Identification of a Fossil Sterane Biomarker in Crude Oil – an Androstane with a Modified Carbon Skeleton

Abstract

Three constitutional isomers of androstane were prepared for comparison with three unknown fossil C19H32 organic biomarkers (“19A”, “19B”, and “19C” in elution order) in geological samples from Oman. 3β‐Methyl‐A‐nor‐androstane was prepared in six steps (8 % overall yield) from testosterone. The key steps of this sequence were an Eschenmoser fragmentation and recyclization of the A ring. A mixture of four stereoisomers of 3‐methyl‐A‐nor‐androstane (4 % overall yield) was prepared by ionic hydrogenation of the olefin after A‐ring recyclization in three steps. 17‐Methyl‐18‐nor‐androstane was synthesized in four steps as a mixture of isomers (58 % overall yield) from dihydrotestosterone with a Wagner–Meerwein shift of the 13β‐methyl group to C‐17 as the key step. Pure 17β‐ and 17α‐methyl‐18‐nor‐13α‐androstane (4 and 2 % overall yield, respectively) were obtained in three additional steps after α‐oxidation of the Wagner–Meerwein rearrangement product and subsequent reduction. The synthesis of 18‐nor‐D‐homo‐13β‐androstane (12 % yield over seven steps from dihydrotestosterone) involved oxidative cleavage of the C–C double bond in the Wagner–Meerwein rearrangement product from the previous synthesis and subsequent recyclization of the D ring followed by reduction. The relative configurations of all final products were confirmed by X‐ray crystallography. A comparison of the synthetic standards with the saturated hydrocarbon fraction of an Oman crude oil by gas chromatography (GC–MS) coinjection on three different GC columns and comparison of mass spectra revealed that unknown compound 19C is 18‐nor‐D‐homo‐13β,14α‐androstane, whereas the isomeric 3‐methyl‐A‐nor‐androstanes and 17‐methyl‐18‐nor‐androstanes elute close to 19A and 19B, respectively, but do not match the unknowns.

A Geomimetic Approach to the Formation and Identification of Fossil Sterane Biomarkers in Crude Oil: 18‐nor‐D‐homo‐Androstane and 5α,14β‐Androstane

“…The D ‐ homo ‐constitution of 19 a / 19 b was furthermore proven by the following derivatization: Hydrogenation of this mixture (after acidic pre‐treatment) yielded three components with m / z : 260 consisting of the saturated hydrocarbons 20 , 2 a , and 2 b (ratio 81, 16, and 3 % from GC). The structures of compounds 2 a and 2 b were unambiguously confirmed by comparison (GC coinjection) with authentic standards previously prepared in our laboratory 4. As mentioned in the introduction, compound 2 a had already been identified to be component “19C” in natural crude oil.…”

“…The structureso fc ompounds 2a and 2b were unambiguously confirmed by comparison( GC coinjection)w ith authentic standardsp reviously prepared in our laboratory. [4] As mentioned in the introduction, compound 2a had already been identified to be component "19C" in naturalc rude oil. Furthermore, GC coinjection experiments confirmed by that hydrocarbon 20 was identicalt o crude oil component "19B" (Figure 1), which is actually the most important key finding of this manuscript.…”

“…In extensives ynthetic studies, [3] we were recently able to identify 5a,13b,14a-18-nor-d-homo-androstane (2a,S cheme 1) as the hitherto unknown sterane "19C" by unequivocal GC coelutione xperiments. [4] Conceivably,e nlargement of the D-ring may have occurred by Wagner-Meerwein rearrangemento f as pecies 1 with al eaving group Xa tC -18,w hichc ould be either ap rotonated alcoholo rah alogen atom activated by aL ewis acid. [5] These precursors could have been formed by marine organismsi na erobic, saline environments by oxidative hydroxylation or halogenation of the C-18 methyl group and are well-known from laboratory synthesis.…”

“…In extensive synthetic studies,3 we were recently able to identify 5α,13β,14α‐18‐ nor ‐ D ‐ homo ‐androstane ( 2 a , Scheme ) as the hitherto unknown sterane “19C” by unequivocal GC co‐elution experiments 4. Conceivably, enlargement of the D‐ring may have occurred by Wagner–Meerwein rearrangement of a species 1 with a leaving group X at C‐18, which could be either a protonated alcohol or a halogen atom activated by a Lewis acid 5.…”

Precambrian Organic Matter

“…The most abundant fossil steroids in these rocks, in particular, can be attributed to the Neoproterozoic rise to prominence of eukaryotic phytoplankton (Knoll et al 2007). However, other steranes that are rare in, or absent from, younger sedimentary sequences were also detected (Bender et al 2013(Bender et al , 2015McCaffrey et al 1994), some of which have been attributed to an origin from sponges (Brocks et al 2016;Love et al 2009).…”