A compound of the formula: wherein R.sup.1 is selected from H, methyl, ethyl, carboxyl protecting groups and hydrophilic moieties, R.sup.2 and R.sup.3 are independently selected from H, methyl, ethyl and carboxyl protecting groups, R.sup.4 is selected from H, methyl, ethyl, hydrophilic moieties and carboxyl protecting groups, and R.sup.5 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.