Over the past twenty years there have been many published reports of a group of abundant C20, C25 and C30 highly branched acyclic alkanes and alkenes in surface sediments. Synthesis and spectral and chromatographic characterisation of 2,6,10-trimethyl-7-(3-methylbutyl) dodecane (C20), 2,6,10,14-tetramethyl-7-(3-methylpentyl) pentadecane (C25) and 2,6,10,14,18-pentamethyl-7-(3-methylpentyl) nonadecane (C30) in the present study showed that these were the parent skeleta of the unknown hydrocarbons. Only the C20 hydrocarbon has been synthesised previously. Intermediates in the synthetic routes were also fully characterised. The synthetic alkanes and alkenes were used to assign structures to hydrocarbons in several sediments from Europe and Africa and a comprehensive overview of the sedimentary occurrences of these new 'biomarker' hydrocarbons is made. Biodegradation of the synthetic alkanes and alkenes using pure cultures of the bacterium Pseudomonas aeruginosa indicated that the high abundance of the hydrocarbons relative to normal alkanes in certain environments is related to their resistance to biodegradation. A group of bicyclic C25 and C30 hydrocarbons have often been reported to co-occur with the acyclic hydrocarbons. Mass spectra of these compounds suggest that they possess 'extended-drimane' carbon skeletons. Synthesis of two bicyclic hydrocarbons, 8 αß (H) 9 αß (H) 11- tetrahydrogeranyldrimane (C25) and 8 αß (H) 9 αß (H) 11-hexahydrofarnesyldrimane (C30), and comparison of these with the sedimentary hydrocarbons indicated that the compounds were similar but not identical. However, from the data presented more likely structures are proposed. The synthesis of certain other geologically-occurring 'irregular' acyclic isoprenoid alkanes is described.

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