Oligosaccharide synthesis

Details Oligosaccharide synthesis Oligosaccharide synthesis

1999-09-02

2003-06-03

Wilson, James O. (Department: 1623)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

C525S333300, C536S124000, C560S183000, C562S508000, C562S510000

Reexamination Certificate

active

06573337

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to methods for synthesis of oligosaccharides, and in particular to methods for solid phase or combinatorial synthesis of oligosaccharides. The invention provides a novel linker-resin, linker-saccharide, or resin-linker-saccharide complex, which in one embodiment enables a saccharide residue to be linked to a soluble or insoluble polymeric support for use as a basis for solid-phase synthesis of oligosaccharides. In a second embodiment, the-complex of the invention enables oligosaccharides to be linked to a solid polymeric support for use as an analytical reagent.
BACKGROUND OF THE INVENTION
Oligosaccharides constitute a major class of bioactive polymers, implicated in biochemical processes (Lasky, 1992; Varki, 1993) as diverse as cellular differentiation, hormone-cell recognition and cell-cell adhesion, especially viral-host cell (Gambaryan et al, 1995) and bacteria-host cell attachment (Boren et al, 1993). Involvement of oligosaccharides in diseases such as cancer, cardiovascular disorders, microbial infections, graft rejection and autoimmune disorders has therefore, been strongly suggested. Conjugation of carbohydrates to bioactive peptides has also been demonstrated to stabilise the peptides against degradation, and, in more specific circumstances, to facilitate peptide transport across biological barriers (Lee, 1989; Fisher, 1991; Rodriguez, 1989). Thus the ability to synthesise oligosaccharides in a facile and efficient manner is now becoming an extremely important area within organic chemistry.
The highly labour intensive solution phase strategies hitherto utilised in oligosaccharide syntheses require an extremely specialised knowledge and a high degree of chemical skill. This situation was mirrored within the area of peptide synthesis, until Merrifield et al proposed and developed Solid Phase Peptide Synthesis (SPPS) over thirty years ago (Merrifield, 1963). In SPPS immobilisation of the first amino acid of the required sequence to an insoluble resin enabled large excesses of reagents to be used to achieve the coupling of the second amino acid. Any unused materials remaining at the end of the coupling step could then be removed simply by washing the resin beads. This technology meant that the chemist could drive each coupling reaction to almost quantitative yields, and since the peptide intermediates formed were still bound to the resin, purification after each acylation step was not required. SPPS enables peptide and polypeptide synthesis to be employed as a routine research and synthetic tool, and permits large-scale combinatorial synthesis of peptides for screening of potential pharmaceutical agents.
For many years chemists have attempted to transpose this solid-phase methodology to oligosaccharide synthesis, with varying degrees of success. The first attempt was approximately 25 years ago (Frechet and Schuerch, 1971; Frechet and Schuerch, 1972; Guthrie et al, 1971; Guthrie et al, 1973). However, the ozone-mediated deprotection product was an aldehyde-substituted glycoside. Danishefsky and coworkers described the solid phase synthesis of the Lewis b Antigen (Randolph et al, 1995) and N-linked glycopeptides (Roberge et al, 1995) by initial attachment of the primary sugar unit of the oligosaccharide to a 1% divinylbenzene-styrene co-polymer support via a silyl ether linkage. The resin-bound sugar moeity was in this instance a glycal, with on-resin activation achieved via epoxidation of the double bond, and the resulting glycal residue acting as a sugar donor through nucleophile ring-opening of the epoxide. Since there are no calorimetric methods available to the sugar chemist to monitor on-resin glycosylations, the only means of assessing the progress of the reaction is by lysis of the oligosaccharide-resin bond and subsequent analysis of the cleavage product, usually by thin layer chromatography. The tetra-n-butylammonium fluoride-mediated deprotection conditions required to cleave Danishefsky's silyl ether linker are both hazardous and slow. This coupled with the requirement for on-resin activation of the tethered glycals, makes the overall strategy and methodology far from ideal.
In an alternative approach, Douglas and coworkers described the synthesis of D-mannopentose using a polyethyleneglycol &ohgr;-monomethylether co-polymer and a succinoyl or an &agr;,&agr;′-dioxyxylyl diether linker (Douglas et al, 1995). The reactions were carried out in solution phase, with removal of unused reactants being achieved by precipitation of the oligosaccharide-polymer complex and subsequent washing. In the latter example, cleavage of the oligosaccharide-polymer bond was achieved through catalytic hydrogenation, which required exposure of the conjugate to 1 atm of H
2
for 48 h to achieve respectable yields. This again is far too slow to allow effective monitoring of individual glycosylation reactions. Yan et al reported sulphoxide-mediated glycosylation on a Merrifield resin, using a thiophenol linker for the attachment of the primary sugar residue (Yan et al, 1994). This method resulted in the construction of (1-6)-linked oligosaccharides, and was suitable for synthesis of both &agr;- and &bgr;-glycosidic linkages. However, the thioglycosidic linkage to the resin dictates that similar sugar donors cannot be employed in this strategy.
Recently Rademann and Schmidt reported the use of trichloroacetimidate sugar donors to a resin bound sugar tethered via an alkyl thiol (Rademann and Schmidt, 1996); once again, however, this method precludes the use of the far superior thioglycoside sugar donors. Meanwhile, Adinolfi et al described the synthesis of disaccharides using a polyethyleneglycol-polystyrene resin, with connection of the first sugar to the polymeric support through a succinate spacer (Adinolfi et al, 1996). However, the acid lability displayed by this linker means that the primary sugar cannot be linked to the resin via the glycosidic position.
The above examples serve to illustrate that the critical element in solid phase synthesis is the nature of the linker between the solid support and the initial synthon. The linker must display excellent stability to the conditions of coupling and deprotection, yet in the case of solid phase oligosaccharide synthesis, it should also be rapidly and efficiently cleaved to allow monitoring of the progress of individual coupling reactions. The cleavage should ideally be achieved by the use of a relatively innocuous chemical reagent.
It is clear, then, that there remains a need in the art for simple, efficient and economical methods for solid-phase synthesis of oligosaccharides.
A hydrazine-labile primary amino-protecting group, N-1-(4,4-dimethyl-2,6-dioxocyclohexylidene)ethyl (Dde), has been reported for protection of lysine side chains during SPPS (Bycroft et al, 1993). This group was modified for use as a carboxy-protecting group in SPPS when the 2-(3-methylbutyryl)dimedone analogue of 2-acetyl-dimedone was condensed with 4-aminobenzylalcohol to afford 4-[N-[1-(4,4-dimethyl-2,6-dioxocyclohexylidene)-3-methyl-butyl]-amino] benzyl ester (ODmab)(Chan et al, 1995).
The two protecting groups were reported to be stable to the deprotecting conditions widely used in SPPS, ie. trifluoroacetic acid (TFA) or 20% piperidine in dimethyl formamide (DMF). The ethyl ester, 4-[N-(1-(4,4-dimethyl-2,6-dioxocyclohexylidene)ethyl)amino]benzyl ester (ODab) showed small but significant instability to 20% piperidine-DMF. Both Dde and ODmab are linked to groups on amino acids, rather than directly to the solid-phase support. Their use in solid-phase oligosaccharide synthesis has not been suggested.
We have now surprisingly found that protecting groups similar to Dde and ODmab can be coupled to a polymeric support, thereby generating a system for the immobilisation of sugars. To this end we have immobilised N- and O-glycosides to the solid support and synthesised oligosaccharides using various sugar donors. The linkers display excellent stability to most acids and secondary/

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