Synthesis of an arabinogalactan-type octa- and two isomeric nonasaccharides. Suitable tuning of protecting groups

Article abstract of DOI:10.1016/S0040-4039(02)02723-5

An arabinogalactan-type double-branched octa- and two isomeric nonasaccharides were synthesized using the (2-naphthyl)methyl (NAP) and the acid sensitive but base stable (methoxydimethyl)methyl (MIP) protecting groups. The β-(1→6)-linked hexagalactan skeleton was synthesized having a benzyl and a (2-naphthyl)methyl (NAP) group at positions 2 of the second and the penultimate galactopyranosyl units, and this made possible sequential introduction of α-L-arabinofuranosyl or α-L-arabinofuranosyl-(1→5)-α-L-arabinofuranosyl side chains.

Full text of DOI:10.1016/S0040-4039(02)02723-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 631–635
Synthesis of an arabinogalactan-type octa- and two isomeric
nonasaccharides. Suitable tuning of protecting groups
Magdolna Csa´va´s,a Aniko´ Borba´s,a Lo´ra´nt Ja´nossyb and Andra´s Lipta´ka,b,
*
aResearch Group for Carbohydrates of the Hungarian Academy of Sciences, PO Box 55, Debrecen H-4010, Hungary
bDepartment of Biochemistry, University of Debrecen, PO Box 55., Debrecen H-4010, Hungary
Received 14 October 2002; revised 20 November 2002; accepted 29 November 2002
Dedicated to Professor Frieder Lichtenthaler on the occasion of his 70th birthday
Abstract—An arabinogalactan-type double-branched octa- and two isomeric nonasaccharides were synthesized using the (2-naph-
thyl)methyl (NAP) and the acid sensitive but base stable (methoxydimethyl)methyl (MIP) protecting groups. The b-(1“6)-linked
hexagalactan skeleton was synthesized having a benzyl and a (2-naphthyl)methyl (NAP) group at positions 2 of the second and
the penultimate galactopyranosyl units, and this made possible sequential introduction of a-
L
-arabinofuranosyl or a-L-arabino-
furanosyl-(1“5)-a- -arabinofuranosyl side chains. © 2003 Elsevier Science Ltd. All rights reserved.
L
Monoclonal antibodies1 are used very often for the
structure elucidation of complex polysaccharides. Their
production or the evaluation of their specificity require
well-defined oligosaccharides which are the epitopes of
the antigens.
4 was prepared starting either from b-
D
-galactopyran-
osyl-(1“6)-
D
-galactose5 or from b-
D-galactopyranosyl-
(1
“6)-1,2:3,4-di-O-isopropylidene-a-
D
-galactopyranose.6
The dioxolane-type isopropylidene protecting groups
(PGs) behave as medium acid stable permanent PGs
and the more acid labile (methoxydimethyl)methyl
ether (MIP) as a temporary PG and compound 4
contains a free OH-2% functionality. Unfortunately the
MIP group is too labile for compound 4 to act as a
glycosyl acceptor in a subsequent glycosylation reaction
due to hydrolysis or its participation in condensation
reactions.7 For the preparation of the building blocks
suitable for the hexagalactan skeleton, OH-2% of com-
pound 4 was either benzylated8 to give compound 5, or
(2-naphthyl)methylated (NAP) to afford compound 6.9
The benzyl as well as the NAP-groups serve as acid and
base stable orthogonal PGs,10 both can be removed by
hydrogenolysis11 but the NAP group can be cleaved
Elucidation of arabinogalactans2,3 isolated from cell-
cultured Echinacea purpurea would become possible by
using among others the synthetic oligosaccharides 1–3.
All of the three target oligosaccharides have the same
hexasaccharide-type b-(1“6)-linked galactosyl skeleton
in which OH-2 of the second as well as of the penulti-
mate galactopyranosyl units are arabinofuranosylated
(Scheme 1).
Our key reaction was the treatment of a free b-D-
galactopyranosyl unit with 2,2-dimethoxypropane in
the presence of an acid catalyst to obtain the 3,4-O-iso-
propylidene-6-O-(methoxydimethyl)methyl-b-
D-
galactopyranosyl building block.4 The key disaccharide
Scheme 1. Structure of the planned branched oligosaccharides (1, 2 and 3).
* Corresponding author. Tel.: 36-52-512-900-2256; fax: 36-52-512-913; e-mail: liptaka@tigris.klte.hu
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)02723-5

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M. Csa´6a´s et al. / Tetrahedron Letters 44 (2003) 631–635
under oxidative12 (DDQ or CAN) conditions as well.
The sensitive MIP group was also removed from com-
pounds 5 and 6 under very mild acid hydrolysis8 to
obtain OH-6% containing disaccharide-type acceptors (7
and 8) (Scheme 2). These were separately glycosylated
by tetra-O-acetyl-a-D-galactopyranosyl bromide to give
two fully protected trisaccharides (9 and 10) (Scheme
3).
The trigalactopyranose-type glycosyl donor was pre-
pared as follows: from compound 10 the three iso-
propylidene groups were removed with 90% aqueous
trifluoroacetic acid and the free OH groups were acetyl-
ated to give compound 11. The OAc-1 of 11 was
removed by treatment with hydrazine acetate and the
resulting 12 was transformed into trichloroacetimidate
derivative (13) having a readily removable ONAP-2%
group.
To obtain the trisaccharide-type acceptor, compound 9
was saponified using NaOCH3 to give compound 14.
Treatment of 14 with 2,2-dimethoxypropane in the
presence of p-toluenesulfonic acid introduced isopropy-
lidene and MIP groups into the terminal galactopyran-
Scheme 3. Synthesis of the glycosyl acceptor trisaccharide. (i)
tetra-O-acetyl-a- -galactopyranosyl bromide, Hg(CN)2, abs.
D
CH2Cl2, rt, 12 h; (ii) Zemple´n-saponification, 3 h; (iii) 2,2-
dimethoxypropane, pTSA, rt, 4 h; (iv) Ac2O, pyr., rt; (v) 96%
CH3COOH, H2O, CH2Cl2, reflux, 6 h.
osyl residue to obtain compound 15. Acetylation
resulted in the fully protected trisaccharide derivative
(16) from which the MIP group was removed upon very
mild acidic hydrolysis and the second building block 17
was isolated.
TMSOTf catalyzed coupling of the two trisaccharide-
type building blocks (13 and 17) afforded the fully
protected hexasaccharide (18) (Scheme 4) in an accept-
able yield (59%). The presence of the two orthogonal
protecting groups (NAP and Bn) at positions 2 of the
second as well as of the penultimate galactopyranosyl
residues makes it possible to use compound 18 for the
synthesis of all three (1, 2 and 3) oligosaccharides.
The orthogonal deprotection started with the removal
of the NAP group of compound 18 using DDQ12a in
dichloromethane–methanol (4:1) solvent system and
compound 19 was isolated in a yield of 70%.
Compound 19 was glycosylated either with 2,3,5-tri-O-
acetyl-a,b-
to give the heptasaccharide (21), or with 2,3,5-tri-O-
acetyl-a- -arabinofuranosyl-(1“5)-2,3-di-O-acetyl-a,b-
-arabinofuranosyl trichloroacetimidate9 (22) to afford
L
-arabinofuranosyl trichloroacetimidate9 (20)
L
L
the octasaccharide 23 (Scheme 5). Removal of the
benzyl group of compound 21 by catalytic hydrogenoly-
sis afforded compound 24 which was arabinosylated at
position 2 of the second galactopyranosyl unit of the
hexameric b-(1“6)-linked galactopyranosyl skeleton
furnishing the fully protected octasaccharide 25. On the
Scheme 2. Digalactosyl building blocks (7 and 8) and synthe-
sis of the glycosyl donor trigalactoside (13). (i) tetra-O-acetyl-
a-D-galactopyranosyl bromide, Hg(CN)2, abs. CH2Cl2, rt; (ii)
90% aq. CF3COOH, rt, 15 min; Ac2O, pyr., 12 h; (iii)
hydrazine acetate (1.3 equiv.), DMF, rt, 5 h; (iv) CCl3CN,
K2CO3, abs. CH2Cl2, 12 h.

M. Csa´6a´s et al. / Tetrahedron Letters 44 (2003) 631–635
633
Scheme 4. Synthesis of the fully protected b-(1“6)-linked hexagalactose skeleton (18) and its transformation into the aglycon (19)
ready for arabinosylation. (i) abs. CH2Cl2, TMSOTf (0.15 equiv.), −45°C, 2 h; (ii) CH2Cl2:MeOH=4:1, DDQ (1.5 equiv.), rt, 5 h.
Scheme 5. Synthesis of the protected branched hepta- (21) and octasaccharides (23) and their hydrogenolysis to the monohydroxy
compounds 24 and 27. (i) abs. CH2Cl2, TMSOTf, −45°C; (ii) Pd/C, H2, EtOAc, rt.
other hand glycosylation of compound 24 with glycosyl
donor 22 yielded the nonasaccharide 26 (Scheme 6).
The fully protected arabinogalactans (25, 26 and 28)
contained the same set of protecting groups, namely
acetyls and dioxolane-type isopropylidene acetals.
Catalytic hydrogenolysis of the benzyl group of
octasaccharide 23 provided the hydroxyl group contain-
ing octasaccharide 27 which was treated with the donor
20 and the second nonasaccharide 28 was obtained.
Deprotection followed common procedures, involving
Zemple´n’s deacetylation and consecutive mild acid
hydrolysis of the isopropylidene groups with tri-

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M. Csa´6a´s et al. / Tetrahedron Letters 44 (2003) 631–635
Scheme 6. Introduction of the arabinofuranosyl- and the diarabinofuranosyl units into position OH-2%. (i) abs. CH2Cl2, TMSOTf
(1 equiv.), −45°C.
fluoroacetic acid resulting in the free octa-(1) and the two
nonasaccharides (2 and 3) (Scheme 7).
In summary we have demonstrated an efficient synthesis
of a branched octa- and two nonasaccharides. Oligosac-
charides remain challenging synthetic targets, the intro-
duction and the use of the MIP acetal and its
combination with NAP and benzyl ethers hopefully will
gain application in the synthesis of other complex
oligosaccharides.
The identity and stereochemical integrity of all com-
pounds described here were confirmed by HPLC and by
1H and 13C NMR spectra. MALDI-TOF mass spectra13
provided further support for the declared structures.
Scheme 7. Synthesis of the fully protected nonasaccharide 28 and deprotection of compounds 25, 26 and 28 into free
oligosaccharides 1, 2, and 3. (i) abs. CH2Cl2, TMSOTf (1 equiv.), −45°C, 2 h, (ii) NaOCH3, MeOH, rt. quant., (iii) 90% aq.
CF3COOH, rt, 15 min.

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635
Acknowledgements
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This work was supported by grants from the Ministry
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