Processes for preparing 4acoeazido nucleoside derivatives

A process for the preparation of a 4'-azido-2',3',5'-triacyl-nucleoside compound (I; B=B1; R is R CO- and R is R CO-) or a 4'-azidonucleoside compounds (I; B is B1 or B2 and R and R are hydrogen and acid addition salts thereof) wherein R and R are independently C1-10 alkyl or phenyl optionally substituted with 1 to 3 substituents selected from the group consisting of alkyl, alkoxy, halogen, nitro or cyano and R is selected from the group consisting of hydrogen, C1-6 alkyl, C1-3 haloalkyl and halogen, comprising contacting a 5'-iodo compound II with a peracid, R C(O)OOH, an acid R C(O)OH and a phase transfer catalyst and interconverting a uridine B1 to a cytosine B2. The present process provides the 4'-azidonucleosides safely and selectively in high purity with increased efficiency.