A compound, pharmaceutical composition and method for the treatment of mammals wherein the active therapeutic agent is a compound having the structure: ##STR00001## wherein: R.sub.1 is an electronegative substituent, R.sub.2 is R.sub.1 or alkyl, R.sub.3 is H or O-alkyl, R.sub.4 and R.sub.5 are the same or different and are alkyl and R.sub.6 is H or OH.