Assembly of a β-(1?→?3)-glucan laminarihexaose on ionic liquid support

Article abstract of DOI:10.1016/j.tetlet.2017.03.035

An efficient method for the preparation of β-(1?→?3)-D-glucan laminarihexaose on ionic liquid (IL)-support is described. A β-(1?→?3)-glucan laminarihexaose was rapidly assembled in 15?h in a stereoselective fashion with an average yield of over 90% per st

Full text of DOI:10.1016/j.tetlet.2017.03.035

Tetrahedron Letters xxx (2017) xxx–xxx
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Tetrahedron Letters
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Assembly of a b-(1 ? 3)-glucan laminarihexaose on ionic liquid support
⇑
Yanhui Wang, Xiaoke Zhao, Qiuyun Kong, Jiajian Yao, Xiangbao Meng, Zhongjun Li
The State Key Laboratory of Natural and Biomimetic Drugs, Department of Chemical Biology, School of Pharmaceutical Sciences, Peking University, Beijing 100191, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient method for the preparation of b-(1 ? 3)-D-glucan laminarihexaose on ionic liquid (IL)-sup-
Received 11 February 2017
Revised 6 March 2017
Accepted 10 March 2017
Available online xxxx
port is described. A b-(1 ? 3)-glucan laminarihexaose was rapidly assembled in 15 h in a stereoselective
fashion with an average yield of over 90% per step using an optimized combination of glycosylating
agents. This ionic liquid support approach provides an efficient and fast means for the assembly of
b-(1 ? 3)-glucans.
Ó 2017 Elsevier Ltd. All rights reserved.
Keywords:
Ionic liquid
Beta glucans
Laminarihexaose
Oligosaccharide
Rapid assembly
Oligosaccharides are widely studied because they possess a
variety of unique structures and functionalities. It is essential to
Because of their immunostimulating, antibacterial and antitumor
activities, synthesis of b-(1 ? 3)-glucans has attracted intensive
interest. And several syntheses of linear b-(1 ? 3)-glucans have
1
gain access to oligosaccharides in sufficient quantities and in pure
2
17–24
form for biological studies. Chemical synthesis of oligosaccharides
been published to date.
is still a challenge due to the need of selective protection and
deprotection of multiple hydroxyl groups. Moreover, traditional
synthesis requires purification by chromatography after each step
Traditionally, b-(1 ? 3)-glucans are synthesized by liquid-
phase reaction and purified by chromatography after each step of
glycosylation. In 2013, the automated solid phase synthesis of b-
3
,4
5
of glycosylation, which is time-consuming and costly.
Solid-
(1 ? 3)-d-glucododecaoside was described. However, a large
phase synthesis, which allows convenient product isolation and
automation, is one of the most effective ways for assembling com-
plex oligosaccharides. However, it has unavoidable limitations
excess of donors and expensive promoters was required to facili-
tate the glycosylation. Herein, we report the first synthesis of Lam-
inarihexaose 1 on ionic liquid support.
5
because of the heterogeneous reaction conditions. Therefore, sev-
eral soluble polymer supports have been used instead in recent
Retrosynthetically, we envisioned that the linear synthesis of
Laminarihexaose 1 could be accomplished by utilizing just two
building blocks, the glycosyl donor 2 and the ionic liquid support
3 (Fig. 1). Thus, the most important factor is the coupling efficiency
for each glycosylation step and the control of the anomeric config-
uration for the newly formed glycosidic bonds, both of which are
imperative in order to reduce the complexity of the reaction.
As reported previously, glycosyl acceptors with a free OH group
6
years, such as fluorinated labels, hydrophobically assisted switch-
ing-phase (HASP) method7 and more recently ionic liquids (ILs).
ILs have attracted growing interest among chemical researchers
over the past few years because of their unique and tunable phys-
ical and chemical properties. As soluble functional supports, the
use of ILs combines the features of solution phase chemistry with
the advantages of chromatography-free purification in oligosac-
8
at C-3 are common intermediates for the synthesis of b-(1 ? 3)-
9
17–22,24
charide synthesis. Several groups have successfully synthesized
glucans.
However, when the adjacent hydroxyl groups at
several classes of oligosaccharides by utilizing ILs as phase-separa-
tion tags.
C-2 and C-4 are both protected by acyl groups, they may show
4
,10–13
19
low reactivity. Moreover,
with glycosyl donors bearing a participating acyl group at O-2.
a-glycoside byproducts may be formed
25
b-(1 ? 3)-Glucans, also known as laminarin polysaccharides,
are a family of homopolysaccharides of glucose widespread in nat-
ure. They are essential constituents of cell walls in fungi and yeasts
Therefore, glycosyl acceptors protected with 4,6-O-benzylidene
and 2-O-acyl groups are comparatively more appropriate for the
1
4–16
17–22,24
as well as major storage polysaccharides in brown seaweeds.
synthesis of such oligosaccharides.
For this work,
trichloroacetimidate 2 was envisioned as a suitable building block
to prepare b-configured glucans. The anomeric leaving group
trichloroacetimidate was chosen for its higher reactivity than
⇑
Corresponding author.
E-mail address: zjli@bjmu.edu.cn (Z. Li).
http://dx.doi.org/10.1016/j.tetlet.2017.03.035
040-4039/Ó 2017 Elsevier Ltd. All rights reserved.
0
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2
Y. Wang et al. / Tetrahedron Letters xxx (2017) xxx–xxx
HO
O HO
HO
O HO
HO
O HO
O
The versatility of functionalized ionic liquid in the synthesis of
HO
O HO
HO
HO
HO
HO
HO
4,10,11
O
O
various oligosaccharides has recently been demonstrated.
In
O
O
O
OH OH
O
OH
OH
OH
this study, modified ionic liquid support 3 was selected since it
OH
OH
HO
has been successfully employed in the synthesis of carbohydrates
1
4
0
in our group. IL support using a,a -dioxyxylyl diether as the linker
O
has several advantages: a) it is stable in the common activation
conditions for trichloroacetimidate and readily removable by
hydrogenolysis; b) it can be coupled via an ether or O-glycosidic
linkage; c) it can be prepared from commercially available
Ph
O
NH
CCl
O
LevO
O
3
N
OBz
N
PF
6
0
4
2
3
a
,
a
-dibromo-p-xylene.
Building block 2 was synthesized in six steps starting from d-
glucose (Scheme 1). Intermediate 5, 6 were prepared according
Fig. 1. Retrosynthetic analysis of Laminarihexaose 1.
2
1
to previously reported procedures. Selective esterification of
the C2 hydroxyl group with benzoyl chloride afforded 7 in 74%
yield. And the C3 hydroxyl group was then protected with a tem-
OH
OH
21
a)
O
O
HO
HO
porary protecting group levulinyl (Lev) to give the key precursor
HO
HO
26
OH
8. Removal of the allyl ether with PdCl
2
was smoothly conducted
buffered solution of acetate. Followed treatment with
trichloroacetonitrile finally afforded trichloroacetimidate building
OH OH
4
O
5
in a
b)
2
7
O
c)
block 2 as anticipated.
Ph
Ph
O
O
O
Ph
O
O
After the synthesis of glycosyl donor 2, we attempted to attach
it to the modified ionic liquid support 3, which was prepared
HO
HO
BzO
O
OH
O
4
according to the previously described procedures.. Generally,
7
6
d)
3 2 2
donor 2 and acceptor 3 were dissolved in dry CH CN/CH Cl at
e)
0 °C, and 0.5 equiv of trimethylsilyl trifluoromethanesulfonate
(
O
O
O
O
Ph
O
TMSOTf) was added as a promoter of the glycosylation reaction.4
O
LevO
LevO
BzO
O
OBz OR
The crude product was obtained by washing with saturated aque-
ous NaHCO and brine to remove all the water-soluble impurities.
Further purification of the product was carried out by concentrat-
ing to form syrup, redissolving in CH Cl and precipitating with
8
3
9
2
R=H
R=Cl
f)
3
CC(NH)
2
2
Scheme 1. Synthesis of glycosylating agent 2. a) allyl alcohol, acetyl chloride,
–70 °C, 53%; b) benzaldehyde dimethylacetal, TsOHꢀH O, anhyd DMF, 60 °C, 39%;
c) Benzoyl chloride, imidazole, CH Cl
laminopyridin, 0 °C, quant.; e) PdCl
isopropyl ether. Thereafter, the solvent was removed partially by
rotary evaporation in vacuo until the remaining solution was about
double to the original CH Cl volume and white precipitate
2 2
appeared, which was immediately collected by centrifugation to
afford the product 10–1 (Scheme 2).
0
2
2
2
, 74%; d) levulinic acid, EDCꢀHCl, 4-dimethy-
2
,
CH
3
COOH/CH
3
COONa/H
2
O, 76%; f)
trichloroacetonitrile, Cs
2
CO
3
2 2
, CH Cl , Ar, 96%.
Following the attachment of a sugar to the IL support, the
IL-supported monosaccharide 10–1 was undergone de-levulinoy-
HO
O
Ph
O
O
O
lation by treatment with N
tion mixture was diluted with CH
saturated aq. NaHCO and brine, dried over Na
2
H
4
ꢀH
2
O and CH
Cl , washed with 1 M HCl aq.,
SO , and then
3
COOH in THF. The reac-
LevO
OBz
2
2
O
TMSOTf/0.5 eq
Ph
O
NH
CCl
3
2
4
O
LevO
+
CH CN/CH Cl (1/10)
N
3
2
2
filtered. Thus, the acceptor 10–2 was obtained after simple evapo-
ration in vacuo, which was sufficiently pure for further reactions.
Unfortunately, the glycosylation of 11–1 with donor 2 was
unsuccessfully under the same condition as in Scheme 2. The C3
hydroxyl group of 11–1 was blocked by TMS ether (yield: 30%)
and the expected IL-supported disaccharide 10–2 was obtained
only in a low yield (60%). (Scheme 3). Therefore, we performed
optimization study on the coupling condition to increase the reac-
tion yield (Table 1).
OBz O
3
o
N PF
6
0
9
C
0%
N
N PF
6
2
3
10-1
Scheme 2. Synthesis of 10–1.
thioglycoside and for its successful application in IL supported
oligosaccharide synthesis.4 The benzoyl ester at C2 position was
utilized to ensure the stereoselectivity of b-glucosidic linkages.
As described above, benzylidene acetal was introduced for the pro-
tection of the C4 and C6 hydroxyl groups,
Lev) group was chosen for the temporary protection of the C3
1
7,18
As shown in Table 1, when triethylsilyl trifluoromethanesul-
fonate (TESOTf) was used as the promoter in place of TMSOTf,
1
5,22,23
while levulinyl
0
(
the TMS ether byproduct 11-1 was not observed, but the yield
hydroxyl group based on its stability to acids and the mild condi-
was still low (Table 1, entry 2). Gratefully, we found that BF
was an appropriate promoter to increase the glycosylation yield,
3
ꢀEt
2
O
tions required for its removal.26
O
O
Ph
O
O
Ph
O
O
O
O
Ph
O
O
O
Ph
O
O
TMSO
O
O
O
LevO
OBz
HO
OBz
OBz
OBz
O
Ph
O
NH
CCl
O
LevO
+
OBz
O
3
N
N
N
N
PF6
PF
6
2
11-1
10-2
N
11-1'
N PF
6
Scheme 3. Synthesis of disaccharide 10–2.
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Y. Wang et al. / Tetrahedron Letters xxx (2017) xxx–xxx
3
Table 1
Optimization of the glycosylation conditions.
Entry
Solvent
Promoter (equiv)
Temp (°C)
Yieldb (%)
1
2
3
4
5
6
7
8
CH
CH
CH
CH
CH
CH
CH
CH
2
2
2
2
2
3
2
2
Cl
Cl
Cl
Cl
Cl
2
2
2
2
2
TMSOTf (0.5)
TESOTf (0.5)
0
0
0
0
0
0
66
61
90
69
23
84
–
BF
3
BF
3
BF
3
BF
3
BF
3
BF
3
ꢀEt
2
ꢀEt
2
ꢀEt
2
ꢀEt
2
ꢀEt
2
ꢀEt
2
O (0.5)
O (0.3)
O (0.1)
O (0.5)
O (0.5)
O (0.5)
a
CN/CH
2
Cl
2
Cl
Cl
2
ꢁ20
2
r.t.
85
a
CH
3
CN/CH
2 2
Cl , 1:10(v/v).
b
0
0
Yield of 10–2; the byproduct was 11-1 (yield: 30%) in entry 1; 11-1 was not found in entries 2–8.
O
(Table 1, entry 6). Moreover, the reaction went efficiently when
the reaction temperature was between 0 °C and room temperature
Ph
O
O
O
n
Lev
O
OBz
(Table 1, entry 8). Thus, following the optimized conditions in
O
Ph
O
NH
CCl
Table 1 entry 3, IL-supported disaccharide 10–2 was prepared,
with its structure and purity confirmed by NMR and MS analysis.
The glycosylation condition for IL-supported synthesis was finally
determined as follows: 2.0 equiv of trichloroacetimidate donor
O
LevO
OBz
O
3
N
a), b)
Ph
N
PF
6
2
10-n
O
with 0.5 equiv of BF
0.5 h. The same post-treatment and precipitation purification tech-
nique was used as in the preparation of 10–1.
3
ꢀEt
2 2 2
O as promoter in dry CH Cl at 0 °C for
c)
O
O
O
H
O
OBz
n
Following the routes illustrated in Scheme 4, b-(1 ? 3) glucan
laminarihexaose 10–6 was synthesized with glycosyl trichloroace-
timidates 2 as donors in consecutive glycosylation reactions. The
N
1
13
structure of 10–6 was supported by H and C NMR spectra and
N
PF
6
11-n
d)
further confirmed by MALDI-TOF-MS analysis, which showed a
+
m/z at 2425.7410 [MꢁPF
6
] . Details of each step are listed in
HO
O HO
HO
O HO
HO
HO
O HO
O
Table 2. Hexamer 10–6 was prepared in average yields of 90–94%
per step. The short reaction times, about 2.5 h per monomer addi-
tion, allowed the synthesis of 10–6 in 53.9% overall yield within
HO
HO
HO
HO
HO
O
O HO
O
O
O
O
O
OH OH
OH
OH
OH
OH
OH
1
5 h. In comparison with the previously published liquid phase
Scheme 4. IL-supported assembly of b-(1 ? 3)-glucan laminarihexaose. a) glyco-
sylation; b) purification; c) removal of Lev group; d) deprotection.
synthesis of b-(1 ? 3)-glucan oligosaccharides, our method is
1
5
much more faster. Moreover, the reaction time is similar to the
automated solid-phase synthesis method for oligosaccharides,
while the equivalents of glycosyl donors (2 equiv) and promoter
1
5
without the formation of TMS ether (Table 1, entries 3–8). H NMR
(0.5 equiv) used were much less.
analysis indicated that the coupling reaction was highly efficient
The purity of 10–6 was analyzed by HPLC as in our previous
work. Due to the unique properties of ionic liquid species, mobile
4
when 0.5 equiv of BF
3
ꢀEt
2
O was used (Table 1, entry 3). As donor
Cl , co-solvent
CN was no longer needed for the glycosylation reaction
2
and acceptor 11–1 were both easily soluble in CH
2
2
phases of 0.1% trifluoroacetic acid (TFA) in acetonitrile and water
1
1
CH
3
were used. As shown in Fig. 2, the glycosylation and purification
Fig. 2. HPLC anlysis of prepared 10–6 following ionic liquid supported purification. Mobile phase A: 0.1% TFA in water, B: 0.1% (TFA) in acetonitrile, flow rate: 1 mL/min by
0–50% B, UV absorbance at 254 nm, on an Agilent SB-C18 column.
2
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4
Y. Wang et al. / Tetrahedron Letters xxx (2017) xxx–xxx
Table 2
IL-supported assembly of b-(1 ? 3)-glucan laminarihexaose.
n
1
Operationb
Product
Timea (min)
Recovery (%)
A and B
C
A and B
C
A and B
C
A and B
C
A and B
C
A and B
C
10–1
11–1
10–2
11–2
10–3
11–3
10–4
11–4
10–5
11–5
10–6
11–6
60 + 30 + 30
30
60 + 30 + 30
30
60 + 30 + 30
30
60 + 30 + 30
30
60 + 30 + 30
30
60 + 30 + 30
30
90
>99
90
>99
90
>99
93
>99
94
>99
90
>99
53.9
2
3
4
5
6
Total
900
a
The time includes the preparation time before glycosylation (60 min),the time of glycosylation (30 min), and the time of purification (30 min).The time for removal of Lev
protecting groups is about 30 min.
b
A:glycosylation. B: purification. C: removal of Lev group.
cycle gave product 10–6 in 70.8% purity, with retention time at
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Eventually, the target Laminarihexaose 1 was obtained after
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easily with sodium methoxide, followed by successful cleavage of
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benzylidene group with a CH
0 °C. After removal of the remaining IL support through catalytic
hydrogenolysis, the fully deprotected b-(1 ? 3)-glucan Laminari-
3 2
COOH/H O (9:1, v/v) system at
7
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1
13
hexaose 1 was finally achieved. The analytical data ( H and
C
NMR spectra) of 1 were identical to that reported for the natural
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laminarihexaose, with its structure also confirmed by HRMS (ESI-
+
FT-ICR) analysis (m/z calcd for C36
found 1013.3166).
H
62NaO31 [M+Na] 1013.3173,
2
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Conclusions
In summary, a rapid synthesis of b-(1 ? 3)-glucan on ionic liq-
uid support was developed. The optimal glycosylating promoter
was screened. Glycosyl trichloroacetimidate 2 and ionic liquid sup-
ported 3 were shown to be a suitable combination for the stereo-
controlled synthesis of 10–6 with an average yield of more than
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target linker-equipped b-(1 ? 3)-glucan Laminarihexaose 1. Our
method for the efficient synthesis of large oligosaccharides on IL
support could be a very useful technique to produce oligosaccha-
rides on a large scale.
2
1
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This work was supported by grants from the National Basic
Research Program of China (Grant No. 2012CB822100) and the
National Natural Science Foundation of China (NSFC No.
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A. Supplementary data
2
2
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2017.03.
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Please cite this article in press as: Wang Y., et al. Tetrahedron Lett. (2017), http://dx.doi.org/10.1016/j.tetlet.2017.03.035