Advances in the Universal NMR Database: Toward the Determination of the Relative Configurations of Large Polypropionates

The relative stereochemical determination of naturally occurring large polyketides has proved to be critical, especially when the polyketides are available only in small quantities. Difficulties are often circumvented by the tedious synthesis either of adapted compound libraries (databases) or of all the presumed structures, because both methods allow efficient NMR comparison. To predict the stereochemistries of these frameworks, we have developed a straightforward method requiring neither the derivatisation of the natural product nor the synthesis of a new database of model stereoisomers. With the aid of C-13 NMR spectroscopy, the relative configurations of the considered polypropionate segments are determined through a statistical UDB analysis of each included tetrad subunit followed by superposition of all predicted relative configurations of these tetrads. Examination of the different tetiads by statistical analysis consists of alternatively ruling out the Delta delta values at positions where gamma and delta effects could interfere. In most cases this statistical C-13 NMR UDB approach leads to the stereochemical assignment of large polypropionate structures in a reliable way. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)