One such subclass is comprised of the compounds of the above formula in which X, R, and R1 are as defined, and R2 is ##STR9## Another preferred subclass comprises those compounds in which R2 is hydrogen and either R is hydroxyl or R1 is ##STR10## A further preferred subclass comprises those compounds in which R2 is hydroxyl and R1 is other than hydroxyl or C1 -C5 alkoxy.