The Diels–Alder adduct between ergosterol acetate and 4-phenyl-1,2,4-triazolin-3,5-dione can be efficiently re-converted into ergosterol by lithium aluminium hydride reduction; using this reaction as a key step, ergosta-5,7,22,24(28)-tetraen-3β-ol has been synthesised in labelled form and shown to be an efficient precursor of ergosterol in yeast.