Regioselective and 1,2-cis-α-Stereoselective Glycosylation Utilizing Glycosyl-Acceptor-Derived Boronic Ester Catalyst

Article abstract of DOI:10.1002/anie.201504182

Regioselective and 1,2-cis-α-stereoselective glycosylations using 1α,2α-anhydro glycosyl donors and diol glycosyl acceptors in the presence of a glycosyl-acceptor-derived boronic ester catalyst. The reactions proceed smoothly to give the corresponding 1,2-cis-α-glycosides with high stereo- and regioselectivities in high yields without any further additives under mild reaction conditions. In addition, the present glycosylation method was successfully applied to the synthesis of an isoflavone glycoside.

Full text of DOI:10.1002/anie.201504182

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201504182
German Edition: DOI: 10.1002/ange.201504182
Carbohydrates
Regioselective and 1,2-cis-a-Stereoselective Glycosylation Utilizing
Glycosyl-Acceptor-Derived Boronic Ester Catalyst
Akira Nakagawa, Masamichi Tanaka, Shun Hanamura, Daisuke Takahashi,* and
Kazunobu Toshima*
Abstract: Regioselective and 1,2-cis-a-stereoselective glycosy-
lations using 1a,2a-anhydro glycosyl donors and diol glycosyl
acceptors in the presence of a glycosyl-acceptor-derived
boronic ester catalyst. The reactions proceed smoothly to
give the corresponding 1,2-cis-a-glycosides with high stereo-
and regioselectivities in high yields without any further
additives under mild reaction conditions. In addition, the
present glycosylation method was successfully applied to the
synthesis of an isoflavone glycoside.
alcohols. In terms of regioselectivity, efficient approaches
utilizing not only highly toxic organotin reagents[9] but also
low toxicity organoboron reagents[10–12] have been developed.
In 1999, Aoyama and co-workers reported[11] a pioneering
regio- and stereoselective Koenigs–Knorr-type glycosylation
for the synthesis of 1,2-trans-glycosides using a stoichiometric
quantity of a silver salt and an arylboronic acid for the
activation of a glycosyl donor and a specific hydroxy group in
the glycosyl acceptor, respectively. Recently, Taylor and co-
workers reported[12] a similar type of regioselective Koenigs–
1,2-cis-a-Glycosides are frequently found in many bio- Knorr glycosylation using a catalytic amount of an organo-
logically active natural products and glycoconjugates, such as
glycolipids, glycoproteins, and proteoglycans. To elucidate the
precise biological roles of these carbohydrates, the chemically
synthesized homogeneous and structurally well-defined car-
bohydrates have attracted much attention in chemistry,
biology, and medicine.[1] In this context, development of
efficient glycosylation methods for the synthesis of 1,2-cis-a-
glycosides is becoming increasingly important in synthetic
organic chemistry. From a synthetic standpoint, the efficiency
of the glycosylation reaction was evaluated based on the high
chemical yield, as well as a/b-stereo- and regioselectivities. In
terms of a/b-stereoselectivity, the synthesis of 1,2-cis-a-glyco-
sides is still a challenging task because of the non-availability
of neighboring-group participation from a 2-O-acyl function-
ality in the glycosyl donor. To overcome this problem,
efficient indirect[2] and direct[3] methods have been developed.
For an example of the indirect method, there is an intra-
molecular aglycon delivery (IAD), which was introduced by
Hindsgaul et al.[4] and extended by Stork et al.,[5] Bols,[6] and
Ito and Ogawa.[7] Among them, in 1992, Bols reported
a silicon-tethered IAD for the stereoselective synthesis of 1,2-
cis-a-glycosides.[6] For an example of the direct method, in
1994, Liu and Danishefsky reported[8] a direct glycosylation of
1a,2a-anhydroglucose and stannylated glycosyl acceptors
using a stoichiometric amount of AgBF4 for the stereoselec-
tive synthesis of 1,2-cis-a-glycosides. However, the chemical
yields of the obtained glycosides were low to moderate, and
unfortunately, the protocol was not applicable to secondary
borinic acid to afford 1,2-trans-glycosides. However, to the
best of our knowledge, there are few regio- and stereoselec-
tive glycosylation methods for the synthesis of 1,2-cis-a-
glycosides. Herein, we report a novel regioselective and 1,2-
cis-a-stereoselective glycosylation of a 1,2-anhydro glycosyl
donor and a diol glycosyl acceptor utilizing a glycosyl-
acceptor-derived boronic ester catalyst without any further
additives under mild reaction conditions.
Our glycosylation strategy is based on the following
features of an arylboronic acid, as illustrated in Figure 1:
Figure 1. Regio- and 1,2-cis-a-stereoselective glycosylation utilizing
a glycosyl-acceptor-derived boronic ester catalyst.
1) The arylboronic acid 1 favorably and reversibly binds to
either a cis-1,2- or 1,3-diol[13] in the glycosyl acceptor 2; 2) the
[*] A. Nakagawa, M. Tanaka, S. Hanamura, Dr. D. Takahashi,
Prof. Dr. K. Toshima
resulting glycosyl-acceptor-derived boronic ester
3
is
Department of Applied Chemistry, Faculty of Science and Technology,
Keio University, 3-14-1 Hiyoshi
Kohoku-ku, Yokohama 223-8522 (Japan)
E-mail: dtak@applc.keio.ac.jp
expected to show sufficient Lewis acidity to activate the 1,2-
anhydro glycosyl donor 4 without any further additives; 3) the
formed oxonium cation intermediate 6, involving a tetracoor-
dinate boronate ester moiety, increases the nucleophilicity of
the boron-bound oxygen atom,[11] and concomitant glycosy-
toshima@applc.keio.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201504182.
lation[14] from the less-hindered B O moiety in the boronate
À
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Communications
ester affords the corresponding boronic ester 7; and 4) diol
exchange reaction between 7 and 2 regenerates 3 and
provides the 1,2-cis-a-glycoside 8.
To investigate our hypothesis, we selected 1a,2a-anhy-
droglucose 11,[15] diethyl l-tartrate (DET; 9), and 4-meth-
oxyboronic acid (1a) as the glycosyl donor, glycosyl acceptor,
and arylboronic acid, respectively (Table 1). After prepara-
Figure 2. Proposed mechanism for the generation of 13.
Table 1: Glycosylations of 11 and diethyl l-tartrate-derived boronic esters
10a–c under various reaction conditions.
was lower than that obtained at À208C because of the
increased yield of 13 (entry 5). In contrast, it was confirmed
that the chemical yield of 12 increased to 76% yield at À408C,
because of the decreased yield of 13 (entry 6). In addition, it
was found that a reaction time of 6 hours gave the highest
yield of 12 (entries 7–10). Thus, it was found that the
glycosylation of 11 and 10a in MeCN at À408C for 6 hours
gave the best result, thus producing 12 in 82% yield.
Next, we examined the glycosylations of 11 with 10b and
10c, which were prepared from 9 with 1b and 1c, respectively,
to investigate the electrostatic effect of the substituents on the
benzene ring in the boronic esters. It was found that when 10b
and 10c were used, the chemical yields of 12 were lower than
that obtained using 10a, which possesses an electron-donating
methoxy group. In addition, 10c, possessing an electron-
withdrawing fluorine group, gave the lowest yield (35%) of 12
(Table 1, entries 11 and 12). According to the chemical
features of the boronic esters, it is reasonable to assume
that the electron-withdrawing group in 10c increases both the
Lewis acidity of the boron atom and the activation rate, but
reduces both the nucleophilicity of the boron-bound oxygen
atom in the boronate ester and the glycosylation rate, whereas
the electron-donating group in 10a reduces both the Lewis
acidity of the boron atom and the activation rate, but
increases both the nucleophilicity of the boron-bound
oxygen atom in the boronate ester and the glycosylation
rate (Figure 3). Taken together, the experimental results and
these features of the boronic esters suggest that the rate-
determining step of this reaction is the glycosylation step.
Next, we examined the glycosylation of 11 and 9 using
a catalytic amount of 10a. After several attempts to optimize
the reaction conditions, it was found that the glycosylation of
11 and 9 in the presence of 10a in MeCN at À208C for
Entry
Solvent
T [8C]
t [h]
Boronic acid
Yield [%]
12[a]
13
1
2
3
4
5
6
7
8
MeCN
toluene
Et2O
À20
À20
À20
À20
0
À40
À40
À40
À40
À40
À40
À40
8
8
8
8
8
8
2
4
6
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1c
68
60
60
49
54
76
45
71
82
69
55
35
10[a]
0
0
CH2Cl2
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
0
20[a]
2[b]
<1[b]
<1[b]
<1[b]
10[b]
<1[b]
<1[b]
9
10
11
12
10
6
6
[a] Yield of isolated product. [b] Determined by LC/MS.
tion of the DET-derived boronic ester 10a by mixing
a stoichiometric amount of 1a and 9 in refluxing toluene for
3 hours, followed by concentration in vacuo, we investigated
the glycosylations of 11 and 10a under several conditions. It
was found for the first time that the glycosylation of 11 and
10a in MeCN at À208C for 8 hours proceeded smoothly to
give the 1,2-cis-a-glycoside 12 in 68% yield with excellent
stereoselectivity along with the diglycoside 13 in 10% yield as
a byproduct (entry 1). The configuration of both of the
1
glycosidic bonds in 13 was confirmed to be a by H NMR
analysis. This result suggested that the boronic ester 15a,
which was formed by the glycosylation of 11 and 10a,
activated 11 and induced sequential a-stereoselective glyco-
sylation to provide 13 (Figure 2).
With this preliminary result in hand, we next examined
the solvent effect on the glycosylation of 11 and 10a by using
toluene, Et2O, and CH2Cl2. It was found that when these
solvents were used, although excellent stereoselectivities
were observed in all cases, chemical yields of 12 were lower
than that of the reaction using MeCN under the same reaction
conditions (Table 1, entries 1–4). These results indicated that
MeCN was the best solvent for this reaction. Next, we
optimized reaction temperature and reaction time. When the
glycosylation was carried out at 08C, the chemical yield of 12
Figure 3. The electrostatic effect of the substituents on the benzene
ring in 10a and 10c in the glycosylations with 11.
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stereoselectivities were observed, and in these cases, the
a(1,4) glycosides 27–30 were obtained in high yields in the
absence of any additives under mild reaction conditions
(Scheme 1B–D).[16]
The observed high regioselectivities may be rationalized
by consideration of the following transition states. In the
glycosylation of 11 and 16 using 21, 11 approaches from the
equatorial face of the boron atom in 21 to minimize steric
hindrance, and generate the oxonium cation involving the
boronate ester. At this stage, since significant steric hindrance
between the anomeric proton of the oxonium cation and the
benzene ring of the boronate ester destabilizes the transition-
state TS-1, glycosylation from the oxygen atom at the 6-
position takes place through the favored TS-2 to give 26
(Figure 5a). In contrast, in the glycosylation of 11 and 17
Figure 4. Proposed catalytic cycle for the glycosylation of 11 and 9
using 10a.
22 hours proceeded effectively to provide 12 as a single
isomer in high yield (82%). This result clearly indicates that
the diol exchange reaction between 9 and the boronic ester
15a proceeded smoothly to provide 10a for re-entry into the
catalytic cycle (Figure 4).
With these favorable results in hand, we next examined
the regioselectivity and generality of the present glycosylation
method using several 1,3-diol sugar acceptors (16–20;
Scheme 1). It was found that when the galactoside 16 and
boronic ester 21 were used, excellent regio- and a-stereose-
lectivities were observed, and only the a(1,6) glycoside 26 was
obtained as a single isomer in good yield (Scheme 1A).[16]
Interestingly, when the glucoside 17, mannoside 18, glucos-
aminide 19, and glucal 20 were employed in the glycosylations
using the corresponding boronic esters 22–25 it was also found
that good to excellent regioselectivities and excellent a-
Figure 5. Proposed rationale for the regioselectivity in the glycosyla-
tions of a) 11 and 21, and b) 11 and 22.
using 22, a similar steric hindrance between the anomeric
proton of the oxonium cation and the benzene ring of the
boronate ester destabilizes TS-3 (Figure 5b). Thus, 27 is
regioselectively obtained through the favored TS-4. These
models are also consistent with the observed regioselectivities
in the glycosylations of 11 and 18 using 23, 11 and 19 using 24,
and 11 and 20 using 25.
To investigate further the generality of this present
method, we next examined the glycosylations of 11 with cis-
1,2-diol sugar acceptors, that is the mannoside 31 and
galactoside 32. When the glycosylation of 11 and 31 using
the boronic ester 33 was conducted, the a(1,3) glycoside 37
was obtained in 70% yield as a single isomer with excellent a-
stereoselectivity (Scheme 2A).[16] When the glycosylation of
11 and 32 using the boronic ester 34 was conducted, it was
found that glycosylation at an axial 4-OH in 34 preferentially
proceeded to give the a(1,4) glycoside 38 in 65% yield with
high regioselectivity [a(1,4)/a(1,3) = 9.3:1] and excellent a-
stereoselectivity (Scheme 2B).[16] Next, we turned our atten-
tion to the type of glycosyl donor used. When the 1a,2a-
anhydrogalactose 35[17] and 1a,2a-anhydroisomaltose 36[8]
were employed as glycosyl donors, the glycosylations with
20 using 25 were found to proceed effectively to afford the
a(1,4) glycosides 39[18] and 40, respectively, with good regio-
Scheme 1. Glycosylations of 11 with several 1,3-diol sugar acceptors
(16–20) using the corresponding glycosyl-acceptor-derived boronic
ester catalysts 21–25. Reagents and conditions: a) 1a (0.2 equiv),
toluene, reflux, 3 h. Bz=benzoyl.
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Scheme 3. Total synthesis of the isoflavone glycoside 41 by glycosyla-
tion using 1a, 11, and 45. Reagents and conditions: a) tBuOK, DMF,
608C, 24 h, 65%; b) DDQ, CH2Cl2/1,4-dioxane/phosphate buffer
(20 mm, pH 7.2; 1:1:1, v/v/v), RT, 12 h, 96%; c) tBu2Si(OTf)2, pyridine,
DMF, À40 to 08C, 2 h, 89%; d) BzCl, pyridine, RT, 1 h, 92%; e) TBAF,
AcOH, THF, RT, 3 h, 91%; f) 1a, toluene, reflux, 3 h; then 11, MeCN/
THF (3:1, v/v), RT, 24 h, 77% for 46, 4% for 47; g) BzCl, DMAP,
pyridine, 408C, 2 h, 98%; h) BCl3, CH2Cl2, À788C, 2 h, 50%;
i) NaOMe, MeOH, 408C, 2 h, 92%. DDQ=2,3-dichloro-5,6-dicyano-
1,4-benzoquinone, DMF=N,N-dimethylformamide, PMB=p-methoxy-
benzyl, TBAF=tetra-n-butylammonium fluoride, Tf =trifluoromethane-
sulfonyl, THF=tetrahydrofuran.
Scheme 2. Glycosylations of 11 and the cis-1,2-diol sugar acceptors 31
and 32 using the corresponding catalysts 33 and 34 (A and B).
Glycosylations of 20 and 1,2-anhydro sugars 35 and 36 using the
catalyst 25 (C and D). Reagents and conditions: a) 1a (0.2 equiv),
toluene, reflux, 3 h.
and excellent a-stereoselectivities (Scheme 2C and D).[16]
These results clearly indicated not only the good to high
regioselectivity and high a-stereoselectivities but also the
high generality of the present glycosylation method.
Finally, we applied the present glycosylation method to
the synthesis of the isoflavone glycoside 41 (Scheme 3). The
isoflavone glycoside was enzymatically synthesized by
Hamada and co-workers in 2008.[19] The synthetic scheme
for 41 is summarized in Scheme 3. First, the daidzein 7-O-b-
glucoside 44 was synthesized by a b-stereoselective glycosy-
lation using 43[20] and 4’-O-benzyl-daidzein (42)[21] in the
presence of tBuOK. The compound 44 was converted into the
glycosyl acceptor 45 in four steps (1. de-p-methoxybenzyla-
tion; 2. silylenation; 3. benzoylation; 4. desilylenation). Next,
we conducted the glycosylation of 45 and 11 using a catalytic
amount of 1a in MeCN/THF (3:1) at room temperature. It
was found that the desired a(1,4) glycoside 46 was obtained in
77% yield with excellent a-stereoselectivity and good regio-
selectivity along with the minor a(1,6) glycoside 47 in 4%
yield. These results also demonstrated the high efficiency and
generality of the present glycosylation method. Next, ben-
zoylation of the free hydroxy groups in 46 provided 48. At this
stage, the a(1,4) linkage in 46 was confirmed on the basis of
the downfield chemical shift changes for the 6’’-H protons in
synthetic 41 was found to be identical in all respects with the
reported data.[19]
In conclusion, we have developed the first regio- and 1,2-
cis-a-stereoselective glycosylation utilizing
a
glycosyl-
acceptor-derived boronic ester catalyst without any further
additives under mild reaction conditions. The use of 1a,2a-
anhydro glycosyl donors and 4-methoxyboronic acid (1a) in
MeCN was found to be effective for the glycosylations with
several diol acceptors. Furthermore, we successfully applied
the present glycosylation method to the synthesis of the
isoflavone glycoside 41. Detailed mechanistic studies of this
method, application to other types of donors, and synthetic
studies of other compounds using the present method are now
in progress in our laboratory.
Acknowledgements
This research was supported in part by the MEXT-supported
Program for the Strategic Research Foundation at Private
Universities, 2012–2016, from the Ministry of Education,
Culture, Sports, Science and Technology of Japan (MEXT).
1
the H NMR spectrum, and a correlation peak at 1’’’-C with
4’’-H in the HMBC spectrum. Finally, removal of the Bn
groups in 48 with BCl3 and subsequent removal of the Bz
groups, gave the isoflavone glycoside 41. 1H NMR, 13C NMR,
and HRMS (ESI-TOF) data for an analytical sample of
Keywords: boron · glycosylation · regioselectivity ·
stereoselectivity · synthetic methods
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How to cite: Angew. Chem. Int. Ed. 2015, 54, 10935–10939
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1
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1
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Received: May 7, 2015
Revised: June 19, 2015
Published online: July 23, 2015
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