The present invention provides a compound of the Formula II: wherein X is: R1 is H, -CN, or F; R2 is H or methyl; R3 is H; and R4 is H, methyl, or ethyl; or R3 and R4 joined together form a cyclopropyl ring; or a pharmaceutically acceptable salt thereof.本發明提供式II化合物:其中X為:R1為H、-CN或F;R2為H或甲基;R3為H;及R4為H、甲基或乙基;或R3及R4結合在一起形成環丙基環;或其醫藥可接受鹽。