The synthesis of hybrids of D-galactose with 1-deoxynojirimycin analogues as glycosidase inhibitors

Article abstract of DOI:10.1002/anie.200462413

(Chemical Equation Presented) Sweet stop: Sugar-azasugar hybrids as enzyme inhibitors and leads for drug discovery are presented. Three hybrids 3 of D-galactose with analogues of 1-deoxynojirimycin were prepared from 3,4,6-tri-O-benzyl-2-nitro-D-galactal

Full text of DOI:10.1002/anie.200462413

Angewandte
Chemie
expected, as is evident from two very important, recent
reviews.[1]
The importance of glycobiology in medicinal and biolog-
ical chemistry is at its height, and there have been several
notable contributions in this area from both biologists and
chemists.[9] Recent advances in understanding the role of
glycosidase inhibitors in treating various diseases, though in
their infancy, are significant in the study of glycobiology.[10]
Among many glycosidase inhibitors, nojirimycin, 1-deoxyno-
jirimycin, and their analogues have received a large amount of
attention.[11] Clearly, hybrid molecules derived from nojiri-
mycin analogues will be of interest from the standpoint of
glycosidase inhibition. From the point of view of drug
discovery, hybrids of d-glucose and some heterocycles have
been designed and synthesized by Smith III, Hirschman, and
co-workers.[12] Also, an interesting hybrid molecule that
contains carbasugars has recently been found to be a better
glycosidase inhibitor than the parent carbasugar.[13] These
reports and our interest in the chemistry of glycals[14] and
nitroglycals[14f,g] prompted us to explore the synthesis of novel
hybrids as possible glycosidase inhibitors.
Enzyme Inhibition
Herein, we report the synthesis of three novel hybrids of
d-galactose with three analogues of 1-deoxynojirimycin: 1-
deoxymannonojirimycin, 1-deoxygulonojirimycin, and 1-
deoxymannohomonojirimycin, with 3,4,6-tri-O-benzyl-2-
nitro-d-galactal (1) used as the starting point. The importance
of 2-nitroglycals in the synthesis of glycopeptides, 2-amino-O-
glycosides, 2-amino-C-glycosides, and other useful molecules
has been well demonstrated by Schmidt and co-workers.[14g,15]
We have also recently reported the synthesis of d-livid-
osaminide[14f] from 3-deoxy-2-nitroglucal and 2-amino-C-
glycosides from 2-nitroglycals.[14g] A general retrosynthetic
analysis of the hybrids of d-galactose and analogues of 1-
deoxynojirimycin is presented in Scheme 1, which illustrates
the importance of 2-nitrogalactal and ring-closing metathesis
in our synthetic endeavors. Treatment of 2-nitrogalactal
derivative 1[15] with vinylmagnesium bromide gave a 1:5
mixture of 2a and 2b (Scheme 1) in 73% yield, which were
readily separated by column chromatography. The minor
compound 2a was found to possess a 1C4 conformation as was
revealed by 1H NMR spectroscopic analysis and NOE
interaction experiments. The 1H NMR spectrum of the
major compound 2b, on the other hand, indicated that it is
a mixture of two inseparable components in a ratio of 1.2:1.
However, reduction of this mixture with LiAlH4 followed by
protection of the free amine with an N-tert-butoxycarboxy
(NHBoc) group gave a mixture of three compounds 3a, 3b,
and 3c that could be readily separated by chromatography.
The structures of compounds 3a and 3c were confirmed by
1H NMR spectroscopic analysis, COSY, and NOE interaction
The Synthesis of Hybrids of d-Galactose with
1-Deoxynojirimycin Analogues as Glycosidase
Inhibitors**
B. Gopal Reddy and Yashwant D. Vankar*
The synthesis of hybrid molecules, which are made up of two
different molecular units, has recently gained importance
because many of them exhibit promising physical, chemical,
and biological properties as well as being novel architec-
tures.[1] These molecules are derived from either natural
products or are a combination of natural products and
synthetic compounds that have established or potentially
significant properties, for example, compounds that could be
used in biological or material science applications. In this
context, natural products such as carbohydrates,[2] peptides,[3]
steroids,[4] and taxoids[5] have been employed in the prepara-
tion of hybrid molecules, and their synthetic counterparts
have included anthraquinones,[6] fullerenes,[7] and b-lactams.[8]
Combinations are chosen with the expectation that the hybrid
molecules will display enhanced or modified properties.
Further work on the synthesis of newer hybrid molecules is
[*] Dr. B. G. Reddy, Prof. Dr. Y. D. Vankar
Department of Chemistry
Indian Institute of Technology — Kanpur
Kanpur 208 016 (India)
Fax: (+91)512-259-0007
4
experiments as having C1 conformations. It was difficult to
assign the stereochemical orientations of the vinyl and
E-mail: vankar@iitk.ac.in
1
NHBoc groups on the basis of the H NMR spectrum of 3b;
[**] We thank the Department of Science and Technology, New Delhi,
India for financial support (Grant no. SP/S1/G-21/2001). We also
thank Dr. D. D. Dhavale, Dr. S. G. Sabharwal, and Mr. Tarun Sharma
of the University of Pune for help in carrying out the enzyme
inhibition studies.
however, the appearance of the tert-butyl group as a clean
singlet at d = 1.42 ppm in the spectrum indicated that 3b is a
single isomer, which was also confirmed by its 13C NMR
spectrum. However, it was the spectral analysis of the bicyclic
compound 7 that provided the basis for the conclusive
assignment of the stereochemistry of 3b.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2005, 44, 2001 –2004
DOI: 10.1002/anie.200462413
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2001

Communications
=
Scheme 1. a) CH2 CHMgBr, THF, À608C–RT; b) LiAlH4, THF, 08C–RT, 0.5 h; c) Boc2O, Et3N, CH2Cl2; d) allylbromide, NaH, DMF, 08C–RT;
e) Grubbs’ catalyst (5 mol%), CH2Cl2, reflux; f) OsO4, NMO, acetone/H2O/tBuOH (1:1:0.4); g) 3m HCl in EtOAc, RT, 1 h; h) Ac2O, Et3N, CH2Cl2,
RT, 3 h; i) 20% Pd(OH)2/C, THF, H2, 8 h; j) Ac2O, pyridine, 4 h; k) NaOMe, MeOH, RT, 15 min; l) amberlite (H+) resin (300 mg), MeOH, 10 min;
m) 3m HCl in MeOH, RT, 1 h; n) dowex (OHÀ) resin; o) conc. HCl (0.5 mL) in MeOH (3 mL), reflux, 17 h. DMF=N,N-dimethylformamide,
Boc=tert-butoxycarbonyl, Bn=benzyl, and Cy=cyclohexyl.
The NHBoc group in 3a is axially oriented and allylation
failed under a variety of conditions possibly because of steric
hindrance. However, compound 3b underwent smooth ally-
lation with allyl bromide in the presence of NaH at 08C–room
temperature over one hour to give compound 4 in 97% yield.
Ring-closing metathesis of the diene 4 by using the first
generation Grubbsꢀ catalyst 9[16] produced the expected
bicyclic compound 5 in 91% yield. Dihydroxylation of 5
was then performed with OsO4/N-methylmorpholine N-oxide
(NMO) to form diol 6 in 93% yield. The cis-dihydroxy groups
of the corresponding triacetate 7. Compound 7 showed
significant NOE interaction correlations between H7, H8,
and H9; H5 and H6; and H3, H4, and H9 (see the Supporting
Information). Hydrogenolysis of the triacetate 7 followed by
acetylation was carried out to obtain the hexaacetate 8, which
represents a hybrid molecule of d-galactose and 1-deoxygu-
lonojirimycin (10).
Compound 3c was also transformed into a bicyclic
compound 15 (Scheme 1), a hybrid of d-galactose and 1-
deoxymannonojirimycin (16), by following the same reaction
sequence as above. The cis-dihydroxy groups of the diol 13
were found to have an a geometry, as confirmed by 1H NMR
1
were found to have a b geometry as revealed by H NMR
spectroscopic analysis and the NOE interaction experiments
2002
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spectroscopic analysis and NOE interaction experiments of
the corresponding diacetate 14 (see the Supporting Informa-
tion). Further hydrogenolysis of the diacetate 14 with Pd-
(OH)2/C and hydrogen gas followed by acetylation yielded
the pentaacetate 15, whose structure was determined by X-
ray crystallographic studies;[17] thus, the structures assigned to
compounds 3c and 13 were further confirmed.
A hybrid molecule containing a homologue of the 1-
deoxynojirimycin analogue was prepared from 2-nitrogalactal
(1; Scheme 2), which was treated with allylzinc bromide at
The trans isomer 18a underwent smooth allylation with allyl
bromide in the presence of NaH to give diene 19 in 98% yield.
An attempted allylation of 18b under similar conditions was
unsuccessful, even with heating. This observation is not
surprising as the orientation of the allyl group at C1 and the
NHBoc group at C2 is cis and so there may be steric hindrance
towards allylation. Ring-closing metathesis of the diene 19
with the Grubbsꢀ catalyst 9 gave the expected bicyclic
compound 20 in 94% yield, which was then dihydroxylated
with OsO4/NMO to give 21 in 85% yield. The signal of the
1
anomeric hydrogen atom of 17b appeared in the H NMR
spectrum as a quintet at d = 3.72–3.75 ppm. However, irrida-
tion of the signal for the allylic methylene groups in 17b at d =
2.29 ppm in a homonuclear decoupling experiment resulted in
the signal for the anomeric hydrogen atom becoming a
doublet with J = 10.0 Hz. This result indicated that hydrogen
atoms H1 and H2 are diaxially oriented. The structure was
further confirmed by NOE interaction experiments that
established a cis relationship between H1, H3, and H5 (see
the Supporting Information). Additionally, the structure was
also confirmed by single-crystal X-ray diffraction analysis of
the triacetate 22.[17] The triacetate 22 was hydrogenolyzed and
acetylated to obtain the hexaacetate 23, a hybrid of d-
galactose and 1-deoxymannohomonojirimycin (10).
The hybrid azasugars 8a, 8b, 15a, and 23a showed
selective inhibition activity towards the a-glycosidases[18] at
millimolar concentrations (Table 1). 1-Deoxymannonojirimy-
Table 1: Inhibitory activity of compounds 8a, 8b, 15a, and 23a.[a]
Entry
Enzymes
IC50 [mM]
8a
8b
15a
23a
1
a-galactosidase
coffee beans
almonds
3.73
NI
4.68
6.22
NI
NI
2.00
NI
2
a-glucosidase
rice
3.70
NI
7.00
4.86
2.20
NI
2.19
ND
yeast
3
4
5
b-glucosidase
almonds
=
À
Scheme 2. a) CH2 CH CH2ZnBr, THF, À608C–RT, 3.5 h, 82%;
b) LiAlH4, THF, 08C–RT, 0.5 h; c) Boc2O, Et3N, CH2Cl2, RT, 2 h, 78%
(over two steps); d) allylbromide, NaH, DMF, 08C–RT, 1 h, 98%;
e) Grubbs’ catalyst (5 mol%), CH2Cl2, reflux, 6 h, 94%; f) OsO4, NMO,
acetone/H2O/tBuOH (1:1:0.4), 14 h, 85%; g) 3m HCl in EtOAc, RT,
1 h; h) Ac2O, Et3N, CH2Cl2, RT, 3 h, 87% (over two steps); i) Pd(OH)2/
C, H2, RT, overnight; j) Ac2O, pyridine, RT, overnight, 98%; k) conc.
HCl (0.5 mL) in MeOH (3 mL), reflux, 17 h; l) dowex (OHÀ) resin,
75%.
NI
NI
NI
NI
NI
a-mannosidase
jack beans
b-galactosidase
bovine liver
almonds
6.80
5.20
2.66
ND
NI
NI
5.39
4.50
NI
NI
NI
[a] Inhibition studies were carried out at millimolar concentrations,
optimal pH of the enzymes, and 378C. NI=no inhibition at <7.00 mm
concentration of the inhibitor, ND=not determined.
À608C. The corresponding 2-nitro-C-allyl glycoside was
produced in 82% yield as a 1:4 mixture of two isomers 17a
and 17b, which were readily separated by column chroma-
tography. The minor isomer 17a was assigned the structure as
shown; NOE interaction correlation data (see the Supporting
cin[11a,d] and 1-deoxygulonojirimycin have been recently iso-
lated,[11a,d] however, enzyme inhibition activity of these
compounds has been studied with only three glycosidases.
Thus, the natural 1-deoxymannonojirimycin inhibits a-galac-
tosidase (coffee beans) at a concentration of 420 mm (IC50).[11d]
In our studies with the same galactosidase, the hybrid
molecule 15a of d-galactose with 1-deoxymannonojirimycin
showed no inhibition even at 7 mm concentrations (entry 1).
However, it showed considerable inhibition of a-glucosidase
(rice) at a concentration of 2.20 mm (entry 2). Conversely, the
hybrid of d-galactose with 1,4-dideoxyhomomannonojirimy-
1
Information) indicated it had a C4 conformation. The major
trans isomer 17b was reduced with LiAlH4 and the free amine
protected as a NHBoc group to give a 1:1 mixture of two
isomers 18a and 18b in 78% yield. Clearly, epimerization of
17b had occurred even under mild reaction conditions (08C
to room temperature). The structures of isomers 18a and 18b
were confirmed by COSY and NOE interaction experiments.
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Communications
De Riccardis, D. Meo, I. Izzo, M. Di Filippo, A. Casapullo, Eur.
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cin 23a showed reasonable inhibition of a-galactosidase
(coffee beans), a-glucosidase (rice), and a-mannosidase
(jack beans; entries 1, 2, and 4) and no inhibition of b-
glucosidase and b-galactosidase (entries 3 and 5). This
observation indicates that compound 23a is more specific
towards the inhibition of a-glycosidases than b-glycosidases.
Likewise, hybrid molecules 8a and 8b also moderately inhibit
a-glucosidase (rice) and a-galactosidase (coffee beans).
These studies indicate that the hybrid molecules 8a, 8b,
15a, and 23a are indeed moderate inhibitors of various
glycosidases. It further suggests that structural variations of
these hybrid molecules could promote glycosidase inhibition.
We are currently investigating this possibility and will report
our results in due course.
In summary, we have synthesized three novel hybrids of
d-galactose and 1-deoxynojirimycin analogues that act as
glycosidase inhibitors. Furthermore, they represent novel
scaffolds for possible use in drug discovery, especially in view
of the reports by Smith III, Hirschmann, and co-workers.[12]
Also, the presence of five hydroxy groups raises the possibility
of generating a library of such hybrid molecules, which
enhances the scope of the present syntheses.
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Received: October 24, 2004
Published online: February 21, 2005
Keywords: carbohydrates · enzymes · inhibitors · metathesis ·
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[18] All the enzymes (except a-galactosidase, b-galactosidase, and b-
glucosidase, which were isolated from almond seeds) and the
corresponding substrates were purchased from Sigma Chemicals
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IC50 values obtained are summarized in Table 1.
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