preferred groups are n nonyl n undecyl n tridecyl and n pentadecyl isobutyl isopentyl isohexyl isoheptyl isooctyl and 2 ethylhexyl cyclobutyl cyclopentyl cyclohexyl and cycloheptyl the compound of the present invention represented by the formula ii can be prepared by allowing arabinofuranosylcytosine represented by the formula i to react with an alkoxycarbonyl halide represented by the formula rocox wherein r is the same as defined above and x a halogen atom in the presence of a base in this reaction an alkoxycarbonyl halide for example an alkoxycarbonyl chloride or bromide is used for example in an amount of equimolar or more preferably in the range from 1 to 5 moles per mole of arabinofuranosylcytosine as the base there may be employed an organic base such as pyridine triethylamine or trimethylamine or an inorganic base such as sodium carbonate sodium hydrogen carbonate potassium carbonate or potassium hydrogen carbonate such a base may be used in an amount at least equimolar preferably 1 to 10 moles per mole of alkoxycarbonylhalide the reaction between arabinofuranosylcytosine and an alkoxycarbonyl halide may advantageously be carried out in a solvent as such a solvent there may suitably be used a solvent which can dissolve arabinofuranosylcytosine typically a polar solvent such as dimethylformamide dimethylacetamide dimethylsulfoxide etc or a cyclic ether such as dioxane or tetrahydrofuran.