Kappa opioid receptor antagonists

Compounds of the formula:are provided, which are selective kappa opioid receptor antagonists, wherein R1 is (C1-C5)alkyl, C3-C6(cycloalkyl)alkyl, C5-C7(cycloalkenyl)alkyl, (C6-C12)aryl, (C6-C12)aralkyl, trans(C4-C5)alkenyl, allyl or furan-2-ylalkyl, R2 is H, OH or O2C(C1-C5)alkyl; R3 is H, (C6-C10)aralkyl, (C1-C5)alkyl or (C1-C5)alkylCO; X is O, S or NY, wherein Y is H or (C1-C5) alkyl; R4 is CH2 (methylene) or C=O (carbonyl), R5 is CH2, C=O or C=NH (imino) and R6 is (C1-C4)alkyl or NH(C1-C4)alkyl, optionally substituted by a non-terminal (C1-C2)alkyl group or by N(R7)(R8) wherein R7 and R8 are individually H or (C1-C3)alkyl, with the proviso that one of R4 or R5 is CH2, and the pharmaceutically acceptable salts thereof.