The Mitsunobu-reaction – which is capable of exchanging a hydroxyl-group to a protonated nucleophile – was first published in 1967 . This discovery proved to be of enormous significance over the years, as a great number of articles have been published since then for the synthetic applications of the reaction [2,5,6]. The method is well-known among carbohydrate chemists as well, its first application in this field was the exchange of free hydroxyl-groups of variuos mono- and oligosaccharides to form leaving or protecting groups at different positions of the substrate . However, carbohydrates that contain a tertiary hydroxyl-group are known to react slowly, if at all, due to the steric hindrance of the host carbon atom . Examples for the exchange of such hydroxyl-groups are extremely scarse in the literature . In order to study the spirocyclisation possibilities, we needed the substitution products of heptulopyranosonic esters, and to this end, we have studied Mitsunobu-reactions of these substrates.
Success of the Mitsunobu-reaction largely depends on the pKa value of the nucleophilic reagent , therefore in the first phase of our research, we attempted to react several types of N- and O-nucleophiles that have a pKa lower than ten, with the glucose derivative containing the tertiary hydroxyle-group at its anomeric centre. As a result, we have successfully managed to synthesize a few O- and N-aryl glycosides, as well as some N-glycosyl-heterocycles. Curiously, alternative ways leading to the same products (which involved the exchange of a bromine atom under regular SN conditions) have failed to provide the corresponding glycosides. Next, we examined the possibility of further transformations of these glycosides into glycosylidene-spiro-heterocycles, which may have a potential antidiabetic effect. These efforts resulted in two new spiro-compounds, but unfortunately, they showed no effect in the inhibition of glycogen-phosphorylase enzyme . Finally, we attempted to use some carbohydrate derivatives, that already have an NH-acidic centre, as nucleophiles in the Mitsunobu-reaction. The detailed results of these studies will be presented on the poster.
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