A compound or a salt thereof having the following formula ##STR00001## wherein R.sup.1 is acyl, R.sup.2 is hydrogen, halogen, unsubstituted alkyl or alkyl substituted by halogen; R.sup.3 is halogen or alkoxy; R.sup.4 is cycloalkyl, unsubstituted alkyl or alkyl substituted by cycloalkyl, aryl or hydroxyl or an ester thereof or alkoxy; R.sup.5 is hydroxyl or an ester thereof, alkoxy or alkoxyalkyl; or R.sup.4 and R.sup.5 are bonded with each other to form a pyrrolidine ring substituted by hydroxyl or an ester thereof, alkoxy or alkoxyalkyl; R.sup.6 is hydroxyl or an ester thereof, alkoxy, alkoxyalkoxy, alkoxyalkoxyalkoxy, mercapto or alkylthio; and A.sub.1 and A.sub.2, are the same or different, and are alkylene, provided that (i) when R.sup.4 and R.sup.5 are bonded to each other to form the pyrrolidine ring substituted by hydroxyl or an ester thereof, R.sup.2 is halogen; (ii) when R.sup.4 and R.sup.5 are bonded to each other to form the pyrrolidine ring substituted by alkoxyalkyl, R.sup.2 is hydrogen; (iii) when R.sup.6 is hydroxyl or an ester thereof, R.sup.4 is isopropyl.