Chiral discrimination of some annelated xanthine derivatives by the dirhodium method

Complete H-1 and C-13 Signal assignments and conformation analyses of the t itle compounds (racemic mixtures) were performed. Most H-1 and C-13 NMR sig nals were resolved in the presence of the enantiomerically pure dirhodium c omplex Rh-2(MTPA)(4) (Rh*) allowing for clear and simple chiral recog- niti on. A detailed interpretation of the signal shifts (Delta delta) and disper sions (Delta nu) in the diastereomeric complexes suggested that the pi -sys tem 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 co nsiderations including back-donation and on the fact that diastereotopic di spersions for protons within methylene groups are very different. Signal sh ifts 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 sprea d all over the molecule.