Mixed intramolecular hydrogen bonding in dihydroxythiophene-based units and boron and technetium chelation

Abstract

Three novel potential metal ion chelating units have been synthesized and characterized: 5-hexylcarbamoyl-3,4- dihydroxythiophene-2-carboxylic acid methyl ester (5), 3-benzyloxy-4-hydroxythiophene-2,5-dicarboxylic acid bis-hexylamide (6), and 3,4-dihydroxythiophene-2,5-dicarboxylic acid bis-hexylamide (7). The crystal structure of 6 was obtained and suggests the presence of three distinct intramolecular hydrogen bonds, namely [N-amide-H center dot center dot center dot O] [O-H center dot center dot center dot O-amide] and [N-amide-H center dot center dot center dot S]. Boron chelation with 5, 6 and 7 through the use of BF3 B(OH)(3) or B(OMe)(3) was probed by H-1, B-11, and C-13 NMR spectroscopy. Technetium (I) chelation with 5, 6 and 7 was also studied via HPLC elutions using [Tc-99m(CO)(3)(OH2)(3)](+).