Synthesis of the pentasaccharide hapten from the glycopeptidolipid antigen of Mycobacterium avium serovar 17

Article abstract of DOI:10.1016/S0040-4039(01)00934-0

Effective synthesis of the pentasaccharide hapten from the glycopeptidolipid antigen of Mycobacterium avium serovar 17 in a p-aminophenyl linker-containing form, using 3+2 block synthesis strategy, is described. A 2+3 block synthesis could not be achieved, although different glycosyl donors (1-Br, 1-SPh, 1-O-C(NH)CCl₃) were used.

Full text of DOI:10.1016/S0040-4039(01)00934-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 5283–5286
Synthesis of the pentasaccharide hapten from the
glycopeptidolipid antigen of Mycobacterium avium serovar 17
Zsolt Varga,a Istva´n Bajza,b Gyula Battac and Andra´s Lipta´ka,b,
*
aDepartment of Biochemistry, University of Debrecen, PO Box 55, Debrecen H-4010, Hungary
bResearch Group for Carbohydrates of the Hungarian Academy of Sciences, PO Box 55, Debrecen H-4010, Hungary
cResearch Group for Antibiotics of the Hungarian Academy of Sciences, PO Box 70, Debrecen H-4010, Hungary
Received 2 May 2001; revised 18 May 2001; accepted 31 May 2001
Abstract—Effective synthesis of the pentasaccharide hapten from the glycopeptidolipid antigen of Mycobacterium avium serovar
17 in a p-aminophenyl linker-containing form, using 3+2 block synthesis strategy, is described. A 2+3 block synthesis could not
be achieved, although different glycosyl donors (1-Br, 1-SPh, 1-O-C(NH)CCl3) were used. © 2001 Elsevier Science Ltd. All rights
reserved.
Besides Mycobacterium tuberculosis1 and M. leprae,2,3
the pathogenic agents of human tuberculosis and lep-
rosy, respectively, another ‘atypical’ mycobacteria may
also cause infections in humans.4 Infections with the M.
avium serocomplex show up in more than 50% of
patients with AIDS in some areas of the world.5–7 This
observation initiated a series of investigations, (i) to
determine the surface antigens of different human
Figure 1.
Keywords: oligosaccharide; block synthesis; M. avium serovar 17; nilic acid; hapten; linker.
* Corresponding author. Tel.: +36-52-512-913; fax: +36-52-512-913; e-mail: liptaka@tigris.klte.hu
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00934-0

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Z. Varga et al. / Tetrahedron Letters 42 (2001) 5283–5286
pathogenic mycobacteria, and (ii) to prepare artificial
antigens for the serodiagnosis of mycobacterial
infections.8,9
In this paper we present an effective synthesis of a
p-aminophenyl
linker-containing
pentasaccharide
which was identified as the GPL type antigen8,9 of M.
avium serovar 17 (Fig. 1). The terminal monosaccharide
unit in this structure is a 3-amino-3,6-dideoxy-b-D-
glucopyranoside10 acylated with 3-hydroxy-2-methylbu-
tyric acid (nilic acid11) at position 3. Nilic acid has two
chiral centers and exists in four isomeric forms. We
report here the synthesis of the (2S,3S) isomer contain-
ing pentasaccharide hapten.
Figure 2.
afforded exclusively the b-disaccharide 6 in 75% yield
(Scheme 2).
For the preparation of the target pentasaccharide using
a 2+3 block synthesis, we proceeded as follows. Acetol-
ysis of 3-azido-3-deoxy-1,2-O-isopropylidene-6-O-(p-
However, in contrast to our expectation, the disaccha-
ride donor 6 did not react with the trisaccharide accep-
tor 9 (Fig. 2), and thus to enhance its reactivity,
thioglycoside 6 was transformed into the bromosugar 7
and the trichloroacetimidoyl derivative 8 (Scheme 2).
However, neither 7 nor 8 reacted sufficiently, probably
due to the extremely low nucleophilicity of the trisac-
charide acceptor.
toluenesulfonyl)-a-
D
-glucofuranose12
1
gave
an
anomeric mixture of the tri-O-acetyl derivative 2.
Deoxygenation of 2 at position 6, leaving the 3-azido
group untouched, was performed with the NaCNBH3/
NaI reagent in HMPT13 yielding 3 (73%). Compound 3
itself is a glycosyl donor but with low reactivity, there-
fore, it was transformed into the glycosyl trichloroace-
timidate 4 according to the procedure of Schmidt et
al.14 (Scheme 1).
The unusual behavior of 9 forced us to change the
original synthetic route to a 3+2 approach. Thus, the
disaccharide donor 8 was reacted with ethyl 2,4-di-O-
benzyl-1-thio-a- -rhamnopyranoside 10 in the presence
L
The TMSOTf-mediated coupling of the trichloroace-
timidate 4 with phenyl 2-O-benzyl-4-O-methyl-1-thio-
a-L-rhamnopyranoside 5 as the glycosyl acceptor
of TMSOTf to give the trisaccharide block 11 in a yield
of 55%. The sterically ‘mismatched’ situation in this
glycosylation led to a relatively low yield of coupling.
The thioglycoside donor 11 was then allowed to react
with the disaccharide acceptor 12 in the presence of
NIS/TfOH to furnish the desired pentasaccharide 13 in
a 69% yield (Scheme 3). Although glycosylations were
performed with donors having non-participating group
at C-2, in both reactions the formation of an a-1,2-
trans linkage was observed exclusively.
To avoid a possible O“N acetyl migration, the two
isolated O-acetyl groups were removed with KOtBu,15
prior to the reduction of the azide, to obtain the diol
14. It is to be noted, that in accord with other literature
examples, removal of the hindered or isolated O-acetyl
protecting groups by the classical Zemple´n procedure
Scheme 1. (a) 0.1 M H2SO4, 60°C, then Ac2O/pyridine, 90%;
(b) NaCNBH3, NaI, HMPT, 70°C, 73%; (c) H2NNH3OAc,
DMF, rt, then CCl3CN, K2CO3 CH2Cl2, rt, 85%.
Scheme 2. (a) TMSOTf, CH2Cl2, −50°C, 75%; (b) Br2, CH2Cl2, 0°C; (c) NBS, acetone–H2O, 0°C, then CCl3CN, K2CO3 CH2Cl2,
rt, 75%.

Z. Varga et al. / Tetrahedron Letters 42 (2001) 5283–5286
5285
Scheme 3. (a) TMSOTf, CH2Cl2, −60°C, 55%; (b) NIS/TfOH, CH2Cl2–THF, −50°C, 69%; (c) KOtBu, dioxane–MeOH, rt, quant.;
(d) Ph3P, THF, 87%; (e) (2S,3S)-nilic acid, BOP/DIPEA, 72%; (f) H2/Pt, EtOAc, rt, then Py, TFA-anhydride 0°C, 78%.
Scheme 4. (a) H2/Pd–C, MeOH–H2O (5:1), 85%.
failed in this case.16 For the effective, selective reduc-
tion of the azido group in the presence of the p-nitro-
phenyl aglycone, Ph3P17 was used. The resulting amine
15 was acylated with (S)-3-hydroxy-(S)-2-methylbutyric
product was converted into the trifluoroacetamido
derivative 17. Finally, the benzyl and benzylidene func-
tions were cleaved by hydrogenolysis in the presence of
Pd–C catalyst to furnish the target pentasaccharide 18†
(Scheme 4). Complete NMR data (CD3OD, BRUKER
DRX 500) of 18 are given in Table 1.
acid (prepared from ethyl (S)-3-hydroxybutyrate11,18
)
using BOP/DIPEA as the coupling reagents. Pentasac-
charide 16 was deprotected as follows. The aromatic
nitro group was reduced with Adam’s catalyst and the
The conjugation of the spacer-armed pentasaccharide
hapten to protein, and the biological testing of the
neoglycoconjugate are in progress and will be reported
elsewhere.
† MALDI-TOF for pentasaccharide C44H67F3N2O23+Na+: 1072.54.

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Z. Varga et al. / Tetrahedron Letters 42 (2001) 5283–5286
Table 1. NMR data of pentasaccharide 18 (l, ppm)
Acknowledgements
Residue
A
1H
13C
98.3
77.4
70.6
72.6
68.1
15.8
The authors would like to thank the Howard Hughes
Medical Institute and the National Research Founda-
tion (OTKA T035128) for financial support.
1
2
3
4
5
CH3
5.64
4.03
4.10
3.63
3.89
1.23
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