A rapid and convenient large-scale strategy for the synthesis of some new isoxazolidinyl nucleosides, as potential antiviral drugs, is reported. In particular, a multistep methodology based either on the 1,3-dipolar cycloaddition approach or on a slight modification of the convertible nucleoside concept was exploited in the preparation of 4′-aza-2′,3′-dideoxynucleoside analogues containing cytosine and 5-methyl-cytosine.