A compound of the formula (1): ##STR00001## wherein R.sup.1 and R.sup.2 are the same or different and represent a hydrogen atom, a hydroxyl protecting group, a phosphate group, or --P(R.sup.3)R.sup.4, wherein R.sup.3 and R.sup.4 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 .alpha. 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.