Efficient Strategy for α-Selective Glycosidation of d -Glucosamine and Its Application to the Synthesis of a Bacterial Capsular Polysaccharide Repeating Unit Containing Multiple α-Linked GlcNAc Residues

Article abstract of DOI:10.1021/acs.orglett.0c00101

An efficient α-selective glycosylation method was developed for the synthesis of 2-deoxy-2-amino-d-glucosides based on synergetic α-directing effects of the TolSCl/AgOTf promotion system and the functional groups at the corresponding azido donor 6-O-position to exert steric β-shielding effect or remote participation in the glycosylation reaction. Its practicability was verified with a wide range of monosaccharide glycosyl acceptors and the first, one-pot synthesis of the challenging pentasaccharide repeating unit of an Acinetobacter baumannii K47 capsular polysaccharide.

Full text of DOI:10.1021/acs.orglett.0c00101

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Letter
Efficient Strategy for α‑Selective Glycosidation of D‑Glucosamine
and Its Application to the Synthesis of a Bacterial Capsular
Polysaccharide Repeating Unit Containing Multiple α‑Linked GlcNAc
Residues
Yanxin Zhang,§ Han Zhang,§ Ying Zhao, Zhongwu Guo, and Jian Gao*
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ABSTRACT: An efficient α-selective glycosylation method was developed for
the synthesis of 2-deoxy-2-amino-D-glucosides based on synergetic α-directing
effects of the TolSCl/AgOTf promotion system and the functional groups at the
corresponding azido donor 6-O-position to exert steric β-shielding effect or
remote participation in the glycosylation reaction. Its practicability was verified
with a wide range of monosaccharide glycosyl acceptors and the first, one-pot synthesis of the challenging pentasaccharide repeating
unit of an Acinetobacter baumannii K47 capsular polysaccharide.
α-Linked 2-amino-2-deoxy-D-glucose or D-glucosamine (α-D-
GlcN) and its N-acetyl derivative (α-D-GlcNAc) are key
components in many natural polysaccharides and glycoconju-
gates.1 For the synthesis of these biomolecules, stereoselective
α-glycosidation of GlcN/GlcNAc is vital. However, in contrast
to the facile synthesis of β-glucosaminosides, which can be
readily achieved by taking advantage of the neighboring group
participation effects, stereoselective construction of α-glucosa-
minosides is more difficult to achieve.
limited application scope, and exclusive α-selectivity has been
rarely realized.2c,7−11 Therefore, more effective and broad
strategies for stereoselective α-glycosidation of GlcN are
demanded.
Inspired by the previous reports,12 we have recently
developed a highly selective α-glucosylation method based
on the synergistic α-directing effects of the toluenesulfenyl
chloride (TolSCl)-silver triflate (AgOTf) promotion system
and the β-shielding or remote participation effect of protecting
groups at the donor 6-O-position.13 This method has exhibited
broad applicability and great potential in one-pot synthesis of
complex oligosaccharides.13 Encouraged by this discovery, we
explored here the stereoselective α-glycosidation of GlcN by a
similar strategy. As shown in Scheme 1, we anticipated that the
glycosidation of 2-azido-2-deoxy-thioglucosides with the 6-O-
position protected by the bulky t-butyldimethylsilyl (TBS) or
To address this issue, a variety of elegant strategies have
been explored for α-selective glycosidation of GlcN. Typically,
GlcN is converted into glycosyl donors with the 2-amino group
protected by a nonparticipating functionality.2 For instance,
Kerns et al.3 used oxazolidinone or its analogues4,5 to protect
the 2-N- and 3-O-positions of GlcN to promote α-
glycosylation. The Nguyen group6 employed a benzylidene
group to protect the 2-amino group of GlcN, which enabled
the donor-nickel catalyst coordination to further assist α-
glycosylation. More often, the 2-amino group is converted into
a nonparticipating azido group, which was first explored by
Lemieux and Paulsen7 over four decades ago, since the azido
group is stable under both acidic and basic conditions and
compatible with various glycosylation methods and orthogonal
protecting tactics. In addition, an azido group can be easily
transformed into amino and acetamido groups via selective
reduction (and N-acetylation) at a later stage of the synthesis.
Consequently, 2-azido derivatives of GlcN have been
extensively studied for α-glycosylation. For example, both the
Boons8 and Hung9 groups have used 2-azido-modified donors
for stereoselective synthesis of α-D-glucosaminosides. In
addition, 2-deoxy-2-azido-thioglucosides have been used as
donors to effect more selective α-glycosylation.10
Scheme 1. Proposed α-selective glycosylation reactions
using 6-O-TBS and Bz protected 2-deoxy-2-azido-
thioglucoside donors
Received: January 8, 2020
In spite of the above-mentioned progress, most of the
current methods have either moderate stereoselectivity or
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the participating benzoyl (Bz) group would facilitate α-
selectivity because β-facial attack by the glycosyl acceptor at
the glycosyl triflates formed from preactivation of the donor
with TolSCl and AgOTf would be shielded due to either steric
hindrance or remote group participation.13a
temperature. Subsequently, the reaction mixture was slowly
warmed to room temperature in 1.5 h and stirred for another
15 min.
The results in Table 1 indicated clearly that the reactions of
1 and 2 with all secondary alcohols (entries 1−8), regardless of
the sugar species and protecting groups, proceeded smoothly
to afford the desired products in very high yields (78−89%)
and outstanding (α:β > 15:1) to exclusive (α-only) α-
selectivity. However, the reactions of 1 and 2 with more
reactive 11 containing a primary alcohol gave significantly
lower α-selectivity (α:β = 5:1 and 3:1, respectively, Table 1,
entry 9) but good yields. Considering that glycosyl acceptors
with less nucleophilic hydroxyl groups are less reactive but can
usually afford more stereoselective outcomes,25 we utilized the
Bz-protected analogue 12 of 11 as an acceptor for the
glycosylation. Delightfully, a significant improvement in α-
selectivity (α:β > 19:1) was observed for the reaction between
12 and 1 or 2 (Table 1, entry 10). Similar results were
obtained with Bz-protected acceptor 13 having an α-anomeric
configuration (α:β > 16:1, Table 1, entry 11). The azido
groups in products 14-35 can be readily converted into free
amino or acetamino groups through chemoselective azido
reduction and N-acetylation, as demonstrated in the synthesis
of a very complex oligosaccharide described below. Con-
sequently, a convenient, reliable and efficient strategy was
established for exclusive or highly selective α-glycosylation of
various glycosyl acceptors using donors 1 and 2.
In natural products, GlcN or GlcNAc is α-linked to different
16
positions of various sugars, such as GlcO‑2
,
14 GlcO‑3 15 GlcO‑4
, ,
17
18
19
20
GalNAcO‑3
,
GlcNAcO‑3
,
GlcNAcO‑4
,
D-RhaO‑4
,
21
GlcAO‑4
,
etc. Accordingly, we prepared monosaccharides 3-
1322 as glycosyl acceptors (Table 1) to examine the
Table 1. Glycosylation of Various Monosaccharides with 1
and 2
To verify the practicability of this new glycosylation method,
we applied it to one-pot assembly of the pentasaccharide
repeating unit 37 (Figure 1) of an Acinetobacter baumannii K47
Figure 1. Structure of A. baumannii K47 CPS and the synthetic target
37.
capsular polysaccharide (CPS).26 Compound 37, which
contained three α-linked GlcNAc residues and a branching
α-GalNAc motif, represents a notable synthetic challenge. To
date, there has been no reported synthesis of this
oligosaccharide. In our designed synthetic target 37, a free
amino group is attached to the glycan downstream end to
facilitate its conjugation with other molecules for biological
studies, such as anti-A. baumannii K47 vaccine development.
Our synthetic plan for 37 is outlined in Scheme 2. We
projected that pentasaccharide 38 as the fully protected form
of 37 could be constructed from building blocks 1, 39, and 40
through preactivation-based iterative one-pot glycosylation.24
The 6-O-TBS and -Bz groups in 1 and 39 were expected to
secure α-selective glycosylations. In turn, trisaccharide 39
would be prepared from α-selective glycosylation of 42 with
41.
a
b
Based on isolated yields. α:β anomeric ratios were determined by
1H NMR analysis of reaction mixture.
glycosylation reactions utilizing 6-O-TBS and Bz-protected 2-
deoxy-2-azido-thioglucosides 1 and 2 (see the SI for their
preparation),23 both of which were designed as α-configuration
with an expectation to get the more comparable results. All of
the glycosylation reactions were carried out with the
preactivation protocol;24 that is, after the donor was activated
with TolSCl/AgOTf (1.0 equiv relative to the donor) in
diethyl ether at −78 °C, the acceptor was added at the same
Our synthesis commenced with the preparation of mono-
and disaccharide building blocks 41−43 (see the SI for their
preparation). Next, preactivation-based glycosylation of 41
with 4227 was achieved using promoter TolSCl/AgOTf at −40
°C with DCM−Et2O (v/v, 10:1) as the solvent (Scheme 3).
B
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Scheme 2. Retrosynthesis of the Target Molecule 37
Scheme 5. One-pot Assembly of the Synthetic Target 37
Scheme 3. Synthesis of Trisaccharide Building Block 39
the product isolated from a fraction of the reaction mixture.
Thereafter, 40 was glycosylated with 47 by means of the same
preactivation procedure to provide 38. For each glycosylation,
1.0 equiv of TolSCl/AgOTf (relative to the donor) was
employed. Pure pentasaccharide 38 (GlcN: JH1′,2′ = 3.5 Hz,
GlcN: JH1′′′,2′′′ = 3.5 Hz) was isolated from the reaction
mixture in an impressive 65% overall yield together with a very
small amount of the β isomer (α:β = 13:1) that was generated
from the second glycosylation step.
Full deprotection of 38 was accomplished by a five-step
protocol. First, the TBS group was chemoselectively removed
with 3HF·Et3N at room temperature. This was followed by
removal of the acyl groups with CH3ONa. Next, all four azido
groups were readily converted into acetamido groups by a one-
pot two-step procedure including reduction of the azido
groups with 1,3-propanedithiol and then selective N-
acetylation of the resultant free amines with acetic anhydride
in CH3OH. The intermediate was purified with a Sephadex
LH-20 column and then subjected to Pd(OH)2/C-catalyzed
hydrogenolysis in CH2Cl2 and CH3OH (1:5) to remove all of
the benzyl groups and to remove the Cbz group concurrently.
This mixed solvent could dissolve both of the starting material
and the product well. Finally, the synthetic target 37 was
obtained in a 44% overall yield after purification with a
Sephadex G-15 column using H2O as the eluent and
lyophilization. Its structure was verified with 1D and 2D
NMR and HR MS data.
In summary, a new, efficient, and highly stereoselective
strategy was developed for the construction of α-GlcN linkages
with 2-deoxy-2-azido-thioglucosides as glycosyl donors. It was
proposed that the high α-selectivity was due to combined α-
directing effects of the TolSCl/AgOTf promotion system to
facilitate the formation of an oxonium triflate intermediate and
protecting groups at the donor 6-O-position to shield the β-
face, leading to favorable attack by aglycones from the α-face.
This mechanism was supported by the observation that less
reactive aglycones afforded better α-selectivity (Table 1).
Further study to reveal the exact mechanism are currently
ongoing in our laboratory. The new glycosylation method was
Only the α-product 44 was obtained from this reaction in an
80% yield, and the resultant new α-glycosyl linkage was verified
by the small anomeric coupling constant of GalN-2 (JH1′,2′
=
1
3.5 Hz) in its H NMR spectrum. It is worth noting that the
reaction at −78 °C gave only a low yield (<10% yield) of 44
because 41 was less reactive and was not effectively activated at
too low temperature. Subsequently, the benzylidene ring in 44
was regioselectively opened by reaction with triethylsilane
(Et3SiH) in the presence of boron trifluoride diethyl etherate
(BF3·Et2O) to afford 39 in a 78% yield.
Similarly, preactivation of 43 with TolSCl/AgOTf at −78 °C
followed by reaction with primary alcohol 45 provided 46 in a
moderate α-selectivity (α:β = 9:1), and the isomers were easily
separated after desilylation with 3HF·Et3N to give pure 40
(JH1,2 = 3.5 Hz) in 79% yield for two steps (Scheme 4). In
Scheme 4. Synthesis of Building Block 40
addition, we also carried out this glycosylation by the
conventional procedure; that is, 43 and 45 were mixed in
diethyl ether before AgOTf and TolSCl were added at −78 °C,
and the reaction was kept at room temperature for 15 min. The
latter gave essentially the same stereoselectivity and yield as the
former.
After building blocks 1, 39, and 40 were available, we moved
on to perform one-pot synthesis of pentasaccharide 37
(Scheme 5). Thus, after 1 was preactivated with TolSCl/
AgOTf in diethyl ether at −78 °C within 15 min, trisaccharide
39 was added at the same temperature. The reaction mixture
was allowed to warm to room temperature in 1.5 h and was
stirred for another 15 min to accomplish the first glycosylation.
This reaction afforded the desired α-isomer 47 only, which was
proved by the NMR spectrum (GlcN-3: JH1′′,2′′ = 3.0 Hz) of
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proved to be efficient, versatile, and broadly useful and
compatible with various protecting groups, as evidenced by the
examples in Table 1. Furthermore, due to its robustness and
high stereoselectivity, this glycosylation method should be also
suitable for one-pot carbohydrate synthesis. As a proof of
principle, it was employed to synthesize the pentasaccharide
repeating unit 37 of A. baumannii K47 CPS that contained
multiple α-linked GlcNAc residues via one-pot [1 + 3 + 1]
glycosylation. It represented the first total synthesis of this
highly challenging pentasaccharide. All of the glycosylation
reactions proceeded smoothly to form α-glycosidic linkages in
excellent yields and stereoselectivity. Thus, the new glyco-
sylation method should be applicable to the synthesis of
various complex oligosaccharides containing α-linked GlcN or
GlcNAc residues.
China (2018GSF118120), and the Young Scholars Program of
Shandong University (2018WLJH41). We thank Haiyan Sui
and Xiaoju Li from State Key Laboratory of Microbial
Technology in Shandong University for NMR measurements.
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ASSOCIATED CONTENT
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The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c00101.
Synthetic procedures, analytical data, and NMR and MS
spectra of all new compounds (PDF)
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AUTHOR INFORMATION
Corresponding Author
■
Jian Gao − National Glycoengineering Research Center,
Shandong Key Laboratory of Carbohydrate Chemistry and
Glycobiology, Shandong University, Qingdao, Shandong
266237, China; orcid.org/0000-0002-5928-412X;
Email: jgao@sdu.edu.cn
Authors
́
(b) Codee, J. D. C.; Stubba, B.; Schiattarella, M.; Overkleeft, H. S.;
Yanxin Zhang − National Glycoengineering Research Center,
Shandong Key Laboratory of Carbohydrate Chemistry and
Glycobiology, Shandong University, Qingdao, Shandong
266237, China
Han Zhang − National Glycoengineering Research Center,
Shandong Key Laboratory of Carbohydrate Chemistry and
Glycobiology, Shandong University, Qingdao, Shandong
266237, China
Ying Zhao − National Glycoengineering Research Center,
Shandong Key Laboratory of Carbohydrate Chemistry and
Glycobiology, Shandong University, Qingdao, Shandong
266237, China
Zhongwu Guo − Department of Chemistry, University of
Florida, Gainesville, Florida 32611, United States;
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Complete contact information is available at:
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Author Contributions
§Y.Z. and H.Z. contributed equally.
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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This work was supported by the National Natural Science
Foundation of China (21702124), the Shandong Provincial
Natural Science Foundation, China (ZR2017MB013), the Key
Research and Development Program of Shandong Province,
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