tands for a methylene group, a dimethylmethylene group, an oxygen atom, a sulfur atom, a sulfinyl group or a sulfonyl group, Each of R3 and R4 stands for (1) a hydrogen atom, (2) an alkyl group which may be substituted with a halogen atom such as a fluorine atom, a chlorine atom and a bromine atom, and so on, (3) a phenyl group which may be substituted with aforementioned substituent or substituen