Identification of a fossil sterane biomarker in crude oil - An androstane with a modified carbon skeleton

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. New old fossil: The so far unknown constitution of a fossil sterane biomarker in crude oil from Oman was elucidated to be 18-nor-D-homo-androstane by comparison with a synthetic reference compound.