Stereospecific synthesis of Δ 24 ‐sterols labelled on the 26‐ or 27‐methyl group

A convenient synthesis of Δ 24 ‐sterols labelled on the 26‐ or 27‐methyl group, exemplified by the synthesis of [26‐ 3 H]‐lanosterol, is described. Oxidation of lanosteryl acetate with SeO 2 gives 3β‐acetoxylanosta‐8, 24‐dien‐26‐al; this is reduced to the corresponding labelled alcohol with KB 3 H 4 and the obtained hydroxymethyl group is reduced to methyl group by hydrogenolysis of its sulfate monoester. 26‐Hydroxylanosterol can be transformed into 27‐hydroxy lanosterol by E→Z olefin inversion, so allowing the synthesis of lanosterol stereospecifically labelled at C‐27. Desmosteryl acetate analogously affords the corresponding 26‐aldehyde, which is easily reduced to the 26‐alcohol. 3 H‐stereospecific synthesis.