A compound having formula I is useful for treating a neurodegenerative disease:or a pharmaceutically acceptable salt or ester thereof, wherein R1, R2, R3, R4, R5, R6, R7, R8 are each independently hydroxyl, C1-4 alkyl, C1-4 alkoxyl, C1-8 alkyl, C2-8 alkenyl, C2-8 alkynyl, C4-8, are specified substituents.
