Two novel decamethylhenicosanes (C31H64) identified in a Maoming Basin shale, China

Two new C31 branched alkanes (botryococcanes) presumably produced by the B race of Botryococcus braunii were isolated and purified from the Maoming Basin shales using column chromatography and preparative gas chromatography and structurally characterized with HR-EI-MS and 1D and 2D NMR. Interpretation of their EI mass spectral and 1D and 2D NMR (HMBC and HSQC) data led to the firm assignments of the two alkanes as diastereoisomeric 2,3,6,7,10,12,15,16,19,20-decamethylhenicosanes (DMHs). The structural assignments were further confirmed by the close match of the measured 13C NMR chemical shifts with those predicted by Lindeman-Adams 13C chemical shift modeling. The skeletons of these two DMHs are virtually identical to that of the recently identified C33 botryococcane/botryococcanone in the same sample. It is proposed that these two DMHs share a precursor C33 botryococcene biochemically formed by condensing two farnesyl diphosphates involving an unusual cyclobutanation, a retro-Prins reaction and a tetramethylation. A photo-mediated geochemical oxidation of the double bond in the ethenyl group connected to the sole quaternary carbon C-10 is also proposed to be responsible for the formation of the co-occurring DMHs and C33 botryococcane/botryococcanone.