Assignment of relative configuration of desoxypropionates by 1H NMR spectroscopy: method development, proof of principle by asymmetric total synthesis of xylarinic acid A and applications.

The determination of the relative configuration of 1,3-dimethyl-substituted alkyl chains is possible by interpretation of (1)H NMR shift differences. Additionally, assignments are feasible in a variety of deuterated solvents, because the corresponding shift differences are not significantly influenced by the solvent. The trends for Δδ values depending on functional groups adjacent to the stereogenic centers are shown. Based on a thorough comparison with literature data, the relative configuration of natural products can be predicted. For this purpose, we derived an empirical rule for the ranges in which Δδ values usually occur. Furthermore, we were able to proof the validity of our method by the successful prediction of the relative configuration for the polyketide natural product xylarinic acid A, which was confirmed by the asymmetric total synthesis of its enantiomer. Based on the proposed simple analysis of published (1)H NMR data and the determination of the relevant chemical-shift differences, we predicted the relative configurations of several previously unassigned natural products.