Stereoselective trimethylsilylation of α- and β-galactopyranoses

Article abstract of DOI:10.1016/j.carres.2019.01.013

Trimethylsilylation of the anomeric hydroxyl groups of tetra-O-benzyl and tetra-O-acetyl galactopyranoses was investigated. Stereoselective formation of β-trimethylsilyl glycoside (β-TMS glycoside) of benzyl protected compound was achieved using N-trimeth

Full text of DOI:10.1016/j.carres.2019.01.013

Accepted Manuscript
Stereoselective trimethylsilylation of α- and β-galactopyranoses
Yuri Asami, Yuka Kawaguchi, Yoshimi Kanie, Hajjaj Abdu-Allah, Katsuhiko Suzuki,
Osamu Kanie
PII:
S0008-6215(18)30703-1
https://doi.org/10.1016/j.carres.2019.01.013
CAR 7672
DOI:
Reference:
To appear in: Carbohydrate Research
Received Date: 1 December 2018
Revised Date: 28 January 2019
Accepted Date: 28 January 2019
Please cite this article as: Y. Asami, Y. Kawaguchi, Y. Kanie, H. Abdu-Allah, K. Suzuki, O. Kanie,
Stereoselective trimethylsilylation of α- and β-galactopyranoses, Carbohydrate Research (2019), doi:
https://doi.org/10.1016/j.carres.2019.01.013.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Stereoselective trimethylsilylation of α- and β-galactopyranoses
Yuri Asami,a Yuka Kawaguchi,b Yoshimi Kanie,c Hajjaj Abdu-Allah,b,d Katsuhiko
Suzuki,e Osamu Kaniea,b*
aGraduate School of Engineering, Tokai University, 4-1-1 Kitakaname, Hiratsuka,
Kanagawa 259-1292 Japan.
bDepartment of Applied Biochemistry, Tokai University, 4-1-1 Kitakaname,
Hiratsuka, Kanagawa 259-1292 Japan.
cTechnology Joint Management Office, Tokai University, 4-1-1 Kitakaname,
Hiratsuka, Kanagawa 259-1292 Japan.
dDepartment of Pharmaceutical Organic Chemistry, Faculty of Pharmacy, Assiut
University, Assiut 71526, Egypt.
eFaculty of Pharmaceutical Sciences, Aomori University, 2-3-1 Kohbata, Aomori
030-0943 Japan.
E
-mail addresses: 7bajm002@mail.u-tokai.ac.jp (YA), kwgcyk922@gmail.com
(YK1), yoskanie@tsc.u-tokai.ac.jp (YK2), hajjaj@aun.edu.eg (HA), suzuki-
katsuhiko@aomori-u.ac.jp (KS).
*Corresponding author:
Osamu Kanie, Professor, Tokai University
Department of Applied Biochemistry, Tokai University
4-1-1 Kitakaname, Hiratsuka, Kanagawa 259-1292 Japan
E-mail: kanie@tokai-u.jp
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ABSTRACT
Trimethylsilylation of the anomeric hydroxyl groups of tetra-O-benzyl and tetra-
O-acetyl galactopyranoses was investigated. Stereoselective formation of β-
trimethylsilyl glycoside (β-TMS glycoside) was achieved using N-trimethylsilyl
diethylamine. In the course of the investigation of the selective synthesis of TMS
galactosides using TMS-imidazole, we observed the formation of an intermediate,
which was converted predominantly into α-TMS glycoside after silica gel column
chromatography.
On
the
other
hand,
a
reaction
using
TMS-
trifluoromethanesulfonate–2,6-lutindine selectively yielded α-TMS glycoside.
Keywords
Trimethylsilyl glycoside • Stereoselectivity • Mixed acetal • Silylation mechanism
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1. Introduction
Construction of selective forms of glycosidic linkages is of particular importance
for obtaining oligosaccharides that are useful as biological investigation tools.
Oligosaccharides are a part of a variety of glycoconjugates playing pivotal roles in
eukaryotic multicellular organisms. Standard method of constructing glycosidic
linkages is the glycosylation reaction in which the preinstalled leaving group at the
anomeric center is activated usually by a Lewis acid, resulting in the formation of
oxocarbenium ion species. The stereoselective attack of a nucleophile following the
SN1 mechanism is, thus, the key to the selective formation of α- and β-glycosides. To
achieve this, acyl neighboring group participation [1,2], in-situ anomerization [3], and
solvent effects [4,5] are usually utilized. More recently, SN2 displacement of an
anomeric triflate at an extremely low temperature with a conformational constraint
was reported [6].
On the other hand, a reverse-type of glycoside formation reaction has been
reported earlier in which the anomeric hydroxyl group acts as a nucleophile and a
leaving group is introduced onto its counterpart [7]. Although various glycosides have
been synthesized by this method, its major disadvantage is that a mixture of anomeric
glycosides is obtained, reflecting the ratio of α- and β-oriented hydroxyl groups used
as the nucleophiles. Several cases of the successful control of stereoselectivity have
been reported. Because of the stability of its axially oriented α-hydroxyl group, 2-
ketodeoxyocturosonic acid (KDO) glycoside and its synthesis are of particular
interest; however, a doubly charged chelate species had to be formed to secure its
configuration [8]. More recently, β-mannosylation, wherein a metal chelate enhances
the kinetic control, was reported [9]. Despite some success, the construction of
thermodynamically less stable glycosides based on the reverse glycosylation approach
is still challenging, and generality has to be compromised. However, several examples
utilizing stereoselectively synthesized trimethylsilyl (TMS) glycosides have been
reported for β-galactoside and α- and β-sialoside formation [10,11]. Realizing the
potential of this reverse glycosylation method, we focused on the stereoselective
formation of TMS glycosides, which can be a key step in the stereospecific synthesis
of glycosides.
Herein, we report α- and β-trimethylsilylation reactions of the anomeric hydroxyl
groups of benzylated and acetylated galactopyranoses.
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2. Results and discussion
2.1. Background of anomeric silylation reaction
A stereoselective formation of α- and β-trichloroacetimidates was reported by
Schmidt et al., wherein kinetic or thermodynamic control was the key for obtaining
the selectivity [12]. Also, stereoselective β-silylation has been known in the
preparation of important synthons, tert-butyldimethylsilyl and tert-butyldiphenylsilyl
galactosamine precursors, for the synthesis of O-linked glycopeptides [13,14]. From
these important findings, we perceived that kinetic and thermodynamic control might
preferentially result in the equatorially and axially oriented TMS-protected
carbohydrates at the anomeric position. To examine the stereochemical outcome in
TMS-protection of the anomeric hydroxyl group, tetra-O-benzyl α- and β-
D-
galactopyranoses (1α, 1β) [15] and tetra-O-acetyl α- and β- -galactopyranoses (2α,
D
2β) [16,17] were selected as substrates and subjected to trimethylsilylation under
various conditions (Scheme 1).
2.2. Selective synthesis of β-TMS glycoside
First, we examined TMS dimethylamine (TMS-NMe2) and TMS diethylamine
(TMS-NEt2) as the silylating reagents to examine the effect of steric factors of the
base using compound 1 (Table 1). As it is evident from entries 1–5, more sterically
demanding base seemed to have a tendency to form β-TMS glycoside (3β). For the
reactions with TMS-NMe2, lower temperature was preferred to yield the α-anomer,
indicating that it was the kinetic product (entries 1–3). Raising the reaction
temperature to room temperature exclusively resulted β-TMS glycoside (entry 3).
This indicates the generation of a possible intermediate leading to a transition state
that prefers quasi-equatorial orientation of the anomeric oxygen, probably due to the
steric issue. The use of more sterically demanding TMS-NEt2 resulted in the
formation of 3β exclusively, regardless of the temperature (entries 4 and 5). 1H NMR
data (δ 4.62 ppm, J1,2 = 7.5 Hz) also supported the stereochemistry of β-silyl
glycoside (Fig. 1A). This β-selective tendency could be due to the 1,3-diaxial
repulsions of the axially oriented O-1 with H-3 and H-5, and the steric impediment of
the 1,2-cis-arranged 2-O-benzyl group. β-TMS glycoside was obtained exclusively
and attempts to selectively synthesize α-TMS glycoside failed under these conditions.
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More harsh conditions using TMSCl and TMSOTf in the presence of Et3N and 2,6-
lutidine, respectively resulted in a mixture of α- and β-TMS glycosides (3α, 3β) as
shown in entries 6 and 7. The use of NaH as a base again resulted in the formation of
3β (entry 8).
The observed stereoselectivity in the silylation of 1 forming equatorially oriented
β-TMS glycoside suggests an involvement of formation of ring-opened aldehyde and
its re-closure reaction, namely anomerization. The kinetic control is considered
important in the stereochemical outcome, which is consistent with the fact that TMS-
NEt2 is unable to silylate the axial secondary and tertiary hydroxyl groups [18,19].
However, it is premature to conclude that this is the only mechanism involved in the
anomeric silylation because stereoselective synthesis of both α- (equatorial) and β-
(axial) TMS glycosides at the tertiary anomeric hydroxyl group of the benzylated
sialic acid derivatives has been reported [11]. A reaction mechanism might involve
silylation of an aldehyde form [20] forming silyloxionium ion species in the case that
TMS amine type reagents are used.
Other commonly used silylating reagents such as TMSCl and TMSOTf, in the
presence of organic bases, produced 3, the stereochemical outcome of which reflected
the anomeric ratio of 1. These results indicate that silylation under these conditions
takes place via a mechanism similar to that with ordinary alcohols without
anomerization of hemiacetal. It seemed that we found a potential method to obtain α-
TMS glycoside; however, we could not obtain 3α under these conditions. The use of
NaH resulted in the exclusive formation of the β anomer (Table 1, entry 8).
2.3. Selective synthesis of α-TMS glycoside
We also carried out silylation of 1 using a commonly used silylation reagent,
TMS-imidazole, which predominantly furnished α-TMS galactoside 3α in 60% yield
after silica gel column chromatography (Table 1, entry 9). The anomeric proton signal
1
in its H NMR spectrum appeared at δ 5.21 with J1,2 = 3.1 Hz for H-1, indicating an
α-glycoside (Fig. 1B). We observed unidentified intermediate formation on
concentration of a reaction mixture.
Isolation of intermediates was not possible despite our attempts. However, the 1H
NMR measurements after the removal of solvent indicated that compound 1
converted into some unknown compounds, which were different from either the
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starting materials or 3 (Fig. 1C and Fig. S1B). We initially thought that the excess
amount of TMS-imidazole might have affected the observed chemical shifts or the
pyranose conformation; however, this possibility was negated by a control experiment
of silylation using methyl 2,3,4-tri-O-benzyl-α-D-glucopyranoside (Fig. S2). MS
analysis of the trimethylsilylation reaction mixture after 40 h showed a prominent ion
with m/z 681, the structure of which is not known. Fig. 2A-i shows a spectrum with
some speculated structures. However, considering the fact that TMS-imidazole
neither affects the chemical shifts nor the conformation, it is unlikely that the
unknown intermediate has a pyranose ring. MS analysis after concentration of the
reaction mixture indicated that the signal corresponding to the loss of the TMS group
(m/z 609) significantly increased (Fig. 2A-ii). MS/MS analyses of these ions were
then performed (Fig. 2B). It was found that a set of common product ions, namely
ions with m/z 523 and m/z 591, was obtained. The ion with m/z 523 is similar to that
often observed in the MS/MS of glycan and is associated with the cleavage of
glycosidic linkage.
We also analyzed the 6-O-silylated compound to examine the adduct in MS and
found that protonated imidazole can form an adduct on a molecule (Fig. S3). This
may support some of the ionic structures in which either imidazole or protonated
imidazole is non-covalently attached to a molecule as indicated in Fig. 2. However,
the NMR spectra (Fig. 1) strongly support the presence of ring-opened forms although
it is inconclusive.
One of the concerns in this reaction is whether O-5 is silylated. Although we
cannot deny the possibility, doubly silylated molecular species were not observed in
any analytical methods. Furthermore, observation of the ion with m/z 523 strengthens
our postulation of the position of the silyl group to be O-1 and not O-5, because 1) O-
5 bearing a bulky silyloxy group is unfavorable to attack C-1 and 2) the silyl group
has to migrate to O-1 after the ring closure in order to complete the observed gas-
phase reaction that is usually unfavorable.
Regarding the ring closure reaction during silica gel column chromatography, we
suspect that the imidazoyl nitrogen is protonated by silanol through hydrogen bonding
that triggers the formation of the silyloxocarbenium ion intermediate, followed by an
intramolecular nucleophilic attack by O-5. We further treated TMS glycoside with
imidazole to examine if TMS glycosides indeed anomerize in the presence of
imidazole (Table 2). It was found that α- and β-TMS glycosides formed compound 5,
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1
as evident from their H NMR spectra. Furthermore, individual α- and β-TMS
glycosides produced one of the stereoisomers predominantly (Table 2, entries 2 and 3).
We thought that these isomeric acyclic compounds might transform back into α- and
β-TMS glycosides on treatment with silica gel, a mildly acidic catalyst. However, a
mixture of α- and β-TMS glycosides was formed without significant preference when
a small amount of silica gel was added to the reaction mixture, regardless of the initial
anomeric configuration of TMS glycosides (Table 2, entries 1–3). This suggests that
the stereochemical outcome of TMS glycosides largely depends on the subtle energy
differences between the isomeric intermediates.
2.4. α-TMS glycoside formation of acetylated compound 2
Next, trimethylsilylation of acetylated compound 2 [16,17] was examined using
TMSOTf and 2,6-lutidine. Encouraged by the fact that the anomeric ratios of
substrate 2 and the formed TMS glycosides were same (Table 1, entry 11), we
examined the same reaction using only the α-isomer. In the case where 2α was
employed, α-TMS glycoside was selectively obtained (Table 1, entry 12). Increasing
the amount of 2,6-lutidine resulted in the loss of selectivity (data not shown).
Regardless of the underlying reaction mechanism, we found the required conditions to
selectively synthesize α- and β-TMS glycosides.
The anomerization of glycosides via their ring-opening and re-closure has been
reported [21–24]. Anomerization of O-, S-, and Se-glycosides into the respective α-
glycosides due probably to the endo-anomeric effect [25] involves the ring constraints
resulting either from the fused 2,3-carbonate in glucosamine derivatives or from the
coordination bridge formed by the Lewis acid, C-6 ester carbonyl, and O-5 in
glucuronic acid derivatives. The observed stereoselectivity in our trimethylsilylation
might reflect a similar mechanism observed for the above glucosamine and glucuronic
acid derivatives. This type of anomerization phenomenon might be common, although
it has not been often studied. Furthermore, the underlying chemistry might provide
insights into understanding the unsolved mechanism of the biosynthetic pathway of
the glycan-containing acyclic N-acetyl galactosamine [26].
2.5. Energy difference between α- and β-TMS glycosides
Finally, we analyzed the isolated benzyl-protected α- and β-TMS glycosides of
galactose based on energy-resolved mass spectrometry (ERMS) for further
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confirming the stereochemistry and showing the usefulness of the method in
investigating the isomeric compounds. ERMS provides information on the energies of
ionized molecules, and is useful to distinguish diastereomeric pairs, including the
anomeric configurational isomers [27–31]. Thus, ERMS analyses of 3α and 3β were
carried out, which revealed that sodiated 3α was more labile than 3β as shown by the
E50 values where a half of the precursor ions was broken down at the amplitude
determining the velocity of the trapped ions thus regulating the activation energy. (Fig.
3). The result of lability of the α-anomer was in agreement with our previous results
for various glycosides, complying with the individual isomeric structures.
3. Conclusion
We developed a method for the stereoselective synthesis of α- and β-TMS
glycosides. β-TMS glycoside was specifically synthesized from the benzyl-protected
substrate using TMS-NEt2 under the kinetic control. α-TMS glycoside was obtained
by using TMS-imidazole as a reagent after column chromatography. We also found
that α-TMS glycoside could be obtained from the acetylated hemiacetal by using
TMSOTf and 2,6-lutidine. TMS glycosides were found to be susceptible to endo-
cleavage when treated with imidazole. Finally, it was proposed that the silylation
mechanism with TMS-NEt2 was initiated by the silylation of the carbonyl group, and
not the hydroxyl group, forming silyloxonium intermediate. Considering the
importance of TMS glycosides for the synthesis of sialic acid derivatives [11] and
trehalose-type conjugates [10] and the disadvantage of the glycoside formation
reaction through alkylation at the anomeric hydroxyl group [7], the present method
may find applications in the stereoselective glycosidation reactions.
4. Experimental
4.1. General methods
CH2Cl2 was freshly distilled over CaH2 prior to use. Thin-layer chromatography
(TLC) was performed on Merck Millipore 105715, Kieselgel 60 F254/0.25 mm thick
plates (Merck Ltd., Tokyo, Japan). Visualization was accomplished with UV light and
1% Ce(SO4)2–1.5% (NH4)6MoO24·4H2O–10% H2SO4 solution or 10% H2SO4 solution,
followed by heating. Silica gel column chromatography was conducted on Wakogel
C–300 (Wako Pure Chemical Industries, Ltd. Osaka, Japan). Methyl 2,3,4-tri-O-
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benzyl-α-
D
-glucopyranoside was purchased from Tokyo Chemical Industry Co., Ltd.
1
(Tokyo, Japan). H NMR (500 MHz) spectra were recorded on an AVANCE 500
spectrometer (Bruker Biospin GmbH, Bremen, Germany) in deuterated solvent using
Me4Si (0.00 ppm) as the internal standard. 13C NMR (125 MHz) spectra were
recorded with the same spectrometer using Me4Si (0.00 ppm) or the solvent peak
(CDCl3; 77.0 ppm or CD3OD; 49.0 ppm) as the internal standard. High-resolution
mass spectra (HRMS) were recorded on Shimadzu LCMS-IT-TOF (ion-trap TOF
mass spectrometer with reflectron) mass spectrometer coupled with ESI interface
(Shimadzu Corp.; Kyoto, Japan). MS/MS experiments were performed by using
amaZon (Bruker Daltonics GmbH, Bremen, Germany) equipped with electrospray
ionization (ESI) interface.
4.2. Typical procedure for the synthesis of TMS glycosides of tetra-O-benzyl-
D
-galactopyranose
4.2.1. Trimethylsilyl 2,3,4,6-tetra-O-benzyl-β-D-galactopyranoside (3β)
To a stirred solution of compound 1 (α:β = 58:42) [15] (22.3 mg, 0.0413 mmol)
dissolved in CH2Cl2 (1 mL) at 0 °C, TMS-NEt2 (77.6 µL, 0.412 mmol) was added
under nitrogen atmosphere. The reaction mixture was stirred at 0 °C for 1 h, and
concentrated to dryness under vacuum. The resulting residue was purified on a
column of silica gel (hexane/EtOAc = 2:1 containing 1% triethylamine) to yield
compound 3β (21.1 mg, 84%). This solvent system was used to recover both anomers.
1
Rf 0.30 (hexane/EtOAc = 7:1). H NMR (500 MHz, CDCl3) δ: 7.37–7.24 (m, 20H,
ArH), 4.94 (d, 1H, CHPh), 4.90 (d, 1H, CHPh), 4.77 (d, 1H, CHPh), 4.72 (dd, 2H,
CH2Ph), 4.61 (d, 1H, CHPh), 4.61 (d, 1H, J1,2 = 7.5 Hz, H-1), 4.42 (dd, 2H, CH2Ph)
3.88 (broad d, 1H, J3,4 = 2.8 Hz, H-4), 3.77 (dd, J2,3 = 9.8 Hz, H-2), 3.59–3.53 (m, 3H,
H-5, H-6a, and H-6b) 3.50 (dd, H-3), 0.18 (s, 9H, SiCH3). HRMS, Calcd. for
C37H44NaO6Si+: 635.2799, observed: 635.2768; Low energy CID MS/MS observed
m/z (>5% intensity at ca half of the precursor ion was consumed): 395 (21.7%), and
545 (5.9%).
4.2.2. Trimethylsilyl 2,3,4,6-tetra-O-benzyl-α-D-galactopyranoside (3α)
TMS-imidazole (57 µL, 0.406 mmol) was added, under nitrogen atmosphere, to a
stirred solution of compound 1 (21.2 mg, 0.0392 mmol) dissolved in CH2Cl2 (1 mL)
at
−40 °C. The reaction mixture was stirred for 40 h, and was then concentrated to
dryness under vacuum. The resulting residue was purified on a column of silica gel
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(hexane/EtOAc = 2:1 containing 1% triethylamine) to yield the α/β mixture of
compound 3 (14.5 mg, 60%). The α-anomer was obtained after silica gel column
chromatography (hexane/EtOAc = 2/1). Rf 0.30 (hexane/EtOAc = 7:1). 1H NMR (500
MHz, CDCl3) δ: 7.39-7.22 (m, 20H, ArH), 5.21 (d, J1,2 = 3.1 Hz, H-1), 4.95 (d, 1H,
CHPh), 4.81 (d, 1H, CHPh), 4.78 (d, 1H, CHPh), 4.74 (d, 1H, CHPh), 4.65 (d, 1H,
CHPh), 4.56 (d, 1H, CHPh), 4.43 (dd, 2H, CH2Ph), 4.08 (m, 1H, H-5), 4.01 (broad d,
1H, J3,4 =2.7 Hz, H-4), 3.98 (dd, 1H, J2,3 = 10.0 Hz, H-2), 3.94 (dd, 1H, H-3), 3.58
(dd, 1H, J5,6a = 7.9, J6a,6b = 9.1 Hz, H-6a), 3.47 (dd, 1H, J5,6b = 5.6 Hz, H-6b), 0.15 (s,
9H, SiCH3). HRMS, Calcd. for C37H44NaO6Si+: 635.2799, observed: 635.2750; Low
energy CID MS/MS observed m/z (>5% intensity at ca half of the precursor ion was
consumed): 395 (30.4%) and 303 (7.0%).
4.2.3. Acyclic compound
The formation of an acyclic compound was confirmed by ESI-MS by taking an
aliquot of the following reaction mixture. Compound 1 (21.0 mg, 39 µmol) and TMS-
imidazole (57 µL, 390 µmol) in CH2Cl2 (1 mL) was stirred at −40 °C for 40 h. An
aliquot of the reaction mixture was taken, diluted with MeOH, and analyzed by ESI-
MS.
4.3. Procedure for the synthesis of α-TMS glycoside of tetra-O-acetyl-
galactopyranose
4.3.1. Trimethylsilyl 2,3,4,6-tetra-O-acetyl-α-
D-
D
-galactopyranoside (4α)
To a stirred solution of a mixture of compound 2 [17] and 1,3,4,6-tetra-O-acetyl-
α-D
-galactopyranose [32] (approximately 3:1 mixture according to 1H NMR, 449 mg,
1.29 mmol) dissolved in CH2Cl2 (3 mL) was added 2,6-lutidine (0.415 ml, 3.57 mmol)
and TMSOTf (0.699 mL, 3.87 mmol), and the solution was stirred at 0 °C for 3 h
under a nitrogen atmosphere. The reaction mixture was diluted with CH2Cl2, washed
with brine, dried over Na2SO4, and then evaporated. The residue was purified on a
column of alumina gel (toluene/EtOAc = 2:1 containing 1% triethylamine) to yield
compound 4α (365 mg, quantitative yield based on the amount of 2 in the mixture). Rf
1
0.75 (toluene/EtOAc = 2:1). H NMR (500 MHz, CDCl3) δ: 5.44 (m, 2H, H-4, H-1),
5.36 (dd, 1H, H-3), 5.07 (dd, 1H, J2,3 = 10.7 Hz, H-2), 4.36 (m, 1H, H-5), 4.08 (m, 2H,
H-6a, H-6b), 2.15, 2.06, 2.04, 1.99 (4s, 12H, 4CH3), 0.16 (s, 9H, SiMe3).
4.4. Energy resolved mass spectrometry (ERMS)
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4.4.1. Conditions for the ERMS experiments
Samples were analyzed using a quadrupole ion-trap mass spectrometer (QIT-MS)
coupled with an electrospray interface (amaZon ETD, Bruker Daltonics GmbH,
Bremen, Germany). Samples dissolved in MeOH were introduced into the ion source
via infusion (flow rate, 60 µL/h). The parameters for analysis were: (1) drying
temperature: 140 °C, (2) nebulizer gas (N2): 8 psi, (3) drying gas (N2): 5.0 L/min, (4)
scan range: m/z 100–1500, (5) Trap Drive level: 100%, (6) ICC target: 400,000, (7)
maximum acquisition time: 200 ms, (8) average: 10 spectra, (9) cut-off: 27% and (10)
collision gas (He): 1.5 × 10−6 mbar.
In our MSn experiments, the end-cap radio frequency (r.f.) amplitude was raised
by 0.05 V increments until the precursor ion could no longer be detected.
Isotopic peaks with [Ii + 1] and [Ii + 2], where In indicated a fragment ion, were
included in the calculations. Standard MS/MS spectra are the extracts of these ERMS
at designated amplitudes.
See reference 28 for data handling.
Acknowledgments
This work was supported in part by Japan Society for the Promotion of Science
(JSPS); Grant-in-Aid for Challenging Exploratory Research 15K14399. H. A.
acknowledges a mission from Egypt-Japan Education Partnership: Human Resource
Development Project.
References
[1] H.S. Isbell, H.L. Frush, Mechanisms for the formation of acetylglycosides and
orthoesters from acetylglycosyl halides, J. Res. Natl. Bur. Standards 43 (1949)
161–71.
[2] P.J. Garegg, Y. Norberg, Observations on silver trifluoromethane sulfonate-
promoted syntheses of 1,2-trans-glycosides from acylated glycosyl bromides,
Acta Chem. Scand. B33 (1979) 116–8.
[3] R.U. Lemieux, K.B. Hendriks, R.V. Stick, K. James, Halide ion catalyzed
glycosidation reactions. Syntheses of α-linked disaccharides, J. Am. Chem. Soc.
97 (1975) 4056–62.
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ACCEPTED MANUSCRIPT
[4] G. Wulff, G. Röble, Results and problems of O-Glycoside synthesis, Angew.
Chem. Int. Ed. 13 (1974) 157–216.
[5] A.J. Ratcliffe, B. Fraser-Reid, Generation α-D-glycopyranosylacetonitrilium
ions. Concerning the reverse anomeric effect, J. Chem. Soc. Perkin Trans. 1
(1990) 747–50.
[6] D. Crich, S. Sun, Formation of β-mannopyranosides of primary alcohols using
the sulfoxide method, J. Org. Chem. 61 (1996) 4506–7.
[7] Y.E. Tsvetkov, W. Klotz, R.R. Schmidt, Anomeric O alkylation, 9.
disaccharide synthesis via anomeric O alkylation, Liebigs Ann. Chem. (1992)
371–5.
[8] R.R. Schmidt, A. Esswein, Simple synthesis of KDO α-glycosides by
anomerically selective O-alkylation, Angew. Chem. Int. Ed. 27 (1988) 1178–80.
[9] H. Nguyen, D. Zhu, X. Li, J. Zhu, Stereoselective construction of β-
mannopyranosides by anomeric O-alkylation: Ssynthesis of the trisaccharide
core of N-linked glycans, Angew. Chem. Int. Ed. Engl. 55 (2016) 4767–71.
[10] K. Hiruma, O. Kanie, C.-H. Wong, Synthesis of analogs of 1,1-linked
galactosyl mannoside as mimetics of sialyl Lewis X tetrasaccharide,
Tetrahedron 54 (1998) 15781–92.
[11] K. Suzuki, S. Daikoku, S.-H. Son, Y. Ito, O. Kanie, Synthetic study of 3-
fluorinated sialic acid derivatives, Carbohydr. Res. 406 (2015) 1–9.
[12] R.R. Schmidt, J. Michel, M. Roos, Glycosyl imidates 12. Direct synthesis of O-
α- and O-β-glycosyl imidates, Liebigs Ann. Chem. 1984 (1984) 1343–57.
[13] Y. Nakahara, H. Iijima, S. Shibayama, T. Ogawa, A highly stereoselective
synthesis of di- and trimeric sialosyl Tn epitope: a partial structure of
glycophorin A, Tetrahedron Lett. 31 (1990) 6897–900.
[14] J. Li, B. Yu, A modular approach to the total synthesis of tunicamycins, Angew.
Chem. Int. Ed. 54 (2015) 6618–21.
[15] R. Panchadhayee, A.K. Misra, N-Bromosuccinimide-mediated conversion of
allyl glycosides to glycosyl hemiacetals, J. Carbohydr. Chem. 29 (2010) 76–83.
[16] E. Kaya, F. Sonmez, M. Kucukislamoglu, M. Nebioglu, Selective anomeric
deacetylation using zinc acetate as catalyst, Chem. Papers 66 (2012) 312–315.
[17] R.U. Lemieux, “Some implications in carbohydrate chemistry of theories
relating to the mechanisms of replacement reactions” in Adv. Carbohydr. Chem.,
Academic Press, 9 (1954) 1–57.
12

ACCEPTED MANUSCRIPT
[18] E.W. Yankee, U. Axen, G.L. Bundy, Total synthesis of 15-
methylprostaglandins, J. Am. Chem. Soc. 96 (1974) 5865–76.
[19] M. Lalonde, T.H. Chan, Use of organosilicon reagents as protective groups in
organic synthesis, Synthesis 9 (1985) 817–45.
[20] R.J.C. Corriu, J.J.E. Moeau, M. Pataud-Sat, Silylamines in organic synthesis.
Reactivity of N,N-bis(silyl)enamines toward electrophiles. A route to substituted
2-aza-1,3-butadienes and pyridines, J. Org. Chem. 55 (1990) 2878–84.
[21] S. Manabe, K. Ishii, D. Hashizume, H. Koshino, Y. Ito, Evidence for endocyclic
cleavage of conformationally restricted glycopyranosides, Chem. Euro. J. 15
(2009) 6894–901.
[22] W. Pilgrim, P.V. Murphy, SnCl4- and TiCl4-catalyzed anomerization of acylated
O- and S-glycosides: Analysis of factors that lead to higher α:β anomer ratios
and reaction rates, J. Org. Chem. 75 (2010) 6747–55.
[23] H. Satoh, S. Manabe, Y. Ito, H.P. Lüthi, T. Laino, J. Hutter, Endocyclic
cleavage in glycosides with 2,3-trans cyclic protecting groups, J. Am. Chem.
Soc. 133 (2011) 5610–9.
[24] A.W. McDonagh, M.F. Mahon, P.V. Murphy, Lewis acid induced
anomerization of Se-glycosides. Application to synthesis of α-Se-GalCer, Org.
Lett. 18 (2016) 552–5.
[25] H. Booth, T. Bruce Grindley, K. A. Khedhair , The anomeric and exo-anomeric
effects in 2-methoxytetrahydropyran, J. Chem. Soc. Chem. Commun. (1982)
1047-8.
[26] E. Vinogradov, K. Bock, A new type of glycosidic linkage: An open-chain
acetal-linked N-acetylgalactosamine in the core part of the lipopolysaccharides
from proteus microorganisms, Angew. Chem. Int. Ed. 38 (1999) 671–4.
[27] S. Daikoku, T. Ako, R. Kato, I. Ohtsuka, O. Kanie, Discrimination of 16
structural isomers of fucosyl galactoside based on energy-resolved mass
spectrometry, J. Am. Soc. Mass Spectrom. 18 (2007) 1873–9.
[28] O. Kanie, A. Kurimoto, Y. Kanie, S. Daikoku, A. Ohtake, K. Suzuki, Analysis
of behavior of sodiated sugar hemiacetals under low-energy collision-induced
dissociation conditions and application to investigating mutarotation and
mechanism of a glycosidase, Proc. Jpn. Acad., Ser. B 85 (2009) 204–15.
13

ACCEPTED MANUSCRIPT
[29] S. Daikoku, T. Ako, A. Kurimoto, O. Kanie, Anomeric information obtained
from a series of synthetic trisaccharides using energy resolved mass spectra, J.
Mass Spectrom. 42 (2007) 714–23.
[30] Y. Shioiri, K. Suzuki, S. Daikoku, A. Kurimoto, Y. Ito, O. Kanie, Stereospecific
generation and analysis of α- and β-hemiacetals of monosaccharides in gas
phase, Carbohydr. Res. 382 (2013) 43–51.
[31] O. Kanie, Y. Shioiri, K. Ogata, W. Uchida, S. Daikoku, K. Suzuki, S. Nakamura,
Y. Ito, Diastereomeric resolution directed towards chirality determination
focussing on gas-phase energetics of coordinated sodium dissociation, Sci. Rep.
6 (2016) 24005, doi:10.1038/srep24005.
[32] M.L. Wolfrom, A. Thompson, M. Inatome, Acylmigration in the
structure, J. Am. Chem. Soc. 79 (1957) 3868–3871.
D-galactose
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Table 1. Synthesis of TMS glycosidea
starting
material
entry
α/βb
reagents
temp. (°C)
time (h)
product
α/βb
yield (%)
1
2
3
4
5
56/44
59/41
57/43
52/48
55/45
TMS-NMe2
TMS-NMe2
TMS-NMe2
TMS-NEt2
TMS-NEt2
−20
−40
r.t.
3
21
0.5
2
31/69 quantc
50/50 90c
1
1
1
1
1
3
3
3
3
3
1/99
1/99
1/99
90c
93c
90c
0
−40
23
6
52/48
TMSCl–Et3N
−40→20
23
55/45 quantc
1
3
TMSOTf–
7
59/41
0
2.5
69/31 90c
1
3
2,6-lutidine
8
9
54/46
53/47
53/47
TMSCl–NaH
r.t.
−40
r.t.
1
1/99
99/1
90c
60d
1
1
1
3
3
3
TMS-imidazole
TMS-imidazole
40
0.5
10
63/37 83d
TMSOTf–
2,6-lutidine
TMSOTf–
2,6-lutidine
11
12
77/23
0
0.5
3
77/23 81
2
2
4
4
100/0e
100/0 quantc
0
aAll reactions were performed in CH2Cl2.
bThe ratio was estimated by 1H NMR.
cYields were estimated based on 1H NMR.
dYield obtained after column chromatography (hexane–EtOAc) in the presence of 1% Et3N
eThe compound was obtained from acetobromogalactose [17].
Table 2. Reaction of TMS glycoside with imidazole to form acyclic compound 5a and its
ring-closure reaction
3
α/βb
5
3
α/βb
entry
silica gel
solvents for ring closure
pro-α/pro-βc
d
d
d
e
1
2
3
4
57/43
95/5
1/99
99/1
71/29
86/14
7/93
hexane-EtOAc
hexane-EtOAc
hexane-EtOAc
hexane-EtOAc
65/35
56/44
57/43
67/33
99/1
aReaction was carried out in CH2Cl2 at r.t. in the presence of imidazole.
bThe ratio was estimated by 1H NMR.
cTentative assignments.
dAliquot of silica gel was added to the reaction mixture after solvent exchange where the same solvent system as
that in column chromatography was used.
eReaction mixture was loaded onto a column of silica gel (hexane-EtOAc).
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Scheme and Figure captions
Scheme 1. Trimethylsilylation reactions of galactopyranoses were investigated.
Figure 1. 1H NMR spectra of α- and β- TMS-glycosides of 2,3,4,6-tri-O-benzyl-
galactopyranoses and the silyaltion reaction mixture using TMS-imidazole after
D
-
concentration. The spectra covering 0–8 ppm are given in supporting information.
Figure 2. MS spectra of the reaction mixture and MS/MS spectra of the observed ions
with TMS-imidazole as the reagent. A: (i) Ion with m/z 681 observed as a major
signal. (ii) Ion with m/z 609 corresponding to the loss of TMS group was observed in
addition to an ion with m/z 609. B: (i) MS/MS of the ion with m/z 681. (ii) MS/MS of
the ion with m/z 609. Possible structures of important ions are shown. Diamond
indicates the precursor ions.
Figure 3. Comparison of E50 values of α- and β-TMS galactosides. Repeated ERMS
experiments and non-linear regression analyses provided E50 values. Mean and SD
values are shown.
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Highlights
•
Trimethylsilylation reactions at anomeric position of acetyl and benzyl protected
galactopyranoses were investigated.
•
•
Trimethylsilylation reaction mechanisms were investigated.
It was found that trimethylsilyl amines and imidazole undergo addition of silyl
group at aldehyde forming silyoxonium intermediate.
•
•
It was also found that trimethylsilyl chloride and triflate proceed through
silylation of anomeric hydroxyl group.
We successfully synthesized α- and β-TMS glycosides of galactopyranoses
stereospecifically.