Synthesis of Biomarkers in Fossil Fuels: C-23 and C-24 Diastereomers of (20R)-4,17β,23,24-Tetramethyl-18,19-dinorcholesta-1,3,5,7,9,11,13-heptaene

A synthesis of the C-23 and C-24 diastereomers of triaromatic dinosteroids useful as maturation biomarkers for marine-sourced petroleum was developed. Conversion of stigmasterol to (20R,22E,24S)-4-methyl-19-norstigmasta-1,3,5(10),22-tetraene, ozonolysis of the stigmasterol sidechain, sodium borohydride reduction, and sequential treatment with methanesulfonyl chloride and sodium iodide furnished (20S)-20-(iodomethyl)-4-methyl-19-norpregna-1,3,5(10)-triene. The alkylation of this iodide with methyl 3,4-dimethyl-2-pentenoate provided the C-23R and C-23S diastereomers of methyl (20R,23ζ)-4-methyl-24-methylene-19-norcholesta-1,3,5(10)-triene-23-carboxylate, reduction with lithium aluminum hydride, and separation gave (20R,23R)- and (20R,23S)-23-(hydroxymethyl)-4-methyl-24-methylene-19-norcholesta-1,3,5(10)-triene. The further reduction of the 24-methylene and the hydroxymethyl group in (20R,23R)-23-(hydroxymethyl)-4-methyl-24-methylene-19-norcholesta-1,3,5(10)-triene provided (20R,23R,24R)- and (20R,23R,24S)-4,23,24-trimethyl-19-norcholesta-1,3,5(10)-triene. In the same fashion, the reduction of the 24-methylene and the hydroxymethyl group in (20R,23S)-23-(hydroxymethyl)-4-methyl-24-methylene-19-norcholesta-1,3,5-(10)-triene provided (20R,23S,24R)- and (20R,23S,24S)-4,23,24-trimethyl-19-norcholesta-1,3,5(10)- triene. A chloranil oxidation of each of these monoaromatic diastereomers to (20R)-4,17β,23,24-tetramethyl-18,19-dinorcholesta-1,3,5,7,9,11,13,15-octaene and a catalytic hydrogenation furnished the C-23 and C-24 diastereomers of the triaromatic biomarker, (20R)-4,17β,23,24-tetramethyl-18,-19-dinorcholesta-1,3,5,7,9,11,13-heptaene.