Synthesis of new N2S2 ligands and Re(V)O(N2S2) analogues of 99mTc renal imaging agents. Characterization by NMR spectroscopy, molecular mechanics calculations, and X-ray crystallography

Two new N2S2 ligands [S-trityl-l-cysteine-acetamide-ethanethiol ethyl ester (Tr-l-CAATH2-Et, 1) and S-trityl-d-penicillamine-acetamide-(2-methyl-2-propanethiol) ethyl ester (Tr-d-TMCAATH2-Et, 2)] were prepared. These ligands, with the electron-withdrawing carboxyl group separated by only two bonds from the NH amine group, belong to monoamide-monoamine-dithiol (MAMA) class of chelates that typically form M(V)O(N2S2) (M=Tc, Re) complexes with the ligand secondary amine pKa of ∼6–7 and both syn and anti isomers. Re(V)O(N2S2) complexes were prepared from 1 and 2 with the dangling ester either syn or anti to the ReO intact or hydrolyzed; the latter, ReO(l-CAATH2) and ReO(d-TMCAATH2) have a carboxyl group needed for renal imaging when M=Tc. The gross structure of syn-ReO(l-CAATH-Et) (3) is typical of Re(V)O(N2S2) complexes. However, within the cys moiety of 3, an intramolecular H-bond between the ester carbonyl oxygen O(4) and the NH (N(2)⋯O(4) distance=2.80 Å) causes the cys chelate ring to adopt an unusual conformation with the αC (C(5)) atom 0.42 Å below the plane defined by the cys S, Re, and N(2) atoms with respect to the ReO group. From molecular mechanics (MM) analysis of ReO(l-CAATH2) models, the lowest-energy syn-ReO(l-CAATH2) structure has a corresponding region similar to that of the X-ray structure for 3, an N(2)H endo with respect to ReO group, a N(2)H⋯O(4) hydrogen bond (2.69 Å N(2)⋯O(4) distance) contributing ∼5 kcal mol−1 to its stability and an αC atom deviating 0.24 Å from the cys S, Re, and N(2) plane. Computed structures with an exo N(2)H are ∼6 kcal mol−1 less stable than the lowest-energy endo analogue. Below pH ∼7, the 1H NMR spectrum of syn-ReO(d-TMCAATH2) exhibited changes in two pH regions, consistent with sequential protonation of the amine and of the carboxylic group. These experimental and MM results reinforce previous hypotheses about the factors favoring one syn form in one ionization state of M(V)O(N2S2) compounds at physiological pH.