The aim of the present thesis was the synthesis and the biological evaluation of new series of fluorinated nucleoside analogues with six-membered carbohydrate moieties of N4-benzoylcytosine and N6-benzoyladenine. Based on the interesting biological properties of unsaturated ketonucleosides and deoxy-pyranonucleosides and considering that the introduction of fluorine into the sugar moiety of a nucleoside increases its biological activity, in the present work, the synthesis of new series of fluorinated keto unsaturated and deoxyketo-pyranonucleoside analogues of N4-benzoylcytosine and N6-benzoyladenine respectively, was described. Initially, the synthesis of a new class of nucleoside analogues possessing a fluoro group in the 3΄-position, a keto group in the 4΄- or 2΄-position and deoxy in the 2΄- or 4΄-position of the sugar moiety respectively, was studied. Then, the synthesis of a new class of fluorinated unsaturated ketonucleosides possessing a fluoro group in the 3΄-position, a keto group in the 2΄- or 4΄-position and a double bond in the 3΄,4΄- or 2΄,3΄-positions of the sugar moiety respectively, was described. Finally, a new class of fluorinated deoxy keto unsaturated nucleosides, possessing a fluoro group in the 3΄-position, deoxy in the 6΄-position, a keto group in the 4΄- or 2΄-position and a double bond in the 2΄,3΄- or 3΄,4΄-positions of the sugar moiety respectively, was synthesized. The above nucleoside analogues contained N4-benzoylcytosine or N6-benzoyladenine respectively, as heterocyclic bases. In the present thesis, the biological studies of the newly synthesized compounds were also accomplished, concerning their antiviral, anticancer and antioxidant activity.