R5 stands for (1) a hydroxyl group, (2) a mercapto group, (3) an amino group, (4) a formyl group, (5) an alkyl group which may be substituted with a halogen atom such as a fluorine atom, a chlorine atom, a bromine atom, and so on, (6) an alkoxy group which may be substituted with aforementioned substituent or substituents, (7) a thioalkoxy group which may be substituted with aforementioned substit