The present invention is directed to compounds of Formula I: or a pharmaceutically acceptable salt thereof, wherein the substituents R1, R3, R6, R7, and b are as defined herein. The invention is also directed to pharmaceutical compositions comprising the compounds, methods of treatment using the compounds, and methods of preparing the compounds.本發明係關於式I化合物:或其醫藥學上可接受之鹽,其中取代基R1、R3、R6、R7及b如本文所定義。本發明亦關於包含該等化合物之醫藥組合物、使用該等化合物治療之方法及製備該等化合物之方法。