Direct aqueous synthesis of non-protected glycosyl sulfoxides; Weak inhibitory activity against glycosidases

Article abstract of DOI:10.1016/j.carres.2015.06.003

A flavinium catalyst, in conjunction with hydrogen peroxide as stoichiometric oxidant, allowed the aqueous conversion of non-protected thioglycosides into the corresponding glycosyl sulfoxides. These glycosyl sulfoxides displayed only very weak inhibitory

Full text of DOI:10.1016/j.carres.2015.06.003

Carbohydrate Research 413 (2015) 123e128
Contents lists available at ScienceDirect
Carbohydrate Research
journal homepage: www.elsevier.com/locate/carres
Direct aqueous synthesis of non-protected glycosyl sulfoxides; weak
inhibitory activity against glycosidases
, b, *
Stewart R. Alexander a, Andrew J.A. Watson a, Antony J. Fairbanks a
a Department of Chemistry, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand
b Biomolecular Interaction Centre, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand
a r t i c l e i n f o
a b s t r a c t
Article history:
A flavinium catalyst, in conjunction with hydrogen peroxide as stoichiometric oxidant, allowed the
aqueous conversion of non-protected thioglycosides into the corresponding glycosyl sulfoxides. These
glycosyl sulfoxides displayed only very weak inhibitory activity against corresponding glycosidases.
© 2015 Elsevier Ltd. All rights reserved.
Received 29 May 2015
Accepted 4 June 2015
Available online 14 June 2015
Keywords:
Glycosyl sulfoxide
Thioglycoside
Oxidation
Biomimetic catalysis
Glycosidase inhibition
1. Introduction
active site. In this respect, whilst it has been known for a long time
that the positive charge of protonated imino sugars is important for
Glycosyl sulfoxides, first introduced by Kahne and co-workers,1
have found significant utility as highly reactive glycosyl donors. In
particular they have been used to perform glycosylation reactions
with large or unreactive glycosyl acceptors in cases where other
donors had failed.2e4 Typically they are produced by oxidation of
the corresponding thioglycoside with meta-chloroperoxybenzoic
acid (m-CPBA), although this reagent does suffer from limitations,
including a propensity to cause over-oxidation to the sulfone, and
also the formation of by-products that are difficult to remove. Other
strategies that have been developed to circumvent the use of m-
CPBA may also result in the formation of undesired by-products,5
use equally difficult to work with reagents,6e8 or complex pro-
cedures.9 The development of a catalytic procedure that can effect
the clean conversion of thioglycosides into the corresponding
glycosyl sulfoxides would be advantageous, particularly if a low
cost and easy to handle ‘green’ stoichiometric oxidant, such as
hydrogen peroxide,10e15 could be used.
their inhibitory activity, more recently sulfonium ions, such as
salacinol,17 together with an increasing number of other zwitter-
ionic species in which sulfur or selenium18 bears a positive
charge,19,20 have also been demonstrated to act as potent inhibitors
of glycosidases.21 Since the anomeric sulfur atom of a glycosyl
sulfoxide bears a partial positive charge, one may analogously
postulate that a glycosyl sulfoxide could possess electrostatic fea-
tures22 that may favour binding to the active site of a glycosidase. In
this vein, the sulfoxides of several thiosugars23,24 have been re-
ported to display weak activity against glycosidases, and the two
previous reports25,26 on the inhibitory activity of glycosyl sulfoxides
also indicated inhibitory activity at the millimolar level. In this
paper, we report the development of a catalytic aqueous oxidation
procedure for the conversion of un-protected thioglycosides to
their corresponding glycosyl sulfoxides using hydrogen peroxide as
the stoichiometric oxidant. Also reported are the activities of some
glycosyl sulfoxides against corresponding glycosidases.
Besides their utility as glycosyl donors, glycosyl sulfoxides may
be interesting synthetic targets for a variety of other reasons.
Electrostatic interactions play an important part of the binding of
glycosidase inhibitors16 to carboxylate residues in the enzyme
2. Results and discussion
2.1. Investigation of catalysis
There has been considerable recent interest in the development
of flavin and flavinium species27 as biomimetic catalysts, which, in
conjunction with hydrogen peroxide or oxygen, may be used for a
* Corresponding author.
E-mail address: antony.fairbanks@canterbury.ac.nz (A.J. Fairbanks).
http://dx.doi.org/10.1016/j.carres.2015.06.003
0008-6215/© 2015 Elsevier Ltd. All rights reserved.

124
S.R. Alexander et al. / Carbohydrate Research 413 (2015) 123e128
variety of chemoselective oxidation processes,28e32 including the
oxidation of thioethers to sulfoxides.33e37 It was reasoned that
flavin-based systems might prove useful for the conversion of thi-
oglycosides into glycosyl sulfoxides and provide an effective and
useful alternative to existing procedures.
Flavinium salt 1 was selected as a candidate catalyst, and was
synthesised as previously reported.38 A variety of known thio-
glycosides 2aef, 4a,39 5a40 and 6a41 were also synthesised as
substrates for oxidation. Although thioglycosides can be made in
one step from the reducing sugars in water, using the methodology
of Shoda,42 the current procedure is not completely stereoselective,
and separation of the mixture of anomers of completely de-
protected thioglycosides proved to be extremely problematic.
Therefore, for the current study conventional literature routes,
starting from the corresponding per-acetylated sugar, were used to
access these substrates.
assessed also as the reaction solvent. Pleasingly conversion of 2b
into 3b in water was complete; no residual starting material could
be observed by NMR. It therefore appeared that water was an
appropriate solvent for this particular catalytic oxidation process.
Extension of the oxidation process to the aryl thioglycosides 2c46,47
and 2d,42,48 the solubility properties of which required the use of
MeOH as reaction solvent, did not result in any improvement in
synthetic efficiency. The process was also found to be considerably
less efficient using protected thioglycosides 2e27 and 2f49 as sub-
strates; in these two cases solubility properties necessitated the use
of MeCN as solvent. In all of these cases, the low conversion simply
reflected the persistence of unreacted starting material, rather than
substrate degradation, the formation of other products, or any is-
sues of low substrate solubility.
Catalyst 1 would therefore appear to be effective in conjunction
with H2O2 for production of completely de-protected glycosyl sulf-
oxides, using water as the reaction solvent. A control reaction of
oxidation of ethyl thioglycoside 2b in water with hydrogen peroxide
in the absence of catalyst 1 did not result in the formation of any
sulfoxide, confirming the essential role of the catalyst (Table 1, entry
7). The reaction was subsequently applied to the gluco 2b,45 manno
4a,39 and galacto 5a40 ethyl thioglycosides, and in all cases the cor-
responding glycosyl sulfoxides 3b, 4b and 5b were produced with
complete conversion of the starting material, and in good to mod-
erate yields50 (Scheme 1). The gluco 3b and galacto 5b ethyl glycosyl
sulfoxides were obtained as mixtures of diastereoisomers, whilst in
contrast the manno ethyl glycosyl sulfoxide 4b was obtained as a
single diastereoisomer, tentatively assigned as the (RS)-isomer on
the basis of previous glycosyl sulfoxide syntheses.51,52 Additionally
the reactionwas found to be equallyapplicable to a disaccharide; the
lacto ethyl thioglycoside 6a41 was converted into the corresponding
glycosyl sulfoxide 6b, as a 1:1 mixture of diastereoisomers, under
identical conditions. Additionally in this latter case the use of other
solvents, such as MeOH, was not possible due to the limited solu-
bility of the non-protected disaccharide.
Un-protected tolyl thioglycoside 2a43,44 was selected as an initial
substrate for a study into the efficacy of the catalytic oxidation
process. Unfortunately the low aqueous solubility of 2a precluded
the use of wateras solvent. However, reaction of 2a in methanol with
1.2 equiv of H2O2 as stoichiometric oxidant, in the presence 1.8% of
catalyst 1, at room temperature for 6 h resulted in 78% conversion to
the corresponding glycosyl sulfoxide 3a (Table 1, entry 1); no over-
oxidation to the glycosyl sulfone was observed. The use of MeCN as
an alternative solvent resulted in the formation of a mixture of
products, so that assessment of reaction efficiency was difficult
(Table 1, entry 2). A study into the effect of catalyst loading indicated
that conversion was more efficient when using 5% catalyst (Table 1
entry 3), though the use of a considerably higher catalyst loading
did not significantly increase the conversion (Table 1, entry 4). A
catalyst loading of 5% was therefore used in all subsequent
experiments.
The reaction was then applied to other thioglycosides. Ethyl
thioglycoside 2b45 was converted into sulfoxide 3b with 93% con-
version in MeOH. However, as 2b was aqueous soluble, water was
Table 1
Oxidation of gluco thioglycosides to glycosyl sulfoxides
Entrya
Substrate
Product
Solvent
Catalyst loading (%)
Conversionb (%)
1
2
3
4
5
6
7
8
2a
2a
2a
2a
2b
2b
2b
2c
2d
2e
2f
3a
3a
3a
3a
3b
3b
3b
3c
3d
3e
3f
MeOH
MeCN
MeOH
MeOH
MeOH
H2O
1.8
1.8
5
20
5
5
0
5
5
78
Not clean
87
92
93
>99
0
30
64
30
H2O
MeOH
MeOH
MeCN
MeCN
9
10
11
5
5
<10
a
Reaction conditions were 6 h at rt with 1.2 equiv. of H2O2 as stoichiometric oxidant.
Percentage assessed by 1H NMR of the crude reaction mixture.
b

S.R. Alexander et al. / Carbohydrate Research 413 (2015) 123e128
125
Scheme 1. Reagents and conditions: (i) 1 (5%), H2O2 (1.2 equiv), H2O, rt, 6 h.
2.2. Investigation of inhibitory activity of glycosyl sulfoxides versus
glycosidases
4. Experimental
4.1. General
The monosaccharide glycosyl sulfoxides were assayed against a
panel of corresponding glycosidases. In order that any effect of
sulfoxide configuration upon biological activity could be estab-
lished it was necessary to separate the diastereoisomeric mixtures
of the gluco 3b and galacto 5b glycosyl sulfoxides before testing.
This was eventually achieved by reverse phase high-performance
liquid chromatography (RP-HPLC), and allowed the production of
pure samples of the (RS)- and (SS)-glycosyl sulfoxides of 3b and 5b.
In all cases, the sulfoxide configuration was tentatively assigned
based on differences in the chemical shifts of H-1, C-1 and H-2 for
the two diastereoisomers in the corresponding NMR spectra.51
Compounds were then assayed against their corresponding
Melting points were recorded on an Electrothermal melting
point apparatus and are uncorrected. Proton and carbon nuclear
magnetic resonance (dH
Technologies 400 MR (400 MHz) or Varian VNMR500 (500 MHz)
spectrometers. All chemical shifts are quoted on the -scale in parts
, dC) spectra were recorded on Agilent
d
per million (ppm) using residual solvent as an internal standard.
High-resolution mass spectra were recorded with a Bruker maXis
3G UHR-TOF mass spectrometer. Thin layer chromatography (TLC)
was carried out on Merck silica gel 60F254 aluminium-backed
plates. Visualisation of the plates was achieved using a UV lamp
(lmax¼254 or 365 nm), and/or 5% w/v ammonium molybdate in 2 M
glycosidases (Fig. 1). Neither diastereomer of the
3b showed inhibitory activity against almond -glucosidase at an
inhibitor concentration of up to 1 mM. However, the -galacto
sulfoxides 5b were found to be inhibitors of Eschericia coli
b-gluco sulfoxide
sulfuric acid. Flash column chromatography was carried out using
Sorbsil C60 40/60 silica. Unless preparative details are provided, all
reagents were commercially available or made following literature
procedures. Glycosidases were purchased from SigmaeAldrich and
were used without further purification. Kinetic parameters were
obtained by fitting experimental data using the non-linear curve
fitting program GraFit 5. Reverse phase high-performance liquid
chromatography (RP-HPLC) was performed on a Dionex P680 HPLC
b
b
b
-
galactosidase; the (SS)-diastereomer SS-5b gave a Ki of 1.0 mM, and
showed slightly stronger inhibition than the (RS)-diastereomer RS-
5b, which displayed a Ki of 1.7 mM. Both this weak inhibitory ac-
tivity at the millimolar level and the slight differences in Ki for the
two diastereomers are consistent with the recently published work
of Varela,26 which demonstrated a small difference in the Ki of the
instrument with a Phenomenex Luna C 18(2) 100 Å column (5
mm,
10ꢀ250 mm) at 40 ꢁC.
two diastereoisomers of a
oxide. Finally the (RS)- -manno sulfoxide 4b was found to be a very
weak inhibitor of jack bean -mannosidase, displaying a Ki of
17 mM. This weak inhibitory activity correlates with previously
b-galacto thio-linked disaccharide sulf-
a
4.2. Ethyl 1-sulfinyl-
b-D-glucopyranoside (3b)
a
Ethyl 1-thio-b-D
-glucopyranoside 2b45 (0.672 g, 3 mmol) was
published data on the inhibitory activity of a-manno benzyl and a-
dissolved in H2O (8.1 mL). 1,10-Ethyleneisoalloxazinium chloride 1
(0.0417 g, 0.15 mmol) was added followed by H2O2 (0.21 mL, 50% w/
w in water, 3.6 mmol), and the reaction mixture was then stirred for
6 h. The reaction mixture was freeze-dried and the residue pro-
duced was pre-adsorbed onto Florisil® and then purified by flash
column chromatography (CH2Cl2/MeOH 7:1) to afford ethyl 1-
manno phenylethyl glycosyl sulfoxides.25
3. Conclusions
The organocatalytic oxidation of un-protected thioglycosides
using flavinium catalyst 1, together with H2O2 as stoichiometric
oxidant, allowed the direct aqueous synthesis of a variety of
glycosyl sulfoxides; no oxidation occurred under those reaction
conditions in the absence of catalyst. Two of the sulfoxides inves-
tigated displayed weak inhibitory activity against corresponding
glycosidases, with Kis in the low mM range.
sulfinyl-b-D-glucopyranoside 3b (0.512 g, 71%) as an orange oil
(1:1 mixture of diastereoisomers); nmax (FTIR) 1042 cmꢂ1 (S]O); dH
(400 MHz, D2O) 1.17e1.28 (6H, m, SCH2CH3S and SCH2CH3R),
2.81e2.94 (2H, m, SCHH0CH3S and SCHH0CH3R), 3.04e3.19 (2H, m,
SCHH0CH3S and SCHH0CH3R), 3.29e3.73 (10H, m, H-2S, H-2R, H-3S,
0
H-3R, H-4S, H-4R, H-5S, H-5R, H-6S and H-6R), 3.83 (2H, dd, J5,6

126
S.R. Alexander et al. / Carbohydrate Research 413 (2015) 123e128
Fig. 1. Inhibition of glycosidases by glycosyl sulfoxides. (a) Lack of inhibition of
from almonds by RS-3b, Km¼8.3 mM 0.9 mM. (c) Inhibition of -galactosidase from E. coli by SS-5b, Km¼0.17 mM 0.02 mM, Ki¼1.0 mM 0.1 mM. (d) Inhibition of
from E. coli by RS-5b, Km¼0.20 mM 0.02 mM, Ki¼1.7 mM 0.1 mM. (e) Inhibition of -mannosidase from jack beans by RS-4b, Km¼4.0 mM 0.3 mM, Ki¼17 mM 1 mM.
b
-glucosidase from almonds by SS-3b, Km¼7.0 mM 0.4 mM. (b) Lack of inhibition of
b
-glucosidase
b
b-galactosidase
a
0
0
0
0
6.5 Hz, J6,6 11.9 Hz, H ꢂ 6S and H ꢂ 6R), 4.17 (1H, d, J1,2 9.8 Hz, H-1R),
4.47 (1H, d, J1,2 9.8 Hz, H-1S); dC (100.5 MHz, D2O) 6.5 (q, SCH2CH3S
and SCH2CH3R), 39.4 (t, SCH2CH3S), 40.1 (t, SCH2CH3R), 60.4 (d, C-6R),
60.7 (d, C-6S), 67.7 (d, C-2R), 68.6 (d, C-4R), 68.8 (d, C-4S), 69.1 (d, C-
2S), 76.9 (d, C-3R), 77.1 (d, C-3S), 80.2 (d, C-5R), 80.7 (d, C-5S), 87.7 (d,
C-1R), 90.4 (d, C-1S); HRMS (ESI-TOF): calcd for C8H16O6SNaþ:
263.0560. Found: 263.0563 (MNaþ).
SCHH0CH3), 3.47e3.55 (1H, m, H-5), 3.60 (1H, dd, J6,6 12.5 Hz, J5,6
0
5.9 Hz, H-6), 3.67 (1H, t, J 9.8 Hz, H-4), 3.73e3.79 (1H, dd, J5,6
2.0 Hz, J6,6 12.5 Hz, H-60), 3.89 (1H, dd, J3,4 9.8 Hz, J2,3 3.5 Hz, H-3a
0
and H-3b), 4.29e4.33 (1H, m, H-2), 4.71 (1H, s, H-1); dC (100.5 MHz,
D2O) 5.2 (q, SCH2CH3), 42.5 (t, SCH2CH3), 60.8 (t, C-6), 65.7 (d, C-4),
66.7 (d, C-2), 70.4 (d, C-3), 78.7 (d, C-5), 92.5 (d, C-1); HRMS (ESI-
TOF): calcd for C8H16O6SNaþ: 263.0560. Found: 263.0557 (MNaþ).
4.3. Ethyl 1-(R)-sulfinyl-
a
-D-mannopyranoside (4b)
4.4. Ethyl 1-sulfinyl-
b-D-galactopyranoside (5b)
Ethyl 1-thio-
a
-D
-mannopyranoside 4a39 (0.574 g, 2.56 mmol)
Ethyl 1-thio-b-D
-galactopyranoside 5a40 (0.672 g, 3 mmol) was
was dissolved in H2O (6.9 mL). 1,10-Ethyleneisoalloxazinium chlo-
ride 1 (0.0356 g, 0.13 mmol) was added followed by H2O2 (0.18 mL,
50% w/w in water, 3.07 mmol), and the reaction mixture was then
stirred for 6 h. The reaction mixture was freeze-dried and the
residue was pre-adsorbed onto Florisil® and then purified by flash
column chromatography (CH2Cl2/MeOH 7:1) to afford ethyl 1-(R)-
dissolved in H2O (8.1 mL). 1,10-Ethyleneisoalloxazinium chloride 1
(0.0417 g, 0.15 mmol) was added followed by H2O2 (0.21 mL, 50% w/
w in water, 3.6 mmol), and the reaction mixture was then stirred for
6 h. The reaction mixture was freeze-dried and the residue was pre-
adsorbed onto Florisil® and then purified by flash column chro-
matography (CH2Cl2/MeOH 7:1) to afford ethyl 1-sulfinyl-b-D-gal-
sulfinyl-
a
-
D
-mannopyranoside 4b (0.303 g, 49%) as a pale orange
actopyranoside 5b (0.456 g, 63%) as a pale orange oil (1:0.7 mixture
of diastereoisomers); nmax (FTIR) 1050 cmꢂ1 (S]O); dH (400 MHz,
D2O) 1.19 (3H, t, J 7.6 Hz, SCH2CH3R), 1.24 (3H, t, J 8.2 Hz, SCH2CH3S),
oil; nmax (FTIR) 1036 cmꢂ1 (S]O); dH (400 MHz, D2O) 1.26 (3H, t, J
7.4 Hz, SCH2CH3), 2.75e2.86 (1H, m, SCHH0CH3), 3.02e3.14 (1H, m,

S.R. Alexander et al. / Carbohydrate Research 413 (2015) 123e128
127
2.82e2.95 (2H, m, SCHH0CH3R and SCHH0CH3S), 3.03e3.17 (2H, m,
4.6.4. Ethyl 1-(R)-sulfinyl-b-D-galactopyranoside (RS-5b)
SCHH0CH3R and SCHH0CH3S), 3.58e3.92 (12H, m, H-2R, H-2S, H-3R,
H-3S, H-4R, H-4S, H-5R, H-5S, H-6R, H-6S, H-60R and H-60S), 4.08 (1H,
d, J1,2 9.8 Hz, H-1R), 4.40 (1H, d, J1,2 9.8 Hz, H-1S); dC (100.5 MHz,
D2O) 6.4 (q, SCH2CH3S), 6.5 (q, SCH2CH3R), 39.4 (t, SCH2CH3S), 40.2
(t, SCH2CH3R), 61.0 (d, C-6S), 61.2 (d, C-6R), 65.0 (d, C-2R), 66.5 (d, C-
2S), 68.6 (d, C-3R), 68.8 (d, C-3S), 73.9 (d, C-4R), 73.9 (d, C-4S), 79.9 (d,
C-5R), 80.2 (d, C-5S), 88.4 (d, C-1R), 91.1 (d, C-1S); HRMS (ESI-TOF):
calcd for C8H16O6SNaþ: 263.0560. Found: 263.0554 (MNaþ).
tR¼6.9 min; dH (400 MHz, D2O) 1.20 (3H, t, J 7.6 Hz, SCH2CH3),
2.85e2.96 (1H, m, SCHH0CH3), 3.07e3.18 (1H, m, SCHH0CH3),
3.62e3.70 (2H, m, H-5 and H-6), 3.72e3.80 (2H, s, H-3 and H-60),
3.84e3.92 (2H, m, H-2 and H-4), 4.09 (1H, d, J1,2 9.8 Hz, H-1).
4.7. Inhibition assays
4.7.1.
-Glucosidase from almonds, obtained as a lyophilised powder
(3.4 mg), was dissolved in sodium phosphate buffer solution
(600 L, 0.05 M, pH 6.8). A stock solution of 60 mM o-nitrophenyl
-glucopyranoside (0.36 g, 1.2 mmol) was prepared in sodium
phosphate buffer (20 mL, 0.05 M, pH 6.8). A stock solution of 10 mM
ethyl 1-(R)-sulfinyl- -glucopyranoside RS-3b (23.6 mg, 0.098
mmoL) was prepared in sodium phosphate buffer (10 mL, 0.05 M,
pH 6.8). A variety of volumes of each solution (100e500 L of
substrate, 0e500 L of inhibitor RS-3b) were mixed, and the
b-Glucosidase
b
4.5. Ethyl
b-D-galactopyranosyl-(1/4)-1-sulfinyl-b-D-
m
b-
glucopyranoside (6b)
D
Ethyl
b-D-galactopyranosyl-(1/4)-1-thio-b-D-glucopyranoside
b-D
6a41 (1.16 g, 3 mmol) was dissolved in MeOH (8.1 mL). 1,10-
Ethyleneisoalloxazinium chloride 1 (0.0417 g, 0.015 mmol) was
added followed by H2O2 (0.21 mL, 50% w/w in water, 3.6 mmol),
and the reaction mixture was then stirred for 6 h. The reaction
mixture was freeze-dried and the residue was pre-adsorbed onto
Florisil® and then purified by flash column chromatography
m
m
resulting solutions made up to 1 mL with sodium phosphate buffer
(0.05 M, pH 6.8). The solution was then incubated at 25 ꢁC for
10 min. Enzyme solution (1
mL, as prepared above) was added, and
(MeCN/MeOH 5:1) to afford ethyl
b-D-galactopyranosyl-(1/4)-1-
the change in absorbance followed at 420 nm. A similar procedure
was followed for ethyl 1-(S)-sulfinyl-b-D-glucopyranoside SS-3b
(21.7 mg, 0.090 mmol).
sulfinyl- -glucopyranoside 6b (0.51 g, 42%) as a colourless solid
b-D
(1:1 mixture of diastereoisomers); mp 175e177 ꢁC (MeOH); nmax
(FTIR) 1056 cmꢂ1 (S]O); dH (400 MHz, D2O) 1.17e1.29 (6H, m,
SCH2CH3a and SCH2CH3b), 2.83e2.94 (2H, m, SCHH0CH3a and
SCHH0CH3b), 3.05e3.17 (2H, m, SCHH0CH3a and SCHH0CH3b), 3.44
(2H, s, H-2Ba and H-2Bb), 3.53e3.84 (20H, m), 3.86e3.94 (2H, m,
H ꢂ 60Aa and H ꢂ 6A0 b), 4.18e4.23 (1H, m, H-1Ab), 4.32e4.39 (2H, m,
H-1Ba and H-1Bb), 4.46e4.51 (1H, m, H-1Aa); dC (100.5 MHz, D2O)
6.5 (q, SCH2CH3a and SCH2CH3b), 39.5 (t, SCH2CH3a), 40.2 (t,
SCH2CH3b), 60.0, 61.0, 61.0, 67.5, 68.5, 68.9, 70.9, 70.9, 72.5, 72.5,
75.3, 75.3, 75.4, 75.6, 77.0, 77.2, 79.0, 79.6 (20ꢀC), 87.5 (d, C-1Ab),
90.3 (d, C-1Aa),102.8 (d, C-1Ba and C-1Bb); HRMS (ESI-TOF): calcd for
4.7.2.
-Galactosidase from E. coli was obtained as a lyophilised
powder (1.68 mg), which was dissolved in a buffer solution (600 L,
pH 7.0) containing TriseHCl (10 mM), MgCl2 (10 mM) and -mer-
captoethanol (1 mM). A portion of the resulting solution (10 L)
was then diluted with the above buffer (40 L, pH 7.0) to obtain an
appropriate concentration of enzyme. A stock solution of 1.2 mM o-
nitrophenol -galactopyranoside (36 mg, 0.12 mmol) was pre-
b-Galactosidase
b
m
b
m
m
b-D
pared in sodium phosphate buffer (100 mL, 0.1 M, pH 7.0) con-
taining 0.15 M NaCl. A similar stock solution of 10 mM ethyl 1-(S)-
C
14H27O11Sþ: 403.1269. Found: 403.1284 (MHþ).
sulfinyl-
prepared in sodium phosphate buffer (10 mL, 0.1 M, pH 7.0). A
variety of volumes of each solution (100e500 L of substrate,
0e500 L of inhibitor SS-5b) were mixed, and the resulting solution
b-D-galactopyranoside SS-5b (27.2 mg, 0.11 mmol) was
4.6. Separation of sulfoxide diastereomers by HPLC
m
Portions of the diastereomeric mixtures of gluco glycosyl sulf-
oxides 3b and galacto glycosyl sulfoxides 5b were separated by
HPLC (eluent: 0.05% TFA in H2O; flow rate: 3.5 mL/min; detection:
UV 210 nm) to afford single diastereomers, which were used in
enzyme assays:
m
made up to 1 mL with sodium phosphate buffer (0.1 M, pH 7.0). The
solution was then incubated at 37 ꢁC for 10 min. Enzyme solution
(1
followed at 420 nm. A similar procedure was followed for ethyl 1-
(R)-sulfinyl- -galactopyranoside RS-5b (62.4 mg, 0.26 mmol,
20 mL of buffer).
mL, as prepared above) was added, and the change in absorbance
b-D
4.6.1. Ethyl 1-(S)-sulfinyl-b-D-glucopyranoside (SS-3b)
tR¼10.9 min; dH (400 MHz, D2O) 1.23 (3H, t, J 7.4 Hz, SCH2CH3),
2.81e2.93 (1H, m, SCHH0CH3), 3.04e3.15 (1H, m, SCHH0CH3), 3.32
(1H, t, J 9.0 Hz, H-4), 3.41e3.53 (2H, m, H-3 and0 H-5), 3.55e3.68
4.7.3.
-Mannosidase from Jack beans was obtained as an ammonium
sulfate suspension, which was diluted with sodium acetate buffer
solution (400 L, 0.1 M, pH 4.5). A stock solution of 25 mM p-
nitrophenol -mannopyranoside (0.151 g, 0.50 mmol) was pre-
pared in sodium acetate buffer (20 mL, 0.1 M, pH 4.5). A similar
stock solution of 40 mM ethyl 1-(R)-sulfinyl- -mannopyranoside
5b (96 mg, 0.40 mmol) was prepared in sodium acetate buffer
(10 mL, 0.1 M, pH 4.5). A variety of volumes of each solution
(100e500 mL of substrate, 0e500 mL of inhibitor 5b) were mixed
a-Mannosidase
a
0
(2H, m, H-2 and H-6), 3.82 (1H, d, J6,6 12.5 Hz, H-6 ), 4.46 (1H, d, J1,2
m
a-D
9.8 Hz, H-1).
4.6.2. Ethyl 1-(R)-sulfinyl-b-D-glucopyranoside (RS-3b)
b-D
tR¼12.0 min; dH (400 MHz, D2O) 1.19 (3H, t, J 7.4 Hz, SCH2CH3),
2.82e2.93 (1H, m, SCHH0CH3), 3.05e3.17 (1H, m, SCHH0CH3), 3.37
(1H, t, J 9.4 Hz, H-4), 3.46e3.55 (2H, m, H-3 and H-5), 3.60 (1H, t, J
0
9.4 Hz, H-2), 3.69 (1H, dd, J6,6 12.5 Hz, J5,6 4.7 Hz, H-6), 3.83 (1H, dd,
and the resulting solutions made up to 1 mL with sodium acetate
J6,6 12.5 Hz, J5,6 1.6 Hz, H-60), 4.16 (1H, d, J1,2 9.8 Hz, H-1).
0
0
buffer (0.1 M, pH 4.5). The solution was then incubated at 37 ꢁC for
10 min. Enzyme solution (2 mL, as prepared above) was added, and
4.6.3. Ethyl 1-(S)-sulfinyl-
b
-D
-galactopyranoside (SS-5b)
the change in absorbance followed at 405 nm.
tR¼7.8 min; dH (400 MHz, D2O) 1.25 (3H, t, J 7.6 Hz, SCH2CH3),
2.82e2.94 (1H, m, SCHH0CH3), 3.05e3.16 (1H, m, SCHH0CH3),
3.60e3.68 (2H, m, H-5 and H-6), 3.69e3.76 (2H, m, H-3 and H-60),
3.80 (1H, t, J 9.6 Hz, H-2), 3.90 (1H, d, J4,5 3.1 Hz, H-4), 4.41 (1H, d, J1,2
9.8 Hz, H-1).
Acknowledgements
We thank Dr. Marie Squire (University of Canterbury) for tech-
nical assistance.

128
S.R. Alexander et al. / Carbohydrate Research 413 (2015) 123e128
ꢀ
ꢀ
ꢀ
ꢀ
25. Polakova M, Sestak S, Lattova E, Petrus L, Mucha J, Tvaroska I, et al. Eur J Med
Chem 2011;46:944e52.
Supplementary data
ꢀ
ꢀ
26. Colomer JP, Mayordomo MAC, De Toro BF, Jimenez-Barbero J, Varela O. Eur J
Supplementary data associated with this article can be found in
the online version, at http://dx.doi.org/10.1016/j.carres.2015.06.
003.
Org Chem 2015:1448e55.
27. For a recent review see: Gelalcha FG Chem Rev 2007;107:3338e61.
28. Murray AT, Matton P, Fairhurst NWG, John MP, Carbery DR. Org Lett 2012;14:
3656e9.
29. Murahashi S-I, Ono S, Imada Y. Angew Chem Int Ed 2002;41:2366e8.
€
30. Bergstad K, Backvall J-E. J Org Chem 1998;63:6650e5.
References
31. Mazzini C, Lebreton J, Furstoss R. J Org Chem 1996;61:8e9.
32. Murahashi S-I, Oda T, Masui Y. J Am Chem Soc 1989;111:5002e3.
33. Imada Y, Kitagawa T, Iwata S, Komiya N, Naota T. Tetrahedron 2014;70:
495e501.
1. Kahne D, Walker S, Cheng Y, Van Engen D. J Am Chem Soc 1989;111:6881e2.
2. Crich D, Dudkin V. J Am Chem Soc 2002;124:2263e6.
3. Nicolaou KC, Mitchell HJ, Rodríguez RM, Fylaktakidou KC, Suzuki H, Conley SR.
Chem Eur J 2000;6:3149e65.
4. Nicolaou KC, Li Y, Sugita K, Monenschein H, Guntupalli P, Mitchell HJ, et al. J Am
Chem Soc 2003;125:15443e54.
5. Kakarla R, Dulina RG, Hatzenbuhler NT, Hui YW, Sofia MJ. J Org Chem 1996;61:
8347e9.
6. Vincent SP, Burkart MD, Tsai CY, Zhang Z, Wong CH. J Org Chem 1999;64:
5264e79.
34. Marsh BJ, Carbery DR. Tetrahedron Lett 2010;51:2362e5.
ꢁ
ꢁꢀ
ꢀ
35. Zurek J, Cibulka R, Dvorakova H, Svoboda J. Tetrahedron Lett 2010;51:1083e6.
36. Imada Y, Ohno T, Naota T. Tetrahedron Lett 2007;48:937e9.
ꢀ
€
37. Linden A, Johansson M, Hermanns N, Backvall J-E.
J Org Chem 2006;71:
3849e53.
38. Li WS, Zhang N, Sayre LM. Tetrahedron 2001;57:4507e22.
ꢀꢁ
39. Saksena R, Zhang J, Kovac P. J Carbohydr Chem 2002;21:453e70.
40. Lemieux RU. Can J Chem 1951;29:1079e91.
41. Tomoo T, Kondo T, Abe H, Tsukamoto S, Isobe M, Goto T. Carbohydr Res
1996;284:207e22.
42. Tanaka T, Matsumoto T, Noguchi M, Kobayashi A, Shoda S-i. Chem Lett 2009;38:
458e9.
43. Weng S-S, Lin Y-D, Chen C-T. Org Lett 2006;8:5633e6.
44. France RR, Compton RG, Davis BG, Fairbanks AJ, Rees NV, Wadhawan JD. Org
Biomol Chem 2004;2:2195e202.
7. Tsukamoto H, Kondo Y. Tetrahedron Lett 2003;44:5247e9.
8. Chen MY, Patkar LN, Chen HT, Lin CC. Carbohydr Res 2003;338:1327e32.
9. Chen MY, Patkar LN, Lin CC. J Org Chem 2004;69:2884e7.
10. Sheldon RA. Chem Commun 2008:3352e65.
€
11. Piera J, Backvall J-E. Angew Chem Int Ed 2008;47:3506e23.
12. Arends IWCE. Angew Chem Int Ed 2006;45:6250e2.
13. Ten Brink G-J, Arends IWCE, Sheldon RA. Chem Rev 2004;104:4105e23.
14. Noyori R, Aoki M, Sato K. Chem Commun 2003:1977e86.
15. Grigoropoulou G, Clark JH, Elings JA. Green Chem 2003;5:1e7.
16. Gloster T, Davies G. Org Biomol Chem 2010;8:305e20.
45. Fried J, Walz DE. J Am Chem Soc 1949;71:140e3.
46. Purves CB. J Am Chem Soc 1929;51:3619e27.
47. Gantt RW, Peltier-Pain P, Cournoyer WJ, Thorson JS. Nat Chem Biol 2011;7:
685e91.
48. Brachet E, Brion J-D, Alami M, Messaoudi S. Chem Eur J 2013;19:15276e80.
49. France RR, Rees NV, Wadhawan JD, Fairbanks AJ, Compton RG. Org Biomol Chem
2004;2:2188e94.
50. Since in all cases NMR analysis indicated that the conversion of the starting
material to product was complete, it is presumed that the somewhat moderate
yields for these reactions result from loss of material during purification of the
highly polar un-protected products.
17. Yoshikawa M, Murakami T, Shimada H, Matsuda H, Yamahara J, Tanabe G, et al.
Tetrahedron Lett 1997;38:8367e70.
18. Johnston BD, Ghavami A, Jensen MT, Svensson B, Pinto BM. J Am Chem Soc
2002;124:8245e50.
19. Eskandari R, Kuntz D, Rose D, Pinto BM. Org Lett 2010;12:1632e5.
20. Nasi R, Patrick B, Sim L, Rose D, Pinto BM. J Org Chem 2008;73:6172e81.
21. For a recent review see: Mohan S, Pinto BM Carbohydr Res 2007;342:1551e80.
22. For an example in which inhibitory activity of a thiosugar was modestly
ꢀ
improved by oxidation to the corresponding sulfoxide see: Ulgar V, Lopez O,
51. Khiar N. Tetrahedron Lett 2000;41:9059e63.
ꢀ
~
Maya I, Fernandez-Bolanos J, Bols M Tetrahedron 2003;59:2801e9.
23. Le Merrer Y, Fuzier M, Dosbaa I, Foglietti M-J, Depezay J-C. Tetrahedron
1997;53:16731e46.
52. Crich D, Mataka J, Zakharov LN, Rheingold AL, Wink DJ.
J Am Chem Soc
2002;124:6028e36.
24. Kajimoto T, Liu K, Pederson R, Zhong Z, Ichikawa Y, Porco J, et al. J Am Chem Soc
1991;113:6187e96.