Steroid carbon skeletons with unusually branched C-3 alkyl side chains in sulphur-rich sediments

摘要

A novel series of thiophenes with C-3 alkylated steroid carbon skeletons has been identified in sediments of the Miocene Monterey Formation (California, USA) and in the Turonian Tarfaya basin (Morocco). Their carbon skeletons are unusual in the sense that the alkyl side-chains at C-3 are almost exclusively isopentyl, 3-methylpentyl, and 2,3-dimethylbutyl moieties whilst n-alkyl (pentyl or hexyl) moieties are almost absent. Although they occur as thiophenes, the number of carbon atoms in the C-3 alkyl side chain and their carbon isotopic compositions point towards an origin from carbohydrates for the C-3 alkyl side chain. However, the branched structures of the C-3 alkyl side chains points to a different biosynthetic pathway, possibly starting from the addition of isopentylpyrophosphate to sterols.