Combinatorial Chemistry & High Throughput Screening - Volume 5, Issue 5, 2002

Oligosaccharides, commonly found on the cell surfaces, are deeply involved in a variety of important biological functions, yet demanding difficulties synthesizing such structures limit the investigation of their functions. Technologies to chemically synthesize these oligosaccharides have dramatically advanced during the last two decades mainly due to the introduction of good anomeric leaving groups. In addition, tactical analyses have been addressed to enhance the overall efficiency of oligosaccharide synthesis. Based on the advancement of solution-phase chemistry, solid-phase technologies are being investigated in connection with the current trend of combinatorial chemistry and high throughput screening. This review summarizes the necessary solution-phase methodologies, the status of solid-phase synthesis of oligosaccharides, and combinatorial synthesis of oligosaccharide libraries.

Glycobiology opens a wide field for new therapeutic approaches. However, the complexity and unavailability of various carbohydrate test compounds has excluded this class of natural products from modern screening systems. Alternatively, glycomimetics are considered to be more drug-like candidates for development. By means of multicomponent condensations (MCCs) utilizing suitable carbohydrate synthons, rapid and effective access to glycoconjugate libraries can be obtained. The flexibility of MCCs allows the assembly of diverse carbohydrate containing libraries. It may be assumed that MCCs containing carbohydrate moieties will play an important role in glycomimetic chemistry and biology.

To meet the growing demands for the development of new molecular entities for discovering new drugs and materials, organic chemists have started looking for new concepts to supplement traditional approaches. In one such approach, the expertise gained over the years in the area of organic synthesis and the rational drug-design concepts are combined together to create 'nature-like' and yet unnatural organic molecules that are expected to provide leads in discovering new molecules. Emulating the basic principles followed by nature to build its vast repertoire of biomolecules, organic chemists are developing many novel multifunctional building blocks. Sugar amino acids constitute an important class of such polyfunctional scaffolds where the carboxyl, amino and hydroxyl groups provide an excellent opportunity for organic chemists to create structural diversities akin to nature's molecular arsenal. Recent advances in the area of combinatorial chemistry give unprecedented technological support for rapid compilations of sugar amino acid-based libraries exploiting the diversities of carbohydrate molecules and well-developed solid-phase peptide synthesis methods.This review chronicles the development of sugar amino acids as a novel class of peptidomimetic building blocks and their applications in generating desired secondary structures in peptides as well as in creating mimics of natural biopolymers.

Many biological processes of vital importance are triggered by the molecular recognition of small carbohydrate units by proteins and receptors thus leading to the belief that carbohydrates could act as candidates for the design of new drugs. We have developed a new useful synthetic approach, which can be applied in a combinatorial manner, giving access to 1,1-di-substituted pyrans projecting amide side chains in both the α- and β-directions. Thus, treatment of the readily accessible hemiketal (1) with TFA followed by trimethylsilyl trifluoromethanesulfonate (TMSOTf) in the presence of a nitrile gives dihydrooxazinones (2) via a new type of modified intramolecular Ritter reaction. The dihydrooxazinones (2) can either be isolated or used directly in reactions with a broad variety of amines. Final deprotection furnishes the 1,1-di-substituted sugar β-peptides having the general structure (4).

A protein stationary phase for frontal affinity chromatography was prepared, containing biotinylated β-galactosidase immobilized to controlled pore glass beads via covalently bonded streptavidin. Single microaffinity columns of approximately 30 pmol of active β-galactosidase were prepared from this material and characterized with a known ligand by frontal analysis. These columns were used to measure the specific interactions between the bound β-galactosidase and a library of modified β-galactopyranosides using electrospray mass spectrometry as the means of detection. The library contained 89 entries, each representing 4 diastereomers for a total of 356 library members. A single entry was analysed revealing differential activity among the 4 isomers. The library was grouped into 10 mixtures of 24-40 members each with each mixture infused under frontal chromatographic conditions. This deconvolution procedure led to the identification of 34 entries containing isomers with Kd values better than 10 μM. A method based on a displacement principle was implemented as a rapid prescreen which served as the basis for a parallel column high throughput screening assay.