The present invention is directed to compounds of formula I: wherein A is n is 0, 1, or 2; m is 0 or 1; R1 is H or C1-6alkyl and R2 is H, C1-6alkyl, C1-6alkaryl, aryl, or heteroaryl; and X is O or NH. Tautomers, enantiomers, and diastereomers, as well as pharmaceutically acceptable salt forms, of compounds of formula I are also within the scope of the invention. Methods of preparing and using the compounds of formula I are also described. wherein A is