alkyl, aryl, or heteroaryl groups having 1-20 carbon atoms, wherein at least one of R and R1 is substituted with at least one fluorine atom; and R2 is an alkyl group, optionally substituted withone ormorefluorine atoms. [0191] Also provided is a compound of the formula [0192] or a pharmaceutically acceptable derivative thereof, where X is an aryl or heteroaryl group; n is an integer from 0 to 6; R