Identification of a novel C₃₃ botryococcane and C₃₃ botryococcanone in the Maoming Basin, 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 University of Science and Technology, Shandong Provincial Key Laboratory of Depositional Mineralization & Sedimentary Minerals, Qingdao 266000, China

^c Isotopomics in Chemical Biology and 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.2018.07.005

Keywords: Botryococcanone; Botryococcane; Botryococcene biosynthesis; NMR; Cyclobutyl cation intermediate; Botryococcus braunii; Maoming oil shale

Abstract:A novel C₃₃ botryococcane in a sedimentary sample from the Maoming Basin in China was found to have a methyl group positioned β to the quaternary carbon C-10 in its structure based on IR, MS and NMR analyses and Lindeman-Adams modelling. This structure is inconsistent with the prediction from the currently accepted botryococcene biosynthetic pathway and different from the skeleton for the C₃₃ botryococcanes suggested in the literature. A C₃₃ botryococcan-24-one with the same carbon skeleton as the C₃₃ botryococcane was also found to co-occur in the sediment sample. The location of the carbonyl group at C-24 is different from that of a previously reported botryococcenone from an Australian collection of race B of Botryococcus braunii where it is positioned at C-15. We propose a biogeochemical pathway to explain the occurrence of this unique skeleton in the Maoming Basin sediment. The biochemical component of the pathway involves electrophilic or nucleophilic attack between two farnesyl diphosphates (FPPs) leading to direct or indirect formation of a C30 botryococcene via a cyclobutane ring intermediate (1′-2-3-2′ cyclisation). A Retro-Prins reaction and subsequent methylation gives rise to C₂₉ and C₃₃ botryococcenes in sequence. The geochemical component involves first in-water-column (photo)-oxidation of the alkene to a C₃₃ botryococcanone and subsequent reduction (hydrogenation) to stabilize the ketone as an alkane.