Compounds of Formula I are described: wherein R1 and R2 are defined herein. The compounds of Formula I are useful in the inhibition of HIV reverse transcriptase, the prophylaxis and treatment of infection by HIV, and the prophylaxis, delay in the onset or progression, and treatment of AIDS. The compounds can be employed as ingredients in pharmaceutical compositions, optionally in combination with other antivirals, immunomodulators, antibiotics or vaccines.