The invention provides compounds of the formula: wherein B is a nucleobase selected from the groups (a) to (d): and the other variables are as defined in the claims, which are of use in the treatment or prophylaxis of hepatitis C virus infection, and related aspects.本發明提供下式化合物:其中B係選自基團(a)至(d)之核鹼基:且其他變量係如申請專利範圍中所定義,其用於治療或預防C型肝炎病毒感染,及相關態樣。