Solid-phase synthesis of Di-N-Acetyl-β-chitobiosyl NAG-thiazoline

Article abstract of DOI:10.2174/157018011796235220

The solid-phase synthesis of di-N-acetyl-β-chitobiosyl NAG (N-acetyl D-glucosamine)-thiazoline 3 was reported. After the 6-O-benzyl NAG-thiazoline 9, NHCbz trichloroacetimidate donors 14, and 21 were synthesized, and solid-phase synthesis was performed using the Wang resin as support. The target di-N-acetyl-β-chitobiosyl NAGthiazoline 3 was obtained by iterative glycosylation reactions, catalytic hydrogenation, acetylation, and deacetylation, respectively. donors, Wang resin, Glycosylation reactions.

Full text of DOI:10.2174/157018011796235220

Letters in Drug Design & Discovery, 2011, 8, 649-651
649
Solid-Phase Synthesis of Di-N-Acetyl-ꢀ-Chitobiosyl NAG-Thiazoline
Gangliang Huang* and Ya Chen
College of Chemistry, Chongqing Normal University, Chongqing, 400047, China
Received May 10, 2011: Revised May 30, 2011: Accepted May 30, 2011
Abstract: The solid-phase synthesis of di-N-acetyl-ꢀ-chitobiosyl NAG (N-acetyl D-glucosamine)-thiazoline 3 was re-
ported. After the 6-O-benzyl NAG-thiazoline 9, NHCbz trichloroacetimidate donors 14, and 21 were synthesized, and
solid-phase synthesis was performed using the Wang resin as support. The target di-N-acetyl-ꢀ-chitobiosyl NAG-
thiazoline 3 was obtained by iterative glycosylation reactions, catalytic hydrogenation, acetylation, and deacetylation, re-
spectively.
Keywords: Di-N-acetyl-ꢀ-chitobiosyl NAG-thiazoline, Allosamidin analogue, Solid-phase synthesis, Trichloroacetimidate
donors, Wang resin, Glycosylation reactions.
INTRODUCTION
acetyl-ꢀ-chitobiosyl NAG-thiazoline 3 with more complex
structure was described as follows.
The fungi produce chitinases to modify chitins as the ma-
jor cell wall components, and the insects require chitinases
for the partial degradation of their old exoskeletons. So, it
indicates the potential utility of chitinases as targets for the
development of antifungal agents and biological insecticides
(namely chitinase inhibitors). The allosamidins are just a
potent class of pseudodisaccharide and pseudotrisaccharide
chitinase inhibitors. The parent compound, allosamidin 1
(Fig. 1), was isolated from Streptomyces fermentations
twenty-four years ago [1]. Recently, Macdonald et al. re-
ported in detail that the allosamidin analogues, i.e. chitobiose
and chitotriose thiazolines (2 and 3), exhibited chitinase in-
hibition activity, and they were synthesized by the conven-
tional organic synthesis method [2].
RESULTS AND DISCUSSION
Treatment of compound 4 (Scheme 1) with Lawesson
reagent
[2,4-bis(4-methoxyphenyl)-1,3-dithia-2,4-diphos-
phetane-2,4-disulfide] led to selective formation of the
thioamide 5, which then was cyclized by displacement of
acetate to provide the thiazoline triacetate 6 [5]. Compound 6
was reacted with sodium methoxide and followed by selec-
tive monotosylation of the primary hydroxyl group to obtain
compound 8 (77 % for the above two steps). Compound 8
was undergone the SN2 displacement of the tosyl group with
benzyloxy negative ion to afford compound 9 in 65 % yield.
Treatment of ꢁ-D-glucosamine hydrochloride salt 10
with benzyloxycarbonyl (Cbz)-Cl in the presence of Na-
HCO3/H2O yielded N-benzyloxycarbonyl protected glu-
cosamine 11 in 88 % yield. Acetylation of compound 11 by
means of Ac2O in pyridine obtained tetraacetate 12 as a mix-
ture of ꢁ/ꢀ isomers in 4:1. The anomeric acetyl group was
selectively removed using hydrazine acetate in DMF to af-
ford hemiacetal 13. Reaction of compound 13 with CCl3CN
in the presence of 1,8-diaza[5.4.0]bicycloundec-7-ene
(DBU) exclusively afforded ꢁ-trichloroacetimidate donor 14
in 85 % yield (Scheme 2).
OH
OH
O
OH
O
O
O
O
HO
HO
N
NMe2
NHAc
NHAc
OH
.
OH
OH
1
OH
OH
O
O
HO
HO
HO
O
O
HO
O
NHAc
n
NHAc
2 (n=0)
(n=1)
S
N
3
Treatment of compound 12 with hydrazine acetate in the
presence of DMF obtained hemiacetal 13, which was used
without further purification. Then, the mixture was reacted
with tert-butyldimethylsilyl (TBDMS)-Cl and imidazole to
yield exclusively the ꢀ-anomer of the corresponding
TBDMS derivative 15. Deacetylation of compound 15 with
NaOMe/MeOH afforded TBDMS 2-deoxy-N-benzyloxycar-
bonylamino-ꢀ-D-glucopyranoside 16 in 96 % yield. Treat-
ment of compound 16 with benzaldehyde dimethylacetal
afforded the 4,6-O-benzylidene derivative 17. Compound 17
was treated with Ac2O and pyridine to obtain acetate 18 in
94 % yield. Regioselective reductive cleavage of ben-
zylidene acetal 18 with CF3COOH/Et3SiH at 0 °C afforded
6-O-Bn acceptor 19 in 85 % yield. Compound 19 was treated
with levulinic acid in the presence of N,N’-
diisopropylcarbodiimide (DIPC) to yield the orthogonally
Fig. (1). The structures of allosamidin 1, chitobiose and chitotriose
thiazolines (2 and 3).
The solid-phase synthesis is a rapid and efficient method
to synthesize oligosaccharides [3,4]. Therefore, we also in-
tend to prepare 2-acetamido-ꢀ-glucosyl NAG (N-acetyl D-
glucosamine)-thiazoline 2 (namely chitobiose thiazoline) and
di-N-acetyl-ꢀ-chitobiosyl NAG-thiazoline 3 (namely chi-
totriose thiazoline) by solid-phase synthesis. So, it is easier
to remove excess reactants or byproducts in the course of
multi-step synthesis. The solid-phase synthesis of di-N-
*Address correspondence to this author at the College of Chemistry,
Chongqing Normal University, Chongqing, 400047, China; Tel: (86)0
13068336573; Fax: (86)0 13068336573; E-mail: huangdoctor226@163.com
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© 2011 Bentham Science Publishers Ltd.

650 Letters in Drug Design & Discovery, 2011, Vol. 8, No. 7
Huang and Chen
OAc
O
OAc
OAc
AcO
AcO
AcO
AcO
O
AcO
AcO
O
Lawesson reagent
OAc
OAc
HN
NHAc
S
S
N
6
HO
HO
5
4
OTs
OBn
O
OH
O
HO
HO
HO
HO
O
BnONa, NaHCO3
p-TsCl, Pyridine
NaOMe, MeOH
S
S
S
N
N
N
8
9
7
Scheme 1. Synthesis of 6-O-benzyl NAG-thiazoline 9.
OH
OH
OAc
HO
HO
O
HO
HO
O
AcO
AcO
O
Ac2O, Pyridine
Cbz-Cl, NaHCO3
OH
OAc
OH
HCl
NHCbz
NHCbz
12
.
NH2
11
10
OAc
OAc
O
AcO
AcO
O
AcO
AcO
.
CCl3CN, DBU
N2H4 HOAc
CCl3
NH
O
OH
NHCbz
13
NHCbz
14
Scheme 2. Synthesis of N-Cbz protected donor 14.
protected glucosamine 20 in 94 % yield. The anomeric
TBDMS group was removed using tetrabutylammonium
fluoride (TBAF) in the presence of acetic acid. Then, the
crude product was reacted with CCl3CN in the presence of
DBU to afford the ꢀ-trichloroacetimidate donor 21 (Scheme
3).
Chlorination of Wang resin with SOCl2 seemed to be ill-
advised, as the resulting hydrogen chloride solution would
likely cleave the acid-labile benzylic ether linkage from the
solid support. Herein, the Wang resin was chlorinated using
triphenylphosphine and triphosgene (BTC) [6] to obtain the
Wang-chlorinated resin 22 in 82 % yield (Scheme 4).
OAc
OAc
.
O
O
a.
N2H4
HOAc AcO
AcO
AcO
AcO
NaOMe, MeOH
OAc
OTBDMS
b. TBDMS-Cl
NHCbz
NHCbz
12
15
Ph
O
OH
O
O
O
PhCH(OMe)2
HO
HO
HO
OTBDMS
OTBDMS
NHCbz
16
NHCbz
17
Ph
OBn
O
O
O
AcO
HO
AcO
O
Ac2O, Pyridine
CF3COOH, Et3SiH
OTBDMS
OTBDMS
NHCbz
19
NHCbz
18
OBn
OBn
O
LevO
O
LevO
AcO
a. TBAF, HOAc
b. CCl3CN, DBU
AcO
Lev-OH, DIPC
OTBDMS
CCl3
O
NHCbz
20
NHCbz
NH
21
Scheme 3. Synthesis of N-Cbz-Donor 21.
O
O
BTC, Ph3P
OH
Cl
Wang resin
Cl
Wang-chlorinated resin 22
=
Scheme 4. Preparation of Wang-chlorinated resin 22.

Solid-Phase Synthesis of Di-N-Acetyl-ꢀ-Chitobiosyl NAG-Thiazoline
Letters in Drug Design & Discovery, 2011, Vol. 8, No. 7 651
OBn
O
OBn
O
orthogonal protecting group, which can be efficiently
cleaved to liberate the free hydroxyl site for further glycosy-
lation. Cleavage of the levulinoyl ester was performed using
hydrazine acetate dissolved in MeOH to obtain the acceptor
25. After the acceptor 25 was glycosylated with the donor
14, resin was washed, filtered, and dried under the vacuum
overnight. The saccharide bound resin was catalytically hy-
drogenated to cleave the Cbz, Wang resin, and Bn (91 %
yield). Then, the resulting mixture was acetylated with
Ac2O/pyridine and deacetylated with NaOMe/MeOH, re-
spectively, to obtain a crude product. The crude product was
purified by size-exclusion chromatography on Biogel P4 to
afford the corresponding target pseudotrisaccharide 3 in 73
% yield for the last three steps. The amino group is protected
with Cbz. Due to the neighboring group participation of Cbz
during the glycosylation reaction, the ꢀ-linkage is easy to
form.
HO
HO
HO
Bu2SnO
O
22,CsF
S
S
N
N
23
9
21, TMSOTf
OBn
OBn
O
O
LevO
AcO
O
O
NHCbz
S
N
24
HOAc
.
N2H4
OBn
OBn
O
O
HO
AcO
O
O
In summary, the solid-phase synthesis of di-N-acetyl-ꢀ-
chitobiosyl NAG-thiazoline 3 with NHCbz protected gly-
cosamine donors has been studied. With Wang resin, good
yields were obtained throughout the iterative assemblies.
NHCbz
S
N
25
TMSOTf
a. 14,
H2, Pd/C
b.
.
c. Ac2O, Pyridine
.
ACKNOWLEDGEMENT
d. NaOMe, MeOH
The present work was supported by Foundation Project
of Chongqing Normal University (No. 10XLZ004), Natural
Science Foundation Project of CQ CSTC (No. CSTC,
2009BB5238), and Chongqing Education Commission
Foundation (No. KJ080810), China.
OH
O
OH
OH
O
O
HO
HO
HO
O
O
HO
NHAc
NHAc
S
N
3
REFERENCES
Scheme 5. Solid-phase synthesis of di-N-acetyl-ꢀ-chitobiosyl
NAG-thiazoline 3.
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way of stannylene methodology [3] to provide the dibenzy-
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was analyzed by high pressure liquid chromatography
(HPLC) after cleavage of Wang resin with trifluoroacetic
acid from building block 24. Levulinoyl ester is used as an
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