A novel method for solid-phase synthesis of oligosaccharides using the N-1-(4,4-dimethyl-2,6-dioxocyclohexylidene)ethyl (Dde) linker

Article abstract of DOI:10.1016/S0040-4039(00)02197-3

The application of the N-1-(4,4-dimethyl-2,6-dioxocyclohexylidene)ethyl (Dde) linker for the solid-phase synthesis of oligosaccharides is described. The oligosaccharide products can be cleaved from the resin by hydrazine, ammonia or primary amines, but the linker is stable under the conditions of oligosaccharide synthesis. The first sugar can be attached to the resin linker via a vinylogous amide bond, or by ether linkage using a p-aminobenzyl alcohol converter.

Full text of DOI:10.1016/S0040-4039(00)02197-3

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 1159–1162
A novel method for solid-phase synthesis of oligosaccharides
using the N-1-(4,4-dimethyl-2,6-dioxocyclohexylidene)ethyl (Dde)
linker
Nicholas Drinnan,a Michael L. West,a Max Broadhurst,a Barrie Kellamb and Istvan Totha,b,c,
*
aAlchemia Pty Ltd, 3 Hi-Tech Court, Brisbane Technology Park, Eight Mile Plains, Queensland 4113, Australia
bSchool of Pharmacy, University of London, 29-39 Brunswick Square, London WC1N 1AX, UK
cSchool of Pharmacy, The University of Queensland, Brisbane, Queensland 4072, Australia
Received 25 August 2000; revised 22 November 2000; accepted 29 November 2000
Abstract—The application of the N-1-(4,4-dimethyl-2,6-dioxocyclohexylidene)ethyl (Dde) linker for the solid-phase synthesis of
oligosaccharides is described. The oligosaccharide products can be cleaved from the resin by hydrazine, ammonia or primary
amines, but the linker is stable under the conditions of oligosaccharide synthesis. The first sugar can be attached to the resin linker
via a vinylogous amide bond, or by ether linkage using a p-aminobenzyl alcohol converter. © 2001 Elsevier Science Ltd. All rights
reserved.
Oligosaccharides constitute a major class of bioactive
polymers and are implicated in numerous biomolecular
processes,1 and are considered to be potential therapeu-
tics or targets for intervention in a variety of human
diseases. However, their development as therapeutics
has been hindered by the lack of appropriate technolo-
gies for their synthesis. Traditional solution-phase syn-
thesis is time consuming, labour intensive and results in
complex mixtures of oligomers, which are difficult to
separate and purify. Hence, attention has been focused
on solid-phase synthesis as a means to provide a facile
and cost effective method of oligosaccharide synthesis.
Unfortunately solid-phase synthesis of oligosaccharides
can be problematic due to low coupling yields, inade-
quate stereocontrol, and the necessary employment of
low loading resin to achieve significant coupling yields.2
glycosylation conditions and protecting group manipu-
lations, and (2) the cleavage conditions should not
effect the structure of the synthesised compounds. To
date, solid-phase linkers have been developed from the
protecting and activating groups that are used in carbo-
hydrate synthesis. The sugar may be connected to the
linker via an ether,4 ester,5 silyl,6 alkyl thio-ether,7 aryl
thio-ether8 or sulphonate linker.9 A tris(alkoxy)benzyl
amine anchoring was also reported recently.10 Most of
these linkers satisfy the above mentioned require-
ments,11 but after cleavage the resin-linker is not re-
usable for further synthesis.12
We have reported a hydrazine-labile primary amino-
protecting group, N-1-(4,4-dimethyl-2,6-dioxocyclo-
hexylidene)ethyl (Dde), employed for solid-phase
carbohydrate synthesis.13 This Dde group has been
used previously for protection of lysine side chains
during solid-phase peptide synthesis (SPPS).14 A Dde
analogue was also used as a protecting group in combi-
natorial amine synthesis, again on a solid support.15
As was recently noted, a particularly desirable goal is
the development of generally applicable methods for
the rapid assembly of oligosaccharides with a long-term
view towards automation.3 A critical element in the
realisation of this goal is the nature of the linker
between the solid support and the initial synthon. It is
necessary that (1) the linker should be stable under
We report here the use of Dde analogue linkers for the
solid-phase synthesis of oligosaccharides. Initially the
dimedone based linker was coupled to the solid-phase.
Dimedone was reacted with glutaric anhydride in the
presence of triethylamine and dimethylaminopyridine in
anhydrous dichloromethane to give vinylogous acid 1
(80% yield after crystallisation), which was subse-
Keywords: solid-phase/oligosaccharide synthesis; dimedone based
linker.
* Corresponding author. E-mail: i.toth@pharmacy.uq.edu.au
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)02197-3

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N. Drinnan et al. / Tetrahedron Letters 42 (2001) 1159–1162
Figure 1.
Table 1.
R1
R2
R3
R4
R5
R6
4a
4b
4c
NH2
ClBnO
ClBnO
BnO
ClBnO
ClBnO
OH
H
H
H
ClBnO
ClBnO
BnO
ClBnO
ClBnO
BnO
SMe
a/b-OCH2C6H4NH2
quently coupled to Novabiochem 4-methylbenzhydryl-
amine (MBHA) resin (substitution: 0.7 mmol/g) to give
compound 2a (with higher than 99.5% coupling
efficiency according to quantitative ninhydrin test).
Resin-linker conjugate 2a was then heated at reflux in
THF/ethanol (1:1 v/v) with p-aminobenzyl alcohol (five-
fold excess) to afford resin-linker-converter conjugate 3
(Fig. 1). Alternatively, glutaric acid was coupled to
MBHA resin (0.42 mmol/g), in presence of 1.2 equiv.
dicyclohexylcarbodiimide (DCC), then the intermediate
was stirred with dimedone in the presence of DCC
resulting in compound 2a, which was reacted with
p-aminobenzyl alcohol, resulting in compound 3.
Figure 2.
In an alternative strategy, protected aminosugar 4a
(Table 1) was stirred with acid 1 (1.2 equiv.) in ethanol
in the presence of triethylamine (1 equiv.) at 60°C,
overnight, to afford sugar-linker conjugate 5. Conju-
gate 5 (Fig. 2) was then coupled to MBHA resin (0.7
mmol/g) to give resin bound sugar 6 (with higher than
99.5% coupling efficiency according to ninhydrin assay)
(Fig. 3). Subsequent cleavage experiments indicated
that no O-acylation had occurred.
Carbohydrates without free amino groups could be
coupled to the resin-linker when a p-aminobenzyl alco-
hol ‘converter’ was built into the system. Resin-linker-
converter conjugate 3 was glycosylated with p-chloro
benzylated mono-saccharide donor 4b (4 equiv.) in
dichloromethane in the presence of dimethyl-
Figure 3.

N. Drinnan et al. / Tetrahedron Letters 42 (2001) 1159–1162
1161
Figure 4.
In summary the Dde based linkers were stable under
carbohydrate reaction conditions. The cleavage of the
Dde based linkers were carried out under mild condi-
tions and did not adversely effect the nature of the
synthesised carbohydrate.
References
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Figure 6.
,
(methylthio)sulphonium triflate (4 equiv.) and 4 A pow-
dered molecular sieves, at ambient temperature to give
resin bound sugar conjugate 7 (Fig. 4). Resin-sugar
conjugate 7 was treated with 2% hydrazine hydrate in
DMF for 2 hours to afford 4c as an anomeric mixture
(90% yield after chromatographic purification).
As a final proof of concept disaccharide 9 was synthe-
sised. The 4-hydroxyl group of the resin-linker-saccha-
ride conjugate 6 was glycosylated with thio-methyl
glycoside 4b (5 equiv.) in the presence of methyl trifl-
,
uoromethanesulphonate (5 equiv.) and 4 A powdered
molecular sieves, at ambient temperature to provide
disaccharide conjugate 8 (Fig. 5). The resin complex 8
was treated with 5% hydrazine hydrate in DMF for 4 h,
to afford an anomeric mixture of disaccharides 9 (88%
yield after chromatography, a/b; 17/3).16(Fig. 6)
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Rees, D. C. Synlett 1998, 817–827.
Quantitative yields employing ammonia solutions as the
cleavage reagent could only be achieved after repeated
cleavages. The initially formed resin-linker conjugate 2b
was then regenerated to give conjugate 2a by treatment
with tetrabutylammonium hydroxide in DMF. These
preliminary results indicated that the resin could be
successfully reused.
13. Toth, I.; Dekany, G.; Kellam, B., 1997 Application
AU544 19970826, PCT 9808799.

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N. Drinnan et al. / Tetrahedron Letters 42 (2001) 1159–1162
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16. Analytical data for compound 9: Gal: 5.58 (d, 1H, J1–2=
3.6 Hz, H%-1), GlcNH2: 4.33 (d, 1H, J1–2=7.8 Hz, H-1),
3.04 (dd, 1H, H-2), ES–MS: C61H61Cl4NO10 (1109.96)=
1110.94 (100), [M+H]+.
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.