A compound of Formula X: or a pharmaceutically acceptable salt thereof, wherein: R 1 is hydrogen, (C 1 -C 6)alkyl, or (C 3 -C 6)cycloalkyl; R 2 is (C 6 -C 18)aryl, (C 6 -C 18)aryl(C 1 -C 6)alkyl, (C 3 -C 18)heteroaryl(C 1 -C 6)alkyl, halo(C 6 -C 18)aryl(C 1 -C 6)alkyl, or (C 1 -C 6)alkoxy(C 6 -C 18)aryl(C 1 -C 6)alkyl; R 3 is N-hydroxyamino-oxo-(C 2 -C 6)alkenyl; R 4 is hydrogen, halogen, N-hydroxyamino-oxo-(C 2 -C 6)alkenyl, or N-hydroxyaminocarbonyl(C 6 -C 18)aryl(C 1 -C 6)alkylene; R 5 is hydrogen, halogen, N-hydroxyamino-oxo-(C 2 -C 6)alkenyl, N-hydroxyaminocarbonyl(C 6 -C 18)aryl(C 1 -C 6)alkylene, or amino(C 6 -C 18)aryl-oxo-(C 2 -C 6)alkenyl; and R 6 is hydrogen, N-hydroxyamino-oxo-(C 2 -C 6)alkenyl, N-hydroxyamino(C 6 -C 18)aryl (C 1 -C 6)alkylene, 4-(N-hydroxyaminocarbonyl)phenyl, or N-hydroxyaminocarbonyl(C 6 -C 18)aryl(C 1 -C 6)alkylene; or wherein: R 1, R 2, R 5 and R 6 are each as defined in above; R 3 is hydrogen or N-hydroxyamino-oxo-(C 2 -C 6)alkenyl; and R 4 is N-hydroxyamino-oxo-(C 2 -C 6)alkenyl, or N-hydroxyaminocarbonyl(C 6 -C 18)aryl(C 1 -C 6)alkylene; or wherein: R 1, R 2, R 4 and R 6 are each as defined in above; R 3 is as defined in above; and R 5 is N-hydroxyamino-oxo-(C 2 -C 6)alkenyl, N-hydroxyaminocarbonyl(C 6 -C 18)aryl(C 1 -C 6)alkylene, or amino(C 6 -C 18)aryl-oxo-(C 2 -C 6)alkenyl; or wherein: R 1, R 2, R 4, R 5 are each as defined in above; R 3 is as defined in above; and R 6 is N-hydroxyamino-oxo(C 2 -C 6)alkenyl, N-hydroxyamino(C 6 -C 18)aryl (C 1 -C 6)alkylene, 4-(N-hydroxyaminocarbonyl)phenyl, or N-hydroxyaminocarbonyl(C 6 -C 18)aryl(C 1 -C 6)alkylene. wherein: R 1 is hydrogen, (C 1 -C 6)alkyl, or (C 3 -C 6)cycloalkyl; R 2 is (C 6 -C 18)aryl, (C 6 -C 18)aryl(C 1 -C 6)alkyl, (C 3 -C 18)heteroaryl(C 1 -C 6)alkyl, halo(C 6 -C 18)aryl(C 1 -C 6)alkyl, or (C 1 -C 6)alkoxy(C 6 -C 18)aryl(C 1 -C 6)alkyl; R 3 is N-hydroxyamino-oxo-(C 2 -C 6)alkenyl; R 4 is hydrogen, halogen, N-hydroxyamino-oxo-(C 2 -C 6)alkenyl, or N-hydroxyaminocarbonyl(C 6 -C 18)aryl(C 1 -C 6)alkylene; R 5 is hydrogen, halogen, N-hydroxyamino-oxo-(C 2 -C 6)alkenyl, N-hydroxyaminocarbonyl(C 6 -C 18)aryl(C 1 -C 6)alkylene, or amino(C 6 -C 18)aryl-oxo-(C 2 -C 6)alkenyl; and R 6 is hydrogen, N-hydroxyamino-oxo-(C 2 -C 6)alkenyl, N-hydroxyamino(C 6 -C 18)aryl (C 1 -C 6)alkylene, 4-(N-hydroxyaminocarbonyl)phenyl, or N-hydroxyaminocarbonyl(C 6 -C 18)aryl(C 1 -C 6)alkylene; R 1 is hydrogen, (C 1 -C 6)alkyl, or (C 3 -C 6)cycloalkyl; R 2 is (C 6 -C 18)aryl, (C 6 -C 18)aryl(C 1 -C 6)alkyl, (C 3 -C 18)heteroaryl(C 1 -C 6)alkyl, halo(C 6 -C 18)aryl(C 1 -C 6)alkyl, or (C 1 -C 6)alkoxy(C 6 -C 18)aryl(C 1 -C 6)alkyl; R 3 is N-hydroxyamino-oxo-(C 2 -C 6)alkenyl; R 4 is hydrogen, halogen, N-hydroxyamino-oxo-(C 2 -C 6)alkenyl, or N-hydroxyaminocarbonyl(C 6 -C 18)aryl(C 1 -C 6)alkylene; R 5 is hydrogen, halogen, N-hydroxyamino-oxo-(C 2 -C 6)alkenyl, N-hydroxyaminocarbonyl(C 6 -C 18)aryl(C 1 -C 6)alkylene, or amino(C 6 -C 18)aryl-oxo-(C 2 -C 6)alkenyl; and R 6 is hydrogen, N-hydroxyamino-oxo-(C 2 -C 6)alkenyl, N-hydroxyamino(C 6 -C 18)aryl (C 1 -C 6)alkylene, 4-(N-hydroxyaminocarbonyl)phenyl, or N-hydroxyaminocarbonyl(C 6 -C 18)aryl(C 1 -C 6)alkylene; or wherein: R 1, R 2, R 5 and R 6 are each as defined in above; R 3 is hydrogen or N-hydroxyamino-oxo-(C 2 -C 6)alkenyl; and R 4 is N-hydroxyamino-oxo-(C 2 -C 6)alkenyl, or N-hydroxyaminocarbonyl(C 6 -C 18)aryl(C 1 -C 6)alkylene; R 1, R 2, R 5 and R 6 are each as defined in above; R 3 is hydrogen or N-hydroxyamino-oxo-(C 2 -C 6)alkenyl; and R 4 is N-hydroxyamino-oxo-(C 2 -C 6)alkenyl, or N-hydroxyaminocarbonyl(C 6 -C 18)aryl(C 1 -C 6)alkylene; or wherein: R 1, R 2, R 4 and R 6 are each as defined in above; R 3 is as defined in above; and R 5 is N-hydroxyamino-oxo-(C 2 -C 6)alkenyl, N-hydroxyaminocarbonyl(C 6 -C 18)aryl(C 1 -C 6)alkylene, or amino(C 6 -C 18)aryl-oxo-(C 2 -C 6)alkenyl; R 1, R 2, R 4 and R 6 are each as defined in above; R 3 is as defined in above; and R 5 is N-hydroxyamino-oxo-(C 2 -C 6)alkenyl, N-hydroxyaminocarbonyl(C 6 -C 18)aryl(C 1 -C 6)alkylene, or amino(C 6 -C 18)aryl-oxo-(C 2 -C 6)alkenyl; or wherein: R 1, R 2, R 4, R 5 are each as defined in above; R 3 is as defined in above; and R 6 is N-hydroxyamino-oxo(C 2 -C 6)alkenyl, N-hydroxyamino(C 6 -C 18)aryl (C 1 -C 6)alkylene, 4-(N-hydroxyaminocarbonyl)phenyl, or N-hydroxyaminocarbonyl(C 6 -C 18)aryl(C 1 -C 6)alkylene. R 1, R 2, R 4, R 5 are each as defined in above; R 3 is as defined in above; and R 6 is N-hydroxyamino-oxo(C 2 -C 6)alkenyl, N-hydroxyamino(C 6 -C 18)aryl (C 1 -C 6)alkylene, 4-(N-hydroxyaminocarbonyl)phenyl, or N-hydroxyaminocarbonyl(C 6 -C 18)aryl(C 1 -C 6)alkylene.