A convenient and efficient synthesis of glycals by zinc nanoparticles

Article abstract of DOI:10.1039/c4ra08028a

A simple and efficient method for the synthesis of pyranoid glycals utilizing the reductive elimination of glycopyranosyl bromides by zinc nanoparticles in an acetate buffer is described. A variety of pyranoid glycals derivatives were obtained, especially for the synthesis of 6-deoxy-4,6-O-benzylidene and disaccharide glycals with good yields.

Full text of DOI:10.1039/c4ra08028a

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DOI: 10.1039/C4RA08028A
A simple and efficient method for synthesis of pyranoid glycals utilizing zinc nanoparticles reductive
elimination glycopyranosyl bromides in acetate buffers is described. A variety of pyranoid glycals
derivatives were obtained, especially for the synthesis of 6-deoxy, 4,6-
glycals with good yields
O-benzylidene and disaccharides
.

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A Convenient and Efficient Synthesis of Glycals by
Zinc Nanoparticles
Cite this: DOI:
10.1039/x0xx00000x
Yun Xu, Wenjun Wang, Yu Cai, Xia Yang, Peng George Wang, and Wei Zhao*
Received 00th January 2014,
Accepted 00th January 2014
DOI: 10.1039/x0xx00000x
A simple and efficient method for synthesis of pyranoid glycals utilizing zinc nanoparticles
reductive elimination glycopyranosyl bromides in acetate buffers is described. A variety of
www.rsc.org/
pyranoid glycals derivatives were obtained, especially for the synthesis of 6-deoxy, 4,6-
benzylidene and disaccharides glycals with good yields
O-
.
The biological functions of glycoconjugates and oligosaccharides powerful reactants in organic synthesis.23 Herein, an approach based
are critical and extensive in cell growth, cell adhesion, cell on a nonꢀtoxic, airꢀ and moistureꢀstable zinc nanoparticles as
communication, inflammatory response, and immune defense.1 reduction agent is developed. Mild reaction conditions and high
The efficient stereoselective construction of glycosidic linkages reactivity of zinc nanoparticles leads to high yield of products, ease
for the preparation of complex oligosaccharides and of workꢀup, and ecologically clean procedure for the construction of
glycoconjugates is a challenging focus in organic synthesis and diverse pyranoid glycal derivatives, especially for the synthesis of 6ꢀ
glycochemistry.2 Due to the presence of the enol ether deoxy,4,6ꢀOꢀbenzylidene and disaccharides glycals.
functionality, among the wide variety of glycosylation strategies,
Zn-nanoparticles
glycals are important intermediates, and they have been
commonly employed in the formation of glycosidic bonds, for
example, glycals are multifunctional donors for Oꢀglycosides,3 Cꢀ
glycosides,4 Sꢀglycosides,5 Nꢀglycosides.6 Meanwhile they serve
as irreplaceable building blocks for 2ꢀdeoxyꢀhexoses and 2ꢀ
deoxyꢀ2ꢀaminoꢀhexoses synthesis7 and natural products such as
palytoxin, halichondrin, spongistatin, etc.8 Glycals were
discovered by Fisher and Zach,9 in spite of their long existence,
they remain a type of great interest sugar because of the
addition10, oxidation11 and rearrangement12 reactions that they
undergo. Recently, the uses of glycals in combinatorial chemistry
have been found.13
O
O
(RO)n
AcOH, AcONa,THF (RO)n
AcO
Br
Scheme 1. Synthesis of glycal from glycopyranosyl bromide
For the synthesis of glycals, we utilized zinc nanoparticles reductive
elimination of glycopyranosyl bromides under acetate buffers, as
depicted in Scheme 1. Our approach presented here is mild with
easily available reagents. In initial studies, acetobromoꢀαꢀDꢀglucose
1 chosen as starting materials was carried out in the presence of zinc
nanoparticles (50 nm, commercial available, see supporting
information) under AcOH/AcONa/THF buffers at room temperature
for 1 h (Scheme 1). TLC analysis indicated complete conversion of
the starting material 1 and formed 224 as the sole product in yield of
90% (Table 1, Entry a). This result motivated us to continue our
study to investigate the substrate scope of the reaction. Thus, the
mild protocol was further applied to synthesis of other glycals with a
variety of substituents. Similarly, the desired products were afforded
for substrates with other groups and the results are summarized in
Table 1.
Over the years, numerous efforts have been made to construct
glycals, including reduction of glycosyl halides by metals such as
ZnꢀAcOH,9 LiꢀNH3,14 (Cp2TiCl)2,15 Cr(II),16 ZnꢀMeOHꢀvitaminꢀ
B12,17 and AlꢀHg18 or using thiophenyl glycoside,19 glycosyl
sulfones,20 glycosyl sulfoxides,21 and electrochemical approach.22
However, many of these aforementioned methods have long suffered
from lack of generality because of several drawbacks. Firstly, use of
moistureꢀsensitive, expensive, and toxic metal reagents generating
intricate laboratory operations may limit the applications of these
protocols. Secondly, harsh reaction conditions such as strong acidic
conditions confine the utilization of acidꢀlabile substrates. Thirdly,
because of the low reactivity of metal dust in some cases, excess
loading of metals and acetic anhydride reagents were employed,
resulting in the following workup tedious such as metalꢀscavenging
and acidꢀbase neutralization to remove the superfluous metals and
acidic reagents. In this regard, a userꢀfriendly, convenient, less toxic
and general protocol for the synthesis of glycal is desirable.
As shown in Table 1, various desired glycals were obtained under
the same condition in good yields. Treatment of the tetraꢀOꢀacetylꢀαꢀ
D
ꢀgalacopyranosyl bromide 3 with zinc nanoparticles in acetate
buffers afforded TriꢀOꢀacetylꢀ
D
ꢀgalactal 424a in 93% isolated yield
and the rapid conversion was achieved in less than 1 h (Table 1,
Entry b). Encouraged by the results obtained with zinc nanoparticles
reductive elimination glycopyranosyl bromide 1 and 3 providing
corresponding glucal 2 and galactal 4, we turned our attention to 6ꢀ
deoxy glycals. As illustrated in Table 1, 3,4ꢀDiꢀOꢀacetylꢀ
Lꢀrhamnal
625 and 3,4ꢀdiꢀOꢀacetylꢀ ꢀfucal 826 were readily afforded in excellent
L
In recent years, owing to numerous advantages of metal
nanoparticles particularly the high surface to volume ratio leading to
high reaction activity, metal nanoparticles have been recognized as
yield 93% and 85%, respectively (Table 1, entries c, and d).
Compared with previous reports with tedious workup,27 in this study,
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VJieowuArrtnicalelONnlainme e
DOI: 10.1039/C4RA08028A
benefited from the high reaction activity of zinc nanoparticles acetate in 91% and 86% isolated yield (Table 1, entries e, and f).
increasing the reaction selectivity and then to give pure and high Furthermore, it’s possible to scale up to commercial applications
yield of desired product, 3,6,2',3',4',6'ꢀHexaꢀOꢀacetylꢀαꢀ
1028 and 3,6,2',3',4',6'ꢀhexaꢀOꢀacetylꢀαꢀ ꢀlactal 1228 were decrease of yield (10, 91%).
successfully afforded after recrystallization in hexane and ethyl
Dꢀcellobial because the reaction could be conducted on a gram scale without a
D
Table 1. Synthesis of various substituted glycals from glycosyl bromidesa.
Product
Entry
a
Substrate
Time(h)
Yieldb (%)
OAc
OAc
O
O
AcO
AcO
AcO
AcO
1
90
AcO
Br
224
1
OAc
OAc
OAc
O
OAc
O
AcO
b
c
93
93
AcO
1.5
2.5
AcO
Br
424a
3
Br
O
O
AcO
AcO
AcO
OAc
AcO
5
625
Br
O
O
OAc
d
85
1
OAc
OAc
OAc
OAc
7
826
OAc
O
OAc
O
OAc
OAc
O
O
AcO
AcO
O
AcO
AcO
O
AcO
91
12
AcO
AcO
e
AcO
AcO
Br
9
1028
OAc
OAc
OAc
OAc
O
OAc
OAc
O
O
O
10
O
86
37
O
f
AcO
AcO
AcO
AcO
AcO
AcO
AcO
Br
1228
11
O
O
Ph
Ph
O
O
O
O
AcO
AcO
g
2
AcO
Br
1427a
13
a Reaction conditions: 1.0 mmol glycopyranosyl bromides, 2.0 mmol zinc nanoparticles, 6.0 mmol AcONa and 0.4 mL AcOH, in 4 mL THF at r.t.
b Isolated yield
To ascertain the substrate tolerance and generality of the reaction, was treated under the same reaction conditions (Scheme 1). After 2 h,
2,3ꢀdiꢀOꢀacetylꢀ4,6ꢀOꢀbenzylideneꢀαꢀDꢀglucopyranosyl bromide 13 TLC detection showed that we obtained 47% yield of 3ꢀOꢀacetylꢀ4,
with bearing acidꢀsensitive 4,6ꢀOꢀbenzylidene group as substrate 6ꢀOꢀbenzylideneꢀ
D
ꢀglucal 1427a (Table 1, Entry g). To improve the
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In summary, we have successfully established an efficient and
practical protocol for the preparation of glycals with zinc
nanoparticles under mild conditions. The high reactivity of zinc
nanoparticles leads to complete and clean reaction simplifying
the process of purification as well as fewer acetic acid solutions
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Acknowledgements
4
We thank the 863 Program of China (No. 2012AA021505) and the
Natural Science Foundation of China (No. 21332006, 21372130) for
financial support.
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Address: College of Pharmacy, State Key Laboratory of Elementoꢀ
Organic Chemistry, and Synergetic Innovation Center of Chemical
Science and Engineering, Nankai University, Tianjin 300071, PR China
Tel/Fax: (+86)ꢀ22ꢀ2350ꢀ6290; Eꢀmail: wzhao@nankai.edu.cn (W. Zhao).
† Electronic Supplementary Information (ESI) available: See
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