In other embodiments of compounds of formula I, R1 and R2 independently or together are a group capable of being hydrolyzed to a hydroxyl group in an aqueous solution at physiological pH or in biological fluids and are formed from 1,2-dicyclohexylethanediol; 1,2-ethanediol; 1,3-propanediol; 2,3-butanediol, 1,4-butanediol; diisopropylethanediol; methanol; ethanol; isopropanol; or 1-butanol.