Approaches to intramolecular sialylation 1. Synthesis of pseudodisaccharide Neu5Ac-(1-2)-Gal with ester-linked monosaccharide residues

Article abstract of DOI:10.1023/A:1024803900642

An efficient approach was developed for ester-linking the sterically hindered carboxy group of N-acetylneuraminic acid with the secondary hydroxy group at C(2) of galactose. Putative precursors of the glycosidically linked disaccharide Neu5Ac-(2-3)-Gal we

Full text of DOI:10.1023/A:1024803900642

1434
Russian Chemical Bulletin, International Edition, Vol. 52, No. 6, pp. 1434—1441, June, 2003
Approaches to intramolecular sialylation
1. Synthesis of pseudodisaccharide Neu5Acꢀ(1ꢀ2)ꢀGal
with esterꢀlinked monosaccharide residues
a
b†
L. O. Kononov,а D. A. Volodin, and G. Magnusson
ꢀ
aN. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
47 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (095) 135 5328. Eꢀmail: kononov@ioc.ac.ru
bOrganic Chemistry 2, Chemical Center, Lund University,
P.O. Box 142, Sꢀ221 00 Lund, Sweden
An efficient approach was developed for esterꢀlinking the sterically hindered carboxy group
of Nꢀacetylneuraminic acid with the secondary hydroxy group at C(2) of galactose. Putative
precursors of the glycosidically linked disaccharide Neu5Acꢀ(2ꢀ3)ꢀGal were prepared.
Key words: sialic acids, Nꢀacetylneuraminic acid, sialooligosaccharides, galactose, esters,
regioselectivity, acylation, glycosylation, NMR spectroscopy.
The development of efficient procedures for the synꢀ
get formation of the glycosidic bond is expected to
more efficiently compete with the side formation of
Neu5Acꢀderived glycal.
thesis of complex oligosaccharides and conjugates conꢀ
taining sialic acid residues, in particular, Nꢀacetylꢀ
neuraminic acid (Neu5Ac), is an important problem of
the synthetic carbohydrate chemistry because these strucꢀ
tures are responsible for various immunological, neuroꢀ
biological, oncological, and other biological processes.1
In recent years, considerable progress has been obꢀ
served in the chemical synthesis of Neu5Ac glycosides
(see the review2). However, the efficiency and stereoꢀ
selectivity of sialylation are often far from the desired
ideal and are sometimes unpredictable. Small (at first
glance) changes in the structures of a glycosyl donor or a
glycosyl acceptor as well as variations in the reaction temꢀ
perature, solvent, or promoter can substantially influence
the results of glycosylation. This is typical of the oligosacꢀ
charide synthesis as a whole.3 In the case of sialylation,
the process is additionally complicated by elimination as
a side reaction giving rise to glycal. Presently, this reacꢀ
tion is considered as the main reason for a decrease in the
yield of sialosides.2 For this reason, sialylation products
are isolated in highest yields where the undesirable forꢀ
mation of glycal is suppressed in one way or another. It
should be noted that even the use of a substantial excess of
an expensive sialyl donor does not necessarily lead
to an increase in the preparative yield of the desired
glycosylation product. One of the possible approaches to
the synthesis of Neu5Ac glycosides involves intramolecuꢀ
lar sialylation (Scheme 1). In this case, glycosylation proꢀ
ceeds as a monomolecular reaction due to which the tarꢀ
Intramolecular glycosylation, which has become popuꢀ
lar in recent years, involves linking of a glycosyl donor
with a glycosyl acceptor by a chemical bond (often through
a bridge), subsequent intramolecular delivery of the glyꢀ
cosyl acceptor to the anomeric center of the glycosyl doꢀ
nor upon activation of the latter (glycosylation proper),
and cleavage of the temporary bond to give the target
oligosaccharides in which the monosaccharide residues
are linked through the newly formed glycosidic bond (see
Scheme 1 and the review4). In many cases,4 intramolecuꢀ
lar glycosylation has advantages over traditional intermoꢀ
lecular glycosylation, allows one to prepare products in
higher yields, and makes it possible to control the conꢀ
figuration of the newly formed glycosidic bond. Generꢀ
ally, intramolecular glycosylation occurs via 5—13ꢀmemꢀ
bered cyclic products. Numerous studies demonstrated
that the size of the ring that formed can play a decisive
role in both the efficiency and stereoselectivity of inꢀ
tramolecular glycosylation. However, no conclusions still
can be made as to the optimum size of the ring, and the
solution of this question is to be found in each particular
case. It should be noted that an attempt (unsuccessful) to
use this approach for performing sialylation was made
only in one investigation5 out of dozens of studies on
intramolecular glycosylation. Intermolecular dimerization
appeared to be the main direction of this reaction, whereas
no intramolecular sialylation products were detected. In
this connection, it is of particular importance to study in
detail the reasons why the aboveꢀmentioned attempt to
† Deceased.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1357—1364, June, 2003.
1066ꢀ5285/03/5206ꢀ1434 $25.00 © 2003 Plenum Publishing Corporation

Approach to intramolecular sialylation
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 6, June, 2003
1435
Scheme 1
intramolecular glycosylation. The present study is the first
in a series of publications devoted to these problems.
In the initial step of the study, we chose the disacchaꢀ
ride Neu5Acꢀ(α2ꢀ3)ꢀGal as the target product. Accordꢀ
ing to the concept of intramolecular sialylation (see
Scheme 1), its first step involves linking of glycosyl doꢀ
nor A to glycosyl acceptor B by a temporary bond (step a).
It is known6 that (α2´ꢀ3)ꢀsialooligosaccharides are prone
to generate (1´ꢀ2)ꢀlactones (for example, compound E in
Scheme 1). Hence, compounds in which the residue of a
protected Neu5Ac derivative is esterꢀlinked directly
through its carboxy group to the notꢀtoꢀbeꢀglycosylated
hydroxy group at C(2) of the galactose residue would be
appropriate to consider as first nonꢀglycosidically linked
"disaccharide" precursors. The remaining hydroxy groups
in the galactose residue should be differently protected so
that the OH groups at C(3) and C(4) could be deprotected
selectively. It is reasonable to use the 3,4ꢀOꢀisopropylidene
group as such an orthogonal group. This group can be
removed in a weakly acidic medium in which ester groups
remain intact.7
For this approach to be employed successfully, it is
necessary first of all to elucidate whether it is possible to
bind the carboxy group of the Neu5Ac residue to the
secondary OH group at C(2) of the galactose residue
through the ester bond and then to selectively deprotect
the OH groups at C(3) and C(4) (which is necessary for
the subsequent successful sialylation at O(3) of the galacꢀ
tose residue) with the retention of the protective group at
O(6) and prevention of migration of the acyl residue
(Neu5Ac). The stability of the temporary intersaccharide
bond under typical glycosylation conditions is also of imꢀ
portance.
We used readily accessible peracetate 1 containing the
free carboxy group8 as the carboxyꢀcontaining Neu5Ac
derivative (Scheme 2). Its condensation with the known9
alcohol 2 in the presence of 1ꢀmesitylenesulfonylꢀ3ꢀniꢀ
troꢀ1,2,4ꢀtriazole (MSNT)10 and Nꢀmethylimidazole
(MeIm) proceeded surprisingly efficiently. The correꢀ
sponding "disaccharide" 3 consisting of esterꢀlinked
monosaccharide residues was isolated in 86% yield.
Its deacetonation with 80% AcOH (85 °C) proceeded
smoothly and was not accompanied by migration of the
Neu5Ac residue (migration of the acetyl groups is well
known11) as evidenced by a lowꢀfield position of the sigꢀ
1
nal for H(2) of the galactose residue in the H NMR
a. Formation of a temporary bond between a glycosyl donor and
a glycosyl acceptor. b. Removal of temporary protective groups.
c. Activation of the anomeric center, formation of the glycosidic
bond. d. Cleavage of the temporary bond, removal of protective
groups.
spectrum of diol 4 prepared in 93% yield. Thus, we demꢀ
onstrated that the neuraminic acid residue can be effiꢀ
ciently and regioselectively bound to the secondary OH
group of the galactose residue.
Preliminary experiments on activation of the anomeric
position of the neuraminic acid residue in compound 4 by
different Lewis acids (FeCl3, BF3•Et2O, or TMSOTf in
CH2Cl2) demonstrated that the silyl protective group is
unstable under these conditions. In all cases, mixtures of
perform "intramolecular" sialylation was unsuccessful, to
reveal the factors responsible for either an intramolecular
or intermolecular reaction direction in these systems, and,
finally, to develop new more efficient approaches to

1436
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 6, June, 2003
Kononov et al.
Scheme 2
signals for H(2) and H(6), which indicates that acyl subꢀ
stituents are bound only to O(2) and O(6) of the galactose
residue. Diol 6 was also prepared according to an alternaꢀ
tive procedure from commercial thiogalactoside 7 and
sialic acid acetate 1 without purification of intermediates.
Treatment of 7 with an excess of 2,2ꢀdimethoxypropane
in the presence of camphorsulfonic acid (CSA) rapidly
afforded alcohol 8 in virtually quantitative yield. This
compound was identified by TLC by comparing it with an
authentic sample.9 Condensation of crude alcohol 8 with
acid 1 in the presence of MSNT gave "disaccharide" 9
whose treatment with dilute HCl (workꢀup of the reaction
mixture in a separating funnel) led to removal of the
2ꢀmethoxyisopropyl (MIP) protective group from O(6) to
give alcohol 10. Subsequent benzoylation of 10 produced
the known benzoate 5 (identified by TLC by comparing it
with an authentic sample) whose deacetonation afforded
the target diol 6 isolated in a total yield of 30%, which
corresponds, on the average, to ∼67% yield in each of
steps 8 + 1 → 9 → 10 → 5.
An attempt to activate the anomeric position in acꢀ
etate 6 by the Zn(OTf)2—TMSCl system12 in MeCN at
20 °C did not lead to a noticeable reaction. Refluxing of
the reaction mixture resulted in the virtually complete
disappearance of the starting acetate 6. The hydrolysis
product of anomeric acetate (11, 30%) and the bicyclic
elimination product (12, 34%) containing the dihydroꢀ
oxazole ring were isolated from the reaction mixture. The
structure of hemiacetal 11 was confirmed by the disapꢀ
pearance of one of the signals of the acetyl groups from the
1H NMR spectrum as well as by upfield shifts of the signals
for H(3)eq (δH 2.57 → 2.29) and C(2) (δC 97.2 → 95.3) of
the Neu5Ac residue. The remaining signals in the NMR
spectra of hemiacetal 11 did not undergo substantial
changes as compared to those observed in the spectrum of
the starting acetate 6. The presence of the C(2)=C(3)
double bond in compound 12 is indicated by the positions
of the NMR signals for H(3), C(2), and C(3) of the
Neu5Ac residue characteristic13 of sialic acid glycals
(Tables 1—3). The formation of the dihydrooxazole ring14
fused with the pyranose ring at positions 4 and 5 of the
glycal is confirmed by the absence of the signal of the
AcNH group (δC ∼23) and the appearance of the signal of
the methyl group (MeC) at δC 14.2 in the 13C NMR
spectrum of compound 12 as well as by a downfield shift of
the signal for C(5) of the Neu5Ac residue (δC 49.1 → 61.9)
and an upfield shift of the signal for H(6) of the Neu5Ac
residue (δH 4.03 → 3.30).
MeIm is Nꢀmethylimidazole
1
products were obtained. Their H NMR spectra have no
signals of the tertꢀbutyldiphenylsilyl group. The main conꢀ
clusion drawn from these experiments is that the successꢀ
ful use of nonꢀglycosidically linked "disaccharides" for
intramolecular sialylation requires that the protective
group at O(6) be more stable under acidic conditions. We
chose the benzoyl group as such a protective group
(Scheme 3).
To summarize, we developed an efficient procedure
for binding the sterically hindered carboxy group of the
Neu5Ac residue to the secondary OH group at C(2) of the
galactose residue by an ester bond. This intersaccharide
ester bond is stable in an acidic medium under different
conditions typical of glycosylation, which holds promise
for successful intramolecular sialylation. This approach
An analog of diol 4 containing the benzoyl group
at O(6) (6) was readily prepared from silyl ether 3.
Desilylation (Bu4NF) followed by benzoylation (BzCl/Py)
afforded benzoate 5 in 67% yield. Deacetonation of the
latter with 80% AcOH (80 °C) gave rise to the target diol 6
1
in 93% yield. The H NMR spectrum of 6 has lowꢀfield

Approach to intramolecular sialylation
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 6, June, 2003
1437
Scheme 3
DMP is dimethoxypropane, CSA is camphorsulfonic acid; MIP = CMe2OMe
can also be used for the preparation of other nonꢀglycoꢀ
sidically linked "disaccharide" precursors of the glycosidꢀ
ically linked disaccharide Neu5Acꢀ(α2ꢀ3)ꢀGal with difꢀ
ferent "bridges". It is also evident that further experiments
should be performed with Neu5Ac derivatives in which
the anomeric position can be activated under much milder
conditions than those used in the present study for the
anomeric acetate.
Experimental
The reactions were performed with the use of anhydrous
solvents purified according to standard procedures and comꢀ
mercial reagents (Aldrich and Fluka). Thinꢀlayer chromatoꢀ
graphy was carried out on plates with silica gel on aluminum foil
(Merck). Column chromatography was performed on silica gel
1
60 (40—63 µm, Merck). The H and 13C NMR spectra were

Table 1. 13C NMR spectra (δC, CDCl3) of the compounds synthesized
Comꢀ
pound
Ra
C(1) C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(9)
Ar
MeCN
MeCOO
C=O
1
Neu 168.4 97.3 36.2 68.7 49.2 72.4 67.8 71.2 62.0
Neu 164.6 97.4 36.0 68.4 49.2 72.9 68.1 71.6 62.2
—
22.95
23.2
20.77, 20.86,
20.92
20.67, 20.77,
20.82, 20.88,
20.97
170.3, 170.4, 170.8,
171.08, 171.15
167.8, 170.2,
170.3, 170.4,
170.8, 171.1
3 b
(HETCOR)
Gal
84.8 73.7 76.7 73.4 76.9 63.0
—
—
—
125.3, 127.6, 127.67, 127.71,
128.2, 128.9, 129.0, 129.1,
129.7, 131.7, 133.3, 133.34,
135.6, 137.8
4 c
Neu 165.7 97.3 36.4 68.2 d 49.1 72.8 67.9 d 71.8 62.5
23.2
23.2
20.8 (2C),
20.9 (2C),
21.1
169.0, 170.1,
170.4, 170.8,
171.0, 171.3
Gal
85.4 73.2 73.2 69.1 78.9 63.3
—
—
—
125.3, 127.6, 127.8, 127.9,
128.2, 128.9, 129.0, 129.65,
129.76, 131.6, 133.1, 133.6,
134.8, 135.6, 137.9
5 e
Neu 164.7 97.3 36.0 68.3 d 49.2 73.0 68.0 d 71.5 62.2
20.6, 20.7,
20.80, 20.86,
20.93
166.3, 167.9, 170.1,
170.3 (2C),
170.8, 171.0
(HETCOR)
Gal
84.8 74.3 76.3 73.5 73.5 64.2
(2 C) (2 C)
—
—
—
125.3, 127.6, 128.4, 128.8,
129.7, 129.9, 131.6, 133.2,
133.5, 142.7
6 f
Neu 165.8 97.2 36.5 68.2 d 49.1 72.9 67.9 d 71.9 62.5
23.1
23.1
14.2
20.76, 20.79,
20.84 (2C),
21.0
166.4, 169.2, 170.1,
170.4, 170.8,
171.0, 171.4
Gal
Neu 166.5 95.3 36.9 69.0 d 49.0 72.6 68.8 d 72.2 62.8
Gal
85.6 72.3 77.9 69.3 d 76.3 63.7
85.5 73.3 72.4 69.1 76.3 63.9
—
—
—
127.7, 128.4, 128.9, 129.8,
129.9, 131.7, 133.2, 133.3
11
20.81, 20.87,
20.89, 21.2
168.5, 170.2, 170.5,
170.8, 172.2
—
—
—
127.7, 128.2, 128.5, 128.90,
128.96, 129.04, 129.7,
131.4, 132.0, 133.3
12 g
Neu 161.1 109.1 147.2 69.4 61.9 76.2 67.7 72.2 62.1
Gal 86.1 72.5 72.3 69.4 76.4 63.6
20.7, 20.8,
21.1
166.5, 167.5,
169.5, 170.7
—
—
—
127.5, 128.2, 128.5, 128.8,
129.0, 129.7, 129.8, 131.4,
133.3, 133.8
a R is a monosaccharide residue. b Other signals (δ): 77.3 (CMe3); 26.8 (CMe3); 110.4 (CMe2); 26.4, 27.6 (CMe2). c Other signals (δ): 77.2 (CMe3); 26.8 (CMe3).
d The assignments can be interchanged. e Other signals (δ): 110.9 (CMe2); 26.3, 27.5 (CMe2). f Other signals (δ): 110.9 (CMe2); 26.3, 27.5 (CMe2).
g Other signals (δ): 77.2 (MeC=N).

Table 2. 1H NMR spectra (δH, CDCl3) of the compounds synthesizeda
Comꢀ
pound
Rb
H(1) H(2)
H(3)
H(4)
H(5)
H(6)
H(7)
H(8)
H(9)
NH MeCN
AcO
Ar
CMe3 CMe2
3 c
(COSY)
Neu
—
—
2.21, 2.58
(both dd)
5.29
(ddd)
4.17 (m) 4.11 (m) 5.42 5.04 (m) 4.06 (m), 5.54 1.90 2.02, 2.06,
(dd)
4.46 (dd) (d)
(s)
2.07, 2.12,
2.16 (all s)
Gal
4.82
(d)
—
4.88
(dd)
—
4.24
4.31
(dd)
5.33 (m)
4.03 (m) 3.96
(m, 2 H)
4.03 (dd) 5.42 5.01 (m) 4.02 (m), 5.36 1.89 2.04, 2.06
—
—
—
7.10—7.80 1.06 1.31, 1.44
(t (dd))
2.12 (m),
2.56 (dd)
(m)
(s)
(both s)
4
Neu
4.21
(COSY)
(q (ddd))
(dd)
4.56 (dd) (d)
(s)
(6 H), 2.13,
2.17 (all s)
Gal
4.81
5.23
3.92
4.15 (d)
3.75
3.96
—
—
—
7.10—7.80 1.06
—
(d) (t (dd)) (dd)
(br.t)
(d, 2 H)
(m)
(s)
5
Neu
—
—
2.18 (m),
2.58 (dd)
5.27
(ddd)
4.15 (m) 4.10 (m) 5.38 4.97—5.17 4.03, 4.42 5.44 1.88 1.99, 2.04
(COSY)
(dd)
(m)
(both dd) (d)
(s)
(6 H), 2.08,
2.15 (all s)
Gal
4.89 4.97—5.17 4.27—4.34 4.27—4.34
4.23
(br.dd)
4.21
4.57, 4.68
(both dd)
—
—
—
7.10—8.10
(m)
—
1.34, 1.50
(both s)
(d)
—
(m)
—
(m)
2.12 (m),
2.57 (dd)
(m)
5.32
6
Neu
4.03 (m) 5.41 4.99 (m) 3.96, 4.57 5.48 1.89 2.03, 2.04,
(COSY)
(ddd)
(ddd)
(dd)
(both dd) (d)
(s)
2.05, 2.11,
2.18 (all s)
Gal
4.82
(d) (t (dd))
5.26
4.01 (m)
4.15 (d) 4.02 (m) 4.61—4.72
(m, 2 H)
—
—
—
7.10—8.10
(m)
—
—
11
(COSY)
Neu
—
—
2.10 (m),
2.29 (m)
5.18 (m) 4.18 (m) 4.32 (dd) 5.42 5.16—5.24 3.88, 4.75 6.31 1.88 1.90, 1.99,
(m)
(m)
(both m) (br.d) (s)
2.08, 2.13
(all s)
Gal
Neu
Gal
4.81 5.16—5.24 3.90 (m)
4.10 (m) 3.90 (m) 4.70 (m)
—
—
—
7.10—8.10
(m)
—
—
—
—
(d)
—
(m)
—
12
(COSY)
6.52 (d)
3.92 (m)
4.86
(dd)
3.95 (m) 3.30 (dd) 5.64
(dd)
5.55
(ddd)
—
4.16, 4.38
(both dd)
—
—
2.08 2.03, 2.06,
(s) 2.17 (all s)
4.84
5.25
4.10
(br.d)
3.98 (m) 4.57, 4.71
(both dd)
—
7.10—8.10
(m)
(d) (t (dd))
a All spectra were recorded for samples with concentrations of 20—30 mg mL–1 (unless otherwise stated); in some cases, the spectral patterns depend on the concentration.
b R is a monosaccharide residue.
c The concentration is 100 mg mL–1
.

1440
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 6, June, 2003
Kononov et al.
1
Table 3. Spinꢀspin coupling constants in the H NMR spectra (J/Hz, CDCl3) of the compounds synthesizeda
Comꢀ Monosacꢀ J1,2
pound charide
residue
J2,3
J3,3´
J3´,4
J3,4
J4,5
J5,6
J5,6´
J6,6´
J6,7
J7,8
J8,9
J8,9´
J9,9´ J5,NH
b
b
b
3
Neu
Gal
Neu
Gal
Neu
Gal
Neu
Gal
Neu
Gal
Neu
Gal
—
9.8
—
10.2
—
9.6
—
10.1
—
—
6.2
—
9.4
—
b
13.6
—
13.1
—
13.1
—
13.0
5.0
—
4.8
—
4.8
—
4.8
11.8 ∼10.7
—
—
2.2
—
2.9
—
2.4
—
2.9
—
1.9
—
1.8
—
4.5
—
4.5
—
4.8
—
4.5
2.4
—
1.9
—
2.6
—
2.4
12.6
—
12.6
—
12.6
—
12.6
9.3
—
9.6
—
9.5
—
9.6
—
b
b
5.9
b
1.5
9.9
—
b
4
9.9
∼5.6
9.9 10.9
—
∼5.6
—
3.5
—
b
—
b
3.4
11.4
3.4
∼1.3
—
7.2
—
8.0
—
b
5
—
11.5
—
b
∼1.3
8.0
6
—
8.5
—
b
11.2
3.4
b
9.9 10.9
b
b
b
b
—
b
—
b
—
b
—
b
—
b
11
12
10.9
b
—
b
—
b
9.2
—
b
b
9.8
—
10.0
—
b
—
b
—
8.3
—
—
5.6
—
—
2.4
—
—
12.6
—
—
9.9
4.0
2.9
8.6 10.4
—
4.9
—
11.7
b
—
—
7.4
—
a Primed digits correspond to lowerꢀfield signals for the protons of the C(3)H2, C(6)H2, and C(9)H2 groups.
b Undetermined.
recorded on a Varian XLꢀ300 instrument (300 and 75.4 MHz,
respectively) in CDCl3. The Н chemical shifts were measured
0.021 mmol) in AcOH (0.8 mL). The reaction solution was
heated at 85 °C (bath temperature) for 30 min and concentrated
to dryness. Then toluene was added to, and distilled from, the
residue (×3). The residue was dried in vacuo to give diol 4
(20.0 mg, 93%), which was used without additional purification.
MS (FAB, detection of positive ions): found, m/z 1012.3439
[M + H]. C49H62NO18SSi. Calculated, m/z 1012.3457 [M + H].
Phenyl 2ꢀOꢀ(5ꢀacetamidoꢀ2,4,7,8,9ꢀpentaꢀOꢀacetylꢀ3,5ꢀ
dideoxyꢀβꢀDꢀglyceroꢀDꢀgalactoꢀnonꢀ2ꢀulopyranosonoyl)ꢀ6ꢀOꢀ
benzoylꢀ3,4ꢀOꢀisopropylideneꢀ1ꢀthioꢀβꢀDꢀgalactopyranoside (5).
A mixture of AcOH (41 µL, 0.72 mmol) and Bu4NF•3H2O
(90.4 mg, 0.29 mmol) was added to a solution of silyl ether 3
(75.3 mg, 0.072 mmol) in THF (1 mL). The reaction mixture
was kept at 20 °C for 30 h, diluted with CH2Cl2 (30 mL), and
poured into a cold saturated NaHCO3 solution (40 mL). The
aqueous layer was extracted with CH2Cl2 (×2). The combined
organic extracts (80 mL) were washed with cold brine (2×20 mL)
and water (1×10 mL), filtered through a cotton plug, and conꢀ
centrated to dryness. Then toluene was added to, and distilled
from, the residue. The residue was dissolved in Py (10 mL) and
cooled (ice water). Then BzCl (166 µL, 1.43 mmol) was added,
the reaction mixture was kept at 20 °C for 24 h, and MeOH
(4 mL) was added. The reaction mixture was kept at 20 °C for
1 h and then concentrated. Toluene was added to, and distilled
from, the residue, and then CH2Cl2 (100 mL) was added to the
residue. The resulting solution was washed with 0.5 M HCl
(2×50 mL). The aqueous phase was reextracted with CH2Cl2
(×2). The combined extracts (200 mL) were washed with a
cold saturated NaHCO3 solution (2×100 mL) and cold brine
(100 mL), filtered through a cotton plug, and concentrated to
dryness. The residue was purified by column chromatography
on silica gel (AcOEt) to yield the target benzoate 5 (44.2 mg,
67%), [α]D23 +0.7 (c 3.7, CHCl3). MS (FAB, detection of posiꢀ
tive ions): found, m/z 918.2863 [M + H]. C43H52NO19S. Calcuꢀ
lated, m/z 918.2854 [M + H].
1
relative to the residual signal of CHCl3 (δ 7.27) and the
13С chemical shifts were measured relative to the signal of CDCl3
(δ 77.0). The assignment of the signals in the NMR spectra was
made based on the DEPTꢀ135 experiments and 2D correlation
1H—1H (COSY) and 13C—1H (HETCOR) spectra. The NMR
spectroscopic data are given in Tables 1 and 2. The optical
rotation was measured on a Perkin—Elmer 141 polarimeter.
Highꢀresolution mass spectra (FAB) were obtained on a JEOL
SXꢀ120 mass spectrometer.
Phenyl 2ꢀOꢀ(5ꢀacetamidoꢀ2,4,7,8,9ꢀpentaꢀOꢀacetylꢀ3,5ꢀ
dideoxyꢀβꢀDꢀglyceroꢀDꢀgalactoꢀnonꢀ2ꢀulopyranosonoyl)ꢀ6ꢀOꢀ
(tertꢀbutyldiphenylsilyl)ꢀ3,4ꢀOꢀisopropylideneꢀ1ꢀthioꢀβꢀDꢀ
galactopyranoside (3). NꢀMethylimidazole (43 µL, 0.54 mmol)
was added to a solution of acid 1 8 (84 mg, 0.16 mmol) in
CH2Cl2 (2.5 mL) under argon. The reaction mixture was transꢀ
ferred under argon to a flask containing MSNT (60 mg,
0.20 mmol) and the content was transferred under the same
conditions to a reaction flask containing alcohol 2 9 (74.2 mg,
0.13 mmol). The two flasks were additionally rinsed with CH2Cl2
(1.0 + 1.5 mL) and the resulting solutions were added to the
reaction mixture, which was kept at 20 °C for 26 h (TLC demꢀ
onstrated that the starting alcohol 2 was absent). The reaction
mixture was diluted with CH2Cl2 (40 mL) and poured onto ice.
The organic phase was twice washed with 0.5 M HCl and then
with a cold saturated NaHCO3 solution and ice water, each
aqueous phase being reextracted with CH2Cl2 (2×10 mL). The
combined organic extracts were filtered through a cotton plug
and concentrated to dryness. The residue was purified by colꢀ
umn chromatography on silica gel (acetone—toluene, 3 : 7) to
obtain the target "disaccharide" ester 3 (121.6 mg, 86%),
23
[α]D +0.5 (c 6.4, CHCl3). MS (FAB, detection of positive
ions): found, m/z 1052.3773 [M + H]. C52H66NO18SSi. Calcuꢀ
lated, m/z 1052.3770 [M + H].
Phenyl 2ꢀOꢀ(5ꢀacetamidoꢀ2,4,7,8,9ꢀpentaꢀOꢀacetylꢀ3,5ꢀ
dideoxyꢀβꢀDꢀglyceroꢀDꢀgalactoꢀnonꢀ2ꢀulopyranosonoyl)ꢀ6ꢀOꢀ
(tertꢀbutyldiphenylsilyl)ꢀ1ꢀthioꢀβꢀDꢀgalactopyranoside (4). Water
(0.2 mL) was added to a solution of acetonide 3 (22.4 mg,
Phenyl 2ꢀOꢀ(5ꢀacetamidoꢀ2,4,7,8,9ꢀpentaꢀOꢀacetylꢀ3,5ꢀ
dideoxyꢀβꢀDꢀglyceroꢀDꢀgalactoꢀnonꢀ2ꢀulopyranosonoyl)ꢀ6ꢀOꢀ
benzoylꢀ1ꢀthioꢀβꢀDꢀgalactopyranoside (6). A. Water (1 mL) was
added to a solution of ester 5 (45.1 mg, 0.049 mmol) in AcOH

Approach to intramolecular sialylation
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 6, June, 2003
1441
(4 mL). The reaction solution was heated at 80 °C (bath temꢀ
perature) for 1 h and concentrated to dryness. Then toluene was
added to, and distilled from, the residue (×3). The residue
was purified by column chromatography on silica gel (acꢀ
etone—toluene, 1 : 1) to yield the target diol 6 as a white powder
2ꢀOꢀ{2ꢀmethylꢀ4,5ꢀdihydrooxazolo[4,5:5´4´](2,6ꢀanhydroꢀ
7,8,9ꢀtriꢀOꢀacetylꢀ3,5ꢀdideoxyꢀDꢀglyceroꢀDꢀtaloꢀnonꢀ2ꢀ
enonoyl)}ꢀ6ꢀOꢀbenzoylꢀ1ꢀthioꢀβꢀDꢀgalactopyranoside (12). Aceꢀ
tonitrile (2 mL) was added to a mixture of diol 6 (28 mg,
0.032 mmol) and Zn(OTf)2 (20.4 mg, 0.056 mmol) under argon.
After dissolution of all components, TMSCl (6 mL) was added
and the reaction mixture was heated at 80 °C (bath temperaꢀ
ture). After 3 h, the starting diol 6 disappeared and two new
products were formed (TLC data). The reaction mixture was
diluted with CH2Cl2 (50 mL) and washed with a cold saturated
NaHCO3 solution (30 mL). The aqueous phase was extracted
with CH2Cl2 (5×10 mL). The combined organic extracts were
filtered through a cotton plug and concentrated to dryness. The
residue was purified by column chromatography on silica gel to
yield compounds 11 (8.1 mg, 30%) and 12 (8.1 mg, 33%).
MS (FAB, detection of positive ions): for compound 11 found,
m/z 836.2432 (29.5%) [M + H], 818.2355 (16%) [M + H – H2O].
23
(39.2 mg, 93%), m.p. 131—132 °C, [α]D –2.9 (c 3.2, CHCl3).
MS (FAB, detection of negative ions): found, m/z 876 [M – H],
834 [M – H – CH2=C=O]. MS (FAB, detection of positive
ions): found, m/z 878 [M + H], 818 [M + H – AcOH],
758 [M + H – 2 AcOH]. C40H48NO19S. Calculated,
m/z 878.2541 [M + H].
B. 2,2ꢀDimethoxypropane (30 mL) and then a catalytic
amount of ( )ꢀcamphorꢀ10ꢀsulfonic acid (5 mg) were added to
phenyl thiogalactoside 7 (273 mg, 1 mmol) under argon. The
resulting suspension was stirred at 20 °C for 24 h and then the
reaction was quenched by adding Et3N (2 mL). The reaction
mixture was concentrated, toluene was added to, and distilled
from, the residue (×3) to obtain alcohol 8 (∼100% yield, TLC
data), which was used in the next step without additional purifiꢀ
cation.
C
38H46NO18S. Calculated, m/z 836.2436 [M
+
H].
C38H44NO17S. Calculated, m/z 818.2330 [M + H – H2O]. For
compound 12 found, m/z 758.2110 [M + H]. C36H40NO15S.
Calculated, m/z 758.2119 [M + H].
NꢀMethylimidazole (320 µL, 4 mmol) was added to a soluꢀ
tion of acid 1 (673.4 mg, 1.3 mmol) in CH2Cl2 (5 mL) under
argon. The reaction mixture was transferred under argon to a
flask containing MSNT (447.3 mg, 1.5 mmol) and the content
was transferred to a reaction flask containing alcohol 8 (which
was synthesized in the previous step). The two flasks were addiꢀ
tionally rinsed with CH2Cl2 (2×2.5 mL) and the resulting soluꢀ
tions were added to the reaction mixture, which was kept under
argon at 20 °C for 4 h. Then the reaction mixture was diluted
with CH2Cl2 (100 mL) and poured onto ice. The organic phase
was washed with 0.5 M HCl (2×300 mL) (in this step, the priꢀ
mary reaction product 9 disappeared and alcohol 10 formed,
TLC data) and a cold saturated NaHCO3 solution (300 mL),
each aqueous phase being reextracted with CH2Cl2 (2×50 mL).
The combined organic extracts were filtered through a cotton
plug and concentrated to dryness. Then toluene was added to,
and distilled from, the residue. The resulting alcohol 10 was
used in the next step without additional purification.
Benzoyl chloride (1.16 mL, 10 mmol) was added to a cooled
(ice water) solution of alcohol 10 in Py (10 mL). The reaction
mixture was kept at 20 °C for 18 h. Then finely crushed ice was
added to the reaction mixture. After 30 min, the mixture was
diluted with CH2Cl2 (200 mL). The organic layer was washed
with 0.5 M HCl (200 mL) and a cold saturated NaHCO3 soluꢀ
tion (2×200 mL), each aqueous phase was reextracted with
CH2Cl2 (2×50 mL). The combined organic extracts were filꢀ
tered through a cotton plug and concentrated to dryness to give
benzoate 5 as a yellow syrup (according to the TLC data, it was
identical with the sample synthesized as described above).
Water (2 mL) was added to a solution of acetonide 5 in
AcOH (8 mL). The reaction solution was heated at 80 °C (bath
temperature) for 2.5 h and concentrated to dryness. Then toluꢀ
ene was added to, and distilled from, the residue (×3). The
residue was purified by column chromatography on silica gel
(acetone—toluene, 3 : 7) to yield the target diol 6 (263 mg, 30%
with respect to the amount of the starting thiogalactoside 7)
identical with the sample synthesized according to method A.
Phenyl 2ꢀOꢀ(5ꢀacetamidoꢀ4,7,8,9ꢀtetraꢀOꢀacetylꢀ3,5ꢀ
dideoxyꢀβꢀDꢀglyceroꢀDꢀgalactoꢀnonꢀ2ꢀulopyranosonoyl)ꢀ6ꢀ
Oꢀbenzoylꢀ1ꢀthioꢀβꢀDꢀgalactopyranoside (11) and phenyl
This study was financially supported by the Russian
Foundation for Basic Research (Project No. 02ꢀ03ꢀ32271)
and the Swedish Natural Science Research Council (NFR,
Sweden).
References
1. Glycosciences. Status and Perspectives, Eds. H.ꢀJ. Gabius and
S. Gabius, Chapman and Hall, Weinheim, 1997, 631 pp.
2. G.ꢀJ. Boons and A. V. Demchenko, Chem. Rev., 2000, 100,
4539 (and references cited therein).
3. B. G. Davis, J. Chem. Soc., Perkin Trans 1, 2000, 2137 (and
references cited therein).
4. K.ꢀH. Jung, M. Müller, and R. R. Schmidt, Chem. Rev.,
2000, 100, 4423 (and references cited therein).
5. Y. E. Tsvetkov and R. R. Schmidt, Carbohydr. Lett., 1996, 1.
6. M. Wilstermann, L. O. Kononov, U. Nilsson, A. K. Ray,
and G. Magnusson, J. Am. Chem. Soc., 1995, 117, 4742 (and
references cited therein).
7. T. W. Green and P. G. M. Wuts, Protective Groups in Organic
Synthesis, J. Wiley and Sons, New York, 1991, p. 123.
8. P. Meindl and H. Tuppy, Monatsh. Chem., 1965, 96, 802.
9. J. Broddefalk, K.ꢀE. Bergquist, and J. Kihlberg, Tetrahedron,
1998, 54, 12047.
10. B. BlankemeyerꢀMenge, M. Nimtz, and R. Frank, Tetraꢀ
hedron Lett., 1990, 31, 1701.
11. R. J. Ferrier and P. M. Collins, Monosaccharide Chemistry,
Penguin Books, 1998, 318 pp.
12. K. Higashi, S. Miyoshi, S. Nakabayashi, H. Yamada, and
Y. Ito, Chem. Pharm. Bull., 1992, 40, 2300.
13. K. Okamoto, T. Kondo, and T. Goto, Bull. Chem. Soc. Jpn.,
1987, 60, 631.
14. E. Schreiner, E. Zbiral, R. G. Kleinedam, and R. Schauer,
Liebigs Ann. Chem., 1991, 129 (and references cited therein).
Received January 13, 2003