Nonnucleoside reverse transcriptase

A compound of formula I, wherein: A is ** ** Formula (a) CHR3> = CHR 2, (b) CH2R3CH2R2 or (c) CHR2; R1 is II and R2 is hydrogen, C1-3 alkyl, C1-3 haloalkyl, C2-6 alkenyl, or CH2ORc; and also, if A is (a) or (b), R2 may also be halogen, NRaRb, ORc or CN; X1 is O or S; X2 is O, or, if A is (a), X2 is O or NRd; R3 is hydrogen or C1-3 alkyl; R4 is halogen, C1-6 alkyl, C3-6 cycloalkyl, C1-6 haloalkyl or C1-6 alkoxy; Ra and Rb are independently hydrogen, C1-3 acyl or C1-6 alkyl; Rc and Rd independently are hydrogen or C1-3 alkyl; Ar is phenyl substituted by 1 to 3 groups independently selected from halogen, cyano, C1-6 haloalkyl, C1-6 alkyl, C1-6 alkoxy and C3-6 cycloalkyl; n is an integer of 1 to 3; or pharmaceutically acceptable salts thereof.