Facile Diastereoseparation of Glycosyl Sulfoxides by Chiral Stationary Phase

Article abstract of DOI:10.1002/chir.22610

Separation of the diastereomers of glycosyl sulfoxides differing in the sulfur chirality has been difficult. This article presents a fast and scalable method for their diastereoseparation using a chiral stationary phase. The usefulness of this method was

Full text of DOI:10.1002/chir.22610

CHIRALITY (2016)
Special Issue Article
Facile Diastereoseparation of Glycosyl Sulfoxides by Chiral Stationary
Phase
1
1
1
2
2
1
*
*
TOHRU TANIGUCHI, MAI ASAHATA, AKIHITO NASU, YUKATSU SHICHIBU, KATSUAKI KONISHI, AND KENJI MONDE
1Faculty of Advanced Life Science, Frontier Research Center for Post-Genome Science and Technology, Hokkaido University, Sapporo, Japan
2Faculty of Environmental Earth Science, Hokkaido University, Sapporo, Japan
ABSTRACT
Separation of the diastereomers of glycosyl sulfoxides differing in the sulfur chi-
rality has been difficult. This article presents a fast and scalable method for their
diastereoseparation using a chiral stationary phase. The usefulness of this method was demon-
strated in a 500-mg scale separation within 20 min, and in the separation of trisaccharyl sulfoxide
diastereomers. Chirality 00:000–000, 2016. © 2016 Wiley Periodicals, Inc.
KEY WORDS: chiral HPLC; sulfur chirality; carbohydrate; Kahne glycosidation
Sulfoxide is an important class of molecules used as a
blockbuster drug and as essential reagents for a wide range
of organic reactions.1 Among various sulfoxides, the chemis-
try of glycosyl sulfoxide (Fig. 1) has extensively been studied
since Kahne invented its use as a substrate for a glycosidation
reaction, now called Kahne glycosidation or sulfoxide
glycosidation.2 This glycosidation has facilitated the synthe-
sis of various natural products and drug candidates including
cyclamycin 0, everninomicin, and moenomycin A,3–6 sup-
ported by its advantages such as its ability to react with bulky
glycosyl acceptors, high stereoselectivity, and relatively mild
reaction condition.7,8 In Kahne glycosidation, glycosyl sulfox-
ides are normally used as a mixture of sulfinyl diastereomers
that is obtained by oxidation of the corresponding glycosyl
sulfides, assuming the reactivity of both diastereomers to be
similar.2 While this assumption may be correct for some
cases, Ferrières et al. reported an example where a pair of di-
astereomers yielded different reaction products.9 Although
whether such a difference can be observed for other pairs is
yet to be revealed, further investigation into the difference
in their reactivities is needed for a possible improvement of
the efficiency of Kahne glycosidation. However, such studies
have been hampered by the difficulties in their separation.
In spite of their expected diastereomeric physical differ-
ences, their behavior on silica-gel thin-layer chromatography
(TLC) is generally quite similar: for example, the differences
in the Rf values of various sulfinyl glycosyl diastereomers
prepared in a recent study were all within 0.02,10 and it is
not rare to see them described as an inseparable
diastereomixture.11 On the other hand, diastereoselective
oxidations are only applicable to limited glycosyl sulfides,
especially to those with tailored protective groups or with
α-anomeric configuration.12,13 Development of a fast and scal-
able method to separate sulfinyl glycosyl diastereomers
should facilitate chemical and biological14,15 studies on their
sulfinyl chirality. Previously, our work on brassicanal C, a nat-
ural product with a sulfinate functional group, has shown that
its racemate was efficiently enantioseparated16 by using a
cellulose-based chiral high-performance liquid chromatogra-
phy (HPLC).17,18 This observation, as well as reported suc-
cessful enantioseparations of chiral sulfoxides by chiral
© 2016 Wiley Periodicals, Inc.
HPLC,19–21 led us to apply chiral stationary phases to
diastereoseparation of glycosyl sulfoxides. Here, through
the first systematic application of chiral stationary phase to a
variety of glycosyl sulfoxides, we demonstrate the versatility
of this method in separating their diastereomers.
MATERIALS AND METHODS
1H NMR (500 MHz) and 13C NMR (125 MHz) spectra were recorded
on a Varian (Palo Alto, CA) Inova instrument at 25°C. Chemical shift
values (δ) are reported in ppm relative to CDCl3 (1H, δ 7.26; 13C, δ
77.00), CD3OD (1H, δ 4.87; 13C, δ 49.15), or tetramethylsilane, while
coupling constant values (J) are in Hz. The following abbreviations were
used for signal multiplicities: s = singlet; d = doublet; t = triplet; m = multi-
plet. Electrospray ionization mass spectrometry was conducted by using
a JEOL (Japan) JMS-T100LP spectrometer. Optical rotation was mea-
sured on a Jasco (Japan) P-1020 polarimeter at the sodium D-line using
a 1-cm optical cell under ambient temperature, and reported as [α]D (con-
centration in grams/100 mL solvent). TLC was performed on 0.2 mm sil-
ica gel plates (Merck, Darmstadt, Germany; 60 F-254). Normal column
chromatography was carried out on silica gel (Kanto 60N, 40–50 μm). An-
alytical chiral HPLC was conducted on a Jasco PU-2086 Plus pump
equipped with a Jasco UV-2075 UV spectrophotomeric detector, using a
CHIRALPAK IA, IB, or IC guard column (0.4 cm φ x 1 cm) and its
analytical column (0.46 cm φ x 25 cm) from Daicel (Japan).22 Large-scale
chiral HPLC was conducted using a uf-3020SZB2 pump (Denso Sangyo,
Japan) equipped with a Shimamura (Japan) YRU-880 midget UV-RI detec-
tor, using a CHIRALFLASH IC column (3.0 cm φ x 10 cm) from Daicel.22
Crystal data were collected on a Bruker (Billerica, MA) SMART Apex II
CCD diffractometer with graphite-monochromated Mo Kα radiation
(λ = 0.71073 Å) at 90 K. The crystal structure was solved by direct
methods (SHELXS-97)23 and refined by full-matrix least-squares methods
on F2 (SHELXL-97)24 with APEX II software. The sulfur, carbon, and
oxygen atoms were refined anisotropically, and the hydrogen atoms were
refined isotropically.
*Correspondence to: T. Taniguchi or K. Monde, Faculty of Advanced Life Sci-
ence, Frontier Research Center for Post-Genome Science and Technology,
Hokkaido University, Kita 21 Nishi 11, Sapporo 001-0021, Japan. E-mail:
ttaniguchi@sci.hokudai.ac.jp; kmonde@sci.hokudai.ac.jp
Received for publication 17 February 2016; Accepted 19 April 2016
DOI: 10.1002/chir.22610
Published online in Wiley Online Library
(wileyonlinelibrary.com).

TANIGUCHI ET AL.
Fig. 1. Structures of aromatic glycosyl sulfoxides and their chirality. As representative structures, sulfoxides of glucose monosaccharide are shown.
(SS)-8: 1H NMR (CDCl3) δ 7.95-7.26 (m, 25H, ArH), 5.95 (t, J = 9.3 Hz,
Phenyl 2,3,4,6-tetra-O-benzoyl-1-sulfinyl-β-D-
glucopyranoside (8)25
1H, H-3), 5.71 (t, J = 9.5 Hz, 1H, H-2), 5.54 (t, J = 9.7 Hz, 1H, H-4), 4.87 (d,
J = 9.8 Hz, 1H, H-1), 4.66 (dd, J = 2.6, 12.5 Hz, 1H, H-6a), 4.42 (dd, J = 4.6,
Phenyl 2,3,4,6-tetra-O-bezoyl-1-thio-β-D-glucopyranoside26 (1.00 g,
12.4 Hz, 1H, H-6b), 4.21 (ddd, J = 2.8, 4.5, 10.0 Hz, 1H, H-5); [α]D À 52.2 (c
1.45 mmol) in CH2Cl2 (17 mL) was added Ac2O (220 μL, 1.6 equiv), silica
1.0, CHCl3).
gel (80 mg), and H2O2 (200 μL, 1.2 equiv. from a 34% aqueous solution)
and stirred at room temperature for 8 h. The mixture was then diluted
RESULTS AND DISCUSSION
Synthesis and Diastereoseparation of Monosaccharyl
Sulfoxides
with EtOAc, filtered to remove the silica gel, washed with 10% Na2S2O3
solution and brine, then dried (MgSO4) and concentrated. Part of the re-
sultant diastereomixture of 8 (ca. 500 mg, dr = 1:1) was directly subjected
to a CHIRALFLASH IC column (3.0 cm φ x 10 cm) from Daicel, providing
242 mg of the first-eluted and 236 mg of the second-eluted diastereomers
First, in order to obtain pairs of monosaccharyl sulfinyl
glycosyl diastereomers, several glycosyl sulfides with
β-anomeric configuration were prepared. Glycosyl sulfoxide
with α-anomeric configuration was not tested because
oxidation of the corresponding sulfide is known to proceed
with high stereoselectivity to yield mostly one diastereo-
mer.12 The glycosyl sulfides were oxidized using mCPBA or
1
(hexane:EtOAc =1.5:1). Their H NMR spectra were consistent with the
data reported in Ref. 25. (RS)-8: 1H NMR (CDCl3) δ 8.00-7.30 (m, 25H,
ArH), 6.02 (t, J = 9.3 Hz, 1H, H-3), 5.92 (t, J = 9.6 Hz, 1H, H-2), 5.63 (t,
J = 9.8 Hz, 1H, H-4), 4.62 (d, J = 9.8 Hz, 1H, H-1), 4.56 (dd, J = 2.7,
12.3 Hz, 1H, H-6a), 4.45 (dd, J = 6.3, 12.3 Hz, 1H, H-6b), 4.14 (ddd,
J = 2.8, 6.4, 10.0 Hz, 1H, H-5); [α]D + 24.8 (c 1.0, CHCl3).
Fig. 2. Separation of the diastereomers of 8 by chiral stationary phase. Chromatograms were obtained using (a) CHIRALPAK IC with ~0.1 mg sample loading or
(b) CHIRALFLASH IC with ~500 mg sample loading, detected at 254 nm. Eluent: hexane-EtOAc 60:40 at the speed of (a) 1 mL/min or (b) 10 mL/min. The UV
absorption of the first-eluted diastereomer by CHIRALFLASH IC exceeded the range of the detection limit of the UV detector. (c) 1H NMR spectra of the first-
and second-eluted diastereomers obtained by CHIRALFLASH IC: 500 MHz, CDCl3.
Chirality DOI 10.1002/chir

FACILE DIASTEREOSEPARATION OF GLYCOSYL SULFOXIDES
Chirality DOI 10.1002/chir

TANIGUCHI ET AL.
H2O2/Ac2O/SiO2,27 which resulted in 1:1–3:1 mixtures of
diastereomers 1–12. Around 0.1 mg of each mixture was
injected into an analytical polysaccharide-based CHIRALPAK
IA, IB, or IC column (4.6 mm φ × 250 mm).18,22 For a quick
screening to see whether these diastereomers can be sepa-
rated, all samples were subjected to a hexane-EtOAc gradient
system using conditions A (hexane:EtOAc =90:10 to 1:99 over
20 min) or B (40:60 to 1:99 over 20 min) rather than optimiz-
ing the conditions for each sample.28 Nevertheless, as
summarized in Table 1, diastereomers of all monosaccaryl
sulfoxides 1–12 were efficiently separated with separation
factors of 1.3 or higher when HPLC column and eluent gradi-
ent condition are adequate. This separation method is applica-
ble to a wide range of monosaccharides (glucose, galactose,
mannose, and glucosamine) and protective groups (methyl,
benzyl, acetyl, benzoyl, pivaloyl, N-phthaloyl), which are com-
monly used in sugar chemistry. A typical chromatogram is
shown in Figure 2a. In most cases, CHIRALPAK IA and IC
achieved highly efficient diastereoseparation. Separation
efficiency can be improved by using a suitable gradient
system, as exemplified by the comparison of the outcomes
of conditions A and B for compounds 5 and 8: for 8, its
separation factor was dramatically improved from 1.2
(condition A) to 2.8 (a constant solvent ratio of hexane:EtOAc
=60:40, condition C). Although a hexane-EtOAc solvent
system is satisfactory, other solvent systems are applicable
for their separation (data not shown).
With the above promising results in hand, we then tested
the feasibility of
a
large-scale separation using
a
CHIRALFLASH IC column (30 mm φ × 100 mm).22 About
500 mg of a diastereomixture of 8 was loaded onto the
column and eluted with a hexane-EtOAc 60:40 solvent system
without gradient at a flow speed of 10 mL/min. The resultant
chromatogram showed an almost perfect separation of the
two diastereomers (Fig. 2b). Only one run of chromatography
within 20 min afforded 242 mg of the first-eluted diastereomer
and 236 mg of the second-eluted, both of whose purities were
1
confirmed by H NMR (Fig. 2c). Thus, this result shows the
practicality of the use of a chiral stationary phase for separa-
tion of sulfinyl diastereomers of glycosyl sulfoxides even at
a preparative scale.29 Judging from the chromatogram in
Figure 2b, this separation method should be applicable to a
larger amount of samples and a faster flow speed to further
increase its robustness.
Stereochemical Determination
We also conducted a stereochemical determination of the
isolated isomers of 8. Although circular dichroism (CD) and
nuclear magnetic resonance (NMR) spectroscopies were pro-
posed to be useful in determining the absolute configuration
of glycosyl sulfoxides attached to an alkyl group,30 their valid-
ity to those attached to an aromatic group is yet to be exam-
ined. Moreover, the benzoyl groups in 8 may perturb the
proposed empirical relationship between CD curve and sulfur
chirality. Therefore, the current study employed chemical
correlation and X-ray crystallography. While working on sev-
eral glycosyl sulfoxides, it was found that one diastereomer
of a perpivaloylated glucose derivative 102,27 formed a fine
crystal suited for X-ray crystallography (Fig. 3a).31 This
established for the first time the stereochemistry of 10 as
shown in Figure 3b. In order to chemically correlate the
stereochemistries of 8 and 10, the pivaloyl groups of each
diastereomer of 10 were hydrolyzed using NaOH/MeOH/
Chirality DOI 10.1002/chir

FACILE DIASTEREOSEPARATION OF GLYCOSYL SULFOXIDES
Fig. 3. (a) ORTEP drawing of (SS)-10. Thermal ellipsoids are shown at the 50% probability level. (b) Stereochemical assignment of 8 through chemical
correlation. Reagents and conditions: (a) NaOH, MeOH, H2O; (b) MeONa, MeOH.
H2O. The deprotection proceeded without epimerization of
the sulfur chirality, and thus the configuration of two isomers
of 1432 was determined. In a similar manner, separated
HPLC. A diastereomixture of 13 was obtained starting with
peracetylation of laminaritriose, followed by glycosidation
using PhSH, and oxidation using mCPBA (dr = 3.5:1). Despite
the small structural difference of the diastereomers of 13, the
pair was clearly separated on CHIRALPAK IA with a separa-
tion factor of 1.4 (Table 1). This result demonstrated the effec-
tiveness of chiral stationary phase even for sulfinyl
diastereomers that would otherwise be difficult to separate.
diastereomers of
8
were converted to 14 using
MeONa/MeOH, from which the first and second eluted
diastereomers of 8 from the CHIRALFLASH IC column were
characterized as SS and RS, respectively (Fig. 3b). In a similar
manner, the absolute configurations of 5 and 11 were
determined, as shown in Table 1. The stereochemistries of
3 and 12 described in Table 1 were previously determined
by chemical correlation and X-ray crystallography.11,33
CONCLUSION
This article presents a new method to separate sulfinyl dia-
stereomers of glycosyl sulfoxides using a chiral stationary
phase. This method is fast and scalable: ~500 mg of a diaste-
reomeric mixture was separated within 20 min. Yet the
robustness of this method can be expanded even more by fur-
ther optimizing conditions for solvent system, flow rate,
Chirality DOI 10.1002/chir
Synthesis and Diastereoseparation of Trisaccharyl
Sulfoxide
Last, in order to see the efficacy of this separation method to
further complex diastereomers, a diastereomixture of a
trisaccharyl sulfoxide 13 was prepared and subjected to chiral

TANIGUCHI ET AL.
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sample amount, column type, and repetitive sample
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but also even oligosaccharyl ones. Although there are cases
where diastereomers of glycosyl sulfoxides are separable by
a normal SiO2 column,8,15 we believe that this approach
complements conventional methods and accelerates studies
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“Promotion for Young Research Talent and Network” from
Northern Advancement Center for Science & Technology
(NOASTEC), the Suhara Memorial Foundation, and a
Grant-in-Aid for Scientific Research from JSPS.
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SUPPORTING INFORMATION
Additional supporting information may be found in the
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Chirality DOI 10.1002/chir