The effect of heteroatom substitution of sulfur for selenium in glucosidase inhibitors on intestinal α-glucosidase activities

Article abstract of DOI:10.1039/c1cc13052h

The synthesis of selenium analogues of de-O-sulfonated ponkoranol, a naturally occurring sulfonium-ion glucosidase inhibitor isolated from Salacia reticulata, and their evaluation as glucosidase inhibitors against two recombinant intestinal enzymes maltas

Full text of DOI:10.1039/c1cc13052h

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COMMUNICATION
The effect of heteroatom substitution of sulfur for selenium in glucosidase
inhibitors on intestinal a-glucosidase activitieswz
Razieh Eskandari,a Kyra Jones,b David R. Roseb and B. Mario Pinto*a
Received 24th May 2011, Accepted 28th June 2011
DOI: 10.1039/c1cc13052h
The synthesis of selenium analogues of de-O-sulfonated ponkoranol,
a naturally occurring sulfonium-ion glucosidase inhibitor isolated
from Salacia reticulata, and their evaluation as glucosidase
inhibitors against two recombinant intestinal enzymes maltase
glucoamylase (MGAM) and sucrase isomaltase (SI) are described.
The root and stem extracts of Salacia reticulata, a large woody
climbing plant found in Sri Lanka and southern India, have
traditionally been used for treatment of type-2 diabetes.1
Human clinical trials on patients with type-2 diabetes,
conducted with the extract of Salacia reticulata, have shown
effective treatment of type-2 diabetes with minimal side
effects.2 Our efforts in recent years have been focused on this
Fig. 1 Components isolated from Salacia species.
novel class of a-glucosidase inhibitors comprising sulfonium
ions as putative mimics of the oxacarbenium ion intermediates
in glucosidase-mediated hydrolysis reactions.3 Thus, we have
4
conformation, closely resembles the proposed H3 conformation
described the synthesis and stereochemical structure elucidation of
of the oxacarbenium-ion like transition state.13
the naturally occurring glucosidase inhibitors salacinol (1),4
We have also reported the synthesis of the 50-stereoisomer
ponkoranol (2),5 kotalanol (3),6 de-O-sulfonated kotalanol
of de-O-sulfonated ponkoranol 6 (Fig. 2), and have shown
(4)6 (Fig. 1) from the plant extract of Salacia reticulata.
that it is a potent (Ki = 15 nM) inhibitor of ntMGAM.7
Interestingly, one of our synthetic compounds, de-O-sulfonated
As part of our continuing interest in evaluating the effect of
ponkoranol (5)7 (Fig. 1), was isolated recently from the same
plant.8
heteroatom substitution in the sugar ring on glucosidase
inhibitory activity, we have also synthesized selenium congeners
The antidiabetic property of these herbal extracts can be
of several candidates as potential glucosidase inhibitors.14–16
attributed, at least in part, to the inhibition of intestinal
a-glucosidases by these sulfonium-ion components.9–11 Crystallo-
Comparison of the proposed intermediate in the glucosidase-
catalyzed reaction and the selenonium ion, inferred from the
graphic studies of the N-terminal domain of human intestinal
X-ray crystal structure of the complex of kotalanol with
ntMGAM,13 shows that the selenonium ion in a T2 confor-
maltase glucoamylase (ntMGAM), a family 31 glycoside
hydrolase (GH31),12 with several candidate inhibitors suggested
3
mation superimposes well on the 4H3 conformation of the
that the permanent positive charge of these entities may mimic
oxacarbenium ion intermediate (Fig. 3).
the oxacarbenium-ion like transition state of the glucosidase-
mediated hydrolysis reactions.13 We also suggested that the
3
ring conformation of the thiocyclitol in kotalanol (3), a T2
We now report the synthesis and enzyme inhibitory activity
of analogues of 5 and 6, in which the sulfur atom has been
replaced by the heavier cognate atom selenium to give 7 and 8,
respectively (Fig. 4).
a Department of Chemistry, Simon Fraser University, Burnaby,
British Colombia, Canada V5A 1S6. E-mail: bpinto@sfu.ca;
Fax: (+1)7787824860; Tel: (+1)7787824152
b Department of Biology, University of Waterloo, Waterloo, Ontario,
Canada N2L 3G1
w This article is part of the ChemComm ‘‘Glycochemistry and Glyco-
biology’’ web themed issue.
z Electronic supplementary information (ESI) available: Experimental
procedures, characterization data and 1H, 13C NMR spectra of
compounds 7, 8, 11 and 13, 2D-NOESY spectra of compounds 13
and a representative Lineweaver–Burk plot. See DOI: 10.1039/
c1cc13052h
Fig. 2 Structure of the 50-stereoisomer of de-O-sulfonated ponkoranol 6.
c
9134 Chem. Commun., 2011, 47, 9134–9136
This journal is The Royal Society of Chemistry 2011

Fig. 3 Superimposition of the ring carbon atoms of the proposed
intermediate in glucosidase-catalyzed reactions (in green) and the
selenonium ion (in blue).
Retrosynthetic analysis indicated that 7 or its analogue 8
could be obtained by alkylation of an appropriately protected
1,4-anhydro-4-seleno-D-arabinitol B at the ring heteroatom
with agent C (Scheme 1).7
The required benzyl (Bn)-protected D-selenoarabinitol
(10)14 and benzyl 6-O-p-toluenesulfonyl-a-D-gluco 9 or manno
pyranoside 127 were obtained by literature methods. The
selenonium salts 11 and 13 were synthesized by alkylation of
10 with 9 or 12 (1.2 equiv.) in hexafluoroisopropanol (HFIP),
containing K2CO3, to give 11 and 13 in 55% and 45% yields,
respectively (Scheme 2). In the case of compound 11, just one
isomer was obtained at the stereogenic selenium atom,
whereas in the case of compound 13, a mixture of isomers
1
Scheme 2 Synthesis of compounds 7 and 8.
was obtained; H NMR spectroscopy showed the presence of
two isomers in the ratio of 10 : 1. The benzyl protecting groups
were removed with boron trichloride at ꢀ78 1C to give the
desired sulfonium salts. During the course of deprotection,
some of the tosylate counterions were exchanged with the
chloride ions. Hence, the deprotected sulfonium salts were
treated with Amberlyst A-26 (chloride form) to completely
exchange the tosylate counterion with the chloride ion.7
Finally, the products were reduced with NaBH4 to provide
the desired selenium analogues of de-O-sulfonated ponkoranol
7 and its 50 epimer 8 in 52% and 45% yields, respectively, over
three steps (Scheme 2).
experiment, which showed a correlation between H-4 and
H-60a, thus indicating that these atoms are syn-facial with
respect to the sulfonium salt ring (Fig. 5). In the case of 11, the
absolute configuration at the selenium centre was assigned by
analogy since a NOESY experiment was not possible owing to
overlapping signals for H-4 and H-60. This assignment is
consistent with our previous work.3
Finally, we comment on the inhibitory activities of the
compounds synthesized in this study and previous study7
against the two recombinant intestinal enzymes, maltase gluco-
amylase (MGAM) and sucrase isomaltase (SI), critical for
postamylase processing of starch-derived oligosaccharides into
glucose. Each of these enzymes exhibits two separate catalytic
units (by gene duplication) at their C- and N-terminal domains,
resulting in ntMGAM, ctMGAM, ntSI, and ctSI enzyme
activities.17–22 There are also various alternative splicing
patterns of ctMGAM in mammals; two spliceforms from mice
are discussed in this communication: ctMGAM-N2 and
ctMGAM-N20.23,24 The experimentally determined inhibition
constants (Kis) for the competitive inhibitors de-O-sulfonated
ponkoranol (5) and its synthetic analogues (6–8) are listed in
Table 1.
The stereochemistry at the selenium centre for the major
isomer of 13 was confirmed with the aid of a 2D-NOESY
Fig. 4 Selenium analogues of de-O-sulfonated ponkoranol and its
50-stereoisomer.
Scheme 1 Retrosynthetic analysis.
Fig. 5 2D-NOESY correlations of selected protons in compound 13.
Chem. Commun., 2011, 47, 9134–9136 9135
c
This journal is The Royal Society of Chemistry 2011

Table 1 Experimentally determined Ki values (mM)a
CtMGAM-N2
ctMGAM-N20
ntMGAM
ctSI
ntSI
5
6
7
8
No inhibition
No inhibition
No inhibition
No inhibition
0.096 ꢁ 0.015
0.138 ꢁ 0.068
0.047 ꢁ 0.014
0.041 ꢁ 0.027
0.043 ꢁ 0.0017
0.015 ꢁ 0.0017
0.038 ꢁ 0.008
0.025 ꢁ 0.014
0.103 ꢁ 0.037
0.132 ꢁ 0.047
0.018 ꢁ 0.004
0.019 ꢁ 0.010
0.302 ꢁ 0.123
0.138 ꢁ 0.012
0.013 ꢁ 0.008
0.010 ꢁ 0.002
a
Analysis of inhibition was performed using maltose as the substrate.
Comparison of the data in Table 1 indicates that substitution of
5 B. D. Johnston, H. H. Jensen and B. M. Pinto, J. Org. Chem.,
2006, 71, 1111–1118.
the ring sulfur atom in de-O-sulfonated ponkoranol (5) and its
50 epimer (6) by selenium results in a 13-fold and 23-fold
improvement, respectively, in inhibition of ntSI. NtSI demon-
strates the broadest substrate specificity of all four subunits
and hydrolyses both a(1–4) and a(1–6) linkages.25 Similarly,
substitution of sulfur by selenium leads to an increase in
inhibitory activity against ctSI (cf. 5 and 6 vs. 7 and 8).
In contrast, little discrimination is observed between the
congeners for inhibition of ntMGAM or ctMGAM-N20.
Interestingly, compounds 5–8 differentiate between the
spliceforms of ctMGAM, and none shows inhibition of
ctMGAM-N2; the reason for this selectivity must await
structural analysis.
6 K. Jayakanthan, S. Mohan and B. M. Pinto, J. Am. Chem. Soc.,
2009, 131, 5621–5626.
7 R. Eskandari, D. A. Kuntz, D. R. Rose and B. M. Pinto, Org.
Lett., 2010, 12, 1632–1635.
8 W. Xie, G. Tanabe, J. Akaki, T. Morikawa, K. Ninomiya,
T. Minematsu, M. Yoshikawa, X. Wu and O. Muraoka, Bioorg.
Med. Chem., 2011, 19, 2015–2022.
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We thank Buford L. Nichols, Roberto Quezada-Calvillo
and Hassan Naim for reagents for recombinant expression,
and Lyann Sim for MGAM and SI enzyme purification. Work
by K.J. was funded by CIHR and the Canadian Digestive
Health Foundation (CDHF). We also thank the Canadian
Institutes of Health Research (111237) and the Heart and
Stroke Foundation of Ontario (NA-6305) for financial
support.
12 L. Sim, R. Quezada-Calvillo, E. E. Sterchi, B. L. Nichols and
D. R. Rose, J. Mol. Biol., 2008, 375, 782–792.
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9136 Chem. Commun., 2011, 47, 9134–9136
This journal is The Royal Society of Chemistry 2011