The present invention relates to compounds useful in the treatment of cancer or other proliferative diseases represented by the formula ##STR00001## wherein: R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5 are hydrogen or lower alkyl; R.sub.6 is selected from the group consisting of hydrogen, alkyl, substituted alkyl, aryl, substituted aryl, cycloalkyl, or heterocyclo; X is hydrogen and Y is hydroxy, or X and Y taken together represent a carbon-carbon bond; and pharmaceutically acceptable salts, solvates, or hydrates thereof. Also included are therapeutic compositions containing the compounds represented by formula I as active ingredients, alone or in combination with other therapeutic agents useful in the treatment of cancer or other proliferative diseases.