Novel Methodologies for the Synthesis of Multivalent Glycoconjugates
Glycoconjugates, such as glycolipids and glycoproteins, play an important role in health and disease. The synthesis and biological evaluation of these glycoconjugates allows for the development of novel carbohydrate-based therapeutics and analytical tools. Traditionally, the conjugation of glycans to other substrates required the installment of an anomeric linker during the total synthesis of the glycan, however, this strategy does not allow for the conjugation of naturally isolated glycans. To address this concern, glycan conjugation methodologies without the need for protecting groups have been developed, including the use of oxyamine conjugation methodologies. In particular, the synthesis of a variety of novel bi-functional oxyamine linkers enabled the rapid assembly of various types of glycoconjugates, including fluorescent- and biotinylated-glycans, glycoproteins and multivalent glycodendrons. The multivalent presentation of glycans to cell surface lectins is often required to induce a measurable biological response. This multivalent binding can be achieved by the presentation of glycans on dendrons as these ‘glycodendrons’ have increased affinity for their corresponding lectin compared to monovalent glycans. Moreover, these glycodendrons have several advantages, including high synthetic control, low cytotoxicity and in addition can be derivatised with molecular probes of choice, which can aid in the biological evaluation of these glycoconjugates. Accordingly, novel biotinylated and fluorescent dendrons were synthesised from a highly convergent second generation dendron core scaffold. These functionalised dendrons then allow for the nonavalent conjugation of carbohydrates, such as Lewis antigens, for their biological evaluation in the selective targeting of lectins. Lewis antigens play an important role in host cell recognition, but these glycans are also involved in disease, such as in cancer metastasis and HIV-infection. The synthesis of Lewis antigens allows for the biological evaluation of these glycans, and moreover, could be employed in the development of novel glycan-based therapeutics and analytical tools. Accordingly, a novel high-yielding and efficient synthesis of a crystalline trisaccharide building block is presented, which can then be utilised in the synthesis of most Type-2 Lewis antigens. In particular, the global deprotection of the crystalline material gave the LewisX glycan antigen, a natural ligand for the C-Type lectin DC-SIGN on dendritic cells and macrophages. Finally, the rapid assembling of complex multivalent glycodendrons is discussed by conjugating the glycan antigens to the functionalised multivalent dendrons through the use of the bi-functional oxyamine linker methodology. In particular, the synthesis of a fluorescent LewisX glycodendron is presented, and to demonstrate a potential biological application of this methodology, the fluorescent LewisX glycodendron is evaluated as a flow cytometry marker for the C-type lectin DC-SIGN on human macrophages.