r alkyl radical having from 1 to 3 carbon atoms; and one of R1 and R2 is ???O, --OH or a --O(CO)R6 group, and the other one is --OH or --O(CO)R6, or R1 is ???O and R2 is H, wherein R6 is a saturated or unsaturated acyclic hydrocarbon group having from 1 to about 20 carbon atoms, or --(CH2)m R7 wherein m is 0-10, and R7 is an aliphatic ring from about 3 to about 7 carbon atoms, or an aryl or hetero