Carbohydrate-Appended 2,2‘-Dipicolylamine Metal Complexes as Potential Imaging Agents

Three discrete carbohydrate-appended 2,2‘-dipicolylamine ligands were complexed to the {M(CO)3}+ (M = 99mTc/Re) core:  2-(bis(2-pyridinylmethyl)amino)ethyl-β-d-glucopyranoside (L1), 2-(bis(2-pyridinylmethyl)amino)ethyl-β-d-xylopyranoside (L2), and 2-(bis(2-pyridinylmethyl)amino)ethyl-α-d-mannopyranoside (L3). An ethylene spacer is used
to separate the carbohydrate moiety and the dipicolylamine (DPA) function in all three ligands. The Re complexes
[Re(L1-3)(CO)3]Br were characterized by 1H and 13C 1D/2D NMR spectroscopies, which confirmed the pendant
nature of the carbohydrate moieties in solution. NMR measurements also established the long-range asymmetric
effect of the carbohydrate functions on the chelating portion of the ligand. One analogue, [Re(L1)(CO)3]Cl, was
characterized in the solid state by X-ray crystallography. Further characterization was provided by IR spectroscopy,
elemental analysis, conductivity, and mass spectrometry. Radiolabeling of L1−L3 with [99mTc(H2O)3(CO)3]+ afforded
high yield compounds of identical character to the Re analogues. The radiolabeled compounds were found to be
stable toward ligand exchange in the presence of a large excess of either cysteine or histidine over a 24-h period.