A compound of formula X: ** Formula ** or a pharmaceutically acceptable salt thereof, (I) in which: R1 is hydrogen, (C1-C6) alkyl, or (C3-C6) cycloalkyl； R2 is (C6-C18) aryl, (C6-C18) aryl (C1-C6) alkyl, (C3-C18) heteroaryl (C1-C6) alkyl, (C6-C18) haloaryl (C1-C6) alkyl, or (C1-C6) alkoxy (C6-C18) aryl (C1-C6) alkyl； R3 is N-hydroxyamino-oxo-alkenyl (C2-C6)； R4 is hydrogen, halogen, N-hydroxyamino-oxo-alkenyl (C2-C6), or N-hydroxyaminocarbonylaryl (C6-C18) alkylene (C1-C6)； R5 is hydrogen, halogen, N-hydroxyamino-oxo-alkenyl (C2-C6), N-hydroxyaminocarbonylaryl (C6-C18) alkylene (C1-C6), or aminoaryl (C6-C18) -oxy-alkenyl (C2-C6) ； and R6 is hydrogen, N-hydroxyamino-oxo-alkenyl (C2-C6), N-hydroxyaminoaryl (C6-C18) alkylene (C1-C6), 4- (N hydroxyaminocarbonyl) phenyl, or N-hydroxyaminocarbonylayl (C6-C18) (C1-C6) alkylene； or (II) in which: R1, R2, R5 and R6 are each as defined in (I) above； R3 is hydrogen or N-hydroxyamino-oxo-alkenyl (C2-C6)； and R4 is N-hydroxyamino-oxo-alkenyl (C2-C6), or N-hydroxyaminocarbonylaryl (C6-C18) alkylene (C1-C6)； or (III) in which: R1, R2, R4 and R6 are each as defined in (I) above； R3 is as defined in (II) above； and R5 is N-hydroxyamino-oxo-alkenyl (C2-C6), N-hydroxyaminocarbonylaryl (C6-C18) alkylene (C1-C6), or amino-aryl (C6-C18) -oxo-alkenyl (C2-C6)； or (IV) in which: R1, R2, R4, R5 are each as defined in (I) above； R3 is as defined in (II) above； and R6 is N-hydroxyamino-oxoalkenyl (C2-C6), N-hydroxyaminoaryl (C6-C18) alkylene (C1-C6), 4- (N-hydroxyaminocarbonyl) phenyl, or N-hydroxyaminocarbonylayl (C6-C18) alkylene (C1 -C6).Un compuesto de fórmula X:**Fórmula** o una sal farmacéuticamente aceptable del mismo, (I) en la que: R1 es hidrógeno, alquilo (C1-C6), o cicloalquilo (C3-C6)； R2 es arilo (C6-C18), arilo (C6-C18) alquilo (C1-C6), heteroarilo (C3-C18) alquilo (C1-C6), haloarilo (C6-C18) - alquilo (C1- C6), o alcoxi (C1-C6) arilo (C6-C18) alquilo (C1-C6)； R3 es N-hidroxiamino-oxo-alquenilo (C2-C6)； R4 es hidrógeno, halógeno,
