Compound “J” in Late Cretaceous/Tertiary terrigenous oils revisited: Structure elucidation of a rearranged oleanane coeluting on GC with 18β(H)-oleanane

Abstract

A C30 pentacyclic triterpane eluting slightly after 18α(H)-oleanane in the m/z 191 mass chromatograms of Late Cretaceous/Tertiary terrigenous oils (peak “J” in the early literature) has been isolated from a Niger Delta oil and identified using NMR spectroscopy as 3β-methyl-24-nor-18α(H)-oleanane. The previous assignment as 18β(H)-oleanane is therefore partly erroneous. 3β-Methyl-24-nor-18α(H)-oleanane affords a larger m/z 412→356 response than the oleananes and the relative contribution of 3β-methyl-24-nor-18α(H)-oleanane to the 412→191 “oleanane peak” can be roughly estimated from comparison of the 412→356/412→191 ratio from the oleanane peak with that of the pure compounds. 3β-Methyl-24-nor-18α(H)-oleanane can be as abundant as 18α(H)-oleanane in oils having a high concentration of early eluting rearranged oleananes. 3β-Methyl-24-nor-19α(H)-taraxastane was also tentatively assigned in the oils on the basis of its mass spectrum as well as its gas chromatography and high performance liquid chromatography retention times. 3β-Methyl-24-nor-gammacerane was tentatively assigned in a similar way in an oil containing gammacerane. All 3β-methyl-24-nor-triterpanes could be formed via dehydration, rearrangement and hydrogenation of triterpenoids having an OH group at C-3.