An unprecedented condensation pathway leading to the formation of phenolic C40 bis-diterpenoids in sediments from the Lower Oligocene of the Rhine Rift Valley

In the course of a detailed palaeoenvironmental study based on biomarker distributions in sediments of Rupelian age (Lower Oligocene from the Rhine Rift Valley), an outcrop sample rich in higher plant fossil remains was collected from a quarry near the city of Burnhaupt-le-Haut (South Alsace, France). In agreement with the macrofossil assemblage, most of the biomarkers were related to terrigenous organic matter, as evidenced by the presence of di- and triterpenoid derivatives from vascular plants. The aromatic hydrocarbon fraction was also shown to contain two predominant series of unknown C40 oxygenated compounds, as indicated by high resolution mass spectrometry. Isolation of the predominant compound from the first series and of two isomers from the second was undertaken to determine the structures from nuclear magnetic resonance (NMR) studies. Thus, we determined that both series of compounds consist of bis-diterpenoids made up of phenolic sub-units related to totarol and sempervirol, possibly originating from Podocarpaceae. Their formation comprises a first step involving oxidative coupling between the two phenolic monomeric sub-units via a free radical mechanism and leading to the formation of an ether bond (first series of compounds), or a carbon-carbon bond (second series of compounds). In the case of the compounds from the second series, a second, acid-catalysed cyclisation step likely results in the formation of a central dibenzofuranyl moiety. It remains unclear, however, whether these condensation/cyclization mechanisms are biologically-mediated, or result from diagenetic reactions.