Synthesis and Characterization of fac ‐Re(CO) 3 ‐aspartic‐ N ‐monoacetic Acid: Structural Analogue of a Potential Renal Tracer, fac ‐ 99m Tc(CO) 3 (ASMA)

The reaction of an aminopolycarboxylate ligand, aspartic‐ N ‐monoacetic acid (ASMA), with [Re(CO) 3 (H 2 O) 3 ] + was examined. The tridentate coordination of ASMA to this Re I (CO) 3 precursor yielded fac ‐Re(CO) 3 (ASMA) as a mixture of diastereomers. The chemistry is analogous to that of the Tc I (CO) 3 complex, which yields fac ‐ 99m Tc(CO) 3 (ASMA) under similar conditions. The formation, structure, and isomerization of fac ‐Re(CO) 3 (ASMA) products were characterized by HPLC, 1 H NMR spectroscopy, and X‐ray crystallography. The two major fac ‐Re(CO) 3 (ASMA) diastereomeric products each have a linear ONO coordination mode with two adjacent five‐membered chelate rings, but they differ in the endo or exo orientation of the uncoordinated acetate group, in agreement with expectations based on previous studies. Conditions have been identified for the expedient isomerization of fac ‐Re(CO) 3 (ASMA) to a mixture consisting primarily of one major product. Because different isomeric species typically have different pharmacokinetic characteristics, these conditions may prove useful for the practical isolation of a single 99m Tc(CO) 3 (ASMA) species, thus allowing the isolation of the isomer that has optimal imaging and pharmacokinetic characteristics. This information will aid in the design of future 99m Tc radiopharmaceuticals.