Process for preparing 3-deoxy-B-D-threo-pentofuranosyl nucleosides

A process for preparing a nucleoside derivative represented by the formula (II'):

• ##STR16## wherein B is a member selected from the group consisting of a purine base residue, a purine base residue having a protective group, a pyrimidine base residue, a pyrimidine base residue having a protective group, an imidazolyl group and an imidazolyl group having a protective group;
• R.sup.4 is a hydroxyl protective group or a hydrogen atom;
• which comprise the step of:
• treating a compound represented by the formula (I'):
• ##STR17## wherein B is as defined above;
• R.sup.1 is a member selected from the group consisting of a PivO-group, a TsO-group, a DMTrO-group, a benzoyl group and a hydroxy group;
• R.sup.2 is a member selected from the group consisting of a MsO-group, a TflO group and a TsO group;
• R.sup.3 is a member selected from the group consisting of a PivO-group, a TsO-group, a DMTrO-group, a benzoyl group and a hydroxy group;
• with a base selected from the group consisting of an alkali metal lower alkoxide, an alkaline earth metal lower alkoxide, sodium hydroxide and potassium hydroxide and a reducing agent selected from the group consisting of an alkali metal borohydride complex, an alkaline earth metal borohydride complex and tetraalkylammonium borohydride, in a mixed solvent containing an alcohol at a temperature of from 0.degree. C. to 100.degree. C. for 5 minutes to 20 hours;
• and isolating said derivative represented by the formula (II') from the reaction mixture.