Process for preparing nucleosides 5-azacytosine and of its derivatives

A process for the preparation of a nucleoside compound 5-azacytosine of formula I: ** ** Formula wherein R1 is hydrogen, hydroxy or halogen; R2 is hydrogen or halogen, comprising: a. 5-azacytosine reacting with a silylating agent to produce a reaction mixture containing 5-azacytosine silylated of formula II: Formula ** ** wherein each R3 is independently a C1-C20 alkyl group or aryl group optionally substituted ; b. 5-azacytosine coupling the silylated of formula II with a protected D-ribofuranose of formula III: Formula ** ** wherein R1 is hydrogen, halogen or OR4; R4 represents a hydroxyl protecting group; in a first organic solvent and in the presence of a sulfonic acid catalyst, wherein the amount of sulfonic acid is less than one molar equivalent with respect to the D-ribofuranose of formula III protected, in particular the sulfonic acid is 10 to 30 mole percent of the molar amount of D-ribofuranose of formula III protected, to obtain a solution of protected nucleoside 5-azacytosine of formula IV: ** ** formula wherein each of R5 and R6 is independently hydrogen or Si (R3) 3, R3 is a C1-C20 alkyl group or aryl group optionally substituted, and R1 'and R2 are as defined above; and c. converting the protected nucleoside of formula (IV) in the 5-azacytosine nucleoside of formula (I) 5-azacytosine.