A compound of the formula (1):wherein R1 and R2 are the same or different and represent a
hydrogen atom, a hydroxyl
protecting group, a
phosphate group, or āP(R3)R4, wherein R3 and R4 are the same or different and represent a hydroxyl group, an amino group, an
alkoxy group having from 1 to 4 carbon atoms, a cyanoalkoxy group having from 1 to 5 carbon atoms or an amino group substituted by an
alkyl group having from 1 to 4 carbon atoms; A represents an alkylene group having from 1 to 4 carbon atoms and B represents a purin-9-yl group, a 2-oxo-pyrimidin-1-yl group, a substituted purin-9-yl group or a substituted 2-oxo-pyrimidin-1-yl group having a
substituent Ī± selected from the group consisting of a hydroxyl group which may be protected, an
alkoxy group having from 1 to 4 carbon atoms, a mercapto group which may be protected, an alkylthio group having from 1 to 4 carbon atoms, an
alkoxy group having from 1 to 4 carbon atoms, an amino group which may be protected, a mono- or di-alkylamino group which may be substituted by an
alkyl group having from 1 to 4 carbon atoms, an
alkyl group having from 1 to 4 carbon atoms and a
halogen atom; or a salt thereof.