A compound of formula (I), or a pharmaceutically acceptable salt thereof, where R1 is a (C1-C6)alkyl or (C3-C6)cycloalkyl group； R2 is a (C1-C6)alkyl, (C3-C6)cycloalkyl, (C1-C6)alkenyl, (C1-C6)fluoroalkyl, (C1-C3)fluoroalkoxy, or (C1-C6)alkoxy(C1-C6)alkyl group, substituted: (i) with one to three hydroxyl groups, or (ii) with an NRaRb group, where Ra and Rb are independently a hydrogen atom or a (C1-C3)alkyl group； or a pyrrolidinylmethyl group substituted with one to three hydroxyl groups； R9 is the same as R2 or hydrogen； the R2 and R9 groups independently being substitutable with an —OCOR3 group, where R3 is a natural or unnatural amino acid derivative or a piperidyl group； alternatively, R2 and R9 together form a heterocyclic compound； X and Y are independently a substitutable phenyl or heteroaryl group, the heteroaryl group being a thienyl, pyridyl, pyrimidinyl, thiazolyl, pyrrolyl, or furanyl group； and R6 is a hydrogen or a (C1-C3)alkyl group.
