A compound of the formula: wherein R1 is selected from H, methyl, ethyl, carboxyl protecting groups and hydrophilic moieties, R2 and R3 are independently selected from H, methyl, ethyl and carboxyl protecting groups, R4 is selected from H, methyl, ethyl, hydrophilic moieties and carboxyl protecting groups, and R5 is an aryl, heteroaryl, alkyl or a combination of these groups and is substituted with a carbonyl group, an aminooxy group or a functional group suitable for participating in a cycloaddition reaction. The compounds of the invention may be useful as bifunctional chelating agents which allow chemoselective attachment to targeting molecules.