oa Confirmation Of ∆2-Sterenes as Precursors of 3-Methylsteranes

Δ2-sterenes have long been postulated to represent the precursors of 3β-alkylsteranes, although this relationship has remained speculative in the absence of proven intermediates. The observed prevalence of C5 and C6 alkyl moieties has also prompted the suggestion that 3β-alkylsteranes may derive from precursors with a sugar substituent at C-3. Recognition of the presence of both Δ2-sterenes and methylated Δ2-sterenes in a Cenomanian-Turonian sediment sequence deposited during OAE2 provides convincing evidence that validate a potential pathway for formation of 3-methylsteranes. The similarity in the proportions of corresponding homologs (C27, C28, and C29 versus C28, C29, and C30) for both series of sterenes supports their assignment as precursor and product. In addition, the carbon number distributions of the methylated Δ2-sterenes are wholly distinct from those of co-occurring 4-methylsteroidal hydrocarbons refuting the possibility that the former are 4-methylster-2-enes. The relative retention times of the methylated Δ2-sterenes are uniform among the three series and consistent with expectations for methylation at C-3, which was confirmed by the presence of 3-methylsteranes after hydrogenation.