The synthesis of fluorinated rare sugars

Together with nucleic acids and proteins, carbohydrates are the third major class of biopolymers. Their biological importance has long time been believed to be limited to energy gain and structural functions. However, carbohydrates are important signal molecules, playing for example an important role in infection processes or immune response. Apart from well-known polymeric carbohydrates such as starch, or monosaccharides like glucose or fructose, there is a large variety of rather unknown carbohydrates, called rare- sugars. Rare sugars cannot be isolated in a profitable way from natural sources, due to their low abundance. Still they are of high interest for pharmaceutical industry as building blocks for drug molecules. Furthermore, rare sugars play a vital role as intermediates in many metabolic processes, such as the so-called pentose-phosphate-pathway (PPP) The PPP yields essential building blocks for DNA and RNA synthesis. Despite the importance of this pathway, little is known about the regulative mechanisms. Usually, 5-30% of glucose are metabolized via the PPP in normal cells. This is dependent on the current needs of the cell in terms of energy or DNA-synthesis. These needs can be altered under a variety of pathological conditions. It could for example be observed that this pathway is downregulated in Alzheimers disease and upregulated in several types of cancer. We hypothesize that cells with an upregulation of the PPP, will tend to take up elevated amounts of the rare sugars involved in the PPP, while cells with a downregulation will take up lower amounts. Further we hypothesize that the same will be true for structurally very similar, fluorinated carbohydrates. In this project, we plan to chemically synthesize fluorinated analogues to the rare sugars D- sedoheptulose, D-ribulose, D-xylulose and D-erythrose, which are intermediates of the PPP. Emerging evidence suggests that the named rare sugars are not only intermediates formed in the PPP, but could possibly directly influence and regulate the PPP. We hypothesize that cells with an altered activation of the PPP, will also show an altered uptake of these rare sugars. A possible future application would be diagnostic imaging. Thus, we will further synthesize suitable precursors for future labeling of these rare sugars, with the radioactive isotope fluorine-18. This isotope is used in medicine to radiolabel organic molecules and the signal derived from the radioactive decay can then be used to monitor the distribution of the labeled compound throughout different tissue types and body parts (this method is named PET imaging). In the future, this method could possibly also be used to localize tissue types or body regions with an altered dependence on the PPP. This again has a potential as diagnostic tool for the identification of pathologies, with an altered flux via the PPP.

Carbohydrate metabolism is essential for all living beings. While glycolysis as the major route for carbohydrate usage is a well studied metabolic route, other less prominent metabolic pathways are poorly understood. There is for example a lack of knowledge concerning the complex, enzymatic equilibria involved in the so-called pentose phosphate pathway (PPP), even though it plays a crucial role during ribose biosynthesis and consequently is essential for DNA biosynthesis. During the course of this project, we suceeded to prepare 5 deoxyfluorinated analogues to important rare sugars which are intermediates of the non-oxidative phase of the PPP (non-ox PPP). These are C3- and C4 deoxyfluoro-variants of sedoheptulose , C1- and C3 deoxyfluorinated ribulose and 3-deoxy-3-fluoroxylulose. All prepared carbohydrates were taken up by human fibroblasts as expected, however their further metabolism differed drastically. All the synthesised deoxyfluorinared pentuloses showed a complex metabolic pattern, while 3-deoxy-3-fluorinated sedoheptulose was not metabolised at all. It was shown not to be a substrate for the respective sedoheptulose kinase (SHPK), while the sedoheptulose derivative deoxyfluorinated at C4 (4DFS) was completely phosphorylated. The resulting 4DFS-7-phosphate was only marginally accepted by transaldolase (TALDO) and transketolase (TKT), which are the major enzymes responsible for sedoheptulose-7-phosphate metabolism during the non-ox PPP. This hints at a high potential of 4DFS as future positron emission tomogrpahy (PET) tracer, due to being it was metabolically trapped inside the cells. As such it might become a useful tool to study, or diagnose diseases which are connected to malfactions of the PPP or to develop drugs against these pathologies. Thus, additionally we designed and synthesised highly reactive precursors that can be used for the late-stage incorporation of the PET-nuclide fluorine-18 in these compounds.