Gold(I)-promoted synthesis of a β-(1,3)-glucan hexadecasaccharide via the highly convergent strategy

Article abstract of DOI:10.1016/j.carres.2019.06.014

Elucidation of the structure-activity relationships of β-(1,3)-glucans is hampered by the difficulty to isolate the β-(1,3)-glucan polysaccharides from natural sources. We describe a gold(I)-promoted approach for the efficient assembly of a β-(1,3)-glucan

Full text of DOI:10.1016/j.carres.2019.06.014

Accepted Manuscript
Gold(I)-promoted synthesis of a β-(1,3)-glucan hexadecasaccharide via the highly
convergent strategy
Zixuan Wang, Qingting Hua, You Yang
PII:
S0008-6215(19)30227-7
https://doi.org/10.1016/j.carres.2019.06.014
CAR 7735
DOI:
Reference:
To appear in: Carbohydrate Research
Received Date: 13 April 2019
Revised Date: 15 May 2019
Accepted Date: 24 June 2019
Please cite this article as: Z. Wang, Q. Hua, Y. Yang, Gold(I)-promoted synthesis of a β-(1,3)-glucan
hexadecasaccharide via the highly convergent strategy, Carbohydrate Research (2019), doi: https://
doi.org/10.1016/j.carres.2019.06.014.
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ACCEPTED MANUSCRIPT
Graphical abstract
A gold(I)-catalyzed highly convergent strategy is described for the efficient synthesis
of a β-(1,3)-glucan hexadecasaccharide.

ACCEPTED MANUSCRIPT
Gold(I)-promoted Synthesis of a β-(1,3)-Glucan
Hexadecasaccharide via the Highly Convergent Strategy
Zixuan Wang, Qingting Hua and You Yang*
Shanghai Key Laboratory of New Drug Design, School of Pharmacy, East China University
of Science and Technology, 130 Meilong Road, Shanghai 200237, China. E-mail:
yangyou@ecust.edu.cn
Abstract:
Elucidation of the structure-activity relationships of β-(1,3)-glucans is hampered by the
difficulty to isolate the β-(1,3)-glucan polysaccharides from natural sources. We describe a
gold(I)-promoted approach for the efficient assembly of a β-(1,3)-glucan hexadecasaccharide
via the orthogonal and consecutive activation of thioglycosides and glucosyl ortho-
hexynylbenzoates in a highly convergent manner. The synthetic hexadecasaccharide serves as
the basis for further evaluation of their biological functions.
Keywords: Glycosylation; β-(1,3)-glucan; polysaccharide; gold; glycosyl ortho-
hexynylbenzoate
Introduction
In nature, β-(1,3)-glucans are widely distributed as the essential constituents of fungi and
seeweeds.1,2 By interaction with the complement receptor type 3 and dectin-1, β-(1,3)-glucans
are regarded as an important type of biological response modifiers that are able to stimulate
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the immune system and exhibit antitumor, antifungal, and antibacterial effects.3-14 The strong
binding between β-(1,3)-glucans and dectin-1 has been explored for searching modulators of
innate immunity.15,16 Glycoconjugate vaccines based on β-(1,3)-glucans can provide
protection in mice against fungal pathogens such as Candida albicans and Aspergillus
fumigatus.17-22 However, biological studies of naturally occurring β-(1,3)-glucans containing
short 6-O-branched β-glucans or their hydrolysates may lead to contradictory results due to
the heterogeneity of natural β-glucans.23-25 As such, synthetic β-(1,3)-glucans are highly
desirable for understanding the mechanism and structure-activity relationships of β-(1,3)-
glucans in the interaction with cell surface receptors.
Chemical synthesis of β-(1,3)-glucans has aroused much interest in carbohydrate
community aiming at development of immunomodulatory agents and antifungal vaccines.26-30
To access the β-(1,3)-glucan polysaccharides, various approaches were developed to produce
linear deca-,27 undeca-,28 dodeca-,27 trideca-,28 hexadeca-,29 and branched
heptadecasaccharides29 in solution phase. Recently, Seeberger and coworkers reported the
automated solid-phase synthesis of dodeca- and branched tridecasaccharides for identifying
antibody epitopes.30 However, synthesis of long β-(1,3)-glucans with more than 15 glucose
units is very rare. Hence, efficient assembly strategies are highly pursued for the synthesis of
ultralong β-(1,3)-glucan polysaccharides.
In terms of the synthesis of β-(1,3)-glucans, the solution-phase convergent [n + n]
strategy is usually considered as one of the most efficient approaches for the block assembly
of β-(1,3)-glucans from the perspective of symmetry. Based on this strategy, at least four
glycosylation steps are required for the assembly of a β-(1,3)-glucan hexadecasaccharide
skeleton. Nevertheless, in the solution-phase synthesis of the β-(1,3)-glucan
hexadecasaccharide via the glycosylation with the trichloroacetimidates and ethyl
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thioglycosides as donors reported by H. Tanaka and coworkers, extension of the sugar chain
did not go above tetrasaccharide donors.29 Here, we report the gold(I)-promoted highly
convergent [8 + 8] synthesis of a β-(1,3)-glucan hexadecasaccharide based on the orthogonal
and consecutive activation of thioglycosides and glucosyl ortho-hexynylbenzoates.
As depicted in Scheme 1, the β-(1,3)-glucan hexadecasaccharide 1 can be derived
from odourless 5-tert-butyl-2-methyl phenyl thioglucoside 2 that might avoid glycoside
transfer,31 in which the orthogonal TBS group is attached to the 3-OH position of the non-
reducing end. The benzoyl groups installed at the 2-OH positions of the glucose units can
ensure the stereoselective formation of β-glucosides via the neighboring-group participation
effect. Based on the highly convergent strategy, the hexadecasaccharide 2 is dissected into
octasaccharide ortho-hexynylbenzoate32 3 and thioglycoside 4, which could be constructed by
coupling of tetrasaccharide ortho-hexynylbenzoate 5 and thioglycoside 6 followed by
functional group transformations. Similarly, tetrasaccharide building blocks 5 and 6 are
further divided into disaccharide ortho-hexynylbenzoate 7 and thioglycoside 8 that can be
obtained by glycosylation of monosaccharide ortho-hexynylbenzoate 9 and thioglycoside 10
and subsequent functional group transformations.
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Scheme 1. Retrosynthetic analysis of the β-(1,3)-glucan hexadecasaccharide 1.
Result and discussion
Starting from the commercially available β-D-glucose pentaacetate 11, alcohol 12 was
prepared in 72% overall yield over six steps according to the literature procedures (Scheme
2).30 Benzylation of alcohol 12 with benzyl bromide in the presence of sodium hydride in a
mixture of THF and DMF gave thioglycoside 13 in 94% yield. Subjection of thioglycoside 13
to an NIS/DTBP/AgOTf system in acetonitrile and water led to the formation of the
corresponding hemiacetal, which was condensed with ortho-hexynylbenzoic acid under the
promotion of EDCI and DMAP to provide glucosyl ortho-hexynylbenzoate 9 in 88% yield
over two steps. Exposure of 13 to HF pyridine in dichloromethane resulted in the cleavage of
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the TBS group, affording glucosyl acceptor 10 in 87% yield without migration of the 2-
benzoyl group into the 3-OH position.
Scheme 2. Synthesis of the monosaccharide building blocks 9 and 10.
At the outset, PPh3AuOTf (0.2 eq.)-catalyzed glycosylation of ortho-hexynylbenzoate
9 with glucosyl acceptor 10 was performed in dichloromethane at room temperature to give
the desired β-linked disaccharide 14 in 70% yield, as determined by analysis of the coupling
constants between H-1 and H-2 of the corresponding glucoses (3JH1,H2 = 8.0, 10.0 Hz,
respectively). Interestingly, by replacing dichloromethane with toluene, the coupling gave a
cleaner reaction and the glycosylation yield was improved to 88% due to the solvent effects33
(Scheme 3). Compound 14 was then transformed into disaccharide ortho-hexynylbenzoate 7
in 63% yield over two steps via selective removal of the anomeric thiol ether with
NIS/DTBP/AgOTf and subsequent condensation with ortho-hexynylbenzoic acid. Removal of
the TBS group in 14 with HF pyridine in dichloromethane generated disaccharide acceptor 8
in a moderate 67% yield accompanied by the recovery of starting material 14 in 11% yield.
However, treatment of 14 with HF pyridine in THF proceeded smoothly to afford
disaccharide acceptor 8 in an excellent 95% yield.
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Scheme 3. Synthesis of the disaccharide building blocks 7 and 8.
The [2 + 2] glycosylation of 7 with 8 catalyzed by PPh3AuOTf (0.1 eq.) was originally
investigated in dichloromethane at room temperature, producing β-linked tetrasaccharide 15
in only 41% yield. Exhilaratingly, when toluene was used as solvent, the PPh3AuOTf (0.1
eq.)-catalyzed glycosylation of 7 with 8 resulted in a very clean reaction, furnishing the β-
linked tetrasaccharide 15 in 83% yield (Scheme 4). The β-glycosidic linkages of
tetrasaccharide 15 were confirmed by analysis of the coupling constants between C-1 and H-1
of the corresponding glucoses (1JC1,H1 = 156.0, 162.0, 162.5 Hz, respectively). In a manner
similar to the functional group transformations from 14 to 7, compound 15 was converted into
tetrasaccharide ortho-hexynylbenzoate 5 in 75% yield over two steps. The TBS group in 15
was removed using HF pyridine in THF to provide tetrasaccharide acceptor 6 in 92% yield.
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Scheme 4. Synthesis of the tetrasaccharide building blocks 5 and 6.
Coupling of tetrasaccharide ortho-hexynylbenzoate 5 with tetrasaccharide acceptor 6
under the promotion of PPh3AuOTf (0.2-0.5 eq.) in toluene at room temperature afforded the
β-linked octasaccharide 16 in around 55% yield with the concomitant hydrolysis of donor 5.
By lowering the temperature to –30 °C, the PPh3AuOTf (0.4 eq.)-promoted [4 + 4]
glycosylation of 5 with 6 provided the β-linked octasaccharide 16 as the only anomer in 79%
yield (Scheme 5). The β-glycosidic linkages of octasaccharide 16 were confirmed by analysis
of the coupling constants between C-1 and H-1 of the corresponding glucoses (1JC1,H1 = 160.0,
161.0, 161.5 Hz, respectively). Steps similar to the 15 → 5 conversion were then employed to
prepare octasaccharide ortho-hexynylbenzoate 3 from 16 (71% over two steps). Cleavage of
the TBS group in 16 with HF pyridine in THF produced octasaccharide acceptor 4 in 92%
yield.
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nBu
BnO
BnO
O
BnO
BnO
O
BnO
BnO
O
O
BnO
BnO
O
BnO
BnO
O
O
BnO
BnO
O
O
BnO
BnO
HO
O
S
O
BnO
BnO
TBSO
O
O
O
+
O
OBz
OBz
OBz
OBz
OBz
O
OBz
OBz
OBz
6
5
Ph3PAuOTf (0.4 eq.),
4Å MS, toluene, –30 °C
79%
BnO
BnO
O
BnO
BnO
O
O
BnO
O
O
BnO
BnO
O
S
BnO
O
BnO
BnO
O
BnO
BnO
O
O
BnO
BnO
O
O
OBz
O
BnO
BnO
TBSO
OBz
O
OBz
O
OBz
OBz
OBz
16
OBz
OBz
1) NIS, DTBP, AgOTf,
CH3CN, H2O, rt
2) ortho-hexynylbenzoic acid,
EDCI, DMAP, CH2Cl2, rt
71% over two steps
nBu
O
BnO
O
BnO
BnO
BnO
O
BnO
O
O
BnO
O
O
BnO
O
BnO
O
O
BnO
BnO
O
O
BnO
O
OBz
BnO
O
BnO
BnO
TBSO
OBz
O
BnO
O
OBz
BnO
O
OBz
OBz
3
OBz
OBz
OBz
BnO
O
BnO
BnO
O
O
BnO
O
BnO
O
BnO
O
O
S
BnO
BnO
O
O
BnO
BnO
O
O
BnO
O
HF Py, THF, rt
92%
OBz
BnO
O
BnO
BnO
HO
OBz
O
BnO
16
OBz
BnO
O
OBz
OBz
OBz
4
OBz
OBz
Scheme 5. Synthesis of the octasaccharide building blocks 3 and 4.
With octasaccharide ortho-hexynylbenzoate 3 and octasaccharide acceptor 4 in hand,
we commenced to couple them into the β-(1,3)-glucan hexadecasaccharide 1 (Scheme 6).
Exhilaratingly, [8 + 8] coupling of 3 with 4 promoted by PPh3AuOTf (0.4 eq.) in toluene at –
40 °C proceeded smoothly to furnish β-linked hexadecasaccharide skeleton 2 as the single
anomer in 89% yield. The desired β-glycosidic linkages of hexadecasaccharide 2 were
confirmed by analysis of the coupling constants between C-1 and H-1 of the corresponding
glucoses (1JC1,H1 = 156.5, 160.5, 161.0, 161.5, 162.0, 162.5, 163.0 Hz, respectively).
Unexpectedly, glycosylation of 2 with methanol under the promotion of NIS and TfOH
afforded compound 17 as a mixture of α-methylated and β-methylated anomers in only 51%
yield. With the assistance of IBr and AgOTf, the glycosylation yield was improved to 93%
albeit accompanied by the formation of almost equal amount of α/β anomers (1JC1,H1 = 174.0,
159.0, 159.6, 160.2, 157.2 Hz, respectively). Similarly, glycosylation of monosaccharide 13
with methanol promoted by IBr and AgOTf gave a mixture of the desired methyl glycoside in
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89% yield (β/α = 3:1), testifying that the neighboring-group participation effect did not play a
decisive role in this type of glycosylation reactions.34 Global deprotection of 17 involving
removal of the TBS group with HF pyridine, saponification with sodium methoxide in
methanol and dichloromethane, hydrogenolysis of the benzyl groups over Pd(OH)2/C in a
mixture of THF, water and acetic acid, provided hexadecasaccharide 1 in 55% yield over
three steps. The structure of 1 was confirmed by analysis of 1H NMR spectrum and MALDI-
1
TOF. The H NMR spectrum of synthetic 1 was found to be in good agreement with those
reported in the literature.29,30,35
Scheme 6. Synthesis of the β-(1,3)-glucan hexadecasaccharide 1.
Conclusion
In conclusion, we have described the gold(I)-promoted synthesis of a β-(1,3)-glucan
hexadecasaccharide via the orthogonal and consecutive activation of thioglycosides and
glucosyl ortho-hexynylbenzoates. The highly convergent synthetic approach required only
four glycosylation steps for procurement of the β-(1,3)-glucan hexadecasaccharide skeleton
starting from the monosaccharide building blocks. The couplings with glucosyl ortho-
hexynylbenzoates as donors and PPh3AuOTf as promoter in toluene proved to be very
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efficient for the construction of β-(1,3)-glucan polysaccharides. The synthetic approach
described here lays the foundation for further biological evaluation of β-(1,3)-glucan
polysaccharides.
Acknowledgements
Financial support from the National Thousand Young Talents Program (Y100-4Q-1701,
YC0140103), the National Natural Science Foundation of China (21871081) and the
Fundamental Research Funds for the Central Universities (50321031916002, 22221818014)
is gratefully acknowledged.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://dx.doi.org/10.1016/
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Highlights：
•
•
Efficient synthesis of a β-(1,3)-glucan hexadecasaccharide.
Orthogonal and consecutive activation of thioglycosides and glucosyl
ortho-hexynylbenzoates
•
Gold(I)-catalyzed highly convergent approach in toluene.