Rearranged oleananes: Structural identification and distribution in a worldwide set of Late Cretaceous/Tertiary oils

Abstract

The three most abundant rearranged oleanoid triterpanes eluting early in gas chromatography (GC) have been isolated from a Niger Delta oil and characterized using nuclear magnetic resonance (NMR) spectroscopy. The NMR data confirmed the identification of two of them, observed previously in the literature, as 1(10 → 5)abeo-3β-methyl-24β-nor-18α(H)-oleanane and 5(4 → 3)abeo-3α(H), 5β(Η), 18α(Η)-oleanane on the basis of X-ray crystallography and comparison with a synthetic standard. The third compound was a novel rearranged oleanane, 3α,5β-dimethyl-23α,25-dinor-10β(H),18α(H)-oleanane, with one of the methyl groups at C-4 in the oleanane structure shifted to C-3 and another shifted from C-10 to C-5. The distribution of these three rearranged oleananes and other, yet unidentified, rearranged oleanoid C 30 triterpanes in 25 Late Cretaceous- or Tertiary-sourced oils from Angola, Canada, Greece, India, Indonesia, Iran, New Zealand, Nigeria and Vietnam was examined by way of reverse phase high pressure liquid chromatography (HPLC), GC–mass spectrometry (GC–MS) and GC–MS–MS. Mass spectra of rearranged oleananes and compounds tentatively assigned as rearranged taraxastanes are presented. Rearranged oleananes and other rearranged triterpanes are probably formed by dehydration and rearrangement of higher plant triterpenoids functionalized at C-3. The distribution of rearranged oleananes and their relative concentration seems to be dependent on the extent of clay catalysis during diagenesis. A diagenetic pathway for formation of the rearranged oleananes is proposed. All 25 oils have almost identical rearranged oleanane/oleanane and diasterane/regular sterane ratio values. Two unidentified early eluting (GC) rearranged oleananes are only abundant in oils with a high proportion of diasteranes. Bicadinanes were found in 20 of the 25 oils.