Determination of labdane-type diterpenoid epimers from Vitex limonifolia

In this study, 10 previously undescribed diterpenoids, named vitexlimolides, were isolated from a methanol extract of Vitex limonifolia. Vitexlimolide A (1) and vitexlimolide F (6) were determined as single compounds and their structures were elucidated through comprehensive spectroscopic analyses, including NMR, MS, UV, and experimental electronic circular dichroism (ECD), combination with computational methods such as DP4+, J-DP4, and ECD calculations for complete structural determination. Vitexlimolide B-E (2-5) were characterized as inseparable epimeric pairs, each possessing an α,β-unsaturated-γ-hydroxy(or methoxy)-γ-lactone moiety, based on ¹H and ¹³C NMR data. A simplified exciton chirality method was used to differentiate the major and minor epimers within 2-5. Furthermore, the structures of vitexolide A (7) and myriadenolide (8), which occur as mixtures of epimeric pairs with incomplete elucidated absolute configurations, were also determined through comprehensive ECD simulations. The current work represents the first report of a practical strategy for distinguishing major and minor components of mixtures of labdane-type diterpenoid epimers bearing a γ-oxygenated butenolide moiety.