Disclosed are compounds of formula A: ##STR00001## and pharmaceutically acceptable salts thereof, wherein R.sub.1, R.sub.2, R.sub.2', R.sub.3, R.sub.21, A.sub.1, A.sub.2, X, and Z are as defined herein. Compounds of formula A are useful in the treatment of diseases and/or conditions related to cell differentiation, such as cancer, inflammation, arthritis, angiogenesis, or the like. Also disclosed are pharmaceutical compositions comprising compounds of the invention and methods of treating the aforementioned conditions using such compounds.