Flow chemistry based strategies towards LPS-substructures

The lipopolysaccharide (LPS) of pathogenic Gram-negative bacteria constitutes the outer part of their cell membranes. Higher carbon sugars like heptoses and octulosonic acids (Kdo, Ko) are important and unique structural features of bacterial LPS and contribute to fundamental binding interactions with the innate and adaptive immune system of the bacteria`s hosts. The chemical synthesis of these higher-carbon sugars and their oligomers is much more complex compared to the more common hexose and pentose sugars, which are mostly commercially available. Modern flow-chemistry based chemical approaches utilizing microreactors are nowadays considered as novel techniques with the great potential to solve a number of central problems currently limiting glycoside and oligosaccharide synthesis. The increased control over reaction parameters like temperature, pressure, (short) reaction times and stoichiometry of reactants compared to the conventional batch reactions is of special importance to reactions with a strong selectivity bias. The miniaturization of reactors to volumes of a few L and automation of series of experiments helps to time-efficiently optimize reaction conditions with minimized necessary starting materials, but also decreasing waste, energy and human contact with hazardous materials. The main target of this proposal is the development of a general and common strategy for the selective introduction of protecting groups to achieve complete differentiation of all hydroxyl-groups of heptoses, Kdo and Ko. The synthetic concept is designed to exploit the benefits from modern flow-chemistry based methods and automated equipment which is available in the laboratory of Prof. Baxendale who is a pioneer in the development of flow- chemistry methodology and equipment. Subsequently, these new building blocks will be evaluated in the assembly of some prototype oligoheptoside-substructures of Burkholderias, a dangerous pathogen of the lung. The targeted oligosaccharides, including a common heptotrisaccharide that almost all enterobacterial strains share in their LPS, will serve as ligands for X-ray crystallography, STD-NMR and lectin binding studies, thus supporting future programs aiming at the elucidation of structure-activity relationships of LPS substructures and pathogen-host recognition.

With the increasing threat of antibiotic resistance, new means of fighting microbial infection become an urgent need for human health. In this respect, the Lipopolysaccharide (LPS) of Gram negative bacteria is an interesting potential target as it exhibits major interactions with the hosts immune system. The inner core region of LPS is rich in the higher carbon sugar L-glycero-D-manno heptose (LD- heptose), a rare sugar not found in human cells. Therefore, since its discovery in the 1980ties LD- heptose containing structures have been synthesized for immunological investigations. However, all these preparative efforts have been complicated by the fact that the parent sugar is not readily available but had to be synthesized via known multistep procedures. Such a repetitive and unrewarding effort constituted a substantial entry-burden into this important field of research.Within this project we succeeded in the development of the first short (4 steps) synthesis of LD- heptose and its crystalline and bench-stable LD-heptose peracetate. The process was demonstrated at > 100 mmol scale without any need for chromatographic purification. Next, we were aiming to demonstrate the value of this (prospective) platform chemical by developing a general methodology to convert it into a variety of glycosyl donor and acceptor molecules, in a versatile and time efficient manner. These compounds are the necessary building blocks for the assembly of larger oligosaccharide structures based on LD-heptose. We are convinced that readily available LD-heptose peracetate will become a valuable starting point for many future synthetic efforts towards bacterial oligosaccharides.As a chemical spin-off, a thorough methodological study on the indium mediated acyloxyallylation, the key step in the above process, was undertaken to increase the understanding of this powerful transformation and thus facilitate its applicability in synthetic programs within and beyond carbohydrate chemistry.