Identification of a novel C-33 botryococcane and C-33 botryococcanone in the Maoming Basin, China

摘要

A novel C-33 botryococcane in a sedimentary sample from the Maoming Basin in China was found to have a methyl group positioned beta 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 C-33 botryococcanes suggested in the literature. A C-33 botryococcan-24-one with the same carbon skeleton as the C-33 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 C-30 botryococcene via a cyclobutane ring intermediate (1'-2-3-2' cyclisation). A Retro-Prins reaction and subsequent methylation gives rise to C-29 and C-33 botryococcenes in sequence. The geochemical component involves first in-water-column (photo)oxidation of the alkene to a C-33 botryococcanone and subsequent reduction (hydrogenation) to stabilize the ketone as an alkane. (C) 2018 Elsevier Ltd. All rights reserved.