The present invention relates to a compound of formula I ##STR00001## wherein (i) when a and c are double bonds, R.sup.2 is absent, b is a single bond, R.sup.1 is ##STR00002## or (ii) when a and c are single bonds, b is a double bond, and R.sup.1 is H, R.sup.2 is ##STR00003## wherein the substituents are as defined in the specification.