Mono- and dicyclic unsaturated triterpenoid hydrocarbons in sediments from Lake Masoko (Tanzania) widely extend the botryococcene family

摘要

A mixture of C-33-C-37 botryococcenes and partially reduced derivatives was isolated from ca. 32,000 year old sediment from Lake Masoko, a freshwater crater lake in the Rungwe Range area (Tanzania). Botryococcenes and derivatives accounted for 246 mu g/g dry sediment and for > 92% of the hydrocarbon fraction; 1D and 2D nuclear magnetic resonance spectroscopy (NMR) and mass spectrometry allowed the structure of the dominant botryococcene (43% of hydrocarbon fraction) to be established, after purification using high performance liquid chromatography (HPLC). The compound is a novel tetraunsaturated dicyclic C-34 botryococcene and is named C-34 masokocene. Overall, the structures of six other novel botryococcenes and four partially reduced derivatives were tentatively assigned. The structures of the new biomarkers, three dicyclic C-34-C-36 botryococcenes (or masokocenes) and seven monocyclic C-34-C-37 analogues are discussed along with their biosynthetic relationship. The high abundance of such polyunsaturated compounds preserved in 32,000 year old sediment from the lake indicates an aquatic ecosystem dominated at the time by the green alga Botryococcus braunii, as well very good preservation of the organic matter. (c) 2008 Elsevier Ltd. All rights reserved.