Two Novel Decamethylhenicosanes (C₃₁H₆₄) Identified in a Maoming Basin Shale, China

Jing Liao^a,b, Hong Lu^a,*, Qiao Feng^b, Youping Zhou^c,*, Quan Shi^d, Ping’an Peng^a, Guoying Sheng^a

^a State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry and Institutions of Earth Science, Chinese Academy of Sciences, Guangzhou 510640, China

^b Shandong Provincial Key Laboratory of Depositional Mineralization and Sedimentary Minerals, Shandong University of Science and Technology, Qingdao 266000, China

^c Isotopomics in Chemical Biology & Shaanxi Key Laboratory of Chemical Additives for Industry, School of Chemistry & Chemical Engineering, Shaanxi University of Science & Technology, Xi’an 710021, China

^d State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China

*Corresponding Authors: Hong Lu − Email: luhong@gig.ac.cn; Youping Zhou − Email: youping.zhou@sust.edu.cn

DOI: 10.1016/j.orggeochem.2018.09.012

Keywords: Maoming oil shale; NMR; Decamethylhenicosanes; Botryococcus braunii

Abstract: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.