There are provided compounds of the formula or a pharmaceutically acceptable salt thereof, wherein Q, R1 and R2 are as defined herein.The compounds of formula I are useful in the treatment of cancer.本發明提供下式化合物,或其醫藥學上可接受之鹽,其中Q、R1及R2如本文所定義。該等式I化合物適用於治療癌症。