Boronic-Acid-Catalyzed Regioselective and 1,2- cis -Stereoselective Glycosylation of Unprotected Sugar Acceptors via SNi-Type Mechanism

Article abstract of DOI:10.1021/jacs.7b12108

Regio- and 1,2-cis-stereoselective chemical glycosylation of unprotected glycosyl acceptors has been in great demand for the efficient synthesis of natural glycosides. However, simultaneously regulating these selectivities has been a longstanding problem in synthetic organic chemistry. In nature, glycosyl transferases catalyze regioselective 1,2-cis-glycosylations via the S_Ni mechanism, yet no useful chemical glycosylations based on this mechanism have been developed. In this paper, we report a highly regio- and 1,2-cis-stereoselective S_Ni-type glycosylation of 1,2-anhydro donors and unprotected sugar acceptors using p-nitrophenylboronic acid (10e) as a catalyst in the presence of water under mild conditions. Highly controlled regio- and 1,2-cis-stereoselectivities were achieved via the combination of boron-mediated carbohydrate recognition and the S_Ni-type mechanism. Mechanistic studies using the KIEs and DFT calculations were consistent with a highly dissociative concerted S_Ni mechanism. This glycosylation method was applied successfully to the direct glycosylation of unprotected natural glycosides and the efficient synthesis of a complex oligosaccharide with minimal protecting groups.

Full text of DOI:10.1021/jacs.7b12108

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Article
Boronic Acid Catalyzed Regio- and 1,2-cis-Stereoselective
N
Glycosylation of Unprotected Sugar Acceptors via Si-type Mechanism
Masamichi Tanaka, Akira Nakagawa, Nobuya Nishi, Kiyoko
Iijima, Ryuichi Sawa, Daisuke Takahashi, and Kazunobu Toshima
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b12108 • Publication Date (Web): 19 Feb 2018
Downloaded from http://pubs.acs.org on February 24, 2018
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Boronic Acid Catalyzed Regio- and 1,2-cis-Stereoselective Glycosyla-
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tion of Unprotected Sugar Acceptors via SNi-type Mechanism
Masamichi Tanaka,† Akira Nakagawa,† Nobuya Nishi,† Kiyoko Iijima,‡ Ryuichi Sawa,‡ Daisuke
Takahashi,*,† and Kazunobu Toshima*,†
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†Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku,
Yokohama, 223-8522, Japan
‡Institute of Microbial Chemistry (BIKAKEN), 3-14-23 Kamiosaki, Shinagawa-ku, Tokyo 141-0021, Japan
KEYWORDS. Glycosylation, 1,2-cis-Glycoside, Boronic acid, SNi reaction, Unprotected sugar
ABSTRACT: Regio- and 1,2-cis-stereoselective chemical glycosylation of unprotected glycosyl acceptors has been in great
demand for the efficient synthesis of natural glycosides. However, simultaneously regulating these selectivities has been a
long-standing problem in synthetic organic chemistry. In nature, glycosyl transferases catalyze regioselective 1,2-cis-
glycosylations via the SNi mechanism, yet no useful chemical glycosylations based on this mechanism have been devel-
oped. In this paper, we report a highly regio- and 1,2-cis-stereoselective SNi-type glycosylation of 1,2-anhydro donors and
unprotected sugar acceptors using p-nitrophenylboronic acid (10e) as a catalyst in the presence of water under mild con-
ditions. Highly controlled regio- and 1,2-cis-stereoselectivities were achieved via the combination of boron-mediated car-
bohydrate recognition and the SNi-type mechanism. Mechanistic studies using the KIEs and DFT calculations were con-
sistent with a highly dissociative concerted SNi mechanism. This glycosylation method was applied successfully to the
direct glycosylation of unprotected natural glycosides and the efficient synthesis of a complex oligosaccharide with mini-
mal protecting groups.
To overcome this obstacle, regio- and stereoselective
chemical glycosylations of unprotected or minimally pro-
INTRODUCTION
Carbohydrates play important roles in many biological
processes,1 including cell proliferation and communica-
tion, tumor metastasis, immune recognition, anticoagula-
tion, and microbial pathogenesis. However, understand-
ing these roles at the molecular level has been rather slow
relative to comparable studies of proteins and nucleic
acids due to the lack of amplification and expression
methods of natural glycans. Therefore, to accelerate bio-
logical research of carbohydrates, great efforts have been
devoted to the development of efficient methods for the
chemical synthesis of homogeneous and structurally de-
fined glycosides.2 While complex natural glycosides have
been synthesized in a highly stereoselective manner,3 the-
se synthetic schemes were highly dependent on protect-
ing group strategies to control the regio- and stereoselec-
tivities of each glycosylation step, leading to a decrease in
step and atom economies4 of the overall process. There-
fore, regio- and stereoselective chemical glycosylation
without any protecting groups has been in great demand.
However, the difficulty in simultaneously regulating the
glycosylation site of an unprotected glycosyl acceptor
with high regioselectivity and controlling the stereochem-
istry of the anomeric center with high stereoselectivity is
one of the biggest obstacles in protecting-group-free gly-
cosylation.
tected sugar acceptors have been developed. The repre-
sentative methods of regioselective glycosylations were
based on the regioselective enhancement of nucleophilici-
ty of the desired hydroxyl group using the recognition
ability of organotin5 or organoboron6 compounds. In 1999,
regio- and 1,2-trans-stereoselective Koenigs-Knorr type
glycosylation of unprotected sugar acceptors via aryl-
boronic activation was reported by Aoyama et al.6a Re-
cently, Taylor et al.6b reported a similar but elegant meth-
od, i.e., borinic acid catalyzed regio- and 1,2-trans-
stereoselective glycosylation of minimally protected sugar
acceptors through the SN2-type mechanism. In a different
approach, Kaji et al. reported regio- and 1,2-trans-
stereoselective glycosylations using arylboronic acid as a
transient masking reagent,7 which decreased the nucleo-
philicity of undesired hydroxyl groups by the formation of
a corresponding cyclic boronic ester. These glycosylation
methods require neighboring group participation from
the 2-O-acyl functionality of the glycosyl donor to pro-
duce a 1,2-trans-glycoside with high stereoselectivity. Re-
cently, Miller et al.8 reported high regio- and 1,2-trans-
stereoselective glycosylation of an unprotected sucrose
and sucrose derivatives with a glycosyl fluoride under
aqueous conditions. The obtained high regio- and 1,2-
trans-stereoselectivities could be attributed to the inter-
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Figure 1. Regio- and 1,2-cis-stereoselective enzymatic and proposed chemical glycosylations of unprotected sugar acceptors.
action between a calcium ion and the sucrose moiety.
However, in these chemical strategies, there are few re-
gio- and 1,2-cis-stereoselective glycosylations of unpro-
tected sugar acceptors.5a,9
Therefore, the SNi-type mechanism could provide a new
platform for stereoselective chemical glycosylation.
In this context, we expected that the combination of the
selective recognition24 of a diol moiety in an unprotected
glycosyl acceptor with a boronic acid and the SNi-type
mechanism could permit regio- and 1,2-cis-stereoselective
glycosylation of unprotected glycosyl acceptors. Herein,
we report the regio- and 1,2-cis-stereoselective glycosyla-
tion of unprotected glycosyl acceptors utilizing a boronic
acid catalyst via the SNi-type mechanism (Figure 1b). A
catalytic amount of arylboronic acid 2 selectively and re-
versibly binds to a diol moiety in an unprotected glycosyl
acceptor 1 to give arylboronic ester 3. The formed Lewis
acidic 3 activates the epoxide of 1,2-anhydro donor 4.
Concomitantly, one of the B-O moieties, which has easy
access to the anomeric position on the SNi-type transition
state, is stereospecifically glycosylated from the same face
as the 2-O-functionality to produce arylboronic ester 5.
An ester exchange reaction between 5 and 1 regenerates 3
and provides the desired 1,2-cis-glycoside 6. To the best of
our knowledge, this is the first example of a regio- and 1,2-
cis-stereoselective SNi-type chemical glycosylation of un-
protected glycosyl acceptors.
The stereoselective synthesis of 1,2-cis-glycosides is chal-
lenging because glycosylations using glycosyl donors that
have non-assisting functionality at C2 generally produce
anomeric mixtures of glycosides.10 Therefore, various
strategies for 1,2-cis-stereoselective glycosylation have
been reported, e.g., intramolecular aglycon delivery
(IAD),11 conformationally restrained methods,12-15 six-
membered ring neighboring group participation,16 hydro-
gen bond mediated aglycon delivery (HAD)17 and remote
participation.18
Recently,
we
reported
1,2-cis-
stereoselective glycosylations utilizing glycosyl-acceptor-
derived boronic19 or borinic20 ester catalysts. However,
these methods have not been applied to regio- and 1,2-cis-
stereoselective glycosylations of unprotected glycosyl ac-
ceptors.
In nature, glycosyl transferases catalyze 1,2-cis-
glycosylations21 of nucleotide sugars and unprotected gly-
cosyl acceptors with excellent regio- and stereoselectivi-
ties via the well-known double-displacement mechanism
or SNi-type mechanism, which has recently been studied
intensively22 (Figure 1a). The SNi-type glycosylation pro-
ceeds with complete stereospecificity due to the synchro-
nicity of the activation of the leaving group in the glycosyl
donor and the front-side nucleophilic attack of the glyco-
syl acceptor without the assistance of the nucleophilic
residue required in the double-displacement mechanism.
However, no stereoselective chemical glycosylation
methods based on the SNi-type mechanism have been
developed except for the solvolysis of a glucosyl fluoride.23
RESULTS AND DISCUSSION
We first selected 3,4,6-tri-O-benzyl-1,2-anhydroglucose
(7) and D-glucal (8) as the glycosyl donor and the unpro-
tected glycosyl acceptor, respectively. Under the reported
reaction conditions,19a a glycosyl acceptor and a catalytic
amount of p-methoxyphenylboronic acid (10a) were re-
fluxed in toluene for 3 hours to prepare the glycosyl-
acceptor-derived boronic ester. A solution of the glycosyl
donor in acetonitrile was then added and the mixture was
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Table 1. Regio- and stereoselective glycosylations of unprotected glycosyl acceptor 8.
1
2
3
4
5
6
7
8
9
Entry
R
X
H2O (equiv.)
Yields
Recovery of
acceptor
α(1,4)
Glycoside
12: 13
α(1,6)
Glycoside
14: 0
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
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60
1
2
8: H
9: Bz
8: H
8: H
8: H
8: H
8: H
8: H
8: H
10a: OMe
10a: OMe
10b: H
0
0
0
0
0
0
5
8: 87
9: 0
13: 63
15: 0
3
12: 42
12: 61
14: 0
8: 58
8: 25
8: 19
8: 0
4
10c: F
14: trace
14: trace
14: trace
14: 6
5
6a
10d: Cl
12: 79
12: 49
12: 92
12: 85
12: 93
10e: NO2
10e: NO2
10e: NO2
10e: NO2
7
8: 0
8
9b
10
5
14: 9
8: 0
14: 6
8: 0
aTrisaccharides 16 and 17 were produced in 8% and 15% yields, respectively. bWithout pre-formation of 11e.
and act as a catalyst effectively in this glycosylation pro-
cess even in the presence of an excess amount of water. In
addition, since this glycosylation proceeds faster than
hydrolysis by water, this glycosylation mechanism may
not be the typical SN1-type mechanism with an oxonium
cation intermediate. Next, when the glycosylation of 7
and 8 using 10e without pre-formation of 11e was exam-
ined, a similar result was obtained (entry 9) indicating
that the pre-formation of 11e was not necessary.
stirred at 0 °C for 20 h. α(1,4) Glycoside 12 was obtained in
only 13% yield (Table 1, entry 1).25 On the other hand, the
use of 3-benzoylated glucal as the glycosyl acceptor under
similar conditions gave the corresponding α(1,4) glycoside
13 in 63% yield (entry 2). This result suggested that the
electron withdrawing effect of the benzoyl group effec-
tively promoted the Lewis acidic boron atom to activate
the 1,2-anhydro donor. Therefore, we focused on increas-
ing the Lewis acidity of the unprotected glycosyl accep-
tor-derived boronic ester and investigated the electronic
effect of the substituent on the benzene ring in the bo-
ronic acid. It was found for the first time that p-
nitrophenylboronic acid (10e) showed the highest reactiv-
ity, producing α(1,4) glycoside 12 and overreacted trisac-
charides 16 and 17 (entries 1 and 3-6). This result suggest-
ed that 9-membered boronic ester 18e, which was formed
by the glycosylation of 7 and 6-membered boronic ester
11e, activated 7 and induced sequential α-stereoselective
glycosylation to provide trisaccharides 16 and 17 (Figure
2a).25
To suppress this overreaction, we hypothesized that the
addition of water could induce rapid hydrolysis of 9-
membered boronic ester 18e, thus inhibiting the activa-
tion of donor 7 by 18e, producing the desired disaccharide
12 and the relatively stable 6-membered boronic ester
catalyst 11e through an equilibrium process (Figure 2b).
To investigate our hypothesis, we examined the glycosyla-
tion of 7 and 8 using p-nitrophenylboronic acid (10e) in
the presence of water. As expected, when 5 and 10 equiva-
lents of water were added to the reaction mixture, the
glycosylation proceeded efficiently to provide 12 in 92%
and 85% yields, respectively, without any accompanying
trisaccharides (entries 7 and 8). These results suggested
that 6-membered boronic ester 11e can be regenerated
Figure 2. Proposed reaction mechanism in the absence
or presence of water.
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Table 2. Regio- and stereoselective glycosylations of several unprotected sugar acceptors and 1,2-anhydro do-
nors.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
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21
22
23
24
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27
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b
c
d
a5 equiv. of H2O was added. 10 equiv. of H2O was added. H2O was not added. 30 equiv. of DMF was added due to the low
e
f
solubility of 23. 60 equiv. of DMSO was added due to the low solubility of 26. Reaction was carried with 7 (1.0 equiv.) and 25
(3.0 equiv.).
The scope and selectivities of the glycosylations method
were also investigated using several unprotected sugar
acceptors and 1,2-anhydro donors (Table 2). 1,2-
Anhydroglucose 7 can be efficiently and selectively cou-
pled to a variety of glucose type acceptors 22β, 23 and 26
to provide α(1,4) glucosides 27-29 in high yields, respec-
tively.25 Furthermore, the use of thioglucoside 24 contain-
ing a leaving group at the anomeric position as a glycosyl
acceptor gave the corresponding α(1,4) glucoside 30.25 On
the other hand, when galactoside 25 was used, α(1,6) glu-
coside 31 was obtained in good yield.25 Unfortunately, the
glycosylation using octyl α-mannoside gave a mixtures of
α-glucosides with low regioselectivity (See SI). This low
selectivity may be resulted from non-selective formation
of 2,3- and 4,6-boronic esters with unprotected mannopy-
ranoside, unlike unprotected gluco- and galactopyra-
nosides, which selectively forms 4,6-boronic es-
ters.24c,d α(1,4) Galactosides 32 and 33 were also obtained
with similar selectivities by using 1,2-anhydrogalactose
19.25 In the case of the β-mannosylation using 1,2-
anhydromannose 20, β(1,6) mannosides 34 and 35 were
regio- and stereoselectivities. To understand this reaction
mechanism, we measured the competitive 13C kinetic iso-
tope effect27 (13C KIE) (Table 3). In general, the 13C KIEs for
SN1 reactions and SN2 reactions are 0.995-1.01 and >1.07,
respectively.28 However, in the cases of glycoside hydro-
lyses and glycosylation reactions, it has been reported
that concerted SN2 reactions become asynchronous SN2
reactions, which proceed via ‘exploded’ transition states
and display the relatively small 13C KIEs due to the stabili-
zation of partial positive charge at the anomeric position
by an oxygen adjacent to the site of displacement. For
examples, the 13C KIEs for the spontaneous hydrolysis of
Table 3. The experimental KIEs for the glycosylation
of β-glucoside 22β and the calculated KIEs for the
transition states at the B3LYP/6-31G* level of theory.
obtained
in
good
yields
with
excellent
β-
stereoselectivities.25,26 In addition, disaccharide donor 21
was converted into the corresponding trisaccharide 36 in
good yields with high regio- and 1,2-cis-stereoselectivities
under similar conditions.25 These results clearly demon-
strated not only the high regio- and 1,2-cis-
stereoselectivities but also the high generality of the pre-
sent glycosylation method. All regioselectivities were con-
sistent with those of previously reported glycosylations of
protected diol acceptors.19
Calculated KIEs
Experimental
Position
Constrained
of KIE
KIEs
Initial TS
TS
0.9986 (72)a
0.9999 (7)a
1.055 (3)a
1-13C
1-2H
1.009b
1.114b
1.014c
1.072c
aThe experimental KIEs were measured at 0 °C and were
converted to 25 °C assuming KIE25°C
= exp{(273/298)
Overall, this catalytic system with a glycosyl-acceptor-
derived boronic ester effectively generated 1,2-cis-
glycosides from unprotected glycosyl acceptors with high
ln(KIE0°C)}. bThe calculations of the KIEs were performed
c
using TS1. The calculations of the KIEs were performed us-
ing constrained TS (See SI).
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17
18
19
20
21
22
23
24
25
26
27
Figure 3. Potential energy surfaces for boronic acid catalyzed glycosylation.
α-glucopyranosyl fluoride, the hydroxide-catalyzed hy-
drolysis of 4-nitrophenyl α-mannopyranoside, and β-
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32
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glucosidase catalyzed hydrolysis of methyl β-
glucopyranoside were 1.032,29a 1.026,29b and 1.032,29c re-
spectively. In addition, the 13C KIEs for the stereospecific
glycosylations with highly dissociative transition states,
such as the trehalose-6-phosphate synthase (OtsA) cata-
lyzed glycosyl transfer reaction (1.012)22a,30 and the macro-
cyclic thiourea catalyzed glycosylation (1.000),27b have
been reported to be close to around 1.00. On the other
13
hand, in the case of the present glycosylation, the C KIE
at the anomeric carbon was determined to be 0.9986(72)
and 0.9999(7) compared to the C6 and C3 carbons, used
as internal standards, respectively, by the quantitative 13C-
NMR technique, indicating that this glycosylation can be
regarded as either a SNi-type reaction, a highly dissocia-
tive concerted SNi reaction or a SN1 with an extremely
short-lived intermediate. Next, we measured the α-
secondary deuterium kinetic isotope effect (α-SDKIE) for
the present glycosylation (Table 3). In the cases of the
epoxide ring opening reactions, it has been reported that
the α-SDKIE for the SN2-type acid-catalyzed methanolysis
of p-nitrostyrene oxide was 1.02,31a and the α-SDKIEs for
the SN1-type acid-catalyzed hydrolyses of 1,2,3,4-
tetrahydronaphthalene oxide and 6-methoxy-1,2,3,4-
tetrahydronaphthalene oxide were 1.0831b and 1.05,31b re-
spectively. On the other hand, the α-SDKIE for the pre-
sent glycosylation was determined to be 1.055(3) by the
Figure 4. Evolution of the bond orders of the most relevant
bonds along the reaction coordinate.
1
quantitative H-NMR technique using deuterium labelled
Figure 5. Predictive model for regioselectivity of the glyco-
sylations using 1,2-anhydro donors.
7, (1-2H)-7. Taken together, the experimental results and
the previously reported data also suggested that the
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present glycosylation proceeded via highly dissociative at the 6 position. In the case of the glycosylation of galac-
1
2
3
4
5
6
7
8
concerted SNi mechanism or a SN1 mechanism.
toside 25, the high α(1,6) selectivity of the product may be
rationalized by similar transition states (See SI). Also, the
observed regioselectivities in the β-mannosylations were
completely opposite to those in the α-glucosylations and
α-galactosylations. The differences in regioselectivities
derived from the structure of the glycosyl donor can be
easily predicted by the following transition state models
(Figure 5). In TS-α(1,4), the glycosyl donor moiety exists
apart from the glycosyl acceptor moiety. Conversely, TS-
α(1,6) is destabilized by the significant overlap between
them. In the case of 1,2-anhydromannose 20, the favorable
direction toward the boronic ester was reversed due to
the opposite stereochemistry of C2. Therefore, TS-β(1,6)
is more stable than TS-β(1,4), leading to the production of
β(1,6) mannosides 34 and 35 with high regioselectivities.
Furthermore, we investigated the reaction mechanism
by DFT calculations using Jaguar version 9.1.32 The quan-
tum chemical level B3LYP/6-31G* was selected for struc-
ture optimization and frequency calculation. Single-point
energy values were corrected at the higher B3LYP/6-
31+G** level. To simplify the substrates, benzyl and octyl
groups were changed to methyl groups. The transition
state was found at C1-O2 = 2.03 Å and C1-O4’ = 2.98 Å
(Figure 3, TS1). The IRC scan of this TS structure indicat-
ed that the glycosylation mechanism of the 1,2-anhydro
donor and glycosyl-acceptor-derived boronic ester was a
concerted SNi mechanism. In addition, using this calcula-
tion data, the 13C KIE at the anomeric carbon and the α-
SDKIE were calculated by QUIVER.33 As the results, the
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13
calculated anomeric C KIE (1.009) was found to be close
Next, we applied our strategy to the direct glycosylation
of unprotected natural glycosides (Scheme 1). The glyco-
sylation using daidzine (44), which has five free hydroxyl
groups including a phenolic one, as an unprotected glyco-
syl acceptor and donor 45 proceeded smoothly to give
desired α(1,4) glycoside 46 in high yield with high regio-
and stereoselectivities. The removal of PMB groups of 46
gave the isoflavone glycoside 47. Furthermore, when pae-
oniflorin (48), which is a herbal medicine, was employed
as an unprotected glycosyl acceptor, desired α(1,4) glyco-
side 49 was obtained in high yield with high regio- and
stereoselectivities. Removal of the benzyl groups of 49
provided α-glycoside 50. These results indicated that us-
ing this glycosylation method, various natural glycosides
can be glycosylated with high regio- and stereoselectivi-
ties under mild conditions without any modification to
the natural glycosides.
to the experimental 13C KIE. On the other hand, the calcu-
lated α-SDKIE (1.114) was not consistent with the experi-
mental α-SDKIE (1.055), because it is known that the cal-
culated α-SDKIE was highly dependent on the anomeric
C-O bond length of the TS structure.29b Certainly, the α-
SDKIE for constrained TS (1.072), which was calculated
by optimization the restricted structure (C1-O2 = 1.88 Å),
was found to be close to the experimental α-SDKIE. Fur-
thermore, the details of the reaction mechanism were
investigated by Mayer natural bond order (NBO)34 analy-
sis of the reaction coordinates (Figure 4). The results
showed that, in the shadowed region, a glycosidic linkage
began to form before the bond breakage between the
anomeric carbon and epoxy oxygen, indicating that this
reaction mechanism is a concerted SNi mechanism.
It should be noted that although the epoxide ring open-
ing reactions in generally have early TSs29b,35 due to the
release of the large strain energy (~27 kcal/mol)36 and low
activation energy, the present reaction has a relatively late
stage TS and high activation energy probably because of
the 1,3-diaxial interaction between aryl group and hydro-
gen atoms (H4’ and H6’) in TS1 derived from a change in
boron geometry from trigonal planar as in the 6 mem-
bered ring boronic ester 39 to tetrahedral for the 6 mem-
bered ring boronate ester TS1. Similar 1,3-diaxial interac-
tion in the 6-membered ring boronate complex was re-
ported by Bowie et al.37
Scheme 1. Direct glycosylation of unprotected natu-
ral glycosides.a
Next, we searched the transition states in the production
of the α(1,6) isomer to rationalize the high regioselectivity
of the glycosylation. This transition state was found at C1-
O2 = 2.08 Å and C1-O6’ = 3.19 Å (Figure 3, TS2). We con-
firmed by IRC scan that the reaction mechanism of glyco-
sylation at the 6 position is also a SNi-type mechanism.
The difference in activation energy of the glycosylations
at the 4 and 6 positions was 2.7 kcal/mol, indicating that
the glycosylation at the 4 position is kinetically more fa-
vorable. These results are in good agreement with the
experimental observations. From transition state struc-
tures, the regioselectivity was attributed to the overlap
between glycosyl donor and glycosyl acceptor moieties
that destabilizes the transition state of the glycosylation
aReagents and conditions: (a) TFA, CH2Cl2, 0 °C, 1 h, 90%;
(b) Pd(OH)2/C, H2, THF, MeOH, rt, 20 min, 95%
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excellent regio- and stereoselectivities in the presence of
Scheme 2. The efficient synthesis of branched α-
glucan 51 via boronic and borinic acid catalyzed re-
gio- and 1,2-cis-α-stereoselective glycosylations.a
1
2
3
water under mild conditions. Finally, deprotection of the
benzyl groups gave branched α-glucan 51. The anomeric
configurations and glycosylated positions were confirmed
by coupling constants of anomeric protons and HMBC
correlations of benzoylated 58 (See SI).
OBn
O
OH
O
4
5
6
7
8
9
a
b
c
BnO
BnO
nC8H17OH
HO
HO
HO
HO
OnC8H17
Octanol (52)
OnC8H17
54
22
CONCLUSION
OBn
OPMB
O
O
BnO
BnO
In conclusion, the highly regio- and 1,2-cis-
stereoselective SNi-type glycosylation of 1,2-anhydro do-
nors and unprotected sugar acceptors has been developed
using p-nitrophenylboronic acid (10e) as a catalyst in the
presence of water under mild conditions. Highly con-
trolled regio- and 1,2-cis-stereoselectivities were achieved
via the combination of the boron-mediated carbohydrate
recognition and SNi-type mechanism. Mechanistic studies
using the KIEs indicated that this glycosylation can be
regarded as a highly dissociative concerted SNi reaction or
a SN1 with an extremely short-lived intermediate. Fur-
thermore, computational analysis supported the feasibil-
ity of the highly dissociative concerted SNi mechanism
and the high regioselectivity of this glycosylation. Our
mechanistic studies showed that the 1,2-cis-glycosylations
based on the similar hypotheses, such as borinic acid
catalyzed glycosylation20 and Me3Al mediated glycosyla-
tion,40 might proceed through SNi mechanism. The pic-
ture that emerges for the present boronic acid catalyzed
glycosylation involves a stereospecific SNi mechanism
induced by the simultaneous activation of glycosyl donor
and glycosyl acceptor. This occurs in a manner similar to
those employed by natural glycosyltransferases that gen-
erate 1,2-cis-O-glycosides through the SNi mechanism,
suggesting the possibility of the development of other SNi
type glycosylations using unprotected glycosyl acceptors.
Moreover, this glycosylation method was applied success-
fully to the direct glycosylation of unprotected natural
glycosides and the efficient synthesis of branched α-
glucan 51 with minimal protecting groups. Further appli-
cations of this glycosylation method to the synthesis of
biologically active oligosaccharides, glycoconjugates, and
natural products are now in progress in our laboratory.
PMBO
PMBO
OH
O
PMBO
HO
e
d
HO
O
O
PMBO
PMBO
O
HO
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
HO
HO
OnC8H17
O
55
O
HO
HO
OnC8H17
56
OBn
O
OBn
OBn
O
BnO
BnO
O
BnO
BnO
HO
HO
BnO
HO
OH
BnO
f
g
O
HO
O
HO
O
O
HO
HO
O
HO
O
O
HO
O
O
HO
HO
HO
OnC8H17
HO
OnC8H17
57
58
OH
O
OH
O
HO
HO
HO
HO
HO
OH
O
HO
F
BOH
O
O
HO
2
HO
53
O
O
HO
HO
OnC8H17
Branched -glucan 51
aReagents and conditions: (a) 7, 53, THF, -60 °C, 24 h, 71%
(36% for 7) (α only); (b) Pd/C, H2, MeOH, rt, 14 h, 99%; (c)
45, 10e, H2O, MeCN, rt, 6 h, 72% (α(1,4) only); (d) 7, 53, THF,
-80 °C, 16 h, 73% (α(1,6) only); (e) TFA, CH2Cl2, rt, 30 min,
quant.; (f) 7, 10e, H2O, MeCN, -10 °C, 24 h, 59% (α(1,4) only)
(73% BRSM); (g) Pd/C, H2, MeOH, rt, 18 h, 83%
Finally, we applied the present glycosylation method to
the synthesis of branched α-glucan 51 (Scheme 2). Similar
oligosaccharides were synthesized by Boons et al.38 and
Demchenko et al.39 In our synthetic strategy, α(1,4) glyco-
sidic linkages would be constructed using cyclic 4,6-
boronic ester catalysts, whereas the other α-glycosidic
linkages would be constructed using acyclic borinic ester
catalysts derived from a borinic acid and highly reactive
primary alcohols in the glycosyl acceptors which does not
have a diol group that can form a cyclic complex. Initially,
α-glucoside 54 was prepared as a single isomer by the
highly 1,2-cis-stereoselective glycosylation of 1,2-
anhydroglucose 7 and octanol (52) using borinic acid
catalyst 53.20a Deprotection of the benzyl groups of 54
gave the unprotected sugar 22α in 99% yield. Next, glyco-
sylation of 45 and 22α using a catalytic amount of boronic
acid 10e proceeded effectively to provide α(1,4) glycoside
55 in 72% yield with excellent regio- and stereoselectivi-
ties. Subsequently, borinic acid catalyzed regio- and ste-
reoselective glycosylation of 7 and 55, which has four free
hydroxyl groups, gave α(1,6) glycoside 56 as a single iso-
mer in 73% yield. Deprotection of the PMB groups of 56
gave trisaccharide 57 which possesses seven free hydroxyl
groups. The next boronic acid catalyzed glycosylation
gave α(1,4) glycoside 58 in 59% yield (73% BRSM) with
ASSOCIATED CONTENT
Supporting Information.
Experimental procedures and compound characterizations.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
dtak@applc.keio.jp
toshima@applc.keio.jp
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
We wish to thank Dr. Yu Tsutsumi (Principal Scientist of
Bruker Biospin K.K.) for helpful support in the quantitative
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13C-NMR experiments. This research was supported in part
by the MEXT-supported Program for the Strategic Research
Foundation at Private Universities, 2012-2016 (No. S1201020),
JSPS KAKENHI Grant Numbers JP16H01161 in Middle Molec-
ular Strategy and JP16K05781 in Scientific Research (C), and
the Sasagawa Scientific Research Grant from the Japan Sci-
ence Society.
(20) (a) Tanaka, M.; Takahashi, D.; Toshima, K. Org. Lett. 2016,
18, 5030-5033. (b) Tanaka, M.; Nashida, J.; Takahashi, D.; Toshi-
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Annu. Rev. Biochem. 2008, 77, 521-555.
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34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 1
291x45mm (300 x 300 DPI)
ACS Paragon Plus Environment

Page 19 of 20
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 2
314x134mm (300 x 300 DPI)
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 20 of 20
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 3
122x38mm (300 x 300 DPI)
ACS Paragon Plus Environment