Upon sodium cyanoborohydride reduction followed by de-O-silylation, theO-methyloxime andN-benzylnitrone of 5′-TBDMS-3′-ketothymidine gave resolvable epimeric mixtures of 1-[2,3-dideoxy-3-(N-methoxyamino)-β-d-threo-and β-d-erythro-pentofuranosyl]thymine and 1-[3-(N-benzyl-N-hydroxyamino)-2,3-dideoxy-β-d-threo- and β-d-erythro-pentofuranosyl]thymine respectively. These compounds were inactive against HIV. On the other hand, 1-[2,3-dideoxy-3-(N-hydroxyamino)-5-O-TBDMS-β-d-threo-pentofuranosyl]thymine, upon treatment with acetone, then de-O-silylation, gave the bicyclonucleoside analogue15, slightly more active against HIVin vitrothan DDI.