The present invention provides novel compounds having the general formula: wherein R1, R2, U, W, X, Y and Z are as described herein, compositions including the compounds and methods of using the compounds.本發明提供具有下通式之新穎化合物:其中R1、R2、U、W、X、Y及Z係如本文所述,包含該等化合物之組合物,及使用該等化合物之方法。