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