The C-12 and C-20 configurations of some neo-clerodane diterpenoids isolated from Teucrium species

Abstract

A convenient and conclusive method for determining the C-12 stereochemistry of neo-clerodan-20,12-olide derivatives, even when only one epimer is available, is by 1H NMR NOE measurements. The C-12 configuration of 26 neo-clerodane diterpenoids isolated from Teucrium species has been re-examined by using this type of experiment. The results indicated that all the previous assignments were correct, except for teupyreinin, where the previously assigned C-12( S) configuration must be amended to C-12( R). This was confirmed by chemical transformations and additional 1H and 13C NMR studies. Furthermore, the NOE experiments allowed the assignment of a C-20( S) configuration for teuflavin, the structure of which had previously been reported without this feature, and indicated that the previously assigned C-20( S) configuration for teupyreinidin must be amended to C-20( R). The validity of using NOE experiments to establish the C-12 and C-20 configurations in these neo-clerodane derivatives has been confirmed by applying it to compounds whose structure had already firmly established by X-ray diffraction analyses.