Towards the Synthesis of Carrageenan Oligosaccharides

The plant cell walls represent almost 50% of the biomass found in plants and are therefore one of the main targets for biotechnological research. Many of their components already have important industrial applications in various fields, such as in the food and biomedical industry and in biofuel production. Therefore, it is necessary to optimize the plant production and its utilization. This will require a better understanding of the cell wall structure and function at the molecular level. Most of the studies achieved on plant cell wall structures and their biosynthesis have been focusing on land plants. Only very few reports are dealing with algae. However, land plants have algae ancestors and getting a better knowledge of algae cell walls could help understand the evolution of plant cell walls. Furthermore, some components specific to algae cell walls are very valuable in the industry. Indeed, the polysaccharides present in the plant cell wall vary depending on the plant species and change during the developmental stage of the plant. This makes it very challenging to address the function of individual components in living cells as well as study the physical properties of each particular molecule. Alternatively, structurally defined oligosaccharides can be used as models for the more complex polysaccharide components. This would enable to investigate a range of properties such as cell wall biosynthesis and protein-carbohydrate interactions, but also the physical properties of the pure oligosaccharides in order to optimize their applications in the industry. Chemical or enzymatic degradation of plant cell wall can provide some oligosaccharides but extensive purification is required and only a limited range of structures has so far been isolated. Chemical synthesis, on the other hand, is capable of producing structurally diverse oligosaccharides of excellent purity and in higher quantities.
This thesis presents the development of a synthetic strategy to produce ten different types of carrageenan oligosaccharides from one single precursor. These molecules are highly sulfated galactans, which are found in the cell wall of red algae and serve as gelling, stabilizing and viscosity-enhancing agents in many sectors ranging from the food industry to pharmaceuticals. A modular approach was chosen to enable the synthesis of carrageenan oligosaccharides with different lengths. Different protecting group patterns were tested to synthesize the oligosaccharide backbone. A protected tetrasaccharide precursor was synthesized and can be used to synthesize all ten carrageenan tetrasaccharides. Optimization of the deprotection steps as well as sulfation was done on a similar disaccharide and one disaccharide carrageenan was synthesized in the end. These steps were further translated successfully to the desired tetrasaccharide.
A work dealing with the development of an intramolecular catalyzed fluoroarylation is presented at the end of this thesis. A range of different allyl substituted aryl boronic acids substrates were synthesized in order to investigate the scope of a silver catalyzed reaction and some preliminary results of the catalytic reaction are presented.