Novel triterpene-derived hydrocarbons of the arborane/fernane series in sediments: Part II

Two ring-B aromatized triterpenes related to the arborane/fernane series were isolated from an Italian Triassic black shale ( 4 and 7) and a French Jurassic laminated bituminous limestone (7). They were identified by MS and NMR spectroscopy. These compounds, 25-norferna-5,7,9-triene 7 (C 29) and 22,25,29,30-tetranor-18β-ferna-5,7,9-triene 4 (C 26), in the case of the Italian sediment, belong to a family which extends to at least C 32 as shown by GC-MS. The presence of higher homologs and ring-A methylated structures in the Italian Triassic shale is analogous with the hopane series, which together with carbon isotopic data obtained by gas chromatography-isotope ratio mass spectrometry (GC-IRMS), strongly support a bacterial origin for most of these compounds. This suggests that 4 and 7 may result from a yet unrecognized rearrangement of sedimentary hopenes which have subsequently undergone ring-B aromatization. As an alternative, the C 29 compound 7 could arise from C 30 precursors not necessarily of bacterial origin (fernenes?). This hypothesis would apply particularly to the case of the Jurassic limestone, which does not show any trace of 4 nor of compounds with an extended side chain or a methyl group on ring A. Related structures also bearing an isopropyl group could derive by microbiological transformations from the corresponding fernen-3β-ols or isoarborinol as can be deduced from the occurrence of ring-A degraded monoaromatic hydrocarbons 3 and 5.