Compounds having the structure ##STR1## and the pharmaceutically acceptable salts thereof, wherein R.sub.1 is hydrogen, halogen, hydroxy, alkanoyloxy, alkoxy, alkylthio, alkyl or trifluoromethyl; R.sub.2 is hydrogen, alkyl or alkenyl; R.sub.3 is hydrogen, halogen or alkyl; R.sub.4 is formyloxy or alkanoyloxy; m is 1 or 2; and n is 0, 1 or 2; have useful physiological activity.