Stereochemical studies of acyclic isoprenoid compounds—II The role of chlorophyll in the derivation of isoprenoid-type acids in a lacustrine sediment

The stereochemistries of a series (I)–(IX) of isprenoid-type acids in the lacustrine Green River Shale (Eocene) have been examined. The absolute configurations of five of the acids present in the free-fatty acid fraction have been determined by high resolution gas Chromatographic analysis of the methyl and (?)-menthyl esters and are compatible with an origin from the phytyl moiety of chlorophyll. The results for two other isoprenoid-type acids, 5,9,13-trimethyltetradecanoic acid (IV) and 6,10,14-trimethylpentadecanoic acid (VI), do not at present preclude a chlorophyll derivation. The 4,8,12,16-tetramethylheptadecanoic (VIII) and 5,9,13,17-tetramethyloctadecanoic (IX) acids each comprise two or more stereoisomers. The stereoisomeric composition of the 2,6,10,14-tetramethylpentadecanoic acid (VI) indicates that it is unlikely that 2,6,10,14-tetramethylpentadecane, pristane, is an intermediate in the geochemical pathways presumed to lead from phytol to this acid.