Sterols: Tritium-labeling and selective oxidations

Evidence is presented which shows that in the preparation of radiolabeled cholesterol and other Δ⁵-sterols via the technique of tritium exchange-labeling of keto steroids and their subsequent conversion to sterol, ³H is introduced primarily into positions 2 and 6 ([2,2,4.6-³H]-cholest-5-en-3β-ol) rather than exclusively at 2 and 4 as the literature claims. The stoppered-vial method using sodium methoxide versus column using basic alumina was the preferred method of labeling. It gave labeled keto steroids of reproducible specific activity indicative of the number of exchangeable protons. Because the initial oxidation step is critical to the preparation of ³H-sterols especially on the small scale, four common oxidants: pyridinium chlorochromate (Corey reagent), silver carbonate on Celite (Fetizon reagent), aluminum isopropoxide (modified Oppenhauer reagent) and chromic acid (Jones reagent) were compared using as starting materials, cholesterol, cholest-8-en-3β-ol, ergosterol, 24-methylenecholesterol, 24(25)-dihydrolanosterol, lanosterol, sitosterol and stigmasterol. The effectiveness of the oxidant was markedly influenced by the molecular features of the sterol.