Clay and acid catalysed isomerisation and cyclisation reactions of highly branched isoprenoid (HBI) alkenes: Implications for sedimentary reactions and distributions

Several highly branched isoprenoid alkenes isolated from the diatom Haslea ostrearia have been subjected to laboratory simulations of early diagenetic reactions under mild acid conditions including addition of K-10 Montmorillonite clay. The products of these reactions have been found to be highly dependent on the degree of unsaturation of the alkenes. For dienes, double bond migration and geometric isomerisation are observed as the only reactions. In contrast, trienes undergo both isomerisation and rapid cyclisation reactions to yield substituted cyclohexenes. Molecular mechanics calculations have been performed to complement the laboratory experiments and to assist in proposing reaction mechanisms. The significance of these findings for the interpretation of the numerous reports of highly branched isoprenoids in Recent and ancient sediments is discussed. Copyright (C) 2000 Elsevier Science Ltd.