Sedimentary 24-n-Propylcholestanes, Molecular Fossils Diagnostic of Marine Algae

A SERIES OF [C.sub.30]-STERANE EPIMERS containing 11 C atoms in the side chain has been empirically established as a widespread fossil marker for marine organic input to sedimentary rock and petroleum [1-3]. This concept has challenged recently [4], and some possible [C.sup.30]-sterane precursors have been tentatively identified in lacustrine sediments [5, 6]. In order to evaluate the origins of these steranes and identify them, we have studied a [C.sup.30]-sterane sourced from marine shale present in a Prudhoe Bay petroleum [2].

This [C.sub.30]-sterane (12 ppm in the Prudhoe Bay oil) was isolated by a lengthy procedure. The oil was fractionally distilled under vacuum on a spinning band column to provide a narrow-boiling fraction containing the [C.sup.30]-steranes. The fraction ws separated by preparative silica gel chromatography, and the saturate fraction was deparaffinated with 5A molecular sieves. Key steps were the removal of certain terpanes (that is, hopanes) with 13X molecular sieves [7] and the removal of sterane epimers by chromatography on alumine [8]. These two steps removed the bulk of the polycyclic biomarkers with physical properties similar to those of the target sterane. Repeated, nonaqueous reversedphase, high-perforance liquid chromatography (HPLC) provided 0.6 mg of 65% pure material. This sample was suitable for comparisons by nuclear magnetic resonance (NMR) with the synthetic materials because all contaminating components were minor constituents, each composing less than 5% of this mixture.

As the standards, we prepared the 5[beta] and 5[alpha] epimers of 24-n-propylcholestane (structures 1bA and 1aA in Fig. 1) and 24-isopropylcholestane (structures 1bD and 1aD) by partial synthesis (Fig. 2) as described [9-12]. Comparisons of the synthetic standards with the isolated [C.sub.30]-sterane by gas chromatography-mass spectrometry (GCMS), [sup.13.C] NMR, and H NMR indicated that it is (24R + 24S)-24-n-propylcholestane (1aA). Its mass spectrum (Fig. 3) is identical to that of synthetic 1aA and shows the expected D-ring cleavage having a mass-to-charge ratio (m/z) 217 and B-ring cleavage (m/z 304) ions characteristic of a sterane with a [C.sub.11.H.sub.23] side chain. The [sup.13.C] NMR spectrum of the isolated material was of sufficient strength to show that it is identical with 1aA. Both contained a peak at 14.6 ppm diagnostic for the presence of the n-propyl group [predicted 14.4 ppm on the basis of Lindeman-Adams parameters [13]]. Finally, a fingerprint comparison of 300-MHz H NRM spectra in the methyl resonance region confirmed the structure assignment (Fig. 4).

Steranes are typically found in thermally mature sediments and petroleums as mixtures of stereoisomers at the C-5, 14, 17, 20, and 24 positions [12, 14]. In order to confirm that the [C.sub.30]-steranes (Fig. 5A) belong to the 24-n-propylcholestane series, we prepared an isomerizate by treatment of 24-n-propyl-5[beta]-cholestane (1bA) with Pd/C at 260[degrees]C for 68 hours [15]. This procedure produced a petroleum-like distribution of sterane epimers (Fig. 5C) nearly identical to and coeluting with the peaks in the m/z = 414 to 217 fragmentogra of the oil....