The invention provides novel compounds having the general formula: wherein R1 to R6, W and X are as described herein and their pharmaceutically acceptable salt, enantiomer or diastereomer thereof, and compositions including the compounds and methods of using the compounds.本發明提供具有以下通式之新穎化合物:其中R1至R6、W及X係如本文所闡述;及其醫藥上可接受之鹽、其鏡像異構物或非鏡像異構物,及包括該等化合物之組合物以及使用該等化合物之方法。