Synthesis of a nitrogen analogue of salacinol and its α-glucosidase inhibitory activity

Article abstract of DOI:10.1248/cpb.49.1503

A nitrogen analogue 4 of the naturally occurring sulfoniumion salacinol (1), a potent α-glucosidase inhibitor isolated from the Ayruvedic medicine Salacia reticulata, was synthesized and its inhibitory activity against α-glucosidase tested. Substitution o

Full text of DOI:10.1248/cpb.49.1503

November 2001
Communications to the Editor
Chem. Pharm. Bull. 49(11) 1503—1505 (2001)
1503
known to function as potent inhibitors of glycosidase because
of their ability to mimic carbohydrates.5) Structural modifica-
tion of 1 represents a promising approach in the search for
new antidiabetic agents. One strategy is to replace the thio-
sugar sulfur in 1 with a nitrogen, which gives the azacyclic
version (4) of 1. The recent report by Ghavami et al.6) on the
synthesis of nitrogen analogues of 1 involving 4 and their
evaluation as glycosidase inhibitors prompts us to report our
own findings. This communication describes the synthesis of
4 and comparison of its a-glucosidase inhibitory activity
with that of 1. The solid-state stereostructure of the related
compound, 1Ј-(1-pyrrolidiniumyl)-2Ј,4Ј-O-isopropylidene-1Ј-
deoxy-L-erythritol-3Ј-sulfate (5) is also presented.
Synthesis of a Nitrogen Analogue of
a
Salacinol and Its -Glucosidase
Inhibitory Activity
Osamu MURAOKA,*,a Shao YING,a Kazuya YOSHIKAI,a
Yoshiharu MATSUURA,a Eriko YAMADA,a
Toshie MINEMATSU,a Genzoh TANABE,a
b
Hisashi MATSUDA,b and Masayuki YOSHIKAWA
School of Pharmaceutical Sciences, Kinki University,a 3–4–1
Kowakae, Higashi-osaka, Osaka 577–8502, Japan and Kyoto
Pharmaceutical University,b 1 Shichono-cho, Misasagi, Yamashina-
ku, Kyoto 607–8412, Japan.
Synthesis of 4 was carried out by applying the ring-open-
ing method developed by Yuasa3a) and Ghavami3b) for the
synthesis of 1. The cyclic sulfate 2a,3a) however, was synthe-
sized starting from D-glucose (D-6), which is much less ex-
pensive than the L-isomer (L-6). Thus compound D-6 was first
converted into 4,6-O-benzylidene-D-glucose (7) according to
the protocol developed by Barili et al.7) Practically pure 7
could be obtained only by the extraction of the ethereal solu-
tion of the crude reaction mixture with water. Work-up and
flash-column chromatography were found unnecessary, since
the yield was raised to 82% from the 72% previously re-
ported.
Received September 17, 2001; accepted September 25, 2001
A nitrogen analogue 4 of the naturally occurring sulfonium
ion salacinol (1), a potent a-glucosidase inhibitor isolated from
the Ayruvedic medicine Salacia reticulata, was synthesized and
its inhibitory activity against a-glucosidase tested. Substitution
of the sulfur atom in 1 with a nitrogen reduced the activity con-
siderably. The solid-state stereostructure of the related com-
pound (5) was determined on the basis of single crystal X-ray
measurement.
Key words cyclic sulfate; erythritol; salacinol; glycosidase inhibitor;
glucosidase inhibitor; azasugar
Effective transformation of 7 into 1,3-O-bezylidene-L-ery-
thritol (8) was accomplished by modification of the standard
protocol.8) Thus compound 7 was first oxidized with sodium
metaperiodate in the presence of excess sodium bicarbonate,
and following sodium borohydride reduction of the resulting
D-erythrose intermediate gave 8 in 94% yield. Protection of
the diol group of 8 with 2-methoxypropene followed by hy-
drogenolysis of the benzylidene moiety of 1,3-O-benzyli-
dene-2,4-O-isopropylidene-L-erythritol (9) provided 2,4-O-
isopropylidene-L-erythritol (10) in quantitative yield. Com-
pound 10 was then treated with thionyl chloride to afford a
1 : 2 diastereomeric mixture of 2,4-O-isopropylidene-L-ery-
thritol-1,3-cyclic sulfite (11). The oxidation of 11 to the
desired 2,4-O-isopropylidene-L-erythritol-1,3-cyclic sulfate
(2a) was carried out efficiently by the addition of sodium bi-
carbonate to the reaction mixture to prevent decomposition
of the product 2a, and substantial improvement of the oxida-
tion yield up to 95% was achieved. Thus the desired cyclic
sulfate 2a was synthesized in 73% overall yields from D-6 via
seven steps.
Salacinol (1) is a potent glycosidase inhibitor isolated
from the aqueous extracts of the roots and stems of Salacia
reticulata WIGHT (known as Kotala himbutu in Singhalese),
which is traditionally used in Sri Lanka and India for the
treatment of diabetes. The a-glucosidase inhibitory activity
of 1 was confirmed to be as strong as that of acarbose, which
is used clinically.1) The structure of 1 is unique in that the
ring sulfonium ion is stabilized by a sulfate counteranion by
forming a spirobicyclic-like configuration comprised of 1-
deoxy-4-thioarabinofranosyl cation and 1-deoxyerythrosyl-3-
sulfate anion, as shown in Chart 1. Due to both the extraordi-
nary high glycosidase inhibitory activity and the intriguing
structure, much attention has been focused on 1 and related
compounds.2)
Total synthesis of 1 was accomplished recently by two
groups3) independently on the basis of the same strategy.
In both cases, the key step in the synthesis was the ring
opening of the cyclic sulfate (2) by nucleophilic attack of
1,4-dideoxy-1,4-epitio-D-arabinitol (3a). Structure–activity
relationships between some related sulfonium compounds
and the glycosidase inhibitory activity have also been re-
ported.3b,4)
The coupling reaction of the cyclic sulfate 2a with pyrroli-
dine (12) as a model compound proceeded selectively to give
59) in 80% yield. The FAB mass spectrometry of 5 run in the
positive mode showed a molecular protonated ion [MϩH]ϩ
peak at m/z 296. The IR spectrum of 5 showed absorptions
On the other hand, polyhydroxylated pyrrolidines [azasug-
ars, e.g. 1,4-dideoxy-1,4-imino-D-arabinitol (3b)] have been
Chart 1
To whom correspondence should be addressed. e-mail: muraoka@phar.kindai.ac.jp
© 2001 Pharmaceutical Society of Japan

1504
Vol. 49, No. 11
Reagents and conditions: (i) PhCH(OCH3)2, p-TsOH, DMF, 50 °C, 82%; (ii) NaIO4, NaHCO3, H2O, THF, r.t. and then NaBH4,
r.t., 94%; (iii) CH3(CH3O)CϭCH2, DMF, 0—5 °C, quant.; (iv) H2, Pd–C, EtOH, r.t., quant; (v) SOCl2, Et3N, CH2Cl2, 0 °C;
(vi) NaIO4, RuCl3, NaHCO3, CCl4, CH3CN, H2O, r.t., 95%
Chart 2
Chart 3
Table 1. IC50 Values (ϫ10Ϫ5 M) of Compounds 1, 4, and 3b against Disac-
charidase
Compound
Maltase
Sucrase
Isomaltase
Trehalase
1a)
0.96
30.6
3.5
0.25
4.4
2.3
0.18
13.6
0.4
—
Ͼ31.5
2.2
4a)
3bb)
a) Rat small inteatinal brush border membrane vesicles were used. b) Mouse
small intestinal mucosal homogenates were used.5a)
4 displayed similar 1H-NMR spectroscopic properties to
those of the model compound 5, with deshielded signals of
five a-protons due to ammonium ion formation detected.
The relative stereochemistry of the side chain on the nitrogen
was determined on the basis of nuclear Overhauser effect
spectroscopy (NOESY) experiments, as shown in Chart 3.
The observed NOE correlations between two ring protons a
to the nitrogen (H-4 and one of the methylene protons H-1a),
and the methylene protons on C-1Ј suggested the depicted
configuration of the side chain. The NOE correlation be-
tween the other methylene proton H-1b and H-3 was also de-
tected.
The glycosidase inhibitory activity of 4 was tested for the
intestinal a-glucosidase in vitro13) and compared with those
of 1 and 3b, as shown in Table 1. Upon substitution of the
sulfur atom with the nitrogen, the inhibitory activities were
reduced considerably. However, the magnitude of the de-
crease in the inhibitory activity against sucrase was not as
great as those against maltase or isomaltase, and 4 sustained
nearly equal inhibitory activity to 3b against sucrase. No in-
hibition was detected against commercially available b-glu-
cosidase originating from almonds. Further investigations on
the origin of both the strong a-glucosidase inhibitory activity
of 1 and selectivity for certain glycosidase enzymes of 1 and
Fig. 1. Perspective View of 5
due to two functionalities at 2770 cmϪ1 (NϩH), and 1261 and
1234 cmϪ1 (OSOϪ3 ). Observed downfield shift signals in its
1H-NMR spectrum appeared at d 3.18—3.32 (H-1), 3.40 (H-
1Јa), and 3.61 (H-1Јb), corresponding to a-protons of the ni-
trogen, also supported the ammonium ion structure. The
structural confirmation was performed on the basis of single-
crystal X-ray measurement10) as shown in Fig. 1, which de-
tected the hydrogen atom on the ammonium nitrogen. In this
molecule, the two ionic centers were far apart from each
other, while 1 constructed the inner salt between these two
moieties.1)
Reaction of 2a with azasugar 3b11) gave the corresponding
N-alkylated product, 1Ј-[(1,4-dideoxy-1,4-imino-D-arabini-
tol)-4-N-ammonium]-2Ј,4Ј-O-isopropylidene-1Ј-deoxy-L-
erythritol-3Ј-sulfate (13) in 64% yield. Deprotection of 13
using 0.01% HCl at room temperature provided the desired
nitrogen analogue 412) of salacinol in 95% yield. Compound

November 2001
1505
related compounds are in progress.
4.12—4.16 (2H, m, H-2Ј, H-3Ј). 13C-NMR (125 MHz, CD3OD) d:
20.0 (q, CH3), 23.9 (t, C-2), 28.1 (q, CH3), 57.5 (t, C-1, C-1Ј), 63.4 (t,
C-4Ј), 69.6 (d, C-2Ј), 71.2 (d, C-3Ј), 101.1 (s, (CH3)2C). FAB-MS m/z:
296 [MϩH]ϩ. HR-FAB-MS m/z: 296.1142 (C11H22O6NS, calcd for
296.1168).
References and Notes
1) Yoshikawa M., Murakami T., Shimada H., Matsuda H., Yamahara J.,
Tanabe G., Muraoka O., Tetrahedron Lett., 38, 8367—8370 (1997).
2) Yoshikawa M., Murakami T., Yashiro K., Matsuda H., Chem. Pharm. 10) Crystal data for 5: C11H21O6NS, MWϭ295.35, orthorhombic, space
Bull., 46, 1339—1340 (1998).
group P212121, aϭ11.222(4), bϭ13.390(2), cϭ9.529(2) Å, Vϭ
1431.9(6) Å3, Zϭ4, m(Mo-Ka)ϭ2.47 cmϪ1, F(000)ϭ632, Dcϭ1.370
g/cm3, crystal dimensions: 0.12ϫ0.20ϫ0.24 mm. A total of 1906 re-
flections were collected using the w–2q scan technique to a maximum
2q value of 55°, and 1008 reflections with IϾ3s(I) were used in the
structure determination. Final R and Rw values were 0.039 and 0.039,
respectively. The maximum and minimum peaks in the difference map
3) a) Yuasa H., Takada J., Hashimoto H., Tetrahedron Lett., 41, 6615—
6618 (2000); b) Ghavami A., Johnston B. D., Pinto B. M., J. Org.
Chem., 66, 2312—2317 (2001).
4) Yuasa H., Takada J., Hashimoto H., Bioorg. Med. Chem. Lett., 11,
1137—1139 (2001), and references cited therein.
5) a) Scofield A. M., Fellows L. E., Nash R. J., Fleet G. W. J., Life Sci.,
39, 645—650 (1986); b) Asano N., Nash R. J., Molyneux R. J., Fleet
were 0.19 eϪ Ϫ3 and Ϫ0.19 eϪ Ϫ3, respectively.
Å Å
G. W. J., Tetrahedron: Asymmetry, 11, 1645—1680 (2000); c) Take- 11) Fleet G. W. J., Witty D. R., Tetrahedron: Asymmetry, 1, 119—136
bayashi M., Hiranuma S., Kanie Y., Kajimoto T., Kanie O., Wong C.-
(1990).
H., J. Org. Chem., 64, 5280—5291 (1999), and references cited 12) Characterization of compound 4. A pale yellow oil. [a]D20 ϩ9.4
therein.
(cϭ2.0, CH3OH). IR (neat) cmϪ1: 2764, 1257, 1223. 1H-NMR (500
MHz, CD3OD) d: 3.29 (1H, dd, Jϭ13.5, 9.0 Hz, H-1Јa), 3.57 (1H, td,
Jϭ6.5, 2.5 Hz, H-4), 3.59 (1H, dd, Jϭ12.0, 4.0 Hz, H-1a), 3.77 (1H,
br d, Jϭ12.0 Hz, H-1b), 3.81 (1H, dd, Jϭ12.0, 3.7 Hz, H-4Јa), 3.83
(1H, dd, Jϭ13.5, 2.5 Hz, H-1Јb), 3.92 (1H, dd, Jϭ12.0, 4.0 Hz, H-
4Јb), 3.94 (2H, d-like, Jϭ6.5 Hz, H-5a, H-5b), 4.00 (1H, br s-like, H-
3), 4.20 (ddd, Jϭ7.0, 4.0, 3.7 Hz, H-3Ј), 4.18—4.22 (m, H-2), 4.28
(1H, ddd, Jϭ9.0, 7.0, 2.5 Hz, H-2Ј). 13C-NMR (125 MHz, CD3OD) d:
60.4 (t, C-5), 61.6 (t, C-4Ј), 61.7 (t, C-1Ј), 62.9 (t, C-1), 67.4 (d, C-2Ј),
75.9 (d, C-2), 77.7 (d, C-3), 78.7 (d, C-4), 81.0 (d, C-3Ј). FAB-MS
m/z: 318 [MϩH]ϩ. HR-FAB-MS m/z: 318.0855 (C9H20O9NS, calcd for
318.0852).
6) Ghavami A., Johnston B. D., Jensen M. T., Svensson B., Pinto B. M., J.
Am. Chem. Soc., 123, 6268—6271 (2001).
7) Barili P. L., Berti G., Catelani G., Cini C., D’Andrea F., Mastrorilli E.,
Carbohydr. Res., 278, 43—57 (1995).
8) a) Foster A. B., Haines A. H., Homer J., Lehmann J., Thomas L. F., J.
Chem. Soc., 1961, 5005—5011; b) MacDonald D. L., Fisher H. O. L.,
Ballou C. E., J. Am. Chem. Soc., 78, 3720—3722 (1956).
9) Characterization of compound 5. Colorless prisms. mp 175—181°C
(decomp.). [a]D20 ϩ41.3 (cϭ0.5, CH3OH). IR (KBr) cmϪ1: 2770, 1261,
1234. 1H-NMR (500 MHz, CD3OD) d: 1.39 (3H, s, CH3), 1.52 (3H, s,
CH3), 2.01—2.15 (4H, br m, H-2), 3.18—3.32 (4H, m, H-1), 3.40 (1H,
dd, Jϭ13.0, 8.0 Hz, H-1Јa), 3.61 (1H, dd, Jϭ13.0, 2.0 Hz, H-1Јb), 3.84 13) a-Glucosidase inhibitory activity of 4 was measured according to the
(1H, dd, Jϭ11.0, 7.5 Hz, H-4Јa), 4.09 (1H, dd, Jϭ11.0, 5.0 Hz, H-4Јb),
method described in the literature.1,2)