Compositions and methods for analyzing cysteine

A compound of Formula I:

• wherein:
• each X is independently C or N;
• each E is independently hydrogen or an electron withdrawing group selected from the group consisting of —CF3, —C(O)—Ra, —SO2—Ra, —CN, and —NO2, with the proviso that at least one E is an electron withdrawing group;
• LG is a leaving group.
• R1 and R2, together with the carbon or nitrogen atoms to which they are attached, form a cycloalkyl, heterocycloalkyl, aryl, or heteroaryl;
• R3 is absent or selected from the group consisting of —Ra, -cycloalkyl, -heterocycloalkyl, -aryl, and -heteroaryl;
• and
• each Ra is independently selected from the group consisting of —H, -alkyl, —O-alkyl, —N-alkyl, -alkenyl, —O-alkenyl, —N-alkenyl, -alkynyl, —O-alkynyl, —N—alkynyl, -aryl, —O-aryl, —N-aryl, -heteroaryl, —O-heteroaryl, —N-heteroaryl, -cycloalkyl, —O-cycloalkyl, —N-cycloalkyl, -heterocycloalkyl, —O—heterocycloalkyl, and —N-heterocycloalkyl;
• wherein the compound of Formula I is not