Silyl-assisted 1,2-cis-α-glucosylation for the synthesis of a triglucoside moiety in high-mannose-type oligosaccharides

Article abstract of DOI:10.1039/c5ra16659d

A highly stereoselective 1,2-cis-α-glucosylation reaction was developed, in which the use of a strongly electron-donating tert-butyldimethylsilyl protecting group on the C-2 hydroxy group of a glycosyl donor enhanced the α-favoured transition state, and thus resulted in high yield and α-selectivity. Synthetic utility of this α-glucosylation was demonstrated by the generation of a triglucoside moiety in high-mannose-type oligosaccharides.

Full text of DOI:10.1039/c5ra16659d

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Silyl-assisted 1,2-cis-a-glucosylation for the
synthesis of a triglucoside moiety in high-
Cite this: RSC Adv., 2015, 5, 75918
mannose-type oligosaccharides†
a
Kiichiro Totani, Yuki Shinoda,a Masaaki Shiba,a Shogo Iwamoto,a Akira Koizumi,a
Received 18th August 2015
Accepted 2nd September 2015
*
Yuji Matsuzakib and Makoto Hiranoa
DOI: 10.1039/c5ra16659d
www.rsc.org/advances
A highly stereoselective 1,2-cis-a-glucosylation reaction was devel- oligosaccharide that regulates the glycoprotein quality control.7
oped, in which the use of a strongly electron-donating tert-butyldi- However, the stereoselective construction of a Glca1-2Glca1-3Glc
methylsilyl protecting group on the C-2 hydroxy group of a glycosyl segment with sequential 1,2-cis-a-glucoside linkages in high
donor enhanced the a-favoured transition state, and thus resulted in yield was non-trivial, as glycosylation to generate the triglucoside
high yield and a-selectivity. Synthetic utility of this a-glucosylation was could only be obtained in a maximum yield of 45%. Therefore,
demonstrated by the generation of a triglucoside moiety in high- we developed a new 1,2-cis-a-glucosylation reaction that is
mannose-type oligosaccharides.
mediated by enhancing the a-favoured transition state via an
electron-donating tert-butyldimethylsilyl (TBS) group8 at the
O2-position of the glycosyl donor. To demonstrate the efficiency
of this glycosylation reaction, we also synthesized Glca1-2Glca1-
3Glc through sequential silyl-assisted a-glucosylation.
For that purpose, we designed glycosyl donor 3 with an
electron-donating protecting group, as well as efficient synthetic
pathways to 3 and acceptor 4 from the common key interme-
diate 5 (Scheme 1). Computational studies by Nukada et al.
showed that the conformational effect of the 4,6-O-benzylidene
1,2-cis-a-Glycoside linkages represent a major subject in oligo-
saccharide synthesis,1 as they are oen encountered in bioactive
natural glycoconjugates,2 e.g. a-glucoside and a-fucoside in N-
linked glycoproteins, or a-N-acetyl-galactosaminide in O-linked
glycoproteins. Moreover, glycolipids frequently contain 1,2-cis-a-
glycosides. In the synthesis of oligosaccharides, the formation of
most 1,2-cis-a-glycosides has been explained by the so-called
thermodynamic anomeric effect,3 even though the origin of this
effect is still controversial.4 Since the anomeric effect is still under
ongoing investigation, it is oen difficult to achieve 1,2-cis-a-
glycosylation in high yield and stereoselectivity. The optimization
of glycosylation conditions is usually a time-consuming task,
because yield and stereoselectivity are affected by numerous
factors such as protecting groups, promoters, solvents, and
temperature. In this context, Ishiwata et al. proposed high-
throughput screening for optimized solvent systems in 1,2-cis-
a-glycosylation using Bn-d7 protecting groups.5 Their results
showed that yield and the a/b selectivity uctuated between 0 and
100%, as well as between 1 : 1 and 20 : 1, respectively. These
ndings illustrate the limitations of the practical use of the
anomeric effect for the formation of 1,2-cis-a-glycoside linkages.
We have previously reported the synthesis of Glc3Man9-
GlcNAc2 (ref. 6) (Scheme 1), which is
a glycoprotein
aDepartment of Materials and Life Science, Seikei University, 3-3-1 Kichijoji-kitamachi,
Musashino-shi, Tokyo 180-8633, Japan. E-mail: ktotani@st.seikei.ac.jp
bGlyco Synthetic Lab., Tokyo Chemical Industry Co., Ltd., 6-15-9 Toshima, Kita-ku,
Tokyo 114-0003, Japan
† Electronic supplementary information (ESI) available: Experimental details and
characterization data for new compounds. See DOI: 10.1039/c5ra16659d
Scheme 1 Retrosynthetic analysis for the formation of triglucoside 1
by sequential silyl-assisted glucosylation.
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group should induce a 4H3 chair conformation for the gluco-type
Subsequently, we prepared a series of glycosyl donors (3a–d)
oxocarbenium ion intermediate.9 We decided to use benzyli- and one acceptor (4; Scheme 3). Although 3a–d can be obtained
dene glycosyl donor 3 to reduce the number of intermediates, as from 5, a one-step reduction from 12 would be more practical.
this thioglycoside should exhibit advantageous stability in such Oxidative deprotection of the MP group in 12 provided 13, fol-
a multi-step oligosaccharide synthesis. A series of glycosyl lowed by the formation of a b-selective thioglycoside using 2-
donors, 3a–d (Scheme 3), with different electron-donating chloro-1,3-dimethyl-2-imidazolinium chloride (DMC)10 to give
groups at the O2-position should allow the examination of 14. For the protection of the 2-OH group in 14, we selected
their inuence on the reactivity and stereoselectivity in the several benign electron-donating ethers as protecting groups.
glycosylation with 4. Combining these results with further Initially, we introduced a benzyl group as a typical ether-type
investigations regarding the solvent effect on the glycosylation protecting group to afford glycosyl donor 3a. Subsequently, we
should allow a discussion of the steric and electronic effects of focused on silyl ether groups, as we expected a strong electron
the protecting group. Deprotection at the O20-position of the donation from such groups. As the steric bulk is another
resulting disaccharide should afford disaccharide acceptor 2, tunable feature of silyl ethers, we introduced triethylsilyl (TES),
followed by a second glycosylation with donor 3 to furnish the TBS, and triisopropylsilyl (TIPS) groups with varying degrees of
targeted trisaccharide 1.
steric demand to afford glycosyl donors 3b, 3c, and 3d,
First, we synthesized key intermediate 5 as a glycosyl donor respectively. Although the introduction of bulky silyl groups
and acceptor (Scheme 2). Allylation of 3-OH in diacetone-D- imposes structural strain on the pyranose ring conformation in
glucose 6 provided 7, which was subjected to acidic cleavage of some cases,11 3b, 3c, and 3d maintained a 4C1 conformation as
the isopropylidene groups to afford pyranosidic 8. Aer per- evident from the J couplings of the pyranose ring protons in the
acetylation of the hydroxyl groups to furnish 9, conversion of 1H NMR spectra. Finally, allyl deprotection of key intermediate
the anomeric acetate into a methoxy phenyl (MP) derivative 5 provided glycosyl acceptor 4, and thus the substrate synthesis
yielded 10. Aer deprotection of the Ac groups in 10, resulting for the glycosylation was completed.
in the formation of 11, the 4,6-OH groups were protected with a
We then proceeded to examine the inuence of protecting the
benzylidene group to generate 12. Finally, acetylation of the O2 groups of donor 3 on the reactivity and stereoselectivity of the
2-OH group in 12 yielded key intermediate 5. This regiose- glycosylation reaction with acceptor 4 (Table 1). The promoter
lectively deprotectable compound is also commercially avail- was selected from a range of metal free reagents, in order to
able from Tokyo Chemical Industry Co. (Tokyo, Japan).
avoid the possibility of affecting the stereoselectivity through
coordination of the reaction intermediate with metal.
a
Although DMTST12 and NIS–TfOH13 are well known, strong,
metal-free activation agents, these active species are prepared
in situ, and small amounts of their unreacted precursors might
be still present in the reaction mixture. To eliminate potential
side reactions with these precursors, we chose MeOTf as a
promoter.14 Considering that the resulting stereoselectivity
originates from the donor structure, non-coordinative CH2Cl2
was selected as the reaction medium. A reaction mixture con-
taining O2-benzylated donor 3a, acceptor 4, and MeOTf at 10 ꢀC
provided disaccharide 15a in 85% yield (entry 1). Although the
desired a-linkage was generated preferentially as reported for
similar 4,6-O-benzylidene-2-O-benzylatedglucosyl donors,15 the
observed stereoselectivity was only moderate (a/b ¼ 82 : 18)
under our reaction conditions. In case of TES-substituted donor
3b, deprotection of the TES group was observed, and hence this
donor was considered unsuitable for the glycosylation condi-
tions (entry 2). Subsequently, the glycosylation with
TBS-substituted 3c was examined, and the formation of the
desired disaccharide 15c was observed in excellent yield (96%)
and a-selectivity (a/b $ 95 : 5; entry 3). Although steric conges-
tion around the anomeric position by the bulky TBS group might
induce decreased reactivity, the required reaction temperature
(0 ꢀC) was lower than that for benzylated donor 3a (10 ꢀC). This
result suggested that the steric congestion imposed by the TBS
group does not block the anomeric position. In contrast, the
presence of the TBS group enhances the reactivity through
stabilization of the cationic intermediate by electron donation.
Using low temperature NMR spectroscopy based on the primary
13C kinetic isotope effect in combination with computational
Scheme 2 Synthesis of key intermediate 5.
Scheme 3 Synthesis of glycosyl donors 3a–d and acceptor 4.
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methods, Crich and co-workers revealed that the activation of a Yamada reported independently the conformational arming
4,6-O-benzylidene-glucosyl and mannosyl donors with Tf2O effect of glycopyranosyl donors via conformational changes of
initially provide stable a-triates in equilibrium with an the donor, mediated by the introduction of multiple bulky silyl
a-contact ion pairs (a-CIP), before a subsequent SN2-like attack groups.19 As our reaction system does not include any confor-
of
a
nucleophile provides the b-glucosides.16 Conversely, mational changes of the donor, the increased reactivity should
formation of the a-glucoside must arise from a b-triate, which be due to a stabilization of the oxocarbenium ion by the electron
was not observed by VT-NMR. They also showed that this can donation from the TBS group. Conversely, the effect of the TBS
occur, if the a-triate exists in a rapidly exchanging equilibrium group on the a-selectivity should lead to (1) a shi of the equi-
with its less stable, but more reactive b-CIP or the b-triate via librium of the active intermediate towards the b-CIP due to the
the solvent separated ion pair (SSIP), i.e. the oxocarbenium ion.16 electron donation from the TBS group, and (2) an induced
Based on these results, the a-selectivity observed for entry 3 may a-glycoside formation due to the steric congestion of the TBS
result in a shi of the equilibrium towards the b-CIP through group at the b-face of the anomeric position. Therefore, we
stabilization of a SSIP by the electron donation from the TBS examined a potential TBS-induced steric congestion around the
group. In fact, Crich and co-workers reported the opposite result anomeric position by comparison of solvent effects on the
for the mannosyl donor, i.e. that a non-participating electron- glycosylation reaction (Table 2). For that purpose, we deter-
withdrawing group at the O2-position destabilizes the forma- mined the stereoselectivity of the glycosylation reaction between
tion of a SSIP and results in a b-selective glycosylation through a donor 3c and acceptor 4 using benign solvents (CH2Cl2, toluene),
stable a-triate,17 which is not in conict with our hypothesis. a b-participating solvent (Et2O),20 and an a-participating solvent
However, the question of how the steric bulk of the silyl group (MeCN).21 The reaction in CH2Cl2 provided the desired disac-
affects the glycosylation still requires to be answered. Therefore, charide 15c in high yield and a-selectivity (entry 1). Toluene also
we examined the effect of the more bulky TIPS group in donor 3d furnished the a-product in high stereoselectivity, albeit in lower
(entry 4). For the corresponding reaction, a highly a-selective yield (entry 2). Although differences in substrate solubility
glucosylation to give disaccharide 15d was observed, even should decrease the reactivity to some extend, generally an
though the decreased reactivity caused a lower yield (50%) aer a-enhancing effect of the TBS group was observed in nonpolar
comparable reaction time. In spite of the steric congestion and aprotic solvents. Subsequently, we used the polar, aprotic,
around the anomeric position induced by the TIPS group, which b-participating solvent Et2O (entry 3), which resulted in high
caused the decreased reactivity, the observed highly stereo- a-selectivity and low yield (20%). This can be explained by a
selective generation of the 1,2-cis-a-glucoside instead of the b-orientation of Et2O with respect to the oxocarbenium ion. This
1,2-trans-b-glucoside is interesting. The electronic effect of the orientation would block the b-face as well as decrease the elec-
silyl group at the O2-position on the stereoselectivity seemed to trophilicity and thus the reactivity. Accordingly, there seems to
be unexpectedly strong.
be sufficient space around the b-face at the anomeric position to
Therefore, we examined the effect of the TBS group in detail. coordinate a solvent molecule, which suggested that the
For this silyl protecting group, applications in carbohydrate conformation of the TBS group does not induce steric over-
chemistry have been reported by Yang and co-workers, who congestion. Using the a-participating, polar, and aprotic
demonstrated that the reactivity of arabinofranosyl thioglyco- solvent MeCN, the reactivity was decreased on account of a
side was enhanced by the introduction of TBS groups at the O3- diminished electrophilicity similar to Et2O, and a yield of 21%
and O5-position,18 even though they did not comment on the was observed (entry 4). Moreover, the stereoselectivity was
origin of the increased reactivity. The groups of Bols and decreased as a result of the a-coordination of MeCN to provide
an a/b ratio of 48 : 52. Results from entries 3 and 4 indicated that
enough space is available around both the a- and b-face at the
anomeric position, suggesting no apparent correlation between
steric bulk of the TBS group and the stereoselectivity.
Table 1 Effect of the protecting group at the C-2 OH-group of
glycosyl donor 3 on the glycosylation with glycosyl acceptor 4a
Table 2 Solvent effect on the glycosylation using glycosyl donor 3 and
acceptor 4a
Entry
Glycosyl donor
Solvent
Product
Yield (%)
a/bf
1
2
3
4
5
6
3c
3c
3c
3c
3a
3a
CH2Cl2
Toluene
Et2O
CH3CN
CH2Cl2
Et2O
15c
15c
15c
15c
15a
15a
96
>95 : 5
>95 : 5
>95 : 5
48 : 52
82 : 18
93 : 7
Entry
Glycosyl donor
R
Product
Yield (%)
85
a/bd
41b
20c
21d
85
1
2
3
4
3a
3b
3c
3d
Bn
15a
15b
15c
15d
82 : 18
—
>95 : 5
>95 : 5
b
TES
TBS
TIPS
—
96
50c
11e
a
a
Reagents and conditions: MeOTf, MS4A, CH2Cl2, 10 ꢀC (for entry 1),
Reagents and conditions: MeOTf, MS4A, CH2Cl2, 0 ꢀC (for entries 1–4),
b
0
ꢀC (for entries 2–4), 32 h. 56% of 4 was recovered. Deprotection of 10 ꢀC (for entries 5 and 6), 32 h. b 45% of 4 was recovered. c 58% of 4 was
c
d
d
e
f
the TES group was observed. 38% of
4
was recovered. The recovered. 63% of 4 was recovered. 83% of 4 was recovered. The
1
1
stereochemistry and stereoselectivity were determined by H NMR.
stereochemistry and stereoselectivity were determined by H NMR.
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Scheme 4 Synthesis of triglucoside 1 by sequential silyl-assisted
glucosylations.
TBS-assisted glucosylation reaction, a TBS group at the 2-OH-
position of the glycosyl donor can be considered as a more
effective approach to induce 1,2-cis-a-glycosylation of 3c with
several glycosyl acceptors were also examined (Table 3). All the
examples of glycosylation with galactosyl acceptor 16 (entry 1),
mannosyl acceptor 17 (entry 2) glucosyl acceptor 18 (entry 3)
Fig. 1 Proposed mechanism for the glycosylation of 3c, wherein
electron donation from the silyl ether of 3c should stabilize cationic
intermediates.
Based on these results, a reaction mechanism for the 1,2-cis- and 19 (entry 4) gave the good to excellent a-selectivities.
a-glucosylation is proposed (Fig. 1). Treatment of 3c with MeOTf
Finally, we used a sequential silyl-assisted a-glucosylation to
should initially generate the a-triate B via the temporary synthesize triglucoside 1 (Scheme 4). The rst silyl-assisted
sulfonium salt A. Electron donation from the TBS group should glycosylation to form an a-1,2-linkage between donor 3c and
stabilize the cationic intermediates C, D and E, and a subse- acceptor 4 provided disaccharide 15c in 96% yield. A subse-
quent SN2-like attack of acceptor 4 at the more reactive b-CIP E quent deprotection of the TBS group with TBAF/AcOH to give
should provide the a-glucoside. We further examined the acceptor 2 proceeded without acetyl migration. The following
solvent effect on the glycosylations using 2-O-Bn donor 3a second silyl-assisted glycosylation of donor 3c with acceptor 2
(Table 2). The observed stereoselectivity in CH2Cl2 (a/b ¼ generated an a-1,3-linkage to afford the desired trisaccharide 1
82 : 18; entry 5) was lower than that in Et2O (a/b ¼ 93 : 7; entry in 80% yield.
6). However, using Et2O decreased the reactivity and yielded 15a
In conclusion, we developed a novel 1,2-cis-a-glucosylation
in only 11%. Thus, the introduction of glucosylation compared reaction that is mediated by a TBS-assisted stabilization of the
to an optimization of the solvent system. For generality of the cationic intermediate, which is favourable for an a-attack of the
glycosyl acceptor. This method was used to generate the Glca1-
2Glca1-3Glc moiety of a high-mannose tetradecasaccharide. In
Table 3 Coupling reactions with the TBS-assisted glucosyl donor 3ca
previous synthetic approaches, generation of Glca1-2Glc via
glycosyl uoride yielded in 90%, whereas subsequent glycosyla-
tion using thioglycoside with the optimized reaction conditions
to obtain such trisaccharides did not exceed yields of 45%.6
Conversely, the present method allows the sequential use of the
same silyl-assisted donor to form both a-1,3 and a-1,2-linkages
with high yield and high 1,2-cis-a-selectivity to increase the effi-
ciency of this synthetic pathway. Thus, regioselectively depro-
tectable feature of donor 3c was useful for further glycosylation at
any positions of the OH-groups. Moreover, 2-OH, 3-OH and 6-OH
gluco-type, 4-OH galacto-type and 2-OH manno-type acceptors
were widely acceptable for this silyl-assisted a-glucosylation. In
principle, our silyl-assisted approach could also be applied to
construct a-fucoside and a-galactoside. Further studies in these
areas are in progress and will be reported in due course.
Entry
Glycosyl acceptor
Product
Yield (%)
a/bd
1
20
77b
93 : 7
2
3
21
22
43c
82
92 : 8
>95 : 5
4
23
96
>95 : 5
Acknowledgements
a
Reagents and conditions: MeOTf, MS4A, CH2Cl2, 10 ꢀC, 32 h. b 22% of
This work was supported by a Grant-in-Aid for Young Scientists
(A) and a Grant-in-Aid for Challenging Exploratory Research
from the Japan Society for the Promotion of Science to K. T. (No.
23681049 and 25560420).
the glycosyl acceptor 16 was recovered. c 50% of the glycosyl acceptor 17
including the Ac-migrated 2,3,4,6-tetra-O-Ac form (ca. 1 : 1) were
d
recovered.
The stereochemistry and stereoselectivity were
determined by 1H NMR.
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