Synthesis of CMP-Nulos and Selectivity studies of NulOTs

This project focuses on carbohydrates used by bacteria to camouflage themselves and avoid detection by the human immune system. These special molecules are part of the family of nonulosonic acids and are displayed by some types of bacteria on their outer shell. Here they play a number of important roles: they are part of structural features such as flagella or decorate the outer-most part of biomolecules as protection. Bacterial nonulosonic acids are remarkably similar to nonulosonic acids created in the human body and as a result, bacteria that display these compounds are often not recognized to be foreign and invading organisms. This leads to a reduced response by the human immune system and chronic infections with dangerous results. An important but under-investigated aspect of bacterial nonuolsonic acids is their modification by bacteria with small appendices, not found in their human equivalent. Little is known about the role of or even how bacteria create these modifications, as well as how these appendices impact their role in pathogenic behaviour. Bacteria displaying nonulosonic acids are among the most dangerous pathogens and some of them have even become resistant to most antibiotics used to treat bacterial infection. In order to develop new anti-infective strategies, this project will explore the modification of bacterial nonulosonic acids and the litte-known enzymes used by bacteria to transfer these compounds on large biomolecules on the outer part of the cell wall. To develop a deeper understanding of biology and chemistry of bacterial nonulosic acids, this project aims to create a portfolio of the most common bacterial nonulosonic acids, incorporating their various modifications, by chemical synthesis. With these comounds in hand, the next step will be to study within living bacteria, the characteristics of the enzymes involved in nonulosonic acid biochemistry and how bacterial characteristics depend on nonulosonic acid modification.

In this project we were interested in exploring synthetic pathways towards the bacterial sugars Pseudaminic and Legionaminic acids. We designed routes which would allow for a late stage introduction of substituents and thus enabling us to synthesize a range of bacterial examples of these compounds. To this end we synthesized appropriate substrates by novel methodologies and subsequently studied a range of addition reactions. These compounds were then intended to be used as reference compounds for the study of multidrug resistant bacteria and as lead compounds for potential vaccine development. The surface of all cells, whether bacterial, plant or animal, are covered in complex sugar containing structures. There they are involved in a range of biological processes including cell-cell communication, pathogen recognition and cell-protein interaction. Examples of the building blocks of these structures, called glycans, are the nonulosonic acids (abbreviated to NulO), a group of sugars that include the sialic or neuraminic acids. They are the only nine-carbon sugars found in prokaryotes and vertebrate animals, they are often involved in host-pathogen interfaces, including viral infections, as well as in recognition and regulatory events. Whereas human and other animals display only a few examples of Neuraminic acid, bacterial pathogens have a wider range of these sugars, including uniquely bacterial forms such as Legionanimic acid and Pseudaminic acid. Bacterial NulOs may help pathogens evade the human immune system due to molecular mimicry of the human body's own neuraminic acid structures. Indeed, a number of multi antibiotic-resistant bacteria display NulOs on their glycans structures. For these micro-organisms, these unique sugars are instrumental for their pathogenicity, however, only few reports detail the exact molecular interaction between mammalian immune pathways and bacterial NulOs. To study these phenomena, syntheses like the ones explored in this project are instrumental to provide further insights into biology and help direct the development of new antibiotic strategies. Over the course of this project several literature examples and newly developed methodologies were applied to first generate advanced intermediates carrying all the requisite substituents and then to elaborate them into the desired compounds. Unfortunately, while the desired compounds could be obtained, only minute quantities could be isolated. Attempts at improving these results were unsuccessful and therefore further biological studies could not be initiated in the context of this project. Nevertheless, important new discoveries in the field of synthetic carbohydrate chemistry were uncovered and published as open access in the Journal of Organic Chemistry and presented at to the public at conferences.