REARRANGED HOPANES IN SEDIMENTS AND PETROLEUM

Two new rearranged hopanoid hydrocarbons have been isolated from a Prudhoe Bay crude, Alaska. 17-alpha(H)-15-alpha-methyl-27-norhopane was determined by X-ray crystallography. It is the first identified member of a new series of rearranged hopanes we propose to call "17-alpha(H)-diahopanes." Analysis by gas chromatography-mass spectrometry-mass spectrometry (GC-MS-MS) of the parents of m/z 191 in several crudes suggests that this compound is a member of a C29-C34 series of 17-alpha(H)-diahopanes common to many crude oils and sediments. In addition, a new member of the 18-alpha(H)-neohopane series has also been elucidated. Determination of 18-alpha(H)-17-alpha-methyl-28,30-dinorhopane [18-alpha(H)-30-norneohopane], which we propose to nickname "C29Ts," hinged upon advanced nuclear magnetic resonance (NMR) techniques (at 500 and 600 MHz) such as proton-detected H-1-C-13 correlated spectra for the C-skeleton and Rotating-frame Overhauser Enhancement Spectroscopy (ROESY) for stereochemistry, as well as several other two-dimensional (2D) NMR techniques. This compound is the second known pseudohomolog of the neohopane series (together with 18-alpha(H),22,29,30-trisnorneohopane, Ts), but the existence of additional pseudohomologs is still not clear. The structures of these rearranged hopanes are consistent with an origin by catalytic rearrangement from hopenes during early diagenesis. Carbon isotopic data collected on Ts, 17-alpha(H)-diahopane, C29Ts, 17-alpha(H)-22,29,30-trisnorhopane (Tm), 17-alpha(H)-30-norhopane, and 17-alpha(H)-hopane isolated from the Prudhoe Bay oil are in the -27 to -28 parts per thousand delta-C-13 range supporting mechanistic arguments based on structures that all are derived from common precursors. These delta-C-13 values are slightly more positive than the whole Prudhoe Bay oil (-30.1 parts per thousand), suggesting that these hopanes may have been derived from heterotrophic or cyanobacteria in the paleoecosystem during deposition of its source rock. Molecular mechanics calculations predict relative thermal stabilities in the order 17-alpha(H)-diahopanes > 18-alpha(H)-neohopanes > 17-alpha(H)-hopanes, suggesting new maturity parameters that may be useful into the late oil window.