Novel aspects of the diagenesis of Δ7-5α-sterenes as revealed by a combined molecular mechanics calculations and laboratory simulations approach

Laboratory experiments concerning the acid catalyzed isomerization of 5α-cholest-7-ene have provided information on the diagenetic routes for this compound (Peakman and Maxwell, 1988b). Repeating these experiments with varying concentrations of trifluoracetic acid yielded more detailed information on the initial and final transformation products. Unidentified compounds with short GC retention times appear in the cholestene mixture, making up about 50% of the mixture in the latest stage of the experiment. The experimentally observed concentration profiles of the identified compounds during the course of the reaction are closely simulated using a new calculation method for the isomerization kinetics. This calculation method uses the stabilities of the cholestene isomers and the proposed intermediates in the isomerization reactions (cholestane tertiary carbocations) to calculate the reaction rate. These stabilities were obtained from molecular mechanics calculations. The good agreement between the calculated and the experimental concentration profiles, along with GC and mass spectral data of the late-stage unknown compounds in the experiment, enabled them to be tentatively identified as a yet unrecognized class of rearranged cholestene isomers, 15-alkyl-5α-androstenes.