The present invention provides compounds having the formula: Formula I wherein R1 is selected from methyl, ethyl, propyl, cyclopropyl, and dimethyl, or a pharmaceutically acceptable salt thereof, along with methods and intermediates for their preparation, and uses thereof.本發明提供具有下式之化合物:式I其中R1係選自甲基、乙基、丙基、環丙基及二甲基,或其醫藥上可接受的鹽,以及其製備方法及中間物,及其用途。