Compounds of formula I, wherein R1, R2, R3, X1, X2 and Ar, are as defined herein or pharmaceutically acceptable salts thereof, inhibit HIV-1 reverse transcriptase and afford a method for prevention and treatment of HIV-1 infections and the treatment of AIDS and/or ARC. The present invention also relates to compositions containing compounds of formula I useful for the prevention and treatment of HIV-1 infections and the treatment of AIDS and/or ARC.