Stereospecific synthesis of Δ24‐sterols labelled on the 26‐ or 27‐methyl group

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Abstract
A convenient synthesis of Δ24‐sterols labelled on the 26‐ or 27‐methyl group, exemplified by the synthesis of [26‐3H]‐lanosterol, is described. Oxidation of lanosteryl acetate with SeO2 gives 3β‐acetoxylanosta‐8, 24‐dien‐26‐al; this is reduced to the corresponding labelled alcohol with KB3H4 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.3H‐stereospecific synthesis.