Stereochemistry, elution order and molecular modeling of four diaergostanes in petroleum

Gas chromatography–mass spectrometry–mass spectrometry (GC–MS–MS) reveals four C28 13β,17α(H)-diasterane (diaergostane) peaks in petroleum from clay-rich source rocks, although only two diasterane peaks are commonly observed for each of the C26, C27, and C29 diasteranes. Co-injection of two laboratory synthesized diaergostane standards having 5β(CH3),13β(H),14β(CH3),17α(H)-20S,24R and -20R,24R stereochemistry allowed assignment of the elution order for the four diaergostane peaks as follows: 20S,24R; 20S,24S; 20R,24S; 20R,24R. Partial least-squares regression analysis of 11 quantum chemical descriptors (e.g., molar volume, polarizability, torsion angles and surface area) determined by molecular modeling of the C27–C30 diasteranes, can be used to construct a model that validates the relative GC retention times of the stereoisomers of these compounds. Similar calculated Gibbs free energies for the lowest-energy conformers of the modeled C28 20S,24R versus 20S,24S and 20R,24S versus 20R,24R diasteranes explain why they occur in similar abundance.