Patent ID: 8481543

Claim:
A compound according to formula I: or a pharmaceutically acceptable salt thereof, wherein: X is an electron-withdrawing group; A is an optionally substituted alkanediyl or an optionally substituted cycloalkanediyl that includes 2 to 5 carbon atoms; and R 1 and R 2 are each individually H, tert-butyl, isopropyl, ethyl, propyl, or optionally substituted cycloalkyl, or R 1 and R 2 together form a substituted or unsubstituted heterocyclic ring system, provided that R 1 and R 2 are not both H or R 1 and R 2 are not both ethyl; or a compound according to formula II: or a pharmaceutically acceptable salt thereof, wherein: X is an electron-withdrawing group; L is a linking group; and represents a cyclic ring structure that optionally includes at least one additional heteroatom.