The present invention comprises compounds useful as antiviral or antitumor agents. The compounds comprise nucleotide analogues that comprise tetrahydrofuranyl or tetrahydrothienyl moeities with quaternary centers at the 3′ position. The nucleotide analogues can be used to inhibit cancer or viruses. Accordingly, the compounds of the present invention are useful for treating, preventing, and/or inhibiting diseases or conditions associated with cancers and viruses. Thus, the present invention also comprising pharmaceutical formulations comprising the compounds and methods of using the compounds and formulations to inhibit viruses or tumors and treat, prevent, or inhibit the foregoing diseases.