Chiral Discrimination of Some Annelated Xanthine Derivatives by the Dirhodium Method

Abstract

Complete 1H and 13C signal assignments and conformation analyses of the title compounds (racemic mixtures) were performed. Most 1H and 13C NMR signals were resolved in the presence of the enantiomerically pure dirhodium complex Rh2(MTPA)4 (Rh*) allowing for clear and simple chiral recognition. A detailed interpretation of the signal shifts (Δδ) and dispersions (Δν) in the diastereomeric complexes suggested that the π-system around the C-4/C-5 bond of the central imidazole ring (B) is a binding site of the xanthine derivatives. This assumption was based on HOMO−LUMO considerations including back-donation and on the fact that diastereotopic dispersions for protons within methylene groups are very different. Signal shifts are in the range of 0.5 to 1 ppm for carbon atoms close to the binding site and fade away at the periphery of the molecule. Dispersions are spread all over the molecule.