Modification of heterocyclic base moiety of naturally occurring nucleosides by additional ring often results in new interesting physicochemical (e.g. enhanced lipophilicity, fluorescence) or biological properties. It has been shown that transformation of pyrimidine nucleosides into their 6-alkyl-2,3-dihydrofurano[2,3-d]pyrimidin-2(1H)-one or 6-alkyl-2,3-dihydropyrrolo[2,3-d]pyrimidin-2(3H,7H)-one derivatives leads to new bicyclic nucleoside analogues which exhibit antiviral activity. The type of activity, its potency and selectivity, depend on structural factors: (i) structure of the sugar or pseudosugar moiety, and (ii) the length of lipophilic alkyl substituent in the 6-position. In our search for new biologically active nucleosides we have obtained a series of furano- and pyrrolo[2,3-d]pyrimidine compounds having as a sugar part 2’-C-β-methyl-β-d-ribofuranosyl substituent, which often increases activity of nucleosides against hepatitis virus type C (HCV). In a related project, we have elaborated transformation of the 6-alkyl-2,3-dihydrofurano[2,3-d]pyrimidin-2(1H)-one system into a new class of heterocycles, 6-(β-d-ribofuranosyl)-5,6,7,8-tetrahydropyrimido[4,5-c]pyridazine derivatives, in the reaction with hydrazine. The series of new compounds of this type included nucleosides of β-d-ribofuranose, 2’-deoxy-β-d-ribofuranose, 2’-C-β-methyl-β-d-ribofuranose, and acylonucleosides of the Acyclovir type, which were additionally modified by alkyl substituents of different length in the 3-position.

All new nucleoside analogues were tested in vitro against a variety of RNA- and DNA-viruses at Rega Institute for Medical Research, Katholieke Universiteit Leuven, Belgium, and the results will presented.

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1. Glycosylation reactions of natural poliphenolic compounds