A (ß-D or ß-L) nucleoside of (2'R) -2'-deoxy-2'-fluoro-2'-C-methyl or a pharmaceutically acceptable salt thereof of the structure: ** Formula ** wherein the Base is a pyrimidine base represented by the following formula ** Formula ** X is O； R1 and R7 are independently H, a monophosphate, a diphosphate or a triphosphate； R3 is H and R4 is NH2 or OH.Un nucleosido (ß-D o ß-L) de (2'R)-2'-desoxi-2'-fluoro-2'-C-metilo o una sal farmaceuticamente aceptable del mismo de la estructura:**Fórmula** en la que la Base es una base pirimidinica representada por la siguiente formula**Fórmula** X es O； R1 y R7 son independientemente H, un monofosfato, un difosfato o un trifosfato； R3 es H y R4 es NH2 u OH.
