To date, no general methods or strategies exist for complex oligosaccharide and glycoconjugate synthesis and consequently the preparation of these compounds is very time consuming. It is obvious that more efficient and general methods are required and several of our projects are focused on solving these problems.
Traditional strategies for oligosaccharide assembly are characterized by protecting group manipulations between each glycosylation step. Such manipulations increase the linearity and decrease the efficiency of oligosaccharide assembly. We have developed highly convergent glycosylation strategies where most of the synthetic effort is directed towards the preparation of the monomeric glycosyl donors and acceptors. These units can then be assembled into a complex oligomer involving a minimum number of synthetic steps. For example, we demonstrated that a tumor-associated hexasaccharide can be assembled in five consecutive glycosylations without any intermediate protecting group manipulations. The key features of the new glycosylation sequence were a combination of two-directional glycosylation approaches with a chemoselective and an orthogonal

glycosylation. These strategies exploit both the differences in reactivities of anomeric leaving groups and the subtle control of nucleophilicities of sugar hydroxyls and silyl ethers.

The stereoselective introduction of glycosidic linkages is one of the most challenging aspects of chemical oligosaccharide synthesis. These linkages are generally introduced by condensing a fully protected glycosyl donor bearing a

leaving group (L) at its anomeric center, with a suitably protected glycosyl acceptor containing often only one free hydroxyl group (Scheme 1) to give a product which can have either an a- or b-anomeric configuration. Many factors determine the a/b-ratio of these reactions. We are engaged in mechanistic studies, which will help us to develop more reliable stereoselective methods of glycosidic bond synthesis.

Over the years, hundreds of protecting groups have been described for hydroxyls, carboxylic acids and amino functionalities. Surprisingly, only a very small sub-set is useful in carbohydrate chemistry and there is a great need for more versatile protecting groups. We are engaged in a project aiming to develop better protecting groups for amino functionalities. For example, we have described an efficient

method to convert an amino functionality of a saccharide into a pyrolle derivative. We showed that this pyrolle is compatible with many manipulations commonly applied in carbohydrate chemistry. In addition, it can be removed under mild conditions (Scheme 2).

Solid supported synthesis is an interesting method as it

minimizes time consumingwork-up and purification procedures.Traditionally, solid supported synthesis was mainly applied to oligopeptide and oligonucleotide synthesis. However, several of the leading research teams are trying to develop method for solid supported oligosaccharide synthesis. We are engaged in a number of very successful research projects, which deal with solid supported oligosaccharide and glycoconjugate synthesis. We are developing new linkers for easy attachment and removal of compounds from a solid support. Furthermore, we are trying to establish the optimun set of reaction conditions and resins for solid supported oligosaccharide synthesis and are also developing two-directional glycosylation strategies (Scheme 3). We use solid supported synthesis for making single compounds as well as chemical libraries.
A major problem of solid supported synthesis is that the rate of reactions are generally reduced compared to solution-based methods. We are addressing this problem by using dendrimer-supported solution synthesis of oligosaccharides. This strategy is based on the fact that saccharides attached to a dendrimer are soluble but can be easily separated from excess reagents by ultra-filtration or size exclusion column chromatography. An important aspect of this glycosylation protocol is the design and synthesis of dendritic molecules, which are compatible with glycosylation conditions.

A major problem of solid supported synthesis is that the rate of reactions are generally reduced compared to solution-based methods. We are addressing this problem by using dendrimer-supported solution synthesis of oligosaccharides. This strategy is based on the fact that saccharides attached to a dendrimer are soluble but can be easily separated from excess reagents by ultra-filtration or size exclusion column chromatography. An important aspect of this glycosylation protocol is the design and synthesis of dendritic molecules, which are compatible with glycosylation conditions.

Combinatorial chemistry is an important technology for the preparation of large numbers of compounds and it is now well established that it offers

a faster approach for the discovery of new drugs, catalysts and materials compared to a conventional one-molecule-at-a-time synthesis. A characteristic of combinatorial synthesis is that a reaction isperformed with many synthetic building blocks at once, in parallel or in mixtures, rather than with just one building block. We are developing solution- and solid-phase methods for the synthesis of oligosaccharide libraries. Combinatorial chemistry is only useful when powerful methods are available to elucidate active structures. An important part of or research effort is directed to find such methods and we are exploiting a combination of affinity column chromatography with a range of analytical techniques.