The Mechanism of Introduction of Alkyl Groups at Carbon 24 of Sterols

Seeds of the stone pine, Pinus pinea, have been shown to contain 24ξ-Δ5-stigmasten-3β-ol as well as smaller amounts of 24ξ-Δ5-ergosten-3β-ol and trans-Δ5,24(28)-stigmastadien-3β-ol. Germination of the seeds in the presence of 2-14C-mevalonate led to labeling of the steroids in a high yield. Isolation and reincubation of the labeled 24-ethylidene compound, trans-Δ5,24(28)-stigmastadien-3β-ol, produced the labeled 24-ethyl derivative, 24ξ-Δ5-stigmasten-3β-ol. The 24-substituted C1 analogue of the latter, 24ξ-Δ5-ergosten-3β-ol, was obtained labeled when the substrate contained the 24-methylene group (synthetic 28-14C-Δ5,24(28)-ergostadien-3β-ol). From germination in the presence of the 28-14C-Δ5,24(28)-ergostadien-3β-ol the 24-ethylidene derivative, 28-14C-trans-Δ5,24(28)-stigmastadien-3β-ol, was also formed. The results demonstrate that a second transfer of a C1 group to the Δ24(28) bond occurs with the formation of a 24-ethylidene group in the biosynthesis of 24-substituted C2 steroids. They also show that the Δ24(28) bond can undergo reduction to give the saturated C-24(28) bond in both the 24-substituted C1 and 24-substituted C2 cases.