and R1 are each independently alkyl, aryl, or heteroaryl groups having 1-20 carbon atoms, where at least one of R and R1 is substituted with at least one fluorine atom; and R2 is an alkyl group, optionally substituted with one or more fluorine atoms. [0193] Also provided is a compound of the formula [0194] or a pharmaceutically acceptable derivative thereof, where X is an aryl or heteroaryl group