Compounds of the general formula ##STR1## wherein R.sub.1 and R.sub.2 each represents methyl or ethyl, or wherein R.sub.1 represents chlorine or methoxy and R.sub.2 represents methyl, and R.sub.3 and R.sub.4 each represents hydrogen, methyl or ethyl.
通式为##STR1##的化合物,其中R.sub.1和R.sub.2分别表示甲基或乙基,或者R.sub.1表示
氯或甲氧基,R.sub.2表示甲基,而R.sub.3和R.sub.4分别表示氢、甲基或乙基。