Compounds represented by the formula:wherein R is C1-6 alkyl; C4-6 cycloalkylalkyl; or C3-6 alkenyl; R′ is H or C1-6 alkyl; X is H or OH; Y is alkyl, cycloalkyl, aryl, heteroaryl, arylalkyl or aroyl; and Z is CH or N; provided that X is H, when Z is CH and R is C4 cycloalkylalkyl or C4 alkenyl; prodrugs thereof; and pharmaceutically acceptable salts thereof are provided. Compounds of the above formula are useful as analgesics for treating pain; as immunomodulators, to modulate the behavioral effects of drugs of abuse and to modulate the development of tolerance and dependence to μ agonists.