Inhibitors of histone deacetylase useful for the treatment or prevention of hiv infection

A compound of the formula:

• wherein X is O, S or SO2;
• is a five-membered heteroaryl ring which is optionally substituted with halo, cyano or C1-3 alkyl;
• R1 is phenyl or heteroaryl, which may be monocyclic or bicyclic, wherein said phenyl and heteroaryl groups are optionally substituted with one to three groups independently selected from the group consisting of halo, oxo, cyano, R5, R6, OR5, OR6 and SO2R5;
• R2 is selected from the group consisting of hydrogen, NHR5, NH(C═O)R5 and NH(C═O)R6;
• R3 is hydrogen or C1-6 alkyl;
• R4 is ethyl, heteroaryl or (C═O)NHR5, wherein said heteroaryl group is optionally substituted with C1-6 alkyl;
• each R5 is independently hydrogen or C1-6 alkyl, which is optionally substituted with NH2, N(CH3)2, NH(CH3) or one to three halo;
• R6 is
• (a) heterocyclyl, which may be monocyclic, bicyclic or tricyclic,
• (b) C3-6 cycloalkyl,
• (c) phenyl, or
• (d) heteroaryl, which may be monocyclic, bicyclic or tricyclic,
• wherein said heterocyclyl, cycloalkyl, phenyl and heteroaryl groups are optionally substituted with one to two groups independently selected from the group consisting of halo, cyano, oxo, R5, and OR5;
• or a pharmaceutically acceptable salt thereof.