Alwarsamy Jeganathan

Sedimentary 12-n-Propylcholestanes, Molecular Fossils Diagnostic of Marine Algae

Certain C30-steranes have been used for identifying sedimentary rocks and crude oils derived from organic matter deposited in marine environments. Analysis of a C30-sterane from Prudhoe Bay oil indicates that these C30-steranes are 24-n-propylcholestanes that apparently are derived from precursor sterols 24-n-propylidene-cholesterols and 24-n-propylcholesterol. These widely occurring sterols are biochemically synthesized in modern oceans by members of an order (Sarcinochrysidales) of chrysophyte algae. These data thus imply that C30-sterane biomarkers in sedimentary rocks and crude oils have a marine origin. Screening of a few organic-rich sedimentary rocks and oils from throughout the Phanerozoic suggests that these C30-steranes first appeared and, therefore, their source algae evolved between Early Ordovician and Devonian.

Paleoenvironmental implications of novel C30 steranes in Precambrian to Cenozoic Age petroleum and bitumen

Petroleums and bitumens from Early Proterozoic (≈ 1800 Ma) to Miocene (≈ 15 Ma) age marine strata contain 24-isopropylcholestanes, a novel group of C30 steroids. The abundance of these compounds, relative to 24-n-propylcholestanes, varies with source rock age. Late Proterozoic (Vendian) and Early Cambrian oils and/or bitumens from Siberia, the Urals, Oman, Australia, and India have a high ratio of 24-isopropylcholestanes to 24-n-propylcholestanes (≥1), while younger and older samples have a lower ratio (≤0.4). Temporal changes in this parameter may reflect the relative abundance of certain Porifera (sponges) and certain marine algae through time. Geochemical indicators such as this, which can constrain the source rock age of a migrated oil, are useful in source rock identification during petroleum exploration.

Analysis and occurrence of C26-steranes in petroleum and source rocks

Abstract The C26-steranes previously reported in oils and source rocks (MOLDOWAN et al., 1985) have been identified as 21-, 24-, and 27-norcholestanes (1A, 1B, and 1C). Various 24-norcholesterols or stanols, possible precursors for the 24-norcholestanes, occur widely at low levels in marine invertebrates and some algae, and 24-norcholestanes occur in marine petroleums of Tertiary through Paleozoic age. There are reports of 27-norcholesterols and stanols in recent sediments, but the precursor organisms have not been identified. The natural occurrence of the 21-norcholestane structure is unprecedented. Unlike 24- and 27-norcholestane, 21-norcholestane is in low concentration or absent in immature rocks and increases substantially relative to the other C26-steranes in thermally mature rocks, oils, and condensates. This suggests an origin involving thermal degradation of a higher molecular weight steroid. The ratio of 21-norcholestane to the total C26-steranes is shown to be an effective maturity parameter in a series of Wyoming (Phosphoria source) and California (Monterey source) oils. Molecular mechanics MM2 steric energy calculations indicate a relative stability order of 21 ⪢ 27 > 24- norcholestane for the major stereoisomers. Authentic 21-, 24-, and 27-nor-5α-cholestanes and 24- and 27-nor-5β-cholestanes were synthesized and subjected to catalytic isomerization over Pd/C to yield the full suite of stereoisomers for each. In immature rocks the 5α,14α,17α(H),20R isomers predominate. Mixtures of 24- and 27-norcholestane in oils and mature rocks show a familiar elution pattern of four major peaks presumed to be 14α,17α(H),20S, 14β,17β(H),20R, 14β,17β(H),20S, and 14α,17α(H),20R in order of elution, as well as putative 20S and 20R rearranged steranes [13β,17α(H)-diasteranes]. However, the 21-norcholestanes are represented by a single peak in mature sediments under normal GC conditions, which is shown to consist of a major 14β,17α(H) peak and a minor 14α,17α(H) peak using an extended injection-temperature-hold-time gas Chromatographic technique.

1-Vinyl-4-methyl-2,6,7-trioxabicyclo[2.2.2]octanes: unsaturated homoenolate anion equivalents

Abstract The metalation of (E)-1-(2-chlorovinyl)- or (E)-1-(2-bromovinyl)-4-methyl-2,6,7-trioxabicyclo[2.2.2]octane with tert-butyllithium produced an unsaturated homoenolate anion equivalent which intercepted alkyl halides, aldehydes, ketones, and lactones to afford β-alkylated or β-acylated orthoesters, which, in turn, furnished β-substituted acrylate esters.

Manganese Triacetate Oxidation of 3β, 3aβ, 6-Trimethyl-3a, 7aβ-dihydro-2(3H), 5(4H)-benzofurandione

Abstract The manganese triacetate oxidation of hindered α1-positions in a model enone, 3β, 3aβ, 6-trimethyl-3a, 7aβ-dihydro-2(3H), 5(4H)-benzofurandione, provided a procedure for the introduction of the α′-hydroxyenone functionality characteristic of the A ring of certain quassinoids.