The present invention directs a compound represented by the following formula (I), or a pharmaceutically acceptable salt thereofwherein R1, R2, R3, R4, R101 and R are the same as those described in the specification.本發明係關於以下式(I)代表之化合物,或其在醫藥上可接受之鹽;其中R1、R2、R3、R4、R101及R與在說明書中所述相同。