Synthesis and biological evaluation of heteroanalogues of Kotalanol and De-O-Sulfonated Kotalanol

Article abstract of DOI:10.1021/ol100080m

(Figure Presented) The synthesis of nitrogen and selenium analogues of kotalanol and de-O-sulfonated kotalanol, naturally occurring sulfonium-ion glucosidase inhibitors isolated from Salacia reticulata, and their evaluation as glucosidase inhibitors again

Full text of DOI:10.1021/ol100080m

ORGANIC
LETTERS
2010
Vol. 12, No. 5
1088-1091
Synthesis and Biological Evaluation of
Heteroanalogues of Kotalanol and
De-O-Sulfonated Kotalanol
Sankar Mohan,† Kumarasamy Jayakanthan,† Ravindranath Nasi,†
Douglas A. Kuntz,‡ David R. Rose,‡,§ and B. Mario Pinto*,†
Department of Chemistry, Simon Fraser UniVersity, Burnaby, British Columbia,
Canada V5A 1S6, Department of Medical Biophysics, UniVersity of Toronto and
DiVision of Molecular and Structural Biology, Ontario Cancer Institute, Toronto, ON,
Canada M5G 2M9, and Department of Biology, UniVersity of Waterloo, Waterloo,
Ontario, Canada N2L 3G1
bpinto@sfu.ca
Received January 13, 2010
ABSTRACT
The synthesis of nitrogen and selenium analogues of kotalanol and de-O-sulfonated kotalanol, naturally occurring sulfonium-ion glucosidase
inhibitors isolated from Salacia reticulata, and their evaluation as glucosidase inhibitors against the N-terminal catalytic domain of human
maltase glucoamylase (ntMGAM) are described.
The aqueous extracts of Salacia reticulata, a climbing shrub
native to Sri Lanka and Southern India, used in Indian
Ayurvedic medicine, have been consumed by patients as a
remedy for the treatment of type-2 diabetes.1 The safety and
efficacy of Salacia extracts have been studied in both rats2
and human patients with type-2 diabetes and a placebo-
control group.3 These studies showed that the extract is an
effective treatment for type-2 diabetes, with no serious acute
toxicity and side effects comparable to the placebo control
group. In recent years, we have focused our synthetic efforts
on a novel class of sulfonium-ion glucosidase inhibitors,
namely salacinol 1,4 kotalanol 25 and de-O-sulfonated
kotalanol 3,6 isolated from the aqueous extracts of Salacia
reticulata (Figure 1). Along with salacinol 1 and kotalanol
2, two other members of this class of compounds, namely
salaprinol 4 and ponkoranol 5, have also been isolated from
Salacia prinoides, another medicinally useful plant that
belongs to the Salacia genus (Figure 1).7 The observed
antidiabetic property of these herbal extracts is attributed,
at least in part, to inhibition of the action of intestinal
R-glucosidases by these sulfonium-ion active components.4-6
† Simon Fraser University.
‡ University of Toronto and Ontario Cancer Institute.
§ University of Waterloo.
(1) (a) Chandrasena, J. P. C. The Chemistry and Pharmacology of Ceylon
and Indian Medicinal Plants; H&C press: Colombo, Sri Lanka, 1935. (b)
Jayaweera, D. M. A. Medicinal Plants Used in Ceylon-Part 1; National
Science Council of Sri Lanka: Colombo, 1981; p 77. (c) Vaidyartanam,
P. S. In Indian Medicinal Plants: a compendium of 500 species, Warrier
P. K., Nambiar V. P. K., Ramankutty, C., Eds.; Orient Longman: Madras,
India, 1993, pp. 47-48.
(2) Shimoda, H.; Fujimura, T.; Makino, K.; Yoshijima, K.; Naitoh, K.;
Ihota, H.; Miwa, Y. J. Food Hygienic Soc. Japan 1999, 40, 198–205.
(3) Jayawardena, M. H. S.; de Alwis, N. M. W.; Hettigoda, V.; Fernando,
D. J. S. J. Ethnopharmacol. 2005, 97, 215–218.
(4) Yoshikawa, M.; Murakami, T.; Shimada, H.; Matsuda, H.; Yamahara,
J.; Tanabe, G.; Muraoka, O. Tetrahedron Lett. 1997, 38, 8367–8370.
(5) Yoshikawa, M.; Murakami, T.; Yashiro, K.; Matsuda, H. Chem.
Pharm. Bull. 1998, 46, 1339–1340.
(6) (a) Ozaki, S.; Oe, H.; Kitamura, S. J. Nat. Prod. 2008, 71, 981–
984. (b) Muraoka, O.; Xie, W.; Tanabe, G.; Amer, M. F. A.; Minematsu,
T.; Yoshikawa, M. Tetrahedron Lett. 2008, 49, 7315–7317.
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10.1021/ol100080m  2010 American Chemical Society
Published on Web 02/09/2010

extended acyclic side chain compared to salacinol 1. We
report here the syntheses of the nitrogen 8 and 9 and selenium
10 and 11 congeners of kotalanol and de-O-sulfonated
kotalanol (Figure 2) and their evaluation as glucosidase
Figure 1. Sulfonium-ion glucosidase inhibitors isolated from
Salacia species and related analogues.
We have synthesized several analogues of salacinol and
studied their structure activity relationship (SAR) with human
intestinal maltase glucoamylase (MGA).8 Some of the
modifications included: replacement of the ring-sulfur het-
eroatom by the cognate atoms nitrogen9,10 and selenium;11
change of the configurations of the stereogenic centers; and
extension of the acyclic side chain.12 Some of these
compounds have shown higher or comparable inhibitory
activities against MGA in Vitro compared to acarbose and
miglitol, two antidiabetic drugs that are currently in use for
the treatment of type-2 diabetes.13,14
The acyclic side chain-extension studies of salacinol led
us to predict the possible stereochemical pattern of the acyclic
side chain in kotalanol 2, for which the absolute stereostruc-
ture was not determined at the time of its isolation. Recently,
we have proved the absolute stereostructure of kotalanol 2
and de-O-sulfonated kotalanol 3 by total syntheses.15 In the
case of salacinol, the substitution of the ring sulfur atom by
nitrogen (ghavamiol, 6,9 IC50 ) high mM range,16 Figure
1) resulted in a dramatic decrease in inhibitory activity
against MGA (compare the Ki value of salacinol, 0.19 µM13),
whereas substitution by selenium (blintol, 7, Ki ) 0.49 µM,13
Figure 1) did not affect its inhibitory activity appreciably.
Figure 2. Heteroanalogues and stereoisomers of kotalanol and de-
O-sulfonated kotalanol.
inhibitors against the amino terminal catalytic domain of
human MGA (ntMGAM).13 Since de-O-sulfonated kotalanol
3 was found to be more active than kotalanol 2 itself,6 we
have also converted two biologically active diastereomers
12 and 13 of kotalanol17 into their corresponding de-O-
sulfonated analogues 14 and 15, respectively (Figure 2), and
studied their inhibitory properties against ntMGAM.
The required para-methoxybenzyl (PMB)-protected D-
iminoarabinitol 1618 and D-selenoarabinitol 1719 were pre-
pared by methods described in our earlier work. The required
cyclic sulfate 18 was obtained from D-perseitol, as reported
earlier.15 The synthesis of the nitrogen analogue 8 of
kotalanol was examined first. The coupling reaction of the
iminoarabinitol 16 with the cyclic sulfate 18 proceeded
smoothly under our optimized reaction conditions (sealed
tube, acetone, K2CO3, 60 °C) as shown in Scheme 1.18 The
coupled product 19 was purified by short column chroma-
tography but was deemed to be unstable, probably due to
the partial removal of PMB protecting groups, as confirmed
by the formation of a more polar spot on TLC. Hence,
without any further characterization, the coupled product 19
was taken on to the next step, namely removal of the PMB
and benzylidene protecting groups using TFA/CH2Cl2, as
shown in Scheme 1.
It is of interest, therefore, to study the effect of heteroatom
substitution on the inhibitory activities of kotalanol 2 and
de-O-sulfonated kotalanol 3, both having a 3-carbon-
(8) For recent reviews, see: (a) Mohan, S.; Pinto, B. M. Carbohydr.
Res. 2007, 342, 1551–1580. (b) Mohan, S.; Pinto, B. M. Collect. Czech.
Chem. Commun. 2009, 74, 1117–1136. (c) Mohan, S.; Pinto, B. M. Nat.
Prod. Rep. 2010, in press.
(9) Ghavami, A.; Johnston, B. D.; Jensen, M. T.; Svensson, B.; Pinto,
Similarly, the selenium analogue 10 of kotalanol was
obtained from selenoarabinitol 17 and the cyclic sulfate 18
using our optimized reaction conditions (sealed tube, HFIP,
K2CO3, 70 °C).18 As observed in previous work from our
laboratory,11 during the coupling reaction of D-selenoara-
binitol 17 with the cyclic sulfate 18, along with the desired
coupled product 20 (40% yield), a considerable amount of
the undesired diastereomer 21 (26% yield), with respect to
the selenium center, was also formed (Scheme 1). The
B. M. J. Am. Chem. Soc. 2001, 123, 6268–6271
.
(10) Muraoka, O.; Ying, S.; Yoshikai, K.; Matsuura, Y.; Yamada, E.;
Minematsu, T.; Tanabe, G.; Matsuda, H.; Yoshikawa, M. Chem. Pharm.
Bull. 2001, 49, 1503–1505
.
(11) Johnston, B. D.; Ghavami, A.; Jensen, M. T.; Svensson, B.; Pinto,
B. M. J. Am. Chem. Soc. 2002, 124, 8245–8250.
(12) (a) Johnston, B. D.; Jensen, H. H.; Pinto, B. M. J. Org. Chem.
2006, 71, 1111–1118. (b) Nasi, R.; Sim, L.; Rose, D. R.; Pinto, B. M. J.
Org. Chem. 2007, 72, 180–186.
(13) Rossi, E. J.; Sim, L.; Kuntz, D. A.; Hahn, D.; Johnston, B. D.;
Ghavami, A.; Szczepina, M. G.; Kumar, N. S.; Sterchi, E. E.; Nichols, B. L.;
Pinto, B. M.; Rose, D. R. FEBS J. 2006, 273, 2673–2683
(14) Sim, L.; Jayakanthan, K.; Mohan, S.; Nasi, R.; Johnston, B. D.;
Pinto, B. M.; Rose, D. R. Biochemistry 2010, 49, 443–451
.
.
(17) Nasi, R.; Patrick, B. O.; Sim, L.; Rose, D. R.; Pinto, B. M. J. Org.
Chem. 2008, 73, 6172–6181.
(15) Jayakanthan, K.; Mohan, S.; Pinto, B. M. J. Am. Chem. Soc. 2009,
131, 5621–5626.
(18) Liu, H.; Sim, L.; Rose, D. R.; Pinto, B. M. J. Org. Chem. 2006,
71, 3007–3013.
(16) Pinto, B. M.; Johnston, B. D.; Ghavami, A.; Szczepina, M. G.;
Liu, H.; Sadalapure, K., US Patent, filed June 25, 2004.
(19) Liu, H.; Pinto, B. M. J. Org. Chem. 2005, 70, 753–755.
Org. Lett., Vol. 12, No. 5, 2010
1089

Scheme 1
.
Synthesis of 8 and 10
Table 1. Experimentally Determined Ki Valuesa
undesired diastereomer 21 was conveniently separated from
the desired coupled product 20 by column chromatography.
Once again, the removal of the PMB and benzylidene
protecting groups was achieved in one pot using TFA/
CH2Cl2. Thus, compounds 20 and 21 upon deprotection gave
10 and 22, respectively, as final products.
The absolute configuration at the stereogenic selenium
center in compound 10 was established by means of a 1D-
NOESY experiment. A correlation between H-4 and H-1′a
confirmed that they are syn-facial. In the case of compound
22, correlation of H-1b with H-3 and also with H-1′a
confirmed that they all are syn facial, thus establishing the
absolute configuration at the selenium center as S (Scheme
1). Compound 22 differs from 10 only with respect to the
configuration at the stereogenic selenium center. Hence, this
compound 22 served as a probe of the importance of the R
configuration at the positively charged ring heteroatom for
inhibitory activity; all of the naturally occurring compounds
1-5 have the R configuration at the stereogenic sulfur center.
In the case of the nitrogen analogue 8, the absolute
configuration at the ammonium center was assigned as R by
analogy with our previous work,9,18 since a NOESY experi-
ment was not possible owing to the broad, overlapping
signals at neutral pH.
With the sulfated compounds in hand, we turned next to
the synthesis of the corresponding de-O-sulfonated ana-
logues. Compounds 8, 10, 12,17 and 1317 were converted
into their corresponding de-O-sulfonated compounds 9, 11,
14, and 15 respectively, in a two step process, first treatment
with 5% methanolic HCl,7 followed by treatment with
Amberlyst-A26 (chloride resin) in MeOH, as shown in
Scheme 2. Similarly, compound 22 was also converted into
the corresponding de-O-sulfonated compound 23 (Table 1).
a Analysis of ntMGAM inhibition was performed using maltose as the
substrate.
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Org. Lett., Vol. 12, No. 5, 2010

30 nM)). Interestingly, substitution of the ring sulfur atom
by nitrogen 8 is detrimental to inhibitory activity (Ki ) 90
µM), whereas it does not have any significant change on
the inhibitory activity of the nitrogen analogue of de-O-
sulfonated kotalanol 9 (Ki ) 61 nM).
Scheme 2. Synthesis of De-O-sulfonated Compounds
The significant decrease in the inhibitory activity of the
nitrogen analogue of kotalanol 8 relative to kotalanol 2
deserves comment. Interestingly, this trend was also observed
with ghavamiol (the nitrogen analogue of salacinol) 69
relative to salacinol 1. We speculate, based on our recent
crystallographic work with salacinol and kotalanol deriva-
tives,14 that the positioning of the sulfate anion of 8 in a
hydrophobic pocket in the active site is more sterically
compromised than in the sulfur congener 2. Relief of this
steric interaction by de-O-sulfonation to give 9 apparently
relieves this interaction, and gives a compound that is just
as active as its sulfur congener 3. We note also that the R
configuration at the stereogenic heteroatom center, as ex-
hibited by all of the natural compounds 1-5 isolated so far,
is essential for inhibitory activity; thus, the inhibitory
activities of compounds 22 and 23, bearing the S configu-
ration at the stereogenic selenium center, are considerably
less than those of their corresponding diastereomers with the
R configuration, 10 and 11, respectively. As predicted, the
de-O-sulfonated compounds, 14 and 15 are found to be more
active compared to the parent compounds, 12 and 13,
respectively. We note also that the compound 2415 is the
most potent inhibitor of ntMGAM in Vitro known to date.
The inhibitory activities of the synthesized compounds
(8-11, 14, 15, 22, and 23) against the maltase activity of
recombinant ntMGAM13 are summarized in Table 1. In
addition, we also report here the enzyme inhibitory activity
of compound 2415 (Table 1), a diastereomer of de-O-
sulfonated kotalanol, that was previously synthesized in our
group. Except for the nitrogen analogue of kotalanol (8), all
of the compounds synthesized in this study show greater
inhibitory activities than acarbose, an antidiabetic agent that
is currently approved for the treatment of type-2 diabetes
(Table 1).13 In general, de-O-sulfonation leads to an increase
in inhibitory activity compared to the parent sulfated
compounds. Interestingly, in the case of the nitrogen analogue
of kotalanol 8, de-O-sulfonation resulted in a very large
increase in inhibitory activity (compare Ki values of com-
pounds 8 and 9, Table 1). Our results also indicate that the
substitution of the ring sulfur atom by the cognate atom
selenium does not confer any significant advantage (kotal-
anol, X ) Se: Ki ) 80 nM. X ) S: Ki ) 190 nM) and de-
O-sulfonated kotalanol (X ) Se: Ki ) 20 nM. X ) S: Ki )
Acknowledgment. We are grateful to the Canadian
Institutes for Health Research (FRN79400) and the Heart
and Stroke Foundation of Ontario (NA-6305) for financial
support. This article is dedicated, with respect and gratitude,
to Professor S. Wolfe for his mentorship.
Supporting Information Available: Experimental pro-
1
cedures, characterization data, and H, 13C NMR spectra of
compounds 8-11, 14, 15, 22, and 23. This material is
available free of charge via the Internet at http://pubs.acs.org.
OL100080M
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