Abstract: The present invention relates to compounds of formula I: (I) or a pharmaceutically acceptable acid addition salt thereof, where; X is C(R3c? or N?; R1 is C2-C6 alkyl, substituted C2-C6 alkyl, C3-C7 cycloalkyl, substituted C3-C7 cycloalkyl, phenyl, substituted phenyl, heterocycle, or substituted heterocycle; R2 is hydrogen, C1-C3 n-alkyl, C3-C6 cycloalkyl-C1-C3 alkyl, or a group of formula II (II) provided that when R1 is C2-C6 alkyl or substituted C2-C6 alkyl, R2 is hydrogen or methyl; R3a, R3b, and, when X is C(R3c)?, R3c, are each independently hydrogen, fluoro, or methyl, provided that no more than one of R3a, R3b, and R3c may be other than hydrogen; R4 is hydrogen or C1-C3 alkyl; R5 is hydrogen, C1-C3 alkyl, or C3-C6 cycloalkylcarbonyl, provided that when R3a is other than hydrogen, R5 is hydrogen; R6 is hydrogen or C1-C6 alkyl; and n is an integer from 1 to 6 inclusively.