Higher order and substituent chemical shift effects in the proton NMR of glycosides.

The full analysis of (1)H NMR spin systems and the charting of substituent chemical shifts offer great potential in the structure elucidation of glycosides. Due to the chiral nature of sugar residues, asymmetric induction causes diastereotopic shifts of aglycon resonance, while on the other hand aromatic substituent-induced shifts affect the sugar portion of the molecule. Therefore, it is possible to deduce critical structural information, such as the site of sugar linkage in bisdesmosides and oligoglycosides, through the analysis of exact chemical shifts, J patterns, and signal multiplicity. The potential of the method is demonstrated for three kaempferol bisdesmosides (1-3) by establishing the shift characteristics for 3-O-, 7-O-, and 6' '-O-glycosidation. Spectral simulation and soft-pulse 1D NMR turn out to be indispensable tools in the course of refined signal and structure assignment.