A novel compound has the following formula: ##STR1## in which R.sub.1 and R.sub.2 are methyl or a group having the formula: ##STR2## provided that one of R.sub.1 and R.sub.2 is methyl and the other is said group, wherein R stands for a hydrogen atom or a lower alkyl group, Z stands for a group ##STR3## in which X.sub.1 and X.sub.2, which may be the same or different, stand for a hydrogen atom, a lower alkyl group, a lower alkoxy group, a trifluoromethyl group or a halogen atom, or a pyridyl group or a group ##STR4## in which Y.sub.1 and Y.sub.2, which may be the same or different stand for a hydrogen atom, a lower alkyl group, a lower alkoxy group, a trifluoromethyl group or a halogen atom, X stands for a nitrogen or carbon atom, and n is an integer of from 2 to 10, provided that when R.sub.2 is the group having the formula II, X is not carbon and Z is not pyridyl.