra- to disialic acids 1-3.
(6) (a) Boons, G.-J.; Demchenko, A. V. Chem. ReV. 2000, 100, 4539–4565.
(b) Ando, H.; Imanura, A. Trends Glycosci. Glycotechnol. 2004, 16, 293–303.
(7) (a) Murase, T.; Ishida, H.; Kiso, M.; Hasegawa, A. Carbohydr. Res. 1988,
184, c1-c4. (b) Hasegawa, A.; Nagahama, T.; Ohki, H.; Hotta, K.; Ishida, H.;
Kiso, M. J. Carbohydr. Chem. 1991, 10, 493–498.
Sialic acids are a family of the most complex monosaccharide
units in naturally occurring oligosaccharides, and are frequently
located at the nonreducing ends of oligosaccharides.1 Recent
progress in glycobiology suggests that R(2,8) and R(2,9) di/
oligosialic and polysialic acids may play important roles in
biological events that occur on the cell surface.2 The R(2,9)
disialic acid unit is attached to lactosaminoglycan in human
teratocarcinoma cells (PA1).3 The R(2,9) polysialic acids have
been reported to be present in C-1300 mouse nurobrastoma cells
(NB41A3).4 In addition, a glycoprotein carrying R(2,9) poly-
sialic acids has been identified in sea urchin sperm flagella.5 A
straightforward chemical synthesis of these oligosaccharides
(8) (a) Ogawa, T.; Sugimoto, M. Carbohydr. Res. 1984, 128, c1-c4. (b)
Shimizu, C.; Achiwa, K. Carbohydr. Res. 1987, 166, 314–316. (c) Okamoto,
K.; Kondo, T.; Goto, T. Tetrahedron Lett. 1986, 27, 5229–5232. (d) Okamoto,
K.; Kondo, T.; Goto, T. Tetrahedron 1988, 44, 1291–1298. (e) Hasegawa, A.;
Ogawa, M.; Ishida, H.; Kiso, M. J. Carbohydr. Chem. 1990, 9, 393–414. (f)
Demchenko, A. V.; Boons, G. J. Chem.sEur. J. 1999, 5, 1278–1283. (g) Lu,
K.-C.; Tseng, S.-Y.; Lin, C.-C. Carbohydr. Res. 2002, 337, 755–760. (h) Ando,
H.; Koike, Y.; Ishida, H.; Kiso, M. Tetrahedron Lett. 2003, 44, 6883–6886.
(9) De Meo, C.; Priyadarshani, U. Carbohydr. Res. 2008, 343, 1540–1552.
(10) (a) Demchenko, A. V.; Boons, G.-J. Tetrahedron Lett. 1998, 39, 3065–
3068. (b) Yu, C.-S.; Niikura, K.; Lin, C.-C.; Wong, C.-H. Angew. Chem., Int.
Ed. 2001, 40, 2900–2903. (c) De Meo, C.; Demchenko, A. V.; Boons, G.-J. J.
Org. Chem. 2001, 66, 5490–5497. (d) Ando, H.; Koike, Y.; Ishida, H.; Kiso, M.
Tetrahedron Lett. 2003, 44, 6883–6886. (e) Adachi, M.; Tanaka, H.; Takahashi,
T. Synlett 2004, 609-614. (f) Lin, C.-C.; Huang, K. T.; Lin, C.-C. Org. Lett.
2005, 7, 4169–4172. (g) Takeda, Y.; Horito, S. Carbohydr. Res. 2005, 340, 211–
220. (h) Tanaka, H.; Nishiura, Y.; Adachi, M.; Takahashi, T. Heterocycles 2006,
67, 107–112. (i) Tanaka, K.; Goi, T.; FukaseK. Synlett 2005, 2958-2962. (j)
Crich, D.; Wenju, L. Org. Lett. 2006, 8, 959–962. (k) Hanashima, S.; Castagner,
B.; Esposito, D.; Nokami, T.; Seeberger, P. H. Org. Lett. 2007, 9, 1777–1779.
(11) (a) Tanaka, H.; Nishiura, Y.; Takahashi, T. J. Am. Chem. Soc. 2006,
128, 7124–7125. (b) Tanaka, H.; Nishiura, Y.; Takahashi, T. J. Am. Chem. Soc.
2008, 130, 17244–17245.
(1) Angat, T.; Varki, A. Chem. ReV. 2002, 102, 439–470.
(2) Sato, C.; Kitajima, K. Trends Glycosci. Glycotechnol. 1999, 11, 371–
390.
(3) Fukuda, M. N.; Dell, A.; Oates, J. E.; Fukuda, M. J. Biol. Chem. 1985,
260, 6623–6632.
(4) Inoue, S.; Poongodi, G. L.; Suresh, N.; Jennings, H. J.; Inoue, Y. J. Biol.
Chem. 2003, 278, 8541–8546.
(5) Miyata, S.; Sato, C.; Kumita, H.; Toriyama, M.; Vacquier, V. D.; Kitajima,
K. Glycobiology 2006, 16, 1229–1241.
10.1021/jo900176e CCC: $40.75
Published on Web 05/04/2009
2009 American Chemical Society
J. Org. Chem. 2009, 74, 4383–4386 4383

SCHEME 1. Strategies for Synthesis of the r(2,9) Di- to Tetrasialic Acids 3, 2, and 1
SCHEME 2. Preparation of the Sialyl Donor and Acceptor 8 and 9
the reaction.12,13 The 5,4-N,O-carbonyl protection permitted
R-sialylation without use of nitrile solvents, but the mechanism
for this unique R-sialyaltion was not examined in detail. The
method described herein is particularly useful in glycosylation
reactions involving the use of a primary alcohol in terms of
both yield and stereoselectivity, compared to nitrile-promoted
R-sialylation. Because of the above findings, we initiated a study
of the synthesis of R(2,9) oligosialic acids using 5,4-N,O-
carbonyl protected sialyl donors.
Our initial approach to this synthesis was a one-pot glyco-
sylation involving the chemoselective sialylation of thioglyco-
side 5 with the S-benzoxazolyl (S-Box) sialyl donor 4 and the
subsequent use of disialic acid 7 as a glycosyl donor (Sch-
eme 1).14 This approach was effective in that manipulation of
the protecting group in the oligosaccharide synthesis could be
minimized. However, although the formation of disaccharide 7
from donor 4 and acceptor 5 proceeded in excellent yield and
with R-selectivity, disaccharide donor 7 underwent glycosylation
of the sialyl acceptor 6 with reduced R-selectivity. These results
suggest that glycosylation with di/oligosialic acid units would
be difficult to carry out in an R stereoselective manner. The
second approach to the synthesis of R(2,9) oligosialic acids
involves glycosylation of oligosialic acids 9, which contain an
8,9 diol at the nonreducing end of the sialic acid with the
monosialic acid unit 8. The 8,9 diols 9-11 would be effective
as acceptors in R(2,9) sialylation reactions, because the C8
hydroxyl group does not adversely interfere with glycosylation
at the C-9 position.8b The 8-O-chloroacetyl and 9-O-benzyl
ꢀ-thiosialoside 8 were used as donors because the protecting
groups at the C8 and C9 hydroxyl groups can be removed
chemoselectively.
Preparation of the building blocks 8 and 9 is shown in Scheme
2. The known tetraacethyl thiosialoside 12 was used as the
starting material. The tetraacetates and the acetamide were
removed under acidic conditions. 5,4-N,O-Carbonyl protection,
followed by regioselective acetalization with benzyl aldehyde
at the C8 and C9 positions afforded benzyliden acetal 13 in
64% yield as a diastereomixture (1:1). Acetylation of the
(12) (a) Farris, M. D.; De Meo, C. Tetrahedron Lett. 2007, 48, 1225–1227.
(b) De Meo, C.; Farris, M.; Ginder, N.; Gulley, B.; Priyadarshani, U.; Woods,
M. Eur. J. Org. Chem. 2008, 21, 3637–3677.
(13) (a) Crich, D.; Li, W. J. Org. Chem. 2007, 72, 2387–2391. (b) Crich,
D.; Li, W. J. Org. Chem. 2007, 72, 7794–7797.
(14) Tanaka, H.; Tateno, Y.; Nishiura, Y.; Takahashi, Y. Org. Lett. 2008,
10, 5597–5600.
4384 J. Org. Chem. Vol. 74, No. 11, 2009

SCHEME 3. Synthesis of the Protected r(2,9) Di- to Tetrasialic Acids 10, 23, and 24
hydroxyl group at the C7 position, followed by the regioselecitve
reductive opening of the acetal provided the benzyl ether 15 in
74% yield. The remaining hydroxyl groups at the C8 position
were protected by reaction with chloroacetic acid to yield the
donor 8 in 92% yield. Treatment of donor 8 and n-octanol with
NIS and a catalytic amount of TfOH in CH2Cl2 provided the
R-sialoside 16 in 80% yield with R:ꢀ ) 97:3. The chloroacetyl
group was removed by treatment with thiourea to give alcohol
17 in 84% yield. Hydrogenolysis of the benzyl ether 17 at the
C9 position provided diol 9 in quantitative yield.
The synthesis of tetra- to di-R(2,9) sialic acids 1-3 was
examined. Treatment of R-thiosialoside 8 and 1.0 equiv of diol
9 with NIS and a catalytic amount of TfOH in the presence of
MS-3A in CH2Cl2 stereoselectively provided R-sialoside 20 in
92% yield with R:ꢀ ) >95:5. On the other hand, sialylation of
the alcohol acceptor 18 under the same reaction conditions
resulted in a significantly reduced yield of R-disialic acid 21
and R-selectivity (53% yield with R:ꢀ ) 83:17). These results
indicate that 8,9 diol would be an excellent choice for use in
R-sialylation preparations with use of the 5,4-N,O-carbonyl
protected sialyl donor 8. In addition, the 8,9 dichloroacetyl donor
19 was adapted for use in R(2,9) sialyaltion. The coupling
product 22 requires only one additional step for the synthesis
of the next 8,9 diol acceptor.15 However, the glycosylation of
acceptor 9 with donor 19 resulted in the formation of side-
reaction products that were difficult to separate from the
coupling product 22. The preparation of the disialoside acceptor
10 was achieved as follows: (1) removal of the chloroacetyl
group with thiourea and (2) hydrogenolysis of the benzyl ether
at the C9 position. Sialylation of the disialoside 10 at the 9
position was achieved by using a slight excess (1.2 equiv) of
donor 8 to provide the protected R(2,9) trisialic acid 23 in 83%
yield with R:ꢀ ) >95:5. Deprotection of the chloroacetyl group
and the benzyl ether afforded the tetraol acceptor 11 in 62%
yield. Tetrasialic acid saccharide formation from 11 with 2.0
equiv of donor 8 under the same reaction conditions provided
tetrasaccahride 24 in 58% yield with R:ꢀ ) >95:5. The coupling
3
constants JC1-H3ax (7.3, 6.5, 5.0, and 4.8 Hz) for 24 indicated
that the configuration of all of the glycosidic linkages was R.16
The regioselectivity for each glycosylation step was confirmed
by 1H NMR analysis of the acetylated product 25 (Scheme 3).
Deprotection of the protected di- to tetrasialic acids 19, 23,
and 24 is shown in Scheme 4. The protected tetrasialic acids
20, 23, and 24 were exposed to basic conditions, to remove the
5,4-N,O-carbonyl protecting group as well as ester groups,
followed by acetylation of the resulting amines. The partially
generated O-acetyl groups were then removed by hydrolysis
under basic conditions. Finally, removal of the benzyl ether at
the C9 hydroxyl group was achieved by hydrogenolysis by using
a palladium catalyst to afford the fully deprotected di- to
tetrasialosides 3, 2, and 1 in 87%, 89%, and 69% overall yields,
respectively.
In conclusion, we describe an efficient method for the
stereoselective synthesis of R(2,9) di- to tetrasialic acids 3, 2,
and 1 via the use of a simple glycosylation and deprotection
procedure. The 5,4-N,O-carbonyl protection of donor 8 was
effective for the R sialylation of the primary alcohol and
acetonitrile was not required for the reaction to proceed. The
8,9 diols 9, 10, and 11 possessing a 5,4-N,O-carbonyl protecting
(15) The 8,9-benzilidene thiosialoside 14 represents an alternative candidate
donor, which can be converted to 8,9 acceptors in a single operation. However,
the use of a mixture of stereoisomers derived from the benzilidene acetal makes
it difficult to analyze the structure of the coupling product. In addition, the
selective synthesis of the acetal derivative as a single diastereomer or separation
of these isomers was difficult. Therefore, thiosialoside 14 was not used as a
donor in the synthesis of these complex oligosaccharides.
3
(16) JC-1,H-3ax of R-sialosides is >4 Hz: Haverkamp, J.; Spoormaker, T.;
Dorland, L.; Vliegenthart, J. F. G.; Schauer, R. J. Am. Chem. Soc. 1979, 101,
4851-485.
J. Org. Chem. Vol. 74, No. 11, 2009 4385

SCHEME 4. Deprotection of the r(2,9) Di- to Tetrasialic Acids 19, 23, and 24
J ) 6.8 Hz), 1.28 (m, 10H), 1.54 (br dt, 2H), 2.02 (s, 3H), 2.08
group acted as an effective acceptor for the synthesis of di- to
(dd, 1H, J ) 12.1, 13.0 Hz), 2.09 (dd, 1H, J ) 12.1, 13.0 Hz),
2.15 (s, 3H), 2.92 (dd, 1H, J ) 3.9, 12.1 Hz), 3.01 (dd, 1H), 3.02
(dd, 1H, J ) 9.6, 12.6 Hz), 3.08 (dd, 1H, J ) 9.7, 10.6 Hz), 3.24
(dt, 1H, J ) 6.8, 8.7 Hz), 3.40 (dd, 1H, J ) 2.4, 9.7 Hz), 3.52-3.59
(m, 2H), 3.70 (m, 1H), 3.74 (m, 1H), 3.81 (s, 3H), 3.83 (m, 1H),
3.85 (s, 3H), 3.92 (m, 1H), 3.93 (m, 1H), 4.07 (dd, 1H, J ) 2.4,
9.7 Hz), 4.16 (m, 1H), 4.18 (d, 1H, J ) 15.5 Hz), 4.23 (dd, 1H, J
) 1.4, 9.7 Hz), 4.38 (d, 1H, J ) 12.1 Hz), 4.39 (d, 1H, J ) 15.5
Hz), 4.61 (d, 1H, J ) 12.1 Hz), 4.98 (dd, 1H, J ) 2.4, 9.2 Hz),
5.33 (dd, 1H, J ) 1.4, 10.1 Hz), 5.39 (br s, 1H), 5.41 (m, 1H),
5.46 (br s, 1H), 7.25-7.38 (m, 5H); 13C NMR (100 MHz, CDCl3)
δ 14.1, 20.7, 20.9, 22.7, 25.9, 29.2, 29.3, 29.5, 31.8, 37.1, 37.4,
41.5, 53.2, 53.6, 57.8, 57.9, 65.3, 65.5, 67.3, 67.3, 68.6, 69.3, 71.0,
73.5, 74.0, 74.3, 76.6, 99.9, 100.2, 128.1, 128.1, 128.6, 137.1, 159.2,
159.3 166.7, 168.7, 169.5, 171.2, 171.4; IR (KBr) 772, 1023, 1376,
1742, 2856, 2924, 3399, 3527 (cm-1); HRMS (ESI-TOF) calcd for
C43H59N2O20ClNa [M + Na]+ 981.3242, found 981.3242.
tetra-R(2,9)-sialic acids 3, 2, and 1.
Experimental Section
General Procedure for Glycosylation with the 5,4-N,O-Carbo-
nyl Protected Thiosialoside. A mixture of donor 8 (40.7 mg, 88.2
µmol, 1.00 equiv), acceptor 9 (53.7 mg, 88.2 µmol, 1.00 equiv),
and pulverized activated MS-3A (44.1 mg, 0.50 g/mmol) in
anhydrous CH2Cl2 (900 µL, 10.0 mL/mmol) was stirred at room
temperature for 30 min under argon to remove traces of water. The
reaction mixture was then cooled to -78 °C. N-Iodosuccinimide
(23.8 mg, 102 µmol, 1.20 equiv) and a catalytic amount of
trifluoromethanesulfonic acid (0.80 µL, 8.82 µmol, 0.10 equiv) were
added to the reaction mixture at -78 °C. The reaction temperature
was gradually increased to -60 °C. After completion of the reaction,
as evidenced by TLC, the reaction mixture was neutralized with
triethylamine and filtered through a pad of Celite. The filtrate was
poured into a mixture of saturated aq NaHCO3 and saturated aq
Na2S2O3 with cooling. The aqueous layer was extracted with two
portions of ethyl acetate. The combined extracts were washed with
saturated aq NaHCO3, saturated aq Na2S2O3, and brine, dried over
MgSO4, filtered, and evaporated in vacuo. The residue was
chromatographed on silica gel with 98:2 chloroform-methanol as
the eluent and further purified by gel permeation chromatography
(GPC) to give disaccharide 20 (77.4 mg, 80.7 µmol, 92%, R/ꢀ )
Acknowledgment. This work was financially supported by
JSPS Research Fellowships for Young Scientists DC2.
Supporting Information Available: Experimental proce-
dures and characterization data for all new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
>95/5). The R/ꢀ ratio was determined by 1H NMR analysis. [R]21
D
-6.33 (c 0.43, CHCl3); 1H NMR (400 MHz, CDCl3) δ 0.88 (t, 3H,
JO900176E
4386 J. Org. Chem. Vol. 74, No. 11, 2009