Synthesis and evaluation of sulfamide-type indolizidines as glycosidase inhibitors

Article abstract of DOI:10.1016/j.bmcl.2008.04.004

A practical synthesis of reducing sulfamide-derived iminosugar glycomimetics related to the indolizidine glycosidase inhibitor family is reported. The polyhydroxylated bicyclic system was built from readily accessible hexofuranose derivatives through a sy

Full text of DOI:10.1016/j.bmcl.2008.04.004

Available online at www.sciencedirect.com
Bioorganic & Medicinal Chemistry Letters 18 (2008) 2805–2808
Synthesis and evaluation of sulfamide-type indolizidines
as glycosidase inhibitors
a
b
b,
*
´
Mahmoud Benltifa, M. Isabel Garcıa Moreno, Carmen Ortiz Mellet,
c
a,
*
´
´
´
Jose M. Garcıa Fernandez and Anne Wadouachi
aLaboratoire des Glucides UMR CNRS 6219, Universite´ de Picardie Jules Verne, 33 Rue Saint-Leu, F-80039 Amiens, France
bDepartamento de Quı´mica Orga´nica, Facultad de Quı´mica, Universidad de Sevilla, E-41071 Sevilla, Spain
cInstituto de Investigaciones Quı´micas, CSIC, Americo Vespucio 49, Isla de la Cartuja, E-41092 Sevilla, Spain
Received 11 March 2008; revised 31 March 2008; accepted 1 April 2008
Available online 4 April 2008
Abstract—A practical synthesis of reducing sulfamide-derived iminosugar glycomimetics related to the indolizidine glycosidase
inhibitor family is reported. The polyhydroxylated bicyclic system was built from readily accessible hexofuranose derivatives
through a synthetic scheme that involves 5,6-cyclic sulfamides. Further intramolecular nucleophilic addition of the sulfamide nitro-
gen atom to the masked aldehyde group of the monosaccharide in the open chain form afforded the target sugar mimics. By starting
from D-glucose and D-mannose precursors, 2-aza-3,3-dioxo-3-thiaindolizidine derivatives with hydroxylation profiles that matched
those of (+)-castanospermine and 6-epi-(+)-castanospermine were obtained. In vitro screening against a panel of glycosidases evi-
denced a high selectivity towards a-mannosidase.
Ó 2008 Elsevier Ltd. All rights reserved.
Glycosidases play a very important role in many biolog-
ical processes, such as intestinal digestion, post-transla-
tional processing of glycoproteins and the lysosomal
catabolism of glycoconjugates. Consequently, com-
pounds that can modify or inhibit these processes bear
strong potential in therapies targeted at, for instance,
cancer, viral infections, diabetes and glycosphingolipid
storage disorders, being also of interest for mechanistic
studies and purification of enzymes.1
frequently behave as broad-range glycosidase inhibitors.
Bicyclic derivatives generally show a higher selectivity
towards isoenzymes as compared with monocyclic ana-
logues. For example (+)-castanospermine (1), an indo-
lizidine alkaloid with
a hydroxylation profile of
structural complementarity with D-glucose, is a potent
and specific inhibitor of a- and b-glucosidases from dif-
ferent organisms and sub-cellular locations, for exam-
ple, lysosomal or digestive.3 Nonetheless, the lack of
anomeric selectivity, probably related to the absence of
a defined configuration at the pseudoanomeric carbon
(C-5), remains particularly problematic in view of clini-
cal applications.
Nitrogen-in-the-ring carbohydrate mimics (iminosugars
and azasugars) have attracted increasing interest for
such purposes. Their glycosidase inhibitory activity has
been related to their capability to become protonated
at physiological pH, thus mimicking the putative glyco-
syloxacarbenium cation postulated as an intermediate of
glycosidase-catalyzed hydrolysis.2
We have previously reported the synthesis of a new fam-
ily of glycomimetics in which the sp3 amine-type nitro-
gen characteristic of azasugars has been replaced by a
neutral or very weakly basic pseudoamide-type nitrogen,
with substantial sp2 character (sp2-azasugars).4 This
structural change dramatically increases the anomeric
effect at aminoacetal centres, thereby stabilizing axially
oriented oxygen substituents. Interestingly, reducing
sp2-azasugar-type castanospermine analogues displayed
a total anomeric selectivity towards a-glucosidase, the
inhibition potency being strongly dependent on the nat-
ure of the cyclic pseudoamide group. Thus, the reducing
However, while glycosidases exhibit an exquisite selec-
tivity towards their respective substrates, iminosugars
Keywords: Cyclic sulfamide; Indolizidine; Mannosidases inhibitor.
*
Corresponding authors. Tel.: +33 0322827527; fax: +33
0322827560; e-mail addresses: mellet@us.es; anne.wadouachi@u-
picardie.fr
0960-894X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2008.04.004

2806
M. Benltifa et al. / Bioorg. Med. Chem. Lett. 18 (2008) 2805–2808
urea and thiourea derivatives 2 and 3 behaved as very
weak inhibitors of yeast a-glucosidase (Ki = 5.7 and
4.2 mM), while the corresponding carbamate and thioc-
arbamates 4 and 5 were the potent inhibitors of this en-
zyme (Ki = 40 and 2.2 lM, respectively).5 Moreover,
modifications at the exocyclic heteroatom in the five-
membered ring (e.g., 6), led to a sharp shift in enzyme
specificity (Ki = 2.2 lM for b-glucosidase from bovine li-
ver), pointing to the existence of critical interactions in
the enzyme–inhibitor complex involving this region of
the molecule6 (Fig. 1). The biological activity was also
very sensitive to the relative orientation of the hydroxyl
groups, as already observed in the natural polyhydroxy-
indolizidine alkaloid series.7
O
S
N
H
N
O
S
N
H
N
O
HO
O
HO
HO
HO
HO
OH
OH
HO
7
8
Figure 2. Structure of the sulfamide-type compounds prepared in this
work.
6
5
O
H
N
HN
S
O
2
O
O
4
S
NH
O
OH
N
H
1
HO
HO
O
3
HO
OH
OH
A
B
In order to further explore the structure–activity relation-
ships (SAR) in this family of compounds, the replace-
ment of the (thio)urea segment in 2 and 3 into a
sulfamide functional group seemed particularly intrigu-
ing. Sulfamides have been used for the design of pharma-
cological agents with many applications.8 The sp2
character of the nitrogen atoms in sulfamides must be sig-
nificantly reduced as compared with (thio)ureas as a con-
sequence of the higher difference in energy between the
implied N and S orbitals. As a consequence, the geometry
of the N atom in sulfamides can vary from trigonal pla-
nar to pyramidal.9 On the other hand, the two oxygen
atoms at the tetrahedral S atom are well suited to estab-
lish dipole–dipole or hydrogen bond interactions with
protein residues located nearby. Actually, a comparative
structural analysis of the interaction between cyclic urea
and cyclic sulfamides with HIV-1 protease revealed nota-
ble structural differences.10 Here we report the synthesis
of the (+)-castanospermine analogue 7 and its 6-epimer
8, the first examples of cyclic sulfamide-type glycomimet-
ics (Fig. 2). Their structure and biological activity against
a panel of glycosidases are discussed.
7 8
,
Figure 3. General retrosynthetic scheme for the preparation of
sulfamide-type castanospermine analogues.
a-D-glucofuranose 9, readily accessible from commercial
D-glucuronolactone.11 The reaction sequence involved
benzylation of the secondary hydroxyl group followed
by removal of the tetrahydropyranyl protecting group
by acid hydrolysis to give the azido-alcohol 10 (Scheme
1). Reduction of the azido group under Staudinger con-
ditions afforded the corresponding 5-amino-5-deoxy
derivative 11, which was transformed into the Boc-pro-
tected cyclic sulfamide 13 by treatment with the Burgess-
type reagent tert-butyl N-(triethylammonium sulfo-
nyl)carbamate 12.12 Debenzylation of 13 by catalytic
hydrogenation (!14) followed by simultaneous hydro-
lysis of the Boc and isopropylidene groups with aqueous
D-Glucuronolactone
Ref 10
The general synthetic strategy implemented to build the
2-aza-3,3-dioxo-3-thiaindolizidine skeleton relies on the
ability of the carbonyl group of a monosaccharide in
its open-chain aldehydo form (Fig. 3B) to act as an elec-
trophile against a pseudoamide nitrogen atom located at
1,5-relative position. The hydroxylation profile in the fi-
nal compound matches that of the monosaccharide tem-
plate. Consequently, the preparation of the target
derivatives 7 and 8 required D-gluco and D-manno pre-
cursors, respectively, selectively functionalized at C-5
(Fig. 3A).
OTHP
O
OH
O
a
b
O
O
N3
N3
O
O
HO
BnO
9
10
O
O O
Boc
OH
S
N
Et3N
N
12
OtBu
H2N
O
O
O
O
S
O
O
N
H
c
O
O
BnO
BnO
11
13
Compound 7 was prepared in five steps from 5-azido-5-
deoxy-1,2-di-O-isopropylidene-6-O-tetrahydropyranyl-
d
O
H
Boc
O
N
S
N
N
1
2
3
O
8
8a
O
Y
X
S
HO
e
O
N
HO
HO
O
HO
OH
5
6
H
7
N
HO
HO
HO
HO
N
O
HO
HO
7
14
HO
CS, 1
OH
2 and 3 X = NH; Y = O, S
4 and 5 X = O; Y = O, S
6 X = O; Y = NHBu
Scheme 1. Reagents and conditions: (a) 1—BnBr, 60% NaH, DMF,
1 h, rt; 2—TsOH, 1:1 CH2Cl2–MeOH, 2 h, rt, 88% overall yield from
9; (b) 1—PPh3, 5:1 dioxane–MeOH; 2—30% NH4OH aq 96%; (c)
THF, 74 °C, 2 h, 30%; (d) H2, Pd/C, 1:4 EtOAc–EtOH, 1 atm, 18 h; (e)
90% TFA–H2O, rt, 30 min, 60% overall yield from 13.
Figure 1. Structure of (+)-castanospermine and some pseudoamide-
type analogues.

M. Benltifa et al. / Bioorg. Med. Chem. Lett. 18 (2008) 2805–2808
2807
trifluoroacetic acid (TFA) proceeded with concomitant
furanose ! piperidine rearrangement13 to afford the tar-
get bicyclic glucomimetic 714 (Scheme 1).
tosidase (green coffee beans), b-galactosidase (bovine
liver), a-mannosidase (Jack beans), b-mannosidase
(snail), isomaltase (yeast), amyloglucosidase (Aspergillus
niger), trehalase (pig kidney) and naringinase (Penicil-
lium decumbes). Deceivingly, compound 7 did not bind
to any of the glucoside-processing enzymes tested. It be-
haved instead as a modest but totally selective competi-
tive inhibitor of a-mannosidase (Ki = 320 lM). This
result is surprising considering that none of the pseudoa-
mide-type (+)-castanospermine analogues 2–6 showed
activity towards mannosyl hydrolases. The five-mem-
bered cyclic sulfamide group seems to be particularly
well adapted for interacting with amino acids at the ac-
tive site of this particular enzyme. The corresponding 6-
epi diastereomer 8, with a hydroxylation profile of struc-
tural complementarity with that of D-mannose, was
twice as potent as an inhibitor of a-mannosidase
(Ki = 150 lM) while keeping the total selectivity. This
further stresses the important role of the sulfamide func-
tionality in the biological activity, since neither the nat-
ural compound 6-epi-(+)-castanospermine nor the
epimers at C-6 of compounds 2–6 are inhibitors of this
enzyme.5a,7
To synthesize the 6-epi-(+)-castanospermine analogue 8,
commercial L-gulono-c-lactone was first converted into
the selectively protected furanose derivative 15 through
a four-step reaction sequence as reported6 (Scheme 2).
The primary hydroxyl group of 15 was subsequently
protected by formation of the corresponding tert-butyl-
dimethylsilyl ether 16. Sequential trifluoromethanesulf-
onylation of the remaining alcohol group, reaction
with sodium azide and desilylation with tetra-n-butyl-
ammonium fluoride afforded the 5-azido-5-deoxy-D-
mannofuranose derivative 17, which was reduced to
the key aminoalcohol precursor 18. Sulfamidation by
reaction with the Burgess-type reagent 12 provided the
cyclic sulfamide 19, which after removal of the acetyl
and isopropylidene-protecting groups afforded the indo-
lizidine mannomimetic 8.15
The structure of compounds 7 and 8 was confirmed by
their MS and their 1H and 13C NMR spectra in
D2O.14,15 Although the a-anomer predominated, both
the a- and b-anomers were now present in the solutions.
This is radically different from that observed for the pre-
viously reported structures 2–6, which existed exclu-
sively in the a-configuration, being in agreement with
the lower sp2-character of the sulfamide nitrogen. The
resonances of the pseudoanomeric carbons C5a and
C5b (d 83 to 74 ppm) were significantly upfield shifted
from the expected values for reducing sugars, confirming
the aminoacetal bicyclic structure.
In summary, we have developed an efficient synthetic
route for the preparation of sulfamide-type indolizidine
glycomimetics from commercial carbohydrate precur-
sors. While the sulfamide group is able to provide stabil-
ity to aminoacetal centres, the reduced sp2 character of
the bridgehead nitrogen in the bicyclic structure allows
the presence of both the a and b-anomers in aqueous
solution. Interestingly, the biological results indicate
that the cyclic sulfamide moiety is well suited to interact
with a-mannosidase, providing total selectivity towards
this enzyme. Further research aiming at revealing the
structural requirements governing such interactions are
currently sought in our laboratories.
The new sulfamide-type indolizidine glycomimetics were
assayed against a panel of glycosidases including a-glu-
cosidase (baker yeast), b-glucosidase (almonds), a-galac-
γ
L-Gulono- -lactone
Acknowledgments
Ref 5
OH
´
We thank the Spanish Ministerio de Educacion y Cien-
cia (Contract Nos. CTQ2007-61180/PPQ and CTQ2006-
´
15515-C02-01/BQU) and the Junta de Andalucıa for
financial support.
OTBDMS
O
a
O
b
OAc
OAc
HO
HO
O
O
O
O
OH
OH
15
16
References and notes
O
O
OAc
OAc
N3
c
H2N
O
1. For reviews, see (a) Borges de Melo, E.; da Silveira
Gome, A.; Carvalho, I. Tetrahedron 2006, 62, 10277; (b)
O
O
O
12
d
O
O
e
´
Pearson, M. S. M.; Mathe-Allainmat, M.; Fargeas, V.;
17
Boc
18
Lebreton, J. Eur. J. Org. Chem. 2005, 2159; (c)
Afarinkia, K.; Bahar, A. Tetrahedron: Asymmetry
2005, 16, 1239; (d) Germain, D. P. Clin. Genet. 2004,
65, 77; (e) Cipolla, L.; La Ferla, B.; Nicotra, F. Curr.
Top. Med. Chem. 2003, 3, 485; (f) Compain, P.; Martin,
O. R. Curr. Top. Med. Chem. 2003, 3, 541; (g) Asano,
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2002, 102, 515; (i) Asano, N.; Nash, R. J.; Molyneux, R.
J.; Fleet, G. W. J. Tetrahedron: Asymmetry 2000,
11, 1645; (j) Simmonds, M. S. J.; Kite, G. C.; Porter,
E. A. Taxonomic Distribution of Iminosugars in
O
S
N
H
N
N
O
S
OAc
HO
N
O
H
HO
HO
O
O
OH
8
19
Scheme 2. Reagents and conditions: (a) TBDMSCl, pyridine, 3 h, rt,
98%; (b) 1—Tf2O, CH2Cl2, ꢀ10 °C, 30 min; 2—NaN3, DMF, 75%
overall yield from 16; 3—TBAF (1M/THF), THF, 2 h 30 min, rt, 85%;
(c) H2, Pd/C, MeOH, 2 h; (d) THF, 74 °C, 2 h, 30%; (e) 1—NaOMe,
1:1 CHCl3–MeOH, 3 h, 96%; 2—90% TFA–H2O, 0 °C, 30 min, 62%.

2808
M. Benltifa et al. / Bioorg. Med. Chem. Lett. 18 (2008) 2805–2808
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of tert-butanol with chlorosulfonyl isocyanurate (CSI) to
form tert-butyl N-(chlorosulfonyl)carbamate which was
finally converted into the inner salt 12. For examples of
preparation and use of Burgess-type reagents in the
synthesis of cyclic sulfamides, see: (a) Atkins, G. M.;
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¨
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as Glycosidase Inhibitors; Stutz, A., Ed.; Wiley-VCH:
¨
Weinheim, Germany, 1999; p 125.
´
4. For recent references, see: (a) Aguilar, M.; Dıaz-Perez, P.;
Garcıa-Moreno, M. I.; Ortiz Mellet, C.; Garcıa Fernan-
´
13. General procedure for the furanose ! indolizidine rear-
rangement of the cyclic sulfamide: The cyclic sulfamide
was treated with TFA–H2O (9:1) at 0 °C for 30 min,
concentrated under reduced pressure, coevaporated
several times with water, neutralized with Amberlite
IRA-68 (HOꢀ) ion-exchange resin and subjected to
´
´
´
´
dez, J. M. J. Org. Chem. 2008, 73, 1995; (b) Garcıa-
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´
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Tetrahedron 2007, 63, 7879; (c) Garcıa-Moreno, M. I.;
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´
Chem. 2004, 1803.
column
chromatography
using
(10:1:1) ! (5:1:1)
´
´
´
5. (a) Garcıa-Moreno, M. I.; Dıaz-Perez, V. M.; Ortiz
Mellet, C.; Fuentes, J.; Dıaz Arribas, J. C.; Can˜ada, F.
CH3CN–H2O–NH4OH as eluent to give the sulfamide-
type indolizidine.
´
´
J.; Garcıa Fernandez, J. M. J. Org. Chem. 2000, 65, 136;
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´
14. Compound 7: a/b ratio 1:0.47; yield 40 mg (60%); Rf 0.43
(6:3:1 CH3CN–H2O–NH4OH). 1H NMR (500 MHz,
D2O) d 5.20 (d, 1H, J5,6 = 3.7 Hz, H-5a), 4.37 (d, 1H,
J5,6 = 8.0 Hz, H-5b), 3.66 (m, 1H, H-8aa), 3.61 (dd, 1H,
J1a,1b = 11.1 Hz, J8a,1a = 6.4 Hz, H-1aa), 3.60 (m, 1H, H-
1ab), 3.56 (m, 1H, H-7b), 3.55 (t, 1H, J6,7 = J7,8 = 9.1 Hz,
H-7a), 3.48 (dd, 1H, H-6a), 3.34 (t, 1H, J8,8a = 9.1 Hz, H-
8a), 3.33 (m, 2H, H-6b, H-8b), 3.26 (dd, 1H,
J8a,1b = 7.8 Hz, H-1ba), 3.24 (m, 2H, H-8ab, H-1bb); 13C
NMR (125.7 MHz, D2O) d 82.3 (C-5b), 75.4 (C-6b), 75.0
(C-8b), 74.8 (C-5a), 73.2 (C-8a), 72.5 (C-7a), 72.0 (C-6a),
58.2 (C-8ab), 55.3 (C-8aa), 43.8 (C-1a), 43.7 (C-1b);
HRMS (FAB): Calcd for C6H12N2O6NaS: 263.031378,
found m/z 263.029947.
15. Compound 8: a/b ratio 1:0.15; yield 8.5 mg (62%); Rf 0.36
(7:3 CH2Cl2–MeOH). 1H NMR (300 MHz, D2O), a
anomer: d 5.22 (d, 1 H, J5,6 = 2.9 Hz, H-5), 3.80 (dd, 1H,
J6,7 = 3.2 Hz, J7,8 = 9.4 Hz, H-7), 3.74 (m, 1H, H-8a),
3.71 (m, 2H, H-8, H-1a), 3.66 (t, J8,8a = 9.4 Hz, H-8),
3.34 (m, 1H, H-1b); 13C NMR (75 MHz, D2O), a
anomer: d 76.7 (C-5), 71.1 (C-6), 70.7 (C-8), 70.2 (C-7),
55.9 (C-8a), 44.3 (C-1); b anomer: d 80.78 (C-5), 73.5,
73.3, 70.2 (C-6, C-7, C-8), 57.2 (C-8a), 44.1 (C-1);
HRMS: Calcd for C6H12N2O6NaS: 263.0314, found m/z
263.0323.
´
´
´
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