A compound of formula (I) or a salt thereof (I) wherein R1 represents a hydrogen atom, a methyl group or a ethyl group R2 represents a hydrogen atom or a fluorine atom R3 represents a hydrogen atom, a methyl group or an ethyl group.
式(I)的化合物或其盐(I),其中R1代表氢原子、甲基基团或乙基基团,R2代表氢原子或
氟原子,R3代表氢原子、甲基基团或乙基基团。