Efficient generation of thiolate sugars from glycosyl Bunte salts and its application to S-glycoside synthesis

Article abstract of DOI:10.1016/j.tetlet.2020.152198

A one-pot aqueous solution method for synthesis of S-glycoside derivatives has been developed. Firstly, unprotected sugars were converted into glycosyl Bunte salts from which thiolate sugars can be generated efficiently using Na₂S. The subsequent addition reaction with vinyl compounds or organic halides has successfully been demonstrated, giving rise to the corresponding S-glycosides.

Full text of DOI:10.1016/j.tetlet.2020.152198

Journal Pre-proofs
Efficient generation of thiolate sugars from glycosyl Bunte salts and its appli‐
cation to S-glycoside synthesis
Yasuhiro Meguro, Masato Noguchi, Gefei Li, Shin-ichiro Shoda
PII:
S0040-4039(20)30663-8
DOI:
Reference:
https://doi.org/10.1016/j.tetlet.2020.152198
TETL 152198
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
26 May 2020
25 June 2020
29 June 2020
Please cite this article as: Meguro, Y., Noguchi, M., Li, G., Shoda, S-i., Efficient generation of thiolate sugars
from glycosyl Bunte salts and its application to S-glycoside synthesis, Tetrahedron Letters (2020), doi: https://
doi.org/10.1016/j.tetlet.2020.152198
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1
Graphical Abstract
Leave this area blank for abstract info.
Efficient generation of thiolate sugars from glycosyl
Bunte salts and its application to S-glycoside synthesis
Yasuhiro Meguro, Gefei Li, Masato Noguchi and Shin-ichiro Shodaꢀꢀ
(
OH)n
R
(
OH)n
O
S
O
Na S
O
2
(
OH)n
S
S
ONa
R
O
S-
O
Glycosyl Bunte salts
(OH)n
R'-X
O
S
R'
ꢀ
ꢀꢀꢀꢀꢀꢀꢀ unprotected sugars

1
Tetrahedron Letters
journal homepage: www.elsevier.com
Efficient generation of thiolate sugars from glycosyl Bunte salts and its application to
S-glycoside synthesis
Yasuhiro Meguro, Masato Noguchi, Gefei Li and Shin-ichiro Shoda
Department of Biomolecular Engineering, Graduate School of Engineering, Tohoku University, 6-6-11-514, Aoba, Aoba-ku, Sendai 980-8579, Japan
ARTICLE INFO
ABSTRACT
Article history:
Received
A one-pot aqueous solution method for synthesis of S-glycoside derivatives has been developed.
Firstly, unprotected sugars were converted into glycosyl Bunte salts from which thiolate sugars
Received in revised form
Accepted
2
can be generated efficiently using Na S. The subsequent addition reaction with vinyl compounds
or organic halides has successfully been demonstrated, giving rise to the corresponding S-
Available online
glycosides.
2
009 Elsevier Ltd. All rights reserved.
Keywords:
S-Glycoside
Glycosyl Bunte salt
Sodium sulfide
Glycosyl thiolate
One-pot aqueous synthesis

Corresponding author.
E-mail address: shinichiro.shoda.a6@tohoku.ac.jp (S. Shoda)

2
In nature, almost all mammalian cell surfaces are modified by covalently linked carbohydrates, expressing
glycoconjugates whose structures have been implicated in bio-related processes such as protein folding, cell
1
signaling, and immunological responses. The linkages that combine a sugar moiety and an aglycon part can be
classified into O-, N-, C- and S-glycosidic bonds. There has been an increase in the interest of biologically active S-
2
glycosides as O-glycoside analogues. It is well known that S-glycosides behave as enzyme inhibitors because S-
3
glycosides are less susceptible to hydrolytic enzymes than O-glycosides. Some S-glycosides have been reported
4
to offer increased antibacterial activities. The development of advanced methods to construct S-glycosidic
linkages has, therefore, been essential in carbohydrate chemistry.
One of the most widely used synthetic intermediates for the preparation of S-linked glycomaterials (e.g.,
5
6
glycoproteins and biomedical glycochips ) are thiolate sugars (glycosyl thiolates). The traditional protocol for
preparing glycosyl thiolates involves a nucleophilic substitution between per-O-acetylated glycosyl bromides and
thionating agents (e.g., potassium thioacetate, sodium thiophosphate, and carbon disulfide/sodium sulfide )
and the subsequent hydrolysis or alcoholysis.
Glycosyl thiolates were also prepared by treating unprotected sugars with HF/H S without protection of the
2
hydroxy groups where a mixture of - and -isomers was
Lawesson’s reagent was employed as a safer
7
8
9
1
0
formed.
1
1
alternative in the direct synthesis of glycosyl thiols using
,4-dioxane as solvent.12 However, these methods could not be applied to oligosaccharides due to their poor
solubilities to organic solvents. In addition, glycosyl thiols were easily transformed into the corresponding
1
1
3
disulfides in the air, resulting in low yields. Thus, effective in situ generation of glycosyl thiolate species has
strongly been demanded.
Recently, it has been reported that unprotected N-acetyl-2-amino sugars can be converted to the
1
4
corresponding glycosyl thiols in excellent yields. In these reactions, glycosyl oxazolines were firstly generated
through the intramolecular dehydration of N-acetyl-2-amino sugars by using 2-chloro-1,3-dimethylimidazolinium
1
5
chloride (DMC) as a dehydrating agent. Thereafter, the resulting sugar oxazolines were treated with thioacetic
acid to afford S-acetyl derivatives which have finally been transformed into the corresponding glycosyl thiols by
methanolysis. However, usage of these methods was limited to N-acetyl-2-amino sugars, and there have been no
successful applications to 2-hydroxy sugars such as glucose and lactose where undesired 1,6-anhydro sugars were
1
4b
obtained as main products.
In
our
previous study, we have discovered a novel class of synthetic
intermediate in sugar chemistry, glycosyl Bunte salts (S-
glycosylthiosulfates) 1 (Figure 1), which can be prepared by the
condensation of unprotected sugars and inexpensive sodium
1
6
thiosulfate by using DMC reagent in aqueous media. In spite of
great potential as a useful synthetic intermediate due to its chemical
duality (S-S cleavage to give glycosyl thiolates 2 or C-S cleavage to
give glycosyl cations 3), there has so far been no reports on the
application of glycosyl Bunte salts. Since glycosyl Bunte salts possess
a thiosulfate moiety including an S-S bond, they can be regarded as
1
3
a masked sulfur species that can be used as a potential precursor

3
for S-glycosides. Here we report an extremely convenient method for generation of glycosyl thiolate species from glycosyl
2
Bunte salts 1 under reductive conditions by using sodium sulfide (Na S). We also demonstrated the addition reaction of
the resulting glycosyl thiolates to vinyl compounds or organic halides, affording various kinds of S-glycoside derivatives.
Regarding the conversion of glycosyl Bunte salts into glycosyl thiolates, there was a possibility of employing a strong
1
7
acid for the S-S bond cleavage. However, this protocol was not considered suitable for oligosaccharide derivatives,
because the cleavage of acid-labile internal O-glycosidic bonds may occur as a side reaction. We have, therefore, chosen a
reductive cleavage of the S-S bond in the thiosulfate moiety with a water-soluble phosphine reagent. Treatment with 1.2
1
8
equivalents of tris(2-carboxyethyl)phosphine hydrochloride (TCEP-HCl) gave a complex mixture whereas desired glucosyl
1
6
thiol was obtained in 70% yield when 3 equivalents of TCEP-HCl was used. In order to reduce the amount of reagent, we
2
continued to find a more appropriate reducing agent, and finally found that the addition of Na S was extremely effective
1
9
for the cleavage of the S-S bond in glycosyl Bunte salts.
With the aim of optimizing reaction conditions, the amount of Na
2
S was investigated by using glucosyl Bunte
S to an aqueous
salt as a model substrate. When the reaction was carried out by adding 1.0 equivalent of Na
2
solution of glucosyl Bunte salt, glucosyl thiol was obtained in 91% yield. Furthermore, the use of 1.5 equivalents
of Na S raised the yield up to 95%, whereas the yield decreased to 45% with use of 0.5 equivalents of Na S. When
the product mixture was subjected to column chromatography, the resulting glucosyl thiolate changed to
2
2
2
0
diglucosyl disulfide under atmospheric oxygen. In order to confirm the formation of glucosyl thiol, we acetylated
all the hydroxy groups and the anomeric thiol group by using Ac O-pyridine. The measurement of the molecular
2
weight of the resulting per-acetylated derivative clearly showed that glucosyl thiol moiety was constructed as the
result of S-S bond cleavage of the glucosyl Bunte salt.
Encouraged by these results, we demonstrated the one-pot generation of several glycosyl thiolates starting
from the corresponding unprotected sugars through glycosyl Bunte salts (Table 1). The yield of the products was
1
determined by HNMR spectra of the reaction mixtures by comparing the integration of the anomeric proton of
glycosyl thiolates with the aromatic protons of sodium mesitylene sulfonate standard. Glucosyl thiolate 4, allosyl
thiolate 5. and xylosyl thiolate 6 have been prepared in good yields starting from the corresponding unprotected
monosaccharides (entries 1-3). This method has successfully been applied to oligosaccharides having acid labile
internal O-glycosidic bonds. Lactose, melibiose, and maltopentaose were converted to the corresponding
thiolates, 7, 8, and 9, respectively, without affecting the internal glycosidic bonds (entries 4-6). We also applied
the present method to N-acetyl-2-amino sugars like N-acetylglucosamine (GlcNAc) and chitobiose to give the
corresponding glycosyl thiolates, 10 and 11, respectively (entries 7 and 8).
It has been reported by other groups that the combined use of DMC and thioacetic acid was effective for
thiolation of N-acetyl-2-amino sugars but caused the by-production of 1,6-anhydro sugars when 2-hydroxy sugars
1
4b
were employed as substrates.
It should be emphasized that the present method is applicable not only to N-
acetyl-2-amino sugars but also to 2-hydroxy sugars.

4
Distinct characteristic of the present method is the
fact that both of the preparation of glycosyl Bunte salts
and that of glycosyl thiolates can be achieved in aqueous
media. Based on this, we demonstrated a one-pot
synthesis of S-glycoside derivatives starting from
unprotected sugars through glycosyl thiolates as
synthetic intermediates (Table 2). Firstly, unprotected sugars were transformed into the corresponding glycosyl Bunte salts
by the DMC-mediated condensation reaction with Na
2
S
2
O
3
, and the resulting mixtures were directly treated with Na
2
S.
Glycosyl thiolates thus generated were used for the subsequent S-ligations of vinyl compounds (H
2
C=CHR) or organic
halides (R’X) without isolation. The resulting S-glycosides could be purified via acetylation and column chromatography
followed by deacetylation, and their molecular weights were determined by mass spectrometry. We did not observe the
production of glycosyl disulfides as byproducts.
1
The HNMR spectra of the products showed a doublet signal around 4.5 ppm due to the anomeric proton. The coupling
constants of these peaks (J1,2=9.5-10.0 Hz) clearly indicated that the stereochemistry of the anomeric position was -type.
The excellent -selectivity can be explained by the following two reasons: 1) the S-S bond cleavage of the thiosulfate moiety
does not affect the anomeric configuration of -glycosyl Bunte salts and 2) mutarotation of the resulting glycosyl thiolates
2
1
13
is negligible under alkaline conditions. In the C NMR, the chemical shift of the signal derived from the anomeric carbon
was approximately 85 ppm, clearly indicating that the products possess an S-glycosidic linkage.22
Glucosyl thiolate generated in situ reacted with acrylonitrile, giving rise to the corresponding adduct 12 in 63% yield
(
entry 1). A primary amide, a secondary amide and a tertiary amide behaved as ,-unsaturated Michael acceptors for
the reaction with glucosyl thiolate, affording the corresponding S-glycosides 13-15 (entries 2-4). Recently, the protecting-
group-free synthesis of glyco-monomers has been attracting attention as a useful building block for preparation of glyco-
2
3
polymers. By using N,N’-methylenebis(acrylamide) as a Michael acceptor, an acrylamide type glucosyl monomer 16 has
been synthesized (entry 5).

5
The addition reactions of glycosyl thiolates to organic halides also proceeded smoothly in aqueous media, affording the
corresponding S-glycosides. Recently, we have found that 4,6-dialkoxy-1,3-5-triazinyl glycosides (DAT-glycosides) can be
2
4
recognized by glycosidases and behave as efficient glycosyl donors for enzymatic glycosylations. Their S-glycoside
analogues have attracted the interest of glycobiologists as a ligand for co-crystallization with glycosidases. When glucosyl
thiolate was reacted with 2-chloro-4,6-dibenzyloxy-1,3,5-triazine as an electrophile, the corresponding dibenzyloxy-
1
,3,5tiriazinyl-S-glucoside 17 was obtained in fairly good yield (entry 6). A propargyl group was introduced as an aglycon
part of glucose by the addition of glucosyl thiolate to propargyl bromide, giving rise to an S-glycoside having an ethynyl
2
5
group 18 (entry 7). An S-glucoside 19 having a sulfur-linked fluorescent tag could be prepared effectively by using 4-
bromomethyl-7-methoxycoumarin as an electrophile (entry 8). As an alkyl iodide, 2-iodoacetamide was examined and
reaction with glucosyl thiolate gave desired S-glucoside 20 in medium yield (entry 9).
In addition to these addition reactions of monosaccharide of glucosyl thiolate, representative one-pot aqueous S-
glycosylations of challenging maltopentaose and sialyllactose have been carried out, giving rise to S-maltopentaoside 21
having an ethynyl group and sialyllactosyl acrylamide monomer 22, respectively (Figure 2). These functional
oligosaccharide building blocks can be transformed into various glycomaterials using the copper(I)-catalyzed azide-alkyne
2
6
cycloaddition and radical polymerization, respectively.
In summary, an efficient method for generation of glycosyl thiolates have been achieved by treating the corresponding
2
glycosyl Bunte salts with inexpensive Na S reagent in aqueous media. A three-step one-pot protocol for stereoselective
synthesis of S-glycosides starting from unprotected sugars has successfully been demonstrated. It should be emphasized
that the present method enables us to prepare various S-glycosides without isolating easily oxidizable glycosyl thiols. The
protecting-group-free and aqueous conditions allow this method tolerant towards a wide range of functional groups and
compatible with the S-glycosylation of biomacromolecules. In recent years, special attention has been paid to the synthesis
2
7
of antibodies with a homogeneous glycoform. Glycosyl Bunte salts will play an important role as a masked thiolate sugar
in developing the artificial post-translational modification of proteins, which will provide further insights into the
glycosciences.
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture,
Sports, Science and Technology, Japan.
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2
2
2
2

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Graphical Abstract
Leave this area blank for abstract info.
Efficient generation of thiolate sugars from glycosyl
Bunte salts and its application to S-glycoside synthesis
Yasuhiro Meguro, Gefei Li, Masato Noguchi and Shin-ichiro Shodaꢀꢀ
(
OH)n
R
(
OH)n
O
S
O
Na S
O
2
(
OH)n
S
S
ONa
R
O
S-
O
Glycosyl Bunte salts
(OH)n
R'-X
O
S
R'
ꢀ
ꢀꢀꢀꢀꢀꢀꢀ unprotected sugars

8
Declaration of interests
☒
The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐
The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests:
Highlights:
One-pot synthesis of thioglycosides in aqueous media
Glycosyl Bunte salts as useful intermediates for thioglycosides synthesis
Thioglycosides can be prepared without using any protecting groups.

9