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

Abstract

AbstractThree 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.