The 4-nitrobenzenesulfonyl group as a convenient N-protecting group for iminosugars-synthesis of oligosaccharide inhibitors of heparanase

Article abstract of DOI:10.1016/j.tetlet.2009.11.042

The 4-nitrobenzenesulfonyl (nosyl) group can be used advantageously for the protection of the ring nitrogen atom of iminosugars. This group is conveniently introduced, is stable to most of the standard carbohydrate transformations and can be removed under mild conditions. The applicability of the nosyl group is demonstrated by the synthesis of sulfated oligosaccharides which are inhibitors of the enzyme, heparanase. The N-(4-nitrobenzenesulfonyl) group is orthogonal with the azido function.

Full text of DOI:10.1016/j.tetlet.2009.11.042

Tetrahedron Letters 51 (2010) 391–395
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Tetrahedron Letters
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The 4-nitrobenzenesulfonyl group as a convenient N-protecting group
for iminosugars—synthesis of oligosaccharide inhibitors of heparanase q
*
Zsuzsánna Csíki, Péter Fügedi
Department of Carbohydrate Chemistry, Chemical Research Center, Hungarian Academy of Sciences, Pusztaszeri út 59-67, H-1025 Budapest, Hungary
a r t i c l e i n f o
a b s t r a c t
Article history:
The 4-nitrobenzenesulfonyl (nosyl) group can be used advantageously for the protection of the ring nitro-
gen atom of iminosugars. This group is conveniently introduced, is stable to most of the standard carbo-
hydrate transformations and can be removed under mild conditions. The applicability of the nosyl group
is demonstrated by the synthesis of sulfated oligosaccharides which are inhibitors of the enzyme, hepa-
ranase. The N-(4-nitrobenzenesulfonyl) group is orthogonal with the azido function.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 12 October 2009
Revised 27 October 2009
Accepted 10 November 2009
Available online 14 November 2009
Keywords:
Iminosugars
4-Nitrobenzenesulfonyl group
Oligosaccharide synthesis
Heparanase
Heparan sulfate proteoglycans (HSPGs) are ubiquitous constitu-
ents of the extracellular matrix and cell membrane structures.1 The
structure of HSPG consists of a protein core to which several linear
heparan sulfate (HS) chains are linked by O-glycosidic bonds. HS
binds to a multitude of proteins thereby influencing a variety of
normal and pathological physiological processes, including tumor
growth and metastasis, tissue repair, angiogenesis, and inflamma-
tion.2 Cleavage of HS chains is expected to alter its interaction with
proteins and thus influence the above biological processes. HS is a
member of the glycosaminoglycan family of polysaccharides; it
diabetic, antiviral, and anticancer agents. Though monosaccharidic
iminosugars, in general, show some specificity to inhibit certain
types of glycosidases, this specificity is still fairly broad, which lim-
its their potential therapeutic applications. One way to increase
specificity is to use larger sized molecules which are closer mimics
of the natural substrates of the enzymes. Thus, oligosaccharides
containing an iminosugar component have been synthesized for
various biological purposes.8
In order to incorporate specificity in iminosugars toward hepa-
ranase we have designed pseudooligosaccharides mimicking the
structure of heparin and heparan sulfate (Fig. 1).9 The syntheses
of related aza-analogs of heparin disaccharides10 as well as inter-
glycosidically S-linked oligosaccharides11 have been reported re-
cently for similar purposes.
consists of alternating uronic acid (either
D-glucuronic or L-iduron-
ic acid) and -glucosamine units which are connected by (1?4)
D
linkages. The enzyme, heparanase, is an endo-b-glucuronidase that
cleaves heparan sulfate side-chains at a limited number of sites.3
The cleavage of heparin and heparan sulfate by heparanase plays
a crucial role in a number of biological processes, including cell
invasion, angiogenesis, inflammation, and tissue remodeling.3a,4
The expression level of heparanase has been correlated with the
survival time of cancer patients.5 The inhibition of heparanase
forms the basis of potential antimetastatic cancer therapy, and it
has therefore been intensively investigated.6
Iminosugars are monosaccharide analogs which possess a nitro-
gen atom instead of oxygen in the ring, and have received signifi-
cant attention as carbohydrate mimics.7 Compounds of this type,
such as 1-deoxynojirimycin, are potent inhibitors of glycosidases,
and have been investigated for their therapeutic potential as anti-
Compounds 1 and 2 contain a
D-glucosamine unit
a-(1?4)-
linked to an azasugar analog of -iduronic acid and
L
D
-glucuronic
acid, respectively. The different substitution patterns of the two
nitrogen atoms in the target compounds necessitate their
OR2
OR2
O
HO
HO
R1HN
O
NaO2C
O
OH
NH
CO2Na
NH
R1HN
O
HO
HO
HO
HO
OH
2
1
R1 = Ac or SO3Na; R2 = H or SO3Na
q
Synthesis of glycosaminoglycan oligosaccharides, part 3. For part 2 see, Ref. 24.
* Corresponding author. Tel.: +36 1 438 1100; fax: +36 1 438 1145.
E-mail address: pfugedi@chemres.hu (P. Fügedi).
Figure 1.
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.11.042

392
Z. Csíki, P. Fügedi / Tetrahedron Letters 51 (2010) 391–395
differentiation by separate protecting groups. The target structures
should be available from the aza-uronic acid glycosyl acceptors 3
preparation of the iminosugars via cyclization of nosylated amino-
alditols.19 Thus, Mitsunobu reaction of the N-nosylated
-glucitol
derivative 7, obtained from 6, gave stereoselectively the L-ido con-
D
and 4, and the 2-azido-2-deoxy-
D-glucosyl donor 5 (Scheme 1).
The benzyloxycarbonyl group is used commonly for the protec-
tion of the ring nitrogen during the synthesis of iminosugar-con-
taining oligosaccharides.8a,8d–f,8h,8j,8n,8o,10 There are, however,
problems associated with its use. One such problem is the ready
formation of a 6-O,N cyclic carbamate derivative under basic con-
ditions8i,8j,8o,12 which prevents the accomplishment of certain
transformations at O-6.8i,8j,8o Removal of the benzyloxycarbonyl
group by catalytic hydrogenation results in N-alkylation under
some conditions.13 A major inconvenience in the use of the benzyl-
oxycarbonyl group is the existence of rotamers at the amide nitro-
gen, which results in line-broadening and duplication of signals in
the NMR spectra.8h,8j,12b In order to obtain good quality 1H NMR
spectra it was necessary to record the spectra at 115 °C in
DMSO-d6 at 400 MHz.14 Although these problems are well known,
no effort seems to have been made to replace the benzyloxycar-
bonyl group. There are only a few examples of other protecting
groups used for N-protection of iminosugars in oligosaccharide
synthesis.15,16
We now report that the 4-nitrobenzenesulfonyl (nosyl, Ns)
group can be used advantageously to protect the ring nitrogen in
iminosugars, and illustrate this by its application in the synthesis
of heparanase inhibitory disaccharides. The 2- and 4-nitro-
benzenesulfonyl, as well as 2,4-dinitrobenzenesulfonyl groups
were introduced by Fukuyama for the protection of amines.17
The nitrobenzenesulfonamides are readily prepared and the nosyl
group can be removed under mild conditions, nevertheless it has
found only limited application in carbohydrate chemistry as yet.18
Due to the ready N-alkylation of nitrobenzenesulfonamides by
the Mitsunobu reaction, the nosyl group is instrumental in the
figured iminosugar 8 in 96% yield (Scheme 2). Alternatively, nosyl-
protected iminosugars are readily prepared by nosylation of the
iminosugars obtained by reductive amination. Thus, the crystalline
N-nosyl 1-deoxynojirimycin derivative 11 was readily prepared
from 10, which was itself obtained from the 5-azido-5-deoxy-D-
glucose derivative 9 by hydrolysis of the isopropylidene acetal fol-
lowed by catalytic hydrogenation.20
In contrast to N-benzyloxycarbonyl derivatives, no duplication
or line-broadening of signals was visible in the 1H NMR spectra
of the N-nitrobenzenesulfonyl derivatives.
The nosyl group proved to be stable during common protecting
group manipulations to transform these compounds into the gly-
cosyl acceptors. Thus reductive ring cleavage of the naphthylmeth-
ylene acetal in 8 with BH3ÁTHF–TMSOTf21 and NaBH3CN–HCl22
afforded the 4-O- (12) and 6-O-(1-naphthyl)methyl (1NAP) ethers
(13), in 93% and 69% yields, respectively (Scheme 3).
A critical step in the synthesis of the target oligosaccharides is
the deprotection in the presence of the highly acid- and base-sen-
sitive sulfate groups. To check the stability of sulfate groups under
the conditions of nosyl group removal, compound 12 was con-
verted into the 6-O-sulfate derivative 14, and its denosylation
was examined (Scheme 4). We found that the nosyl group could
be removed with thiophenol in the presence of different bases
(K2CO3 and Et3N) to yield the free amine 15 leaving the sulfate
group intact.
For the synthesis of the target disaccharides, compound 12 was
converted into the glycosyl acceptor via a one-step oxidation with
PDC-Ac2O-t-BuOH23 to give the tert-butyl uronate 16, followed by
removal of the (1-naphthyl)methyl group with CAN24 to yield 17.
OR2
O
R3
N
NaO2C
RO
OH
NH
RO2C
O
R1HN
HO
HO
OR4
O
RO
HO
HO
1
3
RO
RO
SPh
OR2
O
N3
HO
HO
R1HN
CO2R
N
CO2Na
NH
R3
HO
RO
5
O
HO
OR
OH
4
2
Scheme 1. Retrosynthesis of heparanase inhibitory oligosaccharides.
1Naph
Ns
N
1Naph
a
O
BnO
b
O
OH
OH
O
O
NHNs
NH2
BnO
BnO
O
OBn
OBn
BnO
O
1Naph
6
7
8
HO
N3
OH
NH
OH
N
OBn
O
c
d
HO
BnO
HO
BnO
Ns
O
OH
OH
O
Me
11
Me
9
10
1Naph = 1-naphthyl
Scheme 2. Reagents and conditions: (a) NsCl, Et3N, CH2Cl2, 98%; (b) DEAD, Ph3P, CH2Cl2, 96%; (c) (1) TFA, H2O, 95%; (2) H2, Raney Ni, MeOH, 77%; and (d) NsCl, NaHCO3,
dioxane, H2O, 70%.

Z. Csíki, P. Fügedi / Tetrahedron Letters 51 (2010) 391–395
Ns
393
Ns
Ns
BnO
1NAPO
BnO
BnO
HO
b
a
N
N
N
O
O
1NAPO
BnO
BnO
BnO
HO
1Naph
13
8
12
1NAP = (1-naphthyl)methyl
Scheme 3. Reagents and conditions: (a) BH3ÁTHF, TMSOTf, CH2Cl2, 93%; and (b) NaBH3CN, HCl–Et2O, THF, 69%.
H
N
Ns
N
BnO
BnO
NaO3SO
NaO3SO
b
1NAPO
1NAPO
BnO
BnO
a
c
Ns
N
15
14
BnO
HO
1NAPO
BnO
Ns
N
Ns
N
BnO
BnO
d
12
tBuO2C
tBuO2C
1NAPO
BnO
BnO
HO
17
16
Scheme 4. Reagents and conditions: (a) SO3Ápyr, DMF, 83%; (b) PhSH, K2CO3, DMF, 52%; or PhSH, Et3N, DMF, 93%; (c) PDC, Ac2O, tBuOH, CH2Cl2, 54%; and (d) CAN, CH3CN, H2O,
78%.
The nosylated azasugar derivatives 13, 17, and 18 (obtained
from 11) having free 4-hydroxy groups proved to be good glycosyl
the N-(4-nitrobenzenesulfonyl) group with the azido function,
demonstrated here, could be very useful for further applications
in the syntheses of oligosaccharides and other compounds contain-
ing multiple amino functions.
acceptors. Their glycosylation with 2-azido-2-deoxy-
D-glucose
thioglycoside 19 using Me2S2–Tf2O as promoter25 afforded the
a
-
linked disaccharides 20–22 stereoselectively and in excellent
yields (Scheme 5).
The fully protected disaccharides were readily converted into
the target compounds as illustrated by the transformation of 24
into 27 (Scheme 7).
After hydrolysis of the tert-butyl group the amino alcohol was
N,O-disulfated to give 26. Removal of the nosyl group using thio-
phenol and triethylamine, followed by catalytic hydrogenolysis
afforded the target compound 27.
In summary, we have reported the 4-nitrobenzenesulfonyl
group for the protection of the ring nitrogen in iminosugars. This
group can be introduced conveniently and it proved to be stable
to a series of common carbohydrate transformations including
reductive acetal openings, glycosylations, oxidations, sulfations,
Before attempting to convert these protected disaccharides into
the target sulfated oligosaccharides, the orthogonality of the N-no-
syl group and the azido function was investigated. Reaction of the
azido group in 23 [obtained by removal of the chloroacetyl group
with hydrazinedithiocarboxylate26 (HDTC) from 21], with Me3P
followed by hydrolysis of the phosphinimine afforded the free
amine 24 (Scheme 6).
On the other hand, treatment of 23 with 1,3-propanedithiol in
the presence of Et3N afforded the denosylated derivative in 77%
yield without reducing the azido group.27 The orthogonality of
1NAPO
BnO
ClAcO
BnO
Ns
N
1NAPO
BnO
O
Ns
Ns
O
ClAcO
BnO
N
+
+
SPh
SPh
BnO
O
N3
20
N3
BnO
BnO
HO
BnO
19
13
ClAcO
BnO
BnO
Ns
N
BnO
N
tBuO2C
O
BnO
O
O
tBuO2C
ClAcO
BnO
N3
N3
BnO
BnO
HO
BnO
21
19
17
ClAcO
BnO
BnO
O
ClAcO
BnO
BnO
CO2tBu
N
O
HO
BnO
CO2tBu
N
Ns
N3
+
SPh
O
Ns
N3
OLev
BnO
OLev
19
22
18
ClAc = -C(=O)CH2Cl
Scheme 5. Reagents and conditions: Me2S2–Tf2O, Et2O, CH2Cl2, 20 (86%); 21 (90%); and 22 (75%).

394
Z. Csíki, P. Fügedi / Tetrahedron Letters 51 (2010) 391–395
BnO
N
tBuO2C
O
Ns
HO
O
BnO
NH2
BnO
BnO
b
c
24
BnO
N
tBuO2C
O
O
Ns
RO
BnO
N3
BnO
BnO
21 R = ClAc
BnO
N
a
tBuO2C
O
O
H
23 R = H
HO
BnO
N3
BnO
BnO
25
Scheme 6. Reagents and conditions: (a) HDTC, DMF, 74%; (b) Me3P, THF, then H2O, 79%; and (c) 1,3-propanedithiol, Et3N, pyr, H2O, 77%.
BnO
N
NaO2C
O
BnO
N
tBuO2C
O
O
Ns
O
Ns
NaO3SO
BnO
b
HO
a
BnO
NHSO3Na
BnO
NH2
BnO
BnO
BnO
26
24
OH
N
NaO2C
O
O
H
NaO3SO
HO
NHSO3Na
HO
HO
27
Scheme 7. Reagents and conditions: (a) (1) TFA, CH2Cl2; (2) SO3Ápyr, Et3N, CH2Cl2, 95%; (b) (1) PhSH, Et3N, DMF, 98%; and (2) H2, Pd/C, THF, H2O, 52%.
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and removal of several common protecting groups. The nosyl
group can be removed under mild conditions without affecting
the O- and N-sulfate groups. An additional advantage is that the
NMR spectra of the nosylated compounds are simpler than those
of N-benzyloxycarbonyl-protected examples. The usefulness of
the N-nosyl protection was demonstrated by the synthesis of sul-
fated oligosaccharides which are inhibitors of the enzyme, hepa-
ranase. The orthogonality of the N-(4-nitrobenzenesulfonyl)
group with the azido function was also demonstrated. This prop-
erty seems to be of great value for selective functionalization of
polyamino compounds.
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Acknowledgements
We are grateful to Dr. Pál Szabó and Mária Kajtár-Peredy for
their assistance in recording the MS and NMR spectra, respectively.
Supplementary data
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