Increasing the inhibitory potency of L-arabino-imidazolo-[1,2]-piperidinose towards β-D-glucosidase and β-D-galactosidase

Article abstract of DOI:10.1016/S0040-4039(03)00696-8

The synthesis of some potent inhibitors of two retaining β-glycosidases was achieved by introducing aglycon-mimics into the imidazole moiety of L-arabino azasugar 1. The strongest inhibition was observed with the phenyl-ethyl substituent at C(2) of 1 agai

Full text of DOI:10.1016/S0040-4039(03)00696-8

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 3667–3670
Increasing the inhibitory potency of
L
-arabino-imidazolo-[1,2]-piperidinose towards b-
D-glucosidase
and b- -galactosidase
D
Estelle Dubost, The´ophile Tschamber and Jacques Streith*
Ecole Nationale Supe´rieure de Chimie, Universite´ de Haute-Alsace, 3 rue Alfred Werner, F68093 Mulhouse, France
Received 11 February 2003; revised 11 March 2003; accepted 13 March 2003
Abstract—The synthesis of some potent inhibitors of two retaining b-glycosidases was achieved by introducing aglycon-mimics
into the imidazole moiety of -arabino azasugar 1. The strongest inhibition was observed with the phenyl-ethyl substituent at C(2)
-glucosidase, whereas the hydroxymethyl group at C(2) increased only slightly the inhibitory
L
of 1 against b-
D
-galactosidase and b-
D
properties. © 2003 Published by Elsevier Science Ltd.
Several non-natural tetrazole-, triazole-, and imidazole
derivatives fused to furanoses or pyranoses have been
synthesised as potential transition-state- analogue mim-
ics of glycosidase inhibitors. Indeed, some of them
proved to be selective and potent glycosidase
dases’ active sites. Therefore, it was unlikely that they
would lead to very strong inhibitions. We surmised that
a stronger inhibition may result from additional substi-
tution of the imidazole moiety with some aglycone-
mimics.5 Along this line of thought, we describe herein
the application of Vasella’s methodology to introduce
phenylethyl and hydroxymethyl groups to the imidazole
moiety of azasugar 1,5 as well as the inhibition data of
these novel imidazolosugar derivatives.
inhibitors.1 Along these lines,
L-arabino-imidazolo-pipe-
ridinose 1 together with its seven stereomers had been
prepared in our laboratory, starting from easily avail-
able
Tested against six commonly encountered a- and b-gly-
cosidases, the -arabino stereomer 1 inhibited markedly
both b- -glucosidase of almonds (Ki=1 mM) and b-
galactosidase of Escherichia coli (Ki=1 mM).2 The
ribo and the
less potent inhibition of b-
D- and L-erythrose and from D- and L
-threose.2
L
D
D
-
-
D
D
-xylo stereomers showed a selective but
-glucosidase of almonds
D
(Ki=20 mM in both cases), whereas the five remaining
stereomers were either inactive or of poor activity
only.2 It is worth noticing that nagstatine 2, which is a
powerful inhibitor of N-acetyl-b-D-glucosaminidase of
bovine kidney (IC50=4 nM), is the only carbohydrate
fused to an azole moiety found so far in nature.3,4 All
eight type 1 imidazolo-carbohydrate stereomers are
piperidino–pentose derivatives, whereas the six glycosi-
dases with which they have been tested, normally
catalyse the hydrolysis of pyrano–aldohexose polysac-
charides. Even though the half-chair conformation of
these imidazolosugars do mimic an oxocarbenium tran-
sition state, they lack the hydroxymethyl group which
may be important for optimal docking to most glycosi-
In order to obtain the key
developed a new and more straightforward synthetic
approach. The dithioethyl-acetal of -arabinose 3 per-
mitted sequential protection of all four alcohol groups
L
-arabino intermediate 7, we
Keywords: azasugars; imidazolo-piperidinoses; glycosidase inhibitors.
* Corresponding author. Fax: +33 389 336875; e-mail: j.streith@
uha.fr
L
0040-4039/03/$ - see front matter © 2003 Published by Elsevier Science Ltd.
doi:10.1016/S0040-4039(03)00696-8

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E. Dubost et al. / Tetrahedron Letters 44 (2003) 3667–3670
leading thereby to derivative 4 whose selective depro-
tection gave aldehyde 5. Condensation of 5 with glyoxal
in methanol solution saturated with gaseous ammonia,
according to the procedure of Rothenberg, Dauplaise
and Panzer,7,8 gave the linear imidazole 6. The latter
compound was selectively deprotected (HCl 4N) to
tion of the hydroxymethyl groups to the same carbon
atoms: reaction of the monoiodo derivatives 10 and 12
in THF with ethyl-magnesium bromide, followed by
addition of DMF gave in good yield the formyl deriva-
tives 17 and 21, respectively. Reduction of these alde-
hyde moieties with LiAlH4, followed by hydrogenolysis
over palladium of the three benzyl ether groups, led in
good overall yields to the corresponding hydroxymethyl
target molecules 20 and 23 (Scheme 2).
L
-arabino derivative 7.
Starting from 7, two pathways were chosen, in order to
introduce an iodine atom, either to C(2) or to C(3), by
taking advantage of the known difference of reactivity
of these two imidazole carbon atoms.9 Reaction of 7
with NIS (2.5 equiv.) gave di-iodo compound 8 whose
mesylate cyclised at once to 9, a compound which was
selectively reduced to 2-iodo derivative 10. Alterna-
tively, mesylation of 7 led to 11 via an intramolecular
Walden inversion. Iodination of bicyclic compound 11
required NIS in excess (5 equiv.) and gave 3-iodo
derivative 12 (Scheme 1).
The four substituted imidazolo-
14, 16, 20 and 23 were submitted to inhibition assays
which were performed with b- -galactosidase of
Escherichia coli and b- -glucosidase of almonds,
L-arabino-piperidinoses
D
D
according to a methodology described in detail in a
previous paper.2 The inhibition data, as determined via
Michaelis–Menten kinetics (Table 1), demonstrate: (i)
the very powerful inhibition of b-galactosidase (Ki=4
nM) with imidazolosugar 16, a compound which also
strongly inhibits b-glucosidase (Ki=80 nM); (ii) that all
four imidazolo–piperidino derivatives 14, 16, 20 and 23
are reversible inhibitors. The inhibition data indicate
that substituents at carbon atom C(3) interact unfa-
vourably with retaining b-galactosidase and b-glucosi-
dase, while the substituents at C(2) significantly
increase the inhibition. These results are in agreement
with a molecular modelling as proposed by Vasella and
his collaborators for a b-glucosidase.10 These authors
showed that only substituents at C(2) project into the
aglycon binding subsite of such glycosidases, while
Introduction of the substituents to carbon atoms C(2)
and C(3) was performed as follows. Reaction of the
mono-iodo derivatives 10 and 12 with phenylacetylene
according to the standard Sonogashira methodology
led to 13 and 15, respectively.6 Total hydrogenation of
the acetylene triple bond of the latter two products over
a palladium catalyst was accompanied by hydrogenoly-
sis of the benzyl ether protections, and gave the corre-
sponding phenyl-ethyl imidazolo-sugar target molecules
14 and 16. A second set of reactions led to the introduc-
Scheme 1. Reagents and conditions: (a) HSEt, HCl, 0°C, 15 min, 79%; (b) TrCl, DMAP cat., pyridine, 80°C, 3 h, 97%; (c) BnBr,
NaH, Bu4NI cat., DMF, 0°C to rt, 96%, (d) Hg(ClO4)2×H2O, CaCO3, THF/H2O, rt, 2 h, 66%; (e) glyoxal, MeOH/NH3, −20 to
80°C, 1 h, 62%; (f) HCl 4N, dioxane, 80°C, 2 h, 81%; (g) NIS, CH3CN, rt, 12 h, 91%; (h) MsCl, pyridine, 0–80°C, 87% of 9, and
92% of 11; (i) EtMgBr, CH2Cl2, 0°C to rt, 30 min, 87%; (j) 5 equiv. NIS, CH3CN, 80°C, 24 h, 57%.

E. Dubost et al. / Tetrahedron Letters 44 (2003) 3667–3670
3669
Scheme 2. Reagents and conditions: (a) phenylacetylene, Pd(PPh3)4, CuI, Et3N, DMF, 80°C, 3 h, 92% of 13 and 62% of 15; (b)
H2, Pd(OH)2/C, EtOH/AcOH, rt, 12 h, 68% of 14 and 56% of 16; (c) EtMgBr, THF, DMF, 0°C to rt, 2 h, 88% of 17 and 84%
of 21; (d) LiAlH4, THF, −78 to −30°C, 1 h 30, 85% of 18 and 83% of 22; (e) H2, Pd(OH)2/C, EtOH/AcOH, rt, 12 h, 87% of 20
and 68% of 23.
Table 1. Inhibition constants of C(2)- and C(3)-substituted imidazolo-L-arabino-piperidinoses
1 (mM)
14 (mM)
16 (mM)
20 (mM)
23 (mM)
b-
b-
D
D
-galactosidase (Escherichia coli )
-glucosidase (Almonds)
1
1
2
15
0.004
0.080
70
20
0.6
0.1
substituents at C(3) interact unfavourably with the
active site. We assume that the strong inhibition by the
phenyl-ethyl derivative 16 reflects hydrophobic interac-
tions of the substituents at C(2) with putative aromatic
residues located close to the aglycon binding site of
these glycosidases.
Since the
D
-ribo and D-xylo stereomers also exhibited
inhibition of glycosidases,2 we shall prepare several
imidazolosugars possessing a range of hydrophobic
substituents at their imidazole C(2) carbon atom, in
order to confirm the structure-activity relationship as
determined herein for the -arabino derivative 16.
L

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E. Dubost et al. / Tetrahedron Letters 44 (2003) 3667–3670
Huber, W.; Streith, J. Eur. J. Org. Chem. 2001, 4111–4125.
Acknowledgements
3. Aoyagi, T.; Suda, H.; Uotani, K.; Kojima, F.; Aoyama, T.;
Horiguchi, K.; Hamada, M.; Takeuchi, T. J. Antibiot.
1992, 45, 1404–1408.
4. Aoyama, T.; Naganawa, H.; Suda, H.; Uotani, K.; Aoyagi,
T.; Takeuchi, T. J. Antibiot. 1992, 45, 1557–1558.
5. Panday, N.; Canac, Y.; Vasella, A. Helv. Chim. Acta 2000,
83, 58–79.
6. Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron
Lett. 1975, 16, 4467–4470.
7. Rothenberg, A. S.; Dauplaise, D. L.; Panzer, H. P. Angew.
We thank the Fondation pour l’Ecole Nationale
Supe´rieure de Chimie de Mulhouse and the Fondation
Pierre-et-Jeanne-Spiegel for a Ph.D. grant to E. D. We
are indebted to our enzymologist Dr. Ce´line Tarnus for
having checked the kinetic measurements of the glycosi-
dase’s inhibition assays. Continuous support of the
Centre National de la Recherche Scientifique (UMR
7015) is gratefully acknowledged.
Chem., Int. Ed. 1983, 22, 560.
8. Schaumann, E. E. Houben-Weyl, Methoden der organis-
chen Chemie, E8c, Georg Thieme Verlag: Stuttgart, New
York, 1994; pp. 1–215.
9. Iddon, B. Heterocycles 1985, 23, 417–443.
10. Panday, N.; Vasella, A. Synthesis 1999, 1459–1468.
References
1. Lillelund, V. H.; Jensen, H. H.; Liang, X.; Bols, M. Chem.
Rev. 2002, 102, 515–553.
2. Gessier, F.; Tschamber, T.; Tarnus, C.; Neuburger, M.;